New approach to PEB mechanisms in novolac-DNQ resists: influence of physical and viscoelastic properties

This study focuses on the influence of PEB conditions (temperature) on critical dimension (CD) variations observed in I-line lithographic processes. As PEB effects are also related to the thermal history of the resist film, soft bake (SB) conditions have also been considered as an additional parameter. For given SB and exposure conditions, the CDs remain constant up to a defined PEB temperature at which a clear change can be seen. After this transition, the CD dimensions again remain almost constant as the PEB temperature is further increased. This behavior is in contrast with the established diffusion models, as this phenomenon does not appear to be thermally activated below and above this transition. The transition is shown to correspond to the Tg of the film measured by DSC and depends on the SB conditions. This phenomenon can be observed for different exposure doses and remains independent of the dose. Again this result does not support the diffusion model, but can rather be explained by molecular rearrangements. Results are presented for different resists namely the O.M.M. OIR32MD and OIR643. This work proposes a new approach to PEB mechanisms based on the viscoelastic properties of the resist. The results obtained provide a better understanding and control of CD variations during this process step. They should also be of assistance in good modeling of the lithographic process.