Zone plate lenses are used for focusing and imaging applications at x-ray wavelengths. We have previously reported on the fabrication of nickel zone plates for use as objectives and condensers in soft-x-ray microscopy [1]. In the present contribution we present our recent improvements of the high-aspect-ratio nano-fabrication process, and future projects including fabrication of hard x-ray zone plates.
Zone plates are circular gratings consisting of several hundred concentric zones with radially decreasing zone width. The achievable imaging resolution is approximately equal to the width of the outermost zone and the diffraction efficiency for a given wavelength depends on the zone height and the material. For soft x-rays and up to 3 keV photon energy nickel is a well suited material, and at higher photon energies gold is a good choice. Typical parameters for a soft x-ray zone plate are 20-50 nm outermost zone width, and a zone height of 150-250 nm. For hard x-rays at, e.g., 5 keV the zones must be up to 10 times higher to have a comparable efficiency. The combination of narrow and high zones requires a high-aspect-ratio structuring process.
Our fabrication process is based on a tri-level resist that is structured into a plating mold by e-beam lithography and reactive ion etching. The zone material, e.g., nickel, is deposited by through-mask electroplating. Recent improvements include the use of cross-linked polymers for high mechanical stability of the plating mold [2], pulse-reverse plating and in-situ rate control for uniform and accurate mold filling [3,4]. This has resulted in increased process control and higher efficiency optics. Furthermore, the uniform electrodeposition and accurate mold filling enables stacking of zone plates, which is a way to obtain aspect ratios and diffraction efficiencies beyond the capabilities of one tri-level resist. This can be used for hard x-ray optics and so-called volume zone plates, which have more complex zone profiles [5].
[1] A. Holmberg et al., Microel. Engin. 73-74, 639 (2004).
[2] M. Lindblom et al., Microel. Engin. (2007), in press.
[3] M. Lindblom et al., JVST B 24, 2848 (2006).
[4] A. Holmberg et al., JVST B 24, 2592 (2006).
[5] S. Rehbein et al., Inst. Pure Appl. Phys. (Japan) 7, 103 (2006).
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