Working Group

Micro/Nano Optics

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Micro/Nano Optics

The Micro/Nano Optics Working Group develops novel micro/nano-structures, with a focus on X-ray optics. Exploration of new fabrication strategies, thin film materials development and performance testing of the fabricated optics are all parts of the process flow. We are especially interested in fabrication of high resolution and high efficiency X-ray focusing optics such as the binary Fresnel Zone Plate (FZP)1, multilayer-FZPs2 and kinoform lenses3. In the simplest case, a FZP is a set of concentric rings composed of alternating X-ray transparent and opaque materials. FZPs are usually fabricated via electron beam lithography (EBL) mostly limited to binary, planar structures. Fabrication of some efficient 3D structures using EBL is not very practical due to the complexity of the overlay EBL. We are working on a few alternative fabrication methods which lets us realize some truly three-dimensional optics.

The fabrication methods we are using include Focused Ion Beam (FIB) micromachining and lithography (IBL), Atomic Layer Deposition (ALD), Direct Write Optical Lithography (DW-OL) and Reactive Ion Etching (RIE).

Thin film characterization is an integral part of the process development. Both in-situ and ex-situ Spectroscopic ellipsometry (SE) is utilized heavily for optimization of ALD thin films. In addition, we are collaborating with the central X-ray diffraction (XRD) and transmission electron microscopy (TEM) groups with excellent facilities.

The final performance tests of the fabricated lenses are carried out in a state-of-the-art scanning transmission X-ray microscope (STXM) called, MAXYMUS located in BESSY II synchrotron radiation facility.

Last but not least, the accumulated expertise in various nanofabrication methods can be utilized for realization of a wider array of micro/nano optic devices for many different sub-fields of photonic science and engineering.

1.
K. Keskinbora, C. Grévent, U. Eigenthaler, M. Weigand, and G. Schütz, "Rapid prototyping of Fresnel zone plates via direct Ga+ ion beam lithography for high-resolution x-ray imaging," ACS Nano 7 (11), 9788-9797 (2013).
2.
K. Keskinbora, A.-L. Robisch, M. Mayer, U. Sanli, C. Grévent, C. Wolter, M. Weigand, A. Szeghalmi, Mato Knez, T. Salditt, and G. Schütz, "Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV," Optics Express 22 (15), 18440-18453 (2014).
3.
K. Keskinbora, C. Grévent, M. Hirscher, M. Weigand, and G. Schütz, "Single-step 3D nanofabrication of kinoform optics via gray-scale focused ion beam lithography for efficient X-ray focusing," Advanced Optical Materials 3, 792-800 (2015).

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Fresnel Zone Plate (FZP)

Fresnel zone plates are diffractive optics composed of alternating transparent and opaque rings blocking the Fresnel zones (hence the name) of a light source. If the number of these rings is large enough, the FZP act as a simple lens and focuses a parallel beam (source at infinity) to an Airy pattern. This pattern can be used as probe in the same manner as any scanning probe microscopy. The STXM uses this exact configuration where a sample is scanned through the Airy pattern created by the FZP. The width of the outermost zone Δr determines the width of the Airy pattern and therefore, the resolution. We have used focused ion beam lithography (IBL) for robust fabrication of FZPs (see the Fig. 1) with Δr as small as 30 nm in direct milling mode. This is a very efficient way of prototyping FZPs optimized for different applications in essentially a single lithography step. We are also using FIB to machine multilayer type FZPs out of thick multilayer deposits on fibers.

Fig1: An example of a FZP. a) Overview, b) close-up image of the outermost zones and c) a cross-section thereof. Zoom Image
Fig1: An example of a FZP. a) Overview, b) close-up image of the outermost zones and c) a cross-section thereof.

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Multilayer Fresnel Zone Plates (ML-FZPs)

ML-FZPs are very interesting for hard X-ray focusing as they have essentially limitless aspect ratios. The use of ALD lets us fabricate a very precise multilayer stack following the zone plate law (Fig. 2). Then, this stack is sliced to the desired thickness and polished using FIB micromachining and micromanipulation to deliver an ML-FZP. In addition to the freely selectable aspect ratio, these optics have a second distinct advantage over the conventional lithographic FZPs, they can be made to have a very small Δr thanks to the precision of the ALD. We have fabricated and demonstrated ML-FZPs with half-pitch resolutions down to 16 nm and efficiencies of up to >10 % in the soft X-ray energies.

Fig2: a) An SEM image of an ML-FZP on a TEM grid sliced and polished using a FIB. b) An SEM image showing the ALD layers of the ML-FZP. Zoom Image

Fig2: a) An SEM image of an ML-FZP on a TEM grid sliced and polished using a FIB. b) An SEM image showing the ALD layers of the ML-FZP.

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Kinoform Lenses

Kinoform lenses are diffractive/refractive optics taking advantage of both modalities to ensure maximum efficiency. In theory, a kinoform with ideal surface profile, made out of a non-absorbing but refracting material can have 100 % focusing efficiency. The required surface profile is parabolic and until recently, could only be fabricate with step approximations. We demonstrated that gray-scale direct milling IBL can be used effectively to fabricate kinoform lenses in a single step. To realize kinoforms with smooth surfaces another innovation was to utilize a nanocrystalline alloy of palladium and silicon. Amorphous or single crystalline materials are other good candidates for fabricating kinoforms. For instance in the Fig. 3, a (100) single crystal Si is shown to provide smooth lens surfaces after IBL. The aspect ratio is also quite high. This lens is expected to have more than 50 % focusing efficiency at 1000 eV. This figure is beyond anything possible with conventional FZPs.

Fig3: Overview (a) and close-up (b) SEM images of a kinoform fabricated in (100) single crystal Si substrate fabricated using gray-scale direct-write Ion beam lithography. Zoom Image
Fig3: Overview (a) and close-up (b) SEM images of a kinoform fabricated in (100) single crystal Si substrate fabricated using gray-scale direct-write Ion beam lithography.

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Focused Ion Beam (FIB)

Focused Ion Beam (FIB) instruments are proved to be valuable tools both for materials analysis and for nano-fabrication. As an analytical microscope they have been used in combination with secondary ion mass spectrometry, X-ray analysis or electron backscatter diffraction.

FIB devices (see Fig. 4) create a bright beam of accelerated ions, focusing them to spots in the range of 10 nm and even below. Both for 3D materials analysis and lithography, the FIBs take advantage of the effective interaction between the primary ions and the target atoms. Thus, a sample can be nano/micro-machined with the desired structure without the need for a mask. Therefore, FIBs are especially suited for prototyping of micro/nano-optic devices.

Fig4: The chamber view of a two beam FIB system with critical components marked. Zoom Image
Fig4: The chamber view of a two beam FIB system with critical components marked.

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Atomic Layer Deposition (ALD)

The ALD is based on the chemical vapor deposition (CVD) process, where the chemical reaction is separated into two half-reactions. As shown in the figure 5, the process starts by a pulse of metal-organic precursor gas in to the deposition chamber. Under certain conditions, the gas reacts with the surface species of the substrate in a self-limiting reaction that is terminated when the surface runs out of reactants. The excess gas is purged in the second step with a neutral gas such as N2 or Ar depending on the process requirements. The second reactant is introduced into the chamber in the third step, again reacting with the surface species. The excess of reactant and products are purged in the fourth step, concluding one cycle. In an ideal ALD process one atomic layer of material is deposited in each cycle and the number of cycles determines the thickness of the deposited film. ALD deposited films are highly conformal and can be used for coating and encapsulation of complex geometries.

ALD is especially suited for depositing thin films of ceramic materials such as oxides and nitrides. So far, we have successfully deposited Al2O3, SiO2, Ta2O5, HfO2 and Pt and their multilayers in our group. We have the possibility to extend this to other oxides and nitrides.

Fig5: An example ALD cycle for the deposition of Al2O3. Zoom Image
Fig5: An example ALD cycle for the deposition of Al2O3.

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Direct Write Optical Lithography (DW-OL)

In conventional optical lithography, a resist is exposed with UV light that is projected through a specially prepared mask to emboss the desired pattern. The masks are often very expensive and made for a specific application. Therefore, any design change requires a new mask which quickly increases costs of process development. Direct write optical lithography on the other hand uses a computer controlled, high precision scanning system (either the sample or the light) where a design file dictates the pattern to be written. This is very advantageous in research and development for prototyping purposes or new types of structures are developed constantly. We use the micro-structuring laboratory in our department to do DW-OL for fabricating structures that does not need the precision of the FIB or simply too large for FIB processing. The structures can be either used as masks for subsequent RIE process or can be utilized on their own.

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Reactive Ion Etching (RIE)

Reactive ion etching is a method where a masked material is subjected to a shower of ions often in the presence of reactive gasses for enhanced material removal. It is widely utilized in micro-electromechanical systems (MEMS) processes where high aspect ratio structures are required. Therefore, it is an important method of interest in the X-ray community for fabrication of high aspect ratio Fresnel Zone Plates (FZP). Recently, we are interested in using RIE for extending our fabrication capabilities. Main advantage of RIE compared to FIB is its parallel processing approach where one can fabricate many large structures in parallel. In the figure 6 a portion of a very large array of micro-pillars are shown.

Fig6: An array of micro-pillars etched in Silicon. Zoom Image
Fig6: An array of micro-pillars etched in Silicon.

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Spectroscopic Ellipsometry (SE)

Spectroscopic ellipsometry is a very strong method widely used for measuring thin film thicknesses. The change in polarization parameters (amplitude and phase change) of light that was bounced-off a sample, is analyzed and fit with a proper model in order to extract information about film thickness, roughness, dielectric function and hence the optical constants such as the real and imaginary part of the complex refractive index. Its model based approach also lets estimation of electrical conductivity, carrier concentration etc. Our group has an SE that operates from deep UV to mid-IR ranges covering an important and relevant spectrum for many exciting applications such as plasmonics, as well as the “quality control” of ALD films. With the option of automatic thickness mapping one can see the variation of the film thicknesses over wafers up to 10 cm.

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Scanning Transmission X-ray Microscopy (STXM)

The optics we fabricate, are often aimed towards the X-ray applications. What better way to test your optics than the state-of-the-art STXM MAXYMUS (Fig. 7) that our colleagues from X-ray Microscopy working group (AG Weigand) operate in synchrotron radiation facility BESSY II in Berlin. The MAXYMUS operates in the soft X-ray energy range from 200 to 2000 eV and provides a flexible testing platform for X-ray optics. A half-pitch resolution of down to 15 nm in direct imaging can be achieved. As shown in the schematic here, basically STXM operates as an X-ray lens focuses the incoming light on to a sample, which is raster scanned while total transmitted light is collected and registered via an APD. An order sorting aperture ensures the filtering of undesired diffraction orders. We use MAXYMUS mostly for testing the performance parameters of our optics such as the resolution and diffraction efficiency.

Fig7: Schematic view of the MAXYMUS located at BESSY II facility in Berlin. Zoom Image

Fig7: Schematic view of the MAXYMUS located at BESSY II facility in Berlin.

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Nanostructured Films for Metrology

Thin films of gold and silver, perforated with nanoholes and nanoslits have many applications in plasmonics, metamaterials and hyper lenses etc. Using FIB one can make reasonably large arrays of these elementary nanostructures with minimal time for process development. Here, in the Fig. 8, an array of nanoholes with varying pitches, milled directly into a freestanding bilayer of Au/Si3N4 membrane is shown. The FIB is a perfect method for patterning on unconventional or delicate substrates such as these free standing membranes or optical fiber tips.

Fig8: An array of nanoholes drilled into a free-standing Au/Si3N4 bi-layer using a focused ion beam can be seen. Zoom Image

Fig8: An array of nanoholes drilled into a free-standing Au/Si3N4 bi-layer using a focused ion beam can be seen.

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