Robust micromachining of compliant mechanisms for out-of-plane microsensors

Micro-Electro-Mechanical-Systems (MEMS) take advantage of a wide range of very reliable, and well established existing microelectronics fabrication techniques. Due to the planar nature of these techniques, out-of-plane MEMS devices must be fabricated in-plane and assembled afterwards in order to create out-of-plane three-dimensional structures. Out-of-plane microstructures extend the design space of the MEMS based devices and overcome many limitations of the in-plane processing. Nevertheless, several issues have to be addressed in order to integrate an out-of-plane structure into an existing process. These include robustness, yield, reliability, assembly technique, packaging and so forth. In this thesis we introduce an inorganic based post-CMOS compatible process upon which the mechanical structure for out-of-plane micro sensors and actuators can be fabricated. The hinge-less out-of-plane microstructures (compliant mechanisms) are mechanically robust structures that provide reliable electrical connection to the devices that are rotated out-of-plane. Fabrication of these structures by inorganic materials introduces several challenges to the process that have to be addressed.Fabrication of micromechanical structures by silicon micromachining could significantly modify the topography of the substrate, which results in non-planarity and degradation of the mechanical performance of the structures. On the other hand, the residual stress of the structural layer has profound effect on the final shape of the out-of-plane microstructures. In particular, stress non-uniformity can cause severe structural deformation which deteriorates the device performance (e.g. linearity, sensitivity, and dynamic range), or can make the device assembly difficult or sometimes impossible. To overcome the topography issue related to the freestanding structures and to control the stress profile of their structural layer, we have developed two novel techniques. The first technique is an unconventional planarization process that is achieved by modifying the etch property of the sacrificial layer. The second technique compensates the stress non-uniformity across the thickness of the sputter-deposited films by in-situ control of the film property during the deposition process. The practicality and versatility of these techniques has been illustrated through the fabrication of a functional out-of-plane three-axis thermal accelerometer, which has a significant and growing share in the consumer electronics market.