Etching of tungsten via a combination of thermal oxide formation and wet-chemical oxide dissolution

Abstract

In this work, an etching process for the controlled and partial recess of tungsten metal was developed. The process comprises two steps which can be repeated: a thermal oxidation of the tungsten followed by the oxide dissolution in an acidic or basic solution. During the first step the W metal is heated in the presence of O₃ gas in the temperature range of 210–290 °C forming a WO₃ oxide. During the second step this thermally grown oxide is then selectively dissolved towards the underlying W metal. Both NH₄OH and H₃PO₄ were down selected as the best wet chemical dissolution agents in terms of dissolution rate and selectivity. By utilizing this combined thermal/wet-chemical cyclic etch process, the total W recess can be tuned on the nanoscale based on oxidation temperature and total number of cycles. This process was then applied for the deep recess (∼180 nm) of narrow (∼20 nm) tungsten trenches for the fabrication of the bottom contacts in complementary field-effect transistors (CFET).