X-ray reflectivity

What is XRR analysis used for?

X-ray reflectivity (XRR), also known as X-ray specular reflectivity or X-ray reflectometry, is a common thin film characterization technique. With XRR, it is possible to determine the thickness, density, and roughness of thin films.

XRR is suitable for the characterization of a wide variety of thin films, including metal oxides, metals and organic films. The investigated films may be deposited using different techniques, including, Chemical Vapor Deposition (CVD), Atomic Layer Deposition (ALD), Physical Vapor Deposition (PVD), and other similar techniques. Both single-layer thin films and multilayer film stacks on any typical substrate, such as silicon (Si), gallium nitride (GaN), silicon carbide (SiC), gallium arsenide (GaAs) or indium phosphide (InP) wafers can be analyzed. X-ray reflectivity is suitable for the characterization of crystalline and amorphous thin films. The properties of the material's surface and the layers' interfaces can be determined.

An example of XRR results is shown in Table 1. The investigated sample is an Al2O3 thin film deposited on a silicon wafer with a native oxide layer. The thickness, density and surface roughness of both layers were determined.

Thickness determination with XRR is generally reliable for films ranging from a few nanometers to 150 nanometers in thickness. For films thicker than 150 nm, density and surface roughness can still be accurately measured, but thickness determination becomes less reliable. A smooth surface enhances the accuracy of XRR measurements, with samples having an RMS roughness below 5 nm generally considered well-suited for analysis.

What is the working principle of XRR?

In an XRR analysis, a sample is irradiated with a beam of X-rays with a very shallow “grazing” angle. When the X-rays hit the sample, they interact with the material's electrons and reflect off the surface. The XRR instrument measures the intensity of these reflected X-rays as the incident angle is changed, creating a plot of reflectivity versus angle (Figure 1).

Each material possesses a specific critical angle determined by its electron density. Below this angle, incident X-rays primarily undergo total reflection with minimal penetration (a few nanometers). Above the critical angle, penetration depth increases substantially, while the intensity of reflected X-rays drops sharply.

For samples with thin films or coatings on a substrate, X-rays reflect not only from the top surface but also from the interfaces between different layers (wherever the material density changes). Because X-rays reflecting from deeper interfaces travel a longer path, they interfere with those reflected from surfaces above them. This interference, where the X-ray waves either reinforce or cancel each other out, creates a characteristic oscillation pattern in the measured reflectivity data (Figure 2).

By analyzing the features of the XRR reflectivity plot, the following can be determined:

• Density - Calculated from the position of the critical angle.

• Layer Thickness - Determined from the spacing of the interference oscillations (Kiessig fringes).

• Surface and Interface Roughness - Inferred from how quickly the overall reflectivity decreases with increasing angle and how damped the oscillations are. A smoother surface leads to a sharper drop after the critical angle and more defined oscillations, while roughness causes a more gradual decrease and weaker oscillations.

Need an XRR analysis?

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