The reliability improvement on self-aligned contact CMOS by optimizing poly etching

The advanced 2 μm self-aligned contact CMOS IC with ONOP (oxide-nitride-oxide-polysilicon) structure has similar device dimensions to conventional 1 μm CMOS ICs. A special isolation method of sidewall oxidation with a nitride cap layer is used to isolate the poly and metal. The minimum design rule for contact-to-poly distance is zero, which allows the designer to shrink this CMOS device dimension to near the 1 μm conventional CMOS dimensions, even though the 2 μm process has been used. However, the poly profile must be well controlled in this process and must remain sharp after poly etching, or the top corner of the isolated nitride cap will be etched, resulting in nitride cracking along the top corner of the poly sidewall. A two-step poly profile is shown to be the major cause of isolated nitride cap cracking. In the research, an in-situ poly etch for the ONOP structure was developed which can obtain a sharp poly profile. According to our TSUPREM4 simulation and experimental data, the poly can not be exposed outside the ONO structure over a certain range, or isolated nitride cap cracking will occur