An exposure apparatus exposes a substrate to light via a reticle while the substrate and the reticle are scanned based on information regarding a shape of the reticle. For example, instead of a conventionally used CoWP alloy, a modified alloy may be used, by substituting the cobalt species by a metallic species having a less negative standard electrode potential, such as nickel. When the signal is above a threshold for a continual period of time then the processor issues a trigger signal.