Abstract
Micro electrical mechanical system (MEMS), a technology to fabricate miniaturized sensors and actuators, has been growing at an exciting pace in recent years. Basically, MEMS can be divided into to two sorts. Surface micromachining is an additive process to grow several layers such as polysilicon and dielectrics on the surface of the wafer, ranging up to several microns. In contrast, bulk micromachining is a subtractive process etching into silicon wafers to form the desired structures, which ranges up to tens of microns. In modern MEMS technology, combdrive is a very common building block, which can be used as actuators, variable capacitors, inertial sensors, resonators, etc. As an actuator, combdrive can drive micro lens, gratings, mirrors, and hard disk head locators. The capacitance of combdrive is proportional to engagement length of comb fingers, which enable combdrive to be configured as variable capacitors and inertial sensors. High aspect ratio combdrive using bulk micromachining benefits from thicker structure which results in high capacitance and driving force than its surface micromachining counterpart. Silicon on insulator (SOI) wafers are fabricated using silicon fusion bonding and a following thinning process. Silicon fusion bonding can bond two silicon wafers together, providing strong bond energy, no bubble formation, and fulfilling requirements for MEMS applications. Silicon fusion bonding can basically be divided into three steps: surface treatment, room temperature contact, and high temperature. Fusion bonding with buried cavities is also demonstrated. In this thesis, void-free bonding is achieved and fabricated SOI wafers are used in fabrication of high aspect ratio combdrive. SOI wafers provide buried oxide layer, which can act as sacrificial layers under thick microstructure to release them from substrate. Besides release scheme, another key process to fabricate high aspect ratio combdrive is inductively coupled plasma reactive ion etching (ICPRIE), which removes undesired silicon to form combdrive. Process parameters of ICPRIE are retrieved and “notching”, a special phenomenon in SOI etching, is also investigated. High aspect ratio combdrive is successfully fabricated using SOI wafer and ICPRIE. Combdrive with thinned suspension beam is also demonstrated using multi-level process, which is involved with SOI micromachining, and in addition, dual hardmask. The multi-level high aspect-ratio combdrive has finger thickness about 22μm and suspension beam thickness around 12μm and 17μm, respectively. The multi-level process provides thinned suspension beam for high aspect ratio combdrive, which benefits from reduce beam stiffness and thus reduced driving voltage, while capacitance and displacement of the combdrive retains. A technology platform to fabricate high aspect-ratio multi-level combdrive is well set up in this work, which enables stable combdrive fabrication and will be used for many applications in implementing sensors and actuators, such as inertial sensors and micromachined optics.