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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-04-30.

Permanent Link: http://ufdc.ufl.edu/UFE0041444/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-04-30.
Physical Description: Book
Language: english
Creator: Chen, Ke-Hung
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by Ke-Hung Chen.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Ren, Fan.
Electronic Access: INACCESSIBLE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041444:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041444/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2012-04-30.
Physical Description: Book
Language: english
Creator: Chen, Ke-Hung
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by Ke-Hung Chen.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Ren, Fan.
Electronic Access: INACCESSIBLE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041444:00001


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1 BACKSIDE FABRICATION, SENSOR APPLICATION AND RELIABILITY STUDY OF COMPOUND SEMICONDUCTOR TRANSISTORS By KE-HUNG CHEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORID A IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Ke-Hung Chen

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3 To my family, who alwa ys love and support me

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4 ACKNOWLEDGMENTS Foremost, I would like to express my sincere gratitude to my advisor Professor Fan Ren for the continuous support of my PhD study and research, for his immense knowledge, world first class research environment and enthusiasm. He supplies research resource much better than any gr oup I know in Taiwan. His guidance helped me in all the time of resear ch and writing of this thesis. I also thank my committee members, Professor Jenshan Lin, Professor Kirk Zi egler and Professor Stephen Pearton for their contribution to my propos al presentation and disse rtation defense. I thank Dr. Brent Gila, Mrs. Wen-Hsing Wu, Dr. Byoung Sam Kang, Dr. Travis Anderson, Dr. Lii-Cherng Leu, Dr. Yu-Lin Wang, Byung-Hwan Chu, Erica Douglas, Chien-Fong Lo and Sheng-Chung Hung for giving me lots of suggestions and assisting to my research. Finally, and most important ly, I express my thanks and gratitude to my parents and brothers in Taiwan, who alwa ys give me warm love and support me. I would like to thank my dear wife, I-Fen Wang, who has shared my ups and downs for past ten years. She is my best friend and I love her forever.

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5 TABLE OF CONTENTS page ACKNOWLEDG MENTS .................................................................................................. 4 LIST OF TABLES............................................................................................................ 7 LIST OF FIGURES .......................................................................................................... 8 LIST OF ABBR EVIATIONS ........................................................................................... 11 ABSTRACT................................................................................................................... 16 CHA PTER 1 LITERATURE REVIEW AND MOTI VATION ........................................................... 19 1.1 InAlAs/InGaAs MHEM Ts Degradati on Study .................................................... 19 1.2 AlGaN/GaN High Electr on Mobility Transistor .................................................. 23 1.3 Field-Effect Transistor Ba sed Semiconduc tor Sensors ..................................... 26 1.4 Laser Technol ogy ............................................................................................. 28 1.5 Laser Technology Fo r Glass Appl ication .......................................................... 31 1.6 Study Outline .................................................................................................... 33 2 DEGRADATION OF 150 NM MUSHR OOM GATE INALAS/INGAAS MHEMTS DURING DC STRESSING A ND THERMAL STORAGE ......................................... 38 2.1 Background ....................................................................................................... 38 2.2 Exper iment ........................................................................................................ 39 2.3 Results And Discu ssion .................................................................................... 40 3 C-ERBB-2 SENSING USING ALGAN /GAN HIGH ELECTRON MOBILITY TRANSISTORS FOR BREAST CANCER DETECTION ......................................... 55 3.1 Background ....................................................................................................... 55 3.2 c-erbB-2 Antigen Detection Usi ng AlGaN/GaN Hig h Electron Mobility Transis tors........................................................................................................... 56 4 LOW HG (II) ION CONCENTRATION ELECTRICAL DETECTION WITH ALGAN/GAN HIGH ELECTRON MOBILITY TR ANSISTORS ................................ 63 4.1 Background ....................................................................................................... 63 4.2 Hg (II) Metal Ion Detection Us ing AlGaN/ GaN High Electron Mobility Transis tors........................................................................................................... 64 5 CU-PLATED THROUGH-WAFER VIAS FOR ALGAN/GAN HIGH ELECTRON MOBILITY TRANSI STORS ON SI .......................................................................... 71

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6 5.1 Background ....................................................................................................... 71 5.2 Exper iment ........................................................................................................ 73 5.3 Results And Discu ssion .................................................................................... 74 6 UV EXCIMER LASER DRILLED HIGH ASPECT RATIO SUBMICRON VIA HOLE ...................................................................................................................... 81 6.1 Background ....................................................................................................... 81 6.2 Exper iment ........................................................................................................ 81 6.3 Results And Discu ssion .................................................................................... 82 7 193 NM EXCIMER LASER DRILLING OF GLASS SLICES: DEPENDENCE OF DRILLING RATE A ND VIA HOLE SHAP E ON THE DIAMETER OF THE VIA HOLE...................................................................................................................... 89 7.1 Background ....................................................................................................... 89 7.2 Exper iment ........................................................................................................ 90 7.3 Results And Discu ssion .................................................................................... 91 8 SUMMARY AND FU TURE WORK ....................................................................... 104 LIST OF RE FERENSCES ........................................................................................... 107 BIOGRAPHICAL SKETCH .......................................................................................... 118

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7 LIST OF TABLES Table page 1-1 Comparison of laser technologies showing performance par ameter ranges of ex cimer lasers versus solid stat e laser .............................................................. 34

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8 LIST OF FIGURES Figure page 1-1 Schematic drawing of t he crystal structure o f Wurt zite Ga-face and N-face GaN.................................................................................................................... 34 1-2 Bandgaps of the most important elemental and bi nary cubic semiconductors versus their lattice constant at 300K .................................................................. 35 1-3 Band diagram of normal AlGaN/GaN heteros tructure. ....................................... 36 1-4 Different laser pr ocess techniques ................................................................... 36 1-5 (Top left) A schematic of a flat trench created by using a square mask. (Top right) A schematic of an inclined grov e created by using a diamond shape mask. (Bottom left) A schematic of a rounded trench created by using a circular mask. (Bottom right) A schem atic of a stepped grove created by using a cr oss mask ............................................................................................ 37 2-1 Cross-section schematic of InAlAs/InGaAs MH EMT. ......................................... 44 2-2 Top view of OM image for InAlAs /InGaAs MHEMT device with 2 fingers of 75um gate width (top) and TLM pattern (bottom). .............................................. 45 2-3 The drain current density (IDS) and gate voltage (VGS) of virgin MHEMT as function of drain voltage (VDS) (top) and microwave characteristics of the InAlAs/InGaAs MHEMT device at room temperatur e (bottom)........................... 46 2-4 The drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after thermal stress at 250 oC for 36hrs as function of drain voltage (VDS) (top) and the drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after DC stress at 2.7 V for 36 hrs (bottom).......................................... 47 2-5 Data from a TLM pattern stored at 250 C for 48 hours (top) resistance v.s. gap distance (middle) sheet resistance (bottom) specific contact resistivity. ...... 48 2-6 Data from a TLM pattern on a MHEM T stressed with a DC stress of 27 mA at 165 C (top) resistance v.s. gap distance (middle) sh eet resistance (bottom) specific contact resistivity................................................................................... 49 2-7 Low magnification cross-section view TEM image of InAlAs/InGaAs MHEMT after stored at 250 C for 48 hours (top) Low magnification cross-section view TEM image of InAlAs/InGaAs MHEMT after DC stress for 36hrs (b ottom)......... 50 2-8 The higher magnification TEM images of Ohmic contact at the edge of source and drain contact (top). The higher magnification TEM images of ohmic contact at the edge of the contact (bo ttom).............................................. 51

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9 2-9 The gate current of InAlAs/InGaAs MHEMT device for unstressed, thermal stress (250 oC, 36 hours) and DC stress (VDS 2.7 V, IDS 167 mA/mm, 36 hours)................................................................................................................. 52 2-10 Low magnification cross-section vi ew TEM image (top) and the higher magnification TEM image (bottom) of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm fo r 36 hr ............................................................................... 53 2-11 EDS elemental analysis of the mushroom gate after 165 oC, VDS 3V, JDS 300 mA/mm for 36 hr................................................................................................. 54 3-1 (Top) Plan view photomicrograph of a co mpl eted device with a 5-nm Au film in the gate region. (Bottom) Schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with c-erbB -2 antibody/antigen on thioglycolic acid..................................................................................................................... 59 3-2 I-V characteristics of AlGaN/GaN HE MT sensor before and after exp osure to 0.25 g/ml c-erbB -2 ant igen............................................................................... 60 3-3 Drain current of an AlGaN/GaN HEMT over time for c-erbB-2 antigen from 0.25 g/ml to 17 g/ml ........................................................................................ 61 3-4 Change of drain current versus different concentrations from 0.25 g/ml to 17 g/ml of c-er bB-2 ant igen................................................................................... 62 4-1 Plain view photomicrograph of a completed device with a 5 nm Au film in the gate region (top). A schematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with thio glycolic acid (bottom )...................................... 68 4-2 Photographs of contact angle of water drop on the su rface of bare Au (left) and thioglycolic acid f unctionalized Au (right). .................................................... 69 4-3 Time dependence of the drain current for a HEMT sens or exposed to different concentrations of Hg2+ ion so luti on....................................................... 69 4-4 The difference of drain current for the HEMT sensor exposed to different Hg2+ ion concentration to the DI water................................................................ 70 5-1 Schematic of via in AlGaN/GaN HEMT on Si wa fer. ........................................... 77 5-2 Cross-sectional SEM of dry etch ed via in AlGaN/GaN HEMT on Si wafer......... 77 5-3 Cross-sectional SEMs of dielectric/m etal stack on field (top) or sidewall (center) and Cu/Ti/SiO2 stack at high magn ification (bottom)............................. 78 5-4 Schematic of plat ing sequence for Cu. ............................................................... 79

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10 5-5 Cross-sectional SEM of Cu-plated via after mechanical polishing. The diameter of the openings of the vi as is 50 m.................................................... 79 5-6 Defect type as a func ti on of seed aging ti me...................................................... 80 5-7 Optical plan view micrograph of fr ont-side of plated via wafer showing HEMTs and cont act p ads................................................................................... 80 6-1 (Top) Cross sectional micrographic im age of a via hole d rilled through a Si substrate with a diameter of 90 and 45 m for the entrance and exit hole. (bottom) Top view SEM images of a 5 m diameter via hole drilled on a Si substrate with two different drilling ti mes; 30 sec for the image on the left and 40 sec for the image on the right........................................................................ 86 6-2 (Top) Side view of a series of 160 m diameter via holes drilled on the edge of a glass with different numbers of r epetitive laser puls es. (Bottom) Side view of a series of 80 m diameter via holes drilled on the edge of a glass substrate with different numbers of repetitive laser pulses................................. 87 6-3 A micrographic side view image of via holes drille d from both sides of the glass substrate. The diameters of the entrance holes are 80 and 10 m for the top and bottom via hol e, respectively........................................................... 88 7-1 Schematic of the exci mer laser d r illing syst em................................................... 96 7-2 Drilling rate of the glass as a function of the via hole diameter ........................... 97 7-3 Side view images of drilled holes wit h the dia meters of the entrance holes being 120, 80, 40, and 5 m respecti vely........................................................... 98 7-4 (Top) Time dependent drilling rate of the 120 m diameter via hole. (Bottom) Side view images of the 120 m diameter via holes dri lled for differ ent times.... 99 7-5 (Top) Time dependent drilling rate of the 80 m diameter via hole. (Bottom) Side view images of the 80 m diameter via holes drilled for diffe rent times.... 100 7-6 (Top) Time dependent drilling rate of the 40 m diameter via hole. (Bottom) Side view images of the 40 m diameter via holes drilled for diffe rent times.... 101 7-7 (Top) Time dependent drilling rate of the 5 m diameter via hole. (Bottom) Side view images of the 5 m diameter via holes dril led for diffe rent times...... 102 7-8 Cross-sectional SEM of glass slips wit h 5 m entrance diameter drilled with 2 min drilling time. ................................................................................................ 103

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11 LIST OF ABBREVIATIONS 2DEG two dimensional electron gas AAS atomic absorption spectroscopy AES auger electron spectroscopy Al aluminium Al2O3 aluminium oxide AlGaAs aluminium gallium arsenide AlGaN aluminium gallium nitride AlInAs aluminium indium Arsenide AlN aluminium nitride Ar argon ArF argon fluoride Au gold AuGe gold Germanium BCB benzocyclobutene CaF2 calcium fluoride CCD charge coupled device CCTV closed-circuit television Cl2 chlorine CMP chemical mechanical planarization CNT carbon nanotube CNTFET carbon nanotube field effect transistor CTE coefficient of thermal expansion Cu copper DC direct current

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12 DI de-ionized DNA deoxyribonucleic acid DPSS diode pumped solid state DRIE deep Reactive Ion Etching EDS energy-dispersive X-ray spectroscopy ELISA enzyme-linked immunsorbent assay EP electroplating F2 fluorine FET field effect transistor FIB focused ion beam FLICE femtosecond laser irradiatio n followed by chemical etching fmax maximum frequency of oscillation fT unity current gain fs femtosecond FWHM full width at half maximum Ga gallium GaAs gallium arsenide GaN gallium nitride gm max maximum transconductance H2 hydrogen HAZ heat-affected zone HCl hydrochloric acid He helium HEMTs high electronic mobility transistors Hg mercury

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13 HgCl2 mercury chloride HTOL high temperature operating life test ICP inductively coupled plasma ICP-MS inductively coupled plasma-mass spectroscopy ICs integrated circuits Ids source-drain current IGFET insulated-gate field effect transistor IGs source-gate current In indium InAlAs indium aluminum arsenide InGaAs indium gallium arsenide InP indium phosphide I-V current-voltage IR Infrared radiation ISE ion selective electrode ISFET ion-sensitive field effect transistor I-V current-voltage KCl potassium chloride KOH potassium hydroxide KrF krypton fluoride KSP solubility product LD laser diodes LED light-emitting diodes LIBWE laser induced backside wet etching LOD limit of detection

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14 MBE molecular beam epitaxy MgF2 magnesium fluoride MHEMT metamorphic high elec tronic mobility transistor MMICs microwave monolithic integrated circuits Mo molybdenum MOCVD metal organic chemical vapor deposition MODFET modulation doped FET MOSFET metal oxide semiconduct or field effect transistor MTTF mean team to failure N2 nitrogen Nd neodymium Ne neon NH4OH ammonium hydroxide Ni nickel ns nanosecond PBS phosphate-buffered saline PECVD plasma-Enhanced Chemical Vapor Deposition PMMA polymethyl methacrylate ppb parts per billion ppm parts per million ps picosecond Pt platinum PVD physical vapor deposition Rc contact resistance Rd drain resistance

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15 RF radio frequency Rs sheet resistance or source resistance Rt transfer resistance RTA rapid thermal annealing SDHT selectively doped heterojunction transistor SEM scanning electron micrograph Si silicon SiC silicon carbide SiO2 silicon oxide SiNx silicon nitride Ta2O5 tantalum pentoxide TE thermionic emission TEGFET two-dimensional electron gas FET TEM transmission electron microscopy Ti titanium TLM transmission line measurement UV ultraviolet Vds source-drain voltage VUV vacuum ultraviolet WSix tungsten silicide XeCl xenon chloride XeF xenon fluoride XPS X-ray photoelectron spectroscopy YAG yttrium aluminium garnet

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16 Abstract of Dissertation Pr esented to the Graduate School of the University of Fl orida in Partial Fulf illment of the Requirements for t he Degree of Doctor of Philosophy BACKSIDE FABRICATION, SENSOR APPLICATION AND RELIABILITY STUDY OF COMPOUND SEMICONDUCTOR TRANSISTORS By Ke-Hung Chen May 2010 Chair: Fan Ren Major: Chemical Engineering Reliability studies of InAlAs/InGaAs metamo rphic high electron mobility transistors (MHEMTs) grown on GaAs substrates fo r high frequency/power applications are reported. The MHEMTs were stressed at a drain voltage of 3 V for 36 hrs, as well as undergoing a thermal storage test at 250 C for 48 hrs. The drain current density of the MHEMTs at zero gate bias dr opped about 12.5 % after either the thermal storage test or DC stress. The gate leakage cu rrent of the MHET devices with thermal storage was much higher than that of devices after DC stress. In the latter case, significant gate sinking was observed by transmission electron microscopy. The main degradation mechanism during thermal storage was r eaction of the Ohmic contact with the underlying semiconductor. AlGaN/GaN high electron mobility transis tors (HEMTs) were used to detect cerbB-2 antigen, an important bi omarker for breast cancer early detection. The Au gated region of the HEMT was functionalized with thioglycolic acid and then used to immobilize the c-erbB-2 antibodies The source-drain current (Ids) showed a clear dependence on the cerbB-2 antigen concentrat ion in phosphate-buffered saline (PBS)

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17 solution. The limit of detection (LOD) was 0.25 g/ml, which is less than the c-erbB-2 antigen concentration present in both healthy people and people with breast cancer. This approach showed the promise of early stage breast cancer screening and preclinical disease diagnosis by rapid, noninvasive and portable electronic biological sensors based on AlGaN/GaN HEMT technology. Thioglycolic acid functionalized Au-gat ed AlGaN/GaN based HEMTs were also used to detect mercury (II) ions. The sour ce-drain current of the HEMT sensors monotonically decreased with the mercury (II) ion concentration from 1.5 8 to 4 8 M. The source-drain current reac hed equilibrium around 15 sec after the concentrated Hg ion solution was added to the gate region of the HEMT sensors. The effectiveness of the thioglycolic acid func tionalization was evaluated with a surface contact angle study. The results suggested that portable, fast response, and wirelessbased heavy metal ion detectors can be realized with AlGaN/GaN HEMT-based sensors. The Cu filled backside via holes were used to improve the electrical performance and improve heat dissipation of the AlGaN/GaN HEMTs. The 70 m deep through-wafer backside via holes with a diameter of 50 m were etched by deep Si reactive ion etching system on the backside of AlGaN/GaN HEMTs. The pulsed Cu electroplating process was used to fill up the etched vias with Cu. Mechanical polishing was then performed to planarize the Cu layer. This approach is attractive for increasing the effective thermal conductivity of the composite substrate for high power device applications. The effect of the via hole diameter on the laser drilling ra te of glass as well as the

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18 shape of the drilled via holes were investigat ed. The via holes with a diameter of 120 m showed a 7.5angled, tapered side wall, and the drilling rate was relatively constant at around 17 m/s. For the smaller via holes with diameters ranging from 30 to 80 m, significantly different results were obtained due to laser reflection from the tapered side wall of the via hole leading to t he drilling rate being slightly increased and via hole becoming conical in shape. For the smallest via holes, with an entrance diameter of 10 m, the drilling resulted in a very hi gh aspect ratio, funnel shaped via hole with a significantly reduced drilling rate.

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19 CHAPTER 1 LITERATURE REVIEW AND MOTIVATION 1.1 InAlAs/InGaAs MHEM Ts Degradation Study Silicon (Si) based devices dominate electronics devices and are extensively used in the integrated circuits (ICs) and data storage industry based on several important factors: excellent dielectrics (SiO2 and SiNx), good mobility, trans port properties, low surface recombination velocity (~10 cm/s), low cost, mechanical hardness, good stability, and huge experience base [1]. However, the III-V semiconductors attract at tention due to their superior properties compared to Si in some aspects. Gallium Ar senide (GaAs) has higher electron mobility and is a direct bandgap material that can be used in photonics. Wide energy bandgap Aluminium Gallium Arsenide (AlXGa1-XAs) is lattice matched with GaAs and can be grown on GaAs to form HEMT structures. HEMT is also called modulation doped field effect transistor (MODFET), two-dimensional elec tron gas FET (TEGFET) and selectively doped heterojunction transistor (SDHT). HEMT structure is similar with metal oxide semiconductor FET (MOSFET) and both have structure including gate, source, drain and substrate. The lattice mismatch existing in the heterojunction material will cause traps and greatly reduce device performance. An extr emely thin material with optimal lattice constant can be used to allow larger bandgap difference for heterostructure devices, which is called a pseudomorphic HEMT (p HEMT). Metamorphic HEMT (MHEMT), with a buffer layer between different lattice constant materials, is an advanced HEMT structure compared to pHEMT. InAlAs bu ffer layer has a graded in dium concentration so that it can match the lattice constant of both the GaAs substrate and the InGaAs

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20 channel layer using the MHEMT approach. The metamorphic buffer layer is accommodated for the lattice mismatch bet ween the GaAs substrate and the HEMT layers. The larger conduction band discontinuity between the InAlAs and InGaAs layers allows a more effective charge transfer in to the channel layer and improves carrier confinement [1-10]. InAlAs/InGaAs/InP MHEMTs have numerous performance advantages over the more commonly used GaAs pHEMTs due to the high velocity and carrier density. However, the size of InP substrates is limited for 4 inch and InP substrates are very brittle. Therefore, GaAs substrate has bec ome a much more promising substrate for InAlAs/InGaAs MHEMTs due to numerous advant ages: wafers up to 6-inch, lower cost, less fragility/brittleness, more mature backside pr ocessing, the ability to tailor the lattice constant by varying the indium content and compatibility with smaller via holes for compact chip size [1-3, 5, 8, 10-14]. The good stability is crucial for InAlAs/InGaAs MHEMTs to compete with other technology, so its reliability is extensively studied. The failure criterion for MHEMTs is usually defined as a 10% degradation of maximum transconductance (gm max) or a 20% reduction of the maximum source-drain current (Ids). Normally, high temperature operating life test (HTOL), DC biased accele rated stress test, and environmental test are used to investigate degradation mechanisms. HTOL is to store device in high temperature ambient (up to 250 oC) without bias. DC stress was bias the device at fixed source-drain voltage (VDS, 1~3V) at different temperatures (150~250 oC) to get the median time to failure (MTTF) information. MTTF can be obtained at a specified channel

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21 temperature (e.g. 125 oC) by performing a least-square linear fit of median life time versus inverse channel temperature [3-9, 11-13]. The main degradation mechanisms include gate sinking, ohmic contact degradation, hot electron induc ed degradation and buffer crystalline defects. The gate sinking is defined as the diffusion of gate metal into the underlyi ng semiconductor. This decreases the distance between the metalsemiconductor interface and the active channel. Different gate metal schemes hav e already been employed for InAlAs/InGaAs MHEMTs including Ti/Au, Ti/Pt/Au, Pt/Ti/Pt/Auetc. Pt/Ti/Pt/Au gate performs better than Ti/Pt/Au gate in the following aspects: longer life time, less gm max degradation, less drain resistance increase, less source resi stance increase, and less threshold voltage positive shift [3-7, 9-10, 13-14]. Ohmic contact degradation is caused by thermally activated metal-metal and metal-semiconductor interdiffusion during an nealing and causes contact resistance, source resistance and drain resistance to in crease. Various Ohmic metal schemes have already been reported including AuGe/Ni, Au Ge/Pt, AuGe/Ni/Ti/Pt/Au, AuGe/Pt/Ti/Pt/Au, AuGe/Ni/Au, Ti/AuGe/Au, and non-alloyed Oh mic contacts (i.e. Mo/Au). The Ohmic contact degradation was caused by the diffusi on of Au through the Ti and Pt barriers. By using WSix as an Ohmic contact metal, the di ffusion of In into the contact is suppressed and Ohmic contact will get better stabilit y. The non-annealed Ohmic-recess approach was also used to demonstrate better reli ability performance. The Ohmic contact degradation is main degradation mechanism for HTOL since the pinch-off voltage (Vto) is unchanged after thermal storage. The pinchoff voltage shifted more positively under DC bias stress than HTOL [5-7, 9-10, 12-14].

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22 Hot electron degradation is caused by the electric field between the gate and drain and accelerates the carriers in the channel. The high electric field can supply hot electrons enough energy to overcome the conduc tion band discontinuity. The hot carrier degradation will cause the drain resistance increase faster t han the source resistance and decrease source-drain current, and maxi mum transconductance. The shorter gate length and the increase of the indium content will induce hot electron degradation due to the higher electrical field and smaller channel bandgap. The smaller gate to channel distance or the smaller recess length will lead to a higher electrical field on the drain side and cause the enhancement MHEMTs degr ades faster than depletion MHEMTs. The electron-hole pair is separated by the electric field, trapping the holes close to the gate contact and leading to a negative shift of the threshold voltage. The surface traps in the SiNx passivation layer between gate and drain will degrade power gain and increase drain resistance [4-7, 9, 11-12]. Ambient hydrogen degrades InP and GaAs MHEMTs with Ti/Pt/Au as the gate electrode. The Pt gate metal layers can split hydrogen (H2) into hydrogen atom. Ti inside the gate metal stack forms Ti-H with hydrogen atom and causes an expansion of Ti, which then results in stress applied to the semiconductor under the gate metal. The total gate metalsemiconductor interface potent ial drifts as more hydrogen is absorbed and alters the HEMT pinch off voltage over time. Hydrogen has little impact on the maximum drain current or transconductance [8]. MHEMTs with BCB passivation s howed lower degradation than ones with SiNx passivation. The increase in the parasitic capacitances and intr insic capacitances induced in the passivation process was smaller in BCB compared to in SiNx because of

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23 BCBs lower dielectric constant. The trappi ng effect of induced surface states and degradation of the RF performance are al so smaller with BCB passivation [10]. Currently, the InAlAs/InGaAs MHEMTs r equire a burn-in step to improve the device stability and reduce the device perfo rmance deterioration during usage. By investigating the degradation beh avior of InAlAs/InGaAs MEHT and identifying its failure mechanisms, we can further improve device performance and reduce it s fabrication cost. 1.2 AlGaN/GaN High Electron Mobility Transistor GaN can have either a Wurtzite or Zincbl ende crystal structure, though it normally has a Wurtzite crystal structure wi th a hex agonal Bravis lattice and four atoms per unit cell (lattice constant a0=3.189 c0=5.185 and u=0.376). For Ga -face structure or Gapolarity structure, the crystallization direction follows [ 0001] direction and the top layer is Ga. In other words, Ga on t he top position of the {0001} bilayer corresponds to the [0001] polarity. If the top laye r is an N atom layer, then t he structure is called N-face structure or N-polarity struct ure as shown in Figure 1-1. AlGaN/GaN heterostructures grown on AlGaN nucleation layer by MOCVD are always found to have a Ga face. GaN (bandgap 3.4 eV) is a wide bandgap material and can form ternary or quaternary compound with InN (bandgap 0.65 eV) and AlN ( bandgap 6.2 eV) as shown in Figure 12. GaN is a direct bandgap material and its bandgap can be adjusted to cover visible light range making it an ideal candidate for light-emitting diode (LED) and laser diode (LD) material [16-20]. AlGaAs/ GaAs and AlGaN/ GaN are most commonly used materials for HEMT. When two semiconductor material contact wit h each other, the free electrons will diffuse from the wide bandgap of AlGaN to the narrow bandgap of GaN near the interface. The band will bend due to the fixed Fermi energy and will form a quantum well close to the

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24 interface on the narrow energy band side. A potential barrier confines the electrons in a triangular shaped quantum well and form 2DEG as shown in Figure 1-3. The carriers are limited in the potential barrier and are only allowed to move in two dimension spaces instead of three dimensions. The bandgap difference between AlGaN and GaN form two dimensional electron gas (2DEG) in the heterojunction interface without intentional doping. The mobility will increas e since the electron can move quickly without colliding with an y impurities or dopant ions [16-18, 21-26]. The piezoelectric polarization will be i nduced by the strain due to the lattice mismatch between AlGaN and GaN and is more than five times larger as compared to AlGaAs/GaAs structures. The piezoelectric polari zation increases with increase of strain. The bonds between group III elements (Al, Ga and In) and group V element (N) are not only covalent but also ionic, so spontaneous pol arization (polarizati on at zero strain) will also be induced. The total polarization is the sum of piezoelectric polarization and spontaneous polarization. The total polariz ation will increase under tensile strain and decrease under compressive strain. The s heet carrier concentration of 2DEG will increase with the increase of polarizati on. The sheet carrier concentration for AlGaN/GaN structure can be as high as 1013 cm-2 and is much higher than AlGaAs/GaAs structure, also making Al GaN/GaN HEMTs more promising for high power applications. Besides electron mob ility, GaN devices have the following advantages compared to Si based device: high breakdown voltage, high switching frequency, low power losses, high output power density, high operating voltage, and high input impedance [1618, 20-22, 24, 27].

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25 GaN is a material with outstanding mechanical properties and chemically stability, making it extremely suitable for operation in chemically harsh environments. The high saturation velocity and high temperature (i.e. 500 C) operating characteristics allow GaN to be used in high speed, high pow er, high frequency and high temperature applications [1-2, 19-20]. Present DC and RF performance re cords for GaN transistor are fT 190 GHz, fmax 241 GHz, maximum current handling 30 A, ma ximum drain current density 1.6 A/mm, peak transconductance 424 mS/mm, blocking volt age 8,300 V, breakdown electric field strength 6 MV/cm, breakdown voltage 1,650V, Gain 22 dB at 26 GHz, output power 110 W at 60 V, power density 40 W/mm at 4 GHz, and can deliver power in the millimeterwave range (60, 76 and 94 GHz) [19, 24, 27, 28]. Several methods can be used to boost GaN transistor performance. Catalytic CVD process is used to form the SiNx passivati on layer because it does not damage the GaN surface. The gate length is reduced to 30 nm, which increases the HEMTs speed. Multiple fingers will reduce gate resistance and a T-shaped gate decreases gate-todrain capacitance. Double het erojunction structure can mi tigate short channel effects and result in better substrate isolation. Higher aluminum content in the channel layers will produce higher breakdown voltages. A field plat e can redistribute the electric field in the gate-drain region, reduc e the peak electric field strength and increase the breakdown voltage. Highly doped cap layers c an be added to the epi structure to reduce source resistance. The non-alloyed Ohmic contacts can allow the reduction of the gatedrain spacing, thus further lowe ring the access resistance [24, 27].

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26 Even though AlGaN/GaN HEMTs can be used in high temperature conditions, the poor heat dissipation by substrate (i.e. s apphire) reduces HEMTs device performance. The high thermal conductivity Cu formed in the etched vias can pr ovide low-inductance grounding and improve heat transfer characte ristics for AlGaN/GaN HEMTs used in high power device applications. 1.3 Field-Effect Transistor Based Semiconductor Sensors Field-effect transistor (FET) type sensor s have made great progress recently in metal ion detection and biomaterial detection. T he electric field controls the flow of source drain current (source to drain) by affecting the conductive channel and applied gate voltage. The main concept of the FET based semi conductor sensors is to correlate the target concentration with the change induced by electric field. The ideal FET b ased sensors should be very sensitive to the change caused by the analyte of interest at or nearby the gate surface with molecular receptor s or ion-selective membranes in a short time. The analyte of interest will induce the chemical or electrical change at the gate surface and modulate the current in the c hannel of the FET. FET based sensors can directly translate the analyte-surface interact ion into a readable signal, without the need for elaborate optical components. The insulated-gate field-effe ct transistor (IGFET) has a structure similar to a MOSFET and the gate is electrically isol ated from the source and drain. The ionsensitive FET (ISFET) is similar to IGFET, bu t in the ISFET, the metal gate is replaced by ion-selective membrane, electrolyt e and reference electrode. The ISFET is extensively used as pH sensor or biosensors with SiO2, Si3N4, Al2O3, Ta2O5 and other dielectric as its gate. The various biomolecules such as DNA, proteins, enzymes, and

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27 cells can be monitored and detected by numerous ways including nucleic acid hybridizations, protein-prot ein interactions, antigen-ant ibody binding, and enzymesubstrate reactions [29-32]. Carbon nanotubes (CNTs), 1D-conductive polymer nanomaterials and nanowires, are used as conductive channels in FETs, which are notable candidates for highly sensitive label-free biosensors owing to t heir unique geometries wit h a high surface-tovolume ratio. CNTFETs are expected to have a high sensitivity for biomolecules detection because the proteins are much larger than the diameter of the CNT channels (1-2 nm). CNTFET can also be used as me rcury ions detection sensors with the detection limit of 10 nM [33-37]. AlGaN/GaN HEMTs can be operated at high power, elevated temperature conditions. AlGaN/GaN system is more radiation-hard th an the conventional AlGaAs/GaAs heterostructure due to the higher displacement energies in the nitrides. AlGaN/GaN HEMTs not only have the above-mentioned advantages but also have the following characteristics: low power consumpti on, low quantity of sensing materials, fast response, long lifetime, portable size and te chnology, and wireless feasibility. These advantages make AlGaN/GaN HEMTs a relatively ideal candidate option as sensors [1, 2, 38]. AlGaN/GaN HEMTs can be used to detect breast cancer bio-marker. The sourcedrain current can be transformed to the conc entration of the breast cancer biomarker. So AlGaN/GaN HEMTs sensor can be used as screening method to decide whether it is necessary to do further diagnoses by checki ng the biomarker concentration. Compared

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28 to the traditional method (mammography), HEMT biosensors can reduce cost and prevent exposure of invasive radiation to patients. Not only in biosensing field, AlGaN/GaN HEMTs can also be used to detect metal ion concentration. The mercury (II) ion is toxic and can cause serious problems for the ecosystem as well as to humans. So how to detect mercury (II) ion concentration is important topic. Based on sim ilar idea, the source-drain cu rrent can be transformed to mercury (II) ion concentrati on. So AlGaN/GaN HEMTs can be used as mercury (II) ion detection sensor and bring multiple advantages compared to traditional spectroscopic or electrochemical measurement methods. Spectroscopic methods include atomic absorption spectroscopy, Auger-electron spec troscopy, and inductively coupled plasma mass spectrometry. Electrochemical met hods include ion selective electrodes and polarography. All of these methods exhi bit the same disadvantages which are expensive, time consuming and not pr actical for real-time detection. 1.4 Laser Technology Laser processing is emerging as an alter native to conventional micro-machining methods, such as wet etching and dry etch ing, due to several adv antages. It has more flexibility to produce feature sizes down to mi cron scale as well as the ability to produce various shapes. It has the ability to selectively remove material by adjusting the applied fluence between two materials threshold fluenc e. Unlike photolithography processes, it does not require a flat s ubstrate. Laser processing allows one to skip several processing steps including spin-coating, developing, and stripping compared to lithography and etching. It does not require t he use of vacuum equipment compared to X-ray, electron and ion beam process, and it is a contactless process [39-42].

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29 Excimer lasers and diode pumped solid stat e (DPSS) lasers are the most common lasers used in semiconductor field applicat ions. They have different wavelengths and can produce intense ultraviolet light pulses. This allows di fferent materials including ceramics, semiconductors and polymers, to be processed for a specific application need. Solid state lasers exhibit good beam quality, produce very high repetition rates and have short pulses down to the picosecond (ps) or femtosecond (fs) range [43, 44]. Excimer lasers have minimal heat-affect ed zone (HAZ) and surface contamination because the ablation mechanism is a cold proc ess. While DPSS lasers typically rely on heating to ablate the materi al, the excimer lasers hav e high peak power and small interaction volume resulting in high-energy material ablation with little heat transfer to the surrounding material. Excimer lasers produce a much larger beam than solid state lasers, allowing for relatively large processing area. Excimer lasers generate intense and short ultraviolet pulses directly wit hout the complex converting schemes while DPSS requires a series of nonlinear optics cr ystals to alter the wavelength. The power output for excimer lasers is higher than DP SS power output because the power output will substantially drop for the wavelength conversion for DPSS. Excimer lasers have much smaller beam divergence and significant capability to initiate photochemical reactions. Excimer lasers provide better repr oducibility of results, as well as higher precision and quality [40-41, 43-46]. The excimer lasers gain medium is composed of inert and reactive gases. The gases include helium, neon, krypton and fl uorine (for 157, 193 and 248 nm systems) and xenon (for 308 and 351 nm systems). A dimer molecule is produced by an electrically stimulated reacti on and produces light in the ultraviolet range. The main

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30 performance characteristics of the excimer and solid state lasers are summarized in Table 1-1 [41, 47]. Over the years, four drilling proce sses have been developed, including singleshort, percussion, trepanning and helical drilling as shown in Figure 1-4 [48]. The laser systems can drill shapes such as circles, squares, rectangles, lines and 3D complex patterns. The lines or patterns with a desir ed curvature are fabr icated by employing specially designed masks and stage movement. Ideally, the drilled depth is proportional to the laser drilled irradiation time. For example, if one burst pulse will etch 1 unit of material, then the trench edge depth would be 1 unit and the trench center depth would be D unit by using the D unit diameter ci rcle mask. So, even when using the same square mask, the drilled angle will cause diffe rent shapes of trenches. If the drilled direction is 45 degree then the drilled trenc h would be similar to that drilled by a diamond mask with 0 degree drilled direction. The cross-sect ion view for a square mask with 0 degree drilled direction is rectangular in shape while a 45 degree drilled direction with the same square mask will result in a triangular shape. Figure 1-5 shows four different shapes of grooves/ trenches creat ed by using specially designed masks. A stepped groove can be created by using a cro ss mask. When the sample stage moves in the direction along the center line of the cr oss in the mask, the center groove receives a higher dose of the laser light as compar ed to the areas under the two wings of the cross. Therefore the drill ed depth of the c enter groove will be deeper and a stepped groove can be produced. Similarly, by usi ng a diamond shape mask, an inclined trench can be created. By adjusting the lengths of the two side of t he diamond shape mask,

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31 different slopes of the inclined trenches c an be achieved. The curvature of the rounded trenches can also be changed with an elliptical mask. 1.5 Laser Technology For Glass Application Various las ers including excimer lase rs (157 nm, 193 nm, 248 nm, 308 nm) [46, 49-64], DPSS lasers (355 nm, 1064 nm) [49, 65], and Ti:Sapphire laser (800 nm) [42, 66-68] were used to drill holes, micro vias, c hannels and 3D patterns to investigate the ablation rate, morphology, and impact of fluence, repetition rate, irradiation time, and pulse width on drilling behavior. F2 laser induces strong absorption and defines the smallest features (~100 nm) of any laser process. The rectangular channels with near-vertical walls and flat bottoms, V shape channel and smooth near-vertical walls with flat bottom can be formed with suitable fluence [49-53]. ArF laser produces good qualit y of entrance holes, rectangul ar cavity, the drilled structures with high aspect ratios, smooth and steep sidewalls, and high ablation rates which are better than those drill ed with KrF laser. KrF laser with ns and fs pulse widths were used to study the morphol ogy of drilled holes [53-59]. Laser fluence is a very impor tant parameter in laser micro-machining of glass. In bulk glass, the drilling will spontaneously stop at a certain depth if the laser fluence drops below a specific limit. A drill ed-through hole can be made once the fluence exceeds a certain minimum value. The smaller holes require higher fluence in order to drill through the same thickness of glass. The increase of laser fluence will cause a rougher surface due to higher reactivity of the drilled material. The effect of the laser fluence on the drilling rate is relatively small once the fluence above some point. The excess of energy will be absor bed by ejected material and result in stronger laser

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32 plasma. The entrance diameter of a drilled hole is weakly dependent on the incident fluence. The increase of irradiation time will resu lt in an increase in the size of the exit hole until it reaches a saturated size. The in crease of irradiation time will also generate more debris and recast due to a denser plas ma cloud. The plasma cloud will absorb part of beam energy, reduce the ablation rate and cause melting and vaporization around the drilled area. Compared to 1064 nm IR laser, the 355 nm UV laser with higher fluences will generate lo nger cracks in the drilled channel. At fixed fluence, UV light drills faster than IR li ght and the drilling threshold is al so lower at 355 nm compared with 1064 nm [46, 53, 54, 59, 63-65]. Laser induced backside wet etching (L IBWE) normally use KrF, XeCl and XeF laser to etch UV-transparent materials incl uding fused silica, quartz, Pyrex, sapphire, CaF2 and MgF2. The idea of LIBWE is based on the deposition of laser energy on the sample surface during the ablation of an absorption solution. LIBWE has few pre-/ posttreatments for a target substr ate, large area irradiation, much lower required fluence, very small roughness, and no debris and cracks com pared to directly dr illing method [51, 53, 58, 66-70]. Unlike the absorption solution used in LIBWE, the water in contact with the rear surface of the gla ss can be used to remove debris from the hole and reduce the effects of redeposition and blocking. The high aspect ratio drilled hole can be achieved by focusing on the rear surf ace of the sample [71-73]. The fs Laser Irradiation followed by C hemical Etching (FLICE) technique can be used to form high aspect ratio cylindric al microchannels with moderate fluence. The focus is located hundred m below the surface of the gl ass during the drilling [74, 75].

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33 The laser drilled structure can be used for various applications (e.g. microfluidic devices, microfilters, nanopores, electrical th rough-connections, biophotonic chip and so on). A 193nm wavelength light is well coupled with glass and makes it possible to study how to drill a high aspect ratio of the small via holes as well as study the drilling rate and shape of the via holes as a function of the ent rance diameter of the via hole on glass, which has not yet been repor ted by another group. 1.6 Study Outline The main objective of this research is to focu s on two sections: III-V semiconductor electronic device (Chapter 2, 3, 4 and 5) and excime r laser application study (Chapter 6, and 7). Chapter 2 illu strates the degradation mechanisms of InAlAs/InGaAs MHEMTs devices. Chapter 3 describes the details of fabrication and sensitivity measurements of the breast cancer biomarker, c-erbB-2 antigen, using AlGaN/GaN HEMTs with c-erbB-2 antibody coated gates. Chapter 4 presents detection of mercury ions using AlGaN/GaN HEMTs through a surface coating of carboxyl groups on the gate region. This is an extension of the previous work done by Dr. Hung-Ta Wang. Chapter 5 shows how to form and planar ize Cu in through vias in AlGaN/GaN HEMT devices by electroplating and mechan ical polishing. Chapte r 6 demonstrates the ability to drill high aspect ratio vias in glass drilled by ArF excimer laser. Chapter 7 studies the effect of the via hol e diameter on the laser drilling rate of glass as well as the drilled shape. Chapter 8 summarizes abov e works and suggests future studies.

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34 Table 1-1. Comparison of laser technol ogies showing performance parameter ranges of excimer lasers versus solid state laser [41, 44]. Laser Technologies Parameter Excimer Laser Photon Energy DPSS Laser Available wavelengths 157 nm 193 nm 248 nm 308 nm 351 nm 7.90 eV 6.42 eV 5.00 eV 4.66 eV 4.02 eV 3.53 eV 3.53 eV 2.33 eV 1.17 eV 266 nm 355 nm 532 nm 1064 nm Power Range Up to 540 W 60 W Energy Range Up to 1,100 mJ 0.3 mJ Repetition Rate Variable 1 to 600 Hz Variable 1 to 300 KHz Pulse Length, FWHM 10 to 20 ns fs to 100 ns Beam Profile Homogenous flat-top Near-Gaussian Shot-to-shot stability 0.5 to 1.0 %, rms 5 to 10 %, rms Max. Energy Density 200 J/cm2 2,500 J/cm2 Max. Peak Power Density 1,000 MW/cm2 200 GW/cm2 Figure 1-1. Schematic drawing of the crysta l structure of Wurtzite Ga-face and N-face GaN [16].

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35 Figure 1-2. Bandgaps of the most important elemental and binary cubic semiconductors versus their la ttice constant at 300K [76].

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36 Figure 1-3. Band diagram of norma l AlGaN/GaN heterostructure [77]. Figure 1-4. Different laser process techniques [48].

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37 Figure 1-5. (Top left) A schem atic of a flat trench created by using a square mask. (Top right) A schematic of an inclined grov e created by using a diamond shape mask. (Bottom left) A schematic of a rounded trench created by using a circular mask. (Bottom right) A schematic of a stepped grove created by using a cross mask [41].

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38 CHAPTER 2 DEGRADATION OF 150 NM MUSHROOM GATE INALAS/INGA AS MHEMTS DURING DC STRESSING AND THERMAL STORAGE 2.1 Background Reliability studies of both GaAs and In P-based HEMTs have identif ied numerous degradation mechanisms, includi ng contact problems (especia lly gate sinking), surface states that contribute to gat e lag, hot carrier-induced impact ionization at the gate edge or avalanche breakdown in the semiconductor, mechanical stress due to hydrogen absorption into Ti metallization, fluorine c ontamination, and corrosion (mainly related to Al oxidation) [3, 6, 8, 78-93]. MHEM T technology has been developed using metamorphic buffer layers to grow InAlAs/In GaAs on larger diameter GaAs substrates to overcome the limitations of InP substr ates as described in section 1.1. The commercial applications of MHEMTs are predominantly in low noise mm-wave amplifiers for radio communications, autom otive collision avoidance radar and high bitrate fiber systems [93] .The choice of w hether they can be used in place of InP-based HEMTs depends upon their DC/RF performanc e and the chip cost requirements. A number of studies have shown that MHEMTs can exhibit similar reliability to InP HEMTs, with over 106 hour mean-timeto-failure at 125 C [6, 8]. However, the InAlAs/InGaAs MHEMTs r equire a burn-in step to improve the device stability. Current device designs t end to suffer from device degradation, and a costly burn-in process is typically performed to make the device more stable and eliminate the early degr adation when the devices are placed in service The transistors are generally biased at certain gate and dr ain voltages for 24-60 hours before sending the devices to customers. During the burnin process, the drain current decreases and Ohmic contact resistance increases with time, and level off around 36 hours.

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39 Minimizing the burn-in time or eliminating t he burn-in step is highly desirable to reduce the device fabrication cost. In order to effe ctively identify the failure mechanisms, both a high temperature storage test and DC stress were used in this study. Besides MHEMT devices themselves, transmission line method (TLM) patterns were al so used to isolate the gate effect on the device degradation. 2.2 Experiment The MHEMTs were obtained from a commercial vendor. A schematic of the device structure used is shown in the cross-se ction of Figure 2-1. The MH EMTs were fabricated on lattice matched InGaAs/InAlAs HEMT structures. The epitaxial layer structures including InGaAs, InAlAs, InGaAs and InAlAs were grown on 6-inch semiinsulating GaAs wafer by molecular beam epi taxy (MBE) and used for cap layer, spacer layer, channel layer and buffer layer. The conductive InGaAs layer was used to form better Ohmic contact with Ohmic metal. Ho wever, the conductive InGaAs layer will influence the gate operation. So after form the Ohmic metal, the photo lithography process will be used to open the recess opening. The wet etching process will be used to etch off the InGaAs layer, this etching process is selectively etch InGaAs than InAlAs, which means that the InAlAs layer of the HEMT structure acts as an etch stop. The buffer layer is used to isolate defects from the substrate and create a smooth surface for following growth of the active layers of the transistor. The 150 nm gate length mushroom shaped Ti/Pt-based Scho ttky gate was formed with 1.2 m spacing between both gate/drain and gate/source. The final metal layers are formed and provided for interconnection. The passivation layer is SiNx formed by PECVD. Mesa etch is employed for device isolation. The two finger design with gate width 75 m is shown in the optical micrograph of Figure 2-2 (top). The top layer InGaAs/InAlAs does not

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40 undergo the gate recess process for TLM sample. The TLM patterns also present on the device chip employed 45 x 70 m pads with gaps of 3, 6, 9, 12 and 15 m, as shown in the optical micrograph at the bottom of Figure 2-2. Typical DC and RF characteristics of the MHEMTs prior to stressing are shown in Figure 2-3. The maximum drain-source current density was 270 mA/mm, with a gate current in the hundreds of nA ra nge. The unity current gain, fT, was 94GHz while the maximum frequency of oscillation, fmax, was 124 GHz. The devices were stressed in one of two ways. Some of the MHEMTs were biased at a source-drain voltage of 3 V for 36 hours at 165 oC. Other devices were given a thermal storage test in an oven at 250 C for 36 hours. The DC characteristics were measured before and after both kinds of st ressing using an Agilent 4156C parameter analyzer. Some of the devices were also ex amined by cross-sectional TEM to look for reactions of the contacts with the underly ing semiconductor. EDS elemental analysis was performed to obtain the elemental profiles near the reacted contacts. 2.3 Results And Discussion Figure 2-4 shows the drain current (IDS) of the InAlAs/InGaAs MHEMT after both thermal storage at 250 oC for 36 hrs as function of drain voltage (VDS) (top) and the drain current (IDS) of the InAlAs/InGaAs MHEMT a fter DC stress at 3 V for 36 hrs (bottom). Both forms of stre ss lead to a reduction in drain current density of ~12.5% compared to the unstressed devices (Figure 2-3). Thus, after the burn-in process, the MHEMTs suffer from higher parasitic resistance. To further investigate the origin of t he degradation in drain-source current, the sheet resistance and specific contact resistanc e of the devices were obtained from TLM data as a function of thermal storage time and a function of constant bias voltage stress

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41 time. Using the TLMs to examine the increas e of the parasitic re sistance isolated the effect of the gate sinking on the degradation of drain-source current from the effect of Ohmic metal contact degradation. As shown in Figure 2-5, the total resistance of TLMs increases significantly with time in the first 12 hours of thermal storage at 250 C, while the specific contact resistivity increases much more than sheet resistance and shows the contact between Ohmic metal and semico nductor dominated the degradation during the thermal storage. Similar data for the constant current stressed devices is shown in Figure 2-6. In this case, the sheet resi stance increased around 18%, while specific contact resistance was reduced by 40%. The device resistance increase was dominated by changes in the sheet resistanc e instead of contact resistance. TEM cross-sections of a degraded therma l storage HEMT and a constant current stressed HEMT are illustrated in Figure 2-7 (top) and Figure 27 (bottom), respectively. Both samples showed metal spikes with the Oh mic metal diffusing into epitaxial layer, which were formed during the high temperature Ohmic annealing. For the constant current stressed sample, the density of the spikes was higher around the edge of the source Ohmic contact pad and drain Ohmic cont act pads, as illustrated in the Figure 2-8 (top). Interestingly enough, t he region of the high density spikes in the TEM picture matched the estimated transfer length of the TL M measurement as shown in top plot in Figure 2-6. Thus the formation of the high density spikes can result from the current induced electro-migration. The drain current density of the commerc ial power MHEMTs fabricated with the structure described here is around 0.5 A/mm to 1 A/mm. In the fabr ication, the final metal often has a thickness of 4-6 m, while the Ohmic metal contact has a thickness of

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42 less than 0.3 m. In these devices, the Ohmic me tal is alloyed with the semiconductor and the resistance of the alloyed metal is also larger than the unalloyed metal stack. The 4-6 m thick final metal does not exhibit a problem with a current density of 0.5 A/mm to 1 A/mm. However, as shown by the TEM image of Figure 2-8, the metal thickness of the region at the edge of the Ohmic contact pad is too thin to sustain the current density to avoid electromigration. Fo r example, the Ohmic me tal thickness for in the device shown in Figure 28 is roughly 250 nm. During the burn-in process, 20 mA was used to stress a device with gate width of 75 m (20 mA/75 m = 266 mA/mm). Therefore, the cu rrent density is given as 20mA/ (75 m 0.25 m) = 1 105 A/cm2, which is the current density through the Oh mic pad. Such high current density of current flowing across the thin Ohmic metal then across the metal semiconductor interface into the semiconductor causes t he Ohmic metal diffusion during the burn-in process. This caused the electromigrationinduced voids and the formation of additional metal spikes at the edge of the Ohmic metal contact pads, as shown in Figure 2-8, the source contact pad (left) and drain contact pad (right) after performing the constant current stressed HEMT. The gate ideality factor of the unstressed HEMT extract ed from the gate currentvoltage (I-V) characteristics was ~1.6, indi cative of both recombination as well as thermionic emission electron transport me chanisms being present (Figure 2-9). The gate characteristics of the thermal and DC stressed HEMTs showed significant degradation and gate current increased several order in both forward and reverse bias conditions. Figure 2-10 (top) shows the lo w magnification cross-section view TEM image of a Pt/Ti/Pt/Au mu shroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

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43 A higher magnification TEM image is illustrat ed in Figure 2-10 (botto m); the white area in the gate is Ti layer and the dark area repr esents the Pt layer. Apparently, the bottom Pt of the Pt/Ti/Pt/Au mushr oom gate diffused into the InAl As gate contact layer. EDS elemental analysis was used to analyze the Pt diffusion depth the gate region. As shown in Figure 2-11, around 5-10 nm of Pt diffused into InAlAs layer.

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44 Figure 2-1. Cross-section schem atic of InAlAs/InGaAs MHEMT. InGaAs InAlAs Source Drain Gate InGaAs InAlAs Metamorphic buffer GaAs Substrate

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45 Figure 2-2. Top view of OM image for In AlAs/InGaAs MHEMT device with 2 fingers of 75um gate width (top) and TLM pattern (bottom). 75 m

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46 0.00.30.60.91.21.5 0 100 200 300 VG= 0 V Vstep= -0.1V IDS(mA/mm)VDS(V) 11 01 0 0 0 10 20 30 40 50 Gain (dB)Vds= 1.5 V, VG= 0 V fT= 94.1 GHz fMax= 124 GHzFrequency (GHz) Figure 2-3. The drain current density (IDS) and gate voltage (VGS) of virgin MHEMT as function of drain voltage (VDS) (top) and microwave characteristics of the InAlAs/InGaAs MHEMT device at room temperat ure (bottom).

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47 Figure 2-4. The drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after thermal stress at 250 oC for 36hrs as function of drain voltage (VDS) (top) and the drain current (IDS) and gate voltage (VG) of the InAlAs/InGaAs MHEMT after DC stress at 2.7 V for 36 hrs (bottom).

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48 -505101520 0 20 40 60 80 100 120 LT Resistance (ohm)Gap (m) before stress after 250oC 12hr after 250oC 24hr after 250oC 36hr after 250oC 48hr -1001020304050 0 50 100 150 200 250 300 sheet resistance (ohm/sq)Stress time at 250oC(hours) 01020304050 1E-7 1E-6 1E-5 1E-4 specific contact resistance(ohm-cm2)Stress time at 250oC(hours) Figure 2-5. Data from a TLM pattern stored at 250 C for 48 hours (top) resistance v.s. gap distance (middle) sheet resistance (bo ttom) specific contact resistivity.

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49 -505101520 0 20 40 60 80 100 120 LT Gap (m)Resistance (ohm) before stress after 165oC 12hr at 27mA after 165oC 24hr at 27mA after 165oC 36hr at 27mA after 1650C 48hr at 27mA -1001020304050 0 50 100 150 200 250 300 sheet resistance (ohm/sq)Stress time at 165oC with 27 mA (hours) 01020304050 1E-7 1E-6 1E-5 1E-4 specific contact resistance(ohm-cm2)Stress time at 165oC with 27 mA (hours) Figure 2-6. Data from a TLM pattern on a MHEMT stressed with a DC stress of 27 mA at 165 C (top) resistance v.s. gap dist ance (middle) sheet resistance (bottom) specific contact resistivity.

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50 Figure 2-7. Low magnification cross-se ction view TEM image of InAlAs/InGaAs MHEMT after stored at 250 C for 48 hours (top) Low magnification crosssection view TEM image of InAlAs/InGa As MHEMT after DC stress for 36hrs (bottom).

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51 Figure 2-8. The higher magnification TEM images of ohmic cont act at the edge of source and drain contact (top). T he higher magnification TEM images of Ohmic contact at the edge of the contact (bottom).

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52 -0.6-0.4-0.20.00.20.40.6 1E-12 1E-10 1E-8 1E-6 1E-4 0.01 1 100 Unstress DC stress Thermal stressVGS (V)-IGS (A)1E-12 1E-10 1E-8 1E-6 1E-4 0.01 1 100 IGS (A) Figure 2-9. The gate current of InAlAs/In GaAs MHEMT device for unstressed, thermal stress (250 oC, 36 hours) and DC stress (VDS 2.7 V, IDS 167 mA/mm, 36 hours).

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53 Figure 2-10. Low magnification cross-se ction view TEM image (top) and the higher magnification TEM image (bottom) of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

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54 0102030405060 0 5 10 15 20 Intensity (A.U)Distance (nm) % (Titanium ) % (Arsenic ) % (Platinum) % (Indium ) Figure 2-11. EDS elemental analysi s of the mushroom gate after 165 oC, VDS 3 V, JDS 300 mA/mm for 36 hr.

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55 CHAPTER 3 C-ERBB-2 SENSING USING ALGAN/ GAN HIGH ELECTRON MOBILITY TRANSIST ORS FOR BREAST CANCER DETECTION 3.1 Background Currently, the overwhelming majority of patients are screened for breast cancer by mammography. This procedure involves a high cost to the patient and is invasive (radiation), which limits the frequency of screening. Work by Michaelson et al [98] indicates a 96% survival rate if patient s can be screened every three months. Thus, mortality in breast cancer patients c an be reduced by increasing the frequency of screening. However, this is not presently f easible due to the lack of cheap and reliable technologies that can noninv asively screen breast cancer. There is recent evidence to suggest that salivary testing for biomarkers of breast cancer may be used in conjunction with mammography [99-108] Saliva-based diagnoses for the protein c-erbB-2 hav e tremendous prognosti c potential [106, 109] Soluble fragments of the c-erbB-2 oncoprotein and the cancer antigen 15-3 were found to be significantly higher in the saliva of women who had breast cancer than in those patients with benign tumors [107]. Other studies have shown that epidermal growth factor is a promising marker in saliv a for breast cancer detection [109, 110]. These initial studies indicate that the saliva test is both sensitive and reliabl e and can be potentially useful in initial detection and follow-up screening for breast cancer. However, to fully realize the potential of salivary biomarkers, technologi es are needed to enable facile, sensitive, and specific detection of br east cancer. AlGaN/GaN HEMTs have shown promise for biosensing applications [111-114], since they include a high electron sheet carrier concentration channel induced by piezoelectric polarization of the strained AlGaN layer as discussed in section 1.2 [111-122]. There are positive counter charges

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56 at the HEMT surface layer induced by the elec trons located at the AlGaN/GaN interface. Any slight changes in the ambient can affe ct the surface charge of the HEMT, thus changing the electron concentration in the channel at AlGaN/GaN interface. 3.2 c-erbB-2 Antigen Detection Using AlGaN/GaN High Electron Mobility Transistors The HEMT structures consisted of a 3 m thick undoped GaN buffer, 30 thick Al0.3Ga0.7N spacer, 220 thick Si-doped Al0.3Ga0.7N cap layer. The epi-layers were grown by MOCVD on thick GaN buffers produc ed on Si substrates. Mesa isolation was performed with an Inductively Coupl ed Plasma (ICP) etching with Cl2/Ar based discharges at V DC self-bias, ICP pow er of 300 W at 2 MHz and a process pressure of 5 mTorr. 10 50 m2 Ohmic contacts separated with gaps of 5 m consisted of e-beam deposited Ti/Al/Pt/Au patterned by lift-off and annealed at 850 C, 45 sec under flowing N2. 400-nm-thick 4% Polymethyl Methacrylate (PMMA) was used to encapsulate the source/drain regions, with only the gate region open to allow the liquid solutions to cross the surface. The source-drain current-voltage characteristics were measured at 25 C using an Agile nt 4156C parameter analyzer with the gate region exposed. A plan view photomicrograph of a completed device is shown in Figure 3-1 (top). The Au surface was functionalized with a s pecific bi-functional molecule, thioglycolic acid. We anchored a self-assembled m onolayer of thioglycolic acid, HSCH2COOH, an organic compound and containing both a thio l (mercaptan) and a carboxylic acid functional group on the Au surface in the gate area through strong interaction between gold and the thiol-group of the thioglycolic acid. The devices were first placed in the ozone/UV chamber and then subm erged in 1 mM aqueous solution of thioglycolic acid

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57 at room temperatur e. This resulted in bindin g of the thioglycolic acid to the Au surface in the gate area with the COOH groups availabl e for further chemical linking of other functional groups. XPS and electrical m easurements confirming a high surface coverage and Au-S bonding formation on the GaN surface have been previously published [122]. The device was incubated in a PBS solution of 500 g/ml c-erbB-2 monoclonal antibody for 18 hours before real ti me measurement of c-erbB-2 antigen. Figure 3-1(bottom) shows a schematic device cro ss sectional view with thioglycolic acid followed by c-erbB-2 antibody coating. After incubation with a PBS buffered solu tion containing c-erbB -2 antibody at a concentration of 1 g/ml, the device surface was thoroughly rinsed off with deionized water and dried by a nitrogen blower. The s ource and drain curre nt from the HEMT were measured before and after t he sensor was exposed to 0.25 g/ml of c-erbB-2 antigen at a constant drain bi as voltage of 500 mV, as shown in Figure 3-2. Any slight changes in the ambient of the HEMT affect the surface charges on the AlGaN/GaN. These changes in the surface charge are transduced into a change in the concentration of the 2DEG in the AlGaN/GaN HEMTs, leading to the slight decrease in the conductance for the device after exposure to c-erbB-2 antigen. Figure 3-3 shows real time c-erbB-2 antigen detection in PBS buffer solution using the source and drain current change with cons tant bias of 500 mV No current change can be seen with the addition of buffer solution around 50 seconds, showing the specificity and stability of the device. In clear contrast, the current change showed a rapid response in less than 5 seconds when target 0.25 g/ml cerbB-2 antigen was added to the surface. The abrupt current change due to the exposure of c-erbB-2

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58 antigen in a buffer solution was stabilized afte r the c-erbB-2 antigen thoroughly diffused into the buffer solution. Three different conc entrations (from 0.25 g/ml to 16.7 g/ml) of the exposed target c-erbB-2 antigen in a bu ffer solution were detected. The experiment at each concentration was repeated five time s to calculate the standard deviation of source-drain current response. The limit of detection of this device was 0.25 g/ml cerbB-2 antigen in PBS buffer solution. The source-drain current change was nonlinearly proportional to c-erbB-2 antigen concentration as shown in Figure 3-4. Between each test, the device was rinsed with a wash buffer of 10 M, pH 6.0 phosphate buffer solution containing 10 M KCl to strip the anti body from the antigen. Clinically relevant concentrations of t he c-erbB-2 antigen in the saliva and serum of normal patients are 4-6 g/ml and 60-90 g/ml respectively. For breast cancer patients, the c-erbB-2 ant igen concentrations in the saliva and serum are 9-13 g/ml and 140-210 g/ml, respectively [106 ]. Our detection limit suggests that HEMTs can be easily used for detection of clinically relev ant concentrations of biomarkers. Similar methods can be used for detecting other im portant disease biomarkers and a compact disease diagnosis array can be realiz ed for multiplex disease analysis.

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59 Figure 3-1. (Top) Plan view photomicrograph of a completed device with a 5-nm Au film in the gate region. (Bottom) Schemat ic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with c-er bB-2 antibody/antigen on thioglycolic acid.

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60 Figure 3-2. I-V characteristi cs of AlGaN/GaN HEMT sensor before and after exposure to 0.25 g/ml c-erbB-2 antigen.

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61 Figure 3-3. Drain current of an AlGaN/GaN HEMT over time for c-erbB-2 antigen from 0.25 g/ml to 17 g/ml.

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62 Figure 3-4. Change of drain current vers us different concentrations from 0.25 g/ml to 17 g/ml of c-erbB-2 antigen.

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63 CHAPTER 4 LOW HG (II) ION CONCENTRATION ELEC TRICA L DETECTION WITH ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS 4.1 Background Mercury is one of the extremely to xic metals and its compounds produce irreversible neurological damage to human health [123] Due to its toxic effects, the standard limit of mercury in drinking water is 0.001 mg/g and while that in industrial waste water is 0.01mg/g [124] Hence, it is important to develop methods to detect mercury ions in contaminated waste water to innocuous levels. Sensitive detection of mercury (II) (Hg2+) ions is essential because its toxicity has long been recognized as a chronic environmental problem [123-128] Traditionally, there are several approaches can be used to detect Hg2+ concentration including spectroscopic (AAS, AES, or ICPMS) or electrochemical (ISE, or polarogr aphy) methods. However, these methods have shortcomings in practical use owing to hi gh cost, and large size which impedes on-site detection. There is a need for hand-held port able devices that can detect heavy metal ions with high sensitivity [129-139] AlGaN/GaN HEMTs are an attractive alternative as Hg ion sensor, since they include a high electron sheet carrier conc entration channel induced by piezoelectric polarization of the strained AlGaN layer. A variety of gas, chemical and health-related sensors based on HEMT technology have been demonstrated with proper surface functionalization on the gate area of the HEMTs. Such HEMTs have been used to detect carbon dioxide, hydrogen, chloride io n, prostate-specific antigen (PSA), kidney injury molecule, DNA, and glucose [117, 121, 122, 140-143] Because HEMTs operate over a broad range of temperatures and form the basis of next-generation microwave communication systems, so an integrated sensor/wireless chip is feasible. Bare Au-

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64 gated and thioglycolic acid functionalized Au-gated HEMTs were used to detect mercury (II) ions before [144, 145]. Fast detecti on of less than 5 seconds was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN/GaN HEMT based sensors showed 2.5 times larger re sponse than bare Au gatedbased sensors. The sensors were able to detect mercury(II) ion concentration as low as 10 7 M. The sensors showed an excellent sensing selectiv ity of more than 100 for detecting mercury ions over sodium or magnesium ions [142, 143]. However, in many applications, even lower detection sensitivities are required [144, 145]. 4.2 Hg (II) Metal Ion Detection Using AlGaN/Ga N High Electron Mobility Transistors The HEMT structures consisted of a 2 m thick undoped GaN buffer and 250 thick undoped Al0.28Ga0.72N cap layer. The epi-layers were grown by molecular beam epitaxy system on 2 inch sapphire substrates at SVT associates. The sheet carrier density and mobility of the HE MT sample were 1.1 1013 cm 2 and 1,600 cm2/(V s), respectively. Mesa Isolation, Ohmic me tal deposition, gate metal deposition and functionalization with thioglycolic acid, and PMMA formation were discussed in section 3.2. A plain view photograph of a f abricated sensor array is shown in Figure 4-1 (top). Hg2+ ion solutions ranged from 10 7 to 10 10 M were prepared by dissolving HgCl2 in DI water with pH 2 controlled by adding HC l in DI water. The Ksp of Hg(OH)2 =3 26 and it is necessary to control to lower pH value to achieve low concentration Hg2+ ion solutions. The sourcedrain currentvoltage characte ristics were measured at 25 C using an Agilent 4156C parameter analyzer with the th ioglycolic acid functionalized Au-gated

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65 region exposed to different concentrations of Hg2+ solutions. Ac measurements with modulated 500 mV at 11 Hz were performed to prevent side el ectrochemical reactions A schematic cross-section of the device with Hg2+ ions bound to thioglycolic acid functionalized on the gold gate region and plan view photomicrograph of a completed device is shown in Figure 4-1 (bottom). A se lf-assembled monolayer of thioglycolic acid molecule was adsorbed onto the gold gate d ue to strong interaction between gold and the thiol-group. The extra thioglycolic acid mo lecules were rinsed o ff with DI water. An increase in the hydrophilicity of the treated surface by thioglycolic acid functionalization was confirmed by contact angle measurem ent which showed a change in contact angle from 58.4 to 16.2 after the surface treatment, as shown in Figure 4-2. Figure 4-3 shows the time dependence of the drain current for a HEMT sensor with 20 m 50 m of gate sensing area exposed to different concentrations of Hg2+ ion solution. 15 l of DI water was enough to cover the entire area of the gate area. No drain current changes were observed another 5 l of DI water added to the 15 l of DI water (at 50 seconds). This measurement ru led out effect of change in mass of the buffer solution on the signal. In order to ex pose the gate area of the sensor to target Hg2+ ion concentrations of 1 9 and 1.5 8 M, 5 l of 5 9 M Hg2+ ion concentration was added into the 20 l of DI water already on the gate area at 150 seconds and followed with additional 5 l of 6 8 M Hg2+ ion solution at around 200 seconds. Although the sensor was not sensitive enough to detect the Hg2+ ion concentration of 10 9 M (Figure 43, at 150 seconds ), the current showed a rapid response (drain current dip) when 5 l of 6 8 M Hg2+ ion solution added to reach a target concentration of 1.5 8 M Hg2+ ions. This drain current dip was due to the gate

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66 sensing area instantly exposed to higher Hg2+ ion concentration solution than the target 1.5 8 M Hg2+ ion concentration and the drain current took nearly 20 seconds to stabilize. The 20 seconds of lag time was interpreted for the Hg2+ ions to uniformly diffuse through the entire liquid drop covered on the gate sensing area. As illustrated in Figure 4-3, larger drain current dip was observed for higher Hg2+ ion concentration solution added to the solution already on the sensor. When sensing higher Hg2+ ion concentration, in order to reach the target Hg2+ ion concentration, such as 3.37 8 M, 5 l of Hg2+ ion solution with much higher concentration, 3 7 M, than the target one was needed to be used. This was due to the gate sensing ar ea covered by 20 l of DI water plus several injections 5 l of lower ion concentration so lutions. This drain current dip actually showed our sensor extremely sensitive to the Hg2+ ion solution and the sensor can detect the instantaneous Hg2+ ion concentration right above the gate sensing area. It is possible to eliminat e the drain current di p by employing the microfluidic device to produce sharp transition of Hg2+ ion concentration. The thiolglycolic acid functionalized HEMT sensor can also be repeatably used to detect Hg2+ ion solution and the results was previously published [142] The drain current reduction is due to the chelating with Hg2+ ions of the carboxylic acid functional groups of t he self-assembled monolayer of thioglycolic acid molecules on the gold surface. T he charges of trapped Hg2+ ion in the RCOO (Hg2+) OOCR chelates was hypothesized to change the polarit y of the thioglycolic acid molecules. This can induce negative counter charges on the gate surface of the HEMT sensor, resulting in a reduction in drain current. Upon exposure the HEMT sensor to the higher concentration of Hg2+ ion solution the drain current reduced further. The degree of drain

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67 current dip was more serious and it took long time for the sensor to reach equilibrium. The difference of drain current for t he HEMT sensor exposed to different Hg2+ ion concentration to the DI water is also illustra ted in Figure 4-4. The drain current of each Hg2+ ion concentration was repeated five times. As shown in Figure 4-3 and Figure 4-4, the Hg2+ ion concentration detection limit of t he thioglycolic acid functionalized AlGaN/GaN HEMT sensor is 1.5 8 M. This is about an order of magnitude lower than previously published [142, 143]

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68 Figure 4-1. Plain view photomicrograph of a completed device with a 5 nm Au film in the gate region (top). A sc hematic of AlGaN/GaN HEMT. The Au-coated gate area was functionalized with thioglycolic acid (bottom).

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69 Figure 4-2. Photographs of c ontact angle of water drop on the surface of bare Au (left) and thioglycolic acid functionalized Au (right). Figure 4-3. Time dependence of the drain cu rrent for a HEMT se nsor exposed to different concentrations of Hg2+ ion solution.

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70 Figure 4-4. The difference of drain current for the HEMT sensor exposed to different Hg2+ ion concentration to the DI water.

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71 CHAPTER 5 CU-PLATED THROUGH-WAFER VIAS FO R ALGAN/ GAN HIGH ELECTRON MOBILITY TRANSISTORS ON SI 5.1 Background Through-wafer, backside vias are comm on elements in compound semiconductor power device technologies for providing a low-inductance grounding and improving heat transfer characteristics [146, 147]. The inductance generated by the long Au wire between the bond pads and package can be mini mized by full backside Ohmic contact through vias. Furthermore, a thermallyand electricallygrounded drain does not require an expensive and complicated air-bridge fabrication process and, thus, can simplify the fabrication process and incr ease the device reliability as air-bridge structures regularly deform at high temperature [148, 149]. These were developed for GaAs microwave monolithic integrated circ uits (MMICs) but more recently have been used with AlGaN/GaN HEMTs for high frequency power amplifier technology. These AlGaN/GaN heterostructures are usually grown on SiC due to a smaller lattice mismatch and higher thermal conductivity compared to those grown on sapphire and this leads to a significant reduction in device operating temperature at high power levels [150-153]. Vias can be realized by either dry etching or laser dr illing of the substrate [154-156]. There is also significant interest in use of Si substrates because of lower costs and capability for scaling to larger wafer di ameters [157-164]. Based on the reliability comparisons between GaN/Si and GaN/SiC, it appears that GaN/Si offers similar reliability combined with the cost model of GaAs technology. Specifically, the reduced cost of the materials (economic al substrates), same tooli ng factors as silicon industry (back side processing, die attach technology), and scalable processing for large

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72 diameter wafers allow for ec onomical manufacture of MMICs that require high levels of uniformity across large substrates to allo w proper circuit design of both active and passive components. For power amplifier applications, improving ability to extract heat and the allowed thermal budget of operation is very critical. Thermal simulations employing a 3-D finite element analysis show t hat the Si substrate is an effectiv e heat sink relative to sapphire, but device temperatures are still higher than on SiC substrates [163, 164]. This model is based on the unsteady state energy balance equat ion using rectangular coordinators (x, yand z-axes), where T is the temperature, is time, is density, Cp is the heat capacity, k is thermal conductivity (W/cm K) and PD is the source of power, i. e., the rate of internal power generation. The latter can be deter mined from the product of bias voltage and drain current through the HEMT, divided by the HEMT layer volume. The primary modes of heat transfer are conduction thr ough the layers and convection at the free surfaces. One approach for increasing heat transfer is to use a high thermal conductivity metal such as Cu in vias specifica lly designed for minimizing device operating temperatures at high power levels. This does not necessarily have to provide through wafer electrical connection but rather ar e employed to enhance the effective thermal conductivity of the substrate. This approach includes deep Reactive Ion Etching (DRIE) followed by deposition of the insulation layer, barrier layer and seed layer. Vias were then electroplated with Cu and the metal planarized by mechanical and chemical polish P z T y T x T k) T ( C D 2 2 2 2 2 2 P

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73 processes. This approach is shown to pr oduce via arrays with smooth sidewall and via base, conformal coverage of the seed layers lining the vi a and void-free electroplating (EP) of the vias. 5.2 Experiment The HEMT on Si wafers were grown by MO CVD with conventional precursors in a cold-wall, rotating disc reactor designed from flow dynamic simulations. The growth process was nucleated with an AlN layer to avoid unwanted Ga-Si interactions. The epitaxial stack then consisted of a proprie tary AlGaN transition layer [165, 166], ~800 nm GaN buffer layer, and 16 nm unintentionally doped Al0.26Ga0.74N barrier layer. The nominal growth temperature for the GaN buffer and AlGaN barrier layers was 1030 C. HEMT fabrication has been pub lished previously [157, chapt er 2 and 3] but in brief summary, began with Ti/Al/Ni/Au Ohmic me tallization and RTA in flowing N2 at approximately 825 C. Contact resistance, spec ific contact resistivity, and specific onresistance were 0.45 mm, 510-6 m2, and 2.2 mm, respectively. Immediately following Ohmic anneal, the wafers were passivated with SiNx in a PECVD chamber maintained at a base plat e temperature of 300 C. Inter-device isolation was accomplished by using multiple energy N+ implantation to produce significant lattice damage throughout the thickness of the GaN buffer layer. The ion implantation step maintains a planar geometry in the fabricated device and reduces parasitic leakage paths that may exist in passi vated, mesa-isolated HFETs [167]. Schottky contacts were formed by selectively removing the SiNx passivation layer and s ubsequent deposition of 0.7 m Ni/Au gates. Large periphery devices were air bridged for source interconnection using standard Au electroplating te chniques to a thickness of ~3 m.

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74 Backside vias in the Si were formed by the conventional DRIE process that uses pulsed etch/deposition cycles to prevent sidewall undercut. A schematic of the structure is shown in Figure 5-1 and a SEM image of t he cross section of an etched via is shown in Figure 5-2. Seed layer formation consis ted of deposition of low temperature SiO2 by PECVD to avoid problems with wafer-carri er separation. The thickness was 0.7-1 m. This layer provides electrical isolation. The seed layers were deposited for the Cu plating, namely, Ti (500-650 nm) and Cu (1.5-1.85 m), both of which were deposited by sputtering at room temperature. The sidew all layer thickness is about 33% of the top layer thickness as shown in Figure 5-3, i.e., SiO2 was 0.84 m on the field, but 0.29 m on the sidewall, the Ti was 0.52 m on the field and 0.17 m on the sidewall and the Cu was 1.54 m on the field and 0.50 m on the sidewall. Cu was plated into the via and mechanically polished to planarize. The schemat ic plating sequence is shown in Figure 5-4. 5.3 Results And Discussion The wafers were plated with Cu for various times to understand both the time required and how the via fill proceeded as a function of Cu thickness. It was found that complete coverage was not obtained until at least 5 hours of plating. The average current density during t he plating was 6.7 A cm-2, leading to an average plating rate of 9.3-12.5 m/h into the actual via, with the hi gh end occurring in the initial stages. We then performed a mechanical polish in standard Cu polishing solution to planarize the Cu and cleaved the wafers to get cross-secti on views of the vias. As seen in the optical micrographs in Figure 5-5, the vias with Cu were successfully filled and achieved good planarization of the remaining Cu film. Approximately 15 % of the Cu plugs showed the presence of voids that filled typically 15 %20 % of the volu me of the via. The reasons

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75 are currently investigated for the void formation, including the effect of Cu seed aging time and plating rate. Another i ssue is whether there is an effe ct on the Cu seed layer of the time prior to the CMP being carried out. In separate experiments on Cu seed layers, various types of defects formed on the electr oplated Cu as a function of time after the deposition of the Cu seed layer, leading us to realize an evaluatio n of the films using surface analytical techniques to obtain a fundamental understandi ng of barrier/seed and electroplated copper film behavior as a func tion of time and treatment was necessary. Other researchers successfully used laser re moval of copper oxide from copper, while a wet cleaning mixture of 1:200 NH4OH has also been found to be useful in removing copper oxide films [168] The known copper oxide thickness films were deposited and then treated them to determine which treatment is most effe ctive and the rate of surface contamination removal. The resu lts can be summarized as follows: (i)The time delay between physical vapor deposition (PVD) copper seed layer deposition and copper electroplating was obser ved to influence copper electroplating film defects. (ii) The total defect count increased with delay time between electroplating and seed layer deposition. The defects consist of both embedded defects and voids. Both types are shown in Figure 5-6, where it is seen that the void s are the ones that increase with aging time. Only the latter is a problem because the embedded defects are removed at the chemical mechanical planarization (CMP ) stage. Embedded defects are induced by EP process, removed by CMP, and composed of copper. Voids leave metal interconnect lines void of copper material, prevents current carrying capability of the metal

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76 interconnect and are hypothesized to be due to a lack of wetting duri ng electroplating. All four applied treatments dec reased post electroplating def ects caused by seed aging. These were Cu oxide reduction, single wa fer clean, electrolyte rinse and reverse plating. The time delay between PVD Cu se ed layer deposition an d Cu electroplating increased the density of copper el ectroplating film defects. A decrease in wettability was shown by an increase in contact angle from 40 to 63 over a fourteen days period. The larger contact angle indicates hydrophobic b ehavior while the increase in hydrophobicity suggests a decrease in wettability. This means that the copper is less likely to be adequately wetted by the plating electrolyte as time delay increases. The copper seed reflectance decreases with delay time and is similar to a wafer with 30 copper oxide layer. Hydrogen reduction treatment results in a 42contact angle as compared to other treatments with a 48-50 cont act angle. The seed surface decreases in reflectivity over a fourteen day aging period. All tr eatments tried increa se the reflectivity of the Cu film, suggesting they all remove the Cu oxide. Figure 5-7 shows the front side of the HEMT-on-Si wafer after Cu plating and planarization. The HEMT devices and contact pads are not visibly damaged by the via fill/planarization process.

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77 Figure 5-1. Schematic of via in AlGaN/GaN HEMT on Si wafer. Figure 5-2. Cross-sectional SEM of dry etched via in AlGaN/GaN HEMT on Si wafer.

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78 Figure 5-3. Cross-sectional SEMs of dielectric/metal sta ck on field (top) or sidewall (center) and Cu/Ti/SiO2 stack at high magnification (bottom).

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79 Figure 5-4. Schematic of plating sequence for Cu. Figure 5-5. Cross-sectional SEM of Cu-p lated via after mechanical polishing. The diameter of the openings of the vias is 50 m.

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80 Figure 5-6. Defect type as a function of seed aging time. Figure 5-7. Optical plan view micrograph of front-side of plated via wafer showing HEMTs and contact pads.

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81 CHAPTER 6 UV EXCIMER LASER DRILLED HIGH ASPECT RATIO SUBMI CRON VIA HOLE 6.1 Background Laser drilling has been used for creating holes with high aspect ratios in various types of materials such as semiconductors metals, plastic, and different types of ceramics [56, 64, 156, 169, 170]. The ability to control the lo cation, time, and duration of the energy deposition process as well as the high machining rate make this laser drilling process more appealing than other conventi onal techniques, such as lithography in conjunction with wet chemical or plasma based dry etching [54, 171-174] .The majority of the through-substrate via hole fabrication work has focu sed on diameters in the mm and m range. There are many potential applic ations such as microfluidic arrays, microfilters and nanoporous arrays where there is a need for a technology to fabricate submicron sized via holes. In this chapter, the fabricat ion of via holes in both Si and glass substrates with 300 nm diameter entrance holes was reported by using an UV excimer laser. The effects of both the diameter of the ent rance hole and the number of r epetitive laser pulses on the formation of the submicr on via holes were studied. 6.2 Experiment 4 inch silicon wafers and cover glass sl ips (22 mm 22 mm, Fisher Premium Cover Glass, Fisher Scientific, Inc.) were used in this work and the samples were drilled with a JPSA IX-260 ArF excimer laser sys tem. A convex/convex doubler objective lens with a meniscus corrector was used to correct the spherical aberration and the focal length of the doubler was 10 cm. A multiple hole me tal mask with openings varied from 5 m to 200 m was focused on the process sample surface with a

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82 demagnification of 12.5. The laser pulse durat ion was fixed at 25 ns and the repetition rate was varied from 1 Hz up to 100 Hz in this study. The output pulse energy was 200 mJ. The average power measured at the maximum repetit ion rate was 12 Watt. The energy density (fluence) of the focused processing bea m was in the range 2-4 J/cm2. The sample stage was designed to accommodate 6 inch wafers. The resulting vias were examined by both optical microscopy and SEM. 6.3 Results And Discussion Figure 6-1 (top) shows a photo of a cleaved 400 m thick Si wafer with a laser drilled via hole. The diameter of t he entrance and exit holes are 90 and 45 m, respectively, and the taper angle from top to bottom of the via holes was estimated around 3.2 to 3.5 based on the diameter of the ent rance and exit hole. Although the laser pulse can be considered to transmit it s energy in a single time unit, the laser drilling process consists of three steps. Initially, the drilled material absorbs the laser energy, and the top layer of the drilling material is melted. The molten materials continue to absorb energy, which then leads to vaporization of the drilled material. The resulted saturated vapor pressure (recoil pressure) generated by the sudden expansion of the vaporization of the molten material applies a force on the molten material and laterally pushes the molten material out of th e via holes along the side of the holes [171]. After expelling the molten material, the surfac e of solid metal on the bottom of the via holes is then exposed to the laser light again and starts to absorb laser energy. This process is repeated many times, leading to drilling of the material exposed to the laser beam. The time scale of the melting proce ss and the process of pushing out the molten material by the recoil pressure are much fa ster than the systems laser pulse repetition

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83 rate of 100 Hz used in this experimen t. Consequently, a higher drilling rate was achieved with higher laser pulse repetition ra te to avoid the cooling down of the drilled material. During the drilling proc ess, the rates of expelling, cooling, and re-solidification of the molten material on the sidewalls of the via holes and the deviation from the focal plane for the deep drilling deter mined the tapered angle of via hole. Figure 6-1 (bottom) shows the top SEM images of a 5 m diameter via hole drilled on the Si wafer with two drilling times using a 62.5 m diameter circular mask. T he drilling times were 30 and 45 seconds for the via hole shown on the bottom left and right in Figure 6-1, respectively. The via hole after 30 seconds accumulated drilling time had a depth of around 5 micron. Although the via hole with 45 seconds of accumulated drilling time had a similar depth and shape as the via hole drilled for 30 sec onds, there was an addit ional tiny via hole with a diameter around 300 nm visible inside t he original via hole. So far, there have been no reports on such second via hole format ion inside a larger diameter via during the laser drilling process using a single drilli ng process. It was difficult to find out the exact depth of the 300 nanometer size vi a hole even using focused ion beam (FIB) sectioning since small variations during t he FIB process would miss the tip of the via hole. In order to examine the exact dr illed depth and the shape of the 300 nanometer size via hole, a same sized via hole was drilled on the edge of glass slices. Then the depth of the drilled via holes were examined from the side of the glass slices with an optical microscope. In order to find out the form ation mechanism of this ti ny via hole inside the normal larger via hole, circular via holes with tw o larger diameters and a different number of laser pulses on the edge of the glass slice were drilled. The top row of pictures in Figure

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84 6-2 show the cross-sect ional view of the 160 m diameter via holes drilled on the glass with 70, 80, and 100 pulses. The depth of the via holes increase with the number of the laser pulses applied, and the bottom of via ho le was flat in all cases. For the 80 m diameter via hole, a similar via hole shape wi th a flat bottom as the larger via hole (160 m) was obtained for 70 laser pulses. When a higher number of laser pulses were employed in the 80 m via holes, an additional smaller via hole formed in the center of the big via hole. This additional tiny via hole formation to the laser light was attributed by reflecting from the side walls of the via hol e and refocusing at the center of the bottom surface in the larger via hole. This additi onal dose of the reflec ted laser light enhanced the drilling rate in the center of the larger hole. For the shallow and larger via hole, the most of incoming laser light di rectly exposed to the bottom of the via hole. The portion of the reflected laser light fr om the tapered via hole si de walls was smaller and not focused. Therefore, the bottom of the drilled via kept flat. As the entrance diameter of the via hole became smaller and the depth of t he via hole became larger, the portion of the incoming laser light on the tapered si de wall became larger as compared to the laser light directly exposed on the bottom of the via hole. Once the reflected laser light was focused on the bottom of via hole, the drilling rate significantly increased at the bottom surface due to the additional dose of t he reflected laser light. When the tiny hole was formed, the laser light was trapped inside the tiny via hole, which then behaved as a wave-guide for the laser light. However, the drilled material was difficult to vaporize in this geometry and the possibility of expelling the molten mate rial by the recoil pressure in such a small via hole was even smalle r. Thus the drilling rate suddenly decreased significantly. Figure 6-3 shows a picture of via holes drilled on both sides of a glass

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85 slide. The top and bottom via hole had an entrance diameter of 80 m and 10 m, respectively. The diameter at the tip of the bottom via can be adjusted by changing the drilling time, since the drilling rate of the tiny hole is very slow and controllable. Since the UV excimer laser system can drill arra ys of via holes at the same time, this technique can be readily used to make microfilters or nanopores.

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86 Figure 6-1. (Top) Cross sectional microgr aphic image of a via hole drilled through a Si substrate with a diameter of 90 and 45 m for the entrance and exit hole. (bottom) Top view SEM images of a 5 m diameter via hole drilled on a Si substrate with two different drilling ti mes; 30 sec for the image on the left and 40 sec for the image on the right.

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87 Figure 6-2. (Top) Side view of a series of 160 m diameter via holes drilled on the edge of a glass with different numbers of repet itive laser pulses. (Bottom) Side view of a series of 80 m diameter via holes dril led on the edge of a glass substrate with different number s of repetitive laser pulses.

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88 Figure 6-3. A micrographic side view image of via holes drilled from both sides of the glass substrate. The diameters of the entrance holes are 80 and 10 m for the top and bottom via hole, respectively. 5 m

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89 CHAPTER 7 193 NM EXCIMER LASER DRILLING OF GLASS SLICES: DEPENDENCE OF DRILLING RATE AND VIA HOLE SHAPE ON THE DIAMETER OF THE VIA HOLE 7.1 Background Glass has attractive properties including high optical transparency in a wide wavelength range, low fluorescence, chemical inertness, benign surface, dimensional stability, as well as being electrically and ther mally insulating. Glass also has a similar CTE to that of Si so it can be used to r educe the thermomechanical stresses in flip-chip assemblies. These characteristics make glass a suitable substrate for Si-based electronics package applications. Glass is a brittle material and laser micromachining can be an option once the glass is too thin Pyrex glass has anodic bondability to Si and can be used in the field of microsystems tec hnology. There are several important glass material received most attention including bo rosilicate glass, Pyrex glass, fused Quartz, fused silica, and soda-lime glass [42, 49, 53-56, 60, 63, 175, 176]. Traditionally, there are several methods commonly used to pattern glasses including ultra-sonic drilling and etching. The ultra-sonic drilling process has limitations in forming small diameter via holes in the range of 300 m. Wet etching and dry etching requires photolithographi c masking of the material surface prior to etching. Moreover, wet etching is characterized by an isotropi c etch behavior, tends to under-etch the mask and does not provide a good feature control. Dr y etching exhibits an excellent profile control but has difficulty in producing small holes with high aspect ratio due to mask erosion and low etch rate. Thus these met hods are not appropriate to realize surface structures, which required bot h small holes and high aspect ratio [53-56, 59, 177-180]. Laser drilling has several benefits over t he ultra-sonic drilling and etching methods for engineering of via holes on glass substrates as described in section 1.4. The lasers

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90 with visible or near-visible wavelengths ar e not suitable for micromachining glass due to their weak interaction with glass. The ablatio n rate using a shorter wavelengths laser is much greater than that at longer wavelengths. The ablation threshold also increases with an increase of wavelength due to lo wer material absorption [64, 181]. Effective laser ablation requires a sufficiently high absorption coefficient of the material to be ablated. Transparent glasses ar e difficult to process with most lasers because they poorly interact with visible or IR wavelengths. ArF excimer lasers are preferred to be used for the laser ablation of glass because of sufficiently high absorption at 193 nm. The s hort wavelength with higher photon energy than obtained in the IR range also makes it easier to remo ve the target material. The photon energy at 193 nm is higher than any ot her solid state lasers and al so can break the chemical bonding of most drilled materials. The larger beam size of t he excimer laser can also be used to realize simultaneous drilling of multiple holes, whic h can further save process time and cost [49, 58, 63]. In this chapter, the drilling rate and the shape of the via hole in glass substrates as a function of the diameter of the drilled holes wa s reported with a 193 nm excimer laser drilling system. The effects of the laser light reflected off the side wall of the drilled hole on the shape and drilling rate of the drilled holes were investigated as well. 7.2 Experiment Corning 0211 cover glass slips were used in this work. The laser system and the related parameters are covered in section 6.2. The fluence of t he processing beam in focus was in the range of 3-6 J/cm2. A schematic diagram of the experimental set-up is shown in Figure 7-1. Along the laser beam path, there were turning mirrors, an up collimator and a dichroic beam splitter inst alled on kinematic mounts, a metal mask, and

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91 a convex/convex doubler objective lens. In the beam delivery system, a computer-controlled closed-circuit television (CCTV) was installed to provide a viewin g image of the drilled su rface; the CCTV can be automatically adjusted parfocally and coaxia lly to the focused laser beam spot. The CCTV consisted of a multi-element objective lens with coarse/fine focus barrel, a kinematically mounted mirror, a CCD high re solution camera with lens and corrective optics, and a 15" monitor with electronic crosshair. The sample stage was designed to acco mmodate 6 inch wafers. Two HeNe lasers were used to guide the high-accuracy air-bearing x-y linear-motor sample stage, and the minimum stage movement was 0.1 m/step. The stage had an accuracy of +/3 m over a full range of motion with a stage velo city of 6-8 inches per second; the stage position can be programmed wi th a resolution of +/1 um and stage movement repeatability of +/1 m. In order to examine the dept h and the shape of the via holes, the via holes were drilled along the edge of gl ass slices. The images of the via holes were taken from the side of the glass slices with an optical microscope. 7.3 Results And Discussion Theoretically, the shape of the via holes s hould be independent of the diameter of the via holes until the point w here the diameter is comparabl e to the wavelength of the laser source. The drilling rate and the shapes of the via holes were highly dependent on the opening size of the via hole. Figure 7-2 shows the drilling rate of the glass as a function of the via hole diamet er. The drilling rate was very slow in the range of 0.1 m/sec for the via holes with diameter less than 10 m. The drilling rate quickly increased to 19 m/shot for the via holes wi th diameter of 40 m, and then slightly

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92 decreased to 17 m/shot for larger via holes. As shown in Figure 7-3, the shape of the via holes was highly dependent on the diameter of the entrance hole. The via hol es with the entrance diameter of 120 m showed a tapered side wall with a 7.5-9 degr ee angle after 10 second of laser drilling. The via holes with diameter of 80 m also showed a 7.5-9 degree tapered side wall after 10 second of laser drilling; however, there was an additional smaller via hole observed at the bottom of the big via hole. The via hole with the diameter of 40 m of the entrance hole showed a cone shape vi a hole with a similar 5-6 degree tapered angle. The via hole with diameter of 5 m of the entrance hole ex hibited a very high aspect ratio funnel shape. In order to confirm the relationship between the drilling rate and the shape of the via hole on the diameter of the entrance hole, the drilling rate and shape of the via hole as the function of drilling time have been exami ned. As illustrated in Figure 7-4 (top), the drilling rate for the via hole wi th a diameter of 120 m stayed fairly constant. Figure 7-4 (bottom) shows the side view images of t he via holes for different drilling times; the tapered angle of the via holes remained almo st the same. The only difference between the via holes at different drilling stages wa s that the via holes became deeper as the drilling time increased. The drilling rates and the side view images of the via hole with a 80 m diameter at different drilling times are shown in Figure 7-5. The drilling rates were very similar in the beginning, and then the drilling rates increased to a higher level after 3 second of drilling. As illustrated in Fi gure 7-5 the tapered angle of the via holes stayed the same in the beginning. A smaller via hole formed at t he bottom of the original via hole after 3

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93 second of drilling, as shown in Figure 7-5 (b ottom). This rate increase and smaller via hole formation at 3 second of drilling were due to the additional laser light that was reflected from the tapered side wall of the vi a hole and refocused at the center of the via hole bottom. The laser light that reached the bottom of the via hole consisted of both the laser light directly exposed upon the bottom of the via hole and the laser light reflected off the tapered sidewall to the bottom of the via hole, as marked in white lines in Figure 7-5 (bottom). At the start of the drilling, t he via hole was shallow and the portion of the reflected laser light reaching the bottom of the via hole was minimal as compared to the laser light directly exposed to the bottom of the via hole. Once the via hole became deeper, the portion of the reflected laser light became a significant addition to the laser light directly exposed upon the bottom of th e via hole; the aggregate dose of the laser light increased, which, subs equently, caused an increase in the drilling rate. When the via hole became deeper and some of the re flected laser light became focused upon the center of the via hole, an additional smaller vi a hole began to form at the bottom of the big via hole. Figure 7-6 shows the drilling rates and the side view images of the via hole with a 40 m diameter at different dr illing times. The drilling rate was higher than that of a 80 or 120 m via hole. Unlike the 80 or 120 m via hole, a conical via was formed throughout the entire drilling time This was due to the refl ected laser light dominating the drilling process. The laser light that was reflected from the sidewall of the via hole and focused on the bottom of the via hole, produced both the highest drilling rate and conical shape of the via hole.

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94 Figure 7-7 shows the drilling rate and the cross sectional images of the same via hole at different stages during the drilling. The drilling rates were significantly slower than those of the larger via holes discuss ed previously, and the drilling rates also decreased exponentially. Figure 77 (bottom) illustrates optical images of the via holes drilled with a range of drilli ng times ranging from 1 to 10 minutes. The initial drilling formed the top part, a funnel shape, of the via holes, whic h had a higher drilling rate. However, the depth of the top part of t he via holes barely c hanged throughout the rest of drilling time. Once the t op part of the via holes form ed, the incoming laser light reflected off from the sidewall the via hole, refocused on the center of the bottom of the via hole and drilled a smaller high aspect-rati o via hole. The diameter of the bottom part of the via holes was less than 0.5 m and the aspect-ratio of this smaller via hole was >200. The high aspect-ratio via hole hindered the ability of the dr illed material to be vaporized or expelled despite the high recoil pressure; this in turn, slowed down the drilling rate dramatically. To further confirm the dimens ion of the smaller via, a top view SEM image of 5 m via hole was taken, as shown in Figure 7-8. It clearly sh ows the additional tiny via hole with a diameter around 300-400 nm visible in side the 5 m via hole. Once this high aspect-ratio via hole formed, it served as a wave guide to the laser light propagation inside a circular waveguide. The diameter of this via hole was only 1.5 to 2 times the wavelength of the laser light used. T he TE01 mode, which is fundamental mode, normally appears in a small waveguide, wi th a circular symmetry with maximum intensity in the center should dominate in the light propagat ion within the via hole. As the via hole was developing, it further s queezed the light down t he waveguide and the

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95 intensity profile became sharper as indicat ed by the tapered angle of the via hole.

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96 Figure 7-1. Schematic of t he excimer laser drilling system.

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97 Figure 7-2. Drilling rate of the glass as a function of the via hole diameter

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98 Figure 7-3. Side view images of drilled hole s with the diameters of the entrance holes being 120, 80, 40, and 5 m respectively.

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99 Figure 7-4. (Top) Time depende nt drilling rate of the 120 m diameter via hole. (Bottom) Side view images of the 120 m diameter via holes drilled for different times.

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100 Figure 7-5. (Top) Time depende nt drilling rate of the 80 m diameter via hole. (Bottom) Side view images of the 80 m diameter via holes drilled for different times.

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101 Figure 7-6. (Top) Time dependent drilling rate of the 40 m diameter via hole. (Bottom) Side view images of the 40 m diameter via holes drilled for different times.

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102 Figure 7-7. (Top) Time depende nt drilling rate of the 5 m diameter via hole. (Bottom) Side view images of the 5 m diameter via holes drilled for different times.

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103 Figure 7-8. Cross-sectional SEM of glass slips with 5 m entrance diameter drilled with 2 min drilling time.

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104 CHAPTER 8 SUMMARY AND FUTURE WORK In order to address the needs for eliminat ing the burn-in processes and improving device reliability, the mechanisms of InAlAs/InGaAs MHEMT degradation were investigated. The degradation mechanism of MHEMTs under DC stress at a drain voltage of 2.7 V and current density of 167 mA/mm as well as thermal stress was examined. After DC stress, the P t in the P t/Ti/Pt/Au mushroom gate stack diffused into the InAlAs layer, which was confirmed with TEM images and EDS analysis. The DC stress at a drain volt age of 2.7 V at 165 oC led to drain current degradation and gate sinking. The gate leakage current afte r thermal storage (o ven storage at 250 oC for 36 hrs) increased much larger than that from DC stress. The Rc deterioration, metal spike formation and Ohmic metal diffusion were observed during the thermal storage. In order to alleviate Ohmic contact degradation, non-annealed Ohmic process can be investigated since non-annealed Ohmicrecess approach showed improved MHEMT DC and RF performance [9]. Passivation ma terial should also be studied since the MHEMTs with BCB passivation showed lower degradation than ones with Si3N4 passivation [10]. Through a chemical surface modification, the c-erbB-2 antibody was immobilized in the Au-gated region of an AlGaN/GaN HEMT structure fo r the detection of c-erbB-2 antigen and a limit of detection of ~ 0.25 g/ml was achi eved. This electronic detection of biomolecules was a significant step to wards a compact sensor chip, which can be integrated with a commercial available hand -held wireless transmitter to realize a portable, fast response and high sens itivity breast cancer detector.

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105 In the current study, the c-erbB-2 antigen was dissolved in PBS and human trial should be conducted. Besides th e c-erbB-2 antigen, there ar e three other biomarkers commercially available [107, 109, 110]. The integrated sensors employing these four different antigens can be used to detect the breast cancer on the same time and improve the detection accuracy. TGA functionalized AlGaN/GaN HEMTs s howed an excellent sensitivity to detection Hg2+ ions. A detection limit of 1.5 8 M of the Hg2+ ion was achieved. It is also important to study the assembly and package of sensor devices. For example, the sensor chips can be inserted into microfluidic devices. By integrating the sensor chip with a wireless transceiver, we can make a wireless and portable device for on-site measurement. A backside via hole process using Cu plugs was developed for AlGaN/GaN HEMTs grown on Si substrates to improve the heat dissipation. The diameter of the via hole was 50 m and at least 80 m thick of Cu was required to be electroplated to fill the via. The excess electroplated Cu can be polished away for planarization. The device DC and RF performance improvement should be verified after forming the Cu-plated through-wafer vias for AlGaN/GaN HEMTs on Si. Laser processing was shown as a promisin g alternative to replace conventional micro-machining methods, such as wet etchi ng and dry etching, for microfluidic device and backside via forming application. The effect of the via hole di ameter on the drilling rates and the shapes of the drilled via holes was investigated by glass due to its well coupled with irradiation of ArF excimer la ser. An additional smaller via hole at the bottom of the big via was observed due to t he refocused aggregate light from reflected

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106 tapered sidewall. The drilling via hole shape (e.g. submicron via hole with high aspect ratio) and associated process parameters (e.g irradiation time, ma sk size) were studied and allowed us to drill desired pattern. Ot her patterns of masks, e.g. square and rectangle, can be used to compare the dependence of drilling rate and via hole shape with circle mask. By obtaining required proc ess recipe, e.g. fluence, irradiation time, Rep. Rate, for different pa tterns of masks, the complex desired pattern with vias, channels and submicron holes can be formed with ArF based UV excimer laser.

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107 LIST OF REFERENSCES (1) S.J. Pearton, handout for the course Advanced electronic material processing, 2008. (2) Fan Ren, John C. Zolper, Wide E nergy Bandgap Electronic Devices, ISBN 981238-246-1 (2003). (3) M. Dammann, M. Chertouk, W. Jantz, K. K6hler, and G. Weimann, Microelectronics Reliability, 40 1709 (2000). (4) M.Dammann, A.Leuther, H.Konstanz er, and W.Jantz, JEDEC, 87 (2001). (5) M.Dammann, A.Leuther, F.Benkhelifa, T.Feltgen, and W.J antz, Phys. Stat. sol. 195, 1, 81 (2003). (6) M. Dammann, A. Leuther, R. Quay, M. Meng, H. Konstanzer, W. Jantz, and M. Mikulla, Microelectronics Reliability, 44, 939 (2004). (7) M.Dammann, A. Leuther, A. Tessmann, H.Massler, M.Mikulla and G. Weimann, Electronics Letters, 41, 12, 699 (2005). (8) P.F. Marsh, C.S. Whelan, W.E. Hoke, R.E. Leoni III, and T.E. Kazior, Microelectronics Reliability, 42, 997 (2002). (9) Li-Yang Chen, Shiou-Ying Cheng, Wen-Shiung Lour, Jung-Hui Tsai, Der-Feng Guo, Tsung-Han Tsai, Tzu-Pin Chen, Yi -Chun Liu and Wen-Chau Liu, Semicond. Sci. Technol., 23, 125041 (2008). (10) Yong-Hyun Baek, Jung-Hun Oh, Seok -Gyu Choi, Woo-Suk Sul and Jin-Koo Rhee, Journal of the Korean Phys ical Society, 54, 5, 1868 (2009). (11) M. Chertouk, W.D.Chang, C.G.Yuan, C.H.Chen, and D.W.Tu, 11th GAAS symposium 9 (2003). (12) S.C. Chen, H.C. Chou, Frank Chou, Iris Hsieh, D.W. Tu, Y.C. Wang, C.S. Wu, and S.R. Nelson, JEDEC, 47 (2007). (13) D.C.Dumka, H.Q.Tserng, M.Y.Kao, E.A.Beam, and P.Saunier, IEEE Electron device letters, 24, 3, 135 (2003). (14) Jae Yeob Shim, Hyung Sup Yoon, D ong Min Kang, Ju Yeon Hong and Kyung Ho Lee, ETRI Journal, 27, 6, 686 (2005). (15) Dong Xu, Wendell M.T.Kong, Xiaoping Yang, P.M.Smitth, D. Dugas, P.C.Chao, G.Cueva, L.Mohnkern, P. Seekell, L.Mt.Pleasant, B.Schmanski, K.H.G.Duh, H.Karimy, A. Immorlica, and J.J.Komiak, IEEE Electron device letters, 29, 1, 4 (2008).

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108 (16) O.Ambacher, J.Smart, J.R.Shealy, N.G.Weimann, K. Chu, M.Murphy, W.J.Schaff, and L.F.Eastman, Journal of applied physics, 85, 6, 3222 (1999). (17) O.Ambacher, B.Foutz, J.Smart, J.R.Shealy, N.G. Weimann, K.Chu, M.Murphy, A.J. Sierakowski, W.J.Schaff, and L.F.East man, Journal of applied physics, 87, 1, 334 (2000). (18) R.Dimitrov, M.Murphy, J. Smart, W.Schaff, J.R.Shealy, and L.F. Eastman, Journal of applied physics, 87,7,3375 (2000). (19) Mike Cooke, The advanced semi conductor magazine, 19, 1, 20 (2006). (20) V V Buniatyan and V M Aroutiounian, J. Phys. D: Appl. Phys., 40, 6355 (2007). (21) Masumi Fukuta, History of HEMT Transistors http://www.fqud.fujitsu.com/Hemt/ (2002). (22) Ricardo Borges, GaN High Electron Mobility Transistors (HEMT) Nitronex company website. (23) L.Aucoin, Electronic parts Engineering office 514. (24) Umesh K.Mishra, Likun Shen, Thomas E. Kazior, and Yi-Feng Wu, Proceedings of the IEEE, 96, 2, 287 (2008). (25) Takashi Mimura, IEEE Transactions on Microwave Theory and Techniques, 50, 3, 780 (2002). (26) Takashi Mimura, Japanese Journal of Applied Physics, 44, 12, 8263 (2005). (27) Richard Stevenson, compound semiconductor, 25, (2008). (28) Nor Zaihar Yahaya, Mumtaj B egam Kassim Raethar and Mohammad Awan, Journal of Power Electr onics, 9, 1, 36 (2009). (29) A. Poghossian, S. I ngebrandt, A. Offenhusser, and M. J. Schning, Seminars in Cell & Developmental Biology, 20, 41 (2009). (30) Errachid A, Zine N, Samitier J, Bausells J., Electroanalysis, 16, 1843 (2004). (31) Thedinga E, Kob A, Holst H, Keuer A, Dr echsler S, Niendorf R, et al. Toxicol Appl Pharmacol, 220, 33 (2007). (32) Chang-Soo Lee 1, Sang Kyu Kim 1,2 and Moonil Kim, Sensor s, 9, 7111 (2009). (33) Marco Curreli, Rui Zhang, Fumiaki N. Ishikawa, Hsiao-Kang Chang, Richard J. Cote, Chongwu Zhou, and Mark E. Thompson, IEEE TRANSACTIONS ON NANOTECHNOLOGY, 7, 6, 651(2008). (34) Kenzo Maehashi and Kazuhiko Matsumoto, Sens ors, 9, 5368 (2009).

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118 BIOGRAPHICAL SKETCH Ke-Hung Chen grew up in Kaohs iung, Taiw an. He received his bachelors degree at the National Taiwan University in 2001 and his masters degree at National Tsing Hua University in 2003. He joined MEGIC Corporation from July 2003 to January 2006. He received the basic military training during October~December in 2003. He worked for DuPont Taiwan from February 2006 to July 2007. He began his graduate studies at the University of Florida in August 2007 and joined Professor Fan Rens semiconductor material and device research group to purs ue a doctorate degree. He graduated in the May of 2010 after spending three years being educated in chem ical engineering and material science.