Sacrificial layer SiO/sub 2/ wet etching for micromachining applications

The key process in surface micromachining is the selective etching of sacrificial layers underlying structural layers. Often, this is accomplished with hydrofluoric (HF) etching of phosphosilicate glass (PSG). The etching takes place along tortuous channels that may be 10-100 mu m long and 1-5 mu m wide. Therefore, the etching reaction process is influenced by mass transfer limitations of both the reactants and products, particularly at long times. The etching reaction will quite likely shift from kinetic-controlled to diffusion-controlled as the etch channel develops. The first order model that has been derived assumes one component diffusion, one-dimensional geometry, first order reaction, and a constant diffusion coefficient. Test structures consist of several low pressure chemical vapor deposition (LPCVD) PSG etch channels of widths from 1 to 50 mu m beneath transparent silicon-rich LPCVD silicon nitride. Good agreement has been found between experiment and theory at short times. Diffusion limitations are observed at long times, but the diffusion coefficient and reaction rate constant are not physically consistent; therefore, they are used only as parameters.<>