Thin film characterization techniques

A thorough understanding of a thin film requires a complete study of their thickness, chemical composition, crystallinity, surface morphology, adhesion and stress levels correlated to the primary property being analysed like mechanical, electrical, biological, optical or magnetic performance. In order to understand the reason for the super hardness and excellent biocompatibility of the Ti-Au material system their thickness, chemical composition, crystalline structure, and material morphology need to be recorded and compared to the corresponding change in their mechanical hardness and cell viability results.

Film thickness measurement:

Dekatak Profilometer at Northumbria University

A profilometer is a device used to measure the variation in the Z-axis or thickness (d) of a thin film. In simple terms, the profilometer is made up of two main parts: a sample stage and a detector, and the sample stage moves to slide the sample across the detector to make a Z-axis profile of the surface. The detection of Z-axis height could either be made by using a physical probe like a stylus tip as a detector of height or by a using light to make an optical profile of the surface. In a stylus profilometer, a physical probe is run along the surface of the film to determine the surface height. A step is created by sticking a strip of kapton tape on the substrate prior to deposition that is peeled away later. This creates a step of the exact thickness of the deposited film, which is then measured using the Dekatak XTL probe based profilometer.

Typical profilometer scan with step height measurement

A continuous feedback loop monitors the force by which the film surface pushes against the probe as it moves along. This feedback is used to keep a constant torque on the probe, thereby moving the probe arm as the height of the sample changes. This recreates the surface Z profile. While this method is extremely sensitive and gives excellent resolution of the Z value, the relatively hard probe can scratch the sample surface if it is soft. The Dekatak XTL stylus type profilometer is set up with a 12.5µm tip. Because of its high Z-resolution, high sensitivity, ease of use and immunity to surf ace vibration, the Dekatak XTL stylus profilometer was used for most of the experiments to measure the film thickness.

Surface topology:

TESCAN SEM system at Northumbria university

Optical microscopy struggles to reproduce images of structures smaller than 500 nm, because of white light reaching its diffraction limits. A scanning electron microscope (SEM) employs a beam of electrons, which has a much lower wavelength to capture images of nanoscale features . In a SEM an electron gun produces a steady beam of electrons from a cathode in a column, which is accelerated and shape modified by a series of electromagnetic lenses. On interaction with samples, this beam can either transmit through the samples without interaction or collide with the sample and be reflected back. If the incident electrons collide with loosely bound electrons in the conduction band, then they will emit secondary electrons and this emission rate is highly sensitive to the height difference in the surface. Older generation of SEM`s used an Everhart-Thornley detector which converted the collected secondary electrons into flashes of photons, then multiplied them in a photomultiplier tube to create a 2-D image on a screen.

SEM image of Silver thin film.

Newer versions of SEMs employ a Si-Drift Detector (SDD) which measures the amount of ionization produced by the incoming charged species on a high purity Si screen, resulting in very high count rates and raster speed, thereby achieving high resolution images at higher speeds. Imaging of the samples was done by using a Mira Tescan SEM system.

Chemical composition:

Electron sample interaction

In 1968, Fitzgerald, Keil and Heinrich developed the idea of Energy Dispersive X-ray analysis to study the element composition. Element specific X-Rays are produced in an SEM if the incident e-beam knocks out an electron in the inner shell and an electron from outer shells has to fall down to fill this newly created vacancy. The emitted X-ray is studied for its energy in a technique known as Energy Dispersive X-ray system or EDX. The energy of the X-Ray is measured by a specially arranged Si-Li drifted detector, most commonly installed within a scanning electron microscope as the same electron beam can be used to create an image using secondary electron and the emitted X-rays can be measured to detect the elements presents in the material system. Tescans MIRA SEM system was also equipped with an Oxford instruments X- Max 150 EDX detector to perform EDX measurement.

Hidden analytical SIMS system at Northumbria university

SIMS or Secondary Ion Mass Spectroscopy is another instrumental technique used to analyse the elemental composition of solid thin films. It is one of the destructive analysis techniques in which an energised ion species (several KeV), commonly O2+ or Ar+ is fired from an ion gun onto the sample surface, which on impact blasts out surface atoms and molecules from the thin film. The ejected particles are detected using a mass spectrometer, which measures the mass of the secondary ions ejected. The most important advantage of SIMS is that it has very high sensitivity to most of the elements in the periodic table and hence can detect even the smallest amount of concentration. But the major disadvantage of SIMS is that the quantification of the SIMS result to get exact composition is not very reliable, hence SIMS is more suitable to develop a depth profile of elemental concentration across film thickness. SIMS system assembled by Hidden Analytical equipped with IG20 type gun, and quadruple system based detector with 1-1000 a.m.u range is used for this work.

Surface roughness:

Different techniques are applied to measure the roughness value of substrates, which is usually in the range of 100s of nanometres compared to that of thin films, which are generally under 10 nanometres. For substrates with higher roughness values, non-contact optical focus variation techniques can be used to create a 3-dimensional image of the substrate surface from which surface roughness values can then be calculated. This technique is suitable for non-reflective surfaces like alumina and transparent glass substrates. For this work, the Alicona Infinity focus with 10X lens is used to measure the surface roughness of the substrates.

DI Dimension 3100 AFM system at Northumbria University

For thin film surfaces, atomic force microscopy (AFM) is a more suitable technique for measuring surface roughness. AFM makes use of a silicon cantilever equipped with an ultra-sharp probe of 5-15 nm at its tip. Similar to dekatak stylus profilometer explained earlier, the tip of the AFM also rasters the sample surface, either in continuous contact or intermittently tapped mode.

AFM image of Silver thin film

A laser reflecting from the cantilever onto a photodetector, measures the vertical tip movements and records them as a 3-D image of the surface. AFM has very high resolution because of its small tip dimension and can be used to support the topology of film surfaces generated from SEM techniques. In this work, a Digital instrument Dimension STM 3100 AFM system manufactured by Veeco Metrology group is used to measure and generate the surface 3D plots.

Structural analysis:

Rigaku`s smartlab XRD system at Northumbria University

The same order magnitude of wavelength of X-rays as with the lattice parameters of crystalline structure causes the x-rays to diffract at the crystal planes. X-rays generated by the source material are made incident upon the samples surface, which after incidence get diffracted and form constructive and destructive interferences as per Bragg`s Law. These interferences produce a diffraction pattern and each material has its own standard diffraction pattern. The diffraction pattern generated in the system can be compared to the diffraction patterns saved for the same material in an international database by a committee known as the Inorganic Crystal Structure Database (ICSD). By comparing diffraction patterns, important information about the material can be developed like: preferred orientation of the crystals, grain size, internal strain and the lattice parameters. In this

Typical example of XRD pattern showing different phases of Silver thin film

work, the diffraction patterns for materials are generated using a RIGAKU smartlab XRD system employing a Cu Kα radiation of wavelength, λ = 1.54184 Å. The XRD measurements were made for a 2θ range of 20˚ to 80˚ in steps of 0.02˚. The Patterns were analysed using Origin 8.1, graphing and analysis software from Origin lab and Fityk data processing and nonlinear curve fitting software.

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