Characterization of surface reaction during sulfur hexafluoride etching of silicon in an electron cyclotron resonance (ECR) plasma reactor

This investigation compares a surface chemistry model with an experimental SF{dollar}sb6{dollar} etching chemistry of polysilicon and silicon dioxide in ECR plasmas under low-energy ion bombardment. The surface chemistry model with the assumptions of sequential fluorination of silicon surface is based on a set of reaction parameters which include fluorine concentration, wafer ion flux, wafer ion energy, and wafer temperature. Ion-assisted etching mainly enhances the desorption of SiF{dollar}sb2{dollar} on the silicon surfaces. Sidewall etching comes from the spontaneous etching of silicon with fluorine atoms and its rate is determined by the balance of photoresist deposition and the stripping of the deposition. Patterned 4-inch silicon wafers with a 5000 A polysilicon film over a 1000 A SiO{dollar}sb2{dollar} are used to study the correlations between etch characteristics and reaction parameters, which were determined from optical emission actinometry and Langmuir probe measurements. The inherent capability of the ECR reactor is exploited to control the reaction parameters. Data were obtained by independently varying the microwave power and the wafer chuck position, while holding pressure and flow rates constant. Under these conditions, the fluorine concentration and ion energy were approximately constant at the wafer, while the ion current varied over a range of 0.5 mA/cm{dollar}sp2{dollar} to 20 mA/cm{dollar}sp2{dollar}. The etch rates were strongly correlated with the wafer ion current. In particular, the vertical polysilicon etch rate increased with ion current from 0.5 to 1.5 {dollar}mu{dollar}m/min, while the lateral etch rate decreased from 0.4 to 0.1 {dollar}mu{dollar}m/min for gas compositions of 95%SF{dollar}sb6{dollar}/Ar, pressure of 1 mTorr and photoresist patterns. These trends are also valid for different fluorine concentrations tried. Oxide etch rate appears strongly ion-assisted and varied almost linearly with ion current and fluorine concentration. These data fit well with current surface chemistry models and indicate that reaction parameters are the fundamental variables to the etch process. The models predict that anisotropic etching with high selectivity to oxide ({dollar}sim{dollar}10) can be achieved in SF{dollar}sb6{dollar}/Ar ECR plasmas at low fluorine concentration ({dollar}sim{dollar}10{dollar}sp{lcub}12{rcub}{dollar} atoms/cm{dollar}sp3{dollar}) and medium ion flux (6 mA/cm{dollar}sp2{dollar}) but sacrificing the etch rate ({dollar}sim{dollar}0.2 {dollar}mu{dollar}m/min). Suggestions for further study are presented.