Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

Abstract

The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorinecontaining plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl 2 , but was found to be even more important in pure Cl 2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a "boundary electrode" during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at nearthreshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only likely explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the "sub-threshold" etching was due to photons, predominately at wavelengths < 1700 Å . The photo-assisted etching rate was equal to the ionassisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, photo-assisted etching likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, contributing to the commonly observed sloped sidewalls and microtrenches.

Comparison of surface vacuum ultraviolet emissions with resonance level number densities. II. Rare-gas plasmas and Ar-molecular gas mixtures

“…It has been shown that VUV enhances etching by reducing the magnitude of the threshold ion energy necessary to initiate surface reactions with particular relevance to profile control. 2 However, UV and VUV photons sometimes also have undesirable effects during etching, producing more severe irradiation damage than positive ion bombardment, resulting in defects that lead to degradation of MOS devices by dielectric breakdown. 3 Degradation of low dielectric constant ("low-k") materials used to insulate wiring in high-speed IC interconnects has been attributed to exposure to VUV during etching, stripping and cleaning steps using O 2 , Ar/O 2 , and He/H 2 plasmas.…”

Overview of atomic layer etching in the semiconductor industry

“…33 However, plasma chlorination considerably increases the possibility of background etching due to the presence of radicals, ions, photons, metastables, and low energy electrons, all of which can potentially induce etching. 13 Photons may play a role, as they are known to induce subthreshold etching as described by Shin et al 100 Low energy ions should not etch at subthreshold energies (<16 eV), although molecular dynamic simulations by Brichon et al show a finite silicon etching yield for chlorine ions with kinetic energies as low as 5 eV. 20 Even "nonreactive" species in plasma such as He neutrals can deliver substantial (>10 eV) energy through relaxation of Rydberg states.…”

Predicting synergy in atomic layer etching

“…Ideally, synergy will approach 100% with no etching from either step alone. In practice, nonzero contributions from a and b may be present due to photon-induced etching, 9 physical sputtering, step contamination, and/or competing reactions of conventional etching. To experimentally measure the quantities of a and b, steps A and B can be performed as independent processes.…”