mems packaging for bio medical application

8/6/2019 Mems Packaging for Bio Medical Application

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Proceedings of ICMMB-11:International ConferenceonMechanics in Medicine and BiologyApril 2-5,2000 I Maui, Hawaii

MEMS PACKAGING FOR BIOMEDICAL APPLICATIONSKevin R. King, H. Lynn Kim, Gisela LinMEMS Technology GroupJet Propulsion Laboratory4800 Oak Grove Drive, Mail top 302-231Pasadena, CA 91 109-8099

ABSTRACTMEMS packaging for biomedical applications can beachieved in several ways.One is completeeviceencapsulation. Several examples are discussed, includingthe encapsulation of a MEMS heart cell force transducerin which the encapsulant is composed of four differentmaterials. The second option is a reconfigurablepackaging scheme that consists of an inert protectivepolymer layer that can be moved or broken on command.This type of package facilitates direct sensor access to theenvironment and preliminary results are presented.INTRODUCTIONMicro-electromechanical systemsMEMS)technology provides a relatively new, inexpensive way tomake sensors. By usingsilicon-based ntegrated circuitfabrication techniques chemical, inertial, thermal, ndpressure sensors have been miniaturized [ l] . MEMS hastremendous growth potential especially in the biomedicalsciences. Miniature MEMS biosensors have already beenusedo measure and interact with a variety ofbiomolecules, and wireless communication o and fromsurgically implanted biomedical MEMS devices has beendemonstrated [l] [2]. However, advances in MEMSbiosensor packaging are fewer innumber.Thisoftenneglected but important deviceaspect snecessary ocomplete a system. Especially in wet,orrosiveenvironments, such as biological fluid, reliable packagingof active devices is extremely challenging.

Two methods ofiosensorackaginganeemployed: (1) complete encapsulationfctivecomponents or (2) reconfigurable encapsulationhatallows the active components controlled access o heenvironment. Completencapsulations a morestraightforward approach andhas proven effective forboth standard and MEMS biomedical devices. However,total encapsulation inherently limits the type of sensingthat can be performed. Sensors ultimately need access tothe environment and willhusequire a moresophisticated, interactive package that simultaneouslyprotects the sensor whileallowing the required accesswhen needed. Both approaches will be discussed.

ENCAPSULATION PACKAGINGFor many years, medical devices such as pacemakersconsisted of electronics encased in biocompatible hardshell titanium housings to protect the device from hostilebody fluids. Miniature, in-situ sensor systems operating inharsh liquid environments have also involved completeencapsulation. For example, a swallowable emperaturesensoras been developed by HTI TechnologiesIncorporated (Figure I ) [3]. The pill is entirely coated insilcone rubber and epoxy which enables it to withstandthe acidity and toxicity of the GI tract.

2,nner epoxy she1

.TemperatureSensingCrystalBattery

Figure 1. Diagram of a swallowable temperaturesensor byHTI Technologies Incorporated [3].

In addition to standard dip coating and spray coating,encapsulation may require custom processing. Ziaie, et.al., usedan ultrasonically machined glasscapsule thatwas electrostatically bonded to he substrate to protect thereceiver circuitry and hybrid elements of an implantableMEMS nerve microstimulator rom body fluids [4].Encapsulation may also require more thannematerial.orxample, the force generated by an

individual heart cell wasmeasuredusing a completely


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encapsulated MEMS device. The packaging required forthe chip and supporting electrical interconnects requiredcomplete encapsulation using silicon dioxide, epoxy,silicone rubber sealant, and enamel. The force transducer(Figure 2) was fabricated using a commercially available2ym Complementary Metal-Oxide-Silicon (CMOS)process [5]. A single heart cell is mounted between twosilicon dioxide clamps attached to microbeams. The straingauge is part of an on-chip Wheatstone bridge. When thecell contracts, it activates a strain gauge ocated at hebase of one of the microbeams. The signal from thebridge is amplifiedon chip and recorded using an off-chipdata acquisition system.

Figure 2. SEMphotograph of MEMS heart cell forc etransducer.The available layer stack for the CMOS process isshown in Figure 3 . In this process Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) arefabricatedlong with MEMSicrostructuresiaphotolithography and etching. The metals are used aselectrical interconnects, while the polysilicon anbepatterned into piezoresistive strain gauges and resistors.Since layers of polysilicon and metal are sandwichedbetween the oxide, the polysilicon and metal can bepatterned such that electrical components are fullyencased in the oxide. Thus, the oxide becomes part of the

encapsulation packaging.To create a folded, three-dimensional microstructure,the metal layers were also patterned into freestanding,exposed microbeams which act as mechanical hinges aswell as electrical interconnects [ 6 ] .To encapsulate thesebeams, silicone rubber sealant was applied via a needleprobe. The probe is attached to a 3-axis micromanipulatorand is also used to assemble the microstructure once it hasbeen released from the substrate. Release is achieved yetching the silicon substrate from underneath the oxideafter the chips come back from the foundry.

I Oxide overglass ( 1 km) II Oxide #2 (0.65pm) I

I Oxide #I (0.8pm) II Field oxide (0.6pt-n) I

Silicon Subst ra te

Figure 3. CMOS layer stack. SOOJ of thermally grownSi02 is located between thepolysilicon layersVias are holespatterned n the oxide layers bywhich layers of polysiliconnd metal are connected to hesubstrate andor each other. Etch windows are created bypatterning successive vias on top of each other, therebyleaving the substrate exposed. The oxide microstructurescanbe eleasedusinganywet silicon etchant such asTMAH or KOH. However, due to the delicacy of mostmicrostructures, a dry-phase etchant such as XeF, ispreferred [7]. This isotropic gas-phase etchant offers highselectivity oward silicon dioxide and aluminum, and teliminates any liquid meniscus forces or bubbles that candamagehemicrostructures.Etchings xpedited yusing etch holes (i.e. small etch windows patterned in a

periodic array throughout the microstructure as shown inFigure 2).Part of the device package includes interconnects tobring signals off he chip to a data acquisition system.Thus, before release etching he chip is bonded to a quartz

coverslip on which en lym-thick aluminum lines havebeen patterned. The signal pads on the chip arewirebonded to these lines, and insulated output wires arealso attached. After release etching and device assembly,the wirebonds and the silicon substrate are encapsulatedin epoxy (applied manually). Finally, the aluminum lineson the coverslip are encapsulated in enamel.

Although his encapsulation method s complicatedand labor intensive, t adequately protected the device andallowedhis eviceo function in a nutrient salinesolution environment conducive to keeping the heart cellsalive. Furthermore, it did not have any adverse effect onoverall cell function. Using this device, heart cell forcesin the micronewton range were measured9].RECONFIGURABLE SENSOR PACKAGINGAsllustrated in the previous section, totallyencapsulatedMEMSdeviceshave been used tomakebiologicalmeasurements.owever, to expand thecapability of MEMS iosensors, especially chemical


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~ . .,

sensors, a more sophisticated, interactive package will berequired. At NASA/JPL we are addressing this issue bycreating a new reconfigurable biosensor package that willoperate in liquid environments and allow sensors directaccess to their environment. This type of packaging willallow MEMS biosensors to be an integral component forNASAs advanced life supportystemsndastrobiological studies that will require a host ofenvironmental, chemical, and microbial direct sensingcapability.

The reconfigurable package uses a protectivepolymer door that will open to heenvironment oncommand. As illustrated in Figure 4, a protective polymercovers the inlet until the underlying heater or actuator isactivated. Upon activation the polymer will be movedeither by mechanical, thermal, or chemical means.Different microactuation methods are currentlybeingassessed such as shape memoryalloys and magneticmicroactuators. Inert polymers are being considered forthe protection layer, such as teflon and viton.

I Protectiveolymer

- .actuatorFigure 4 . Schematic drawing of reconjgurable packageunder investigationat NASA/JPL. The nlet leads to amicrojlu idic circuit contain ing a ensor.

To date we have routinely spun thin films of TeflonAF (Dupont) onto silicon substrates. Typically the filmsare spun for 15 seconds at 2000 RPM.The ilms areroughly 0.8pm thick and are cured at 2OOOC for 2 hours.As expected, the films did not degrade when immersed insolvents. If not properly cured, we found that the filmswould delaminate when exposed to solvents. If the filmdelaminates, it comes off as single sheet.

In addition to proper curing, we found that exposingthe substrate toHMDSvaporprior ospinning on theteflon increased the teflon film adhesion. Films have beenpatterned using photoresist as a maskAZ214photoresist). Photoresist will not adhere to teflon unlessthe teflon surface is first roughened. Teflon surfaceroughening is achieved via an oxygen plasma at 200W or5 minutes. The photoresist is spun on andpatternedimmediately afterwards, and the teflon is patterned usingan argon plasma. 250W for 30 minutes was needed o

pattern a 0.8pm-thick teflon film. A microchannel testpatternetched into teflon s shown in Figure 5. Thephotoresist was approximately lpm thick. Etching timeswill vary due to nonuniformities in the film. Teflon filmthicknesscanvarybyasmuchas k 0.2pm within alOOym radius from a given point.

Figure 5 . Opticalmicroscopehotograph of tejlonmicrochannels patterne d using pho tores ist. Channels are3 5 p wide.

In a separate experiment we ttemptedo breakteflon-coated membranes using metal heating wires. 300Achromium and 2000A gold were evaporated onto lpm ofthermally grown silicon dioxide. Ther/Auaspatterned into heating wires using a liftoff process andwas ubsequentlycoveredwith an unpatterned 0.8pmfilm of teflon. The metal heating wire is used to create alarge thermalgradient across the silicon dioxide, initiatingcracks. The tensile stress in the oxide is then utilized increating out-of-plane motion, which tears the teflon. Toform the membrane, the silicon substrate was etched fromthe backside using an SF, plasma in an STS deep-trenchetcher. Membrane size varies from 200pm square to Immsquare. A representative test structure is shown in Figure6 .

Figure 6. Optical microscope photo of tejlon membranetest structure. Membrane shown is 500p x 5 0 0 p . Theheating wire is 3 0 p wide.


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. , , .Current was applied to the heating wire by touching

two tungsten needle probes to each pad at a probe station.The test structure was incorporated into a circuit as.shownin Figure 7. Preliminary results indicate that hundreds ofmilliamps are required to initiate oxidecracking.Thetensile stress in the oxide film caused the attached teflonfilm to lift out of the plane of the wafer, but the amount offorce was not h g h enough to tear the teflon.Highertensile stress is needed and can be introduced either intothe oxide or by adding a material layer with high tensilestress such as platinum or silicon nitride. Also, patterningthe teflon into "doors" may facilitate lifting of the teflonduring actuation. Both of hese options are being pursued.

4 oltageAppliedFigure 7. Schematic diagra m of test circuit fo r membranetest structures.

Other methods of polymer actuation are also beingconsidered, such as piezoelectric actuation.Electrochemical actuation may also be a viablealternative. Conjugated polymer actuators expand andcontract when the polymer is electrochemically oxidizedand reduced. They have proven to be robust actuators thatfunction in ionic solutions (such as aline ndotherbiological fluids) [9].Electrochemistry may also be used odissolveorburst the membrane. Electrochemical dissolution of thin

metalmembranes covering microreservoirs has eendemonstrated [101. The transformation of liquid intovapor can be achieved electrochemicallyswell.Sufficient vapor pressure will tear polymer membranes.CONCLUSIONSIn this paper we describe several options for MEMSpackaging for biomedical applications. Completeencapsulation, while more straightforward, may be not besuitable for devices that needccesso the liquidenvironment to be assessed. To this end, we are pursuinga reconfigurable packaging scheme hat consists ofaninert protective polymer layer that can be moved orbroken on command. To date, we have patterned hin

films of Teflon AF using photoresist asa mask. Breakingor ifting a teflon-coatedmembrane has proven tobemore hallenging, but preliminary tests indicate thatperhaps patterned eflon combined with more actuationforce should yield favorable results.ACKNOWLEDGMENTSThe research described in this paper was carried outat he Jet Propulsion Laboratory, California Institute ofTechnology,ndheUniversityf California, LosAngeles. This work was sponsoredby the American HeartAssociation, Greater Los Angeles Affiliate, Grant-in-Aid#lo59 GI-3, the National Institute of Health, #HL-47065,and the National Aeronautics and Space AdministrationCenter for SpaceMicroelectronics Technology underNASA UPN-632.

REFERENCES[ l ] Gregory T. A. Kovacs, Micromachined TransducersSourcebook,WCB/McDraw-Hill, Boston, 1998.[2] T. Akin,K.Najafi,.radley, "A WirelessImplantable Multichannel Digital Neural RecordingSystem for a Micromachined Sieve Electrode," IEEEJournal of Solid State Circuits, vo1.33, no. 1, pp. 109- 118, January 1998.[3] http: / lm.hti tech.com/med.htm[4] B. Ziaie, J. A. Von Arx, M. R. Dokmeci, K. Najafi,"A Hermetic Glass-Silicon Micropackage with High-DensityOn-Chipeedthroughs for Sensors andActuators," Journal of MicroelectromechanicalSystems, Vol. 5, No. 3, pp. 166- 179, 1996.

[5] MOSIS foundry service using Orbit Semiconductor's2pm CMOSrocess.USCLnformationciencesInstitute, 4676 Admiralty Way, Marina Del Rey, CA27709. http:llwww.mosis.orgl[6] Hoffman, E., B.Warneke, E. J.. Kruglick, J.Weigold,K. S. J.ister, "3D Structures withPiezoresistive Standard CMOS," Proceedings IEEEMEMS-95Workshop on MicroElectroMechanicalSystems, Amsterdam, The Netherlands, January 29 -February 2, pp. 288 - 293,1995.[7] P. B. Chu, J. T. Chen, R. Yeh, G . Lin, C. P. Huang,B. A.Warneke, and K. S. J.Pister, ControlledPulse-Etching with Xenon Difluoride," Transducers

'97, Chicago,Illinois, pp. 665 - 668, June 16 - 19,1997.[8] G. Lin,MEMSorceransducer System forCellularForceMeasurements,"PhDDissertation,UCLA School of Engineering and Applied Science,March 1998.[9] E. Smela,.allenbach,. Holdenried,"Electrochemically Driven Polypyrrole Bilayers forMoving and Positioning Bulk MicromachinediliconPlates," Journal of Microelectromechanical Systems,[101J. Santini,M. Cima, R.Langer, "A Controlled-releaseMicrochip," Nature, vol. 397, pp. 335 - 338, 1999.

Vol. 8,NO.4, pp. 373 - 383, 1999.

mems packaging for bio medical application

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