Low-temperature BPSG reflow compatible with surface micromachining

Borophosphosilicate glass (BPSG) demonstrates superior reflow properties at low reflow temperature compared with phosphosilicate glass (PSG). However, the latter is conventionally used in surface micromachining, because of the high underetch rate in HF. This paper reports on the BPSG composition required to achieve both satisfactorily reflow at 850 degrees C and adequate etch rate in BHF to enable the application of BPSG as reflow and sacrificial material. The effect of the boron content in the BPSG and the application of a HMDS film before low-temperature reflow on the reflow performance have been investigated using LPCVD deposition and reflow in a wet oxidation furnace.