Materials impacts on micro-opto-electro-mechanical systems

Micro-Optical-Electro-Mechanical Systems (MOEMS) employ batch fabrication processes to construct miniature devices with macroscopic functionality. Surface micromachined MOEMS structures are manufactured by the deposition and patterning of thin films. In marked contrast with conventional fabrication processes land bulk micromachining, the thin film materials used in surface micromachined structures are formed as the device is processed. In general, the material properties of thin films are not controlled during deposition, and are only measured after processing is completed. Characterization methods include wafer curvature measurements and a variety of test structures. None of the thin film characterization techniques currently employed is entirely satisfactory and all methods rely on process repeatability to be useful. The ultimate optical performance of MOEMS depends directly on the materials properties of the thin films employed. Differing residual material stresses and residual stress gradients cause curvature which degrades the optical performance of nominally flat reflecting surfaces. For multilayer structures like most micromirrors curvature due to the bimetallic effect can not be ignored. Recent work in which foundry processes were used to fabricate low-cost deformable mirrors (MEM-DMs) for adaptive optics illustrates the impact of residual material stress on system level optical performance. In other MOEMS devices residual material stress can be exploited to produce unique structures. More precise monitoring and control of film stress during deposition remains as a challenge for MEMS and MOEMS. This paper will address the principal materials issues for MOEMS and suggest both design and process control solutions.