Hybrid lithography for x-ray diffractive optical elements

X-ray diffractive optical elements (DOEs) are used in x-ray astronomy, interferometry, extreme UV lithography, and plasma diagnostics for laser-fusion targets. To fabricate DOEs, microlithographic techniques determine the minimum feature size and functional capabilities critical to the elements' function. However, DOE requirements for membrane-based metal nanostructures with high aspect ratio and fidelity present challenges for lithography. Over the past 50 years, microlithography has been developed for the advancement of very small semiconductor and metal device structures, but only in the last two decades has it started to meet the requirements of x-ray DOEs.¹

Existing lithographic techniques for nanopatterning include electron-beam lithography,^{2, 3} interference lithography,⁴ and focused-ion-beam technology.⁵ Using interference lithography, researchers from the Massachusetts Institute of Technology's Space Nanotechnology Laboratory have fabricated a large number of x-ray transmission gratings for laser plasma fusion and space flight missions.⁶ However, this approach is limited to periodic geometries and (as with all the aforementioned techniques) often requires multilayer resist processes to obtain nanometer structures with high aspect ratio.

Alternatively, we report a strategy for low-volume x-ray DOE nanofabrication that combines electron-beam, x-ray, and proximity optical lithographies on a silicon carbide (SiC) membrane. The idea is to use the strengths of each appropriate lithographic technique for the various processing steps. We prepared the membrane using a home-made plasma-enhanced chemical vapor deposition system with a temperature of up to 1000°C, making the membrane impervious to radiation attack during laser fusion experiments.

There are three main steps in our approach (see Figure 1). The first is to pattern fine complex structures with low aspect ratio on a master x-ray mask using 100kV electron-beam lithography, followed by gold electroplating. Here we used a 1μm-thick SiC membrane as a substrate, and primary electrons could pass completely through the resist and membrane. This resulted in a reduction in the proximity effect, and we obtained high-fidelity patterns (see Figure 2). However, this first process is time-consuming: when using a poly(methyl methacrylate) (PMMA) resist, it took us at least eight hours to pattern a grating of 1×1mm² with a period of 200nm.

Figure 1. The fabrication process for x-ray diffractive optical elements (DOEs) based on a silicon carbide (SiC) membrane. First, electron beam lithography patterns x-ray masks (a, b). Next, x-ray lithography replicates the DOEs (c–d), and finally, proximity optical lithography patterns a large-scale gold (Au) mesh (e, f). PMMA: poly(methyl methacrylate). Cr: Chromium.

Figure 2. A scanning electron microscopy (SEM) image of nanostructures patterned on a 1μm-thick SiC membrane using electron-beam lithography. A fully resolved fine line (50nm wide) was written between two large pads with no proximity correction.

The second step is to efficiently replicate DOEs (daughters) using x-ray lithography. This step is suitable for metallic, semiconducting, and insulating substrates with high resolution and large process latitude (the variations in linewidth with respect to the processing conditions) and determines the minimum feature size of the DOEs. We used the Beijing synchrotron radiation x-ray lithography beamline, and the large-area nanopatterning required exposure time of just a few minutes, making it much shorter than our first patterning step. More importantly, we could amplify the aspect ratio of the DOEs with a single lithography exposure because the greater depth of focus enabled incident x-rays to penetrate thick resists with negligible diffraction effects (see Figure 3). The current generation of x-ray DOEs has a maximum aspect ratio of 14 with critical dimension of 200nm, but we anticipate improving on these results.

Figure 3. SEM image of gold zone structures replicated by x-ray lithography. The zone width is 200nm and the zone height is 2.8μm.

Freestanding structures are often needed to support laser fusion applications. Thus, the final step in our approach was to fabricate a large-scale gold mesh as the supporting structure using low-cost proximity optical lithography and then gold electroplating (see Figure 4). The required exposure time was a few seconds, and the mesh thickness was larger than 2.5μm. As a result, we obtained a geometric transmission of greater than 60% for the freestanding structures.

Figure 4. SEM image of large-scale gold mesh patterned by proximity optical lithography. Shown in the inset is a more detailed view of the freestanding gold structures.

Our hybrid lithographic approach enables high-aspect-ratio metal nanometer structures in reasonable time and at low cost, and therefore is well-suited to both laboratory and low-volume production applications, where testing and debugging of novel device designs are required. Having optimized the resist, processing, tools, and metrology of the three lithographic techniques described, we can fabricate working x-ray DOEs with feature sizes down to 100nm for laser fusion applications. In the x-ray DOEs we have already shipped to customers, the ninth-order diffraction peak can be observed for both the 3333 and 5000 lines/mm x-ray gold transmission gratings,⁷ indicating that the gold structures with vertical sidewalls have precise feature size and line edge control. Furthermore, the exposure conditions of the second and third steps in our approach do not depend on using a specific type of substrate. Therefore, our method could also be used to fabricate electronic, sensory, and optical devices.

In future work we plan to fabricate new types of x-ray DOEs, such as Cantor dust zone plates⁸ and modified photon sieves.⁹In particular, we aim to improve the aspect ratio for DOEs in the hard x-ray region.