XRR

X-ray reflectivity (XRR) is a non-destructive characterization technique used to study thin films and multilayers by measuring the intensity of X-rays reflected from a surface as a function of incident angle or momentum transfer. By analyzing the specular reflectivity, XRR provides information about layer thickness, density, and interface roughness, as well as the overall structure of layered materials.

XRR relies on the interference of X-rays reflected at electron density interfaces. In terms of momentum transfer qz = (4π/λ) sin θ, the reflectivity pattern contains oscillations known as Kiessig fringes, whose period relates to the total film thickness, while the critical angle depends on the mean electron density. The data are commonly modeled using Parratt recursion or related formalisms, with roughness incorporated via Nevot-Cudahy factors to account for interface roughness.

Experimentally, XRR is performed with a collimated X-ray beam and a goniometer to vary the incident angle in specular geometry (θi = θf). Detectors record reflected intensity versus angle. Common sources include Cu Kα radiation (λ ≈ 1.54 Å) and sources based on Mo or Ti targets. In-situ XRR can monitor film growth and interface formation during deposition, enabling real-time assessment of layer formation and roughness evolution.

Applications of XRR include characterization of metal, oxide, polymer, and organic multilayers, measurement of film density and interfacial roughness, and assessment of interface abruptness. XRR is widely used in semiconductor, data storage, and coating industries, and it complements other structural techniques such as X-ray diffraction and complementary reflectivity measurements at off-specular angles.