Effective exposure dose measurement technologies: multitone flood exposureby unresolved gratings with multilevel opening ratio

Accurate measurement techniques for effective exposure dose in optical micr olithography have been developed. The effective exposure dose can be obtain ed 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-m eter 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 d evelopment changes according to the duty ratio. Therefore, the effective ex posure dose can be obtained by grasping the duty ratio of the grating patte rns in the effective dose-meter corresponding to the position that the phot oresist has completely cleared. The effective exposure dose also can be obt ained by measuring the residual thickness of the photoresist at a predeterm ined 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 t he intrawafer process variation on the accuracy to be ignored. High dose re solution of less than 0.5% can be achieved completely focus free. The varia tion of the effective exposure dose in a wafer in the current lithography p rocess was measured to demonstrate this technology. (C) 2001 Society of Pho to-Optical Instrumentation Engineers.