X-ray Reflectivity (XRR)

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

Ideal Uses of XRR

Highly accurate film thickness and density measurement
Measure film or interface roughness
Measure film uniformity across wafers
Measuring pore density and pore size of low-k films

Strengths

Whole wafer analysis (up to 300 mm) 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 ~5 nm in order to determine thickness. Surface roughness and density may still be determined on rougher samples
Maximum film thickness ~300 nm

XRR 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: 15-100 Ǻ minimum layer thickness
Depth Resolution: ~1% of measured thickness
Imaging/Mapping: Yes
Lateral Resolution/Probe Size: ~1 cm