Abstract: A previous sacrificial layer etching model treats the diffusion coefficient D as a constant through the etching process.This model fits the experimental data well during a short initial period of the etching time,but it deviates very seriously as the etching progresses.In order to explain this phenomenon and predict the etching process accurately,a modified model is proposed that treats the diffusion coefficient of HF as a function of the solution concentration.In the modified model,a decrease in the HF concentration will cause an increase of the HF diffusion coefficient,which will partly compensate for the decrease in concentration because of the long diffusion distance. In the modified model,the diffusion coefficient is also a function of temperature.In this way,the modified model matches the experimental data very well.These results provide new insight for understanding not only the mechanism of sacrificial layer etching,but also the solution diffusion in complex structures.The observed phenomenon should be applicable to other kinds of sacrificial layer etching if they are diffusion limited.

Key words: MEMS, sacrificial layer etching, etching rate coefficient, diffusion coefficient