Patent ID: 7646471

Claim:
A plurality of lithographic projection apparatus comprising: a radiation system configured to provide a projection beam of radiation; a support structure configured to support a patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; a substrate table configured to hold a substrate; and a projection system configured to project the patterned beam onto a target portion of the substrate, wherein each lithographic apparatus includes a level sensor comprising: a first reflector positioned to direct light from a light source towards a wafer surface; and a second reflector positioned to direct light reflected from the wafer surface to a detector, wherein the first reflectors of the level sensors of the plurality of lithographic apparatus are substantially identical to one another, wherein the second reflectors of the level sensors of the plurality of lithographic apparatus are substantially identical to one another, wherein the first and the second reflectors are configured to incur a minimal process dependent apparent surface depression.