Focus estimation using scatterometry for 65/45-nm-node hole processes

A highly accurate focus estimation technique that uses scatterometry has been developed for hole photolithography processes. In this technique, scatterometry is used to measure the resist depths, the sidewall angle, and the top and bottom widths of a hole pattern. The relation between these four parameters is then used to estimate the focus and dose by nonlinear regression and the maximum likelihood method. The focus can be estimated very accurately even when the photoresist shape varies nonlinearly with focus and dose. We confirmed that this method can estimate the focus within an error of ±15 nm.