A Study of Self-Bias Formation in Low Electron Temperature Plasmas

The formation of self-bias voltage in low electron temperature plasma was studied. The low electron temperature plasma was generated by varying the grid voltage installed in the inductively coupled plasma (ICP). At a constant electron density of 1×10¹⁰ cm^-3 and constant 12.5 MHz bias power (5 W), the self-bias voltage was measured to be -7.5 V at an electron temperature of 0.4 eV, and -9.9 V at 2.4 eV. Experimental results show that the substrate self-bias voltage varies with electron temperature. This difference arises from the sheath length. Since the sheath length is inversely proportional to electron temperature, the lower electron temperature plasma has longer sheath lengths. Therefore, the sheath capacitance increases as electron temperature decreases. These results suggest that in plasmas with lower electron temperatures, the ion energy is expected to be lower under the same RF bias power.