Optical Response Near the Soft X-Ray Absorption Edges and Structural Studies of Low Optical Contrast System Using Soft X-Ray Resonant Reflectivity

Fine structure features of energy-dependent atomic scattering factor near the atomic absorption edge, are used for structural analysis of low-Z containing thin film structures. The scattering contrast undergoes large and abrupt change as the incident photon energy approaches the natural frequency of the atom and is sensitive to variation in atomic composition and atomic density. Soft X-ray resonant reflectivity is utilized for determination of composition at the buried interfaces with subnanometer sensitivity. This is demonstrated through characterization of Mo/Si multilayers near Si L-edge. We also demonstrate the possibility of probing variation of atomic density in thin films, through the characterization of Fe/B4C structure, near B K-edge. Sensitivity of soft X-ray resonant reflectivity to native oxide is demonstrated through characterization of BN films near B K-edge. 1. Introduction Hard X-ray reflectivity (XRR) probes electron density profile (EDP) and is an important nondestructive research tool to probe interfaces in thin film multilayer structure [1]. The large dynamic range and high momentum scattering vector range of XRR (incident X-ray energy is far away from absorption edge) allow determination of structural parameters such as thickness, roughness, density, and interface interdiffusion profile in thin film structures with high electron density contrast. XRR determines electron density of a layer. Using XRR measurements it is difficult to extract the layer composition from the electron density for following conditions: (i) layers in a multilayer containing an unknown amount of impurity, usually incorporated during film deposition, (ii) layers contain several components (e.g., C impurity in a Si layer) with nearly equal electron density, (iii) formation of low electron density contrast compound at the buried interfaces, (iv) formation of native oxide and change in compositions of oxide, with poor electron density contrast, (v) significant change in porosity of a layer. For example, in a Pt/C multilayer, even a 15% change in the electron density of the C-layers does not produce a significant change in XRR [2]. In high reflectivity X-ray multilayer structures, where strong standing wave fields are generated near the Bragg angle, XRR is combined with X-ray standing wave (XSW) to address this problem. Fluorescence from different elements in the multilayer is simultaneously measured along with the reflectivity. Combined XSW-XRR can determine interface roughness and layer composition. Composition can be determined [3] with a precision of ~2% with