Patent ID: 7482102

Claim:
A photomask having a monitoring pattern configured to obtain information required for adjusting an optical system of a projection lithography tool, the monitoring pattern comprising: a mask substrate; and a first asymmetrical diffraction grating delineated on the mask substrate, configured to generate a positive first order diffracted light and a negative first order diffracted light in different diffraction efficiencies, the first asymmetrical diffraction grating including: a plurality of first probing-phase shifters formed of semi-transparent material, disposed periodically on the mask substrate in parallel, each of the first probing-phase shifters is implemented by a first asymmetrically recessed ridge having a geometry defined by: a strip-shaped first entrance face contacting with the mask substrate, extending along the longitudinal direction of the first asymmetrically recessed ridge, a strip-shaped first exit face opposite to the first entrance face implemented by a top surface of a lower step of the first asymmetrically recessed ridge, the lower step being displaced on one side of the first asymmetrically recessed ridge in a plan view, extending along the longitudinal direction, and a strip-shaped first light-shielding face implemented by a top surface of an upper step of the first asymmetrically recessed ridge, the upper step has a higher horizontal level than the lower step, the upper step being adjacent to the lower step and displaced on another side of the first asymmetrically recessed ridge in a plan view, extending along the longitudinal direction; and an opaque strip disposed on the first light-shielding face for each of the first probing-phase shifters, wherein a phase difference between the light transmitting through the first probing-phase shifters and the light not transmitting through the first probing-phase shifters is an integer multiple of a value larger than zero degrees and lower than 180 degrees, and a width of the first exit face measured perpendicular to the longitudinal direction is 1/R times an interval between the first probing-phase shifters, where R is an attenuation constant of the light leaving from the first exit face, after the light propagates through each of the first probing-phase shifters.