Micro-machining of [100] Si using a novel ultra-violet induced anisotropic etching in HNA solution

Anisotropic etching of <100> silicon is achieved, for the first time, in the presence of ultra-violet exposure in a solution containing hydrofluoric/ nitric/ acetic acids (HNA). The HNA solution is regularly used for polishing silicon and etching polysilicon due to its isotropic etching property. In the technique proposed in this paper, called UV-HNA, the etching of silicon is enhanced in the direction determined by UV exposure. A mixture of HF/HNO₃/HCOOH with a relative composition of 1:15:5 seems suitable for revealing [111] planes with an etch rate of 10 μm/hr at 35°C. Some anomalous behavior of etching in the presence of UV exposure is discussed. Bottom of the etched craters is hillock-free and etch rates as high as 60 μm/hr can be achieved using higher concentration of HF acid in HNA solution. In the latter case the etching is less anisotropic and mask undercut is observed.