TMAH/IPA ANISOTROPIC ETCHING CHARACTERISTICS

The main advantages of tetramethyl ammonium hydroxide (TMAH)-based solutions is their full compatibility with IC technologies. In this work a new etching system of TMAH/IPA (isopropyl alcohol) is suggested. The influence of the addition of IPA to TMAH solutions on their etching characteristics is presented. The etch rates of (100) oriented silicon crystal planes decreases linearly with decreasing the IPA concentration for all experimental conditions. Etch rates for TMAH/IPA solutions (10-45 mum/h) are lower than those for KOH/IPA solutions (30-100 mum/h) but they are still applicable for micromachining purposes. The etch rates of most commonly used masking layers in IC technologies has been investigated. Low-pressure chemical vapour deposited (LPCVD) Si3N4 and thermally-grown SiO2 have excellent stability in TMAH/IPA solutions. Low-temperature deposited silicon oxide (LTO) etch rates are low enough to be used as masking layers in anisotropic etching processes. Quality of etched surfaces is mainly dependent on TMAH wt.% concentration. For pure TMAH solutions the observed undercutting ratio (5-7) is much larger than in KOH case. The addition of IPA to TMAH solutions reduce the undercutting by a factor of more than 2 and leads to smoother surfaces of sidewalls etched planes. We have studied briefly the p++ etch-stop characteristics by means of heavily boron-implanted layers. The etching selectivity with respect to high boron-doped silicon is improved in TMAH/IPA solutions. Implant doses used in our experiments (2 x 10(16) ion/cm2) stands the etching during more than 90 min.