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Parallel optical interconnects on-board level requires low propagation loss in wavelength range between 850 and 1550 nm to be compatible with datacom and telecom optical engines. For highest integration density tight waveguide bends and a scalable number of optical layers should be manufacturable for 2D interfaces to optical fiber array connectors and photonic assembly I/O’s. We developed a glass waveguide panel process for double-sided processing of commercial available display glass by applying a two-step thermal ion-exchange process for low-loss multi-mode graded-index waveguides. Multiple glass waveguide panels can be embedded between electrical layers. The generic concept enables fabrication of high-density integration (HDI) electro-optical circuit boards (EOCB) with high number of optical and electrical layers. Waveguides with high NA of 0.3 for low bend losses could be achieved in glass with propagation loss of 0.05 dB/cm for all key wavelengths. Four of those glass waveguide panels were embedded in an EOCB demonstrator with size of 280 x 233 mm2 providing eight optical layers with 96 channels in an area of 2.8 x 1.5 mm2. To the best of our knowledge it’s the highest number of layers that has ever been demonstrated for an EOCB.
© 2015 Optical Society of America
1. Introduction
Optical interconnections have gained interest over the last years and several approaches have been presented for the integration of optical interconnects into the printed circuit board (PCB). The use of a polymer optical waveguide layer appears to be the prevailing solution to route optical signals on the PCB. Most research focuses on embedding one or two optical layers in the PCB [1, 2]. For server applications there will be the need to route multiple optical layers from one point to another and optical fan out circuit designs will be required similar to electrical pad areas, which requires many electrical layers. One example is the icPhotonics optical module having a 2D optical port array with bi-directional 168-channels each with 8 Gb/s and operating at wavelength of 1000 nm. So far the device was demonstrated with an off-chip communication in total of 1.344 Tb/s full duplex. Because of immense data-throughput such high-performance optical components having 2D optical 12x14 port array interfaces with 250 µm channel pitch [3]. For interconnection to integrated optical waveguides the twelve waveguide rows could be deflected by a mirror to two opposite sides and optically interconnected to a six layer optical PCB. Optical interconnects can route the signals to another components or optical fiber ribbon connectors. In our work we address two problems. Firstly, an alternative to polymer waveguides having bulk absorption losses of e.g. 0.20 dB/cm at the wavelength of 1310 nm and 0.82 dB/cm at 1550 nm [4], with the exception of certain perfluorinated polymers because of a change to the bond vibration overtones [5]. Secondly, integration of multiple optical layers into the same PCB for very high channel density. Requirements for the glass waveguides are low optical propagation loss in bends and straights for wavelength ranges of around 850 nm up to 1550 nm for datacom and telecom application. The concept behind this is the planar integration of waveguides by thermal ion-exchange on both sides of display glass and embedding of multiple dual-layer glass waveguide panels in the core of a PCB. The generic PCB fabrication process consists of stacking multiple optical and electrical packages for HDI-EOCB fabrication with low-built in stress utilizing a low temperature lamination process. In previous work a glass waveguide panel with single optical layer was fabricated, laminated into the PCB and connectorized with an MT optical fiber receptacle for pluggable fiber to waveguide interconnection [6, 7]. The paper describes the design, fabrication and characterization of an optical octa-layer and electrical dual-layer EOCB in detail. For planar optical waveguide integration in display glass different process parameters are evaluated for reaching low loss and high NA optical waveguides in 550 µm thick alkali-aluminosilicate glass, which is used primarily as cover glass for portable electronic devices.
2. Optical octa-layer test-bed PCB design
To integrate the optical waveguides on both surface sides of sheet glass by a two-step thermal ion-exchange the process has to be suitable for low-cost batch processing of large area glass panels. On of the challenges is to handle the thin glass sheets. Multiple of such processed dual-layer glass waveguide panels have to be integrated between base material like FR4, prepregs, adheasive and copper layers for HDI-EOCB fabrication. Fraunhofer IZM developed a glass waveguide panel process for area of 228 x 305 mm2 (9” x 12”). Glass sheets containing sodium ions are subjected to a masked diffusion process with silver ions for planar waveguide integration and following maskless processing in pure sodium salt melt for waveguide profile forming. In the work we used Corning Gorilla Glass 1 (distributed by Schröder Spezialglas, Germany) only available in a thickness of 550 µm. The distance between top and bottom glass waveguide layer is dependent on the glass thickness. The goal is to achieve a vertical pitch of 250 µm or a multiple of 250 µm for channel matching to standard MT-ferrules having up to 72 channels in a 12 by 6 array with 250 µm horizontal and vertical channel pitch. In case the maximum of the planar integrated ion-exchanged waveguides is 25 µm below the glass surface the channel to channel pitch in 550 µm thin glass is two times 250 µm. For a 300 µm glass thickness the vertical channel pitch can be 250 µm. High alignment accuracy is required for the double-sided lithography applying visual alignment and laser direct imaging (LDI). For that alignment marks (point with diameter of 1 mm) are patterned during top side lithography [Fig. 1(b)] on which the bottom side layout [Fig. 1(d)] is aligned. Also, marks with line thicknesses of 2 mm for layer registration during PCB lamination are patterned directly on glass during the top side lithography [Fig. 1(b)]. By embedding four of those dual-layer glass waveguide panels, all glass layers have to be accurately aligned to each other and the distance in-between have to be 200 µm or 450 µm reaching a vertical waveguide pitch of 250 µm or 500 µm accordantly. It’s the first time that multiple glass waveguide layers are embedded in the same PCB and full process demonstration has the primary priority at this stage.
Fig. 1 Full EOCB test-bed design (a), optical trace geometry and alignment marks LDI layout for litho#1 front-side glass panel (b), window etching LDI layout for litho#1 back-side glass panel (c), optical trace geometry LDI layout for litho#2 back-side glass panel (d) and etch cover LDI layout for litho#3 front-side glass panel (e).
The optical trace geometry for a glass waveguide panel with an area of 305 x 228 mm2 comprises 17 waveguide groups including arrays of 12 straight waveguides with a pitch of 125 and 250 µm, straights with crossing stubs at an angle of 20°, 45°, 60°, 75° and 90°, splitters, cascading bends with radius of curvature of 10, 15 and 20 mm and 90° arc bends with radius of curvature in range of 1 mm up to 40 mm [Fig. 1(b)]. The optical trace geometry of the top side of the glass is mirrored and integrated from the bottom side, too [see Fig. 1(d)]. The waveguide layout and EOCB test-bed design was optimized for optical insertion loss characterization in the lab. Based on our experiments we found out that it’s straight forward to fully process the glass before embedding which involves waveguide integration, patterning of alignment marks and glass cutting including end-face preparation. After finished waveguide process the waveguide panels have a size of 170 x 245 mm2. The waveguide layout overlaps that area by about 5 mm and is positioned in the center of the 228 x 305 mm2 glass panel. After laser scribing and mechanical breaking the cut is through the waveguides ensuring in-plane coupling interfaces at the edge of the glass waveguide panel in optical quality without the need for additional polishing. For laser separation alignment marks (square 0.5 x 0.5 mm2) are patterned on the panel. Beside different alignment marks two partner logos are located inside a frame. The outer dimensions of the EOCB is 233 x 285 mm2 and therefore larger as the waveguide panel itself which is center located with cut-outs on three sides for access to the glass waveguides coupling interfaces. The design of the EOCB test bed’s outer shape is defined by the trace geometry and size of bulky insertion loss measurement arrangement for optical waveguide characterization in the lab. Figure 1(a) shows the full EOCB design with all optical, electrical and FR4 layers with optical trace geometry in blue, glass panel and marks in red, FR4 in green, electrical traces in orange and yellow.
3. Double-sided glass waveguide panel fabrication
A diffusion process is applied to fabricate optical waveguides in glass panels according to a defined waveguide pattern. For that, a diffusion barrier is deposited on both glass surface sides. We are using an aluminum layer of 400 nm thickness which is DC-sputtered by Creavac Creamet 600 physical vapor deposition (PVD) equipment [Fig. 2]. Then two lithography and wet-chemical processing steps (litho#1, litho#2) are required to pattern the aluminum mask on front and back-side of the panel. For lithography we use dip coating for double-sided photoresist deposition [Fig. 2]. First the exposure of front-side optical trace geometry and alignment marks [Fig. 1(b)] is carried out on an Orbotec Paragon Ultra 200 laser direct imaging (LDI) system [Fig. 2]. Then the glass panel is flipped and back-side is exposed with a very simple design of four rectangles [Fig. 1(c)]. These are for etching windows in the back-side aluminum layer underneath the LDI point fiducials for later alignment of back-side optical trace geometry through the transparent glass. The exposed glass panel is developed and the aluminum layer on both sides is structured with acid treatment. After resist removal a second lithography step (litho#2) follows for completing mask etching on back-side starting with dip coating. In the LDI the optical trace geometry of back-side [Fig. 1(d)] is exposed by aligning the layout using the front-side alignment marks visible through the etched windows. Then the panels are placed for a second time in the developer, aluminum acid and remover before the mask layer is completed on back-side as well. The double-sided patterned aluminum mask serves as a diffusion barrier and the glass panel is completed in readiness for planar waveguide fabrication. During the diffusion process the glass panel is vertically lowered into the furnace containing salt melt. This technique allows batch-processing for volume production. The size of the furnace constrains the glass panel to a size of 228 x 305 mm2. For the salt melt we use a diluted AgNO3 mixture in the first diffusion step. The sodium ions in the glass will be exchanged with the silver ions in the mixture, which causes a localized increase in the refractive index of the glass. The highest refractive index change occurs at the glass surface, with the refractive index decreasing with depth below surface to that of the bulk glass. The first diffusion step is a depth diffusion of silver ions. The concentration gradient of the silver ions below the glass panel surface is proportional to the resulting refractive index gradient. The diffusion process with mask forms an isotropic refractive index profile in the glass. Before aluminum is removed from the glass surfaces resist is patterned to protect alignment markers and logo’s [Fig. 1(e)]. A third process cycle (litho#3) involving resist-dip coating, LDI exposure, development, aluminum etching and resist removal. The concentration of the silver ions in the surface area is reduced in a second diffusion step until the refraction index maximum (waveguide core center) is effectively shifted to a certain depth below the glass surface. For glass separation we use a MDI LD600-H for the CO2-laser scribing of glass. Afterwards the glass waveguide panels are manually broken with optical quality in the waveguide end-faces. In principle an automated process can be applied as widely used to separate Liquid Crystal Display (LCD) panels. After that process the size of this panel is 170 x 245 mm2.
The process parameters for ion-exchange have to be adjusted accordingly to the glass composition because of strong differences in diffusion characteristics and index change. Process definition is required for the first diffusion regarding the mask opening width (w1), process time (t1), temperature (T1) and silver concentration (C1) and for second diffusion regarding process time (t2) and temperature (T2). Process temperature can be performed in the range between 325°C and 400°C above melt temperature and decomposition temperature of the salt. Waveguide opening width is defined with 30 µm because of applying standard PCB processing for lithography. For the process a combined simulation of the waveguide process and the electromagnetic field distribution has been developed to determine waveguide process parameters for the selected glass. The diffusion characteristics are dependent on the specific glass. The characteristics were determined in an experimental study and summarized in a diffusion database. The 2D waveguide process model was developed based on the COMSOL Multiphysics finite element simulation software package [8].
The process is successfully running for different combinations of process time and silver salt melt concentrations. Glass waveguide panels were fabricated with different process parameter sets (set 1 to 7) and characterized for comparing the waveguide performance. Seven different waveguide profiles could be measured vertically and horizontally through the waveguide cross-section by refractive near field (RNF) method at wavelengths of 678 nm as shown in Fig. 3.
Fig. 3 Refractive index plots x-direction (a) and y-direction (b) for waveguides fabricated with different process parameter sets.
The highest numerical aperture of 0.3 was measured for a waveguide with long diffusion time (largest t1) and high silver concentration in the salt melt (highest C1) for process parameter set 7. Also high NA of 0.27 and 0.28 was achieved with process parameter set 1 and set 6 but the distance of the refractive index maximum to the glass surface is only 15 µm and 14 µm instead of 30 µm for set 7. In the case of long processing time for t1 the outer waveguide dimension increased intensely. The evaluation results and process parameter relation of the different sets are summarized in Table 1.
Table 1. Waveguides characteristics dependent on process parameters
A specific advantage of the two step, all thermal diffusion process is the potential of double-sided glass waveguide panel fabrication with only one additional lithography step (litho#2) for back-side mask patterning compared to one-sided panel process. The thermal diffusion process results in an elliptically graded-index waveguide profile, with horizontal waveguide dimensions (along x-axis) with approximately double width of vertical waveguide dimension (along y-axis) due to the isotropic diffusion profile. As a result of the planar waveguide integration, the glass surface remains smooth and the outer dimensions of the glass are unaffected. Four similar panels of the same set were fabricated for embedding into a printed circuit board. A part of a double-sided multi-mode glass waveguide panel with aluminum fiducials is shown in Fig. 4.
Fig. 4 Detail view of glass waveguide panel with rectangle alignment markers (500 x 500 µm2) for laser cutting, point and line markers for layer alignment during lamination and straight waveguide groups.
4. Waveguide characterization
The processed waveguides depend in terms of dimension, NA and position of refractive index maximum related to glass surface according on the process parameters. Thus the efficiency for fiber-to-waveguide and waveguide-to-fiber coupling as well as waveguide propagation losses have to be studied in detail for choosing appropriate process parameters for EOCB waveguide panel fabrication. The different waveguide performance of the sets was studied by insertion loss characterization of straight waveguides with channel waveguide pitch of 250 µm and waveguide length of 170 mm. Waveguide number 1 to 12 are the waveguides on front-side and waveguide number 13 to 24 are located on back-side of the double-sided waveguide panels.
Figure 5 shows all insertion loss results for measurements which were launched at wavelength of 850 nm and 1310 nm with ArdenPhotonics Mode-Controller (meet Encircled Flux requirements of IEC 61280-4-1) and 50/125µm NA = 0.2 GI-MMF and detected with 50/125µm NA = 0.2 GI-MMF with 5 µm air gap between fiber and waveguide end-faces. Obvious differences between front-side and back-side waveguide performance could not be observed in almost all cases. Only waveguides processed with parameters of set 2 and 3 exhibit a loss increase for most channels of the back-side located waveguide array (waveguide number 13 to 24). Those waveguides characterize a low NA, smaller diffusion depth and small distance of the refractive index maximum to the glass surface. In contrast, waveguides with high NA and larger dimensions like waveguides processed with parameters of set 1, 6 and 7 show reduced insertion loss values. For both wavelengths the best insertion loss values are below 4 dB and best results are measured at wavelength of 1310 nm. Similar results can be expected at wavelength of 1550 nm as studied for single-mode glass waveguides in previous work [8]. To understand the dependency for optical losses in detail a cut-back characterization for waveguides processed with set 1 and set 7 were performed and results are summarized in Table 2.
Fig. 5 Insertion loss measured at wavelength of 850 nm (a) and 1310 nm (b) on a group of 12 straight waveguides with channel waveguide pitch of 250 µm dependent on process parameters (set 1 to 7).
Table 2. Cut-back characterization results for waveguides fabricated by process set 1 and 7
The cut-back method was performed with measuring the waveguides on the same side and opposite side as the laser cutting was performed on the glass samples. For set 1 the better results were observed for waveguides located on same side as the cut. In contrast for set 7 separations on opposite side brings better results. It turns out that the quality of the scribing and breaking process for panel separation is dependent on the waveguide process. Different optical measurements were done for each set. The lowest propagation loss at wavelengths of 1310 nm was measured with 0.03 dB/cm for set 7. The propagation loss at wavelengths of 850 nm and 980 nm are a little higher with 0.05 dB/cm. Coupling loss for a 50/125µm NA = 0.2 GI-MMF for set 7 is in the range of 1.9 to 2.3 dB with an air gap of 5 µm without index matching. The results of set 1 show higher propagation losses. Coupling in a fiber with core diameter of 200 µm (1310nm 50-200 MMF) shows different behavior between set 1 and 7. The low NA limitation of waveguides in Schott D263Teco glass, which was fabricated in earlier work, characterizes high losses in tight bends with minimum radiuses of around 30 mm [6]. With the glass under investigation we could achieve much higher waveguide NA and lower waveguide insertion loss for waveguides processed with parameters of set 1, 5 and 7 with almost same loss for a 90° arc bend down to bend radius of 10 mm. For all sets the waveguide insertion loss of 90° arc bends are plotted in Fig. 6. The measurement variations are caused by waveguide and end facet imperfections. For waveguides fabricated with parameters of set 7 high cross-talk was measured with 12.7 dB for pitch of 125 µm, because of ~200 µm waveguide width An acceptable cross-talk value of 45.2 dB was measured for larger pitch of 250 µm. The launch condition for bend and cross-talk performance characterization was same as applied for straight waveguides. Without optimization of glass composition or waveguide process the presented results show the high potential for optical interconnects in selected commercial available display glass. The waveguides meet all requirements for short reach board-level optical interconnects like high NA for tight bends, low propagation loss, low loss at all key wavelength proven at 850, 980 and 1310 nm and low coupling loss to standard 50/125µm NA = 0.2 graded-index MMF.
Fig. 6 Insertion loss measured at wavelength of 850 nm (a) and 1310 nm (b) on 90° arc bends with different bend radius dependent on process parameters (set 1 to 7).
5. Development of generic EOCB fabrication process
Fraunhofer IZM/TU Berlin successfully fabricated different sets of multi-mode glass waveguide panels with aluminum fiducials for alignment and laser cut end-faces for light coupling. Then the panels were delivered to ILFA GmbH for EOCB fabrication. ILFA GmbH successfully developed a generic EOCB process in which optical and electrical packages will be fabricated by applying standard PCB process routines. Then the different packages can be stacked regarding the customer demands in symmetrical or mixed stack-up configurations. This unique process benefits from a low temperature lamination technique with low built-in stress although combining materials with different coefficients of thermal expansion (CTE) like FR4, copper and glass. In this work an EOCB with four glass waveguide panels in the center and covered with FR4 and copper layers was fabricated. The glass panels with thicknesses of 550 µm and an area of 245 x 170 mm2 were inlayed in a FR4 frame having a large cut-out. In parallel, intermediate layers made of two adhesive foils and one FR4 layer were prepared by cold lamination, building the optical packages as shown in Fig. 7. In parallel, the electrical packages were processed by via drilling, plating and electrical trace etching followed by lamination of FR4 layers. This process can be repeated multiple times for electrical multi-layer stack-up fabrication. Optical and electrical packages were visually aligned to each other (e.g. aluminum fiducials on glass) during the stack-up process. In that work we simplified the electrical packages and fabricated an electrical package with one copper layer. The electrical packages are then laminated by adhesive foils to the optical packages resulting in a final PCB thickness of 4 mm. Finally vias were drilled outside the glass area, plated and the outer electrical traces are etched for defining pad arrays in the EOCB. It was found that both the pure laser drilling, as well as the pure mechanical drilling in the overall hybrid composites (glass and FR4) have major drawbacks that need to be evaluated as a “show stopper” for the glass based EOCB fabrication technology. For that a laser TGV approach in combination with material plugging of the TGVs and mechanical drilling was successfully developed in previous work [9]. The last process step was the solder mask application, stencil printing, pad surface finishing and milling of windows (cut-outs) to achieve optical access to the glass waveguide coupling interfaces.
The thin glasses supplied by Fraunhofer IZM/TU Berlin were successfully processed by ILFA GmbH. The EOCB is fully functional and demonstrates the integration of four glasses within one stack-up as shown in Fig. 8. The glass is completely intact and has no built-in tensions. The logos of the partners patterned on the top glass panel are visible through cut-outs in the electrical packages. An LED backlight was installed for illumination.
Fig. 8 Fabricated EOCB (a) with access on the edges to glass waveguides (b), illuminated partner logos patterned on glass (c), electrical BGA pad array (d) and embedded four double-sided glass panels (e).
6. EOCB characterization and discussion
After successful EOCB fabrication the demonstrator was evaluated in detail. The thickness of each layer was measured on a cross-section view using a microscope as shown in Fig. 9. The thickness of the glass is 550 µm. The intermediate layers between glass panels have thicknesses of around 184 µm which is exactly the same as the thickness of the selected intermediate layers, illustrating the accuracy which can be achieved in vertical direction (y-axis) with the introduced lamination technology.
Fig. 9 Side-view of the 4 mm thick EOCB with cross-section details of the stack-up around the glass and cross-section of the optical glass waveguide coupling interface.
To achieve a 200 µm intermediate layer distance for 250 µm vertical waveguide pitch, the thickness can be easily adapted. In horizontal direction (x-axis) the glass layers are misaligned by about −166 µm between optical layer 1 (L1) and optical layer 3 (L3), −133 µm between L1 and L5 and 0 µm between L1 and L7 as shown in Fig. 9. The misalignment of waveguides in same glass layer results from the back-side alignment process with the LDI during diffusion mask exposure and is measured to be −6 µm between L1 and L2, −4 µm between layer 3 and 4, 4 µm between layer 5 and 6 and −16 µm between layer 7 and 8. We also observed an offset of the waveguide facet plane between the embedded double-sided waveguide panels. In z-direction the misalignment of the glass edge was characterized by 130 µm between L1 and L3, 155 µm between L1 and L5 and 500 µm between L1 and L7. For multi-mode waveguides the misalignment of waveguides between front-side and back-side of the same panel are acceptable and optical coupling by a 2D array connector would be feasible with little increase in coupling loss due to accurate glass thickness, waveguide position and slight misalignment during the LDI process. Also the applied cold lamination technique ensures uniform adhesion thickness and distance between glass layers in y-direction. In contrast, the alignment between the glass panels in x or z-direction by visual alignment shows unacceptable high misalignment. For further processing, an improvement of visual alignment has to be addressed or pin registration by drilling holes into the glass panels after waveguide processing has to be introduced to the manufacturing cycle. Beside the misalignment of glass layers, a variation of glass panel sizes because of position tolerances during panel cutting is another potential risk. We measured an offset of + 12 up to + 106 µm of the cut glass panel to the waveguide layout. The existing offset can be explained by the separation process but the deviation has an impact for optical multi-layer coupling and had to be reduced by process improvement.
The fabricated EOCB was placed on our measurement arrangement for insertion loss characterization. The outline of the EOCB was adapted for the PI Lightline F-206 6-axis translation stages holding the optical fibers for launching and detecting the optical signal for insertion loss characterization of the embedded glass waveguides as shown in Fig. 10.
Two arrays of 12 waveguides were measured for all eight optical layers (L1-L8). All 96 optical channels were characterized and results are plotted in Fig. 11 for 125 and 250 µm channel waveguide pitch. All waveguides were launched at wavelength of 1310 nm with ArdenPhotonics Mode-Controller (meet Encircled Flux requirements of IEC 61280-4-1) and 50/125µm NA = 0.2 GI-MMF and detected with 50/125µm NA = 0.2 GI-MMF with 5 µm air gap between fiber and waveguide end-faces. The mean value for 250 µm pitch for insertion loss at wavelength of 1310 nm is 3.3 ± 0.33 dB and cross-talk was measured with 34.8 dB. Similar results could achieved for the smaller pitch of 125 µm. The mean value for insertion loss of 96 measured channels is 3.3 ± 0.32 dB and cross-talk is 32.0 dB. For the demonstrator the process parameters of set 1 were used for waveguide fabrication. The improvement in cross-talk performance (125 µm waveguide pitch) compared to set 7 can be explained by smaller waveguide width of ~120 µm. The expected loss value of these waveguides based on the characterization results before lamination [Table 2] is 3.1 dB. The integrated waveguides of the demonstrator show an insertion loss increase of only 0.2 dB.
Fig. 11 2D waveguide array characterization with 12 channels and eight layers for 250 µm channel waveguide pitch (a) and 125 µm channel waveguide pitch (b).
The multi-mode glass waveguides are located close to the glass surface. The measurements indicated no significant loss increase as result of the lamination process. Light propagation in the EOCB is presented with very low loss. Eight optical layers with 96 channels are embedded in the EOCB with a cross-section area of 2.8 x 3 mm2 for 250 µm channel waveguide pitch and 2.8 x 1.5 mm2 for 125 µm channel waveguide pitch. Graded-index glass waveguides have been operated with data rates of up to 32 Gb/s [7] which gives a possible data traffic of 3.072 Tb/s for such 96 channel parallel optical link.
7. Conclusion
The process development and waveguide characterization of double-sided glass waveguide panels and embedding of multiple glass panels by standard PCB fabrication routines was in the focus of our work. Low loss waveguides with high NA for tight bends with radius of 10 mm could be demonstrated successfully. The glass waveguides have very low loss at all key wavelengths for datacom and telecom application. An EOCB demonstrator with eight optical layers and two electrical layers was successfully fabricated and optical waveguide loss and channel position accuracy studied in detail. To best of our knowledge it’s the highest number of waveguide layers that has ever been demonstrated for an EOCB.
Acknowledgment
Some of the research leading to these results has received funding from Bundesministerium für Wirtschaft und Energie (ZIM program) under grant number KF3030301PR2/KF2172431PR2.
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