Abstract

Freescale Semiconductor is employing a new, multi-layer integrated seal (IS) on its next generation accelerometers. The IS, which encloses the moveable sensing element, consists of alternating layers of poly-Si and PSG. A technique needed to be developed to remove the integrated seal in order to permit failure analysis. Mechanical methods were attempted first, but these resulted in severe damage to the sensing element. Chemical deprocessing was considered, but eventually abandoned because there seemed to be no way to protect the sensing elements from the wet etchants that would be used on the IS. Eventually, Reactive Ion Etching (RIE) with an Inductively Coupled Plasma (ICP) source proved to be a successful means for removing the IS without impacting the sensing element. As the design of the integrated seal underwent multiple redesigns, the removal process was successfully modified multiple times to comply with these changes. By using the right gases in the correct order, a high level of selectivity was maintained, allowing for removal of successive layers of different materials (poly-Si, PSG) without harming the sensing element. After removal of some IS designs, a wispy residue was observed on the sensing element and remaining IS support pillars. Chemical analysis identified this material as a by-product of the RIE process, and methods were devised to eliminate it.

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