The Investigation On Dual Frequency Capacitively Coupled Plasma By Using Optical Emission Spectroscopy

Capacitively coupled plasma (CCP) driven by dual frequency is currently becoming an important plasma source as a fine etching tool for manufacturing the ultralarge scale integrated (ULSI) circuits. In this dual frequency CCP reactor, one radio frequency (RF) is chosen to be much higher than the other, such an arrangement as to achieve an independent high flux and energy of ion onto the substrate. In this paper, the rotational and vibrational temperatures of nitrogen gas and ion, the Ar plasma nonuniformity and the dissociation of CHF3 in a dual frequency CCP are investigated by using optical emission spectroscopy (OES).1. The influence of exciting frequencies on the rotational and vibrational temperatures of nitrogen gas and ion in a CCP are investigated. The rotational and vibrational temperatures are acquired by comparing the measured and calculated spectra of selected transitions.It is observed that N₂ rotational temperature minimum around 13MHz is the combined effect of ion-dominated heating and electron-dominated heating in the plasma. +N 2rotational temperature almost drops linearly from about 550K to about 350K with increasing exciting frequency up to 13MHz, and then becomes unchanged with further increase of frequency. Also, N 2+rotational temperature is much higher than the corresponding N₂ rotational temperature in the plasma driven by low frequencies. These experimental phenomena may be attributed to the effective ion heating and/or possible resonant heating in the bulk plasma under the low frequency field.The N₂ and N₂⁺ vibrational temperatures almost increase linearly with increasing exciting frequency up to 23MHz, and after that increase slowly or even decrease. The pressure corresponding to the maximum point of N₂ vibrational temperature decreases with increasing exciting frequency. These experimental phenomena is attributed to the electron density increasing whereas the electron temperature decreasing with exciting frequency rising.2. The changes of the rotational and vibrational temperatures with the high frequency (HF) and low frequency (LF) input powers as well as the discharge pressure in dual frequency (41MHz/2MHz) capacitively coupled plasma discharges are investigated.The increment rate of N₂⁺ rotational temperature with increasing LF power are larger than that of HF power, because the N₂⁺ can be effective heated by low frequency fields.Due to the HF power and LF power having different influences on the electron temperature, the increment rate of N 2+ vibrational temperature with increasing LF power is larger than that of HF power. While the influence of HF power on vibrational temperature is just opposite to the situation of the LF, the increment rate of N₂ vibrational temperature with increasing HF power is larger than that of LF power. With the discharge pressure increasing, the difference between the vibrational and rotational temperature decrease.3. The Ar plasma nonuniformity in a single and dual frequency CCP is studied by using optical emission spectroscopy with Able inversion method.In a single frequency CCP, plasma nonuniformity is mainly caused by edge effect. The edge effect decreases with increasing exciting frequency. This is due to discharge voltage decreases with increasing exciting frequency. Additionally, the increasing RF power can make discharge voltage increase to enhance the edge effect, and lead the plasma nonuniformity degree increases. With the discharge pressure increasing, the plasma nonuniformity degree decreases in 2MHz discharge, while it increases in 13.56MHz and 41MHz discharge.In a dual frequency CCP, the situation is rather complex. The increase of discharge pressure does not change the shape of Ar line the intensity profiles along the vertical, while the HF and LF power does. It is due to the HF and LF power can change the distribution of plasma density and electron temperature along the vertical, while the pressure not. The plasma nonuniformity is also mainly caused by edge effect. There are much small nonuniformity in a dual frequency CCP than that of a single frequency CCP. It may be due to the HF and LF power is not uncoupled completely.4. The dissociation of CHF3 in a CCP driven by dual frequency sources (41MHz/2MHz) is experimentally investigated by using optical emission spectroscopy technique.It is observed that the increment rate of the relative density of F and H and the ratio of F/CF2 with increasing LF power are larger than that of HF power. It is caused by the different kinetics absorbed from sheaths of low and high frequency, where electrons can acquire more bouncing kinetics from low frequency oscillating sheath in favor of ionization and dissociation of parent gas and radicals in the plasma.It is also observed the relative density of F and H, and the ratio of F/CF2 decrease with increasing discharge pressure. This is due to the electron temperature decrease with increasing discharge pressure.