| AVS 54th International Symposium | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS-ThP |
| Session: | Plasma Science and Technology Poster Session |
| Presenter: | J.C. Wolfe, University of Houston |
| Authors: | M. Bhargava, University of Houston A.K. Srivastava, Axcelis Technologies W. Donner, University of Houston J.C. Wolfe, University of Houston |
| Correspondent: | Click to Email |
The complete, damage-free and efficient removal of high dose ion-implanted (HDI) photoresist is one of the most challenging issues in integrated circuit manufacturing. The problem arises because a crust of implanted metal ions and vitrified carbon forms on the resist surface that is much less reactive to plasma ashing chemistries than unimplanted resist. The throughput limitation implied by this intrinsically low ash rate is compounded by the need to limit the wafer temperature to avoid popping, the explosive ejection of macroscopic crust particles due to thermally induced volatilization of the unimplanted resist layer beneath the crust. This paper describes the application of a high power O2/N2 plasma system to ashing of HDI photoresist. Reactant gas (typically O2:N2=9:1) is activated by a 2.5 kW, 2.45 GHz surface wave discharge in an air cooled quartz process tube 6 mm in diameter. The directional flow (3slm) of process gas at 80 Torr pressure produces a plasma jet that emerges from the end of the discharge tube and impinges on a scanning wafer. The wafers are held by a vacuum chuck and rastered by an in-vacuum motor assembly at speeds up to 105 cm/s. The jet, about 1 cm in diameter, carries a thermal power of 500 W. Test wafers (200 mm) were coated with 1.0 µm thick I-line resist and implanted with an arsenic dose of 5x1015/cm2 at 40 keV and hard-baked at 120 °C. In our approach, the jet delivers hot, reactive species to the resist surface while the wafer temperature is held below the hard-bake temperature to prevent popping. Remarkably, it is then possible to selectively remove the crust from the unimplanted layer. Once the crust is removed, the base resist is rapidly ashed with a high temperature (low speed) scan. A light haze is formed on the wafer surface due to the reaction of atmospheric water vapor with the As2O3 particles that form during the ash process. This haze can be completely removed with a DI water rinse, after which, SEM and XPS analysis indicates no ash residues. Charge damage, interface trap density, and stress induced leakage were shown to be at or below values for the other commercial plasma ashing tools, which are known to provide damage-free ashing solutions. Silicon loss studies, in progress, will be reported at the conference. A conservative estimate of time-to-clear for a 300mm, 2-jet system is about 80s.