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Within the microelectronics, optics, photovoltaics, coatings, and glass industries, researchers often require details on a wide range of thin-film material properties; particularly the elemental composition of these products. When presented with this information, researchers and engineers in these industries can accurately choose the most appropriate coatings, modified surfaces, and supporting materials for their products.
Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS)
Scanning electron microscopy (SEM) is a well-known tool that engineers, scientists, tribologists, and lubricant engineers often utilize to analyze the morphology, defects, and wear behavior of certain materials.
The combination of SEM with energy-dispersive X-ray spectroscopy (EDS) further enhances the analytical capabilities by providing users with a more powerful method of obtaining information on the elemental composition in near-surface regions, as well as the morphological and topographical details of thin-film samples.
Several studies have cited the sensitivity of SEM-EDS in characterizing the physical, chemical, and mechanical properties of various mineral thin films. For example, a 2012 study utilized SEM-EDS to investigate the thermal stability of hard chromium nitride (CrN) thin films that were deposited on a silicon substrate. The combined SEM-EDS approach allowed the researchers to visualize an apparition of oxygen present within the CrN films. This oxygen presence was subsequently replaced by nitrogen until the chromium (III) oxide (Cr2O3) phase was formed. SEM-EDS data, therefore, provided valuable information regarding the distribution of these phases within the microstructure of the CrN films.
X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS), also referred to as electron spectroscopy for chemical analysis (ESCA), is one of the most commonly used techniques in the surface engineering and tribology industries.
In XPS, the sample surface is excited by mono-energetic aluminum (Al) X-rays, which cause the sample’s surface to emit photoelectrons. The energy of the emitted photoelectrons is then analyzed to provide valuable quantitative and chemical state information on the material being studied.
When used for the elemental analysis of thin films, XPS has provided clear information on the film’s stability and long-term reliability, which are important factors to consider prior to utilizing these constructed films for photovoltaic cells and thin-film light-emitting diode (LED) products.
X-ray Diffraction (XRD)
When used for material characterization, X-ray diffraction (XRD) offers a non-destructive technique that does not require special sample preparation before analysis. Furthermore, XRD allows users to obtain structural information on relatively large areas of the sample without causing any irradiation damage in the process. The application of XRD for the analysis of thin films present in semiconductors, electrodes and piezoelectric products has provided useful information on the crystallite size, lattice strain, reflectivity measurements, elemental composition, relaxation, thickness, roughness, density and pore/particle size distribution.
A 2019 study published in the Journal of Alloys and Compounds discussed the utilization of XRD to evaluate the conductivity effects that occurred when nanostructured Al was incorporated into zinc sulfide (ZnS) thin films. More specifically, the researchers used XRD to investigate the crystal phases of the film sample and determine how these structural characteristics and properties played a role in the optical, electrical, and electrochemical properties of the ZnS thin films. Analysis of the XRD patterns revealed that Al-doping reduced the crystallite size of the ZnS samples, which thereby contributed to a reduced dislocation density and lattice strain of the ZnS films.
Colored Picosecond Acoustic (APiC) Technique
The colored picosecond acoustic (APiC) technique utilizes ultrafast laser technology. This technique has demonstrated its ability to characterize specific properties of thin films, such as the adhesion quality present at the different interfaces between the film and substrate. APiC begins with shining a fast laser light pulse onto the sample, which is followed by the application of acoustic (hypersonic) waves in the same area. These sound waves propagate within the stack of thin films which causes an echo to be emitted and ultimately analyzed to determine the thickness and other material characteristics of the film sample.
A 2018 study recently investigated the potential use of APiC as a characterization method for thin-film samples composed of metals, dielectrics, and semiconductors. In their work, ultra-high frequency acoustic waves provided useful information on material thickness, as well as the elasticity and adhesion properties of nickel (Ni) thin films deposited on a silicon (Si) substrate, without requiring any contact or destruction to occur throughout the analysis process.
References and Further Reading
- “Surface Characterization Techniques: An Overview” – NASA
- Mammeri, F. Z., Chekour, L., Rouag, N. (2013). Characterization of Nitride Thin Films Using SEM and EDX. Proceedings of the 2nd International Congress APMAS2012; April 26-29, 2012. DOI: 10.12693/APhysPolA.123.294.
- “XPS/ESCA” – Physical Electronics
- Chu, K., Bae, K. D., Song, B. G., Kim, J., Park, Y. Y., Xianyu, W., Lee, C. S., & Sohn, Y. (2018). Quantitative analysis of nano-defects in thin film encapsulation layer by Cu electrodeposition. Applied Surface Science 453; 31-36. DOI: 10.1016/j.apsusc.2018.05.078.
- “X-ray thin-film measurement techniques” – Riagku
- Azmand, A., & Kafashan, H. (2019. Al-doped ZnS thin films: Physical and electrochemical characterizations. Journal of Alloys and Compounds 779; 301-313. DOI: 10.1016/j.jallcom.2018.11.268.
- Devos, A., & Emery, P. (2018). Thin-film adhesion characterization by Colored Picosecond Acoustics. Surface and Coatings Technology 352(25); 406-410. DOI: 10.1016/j.surfcoat.2018.07.097.