A study of AA CD uniformity loading optimization at 28nm node

At 28nm technological node, shallow trench top CD becomes narrower for tighten transistors density. Smaller space CD makes process control more difficult, especially uniformity control. On the early stage of our STI etching process developing, dense/ISO structure has opposition within wafer CD distribution map, which leads to device compromise. From our study, dense/ISO uniformity loading comes from accumulation of the whole pattern transfer process, mainly DARC/APF etching step. The etch scheme is studied in this paper, focused on PR remain, DARC profile integrity optimization and soft mask(APF) profile control, which can affect loading greatly and need precisely control.