Calibration of standards for precision pitch measurement in the nanometer region by combined scanning tunneling microscopy and x-ray interferometry

摘要

The aim of the paper is to demonstrate thesuccessful operation of a combination of a scanning tunnelingmicroscope (STM), as a rominent representative of scanningprobe microscopes (SPMs), with an x-ray interferometer (XRI),X-ray interferometry provides a means of calibrating nanometricdisplacement transducers and related instruments using siliconlattice parameters as secondary length standards. On the otherhand, SPMs could be used to calibrate standards e.g. diffractiongratings for dimensional metrology. The extent of themeasuring ranges covered by these instruments is wide, rangingfrom the diameters of atoms to dimentsions of 100 μm.Somespecial instruments, so-called metrology SPMs. Arecalibrated against a laser interferometer and are affected by thelaser interferometer's nonlinearity which is in the range of 1-2nm. This uncertainty is a limit to the SPM and to the samplescalibrated. Further improvement is necessary. If use is made ofthe high resolution of an x-ray interferometer when measuringthe displacement of a sample under an SPM. a higher accuracyshould be achievable and it should be possible to calibratetransfer standards with subnanometre accuracy. Here we report,for the first time, results of pitch measurements obtained for astructure on a sample, not of ideal shape but available for thisexperiment. The translation of the sample and the measurementof its profile were performed by a specially developed scanningXRI which uses the almost homogeneous silicon lattice spacing(Δd/d <10-7) as a (secondary) length standard for subnanometremeasurements (the so-called 'Angstrom ruler').