Advances in the electronics sector, medicine and material sciences have increased the need for inspection tools that are able to resolve fine images of surface topography. Nowadays, the measurement limit reaches values far below 100 nanometer, towards the subnanometre level. For some applications, a high image resolution is sufficient. For industrial applications, however, also accurate quantitative information about the surface is required. Atomic force microscopes (AFMs) are measurement instruments, which are commonly used for the characterisation of surface topography at nanometre level. Commercial devices, however, show significant measurement uncertainty due to thermal drift of the frame and due to creep, hysterisis and nonlinear response of the piezoscanner. Moreover, regular calibration is needed in order to maintain traceability to the length standard.Metrological atomic force microscopes (mAFMs) have a frame of interferometers as a metrology system. Since the wavelength of the interferometer laser source is related to the definition of the metre, these devices are able to produce measurements directly traceable to the length standard. Therefore, metrological AFMs currently represent the primary standard for nanometre level measurements in a number of countries. These devices are able to transfer their traceability to commercial AFMs, used in industry. The calibration procedure typically makes use of transfer standards. These nanostructured samples are measured by means of a mAFM and the commercial AFM of interest. By comparing the images, it is possible to calibrate the commercial AFM. In collaboration with the Federal Public Service Economy, SMEs, Self-Employed and Energy (Metrology - National Standards, SMD-ENS), a mAFM has been developed in this research, with a target accuracy of +/-1 nanometer (coverage factor k=2) over a range of 100 micrometer (in x-,y- and z-directions). The aimed measurement range specifications in x- and y-directions are typical for widely used medium range AFM applications. However, in contrast to most mAFMs with a medium measurement range, the device presented in this research has an increased z-range (100 micrometer instead of 20 micrometer). This makes it suitable for three dimensional measurements, which are gaining importance in nanometrology. Critical components of the presented mAFM also show a highly dedicated design in order to achieve mechanical and thermal stability. Furthermore, the design follows the Abbe principle for obtaining maximum accuracy. An evaluation has been done by means of a detailed uncertainty budget and a series of simulations and experiments.
Table of Contents:
-General design of the mAFM
-Sources of errors
-Component alignment and mAFM controller
-System overview and performance