Flexible and high-resolution surface metrology based on stitching interference microscopy

Abstract

Small optics of 10 mm-scale are widely used in miniature cameras with increasingly sophisticated surface shape. The surface features ranging from low to high frequency need to be measured efficiently for in-process quality control. Wavefront interferometry is applicable with specifically designed null optics for each surface, but not preferred for small optics because the surface radius is approaching the shortest measurable radius limited by the interferometer optics. We present a strategy for flexible surface metrology based on stitching interference microscopy. The full aperture surface is divided into a series of small subapertures with reduced size and slope which are then measurable with a standard interference microscope. The measurement principle is introduced and a method for subaperture design is proposed by checking the subaperture size and slope after coordinate transformation. A simultaneous stitching algorithm is proposed to minimize the overlapping deviations with overlapping points and subaperture configurations determined automatically in a way of alternating optimization. Instrumentation is easy by integrating a multi-axis platform with a commercial interference microscope. Both the lateral and the vertical scales need to be calibrated to give accurate subaperture measurement of three coordinates. The capability of the instrument is finally verified through a series of experiments compared with cross tests such as the null test and the stylus profilometry. It shows high flexibility to measure a variety of complex surfaces including the high-order even asphere, the “gull-wing” surface and microstructured curved surfaces without using any auxiliary null optics. The instrument enables “one-stop” measurement of surface form, waviness and roughness simultaneously.