Plasma Based Dry Release of MEMS Devices

Abstract

Microelectromechanical Systems (MEMS) are constructed to achieve a certain engineering function or functions by electromechanical or electrochemical means. Very intricate moveable structures can be fabricated using the sacrificial layer (Madou, 2002). These moveable suspended structures are micro-bridges fabricated using the sacrificial layer. These structures can be cantilever beam bridges fixed at one end or membrane bridges fixed at both ends. In the field of RF MEMS these two types of bridges are used to fabricate the series or shunt switches. In a fabrication process, the final release of a MEMS device is the most crucial step. Surface micromachining process relies on both wet and dry etching techniques. Wet etching has been widely used for pattern delineation. In wet etching, liquid etchant dissolves away the exposed film and attacks isotropically, resulting in loss of pattern definition due to undercutting and rounding of film features. This chapter suggests a solution to the problem of stiction by avoiding the wet release and in the absence of Critical Point Dryer (CPD). A dry release technique is presented for the RF MEMS structures, that combines the removal of sacrificial layer through wet etching and its substitution with standard photoresist. After coating, this photoresist acts as a supporting layer under the structure and rejects the structure to attack the substrate (Orpana & Korhonen, 1991). During this complete process wafers are not allowed to dry at any moment of time, otherwise structures may be permanently bonded with the substrate. The supporting layer is removed by oxygen plasma using the Reactive Ion Etching (RIE). In the dry etching, residual waste is off concern which effect the reliability of the MEMS structures. Motivation for this unique process was that some left over residues were observed after the single step or traditional RIE process. Secondly, this process was more cost effective as compared to a wet release CPD technique using CO2 dryer. The process not only produced less residual waste but achieved a clean dry release.