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International audience; Variable filters are key components for compact spectral imagers. In this paper, we present a method for the fabrication of linearly variable filters based on Bühler HELIOS machine (plasma assisted reactive magnetron sputtering). These filters are obtained by producing a variation of the thickness of all the layers of the coating, using adapted masks placed in between the sputtering targets for the low and high refractive index materials and the substrates. Variable band pass filter from 550 nm up to 1000 nm is demonstrated.

To monitor complex thin film filter designs, multiple witness glass strategy is often required. We present a strategy of design division into sub-stacks in combination with mixed monitoring techniques to increase thin-film filter accuracy.

Optical monitoring of thin film interference filters is of primary importance for two main reasons: possible error compensation and greater thickness accuracy of the deposited layers compared to non-optical methods. For many designs, the latter reason is the most crucial, because for complex designs with a large number of layers, several witness glasses are used for monitoring and error compensation with a classical monitoring approach is no longer possible for the whole filter. One optical monitoring technique that seems to maintain some form of error compensation, even when changing witness glass, is broadband optical monitoring, as it is possible to record the determined thicknesses as the layers are deposited and re-refine the target curves for remaining layers or recalculate the thicknesses of remaining layers. In addition, this method, if used properly, can, in some cases, provide greater accuracy for the thickness of deposited layers than monochromatic monitoring. In this paper, we discuss the process of determining a strategy for broadband monitoring with the goal of minimizing thickness errors for each layer of a given thin film design.

International audience; In this paper we study the wavelength selection process for optical monitoring of thin film filters. We first discuss the technical limitations of monitoring systems as well as the criteria defining the sensitivity of different wavelengths to thickness errors. We then present an approach that considers the best monitoring wavelength for each individual layer with a monitoring strategy selection process that can be fully automated. We finally validate experimentally the proposed approach on several optical filters of increasing complexity. Optical interference filters with close to theoretical performances are demonstrated.

Optical monitoring is a go to method for complex filters deposition; however, it can be easily shown that filter performance is dependent on monitoring strategy. When a non-quarter wave design needs to be deposited, usually one or minimal number of monitoring wavelengths is selected. This allows to use correction algorithms based on swing that are compatible with level-cut monitoring to a great extent. This approach has a significant drawback that it is very difficult to find one or few wavelengths that can be used for all layers of a complex filter. We present a different approach that relies on the selection of the best monitoring wavelength for each layer using pre-defined criteria that secure minimized thickness errors for each individual layer. Wavelength selection process uses several important criteria, such as monitoring wavelength sensitivity to errors in previous layers, transmittance evolution speed versus layer thickness growth, noise of measurement setup… We show that an additional important criteria is spectral resolution of the optical monitoring system and its impact on filter’s spectral response after each layer. Last, to ensure stable deposition we show that some precautions must be made to avoid false turning point detections. Using a binary approach for each criteria (pass or fail), monitoring wavelengths then can be selected automatically based on criteria defined above. In this work we demonstrate that such an approach can be implemented on stable deposition technique such as plasma assisted reactive magnetron sputtering (Buhler HELIOS machine) for different types of filters with various complexities. We illustrate our results for example on an 8-layer beamsplitter, a 37 layer D65 compensation filter, or a 100 layer custom shape spectral filter that are all very sensitive even to small thickness errors. Similar or better spectral performances are achieved compared with classical optical monitoring approaches but with an automatically determined optical monitoring strategy.

Variable filters, in which the bandpass central wavelength shifts along a dimension of the component, is a promising way to simplify and to miniaturize imaging spectrophotometer systems for spatial observation. In this paper, we present a new method for the fabrication of variable filters with different spatial profiles based on plasma assisted reactive magnetron sputtering. Variable filters are based on the variation of the thickness of the layers deposited on the substrate. The layers are manufactured using a Buhler HELIOS machine. To insure a non-uniformity of the deposited layers, adapted masks are placed in front of the targets for the low and high refractive index materials. The uniformity of these layers is measured via a homemade set-up allowing a local measurement of the transmission over the component surface. We demonstrate the fabrication of a narrow variable filter where all the thicknesses of all layers are changing, resulting in an overall shift of the filter.

Variable filters, which central wavelength of the bandpass shifts along one dimension of the component, is a promising way to simplify and to miniaturize imaging spectrophotometer systems for spatial observation. In fact, by combining a dispersive element and a variable filter, it is possible to realize a spectral and a spatial filtering based on pushbroom technique. While there were numerous methods developed in order to produce variable filters, at this date, there is no simple, reliable and repeatable method. In this paper, we present a new method for the fabrication of variable filters based on plasma assisted reactive magnetron sputtering (PARMS). Variable filters are obtained upon the variation of the thickness of some or all the layers deposited on the substrate. The layers are deposited using a Buhler HELIOS machine in order to benefit from the high stability of the deposition process. To insure a non-uniformity of the deposited layers, adapted masks are placed in front of the targets for the low and high refractive index materials. The uniformity of these layers is measured via a custom set-up allowing a local measurement of the transmission to perform a mapping of the transmission over the whole component aperture. In this paper, we present the fabrication of layers and filters with variable thickness. Various types of thickness gradients are presented.

We present a thorough study of a bandpass filter for the near-infrared (IR) region, with bandwidth below 20 nm, high transmission, and a broad rejection band [350-1100] nm. This filter is angularly tunable between 0 and 50° and shows a shift of the central wavelength from 970 nm down to 880 nm, without major changes in its bandwidth and spectral shape for unpolarized light. We first provide a description of the design procedure and then carry out an experimental demonstration by using plasma-enhanced reactive magnetron sputtering. We finally present an accurate characterization of this class of filter by using a custom optical system.

We discuss the coating industry needs for equipment for the past 45 Years. Recent developments for sensor application in consumer electronic products regarding direct coating of CMOS and glass wafer with interference filters are presented.

HfO 2 films, using metal hafnium as starting material, are deposited by plasma-ion assisted electron evaporation with different Advanced Plasma Source (APS) bias voltages. The refractive index and extinction coefficient are calculated, the chemical state and composition, as well as the stress and aging behavior is investigated. Laser induced damage threshold (LIDT) and damage mechanism are also evaluated and discussed. Optical, structural, mechanical and laser induced damage properties of HfO 2 films are found to be sensitive to APS bias voltage. The film stress can be tuned by varying the APS bias voltage. Damage morphologies indicate the LIDT of the HfO 2 films at 1064 nm and 532 nm are dominated by the nodular-defect density in coatings, while the 355 nm LIDT is dominated by the film absorption. HfO 2 films with higher 1064 nm LIDT than samples evaporated from hafnia are achieved with bias voltage of 100 V.

Silicon layers with low extinction coefficient and moderate compressive stress were deposited by plasma assisted reactive magnetron sputtering. High transmittance band pass filter with low angle shift for wavelength below 1000nm were produced.

High throughput and accuracy of a PIAD coating system with a large dome and an advanced RF-plasma source is demonstrated. Thickness monitoring was performed with a newly developed wideband monitoring system.

A newly developed wideband optical monitoring system is based on a fast triggered spectrometer with a high dynamic array detector with low signal noise. It is useful for fast in-situ transmittance measurements on the rotating substrate holder during the deposition. The spectra can be stored and used for reverse engineering analysis. Layer thickness control with monochromatic monitoring strategies can be applied by using a selected single wavelength from the array detector. The high dynamic detector supports the classical turning point monitoring as well as trigger-point cut-offs with online corrected end points. The basic system design and functionalities are described. The results of multilayer systems demonstrate the performance of the new developed monitoring system.