Optical interference filters are key components for space applications with more and more stringent specifications. As of today, it is common to design and fabricate filters with several hundreds of layers associated with several tens of microns of total thicknesses. This is particularly the case for narrow bandpass filters, notch filters, dichroic… Despite the significant improvement in the stability of the deposition machines and processes, precisely controlling the thickness of each layer of the stack remains one of the limiting step to achieve close to theoretical performances. This can be achieved through in-situ monitoring of the optical performances during deposition at a single wavelength (monochromatic optical monitoring) or over a broad spectral range (broadband optical monitoring). As of today, it is not clear what technique must be used depending on the type of filter and what are the limits of each of them. In this paper, we will review the different advantages and limits of each technique and how to implement them and improve their sensitivities according to the type of components. We will demonstrate various examples of thin-film components fabricated by Plasma Assisted Reactive Magnetron Sputtering (Bühler/Leybold optics HELIOS) optically monitored using Bühler broadband and monochromatic optical monitoring systems. We will conclude on the actual capabilities in terms of spectral performances of complex multilayer filters.
We have assessed and reduced the particle count in coatings from a magnetron sputter coater used for production of optical coatings for applications in photonics and semiconductor industry. Results for particle levels in single layers from Al2O3, SiO2, Nb2O5, Ta2O5, and TiO2 showed semiconductor grade particle levels for the upgraded deposition system. Moreover, particle levels were also investigated for optical filter stacks deposited on bare Si, glass substrates and actual CMOS device wafers, which were used for the manufacturing of hyperspectral imaging (HSI) sensors. In all cases, low particle counts were detected in the optical filters as expected from the results obtained for the single layer. It could be shown that the coating on the device wafers had no negative impact on the production yield of the HSI sensors.
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 (Bühler 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, 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 Bühler 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 thickness. Various types of thickness gradients are presented.
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 Bühler 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 Bühler 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.
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 Bühler 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.
Manufacturing all dielectric mirror coating with reflectivity values of more than 99.99 % is still a challenge to achieve. Losses caused either be transmittance, absorption or scattering have to be maintained well below 100ppm. Increasing the layer number for minimizing the transmittance losses usually increases the scattering by the growth of the roughness. High energy processes are required to minimize or avoid this behavior, but which are a challenge for avoiding unwanted contamination and interface absorption due to unwanted sputtering. As high energy process we used for the preparation of high reflecting dielectric mirrors plasma assisted reactive magnetron sputtering with a Helios 800 system. The machine was equipped with 3 cathode position for low and high index materials. We used metallic tantalum and hafnium targets for the preparation of the high index, silicon and silica targets for the low index. Metallic targets were powered with mid frequency, whereas the quartz target was sputtered by RF. As substrate we used either super polished fused silica or standard silicon wafer. The optical properties of the substrates we characterized by CRD, Laser calorimetry and spectrophotometric measurements. All combination allowed us to reach reflectivity values above 99.99%, with total deficit levels as low as 36ppm.
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.
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.
For the production of high performance multilayer systems with tight specifications and large numbers of layers optical
monitoring is essential. Substantial progress was achieved by the introduction of direct monitoring on the rotating
substrate holder. Pre production analysis by computer simulation of coating processes helps to optimise monitoring
strategies and reduces the effort for expensive and time consuming test runs significantly. However not in any case we
can find error compensating monitoring strategies. Also we have to deal with error accumulation effects especially with
multi layer systems with large number of layers. Changing the monitor glass after the layer stack is deposited partly is
a useful method to discontinue accumulation or to simplify the monitoring strategy.
A testglass changer which helps to suppress error accumulation was developed and automized. The testglasses are
located on the rotating substrate holder which may be a calotte or a plane substrate holder. It combines the advantages
of direct monitoring with the flexibility to change testglasses in a fully automatic process. The basic principle will be
described. Results of multilayer systems demonstrate the benefits of the newly developed testglass changer.
For the production of high performance multilayer systems optical monitoring is essential. Substantial progress was
achieved by the introduction of direct monitoring on the rotating substrate holder. It is a complex task to develop a
stable monitoring strategy for multilayers with a large number of layers and irregular thicknesses. The verification
and improvement requires the feedback of more or less numerous real coating runs. This expensive and time
consuming trial and error method can be reduced significantly by computer simulation of coating runs.
A new software tool which simulates the coating process with monochromatic optical monitoring is introduced.
Process instabilities are described by systematic and random errors of the deposition rate, refractive indices, etc. For
the simulation of the monitoring curve real monochromatic bandwidth, signal noise, measurement frequency, etc. are
taken into account. A UV-IR cut filter and a single notch filter design were simulated with virtual deposition runs. In
both cases the simulation results were confirmed by real coating processes.
For Notch Filters, Rugate designs with a small index contrast and apodisation are well known in the literature. The
required deposition of gradient index layers or so called flip flop structures is very complicated and difficult to
manufacture. Higher order H/L stacks of coating materials with high index contrast result in very thick layer stacks. In
our approach we replace the second refractive index by equivalent layers consisting of H/L materials with high index
contrast. This leads to a combination of thick (>100nm) and very thin layers. Stable coating processes with dense layers
are strict requirements. Another challenge is the accurate thickness control of very thin layers in the nanometer range. Single notch filters were produced with PIAD and broad-band optical monitoring. The most challenging filters were demonstrated with magnetron sputtering and monochromatic optical monitoring. Some outstanding results of single and multiple notch filter coatings will be presented.
The deposition of oxide coatings with excellent optical and mechanical properties requires powerful plasma or ion
sources. We investigated the layer performance of oxide coatings using a large aperture radio frequency powered plasma
source for plasma ion assisted application and related the achieved coating performance to beam parameter of the source.
The coatings exhibit low compressive stress values, high refractive indices and low absorption values. Using the new
type of source in combination with direct optical monitoring on the dome for the production of challenging interference
filters shows a huge potential in terms of stability, running costs and easy maintenance. Examples of application in the
visible spectral region will be given.
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.
The production capabilities for optical multilayer coatings were improved significantly in the last decade. So called "shift free" coatings have become a standard in the coating production. Direct optical monitoring plays a key role to improve the layer thickness accuracy and takes advantage of error compensation effects. For the production of DWDM filters direct monitoring was introduced in the last decade. Continuous measurement is applied on relatively small substrate areas. (Ø < 200 mm). The paper reports substantial progress which has been achieved for coating systems with large area substrate holders (up to Ø 1050 mm). The stationary light spot of a single wavelength optical monitor is far out of centre of the rotating substrate holder. Intermittent monitoring on a substrate or a witness is applied. This technique enables rapid prototyping with tight specifications and high yields in large area batch coaters. Application results of challenging optical multilayer systems are demonstrating clearly the potential of this powerful monitoring technique. The monitoring capability was investigated for a lot of different layer systems such as dielectric mirrors, anti-reflection coatings, sophisticated edge filters, polarizer coatings, beam splitters and multiple cavity band-pass filters. Strong coincidence of theory with experiment was achieved with PIAD and magnetron sputtering. Reproducibility experiments have clearly shown the benefits of this monitoring technique.
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 SiO2, Al2O3, MgF2 and LaF3 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, MgF2 and LaF3, conventionally coated at very high temperatures, have been performed to demonstrate the improved capabilities of the optimized SYRUSpro DUV for DUV applications with all the new features.
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.
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.
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.
Plasma-IAD with the APS (advanced plasma source) has been introduced into the market in 1992. Up to now this technique is used worldwide in almost 100 coating systems. A large number of different layer systems has been investigated in R&D and applied in production. For ophthalmic applications plasma-IAD with the APS is used for antireflection systems as well as for wear resistant coatings onto organic substrates. New processes which combine the AR coating and the hardcoating on ophthalmic lenses have been successfully introduced into mass production. Plasma-IAD is also used for laser protection coatings onto plastic substrates. The ability for the production of shift free multilayer coatings is utilized in manufacture for many applications such as steep edge filters for color separation, rugate filters for laser protection, narrow-bandpass filters in the NIR region for wavelength multiplexing in the field of fiber optic communication or for radiometers in the UV-B region. A review of the development of some important APS based coating processes and applications shows the flexibility of plasma-IAD with the APS. Actual evaluations of the optical constants of Ta2O5 layers deposited with APS assistance show a low extinction coefficient and a stable refractive index. New results of the performance, temperature behavior and long term stability of some interference filters confirm high packing density and low absorption of the films produced with plasma-IAD. The result of a reproducibility experiment demonstrates high process stability and high monitoring accuracy.
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 YF3 and YbF3. Optics with total absorption of about 0.1% and with damage thresholds of up to 30 J/cm2 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%.
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