HYDROFLUORIC-ACID ETCHING OF SILICON DIOXIDE SACRIFICIAL LAYERS .2. MODELING

Modeling of sacrificial layer etching can be done by combining the dif fusion rates of etchants in solution and the chemical reaction rate. S teady-state diffusion is estimated for HF diffusion coefficients in wa ter on the range of 5 X 10(-6) to 5 X 10(-5) cm2/s as observed from an independent experiment. Likewise, the chemical reaction rates (or ini tial etch rates) for each silicon dioxide/hydrofluoric acid system as a function of HF concentration are obtained from independent experimen ts. The etching kinetics of SiO2 with HF are nonlinear. An analytical method is used with power law kinetic reaction rate expressions and Ne wton's method to linearize the problem. This allows the calculation Of the surface HF concentration as a function of etching time. By equati ng the diffusive and reactive fluxes at the SiO2 action surface, the s urface HF concentration is calculated. Applying a forward Euler integr ated etch rate relationship, the surface HF concentration can be used to plot the etch front location as a function of time.