Independent control of ion density and ion bombardment energy in a dual RF excitation plasma

A dual RF excited discharge is described. The dual RF excitation system provides a method to control the substrate self-bias without affecting the state of the discharge. The substrate can be RF-biased utilizing an appropriate excitation frequency and power significantly less than the plasma generating RF power. The substrate self-bias dependence on various system parameters, including substrate excitation frequency, pressure, plasma generating upper electrode RF power, substrate material, and process gas compositions, is described. For a simplified model, a linear relationship between self-bias and RF power is derived using the space-charge limited assumption. The effect of substrate bias on the thermal-oxide etch rate has been studied. The results show good correlation between the ion bombardment energy, i.e., the potential difference across the substrate dark space, and the SiO ₂ etch rate. The SiO₂ etch rate in a CF₄ plasma increases linearly with the ion bombardment energy, having a threshold etch energy of ~19 V