HYDRATION MODEL FOR THE MOLARITY DEPENDENCE OF THE ETCH RATE OF SI IN AQUEOUS ALKALI HYDROXIDES

We present a model to explain the peak observed in the Si etch rate vs. molarity for several alkali-hydroxide solutions. This model requires both free water molecules (H2O)f and hydroxyl ions (OH-) to accomplish etching. As the molarity increases, the OH- concentration increases, while hydration effects steadily reduce the (H2O)f concentration. These two competing effects produce a peak in the etch rate, the location of which is sensitive to the mean hydration number of the particular solution. Results for KOH, NaOH, and LiOH are generally accounted for by the model using mean hydration numbers obtained from independent chemical activity data in the literature. Data for aqueous KOH solutions only (unstirred and stirred) show that stirring shifts the peak etch rate to slightly lower molarity and raises the peak etch rate somewhat, but the behavior is by and large unchanged. Also, at low molarity from 0.1 to 2M the stirred etch rate is measured to be roughly constant, then rises to a peak near 5M. We have attempted to use the present model to account for these more detailed effects and to obtain additional information about the possible etching mechanism applicable at low molarity, where free water alone appears to be the dominant (or rate-limiting) etching species. The very large dependence of the etch rate on crystal orientation at high molarity may be due to geometrical effects in the way the hydration complexes interact with the different crystal surfaces.