Surface measurement: essential for technology
Surface measurement – synonymous with surface metrology – determines surface topography, which is essential for confirming a surface’s suitability for its function. Surface measurement conceptually includes surface shape, surface finish, surface profile roughness (Ra), or in surface area roughness (Sa), surface texture, asperity and structural characterization.
For example, engine parts may be exposed to lubricants to prevent potential wear, and these surfaces require precise engineering— at a microscopic level— to ensure that the surface roughness holds enough of the lubricants between the parts under compression, while it is smooth enough not to make metal to metal contact. For manufacturing and design purposes, measurement is critical to ensure that the finished material meets the design specification.
In the image above, a microscopic surface is measured in three dimensions using an interference microscope. For scale, the 3-D surface measurement above maps features within a 22 nanometer range of height, and the indicated pit defect is less than 12 nanometers deep. A nanometer is one one-thousandth of a micron (µm). There are about 80 microns (80,000 nm) in the thickness of a human hair. The area of the measured surface is 449 × 335 microns.
Surface roughness measurement for defect analysis
Defects may occur either in material surfaces during processing or after use, and defect analysis is often essential for providing the information to improve effectiveness, efficiency and durability of surfaces. For example, a product that requires long life in adverse conditions is prosthetic joints, such as hip joints. Being able to measure the surface material for wear, scratches, and the shape of a prosthetic joint after it has been removed for replacement can be beneficial for future hip replacement procedures. Optical surface measurement techniques have been used to measure these and other medical-quality surfaces such as stents, dental implants and artificial bone.
In the image of a 3-D surface map above, several pits appear in a step height calibration standard, which is made of quartz and then chrome plated. This type of standard is often used for calibration of profilometers of all types. The pits may be the result of impacts, wear or chemical effects. If enough of these pits were present, the surface’s suitability as a step height standard would be compromised. Depending on the application, the determination of pits versus asperities (bumps) is critical to the performance of the surface. While computer hard disk surfaces can accommodate a certain number of pits, asperities can cause failures due to low flying height of the disk read/write heads. Optical profilers must be able to resolve the defect sufficiently to determine its polarity (pit or bump) and to characterize its height or depth.
Surface measurement for process control
Manufacturers need to control processes to produce a consistent, reliable product. Where precision surface engineering is required, surface measurement may be a key part of maintaining control of the process, by checking output to see that the process is not outside of specification.
Surface roughness measurement concerns
Options for surface roughness measurement are essentially delimited by the precision required in the result. For example, surface roughness could be assessed by eye and touch, by comparing a test sample to a standard sample, but this is not a measurement: it is subjective. Touch sensitivity and visual resolution limit the effectiveness of these assessments to features that are a few microns high and wide, at the extreme limit.
Surface topography measurements, at a precision-engineering level, are separated generally into the scale of the features that are examined.
Surface shape is the overall geometry of the area of interest. “Area of interest” varies according to the application. For example, if you were measuring the topography of a micro-lens, “shape” might be described as a measurement of the lens’s curvature, as compared to a manufacturing specification. Ultimately, this is a measure of the lens’s ability to focus correctly. Other shapes of interest in materials sciences and engineering are planes, spheres, toroids, cylinders, parabolas and aspherical and free-form curves.
Measuring surface shape requires a measured field of view large enough to include shape geometries.
Surface roughness – also known as surface profile Ra—is a measurement of surface finish – it is topography at a scale that might be considered “texture” on the surface. Surface roughness is a quantitative calculation of the relative roughness of a linear profile or area, expressed as a single numeric parameter (Ra).
In three dimensional optical profilometry, roughness is usually expressed as surface area roughness (Sa). Profile roughness (Ra) can be extracted as a line through an area. Interestingly, Sa is also able to report average Ra through a surface by averaging several profiles.
Surface finish typically refers to a level of polishing or texture intended for, or resulting on, a surface.
On a lens, it is usually desirable to have as little roughness on a lens surface as possible, if light is being guided, so that light is scattered as little as possible — but this roughness (or smoothness, ideally) doesn’t include the general curvature of the surface shape.
In other applications, roughness may need to be optimized: an adhesive may need a certain amount of roughness to permit air pockets for hardening, while presenting enough surface area to bond.
Optical profilers and stylus profilers are suitable for measuring most surface roughness applications.
Features that can be characterized individually fall into the category of asperities. In precision engineering applications, asperities often refer to sub-micron variations in height and shape.
Optical profilers with high lateral resolution and accurate height resolution, as well as high-end stylus profilers can measure asperities in some cases. Atomic force microscopes (AFM) and electron scanning microscopes (SEM, TEM) have higher resolution and are typically used for asperity measurements, but can be destructive of the surface, depending on the materials – and their small fields of view present difficulty in finding the defects.
Surface measurement with optical profilers:
- Optical profilometers measure area as well as height
- Surface height measurement accuracy that reaches sub-Angstrom repeatability
- Non-contact measurement: No diamond-tipped stylus to damage or alter fragile or soft surfaces
- 3-D measurements can calculate volumes of bumps, mesas or voids
- Better able to measure co-planarity of discontiguous areas in the field of view than contact profiling methods
- Large field of view of Filmetrics profilers offers more surface information than others