Non-contacting straightness measurement to nanometre accuracy

Abstract

A straightness reference is established by using a laser beam which is straight to 4 nm over a distance of 300 mm. The straightness reference has been verified by developing a two-dimensional profilometer system that can measure surface flatness and form with a precision of ± 2 nm without reference to master surfaces. This profilometer measures surface slopes with a resolution of 0.001 arc second and scans optical surfaces in non-contacting mode by using a penta-prism. Over a distance of 200 mm line profiles are repeatable to ± 1 nm and are obtained by integrating the slope values with respect to distance. To confirm the performance of the slope integration method, the profiles measured over 100 mm distances have been compared with those obtained by standard interferometric methods and they agree to within ± 5 nm. The slope integration method can be developed further to measure straightness of lathe beds, rollers and float glass sheets over at least 3 m distances using an oblique incidence scanning scheme. Although this scheme is capable of assessing specular and non-specular surfaces, its angular range is limited to about 1° in one dimension. Two-dimensional scanning of surfaces over an area of 500 mm × 500 mm can be catered for with angular variations up to 5 arc minutes. The laser beam straightness reference can be further exploited to develop a hybrid system to perform metrology of surfaces whose profile height can vary by more than 50 mm. This new scheme will employ electro-optical devices to guide a carriage with respect to the straightness reference. The scheme overcomes slide motion errors. The electro-optical controller will be used to guide and control a carriage using the laser beam to form an accurate slide to work over 1 m distances. New stylus and non-contacting probes with large height ranges can be attached to the carriage to perform accurate metrology.