IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 26, NO. 3, AUGUST 2003 277 Chip-Level Vacuum Packaging of Micromachines Using NanoGetters Douglas R. Sparks, S. Massoud-Ansari, and Nader Najafi, Member, IEEE Abstract—A new approach to vacuum packaging of micro- machined resonant, tunneling, and display devices is covered in this paper. A multi-layer, thin-film getter, called a NanoGetter, which is particle free and does not increase the chip size of the microsystem has been
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  IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 26, NO. 3, AUGUST 2003 277 Chip-Level Vacuum Packaging of MicromachinesUsing NanoGetters Douglas R. Sparks, S. Massoud-Ansari, and Nader Najafi  , Member, IEEE   Abstract— A new approach to vacuum packaging of micro-machined resonant, tunneling, and display devices is covered inthis paper. A multi-layer, thin-film getter, called a NanoGetter,which is particle free and does not increase the chip size of themicrosystem has been developed and integrated into conven-tional wafer-to-wafer bonding processes. Hermetic electricalfeedthroughs are also provided as part of this total-solutiontechnology. Experimental data taken with silicon resonatorsis presented in which Q values in excess of 21,000 have beenobtained. Applications for this technology include gyroscopes,accelerometers, displays, flow sensors, density meters, infrared(IR) sensors, microvacuum tubes, radio frequency microelec-tromechanical systems (RF-MEMS) and pressure sensors.  Index Terms— Chip-scale package, getter, hermetic electricallead transfer, MEMS, resonator, vacuum packaging. I. I NTRODUCTION C HIP-LEVEL vacuum packaging through wafer-to-waferbonding has been undertaken for fragile micromachineddevices such as accelerometers, gyroscopes [1]–[3]. Many microelectromechanical system (MEMS) devices, such asresonators and tunneling sensors, rely on vacuum packaging forimproved performance [4]–[10]. Fig. 1 shows an example of a density meter chip packaged in this manner [8]. Devices thatmust operate under vacuum start development in a laboratorybell jar [11], [12]. If functionality is demonstrated, packaging often progresses to a solder or weld sealed ceramic or metalpackage [13]. If the microsystem has high-volume applicationsthen chip-level vacuum packaging, preferably performed at thewafer level is developed. Traditional vacuum wafer bondingmethods include anodic, glass frit, eutectic, solder, reactiveand fusion bonding [4], [14]–[17]. One problem encountered with existing wafer-to-wafer vacuum bonding is relativelyhigh cavity pressures that change with time and temperature.Anodic bonding is known to generate oxygen and result incavity pressures of 100 to 400 Torr (13–53 KPa) [18]. Solderbonding produces cavity pressures between 2 Torr (266 Pa) dueto surface desorption of gases. Baking wafers prior to solderreflow does reduce the amount of adsorbed water but onlylowers the microcavity pressure from 2 Torr to 1 Torr (133Pa ) [17]. While the pressure of the vacuum wafer bondingsystem can get down to the microTorr level, ultimately surfacedesorption after sealing limits the cavity pressure.Whilebakingofsurfacescansucceedindesorbinggasesfromsurfaces [17], [19], [20], temperature and time limits the effec- Manuscript received December 1, 2002; revised June 30, 2003.The authors are with Integrated Sensing Systems, Inc., Ypsilanti, MI 48198USA ( Object Identifier 10.1109/TADVP.2003.817964Fig. 1. Chip-scale MEMS package. tiveness of this method in obtaining ultra low cavity pressuresin microsystems. Shallow complementary metal oxide semi-conductor (CMOS) junctions, thin film alloying and cantileverwarpage limit prebonding bake temperatures. To overcome thesurface desorption limit found with wafer bonding, getters havebeen employed. Metallic getters have been used for decadesdating back to vacuum tubes to obtain lower pressures in her-metic packages [21]. Pure metals and alloys of Ba, Al, Ti, Zr, V,Fe, and other reactive metals are used in cathode ray tubes, flatpanel displays, particle accelerators, semiconductor processingequipment and other vacuum equipment to lower the pressure[22]–[25]. These metals trap various gases through oxide and hydride formation and by simple surface adsorption. Captureof oxygen, nitrogen and hydrocarbons requires elevated tem-peratures (200 to 550 ), while trapping of hydrogen by themetalsoccurs atroomtemperature. Esashiandothers[18],[26], [27] first applied getters to MEMS devices in the mid 1990’s.In these early studies, NonEvaporable Getters (NEGs) eitherin tablet or strip form were placed in an extra micromachinedcavityoradjacenttothechipinaceramicpackage.Tomaximizesurface area, the NEG is often fabricated using powder metal-lurgy techniques in which the sintering of the metal particles is just initiated, leaving gaps between metal beads. A high tem-perature activation step in vacuum or a hydrogen containing re-ducingambientisrequiredtoremovethesurfaceoxidelayerthatforms on the metal particles during the sintering process. Thisactivation step is accomplished by either annealing the wholepackage or by Joule heating of the NEG strip. One problemencountered with sintered getters is particle generation. WhenNEGmetalstripsareemployed, theyaretypicallycutintosmall 1521-3323/03$17.00 © 2003 IEEE  278 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 26, NO. 3, AUGUST 2003 segments and hand placed into a microcavity prior to waferbonding. The strips often bend during cutting, requiring addi-tional manual handling to straighten the pieces. Particles aregenerated during handling and the cutting process. The 2 to 3micron diameter metal particles can cause electrical shorts, im-pede motion and shift resonant frequencies. A frequency shiftoccurs due to a mass change in the resonator caused by the at-tached particle. Fig. 2 shows an example of one way that anNEG can be integrated into a microsystem. In this illustrationthe NEG is located in a second cavity above the micromachine.An opening in the silicon diaphragm separating the NEG fromthe resonator provides access between the NEG and resonatorchamber. Particles that shed from the sintered NEG can alsomigrate through this opening to the resonating or tunneling el-ement cavity. Fig. 3 shows a photomicrograph of a pressuresensor inwhichaglass cavityunderneathapressure sensorcon-tains the sintered getter strip [28]. For side-by-side cavity de-signs the die size area is essentially doubled, while for a ver-tical integration, such as is illustrated in Fig. 2, the chip thick-ness is increased. The size penalty and need for pick and placeNEGloadingalso preventstheNEGmethodfrom findinguseinhigh-volume MEMS products. This paper will describe a newmethodofchip-levelvacuumpackagingMEMSdeviceswithanimproved, particle-free getter that does not increase the die sizeof the chip.II. E XPERIMENTAL  A. MEMS Processing The micromachined resonators used in this study are solidor hollow silicon resonators, shown at the bottom left of Fig. 4[8]. The hollow resonators are used in producing fluidic densitysensors or Coriolis mass flow sensors [8], [12]. The resonators are formed by anodically bonding a patterned silicon wafer to ametallized, etched glass wafer. Metal electrodes are formed ontheglasswaferstoactasdriveandsenseelements.Afinalwaferbonding process, including hermetic electrical lead transfer, isperformed next to seal the resonator in a vacuum.  B. Vacuum Packaging Initially an NEG was integrated into the MEMS process flowand a device as shown in Fig. 2 was produced. Due to the par-ticle and handling problems associated with NEGs that werementioned in the introduction section, a new vacuum packagingmethod and getter were developed. The wafer bond packagingprocess for this new approach is illustrated in Fig. 5. A cap-ping wafer, generally either silicon or glass is patterned andetched to form both a cavity that will enclose the active micro-machineandopenupaccesstotheelectricalbondpads.NexttheNanogetter and the sealing material are placed on the cap waferand activated. The NanoGetter is comprised of a proprietary,patent pending, multi-layer structure and as the name impliesthe thickness of the thin film layers are in the nanometer-5 nmto 500 nm-range. Since thin film deposition techniques are em-ployed in a cleanroom environment, the NanoGetter is virtu-ally particle free compared to conventional NEGs formed usingpowder metallurgy. A surface texture comparison between the Fig. 2. Cross-sectional illustration of one of the previous methods of integrating NEGs into a vacuum microsystem.Fig. 3. Silicon-on-glass pressure sensor, vacuum sealed with an NEG. NEGandaNanoGetterat200X,isshowninFig.6.Thethinfilmdeposition method also enhances the ability to easily integratethe NanoGetter into a typical MEMS process flow at the waferlevel. For a conventionally wafer bonded device the processshown in Fig. 5 would look the same minus the NanoGetter de-position step. Fig. 7 shows the cap and active MEMS wafersprior to bonding.In themajorityof vacuum packagedmicrosys-temsthecappingwaferispartofapassiveenclosure.AddingtheNanoGetter does not impact the chip size. Vacuum wafer-to-wafer bonding is performed next in this process as illustratedat the bottom of Fig. 5. A vacuum wafer bonding system, anElectronic Visions 501S, was employed in this study. For thedevices under discussion in this paper the top cap was etchedsilicon, bonded with glass frit to the bottom Pyrex wafer whichhad either a solid or hollow tube silicon resonators on them. C. Electrical Testing Procedures An HP 4194A Phase-Gain Analyzer was employed to taketest results on vacuum quality, resonant frequency, and gain.The Q was calculated using the frequency bandwidth where thegain for each of both frequencies is 3 db down from the peak gain. Uncapped resonators were tested in a vacuum chamber.A Varian V-250 turbo pump was used to pull a vacuum in thissystem.  SPARKS  et al. : CHIP-LEVEL VACUUM PACKAGING 279 Fig. 4. Uncappedresonator,bottom leftandcapped chipalong withthe fluidicheader the MEMS chip is attached to, top left.Fig. 5. Process flow for wafer bonding, including the addition of the thin filmgetter used to form the chip-level vacuum package. III. R ESULTS AND  D ISCUSION Through wafer-to-wafer bonding and the use of NanoGet-ters a vacuum level under 850 micro-Torr, resulting in Q valuesgreater than 21000 for silicon resonators can be obtained. Qvalues have ranged from 5000 to over 21000 over a period of 8months on many separate wafer lots. Fig. 8 shows a gain versusfrequency plot for a U-shaped single crystal silicon tube thatvibrates in the vertical direction. To determine what pressurethe high Q values corresponded to a resonator was decappedand tested in a vacuum chamber. Fig. 9 shows how the Q variedwithpressurefortherelativelywide,U-shapedresonator.Withaturbo pump, the chamber pressure could only be pumped downto 790 microTorr (0.10 Pa), which resulted in a Q of 10350 forthe die used to generate the data in Fig. 9. Plotting 1/Q versuspressure results in a linear relationship and an intercept or max-imum Q value of 13065 for this particular device. Only a slight (a)(b)Fig. 6. Pair of 200X optical photomicrographs of (a) an NEG surface and (b)a thin film NanoGetter surface. increase in Q would be expected at pressures below 790 mi-croTorr. Without the NanoGetter a cavity pressure of 1.4 Torris obtained with glass frit sealing due to squeeze-film dampingcaused by desorbed, trapped gas. A Q value of 40 is obtainedfor this wide resonator when vacuum packaged without a getter.The microcavity pressure has been reduced by more than 3 or-ders of magnitude through the use of the getter. The key to ob-taining high Q values in a chip-scale package is the NanoGetterplaced in the cavity of the cap wafer as illustrated in Fig. 5. Thisgetter technology has been applied to chip-scale vacuum pack-agingofcommerciallyavailablemicrofluidicdensityandchem-ical concentration meters [8] and micromachined Coriolis massflow sensors of different flow rate ranges and hence resonatorsizes [12].Glass frit and anodic sealing have been used for many yearsin automotive and medical MEMS applications and so havehad to pass extensive reliability testing [4], [29]. Glass frit sealing has been coupled with silicon, Pyrex, CMOS integratedMEMS, capacitive and piezoresistive devices [29]. To confirmthe long-term hermeticity of the glass seal, 46 parts were  280 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 26, NO. 3, AUGUST 2003 Fig. 7. Active MEMS resonator wafer, left, and the cap wafer, right, prior towafer bonding.Fig. 8. Q-plot for a silicon resonator vacuum packaging using the NanoGetter.Fig. 9. Q versus pressure plot using a vacuum chamber. subjected to a prolonged 95 bake. No loss of vacuum wasnoted in any of the 46 parts for over one year. Resonators haveshown no significant change in Q after 750 h at 95 . Otherlong-term studies of chip-packaged resonators (gyroscopes)without getters have shown a gradual degradation in vacuumquality due to desorption after several hundred hours at 85 to100 [30]. For commercially available modules employingNanoGetter technology [8] thermal shock from 0 to 125 ,biased humidity testing at 85 , 90%RH, vibration andsystem level drop testing have also been performed. No loss Fig. 10. Chip-level and side-brazed ceramic package using NanoGettersdeposited on the underside of the lid.Fig. 11. Application of NanoGetters to a piezoresistive pressure sensor. of hermeticity or getter related particle generation has beenobserved.Sinceseveral vacuum sealing technologies existand each canresult in different residual gases of varying pressures, it wouldbe ideal to have a flexible getter approach. Anodic bonding isknown to generate relatively large amounts of oxygen as thesodium in the glass is ionized under bias at high temperature[18]. A thicker getter layer, than needed for other bondingmethods, is required to chemisorb the high concentrationof oxygen produced by the anodic bond process. Glass fritand solder can outgas water or carbon dioxide, requiring acompound that absorbs hydrogen and carbon. Reactive sealingusing an oxide, nitride or polysilicon deposition process [32]would require the ability to getter hydrogen left behind afterchemical vapor deposition processing, therefore differentNanogetters have been developed for these applications. Caremust be taken in MEMS design with reactive encapsulation toinsure that the getter surface is not coated during sealing. ThisNanoGetter technology offers the flexibility to customize theNanoGetter layers used and the thickness employed which isof great advantage in changing the getter for different MEMSencapsulation processes. The deposition of NanoGetters is notlimited to MEMS chips. Both MEMS die and conventionalmetal and ceramic vacuum packages, see Fig. 10, can incorpo-rate a patterned NanoGetter on either the lid or other surfaces.
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