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MEMS-based gray-scale lithography

Micro-electro-mechanical systems (MEMS) fabrication technologies originated directly from integrated circuit (IC) fabrication. IC devices require only two-dimensional or planar structures to be fabricated, because there are no mechanical operations taking place. Therefore structures fabricated for MEMS devices have been traditionally designed with nominally vertical sidewalls (dry anisotropic etching), undercut sidewalls (wet isotropic etching), or sidewalls that have limited angles due to the crystallographic orientation of the substrate (wet anisotropic etching). Assorted novel techniques have been developed in the MEMS field that can achieve 3-D structures. Such techniques include SU-8 molding using microstereolithography, directing ions in DRIE using buried dielectric layers, and by inclined rotating UV lithography and electroplating. These techniques however are very time consuming and/or are limited in the structures that can be created, especially for large MEMS structures. A technique called gray-scale lithography, typically in diffractive optics, is applied using a one level development process to create 3-D structures in photoresist. This technique utilizes planar processing and provides additional flexibility that is not supported in conventional IC, fabrication technologies. The key components for the development of gray-scale lithography in MEMS applications are presented and include the design of the optical mask and the use of a photolithography stepper system.