What is X-ray reflectometry (XRR) and why is it important?

Precise analysis of thin film thickness and surface roughness is essential to enhancing the electrical, optical, mechanical, and chemical properties of advanced materials.

However, many thin film analysis methods have severe limitations for researchers:

• Ellipsometry is unsuitable for metals analysis
• SEM and TEM require cutting of samples
• TEM can be slow and come with high operating costs

X-ray reflectometry (XRR) offers a powerful alternative: a rapid, non-destructive technique for analyzing thin films across a wide range of materials, including metals and amorphous layers.  Combined with grazing incidence X-ray diffraction (GI-XRD) on Aeris, XRR analysis can give researchers a more comprehensive view of their materials, streamlining thin film research and development (R&D).

In this blog, we’ll discuss what XRR is and how it can be used to accelerate thin film R&D. First, what is XRR and what is it used for?

What is XRR?

XRR, or X-ray reflectometry, is an analytical technique used in chemistry, physics, and materials science to characterize surfaces, thin films, and multilayers. It works by directing X-ray beams at a flat surface at a very shallow incidence angle and measuring the intensity of the X-rays reflected off the surface in the specular direction, where the reflected angle is equal to the incident angle.

At every interface where electron density changes, such as the boundary between a thin film and its substrate, or between two different film layers, a portion of the X-ray beam is reflected. The interference of these partial reflections creates a characteristic oscillation pattern in the reflectivity curve.

If an interface is perfectly sharp and smooth, the reflected intensity will align with the theoretical prediction for a perfectly flat, abrupt interface according to the law of Fresnel reflectivity.

If it is not perfectly sharp and smooth, the deviations from this prediction can be analyzed to form a depth profile of electron density through the film, revealing layer thicknesses, interfacial roughness, and density.

What is XRR used for?

XRR is used primarily in research and development within materials science to characterize the structural and physical properties of thin films and surfaces, helping to optimize material selection and process development.

It can also be used during quality control (QC) for specialized materials such as semiconductors; for instance, ensuring exact layer tolerances for transistors, memory chips, and optical coatings.

XRR analysis is relevant for a broad range of applications, including:

• Semiconductors and electronics
• Energy and functional materials
• Optical and protective coatings

Within these workflows, XRR analysis is used for three main functions.

1. Measuring film thickness

Film thickness directly impacts a thin film’s physical, electrical, and optical properties. This is especially important to monitor in microelectronics, where even sub-nanometer variations can alter performance, conductivity, and reflectivity.

For example, if the oxide layer on a microchip is too thin, charge leakage increases, raising power consumption and device temperature, risking device failure. If it’s too thick, the transistor switches too slowly, slowing down the device.

2. Determining material density

Alongside composition, thin film density is a key factor that influences its refractive index and optical behavior. Denser films are needed for precision optical applications like anti-reflection coatings and laser mirrors, while less dense films may contain microscopic voids or defects that disrupt electrical performance and contribute to resistivity.

Density also affects mechanical strength, adhesion, and resistance to corrosion, making it an important parameter to monitor – for instance, when developing barrier coatings.

3. Evaluating surface roughness

Surface roughness affects many key properties of thin film materials: firstly, conductivity. Excessive roughness can cause increased electron scattering, raising resistivity, which can increase contact resistance in battery electrodes.

Roughness is also important to control when determining a material’s optical properties. Surface roughness scatters light in all directions, which can be helpful in applications such as architectural glass and certain display screens. By contrast, a very smooth surface prevents light from scattering, enabling thin film stacks to reflect specific wavelengths efficiently, which is ideal for high-reflectivity mirrors.

Finally, controlling surface roughness can help fine-tune adhesion by providing greater surface area and mechanical interlocking for adhesive coatings; it can also help optimize corrosion resistance.

What is the difference between XRD and XRR analysis?

XRD (X-ray diffraction) and XRR analysis are complementary techniques that can both be achieved on an advanced X-ray diffractometer like Malvern Panalytical’s Empyrean or Aeris systems. Where XRD identifies the crystal structure and phases of a material, XRR measures surface characteristics like thin film thickness, density, and roughness.

Unlike XRR measurement, in which X-rays are directed at a sample at near-zero angles, conventional XRD works by firing X-rays directly at a crystalline sample. These X-rays penetrate deeply and are diffracted by repeating parallel planes of atoms before scattering at specific angles, with the wavelengths aligning in phase – a process called constructive interference.

The detector measures the intensity of these scattered X-rays and sends it to software that plots it as a graph of peaks, creating a unique “fingerprint” of the material’s internal structure and how atoms are arranged within the crystal lattice.

What is grazing incidence XRD (GIXRD)?

Grazing incidence XRD, or GIXRD, is a type of XRD analysis that is adapted specifically for characterizing the top layer of a material. Where conventional XRD probes deeply into bulk materials and powders, GIXRD uses a fixed, shallower incident angle, restricting X-ray penetration to the top few nanometers.

This type of XRD analysis is ideal for analyzing thin films and surface coatings and can also be achieved using the Aeris system.

XRR vs. GIXRD vs. XRD: Summary table

XRR and XRD are different but complementary X-ray technologies used to probe the characteristics of advanced materials. Here’s a quick breakdown of the differences between XRR vs. XRD and GIXRD.

	XRR	GIXRD	Conventional XRD
Measures	Thin film thickness, roughness, density	Crystal structure, phase, and strain in surface layers and thin films	Crystal structure, phase, crystallite size, and strain in bulk materials
Sample state	Crystalline, amorphous, liquid	Primarily crystalline thin films and polycrystalline solids	Primarily crystalline powders, polycrystalline solids, and single crystals
Incident angle range	Very low, typically 0.1°–5°	Fixed and shallow, typically 0.2°–1.0°	Varies dynamically during scan, e.g. 2°–70°
Tells you	Layer thickness, density, surface roughness	Crystal phase and structure of the film or surface layer	Crystal phase and structure of the bulk material

3 benefits of using XRR on Aeris

Aeris is the compact XRD system already trusted by labs worldwide to deliver rapid, accurate results. Now, Aeris is available with integrated XRR analysis capabilities, able to measure thin film thickness (1–200 nm), density, and surface roughness with ease and efficiency.

This brings many benefits to your lab, including:

1. Fast, alignment-free XRR that anyone can use

With no alignment, no dedicated sample stage, and no specialist training required, Aeris enables any researcher to run XRR measurements immediately, reducing user errors and keeping your workflow moving.

2. Non-destructive analysis that won’t break your budget

XRR is a non-destructive analytical technique that doesn’t require sectioning or depth profiling of your samples. This is especially useful when working with precious metals, rare substrates, or limited deposition runs.

XRR capabilities can be added to your existing Aeris system provided that it is equipped with a PIXcel^3D detector, allowing you to protect your investment and increase the variety of analysis that your compact XRD system can perform. There’s no need for a separate XRR system, reducing cost and training burden.

3. High throughput, by design

Compatible with Aeris’ 6-position sample changer, XRR on Aeris supports automated batch measurements, allowing for fast, actionable results with minimal user interaction, freeing you up to do other tasks

Combine GIXRD and XRR analysis to supercharge your thin films workflow

To get a detailed understanding of the way your thin film materials will perform, you need in-depth insights from a range of analytical techniques. Combining XRR analysis with GIXRD analysis on Aeris characterizes crystallographic phase alongside thickness, density, and surface roughness, helping you build a complete picture of your thin films on a single instrument – all without slowing down your existing workflow.

Find out more about XRR measurements on Aeris here.