Profiling is the act of measuring the dimensional characteristics of a surface and has been facilitated by several techniques. One of the more basic techniques is known as stylus profilometry, where a stylus attached to a cantilever is physically moved across the surface of interest. The cantilever moves sympathetically with the physical features of the surface; signal transducers then translate this movement into a trace, or profile, of the surface, which can then be displayed using appropriate software (Euan, 1996). A significant downside of this technique is the two-dimensional nature of the profile, which limits the usability of the technique. Additionally, as the stylus must be physically moved across the surface, fragile coatings may be damaged or otherwise changed, affecting the physicochemistry and
rendering samples potentially unusable for further analysis or use.
A technique known as atomic force microscopy is commonly used to assess physical features of surfaces. AFM uses a similar configuration as stylus profilometry, wherein a probe is attached to a cantilever. However, whereas stylus profilometry relies on the physical resistance against the movement of the stylus to obtain sympathetic movements in the cantilever, AFM is able to detect resistance provided by the atomic force. The advantage of this is twofold. Firstly, the AFM is able to obtain information about the surface
without affecting the physicochemistry, enabling the same surface to be used in subsequent analyses. Secondly, AFM is many times more sensitive than stylus profilometry, able to resolve surface features on the nanometre scale.
However, this leads to the technique taking a long time to analyse samples, making it less suitable for high-throughput work, or samples with a larger area (Binnig et al., 1986; Xia and Whitesides, 1998; Diebold, 2002; Lee et al., 2008).
Another such technique is optical profilometry, which is a contact, non-destructive profiling technique for surfaces based on the principles of superimposition and interference. When two beams of light with similar wavelengths are combined, or superimposed, they cause constructive and
74 | P a g e destructive interference. This manifests as an interference pattern consisting of intermittent ‘fringes’ (Figure 16).
Maximum interference is caused when two waves of identical wavelength in the same phase are superimposed. Optical profilometry exploits this property to compile a three dimensional map of the surface. Typical configurations are comprised of a light source, beam splitter, digital camera, objective, stage and reference mirror (Figure 17).
Figure 16: Classical interference pattern visualised via optical profilometry. The principles of this technique require the detection of superimposed light waves, combine when exactly in phase. This is realised as intermittent fringes as the maxima and minima of the waves combine.
75 | P a g e From the beam splitter, the focal plane and reference mirror are equidistant.
When a sample is in the focal plane (i.e. in focus), light is reflected both from the sample and reference mirror in the same phase and since they come from the same source, the same wavelength. Therefore, an interference pattern is created. As the sample is moved through a ‘scan length’ in the vertical axis, the interference pattern changes depending on the part of the
Reference mirror
Light source
Digital camera
Beam splitter
Focal plane Test sample
Scan Direction
Stage
Figure 17: Schematic diagram of white-light profilometer. A digital camera, light source, and unidirectional reflector are encased in a housing. The light beam travels from the source towards the beam splitter, where it splits towards a reference mirror and the stage. The beam has a fixed focal plane, which is at the same distance as the reference mirror. In this way, the two superimposed light beams are in phase when a sample is in the focal plane; the sample is scanned through the vertical of this focal plane, so that the interference maxima are detected for the whole sample by a digital camera. Computer software then consolidates this information into a 3D map of the sample.
Objective
76 | P a g e sample in the focal plane. Each pixel in the digital camera corresponds to a specific set of x, y coordinates. Computer software then detects and records the z coordinate at which maximum interference is detected by each pixel and this then composited into a surface map of the sample (Fischer et al., 1999; Whitehouse, 2000). The surface map can be adjusted using post-processing effects, which are typically used to ‘fill-in’ void pixels (areas that have received no data) and to remove ‘tilt’ from a surface. Removing tilt is analogous to levelling the map, so that the curvature of a sample is ignored, and the surface topography stands out.
In addition to a visual map of the sample area, various quantitative data are obtained describing the deviations from an ideally flat surface. A summary of the values used in this work is shown in Table 1.
Table 1: Parameters measured by optical profilometry relevant to this work. The parameters detailed here were used in this work to measure appropriate features of the coatings, namely the roughness and thickness.
Abbreviation Description Explanation
Sa Area
average roughness
Indicates the arithmetic mean of the deviations in the texture from a hypothetical ideal flat surface, while rejecting extreme outliers. It is an indication of roughness across the area of the surface. distance found over the area of the surface. Is the arithmetic mean of the five largest peak and valley
deviations from the hypothetical ideal flat surface – represents the
extremes of the surface. Can be used to identify the size of shelves or steps.
Rz/Rdz Profile
peak-to-valley distance
As Sz, but for a line profile laid across the surface.
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