Numerical Simulation Of The Breakdown Process Of Capacitively Coupled Plasma

Capacitively coupled plasma（CCP）has been widely used in etching,deposition,clean-ing,et al.of the semiconductor industry.For CCP or other plasma sources,the gas break-down is an indispensable process to form the plasma.Studying this process is of great signif-icance for understanding the formation of plasma,optimizing plasma sources,and expanding the gas discharge theory.However,due to the extremely short time,it is difficult to diagnose the breakdown process in the experiment.Therefore,seldom studies have been applied to the breakdown process of CCP.By adding the secondary electron emission model,different external circuit models,particle merging algorithm,and various physical quantities diagno-sis methods to the Particle-In-Cell/Monte-Carlo collision（PIC/MCC）model,the breakdown process of CCP under different discharge conditions has been studied.（1）The breakdown process of argon in CCP driven by dual-frequency is studied with the method of one-dimensional PIC/MCC coupled with a simplified external circuit model.The results show that the whole breakdown process can be divided into three phases:the pre-breakdown phase,the breakdown phase,and the post-breakdown phase.The pre-breakdown is the electron avalanche phase,which is driven by the uniform electric field,and the electron density increases exponentially.The electron-induced secondary electron emission is crucial for the electron avalanche.The breakdown is the phase of sheath formation,in which the unstable sheath maximizes the electron heating rate.The electrical characteristics of CCP are thoroughly changed by the formation of the sheath,resulting in the dramatic CCP voltage dropping declining in this phase.The post-breakdown is the plasma stabilizing phase.The gradual declining ionization rate is gradually approach to the slowly rising electron loss rate at the boundary.At the same time,the power absorption gradually decreases and reaches a stable value in this phase,which balances the discharge system.（2）The breakdown process of CF₄in the RF-driven CCP is studied by the method of one-dimensional PIC/MCC coupled with a simplified external circuit model.The re-sults show that the breakdown process of CF₄almost shares the same characteristics as the breakdown of argon in the pre-breakdown phase,the breakdown,and the initial time of the post-breakdown.In the later period of the post-breakdown phase,the increase of negative ion density gradually transforms the discharge from electropositive to electronegative,and the electron heating mode also changes fromαmode to drift-ambipolar mode.The heating power in the drift-bipolar mode is much higher than in the previousαmode,resulting in fur-ther growth in particle generation rate.The slow growth of the recombination rate gradually increases the ion loss rate.After hundreds of microseconds of evolution,the ion loss rate gradually reaches a balance with its generation rate,and the entire discharge enters a steady state.（3）The breakdown conditions of argon in an RF-driven CCP are studied by a one-dimensional PIC/MCC model coupled with a simplified external circuit.The results show that both the higher electron-induced secondary electron emission coefficients and higher driving frequencies can significantly expand the range of breakdown-able gas pressure.Un-der high frequency driving,in the extremely low-pressure region,there are several sus-tainable and unsustainable discharges modes that are quite different from conventional dis-charges.The electron density of these sustainable anomalous discharges are extremely low with a spatial distribution opposite to that of conventional discharges.（4）The breakdown process of CCP under an L-Type matching circuit is studied by a one-dimensional PIC/MCC model coupled with a complex external circuit.The results show that there is a short electrical signal stabilization process in the initial time of discharge due to the charge effect of the energy storage device.The whole discharge system can be treated as a linear system in the pre-breakdown phase.During the breakdown process,the forma-tion of the sheath instantly increases the equivalent capacitance of CCP,which changes the amplitude of electrical signals in the matching circuit.In the post-breakdown phase,the gradually thinning sheath further increases the equivalent capacitance of CCP,and its os-cillating characteristics directly lead to higher harmonics of the CCP current.By adjusting the capacitance of the matching circuit elements,the breakdown characteristics can be well-modulated.