SBIR Phase I: Advanced Two-Color Interferometer for Turbulence Correction

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9861111 This Small Business Innovation Research Phase I project focuses on reducing the minimum feature size that can be written lithographically to integrated circuits (chips!). Critical dimensions below 0.1 um require improved-precision interferometry to measure the position of the stage that holds the wafer. Although the stability of current laser gauges can be improved to attain a precision of less than 1 nm in constant-density air, these gauges cannot correct for air-density variations (turbulence). As a result, turbulence presently limits overlay precision to 20-30 nm. This imprecision is unacceptably large for 0.1 um design-rule lithography, for which scaling estimates show that the stage-position measurement should contribute no more than ~2.5 nm to the overlay error budget. We propose to develop a two-color stand-alone laser gauge that measures both the turbulence correction and the absolute stage position simultaneously. A major advantage is that one of the colors can be the He-Ne 633-nm line, which is used in nearly all current commercial laser gauges. This research will lead to a low-cost ( ~$60,000), ultra-compact instrument that will decrease the contribution of the laser gauge to the overlay error to 2 nm. Phase I will prove the feasibility of the two-color technique, and Phase II will be a proof-of-principle demonstration. Since the elimination of turbulence effects on laser-gauge precision will become progressively more critical as the required resolution with which steppers must overlay successive lithographic wafer exposures improves, commercial demand for the proposed instrument by stepper manufacturers is certain to be strong, with an anticipated market of $60 million by the year 2004. The proposed instrument will then facilitate the development of steppers with sub-0.1 um minimum feature sizes and improve their resolution.