Fabrication of Single-Crystal Silicon Microstructure by Anodic Reaction in HF Solution

Some silicon micromechanical structures useful in sensors and actuators have been fabricated by electropolishing or porous silicon formation technique by anodic reaction in HF solution. The microstructures were lightly doped single crystal silicon and the formation was isotropic, independent of crystal directions. Porous silicon layer(PSL) was formed selectively in region of silicon structure by anodic reaction in concentrated HF(20-48%) solution. Characteristics of the formed PSL were investigated along with change of the reaction voltage, HF concentration and the reaction time. PSL was formed only in region. The porosity of the PSL was decreased with the increase of HF concentration and independent of reaction voltage. For the case of structures, the etched surface of silicon was fairly smooth and a cusp was not found. The thickness of the micrestructures was the same as that of the epitaxial n-Si layer and good uniformity. We have fabricated acceleration sensors by anodic reaction in HF solution(5 wt%) and planar technology. The process was compatible with conventional IC fabrication technique. Various micromechanical structures, such as rotors of motor, gears and linear actuator, were also fabricated by the technique and examined by SEM photographs.