A stage apparatus for an e-beam inspection apparatus comprising: an object table (3) comprising an supporting surface, the object table configured to support a substrate (190) on the supporting surface; a positioning device (180) configured to a position the object table; a position measurement system (5) comprising a position sensor (8-10) configured to measure a height position of the object table parallel to a first axis, the first axis being substantially perpendicular to the supporting surface, the position sensor comprising an interferometer measurement system having an interferometer sensor (9, 10, 22), wherein a measurement beam (11, 15) of the interferometer sensor is configured to irradiate a reflective surface (13, 17) of the object table in a measurement direction, the measurement direction having a first component parallel to the first axis and a second component parallel to a second axis, the second axis being substantially perpendicular to the first axis.