Phase control in multiexposure spatial frequency multiplication

Multiexposure spatial frequency multiplication is a technique that allows the spatial frequency of grating patterns to be increased by integer factors 2,3,4,… by applying a nonlinear development process between patterning steps. One of the main technical issues with this technique is how to accurately place subsequent patterns on a substrate with respect to previously established patterns, which is referred to as phase control of the overlay. The authors report a technique that achieves accurate phase control over large areas during spatial frequency multiplication by utilizing a surrounding alignment grating. Three key factors—the angle, period, and phase of the alignment grating—have been accurately measured and utilized to position subsequent patterns with respect to previous patterns. Some factors that can dramatically diminish the accuracy of phase control, such as particle-induced substrate distortion and nonlinear distortion of the alignment grating, have also been considered and minimized in order to improve the accuracy of phase control. For spatial frequency doubling with a 574 nm principal pitch, the authors achieved overlay phase errors with a mean of −1.0 nm±2.8 nm(1σ) between level 1 and level 2 grating patterns over a 25×32.5 mm2 area.