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 are 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 SiO2 etch rate. The SiO2 etch rate in a CF4 plasma increases linearly with the ion bombardment energy having a threshold etch energy of approximately 19 V.