Investigation of varying end-capacitance in external antenna for inductively coupled plasma

Abstract

Summary form only given. Modern inductively coupled plasma (ICP) gained a great interest in plasma-assisted material processing in semiconductor industry. This implied by key properties of ICP having high-density, uniform plasma at low pressure. We report the effect of endcapacitance dependency for ICP. The efficiency of ICP is normally measured via inductive coupling efficiency between antenna coil and the plasma 1. Our discharge chamber had 200mm diameter and consisted of three-turn external cylindrical antenna coil through pi-type matching network. End-capacitance is used to suppress electrostatic coupling to plasma. The suppression is one of key issue from material processing in field of semiconductor industry, as it causes sputtering of dielectric materials. In this work, the plasma density and the antenna voltages are measured by changing the end-capacitance. The end-capacitance herein is a vacuum variable capacitance (VVC) ranging from 50 pF to 500 pF. While adjusting the value of capacitor, V-I value flowing through antenna was measured. The test RF discharge parameters are 13.56 MHz, applied up to 4000 W, gas pressure was maintained constant 0.2 Torr. For changing endcapacitance we measured the voltage at two points, before and after antenna coil namely Vin and Vout, using a highvoltage probe. E-H mode transition power and ion current are measured with respect to range of end-capacitance. Ion Current was measured using a floating harmonic method. With increasing the end-capacitance, the ratio of Vout/Vin has increased simultaneously. Less RF power is required for the E-H mode transition when VVC is tuned to higher capacitance. The inductive coupling is more efficient for the case where E-H mode transition is lower. The measured ion current is 2 times higher for changing Vout/Vin ratio from 0.33 to 7.15. The plasma density of ICP is associated with the resonance of inductance and capacitance of the antenna coil. The impedance of antenna, which is adjustable by changing the end-capacitance, determines the plasma-potential oscillation, voltage and current on the coil. With endcapacitance optimization, the plasma density is increased by lowering the antenna wall loss and accelerating ions by selfbias DC voltages. Varying the end-capacitance of antenna and to find optimum capacitance for antenna design has enabled to build effective ICP system.