Thin films and coatings are used for many purposes and in a variety of industries such as electro-optical switches, solar energy, and biocompatible coatings on metallic implants. The remote sensing, non-contact and analytical characteristics of optical spectroscopy techniques are ideal for process control and end-point detection during coating or film application and quality testing within these industries.

Avantes offers several solutions that are ideal for the thin-film and coating industry, including proprietary software designed for the needs of this market. Read more to explore how spectroscopy assists in measuring the optical thickness of a coating, absorption and fluorescence response of thin films, and chemical composition of plasmas and vapours present during deposition.

Coatings and Thin Film Deposition

Thin films are just that: a thin film of material between a few nanometres to about 100 micrometres thick, or the thickness of a few atoms, deposited on a ‘substrate’ surface or on top of previously deposited layers. This technology is at the heart of any high-tech optical device. There are two broad categories of manufacturing methods used in thin film, chemical deposition and physical vapour deposition.

During chemical deposition methods, volatile materials produce a chemical change on the substrate surface. Physical methods, on the other hand, involve the release of a coating material from a source, in the form of an aerosol, or plasma which is created by vacuum evaporation or sources such as ion beam, magnetron or cathodic arc.  Coating chambers distribute this material homogenously onto a substrate over a period of time ranging from several hours to several days. Both chemical and physical methods are a controlled synthesis which can be monitored and tailored using spectroscopic techniques. The advantage of spectroscopy is the ability to remotely and non-invasively measure the conditions in the vacuum chamber in real time during the manufacturing process.

Electrochemical Fluorescence Switching

Increasingly, researchers are utilizing fluorescence techniques to characterize thin films.   Research published in the Journal of Macromolecular Rapid Communications studied electrochemical fluorescence switching by exploiting the electrochemical potential of molecular tetrazines blended in an electrolyte polymer film. The fluorescent switching is a reversible shift between two stable states. The fluorescent state can be turned on and off when an ion-conducting layer reacts to an external stimulus, such as light, to modify the redox state of the tetrazine molecule itself.

The fluorescence switching device was made of two layers, the highly-fluorescent polymer and a conducting medium, sandwiched between two transparent indium tin oxide (ITO) electrodes. During the research, the quantum yields of these fluorescent polymers under varying conditions were calculated by using spectral data collected by the AvaSpec-ULS2048L-USB2 spectrometer. This work with electrochemical fluorescence will contribute to advances in graphic displays and telecommunications devices. 

Thin Films and Coatings Figure 1

Dye-Sensitized Solar Cells 

Titanium dioxide has enjoyed wide use in many applications, including environmental purification, gas sensing, and most recently, solar energy. The recent innovation of dye-sensitized solar cells (DSSCs) are finding prevalent application in the solar energy industry due to their power conversion properties, low cost, and ease of fabrication. Dye-sensitized solar cells consist of a porous layer of titanium-dioxide nanoparticles impregnated with a molecular dye that absorbs sunlight. The molecular dye is arranged in a 3D lattice formed by the semiconductor material which is responsible for transporting the charge generated by exciting electrons in the dye which then flow into the titanium dioxide nanoparticles. where the charges are separated creating an electric current.

Researchers from the University of Malaysia published a novel method for the preparation of Titanium Oxide thin films on an indium tin oxide substrate. They followed fabrication by aerosol-assisted chemical deposition with a hydrogen chloride post treatment and gold particle deposition. This removed sodium ions detrimental to photoactivity. The end result was a Dye-Sensitized Solar Cell which boasted a fivefold increase in photocurrent density and sevenfold increase in DSSC conversion efficiency. During experimentation, researchers used the Avantes AvaSpec-ULS2048L-EVO spectrometer to verify results.

Thin Films and Coatings Figure 2

Biomedical Implants

The bio-compatibility of medical implants is a crucial concern in surgical settings. Metallic implants are subject to corrosion and wear and it can be difficult to achieve a strong bond between the implant and the surround tissue. In these cases, biocompatible coatings, such as calcium phosphate, improve the likelihood of the implant adhering and reduces the risk of rejection or complications. For implants made with titanium or nickel alloys, the coating will also prevent the release of heavy metal ions into host tissue from the surface of the substrate.

Biocompatible coatings should be dense, pore free, and adhere strongly to the substrate: qualities that calcium phosphate offer. The rf-magnetron sputtering deposition process, however, produces coatings of variable composition, resulting in either an amorphous or a stoichiometric crystalline structure.  Researchers at the Tomsk Polytechnic University in Tomsk, Russia carried out experiments in order to gain a better understanding of the film growth mechanisms that determine coating composition. To define the mechanisms of film growth, the researchers investigated parameters related to rf-magnetron sputtering deposition, including the rf-power, the working gas atmosphere, deposition time, substrate position, and plasma composition.

The chemical composition of the plasma was analyzed using Optical Emission Spectroscopy (OES) in the 200-860 nm wavelength range using an AvaSpec-ULS4096CL-EVO, and found to greatly affect the composition of the resultant coatings. The content of calcium and phosphorous in the coating was dependent on molecular ions in the plasma.

Ultimately, this work sought to control the crystallinity and composition of the calcium phosphate coating through varying deposition parameters. Researchers were able to reliably produce coatings with a Ca/P ratio from 1.53 to 3.88 and either crystalline or amorphous structure. Having the ability to control the crystallinity of the coating can directly affect the lifetime of an implant in the body.

Thin Films and Coatings Figure 3

Thin Films Coatings and Beyond in the Modern World

Thin films and coatings are a crucial component in the manufacture of virtually all hi-tech products available on the market today. They are important in renewable energy production as a component of photovoltaic solar cells and make biocompatible implants safer and more effective.

Avantes has two decades of experience in supporting the semiconductor, thin-film, and coating markets. To learn more about our instruments for thin-film measurements and explore their usefulness for your application, contact one of our sales engineers today.

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