Improvement of the porous silicon sacrificial-layer etching for micromachining applications

Using porous silicon (PS) as a sacrificial layer, freestanding structures with a large distance to the bulk can be obtained. The very high specific surface area of PS allows its removal using dilute alkaline solutions. However, a critical equilibrium between a non-violent and a complete etch reaction is necessary. A solution to this problem is a multi-step etching process using solutions of different concentrations. To avoid this disadvantage, different technological conditions are studied in order to optimize the removal of the PS by using a one-step process with a 0.1% KOH solution. The effects of adding ethanol, ultrasonic stirring and polarization during the etching process are evaluated. Optimization of the etching procedure allows good-quality freestanding polysilicon structures featuring a smooth substrate surface to be obtained. © 1997 Elsevier Science S.A.