We have demonstrated an entire series of capacitive sensors using a multi-layer parylene/photoresist surface micromachining technology. The developed sensors are designed for total integration into parylene-based microfluidic systems for real-time system monitoring. Sensors have been demonstrated for the following applications: in-line pressure sensing (range: 0-35 kPa, resolution: 0.03 kPa); liquid front position and/or volumetric measurements (range: 0->50 pL, resolution: <5 pL); and dielectric measurements, which can be used to deduce fluid properties such as liquid composition. The reported sensor technology demonstrates versatility, high sensitivity, small footprints, and easy integration

Shih, Jason

Tai, Yu-Chong

Xie, Jun

Caltech Authors - Main

203.2 
SURF ACE MICROMACHINED AND INTEGRATED CAPACITIVE SENSORS FOR 
MICROFLUIDIC APPLICATIONS 
Jason Shih, Jun Xie, Yu-Chong Tai 
Department of Electrical Engineering 
California Institute of Technology, CA 91125, US 
Tel.: (626) 395-1364, Fax: (626) 584-9104, email:jshih@touch.caltech.edu 
ABSTRACT 
We have demonstrated an entire series of 
capacitive sensors using a multi-layer 
parylene/photoresist surface micromachining technology. 
The developed sensors are designed for total integration 
into parylene-based microfluidic systems for real-time 
system monitoring. Sensors have been demonstrated for 
the following applications: l) in-line pressure sensing 
(range: 0 - 35 kPa, resolution: 0.03 kPa); 2) liquid front 
position and/or volumetric measurements (range: 0 - >50 
pL, resolution: < 5 pL); and 3) dielectric measurements, 
which can be used to deduce fluid properties such as 
liquid composition. The reported sensor technology 
demonstrates versatility, high sensitivity, small footprints, 
and easy integration. 
INTRODUCTION 
The total integration of many microfluidic 
components has been a long sought goal in the 
microfluidics community. The incompatibility of 
different processes renders many demonstrated devices as 
stand-alone. The multilayer parylene/photoresist surface 
micromachining technology utilized here, allows for the 
integration of different microfluidic components onto a 
single chip. Previously demonstrated microfluidic 
devices utilizing this technology include pumps, check 
valves [l], and a mass flow controller [2]. In addition, 
microfluidic systems such as an electrolysis-based system 
for electrospray ionization mass spectroscopy (ESl-MS) 
have been developed [3]. Because of the common 
technology used to create these devices, total integration 
is possible. But with the exception of a hot-wire type 
thermal flow sensor used in the mass flow controller [2], a 
sensor technology had yet to be developed. Sensors are 
necessary for feedback control and for the monitoring of 
the microfluidic system. Only then can the goal of 
autonomous lab-on-a-chip systems be realized. The 
presented capacitance based sensors fill this need. 
Previously, capacitive sensors have been applied 
to microfluidic applications in the form of pressure 
sensors and flow sensors based on a differential pressure 
principle [6]. Capacitive sensing has also been utilized in 
macrofluidic applications such as fluid level sensing, 
determination of ion concentration [7], and for measuring 
the makeup of mixtures ( eg. oil and water). The 
presented sensors exhibit all the functionality of the 
previously mentioned sensors, but wilh the added 
flexibility and ease of integration of our developed 
parylene/photoresist surface-micromachining technology. 
a 
b 
c 
Figure I: Micrographs and Cross Sectional Views 
(Oriented Along Channel) of the Fabricated Sensors, a. 
Pressure Sensor, b. Parallel Plate Configuration, c. 
Interdigitated Configuration 
DESIGN 
Three different sensor configurations are 
investigated. These include a membrane based pressure 
sensor; a parallel plate configuration for dielectric 
measurements; and an interdigitated configuration for use 
in volumetric and dielectric measurements. 
The pressure sensor is realized by creating a 
cavity beneath the channel. The upper electrode is 
sandwiched between the floor of the channel and another 
parylene layer, forming a 200 µrn diameter composite 
membrane. The bottom electrode is fixed on the 
substrate. The fabricated device and schematic cross 
section are shown in figure 1 a. Changes in the local 
channel pressure will cause a deflection of the 
membrane/electrode. The capacitance therefore varies as 
a function of the local pressure in the channel. The air 
gap is not sealed, making this a differential pressure 
sensor. Since the capacitive element is completely 
underneath the channel, the pressure reading is 
independent of the fluid being carried. 
The parallel plate electrode configuration 
consists of two electrodes, one placed in the roof of the 
channel and the other placed in the floor of the channel. 
This configuration is shown in figure I b. The channel 
height is 4 µm and the width of the channel and electrodes 
is 100 µm. Each electrode is 2 mm long. Both the 
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0-7803-7731-1/03/$17.00 ©2003 IEEE 388 
electrodes are electrically isolated from the fluid with a l 
µm parylene layer. Changes in the dielectric constant of 
the channel fluid will register as a capacitance change. 
Properties such as the mixing ratio of two pure liquids can 
be determined through appropriate calibration since the 
effective dielectric constant is a function of the· mixing 
ratio. Also, pure liquids can be identified provided they 
have different dielectric constants. Besides determining 
fluid properties; such a sensor can be used to easily 
determine whether a particular channel is filled. 
The interdigitated electrode configuration 
consists of two interlocking comb-shaped electrodes as 
seen in figure le. Both electrodes are placed directly 
beneath the floor of the channel. The spacing between 
adjacent fingers is 15 µm and the electrodes overlap by 
100 µm, also the width of the channel. The channel 
height is 4 µm and the total sensor length is 2 mm. Again, 
the electrodes are insulated from the channel with a I µm 
parylene layer. The fringing electric field reaches into the 
channel so changes in the dielectric constant of the 
channel fluid are registered as a capacitance change. 
Besides using the interdigitated configuration for the 
same applications mentioned for the parallel plate 
configuration, it can also be used to monitor fluid front 
position. As a column of liquid flows into an empty 
channel, a predictable increase in capacitance is seen. 
This capacitance is a function of the fluid front position, 
which gives a direct measurement of volume. 
Compared to the parallel plate configuration, the 
interdigitated configuration is simpler from a fabrication 
standpoint since only one layer of metal is needed. Also, 
the interdigitated configuration does not obscure the 
channel, therefore making it more desirable for calibration 
purposes. But, since the interdigitated sensor exploits the 
fringing electric fields, a weaker signal is expected. 
FABRICATION 
The sensors are fabricated using a multilayer 
parylene/photoresist surface micromachining technology. 
Four parylene layers and two sacrificial photoresist layers 
are used in the fabrication of these sensors. Because of 
the way these sensors are designed, they are automatically 
integrated into microchannels. Cr/ Au is used for the 
various electrodes and a sputtered silicon layer is used to 
ensure a freestanding pressure sensor membrane, a 
method described in [4]. 
The full fabrication process flow is shown in' 
figure 2. The process begins with the oxidation of a 
silicon wafer. The oxide is patterned and ORIE used to 
etch the backside fluidic ports that will be used to 
introduce liquids. A Cr/Au layer (100 Al 3000 A) is 
deposited and patterned. This metal layer defines the 
bottom electrodes of the pressure sensor and parallel plate 
configuration. This layer is also used to define both 
electrodes in the interdigitated configuration. After a I 
µm parylene layer is deposited, a 4 µm photoresist 
sacrificial layer defines the capacitive air gap in th~ 
pressure sensor. This sacrificial layer also defines the 
203.2 
tfl..._ ___ flU11.. ________ FJ 
Front and B:u;kside Oxide Patterning 
Backside ORIE Etching 
lst Cr/Au Deposition and Patterning (100 A/ 3000 A) 
1st Parylene Deposition and Patterning (I µm} 
03'1---~~-·n..J'l~~----~~---Pb 
Sputter Si and Patterning (3000 A) 
Isl Sacrificial Photorcsist and Patterning (4 µm) 
2nd Parylenc Deposition and Patterniiig (I µm) 
2nd Cr/Au Deposition and Patterning (100 A/ 1500 A) 
3rd Parylene Deposition and Patterning (I µm) 
2nd Sacrificial Photoresist Patterning (5 µm) 
4th Parylene Deposition and Patterning (I µm) 
ORIE Open Backside Inlet and Outlet 
Acetone Release Sacrificial Photoresist 
BrF3 Release Sac..Tificial Si 
DD D 
-
D 
Parylene Substrate Oxide 2nd Cr/ Au I st Cr/ Au Photoresist Sputtered Si 
Figure 2: Full Process Flow 
microchannel in which the parallel plate and interdigitated 
sensors are integrated. After another 1 µrn paf)'.lene lay~r, 
a second Cr/Au layer (JOO Al 1500 A) layer is deposited 
and patterned to produce the top electrodes of the pressure 
sensor and parallel plate configuration. A third I µm 
parylene layer is deposited and a 5 µm photoresist 
sacrificial layer is deposited and patterned to define the 
channel above the pressure sensor. Finally, ORIE is used 
to etch the fluidic ports completely through the wafer and 
the photoresist is released by acetone. Upon drying of the 
acetone, the pressure sensor membrane Will often stick 
down because of stiction. Gas phase BrF3 is used to etch 
away the sputtered silicon and produce a freestanding 
membrane. 
RESULTS AND DISCUSSION 
Pressure Sensor 
The pressure sensor is calibrated using a 
regulated pressure source. Pressure is applied to both 
ends of the channel simultaneously by means of a custom-
built Plexiglas jig, thus ensuring the channel pressure is at 
the desired value. The pressure is varied and the 
changing capacitance is measured. Capacitances are 
measured using a commercially available IC 
(Microsensors UCR MS3110). Calibrations of the 
pressure sensor are done using both water and air as the 
channel fluid. The results are shown in figure 3. The 
sensor resolution is estimated to be 0.03 kPa and the 
accuracy to be I. 0 kPa. 
TRANSDUCERS '03 
The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, June 8-12, 2003 
389 
203.2 
200 
~ 150 
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u 
5 
•• 
•• 
······-':----- ··-~-·---' ... ····!··· 
: . 
~ 
··•·:·······:······· 
···----:------· ······--'-· 
JO 15 20 25 30 
Pressure (kPaJ 
I 
35 
Figure 3: Pressure Sensor Calibration with Air and Water 
in the Channel. 
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0' 0 t--~~::;;;;::+;;;;;;;;:;~4::::'.---------i 
-100 -50 0 50 100 
x (um) 
Figure 4: The Membrane Profile at Different Pressures 
(From Top to Bottom (kPa): 0.00, 3.45, 7.03, 10.62, 
13.65, 17.23, 24.26) 
The response of the sensor shows a roughly 
linear increase in capacitance at pressures below 20 kPa. 
At higher pressures, the sensitivity decreases and the 
capacitance begins to plateau. This is likely caused by the 
membrane bottoming out. To investigate the pressure 
sensor behavior more thoroughly, the deflection of the top 
electrode is measured at various applied pressures using a 
WYKO NT2000 Optical Profiler. The membrane profile 
at different pressures is shown in Figure 4. 
· The initial membrane is not flat, as would be 
expected. Instead, the membrane exhibits a large 
downward deflection with the center of the membrane 
slightly bowed upward. This signals that the membrane 
has been distorted during the fabrication process. Most 
likely this deformation occurred during the drying 
process. After photoresist dissolution, stiction occurs 
betweens the pressure membrane and the floor of the 
capacitive air gap. The forces exerted on the membrane 
during drying may have been high enough to permanently 
deform the membrane. 
In the fabrication of the pressure sensor, a 4 µm 
photoresist layer is used to define the air gap. Our 
measurements, as seen in figure 4, indicate a total 
deflection of only l .S µm. This discrepancy is once again 
due to the distortion of the membrane. This distortion 
effectively decreases the pressure range of this sensor. 
Our n1easurements also indicate that the membrane 
bottoms out at approximately 20 kPa. This coincides 
precisely with the measured sensor response. 
A test of the pressure sensor is done by applying 
pressure to one end of the microchannel and leaving the 
other end open. The pressure sensor is located in the 
center of this channel. By varying the applied pressure 
and at the same time recording the pressure sensor output 
and flow rate, we are able to demonstrate the capability of 
this sensor to monitor the local pressure inside the 
channel. This also functions as flow sensor based on a 
differential-pressure principle. 
The applied pressure and the measured channel 
pressure are shown in figure 5, and are both related to the 
corresponding flow rate. The flow rate vs. applied 
pressure data is fit using equation 2, as found in [5]. 
wh 3 l:!.P Q = 0 (1 + 2aAf>) 
12vL 
(2) 
The 1+2a~P accounts for bulging of the microchannel. 
The measured data is fit assuming an air viscosity of 
1. 73e-5 kg/s-m. By fitting our data, a is found to be 
0.0065 /kPa. The measured pressure in the center of the 
channel is found to be precisely 0.41 of the applied 
pressure. The line corresponding to 0.41 of the applied 
pressure is shown in figure S. A similar test is done using 
ethanol as the channel fluid. The results are shown in 
figure 6. The pressure at the center of the channel is 
found to be approximately 0.46 of the applied pressure in 
this test. 
a= 0.0065 /kPa 
u = l. 73e-5 kg/s-n1 · ,., ,.,• 
········~·······--;·····.:.·.,~­
: ..... 
. :· -. . .. -: . --.. ---:·.: .... 
' ' ,... ... 
' : ..... : 
, 
• Applied Pressure 
- - - Theory(Eqn 2) 
·:·. ·-~~-.: ;.~~~-- -· .. ·:· .. -- -
• }vfeasured Pressure 
--Theory (0.41 P .... u.) 
O-t«-=+--1--+--+~..-+--+-t---+----t-..-"=i 
0 10 20 30 40 50 60 
Pressure (kPa) 
Figure 5: Monitoring of Pressure in the Channel (Air as 
the Fluid) 
40 a= 0.0311kPa 
'S 
a 30 ~ 
, 
u = l.2e-3 kg/s-1n " ··-··-•·····--·--····:---~-~---···· 
·' 
... ,., ~ 
" 20 2l 
] 10 
"-< 
--.. -----... ~ .. --. ----... ~ ... ·,..: .. 
; ... ,.. • Applied Prest-:ure 
, ...... fl .. ; --- Theory(Eqn2) 
.... - . - -~ ~:~ ._. :..·-- - . - - - - . : .... -- • rvieasured Pressure 
... ... ; --Theory (0.46 P IPfli•d) 
0-t«-=--+------f~-+~-+-~~~....--~~--, 
0 
Figure 6: 
5 10 15 20 
Pressure (kPa) 
Monitoring of Pressure in the Channel (Ethanol 
as the Fluid) 
Volumetric Sensor 
The volumetric sensor is calibrated by slowly 
introducing water into the interdigitated sensor. The 
introduction of the liquid is captured on video and 
correlated with the capacitance measurements. The 
measurements shown in figure 7 correspond to the 
TRANSDUCERS '03 
The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, June 8-12, 2003 
390 
capacitance as the fluid front passes over each successive 
finger. Since the cross section of the channel is fixed, this 
fluid front position corresponds directly to a volume. The 
spacing between each finger is 15 µm which corresponds 
to 6 pL. The capacitance change as a ~ction of volume 
is shown in figure 7. 
The sensor shows very high linearity in this short 
section of the sensor. A resolution of< 5 pL should be 
attainable. 
Dielectric Measurements 
One simple use of the parallel plate and 
interdigitated sensors is to detect the presence of a fluid in 
a channel. For example, the capacitance of our sensor 
would increase significantly if there is water in the 
channel as opposed to air. For the interdigitated sensor 
the increase in capacitance is measured to be I.I I pF and 
for the parallel plate configuration it is measured to be 
3.38 pF. Since different fluids have different dielectric 
constants, it is also possible to distinguish a large variety 
of fluids, provided their dielectric constants are different. 
The composition of a mixture of liquids can also 
be determined if the two liquids mix predictably. Since 
the effective dielectric constant is a function of the 
distribution of the two components as well as the 
individual dielectric constants, the distribution must be 
predictable for this method to work. A proof of concept is 
done using mixtures of Isopropyl Alcohol and water. The 
measured interdigitated capacitance increases for varying 
mixtures is shown in figure 8. Tbe capacitance is 
expressed as an increase over an empty channel. 
CONCLUSION 
The demonstrated sensors show great promise 
for microfluidic applications because of their high 
sensitivity and easy integration. The versatility of the 
parylene/photoresist surface micromachining technology 
allows for the integration of these sensors with other 
demonstrated devices for the creation of entire 
microfluidic systems. 
ACKNOWLEDGEMENTS 
This work is supported in part by NSF ERC 
Center at Caltech (Grant No. EEC-9402726), NIH (Grant 
No. 5ROIRR06217-10), and DARPAIMTO Bioflips 
Pro grain. 
REFERENCES 
[I] J. Xie, X. Yang, X.Q. Wang, _Y.C. Tai, "Surface 
Micromachined Leakage Proof Parylene Check Valve." 
14th IEEE International Conference on Micro Electro 
Mechanical Systems (MEMS '01). 2001. pg. 539-542. 
80 
~60 
v 
u § 40 
"[) 
~ 
gr 20 
u 
0 
() 
203.2 
. ., . 
. . . . ·:- .... -:·.... . .. -~ .... -:· .... 
. . . 
. . . 
. . . 
. . . . 
·····: ····-:·····:·····f····T···· 
. . . . 
. . . . 
...... ···-~- .. -~-- .............. ~-
. . . . . . 
. . . ' . . . 
. . . . . . 
. . . . . . 
. . . 
10 20 30 40 50 
Volume (pL) 
Figure 7: Capacitance as a Function of Volume for the 
Interdigitated Sensor 
1.2 
r;:;' 
Co.& l 0.4 
u 
. . . . . . . . 
...... ; ..... ; .... -:- .... ; .... -~ . . . ·: ..... ~ ..... 
. . . . . 
···-~·-···1····r··-·1·····r····1·····r····~ ····~----· 
-··-r··-·1····T····1··--·f····1·····r··-·r····: ····· 
:::::c::r:::r::::::::r:r::r::i __ ::-t::::-
0 0 +----+-+--+-+--+--t---+--t---+---1 
0.0 0.2 0.4 0.6 0.8 1.0 
IP A Percentage 
Figure 8: Capacitance as a Function of IPA Percentage 
for the Interdigitated Sensor 
[2] J. Xie, J. Shih, Y.C. Tai. "Integrated Surface-
Micromachined Mass Flow Controller." 16th IEEE 
International Conference on Micro Electro Mechanical 
Systems (MEMS '03). 2003. pg. 20-23. 
[3] J. Xie, Q. He, Y.C. Tai, J. Liu, T. Lee. "Electrolysis-
Based On-Chip Dispensing System for ESI-MS." 16th 
IEEE International Conference on Micro Electro 
Mechanical Systems (MEMS '03). 2003. pg. 443-446. 
(4] T.J. Yao, X. Yang, Y.C. Tai. "BrF3 Dry Release 
Technologies for Large Freestanding Parylene MEMS." 
International Conference on Solid-State Sensors aod 
Actuators (Transducers '01). 2001. pg. 652-655. 
[5] S. Wu, J. Mai, Y. Zahar, Y.C. Tai, C.M. Ho. "A 
Suspended microchannel with Integrated Temperature 
Sensors for High-Pressure Flow Studies." 11th IEEE 
International Workshop on Micro Electro Mechanical 
Systems (MEMS '98). 1998. pg. 87-92. 
[6] R.E. Oosterbroek, T.S.J. Lammerink, J.W. 
Berenschot, A. van den Berg, Elwenspoek. "Designing, 
Realization And Characterization Of A Novel Capacitive 
Pressure/Flow Sensor." International Conference on 
Solid-State Sensors and Actuators (Transducers '97). 
1997. pg.151-154. 
(7] K. Shida, A.B.M. Ismail. "Measurement of 
Concentration of Electrolytic Solution Based on Novel 
Non-Contact Multi-Sensing Techniques." IEEE 
Instrumentation and Measurement Technology 
Conference, 1999 (IMTC'99). 1999. pg. 1372- 1376. 
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391 


Surface micromachined and integrated capacitive sensors for microfluidic applications

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Surface micromachined and integrated capacitive sensors for microfluidic applications

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