Etching with atomic precision by using low electron temperature plasma

Abstract

There has been a steady increase in sub-nm precision requirement for many critical plasma etching processes in the semiconductor industry. In addition to high selectivity and low controllable etch rate, an important requirement of atomic precision etch processes is no (or minimal) damage to the remaining material surface. It has traditionally not been possible to avoid damage in conventional radio-frequency (RF) plasma processing systems, even during layer-by-layer or ‘atomic layer’ etch. To meet these increasingly stringent requirements, it is necessary to have an accurate control over ion energy and ion/radical composition during plasma processing. In this work, a new plasma etch system designed to facilitate atomic precision plasma processing is presented. An electron sheet beam parallel to the substrate surface is used to produce a plasma in this system. This plasma has a significantly lower electron temperature Te ~ 0.3 eV and ion energy Ei < 3 eV (without applied bias) compared to inductively and capacitively coupled RF plasmas. Electron beam plasmas also have a higher ion–to–radical ratio compared to RF plasmas, so this plasma etch system employs an independent radical source for accurate control over relative ion and radical concentrations. A low frequency RF bias capability that allows control of ion energy in the 2–50 eV range is another important component of this plasma etch system. The results of etching of a variety of materials and structures in this low-electron temperature plasma system are presented in this study: (1) layer-by-layer etching of p-Si at Ei ~ 25–50 eV using electrical and gas cycling is demonstrated; (2) continuous etching of epi-grown µ-Si in Cl2-based plasmas is performed, showing that surface damage can be minimized by keeping Ei < 10 eV. Also presented are the results of molecular dynamics modeling of atomic precision etching at low Ei.