A method, structure, system of aligning a substrate to a photomask. The method comprising: directing light through a clear region of the photomask in a photolithography tool, through a lens of the tool and onto a set of at least three diffraction mirror arrays on the substrate, each diffraction mirror array of the set of at least three diffraction minor arrays comprising a single row of mirrors, all mirrors in any particular diffraction mirror array spaced apart a same distance, mirrors in different diffraction mirror arrays spaced apart different distances; measuring an intensity of light diffracted from the set of at least three diffraction mirror arrays onto an array of photo detectors; and adjusting a temperature of the photomask or photomask and lens based on the measured intensity of light.