This paper reports the application of selective parylene C deposition for the waterproof coating of underwater shear-stress sensors. The selective deposition has been achieved by electrically heating the sensing element during deposition, utilizing the dependence of parylene deposition rate on substrate temperature. The electrical power supplied was only 20 mW to raise the temperature of the sensing element 210°C above ambient due to the sensor's excellent thermal isolation. After selective parylene deposition, the sensing element was still exposed while other areas were covered by parylene. More effective interaction between the sensor and water was realized. With selective parylene deposition, excellent waterproof and high shear-stress sensitivity can be achieved at the same time

Tai, Yu-Chong

Xu, Yong

Caltech Authors - Main

3E35.P 
SELECTIVE DEPOSITION OF PARYLENE C FOR UNDERWATER SHEAR-STRESS 
SENSORS 
Yong Xu (I) ,  Yu-Chong Tai ( 2 )  
‘”Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202, USA 
‘Z’Electrical Engineering, California Institute of Technology, Pasadena, CA 91 125, USA 
ABSTRACT 
This paper reports the application of selective 
parylene C deposition for the waterproof coating of 
underwater shear-stress sensors. The selective deposition 
has been achieved by electrically heating the sensing 
element during deposition, utilizing the dependence of 
parylene deposition rate on substrate temperature. The 
electrical power supplied was only 20 mW to raise the 
temperature of the sensing element 210 “C above 
ambient due to the sensor’s excellent thermal isolation. 
After selective parylene deposition, the sensing element 
was still exposed while other areas were covered by 
parylene. More effective interaction between the sensor 
and water was realized. With selective parylene 
deposition, excellent waterproof and high shear-stress 
sensitivity can he achieved at the same time. 
INTRODUCTION 
Parylene is the generic name for members of unique 
family of thermoplastic polymers that are vapor phase 
deposited by sublimating dimer of para-xylylene. The 
three most common types of parylene are referred to as: 
parylene N, parylene C, and parylene D. Parylene has 
found numerous applications in micromachined 
transducers. Especially, it is used as protective coating 
against moistures, corrosives, and even body fluids 
because parylene exhibits excellent harrier properties [I]. 
In many cases, windows need to be opened to achieve a 
better interaction between transducers and environments. 
Usually, oxygen plasma is used to etch parylene and 
photoresist is chosen as mask material. The etch rates of 
parylene and photoresist in oxygen plasma are 
approximately same. This makes the patterning of thick 
parylene a troublesome process. Moreover, sometimes 
the parylene coating has to he carried out after packaging, 
in which case the conventional photolithography and 
oxygen plasma process is not applicable. 
One example is the shear-stress sensor for 
underwater applications [2]. The structure of the sensor, 
which is schematically shown in Fig. 1, is a polysilicon 
resistor seating atop a nitride diaphragm with a vacuum. 
cavity underneath. During operation, the polysilicon 
resistor is heated electrically, while the fluid flowing 
over tends to cool it down. The input power of the 
sensor will change with the wall shear stress of the 
ambient flow field and this change can he readily 
detected electrically. The power is dissipated in two 
ways: conduction loss to substrate and convection loss to 
the fluid. The heat loss through radiation is negligible at 
the normal operation temperature. To increase the 
sensitivity and reduce the power consumption, we want 
most of the beat to be transferred to the fluid by 
convections. Namely, it is desirable to minimize the heat 
loss to substrate. This is achieved by the vacuum cavity 
underneath the polysilicon beater. 
Nitride passivation Polysilicon resistor 
Nitride diaphragm \ PaTleny C I Metal lead 
Figure 1. Cross section’ of the underwater shear-stress 
sensor uniformly coated with parylene C .  
0 5 l0  15 
Power (mw) 
Figure 2. Temperature increase in water as a function of 
input power of one sensor coated with different thick 
parylene layers. 
To operate under water, a waterproof layer needs to 
be deposited to encapsulate the metal wires. Parylene C 
is chosen as the waterproof material doe to its small 
moisture transmission rate and its capability to he 
conveniently vapor phase deposited [2]. This waterproof 
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layer (as shown in Fig. l),  on the other hand, limits the 
convective heat transfer from the sensor to water, 
resulting in the decrease of shear-stress sensitivity. 
Shown in the Fig. 2 is the temperature increase vs. power 
curves measured in water of the same sensor coated with 
different thick parylene C layers. The curves in Fig. 2 
are derived from I-V measurements of the sensor at zero 
shear stress. This static thermal characteristic is very 
easy to obtain, but provides valuable information ahout 
the performance of the shear-stress sensor. It can be 
observed in Fig. 2 that as the parylene layer thickness 
increases, the curve shifts to the left. This means that 
less power is dissipated to water for thicker parylene 
layers, resulting in a lower sensitivity. 
To achieve high sensitivity, the parylene layer 
should be as thin as possible. On the other hand, the 
parylene layer should be relatively thick for waterproof 
purpose. There seems to be a contradiction here. 
Nevertheless, if we observe the cross section of the 
sensor in Fig. 1 carefully, we can find out that the 
sensing element itself has already been encapsulated by 
silicon nitride, which is a superior harrier material. 
Therefore, there is no need to deposit parylene on the 
sensing element. Windows can be opened on the 
diaphragm to achieve a better thermal interaction 
between sensor and water. However, in this case we 
need to coat parylene after dicing and wire bonding the 
chip, which precludes the conventional photolithography 
and oxygen plasma etching process. Selective parylene 
deposition is employed here to expose sensing elements 
without using photolithography. 
EXPERIMENT 
Pyrolysis: 690°C 
(Dimer) 
Col Vacuum 
-70°C Pump 
Figure 3. Simplified parylene deposition process. 
Adapted from [3]. 
The deposition process of parylene is illustrated in 
Fig. 3. First, solid dimer is vaporized at about 170 "C 
and then followed by the pyrolysis of the dimer at about 
690 "C to yield monomer. Next, monomer enters the 
deposition chamber and deposits on any surfaces 
exposed. However, the deposition will only occur at 
surfaces where the temperature is below the ceiling 
temperature of polymerization. Therefore, the selective 
parylene deposition can be simply achieved by passing a 
current to the sensing element to electrically heat it 
during the deposition [4]. 
With a cavity undemeath, only very small power is 
needed to heat up the shear-stress sensor as shown in Fig. 
4. In the experiment, the input power is about 20 mW to 
achieve an average temperature increase of 210 "C above 
ambient temperature. The 20 mW power can be easily 
provided by a battery. It is worth noting that the 
temperature distribution on the polysilicon resistor is not 
uniform. What Fig. 4 shows is the average temperature 
increase of the polysilicon resistor. Another issue we 
need to consider is the substrate temperature increase. If 
the substrate temperature is too high, no deposition will 
occur. A polysilicon temperature sensor is integrated on 
the substrate to monitor the temperature. As shown in 
Fig. 5 ,  the temperature increase of the substrate is less 
than 3 "C after supplying electrical power to the sensor. 
Therefore, no substrate cooling is required during the 
deposition. 
305 
E 250 
2 2w 
.- 2 150 
g la, 
c 
0 5 10 15 20 25 30 
I Power (mw) 
Figure 4. Temperature increase of the sensing element as 
a function of input power in vacuum. 
. * *  
2.5 . .  
0.5 
Figure 5.  Temperature increase of the substrate in 
vacuum after turning on the power. 
The selective parylene C deposition was camed out 
using a regular deposition system without any 
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modification. A chip with multiple shear-stress sensors 
[2] was first attached to a printed circuits board, then 
wire bonded and soldered. Before loading this chip to 
the deposition chamber (Fig. 3 ) ,  a battery was connected 
to one sensor on the chip to electrically heat it up. Other 
sensors on the chip were not powered and remained at 
room temperature. These sensors will he compared with 
the one selectively coated with parylene C. .The selective 
deposition of 5.2 pm parylene C was finished within 3 
hours. 
RESULTS AND DISCUSSIONS 
Figure 6 shows the picture of the sensor selectively 
coated with 5.2 pm parylene C taken under a microscope. 
The interference rings caused by the thickness variation 
can he clearly observed. Very interestingly, these rings 
also represent the isothermal curves on the diaphragm, 
since parylene should have same thickness on isothermal 
curves. The isothermal curves on the diaphragm are 
difficult to obtain using infrared cameras due to the 
infrared radiation reflected back from the bottom of the 
cavity 
Figure 6. Micrograph of the shear-stress sensor 
selectively coated with 5.2 pm parylene C. The 
diaphragm dimension is 210x210 pm2. 
The surface profiles before and after 5.2 pm 
selective parylene C deposition are shown in Fig. 7 (a) 
and (b). These curves were obtained by scanning along 
the center of the diaphragm (along the dashed line shown 
in Fig. 6) using a stylus surface profiler. In Figure 7 (a), 
the spike in the center is caused by the polysilicon 
resistor (0.6 pm) while the two small spikes in the edge 
are due to the hird's beak of local oxidation [2]. Figure 7 
(c) shows the thickness of parylene as a function of 
position after selectively depositing 5.2 pm parylene C 
by subtracting (a) from (b). Outside the cavity, the 
parylene thickness is 5.2 pm. Starting from the edge of 
the cavity, the thickness decreases sharply. 
1.2 
Polysilicon iim 
g 0 2  
4.2 
Position (pm) 
I ,  I 
% J  
Position (pm) 
0 100 200 300 400 
Position (sm) 
(C) 
Figure 7. (a) Surface profile before parylene deposition; 
(b) surface profile after 5.2 pm selective parylene 
deposition; (c) thickness of parylene across the center of 
the diaphragm. 
At steady state, the temperature along the center of 
the diaphragm (the dashed line in Fig. 6) can be 
described by the following I-D equation due to the 
symmetry: 
where h is the.convective heat transfer coefficient of 
ambient fluid, k i s  the thermal conductivity of diaphragm 
material, d i s  diaphragm thickness, and T, is the ambient 
temperature. Because the deposition is camed out in 
vacuum, the convection can be ignored (H). Namely, 
the right side of the equation is equal to zero. Therefore, 
the temperature along the center of the diaphragm is a 
linear function of the position in vacuum. It is worth 
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noting that thickness of parylene across the center of the 
diaphragm is not constant as shown in Fig. 7 (c). 
Nevertheless, the thickness of silicon nitride is 4 p n ,  
comparable with the parylene laye;, and the thermal 
conductivity of low stress silicon nitride (3.2 WImK) is 
much larger than that of parylene C (0.082 W/mK). The 
effect of the parylene layer on thermal conduction can he 
neglected to a first order approximation. The conclusion 
that the temperature along the center of the diaphragm is 
a linear function of the position is still valid. Based on 
this, we can derive the parylene thickness as a function 
of temperature from Fig. 7 (c). It can be estimated that 
when the temperature is above 140 "C, the parylene C 
deposition rate is negligible. 
To characterize the performance of the underwater 
shear-stress sensor, we can perform the shear-stress 
calibration in a well-characterized water tunnel. On the 
other hand, we can obtain the temperature vs. power 
relationship at zero shear stress easily by perfnrming an 
I-V curve measurement. As mentioned previously, this 
static thermal characteristic provides valuable 
information on the performance of the shear-stress sensor. 
Figure 8 compares the static thermal characteristics of 
,two sensors, one uniformly coated with 5.2 p n  parylene 
C (Sl) and the other selectively coated with 5.2 um 
parylene C (SZ), in water and vacuum respectively 
5 XI E 
Power (mW) 
Figure 8. Temperature increase as a function of power 
consumption of two sensors (S1 and S2) in vacuum and 
water respectively. S1: uniformly coated with 5.2 km 
parylene C; S2: selectively coated with 5.2 pm parylene 
C 
The heat loss (power consumption) of the sensor 
consists of two parts: convective heat loss to ambient 
fluid y,,  and conductive heat loss to substrate through 
nitride and parylene thin films q2. In vacuum, the 
convective heat loss can be neglected and the total heat 
loss is only the conductive heat loss q2. When the sensor 
is immersed in water, the total heat loss is the sum of q, 
and y2. To the first order approximation, q 2  can he 
considered as constants at the same temperature. 
Therefore, the heat convectively transferred to water (4,) 
can be estimated by the horizontal difference of- the 
sensor's temperature-power curves measured in vacuum 
and water respectively. It can be observed in Fig. 8 that 
the sensor selectively coated with parylene C ($42) has 
more heat transferred to water than the sensor uniformly 
coated with parylene C (Sl). For instance, with an 
operation temperature 40 "C above ambient, S2 has ahout 
5.3 mW power transferred to water while S1 has only 
about 2.6 mW. Therefore, the sensor with selective 
parylene deposition has better thermal interaction with 
water and is expected to have higher sensitivity. 
CONCLUSIONS 
The selective parylene coating has been achieved 
simply by electrically heating the sensor during 
deposition. Due to the sensor's excellent thermal 
isolation, only very small power (20 mW) was needed 
and the substrate temperature increase was minimized 
(<3 "C). The selective deposition process was carried out 
with a regular parylene coating system. We also 
obtained the isothermal curves on the sensor diaphragm. 
The ceiling temperature of parylene C, above which the 
deposition rate is negligible, was estimated to be 140 "C. 
The temperature increase as a function of input power 
was also measured and compared for sensors selectively 
and uniformly coated with parylene C. It is concluded 
that with selective parylene deposition, excellent 
waterproof and high shear-stress sensitivity can he 
achieved at the same time. The same method can also be 
employed in many other applications such as the 
packaging of some hiosensors. 
ACKNOWLEDGMENTS 
This project was partly supported by the Office of 
Naval Research through an SBIR grant. The authors 
would also like to acknowledge the help of MI. Hongyu 
Yu and Dr. Qiao Lin. 
REFERENCES 
[l] "Parylene Data Sheet," Specialty Coating System, 
Indianapolis. 
[2] Y. Xu, F. Jiang, Q. Lin, J. Clendenen, S .  Tung, and 
Y:C. Tai, "Underwater Shear-Stress Sensor," 
presented at IEEE International Conference on 
Micro Electro Mechanical Systems (MEMS), Las 
Vegas, Nevada, 2002. 
Mechanical Systems," Ph.D. thesis, California 
Institute of Technology, 2000 
[4] E. M. Charlson, E. J. Charlson, and R. Sabeti, 
"Temperature Selective Deposition of Parylene-C," 
ZEEE Transsqcrions on Biomedcal Engineering, vol. 
[3] X. Q. Wang, "Integrated Parylene Micro Electro 
39, pp. 202-206, 1992. 
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