Driving frequency effect on the floating probe measurement of plasma property

Abstract

Monitoring of plasma property becomes important to achieve the precise etching, deposition and other dry processes to fabricate nano scale semiconductor devices. To do this, the floating probe method for in-situ plasma monitoring tool has been developed under the consideration of less plasma interaction. Due to the electron energy distribution function (EEDF) the electron current flown through the electrical probe nonlinearly increases with the sheath potential between the probe and the plasma. Because of this nonlinearity, the probe current is a distorted sinusoidal current with many high frequency harmonics when a simple sinusoidal ac bias is applied to the floating probe. Because the amplitudes of these harmonics of the probe current strongly depend on the electron temperature and the density of plasma, the electron temperature and the plasma density can be obtained by means of the amplitude of current harmonics and its ratio. Due to the stray capacitance between the probe circuit and ground, the displacement current exists in the probe circuit. For more accurate measurement, the displacement current which consists in the probe current needs to be eliminated. In this study, the amplitude of sheath conduction current and the stray capacitance are obtained by means of the amplitude change due to the frequency dependency of displacement current. Comparing with the typical Langmuir probe diagnostic method, the floating probe with 500p/F of capacitance overestimats the electron temperature about 1eV higher and the plasma density about 150% higher when a 5kHz ac bias is applied to the probe. When the effect of displacement current is eliminated by means of the currents at 5kHz and 7kHz of bias frequency, by contrast, the floating probe with 500p/F of capacitance measured the electron temperature with 0.2eV of error and the plasma density with about 10% of error.