Thin films are two-dimensional (2D) material layers deposited on a bulk substrate, possessing a thickness of a few nanometers to impart properties that cannot be realized by base materials. The unique properties of thin films result from their preparation method.
Image Credit: ktsdesign/Shutterstock.com
The thickness of a thin film determines its electrical, optical, mechanical, and thermal properties, which are of critical importance in nanomaterials. Hence, it is essential to measure and regulate their thickness in various industries. However, their thicknesses range from a few atoms to microns, making them difficult to measure using conventional methods.
Selection of Measurement Technique
Different techniques can be used to measure the thickness of the materials. The method selection depends on several factors, including the transparency of the material in the optical region, additional information required apart from the film thickness, and budget.
Thus, it is essential to be aware of the nature of the material and thickness range to determine the measurement technique that should be adopted. In addition to the thickness of the thin film, information such as the refractive index (RI), surface roughness, density, and structural properties can be determined based on the method adopted.
Mechanical Methods to Measure Thin Film Thickness
Stylus profilometry and interferometry are two mechanical methods used to measure film thickness. Both methods require the presence of a groove or step between the surface of the film and the substrate.
The grooves are produced either by masking portions of the substrate or by removing parts of the deposited film. In addition, interferometry requires a highly reflective surface to obtain interference fringes. In both methods, the film thickness is measured at a specific point. As a result, the uniformity of the film is a critical parameter that determines the accuracy of thickness.
Non-Destructive, Non-Contact Methods to Measure Thin Film Thickness
Ellipsometry is a technique that facilitates the simultaneous measurement of the thickness of up to 1000Å in thin films and their optical properties (RI and extinction coefficient). This method is widely used in the electronics and semiconductor industries.
Although ellipsometry is a non-destructive and non-contact method, it does not accurately measure the thickness of thin films based on transparent substrates, which are often used in optics because of the difficulty in finding the null point. The only solution to this problem is to grind the back of the substrate, which makes this technique a destructive method. Hence, ellipsometry cannot be used in optics.
Reflectometry measures the intensity of reflected light as a function of wavelength. This method helps obtain spectra for single- and multi-layered thin films. The thickness of a small spot on the substrate can be measured via reflectometry using a microscope.
The intensity of the reflected light as a function of wavelength can have a complex shape in the spectra depending on the film's RI, thickness, and other properties. Evaluating the spectra requires powerful software to extract the desired information.
X-Ray reflectivity (XRR) is used to measure the thickness of multi-layered materials. In addition to determining the thickness and surface roughness of the film, XRR provides information on the thickness and density of the individual layers. This method is suitable for materials with thicknesses of up to 100 nm.
Two prerequisites are required for the successful measurement of thickness via XRR. First, the material thickness should be at least one order of magnitude greater than the surface roughness. Secondly, the composition and sample structure should be known to avoid errors.
Scanning Electron Microscopy
Thickness measurement using scanning electron microscopy (SEM) is suitable for semiconducting thin films ranging from 100 nm to 100 μm. In addition to measuring the thickness of single- and multi-layered thin films, SEM provides information about the elemental composition and surface morphology when attached to an energy dispersive spectroscopy (EDS) detector.
Transmission Electron Microscopy
The thicknesses of the conducting and semiconducting thin films are measured by transmission electron microscopy (TEM). This method helps to measure the thickness of both single- and multi-layered thin films with thicknesses of up to 100 nm. Similar to SEM, attaching an EDS detector to the TEM helps obtain the elemental composition and structural information about the thin film.
Applications of Thin Films
Thin-film technology has been used in many applications, including electronic and semiconductor devices such as micro-electromechanical systems (MEMS) and light-emitting diodes (LEDs), photovoltaic solar cells, optical coatings, manufacturing thin-film batteries, increasing the cost efficiency of photovoltaic systems, resisting chemical degradation, and manufacturing antireflective, reflective, and self-cleaning glass.
In summary, the properties of thin films vary from those of bulk materials because of their small thickness. The large surface-area-to-volume ratio of thin films imparts a unique physical structure during the growth process. The technique adopted to measure the thickness of thin films depends on the different properties of the material, including the RI, surface roughness, and other information required to be retrieved.
References and Further Reading
Moosakhani, S. (2022) Thin-film thickness measurements – guidelines for method selection, Measurlabs. Measurlabs. Available at: https://measurlabs.com/blog/thin-film-thickness-measurements/
3.5.2 Important Techniques (2023) 3.5.2 important techniques. Available at: https://www.tf.uni-kiel.de/matwis/amat/semitech_en/kap_3/backbone/r3_5_2.html
Piegari, A., Masetti, E. 1985. Thin film thickness measurement: a comparison of various techniques. Thin solid films. https://doi.org/10.1016/0040-6090(85)90273-1
Kumar, S., Aswal, D. K. 2020. Thin Film and Significance of Its Thickness. Recent Advances in Thin Films, 1-12. https://doi.org/10.1007/978-981-15-6116-0_1.
Thomas, D.L. (2022) How thin is a thin film?, AZoM.com. Available at: https://www.azom.com/article.aspx?ArticleID=17334
Thin Film Nanotechnology (2023) Thin Film Nanotechnology | TORR INTERNATIONALSERVICES LLC. Available at: https://www.torr.com/thin-film-nanotechnology