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

摘要

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 (NH₄OH) solution generated at the point of use, is presented. The apparatus of the present invention supports generation of high purity aqueous NH₄OH solution from ammonia NH₃ gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of NH₃, within a hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NH₃ gas from aqueous NH₄OH 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 NH₃ gas and H₂O precursors of NH₄OH 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.