Masking effect of copper during anisotropic etching of silicon in buffered hydrofluoric acid solutions

The etching characteristics of silicon samples of (100) orientation in buffered hydrofluoric acid (BHF) solutions containing different levels of dissolved copper were studied systematically using atomic force microscopy, total reflection x-ray fluorescence, inductively coupled plasma mass spectroscopy and transmission electron microscopy techniques. It was found that the presence of trace amounts of copper ions in BHF solutions can cause not only metallic contamination but also very severe surface roughness. Copper deposits onto the silicon surfaces in the form of submonoatomic layer clusters which act as masks during silicon anisotropic etching in BHF solutions. Surface roughness as high as 12 nm was generated in 30 min of etching in buffered hydrofluoric acid solution containing 100 ppb Cu²⁺. Cross-sectional TEM micrographs showed that silicon was etched anisotropically in BHF solutions containing Cu ²⁺ ions. Etching rates as high as 1.0 nm/min were obtained on the (100) planes of p-type silicon in the presence of a 100 ppb Cu²⁺. The size of the etching structures depends on the Cu²⁺ concentration in solution and the etching time.