The Turner mechanism of porous silicon formation during stain etching was developed and accepted without surface-sensitive data and without an understanding that nanostructures are being formed. Here it is shown that an oxide intermediate does not play a role in the formation of nanocrystalline porous Si films. Furthermore, a mechanistic understanding of etching and nanostructure formation leads to the formulation of seven rules for the rational design of stain etchants. These rules are used to develop three new formulations of stain etchants containing Fe3+, VO2+, and Ce4+, which are demonstrated to effectively produce porous silicon. These new formulations represent a significant advance in stain etching as they avoid many of the problems associated with common nitrate-/nitrite-based stain etchants including no need for “activation”, short induction times, and the reproducible production of homogeneous films of unprecedented thickness.

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Charge Transfer and Nanostructure Formation During Electroless Etching of Silicon

In this review paper, we present a comparative analysis of the electrochemical dissolution of III-V (InP, GaAs, GaN), II-VI (ZnSe, CdSe) and SiC semiconductor compounds. The resulting morphologies are discussed including those of porous layers and networks of low-dimensional structures such as nanowires, nanobelts, nanomembranes etc. Self-organized phenomena in anodic etching are disclosed, leading to the formation of controlled porous patterns and quasi-ordered distribution of pores. Results of templated electrochemical deposition of metal nanowires, nanotubes and nanodots are summarized. Porosification of some compounds is shown to improve luminescence characteristics as well as to enhance photoconductivity, second harmonic generation and Terahertz emission. Possible applications of porous semiconductor compounds in various areas are discussed.

Porous semiconductor compounds

In this work, we reported on the development of lithography-free technology for the fabrication of nanopatterned Si substrates. The combination of two phenomena, the solid-state dewetting process and metal-assisted wet chemical etching, allowed for fabrication of Si nanocolumns on large areas in a relatively simple way. The process of dewetting the thin metal layer enabled formation of nickel nanoislands, which were used as a shadow mask in the deposition of a catalytic metal pattern. Application of the two-stage dewetting process with the repetition of the metal deposition and annealing step enabled us to obtain a significant increase in the surface coverage ratio and the surface density of the nanoislands. As a catalytic metal, a gold layer was applied in the metal-assisted wet chemical etching process. The obtained columnar nanostructures showed a great verticality and had a high aspect ratio. In the conducted studies, the maximum etching rate (at RT) was higher than 1.2 μm min−1. The etching rate increased with increasing concentration of oxidizing (H2O2) and etching (HF) agent, with a tendency to saturate for more concentrated solutions. The etching rate was significantly higher for Si substrates with a crystallographic orientation (115) than for (111), but there was no privileged direction of etching except for the direction vertical to the substrate. With increasing layer thickness of the catalytic metal a decrease in the metal-assisted wet chemical etching process efficiency was observed. The developed technology allows for fabrication of patterned substrates with a wide range of lateral dimension of nanocolumns and their density.

/pdf/nanostructuring-of-si-substrates-by-a-metal-assisted-200xsqgube.pdf

Nanostructuring of Si substrates by a metal-assisted chemical etching and dewetting process

During the post-processing of silicon (Si) microsystems in hydrofluoric acid (HF)-based solutions, a galvanic couple is formed between the Si (anode) and metallic overlayers (cathode), such as gold (Au). Electrochemical etching (corrosion) of the exposed Si results in a porous silicon (PS) film and substantial degradation of mechanical and electrical properties occur. Focused ion beam milling (FIB) of micro-scale silicon-on-insulator (SOI) devices post-processed using HF solutions is used to determine the depth uniformity of the PS as a function of the geometry of the device. As the dopant concentration of the Si is critical to corrosion, dynamic SIMS is employed to assess the dopant concentration profile in SOI. As a means to model corrosion, we present a finite element method (FEM) enabled simulation to model the galvanic corrosion process on Si microsystems exposed to HF. The model uses an analogy to heat transfer to represent electrical conduction and accounts for electrochemical kinetics using the Tafel equation to represent empirical electrochemical measurements of Au and Si in HF. The model reproduces the current-limited condition resulting from the finite surface area of metal relative to silicon and predicts the uniform etch rate across the device for surfactant-enhanced HF solutions as seen in FIB. This work can be extended to applications where forming PS using a galvanic method may be advantageous.

https://iopscience.iop.org/article/10.1088/0960-1317/20/8/085017/pdf

Galvanically coupled gold/silicon-on-insulator microstructures in hydrofluoric acid electrolytes: finite element simulation and morphological analysis of electrochemical corrosion

This paper reports selective acetone sensing portrayed by functionalized nano-porous silicon (PS). Functionalization of nanostructure was performed using hexamethyldisilazane (HMDS). Nano-PS fabrication and morphology analysis were done using electrochemical etching and SEM respectively. Optical properties of PS were analyzed using Raman and Photoluminescence (PL) spectroscopy. Parallel aluminium electrodes were formed on nano-PS surface using thermal evaporation technique. These electrodes were used for recording electrical measurements from porous surface in presence of volatile organic compounds (VOCs) and moisture. Resistance variations due to interaction of sensing surface with different concentrations of ethanol, acetone, moisture and isopropyl alcohol vapors were monitored at room temperature. Various sensor parameters were calculated like sensitivity, response- and recovery-time. Sensor based on functionalized nano-PS was found to be most responsive for acetone in comparison to other VOCs or moisture. Since, acetone is one of the important metabolite components in the circulated blood of the human body therefore, its concentration determination is necessary. The approach for selective sensing presented in this work is simple and low cost.

Selective acetone electrical detection using functionalized nano-porous silicon

This work describes the characterization and optimization of anisotropic formation of porous silicon in large volumes (0.5?1 mm3) of silicon by an electroless wet etching technique. The main goal is to use porous silicon as a sacrificial volume for bulk micromachining processes, especially in cases where etching of the full wafer thickness is needed. The porous silicon volume is formed by a metal-assisted etching in a wet chemical solution composed of hydrogen peroxide (30%), hydrofluoric acid (40%) and ethanol. This paper focuses on optimizing the etching conditions in terms of maximizing the etching rate and reproducibility of the etching. In addition to that, a study of the morphology of the pore that is obtained by this technique is presented. The results from the characterization of the process are applied to the fabrication of boron?silica?glass cantilevers that serve as a platform for bio-chemical sensors. The porous silicon volume is formed in an early step of the fabrication process, allowing easy handling of the wafer during all of the micromachining processes in the process flow. In the final process step, the porous silicon is quickly etched by immersing the wafer in a KOH solution.

https://iopscience.iop.org/article/10.1088/0960-1317/20/1/015034/pdf

Jordi Teva

Papers

Electroless porous silicon formation applied to fabrication of boron- silica-glass cantilevers