If you need to characterize features at micrometre to sub micrometre on natural or manufactured surfaces, an optical profilometer will provide 3D surface metrology without any – potentially destructive – surface contact. This is a crucial advantage of optical profilometry over tactile (stylus-based) techniques.
Other advantages of optical profilometers are speed, reliability and large field of view. Depending on the required resolution and the properties of the material, different optical techniques are available. ST Instruments’ portfolio includes a range of optical surface profilers using one or multiple technologies such as confocal microscopy, interferometry and/or focus variation. These instruments are produced by Sensofar and KLA/Filmetrics.
All optical profilometers use light as a ruler, but there are different techniques to do so.
Confocal scanning blocks out-of-focus light using an aperture at the confocal plane. The surface height of smooth to very rough surfaces can be measured with sampling as low as 0.10 µm. Confocal algorithms provide vertical repeatability on the nanometre scale.
Optical interferometry gathers information on the surface topology of a sample by interference between a reference arm and a sample. This technique provides vertical resolution at nanometre scale.
Focus variation scans an object, creating a set of images which are combined to one 3D image using algorithms that determine which points are in focus in each separate scan. The technique is especially suited for measuring the shape of large, rough surfaces. All three techniques are combined in the Sensofar 3-in-1 S-line.
As in many modern techniques, the performance of an optical profilometer is for a large part determined by its software: the quality of the software running the analyses determines the accuracy and versatility of an instrument. Sensofar developed software that will add a Continuous Confocal mode, which avoids the discrete acquisition of data by a continuous scanning along the Z-axis, increasing the scanning speed. Furthermore, the Confocal Fusion mode will combine the best from Confocal scanning and Focus variation techniques by using a smart algorithm.
Surfaces are defined by their primary shape (topography) and the degree of structure, waviness and roughness. The latter might be categorized as finish, haptic, texture, defects, marks, micro-wear, etcetera. Alternatively, surface characteristics like critical dimensions, step-height, peak-to-valley, volume or slope, or even map coating thickness can be measured. Optical Profilometry qualifies and quantifies the relative contribution of these different characteristics to a surface.
Measurements of the initial deflection of a nano pressure sensor for biological applications
Nano-pressure sensors are designed to be used inside a living cell. The sensor chips measure 6 x 10 µm and are comprised of a mechanical sensor defined by two polysilicon membranes separated by a vacuum gap, and an optical reference area. The membranes are partially transparent for some wavelengths and form a Fabry-Pèrot resonater.
Before use in a cell, initial membrane deflection measurements are carried out, often using Scanning Electronic Microscopy (SEM), which means the samples must be under vacuum pressure. This may alter their initial state. Using a Sensofar optical profilometer, it is possible to measure in a quick and non-intrusive way the deflection of the membranes after manufacturing.
The effect of salt deposition on the performance of floating PVs
Off shore windfarms may soon be followed by off shore solar farms. But how will photovoltaic cells respond to the salt that is going to be deposited on their surface? An optical profilometer using white light has been used to obtain thicknesses of salt depositions on glass slides exposed to water with different salt concentrations.
Salt crystals were visualized, and using confocal techniques, three dimensional profiles were then processed in order to obtain information such as crystal height (and an approximation of thickness) as well as salient features.
Advanced inspection and analysis with rotational module
Measuring the surface of complex shapes, e.g. in engineering, with an optical profilometer is made possible by the S Neox five axis motorized rotational stage. This allows users to take 3D surface measurements at different positions of rotation and elevation, generating a group of individual measurements. The SensoFIVE software merges all of the surfaces providing a sample surface with high accuracy. This method also makes it possible to measure complex surfaces which contain steep angles. These are otherwise very difficult to sample due to shadowing effects.