An appropriate microscope set-up is essential in order to be able to optically detect small structures down to the sub-micrometre range. Only with such a setup can details smaller than 1 µm be recognised, which is due to the high aperture of the beam path. The aperture is also known as the light opening or aperture.
However, a high aperture also leads to a shallow depth of field in the images, which is often even significantly smaller than the structural depths that are to be measured. This effect is also known from photography: With a large aperture, the depth of field is very small and with a small aperture, it is very large. A depth scan is therefore absolutely essential for these high resolutions.
Based on this basic microscope set-up, three measurement methods have been developed: focus variation, white light interferometry and confocal microscopy.
Alternative optical measurement methods that work with smaller apertures in order to capture the entire structure in depth include light stripe projection, triangulation or stereoscopy. However, these methods offer a lower resolution and therefore capture the structures less precisely.
The method of focus variation utilises the shallow depth of field of the optics to obtain information about the depth.
The scan is performed in detail as follows: During the scan, the structure to be captured is recorded from different focal positions. You start above the structure and take an image at fixed intervals – every 2 µm, for example. The contrast value along the z-axis is then analysed for each pixel of the image plane. The z-position of a pixel corresponds to the point at which the contrast is highest. Once the z-positions of all pixels in the image plane have been determined, a 3D model can be created.
A major advantage of focus variation compared to white light interferometry and confocal microscopy is that measurements can be carried out with ring illumination. This means that even steep edges can be illuminated in such a way that light is reflected back into the microscope objective. However, this is precisely the prerequisite for detecting anything at all.
White light interferometry and confocal microscopy, on the other hand, only work with coaxial illumination. Here, steep edges appear black, which means that the structure cannot be captured at these points. The software of these microscopes interpolates, i.e. ‘estimates’, the surface profile. As a result, the scoop volume is incorrectly determined.
The focus variation has the following limitation: The method only provides measured values at points where a contrast can be determined. A mirror-like surface, for example, does not provide any measured values. However, such surfaces can be pre-treated, for example by making them dirty or roughening them. The DotScope is very sensitive, so the roughness of a chrome surface alone is often enough to provide sufficient information. A mirror surface could, for example, be soiled with a simple fingerprint so that it can then be measured.
With white light interferometers, an interference effect is used to determine depth. This also involves scanning in the depth direction. As soon as the structure is at the measuring height, strong fluctuations in brightness occur (if light is reflected back, see above). These fluctuations are very short-wave, which is why images must be taken at a distance of approx. 0.1 µm in order to detect them reliably. In contrast to focus variation, this step size can hardly be changed. For example, 1000 images must be taken for a scan of 100 µm with the interferometer, whereas with focus variation typically 20 images are taken with a 5x objective and 100 images with a 20x objective. This means that the white light interferometer has to record, transmit and process many more images. In addition, the interferometer is 10 to 50 times more sensitive to mechanical vibrations that lead to faulty detections.
Focus variation is therefore the simpler and more robust method, which also enables more complete detection of typical printing rollers.
