IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 30, NO. 1, FEBRUARY 2007 19

Wafer-Level Packaging of MicromechanicalResonators

Paul Jayachandran Joseph, Pejman Monajemi, Member, IEEE, Farrokh Ayazi, Senior Member, IEEE, andPaul A. Kohl, Member, IEEE

Abstract—An approach to low-cost, wafer-level packaging ofmicroelectromechanical systems (MEMS), e.g., microresonators,is reported. The process does not require wafer-to-wafer bondingand can be applied to a wide range of MEMS devices. A sacrificialpolymer-placeholder is first patterned on top of the MEMS com-ponent of interest, followed by overcoating with a low dielectricconstant polymer overcoat. The sacrificial polymer decomposesat elevated temperature, and the volatile products from the sac-rificial material permeate through the overcoat polymer leavingan embedded air-cavity around the MEMS structure. Thus, thedevice is released from the sacrificial polymeric material, housedin a protective overcoat. The protected MEMS device can then behandled and packaged like an integrated circuit. The electricalcharacteristics of the microresonators before and after packagingwere essentially the same, showing the packaging scheme does notalter the device performance. This approach is applicable to bothsurface and bulk micromachined devices.

Index Terms—Encapsulation, microelectromechanical, micro-resonators, packaging, polymer-based, silicon, wafer-level.

I. INTRODUCTION

THE development of a low-cost, high-volume packagingtechnique for microelectromechanical systems (MEMS) is

an important challenge since packaging can constitute a signif-icant fraction of the overall manufacturing cost. Micromechan-ical devices such as resonators, gyroscopes, and accelerometersare packaged so as to provide physical protection and electricalconnections to the movable device. Encapsulation provides amore robust device for transportation and handling without al-tering the performance of the devices. Several MEMS pack-aging techniques have been reported involving wafer-to-waferbonding and thin-film sacrificial-based sealing methods [1]–[8].

The integrated circuit packaging industry relies on standard,low-cost materials, and automated tooling to package devicessuch as lead-frame packaging. In order for MEMS technologiesto achieve high-volume, commodity status (and impact), apackaging methodology with similar attributes is necessary.In this paper, we demonstrate an approach to low-cost MEMSpackaging by use of sacrificial polymers. The encapsulation

Manuscript received December 8, 2005; revised March 28, 2006 and May 2,2006.

P. J. Joseph and P. A. Kohl are with the School of Chemical and BiomolecularEngineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail:paul.kohl@chbe.gatech.edu).

P. Monajemi is with Silicon Clocks, Inc., Fremont, CA 94538 USA.F. Ayazi is with the School of Electrical and Computer Engineering, Georgia

Institute of Technology, Atlanta, GA 30332 USA.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TADVP.2006.890220

of the MEMS device would allow safe transportation and givethe MEMS device attributes much like an integrated circuit.A wide variety of MEMS devices (for example, resonators,RF-MEMS switches, and accelerometers) may be applicableto this packaging approach. In this approach, a sacrificialpolymer is first patterned on the MEMS structure, with anexact geometry to encase the MEMS device, followed by over-coating with a protective polymer. Once the overcoat polymeris in place, the sacrificial material is thermally decomposedin a tube furnace, and its volatile, gaseous products permeatethrough the overcoat polymer releasing the MEMS device. Theadvantages of this encapsulation approach compared to otherMEMS packaging techniques are that it is a low-temperatureprocess that can be used for packaging a wide variety ofMEMS including metallic structures, it produces a low-profileencapsulating cover, and can be performed on any substrate.Thermal decomposition of sacrificial polymer is performedthrough a solid perforation-free capsule, which eliminates thesteps needed in some other sacrificial-based techniques to seala perforated [4], [5] or porous [7] cover. It does not requirehigh-temperature deposition and etching of sacrificial materials[2], [5], [7] and is stiction-free. The overcoat geometry can bescaled according to application to tailor different sizes frommicroscale to millimeter-scale. The polymer packaging doesnot require wafer-to-cap alignment and bonding [1]–[3], [8].

Sacrificial polymers can serve as temporary placeholdersand can be photochemically or physically patterned to thedesired shapes and geometry of the area to be encapsulated[9]–[13]. After patterning, the devices were encapsulated by apolymer overcoat whose decomposition temperature is higherthan the sacrificial polymer. Once the overcoat has been cured,the structure is heated to the decomposition temperature ofthe sacrificial polymer which creates a buried air-cavity. Pre-viously, air-cavities have been used in electrical interconnectsbecause they lower the effective dielectric constant of thematrix [9]–[13]. The formation of overcoated air-cavities viasacrificial polymers can be easily adapted to other devices.The same approach was used to from air-gaps in compliantsea-of-leads chip-to-substrate interconnects designed to meetfuture I/O needs [14]. The fabricated air-gaps were used forout-of-plane ( -axis compliance) mechanical performance.The inclusion of air-cladding regions around optical waveg-uides provided high core-to-cladding refractive index contrastproviding optical confinement [15]. A critical attribute of thetemperature for forming the air-gap is that the electrical andoptical structures are not distorted during the fabrication. Afunctional overcoat was formed over air channels made byuse of a sacrificial polymer as part of a microfuel cell [16]. In

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20 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 30, NO. 1, FEBRUARY 2007

this case, the overcoat dielectric served as the proton exchangemembrane of the fuel cell. Several other approaches to theformation of air-gaps have been reported. Suh et al. used adendritic material as a dry-release sacrificial material in thefabrication of cantilever beams [17]. Gleason and Lee reportedthe fabrication of air gaps using the hot-filament chemicalvapor deposition of polyoxymethylene as a sacrificial layer[18]. Harnett et al. demonstrated the use of polycarbonatesas a sacrificial material in fabricating nanofluidic devices byelectron beam lithography [19]. The fabrication of buried airchannels is useful for the creation of vias in multilevel wiringboards, microdisplay boards with high resolution, and ink-jetprinter heads.

Microfluidic reactors incorporating air gaps can be usedfor miniature-scale chemical syntheses, medical diagnostics,and microchemical analysis and sensors [20], [21]. Chuanget al. reported the fabrication of embedded microchannels andmicrofluidic structures by employing dosage-controlled UVexposure on a thick SU-8 resist and antireflection coating on thebottom surface to prevent the reflection UV light from inducingexposure [22]. These applications require the formation ofburied microchannels in several different materials at a varietyof temperatures.

Micromechanical beam resonators are used in a variety ofsensing and frequency filtering systems and also can find appli-cation in resonant sensors such as chemical sensors, pressuresensors, gyroscopes, and accelerometers [23]. The packagingof resonators is important, as the resonant beam is sensitiveto moisture, dust, and other particles in the atmosphere whichinterfere with beam movement. Thus, encapsulation of these de-vices is important to facilitate the handling, transportation, andusage of these devices in nonvacuum conditions. This paper re-ports a general-purpose packaging method for MEMS devices.The packaging method is demonstrated on micromechanicalbeam resonators fabricated on 100-mm silicon-on-insulator(SOI) substrates. The Q-factor and the frequency of the res-onators before and after packaging are reported, and show thatthe resonator packaging does not alter the device performance.Polymer encapsulation is an intermediate packaging scheme,and can be sufficient for MEMS devices that do not requirehermetic packaging, such as accelerometers, tunable capaci-tors, or magnetic sensors. Hermiticity and vacuum operationcan be obtained by coating thin-film metal overcoats onto thepolymer package. Chromium, gold, aluminum, and titaniumwith thickness of 0.5–4 m have been deposited on top ofAvatrel to form a metal–organic membrane [24].

II. EXPERIMENTAL

The overcoat polymer was Avatrel 2195, an epoxy-basedpolynorbornene copolymer (Promerus, LLC, Brecksville, OH).Avatrel is a negative-tone photosensitive material. The sac-rificial polymer was Unity 2303, a carbonate-based polymer(Promerus, LLC). The process steps and materials used in thepackaging of the MEMS structures will be presented in the nextsection.

TABLE IDESIGN PARAMETERS OF THE FABRICATED MICRORESONATORS

Fig. 1. Fabrication sequence for the formation of MEMS package via reac-tive-ion etching of nonphotosensitive sacrificial polymers.

III. POLYMER-PACKAGING PROCESS DETAILS

The polymer-based packaging of microresonators wasachieved by two approaches: patterning via reactive-ion etching(RIE) the sacrificial polymer and patterning via photosensitivesacrificial polymer. The fabricated microresonator’s designparameters like physical layout size, design resonant frequency,and mode of excitation are listed in Table I.

The fabrication sequence for the formation of MEMSpackage via reactive-ion etching of nonphotosensitive sacri-ficial polymers is shown in Fig. 1. First, a nonphotosensitiveUnity 2303 was spin-coated on to the wafer where an array ofmicromechanical beam resonators are prefabricated for packing(Step a). The Unity film was soft-baked at 110 C for 10 minon a hotplate, and the thickness of the film was measured.Next, plasma-enhanced chemical vapor deposition (PECVD)silicon dioxide (1 m) was deposited on the Unity sacrificialpolymer film at 100 C. The SiO was used as a hard mask

JOSEPH et al.: WAFER-LEVEL PACKAGING OF MICROMECHANICAL RESONATORS 21

Fig. 2. Fabrication sequence for the formation of MEMS package via photopat-terning photosensitive sacrificial polymers.

for subsequent patterning of the sacrificial polymer. Metal,such as Ti, Al, or Ti–Au can also be used for this purpose. Thehard mask was patterned by standard photolithography usingShipley Microposit 1827 photoresist and MF 319 developer(Step b). The SiO was wet etched using buffered oxide etch,BOE (1:6 HF/H O). Reactive-ion etching was used to etch theexposed Unity polymer followed by etching of the remainingSiO hard mask (Steps c and d). The RIE polymer etch condi-tions are as follows: 15 sccm Ar, 40 sccm O , power 400 W,pressure 250 mTorr at 35 C. A thin layer of PECVD SiOwas deposited on top of the Unity patterns before encapsulationto provide good mechanical strength of the air-cavity after thesacrificial polymer was removed by decomposition (g). TheUnity patterns were encapsulated using Avatrel polymer over-coat. The Avatrel was patterned to open the bond pads (Stepse and f). After encapsulation of the patterned Unity polymer,decomposition was carried out in a Lindbergh tube furnace in anitrogen atmosphere (Step g) at 280 C. The decomposed Unitypolymers produce gaseous products that permeated through theAvatrel overcoat polymer releasing the MEMS device enablingfurther electrical characterization and testing of the deviceperformance after packaging.

The fabrication sequence for the formation of MEMSpackage by use of a photosensitive sacrificial polymer isoutlined in Fig. 2. First, a thick sacrificial polymer layer (ap-proximately 10 m) of Unity 2303P was spin-coated (Step a).The wafer was then soft-baked on a hotplate at 110 C for 10min, and the thickness of the polymer film was measured. The

deep UV exposures (248 nm, 1 J/cm ) were performed usinga Karl Suss MJB 3 mask aligner by selectively masking theresonator beams (Step b). Exposure doses were measured byusing a Karl Suss Ultra-Violet intensity meter (model 1000)radiometer with a 248-nm probe. The film was bake-developedat 110 C for 8–10 min to decompose the exposed area (Step c).A thin protection layer ( m SiO ) was deposited before en-capsulation to provide additional mechanical strength. This wasfollowed by the encapsulation of the sacrificial material usinga polymer overcoat, Avatrel and subsequent photopatterning(Steps d and e). The unexposed area of the sacrificial polymerthat is covering the beams was decomposed by heating at 170 Cto 260 C to form air-cavities where the beam resonators wereconcealed (Step f).

The sacrificial material used in the RIE and photopat-terned methods was a polycarbonate-based polymer [25]. Thenonphotosensitive version was formulated by dissolving thepolymer in a solvent without the photoactive compound. Thenonphotosensitive version of the polycarbonates decomposesbetween 250 C and 300 C and the absence of the photosen-sitive compounds reduces the chances of residue. The polycar-bonates can be photopatterned by the addition of a photo-acidgenerator (PAG), to the polymer solution. Introduction of aPAG such as diphenyliodonium or triphenylsulphonium salts,into the polycarbonates, results in a UV-sensitive material.Upon irradiation to UV light, the onium salts undergo a pho-tolysis reaction. The organic cation of the PAG decomposesand a strong acid is generated. The protonic acid initiates andcatalyzes the degradation of polycarbonate in the subsequentheating step at a lower temperature than the nonphotosensitivepolymer thus enabling photopatterning via bake development.The remaining Unity can be decomposed thermolytically byactivating the PAG at 180 C. A detailed study related tothe sacrificial photosensitive polycarbonates, the operatingmechanism of the acid-catalyzed decomposition, and residueanalysis has been reported [25]. Incorporation of the PAG intothe formulation simplifies the photopatterning process and alsolowers the decomposition temperature to 100 C–120 C forUV exposure and 180 C for thermally activated PAG. Negli-gible residue is formed when the photosensitive polycarbonateis used after decomposition at 170 C–180 C range. Whennonphotosensitive polycarbonate was decomposed at 260 C,no visible residue was observed under an optical microscope.

After exposure to 1 J/cm irradiation at 248 nm, the photosen-sitive polymers patterns were decomposed at two different tem-peratures, 260 C and 180 C for 2 h. The activation and thermaldecomposition of the PAG is shown in Fig. 3. Energy disper-sive X-ray fluorescence (EDS) was used to analyze the residueafter thermal decomposition of the polymer. From the EDS datashown in Table II, it is clear that the sample decomposed at260 C had thinner residues than the one decomposed at 180 C.At high temperatures, lower fluorine content (3.54 %) was de-tected. The fluorine is a by-product of the pentaflourobenzenemoieties in the PAG that remain after the decomposition of thePAG. The presence of pentafluorobenzene, as shown in Fig. 3,was detected by the mass spectral characterization studies in aprevious study [25]. When the samples were not UV exposedand decomposed at 260 C for 2 h, the fluorine was 2.25%.

22 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 30, NO. 1, FEBRUARY 2007

TABLE IIEDS RESULTS OF THE ELEMENTAL COMPOSITION OF THE RESIDUAL MATERIAL

REMAIN AFTER DECOMPOSITION OF SACRIFICIAL MATERIALS

Fig. 3. Photolytic/thermolytic decomposition mechanism of the PAG.

The PAG goes through the same decomposition mechanism,Fig. 3, whether decomposed photolytically or thermolytically.The UV-exposed sample gave higher fluorine content than thenon-UV-exposed sample when the samples were both heatedat 260 C for 2 h. It appears that the more efficient acid gen-eration method using both photolytic (when UV exposed) andthermolytic (when the temperature reaches the decompositiontemperature of the PAG) increases the rate of acid generationand fluorine-based byproduct residue. The carbon and oxygencontent were lower in samples that were heated at 260 C, com-pared to those heated to 180 C. As can be seen from the datawhen the samples decomposed at 260 C, there is higher per-centage of Si as compared to those decomposed at 180 C. Athigher temperature, the thermal decomposition of the polymerand PAG into small, volatile products is more efficient.

Fig. 4. Representative SEM image of the (a) unpackaged 2.6-MHz SOI beammicroresonator and (b) and (c) images of the Unity 2303 patterns on the mi-croresonators via etching method.

Fig. 5. (a) SEM image of a single fully packaged microresonator and (b) itscross-sectional image via photopatterning method.

Fig. 4 shows SEM micrographs of the unpackaged resonator,array of resonators patterned with Unity 2303, and a 2.6-MHzresonator patterned with Unity 2303 via RIE. Fig. 5 shows anexpanded view of a single packaged microresonator and a cross-sectional image of the resonator surrounded by an air-cavity.

JOSEPH et al.: WAFER-LEVEL PACKAGING OF MICROMECHANICAL RESONATORS 23

Fig. 6. Frequency response of the 2.6-MHz SOI beam microresonator (a) be-fore and (b) after packaging in vacuum chamber heated at 200 C after 2 h ofwaiting time; the IF bandwidth is 10 Hz, the ac voltage is 0 dBm, and the polar-ization voltage is 50 V.

IV. ELECTRICAL CHARACTERIZATION AND

PERMEABILITY EVALUATION

The microresonators packaged via RIE patterning (Fig. 1)were tested under vacuum. The resonator pads were directlyprobed, and an ac signal was applied using an Agilent networkanalyzer. A dc polarization voltage of 50 to 80 V was appliedto the substrate by direct probing. Fig. 6 shows the frequencyresponse of a sample 2.6-MHz microresonator in vacuum be-fore and after packaging. A Q-factor of 4500 was measuredfor a sample resonator before and after packaging. The Q ofthese beam resonators in vacuum is limited by support lossand thermoelastic damping [26], [27]. Some of resonators weretested one month after polymer encapsulation, and no changewas observed compared to resonators that were tested rightafter encapsulation.

Fig. 7. Plot of Avatrel thickness versus beam oscillation time upon heating atdifferent temperatures.

The beam resonator shown in Fig. 5 is completely encapsu-lated (overcoated) by Avatrel polymer. There is no vent hole fordegassing the resonator cavity. Thus, in order to create a vacuumin the enclosed cavity surrounding the beam resonator in Fig. 5,the sample was placed in an evacuated chamber and the air wasallowed to permeate through the overcoat material. The vacuumchamber was maintained at 0.1 mTorr, and the oscillation oc-curred when the pressure surrounding the resonator decreasedbelow 4 Torr. For a 20- m-thick Avatrel overcoat, it took 2 hfor the gas pressure surrounding the resonator beam to decreaseto a value where the beam could oscillate. The permeability ofthe overcoat Avatrel increases at elevated temperature. Whenthe packaged resonator was heated to 150 C and degassed ina vacuum chamber, it only required 1.5 h for the beam to res-onate due the higher permeability of air through the overcoat(i.e., degassing of the cavity). A series of resonators were pack-aged with different Avatrel overcoat thicknesses and degassedin a vacuum chamber. Fig. 7 shows a plot of Avatrel thicknessversus time in the vacuum chamber to achieve beam oscillationat different temperatures (25 C, 100 C, and 150 C). The timeto degas the enclosed cavity decreased at higher temperaturesand decreased with thinner overcoats. It is clear from the plotthat the onset beam oscillation time is reduced at lower thick-ness of Avatrel and upon heating the substrate at higher temper-atures [24].

Permeability of a gas (nitrogen) with total mass of , in-side an air-cavity encapsulated with Avatrel overcoat with thick-ness, , and area, , with pressure difference of ,is defined as [28]

(1)

where is the gas flow rate through the polymer cap. Withdifferent Avatrel overcoat thicknesses (20, 10, 7, and 4 m), thepermeability of nitrogen gas through the Avatrel overcoat forpackaged resonators was calculated to be 0.89, 1.29, 1.57, and1.40 ( g/cm-s-Torr), respectively (Table III).

During polymer encapsulation, the buffer oxide layer thick-ness is between 0.5 to 1 m [29], while Avatrel overcoat is usu-ally thicker than 10 m. Therefore, changing the SiO bufferthickness from 0.5 to 1 m did not change the measured perme-ability of Avatrel for different batches of resonators.

24 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 30, NO. 1, FEBRUARY 2007

TABLE IIIPERMEABILITY AT DIFFERENT AVATREL THICKNESSES

Fig. 8. SEM images of the Unity 2303 patterns on the microresonators (a andb) via photopatterning method.

V. EVALUATION OF PACKAGE PERFORMANCE

The microresonators packaged by using the photosensitivepolymer were tested while in wafer-form inside the vacuumprobe station. An array of photopatterned resonators using

10 m of Unity 2303 were fabricated, Fig. 8, and tested.Fig. 9 shows the array after packaging and a top view of asingle packaged resonator. Fig. 10 shows an optical micrographof the top of packaged resonator. The Avatrel overcoat istransparent so that the released resonator beam can be seenunder the Avatrel. The dc polarization voltage used to test theresonators was 50 to 80 V. A network analyzer was used ina two-port configuration. The operating performance of the

Fig. 9. SEM images of (a) an array of packaged microresonator and (b) a singlemicroresonator via photopatterning method.

microresonator was quantified by measuring the mechanicalquality factor Q

(2)

where is the overall energy applied to the resonator,is the dissipated energy, is the resonance frequency,

and BW is the measured bandwidth. Table IV compares theresonant frequency and the Q factor of the resonators beforeand after packaging. On average, the fundamental resonantfrequencies of the tested microresonators decreased about 1.3%after packaging. This experiment proves that thermal decom-position of the Unity sacrificial polymer and the packagingprocess does not alter the device performance significantly.Some degradation in Q factor after encapsulation is believed tobe due to small residues of the sacrificial polymer on the beamsurface, which can increase the surface loss.

The packaging yield (fraction of the packaged beams whichresonated) was greater than 80% in our laboratory environment,The reduction in yield after packaging may be due to the non-ideal shape of the air-cavity in some resonators because of themisalignment or lithography errors, and not due to the pack-aging process. Additional studies addressing vacuum encapsu-

JOSEPH et al.: WAFER-LEVEL PACKAGING OF MICROMECHANICAL RESONATORS 25

TABLE IVRF RESPONSES BEFORE AND AFTER PACKAGING OF MICRORESONATORS. f (f ): FUNDAMENTAL RESONANT

FREQUENCY OF THE UNPACKAGED (PACKAGED) RESONATOR Q (Q ): QUALITY FACTOR (Q )OF THE UNPACKAGED (PACKAGED) RESONATOR

Fig. 10. Optical micrographs of the top view of the packaged microresonator(a and b) revealing the clean mcroresonator beams.

lation of the microresonators and other MEMS device are un-derway and will be the subject for future reports.

Polymer encapsulation is an intermediate packaging schemeand should be performed right before the final hermetic pack-aging and assembly. The sacrificial polymer volatiles permeateat elevated temperatures of 200 C–280 C. As shown in Fig. 7,permeability of Avatrel overcoat increases with temperature.This is because of increased diffusivity of nitrogen atoms andan increase in polymer chain separation. Gas permeability of a10- m-thick Avatrel overcoat is significantly higher at decom-position temperatures than the same value at room temperature.Therefore, diffusion of water molecules inside the package ismuch slower than outgassing of nitrogen atoms at the decom-position temperature. The packaged microresonators should notbe exposed to a hot and humid environment prior to subsequentpackaging steps.

VI. CONCLUSION

A low-cost, high-volume polymer-based wafer-level pack-aging of microresonator devices was demonstrated. The pack-aging technique does not involve wafer bonding or wet-etchingof SiO sacrificial thin layers. The packaging technology in-volves the thermal decomposition of a sacrificial polymer thatresults in organic volatiles upon heating and permeation througha dielectric overcoat. A Q factor of about 4500 was measured be-fore and after packaging for a typical 2.6-MHz beam resonators.This wafer-level packaging can be applied to a wide variety ofMEMS devices including gyroscopes, accelerometers, tunablecapacitors, and RF MEMS switches.

ACKNOWLEDGMENT

The authors would like to thank C. Apanius, J. Krotine,E. Elce, R. Shick, (Promerus. LLC, Brecksville, OH) for theirintellectual and material contributions. They would also liketo thank C. Lang for the fruitful discussions related to perme-ability calculations.

REFERENCES

[1] B. Ziaei, “A hermetic glass silicon micropackage with high-densityon-chip feedthroughs for sensors and actuators,” J. Microelectromech.Syst., vol. 5, pp. 166–179, 1996.

[2] L. Lin, “MEMS post-packaging by localized heating and bonding,”IEEE Trans. Adv. Packag., vol. 23, no. 4, pp. 608–616, Nov. 2000.

[3] Y. Mei, “A robust gold-silicon eutectic wafer bonding technology forvacuum packaging,” in Tech. Dig., Solid-State Sensor Actuator Work-shop, Hilton Head Island, SC, Jun. 2002, pp. 86–89.

[4] B. H. Stark, “A wafer-level vacuum packaging technology utilizingelectroplated nickel,” in Proc. ASME Congr., MEMS Nanotechnol.Symp., New Orleans, LA, Nov. 17–22, 2002.

[5] R. N. Chandler, W. T. Park, H. Li, G. Yama, A. Partridege, M. Lutz, andT. W. Kenny, “Single wafer encapsulation of MEMS devices,” IEEETrans. Adv. Packag., vol. 26, pp. 227–232, 2003.

[6] K. Najafi, “Micropackaging technologies for integrated microsystems:Applications to MEMS and MOEMS,” in Proc. SPIE MOEMS/MEMS,San Jose, CA, Jan. 2003, pp. 1–19.

[7] R. He, L. Fan, M. C. Wu, and C.-J. Kim, “Porous polysilicon shellformed by electrochemical etching for on-chip vacuum encapsulation,”in Proc. Solid-State Sensor, Actuator Microsystems Workshop, HiltonHead Island, SC, 2004, pp. 332–335.

[8] H. A. Yang, M. Wu, and W. Fang, “Localized induction heating solderbonding for wafer level MEMS packaging,” J. Micromech. Microeng.,vol. 15, pp. 394–399, 2005.

[9] P. A. Kohl, Q. Zhao, K. S. Patel, D. Schmidt, S. A. Bidstrup-Allen,R. Shick, and S. Jayaraman, “Air-gaps for electrical interconnections,”Electrochem. Solid State Lett., vol. 1, p. 49, 1998.

26 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 30, NO. 1, FEBRUARY 2007

[10] P. A. Kohl, D. M. Bhusari, M. Wedlake, C. Case, B. C. Lee, R. J. Gut-mann, and R. Shick, “Air-gaps in 0.3 �m electrical interconnections,”IEEE Electron Device Lett., vol. 21, no. 12, pp. 557–559, Dec. 2000.

[11] H. A. Reed, C. E. White, V. Rao, S. A. Bidstrup-Allen, C. L. Hen-derson, and P. A. Kohl, “Fabrication of microchannels using polycar-bonates as sacrificial materials,” J. Micromech. Microeng., vol. 11, no.6, pp. 733–737, 2001.

[12] D. Bhusari, H. A. Reed, M. Wedlake, A. Padovani, S. A. Bidstrup-Allen, and P. A. Kohl, “Fabrication of air-channel structures for mi-crofluidic, microelectromechanical, and microelectronic applications,”J. Micromech. Microeng., vol. 10, no. 3, pp. 400–408, 2001.

[13] M. B. Anand, M. Yamada, and H. Shibata, “Use of gas as low k in-terlayer dielectric in LSI’s: Demonstration of feasibility,” IEEE Trans.Electron Devices, vol. 44, no. 11, pp. 1965–1971, Nov. 1997.

[14] M. S. Bakir, H. A. Reed, A. V. Mule, J. P. Jayachandran, P. A. Kohl,K. P. Martin, T. K. Gaylord, and J. D. Meindl, “Chip-to-module inter-connections using “sea of leads” technology,” Mater. Res. Soc. Bull.,vol. 28, no. 1, pp. 61–67, 2003.

[15] A. V. Mule’, R. Villalaz, P. J. Joseph, A. Naeemi, P. A. Kohl, T. K.Gaylord, and J. D. Meindl, “Polylithic integration of electrical and op-tical interconnect technologies for gigascale fiber-to the-chip commu-nication,” IEEE Trans. Adv. Packag., vol. 23, no. 3, pp. 421–433, Aug.2005.

[16] D. Bhusari, J. Li, P. J. Joseph, C. W. Moore, and P. A. Kohl, “Devel-opment of P-doped SiO as proton exchange membrane for micro-fuelcells,” Electrochem. Solid State Lett., vol. 8, no. 11, pp. A588–A591,2005.

[17] H.-J. Suh, P. Bharathi, D. J. Beebe, and J. S. Moore, “Dendrimic ma-terials as dry-release sacrificial layer,” J. Microelectromech. Syst., vol.9, no. 2, pp. 198–205, 2000.

[18] L. S. Lee and K. K. Gleason, “Hot-filament chemical vapor deposi-tion of polyoxymethylene as a sacrificial layer for fabricating air-gaps,”Electrochem. Solid State Lett., vol. 4, no. 11, pp. G81–G84, 2001.

[19] C. K. Harnett, G. W. Coates, and H. G. Craighead, “Heat-depolymeriz-able polycarbonates as electron beam patternable sacrificial layers fornanofluidics,” J. Vac. Sci. Technol. B., vol. 19, no. 6, pp. 2842–2845,2001.

[20] R. F. Service, “Microchip arrays put DNA on the spot,” Science, vol.282, pp. 396–399, 1998.

[21] J. P. Gravesen, J. Branejerg, and O. S. Jensen, “Microfluidics—A re-view,” J. Micromech. Microeng., vol. 3, pp. 168–182, 1993.

[22] Y.-J. Chuang, F.-G. Tseng, J.-H. Cheng, and W.-K. Lin, “A novel fab-rication method of embedded micro-channels by using SU-8 thick-filmphotoresists,” Sens. Actuators A, vol. 103, pp. 64–69, 2003.

[23] N. Yazdi, F. Ayazi, and K. Najafi, “Micromachined inertial sensors,”Proc. IEEE, vol. 86, no. 8, pp. 1640–1659, Aug. 1998.

[24] P. Monajemi, P. Joseph, P. Kohl, and F. Ayazi, “Wafer-Level MEMSpackaging via thermally-released metal-organic membranes,” J. Mi-cromech. Microeng., vol. 16, no. 4, pp. 742–750, 2007.

[25] J. P. Jayachandran, H. A. Reed, HongshiZhen, L. F. Rhodes, C. L.Henderson, S. A. Bidstrup-Allen, and P. A. Kohl, “Air-channel fab-rication for microelectromechanical Systems via photosensitive poly-carbonates,” J. Microelectromech. Syst., vol. 12, no. 2, pp. 147–159,2003.

[26] S. Pourkamali, A. Hashimura, R. Abdolvand, G. Ho, A. Erbil, andF. Ayazi, “High-Q single crystal silicon HARPSS capacitive beamresonators with sub-micron transduction gaps,” J. Microelectromech.Syst., vol. 12, no. 4, pp. 487–496, 2003.

[27] Z. Hao, A. Erbil, and F. Ayazi, “An analytical model for support lossin micromachined beam resonators with in-plane flexural vibrations,”Sens. Actuators A, vol. 109, pp. 156–164, 2003.

[28] C. Nagel, K. Gunter-Schade, D. Fritsch, T. Strunskus, and F. Faupel,“Free volume and transport properties in highly selective polymermembranes,” Macromolecules, vol. 35, pp. 2071–2077, 2002.

[29] P. J. Joseph, H. A. Reed, S. A. Allen, and P. A. Kohl, “Improved fab-rication of micro air-channels by incorporation of a structural barrier,”J. Micromech. Microeng., vol. 15, pp. 35–42, 2005.

Paul Jayachandran Joseph received the Ph.D. de-gree in physical-organic chemistry from the Univer-sity of Madras, Chennai, India, in 1997.

From 1994 to 1997, he was a Council of Scientificand Industrial Research–Senior Research Fellow atthe Department of Physical Chemistry, University ofMadras, where he carried out an extensive researchwork in the field of Phase Transfer Catalysis. From1997 to 2000, he was a Visiting Scientist for theNational Science Council of Taiwan, Taiwan, R.O.C.Since 2001, he has been a Research Scientist at

the School of Chemical and Biomolecular Engineering, Georgia Instituteof Technology (Georgia Tech), Atlanta. His research focuses towards thedevelopment of new sacrificial polymeric materials and its application inthe field of microelectronics, microfluidics, and microelectromechanicalsystems. Since November 2005, he has been a Faculty Research Scientist andBiomedical Engineering Research Coordinator at the Georgia Tech-Micro-electronics Research Center (GT-MiRC). His current research interests are inNanobiotechnology, Bio-MEMS, and Biosensors’ applications as Diagnosticsand Detection Systems. He is also serving as a Director of Communications atthe NanoTech Executive Board of GT-MiRC. He has authored or coauthoredsome 45 publications in journals and conference proceedings.

Pejman Monajemi (S’05–M’07) received theB.S.E.E. and M.S.E.E. degrees from Sharif Univer-sity, Tehran, Iran, in 1994 and 1997, and the M.S. andPh.D. degrees in electrical and computer engineeringfrom the Georgia Institute of Technology, Atlanta,in 2003 and 2006, respectively. His thesis workinvolved design, fabrication, and wafer-level pack-aging of micromechanical resonators, RF varactors,and inertial sensors.

From 2005 to 2006, he was with Jacket Micro De-vices, Inc., Atlanta, as an RF design engineer. He is

currently a Senior Process Engineer at Silicon Clocks, Inc., Fremont, California.

Farrokh Ayazi (S’96–M’99–SM’05) receivedthe B.S. degree in electrical engineering from theUniversity of Tehran, Iran, in 1994 and the M.S.and Ph.D. degrees in electrical engineering from theUniversity of Michigan, Ann Arbor, in 1997 and2000, respectively.

He is an Associate Professor in the School ofElectrical and Computer Engineering at the GeorgiaInstitute of Technology (Georgia Tech), Atlanta. Hejoined the faculty of Georgia Tech. in December1999. His research interests are in the areas of

integrated micro- and nanoelectromechanical resonators, RF MEMS, MEMSinertial sensors, VLSI analog circuits and techniques, and microfabricationtechnologies.

Prof. Ayazi is a 2004 recipient of the NSF CAREER Award, the 2004 RichardM. Bass Outstanding Teacher Award (determined by the vote of the ECE seniorclass), and the Georgia Tech College of Engineering Cutting Edge ResearchAward for 2001–2002. He received a Rackham Predoctoral Fellowship fromthe University of Michigan for 1998–1999. He is an editor for the IEEE/ASMEJOURNAL OF MICROELECTROMECHANICAL SYSTEMS and serves on the technicalprogram committee of the IEEE International Solid State Circuits Conference(ISSCC).

Paul A. Kohl (M’03) received the Ph.D. degree inchemistry from The University of Texas, Austin, in1978.

He is Regents’ Professor of Chemical Engineeringat the Georgia Institute of Technology (GeorgiaTech), Atlanta. He was with AT&T Bell Laboratoriesfrom 1978 to 1989. At Bell Laboratories, he wasinvolved in new materials and processing methodsfor semiconductor devices. In 1989, he joined thefaculty of Georgia Tech. His research interestsinclude ultralow-k dielectric materials, interconnects

for microelectronic devices, and electrochemical energy conversion devices.He has more than 150 journal publications, and 40 patents.