Modeling of Si etching under effects of plasma molding in two-frequency capacitively coupled plasma in SF₆/O₂ for MEMS fabrication

We numerically investigated Si deep etching with several hundreds of micrometers such as that used in microelectromechanical system fabrication. This was carried out in SF₆(83%) O₂ at 300 mtorr in two-frequency capacitively coupled plasma using an extended vertically integrated computer-aided design for device processing (VicAddress). We estimated the local characteristics of plasma molding, including potential distribution and flux ion velocity distribution that are adjacent to an artificial microscale hole pattern. The sheath thickness is comparable to or even smaller than the size of the hole, and the sheath tends to wrap around the hole on a Si wafer. The distorted sheath field directly affects the incident flux and velocity distributions of ions. The angular distribution of SF₅⁺ ions at the edge of the hole is strongly distorted from the normal incidence. That is, the ion flux becomes radially nonuniform in the vicinity of the hole pattern. The feature-profile evolution by radicals and ions under the presence of plasma molding indicates that the etching is enhanced particularly at the bottom corner due to the removal of the passivation (SiO_x F_y) layer by energetic ion, resulting in the suppression of anisotropy of the etch profile.