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A thin film is defined as a layered material between a sub-nanometer and several micrometers and is represented in all kinds of coatings from conformal and spray to atomically thin coatings produced by techniques such as atomic layer deposition (ALD). Given that thin films can be created in a number of ways and are present in many applications, it is only natural that there are many ways to characterize them. In this article, we take a look at a few of them
X-ray Diffraction (XRD)
XRD is an excellent technique for studying thin films with a crystalline structure, such as those composed of inorganic/solid-state materials. XRD is a technique used to determine the crystalline structure and atomic spacing of a thin film, and the produced spectral pattern is compared against known references.
XRD fires collimated X-rays at a crystalline sample, and the light becomes diffracted according to Braggs law by the crystal planes within the thin film. The X-rays are produced by a cathode source and are monochromated. Upon hitting the planes of the thin film, constructive interference is created, causing the sample to be scanned through an angle of 2θ and allowing all of the dimensions of the film to be observed.
Energy Dispersive Analysis of X-rays (EDAX)
Energy Dispersive Analysis of X-rays (EDAX or EDS), also known as electron probe micro-analysis (EPMA), is the most common non-destructive technique used to analyze a thin film’s composition. They are often used in conjunction with, and incorporated into, SEM and TEM instruments.
EDAX is an X-ray technique that identifies the elemental composition of a thin film. EDAX produces a spectrum which shows a range of peaks where each peak corresponds to the elements of the thin film.
EDAX fires a beam of X-rays onto the thin film and excites the electrons from the ground state and ejects them out of the nucleus of the different elements. The ejection of the electron creates a hole in its place. The hole is then filled by an electron from a higher energy level, and the difference in energy between the higher and lower states is emitted as an X-ray. The X-rays are then measured by an energy-dispersive spectrometer that can deduce the elemental composition from the emitted energy values.
Scanning Electron Microscopy (SEM)
SEM is a tool used to characterize the morphology and composition of thin films. Electrons are fired from an electron gun and pass through the thin film. The energies of the electrons are concentrated and focused on using a series of lenses. The electron beam then passes through a pair of scanning coils and deflector plates in the final lens. After the electrons are focused, they are then directed towards the thin film.
When the electrons interact with the sample, their energy diminishes due to scattering and absorption. The exchange of energy between the electrons and the sample causes high energy electrons to reflect through elastic scattering. Secondary electrons are also released by inelastic scattering and the emission of electromagnetic radiation, both of which are detected. The image is a distribution of the signal intensity, which is digitally captured and allows for the structure of the thin film to be determined.
Transmission Electron Microscopy (TEM)
TEM is similar to optical microscopy, in that it uses a series of mirrors to take an image of the surface of a thin film. However, it possesses a much higher magnification than optical microscopes can produce.
In a TEM, electrons fired from an electron gun and accelerated towards the sample (positioned on a copper grid) at high-voltages using condenser lenses. TEM is a useful technique for thin films, as any sample in a TEM is required to be thin for the process to work. The scattered electrons are transmitted to form a diffraction pattern in a back focal plane. A magnified image is also produced in an image plane. TEM uses additional lenses so that the pattern can be projected on to a fluorescent screen, which can then be viewed. The images produced can yield important information about the structure and composition of a thin film.
Atomic Force Microscopy (AFM)
AFM is a versatile technique and can be used to determine the chemical structure, morphology, and growth of a thin film. Thin films of varying composition can be characterized due to the different number of tips (attached to a cantilever) and modes that can be employed.
As the tip approaches the surface of the thin film, the van der Waals attraction between the film and the tip causes the cantilever to move towards the film and tap it. A laser beam is used to detect any movement from the cantilever. The laser beam deflects off of the cantilever as it moves, and the positional change is recorded by the laser hitting a position-sensitive photo-diode (PSPD). AFM uses a feedback loop system to generate a high-resolution topographic map of the thin film, once all deflections of the cantilever have been measured.
Sources and Further Reading
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