Effect of Secondary Electron Emission From Various Targets During Variable Pulse Length PIII

Abstract

Plasma Immersion Ion Implantation(PIII) is a versatile material processing technique [1] , [2] . In PIII a solid target is immersed in plasma, and negative-polarity high-voltage(NPHV) pulses are applied to the target. During a NPHV pulse electrons are repelled from the near-target region, resulting in a positive ion sheath surrounding the target; ions traversing the sheath are implanted into the target surface. Lieberman et al. [2] developed a PIII sheath model which captures basic aspects of the sheath evolution during PIII pulses. However, some key assumptions used in this model do not hold. In particular, experiments show that the plasma density and electron temperature do not remain constant during the NPHV pulses, due to various effects, including the effect of secondary electrons ejected from the target. The ejected electrons accelerate back through the sheath, depositing their energy in the bulk plasma; this can significantly increase the plasma density and electron temperature above the nominal steady-state values measured before the PIII pulse [3] , [4] , [5] . To further elucidate the effect of secondary electrons, we report on recent PIII experiments with variable pulse lengths(10us-100us) on a range of target materials including silicon, stainless steel, brass, and tungsten alloys with a range of NPHV pulse amplitudes(1-5 kV). We discuss the results of these experiments and the various mechanisms involved.