Monitoring lithographic focus and tilting performance by off-line overlay measurement tools

In this work we present a novel bar-in-bar (BIB) pattern to monitor the foc us and tilting of exposure tools and production wafers. The inner and outer bars contain various hole sizes. When defocused, the shrinkage of the smal ler patterns is more significant than that of the larger ones, thus causing the center of gravity to shift. Through the organization of the bar patter ns, the centers of inner and outer bars shift in opposite directions when d efocused. An overlay measurement tool can be used to easily measure the shi ft between the centers of inner and outer bars. Therefore, a second-order p olynomial equation can precisely fit the measured BIB shift. In addition, a n accurate and reliable focus value can be obtained with a maximum error of less than 0.05 mum by simply differentiating the fitting equation. The nov el BIB has many applications, such as measuring field curvatures for exposu re tools and determining best focus related information for production wafe rs. (C) 2001 American Vacuum Society.