Studying the Etch Rates and Selectivity of SiO2 and Al in BHF Solutions

One limitation to fabricating MEMS devices in some academic labs is the lack of polysilicon deposition technology. This limits the devices that can be fabricated because polysilicon is a common structural material in MEMS devices. In order to enhance the MEMS fabrication capabilities, expanding the use of Al as a structural layer has been researched. The main difficulty is the lack of a selective wet etch between the Al structural layer and the SiO₂ sacrificial layer. In this study, seven etching solutions were studied on SiO₂ and Al for their etch rates and selectivity. Two were buffered hydrofluoric acid (BHF) solutions with different concentrations of NH₄F. Four were BHF solutions with propylene glycol or glycerin at different concentrations. The seventh solution was a commercial solution, Pad Etch 4. Among the seven etching solutions, 5:1 BHF gave the best selectivity between SiO₂ and Al. Increasing the NH₄F concentration from 5 to 7 parts did not increase the selectivity, but selectivity increased by adding NH₄F in HF solution. Adding propylene glycol or glycerin to the 7:1 BHF solution did not increase the selectivity. When glycerin was added to 7:1 BHF solution it provided superior selectivity than was obtained from adding propylene glycol to 7:1 BHF. This paper will present the etch rates and selectivity of the 7 solutions along with comparisons with other published results.