An optical heterodyne alignment system used in an SR stepper, the SS-1, is capable of high resolution, but its practical accuracy is affected by multiple reflections between the mask and the wafer. These multiple reflections cause a significant phase modulation when there is an inclination error in the heterodyne optics. In order to attain high alignment accuracy, an opaque film coating and an anti-reflection coating have been applied to an X-ray mask. The multiple reflections between the mask alignment mark and the wafer surface can be reduced by coating chrome films on X-ray mask alignment mark area. The mask distortion of less than 30 nm (3 (sigma) ), caused by forming 500-angstroms thick chrome films on a chip is obtained in the measurement. As a result, an alignment accuracy of 23 nm (3 (sigma) ) is achieved by a double-exposure experiment with the X-ray mask.