Non-lithographically microfabricatedcapacitive pressure sensor for biomedicalapplications

D. Brox, A.R. Mohammadi and K. Takahata

A novel micromachined capacitive pressure sensor fabricated on ametal substrate without using a photolithographic process is reported.The device is constructed by thermal bonding of a Parylene membraneonto a 1.5 × 1.5 × 0.2 mm3 stainless-steel chip with a shallow cavitycreated using micro-electro-discharge machining. The fabricationapproach enables rapid, low-cost manufacturing of the device withbiocompatibility. The sensor is designed to provide a gauge pressurerange of 200 mmHg or greater to be potentially suitable for in vivoblood-pressure sensing applications. A highly linear response of2 fF/mmHg is demonstrated with the fabricated devices. The tempera-ture coefficient of the sensor is observed to be 250 ppm/8C.

Introduction: The construction of miniaturised capacitive pressuresensors has been founded on the micro-electro-mechanical systems(MEMS) technology using Si as the primary working material [1–3].In this approach, the sensor fabrication typically requires surface micro-machining and/or bulk micromachining (including wafer bonding)based on photolithographic processes to define cavities in the Si substratesand seal them with a deflectable electrode membrane hermetically [1].The device manufacturing involves a number of lithographic steps thatare time consuming and expensive. In addition, they are often unsuitedfor biomedical applications because of biocompatibility issues [4].Rather than using Si to form sensor membranes, groups have utilisedpolymers such as PDMS [5], Polyimide [6], and Parylene [7] asmembrane materials, although lithography-based techniques are stillrelied upon for the device fabrication. This Letter reports a capacitivegauge pressure sensor that is constructed by hermetic sealing of a cavitycreated in a stainless-steel chip with a deflectable membrane achievedthrough Parylene thermal bonding [8], eliminating the need for litho-graphy from the fabrication while ensuring the potential biocompatibilityof the device.

copper sheet

pressureAu

air

Ti

Parylenecapacitance

reading

cavity

Au(50nm)

Ti(2mm)

Parylene(1mm)

5mm

200mm

1.0mm

stainless steel chip

stainless steel chip

1.5mm

a

b

Fig. 1 Conceptual diagram of sensor

a Components of capacitive pressure sensor before Parylene bondingb Completed form of sensor in operation of pressure measurement

Design and fabrication: The developed pressure sensor consists of a1.5 × 1.5 × 0.2 mm3 stainless-steel substrate chip with a 1 × 1 mm2

cavity capped by a 3 mm-thick membrane of a Parylene-metal multilayerthat hermetically seals the cavity at atmospheric pressure (Fig. 1). Themetal layer of the membrane and the bottom of the cavity form thesensing capacitor. Micro-electro-discharge-machining (mEDM) is usedto create the cavity to a depth of 5 mm in a stainless-steel plate, aswell as to cut the chip component out of the plate (Fig. 2a). Prior tomEDM, the stainless-steel plate is mechanically polished to have asurface smoothness of �150 nm. In the cavity, four 300 mm-diameterholes with 70 mm depth are mEDMed to increase the volume of thecavity space and the deflection of the membrane (discussed later). Ona separate 100 mm-thick Cu sheet (surface smoothness ,50 nm),layers of 50 nm Au (with a 5 nm Ti adhesion layer) and 2 mm Ti aredeposited by e-beam evaporation, followed by coating of 1 mm-thickParylene-C film on both the Ti/Au/Cu sheet and the stainless steelchip (Fig. 1a). The Parylene surfaces of the two components are thenthermally bonded together in air at 1758C for 35 minutes under

ELECTRONICS LETTERS 1st September 2011 Vol

approximately 50 MPa of pressure applied using a custom mechanicalfixture [8]. After bonding, oxygen plasma and wet etching are performedto remove the Parylene film and the Ti/Au layer, respectively, exceptwhere the chip is bonded to the Parylene/Ti/Au/Cu sheet. Wetetching of the entire Cu sheet and the 5 nm Ti adhesion layer remainingon the chip leaves the Parylene/Ti/Au membrane suspended above thecavity, with the Ti/Au layer electrically insulated from the stainless steelsubstrate, thereby forming a capacitive pressure sensor (Fig. 2b).

stainlesssteel chip 70mm

deephole

300mm 500mm

5mm deepcavity

cavityedges

a b

cavity region

Au electrodeAu wire

Fig. 2 Fabricated sensor and its substrate

a SEM image of stainless-steel chip with mEDMed cavityb Optical image of fabricated sensor with Au signal lead

Measurement results and discussion: To characterise the fabricatedsensors, electrical connections to the sensor chip were made by wirebonding to the Au surface and silver pasting a lead to the stainlesssteel body (Figs. 1b and 2b). The capacitance between the two leadswas measured using an LCR meter (HP 4275A). The ambient pressureof the sensor was varied by enclosing it in a custom chamber the internalpressure of which was controlled while reading the reference pressuresensor (PX-26, Omega Engineering Inc., QC, Canada) coupled to thechamber. A capacitive response of �2 fF/mmHg was observed over200 mmHg or more with very high linearity (Fig. 3a). The base capaci-tance was measured to be approximately 20 pF. Assuming that the totalthickness of the bonded Parylene layer outside of the cavity is 2 mm andthat the membrane is perfectly suspended above the 5 mm-deep cavity,the base capacitance can be calculated to be 21.47 pF (the sum ofvariable 1.54 pF for the cavity region and fixed 19.93 pF for theoutside of the cavity), which is in reasonable agreement with themeasured value.

0 50 100gauge pressure, mmHg

a b

capa

cita

nce

chan

ge, p

F

capa

cita

nce,

pF

gauge pressure, mmHg150 200 250 0 50 100 150 200 250

0.45 20.35

20.30

20.25

20.20

20.15

20.10

45°C

35°C20.05

20.0

19.95

19.90

19.85

0.40

0.35

0.30

0.25 2 fF/mmHg

0.20

0.15

0.10

0.05

0

Fig. 3 Testing results

a Measured change in sensor capacitance against pressure at room temperatureb Measured sensor response at elevated temperatures

For small deflections (,5 mm) of the membrane, it is expected thatthe deflection by an external pressure will be balanced by an equalpressure inside the cavity. This condition will be a good approximationif the membrane is thin enough so that its rigidity can be neglected.Assuming an ideal-gas case at atmospheric pressure and constanttemperature inside and outside of the cavity, the above conditionmeans that, for example, a 200 mmHg increase in the ambient pressureonly results in a 21% reduction in the cavity volume. Having the fourholes in the cavity raises the cavity volume from 0.005 mm3 to0.0248 mm3. The 21% volume reduction in this case corresponds to aloss of volume of an amount almost identical to the original volumeof the cavity (0.005 mm3). This implies that almost the entire area ofthe membrane is forced into contact with the bottom surface of thecavity at 200 mmHg, which may be an approximate level of themaximum pressure that the sensor responds to under touch-modeoperation [9]. At this extreme condition, the cavity area is estimated toadd 11.7 pF from its base value. The measured capacitance increaseby applying 200 mmHg was �0.4 pF (Fig. 3a), much less than theabove estimate. The most likely cause of this discrepancy is that partof the membrane was already in contact with the bottom surface of

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the cavity at atmospheric pressure. This could be attributed to fabrica-tion/material-related factors such as the non-uniformity of the mem-brane (or its base Cu layer), the same for the chip substrate, and lackof the membrane’s rigidity, which need further optimisation. Thetouch-mode capacitive pressure sensors typically exhibit improvedlinearity in the response over non-contact-mode sensors [9]. Althoughthe sensitivity is in general compromised in touch-mode configurations,the high-linearity feature may be advantageous in certain sensingapplications.

Further tests were performed to characterise the temperature depen-dence of the sensor. The sensitivity was found to be relatively constantover a temperature range covering typically possible values in the humanbody (35–458C) (Fig. 3b). Shifts in the base capacitance were observedwith an average coefficient of 250 ppm/8C. This shift could be related toa temperature dependence of the permittivity of Parylene-C (which riseswith temperature [10]); this effect may be minimised using other typesof Parylene (e.g. type N) that have dielectric constants with less thermalsensitivity [10].

Conclusion: A micromachined capacitive pressure sensor formed on astainless steel chip has been developed through a non-lithographicfabrication approach. The sensor exhibited a highly linear response of�2 fF/mmHg, or a sensitivity of �100 ppm/mmHg, over a gaugepressure range of 200 mmHg that is potentially useful for blood-pressuremonitoring applications. The linearity of the sensor response wasobserved to remain over a temperature range covering possible human-body temperatures. Further design/fabrication optimisation will followdirected towards performance improvement and in vivo applications ofthe sensor.

Acknowledgments: This work was partially supported by the NaturalSciences and Engineering Research Council of Canada, the CanadaFoundation for Innovation, the British Columbia KnowledgeDevelopment Fund, and CMC Microsystems. D. Brox acknowledgesfinancial support from the University of British Columbia. Theauthors thank M. S. M. Ali for assistance in use of the mEDM system.

ELECTRONICS

# The Institution of Engineering and Technology 201115 July 2011doi: 10.1049/el.2011.2230One or more of the Figures in this Letter are available in colour online.

D. Brox, A.R. Mohammadi and K. Takahata (Department of Electricaland Computer Engineering, University of British Columbia, 2332 MainMall, Vancouver, BC V6T 1Z4, Canada)

E-mail: takahata@ece.ubc.ca

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