Experimental investigation of anisotropy in isotropic silicon etching

Isotropic etching of silicon in HF-based solutions exhibits some level of anisotropy. We study this anisotropy in detail by etching silicon via circular mask openings for wafers of different orientations. The in-plane shape of the cavities is analyzed with high precision as a function of the etching time and opening size. Fourier expansion of the cavity shape is used to analyze different anisotropy components in relation to the crystal symmetry. It is found that the anisotropy pattern is in agreement with the crystal symmetry with a precision better than 0.4%. The relative anisotropy does not depend on the etching time and increases with the reduction of the opening size. For radii of mask holes a > 4 µm all the Fourier coefficients demonstrate a universal behavior increasing in absolute value linearly with a-1/3. The smaller holes exhibit saturation of these coefficients. The maximal anisotropy is about 9% for both (1 0 0) and (1 1 0) wafers but only 1.5% for (1 1 1).