Particle-in-cell/Monte Carlo Simulation Of Single-and Dual-frequency Capacitively Coupled Chlorine Discharges

Chlorine is widely used in plasma etching of both semiconductors and metals. In this study, we first demonstrate the oopdl (object oriented plasma device for one dimension) particle-in-cell/Monte Carlo simulation tool for the capacitively coupled chlorine discharge with a comprehensive reaction set. Then we use this code to explore typical capacitively coupled chlorine discharges. Actually, we apply a hybrid approach consisting of a particle-in-cell/Monte Carlo simulation and a volume averaged global model. The simulation results are compared with available experimental measurements and good agreement is achieved. We explore typical single-frequency capacitively coupled chlorine discharges driven by a voltage source and a current source, respectively. The effect of gas pressure, driving current, driving frequency and secondary electrons on the discharge is systematically investigated. The feedstock gas pressure is varied from5to100mTorr, the driving current is varied from20to80A/m2, the driving frequency is varied from13.56to60MHz and the secondary electron emission coefficient is varied from0.0to0.4. Key plasma parameters including the particle density, the electron heating rate, the effective electron temperature, the electron energy probability function (EEPF), the ion energy distribution (IED) and ion angular distribution (IAD) of both Cl+and Cl2+ions are explored and their variations with control parameters are analyzed and compared with other discharges.As the gas pressure increases from5to100mTorr, the electron heating mechanism evolves from both stochastic and Ohmic heating to predominantly Ohmic heating. Also, the electron heating outweighs the ion heating at high pressure. The density profile for Cl2and Cl-ions becomes flat in the bulk region and the electronegativity increases with increasing pressure. The creation of Cl+ions in the sheath region is mainly due to conversion from Cl2ions to Cl+ions through non-resonant charge exchange, while in the bulk region the creation of Cl-ions is mainly ascribed to electron impact ionization processes. With increasing driving current, the EEPF shows a transition from a Druyvesteyn like to a Maxwellian like and then to a bi-Maxwellian profile, which is mainly caused by the enhanced stochastic heating in the sheath region and diminished Ohmic heating in the bulk region. As the driving frequency increases at fixed absorbed power, the displacement current increases and a lower rf voltage is required to keep the power invariant. Thus, the average sheath potential decreases, such that the IEDs of Cl2+and Cl ions shift to the low-energy region and the IADs of Cl2+and Cl ions extend to large angles. The effect of the secondary electrons on the discharge can be witnessed mainly from the variations of the electron heating rate and the EEPF. The IEDs, IADs and neutral energy distributions show little dependence on the secondary electrons.Furthermore, we extend our study to dual-frequency capacitively coupled chlorine discharge by adding a low-frequency current source and explore the effect of the low-frequency source on the discharge. The low-frequency current density is increased from0to4A/m2. The flux of Cl2+ions to the surface increases only slightly while the average energy of Cl2+ions to the surface increases almost linearly with increasing low-frequency current. This shows that it is possible to independently control the flux and energy of Cl2ions by varying the low-frequency current in a dual-frequency capacitively coupled chlorine discharge. However, the increase of the flux of Cl+ions with increasing low-frequency current, which is mainly due to the increased dissociation fraction of the background gas caused by extra power supplied by the low-frequency source, is undesirable. In addition, the effect of secondary electron emission in the dual-frequency discharge is found to be stronger than in the single-frequency discharge and this intensified effect is ascribed to the added low-frequency source.The IEDs and IADs of Cl2+and Cl+ions in the dual-frequency capacitively coupled chlorine discharge are explored in detail due to their significance in materials processing at the surface. Since the ion transit time is less than the low-frequency period, the ions respond to the instantaneous electric field in the sheath region, which leads to bimodal profile for the IEDs of Cl2|and Cl+ions. When transiting the sheath, the Cl+ions experience a more collisional sheath than the Cl2|ions. The IADs of Cl2|and Cl+ions at the surface are almost anisotropic. However, a secondary peak is found in the IAD of Cl+ions, which can be ascribed to dissociative ionization reactions.