Within the Dutch standards laboratory (NMi Van Swinden Laboratorium) a traceable atomic force microscope (AFM) is currently being developed where we aim at a measurement uncertainty of 1 nm. As part of this development novel methods have been developed for the calibration of this instrument. The SPM has been constructed using a commercial AFM head that has been embedded in a metrology frame using an accurate 3D translation stage. The position of the AFM probe relative to the sample is determined along three orthogonal measurement axes by three individual laser interferometers. Due to the properties of the 3D translation stage the Abbe offset between the probe position and the measurement axes should remain below 0.1 mm in order to realize an overall measurement uncertainty of 1 nm. Since the laser interferometers use a four-pass optical configuration the measurement axis is defined by the virtual centre of the four positions of the laserspots onto the plane retro mirror. In order to accurately determine this virtual centre and therefore the position of the measurement axis a device has been designed to measure the four laserspot positions of each axis within the SPM. Since the AFM probe position is determined in 3D accurate calibration of the angles between the measurement axes is important if the measurement error is to be minimized to 1 nm. Different calibration setups to accurately determine these angles will be presented. Additionally, a method for the alignment of the optical axes of the laser interferometers with respect to the end mirrors of the interferometers will be discussed. In order to calculate the overall measurement uncertainty of the scanning probe microscope we decided to use the virtual measurement machine concept since a purely analytical approach for these types of measurement instruments is nearly impossible (Schwenke et al 2000 Ann. CIRP 49 395–8). The model or virtual SPM that has been developed for the simulation of the instrumental characteristics will be presented.
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