2.1

Preparation of plate mass production

An important aspect of SPO is that the individual curvature of the plate, as required to form a concentric optic, is generated during the stacking process as described later in this paper, which means per ring of Athena (15-20 rings) only two types of mirror plates are required, which differ in length but are otherwise identical. All other plate parameters can be kept the same, greatly facilitating their production process, the logistics and reducing the cost. SPO uses a semiconductor mass production process to generate rectangular silicon plates with a wedged silicon oxide layer. We currently produce for the three development radii three types of geometries of plates (see Figure 1), where fully automated ribbing and dicing machines are used to machine grooves into oxidized 12” wafers, leaving ribs and a thin membrane. Etching is performed to remove micro-cracks in the surfaces. The outside of the membrane is the x-ray mirror and on the inside the ribs are used as interconnects to bond during stacking to the next mirror. As the ribs are an integral part of the former wafer, the spacing of the mirrors is fixed to the original wafer thickness of 775 μm. We have now demonstrated a production capacity of several thousand plates per year and we can control the dimensions of the plates through programming the dicing machines. The mirror thickness used for development is 150 to 170 μm, the rib width 170 μm and the rib pitch is 1 to 2.3 mm. The wider rib pitch is especially important to increase the field of view of the telescope as for example required by the WFI detector: this was demonstrated first on the inner radii plates. We plan for the near future to increase the rib spacing also on the middle and outer radii, after having first improved the performance of the stacks, as discussed later in this paper.

Figure 1

– Cross section of a silicon pore optic mirror plate, showing the ribs and the membrane (Left). The right image shows the three types of plate used for the process development at the outermost radius (r=1500 mm, left), the middle radius (r=737 mm, middle) and the innermost radius (r=250 mm, right). Each plate type demonstrates different aspects of the technological parameter space, as required to pass the Athena MAR. The outer plates are very wide with a 1 mm wide rib spacing. The innermost radius plates are very long and demonstrate the ability to increase the rib pitch to 2.3 mm and also the capability to vary the rib position within a plate, such that outer ribs align when plates are bent and stacked. Note that these plate types are technology demonstrators and do not represent the final Athena geometry.

After having increased the mirror plate production capacity at the first plate supplier, in the past year we were able to add a second source of mirror plates. This is important for programmatic reasons, in view of the flight model production. The second supplier has recently produced the first batch of middle radius plates that have been successfully stack (see Figure 2).

Figure 2

– Photographs of the very first stacks where plates of two different suppliers (A, B) have been stacked on top of each other, demonstrating their compatibility. The left image shows the stack from the side with the different laser marked plate identifier highlighting the two suppliers. The right image shows the front, where no visible difference can been seen.

2.2

Plate process development

We use as starting material for the mirror plate highest grade 12” wafers with a residual total thickness variation of < 200 nm over the 300 mm diameter surface, measured by the wafer suppliers, being on the order of 20 nm on the area of a plate. During the plate production process we alter the surface by growing a silicon oxide of ~ 2 μm into both sides of the wafer, which we wedge [18] down to several hundred nm with a relative accuracy better than 3%, to be able to later stack plates into a focusing geometry. Optical thin film metrology can accurately measure the oxide layer, and the uncoated wedged plate quality has been determined to be of sufficient quality to meet the requirements of Athena. To advance the plate quality, especially the wedging process, further and to increase the margins in the error budget, we require a system that allows measuring the combined thickness variation of the silicon oxide and the silicon in one go, which is not trivial. We are currently commissioning a system that combines visible and infrared light to measure the absolute thickness variation, which will become operational in the second half of 2018. This system will then be used in the coming year to guide the plate suppliers in the improvement of the wedging process.

With the support of SRON and ESA we are also testing different plate process improvements that can benefit Athena in terms of angular resolution, effective area and straylight reduction. One example is the roughening of the ribbed side of the plates, such that the back of the mirror and the side of the ribs have a significantly reduced reflectivity for x-rays, reducing straylight. Figure 3 (left) shows a microscope image of the ribbed side of a test plate, directly after ribbing. We then apply a chemical process which roughens the backside of the mirror and the side of the ribs. In optical wavelengths the silicon appears now darker, as light is absorbed in the etch pits (Figure 3 right). We also tested in x-rays, using the Four-Crystal-Monochromator (FCM) beamline of the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY II synchrotron radiation facility in Berlin, Germany [19]. We performed reflectivity measurements in θ/2θ configuration on the side of the ribs and the backside of the mirror (membrane) at 2.5 keV and at 10 keV. The results (see Figure 4), comparing the reflectivity of a regular plate to the roughened one, show a reduction of reflectivity by several orders of magnitude, for both measured energies.

Figure 3

– Microscope images of a middle radius plate, showing the ribbed side. The left image shows the plate before roughening of the non-reflective side. The right image shows the plate after processing.

Figure 4

– Reflectometry comparing the ratio the reflectivity of a regular to a roughened plate as a function of grazing incidence angle, at two different energies (2.5 keV left, 10 keV right). The data was recorded at the FCM beamline of PTB at BESSY II. The reflectivity of both the ribbed side and the backside of the silicon mirror can be reduced by several orders of magnitude as a result of the blackening process.

This means, if Athena requires it, we would have a technique to adjust the reflectivity of the backside of plates and if necessary, can include it in time for the MAR in the mass production process. Other areas of plate development address thinning the membrane to gain effective area and widening the rib spacing to increase the field of view.

3.1

Preparation of coated plate mass production

In April 2018 a large drum coating machine using magnetron sputtering to deposit thin film coatings on the mirror plates has been delivered and installed at cosine, next to the stacking robots. This marks an important step to demonstrate the mass coating capability at the MAR, as so far all representative coated stacks have been produced with smaller scale coating systems. The industrial system has been custom designed and holds up to three magnetrons. It also has a plasma cleaning system and can coat up to 150 plates in one fully automated run, with a capacity of about 100.000 plates per year.

The plates are supplied with lithographic masking by the plate suppliers and are loaded into frames that are hung into a carrousel inside the coating machine, through a large door from the cleanrooms (see Figure 5). The machine itself is located in a grey-room and can be serviced and cleaned from there. The system is then evacuated and automatically performs the coating run. After several hours, depending on the coating recipe, the machine is vented and the coated mirror plates undergo a lift-off process before they are being stacked. We are currently in the process of performing on-site acceptance tests and will then continue to commission the system until the end of the year. We will then use the coating system to produce the coated stacks required for the QM.

Figure 5

– Photographs of the newly installed drum coating machine for mass production of coated SPO plates. The left image shows the backside of the machine inside a grey room for easy access and service. The machine has a plasma cleaner, two magnetrons and space for a third one. The right image shows the front side of the vacuum chamber as seen from inside the cleanroom of cosine. The large door can be opened to load and unload carriers holding up to 150 SPO plates. The system is then closed, evacuated and the carousel moves the plates past the magnetrons to sputter single or multi-layers. The system is fully automated and the recipe can be set via the computer system shown in the right image.

3.2

Coating process development

The coating process itself is currently being developed by DTU Space [20-22] and is then applied and tested on the industrial coating machine described above. First results of single, bi- and multi-layers of Ir and B⁴C coated on silicon using the industrial coating machine show very good results, demonstrating that the research grade quality can be matched with the mass production systems being prepared for Athena. In order to follow the developments of DTU on the Athena baseline coating design, we are in the process of also procuring a SiC target. In 2018 we will complete the commissioning of the coating machine, by DTU carrying out studies on the uniformity of the coating, the plasma cleaning, collimation effects, film structure and preliminary composition studies. In parallel to the coating process development we also work with the plate suppliers and DTU on the improvement of the lithographic masking process. The plates only need to be coated in between the ribs, to leave the bonding areas to be silicon oxide. For small volume production the currently used process is sufficient, however for larger volumes dedicated lithography equipment and suitably sized lift-off baths will have to be developed, commissioned and tested.

4.1

Preparation of stack mass production

In 2017 we performed a first production run for second generation f= 12 m middle radii mirror modules, to meet the requirement to supply 4 mirror modules [23] for the two parallel Assembly, Integration and Verification (AIT) activities [24, 25] of ESA, another part of ESA’s TDP plan, to develop methods to co-align and integrate mirror modules into the Athena optical bench. Such a production run allows testing all parts of the stacking chain, from plate inspection, cleaning to stacking on two different types of mandrels, using several stacking robot operators working in shifts, to x-ray testing them at XPBF 1 (X-ray pencil beam facility) [26, 27] or the new XPBF 2.0 (X-ray parallel beam facility) [28, 29], both operated by PTB at the BESSY II synchrotron radiation facility. As preparation for the production run we had in the first half of 2017 upgraded the stacking facilities, water supplies, control equipment and trained additional stacking robot operators. We also reduced the stacking times from 45 minutes per plate to less than 20 minutes. Furthermore we upgraded the data analysis pipelines in order to be quicker in the characterization of stacks and mirror modules. We successfully produced a total of 18 stacks with a height of 35 plates, consuming more than 600 plates within a few months. Of these, 16 were integrated into 4 mirror modules (see Figure 6), within a period of 6 weeks.

Figure 6

– Photos of two of the four mirror modules, differentiated by their bracket type, produced during the production run in 2017. The left image shows a mirror module prepared for the AIT activities led by TAS Switzerland, the right photo shows a Media Lario module. Each mirror module consists of 2 pairs, called an XOU, of a primary and a secondary mirror stack.

From the production run we also learned valuable lessons which are being addressed this year to prepare for the MAR in 2021:

Figure 7

– Motorised 2^nd generation mirror module alignment and assembly jig, outside of the vacuum chamber of XPBF 2.0 of PTB at BESSY II. It has three hexapods to co-align in vacuum and under x-ray illumination 3 stacks to the fourth one. After alignment, the chamber is vented and the jig can be moved outside of the chamber to inject glue to fixate the brackets to the stacks.

4.2

Stacking process development

We had produced a very first set of f= 12 m middle radii test stacks late 2016 to test the new production equipment. The x-ray test resulted in a ~40” HEW PSF (see XOU-0038 in Figure 8), as is to be expected when commissioning new production systems where all relevant parts such as plates, mandrels and dies had to be adjusted to accommodate the focal length change. After we had further upgraded the production facilities as described above, we could use several months in 2017 to improve the stacking process parameters, resulting in a performance improvement of a factor of ~2 (XOU-0044 in Figure 8), catching up with the f=20 m angular resolution (XOU-0035). An in-depth review of the stacking results carried out early 2018 has shown the plate pre-forming equipment to cause figure deviations at the edges of the plates and we are now in the process of adjusting the tooling to improve the performance further, resulting later this year in 3^rd generation optics. This process improvement is part of ESA’s TDP, where a dedicated activity focusses on improving the angular resolution in several iterations.

Figure 8

– The half energy width (HEW) of 20 plates of several XOUs as measured in double reflection at XPBF 1 in the PTB laboratory at BESSY II. XOU-0035 is the last f=20 m optics produced in July 2016, measured at 2.8 keV. XOU-0038 is the first generation f=12 m optics, XOU-0044 is of the 2^nd generation f=12 m optics, all measured at 1.0 keV. We show data of 10%, 30%, 70% and 100% of the width of the XOU area illuminated by the x-ray beam. This year we are further improving on the stacking process to produce a 3^rd generation XOU.

The outer radii development runs in parallel to the middle radii. We have recently reached an important milestone of producing the very first stacks with a nominal height of 35 plates (see Figure 9). The development now focuses on improving the cleanliness of the plates and on stacking iridium coated plates, required to measure the stacks in x-rays at an energy of 1 keV, as the grazing incidence angle of 1.78 deg at r=1500 mm would result in a very low silicon reflectivity. In 2018 we are going to produce several 35 plate stacks to be tested in double reflection and to be finally integrated into a first outer radius mirror module.

Figure 9

– Photographs of the very first 35 plate outer radius stack, produced in May 2018.

The inner radii development had demonstrated a first 35 plate stack late 2016 and has essentially been on hold since then, to give priority to the other activities discussed in this paper. We are in the process of setting up an additional stacking robot and will start the production of inner radii stacks later in 2018, such that all three representative types of mirror modules can be produced in 2019 in preparation for the Qualification Model demonstration required for the MAR in 2021.