Focus measurement with a simple pattern design

The increasingly smaller depth of focus of advanced lithographic tools requ ires that the position of best focus be determined to ensure accuracy and e fficiency. We present what we believe is a novel bar in bar that is drawn o n a conventional chrome binary mask to translate focal errors into center-t o-center shifts of outer and inner bars. An overlay measurement tool can ea sily measure this shift. A symmetrical center-to-center shift against best focus is created during defocus, and this shift can be well fitted by a sec ond-order polynomial equation. Simply differentiating the fitted equation l eads to an accurate and reliable focus value, with a maximum error of less than 0.05 mum. The proposed technique can also be employed to evaluate the tilt, field curvature, and astigmatism of advanced lithographic tools. (C) 2001 Optical Society of America.