The major problems for achieving 1μm technology in optical lithography are mainly caused by the surface topography, reflectivity and defocusing. By using a multilayer resist process, these problems can be resolved. In this paper, the trilayer resist process of AZ 1370/$SiO_2$/AZ 4210 structure is studied. The bottom layer is coated with a thick layer (∼2μm) of AZ 4210 resist and baked over 210℃ for planarization.
The baking of the bottom layer over 210℃ enhance the absorption ratio over 70% with 436 nm (g-line), which can reduce standing wave problem in the top resist layer: Thin intermediate layer (-1700 A˚) is formed with SOG (spin on glass) or PECVD (plasma enhanced chemical vapor deposition) oxide.
The top layer is coated with AZ 1370 for fine pattern delineation. After exposure and development of top resist layer, the intermediate layer is etched by reactive ion etching using $C_2F_6$ and $CHF_3$ gas mixture. The planarized bottom layer is then etched by reactive ion etching in a gas mixture of $O_2$ and $SF_6$ or $C_2CIF_5$.
In defining 1μm features, this process makes good line width control and high aspect ratio over 1μm step, and which is confirmed by Scanning Electron Microscopy.
