Hydrogenation-Assisted Lateral Micromachining of (111) Silicon Wafers

Abstract

Micromachining of (111) silicon wafers by means of a plasma hydrogenation and chemical etching sequence is achieved. Vertical etching is used to define the depth of the craters as well as the thickness of the final suspended silicon body. After protecting the 3-D structure by a thermally grown oxide, a hydrogenation step is used to remove the oxide layer from the bottom of the crater, allowing a lateral underetching. Final exposure of the processed silicon to a KOH solution, etches silicon in a lateral fashion and in the exposed places. A lateral aspect ratio of four to six has been achieved. The evolution of suspended structures on (111) wafers, suitable for sensor fabrication, is feasible without a need to a 3-D lithography. Using this technique suspended interdigital structures have been realized with a depth up to 70 mum. In addition, ultrathin fully suspended structures have been successfully fabricated. A preliminary capacitive accelerometer has been realized and tested on (111) substrate.