The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices.

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Plasma etching: Yesterday, today, and tomorrow

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

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Black silicon: fabrication methods, properties and solar energy applications

An intensive study has been performed to understand and tune deep reactive ion etch (DRIE) processes for optimum results with respect to the silicon etch rate, etch profile and mask etch selectivity (in order of priority) using state-of-the-art dual power source DRIE equipment. The
research compares pulsed-mode DRIE processes (e.g. Bosch technique) and mixed-mode DRIE processes (e.g. cryostat technique). In both techniques, an inhibitor is added to
fluorine-based plasma to achieve directional etching, which is formed out of an oxide-forming (O2) or a fluorocarbon (FC) gas (C4F8 or CHF3). The inhibitor can be introduced together with the etch gas, which is named a mixed-mode DRIE process, or the inhibitor can be added in a
time-multiplexed manner, which will be termed a pulsed-mode DRIE process. Next, the most convenient mode of operation found in this study is highlighted including some remarks to
ensure proper etching (i.e. step synchronization in pulsed-mode operation and heat control of the wafer). First of all, for the fabrication ......
 Enjoy reading . Henri Jansen 18 June 2008

https://iopscience.iop.org/article/10.1088/0960-1317/19/3/033001/pdf

Black silicon method X: a review on high speed and selective plasma etching of silicon with profile control: an in-depth comparison between Bosch and cryostat DRIE processes as a roadmap to next generation equipment

The evolution of silicon cryoetching is reported in this topical review, from its very first introduction by a Japanese team to today's advanced technologies. The main advances in terms of the performance and comprehension of the mechanisms are chronologically presented. After presenting the principle of silicon cryoetching, the main defects encountered in cryoetching (such as undercut, bowing and crystal orientation dependent etching) are presented and discussed. Mechanisms involved in SiOxFy passivation layer growth in standard cryoetching are investigated through several in situ characterization experiments. The STiGer process and alternative cryoetching processes for high-aspect-ratio structures are also proposed to enhance the process robustness. The over-passivation regime, which can provide self-organized columnar microstructures, is presented and discussed. Finally, advanced technologies, such as the cryoetching of sub-20 nm features and porous OSG low-k cryoetching, are described.

https://iopscience.iop.org/article/10.1088/0022-3727/47/12/123001/pdf

Plasma cryogenic etching of silicon: from the early days to today's advanced technologies

This article presents an overview of the fabrication methods of black silicon, their resulting morphologies, and a quantitative comparison of their optoelectronic properties. To perform this quantitative comparison, different groups working on black silicon solar cells have cooperated for this study. The optical absorption and the minority carrier lifetime are used as benchmark parameters. The differences in the fabrication processes plasma etching, chemical etching, or laser processing are discussed and compared with numerical models. Guidelines to optimize the relevant physical parameters, such as the correlation length, optimal height of the nanostructures, and the surface defect densities for optoelectronic applications are given.

Black Silicon Photovoltaics

Passivation mechanisms of Si trenches involved in SF6/O2 cryogenic plasma etching were investigated in order to better control the process and avoid defects. Trench sidewalls and profiles were ex situ characterized geometrically by SEM and chemically by spatially resolved XPS experiments. These measurements reveal that the passivating layer is removed during the increase of the wafer chuck temperature leading to a very clean surface of the sidewalls after processing. Nearly no SiO2 formation on the sidewalls was observed after the very low temperature etching (−100 °C). A two-step process was defined to rebuild the passivating layer after its destruction and continue the trench etching. The necessary conditions for properly rebuilding the passivating layer give precious information about its chemical composition. These experiments clearly show that sulfur is not a necessary element to form an efficient passivating layer.

https://iopscience.iop.org/article/10.1088/0960-1317/14/2/004/pdf

Passivation mechanisms in cryogenic SF6/O2 etching process

The SiOxFy passivation layer created on structure sidewalls during silicon cryoetching is investigated. This SiOxFy passivation layer formation strongly depends on O2 content, temperature and bias. It is a fragile layer, which mostly disappears when the wafer is warmed up to ambient temperature. A mass spectrometer was used to analyze the desorbed species during the warm-up and using this instrument allowed us to find a large signal increase in SiF3+ between −80°C and −50°C. SiF4 etching products can participate in the formation of the passivation layer as it is shown by a series of test experiments. SiF4∕O2 plasmas are used to form a thin SiOxFy layer on a cooled silicon wafer. Thickness and optical index of this thin film can be determined by in situ spectroscopic ellipsometry. It is shown that the passivation layer spontaneously desorbs when the silicon wafer temperature increases in good agreement with the mass spectrometry analysis. Two physical mechanisms are proposed to explain the SiOxFy passivati...

SiOxFy passivation layer in silicon cryoetching

An inductively coupled SF6/O2 plasma is used to form a columnar microstructure (CMS) on silicon samples cooled at very low temperature (~ −100 °C). The formation of this CMS is studied as a function of bias voltage, temperature, RF power and gas pressure. The characteristic mean diameter and mean height of the microstructure are evaluated by image processing tools from SEM micrographs. A crystallographic effect is also observed at very low temperature, which induces a needle-shaped structure. A physical mechanism is proposed to explain the formation of this CMS.

Silicon columnar microstructures induced by an SF6/O2 plasma

In silicon etching in SF6∕O2 plasmas, an oxidation threshold appears when the oxygen content is large enough. A SiOxFy passivation layer is formed under such conditions. This threshold is reached at lower oxygen proportions if the substrate is cooled down to cryogenic temperatures. In this article, we present a mass spectrometry study of this oxidation threshold in different experimental conditions (temperature, source rf power, self-bias) on bare silicon wafers. The presence of the threshold is clearly evident in the signals of many ions, for example, SiF3+, F+, and SOF2+. This helps us to determine the main reactions which can occur in the SF6∕O2 plasma in our experimental conditions. This threshold appears for higher oxygen proportions when either the source power or the chuck self-bias is increased. The ion bombardment transfers energy to the surface and makes the film desorb. A model, describing the oxygen coverage as a function of the parameters mentioned above, is proposed to interpret these result...

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Oxidation threshold in silicon etching at cryogenic temperatures

Two types of silicon cryoetching processes are proposed. The first one consists of alternating SF 6 plasma isotropic etching steps and SiF 4 /O 2 passivation steps at low temperature of the silicon substrate. This process only works at very low temperature and avoids reactor wall contamination. A second process alternating long anisotropic SF 6 /O 2 plasma steps with short SiF 4 /O 2 over-passivating steps is investigated. This second process gives an etch rate as high as the one obtained in standard cryoetching and allows blocking of the undercut evolution. These two processes offer many advantages in terms of cleanliness, robustness, and performance.

https://iopscience.iop.org/article/10.1149/1.2826280/pdf

Xavier Mellhaoui

Papers

Passivation mechanisms in cryogenic SF6/O2 etching process

SiOxFy passivation layer in silicon cryoetching

Silicon columnar microstructures induced by an SF6/O2 plasma

Oxidation threshold in silicon etching at cryogenic temperatures

Two Cryogenic Processes Involving SF6, O2, and SiF4 for Silicon Deep Etching