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Specular X-ray Reflectivity (XRR)

Specular X-ray reflectivity (XRR), a technique related to X-ray diffraction (XRD), is becoming a widely used tool for the characterization of thin-film and multilayer structures. X-ray scattering at very small diffraction angles allows characterization of the electron density profiles of thin films down to a few tens of Ǻ thick. Using a simulation of the reflectivity pattern, a highly accurate measurement of thickness, interface roughness, and layer density for either crystalline or amorphous thin films and multilayers can be obtained. No prior knowledge or assumptions regarding the optical properties of the films are required, unlike optical ellipsometry

Main Applications of Specular X-Ray Reflectivity (XRR)

  • Determination of thickness, interface roughness, and density for thin films and multilayers
Ideal Uses
  • Highly accurate film thickness and density measurement
  • Measuring film or interface roughness
  • Measuring film uniformity across wafers
  • Measuring pore density and pore size of low-k films
Technical Specifications

Signal Detected: Reflected X-rays
Elements Detected: No elements are specifically detected, rather the electron density of the layers is measured. This, combined with knowledge of the layer composition, allows an exact determination of layer thickness and density.
Detection Limits: 15-100Ǻ minimum layer thickness
Depth Resolution: ~1% of measured thickness
Imaging/Mapping: Yes
Lateral Resolution/Probe Size: ~1cm

Strengths
  • Whole wafer analysis (up to 300mm) as well as irregular and large samples
  • Mapping of full wafers
  • Conductor and insulator analysis
  • Optical properties of the film are not required to be known for accurate thickness determination
  • Minimal or no sample preparation requirements
  • Ambient conditions for all analysis
Limitations
  • Some knowledge of the expected basic sample structure is needed in order to provide accurate density results (e.g. the order of the layers present and their approximate compositions)
  • Surface and interfacial roughnesses must be lower than ~5nm in order to determine thickness. Surface roughness and density may still be determined on rougher samples
  • Maximum film thickness ~300nm