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 (H 2 O) f and hydroxyl ions (OH) to accomplish etching. As the molarity increases, the OH − concentration increases, while hydration effects steadily reduce the (H 2 O) 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 some-what, 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