Calibration of high-resolution X-ray tomography with atomic force microscopy

For two-dimensional x-ray imaging of thin films, the technique of scanning transmission x-ray microscopy (STXM) has achieved images with feature sizes as small as 40 nm in recent years. However, calibration of three-dimension al tomographic images that are produced with STXM data at this scale has no t yet been described in the scientific literature, and the calibration proc edure has novel problems that have not been encountered by x-ray tomography carried out at a larger scale. In x-ray microtomography, for example, one always has the option of using optical imaging on a section of the object t o verify the x-ray projection measurements. with STXM, on the other hand, t he sample Features are too small to be resolved by light at optical wavelen gths. This fact implies that one must rely on procedures with higher resolu tion, such as atomic force microscopy (AFM), for the calibration. Such proc edures, however, generally depend on a highly destructive sectioning of the sample, and are difficult to interpret because they give surface informati on rather than depth information. In this article, a procedure for calibrat ion is described that overcomes these limitations and achieves a calibratio n of an STXM tomography image with an AFM image and a scanning electron mic roscopy image of the same object. A Ge star-shaped pattern was imaged at a synchrotron with a scanning transm ission x-ray microscope. Nineteen high-resolution projection images of 200x 200 pixels were tomographically reconstructed into a three-dimensional imag e. Features in two-dimensional images as small as 40 nm and features as sma ll as 80 nm in the three-dimensional reconstruction were resolved. Transver se length scales based on atomic force microscopy, scanning electron micros copy, x-ray transmission and tomographic reconstruction agreed to within 10 nm. Toward the center of the sample, the pattern thickness calculated from projection images was (51 +/- 15) nm vs (80 +/- 52) nm for tomographic rec onstruction, where the uncertainties are evaluated at the level of two stan dard deviations.