Al to Al bonding was successfully demonstrated for hermetic sealing of MEMS devices and three-dimensional interconnects. On a MEMS device wafer, 2 µm thick Al (with 2% Cu) was patterned at the perimeters of the individual dies as well as the input/output bond pads. On a cap wafer, after forming polycrystalline-Si filled vias, the seal rings and bond pads were also patterned with the Al described above. The two wafers were then bonded at ~ 450 °C with various bond forces up to 80 kN. The leak detection on the capped device showed the superb hermeticity of ~10 -12 cm 3 atm/sec He leak rate with an Al seal width as narrow as 3 µm. And the electrical contact resistance of the Al to Al bonded interface measured less than 1 Ω.

Various categories of bonding technologies were investigated for MEMS encapsulation applications. The bonding processes presented in this paper include Al to Al, Si to Si, and metal (Al or Au) to Si. Among the above different bonding schemes, the Al to Al bonding gave the highest process yield and bond strength. In addition, actual MEMS accelerometers were successfully integrated with Al to Al bonding with high yield all the way through plastic over-mold packaging and assembly.

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The technique of transferring patterned ion-cut layers from one Si wafer to another was demonstrated. The starting silicon wafer was masked with checkerboard and line patterns with a 3 m thick polymethylmethacrylate/photoresist and was implanted with 5ϫ10 16 H ϩ ions/cm 2 at 150 keV. After stripping off the mask, the wafer was bonded to an oxide-coated receptor wafer through low-temperature direct wafer bonding. Heat treatment of this bonded pair showed that the hydrogen-induced silicon surface layer cleavage ͑ion cut͒ could propagate throughout about 16 mϫ16 m of nonimplanted material with implanted regions only 4 m wide. Mask width, spacing, and implantation profiles through the mask shape were shown to have effects on the internal microfracturing mechanisms. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00645-7͔Three-dimensional electronic device integration offers significant opportunities for future system improvement in microprocessors and memories. 1,2 This prospect might be implemented with the hydrogen-induced silicon layer cleavage process, which has already been reported for capacitor patterns ͑passive devices͒. 3 To cleave the implanted layer, a minimum dose of a few times 10 16 /cm 2 of implanted hydrogen is needed. 4 This large dose of hydrogen most likely will damage the devices fabricated on the silicon prior to the ion-cut process. In this study, we introduce a patterned ioncut process in which active regions of the wafer are protected from the hydrogen implantation.In this study, Czochralski-grown, ͑100͒, n-type ( ϭ5 -50 ⍀ cm), 100 mm silicon wafers were used. The Si donor wafer was coated with a layer of KTI 950K 9% polymethylmethacrylate ͑PMMA͒ and a layer of Shipley 1400-30 photoresist with a total thickness of 3 m, followed by patterning of various sizes of squares and lines for the implantation mask with different openings for the hydrogen implantation ͑see Fig. 1͒. This patterned wafer was then implanted with H ϩ ions at 150 keV with a dose of 5ϫ10 16 cm Ϫ2 . Dur-ing implantation, the wafer was kept at ambient temperature. A 3 m thick ion mask layer ͑including PMMA and the photoresist͒ was applied to prevent the hydrogen ions from reaching the silicon wafer surface, resulting in hydrogen-ion implantation only in the openings. After the implantation, the ion mask was removed by oxygen plasma ashing. To determine the cracking temperature, the patterns were annealed at temperatures from 400 to 600°C in forming gas for 5 min after the removal of the ion mask. It was found that blistering occurred between 500 and 550°C. It was clear under the microscope that all the blisters were confined to the implanted regions. This observation confirms the effectiveness of the implant mask for the protected regions.On the receptor wafer, a layer of thermal oxide 200 nm thick was grown. The two wafers were bonded directly faceto-face at room temperature or at slightly elevated temperature after standard RCA cleaning of the implanted wafer. The bonded pair was then heated in a rapid thermal annealer un...

A flow sensor capable of measuring flow rate, fluid type, fluid pressure, fluid presence, and flow direction has been demonstrated. This research activity demonstrates a very simple cap fabrication and assembly process that enabled the realization of a flow sensor with multiple sensing capabilities. A surface micromachined process is employed to fabricate the pressure sensors as well as the cap that defines the fluid channel. The sensor was configured in a way to successfully demonstrate two differential pressure sensing modes (i.e. sensors within the cavity and sensors in the flow channel). Both flow rate and viscosity measurements are based on the differential pressure sensing principle. Fluid presence is determined using an interdigitated structure which can also sense the fluid type based on permittivity of the fluid. The flow rate measured successfully has been as low as 0.01 ml/hr and showed very good linearity when compared to the theoretical model. The diameter of the pressure sensor diaphragm utilized was as low as 100 microns and depended on the flow rate required to measure.