Improved photoresist integrity by UV photostabilization for high dose, high energy ion implants

High energy ion implantation applications are moving beyond the original low dose requirements of retrograde and triple wells. Collector regions for bipolar and BiCMOS circuits are increasingly formed by high energy implantation. Compared to "the previous method of low energy implantation followed by epitaxial layer growth, high energy implantation saves both process steps and cost. Since collectors require low resistivity to give high output currents, relatively high implant doses (2×10¹⁴cm^-2- 2×10¹⁵cm^-2) are needed for a successful epi-replacement process. In the MeV energy range, softbaked photoresist is very susceptible to lifting and popping at doses >2×10¹⁴cm ^-2, causing resist failure and high particle contamination. We examined the stability of 2.7-3.5μm photoresist during phosphorus implantation at energies of 900-1200 keV. Photoresist pretreatment with heat and UV light prior to implantation greatly increases the dose that can be implanted before resist hardening or lifting occurs, as compared to hardbaked or softbaked pretreatment. These results are explained in terms of UV-induced cross-linking of the polymer chains in the resist. Heating the resist to >200°C during UV treatment is crucial for maximizing resist integrity, a high oxygen ambient is also beneficial. Our results indicate that UV pretreatment is an important process technology for moderate and high dose MeV implants.