Sub-Nanometer Resolution for the Inspection of Reflective Surfaces Using White Light

The quality control of highly reflective surfaces requires a measurement method which is able to resolve the surface shape in the nanometer range. Different methods have been developed in the past, e. g. based on interferometry or by tactile coordinate measurement machines. However, most of them do not match the industrial need for a fast method which is insensitive to environmental disturbance. The newly developed method using the reflection of fringe pattern by the surface under test, and therefore called "Fringe Reflection Technique (FRT)", overcomes the difficulties of known measurement methods. In this method a pattern of straight fringes is generated by a monitor. The mirrored pattern is observed by a camera via the object surface under test. Any deviation of the surface against the ideal, i.e. the mathematically accurate surface will yield a distortion of the pattern. This distortion is analyzed by an image processing system, called the Fringe Processor. The surface topology is delivered by local surface gradients which can be integrated to object shape or differentiated to local curvature. The resolution of the system can be adapted to the measurement requirements in a wide range from micrometer down to sub-nanometer. Anyhow, the system is stable against environmental disturbances. It works without vibration isolation in rooms without any climate control. It is possible to measure freeform surfaces with no constraints on object geometry. The measurement of a silicon mirror surface produced by diamond turning in a high precision tool machine serves as one example. The surface shape could be determined with a resolution below one nanometer. The measurements match the results of an interferometer and are better in certain areas.