Numerical Study On The Plasma-surface Interaction In Pulsed Plasma Processings

Three pulsed plasma processings, i.e. the modulated pulsed power magnetron sputtering (MPPMS), the plasma-based ion implantation (PBII) and the plasma-based low-energy ion implantation (PBLEII) have been taken as examples, the energy, flux and uniformity of the ion flow in these pulsed plasma processings have been numerically studied in order to improve the development of these processings and to demonstrate the advantages of pulsed plasma characteristics in the surface modification. The influence of the pulsed plasma discharge characteristics on the ion deposition energy in MPPMS, the multi-pulse sheath dynamics on the ion implantation current density in PBII and the plasma characteristics on the ion implantation current uniformity in PBLEII have been numerically studied for purpose of understanding the plasma-surface interaction in pulsed plasma processings, investigating the processing mechanisms, optimizing the processing parameters, developing the industrial applications and playing a model role for the modeling work of other pulsed plasma processings.A high ionization fraction of sputtered species up to 90%can be achieved by MPPMS, and the microstructure of the films deposited with energetic ions is significantly improved compared with conventional magnetron sputtering. The discharge characteristic is the critical issue of MPPMS, which determines the ion deposition energy, and therefore the microstructure, preferred orientation and grain size of deposited thin films. Based on the particle balance and the energy balance in the ionization region, and considering the loss of electrons by cross-B diffusion, a global plasma model has been developed to describe the MPPMS discharge process, and the temporal variations of the plasma parameters in the ionization region are obtained. The modeling results show that, as increasing the working pressure from 0.1 to 0.7 Pa, the modeled electron density during the strongly ionized period increases from 8×1018 m-3 to 2×1019 m-3, the modeled electron temperature decreases from 6-8 eV to about 4 eV, the effective power transfer coefficient, which represents the power fraction that effectively heats the electrons and maintains the discharge, decreases from 0.07-0.09 to 0.06. Using the modeled plasma parameters to evaluate the kinetic energy of arriving ions and the substrate temperature, the variations of processing parameters which decrease both values lead to a weakened diffusion ability of adatom and a reduced input energy to the substrate, corresponding to the observed transition of the deposited Cu thin films from a void free structure with a wide distribution of grain size into an underdense structure with a fine fiber texture in the extended structure zone diagram, the increase of intercolumnar voids and surface roughness, as well as the decrease of grain sizes. The microstructure transition of deposited thin films is well-explained by the modeling results, suggesting that the primary plasma processes are properly incorporated in the model. The results contribute to the understanding of the influence of MPPMS discharge characteristics on the ion deposition energy, as well as the microstructure of deposited thin films.In the PBII processing, a series of negative pulsed biases are applied onto the sample immersed in the plasma, to achieve a simultaneous modification on each surface of the sample, the modification efficiency is much higher than conventional ion beam implantation processing. The pulsed sheath dynamics is the critical issue of PBII, which determines the ion implantation current density, and therefore the modification efficiency. A magnetized plasma diffusion fluid model is established using the ion continuity equation, the ion motion equation and the measured plasma diffusion coefficient. Together with a magnetized sheath collisional fluid model, the full pulse period including the sheath dynamics during pulse-on time and the plasma recovery during pulse-off time can be described, and the models are verified to be accurate by comparing with experimentally measured electron density profiles. Under typical PBII processing parameters, by considering the plasma diffusion, the steady-state sheath thickness increases from 0.05 to 0.06 m and the corresponding ion implantation current density decreases from about 4.2 A/m2 to 2.8 A/m2. The variations of processing parameters which accelerate the plasma diffusion reduce the steady-state sheath thickness and increase the ion implantation current density, and vice versa. Increasing the pulse frequency from 1 to 100 kHz under typical PBII processing parameters significantly increases the average ion implantation current density, and the limiting factor which affects the modification efficiency is converted from duty cycle to plasma diffusion. The modification efficiency increases linearly with the bulk plasma density and decreases significantly under the transverse magnetic field, while other processing parameters have no such significant impact.Using a high density plasma source and a low-energy pulsed bias, the sheath thickness in PBLEII processing is considerably decreased and a conformal modification effect can be achieved. The plasma characteristic is the critical issue of PBLEII, which determines the ion implantation current uniformity, and therefore the conformality of the modification effect. The plasma characteristics and the conformal modification by PBLEII are numerically modeled and experimentally investigated in order to systematically explore the processing mechanism of PBLEII and optimize the processing conditions. A global plasma model is used to simulate the electron cyclotron resonance microwave plasma discharge process and the modeling results were verified by the experimental diagnostics. The magnetized plasma diffusion fluid model is used to calculate the plasma downstream diffusion process with a high density of 1011-1012 ions/cm3 to the processing chamber along the divergent magnetic field, shows that a uniform plasma can be obtained at the sample surface in PBLEII, which is beneficial to achieve a uniform modification. The decrease of working pressure reduces the collisions between the plasma and the background gas, therefore the plasma density at the sample surface is increased. A magnetized sheath collisional fluid model is adopted to calculate the sheath expansion and the low-energy ion implantation at the surface during the pulse-on time. Under the working pressure of 5×10-2 Pa, by increasing the plasma source density from 2×1011 ions/cm3 to 5×1011 ions/cm3, the plasma density near the sample surface increases from 1×1010 ions/cm3 to 2.5×1010 ions/cm3, the implantation current density and the modification efficiency are increased, the implantation uniformity is improved as well. Adopting the calculated ion implantation current density as the boundary condition, a nonlinear kinetic discrete model is established to describe the nitrogen transport in the austenitic stainless steel, the results are verified by EPMA measurement. The nitriding depth distributions are calculated to estimate the modification uniformity. It is found that the conformal modification of PBLEII processing can be achieved at the optimized processing parameters of working pressure 5×10-2 Pa and microwave power 300 W.