Modeling of inner surface modification of a cylindrical tube by plasma-based low-energy ion implantation

The inner surface modification process by plasma-based low-energy ion implantation (PBLEII) with an electron cyclotron resonance (ECR) microwave plasma source located at the central axis of a cylindrical tube is modeled to optimize the low-energy ion implantation parameters for industrial applications. In this paper, a magnetized plasma diffusion fluid model has been established to describe the plasma nonuniformity caused by plasma diffusion under an axial magnetic field during the pulse-off time of low pulsed negative bias. Using this plasma density distribution as the initial condition, a sheath collisional fluid model is built up to describe the sheath evolution and ion implantation during the pulse-on time. The plasma nonuniformity at the end of the pulse-off time is more apparent along the radial direction compared with that in the axial direction due to the geometry of the linear plasma source in the center and the difference between perpendicular and parallel plasma diffusion coefficients with respect to the magnetic field. The normalized nitrogen plasma densities on the inner and outer surfaces of the tube are observed to be about 0.39 and 0.24, respectively, of which the value is 1 at the central plasma source. After a 5 μs pulse-on time, in the area less than 2 cm from the end of the tube, the nitrogen ion implantation energy decreases from 1.5 keV to 1.3 keV and the ion implantation angle increases from several degrees to more than 40°; both variations reduce the nitrogen ion implantation depth. However, the nitrogen ion implantation dose peaks of about 2×10¹⁰-7×10¹⁰ ions/cm² in this area are 2-4 times higher than that of 1.18×10¹⁰ ions/cm² and 1.63×10¹⁰ ions/cm² on the inner and outer surfaces of the tube. The sufficient ion implantation dose ensures an acceptable modification effect near the end of the tube under the low energy and large angle conditions for nitrogen ion implantation, because the modification effect is mainly determined by the ion implantation dose, just as the mass transfer process in PBLEII is dominated by low-energy ion implantation and thermal diffusion. Therefore, a comparatively uniform surface modification by the low-energy nitrogen ion implantation is achieved along the cylindrical tube on both the inner and outer surfaces.