On the Formation of Black Silicon Features by Plasma-Less Etching of Silicon in Molecular Fluorine Gas

Abstract

In this paper, we study the plasma-less etching of crystalline silicon (c-Si) by F2/N2 gas mixture at moderately elevated temperatures. The etching is performed in an inline etching tool, which is specifically developed to lower costs for products needing a high volume manufacturing etching platform such as silicon photovoltaics. Specifically, the current study focuses on developing an effective front-side texturing process on Si(100) wafers. Statistical variation of the tool parameters is performed to achieve high etching rates and low surface reflection of the textured silicon surface. It is observed that the rate and anisotropy of the etching process are strongly defined by the interaction effects between process parameters such as substrate temperature, F2 concentration, and process duration. The etching forms features of sub-micron dimensions on c-Si surface. By maintaining the anisotropic nature of etching, weighted surface reflection (Rw) as low as Rw < 2% in Si(100) is achievable. The lowering of Rw is mainly due to the formation of deep, density grade nanostructures, so-called black silicon, with lateral dimensions that are smaller to the major wavelength ranges of interest in silicon photovoltaics.

Atmospheric Pressure Dry Etching of Polysilicon Layers for Highly Reverse Bias‐Stable TOPCon Solar Cells

“…The single-sided etching process is performed using the ADE tool, which previously has been used extensively for etching and texturing of crystalline silicon (c-Si) surfaces. [14] The schematic of the process is shown in Figure 1.…”

“…More details about the etching tool and its use in c-Si etching can be read in previous studies. [14][15][16]…”

“…In Figure 2, a slower etching at the start of the annealed layers can possibly be attributed to a high selectivity of F 2 gas to silicon in comparison with the native oxide layer. [ 14 ] Although HF dip was performed after LPCVD deposition, even a short waiting time (≈30 s) after HF dip is reported to grow native oxide layer on c‐Si surface. [ 17 ] The experimental conditions in our experiments are similar, where the waiting time between HF dip and ADE process is even much longer (up to 1 h).…”

“…In summary, high ERs >3 μm min −1 could be reached for both layers with the applied process parameters, which could be further increased at higher F 2 flows and concentration. In addition, increasing process temperature is also known to exponentially increase the silicon ER, [ 14 ] which suggests further potential in increasing the process throughput and lowering the process costs in high volume manufacturing. However, even with the process utilizing 10% F 2 concentration at 225 °C, the deposited silicon layer of around 200 nm thickness can be etched in just a couple of seconds.…”

“…In this work, we propose an atmospheric pressure dry‐etching (ADE) process [ 14 ] to perform single‐sided etching of amorphous/polycrystalline silicon (a‐Si/poly‐Si) layers for fabrication of industrial TOPCon solar cells on n‐type substrates. The process uses diluted molecular fluorine (F 2 ) as the etching gas to enable high throughput inline etching of silicon layers in atmospheric pressure conditions, avoiding the need of ion induced excitation.…”