Dr. Albrecht Brandenburg, Fraunhofer IPM: Superficially clean
Many modern products rely on residue-free and immaculate surfaces. Fraunhofer IPM has developed measuring systems to detect impurities or defects on surfaces in the production line based on different technologies according to the task in hand. Dr.-Ing. Albrecht Brandenburg is group manager “Optical Surface Analytics”.
1. An innovative imaging fluorescence detection system recognizes contamination on large surface areas. Could you tell us briefly how fluorescence detection works?
Dr. Albrecht Brandenburg: With fluorescence, light absorption causes electrons to jump from their ground state to a higher energy, excited state. These excited states are not stable and, as a result, the electron passes through several intermediate steps and returns to its original ground state. The energy that is released can be in the form of heat, through radiationless deactivation, or fluorescent light as a result of photon emission.
The probability that photons are emitted when energy is released is called quantum efficiency. Since some energy is always emitted without radiation, the energy of the emitted light is lower than the energy that was originally absorbed. Hence, the wavelength of the emitted light is greater than the wavelength of the absorbed light.
2. What's special about your new method?
Dr. Albrecht Brandenburg: Conventional methods to test for cleanliness usually need samples to be taken beforehand. The new feature with our method is that we have developed a way of testing for cleanliness as an inline control without the need for prior sampling; in other words, with imaging fluorescence analysis we can conduct cleanliness checks directly in the machine and evaluate the result immediately.
3. How "large" must the smallest contamination be for it to be detected?
Dr. Albrecht Brandenburg: With our fluorescence method we can detect very small quantities of undesired residue. The system can already detect just a few micrograms per square centimeter surface area with a camera. Particles in the micrometer range have been clearly detected with this method.
4. Are scanning methods generally better than parallel detection using LEDs and a CCD camera?
Dr. Albrecht Brandenburg: By using the laser scanner we can detect disruptive contamination on surfaces in the square meter range. In the past, imaging fluorescence detection systems were suitable only for small surface areas. This size limitation was due to LED illumination with parallel detection of the entire surface area using a CCD camera.
We have now developed a scanning system to investigate larger surface areas in which mirrors are used to deflect the beam of a UV laser over a scanning area of +/– 20 °. At a distance of 60 cm from the surface to be investigated we can examine component surface areas up to 50 cm × 50 cm. The fluorescence that is excited on the surface is then detected.
Software aggregates the data from the individual measuring points into an image of the surface. This image is processed so that, for example, areas with a higher intensity – that is, with more contamination – can be detected and evaluated automatically.
5. Is there a difference in terms of the speed of the two methods? This can be an especially important issue in production.
Dr. Albrecht Brandenburg: We measure contamination or defects on surface areas of several square centimeters within milliseconds. This is how we are able to measure directly in the production line.
6. In which applications can we expect to see cleanliness testing with laser scanners in the future?
Dr. Albrecht Brandenburg: Testing the cleanliness of metal surfaces is an important area for which there is high demand. Other functional surfaces and photolithographic systems are other potential areas. The system can be used in component production in mechanical engineering, in the automotive industry, or in medical engineering. In fact, everywhere where the cleanliness of the surface of a component is especially important. Production processes can be reliably tested and documented with fluorescence measurement techniques, which would then enable them to be continuously optimized.
Thank you for the interview.