Key points in 14 nm photolithographic process development, challenges and process window capability

After 20 nm, the 14 nm process has become a major technology node that may stay in market for some extended period of time. In the 14 nm, 2 major technologies have been introduced to the 193 nm immersion photolithography, i.e., the source-mask co-optimization (SMO) and the negative tone developing (NTD). The former will maximize the process window for a given set of design rules, while the latter will further improve the process window through the utilization of bright field imaging together with the use of attenuated phase shifting mask (Att-PSM). Though SMO has a distinct advantage in balancing the process window for various patterns when compared to the parameterized illumination condition, e.g., cross poles or annular, it may cause severe troubles if the input pattern group does not have good representation of all critical designs. Therefore, some trade-offs must be present in order to produce a more accommodating process. The use of negative tone developing, on one hand, has significant advantage in printing vias and trenches through converting the low contrast dark field imaging to the higher contrast bright field imaging. On the other hand, the chemistry of NTD is different compared to the traditional positive tone developing in the fact that it needs more chemical reactions or the acid diffusion-reaction process has to take place on a greater scale. More diffusion-reaction process will mean longer photo acid effective diffusion length, which will damage imaging resolution. This paper will discuss various aspects of 14 nm photolithography process.