Phase calibration for attenuating phase-shift masks

Abstract

As metrology tolerances for photomasks become increasingly tight, the issues of measurement methods and calibration standards become more and more important. This is as true for optical phase metrology as it is for dimensional metrology. To first order, an optical phase standard can be defined by etching a step or trench into a transparent medium like fused silica with a well-characterized index of refraction. The normal-incidence phase shift of such a structure can be calculated with accuracy limited mainly by our ability to measure the physical depth of the trench. Other methods exist for measuring optical phase shifts in alternating aperture masks. For example a Levenson grating exhibits symmetrical behavior through focus only if the phase difference between the phase shifted and unshifted openings is exactly 180°. A measured asymmetry between the phase shifted and unshifted clear features in such a grating can be used to quantitatively measure the deviation from 180° optical phase. This approach is particularly attractive because the measurement can be made on an aerial image metrology system with the numerical aperture and illumination conditions used by the wafer stepper that will eventually project the mask. Calibration of optical phase in attenuating phase masks is more difficult. Until now there has been no accurate way to create and verify a calibration artifact In this paper we discuss a new phase metrology pattern for attenuating phase masks. The phase error of this pattern can be determined to high accuracy by aerial image measurements. This pattern can be used to create an optical phase standard for calibrating phase metrology equipment for attenuated phase masks, or as a witness pattern on a product mask to verify the phase accuracy of that mask.