METHOD AND SYSTEM FOR DETERMINING THE SEPARATION DISTANCE BETWEEN A BODY AND THE SURFACE OF AN OBJECT BY MEANS OF LOW COHERENCE OPTICAL INTERFEROMETRY TECHNIQUES UNDER DISTORTION DUE TO SUB-SAMPLING

摘要

A method and a system are described for determining the separation distance between an object or a material and a processing tool or an instrument for measuring the object or material, comprising:- generating a measurement low coherence optical radiation beam, leading the measurement beam towards the object and leading the reflected or diffused measurement beam from the object towards an optical interferometric sensor arrangement in a first direction of incidence,- generating a reference low coherence optical radiation beam, and leading the reference beam towards the optical interferometric sensor means in a second direction of incidence at a preset angle of incidence with respect to the first direction of incidence of the measurement beam;- superimposing the measurement beam and the reference beam on a common region of incidence of the sensor means;- detecting the position of a pattern of interference fringes between the measurement beam and the reference beam on the region of incidence; and- determining a difference in optical length between the measurement optical path and the reference optical path on the basis of the position of the pattern of interference fringes along an illumination axis of the region of incidence, which is indicative of a difference between (a) the current separation distance between the processing tool or measuring instrument and the surface of the object and (b) the predetermined nominal separation distance,wherein the sensor means comprise an arrangement of photodetectors along the illumination axis, and the angle of incidence is controlled in such a way that the spatial frequency of the pattern of interference fringes is greater than the spatial frequency of the photodetectors.