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RENESAS factories are starting to use scatterometry for inline focus measurement. This paper presents the development of the method used for nondestructive, high accuracy, and high-speed focus measurement on production wafers. Focus change results in a subtle variation of the photoresist shape, and this phenomenon is parameterized by using a new eight-layer model. Partial least squares regression methods are used to calculate focus from scatterometry measurement results. The measurement error for a focus variation of 0.1 mum is within 30 nm. This method enables the focus offset to be corrected more frequently without increasing the aligner machine downtime and reduces the depth of focus required because of aligner fluctuation. With it, we will be able to get sufficient focus margins for mass production of devices beyond the 65-nm-node devices.
Date of Publication: Aug. 2007