High purity, environmentally clean method and apparatus, for high rate, liquid anisotropic etching of single crystal silicon or etching of polycrystalline silicon, using an overpressure of ammonia gas above aqueous ammonium hydroxide

Abstract

A high purity, non-toxic, environmentally friendly method for anisotropically etching single crystal silicon and etching polysilicon, suitable for microelectronics, optoelectronics and microelectromechanical (MEMS) device fabrication, using high purity aqueous ammonium hydroxide (NHOH) solution generated at the point of use, is presented. The apparatus of the present invention supports generation of high purity aqueous NHOH solution from ammonia NHgas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of NH, within a hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NHgas from aqueous NHOH solution for achieving a high anisotropic etching rate. Other liquid anisotropic etching methods for silicon may use tetramethylammonium hydroxide (TMAH). In contrast to carbon containing TMAH, the NHgas and HO precursors of NHOH etchant eliminate risk for solid residues to be deposited on silicon due to being composed entirely of elements having a gaseous form at room temperature.