On-target fluence variation in a multi-magnification laser ablation system

An algorithm of calculating fluence on-target in a mask-projection ablation system using an excimer laser will be reported. The algorithm extrapolates and obtains fluencies at multiple magnifications from one measurement at one magnification. To further enhance the capability, one can extend the algorithm to include one full set of fluencies under various pulse energies at one magnification. Our test results illustrate that the correct fluence at any magnification can be simply “dialed in” even if the fluence itself is not directly measured. It provides a practical method of determining the ablation fluence, with acceptable precision, for those end users who need to change magnifications of the optical system frequently. Due to the beam mode variation with respect to high voltage (and thus pulse energy) in an excimer laser, the laser pulse energy and fluence on target are not linearly proportional to each other. Furthermore, the intuitive magnification-squared dependence of the fluence actually breaks down because the beam spatial mode generally is not constant along the beam path. Beam divergence and the effect of illumination-forming lens(es) before the mask are the only parameters included in our algorithm. A best-fit procedure can potentially increase the precision of these two parameters and thus of the algorithm.