A PARYLENE MEMS FLOW SENSING ARRAY
Ellis Meng and Yu-Chong Tai
Department of Electrical Engineering, Caltech Micromachining Lab
California Institute of Technology, 136-93, Pasadena, CA 91126, USA
ABSTRACT
We have demonstrated the first parylene MEMS thermal
sensor array capable of detecting small flows down to 0.5µl/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 flowsensing using a single device are studied and compared.
Multi-mode testing using hot film, calorimetric, and timeof-flight techniques at low overheat ratios has beenperformed. Thus, this device is biocompatible in bothconstruction and operation.

INTRODUCTION
Thermal flow sensing techniques have long been popular
for their simplicity in both structure and electronicimplementation. Despite the well-known problems ofsensor drift and dependence on chemical properties of thefluid, 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 andconductivity) [1, 2].

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 from theoriginal in [8]. All process steps are low-temperature andthus compatible with parylene technology.

Using only an array of resistive sensors, it is possible todetect thermal properties as well as operate under multiple
sensing methods. The three most popular thermal flow
sensing methods are hot-film, calorimetric, and time-offlight [3-6]. All three methods are implemented anddemonstrated in this device.

DESIGN AND FABRICATION
A linear array of seven metallic sensing elements issandwiched in a membrane on one side of a bulkmicromachined channel. The channel dimensions are
1 mm × 0.5 mm × 8 mm and the sensors are spaced 500µm apart, center-to-center. By utilizing an array layout, it
is possible to operate the device in many different modes
and determine the flow direction 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 aparylene membrane. Parylene is chosen as the membranematerial for its transparency, barrier properties, andbiocompatibility. Platinum is also biocompatible andcompatible with parylene technology. In addition, itexhibits excellent corrosion and oxidation resistance and isthermally stable over a wide temperature range. Special
silicon couplers [7] are also used to make fluidic
connections possible to the device.

Figure 2 Sensor Process FlowFirst, thermally oxidized wafers (1.8 µm thick) are covered
with a 2 µm thick coating of parylene. Contact openings
are made and then sensors are patterned. Successful sensor
formation requires special lithography technique to create a
two-layer undercut photoresist lift-off mask. A 1000 Å
thick platinum film with a 200 Å of Ti/W for adhesion issputter deposited onto this mask and defined using lift-off.
A second layer of protective parylene (4 µm) is depositedover the sensors and the contact pads are selectively
uncovered using oxygen plasma etching. These pads areplated with gold using an electroless plating formulation(Stapleton Technologies Inc.) to facilitate packaging thedevice. On the back side of the wafer, the channel ispatterned and etched through to the oxide etch stop using
deep reactive ion etching of silicon. The oxide layer is thenremoved by buffered hydrofluoric acid vapor etching.
Individual devices are then separated by dicing.

well known and unfortunate that thin film platinum has amuch lower TCR than that of the bulk material( 3.92 × 10-3 °C ). For biological applications, large
increases in temperature cannot be tolerated. However, tomaximize frequency response and sensitivity, the overheat
ratio should be made as high as possible. Given thesefactors, tradeoffs are necessary and a maximum overheattemperature of 25 °C is imposed when choosing sensor
operation points.

Figure 3 Fabricated Sensor Array Suspended Over Channel

PACKAGING
Both fluidic and electrical connections are required tooperate the device. First a glass backing plate withmechanically drilled inlet and outlet holes is glued to theback side of each device to form an enclosed channel. Thisstructure is ready to be mounted on a custom printed circuitboard. A slot is conventionally milled into the PCB prior tomounting 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 isperformed.
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 5 Back Side of Packaged Sensor Mounted on Milled PCBShowing Fluidic Connections

Figure 6 Temperature Calibration Data

Figure 4 Front Side of Packaged Sensor Wire Bonded to PCB

EXPERIMENTAL
Calibration Experiments. Sensor temperature calibration
data is obtained by measuring sensor resistances (HP34970A) in a calibrated oven at known temperatures (DeltaDesign 9010). The calculated TCR based on thesemeasurements, shown in Figure 6, is 1.0×10-3 °C . It is

Flow Testing. Flow has been monitored using threedifferent techniques. For each test, flow sensors wereplaced in thermally isolating enclosures to minimize
fluctuations in sensor output due to ambient temperature
variations. Precise flows of filtered deionized water weredelivered 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 atconstant current and placed in a half bridge for temperaturecompensation purposes. The bridge is zeroed using aprecision, multi-turn potentiometer. Constant temperature
operation is also possible but requires addition feedback

circuitry. For simplicity, only constant current operation isstudied here. The bridge output vs. flow rate was observed
for three different overheat ratios and is given in Figure 8.

Figure 9 Calorimetric Mode Testing Results

Figure 7 Sensor Testing SetupCalorimetric. Calorimetric sensing involves monitoring
the temperature profile displacement around a heating
element in the presence of flow. Structurally speaking, acalorimetric device must consist of at least one heater,
upstream sensor, and downstream sensor. Conveniently,
one of the resistive sensing elements in the array can beconverted into a heater.
Asymmetric or symmetricarrangements of sensors can be used. Here, the output ofsensors placed in a bridge and arranged asymmetricallyaround a heater is shown in Figure 9. A detail of results atlower flow rates is also shown in Figure 10.
Time-of-Flight. Time-of-flight flow sensing is a thermal
tracing technique where a heat pulse generated at anupstream heater is detected downstream. The item ofinterest is the time where the maximum response of thedetected signal occurs, otherwise known as the top time. A
typical sensed profile for 30 µl/min 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.

Figure 8 Hot-Film Mode Testing Results

Figure 10 Calorimetric Mode Testing Results at Low Flow Rates

Figure 11 Typical Detected Heat Pulse for 30 µl/min Flow

Figure 12 Time-of-Flight Mode Testing Results for Applied Heat
Pulses of 3V with 1s Duration

tends to require a large applied heat pulse and suffers fromlow resolution but is useful over large ranges.

CONCLUSION
A thermal flow sensing array capable of multi-mode
operation has been designed, fabricated, and tested. Thissensor is biocompatible both in terms of operation andmaterials used. Of the three operational modes examined,
the calorimetric method provides the most sensitivity over alarge range of flow rates.

Acknowledgements

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 thesensor signal with respect to the flow rate. In the case of
non-linear flow characteristics, the sensitivity at zero flow
( S0 ) is used and is expressed as:

S0 =dVdQ Q →0(1)where V is the sensor voltage output and Q is thevolumetric flow rate. Sensitivity values for hot film andcalorimetric techniques were calculated and aresummarized in Table 1.
Table 1 Summary of Sensor DataHot-Film
Power(mW)Overheat
Temperature (°C)
Overheat Ratio(%)
Sensitivity(µV/µl/min)CalorimetricTOF

3.3

9.2

15.5

9.2

23.5

1015201524

1.0

1.52.0

1.5

2.4

3.45

5.11

2.67

71.26

N/A

From these results, calorimetric methods clearly yield thebest sensitivity by an order of magnitude in comparison tohot-film techniques. For flow rates lower than 10 µl/min,calorimetric sensing is preferred. However, at a certain
point above 50 µl/min, the maximum temperature
difference between the sensor pair is reached and theresponse starts to decrease. The tail, or cooling part, of thecalorimetric response can be used to increase operational
flow range at the expense of decreased sensitivity [9, 10].
A similar nonlinear response is seen in the hot-film results
starting roughly at 100 µl/min. Time-of-flight flow sensing

This work was funded in part by the Engineering ResearchCenters Program of the National Science Foundation underAward 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.

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