Paper MN-MoM12
Process Characterization of Vapour Phase Sacrificial Etching

Monday, October 15, 2007, 11:40 am, Room 615

The manufacture of MEMS devices has primarily used processes and techniques developed for the semiconductor industry. The process characterization is well established for these methods and are then adapted to the MEMS structure. One process unique to MEMS manufacture is isotropic etching of a sacrificial layer. Historically these processes have been developed using wet etch methods, stagnant gas techniques or gas flow processes with limited process capability. Wet processing and stagnant gas processes employ a one process fits all approach. However, it is seen that different MEMS structures require significantly different process optimization and control. Using memsstar systems for etching, based on controlled continuous flow technology CCFT the process is optimised to the structure being etched. In this example for XeF₂ etching, a carrier gas is employed to transport a precise flow of XeF₂ to the process chamber. The flow of the carrier gas determines the flow of the XeF₂. When etching a structure with a large open access to the sacrificial material the etch is seen to be transport limited. The etch rate is dependent on the flow of XeF₂ into the chamber, the higher the flow the higher the etch rate. When the open access to the sacrificial material is very limited the etch is seen to be reaction limited. In this case the etch rate is dependent on the partial pressure of the XeF₂, the higher the partial pressure the higher the etch rate. Using controlled continuous flow of the process gases combined with fine chamber pressure control the sacrificial etch process can be tuned to the MEMS structure being manufactured. Experimentation with different structures is discussed to show that the etch process performance and process window varies depending upon the mechanical materials and dimensions.