A Study on Gap Measurement with Scanning Optical Encoder

Abstract

Precision stage systems for semiconductor manufacturing machines or precision machining tools usually need 6 DOF measurements. In many case these systems have multi beam laser interferometers despite their complexity and high-cost. In recent years, optical linear encoders spread over precision machines, because of their air turbulence tolerance and cost-effectiveness. However the linear encoders are not compatible for the 6 DOF measurements. In order to correct their Abbe errors, the stage rotation measurements are necessary. Moreover the collocation of 3 axes measurement is important because the stage material is not perfectly stiff. In this study we developed a new optical encoder using small scanner. This metrology produces the position information along to the grating surface and the gap information which is perpendicular to the grating surface. This paper provides the concept of gap measurement and some test results. We achieved 1nm resolution using 4μm grating.