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Wafer-level vacuum packaging for MEMSR. Gooch,a)T. Schimert, W. McCardel, and B. RitcheyRaytheon Systems Company, MS 35, Dallas, Texas 75243D. Gilmour and W. KoziarzAir Force Research Laboratory/IFTE, Rome, New York 13441-4505Received 12 October 1998; accepted 26 April 1999!Vacuum packaging of high performance infrared IR! MEMS uncooled detectors and arrays, inertialMEMS accelerometers and gyros, and radio frequency rf! MEMS resonators is a key issue in thetechnology development path to low cost, high volume MEMS production. Wafer-level vacuumpackaging transfers the packaging operation into the wafer fab. It is a product neutral enablingtechnology for commercialization of MEMS for home, industry, automotive, and environmentalmonitoring applications. 4 in. wafer-level vacuum packaging has been demonstrated using IRMEMS bolometers and results will be presented in this article. In addition to the wafer-levelpackaging results, vacuum package reliability results obtained on component-level ceramic vacuumpackages will also be presented. 1999 American Vacuum Society. S0734-210199!21204-3#I. INTRODUCTIONVacuum packaging of high performance infrared IR!MEMS uncooled detectors and arrays,1as well as, inertialMEMS accelerometers and gyros, and radio frequency rf!MEMS resonators is a key issue in the technology develop-ment path to low cost, high volume MEMS production. Mostuncooled detector technologies employ a thin, thermally re-sponsive suspended membrane pixel design thermally iso-lated from the supporting substrate by long thin thermal iso-lation legs. The typical suspended membrane thermalisolation measured in terms of thermal resistance is RTH.13107K/W i.e., thermal conductance GTH,131027W/K.This level of thermal isolation requires a vacuum environ-ment with sub 10 mTorr vacuum to eliminate thermal lossthrough gas conductance in the package.In this article, recent results on wafer-level vacuum pack-aging of IR MEMS uncooled detectors are presented andcontrasted with a component-level ceramic vacuum packageapproach. A comparison of component-level and wafer-levelpackage approaches is shown in Fig. 1. The component-levelvacuum packaging process involves dicing up the MEMSwafer and mounting MEMS die in individual ceramic pack-ages which are subsequently sealed and tested. In the wafer-level packaging approach, the MEMS die are sealed in asingle sealing process and subsequently tested at the waferlevel. The wafer is diced into individual vacuum packageddie only after testing is completed. The key advantages ofthis wafer level approach to vacuum packaging are substan-tially lower cost and higher volume throughput relative to thecomponent-level vacuum packaging approach. Wafer-levelpackaging also offers key advantages in terms of miniatur-ization and system integration for low cost microsensor ap-plications.Results for wafer-level vacuum packaged a-Si microbo-lometer arrays are presented. Wafer-level packaged arrayswith sub 10 mTorr vacuum have been demonstrated. Resultsfrom ongoing testing of sealed wafer-level packages indicatepackage lifetimes of seven months with no degradation ofpackage vacuum thus far. A 4 in. wafer-level vacuum pack-age process with 78% seal yield across the wafer has beendemonstrated.Reliability results for an 84-pin ceramic vacuum packagewith 0.7in.30.7in. cavity and a 16-pin CERDIP with0.2in.30.3in. cavity are also presented. The ceramic pack-ages are sealed with an antireflection-coated Si or Ge lid.Sub 10 mTorr vacuum has been demonstrated for packageda-Si microbolometer arrays. In addition, ceramic vacuumpackaged microbolometer arrays have been subjected tohigh-temperature stability bake testing at 150C for 1500 hat the Air Force Research Laboratory, Rome, NY, with nodegradation in package vacuum. The ceramic vacuum pack-age has been used to reliably package a-Si microbolometerarrays with thermal resistance RTH.73107K/W indicatingthermal isolation in the package approaching the radiationlimit.II. WAFER-LEVEL VACUUM PACKAGINGWafer-level vacuum packaging is under development forlow cost, high volume IR MEMS inertial MEMS, and rfMEMS applications. Initial development was carried out on1 in. parts sawed from 4 in. wafers. The wafer-level packag-ing process has now been scaled up to 4 in. wafers. Recently,a 4 in. wafer-level vacuum packaging demonstration usinga-Si microbolometer array wafers has been carried out withsealing yield of 78% across the wafer.A 1 in. wafer-level package is shown in Fig. 2a!. The 1in. part has six individually packaged a-Si microbolometerdie each with a seal ring as shown in Fig. 2b! A 1 in. wafercontains four two-channel microbolometer package die andtwo four-channel package die, respectively, used in IR gassensor applications.1The sealed bolometer cavity is ad-dressed using a metal interconnect running under the seal asshown in Fig. 2b!. The interconnects are electrically isolateda!Electronic mail: r-gooch22952295J. Vac. Sci. Technol. A 174, Jul/Aug 19990734-2101/99/174/2295/5/$15.001999 American Vacuum Societyfrom the seal ring by an insulating SiN layer. A 4 in. mi-crobolometer wafer showing the seal ring architecture isshown in Fig. 2c!.Wafer-level vacuum package results demonstrating pack-age vacuum ,10 mTorr are shown in Fig. 3. To obtain ac-curate calibration of the cell pressure in the wafer-levelvacuum packaged die, a hole was drilled in one of the sealedpackages and the package was placed in a vacuum Dewar toobtain the calibration curve of microbolometer signal versusvacuum pressure shown in the figure. Using the curve, thesignal in the sealed package 54 mV! corresponds to a sealedvacuum level of 9 mTorr.Vacuum level stability measurements of the six packageddie in a 1 in. wafer-level package Fig. 2b!# have been car-ried out over a seven month period with no degradation inmicrobolometer signal in any of the packaged die. Thevacuum stability results for wafer-level package No. 123 areshown in Fig. 4. The microbolometer signal in five of the sixpackage die is shown to remain steady over the seven monthtest period indicating that wafer-level package vacuum hasnot deteriorated. A short developed in package die No. 1making data unavailable after the third measurement. Theseresults are the first demonstration of a long term reliableuncooled detector wafer-level vacuum package. The packagewill continue to be monitored to evaluate wafer-levelvacuum package integrity over an extended time period.The wafer-level packaging process has been scaled up to4 in. wafers Fig. 2c!# with a recent 4 in. a-Si bolometerwafer-level vacuum packaging demonstration exhibitingsealing yield of 78% for functional die across the wafer.Individual die sawed from sealed 4 in. wafers are shown inFig. 9 and are discussed below in Sec. III.III. RELIABILITY STUDIES FOR COMPONENT-LEVEL CERAMIC VACUUM PACKAGINGIn this section, vacuum package stability and reliabilitystudies obtained from both high-temperature bake stabilityand long-term vacuum stability tests on component-level ce-ramic packages will be presented. The 84-pin ceramicvacuum package developed for large area MEMS, is shownin Fig. 5. An open 84-pin alumina ceramic package with0.7in.30.7in. cavity area is shown in Fig. 5a! with two256378a-Si microbolometer arrays mounted in the cavity.The package with a solder-sealed antireflection AR!-coatedGe window 812mm spectral bandpass! is shown in Fig.5b!. The Ge window is used because of its transparency inthe 812mm spectral band. Also, the low coefficient of ther-mal expansion CTE! mismatch between the Al2O3ceramicpackage (CTEAl2O3ceramic;731026C21) and the Ge window(CTEGe;6.331026C21) is a critical requirement for a highyield, reliable solder seal process using this large package.Vacuum packaging results obtained on a sealed 84-pinFIG. 1. Comparison of component level and wafer-level vacuum packagingapproaches.FIG. 2. a! 1 in. wafer-level vacuum package. Seal ring architecture for b! 1in. wafer, and c! 4 in. wafer.2296Goochet al.: Wafer-level vacuum packaging for MEMS2296J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999package are shown in Fig. 6. As with the wafer-level pack-age, vacuum calibration was performed after the packagewas sealed and tested by drilling a hole in the lid of thesealed package and placing the package in a vacuum Dewarto obtain a calibration curve of microbolometer signal versusvacuum level. A large area 84-pin ceramic vacuum packagepost-seal calibration curve of microbolometer signal versusvacuum level is shown in Fig. 6. The bolometer signal levelmeasured for the sealed package in this case is 101 mV.From the calibration curve in the figure, this corresponds tosealed package vacuum of 3 mTorr.Two 84-pin ceramic vacuum packages containing thetwin 256378 arrays have been subjected to high-temperaturebake stability testing at the Air Force Research Laboratory,Rome, NY. In the testing, packages 593 and 595 were sub-jected to 150C bake for 1500 h. The results, shown in Fig.7, display the bolometer signal level for two channels 26,218! in each package. The packages were tested after a totalof 100 h at 150C bake and retested after a total of 350,1000, and 1500 h, respectively, at 150C bake. After 100 h,the detector channels in both showed a slight increase inbolometer signal. After 1500 h, the two detector channels inpackage 593 were essentially unchanged. In package 595,there is a slight drop in signal after 350 h due to partialdelamination of the antireflection coating. However, the finalsignal levels for the two channels are comparable to the ini-tial signal levels indicating no vacuum degradation in thepackage.It is noted that the a-Si microbolometer signal levels inFig. 7 are two to three times larger that that shown in Fig. 6.Both sets of measurements were carried out under identicalconditions. The improved signal levels in Fig. 7 are due torecent enhancement in a-Si microbolometer pixel thermalisolation. In these 50mm350mm pixel elements, substan-tially enhanced thermal resistance Rth.73107K/W ther-mal conductance Gth,1.431028W/K) has been achievedcompared with Rth;(24)3107K/W reported previously.1Finally, with regard to long-term package vacuum, it isnoted that the oldest of 84-pin ceramic vacuum packages arenow more that 12 months old and show no signs of vacuumdegradation. Furthermore, results from long-term 29 month!vacuum package stability tests carried out using previouslydeveloped component-level 16-pin CERDIP vacuum pack-FIG. 3. 1 in. wafer-level vacuum package with ,10 mTorr vacuum.FIG. 4. Wafer-level package stability results over seven month period Package 123!.FIG. 5. 84-pin ceramic package a! showing mounted arrays, b! sealed.2297Goochet al.: Wafer-level vacuum packaging for MEMS2297JVST A - Vacuum, Surfaces, and Filmsages sealed with AR-coated silicon lids is shown in Fig. 8. Inthe figure, the ratio of final microbolometer signal after 29months! to the initial signal is shown and remains essentiallyunity for the 17 packages tested indicating no degradation inmicrobolometer signal, and hence package vacuum, over the29 month period.Figure 9 shows six WLVP die which were sawed fromsealed 4 in. wafers and mounted in test packages to carrythem through the same set of environmental tests as de-scribed for ceramic packaged bolometer arrays. Evaluation isongoing at present. All die have survived unchanged afterseveral hundred hours of 150C baking.IV. SUMMARYVacuum packaging of high performance IR MEMS un-cooled detectors and arrays, as well as, inertial MEMS ac-celerometers and gyros, and rf MEMS resonators is a keyissue in the technology development path to low cost, highvolume MEMS production. In this article, recent results onwafer-level vacuum packaging of IR MEMS uncooled detec-tors were presented and contrasted with a component-levelceramic vacuum package approach. In the wafer-level pack-aging approach, the MEMS die sealed in a single sealingprocess and subsequently tested at the wafer level. The waferis diced into individual vacuum packaged die only after test-ing is completed. The key advantages of this wafer levelapproach to vacuum packaging are substantially lower costand higher volume throughput relative to the component-level vacuum packaging approach. Wafer-level packagingalso offers key advantages in terms of miniaturization andsystem integration for low cost microsensor applications.Results for wafer-level vacuum packaged a-Si microbo-lometer arrays were presented. Wafer-level packaged arrayswith sub 10 mTorr vacuum were demonstrated. Results fromongoing testing of sealed wafer-level packages indicatepackage lifetimes of seven months with no degradation ofpackage vacuum thus far. A 4 in. wafer-level vacuum pack-age process with 78% seal yield across the wafer was dem-onstrated.Reliability results for an 84-pin ceramic vacuum packagewith 0.7in.30.7in. cavity and a 16-pin CERDIP with0.2in.30.3in. cavity were also presented. The ceramic pack-ages were sealed