Negative tone resist for phase-shifting mask technology:: A progress report

With the objective to make smaller device structures at a given illumination wavelength the semiconductor industry is more and more trying to implement Phase-Shifting Masks (PSMs) as resolution enhancement technique for DUV lithography. However, with positive photoresists there is a phase edge problem. Using negative resists is the easiest approach to solve the phase edge problem. This is one of the reasons why negative resists are becoming more and more attractive for leading edge lithography(1). Therefore, we are developing a novel negative resist with 248/193nm crossover capability. Most experiments were done in imitation of the CARL(R) bilayer resist process(2). The goal was to use established resist techniques and polymer materials, and just to change the generators and additives to get tone reversal. Using a photoacid generator (PAG) as additive leads to positive tone. In contrary with a photobase generator (PBG) and thermoacid generator (TAG) combination a negative tone behavior is observed. Comprehensively, this blending concept allows the use of similar working polymers in both, positive and negative resists. The generator efficiencies were studied as well as the diffusion behavior of resist components during resist processing. Especially, process factors like baking conditions were investigated with the objective to control diffusion and limit resist outgassing in a high activation energy resist platform. Furthermore, in adaptation of the CARL(R) process, a separate liquid silylation step was integrated and investigated for various process conditions. In our paper we will discuss the characteristics and the lithographic capabilities of the novel methacrylate based negative resists. First promising results are based on DUV (248/193nm) and ebeam exposures. Recent results with our positive version indicate the same outstanding possibilities(3). We expect a similar performance for the negative pendant in the near future.