Patent ID: 8068213
Filing Date: 2011-11-29
Classification: B64D,G03F

Abstract:
1. A method for projecting an image of a photomask on a wafer, comprising: installing a photomask having a device pattern and an asymmetrical diffraction grating implemented by probing-phase shifters into a projection lithography tool, the asymmetrical diffraction grating being configured to generate a positive first order diffracted light and a negative first order diffracted light in different diffraction efficiencies, the asymmetrical diffraction grating including a plurality of probing-phase shifters formed of semi-transparent material, disposed periodically on the mask substrate in parallel, each of the probing-phase shifters being implemented by an asymmetrically recessed ridge, a geometry of the asymmetrically recessed ridge being defined by: obtaining image information of projected images of the asymmetrical diffraction gratings on monitoring wafers, by varying the positions of the monitoring wafers along an optical axis of an optical system of the projection lithography tool; calculating an approximate function prescribing a relationship between the positions of the monitoring wafers along the optical axis and the image information; calculating a most suitable focal position for a working wafer; adjusting a position of the working wafer to the most suitable focal position in the optical system; and projecting the device pattern on the working wafer, while keeping the adjusted position of the working wafer, wherein a phase difference between light transmitting through the probing-phase shifters and light not transmitting through the probing-phase shifters is integer multiples of a value larger than zero and lower than 180 degrees, and a width of the exit face measured perpendicular to the longitudinal direction is 1/R times an interval between the probing-phase shifters, where R is an attenuation constant of light leaving from the exit face, after the light propagates through each of the probing-phase shifters.