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  • Basics of multilayer optics
  • Production & characterization of optics
  • Basics of IµS
  • Production & characterization of IµS
Bragg equation, modified for multilayers

Multilayer optics - the physics behind

Nowadays, a large variety of thin film based reflective optics exists for home-lab X-ray instruments [1,2] and synchrotrons [3,4]. The most widespread optics at synchrotrons are total reflection optics, where a single layer reflects the incident beam under a very flat angle – typically below 2°. On the other hand, Bragg diffraction on single crystals was used for directing synchrotron beams. More and more multilayer optics are now also used for shaping and reflecting the synchrotron beam in a defined manner. Additionally the energy spectrum of the reflected beam can strongly be influenced. The reflection profile of the multilayer can be designed exactly to the user’s requirements.

In modern home laboratories highly brilliant X-ray sources with small anode spots are combined with multilayer optics for beam conditioning. The optics should redirect, monochromatize, focus or collimate the X-ray beam.

For X-rays the real part of the refractive index n is nearly 1, so that X-rays cannot be shaped by lenses. The refractive index n = 1-δ+iβ consists of the dispersion δ and the absorption β. A possibility of controlled X-rays beam shaping exists by using Bragg’s diffraction.

According to Bragg’s condition, radiation of a certain wavelength,which illuminates a periodic structure with lattice constant d, is deflected under the angle θ.  Different orders m are deflected in different directions. The perfection of the lattice as well as the lattice materials determine the reflectivity R.

 

Parabolically shaped substrate with laterally graded multilayer collimate X-rays.

 

Optimized periodic structures of high reflectivity with adjustable d-values can be obtained by multilayer deposition. These multilayers can consist of up to several hundred layer pairs of two materials with single layers of a few nanometers thickness. The first order (m=1) can show a very high reflectivity of up to 95%, and the peak widths and energy bandwidths can be 1-2 orders of magnitude larger than for single crystals. The multilayers consist of a heavy reflector and a light spacer, usually with a high dispersion contrast at the interface. For a good X-ray multilayer, the absorption should be low to achieve high reflectivity and reasonably sharp Bragg reflections.

Using multilayers with a lateral layer thickness profile that are deposited onto curved substrates, optical elements can created that redirect and shape X-rays. Such so-called Göbel Mirrors consist, for example, of a parabolically curved substrate, and a multilayer coating with d-values typically in the range 3 – 5 nm. Due to the parabolically curved shape in conjunction with the d-value gradient, the divergent beams emitted from an X-ray source are transformed into a parallel beam bundle using Bragg diffraction.

References

[1] M. Schuster, H. Göbel, L. Brügemann, D. Bahr, F. Burgäzy, C. Michaelsen, M. Störmer, P. Ricardo, R. Dietsch, T. Holz, and H. Mai, “Laterally graded multilayer optics for x-ray analysis” in Proc. SPIE 3767, (1999) 183

[2] C. Michaelsen, J. Wiesmann, C. Hoffmann, K. Wulf, L. Brügemann, and A. Storm, “Recent developments of multilayer mirror optics for laboratorary x-ray instrumentation” in Proc. SPIE 4782 (2002) 143

[3] M. Störmer, A. Liard-Cloup, F. Felten, S. Jacobi, B. Steeg, J. Feldhaus, R. Bormann, “Investigations of large x-ray optics for free electron lasers” in Proc. SPIE 5533 (2004) 58

[4] B. Steeg, L. Juha, J. Feldhaus, S. Jacobi, R. Sobierajski, C. Michaelsen, A. Andrejczuk, and J. Krzywinski, “Total reflection amorphous carbon mirrors for vacuum ultraviolet free electron lasers“, Appl. Phys. Lett. 5, 84 (2004) 657

 

The shape of a Göbel Mirror is measured by optical profilometry.

Production and characterization of multilayer optics

The production can be devided into two major steps: The substrate and the deposition.

The substrates

For synchrotron applications we mainly use prefigured perfectly shaped ultra-low roughness substrates from other specialized companies.  For most of the optics for lab-instrumentation, the manufacturing process includes the preparation of shaped substrates by bending silicon wafers. Their quality is tested with optical profilometry methods. The height deviations onto the substrate must be within several hundreds of nanometers.

 

Linear sputter coating unit with 4 different target materials

The deposition

The most important part of the optics production is the following deposition of multilayers (2 materials, up to several 100 pairs). We use the very reliable and reproducible method of magnetron sputtering.

We have experience with a lot of material combinations for single layer or multilayer coatings, e.g.: C, B4C, BN, SiC, Sc, Ti, V, Cr, Ni, Ge, Mo, Ru, Rh, Pd, La, Ta, W, PT, Au, Ru/C, Cr/B4C, W/Si, La/B4C, Mo/B4C, …
We are able to deposit stripes of single layer films up to a length of 150 cm as well as laterally and depth graded multilayers over a length of up to 50 cm.

The coatings are characterized by excellent homogeneities, very good adhesion, high density, low roughness and high reflectivities.

 

Thickness distribution of a total reflection mirror with a length of 150 cm, measured by XRR

The characterization

Typically, coating parameters such as layer thickness, uniformity density, roughness, reflectivity and gamma ratio for the multilayers are characterized using X-ray reflectometry (XRR) at Cu-Kα (8048 eV) at different positions on each mirror coating.

For multilayers, the quantification of the 1st Bragg order allows the determination of the maximum of reflectivity, integral intensity and energy resolution. The XRR measurements allow a deduction of the interface roughness and the material density of the layers. The aim of multilayer film deposition for X-ray optics is to obtain interfaces which are as sharp and smooth as possible, and to achieve optimal density contrast between single layers. The Bragg peak angles can be used to calculate the double layer thickness period, the so-called d-spacing.

The thickness distribution of the layers along the substrate length must be within the needed specification of typically ±1%. This means that we need a precision along the substrate within the pm range.  During development of multilayers transmission electron microscopy is needed to measure the quality of interfaces and to investigate if unwanted interdiffusion or roughness is created.

 

IµS - Incoatec Microfocus Source

The IµS is a high-brilliant X-ray source which consists of:

  • a multilayer optics
  • a microfocus sealed X-ray tube and tube housing
  • a high voltage generator

 

The IµS is compliant with Machinery Directive 2006/42/EC.
It can be fully implemented into Bruker AXS software and safety system. Our customized engineering enables an individual mechanical adaptation as well as an integration in all common safety circuits.

Optics - the main principle

Multilayers are best suited for beam formation and monochromatization of X-rays. Applying Bragg‘s law X-rays are collected in a solid angle of approximately 1 mrad e.g. at W/C multilayers and are redirected with a reflectivity larger than 80 % while simultaneously suppressing Kα-radiation. To take account for the varying incident angles, the multilayer requires a lateral gradient of the layer thickness. It is possible to focus the incident beam with an elliptically shaped substrate. A collimation is achieved with a parabolically shaped substrate. A 2-dim shaping of the beam is possible by combining and fixing two multilayer mirrors side-by-side in an L-shape. This assembly is called Montel optics. In this geometry the beams are doubly reflected and thus the monochromatization effect is squared.


The graphic illustrates that the multilayer optics are an ideal combination for X-ray sources with a focal spot diameter well below 100 µm on the anode. With larger sources the range of incident angles increases. Therefore, the multilayer optics can only reflect the incident beam partially as the other X-rays do not fulfil Bragg‘s law. Generally, the optics can be designed to meet customer‘s needs. Hence, there are optics exhibiting e.g. low divergencies or high flux densities with spots on the sample varying from some 10 µm up to the mm range.

Tube - the main principle

The IµS provides a highly brilliant X-ray beam in a power range of 10-50 W. It reaches an amazing performance by using air-cooling and a low-power sealed tube. The IµS is a microfocus source as the focal spot of the electron beam on the anode only has a diameter of 20-50 µm. Incoatec offers all typical anode materials like Cu, Mo, Ag and Cr.


Due to the higher surface-to-volume ratio of the focal spot compared to the old 1-2 kW sealed tubes, the IµS has an improved heat conductivity and thus allows significantly increased power densities. With values larger than 5 kW/mm2 the performance of the IµS is comparable to 5 kW-class rotating anodes (see graphic).


The brilliance cannot be increased by the optics, therefore it is of utmost importance to achieve the highest possible brilliance within the tube. Consequently, it is best to combine the tube with a multilayer optics as opposed to other types of optics, as this ensures that the small focal spot is directed to the sample with the highest possible brilliance conservation.

Microfocus Source IµS - production & characterization

The production and assembly of all key components are carried out in-house. The IµS is equipped with the latest generation of Montel Optics, the so-called Quazar Optics, which can be up to 15 cm in length.The multilayer optics are mounted and pre-aligned in our patented optics housing garantueeing high stability.

 

 

 

The microfocus source consists of two parts: firstly an X-ray tube mounted and aligned inside the cooling body and secondly an upper housing part containing the fans, the electronic controls and the safety shutter system. The picture left shows how these two parts are assembled. Due to the low weight it is possible to mount the IµS on all kinds of standard goniometers and positioning stages, making a customer-specific integration into existing setups possible. This type of installation has been carried out successfully for numerous customers.

 

 

For the electronic control of the IµS an intelligent X-ray generator has been developed that is also produced in-house.

From the first IµS model on, we have implemented a sensor inside the optics housing that closes the shutter if the multilayer is not used in vacuum (the brilliant X-ray beam would otherwise destroy the optics by producing ozone). Our IµS High-Brilliance is even more advanced as the generator collects additional information on the properties of the tube and the optics. Additionally, parameters such as the ramp rate of the tube or system failures are monitored. This monitoring system ensures a better and faster customer support. Furthermore, for safety reasons the shutter between source and optics can only be opened if the optics housing is mounted correctly. Customer-specific wishes such as individual safety circuits or motorized optics alignment are fulfilled by our experienced electronics group.

 

 

After manufacturing every IµS is tested in our X-ray lab. The beam profile is measured with a calibrated detector and the intensity of the beam is checked and recorded. This value is the benchmark which needs to be achieved at the customer’s site after the IµS has been installed. Our complete production chain ensures that every single IµS is most accurately manufactured and characterized before leaving our company. Our in-house development is a guarantee for the customer that the IµS product range is continuously being improved and special customer requirements are met quickly at a good price performance ratio.