Optical EngineeringEffective exposure dose measurement technologies: multitone flood exposure by unresolved gratings with multilevel opening ratio
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Accurate measurement techniques for effective exposure dose in optical microlithography have been developed. The effective exposure dose can be obtained by a dose monitor mark in a photomask, called the effective dose-meter, consisting of plural segments including grating patterns with a pitch below the resolution limit and different duty ratios. Since the effective dose-meter does not resolve on a wafer but makes flood exposure with the dose as a function of the duty ratio, residual thickness of the photoresist after development changes according to the duty ratio. Therefore, the effective exposure dose can be obtained by grasping the duty ratio of the grating patterns in the effective dose- meter corresponding to the position that the photoresist has completely cleared. The effective exposure dose also can be obtained by measuring the residual thickness of the photoresist at a predetermined segment. A calibration technique utilizing an aerial image measurement system also has been proposed to avoid the influence of intrawafer process variation. The effective dose-meter is small enough for the influence of the intrawafer process variation on the accuracy to be ignored. High dose resolution of less than 0.5% can be achieved completely focus free. The variation of the effective exposure dose in a wafer in the current lithography process was measured to demonstrate this technology.