Optimization of CD control in DUV positive resists: influence of photoresist viscoelastic properties on PEB conditions

For the achievement of the 0.25 micrometers design rule technology, the CD variations must not exceed +/- 25 nm; this has to be ensured by a high performance reproducible process. The resolution capability of DUV lithography at 248 nm has already been demonstrated. However, positive tone DUV materials and their associated processes are still under optimization. Among the various process steps to be optimized, the baking steps are of great importance. Taking into account the conclusions drawn on SB mechanisms in a previous work, the role of PEB has been studied in order to obtain better linewidth control and process reproducibility. For this study, the BASF ST2 resist has been chosen as a typical example of advanced DUV positive resists. The lithographic experiments have been performed in a 200 mm DUV lithographic cell, including an ASM-L 5500/90 DUV stepper and a MARK 8 TEL track. Two main conclusions can be drawn. Firstly, the experimental results obtained clearly demonstrate that the PEB temperature is directly correlated with the viscoelastic properties (Tg) of the resist material: two diffusion regimes, below and above Tg, can be observed. Secondly, the process latitude as well as CD reproducibility are increased for PEB temperatures below Tg.