Focusing technique suitable for use with an unpatterned specular

Abstract

A microlithographic stepper, employing a Half-Field Dyson projection optical system, achieves focusing of an image of a first-layer reticle pattern on a completely unpatterned reflective wafer surface by including a repetitive diffraction pattern on the reticle which has a configuration in which a particular ordinal diffraction order is normally missing. In response to being simultaneously illuminated with each of two incident monochromatic beams of light, diffraction orders generated by the repetitive diffraction pattern are imaged on the reflective wafer surface and then reflected back to and reimaged on the repetitive diffraction pattern on the reticle. Diffraction orders generated on the first encounter with the repetitive diffraction pattern generate light in the originally missing ordinal diffraction order on the second encounter. If more than one diffraction order from the first encounter contribute toward generating this missing-order light, then the light intensity of the originally missing order will depend on focus position of the wafer. Best focus will generate either maximum or minimum light intensity of the originally missing order. Using a single wavelength, a single repetitive diffraction pattern period, and a single incidence angle, provides a limited unambiguous focus range. However, by using a second wavelength, a second repetitive diffraction pattern period, or a second incidence angle, the unambiguous focus range is substantially extended. Each of these separate incident illuminating beams gives rise to its own set of reflected diffraction-order light. Thus, each of the two monochromatic beams of light gives rise to reflected diffraction-order light from each of two sets that occupy the position of the normally-missing particular ordinal diffraction order. Detection of interference maxima and minima in the intensity of this light, derived either by moving the wafer surface through in-focus or by electro-optic means responsive to this light being orthogonally polarized, permits best focus to be achieved.