Patent ID: 7580136

Claim:
A height position detector for a work held on a chuck table, for detecting the height position of an upper surface of a work held on a chuck table, comprising: laser beam oscillating means for oscillating a laser beam; annular spot forming means by which a spot of said laser beam oscillated by said laser beam oscillating means is formed into an annular shape; a first beam splitter by which said laser beam with the spot formed into said annular shape by said annular spot forming means is guided into a first path; a light condenser by which said laser beam guided into said first path is condensed so as to irradiate said work held on said chuck table therewith; a pinhole mask disposed in a second path into which said laser beam reflected by said work held on said chuck table is split by said first beam splitter; a second beam splitter by which said reflected light having passed through said pinhole mask is split into a third path and a fourth path; a first light receiving element for receiving said reflected light split into said third path by said second beam splitter; a second light receiving element for receiving said reflected light split into said fourth path by said second beam splitter; light reception region restricting means which is disposed in said fourth path and which restricts a reflected light reception region where said reflected light is received by said second light receiving element; and control means by which the height position of said upper surface of said work held on said chuck table is determined based on the ratio between the quantity of light received by said first light receiving element and the quantity of light received by said second light receiving element.