Polymer-based wafer-level packaging of micromachined HARPSS devices

This thesis reports on a new low-cost wafer-level packaging technology for microelectromechanical systems (MEMS). The MEMS process is based on a revised version of High Aspect Ratio Polysilicon and Single Crystal Silicon (HARPSS) technology. The packaging technique is based on thermal decomposition of a sacrificial polymer through a polymer overcoat followed by metal coating to create resizable MEMS packages. The sacrificial polymer is created on top of the active component including beams, seismic mass, and electrodes by photodefining, dispensing, etching, or molding. The low loss polymer overcoat is patterned by photodefinition to provide access to the bond pads (or probing pads). The sacrificial polymer decomposes at temperatures around 200-280°C and the volatile products permeate through the overcoat polymer leaving an embedded air-cavity. For MEMS devices that do not need hermetic packaging, the encapsulated device can then be handled and packaged like an integrated circuit. For devices that are sensitive to humidity or need vacuum environment, hermeticity is obtained by deposition and patterning thin-film metals such as aluminum, chromium, copper, or gold. To demonstrate the potential of this technology, different types of capacitive MEMS devices have been designed, fabricated, packaged, and characterized. This includes beam resonators, RF tunable capacitors, accelerometers, and gyroscopes. The MEMS design includes mechanical, thermal, and electromagnetic analysis to obtain a thorough understanding of the MEMS and the associated packaging. The device performance, before and after packaging is compared and the correlation to the model is presented. In the process of this work, new fabrication techniques are explored and reported.; Many MEMS packaging methods are reported for only a special device. The main packaging methods include wafer-to-cap bonding and sacrificial-layer-based sealing. The wafer bonding schemes are the most reliable methods, but are costly and not size efficient. The sacrificial-layer-based sealing schemes are device-dependent, costly, and mainly high-temperature. These methods require perforation in the package to release the sacrificial film, followed by multiple steps to seal the holes. The elevated temperature during packaging sequence introduces stress on the MEMS device and can degrade the performance. The advantages of the presented approach compared to other MEMS packaging techniques are that it is a low-temperature process that can be used for packaging a wide variety of MEMS including metallic structures, it produces a lowprofile encapsulating cover, and can be performed on any substrate. Thermal decomposition of sacrificial polymer is performed through a solid perforation-free capsule, which eliminates the steps needed in some other sacrificial-film-based techniques to seal a perforated or porous cover. It does not require high temperature deposition and etching of sacrificial materials and is stiction-free. The overcoat geometry can be scaled as needed by the application to tailor different sizes from microscale to millimeter-scale. The packaging does not require wafer-to-cap alignment and bonding. This method does not impose any limitation on MEMS size, topology, or substrate. New MEMS and package characterization methods including evaluation of the package permeability, stress, and cavity pressure are presented.