Piezoresistive MEMS Underwater Shear Stress ... - Stanford University

PIEZORESISTIVE MEMS UNDERWATER SHEAR STRESS SENSORS A.A. Barlian, R. Narain, J.T. Li, C.E. Quance, A.C. Ho, V. Mukundan, B.L. Pruitt Stanford University, Stanford, California, USA ABSTRACT We report on the design and performance of underwater piezoresistive floating-element shear stress sensors for direct dynamic measurements. Our design utilizes sidewallimplanted piezoresistors to measure lateral force and infer shear stress, and traditional top-implanted piezoresistors to detect normal forces and pressure transients. A gravity-driven flume was used to test the sensors. FEMLAB simulation and microscale Particle Image Velocimetry experiments were used to characterize the flow disturbance over different gap sizes. The results show no detectable disturbance of the flow over the range of sensor gap sizes evaluated (5-20µm).

1. INTRODUCTION Micro Electromechanical Systems (MEMS) shear stress sensors offer the potential to make measurements in fluid with unprecedented sensitivity, spatial, and temporal resolution. Most MEMS shear stress sensors have been developed for measurements in air [1-5] and utilize indirect methods [4-6], which require a priori knowledge of flow profiles, in situ calibration under identical conditions, and are limited by heat transfer when used in liquid (hot wire/film anemometry). Naughton et al. highlighted the need for further work on MEMS scale direct measurement methods [7]. These sensors are designed to study the hydrodynamics of wavy flow profiles and surface roughness effects in coral reef environments, as well as in oscillatory flowing cell-culture, and cardiovascular mockups. The sensors will also allow detection of flow reversals in turbulent flow and normal force due to flow separation. We present results of underwater testing in an open water channel. Micro Particle Image Velocimetry (µPIV) experiments were done to characterize the effect of gap size on flow. The results were then compared to finite element simulations (FEMLAB). Parylene and triplex layers of SiO2Si3N4-SiO2 were investigated as a passivation scheme against water.

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Figure 1: (Left) Piezoresistive floating-element shear stress sensor. (Top Right) SEM image of a 500-µm floating element with 15 µm tether width. (Bottom Right) The side-wall implanted piezoresistors on one of the tethers

3. FABRICATION Shear Stress Sensors

2. DESIGN Our design is based on the floating element concept [1-3] shown in Fig.1. It consists of a plate element suspended by four tethers. Piezoresistors are placed at the root of each tether and shear stress is inferred from beam deflection and resulting strains. Each sensor measures normal and lateral forces simultaneously. The orientations of the piezoresistors were

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chosen such that two are sensitive to lateral (along the flow direction), while the other two are sensitive to out-of-plane deflections. Sensors with varying tether, plate geometry and gap sizes were fabricated to evaluate parametric effects at the microscale, including: dimensions of the tethers (264-1236 µm long, 7-15 µm wide, and 7-12 µm thick), plates (40-1030 µm square), and their ratios (0.1-5); gap sizes (5-20 µm); and geometry (squares, rectangles). Fabrication details and the bench-top calibration methods are reported elsewhere [8]. The in-plane and out-of-plane sensitivities due to lateral force were found to be 0.063 and 0.008 mV/Pa using a Wheatstone bridge configuration (confirming low crosstalk), respectively. The out-of-plane sensitivity of the top-implanted piezoresistor to normal force was found to be 0.04 mV/Pa. Hydrogen annealing to smooth the DRIE scalloped silicon sidewalls [9] before implant also reduced 1/f noise in oblique-implanted piezoresistors by almost an order of magnitude [8].

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The fabrication process starts with 4” n-type (Ph-doped) double-polished (100) Silicon-on-Insulator (SOI) wafers with device layer thickness of 7, 10, and 12 µm for varied range of flow sensitivity. Ion implantation (Boron) forms the top piezoresistors and the conducting regions (Fig.2a). Low Temperature Oxide (LTO) was deposited in preparation of the

MEMS 2006, Istanbul, Turkey, 22-26 January 2006.

side-wall implant. LTO and silicon were then etched to pattern the geometry of the sensor using Reactive Ion Etching (RIE) and Deep Reactive Ion Etch (DRIE), respectively (Fig. 2b). The wafers were hydrogen-annealed to smooth the side wall from the resulting scallops due to DRIE process [8, 9]. The top surface was patterned near the root for an angled ion implant (Boron) at 20° angle from normal axis (Fig. 2c). Passivation oxide was thermally grown, patterned and etched using 6:1 Buffered Oxide Etch / BOE (34% NH4F, 7% HF, and 59% water) to open vias for aluminum to the conducting region. Aluminum was sputtered, patterned, and etched back using aluminum etch (72% Phosphoric Acid, 3% Acetic Acid, 3% Nitric Acid, and 12% water). The sensors were then released from the backside using DRIE process and 6:1 BOE to etch silicon and the buried oxide, respectively (Fig. 2d). Finally, the wafers were forming gas (hydrogen) annealed for 2 hours at 400°C.

Figure 3: Polymer flip-chip flexible interconnects (left). Microscope image of the gold trace and the conductive polymer on top of the 500µm bond pads (top right). A sensor chip and flexible interconnect on nylon plug fixture which is mounted flush with bottom of the flume (bottom right). The resulting flexible interconnect is flipped and contacted onto the bond pads on the sensor chip and thermally bonded at 180°C (about 30 °C above the thermoplastic conductive polymer melting temperature), while light pressure was applied. The temperature was held at 180°C for three minutes before it was cooled down. The base resistance of the sidewall-implanted piezoresistor was 0.85 kȍ and the added resistance from the interconnects was ~0.05 kȍ. A shear sensor chip integrated with flexible interconnect was then mounted on a microscope slide while the large bond pads on the interconnects were contacted with pogo pins installed on a nylon fixture. The nylon fixture flush mounts the sensor in a water channel and isolates the electronics.

4. EXPERIMENTAL SETUP AND RESULTS Underwater Test

Figure 2: Fabrication steps of the shear stress sensors. Note that the implant for conducting region and passivation oxide are not shown in the figure. Polymer Flip-chip Flexible Interconnect Polymer flip-chip flexible interconnect [10] fabrication starts with DuPont HN125 kapton film (125µm-thick) taped onto a silicon wafer. Cr/Au (35/350nm) was evaporated and metal traces were patterned by wet etch (Transene gold etch). The metal traces were 100 µm wide and bond pads are 2 mm (for contact to pogo pins) and 500 µm (for chip bonding). Next, conductive polymer (Epo-Tek-K/5022-115BE) were deposited, squeegeed, cured in a convection oven at 110°C for 15 minutes, and patterned by lift-off method using 15µmthick photoresist (SPR220-7). Figure 3 shows the resulting polymer flip-chip flexible interconnects.

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A gravity-driven water flume (length, L = 14’, width, W= 4’, and sidewall height, H = 1’) was used to test the performance of the sensors underwater (Fig. 4). The tilt angle of the flume was fixed at 0.0025 radians. Water is circulated by two variable speed pumps and the flow was fully developed, steady, and uniform at any water channel cross section. Theoretical bottom wall shear stress, a function of the flow rate, is inferred from W S f UgR H Q , where Sf, ȡ, g, RH, and Q are air-water interface slope, water density, gravity, the water channel hydraulic radius, and flow rate, respectively. Output from sidewall-implanted piezoresistors was captured at 1.1 Hz for 180 seconds. The data were timeaveraged, excluding the first 60 seconds to allow for settling. Temperature of the water near the bottom wall of the channel was measured using a thermocouple located 5 cm to the left of the sensor. Change in resistance due to temperature sensitivity was adjusted from the output based on the temperature

coefficient of sensitivity of 0.0081 kŸ/°C [8]. Resistance measurements were captured using an HP34401A Digital Multi Meter (DMM). The performance of the shear stress sensor is shown in Fig. 5.

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Microscale Particle Image Velocimetry (µPIV) The sensor was placed on a side wall of a molded PDMS mini channel with dimension of 2 mm x 2 mm (Fig 6). A Leica DM IRB inverted microscope and CCD Leica DFC350 FX were used to visualize the plane perpendicular to the center of the sensor’s plate element. Fluorescent beads (0.7µm, Duke Scientific Inc.) were diluted to 0.01% solid by volume and dispensed into the channel at a constant rate by a syringe pump (Harvard Apparatus HA11WD). Recorded images of the particles at 15 frames/second for 6 seconds and an ensemble averaging algorithm were used to calculate the velocity vectors using interrogation regions of 50 x 50 pixels with 50% overlap. Mercury Lamp

Figure 4: Gravity driven water flume

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Figure 6: Schematic diagram of the experimental setup. The test was run for sensors with 5, 10, 15, and 20-µm gaps. Typical flow velocity ~ 1 mm/s.

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The experiment was done under steady and fully developed flow (constant pressure). We find no significant disturbance to the flow profile near the sensors due to the gap. The results are in agreement with a 2-D FEMLAB simulation (Fig. 7), which shows that there are streamlines in the gap, however their magnitude is approximately 1000X less than the average velocity over the sensors.

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Figure 5: Plot of the normalized change in resistance with respect to varying bottom wall shear stress. The base resistance value was 0.9 kȍ. The base fit line shows a sensitivity (ǻR/R/Pa) of -0.0156 Pa-1 The scatter in the experimental results was due to uncertainties in the measurements. Uncertainty in the slope of the water flume measurements was ±0.0001 radians, which corresponds to about ±0.02 Pa of shear stress. Uncertainty in the measurements of the height of the water film (±300 µm) and the local variation of the temperature at the sensor surface due to convection, which affects the local density of water, were considered negligible in the calculation of theoretical shear stress. The uncertainty bars are the standard deviation of the averaged value of the normalized change in resistance for a given flow rate (shear stress). The experimental results are still much larger than our predicted normalized change in resistance values based on beam mechanics. We are

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Reliability Test Triplex layers of SiO2-Si3N4-SiO2 [11] and Parylene C [6] are under study as passivation schemes underwater. Some sensors were coated with triplex layers of SiO2-Si3N4-SiO2 (135nm/715nm/130nm) and some were coated with 1.5 µm of Parylene C (PDS 2010). A surface mount board and the parylene coated sensor were mounted on a microscope slide, wirebonded, and the sensing region was submerged underwater. A simple voltage divider circuit with bias voltage of 10V was constructed, with one fixed resistor (1 kŸ) and one piezoresistor (1.7 kŸ). Measurements were done using a GPIB controlled HP34401A DMM.

Parylene C (1.5 µm) was shown to be reliable as a passivation layer against Aluminum electrolysis for at least 8 days. The test failure was due to corrosion of wirebonds above the waterline. Triplex-layers-passivated sensors immersed underwater with a 10V bias survived for at least 36 hours. The drift of the piezoresistor during the 8-day period was less than 2%, which corresponds to ǻR/R of -0.0331.

Facility (a member of the National Nanotechnology Infrastructure Network) which is supported by the National Science Foundation under Grant ECS-9731293, its lab members, and the industrial members of the Stanford Center for Integrated Systems. The authors would like to thank Dr. Fong and Venayagamoorthy and Profs. Koseff and Monismith for input on underwater experiments and Prof. Meng (USC) for parylene deposition.

7. REFERENCES

Figure 7: The results for 10-µm gap size FEMLAB simulation (top) and its corresponding µPIV experimental results (bottom).

5. CONCLUSIONS AND FUTURE WORK A piezoresistive MEMS shear stress sensor for underwater applications has been designed, fabricated, calibrated, and tested in a water channel. Key fabrication techniques included oblique implant of piezoresistors and hydrogen anneal to smooth scalloped silicon surface resulted from DRIE process. Ultimately, arrays of Wheatstone bridges with temperature compensation, amplification, and multiplexing will be utilized for future experiments. Improvements of the design of the sensors and electrical through-wafer interconnects need to be implemented on future designs to obtain the desired reliability.

6. ACKNOWLODGEMENTS This work was supported by NSF CTS-0428889. Fabrication work was performed in part at the Stanford Nanofabrication

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[1] S. Horowitz, et al., "A Wafer-Bonded, Floating Element Shear-Stress Sensor Using a Geometric Moire Optical Transduction Technique," Solid-State Sensor, Actuator and Microsystems Workshop, Hilton Head Island, South Carolina, USA, 2004. [2] A. Padmanabhan, et al., "A wafer-bonded floatingelement shear stress microsensor with optical position sensing by photodiodes," Microelectromechanical Systems, Journal of, vol. 5, pp. 307-315, 1996. [3] J. Zhe, et al., "A microfabricated wall shear-stress sensor with capacitative sensing," Microelectromechanical Systems, Journal of, vol. 14, pp. 167-175, 2005. [4] C. Liu, et al., "A micromachined flow shear-stress sensor based on thermal transfer principles," Microelectromechanical Systems, Journal of, vol. 8, pp. 90-99, 1999. [5] F. Jiang, et al., "Flexible shear stress sensor skin for aerodynamics applications," Micro Electro Mechanical Systems, 2000. MEMS 2000. The Thirteenth Annual International Conference on, 2000. [6] Y. Xu, et al., "Underwater flexible shear-stress sensor skins," Micro Electro Mechanical Systems, 2004. 17th IEEE International Conference on. (MEMS), 2004. [7] J. W. Naughton and M. Sheplak, "Modern developments in shear-stress measurement," Progress in Aerospace Sciences, vol. 38, pp. 515-570, 2002. [8] A. A. Barlian, et al., "Design, Fabrication, and Characterization of Piezoresistive MEMS Shear Stress Sensors," 2005 ASME International Mechanical Engineering Congress and Exposition (IMECE 2005), Orlando, Florida, 2005. [9] M.-C. M. Lee, et al., "Silicon Profile Transformation and Sidewall Roughness Reduction Using Hydrogen Annealing," IEEE International Conference on MEMS, Miami, FL, USA, 2005. [10] C. Li, et al., "Polymer flip-chip bonding of pressure sensors on flexible Kapton film for neonatal catheters," Micro Electro Mechanical Systems, 2004. 17th IEEE International Conference on. (MEMS), 2004. [11] G. Schmitt, et al., "Passivation and corrosion of microelectrode arrays," Electrochimica Acta, vol. 44, pp. 3865-3883, 1999.