Optimization of a plasma immersion ion implantation process for shallow junctions in silicon

Abstract

A plasma immersion ion implantation (PIII) process has been developed for realizing shallow doping profiles of phosphorus and boron in silicon using an in-house built dual chamber cluster tool. High Si etch rates observed in a 5% PH3 in H2 plasma have been ascribed to high concentration of H(α) radicals. Therefore, subsequent work was carried out with 5% PH3 in He, leading to much smaller etch rates. By optical emission spectroscopy, the radical species H(α), PH*2, and PH* have been identified. The concentration of all three species increased with pressure. Also, ion concentrations increased with pressure as evidenced by Langmuir data, with a maximum occurring at 0.12 mbar. The duty cycle of pulsed DC bias has a significant bearing on both the implantation and the etching process as it controls the leakage of positive charge collected at the surface of the silicon wafer during pulse on-time generated primarily due to secondary electron emission. The P implant process was optimized for a duty cycle of 10% or less at a pressure of 0.12 mbar with implant times as low as 30 s. Secondary ion mass spectroscopy showed a P dopant depth of 145 nm after rapid thermal annealing (RTA) at 950 °C for 5 s, resulting in a sheet resistance of 77 Ω/◻. Si n+/p diodes fabricated with phosphorus implantation using optimized PIII and RTA conditions exhibit Jon/Joff > 106 with an ideality factor of nearly 1.2. Using similar conditions, shallow doping profiles of B in silicon have also been realized.