by M. David Levenson, Editor-in-Chief, Microlithography World
In an interview in Milpitas, CA after SEMICON West, Edward Charrier, VP/GM of KLA-Tencor’s process control information division, described the latest improvements in Prolith, the venerable litho simulation tool.
Prolith 11 supports the most likely double patterning option for the 32nm node, “litho-etch-litho etch” (LELE), according to Charrier. “Prior to Prolith 11, computational lithography studies assumed that the two exposure steps could be considered independently,” he reported, “but embedded topography from the first pass can disrupt the second exposure.”
Prolith 11 calculates the electric fields inside the resist/hardmask stack using its extensive catalog of material parameters and the topography inputted by the user, preferably from scatterometry or CD-SEM measurements of the results of the first step. The exposure and development of the second resist film is simulated including the patterned hardmask, substrate topography, and reflectivity. For accuracy at 32nm, EMF effects at the wafer due to the nonuniform film stack have to be included (see figure below). According to Charrier, there are major differences compared to assuming planarity that result in shifts in the patterns.
A comparison of the electric field in the resist film caused by a plane perpendicular wave at the second pass exposure clearly highlights the complexity of introducing topography. (Source: KLA-Tencor)
Prolith has long strived to provide portable resist models that can be plugged into new situations and predict CDs and profiles accurately. Charrier reported major progress in the last few years in resist characterization and modeling, making 1nm accuracy possible through focus and dose. Of course, the physical models used by Prolith are slower than the heuristics employed by OPC engines, but he insists they are now fast enough to use on the small clips of circuit patterns of interest to R&D and process engineers. LithoWare, the Linux version of Prolith intended for layout and OPC designers, runs the same models more quickly on clusters of up to 120 processors.
Double patterning undoubtedly leads to increases in complexity and cost, but now computational tools are emerging to help with the decision-making. One can only hope that they achieve the same “predictive accuracy” and ease of use that characterized Prolith in previous eras. — M.D.L.