Experimental Diagnosis Study On Capacitively Coupled Plasma Driven By Single Or Dual Radio Frequency

The low pressure radio frequency capacitively coupled plasma (CCP) is widely used in the manufacturing of semiconductor devices, especially for the anisotropic etching of dielectric materials. Due to the continuous shrinkage of feature size and large area processing uniformity, microelectronics engineering needs more accurate control of plasma technology and reactors. Therefore the CCP discharge is widely studied and focused, such as the dual-frequency capacitively coupled plasma (DF-CCP). It is expected in DF-CCP that the higher frequency source controls the plasma density while the lower frequency source controls the ion bombardment energy and then the independent control of ion flux and ion energy is achieved. Due to the nonlinearity and complexity of plasma, there are still problems to be investigated in CCP discharges.By using Langmuir probe, floating double probe and optical emission spectroscopy we investigated the characteristics of CCP discharges.1) We proposed a collisional-radiative model for low pressure argon gas which contains 22 effective energy levels and studied the behavior of various excited states. The main processes contained in the model are electron impact excitation, ionization and radiative decay. Under low pressure condition the diffusion loss in an important loss mechanism. It is found that excited states densities increase with increasing electron temperature and density. By combining the model with optical emission spectroscopy and Langmuir probe measurement, the 13.56 MHz single frequency CCP discharge diagnosis is carried out. It is found that under our experimental condition the two methods are in agreement.2) Since the strong perturbation arisen from low frequency and also other harmonics, the traditional Langmuir probe can not work in DF-CCP with lower frequencies, such as 2 MHz. Here the DF-CCPs with different frequency configurations, i.e., 13.56/2、27/2、41/2、60/2 MHz in Ar plasma under 5 Pa pressure are investigated by using floating double probe and optical emission spectroscopy. The electron temperature increases immediately with low frequency power addition while electron density first decreases slightly and then keep increasing. This trend weakens as high frequency increases from 13.56 to 60 MHz which indicates the higher frequency is helpful to the decoupling of DF-CCP. With low frequency addition, the Ar 750.4 nm emission line intensity first increases slightly and then decreases continuously in 27/2、41/2、60/2 MHz discharge, which is in conflict with the probe measurement. It is investigated that the discrepancy between probe and optical emission spectroscopy is due to the change of electron energy distribution with increasing high frequency.In 41/2 MHz DF-CCP of Ar, the electron temperature decreases with pressure generally while electron density first increases and then decreases. Under higher gas pressure and larger low frequency power condition, the electron temperature decreases substantially. This phenomenon is attributed to mode transition of the discharge.3) The 60/2 MHz and 60/13.56 MHz DF-CCPs in N2 are investigated by using floating double probe and optical emission spectroscopy. Under higher pressure and large low frequency power, there is an abnormal enhancement of electron density and N2+ emission intensity. The difference for 2 and 13.56 MHz could be attributed to the discharge mode transition (αmode &γmode) induced by 2 MHz. By analyzing behaviors of electron density, temperature and N2+ emission, it is concluded that N2+(B) state is mainly contributed by electron impact excitation of N2+(X).By comparing the measured and calculated spectra we acquire N2 and N2+ rotational temperatures in N2 DF-CCPs with frequency configuration of 60/2 MHz and 60/13.56 MHz. it is shown that both temperatures increase with gas pressure and low frequency power. In 60/2 MHz DF-CCP, the N2+ ion rotational temperature is much higher than that in 60/13.56 MHz case. Influences of different low frequencies are investigated and the higher ion rotational temperature in 60/2 MHz DF-CCP is attributed to the effective heating of ion which could correspond to the 2 MHz low frequency field.