Development of a metrological atomic force microscope with minimized Abbe error and differential interferometer-based real-time position control

A metrological atomic force microscope (mAFM) has been developed at LNE. It will be dedicated to traceable dimensional measurements and calibrations of transfer standards with a maximum size of 25 mm x 25 mm x 7 mm. The displacement range is 60 mu m for the X and Y axes and 15 mu m for the Z axis. The instrument uses four laser differential interferometers in an original Abbe-compliant arrangement to measure the position of the tip relative to the sample and to be directly traceable to the SI. The expected uncertainty for the measurement of the tip/sample relative position is 1 nm for the whole range without taking into account the tip contribution. To fulfill this specification, the design of the instrument has been optimized to minimize Abbe errors, to reduce the metrology loop length, to limit the drifts due to thermal dilatation and to improve the stability of interferometric position measurement in ambient air. To limit Abbe errors, a dedicated three-axis flexure stage has been developed to reduce parasitic rotational motion at the level of 1 mu rad for the whole range. This stage is driven by piezo-actuators. The instrument controller is based on a FPGA combined with an embedded PXI controller to perform real-time control of the XYZ position. We present the design of the instrument and the very first results.