Fresnel Zone Plate for X-ray Microscopy

Many experimental stations at synchrotron beamlines require X-ray nanofocusing and high spatial resolution X-ray imaging. Fresnel zone plates provide very high spatial resolution, only limited by the width of their outermost zone. The optimum material and height varies depending on the photon energy of the X-ray beam. In our group, state-of-the-art nanofabrication techniques are used to produce these optical elements.

We use a nanofabrication process based on high aspect ratio structuring of PMMA molds using high energy electron-beam lithography and subsequent filling of these molds with metal by electroplating [1,2]. For soft x-rays (E < 4 keV) we mainly use nickel as zone plate material while for higher photon energies gold provides better efficiency due to its higher density (see Figure 1).

Figure 1: Electroplated gold zone plate for multi-keV x-rays with 50 nm wide and 500 nm high structures (left and center). Nickel zone plate for soft x-rays with 25 nm outermost zone width (right).
Figure 1: Electroplated gold zone plate for multi-keV x-rays with 50 nm wide and 500 nm high structures (left and center). Nickel zone plate for soft x-rays with 25 nm outermost zone width (right).

We provide nickel and gold Fresnel zone plates to many research groups at synchrotrons worldwide, and the performance of our devices has been proven in many experiments. Special designs and dimensions can be produced - depending on the specific requirements of the experimental end stations.

Publications

  1. S. Gorelick, V.A. Guzenko, J. Vila-Comamala and C. David, Direct e-beam writing of dense and high aspect ratio nanostructures in thick layers of PMMA for electroplating Nanotechnology 21 (2010) p. 295303
  2. S. Gorelick, J. Vila-Comamala, V.A. Guzenko, R. Barrett, M. Salomé and C. David, High efficiency Fresnel zone plates for hard X-rays by 100 keV e-beam lithography and electroplating Journal of Synchrotron Radiation 18 (2011) p. 442 - 446

To further increase the resolution of Fresnel zone plates beyond the limits of electron-beam lithography, we developed  technique based on the coating of a template structure with a metal layer [1]. The electron-beam written template is coated uniformly with iridium using an atomic layer deposition (ALD) process (see figure 2). As iridium has a much higher x-ray refractive index as the template, we obtain a doubling of the effective zone density and subsequent improvement of the resolution by a factor of two compared to the template structure.

Zone-Doubling Fabrication
Figure 2. Zone doubling fabricaton approach. A sparse low absorbing template is coated with a high absorbing metal by atomic layer depostion.

The method was used to fabricate iridium FZPs with line widths down to 12 nm. The imaging properties of the devices were experimentally demonstrated in the scanning soft x-ray microscope PolLux at the SLS using a test object consisting of GaAs/InGaAs line pairs of varying thickness, as shown in figure 3 (left). Line pairs down to 9 nm have been be resolved [2].

This method can also be applied to produce Fresnel zone plates for the multi keV photon range. Using high energy electron-beam lithography electron-beam lithography, we can fabricate HSQ template structures with aspect rations beyond 20. After ALD coating, we obtained Iridium zone plates with outermost zones that are 550 nm high and only 20 nm wide, as shown in figure 3 (middle). This type of  Fresnel zone plates shows excellent resolution and good efficiency at high photon energies and can be use for full-field transmission X-ray microscopy providing sptial resolutions down to 20 nm, as shown in Figure 3 (middle) [3,4].

Zone-doubling examples
Figure 3. Scanning x-ray microscope image of a test object consisting of a GaAs/InGaAs line pairs from 40 nm down to 9 nm thickness obtained with a photon energy of 1.2 keV (left). FIB cross section of a line doubled iridium zone plate with 20 nm wide and 550 nm high zone plate structures (middle). Full-field transmission X-ray microscopy image with spatial resolution of 20 nm, taken at the Advanced Photon Source (right).

Publications

  1. K. Jefimovs, J. Vila-Comamala, T. Pilvi, J. Raabe, M. Ritala, and C. David, A zone doubling technique to produce ultra-high resolution x-ray optics Physical Review Letters 99 (2007) p. 264801
  2. J. Vila-Comamala, K. Jefimovs, T. Pilvi, J. Raabe, R.H. Fink, M. Senoner, A. Maaßdorf, M. Ritala, and C. David, Advanced Thin Film Technology for Ultrahigh Resolution X-Ray Microscopy Ultramicroscopy 109 (2009) p. 1360–1364
  3. J. Vila-Comamala, S. Gorelick, E. Färm, C.M. Kewish, A. Diaz, R. Barrett, V.A. Guzenko, M. Ritala, and C. David Ultra-high resolution zone-doubled diffractive X-ray optics for the multi-keV regime Optics Express 19 (2011) p. 175 - 184
  4. J. Vila-Comamala, Y. Pan, J.J. Lombardo, W.M. Harris, W.K.S. Chiu, C. David, Y. Wang, Zone-doubled Fresnel Zone Plates for High-Resolution Hard X-ray Full-Field Transmission Microscopy Journal of Synchrotron Radiation 19 (2012) p. 705-709

To further increase the diffraction efficiency of high spatial resolution Fresnel Zone Plate, we developed a scheme for the stacking of two Fresnel zone plates at both side of the support membrane to effective double its height, as show in figure 4. By patterning both sides of the same support membrane, we can provide a pair of stacked zone plates as a monolithic device.

Within a collaboration funded by the Nanoscopium and Anatomix beamlines of Synchrotron SOLEIL, we used this approach to produce double-sided zone plates with 30 nm outermost zone width and 1200 nm effective zone height, corresponding to an aspect ratio of 40 [1]. The devices showed an enhanced diffraction efficiency of 10% at 9 keV photon energy (see Figure 4). The shape of the focal spot was reconstructed from a ptychographic data set [2], revealing a spot size matching the expected value of 30 nm FWHM.

Doubied Side Fresnel Zone Plates
Figure 4. Fabrication process of a double-sided Fresnel zone plates (left), the structures is patterned at both sides of the support membranes. FIB cross section of a double-sided Fresnel zone plate with 30 nm outermost zone width, the height of the two sides adds up to 1200 nm (middle). Focus with a size of 30 nm of the a double-sided Fresnel zone plate obtained from a ptychographic reconstruction at 9 keC photon energy (right).

Publications

  1. I. Mohacsi, I. Vartiainen, M. Guizar-Sicairos, P. Karvinen, C. Kewish, A. Somogyi, V.A. Guzenko, E. Müller, E. Färm, M. Ritala, and C. David, Double-sided diffractive X-ray optics for hard X-ray microscopy Optics Express 23 (2015) p. 776
  2. J. Vila-Comamala, A. Diaz, M. Guizar-Sicairos, A. Mantion, C. M. Kewish, A. Menzel, O. Bunk and C. David , Characterization of high-resolution diffractive x-ray optics by ptychographic coherent diffractive imaging, Optics Express 19 (2011) p. 21333
  3. Yurgens, V., Koch, F., Scheel, M., Weitkamp, T., & David, C. (2020). Measurement and compensation of misalignment in double-sided hard X-ray Fresnel zone plates. Journal of Synchrotron Radiation, 27, 583-589.

The short wavelengths of X-rays promise to reach spatial resolutions in the deep single-digit nanometer regime, providing unprecedented access to physical phenomena, such as magnetism at fundamental length scales. Despite considerable efforts in soft X-ray microscopy techniques, a two-dimensional resolution of ten nanometers has not yet been surpassed in scanning or full-field X-ray microscopy.

The developments in lithography-based diffractive optics by the X-ray Optics and Applications group resulted in Fresnel zone plate lenses with structural sizes that can be as small as 6.4 nm [1]. These lenses, combined with the extreme stability and precision of the PolLux and Hermes scanning X-ray microscopes and a careful design of the experimental geometry, made the imaging of seven nanometer-sized structures possible. By combining this highly precise microscopy technique with the X-ray magnetic circular dichroism effect, the team reveals dimensionality effects in an ensemble of interacting magnetic nanoparticles.

7nm Spatial Resolution
Figure 5. High resolution scanning X-ray microscopy at the PolLux beamline (left). Scanning X-ray microscopy of a test sample with spatial resolution better than 10 nm (middle). Fourier Ring Correlation was used to estimate a spatial resolution of 7 nm (right).

Publications

  1. Rösner, B., Koch, F., Döring, F., Bosgra, J., Guzenko, V. A., Kirk, E., … David, C. (2018). Exploiting atomic layer deposition for fabricating sub-10 nm X-ray lenses. Microelectronic Engineering, 191, 91-96
     
  2. Benedikt Rösner, Simone Finizio, Frieder Koch, Florian Döring, Vitaliy Guzenko, Manuel Langer, Eugenie Kirk, Benjamin Watts, Markus Meyer, Joshua Lorona Ornelas, Andreas spaeth, Stefan Stanescu, Sufal Swaraj, Rachid Belkhou, Takashi Ishikawa, Thomas Keller, Boris Gross, Martino Poggio, Rainer Fink, Jörg Raabe, Armin Kleibert, and Christian David, (2020) Soft x-ray microscopy with single-digit nanometer resolution, Accepted, https://doi.org/10.1364/OPTICA.399885