Characterization of the anisotropic etching of silicon in two-component alkaline solution

Abstract

The characteristics of wet chemical etching processes were compared for crystalline Si in aqueous solutions of pure NaOH and NaOH with added oxidizing agent NaOCl (sodium-hypochlorite). Due to the addition of NaOCl to the reference NaOH solution, the etch rate decreased for the {1 0 0} and {1 1 0} planes and increased for the {1 1 1} plane. The difference between the activation energy of these etching processes for the three different crystallographic directions became rather small due to the combined effect of the oxidation and the subsequent etching sub-processes. This means that the temperature dependence of the number of kinks as chemical oxidation sites is very similar for the different planes. For the {1 1 1} plane the oxidation sub-process has a higher efficiency resulting in an increased etch rate whereas the etch rate decays for the {1 0 0} and {1 1 0} planes due to enhanced passivation, and these two phenomena contribute to the decrease of the anisotropy. Due to the addition of NaOCl, the oxidation sub-process is more isotropic but still site dependent. In the practice, the etched surface remains of good quality without defect development. In the presence of the oxidizing agent, the difference of the etch rates in the ⟨1 0 0⟩ and ⟨1 1 0⟩ directions is reversed to R⟨100⟩ < R⟨110⟩. This fact can be exploited in the formation of novel 3D structures on the (1 0 0) wafers by forming a square-masking pattern aligned precisely to the ⟨1 0 0⟩ directions.