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Got a Litho Question? Ask the Experts Q How does diffusion during post-exposure bake affect resist linewidth?
A Diffusion during post-exposure bake (PEB) can serve many purposes, including smoothing away standing waves and allowing acid in chemically amplified resists to migrate to reaction sites. One detrimental affect of diffusion is the possibility for a change in the critical dimension (CD) of the feature. In this respect, diffusion acts a lot like going out of focus: it degrades the gradient of chemical species between the exposed and unexposed areas. Like defocus, its affect on CD depends both on the feature type and the exposure dose. For example, consider a dense array of lines and spaces. When underexposed, going out of focus tends to make the line larger, while the opposite occurs when overexposed. For most isolated lines, going out of focus will always make the line smaller (unless you are seriously underexposed). Increased diffusion has nearly the same effect on the CD for dense and isolated lines. For chemically amplified resists, the picture is more complicated. Increased diffusion may also lead to increased amplification reactions, which tend always to make the lines smaller for a positive resist. If, however, one thought of diffusion as independent from the reactions that occur during PEB for a chemically amplified resist (a useful mental exercise, if nothing else), then the same ideas described above will apply.
Q Can PROLITH be used to calculate the
overlapping process window of horizontal and vertical lines in the presence of astigmatism?
A
PROLITH can certainly simulate the effects of astigmatism on the process window. By entering the coefficients of the 36 term Zernike polynomial, just about any aberration can be simulated. By running a focus-exposure matrix as a simulation set, the process window will automatically be calculated. This can, of course, be repeated for both vertical and horizontal lines (the "rotate mask" feature is very convenient for this purpose). However, PROLITH cannot be used to overlap the two simulated process windows. Klarity ProDATA, KLA-Tencor’s lithography data analysis software solution, is needed to accomplish this. A simple drag and drop operation can be used to take the simulated focus exposure data and add it to Klarity ProDATA. Repeating this step for both horizontal and vertical lines will put both data sets into ProDATA. Then, a data group with these two data sets can be created in Klarity ProDATA and the overlapping process window feature can be selected.
Do you have a lithography question? Just e-mail lithocolumn@kla-tencor.com and have your questions answered by Chris Mack or another of our experts.
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