Cryogenic etching of a-Si, SiO₂, and Si₃N₄ materials by CHF₃/Ar inductively coupled plasma is investigated in a range of temperature from −140 to +20 °C. Samples of the three different materials are placed together on the same silicon carrier wafer. Depending on the experimental conditions, etching or deposition regimes were obtained on the samples. The thickness variation was measured by spectroscopic ellipsometry. A process window between −120 and −80 °C was found in which the Si₃N₄ surface is etched while CF_x deposition is obtained on a-Si and SiO₂ surfaces, resulting in the infinite etching selectivity of Si₃N₄ to the other materials. At high enough self-bias (−120 V) and very low temperature (<−130 °C), Si₃N₄ etch is reduced down to a very low value, while a-Si and SiO₂ are still being etched, which inverses the selectivity between Si₃N₄ and the two other materials. EDX analyses of a Si₃N₄/a-Si/SiO₂ layer stack after the same etching process carried out at 20 and −100 °C confirm the presence of carbon and fluorine on a-Si at low temperature, showing the effect of the low temperature to switch from the etching to deposition regime on this material.

A silicon oxyfluoride layer was deposited on a-Si samples using SiF ₄ /O ₂ plasma at different temperatures between −100°C and −40 °C. In situ X-ray photoelectron spectroscopy measurements were then performed to characterize the deposited layer. The sample was then brought back to room temperature and analyzed again. It has been shown that a temperature below −65 °C is needed to significantly enhance the physisorption of SiF _x species. Hence, in this condition, a F-rich oxyfluoride layer, stable at low temperature only, is physisorbed. Above this threshold temperature, the native silicon oxide layer is fluorinated and the proportion of O in the deposited layer is higher and remains stable even when the sample is brought back to room temperature.

This article first presents quasi-in situ XPS measurements on Si3N4 and a-Si samples after exposure to an SiF4/O2 plasma at different cryogenic temperatures. A different behavior is observed between the two materials at −65 °C, which has led to the development of a time-multiplexed process for nanoscale etching. This study clearly shows the possibility to switch from a deposition regime to an etching regime by decreasing the temperature. The threshold temperature between these regimes being different for both materials, it was possible to perform selective etching of Si3N4 over a-Si by wisely choosing the temperature.

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.

Ultraviolet (UV) absorption spectroscopy is used to monitor the CF radical density in CF4 inductively coupled plasma (ICP) plasmas as a function of the substrate temperature. The CF density decreases dramatically when the wafer temperature is reduced from 20 to −130 °C by applying identical plasma conditions, demonstrating that the CF surface sticking coefficient increases as the surface temperature is reduced. This suggests that CF4 plasma could be used to form sidewall passivation layers and perform anisotropic etching at cryogenic temperature, which is impossible at room temperature. Subsequently, a cyclical Bosch type etching process of silicon was evaluated at −100 °C using CF4 plasma to passivate the trench sidewalls. Anisotropic etch profiles were obtained with an etch rate of 4.4 μm/min. Compared to a typical Bosch process using highly polymerizing c-C4F8 plasma, chamber wall contamination could be significantly reduced, alleviating a major issue of this cyclic process. Furthermore, CF4 has a 28% lower global warming potential than c-C4F8.

The present study describes a novel strategy for preparing thin Silicon 2D and 3D electrodes for lithium ion batteries by a spin coating method. A homogeneous and stable suspension of Si nanoparticles (SiNPs) was prepared by dispersing the nanoparticles in 1-methyl-2-pyrrolidone (NMP) or in the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI). This proposed methodology was successfully employed to prepare 2D and 3D with different aspect ratios electrodes. Both 2D and 3D materials were then used as anode materials. The 2D SiNPs anodes exhibit a high reversible capacity, which is close to 3500 mAh·g⁻¹ at C/10. For a higher discharge rate, the capacity of the 2D anode is considerably improved by dispersing the nanoparticles in Pyr14TFSI instead of NMP solvent. In order to further improve the anode performances, graphene particles were added to the SiNPs suspension. The anodes prepared using this suspension method exhibit relatively low columbic efficiency during the first few cycles (less than 30%) and low reversible capacity (2800 mAh·g⁻¹ at C/10). The 3D SiNPs (NMP) electrode shows a higher intensity during cyclic voltammograms and a better stability under galvanostatic cycling than the 2D SiNPs (NMP) electrode.