A scanner of photolithographic equipment has a reticle masking device capable of forming an aperture, in the shape of a slit, without inducing vibrations in the scanner. The reticle masking device forms the slit electronically, without the use of motive power, and can likewise vary the width of the slit in a direction parallel to the direction of movement of a reticle stage. In particular, the width of the slit is increase at the beginning and decreased at the end of a scan. Preferably, the reticle masking device is a liquid crystal display. It is thus possible to isolate and compensate for vibrations induced by the movement of the reticle stage or wafer stage during a scan, and thus to prevent flaws in the exposure process due to relative positional errors and thereby enhance the production yield.