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Thin films are layers of materials with a thickness ranging from a few nanometers to a few micrometers. They are used in a variety of applications such as coatings for cutting tools, solar cells, optical and decorative coatings, and as diffusion layers in integrated circuits. Thin films are grown by different deposition techniques like physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD, a vapor of the material is deposited by physical means like by energetic ion bombardment (sputtering) or by heating (evaporation). Sputtering adds benefits to the deposition process, due to the presence of plasma. In CVD, chemical reactions take place at the surface of the substrate before the deposition.
Some of the techniques that are used for thin film growth are low-pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), plasma enhanced CVD (PECVD), metal organic CVD (MOCVD) and liquid phase epitaxy (LPE). In a thin film deposition process, energy is supplied to the reactant gases of a solid source to obtain thin films. The energy is supplied either in the form of heat, radiation, magnetic energy, electrical energy, or a combination of these sources.
Plasma is a quasi-neutral gas consisting of neutral and charged particles which exhibits a collective behavior. Nucleation and growth are the two processes by which thin films are formed on a substrate. Nucleation depends on the interaction between the substrate and the adatoms (atoms that are added). It is also affected by the surface mobility of the species which depends on the deposition rate, substrate temperature and the energy of the arriving atoms.
A wide variety of characterization techniques are used to assess the quality of the thin films. Scanning electron microscope (SEM) and atomic force microscopy (AFM) in non-contact mode are used to determine the structural properties of the polycrystalline films while X-ray diffraction (XRD) and transmission electron microscopy (TEM) for the presence of crystalline phases. Energy dispersive analysis of X-rays (EDAX) provided with SEM is used for composition measurements.
X-Ray Diffraction (XRD)
XRD is used for studying crystal structures of thin films and solids. In XRD, a collimated beam of X-rays is incident on a specimen and is diffracted by the crystalline phases in the specimen satisfying the Bragg’s law i.e., nλ=2dsinθ, where d is the spacing between the atomic planes in the crystalline phase, λ is the X-ray wavelength, and the intensity of the diffracted X-rays is measured as a function of the diffraction angle θ. The diffraction pattern is used to identify the crystalline phases present in thin films.
Scanning Electron Microscope (SEM)
SEM is used to determine the structural properties of the films. In SEM, electrons are emitted from an electron gun and passed through a series of lenses to be focussed and scanned across the sample. The electron beams have energies ranging from 0.5keV to 30keV. When the electron beam interacts with the sample, the electrons lose energy by repeated random absorption and scattering. The energy exchange between the sample and the electron beam results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons and emission of electromagnetic radiation. Each of these signals is detected by their respective detectors.
The electron beam is raster scanned on the sample. The raster scanning of the cathode ray tube (CRT) display is synchronized with that of the beam on the specimen in the microscope. The resulting image is a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. Energy dispersive analysis of X-rays (EDAX) is one of the most common non-destructive techniques used for the compositional analysis of thin films. It is generally attached with the SEM. It measures the entire energy spectrum simultaneously with an energy sensitive detector.
Transmission Electron Microscopy (TEM)
TEM has a similar principle to that of optical microscopes. It is generally used for higher magnification imaging. In TEM, the sample is placed on a small copper grid (few mm in diameter). The sample must be adequately thin (a few tens to a few hundred nm) to be transparent to electrons. The transmitted and scattered electrons form a diffraction pattern in the back focal plane and a magnified image in the image plane. The ability to form a diffraction pattern yields structural information about the sample. Selected area electron diffraction (SAED) is a crystallographic experimental technique which is performed inside a TEM. To obtain a SAED pattern the wavelength of high-energy electrons is incident on a very thin and semiconducting film. The atoms in the film act as a diffracting grating to the electrons and are diffracted. As a result, the image on the screen is a series of spots known as the selected area diffraction pattern. Each spot corresponds to a satisfied diffraction condition of the sample’s crystal structure.
Atomic Force Microscopy (AFM)
AFM is used to investigate the morphology and growth structure of thin films. AFM consists of a sharp tip mounted on one end of the cantilever. The cantilever motion causes the reflected light to impinge on different segments of the photodiode. There are three common modes of operation of AFM. They are contact, non-contact and tapping mode. In contact mode, the probe tip is dragged across the sample surface. The image obtained is a topographical map of the sample surface. In non-contact mode, the instrument senses the Vander Waal attractive forces between the probe tip and the sample surface. These forces are considerably weaker than the contact mode forces. Non-contact mode gives lower resolution than the contact and tapping mode. In tapping mode, limitations of the conventional scanning modes are overcome by alternately placing the tip in contact with the surface to provide high resolution. Then the tip is lifted off the surface to avoid dragging across the sample surface. It works well for soft, fragile or adhesive samples. Tapping modes allow high-resolution topographic images of sample surfaces that are easily damaged or difficult to image by other AFM techniques.
Further Reading and References
- Kazmerski, Lawrence L. "Analysis and characterization of thin films: A tutorial." Solar cells 24, no. 3-4 (1988): 387-418.
- Peter Rointan Framroze Bunshah, “Handbook of deposition technologies for films and coatings: science, Technology and Applications”, Noyes Publications, USA (1994).
- David Brandon, Wayne D. Kaplan, “Microstructural Characterization of Materials”, John Wiley & Sons ltd., England (2008).
- Milton Ohring, “Materials Science of Thin Films: Deposition & Structure”, Academic Press Inc., USA (2002).