Sub-millisecond post exposure bake of chemically amplified resists by CO₂ laser heat treatment

Chemically amplified photoresists require a post exposure bake (PEB), typically on a hot plate at 90-150°C for 30-120 seconds, to catalytically deprotect the polymer backbone. During PEB, excessive diffusion of the photo-generated acid results in loss of line edge definition, blurring of latent images and changes in the line edge roughness. Both acid diffusion and deprotection are thermally activated processes, with the relative rates affected by the time/temperature profile of the PEB. In this work, we introduce an alternate PEB method involving 500 μs time scale heating over a temperature range of 130°C to 450°C using a continuous wave CO₂ laser. A methodology is developed for characterizing this laser PEB and comparing the behavior with conventional hot plate PEB. The thermal stability of several polymer and photoacid generator (PAG) resist systems were studied and shown to be stable at these high temperatures due to the short heating duration. Sensitivity of resists under hot plate and laser PEB were measured. Under moderate temperatures, the laser PEB sensitivity can exceed that of hot plate PEB by an order of magnitude. Quantitative determination of the acid diffusion was obtained using resist bilayers (PAG loaded/PAG free). Despite the five orders of magnitude difference in PEB time, systems with l-PEB and hot-plate PEB exhibit comparable imaging quality under deep ultraviolet exposure.