An optical proximity model for negative toned developing photoresists
At 14 nm technological node and beyond, negative toned developing (NTD) has become needed process for trench and contact/via photolithographic processes owning to its higher utilization of light energy compared with positive toned developing (PTD) process. However, the efficiency gain for the NTD process only happens in semi-dense to isolated pitches where more light can pass through the mask. For dense pitches, such as the pitches ranging from 76–100 nm, the NTD process has no advantage compared to the PTD process. This is because in the PTD process, the photochemical reaction is only needed to weaken the targeted photoresist volume so that the targeted area can collapse and be removed by the developer to form photoresist image. In the case of negative toned developing method, however, the targeted photoresist portion has to be fully reacted, or its dissolution properties in the solvent has to be fully changed in order to remain after solvent developing. As a confirmation of the model, after developing, the profile of the NTD resists will shrink in thickness and size. This indicates that, for the same amount of light, there will be more chemical amplification in NTD photoresist than in PTD photoresist. We will present a model based on the above idea.