Tapered Windows in SiO2: The Effect of NH 4 F : HF Dilution and Etching Temperature

Etching of windows in thermally grown silicon dioxide to achieve an approximately 45° taper edge profile has been empirically optimized using NH 4 F / HF compositions and etching bath temperatures. Steeper slopes or much shallower tapers are obtainable by suitable adjustments of these two experimental parameters. The mechanism by which tapering occurs may best be described as a controlled separation of the photoresist/oxide interface which relates the lateral etch rate to the bulk etch rate. Depending on etching conditions, each beveled window edge consists of two or three distinct regions which comprise the final shape of the sloping taper. The dissolution reaction in all instances was found to be activation controlled with an apparent activation energy of 9.9 kcal/mole. Curves of etch rates as a function of temperature and HF molarity are presented and a quantitative relationship for etch rate in terms of these two parameters is given.