We have demonstrated the first parylene MEMS thermal sensor array capable of detecting small flows down to 0.5 µl/min. The device is fabricated using low-temperature parylene/platinum technology and consists of metallic sensors suspended on a membrane over a bulk micromachined silicon channel. For the first time, insightful results from three different methods of flow sensing using a single device are studied and compared. Multi-mode testing using hot film, calorimetric, and time-of-flight techniques at low overheat ratios has been performed. Thus, this device is biocompatible in both construction and operation

Meng, Ellis

Tai, Yu-Chong

Ellis Meng

Yu-chong Tai

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A Parylene MEMS Flow Sensing Array

Caltech Authors - Main

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A PARYLENE MEMS FLOW SENSING ARRAY 
Ellis Meng and Yu-Chong Tai 
Department of Electrical Engineering, Caltech Micromachining Lab 
Califomia Institute of Technology, 136-93, Pasadena, CA 91 126, USA 
ABSTRACT 
We have demonstrated the first parylene MEMS thermal 
sensor array capable of detecting small flows down to 0.5 
pVmin. The device is fabricated using low-temperature 
paryleneiplatinum technology and consists of metallic 
sensors suspended on a membrane over a bulk 
micromachined silicon channel. For the first time, 
insightful results from three different 'methods of flow 
sensing using a single device are studied and compared. 
Multi-mode testing using hot film, calorimetric, and time- 
of-flight techniques at low' overheat ratios has been 
performed. Thus, this device is biocompatible in both 
construction and operation. . 
' e  
INTRODUCTION 
Thermal flow sensing, techniques have long been popular 
for their simplicity in both structure and electronic 
implementation. . Despite the well-known. problems of 
sensor drift and dependence on chemical properties of the 
fluid, interest in thermal sensing for . microfluidics 
applications continues to grow. Recent developments 
include the use of thermal sensors to determine the thermal 
properties of the sensed fluid (thermal diffusivity and 
conductivity) [ l ,  21: 
Using only an array of resistive sensors, it is possible to 
detect thermal properties as well as operate under multiple 
sensing methods. The three most popular thermal flow 
sensing methods are hot-film, calorimetric, and time-of- 
flight [3-61. All three methods are implemented and 
demonstrated in this device. 
DESIGN AND FABRICATION 
A linear array' of seven metallic sensing elements is 
sandwiched in a membrane on one side of a bulk- 
micromachined channel. The channel dimensions are 
1 hm x 0.5 mm x 8 mm and the sensors are spaced 500 
pm apart, cenier-to-center. BY utilizing an array layout, it 
is possible to operate the device in many different modes 
and determine .the 'flow d&ection as well as the thermal 
profile in the fluid. Sensor structure can be seen in Figure 
1 and consists of platinum sensing elements embedded in a 
parylene membrane. Parylene is chosen as the membrane 
material for its transparency, barrier properties, and 
biocompatibility. Platinum is also biocompatible and 
compatible with' parylene technology. In addition, it 
exhibits excellent corrosion and oxidation resistance and is 
thermally stable over a wide temperature range. Special 
silicon couplers [7] are also used to make fluidic 
connections possible to the device. 
.-._-___- .... 1- -I.._---_____- 
Figure 1 3D Views of Sensor Construction 
The fabrication steps for this multi-mode flow sensing 
array are shown in Figure 2 and is modified &om the 
original in [PI. All process steps are low-temperature and 
thus compatible with parylene technology. 
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-, . 5  ....... &Zz3 
I I . . . . . . . . . . . . . . . . . . . . . . .  
5:- U- __.-~ 
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Una h-w 1 Ybwk Blh 
Figure 2 Sensor Process Flow 
I. .. .A:.-. ...... . ~. Lc---l ..._-. ._-_. .. 
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c.M::.'-. :::.=3 
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First, thermally oxidized wafers (1.8 pm thick) are covered 
with a 2 pm thick coating of parylene. Contact openings 
are made and then sensors are patterned. Successhl sensor 
formation requires special lithography technique to create a 
two-layer undercut photoresist lift-off mask. A 1000 A 
thick platinum film with a 200 A of TiAN for adhesion is 
sputter deposited onto this mask and defined using lift-off. 
A second layer of protective parylene (4 pm) is deposited 
over the sensors and the contact pads are selectively 
uncovered using oxygen plasma etching. These pads are 
plated with gold using an electroless plating formulation 
(Stapleton Technologies Inc.) to facilitate packaging the 
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2E72.P 
device. On the back side of the wafer, the channel is 
pattemed and etched through to the oxide etch stop using 
deep reactive ion etching of silicon. The oxide layer is then 
removed by buffered hydrofluoric acid vapor etching. 
Individual devices are then separated by dicing. 
Figure 3 Fabricated Sensor Array Suspended Over Channel 
PACKAGING 
Both fluidic and electrical connections are required to 
operate the device. First a glass backing plate with 
mechanically drilled inlet and outlet boles is glued to the 
back side of each device to form an enclosed channel. This 
structure is ready to be mounted on a custom printed circuit 
board. A slot is conventionally milled into the PCB prior to 
mounting the device to allow for fluidic access. Packaging 
is designed such that electrical and fluidic connections are 
separate and are accessed on opposite sides of the PCB. 
Once the device is glued down to the PCB, wire bonding is 
performed. Then bulk-micromachined silicon fluidic 
couplers with PEEK tubing (Upchurch Scientific) are glued 
into place. These couplers are described extensively in [7]. 
Front and back views of fabricated and packaged devices 
are shown in Figure 4 and Figure 5. 
Figure 4 Front Side of Packaged Sensor Wire Bonded to PCB 
EXPERIMENTAL 
Calibration Experiments. Sensor temperature calibration 
data is obtained by measuring sensor resistances (HP 
34970A) in a calibrated oven at known temperatures (Delta 
Design 9010). The calculated TCR based on these 
measurements, shown in Figure 6, is l.OxlO" /"C , It is 
well known and unfortunate that thin film platinum has a 
much lower TCR than that of $e bulk material 
( 3 . 9 2 ~ 1 0 . ~  /"C ). For biological applications, large 
increases in temperature cannot be tolerated. However, to 
maximize frequency response and sensitivity, the overheat 
ratio should be made as high as possible. Given these 
factors, tradeoffs are oecessluy and a maximum overheat 
temperature of  25 "C is imposed when choosing sensor 
operation points. 
Figure 5 Back Side of Packaged Sensor Mounted on Milled PCB 
Showing Fluidic Connections 
0.c. 
0.WW , , , . , . , , . ~. 1 
11 I II) lo I O  $0 ,m 
TemprUun I'CI 
Figure 6 Temperature Calibration Data 
Flow Testing. Flow has been monitored using three 
different techniques. For each test, flow sensors were 
placed in thermally isolating enclosures to minimize 
fluctuations in sensor output due to ambient temperature 
variations. Precise flows of filtered deionized water were 
delivered to the sensors using precision instrument syringes 
(Hamilton Company) driven by a syringe pump. The 
testing setup described is depicted in Figure 7. 
Hot-Film. In hot-film operation, heat loss from a hot body 
exposed to fluid flow is measured. For metallic sensors, 
the response is in part governed by the TCR of the sensing 
material. In the hot-film mode, sensors were biased at 
constant current and placed in a half bridge for temperamre 
compensation purposes. The bridge is zeroed using a 
precision, multi-tuin potentiometer. Constant temperature 
operation is also possible but requires addition feedback 
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circuitry. For simplicity, only constant current operation is 
studied here. The bridge output vs. flow rate was observed 
for three different overheat ratios and is given in Figure 8. 
PC 
Figure I Sensor Testing Setup 
Calorimetric. Calorimetric sensing involves monitoring 
the temperature profile displacement around a heating 
element in the presence of flow. Structurally speaking, a 
calorimetric device must consist of at least one heater, 
 upstream sensor, and' downstream sensor. Conveniently, 
one of the resistive sensing elements in the array can be 
converted into a heater. Asymmetric or symmetric 
arrangements of sensors can he used.. Here, the output of 
sensors placed in a bridge and arranged asymmetrically 
around a heater is shown in Figure 9. A detail of results at 
lower flow rates is also shown in Figure IO. 
Time-of-Flight. Time-of-flight flow sensing is a thermal 
tracing technique where a heat pulse generated at 'an 
upstream heater is detected downstream. The item of 
interest is the time where the maximum response of the 
detected signal occurs, otherwise known as the top time. A 
typical sensed profile for 30 plimin flow is shown in Figure 
11. Testing results (Figure 12) displayed the expected 
inverse relationship between top time and flow'rate. At 
low flow rates, diffusivity dominates the time-of-flight 
response. Heater-sensor spacing and average flow velocity 
come into play at higher flow rates. Superimposed 
responses at different flow rates are displayed in Figure 13. 
' 
" 
- 7  
F l a  Ram Wml 
Figure 8 Hot-Film Mode Testing Results 
0 0  . , . , . , . , . , . , I  
0 I 4D ,I m )I m 
Fiw RN "1 
Figure 9 Calorimetric Mode Testing Results 
Flo* RN Wm] 
Figure 10 Calorimetric Mode Testing Results at Low Flow Rates 
rim. [SI 
Figure 11 Typical Detected Heat Pulse for 30 pVmin Flow 
1 . , . , . , . , . ,  , I  
; * 1 . 4 6 * I s  
Flar Rat. bum, 
Figure 12 Time-of-Flight Mode Testing Results for Applied Heat 
Pulses of 3V with 1s Duration 
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*.z - 
l.0- 
0.8 - 
0.6- 
E. 
5 0.4: 
0.2 - 
0.0 1 
I ,  I 
0 1 2 3 4 5 S 
Time [SI 
Figure 13 Superimposed Sensor Responses at Different Flow 
Rates in Time-of-Flight Mode Testing 
DISCUSSION 
It is useful to compare the sensitivity of the various testing 
methods. Sensitivity is defined as the derivative of the 
sensor signal with respect to the flow rate. In the case of 
non-linear flow characteristics, the sensitivity at zero flow 
(So) is used and is expressed as: 
where V is the sensor voltage output and Q is the 
volumetric flow rate. Sensitivity values for hot film and 
calorimetric techniques were calculated and are 
summarized in Table 1. 
Table 1 Summary of Sensor Data 
TOF Calori- metric Hot-Film 
3.3 9.2 15.5 9.2 23.5 Power 
(mWI . ,  
IO 15 20 15 24 Overheat Temperature ("C) 
OverheatRatio 1.0 1.5 2.0 1.5 2.4 
(%) 
3.45 5 .11  2.67 71.26 NIA Sensitivity (uV/uVmin) 
From these results, calorimetric methods clearly yield the 
best sensitivity by an order of magnitude in comparison to 
hot-film techniques. For flow rates lower than 10 @min, 
calorimetric sensing is preferred. However, at a certain 
point above 50 plimin, the maximum temperature 
difference between the sensor pair is reached and the 
response starts to decrease. The tail, or cooling part, of  the 
calorimetric response can he used to increase operational 
flow range at the expense of decreased sensitivity [9, IO]. 
A similar nonlinear response is seen in the hot-film results 
starting roughly at 100 pllmin. Time-of-flight flow sensing 
tends to require a large applied heat pulse and suffers from 
low resolution but is useful over large ranges. 
CONCLUSION 
A thermal flow sensing array capable of multi-mode 
operation has been designed, fabricated, and tested. This 
sensor is biocompatible'hoth in terms of operation and 
materials used. Of the three operational modes examined, 
the calorimetric method provides the most sensitivity over a 
large range of flow rates. 
Acknowledgements 
This work was funded in part by the Engineering Research 
Centers Program of the National Science Foundation under 
Award Number EEC-9402726. We would like to thank Mr 
Trevor Roper for assistance in fabrication and Mr. Tuan 
Hoang for help with experiments and proofreading. 
References 
[ I ]  van Baar, J.J., R.W. Wiegerink,T.S.J. Lammerink, G.J.M. 
Krijnen, and M. Elwenspoek, Micromachined structures for 
the thermal measurements offluid andflow parameters. 
Jonrnal of Micromechanics and Microengineering. 2001. 
~ - 
ll(4):p. 311-318. 
121 van Kuiik. J.. T.S.J. Lammerink. H.-E. de Bree. M L 1  ~. I 
Elwenspoek, and J.H.J. Fluitman, Multi-parameter detection 
influidflows. Sensors & Actuators A, 1995.47(1-3): p. 369- 
372. 
[3] van Oudheusden, B.W., Silicon thermalflow sensors. 
Sensors and Actuators A, 1992.30(1-2): p. 5-26. 
[4] Yang, C.Q. and H. Soeberg, Monolithic Flow Sensor For 
Measuring Milliliter Per Minute Liquid Flow. Sensors and 
Actuators A, 1992. 33(3): p. 143-153. 
151 Nguyen, N.-T. and W. Dotzel, Asymmetrical locations of 
heaters and sensors relative to each other using heater 
arrays: a novel method for designing multi-range 
electrocaloric moss-flow sensors. Sensors and Actuators A, 
1997. 62(1-3): p. 506-512. 
[6] Nguyen, N.-T., Thermal Mass Flow Sensors, in The 
Measuremenl,'lnstrumentation, and Sensors Handbook, J.G. 
Wehster, Editor. 1999, CRC Press: Boca Raton, Florida. 
[7] Meng, E., S.  Wu, and Y.-C. Tai, Silicon couplers for 
microfluidic applications. Fresenius Journal of Analytical 
Themishy, 2001.371(2): p. 270-275. 
[8] Meng, E., S.  Gassmann, and Y.-C. Tai, A MEMS Bo& Fluid 
Flow Sensor, in Micro Total Analysis Systems 2001.2001: . .  
Monterey, CA. p. 167-168. 
191 van Oudheusden, B.W. and J.H. Huiisinn, Intearafedklow .. 
Friction Sensor. Sensors and Actuators,i988. i5(2): p. 135- 
144. 
Flow Sensor On a Thin Dielectric Membrane. Sensors and 
Actuators A, 1993.37-8: p. 33-37. 
[IO] Moser, D. and H. Bakes, A High-Sensitivify CMOS Gas- 
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A parylene MEMS flow sensing array

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A parylene MEMS flow sensing array

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