Silicon wafers with optically specular surfaces formed by chemical polishing

Abstract

This letter investigates chemical polishing with a hydrofluoric acid, nitric acid, and acetic acid (HNA) mixture as an alternative to chemical mechanical polishing (CMP) to produce smooth surfaces on both slurry- and diamond-cut silicon solar wafers. With 30 µm of silicon etched from each side, as-cut wafers appear mirror-like to the naked eye. A quantitative analysis of the specularity of HNA-polished wafers indicates that 97 % of light reflected from slurry-cut wafers falls within ±10 mrad of the specular beam and is isotropically distributed. Conversely, HNA-polished diamond-cut wafers retain a history of the wafer-sawing process: the reflected light is anisotropic with 99.4 % of light within ±10 mrad of the specular beam in the sawing direction but only 89.1 % within ±10 mrad in the perpendicular direction. Topographical characterization by optical profilometry and atomic force microscopy measurements reveals that HNA-polished slurry-cut wafers are spatially uniform with a surface roughness of 45 nm. Diamond-cut wafers have a roughness of only 18 nm but also have residual sawing grooves tens of micrometers across—these are responsible for the anisotropic scattering of light. The HNA-polished wafers are appropriate alternatives to CMP wafers for high-efficiency solar cells, including interdigitated-back-contact and tandem cells that require single-side polished wafers, as well as for other optical applications such as process monitoring with characterization techniques that require planar substrates.