Development of a metrological atomic force microscope for nano-scale standards calibration

The rapid advancement of nanotechnology is increasingly demanding measurements carried out at nano-scale be more accurate, comparable and traceable to the international standards of units (the SI). The Atomic Force Microscope (AFM) is a very powerful tool for the measurement of surface texture and micro-/nano-structures, with wide applications in nanotechnology. However, the traceability and accuracy of quantitative measurements made by commercial AFMs are often questionable and large discrepancies among them have been reported. This paper describes a metrological AFM developed at the National Metrology Centre (NMC) which has a very large scanning range with nanometre uncertainty. In order to achieve direct traceability to the SI, the system was constructed by integrating an AFM probe with a 3-axis nano-translation stage furnished with high performance autocollimators and laser interferometers along its X, Y and Z axes on a metrological frame. The large scanning range (25 mm X 25 mm X 5 mm) enables the system to be used for surface inspection on much larger samples than those allowed in normal commercial AFMs. Details of the system design and operation will be described in the paper. The uncertainty evaluation was done using certified step height, 1D/2D lateral pitch gratings. The experimental results show that the system is capable of achieving an uncertainty in the order of a few nanometres, which demonstrates that the system is suitable for providing traceability to commercial scanning probe microscopes (SPMs) including AFMs through calibrated transfer artefacts.