title: Focus measurement with a simple pattern design.
abstract: The increasingly smaller depth of focus of advanced lithographic tools requires that the position of best focus be determined to ensure accuracy and efficiency. We present what we believe is a novel bar in bar that is drawn on a conventional chrome binary mask to translate focal errors into center-to-center shifts of outer and inner bars. An overlay measurement tool can easily 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 second-order polynomial equation. Simply differentiating the fitted equation leads to an accurate and reliable focus value, with a maximum error of less than 0.05 microm. The proposed technique can also be employed to evaluate the tilt, field curvature, and astigmatism of advanced lithographic tools.
authors: Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan.
authors: C Y
authors: CY
authors: Ku
authors: T F
authors: TF
authors: Lei
authors: H K
authors: HK
authors: Lin