IBL-Based Nanofabrication of Optical Elements for SXR- and EUV-Applications

Introduction

Diffractive optical elements are important components for applications in soft x-ray (SXR) and extreme ultraviolet radiation (EUV). At present, the standard fabrication method for such optics is based on electron beam lithography (EBL) followed by nanostructuring. This requires a series of complex processes including exposure, reactive ion-etching and electro-plating [1]. We report on experiments showing the single-step fabrication of such elements using ion beam lithography (IBL). Both transmission and reflection gratings were fabricated and successfully implemented as spectrometers at a laboratory soft x-ray source. Additionally, first steps towards zone plate fabrication are described.

Ion Beam Lithography

Presently, diffractive optical elements for SXR- and EUV- applications are almost exclusively fabricated by electron beam lithography, followed by complex nanostructuring processes (figure 1). We aim to develop an alternative fabrication method with shorter process paths: ion beam lithography. Structures are directly milled into a substrate by focused ion beam (FIB) sputtering; no post-processing of the milled structures is needed. This makes fabrication faster and more flexible.

Figure 1: Electron beam lithography process versus ion beam lithography process. a. Traditional fabrication using EBL for exposing the pattern and nanostructuring for transferring the desired structure into a substrate. b. IBL-based process; exposure and structure transfer is combined in a single step.

Suitable optical elements for next generation x-ray sources operating at shorter wavelength will strongly depend on new materials with sufficient absorption and phase shift. IBL offers a solution for this problem, as the sputtering process - in contrast to electroplating - can be used for a large number of different materials.

Early Patterning Experiments

For first experiments we used a standard FIB system (Zeiss XB1540) with 30 keV Ga ions, normally used for TEM lamellae preparation. At this point, the FIB microscope was not equipped with a special patterning attachment. Therefore, the addressable pattern size was limited to 1024*1024 pixel. Reflection as well as transmission gratings were fabricated and tested in slit-grating spectrographs (SGS) at a laser-induced ethanol-jet laser plasma [2].

Drift Correction

Compared to EBL-based processes, where exposure times are in the range of minutes, IBL-exposures take several hours. During these long-time exposures, the system has to be kept stable regarding stage position, ion beam current and gun emission characteristics. We are using a standard FIB-microscope with a 16 bit Raith Elphy Plus lithographic attachment and a non-interferometric stage. Due to that, drift correction is implemented via cross-correlation-based recognition of a directly written mark, followed by shifting the ion beam with an offset voltage applied to the scanning interface. Using a 5µm sized mark, average placement accuracy of 24 nm can be achieved with a standard algorithm (figure 2). Further characterisation of the cross-correlation algorithms are topic of current research.

Figure 2: Vernier pattern on 50 nm Cr-coated Si: ±90 nm deviation can be measured in 10 nm steps.

Fresnel Zone Plates

Up to now, process development has been performed on coated Si wafers. Transferring the patterning strategies to coated Si3N4 membranes is planned shortly. A first zone plate with an outermost zone width of 129 nm was fabricated in 50 nm Cr-coated Si. Fabrication was done in 9 h at a dwell time of 12 ms. After each of the 50 rings, drift correction was performed on a directly-milled, 5 µm cross-mark (figure 3). The next steps will be the transfer of the Si wafer process to Si3N4 membranes. Due to shorter process paths and higher flexibility regarding material choice, ion beam lithography is a promising alternative to existing fabrication methods.

Figure 3: Zone plate MZP10-004. a., c., d. Outer and inner regions of the zone plate. Outermost zone width: 129 nm. Number of rings: 50. Total size: 50 µm. Curved dwell time: 12 ms. Total writing time: 9 h. b. Mark used for the drift correction algorithm. Scanned and corrected after each zone.

References:

[1] Reinspach, J., Lindblom, M., von Hofsten, O., Bertilson, M., Hertz, H. M., and Holmberg, A. (2009) J. Vac. Sci. Technol. B 27, 2593–2596
[2] Lenz, J., Wilhein, T., and Irsen, S. (2009) Appl. Phys. Lett. 95, 191118