Determination of the short wavelength cutoff of interferential and confocal microscopes

For the characterisation of a surface the spatial short wavelength cutoff of the measuring instrument is an important parameter that has to be known, especially when different techniques are to be compared. The definition of the transfer characteristic by a transmission bandwidth with lambda s and lambda c-limits as defined in the GPS-standards for surface contact stylus measurement is transferred here to the optical instruments, which are widely used for the same task as the mechanical instruments. In order to find out the short wavelength limit in the sense of ISO 3274, a set of gratings is used that contains areas with different pitch values all having the same height. The range of pitch values spans from 4 to 800 mu m with a 2-4-8-subdivision. In the first step the user measures the depth of a groove for which the wavelength is well within the transfer band. This is like the use of a depth measurement standard. In the second step that grating section has to be found out, for which the measured depth is half of the long wavelength depth. The specimen is available with a height of 90 nm and nearly 4 mu m. The manufacturing process, which takes the metrological design aspects in account, is described afterwards. After some example measurements with different instruments an outlook to other applications of this set of gratings is given. Especially, the use of this specimen in the frame of ISO standardisation for optical instruments is indicated. (C) 2007 Elsevier B.V. All rights reserved.