Two dimensional computer simulation of plasma immersion ion implantation

Abstract

The biggest advantage of plasma immersion ion implantation (PIII) is the capability of treating objects with irregular geometry without complex manipulation of the target holder. The effectiveness of this approach relies on the uniformity of the incident ion dose. Unfortunately, perfect dose uniformity is usually difficult to achieve when treating samples of complex shape. The problems arise from the non-uniform plasma density and expansion of plasma sheath. A particle-in-cell computer simulation is used to study the time-dependent evolution of the plasma sheath surrounding two-dimensional objects during process of plasma immersion ion implantation. Before starting the implantation phase, steady-state nitrogen plasma is established inside the simulation volume by using ionization of gas precursor with primary electrons. The plasma self-consistently evolves to a non-uniform density distribution, which is used as initial density distribution for the implantation phase. As a result, we can obtain a more realistic description of the plasma sheath expansion and dynamics. Ion current density on the target, average impact energy, and trajectories of the implanted ions were calculated for three geometrical shapes. Large deviations from the uniform dose distribution have been observed for targets with irregular shapes. In addition, effect of secondary electron emission has been included in our simulation and no qualitative modifications to the sheath dynamics have been noticed. However, the energetic secondary electrons change drastically the plasma net balance and also pose significant X-ray hazard. Finally, an axial magnetic field has been added to the calculations and the possibility for magnetic insulation of secondary electrons has been proven.

Magnetic suppression of secondary electrons in plasma immersion ion implantation

“…Since electrons are free to move in the direction parallel to the magnetic field lines, the virtual cathode will be maintained if it is formed faster than the axial transit time. If this condition is not satisfied, secondary electrons are emitted as two confined beams along the field lines.Numerical simulations with low density nitrogen plasmas have been performed, showing that an electron layer would indeed be formed, 8,9 but to our knowledge, this magnetic insulation method for suppressing secondary electrons in PIII has not been demonstrated experimentally.In this work, secondary electrons emitted during ion implantation experiments in a vacuum arc aluminum plasma are directly measured by two Faraday cups with and without the presence of a magnetic field parallel to a copper target surface. Unlike most of the vacuum arc systems used for ion implantation, which have curved magnetic filters, our equipment has a straight magnetic duct and implanted surfaces are oriented parallel to the plasma stream and magnetic field, in order to avoid macroparticle contamination and minimize deposition.…”

“…Numerical simulations with low density nitrogen plasmas have been performed, showing that an electron layer would indeed be formed, 8,9 but to our knowledge, this magnetic insulation method for suppressing secondary electrons in PIII has not been demonstrated experimentally.…”

“…An oscillation of about 25 kHz persisting throughout the discharge is attributed to drift waves, 12 with no significant negative perturbation being seen beyond the amplitude of this oscillation. The numerical simulations done so far by Rej et al 8 and Kostov et al 9 assumed low density ͑n ϳ 10 9 cm −3 ͒ plasmas with sheath thicknesses of several centimeters, in which the confined electron layer is formed a few millimeters from the electrode's surface. Electrons in this virtual cathode can escape through the ends of the electrode along field lines.…”

Numerical Simulation Study on the Effect of CO2 Laser on the Plasma Behavior

“…When appropriate methods are used, relatively small systems of thousand superparticles can indeed simulate accurately the collective behavior of real plasmas. The development of new algorithms and the availability of more powerful computers has allowed particle simulations to progress from simple onedimensional, electrostatic problems to more complex and realistic situations involving electromagnetic fields in multiple dimensions and up to 106 particles [1]. Any computer simulation one, two or three-dimensional starts with some initial particle distribution, i.e.…”

Magnetic field effects on secondary electron emission during ion implantation in a nitrogen plasma

“…Numerical simulations showed that under certain conditions an electron layer would indeed be formed. 10,11 The crucial point for suppression is that this virtual cathode must be formed faster than it is diffused away along the field lines. If the longitudinal confinement time is not long enough, two beams of transversally confined electrons would be formed along the two directions of the field line.…”