Nanofabrication and diffractive optics for high-resolution x-ray applications

Abstract

Short wavelength x-ray radiation microscopy is well suited for a number of material and life science studies. The x-ray microscope ͑XM1͒ at the Advanced Light Source Synchrotron in Berkeley, California uses two diffractive Fresnel zone plate lenses. The first is a large condenser lens, which collects soft x-ray radiation from a bending magnet, focuses it, and serves as a linear monochromator. The second is the objective zone plate lens, which magnifies the image of the specimen onto a high-efficiency charge coupled device detector. The objective lens determines the numerical aperture and ultimate resolution. New objective lens zone plates with a minimum linewidth of 25 nm and excellent linewidth control have been fabricated using Berkeley Lab's 100 keV Nanowriter electron beam lithography tool, a calixarene high-resolution negative resist, and gold electroplating. Although the condenser zone plate is less critical to the resolution of the instrument, its efficiency determines the flux on the sample and ultimately the exposure time. A new condenser zone plate was fabricated and has a 9 mm diameter, 44 000 zones, and a minimum zone width of 54 nm ͑optimally the condenser and objective should have the same zone width͒. It is also fabricated with the Nanowriter at 100 keV using poly͑methylmethacrylate͒ resist and nickel electroplating. The phase shift through the nickel absorber material enhances the diffraction efficiency over an amplitude only zone plate. To evaluate the microscope's performance transmission test patterns have been made and imaged. Lineout data show modulation for 30 nm lines and 60 ͑1:2͒ spaces to be almost 100%. These new diffractive optical elements represent a significant advancement in the field of high-resolution soft x-ray microscopy. Diffractive optical elements have been used to measure the wave front error of an extreme ultraviolet projection optical system. The reference wave is generated by the spherical wave generated by diffraction from a small freestanding pinhole.

Hard X-ray Phase-Contrast Tomographic Nanoimaging

“…The ideal illumination should be as homogeneous and as intense as possible and its numerical aperture should be matched to that of the objective lens in order to obtain optimum resolution. Condensing x-rays with Fresnel zone plates (FZPs) [4], tapered capillaries [5], mirrors [6] or combinations of these devices [7] is a common solution. These optics focus the beam into a Gaussian-shaped spot, normally smaller than the field of view of the microscope, which therefore requires the condenser to be scanned along the direction transverse to the beam to partially overcome this inhomogeneous illumination.…”

Resist Requirements and Limitations for Nanoscale Electron-Beam Patterning

“…If we confine ourselves to a discussion of high-energy (> 50 keV) electrons, then forward scattering is a negligible factor in controlling the form of the deposited energy profile [27,28], as expected. 90% of a 100 keV beam is contained within a diameter of 8 nm at the base of a 100 nm thick resist [28].…”

Sub-38 nm resolution tabletop microscopy with 13 nm wavelength laser light

“…The condenser and objective zone plates were fabricated by electron beam lithography 15 in a 120 nm thick nickel film supported by a 100 nm Si 3 N 4 membrane that is ϳ40% transparent to the incident 13 nm light. The condenser zone plate has a diameter of 5 mm and consists of 12,500 zones of decreasing width down to 100 nm.…”