Fraunhofer Institute for Surface Engineering and Thin Films IST
Dielectric beam splitter of approx. 100 individual layers with a reflection in the range of 750 - 850 nm and a transparency in the range of 450 - 745 nm.
Precision at the nanometer scale
Challenges in modern optics
In precision optics, a fraction of a nanometer can make the difference between success and failure. But rising performance requirements and complex multilayer systems are pushing traditional coating methods to their limits – making new technologies indispensable.
Precision on the nanometer scale is crucial in precision optics. Even slight deviations in layer thickness can significantly influence the spectral properties of optical filters: bandgaps shift, and phase and polarization properties change. In complex multilayer systems with hundreds to thousands of layers, such deviations accumulate and lead to spectral drifts and wavefront errors.
Increasing demands on materials and processes
As the demands of modern applications grow, so do the technical challenges. These include the need for maximum homogeneity across wafers and batches, controlled layer stress management to prevent so-called ”bow” deformation of thin substrates, and low-particle processes to ensure yield and laser resistance. At the same time, production processes must enable high deposition rates and long, stable production runs.
New generations of devices further intensify these requirements: Thin glass or silicon wafers with thicknesses ranging from 100 to 250 µm, filter-on-chip devices, or complex multilayer systems with total thicknesses of up to 100 µm demand precisely defined gradient profiles and tolerances in the parts-per-thousand range.
Limitations of conventional methods
Many conventional coating methods reach their limits here. Planar cathodes change their geometry during operation due to erosion, which can lead to variations in coating rate and distribution. Partially reactive sputtering is susceptible to fluctuations caused by reactive gases, while manually flipping substrates poses additional risks, particularly with thin wafers. Other methods, such as atomic layer deposition (ALD), are often too slow for thick multilayer stacks.
New system concepts and process control methods are therefore needed: geometrically stable sources, reactive-gas-free cathode processes with separate post-reaction, simultaneous double-sided coating, and model- and data-based in-situ control. These enable reproducible high precision on an industrial scale – even with increasing production volumes.
Coated aspherical plane/convex lenses.
”The challenge lies not only in the precision of individual layers, but in the reproducible fabrication of complex multilayer systems over long production cycles. To achieve this, we need system designs that combine stability, process control, and productivity.”
Dr. Philipp Farr, Group Manager Precision Optical Coatings