Seok-Hwan Lee

Frequency and electrode shape effects on etch rate uniformity in a dual-frequency capacitive reactor

SiO2 was etched on 300 mm wafers in a dual-frequency capacitive plasma reactor to study etch rate nonuniformity as a function of driving frequency and power. It is shown that the etch rate profile shape varies significantly with the driving frequency. It also is shown that for different driving frequencies, the behavior of etch rate profile shape with the power is quite different, namely: (i) for lower frequency (27 MHz), the shape almost does not change with the power; (ii) for higher frequency (100 MHz), the shape considerably varies with the power. These results clearly indicate that the main reason for the etch rate nonuniformity in high-frequency capacitive reactors is the plasma nonuniformity caused by electromagnetic (standing wave and skin) effects. Using a specially shaped top electrode rather than the traditional flat one is shown to considerably improve the etch rate uniformity.

Improvement of dynamic range of electron energy probability function from two asymmetrical collecting area probe data filtered by Savitzky-Golay and Blackman window methods

The electron energy probability function (EEPF) measured by Langmuir probe is required to be reasonable in low energy regime and have large dynamic range (DR) in high energy regime to investigate the kinetics of low pressure plasma. However the internal resistance (Rint) in bias circuit of probe and the adaption of digital smoothing filter to increase DR destruct these requirements by distorting the EEPF in low energy regime. Rint is sum of the resistances due to the chamber wall sheath and surface of chamber wall. The existence of Rint gives distortion of measured EEPF in low energy regime by overestimating measured probe voltage. Adapting digital smoothing filter gives additional distortion of EEPF in low energy regime since it flattens the peak shape near zero electron energy. A new method is proposed to acquire EEPF which has reasonable value in low energy regime and large DR in high energy regime. The overestimated probe voltage is corrected by removing the effect of Rint which is determined from two sets of plasma potential (Vp) and electron saturation current (Ipe*). The Savitzky-Golay and Blackman window filters are adapted to the I-V characteristics of larger collecting area probe, which has larger signal-to-noise ratio. The two digital smoothing filters are optimized to maximize the strengths of each filter by considering the property of EEPF in low and high energy regime. The verification and capability evaluation of the proposed method are performed by comparing the EEPF measured from optical emission spectroscopy (OES) and conventional method based on single Langmuir probe. The method enhances DR of measured EEPF about 35 ~ 40 dB in comparison with the EEPF from conventional method, especially at two energy regions near zero electron energy and high energy. There are two requirements for proposed method. The distance between two probes is small enough to maintain that ΔVp due to the difference of measurement position is smaller than ΔVp due to Rint where ΔVp is the difference of Vp between two probes. Also signal-to-noise ratio of larger collecting area probe should be larger than 55 dB to ensure the performance of Savitzky-Golay method in low energy regime.

Determination of electron energy distribution function shape for non-Maxwellian plasmas using floating harmonics method

A new floating harmonics method is developed to determine the electron energy distribution shape in the non-Maxwellian plasmas. The slope and curvature of the electron energy probability function (EEPF) at the floating potential can be obtained by measuring amplitude ratios among the first three sideband harmonics of the probe electron current. Together with the generalized EEPF formula, the EEPF shapes of the non-Maxwellian plasmas are able to be determined from the slope and curvature. Here, the new method is experimentally verified and its results are compared with EEPF measurements using the Langmuir probe (LP) method. The effective electron temperature and the EEPF shape parameter obtained by the method give good agreements with those of the LP measurements.

Standing wave effect on plasma distribution in an inductively coupled plasma source with a short antenna

The effect of a standing wave on plasma density is observed in an inductively coupled plasma (ICP) with a single turn antenna whose circumferential length is much shorter than the wavelength of the driving frequency. The experiment is performed in an argon plasma operated at 5 mTorr with 13.56 MHz power applied to the single turn antenna with diameter of 300 mm. Measured plasma density near the ground termination of the antenna is higher than near the power input, which degrades the uniformity in the ICP source. It is analysed with the standing wave effect of the current and voltage on the antenna, which is carried out with a transmission line model based on the transformer circuit model of the ICP. The wavelength and the distribution of the inductively and capacitively coupled powers along the antenna are obtained from the model with the measured impedance of the antenna. The expected density distribution agrees well with the measured one, revealing that the wavelength is shortened and the nonuniformity of the plasma density becomes severe with increase of the input power.

Monitoring Ion Energy Distribution in Capacitively Coupled Plasmas Using Non-invasive Radio-Frequency Voltage Measurements

A non-invasive method for ion energy distribution measurement at a RF biased surface is proposed for monitoring the property of ion bombardments in capacitively coupled plasma sources. To obtain the ion energy distribution, the measured electrode voltage is analyzed based on the circuit model which is developed with the linearized sheath capacitance on the assumption that the RF driven sheath behaves like a simple diode for a bias power whose frequency is much lower than the ion plasma frequency. The method is verified by comparing the ion energy distribution function obtained from the proposed model with the experimental result taken from the ion energy analyzer in a dual cathode capacitively coupled plasma source driven by a 100 MHz source power and a 400 kHz bias power.