Numerical Study On Sheath Characteristics Of Plasma-Based Low-Energy Ion Implantation For Inner Surface Modification

For inner surface modification of metallic tube by plasma-based low-energy ion implantation (PBLEII), a PBLEII inner surface modification system which based on the electron cyclotron resonance microwave plasma is developed in this paper. Using magnetized sheath collisional fluid model, the ion implantation energy, angle, dose, and sheath evolution characteristics under additional magnetic field were studied. The dependence of critical radius of tube on plasma density, negative voltage bias, plasma source radius and processing pressure were studied and the optimization of the process parameters for the inner surface modification by PBLEII is realized. Using the developed low-pressure non-steady diffusion fluid model, the pulsed sheath dynamics and plasma recovery characteristics during pulse-off time of PBLEII inner surface modification under low negative voltage pulses and plasma bipolar diffusion were studied. The numerical study on sheath characteristics of plasma-based low-energy ion implantation for inner surface modification is made, in order to provide the theoretical basis and guidance for setting the technical requirement and process specification of complex surface modification by PBLEII.The developed PBLEII device for inner surface modification of the tube includes vacuum chamber, linear ECR microwave plasma source, low-energy ion implantation power, auxiliary heating source, etc.. The linear ECR microwave plasma source is consist of microwave source, coaxial line waveguide, grounded grid electrode and magnetic field coil outside the vacuum chamber. Radiation slots were slotted along the spiral line on the outer conductor of the waveguide, and the2.45GHz microwave was obtained between the coaxial line and the grid electrode by microwave source along the slots. Under the effect of0.0875T magnetic field produced by magnetic field coil, the plasma of1010～1011ions cm-3density can be generated uniformly along the axial and tangential directions, low-energy ion implantation power applied-0.4～-2kV negative voltage pulses onto the tube, ions in the sheath of the inner surface were accelerated by the pulses and realized the low-energy ion implantation. Using equation of ion continuity, equation of ion motion which is consist of magnetic field, Poisson’s equation and Boltzmann relation of electron, the magnetized sheath collisional fluid model is established, the effect of magnetic field on the parameters of low-energy ion implantation and the uniformity of implantation parameters along axial direction were studied, calculating plasma density1010ions cm-3, negative voltage pulses steady value-2kV, plasma source radius2.0cm, processing pressure10-2Pa, low-energy ion implanting the inner surface of the tube with6.0cm radius, the axial uniform magnetic field of0.0875T only increased the ion implantation angle inside the tube for0～6°, and had no disadvantageous effect on the inner surface modification process, the effect of magnetic field can be ignored. On the inner surface where far away from the ends of the tube, the ion implantation angle is less than10°, the ion implantation energy is about1600eV, but in the area where less than2.0cm from the ends of the tube, the ion implanted with a larger angle of10～60°and a smaller energy of400-1600eV. The ion implantation dose is uniform far away from the ends of the tube, the necessary time to achieve the implantation dose threshold1017ions cm-2is about5.0h, a implantation dose peak appeared at the position0.5cm away from the ends of the tube, and the necessary time to achieve the implantation dose threshold1017ions cm-2is about3.1h. In the mass transfer process which combined by low-energy ion implantation and thermal diffusion, the modification effect was determined by the ion implantation dose and therefore, an acceptable modification effect can be obtained onto the area at the ends of the tube due to the sufficient ion implantation dose.Using the sheath collisional fluid model, the relationship of the radius of tube which can be treated by PBLEII and the processing parameters were studied, calculating plasma density1010ions cm-3, negative voltage pulses steady value-0.4～-2kV, plasma source radius1.0-2.0cm, processing pressure10-2～10-1Pa, modification time10h can provide1017ions cm-2implantation dose threshold for the radius of less than10.0cm tube, low negative voltage pulses of-0.4～-2kV can realize effective implantation of radius1.0cm tube theoretically; negative voltage pulses steady value, plasma source radius and processing pressure are the main influencing factors of the maximum radius of the tube, plasma density1010ions cm-3negative voltage pulses steady value-2kV, plasma source radius2.0cm, processing pressure10-2Pa, the maximum radius of tube is26.5cm; negative voltage pulses steady value and plasma source radius are the main influencing factors of the minimum radius of the tube, negative voltage pulses steady value-2kV, plasma source radius0.93cm, the minimum radius of the tube is1.00cm. For the high plasma density of1010～1011ions cm-3and low negative voltage pulses of-0.4～-2kV is used in the PBLEII process for inner surface modification, an acceptable effect can be obtained for small size tubes.Based on the plasma ambipolar diffusion mechanism under relatively lower pressure, assuming that during the plasma non-steady diffusion process the plasma is quasi-equilibrium and quasi-neutrality in every moment, the ion instant flux is described by the low-pressure steady ion mobility proposed by Lieberman,combined with the fluid motion equation which described the continuous ion flux and Boltzmann relation of electron,the low-pressure non-steady diffusion fluid model is established, the effect of plasma diffusion on the sheath expansion, plasma recovery behavior and the effect of duty cycle on the ion implantation dose were studied. Calculating plasma density1010ions cm-3, electron temperature8eV, negative voltage pulses steady value-2kV, pulse width10μs, plasma source radius2.0cm, processing pressure10-1Pa, low-energy ion implanting the inner surface of tube with6.0cm radius, the non-steady diffusion of plasma can promote the sheath expansion, and the sheath size increased about0.31cm from1.96cm at the end of the pulse; high electron temperature and low processing pressure accelerate the plasma recovery process, which recovers to95%of the steady state just1.3μs after the end of the pulse, at2.8μs there is an overshoot phenomenon and after10.0μs the recovery gradually tends to steady state; the effect of duty cycle on the ion implantation energy is little, the duty cycle higher than0.8results in the plasma incomplete recovery, but high duty cycle can increase the ion implantation efficiency substantially. When the duty cycle is0.3, the average implanted ion current density is only0.86×10-4A/cm2, but when the duty cycle is0.9, the average implanted ion current density is up to2.60×10-4A/cm2. The duty cycle of0.9can provide the inner surface of the tube an ultrahigh ion implantation dose of5.8×1019ions cm-2during10h modification time. For the inner surface modification process by PBLEII, increasing the duty cycle of negative voltage pulses is an effective approach to increase the ion implantation dose.