Mock CMOS: An Inexpensive, Fast, and Versatile Microfabrication Technique Using One Metal and One Silicon Dioxide Film

Abstract

A versatile fabrication process that allows users to quickly construct micromachined structures using a one metal, one silicon dioxide film stack on a silicon wafer is presented in this technical report. This simplified process, called Mock CMOS, (a) starts from pre-processed wafers and requires only one photolithography step, (b) provides a conductor material for actuating electrostatic and thermal devices, (c) avoids electrical shorting between metal microstructures or to the silicon substrate by using silicon dioxide as an insulator, and (d) allows quick prototyping of MEMS structures similar to those designed at Carnegie Mellon University (CMU). The CMOS-MEMS process at CMU is a post-CMOS fabrication process in which the etching masks are provided by the interconnect metal layers in the standard CMOS process. Prototyping of sensor and actuator devices in the CMOS-MEMS process requires considerable cost, waiting time for chip fabrication, and possible further iterations until satisfactory device performance is attained. By using the three fundamental materials of metal, oxide, and silicon, a designer can create microelectromechanical devices in the Mock CMOS process, similar to the standard CMOS-MEMS microstructures, yet with a significantly reduced turnaround time. By removing the CMOS component and limiting the process to one metal and one oxide layer, a designer can focus on the mechanical aspects of a microstructure with the capability to layout multiple device variations of arbitrary size onto a four inch wafer. The deposition of two layers (aluminum and silicon dioxide) on a batch of silicon wafers, along with lithography material costs, brings the price for Mock CMOS to $0.05/mm2. Devices successfully created include surface-normal and lateral electrostatic and thermal actuators, the majority of which were designed by forty students in an Introduction to MEMS course in Fall 2001 at Carnegie Mellon University.