Photoresist characterization using double exposures with interference lithography
The sinusoidal dose distribution resulting from an IL exposure can be completely defined by the period, , average dose, , and dose modulation, .
In this top view of the double-exposure process, the diagonal lines represent the intensity maxima of the two separate exposures. The coordinate system is defined after the exposures relative to the orientation of the two exposures.
Typical double-exposure result for PS4 with . In the high modulation region where , the modulation is sufficient for lines to develop fully. In the low modulation region where , the modulation too low for lines to develop. The length of the developed region, , and can both be measured as marked.
Comparison of duty cycle-average dose results for both single and double exposures with PFI-88 (top) and PS4 (bottom). Excellent agreement is seen between the results of single and double exposures. The results for PFI-88 were fit to a simple model, and the dose normalized based on the fit, , with , . The results for PS4 were fit to a modified binary model, , using , , . Exposure dose for both resists was normalized by dividing by .
The minimum dose for which lines will develop as a function of dose for PS4 photoresist. The required dose modulation varies with spatial period. At 500 nm, the minimum modulation follows the predictions of the binary resist model.
It is possible to directly identify the dose where linewidth varies little with changes in dose modulation. Here, in a 500 nm period PS4 double exposure, the linewidth can be seen to be constant from to about .
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