Inline Ellipsometry for Roll-to-Roll Coating Inspection

Researchers at TU Wien have developed an inline imaging-ellipsometer designed for roll-to-roll (R2R) manufacturing environments. The system integrates a polarized industrial camera from Allied Vision to measure coating thickness and uniformity across moving substrates in real time, enabling continuous thin-film inspection at production line speeds.

Thin-Film Inspection Challenges in Roll-to-Roll Manufacturing
Roll-to-roll production processes deposit functional coatings on flexible substrates such as polymer films or metal foils at speeds exceeding 100 m per minute. These coatings are used in manufacturing applications including lithium-ion battery electrodes, flexible electronics, and display components.

Quality control in such high-throughput environments remains challenging. Conventional inspection methods typically rely on sampling or offline metrology systems. As a result, deviations such as viscosity variations, temperature fluctuations, or coating misalignment may remain undetected until an entire roll has already been processed.

The imaging-ellipsometry system developed at TU Wien addresses this limitation by enabling continuous monitoring of coating thickness and uniformity across the full substrate width during production.

Optical Architecture for Inline Ellipsometry
Ellipsometry determines film thickness and optical properties by measuring the change in polarization of reflected light. While widely used in laboratory metrology, conventional ellipsometers rely on large collimated optical paths and mechanical scanning, limiting their use in industrial production lines.

The TU Wien system replaces conventional refractive optics with large-aperture Fresnel lenses. Fresnel optics reduce both the physical size and cost of the optical setup while maintaining the controlled angle of incidence required for ellipsometric measurements.

This configuration enables full-width inspection of moving foils, demonstrated on a production setup handling substrates with a width of 300 mm. The architecture allows the system to operate directly within a roll-to-roll coating line.

Polarization Imaging with Industrial Machine Vision
At the core of the system is a polarized machine-vision camera from Allied Vision. The camera is based on Sony’s IMX250MZR CMOS sensor with a resolution of 2448 × 2048 pixels and a global shutter architecture. Each pixel has a size of 3.45 µm, and the sensor operates at frame rates up to 75 frames per second.

The sensor integrates a nanowire polarizer array that captures four polarization orientations—0°, 45°, 90°, and 135°—simultaneously. The polarization filters are arranged in 2 × 2 pixel groups, enabling the camera to record multiple polarization states within a single exposure.

Illumination is provided by a user-selectable LED source operating within a spectral window of 350 nm to 1100 nm. A fixed linear polarizer at 45° is used in the illumination path. By measuring the intensity ratios of the reflected polarization components, software algorithms calculate spatially resolved coating thickness maps for each frame acquired at line speed.

Industrial Validation on Functional Coatings
The imaging-ellipsometer was evaluated on materials commonly used in industrial coating processes. Test measurements included PEDOT layers on PET foil, which are used in organic electronics and transparent electrode applications.

Additional tests were conducted on electrolyte coatings applied to copper foil for battery manufacturing. In both cases, the system generated quantitative thickness maps while the substrate moved at production speed.

Polarimetric analysis enabled the detection of coating non-uniformities, edge defects, and surface irregularities that may not be visible in conventional optical inspection systems. Such early detection can improve process control and reduce material waste in high-volume production environments.

Mechanical Stability and Noise Reduction
Inline ellipsometry measurements are typically affected by mechanical and optical disturbances such as foil vibration, substrate birefringence, and backside reflections.

The system architecture addresses these effects by performing measurements directly on the roll geometry of the substrate. The on-roll measurement configuration stabilizes the effective angle of incidence and reduces vibration.

Backside reflections are redirected away from the detector, while the LED illumination uses a spectral bandwidth of 20–60 nm. This bandwidth prevents coherent interference between front- and backside reflections, eliminating interference artifacts in the thickness measurement.

Scaling for Industrial Roll-to-Roll Production
The measurement concept can be scaled to wider substrates used in industrial roll-to-roll manufacturing. The system architecture supports foil widths up to approximately two meters, which are typical for battery electrode and flexible electronics production lines.

The measurable thickness range extends from nanometer-scale highly absorbing layers to micrometer-scale transparent coatings. Because the system operates at a fixed wavelength and illumination angle and contains no moving parts, integration into production lines can be achieved with relatively simple mechanical installation.

The research results were reported in Flexible and Printed Electronics (2026) by Ferdinand Bammer and colleagues, describing real-time imaging-ellipsometry for roll-to-roll coating inspection.

www.alliedvision.com