Characterization of thin films – what do we want to know?

Published February 1, 2023

Heta Nieminen

Heta Nieminen

Doctoral researcher, University of Helsinki

Thin films are one of the biggest enablers of the high-tech society we nowadays live in. Thin films constitute all the important components of our integrated circuits, provide clean energy solutions, for example in the form of solar cells, and have many applications in optics, for example as anti-reflective coatings. The devices that we can design with thin film technologies provide fast and efficient usage of unbelievably large amounts of data. All this success condenses to the success of high-quality thin film materials.

What are thin films?

A thin film is a thin layer of material on top of a growth surface. The surface is very often a silicon wafer or another thin film. Thin, in the context of this blog text, means lowering one dimension of the layer down to a nanometer scale. This means down to 10E-9 m, in other words, 0.000000001 m. A thin film can be, for example, 5, 20, or 100 nm thick in devices nowadays. For comparison, a sheet of paper is roughly 76,000 to 180,000 nm thick.

As the reader might have guessed, we cannot see the quality of films this thin by eye. Usually, only a color change on the substrate can be observed, and in order to figure out the quality of the film it has to be characterized carefully. For most applications, film thickness, composition, and purity are crucial characteristics to analyze. These characteristics define the electrical and optical properties of the film, which very often provide the correct functionality for the film.

How are thin films analyzed?

Physical qualities, such as thickness, can be obtained via several analysis routes. One very useful tool for determining film thicknesses is x-ray reflectometry (XRR), as it can, in the best cases, also define the density and the roughness of the film. With XRR, also multilayered structures can be analyzed.

By using optical methods, such as spectroscopic ellipsometry (SE), refractive index and other dielectric properties can be obtained quickly, at the same time with the film thickness. Some methods, like energy-dispersive x-ray spectrometry (EDS), measure the characteristic x-ray emission lines of each element and link their intensities to the concentrations of the elements. These can be further converted to film thicknesses by using the density of the film material.

Mass gain on a growth surface can be monitored with a certain super accurate scale, quartz crystal microbalance (QCM), already during the thin film growth. Microscopy techniques, like scanning electron microscopy (SEM), can be used for the thickness evaluation if the sample can be imaged from a side view.

Methods for thin film composition analysis

Another important aspect of film characterization is film composition. Properties of materials change substantially when going from oxides to nitrides or carbides, not to mention metallic thin films. A very accurate way to study film composition is to use time-of-flight elastic recoil detection analysis (TOF-ERDA). This characterization technique can detect all elements on the film with an accuracy of less than 0.5 atomic percent, while at the same time providing elemental depth profiles. Hence, the method is excellent in detecting any possible impurities on or in the film. To some extent less accurate, but perhaps easier to access, EDS can and is commonly used to define film compositions. The technique is more reliable on heavy elements.

When more knowledge of the thin film is desired, the physical ordering of the atoms as well as the chemical environment can be defined. For example, x-ray diffraction (XRD) is widely used for defining the phase of a crystalline material. If the crystallinity is crucial, for example when the ordering of the atoms should match the one of the substrate, low-energy electron diffraction (LEED) can be used for detailed analysis. X-ray photoelectron spectroscopy (XPS) can, on the other hand, specify the chemical environment, or bonding, of each element. XPS is very useful also in detecting byproducts on the film thanks to its low detection limit of less than 1 atomic percent.

Method comparison guidesheet for thin film thickness analyses

In-depth analysis requires a combination of methods

Thin film analysis can provide a very deep state of understanding of the material and its properties. However, this can only be achieved by multiple different methods, rarely available from one analysis laboratory alone. Depending on the case, of course, less information might already be enough, but detailed analysis is especially important during process advancement or whenever new materials are developed.

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