Sheath overlap during large scale plasma source ion implantation

Summary form only given. We are investigating experimentally the effects of sheath overlap during plasma source ion implantation of a large workpiece of complex geometry. The workpiece consists of 1000 aluminum, automotive piston surrogates mounted on 4 racks; total surface area is over 16 m/sup 2/. The 4 racks are positioned parallel to each other, 0.25 m apart, in an 8 m/sup 3/ vacuum chamber. The racks of pistons are immersed in a capacitive RF plasma, with an argon gas pressure of 0.1-0.6 mtorr. We implant the pistons with the plasma ions by repeatedly pulse biasing the workpiece to 20 kV for 20 /spl mu/sec. The plasma behavior during the pulse is monitored with a Langmuir probe placed in between 2 racks of pistons. At plasma densities less than 10/sup 15/ m/sup -3/ and low gas pressures, the sheaths between the racks of pistons overlap, resulting in non-uniform implantation of the plasma ions. In addition, sheath overlap causes a drop in the plasma potential between the racks of pistons. This reduces both the measured ion current to the workpiece and the energy of the implanted ions. At gas pressures of 0.5-0.6 mtorr, we observe the creation of a high density DC discharge in the potential well. The discharge is induced by ionization of the source gas by secondary electrons. While this produces a two order-of-magnitude increase in the measured ion current, the implantation energy is still low. We model the sheath behavior with the two-dimensional, particle-in-cell code XPDP2.