DRIE for MEMS devices

BY MICHEL PUECH, JEAN-MARC THEVENOUD, JEAN-MARC GRUFFAT, et al, Alcatel Micromachining Systems

MEMS products are finding more applications in consumer markets like automotive, telecommunications, information technology, health care and environment. Because they address a high-volume, yet competitive market environment, MEMS products — including gyroscopes, accelerometers, silicon microphones, ink jet printer heads, tire pressure sensors, biomedical integrated devices, etc, — need to be produced at the lowest possible cost.

Minimizing the cost of MEMS production is a key success factor for the industry. DRIE equipment manufacturers are optimizing the cost of tools and maximizing their throughput. In the case of DRIE, one important process parameter in favor of a higher throughput is the etching rate. A permanent etch rate improvement program has already produced significant progress. Process yield is also a key factor to provide the highest possible number of devices-per-wafer. Solutions like wafer bevel-edge protection and fine control of tilt profile angle have been successfully designed. A higher degree of automation is also necessary for reaching the most productive wafer handling solutions. Cluster tool platforms have been implemented to accommodate the cost-effective DRIE process modules for higher yield and higher throughput.

High Etch Rate

An efficient DRIE tool, able to produce high silicon etching rates that achieve high selectivity with respect to the photo-resist mask, requires high plasma density for a high dissociation of etching gas molecules together with a low plasma potential for high selectivity. Standard RIE systems are not the best candidates because of the inherent high DC bias voltage implying a low selectivity. Within the high density plasma (HDP) sources, inductively coupled plasma (ICP), is one of the best candidates because if its capacity to generate HDP (1011 to 1012 ions/cm-3), providing efficient gas dissociation and low plasma potential at relatively high pressure. The fluorine radicals produced in the ICP plasma are transported within the gas phase to the silicon surface where they react with the silicon to produce volatile SiF4 molecules evacuated by the pumping system according to the equation: Si(s) + 4F(g)

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