Post Exposure Bake (PEB), also known as diffusion bake or deferred bake, has been widely accepted in the industry as a means of reducing standing wave effects in positive photoresists. The basic process involves a bake cycle after exposure, but prior to development. A typical sequence is a soft bake of 50°C - 70°C, exposure, a PEB of 90 -110°C, followed by development. We investigated the presumed role of diffusion in the PEB process through experiments with different size sensitizer molecules and the possible role of phase transition. These investigations revealed a previously unappreciated degree of freedom in the PEB process. A new process variation is presented using AZ 4000 positive photoresist where the PEB is conducted at a much higher temperature, for example, 150°C. We named the process, High Temperature Post Exposure Bake (HTPEB). Though developing conditions must be adjusted, we have found several benefits from use of the HTPEB process: o Improved contrast, photolithographic process latitude and working resolution. o Thermal stability at least equal to the HTPEB temperature. o Improved control of standing waves and other reflective phenomena. o Elimination of micro-peeling and residue problems common to metal ion free developers. Though not presented in this paper, we have demonstrated in our laboratory one practical use of the HTPEB process in opening 1.25μm contact holes having controllable wall angles in the 50° and 70° range.