Investigation of energy purity of Sub-10 keV B+ implants on a Varian VIISion PLUS ion implanter

The continuing decrease in device dimensions places increasing requirements for production-worthy ion beam currents at energies suitable for the fabrication of shallow junctions. The use of electrostatic optical elements in the design of ion implanters may potentially produce higher-energy ions, which would compromise the desired junction depth. In this paper, we present results from a study of the effect of photoresist load on the energy purity of 2 and 5 keV B ⁺ ion beams produced by a Varian VIISion 80 PLUS high-current ion implanter which uses a novel dual Einzel lens design to improve beam transmission at low beam energies. Implants are performed into preamorphized wafers and into single-crystalline wafers under channeling conditions. Peak fitting to the SIMS depth profiles of the implanted ion distributions is used to assess the beam energy purity. The dependence of beam purity on ion energy, ion dose, photoresist load, Einzel lens voltage and disc tilt angle has been investigated. Results from preamorphized wafers indicate typical impurity of the order of 0.1% for 5 keV B⁺ and as low as 40 ppm for 2 keV B⁺ implants with maximum photoresist load. Channeled implants exhibit no dependence of the implant profile on photoresist load