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Figure 1: Cross section of an Iridium Fresnel zone plate with 12 nm zone width produced by atomic layer deposition onto a template structure.
To further increase the resolution of Fresnel zone plates beyond the limits of electron-beam lithography, we have developed a novel technique based on the coating of a template structure with a metal layer. The electron-beam written template is coated uniformly with iridium using an atomic layer deposition (ALD) process (see figure 1). 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 [1].
Figure 2: Scanning x-ray microscope image of a test object consisting of a GaAs/InGaAs line pairs from 40 nm down to 9 nm thickness. Photon energy: 1.2 keV.
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 (see Fig. 3). Line pairs down to 9 nm have been be resolved [2], which represents the best resolution to date achieved in soft x-ray microscopy.
Figure 3: FIB cross section of a line doubled iridium zone plate with 20 nm wide and 550 nm high zone plate structures.
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 (see Figure 3). This type of zone plates shows excellent resolution and good efficiency at high photon energies (see Figure 4) [3,4].
Figure 4: Full-field microscopy image taken at the Advanced Photon Source [4]. A resolution of 20nm>
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
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
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
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
Dr. Christian David
christian.david@psi.ch