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Underwater Shear Stress Sensor
Yong Xu, Fukang Jiang, Qiao Lin, Jason Clendenen, Steve Tung and Yu-Chong Tai

Abstract. A micromachined, vacuum-cavity insulated, thermal shear stress sensor is developed for underwater applications. The two major challenges for underwater application, namely the waterproof coating and pressure sensitivity, are specially studied for our device.

Shear stress measurement is of crucial importance for a lot of fluid dynamic monitoring/diagnostics applications [1,2,3]. Historically, however, almost all the MEMS effort has been spent on developing sensors in air, rather than in liquid (e.g. water). Here, for the first time, we report the development of a micromachined thermal shear stress sensor for underwater applications. As shown in Figure 1, the sensor is a polysilicon resistor sitting on a diaphragm with a vacuum cavity underneath, which provides excellent thermal isolation to reduce the heat loss to substrate. The input power of the resistor changes with the wall shear stress of the ambient fluid and this change can be readily detected electronically.

This design is based on our aerial shear stress sensor [2,3]. However, there are two major difficult challenges for the underwater applications. The first challenge is to develop a compatible waterproof coating to enable the sensor to operate underwater for longer than a month. The second challenge is to minimize the sensorsÕ pressure sensitivity for the operation under 10 ft of water, which imposes a 4.3 psi pressure range. For waterproof coating, LTO was originally investigated, but then abandoned because the sensor will be integrated onto a flexible skin [4] and tests show that LTO cracks easily to break metal wires. Then, Parylene was investigated and proved to be the right waterproof material because it is flexible, resistant to water transmittance, and easily CVD-deposited at room temperature [5]. Underwater tests show that, when operated at 55 °C, sensors coated with 2 mm Parylene-N can survive in water at least for a month. Longer surviving time is expected with Parylene-C, which has even smaller moisture vapor transmission. For pressure sensitivity, ideally the total strain across a clamped-edge square diaphragm should be zero. However, it is not the real case due to a lot of reasons, e.g., the intrinsic stress of nitride, the non-ideal boundary condition. So the applicable way is to decrease the size or increase the thickness of diaphragm. This, however, leads to more heat loss to substrate, and decreases the shear stress sensitivity. Here we report a comparative study on the effect of diaphragm dimension, which has not been addressed systematically before.

For example, Figure 2 shows a fabricated sensor chip with 2 rows of sensors with different diaphragm dimensions. We keep the length, L, a constant (210 mm), but the width, W, varies as 210 mm, 150 mm, 100 mm, 75 mm, and 45 mm. All the sensors have a large effective nitride thickness, t, of 4.0 mm. For our sensors, the total heat loss includes conduction loss to substrate (q1, q2, q3, and q4) and convection loss to fluid (q5) as shown in Figure 1. To achieve a high sensitivity, a larger q5 is desired. Figure 3 shows the static thermal characteristics (with zero shear stress) of the 210 mm wide sensor in vacuum, air and water respectively. In air, q5 only accounts for ~5% of the total heat loss. In water, q5 has much larger percentage (>45%) as expected. Figure 4 illustrates the static thermal characteristics of the five sensors in vacuum and water. Even for 45 mm wide sensor, there is still more than 15% of the power transferred to water. The pressure sensitivity is illustrated in Figure 5. Interestingly, for our sensor specifications (a pressure variation of 4.3 psi), 75mm wide sensor is the optimum design. The 45 mm sensor shows increase of resistance, which is due to the dominance of the transverse strain when W is much smaller than L. Figure 6 shows the calibration curve of a 1.8 mm thick, 210 mm´210 mm sensor in water. A sensitivity of 0.147V/Pascal is observed.


[1] R. J. Goldstein, Fluid Mechanics Measurements, Taylor & Francis, pp. 575-648, 1996.
[2] C. Liu, Y. C. Tai, J. B. Huang, and C. M. Ho, ASMEÕ94, pp. 9-15
[3] F. Jiang, Y. C. Tai, B. Gupta, R. Goodman, S. Tung, J. B. Huang, and C. M. Ho, MEMS '96, pp. 110-115
[4] F. Jiang, Y. Xu, T. Weng, Z. Han, Y.C. Tai , A. Huang, C. M. Ho , and S. Newbern, MEMS '00, pp. 364-369
[5] http://www.paryleneinc.com