To address the increasing need to precisely control the thickness of plastic bottles, tubes, and foils, scientists at the Institute of Production Engineering and Photonic Technologies of TU Wien (TUW) in Austria have devised an ellipsometric inline measurement tool harnessing the advanced technologies of SVS-Vistek polarization cameras.
Boasting frame rates of 24 frames-per-second (fps) and the ability to filter reflected light and make hidden details visible, the cameras enable both fast offline as well as real-time inline thickness measuring control.
Quality Control with Ellipsometry
Ellipsometry is a non-destructive, contactless optical technique that measures the polarization of light reflected from a surface. Thin films in semiconductors, metals, polymers, and dielectric films have their thicknesses, optical characteristics, and compositions determined via ellipsometry. In the food, pharmaceutical and beverage industries, thickness of plastic containers is measured by ellipsometry to ensure products are safely guarded from mechanical damage, while also ensuring environmental factors like moisture or impurities don’t contaminate what is inside.
Tested on Bottles, Tubes and Foils
As proof of concept, the scientists at TUW tested their new ellipsometric tool on coated PET-bottles, PDMS-coated glass tubes, and PET-foils to determine layer thicknesses within a range of 0–500 nm. PET plastic, or polyethylene terephthalate, is a polyester resin commonly used to make bottles and containers for food, beverages, and other products. Polydimethylsiloxane (PDMS) coating is a hydrophobic polymer applied to solid surfaces to make them water-resistant.
Analyzing images from a SVS-Vistek exo250ZGE polarized GigE Vision camera, the ellipsometric tool measured the change of polarization imposed on the reflection of each sample, therefore determining the layer thickness of either single or multiple coatings. Featuring a Sony IMX250 5-megapixel (2448 x 2048) Polarization sensor with global shutter and a four-directional on-chip Polarizer, the SVS-Vistek exo250ZGE camera captured the degree of polarization and planes of polarizing from a single image, allowing for detailed structural analysis. Polarization enables the camera to detect stress, birefringence, through-reflection and glare from glass, plastic, and metal surfaces.
For illumination, an LED was chosen in the range 350–1100 nm with reflected light imaged on the camera sensor. Collimated light with linear polarization of 45 degrees was directed to each cylindrical sample and reflected as a diverging beam. Imaging was performed using two confocal lenses with the resulting magnification f 2/f 1 = 0.12. A quarter-wave plate was positioned either directly in front of the camera sensor or in the common focal point of the two lenses, where an additional small aperture could be placed to increase the angle-sensitivity of the system.
Versatile, Inexpensive Solution
According to the scientists, their versatile setup can easily be installed on various production lines across multiple industries. For instance, in the production of flexible electronics based on printing roll-to-roll (R2R), PET foil thickness can be measured directly on the roll by the ellipsometric inline measurement tool. Traditional inline-ellipsometry of PET foil is difficult owing to foil vibrations and reflections from the backside of the foil that superposes an unknown polarization component. Direct measurement on a roll, however, achieves a very stable measurement and avoids problems with foil waviness and vibrations. When foil is moving at a typical velocity of 25 mm/second, thickness distribution of a complete coating can be acquired by configuring the SVS-Vistek camera speed to 24 fps.
Future research work at TUW will explore installations of the ellipsometer in different inline-applications. Simple and inexpensive components, coupled with a lack of moving parts on a fixed wavelength with a fixed illumination angle, is expected to accelerate its adoption in crucial measurement tasks.
Learn more about SVS-Vistek cameras at http://www.svs-vistek.com.
