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Abstract:

In recent years there has been an increasing interest for thin-film packaging techniques that can seal MEM devices at the wafer level. When these packages are fabricated using standard surface micromachining techniques, the need for wafer bonding equipment is eliminated. These packages not only ensure a stable environment for the devices, but also protect the packaged devices during further processing like dicing for example.This thesis presents the development of a thin-film wafer level packaging process flow with the use of polycrystalline silicon-germanium thin films. Poly-SiGe layers are developed with a low residual stress, a low stress gradient and a low electrical resistivity. μc-SiGe is identified as a locally porous layer that can be used in the packaging process to avoid deposition inside the cavity. Since poly-SiGe can be deposited at temperatures around 450C, the developed packaging technology is CMOS compatible. This means that MEMS and its package can be deposited directly on top of a CMOS substrate. The contact between CMOS and MEMS is therefor characterized and optimized. A model is proposed to accurately extract the contact resistivity from measurements using a Stacked Greek Cross Resistor structure. An Ar soft sputter etch in combination with a Ti-TiN interlayer results in a contact resistivity of 5 10−7 Ohm cm2 for a 2×2 μm2 contact size. This value is comparable with the state-of-the-art but the within wafer uniformity is one order of magnitude better than previously reported processes.Also the hermeticity of thin-film packages is evaluated. This is done using gross and fine leak tests. The packages were leaky for He. In air the packages sealed with AlCu have a leak rate that is lower than 5.73 10−16 mbar l/s. A further investigation is done using an encapsulated resonator to characterize the pressure inside the cavity. Resonator measurements show that the pressure inside a AlCu sealed cavity is near atmospheric pressure (1atm). This high pressure inside the package is unexpected, but highly suitable to encapsulate accelerometers. Accelerometers were thus packaged using AlCu as a sealing layer as a proof of concept. After encapsulation, the accelerometers show to be free-moving and functional. Also measurements at low (0, 5, and 15 C) and high (125C) temperatures indicate a stable environment. The medium-term hermeticity was checked for 6 weeks. For the first time shear tests are performed on thin-film packages to evaluate their mechanical strength. Different variations in design and processing of the anchors are tested. By using a Ti-TiN ’glue layer’ almost all structures meet the MIL-STD 883E standard (method 2019.5) specifications. If the ratio determined by the total SiGe-SiGe anchor area and the SiO2-SiGe anchor area is higher than 1, the thin film package reaches the MIL-standard requirements. These packages are able to resist a VHF release process. For the failing packages the cause is identified and understood.