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 focus and tilting of exposure tools and production wafers. The inner and outer bars contain various hole sizes. When defocused, the shrinkage of the smaller patterns is more significant than that of the larger ones, thus causing the center of gravity to shift. Through the organization of the bar patterns, the centers of inner and outer bars shift in opposite directions when defocused. An overlay measurement tool can be used to easily measure the shift between the centers of inner and outer bars. Therefore, a second-order polynomial equation can precisely fit the measured BIB shift. In addition, an accurate and reliable focus value can be obtained with a maximum error of less than 0.05 μm by simply differentiating the fitting equation. The novel BIB has many applications, such as measuring field curvatures for exposure tools and determining best focus related information for production wafers.