HYDROFLUORIC-ACID ETCHING OF SILICON DIOXIDE SACRIFICIAL LAYERS .1. EXPERIMENTAL-OBSERVATIONS

The key process in silicon surface micromachining is the selective etching of silicon dioxide sacrificial layers with hydrofluoric acid. This paper discusses experimental sacrificial layer etching results. The etching reaction shifts from kinetic controlled to diffusion controlled as the etch channel dissolves. Test structures consist of several low pressure chemical vapor deposited (LPCVD) phosphosilicate glass (PSG) isolated etch channels having widths ranging from 2 to 50 mum underneath transparent silicon-rich LPCVD silicon nitride or thin (1500 angstrom) LPCVD polycrystalline silicon structural layers. Etching is monitored at timed intervals through this structural layer. Diffusion-limitations are observed at long times. After 20 min in concentrated HF, etch fronts Eave moved 25% less than the initial etch rate would suggest. In addition, increasing the phosphorus content in PSG thin films, increasing the HF concentration, and the addition of HCl to HF solutions increases the initial etch rate of silicon dioxide sacrificial layers. Buffered HF and surfactant buffered HF did not enhance the etching of silicon dioxide sacrificial layers. The addition of fluosilicic acid to HF decreased the initial etch rate of silicon dioxide but did not affect the diffusion rate of HF in water for low concentrations of H2SiF6. Etch channel width, sacrificial layer thickness, structural material choice and stress condition, and applied stress did not affect the sacrificial layer etch process rate.