Back-end processing of scanning mirrors with scratch drive actuators

The use of micro-electro-mechanical system (MEMS) processes in fabrication of micr- electro-mechanical devices for optical applications has been widespread. The invention of digital light processing (DLP) technology by Texas Instruments Inc. popularized the use of micro-mirrors. One of the applications of the micro-mirrors is in the form of scanners for biomedical imaging. The small size of mirrors and actuators makes them a good candidate for in vivo measurements/imaging. Various techniques in bulk- and surface-micromachining, and various actuators have been used to fabricate scanning mirrors. The scanning mirrors used in this work make use of scratch drive actuators (SDA) for their scanning motion. Both surface- and bulk-micromachining technologies are used to fabricate the devices. Surface-micromachining (Multi User MEMS Processes or MUMPs®) is used to fabricate scanning mirrors, and bulk-micromachining is used to separate dies, to create sloped sidewalls for efficient packaging and to make grooves for optical fibers. This work describes the techniques used in the post-MUMPs processing of the devices. The main features of the post-MUMPs processing are substrate etching and formation of insulating links. The presence of devices on the front side necessitates the use of a protection method in the substrate etching step. Polymers have drawn attention in recent years as protective materials due to their chemical stability as well as the ease of use. This thesis work utilizes poly dimethylsiloxane (PDMS) as a protection material, and examines the effect of PDMS process conditions on the quality of the protection. Protection for at least 10 hours was achieved in this work. The scanning mirror devices of this thesis work require non-conductive links between the actuators and the mirror. Thick photoresists such as AZ 4620, AZ 9260, SU-8 etc, are a good choice due to the ease in their patterning as well as their excellent insulating properties. This thesis documents the processes followed to achieve optimal link structures using thick photoresist (AZ 9260). The resist was optimally hard baked to make it chemically inert to common organic solvents