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

Abstract

Etching of windows in thermal ly grown silicon dioxide to achieve an ap-proximately 45 ~ taper edge profile has been empir ical ly optimized using NH4F/HF compositions and etching bath temperatures. Steeper slopes or much shal lower tapers are obtainable by suitable adjustments ot these two exper i-mental parameters. The mechanism by which tapering occurs may best be described as a control led separation of the photoresist/oxide interface which relates the lateral etch rate to the bulk etch rate. Depending on etching con-ditions, 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 control led with an apparent act iva-tion energy of 9.9 kcal/mole. Curves of etch rates as a function of temperature and HF molar i ty are presented and a quantitat ive relationship for etch rate in terms of these two parameters i given. It has recently been shown (1-5) that dilute solu-