The cryogenic process is well known to etch high aspect ratio features in silicon with smooth sidewalls. A time-multiplexed cryogenic process, called STiGer, was developed in 2006 and patented. Like the Bosch process, it consists in repeating cycles composed of an isotropic etching step followed by a passivation step. If the etching step is similar for both processes, the passivation step is a SiF₄/O₂ plasma that efficiently deposits a SiO_xF_y layer on the sidewalls only if the substrate is cooled at cryogenic temperature. In this paper, it is shown that the STiGer process can achieve profiles and performances equivalent to the Bosch process. However, since sidewall passivation is achieved with polymer free plasma chemistry, less frequent chamber cleaning is necessary, which contributes to increase the throughput.

The Bosch process was studied at a substrate temperature of −100 °C and compared to etchings performed at room temperature, as in the general case. The tests were realized using an inductively coupled plasma reactor by varying C₄F₈ passivating gas flow injections both at +20 °C and −100 °C. It was observed that the Bosch process is effectively temperature dependent and that the necessary C₄F₈ passivating gas flow can be reduced to obtain similar anisotropic profiles at −100 °C compared to the ambient temperature process. For example, in one of the studied cases, a fluorocarbon injection of 8 sccm was sufficient to obtain an anisotropic etch rate of up to 4.4 μm min⁻¹ at −100 °C whereas the profile obtained at +20 °C using the same parameters presents lateral etching defects with a reduced etch rate of 2.4 μm min⁻¹. At this point, the C₄F₈ flow must be increased to 12 sccm (50% more) to retrieve an anisotropic profile with an etch rate of 4.0 μm min⁻¹. In the case of cryogenic Bosch (cryo-Bosch) processing, C₄F₈ feed dosing has a greater influence on the passivation regime which affects the subsequent etching result but it can be easily refined through the optimization of process parameters. An in-situ ellipsometry study of the deposition rate of C₄F₈ on both polycrystalline silicon (p-Si) and SiO₂ substrates was realized by varying the gas flow at −100 °C and +20 °C. This study shows that the deposited fluorocarbon material is approximately a hundred times thicker at cryogenic temperatures using the same process parameters. Scanning electron microscopy (SEM) observation of these samples are in adequacy with the ellipsometry results. Cryo–Bosch etching also results in a slightly higher etch rate compared to room temperature processing when analyzing similar anisotropic profiles. Si:SiO₂ etching selectivity is significantly increased at −100 °C although the aspect-ratio dependent etching phenomenon is more important.

A global zero-dimensional (0D) model has been developed to describe the Cl2/Ar plasma discharge in dynamic mode. Our model computes the time evolution of the plasma composition under conditions similar to fast-paced plasma processes, such as atomic layer etching (ALE), characterized by alternations in the feed gas. The study focuses on calculating the densities of charged and neutral species for various gas switch durations, (ts). Simulations demonstrate the impact of gas switching time (ts) on the temporal evolution of Cl2, Cl, and ion densities, as well as the electron temperature (Te) during the gas switch. A parametric study reveals that the temporal evolution of Te can be represented by a semiempirical exponential law during the transition from a pure Cl2 plasma to Ar as a function of (ts). During the gas switch, the extinction of chlorinated species, which plays a crucial role in the adsorption step in ALE, persist during the argon plasma phase. The duration of this extinction decreases with longer ts. Finally, our model shows a good reproducibility of ALE cycles modeled for the chosen input parameters, including the densities of neutral and charged species and Te, relative to the ALE period. This work aims to provide insights into the kinetics of transient plasmas occurring in the ALE cycle, the importance of purging, and lifetime residual species, such as residual chlorine in a plasma with pure argon.

Atomic layer etching (ALE) of GaN on silicon substrates has been investigated using fluorine-based chemistry. The ALE process used for this study consists of a modification step using SF6 plasma and a removal step using argon plasma. Two configurations were studied in which the distance between the sample and the plasma source was modified. The energy scan, synergy, selective etching of GaFx by Ar+ ion bombardment, and self-limiting etching regime of the ALE of GaN were first investigated. An etch per cycle of 0.50 nm/cycle averaged over 100 cycles was reached for GaN ALE. The self-limiting regime was also obtained for both SF6 and argon plasma steps. Moreover, a synergy rate as high as 78% was achieved. It was found that the GaFx-modified layer can be selectively sputtered by applying a self-bias voltage while the argon ion energy is maintained below the sputtering threshold of GaN.