Microbeam high-resolution diffraction and x-ray standing wave methods applied to semiconductor structures

A new approach to conditioning x-ray microbeams for high angular resolution x-ray diffraction and scattering techniques is introduced. We combined focusing optics (one-bounce imaging capillary) and post-focusing collimating optics (miniature Si(004) channel-cut crystal) to generate an x-ray microbeam with a size of 10μm and ultimate angular resolution of 14μrad. The microbeam was used to analyse the strain in sub-micron thick InGaAsP epitaxial layers grown on an InP(100) substrate by the selective area growth technique in narrow openings between the oxide stripes. For the structures for which the diffraction peaks from the substrate and the film overlap, the x-ray standing wave technique was applied for precise measurements of the strain with a d/d resolution of better than 10−4. High angular resolution x-ray diffraction (HRXRD) and scattering techniques have been among the major characterization tools for the semiconductor industry for more than two decades, providing important information about strain, composition, mosaic structure and defect density in thin epitaxial layers [1] and, in the case of the x-ray standing wave (XSW) method, about interface atomic structure and the location of doping and absorbate atoms relative to the host lattice [2]. In general, these techniques require an incident x-ray beam with an angular divergence less than the width of the Darwin reflection curve, i.e. <50μrad for perfect crystals. Recent advances in technology towards further miniaturization of active electronic elements make possible production of semiconductor structure with dimensions from several microns (e.g. lasers and modulators in modern optoelectronics) to several nanometres (quantum wires and dots). Effective control of the main characteristics of these structures requires adequate characterization tools, i.e. x-ray techniques combining both high spatial and high angular resolution. Sub-micron size beams have become available recently at synchrotron radiation (SR) facilities due to the remarkable progress in x-ray focusing by using a variety of different focusing optics [3]. However, any focusing optics necessarily creates convergent x-ray beams with convergence angles of a few milliradians, deterring the development of microbeam high angular resolution techniques. There are several approaches to overcome this problem. The first one is based on using a pinhole of a few microns size and on taking advantage of the excellent angular collimation of the beam provided by 3rd generation SR sources [4, 5]. The pinhole, however, significantly deteriorates the angular resolution due to diffraction effects, and the intensity of the microbeam is limited by the flux density of the incident beam. A highly collimated monochromatic x-ray microbeam of 7μm by 5μm with both vertical and horizontal divergence of 5 to 7μrad was produced by collimating an undulator beam by slits and compressing it further with multi-crystal optics [6]. 0022-3727/04/040009+04$30.00 © 2004 IOP Publishing Ltd Printed in the UK L9