Process Robustness against Photoresist Outgassing in Single-Wafer High-Energy Implanters

Abstract

In recent years, deeper photodiode (PD) structures in advanced image sensors have been pursued to obtain higher light sensitivity. In order to form the deep PD structure, the ultra-high energy implantation is required. For example, 4MeV Phosphorus implantation is required to form a PD with depth of 3 m and 3MeV Boron is required for the cell isolation as well. Therefore, masking photoresist (PR) should be thick enough to stop high energy ions inside and its aspect ratio is significantly high up to 30 or more. The reliable zero angle implantation should be secured for an effective implantation into the deep trench structure. It is well known that the dopant profile shift by the channeling effect is inevitable for the zero degree implant especially for the high energy implantation. In order to obtain the reliable dopant depth profile, the implant angle accuracy should be secured. However, because the ion beam trajectory could be varied when the charge state of the ions is changed by reaction with residual molecules or atoms generated by PR out gassing, the PR out gassing effect is not negligible for high energy implantation into thick PR coated wafers. Sumitomo Heavy Industries Ion Technology's (SMIT) single-wafer ultra-high energy implanter, the S-UHE, is designed to minimize the trajectory variation or the dopant profile shift by the PR out gassing with the unique technology, that is, the symmetric beam line and the final energy filter. The angle integrity in horizontal direction is realized by the symmetric beamline, and the energy contamination free implantation which leads to the angle integrity in vertical direction is secured by the final energy filter. Phosphorus implantation was performed at zero degree with the S-UHE onto a PR coated wafer with a striped bare region. The dopant depth profiles were measured by SIMS at different horizontal positions. Figure 1 shows the same dose and channeling profile at three positions. The profiles of PR coated wafer are completely identical. Process robustness against the PR outgassing is secured by utilizing the symmetric beam line and the final energy filter in the S-UHE.