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August 29, 2005

 

 

New Zone Plate for Soft X-Ray Microscopy at 15-nm Spatial Resolution

Analytical tools that combine spatial resolution with elemental and chemical identification at the nanometer scale along with large penetration depth are indispensable for the life and physical sciences. The XM-1 soft x-ray microscope at the ALS produces images that not only reveal structures but can identify their chemical elements and measure magnetic and other properties as well. Now a new method for creating optical devices with nanoscale accuracy has allowed researchers in Berkeley Lab's Center for X-Ray Optics (CXRO), which built and operates the XM-1, to achieve an extraordinary resolution of better than 15 nm, with the promise of even higher resolution in the near future.

beamline layout

CXRO XM-1 full-field imaging microscope at ALS Beamline 6.1.2.

Seeing Better with X Rays

X-ray microscopes promise images with 10-nanometer detail throughout the volume of the sample (3-dimensional imaging). Such a capability would benefit both life scientists peering into cells and materials scientists doing the same for solid materials. Realizing these promises has been a gradual process, owing in part to the difficulty of making lenses that can image x rays the way transparent glass and other materials focus light refractively. Two other optical properties, reflection and diffraction, offer alternatives. Polished metal mirrors can reflect x rays fairly efficiently, and curving the mirrors gives a focusing effect, but for the finest resolution, devices known as Fresnel zone plates have been the choice.

Looking somewhat like a microscopic archery target, zone plates consist of alternating transparent and opaque concentric “zones” of decreasing width from center to outer edge. X rays passing through a zone plate are diffracted inward to form a spot approximately equal to the width of the outermost zone. Chao et al. have adapted a nanofabrication technique based on the use of a finely focused electron beam to “write” the zone plate pattern. With their “overlay” techniques, they made a zone plate with an outer zone width of 15 nm and tested it in an x-ray microscope. They foresee refinements of their technique that will result in zone plates with 10-nm zone widths, once regarded as the practical resolution limit for x-ray microscopy.

Since x rays cannot be focused by conventional refractive lenses, the XM-1 uses Fresnel zone plates, disks of concentric rings of metal of decreasing width that diffract the soft x rays to form an image. An objective lens called a “micro” zone plate (MZP) projects a full-field image of the sample onto a CCD area detector. CXRO fabricates its own zone plates with its Nanowriter electron-beam lithography tool. An energetic beam of electrons just 7 nm wide carves preprogrammed patterns in a resist-coated silicon wafer. The carved-out circular patterns in the resist are then replaced with opaque gold to form an object.

nanofabrication

Top: In the overlay nanofabrication technique, generating separate e-beam lithography patterns for alternating opaque zones reduces the smearing due to electron scattering when writing closely spaced features. Bottom: Scanning electron micrograph of a micro zone plate made using the overlay technique demonstrates achievement of a 30-nm zone period (distance from centers of outermost opaque zones) with high quality (e.g., placement accuracy of 1.7 nm).

Since the spatial resolution is approximately the width of the outermost zone, high resolution depends on the ability to make narrow outermost zones, with a placement accuracy better than one-third the width of the zones themselves. The Nanowriter is capable of placement accuracy to within 2 nm, but unfortunately, electron scattering spreads even a tightly focused beam when it hits the resist. The exposure due to scattering from neighboring zones, combined with inherent limits in the resist resolution, has made it impossible to maintain high contrast and optical separation for zones narrower than those in the XM-1’s earlier objective lens of 25 nm.

To overcome this limit, the CXRO researchers adapted an overlay technique used in the semiconductor industry by combining two different zone-plate patterns. Opaque zones are typically given even numbers, so in this scheme the first pattern contains zones 2, 6, 10, 14, and so on, and the second contains zones 4, 8, 12, 16, and so on. The first pattern is carved into the resist-coated wafer; then the zones formed by the electron patterning are filled with gold. The wafer is coated with resist again to make the second pattern.

modulation transfer

Soft x-ray images taken with the CXRO XM-1 full-field imaging microscope at ALS Beamline 6.1.2 vividly display the improved resolution achievable when using the new micro zone plate with a 15-nm outer zone width (ΔrMZP).

When combined, the critical outer zones of the combined patterns were less than 15 nm apart, accurately placed to within less than 2 nm. However, the opaque zones were broken by tiny gaps, and they were wider (and the transparent zones between them narrower) than they should have been, somewhat reducing the zone plate's efficiency.

The experimental MZP was then used to obtain images sharper than any previously achieved with an x-ray microscope. Not only were images of test patterns better defined than those made with the XM-1’s current 25-nm MZP, the new MZP was able to obtain sharp images of lines a mere 15 nm apart—where the older zone plate had seen only a featureless field of gray. The contrast (modulation) between light and dark features of the pattern was also remarkably good.

modulation transfer functions

The calculated modulation transfer functions of the microscope with an older micro zone plate whose outer zone width (ΔrMZP) is 25 nm (left line) and the new one with ΔrMZP = 15 nm (right line). The theoretical resolution for the two lenses are 19 nm and 12 nm, respectively. Squares are experimental data indicating the degree of modulation obtained for various test patterns imaged using the older zone plate.

The benefits of the advance may not be limited to x-ray microscopes at synchrotron light sources. The CXRO researchers suggest that before too many years, new sources of bright, soft x rays, such as compact, laser-based x-ray sources, will make it possible to fit x-ray microscopes on the bench top. Nanoscience and nanotechnology will be both the beneficiaries and the driving forces behind the widening horizon for nanoscale analysis.

Research conducted by W. Chao and D.T. Attwood (Berkeley Lab and University of California, Berkeley) and B.D. Harteneck, J.A. Liddle, and E.H. Anderson (Berkeley Lab).

Research Funding: National Science Foundation, U.S. Department of Energy, Office of Basic Energy Sciences (BES), and Defense Advanced Research Projects Agency. Operation of the ALS is supported by BES.

Publication about this research: W. Chao, B.D. Harteneck, J.A. Liddle, E.H. Anderson, and  D.T. Attwood, “Soft x-ray microscopy at a resolution better than 15 nm,” Nature 435, 1210 (2005).

ALSNews Vol. 256, August 31, 2005

 

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