Atomic layer etching removal of damaged layers in a contact hole for low sheet resistance

Abstract

A damaged layer remains on silicon substrates after high-aspect-ratio contact (HARC) etching when using a fluorocarbon gas. Atomic layer etching (ALET) is a technique that can be applied to remove the damaged layer of silicon, removing about 1.36 Å per etch cycle. The characteristics of contact damage removal by ALET are investigated and compared with the conventional damage removal technique of low-power CF4 plasma etching. The low-power CF4 plasma etching technique not only has inadequate etch depth control, but also introduces secondary damage by implanting impurities about 25 Å into the contact bottom of the silicon surface. However, ALET allows contact damage to be removed effectively without introducing secondary damage to the substrate, and with precision etch depth control at the angstrom scale. When ALET is applied subsequent to low-power CF4 plasma etching, the fluorine- and carbon-damaged silicon is effectively removed in about 10 cycles. The sheet resistance of HARC etched silicon decreases from 142 to 137 Ω/◻ after using low-power CF4 plasma etching, and subsequent ALET treatment further decreases the sheet resistance to 129 Ω/◻, which is close to the reference value of 124 Ω/◻.