Numerical investigation of relationship between micro-scale pattern, interfacial plasma structure and feature profile during deep-Si etching in two-frequency capacitively coupled plasmas in SF6/O2

Abstract

The feature profile of deep-Si etching in the presence of plasma moulding in two-frequency capacitively coupled plasmas in SF6/O2 has been numerically simulated to investigate the relationship between the micro-scale pattern on a Si wafer, the interfacial structure of the plasma (i.e. the sheath) and the etching profile. We have also discussed the deep-Si etching profile as functions of the SF6/O2 gas mixture ratio and the total gas pressure. The result shows that the plasma moulding has no influence on the sheath structure when the sheath thickness is an order of magnitude longer than the width of the hole pattern. In the presence of a large number of F radicals, the chemical etching of Si causes isotropic etching with a significant undercut profile, reducing the plasma moulding's influence on the etching profile due to the F radicals' much higher Si etching rate compared with the rate of etching by ions. On the other hand, with the increase in the oxygen mixture ratio, a passivation layer grows up on the inside wall, and the removal of the layer at the bottom corner is strengthened by the distorted ion by plasma moulding. Thus, the etching is enhanced particularly at the bottom corner. As the pattern width decreases and the oxygen mixture ratio increases, the etching profile becomes anisotropic by the formation of the passivation layer at the sidewall. In addition, the bottom profile becomes flat because of the uniform ion flux under the reduced influence of the plasma moulding.