Calibration of standards for precision pitch measurement in the nanometre region by combined scanning tunnelling microscopy and X-ray interferometry

The aim of the paper is to demonstrate the successful operation of a combination of a scanning tunnelling microscope (STM), as a prominent representative of scanning probe microscopes (SPMs), with an x-ray interferometer (XRI). X-ray interferometry provides a means of calibrating nanometric displacement transducers and related instruments using silicon lattice parameters as secondary length standards. On the other hand, SPMs could be used to calibrate standards e.g. diffraction gratings for dimensional metrology. The extent of the measuring ranges covered by these instruments is wide, ranging from the diameters of atoms to dimensions of 100 mu m. Some special instruments, so-called metrology SPMs, are calibrated against a laser interferometer and are affected by the laser interferometer's nonlinearity which is in the range of 1-2 nm. This uncertainty is a limit to the SPM and to the samples calibrated. Further improvement is necessary. If use is made of the high resolution of an x-ray interferometer when measuring the displacement of a sample under an SPM, a higher accuracy should be achievable and it should be possible to calibrate transfer standards with subnanometre accuracy. Here we report, for the first time, results of pitch measurements obtained for a structure on a sample, not of ideal shape but available for this experiment. The translation of the sample and the measurement of its profile were performed by a specially developed scanning XRI which uses the almost homogeneous silicon lattice spacing (Delta d/d < 10(-7)) as a (secondary) length standard for subnanometre measurements (the so-called 'Angstrom ruler').