Characterization of a MEMS Optical Floating-element Shear Stress Sensor

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Characterization of a MEMS Optical Floating-element Shear Stress Sensor
Pena, Jhoanna
Nishida, Toshikazu ( Mentor )
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Gainesville, Fla.
University of Florida
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Characterization of a MEMS Optical Floating-element Shear Stress Sensor

Jhoanna Peha


Microelectromechanical systems (MEMS), a new field of technological advances, consists of integrated devices

or systems that combine electrical and mechanical components. Such systems are commonly fabricated using

silicon, the most dominant material in MEMS devices [1]. These systems can sense, control and actuate on the

micro scale. MEMS applications include accelerometers, pressure, chemical and flow sensors, and optical

scanners. We have explored an optical floating-element fluid shear stress sensor that permits the

direct measurement of wall shear stress based on geometric Moire interferometry. The characterization of this

device includes sampling the displacement of the sensor with variable mean shear stress, and the calculation of

the actual mechanical displacement to check for accuracy of linear output from the sample data.


The Flow of fluid over a surface is a phenomenon that occurs anywhere, for instance in our blood vessels, on

the body of the car, and in a pipe. The motion of a fluid with respect to a surface creates a boundary layer. Flow

in this layer may be laminar or turbulent which is determined by the Reynolds number. Fox and McDonald

defined Reynolds number as "the ratio of inertia to viscous forces" [2], however there is not unique value at

which transition from laminar to turbulent flow occurs in a boundary layer. Laminar flows refer to flows that go

slowly and smoothly, where particles of fluid move parallel to a surface wall. On the other hand, turbulent flows

refer to flows with fast velocities that are highly unsteady due to more interaction between the surface and the

fluid. The detection and control of turbulent flows has been of importance not only in the fluid mechanics but also

in the chemical and environmental community because of negative effects on viscosity, drag, and shear stress.

The development of an optical floating-element shear stress sensor in this work was motivated by a need for a

direct measurement of shear stress in a water recovery system. Such systems are important for the treatment

of wastewater to produce potable water. Measurements of time resolved shear stress give information on

flow separation, transition from laminar to turbulent flow, and re-attachment. In this project, we explored and

tested the functionality of a MEMS optical floating-element flow shear stress sensor (Fig.1). Schmidt defined

a floating-element shear stress sensor as "an element that is free to displace laterally against the restoring force

set up by springs, flush mounted into the wall" [3]. The floating element sensor is an example of direct

force measurement. Geometric Moire interferometry was used as the optical detection principle to measure

the displacement of the floating element.

Figure: Microscopic top view of the floating-element.

A geometric Moire interference pattern is formed when light impinges on two overlapping gratings with

different spacing of pitch. A grating is defined as a repetitive series of dark and light bars. When superimposing

two sets of gratings of slightly different pitch creates a new image with black as well as illuminated sections of

a specific period known as the Moird effect. When the top grating attached to a floating element is displaced due

to shear stress, it will create a linear amplified fringe shift that can be used to calculate the displacement of

the floating element. Therefore, the sensor provides a direct indication of the impinging flow. The optical

detection scheme is based on the detection of the Moird fringe pattern motion using a CCD imaging array.

Further, this paper will explain in more detail wall shear stress, the Moird effect, and the sensor design

specification in the background section followed by the settings of the actual experiment that includes sampling

the displacement of the sensor with variable mean shear stress and the calculation of the actual

mechanical displacement for a given shear stress. At last, the results of the experiment will be discussed.


Wall Shear Stress

The motion of a fluid with respect to a surface creates a boundary layer (Fig.2) due to the no-

slip boundary condition. The no-slip boundary condition describes the absence of fluid motion on a

fixed surface. When the fluid comes in contact with the surface, it creates a force [4]. The

force perpendicular to the surface is pressure while the force parallel to the surface is known as

shear stress. Measurement of time resolved shear stress gives information on the condition of the

flow laminarr or turbulent), flow separation, and re-attachment. In other words, being able to

measure skin-friction will help to measure, analyze, and control flow as it interacts with the

surface. Further, if we want to measure shear stress in a fluid pipe, it is necessary to have a small

device for high spatial and temporal resolution [5]. Therefore silicon-micromachined flow sensors

have been developed to measure flow, shear stress and accurately capture the turbulent

flow fluctuations.

..................................... , .... U Freestreamrn --

i nlet - -
............................... - . ...............................----
............................... F lo w .y/.e .

Shear stress Wall-Mounted
Shear stress
Figure 2: Boundary Layer, courtesy of V. Chandrasekaran.

Moire Effect

The Moire effect is one of the optical detection techniques used as a measurement tool for

the determination of relative displacement [6]. According to Post and Ifju, "The Moire fringes are

broad dark and light bands formed by the superposition of two amplitude gratings each comprised

with opaque bars and clear spaces" [7]. If we have a set of fringes (Fig. 3), and we overlap them (Fig.

4), we create a pattern whose displacement is proportional to the physical displacement. Such a

Moire pattern originates from the geometric distribution of light passing through the gratings.

a b
Figure 3: Gratings of 19mm (a) and 20mm (b) pitch [8].

Figure 4: Moire Pattern of 19mm and 20mm pitches [8].

The displacement of the fringes is determined as follows [7]:

A =6[G/gl],

where G is the fringe pitch, gl is the pitch of movable grating, and G/g1 is the

amplification factor.

The Moire effect may be considered as a spatial amplifier as can be seen by the amplification factor

from the above formula.

The optical floating-element sensor consists of a floating micro-machined silicon plate held by

four tethers patterned with a grating on the bottom surface and a lower Pyrex plate also patterned with

a set of gratings on the top surface (Fig. 5). When the device is illuminated from the bottom of the

Pyrex, the movement of the upper plate in contrast with the bottom one will produce a shift in the

two gratings. Since light is transmitted through the superposed top and bottom gratings, a Moire

fringe pattern is created. When the floating element is displaced due to the shear stress, it will create

a linear amplified fringe shift that can be used to calculate the displacement of the floating element.

The MEMS optical flow-element shear stress sensor was previously developed in the

Interdisciplinary Microsystems group [8].

BOX (0.4 ?m) SOI (A) SOl (C)

Si(10 ?m) Al(025 ?m) Pyrex

(B) -*******
Pyrex (B)

Figure 5: A Schematic of the fabrication sequence: (A) Etch 2 pm recess on SOX

wafers and deposit and pattern device gratings (0.25 pm - Al). (B) Deposit 0.25

mm of aluminum on the Pyrex wafer and pattern handle gratings. (C) Anodically

bond Pyrex and SOI wafers. (D) Aligned DRIE up to the recess to release the

floating element [8].

Sensor Design Specification

In order to measure shear stress, for instance in a fluid pipe, it is necessary to have a small device

for spatial and temporal resolution. Therefore micromachined silicon flow sensors have been

developed to measure flow, shear stress and accurately capture the turbulent flow fluctuations

[5]. Silicon micromachining is a process that creates microscopic mechanical parts out of the

silicon substrate, for instance the tethers of this sensor. According to Naughton and

Sheplak, "micromachined wall shear stress sensors can be grouped into two distinct

measurement classes: direct techniques such as floating-element type devices, or indirect

techniques such as thermal and optical sensors" [5]. The sensor specified below uses direct

techniques to measure the force produce by the shear stress on the floating element. The MEMS

optical floating-element flow shear stress sensor consists of a 1280pm x 400pm silicon floating

element of 10pm thickness, suspended 2.0pm above the surface of a Pyrex wafer by four tethers (Fig.

6). The Moire fringes are realized by pattern aluminum lines with varying pitch on both the bottom of

the floating element and on the Pyrex wafer [8].

Tether -

Aluminum --

Transmitted Floauing clement
Moirc fringe
v Silicon

Incident light

Figure 6: Top-view and cross sectional schematic of the first-generation optical shear stress sensor



For this experiment, a sensor with a set of gratings of 9.9pm and 10pm was used. The sensor die

was flush-mounted in a Lucite plug with back-side imaging optics and a Thomson-CSF TH78CE13

linescan CCD camera (Fig. 7). The CCD camera contains an array of 1 x 1024 pixels, each having a

width of 10pm. The package was mounted in a 2-D flow cell that provides a variable mean shear

stress. The Moire fringe was captured using the CCD camera. First, we started by grabbing a live image

of the sensor with no flow (zero shear stress) using a frame grabber. Then, flow was turned on

and several images were taken at different pressure levels. Once all the images were collected, it

was necessary to calculate the displacement of the floating element by taking the no-flow image

and comparing it with the ones of active flow. Using Matlab, a least squares curve fit was applied to

the recorded intensity pattern obtained for a given shear stress input. The following formula was

applied for the least-squares curve fit [8]:

y(x) = AO + Al*sin(A2*x+A3)

where AO is the offset, Al is the amplitude, A2 is the frequency, and A3 is the phase.

The least squares method assumes that the best-fit curve is the curve that has the minimal sum of

least square errors from a given set of data. The intensity frequency for such a curve needed

was determined using the least square procedure to fit a sinusoidal intensity pattern to the

measured pattern (zero shear stress) and then determine the amplitude, DC offset, frequency, and

phase of the sinusoid. Once the frequency was determined, then it was used as a constant for

the remaining sample data.

Floating Sensor Package

KLamninar flow crl

Incident Light

Figure 7: Schematic diagram of optical testbed for sensor characterization.

The same procedure is used for a each shear stress and the corresponding intensity pattern to

obtain new phase of the sinusoid. By looking at the phase shift, that is the phase between the no-

flow pattern and the rest of the data, the number of pixels shifted by the Moire fringe is calculated.

Based on the Moire optical amplification, the corresponding mechanical displacement of the

floating element was determined using the following formula

Sensor Displacement= [AMoir6 shift *gJ]/ G

where DMoird shift= (02-01)/ frequency and 1/G= (1/g9)-(1/g2).

The sensor displacement gives a direct measurement of the wall shear stress. The displacement, A,

of floating element as a function of wall shear stress is given as follows [5]:

A= Tw (LeWe/ 4Et)(Lt/Wt)3[1 + 2(LtWt/LeWe)]

where Tw = shear stress, Lt= tether length, Wt= tether width, t= tether thickness, and E=elastic

modulus of tether (Fig. 8).

Flow -w
P- X t

^ Floating Flet-netitr


Figure 8: Schematic plan view and cross-section of a typical floating-element sensor [5].


The recorded Moire intensity pattern for a shear stress of 0 Pa and 1.6 Pa is shown in grayscale in Fig.

9 and the relative intensities are shown vs. pixel number in fig. 10. The Moire pattern was found to

have a spatial period of 943pm, before the 5x optical amplification, compared to the physical

grating period of 9.9pm. The Moire amplification for the sensor was found to be 95.3 compared to

the theoretical value of designed 100.

Figure 9: Moire fringe pattern for shear stress of 0 Pa (a) and 1.6 Pa (b) as seen by

1024 pixel linescan camera. Successive frames from the camera are stacked vertically

within each rectangle.


I-I - x

5 OPa
---- Fit - 0 Pa

- Fit - 1.6 Pa

50 100 150 200 250 300 350 400 450
Pixel Number
Figure 10: Measured relative pixel intensity overlaid with least squares curve
fit for 0 Pa and 1.6 Pa of shear stress.

Furthermore, the number of pixels shifted by the Moire pattern was determined for a range of
applied shear stress. The results are shown in Fig. 11 and 12. The mechanical sensitivity found from
the slope of this curve is 0.56pm/Pa, while the optical sensitivity of the Moire fringe after the 5x
optical amplification is 266.84pm/Pa.





0 0.4 0.8 1.2 1.6
Shear Stress II'a|
Figure 11: Mechanical displacement of the floating element of the shear-stress sensor.


0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Sthar Stress (Pa)

Figure 12: Number of pixels shifted by the Moir6 fringe in pixels for a given shear stress.


Using MEMS technology for development and packaging, an optical flow shear stress sensor utilizing a

geometric Moird based transduction technique was statically characterized with applied shear stress from 0.1Pa

to 1.6Pa. In general, a linear trend was seen between the mechanical and optical displacement and the applied

shear stress, showing the potential utility of this sensor and transduction technique. Future work includes

rigorous dynamic and static characterization, noise floor studies, and sensitivity analysis to non-shear stress inputs.
S -7'S

rigorous dynamic and static characterization, noise floor studies, and sensitivity analysis to non-shear stress inputs.


Approximately The author would like to thank Professor Toshikazu Nishida, Professor Mark Sheplak, Professor

Lou Cattafesta, Steve Horowitz, Venkat Chandrasekaran, and Julio Castro for their assistance and cooperation in

this project.


1. Gardner, J., Varadan, V., Awadelkarim, 0., Microsensors MEMS and Smart Devices, John Wiley & Sons, New

York, 2001.

2. Fox, R. McDonald, A., Introduction to Fluid Mechanics, John Wiley & Sons, New York, 1992.

3. Schmidt, M., Howe, R., Senturia, S., Haritonidis, J., Design and Calibration of a Microfabricated Floating-

Element Shear-Stress Sensor, IEEE Transactions on Electron Devices, Vol. 35, No. 6, pp. 750,1988.

4. Padmanabhan, A., Goldgerg, H., Breuer, K., Schmidt, M., A Wafer-Bonded floating element shear stress

microsensor with optical position sensing by photodiodes, Journal of Microelectromechanical Systems, Vol. 5, No.

4, pp. 307,1996.

5. Naughton W., J., Sheplak, M., Modern developments in shear-stress measurement, Progress in Aerospace

Sciences, pp.516-520, 2002.

6. Kuhnert, R., Bernd, M., Pattern sequential analysis of second-order Moire Interferograms, Physical

Research, Akademic- Verlag Berlin, pp.101-102, 1989.

7. Post, D., Han, B., Ifju, P., High Sensitivity Moire, Springer, New York, pp.103-105, 1994.

8. The MEMS optical floating-element shear stress sensor was designed by Mark Sheplak and Toshikazu Nishida,

with Steve Horowitz and Hamed Kourouma under NASA Langley Research Center support and monitored by Dr.

K. Tedjojuwono. It was fabricated at the Stanford Nanofabrication facility by Venkat Chandrasekaran and

post-procedure at ARL by Steve Horowitz, Kourouma, H., Thesis on Design and Analysis of an optical

detection scheme for micromachined floating-element shear stress sensors, University of Florida, 2002.


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