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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.

We developed a novel sputter process for deposition of amorphous layers from TiO2 with low defects levels for precision optics and photonics applications. The layer properties were investigated by XSEM, GIXRD, and particle measurements.

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.

Completing our suite of deposition equipment, we are developing a new Ion Beam Sputtering (IBS) System with different substrate configurations: the High Throughput version (HT) and the High Precision version (HP). The HT version enables the coating of 4 planets of up to 350mm diameter substrates, whereas the HP version allows coating of substrates up to 600mm diameter in a single planet configuration. The IBS system is configured with a Buhler proprietary Optical Monitoring System for layer termination, a large 22cm RF sputtering source, and a LION plasma source for assist. In this presentation the optical performance of this IBS coatings, including LIDT, absorption, total loss and residual coating stress, will be discussed and compared to the other available deposition techniques, such as Plasma Assisted Reactive Magnetron Sputtering, and Plasma Ion Assisted deposition (PIAD). Preliminary results of a 1064nm mirror show less than 5ppm absorption, reflectivity’s of 99.997%, and no visible damage in CW LIDT testing up to 10MW/cm2. Pulsed laser damage testing is in process and will be reported. These results will be compared to the coatings being done using PARMS and Evaporation.

Random effects in the repeatability of refractive index and absorption edge position of tantalum pentoxide layers prepared by plasma-ion-assisted electron-beam evaporation, ion beam sputtering, and magnetron sputtering are investigated and quantified. Standard deviations in refractive index between 4 ∗ 10 -4 and 4 ∗ 10 -3 have been obtained. Here, lowest standard deviations in refractive index close to our detection threshold could be achieved by both ion beam sputtering and plasma-ion-assisted deposition. In relation to the corresponding mean values, the standard deviations in band-edge position and refractive index are of similar order.

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.

Random effects in the reproducibility of refractive index and absorption edge position of Ta2O5 layers prepared by plasma-ion assisted electron beam evaporation, ion beam sputtering, and magnetron sputtering are investigated and quantified.

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.

A new, highly efficient impedance matching network is introduced , increasing the LION300 RF-source efficiency by approximately 20% and enabling considerably higher SiO2 rates for the production of shift-free and low scattering interference filters. Results of single layer coatings and a UV-IR cut filter are presented.

The cut-off and layer thickness accuracy of single layers is discussed. Error compensation effects of complex multilayer systems are investigated with computer simulated process runs and confirmed by practical coating experiments.

Plasma assisted reactive magnetron sputtering is used to produce demanding interference filters.Using radio frequency powered magnetrons can improve the layer quality in respect to defect density and laser induced damage threshold.

The performance of a PIAD process with RF-plasma source in a large box coater is investigated in respect to high performance coatings. UV-IR cut and BP-filters with low losses are presented.

The success of fabrication of optical coatings depends on a proper choice of a theoretical coating design and on the choice of monitoring strategy that provides low thickness errors for the chosen design. Software tools described in this presentation help an optical coating engineer to investigate a potential manufacturability of a given theoretical design when various monochromatic monitoring strategies are applied. This may help to reduce or even eliminate test deposition runs required for a successful coating fabrication.

Single wavelength optical monitoring in intermittent mode was investigated. The achieved coincidence between theory and experiment is outstanding. This monitoring technique enables rapid prototyping with tight specifications and high yield in large box coaters.

The performance of a new large rf-plasma source for large box coaters is investigated in respect to layer performance and achievable growth rates. The optical constants of SiO2 and TiO2 thin films are presented.

Future production of high quality laser components asks for both, the precise preparation of low loss multilayer stacks and a clean room compatible innovative deposition process. In addition a cost-effective thin film filter production is required in order to transfer new developments like ultra fast pulse laser technique from research to economical products and applications. One of the most promising candidates is magnetron sputtering due to the potential of excellent film properties, a fully automatic clean room compatible manufacturing process and a high productivity. Similar film qualities as with ion beam sputtering are realised but with a more than 10 times higher productivity. The reproducible production of multilayer broad band mirrors with controlled group delay dispersion and low losses is still a big challenge. Promising results were achieved on the basis of silica and niobia and will be presented.

The requirements to produce high performance coatings increase dramatically when moving from 248 nm to 193 nm. The quality of DUV thin film components is mainly determined by the optical properties of the applied layer materials. The reduction of losses due to scattering and absorption of dielectric materials is essential for excellent properties of the coating results. The most common oxide and fluoride materials SiO 2 , Al 2 O 3 , MgF 2 and LaF 3 have been investigated and optimized. Plasma ion assisted deposition was applied for the deposition of the oxide materials, using improved coating equipment such as the modified APSpro (advanced plasma source). The paper reports the results of DUV coatings using plasma ion assisted deposition for the oxide materials. Single layers of silica and alumina and multilayer systems with both materials were investigated. In addition, MgF 2 and LaF 3 , conventionally coated at very high temperatures, have been performed to demonstrate the improved capabilities of the optimized SYRUS pro DUV for DUV applications with all the new features.

Plasma- or ion-assisted coating processes represent the state of the art for the production of high quality interference coatings. To meet the special filter demands of the telecom industry, special types of coating equipment have been developed with outstanding capabilities for producing complex layer systems. However, their limited productivity makes them unsuitable for cost-effective filter production on traditional sized substrates. Therefore, the challenge for future coating equipment is to achieve the same order of accuracy in process control on large substrate areas together with short production cycles and superb layer properties. The existing plasma and ion sources are one limitation for achieving short production cycles. It is their limited performance that restricts the obtainable coating rate for dense, shift-free interference coatings. The direct optical monitoring is, together with the applied coating technology, the key for the production of complex layer systems. This has been shown in many applications, but is still a challenge on large scale.

The paper deals with single wavelength optical monitoring in batch type coating systems. Indirect monitoring on stationary test slides and direct monitoring on a fast rotating substrate holder was investigated. Application results will be presented.

The performance of a tandem plasma source solution for large box coaters is investigated for layer performance and achievable growth rates. A new fully automated sputter coating system for producing complex interference filters is presented.

Plasma Ion Assisted Deposition (PIAD) with the Advanced Plasma Source (APS) results in 50 GHz narrow band pass filters with FWHM of 0.33 nm at a wavelength of 1550nm. The pass bandwidth was greater than 0.22 nm and the temperature shift could be adjusted between 1 pm/°C and –2 pm/°C.

Plasma-IAD with the APS has been applied for a large number of different layer systems in production an ind R and D. The ability for the production of shift free multilayer coatings for the visible and NIR spectral range is utilized in manufacture for many applications such as steep edge filters for color separation, rugate filters for laser protection and narrow-bandpass filters for wavelength division multiplexing. An overview was given. Shift free narrowband filters for the UV-B region were published in 1996. The paper reports the result of UV coatings using plasma ion assisted deposition. Tantala/silica and hafnia/silica combinations have been used for multilayer coatings in the UV-A and UV-B spectral range. Single layers of silica and alumina and multilayer systems with both materials were investigated in the UV-B and UV-C region. The coatings were characterized by obtained transmittance and reflectance curves as well as absorption and scattering measurements. The temperature stability results are compared with coatings in the visible and NIR spectral range published.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Rare earth fluorides have been investigated as possible coating materials for carbon dioxide laser optic components. AR and PR coatings on ZnSe substrates were produced by means of ion assisted deposition (IAD) of YF 3 and YbF 3 . Optics with total absorption of about 0.1% and with damage thresholds of up to 30 J/cm 2 were produced. Effects of IAD on stoichiometry and chemical state of the rare earth ions have been examined by x-ray photoelectron spectroscopy (XPS). In-situ XPS revealed that ion bombardment, as used in IAD, and sputter cleaning result in fluorine deficient films. Oxygen incorporation during preparation could be kept below 1 - 5at%.

Rare earth fluorides have been investigated as possible substitute materials for standard thorium-fluoride layers in CO2-laser optic components.