Focusing of 8 keV x rays to a spot size of 150 and 90 nm full width at half maximum have been demonstrated at the first-and third-order foci, respectively, of a phase zone plate ͑PZP͒. The PZP has a numerical aperture of 1.5 mrad and focusing efficiency of 13% for 8 keV x rays. A flux density gain of 121 000 was obtained at the first-order focus. In this article, the fabrication of the PZP and its experimental characterization are presented and some special applications are discussed.

The adoption of a novel method for producing fine features by 1 nm proximity x-ray lithography would solve all of the current technical limitations to its extensibility. These limitations include the fabrication of fine features on masks and the maintenance of narrow mask-wafer gaps. Previously, with demagnification by bias, we described line features of 43 nm width produced with comparatively large clear mask features and large mask-wafer gaps. The method is generally applicable and has been shown to be extensible to beyond 25 nm printed features sizes on the wafer. The demagnification, ×1-×6, is a result of Fresnel diffraction and occurs without lenses or mirrors. The method takes advantage of the modern control of resist processing and has good exposure stability. We now expand on the optimization of the process by defining and explaining the critical condition and by demonstrating the consistency of various types of simulation. The simulations demonstrate the effects of the gap width, non-symmetric rectangular masks, spectral bandwidth, outriggers, T junctions, blur, etc. In two-dimensional images, the spectral bandwidth allows sharp features due to interference and effectively eliminates ripple parallel to the longer dimension. Demagnification by exposure near the critical condition extends the most mature of the next generation lithographies which we define generically-following actual current lithographic practice-in terms of the departure from the classical requirement for fidelity in the reproduction of masks. Specifically, for 1 nm proximity lithography, demagnification of critical features greatly facilitates the printing of fine features.

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature 'bias'. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local 'demagnification'. This demagnification (×3 − ×6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a ∼50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (>100 nm) and large gaps (30-15 µm). The method is demonstrated for demagnification values down to ×3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.

It is time to revisit X-ray. By enhancing, in the Near Field, Proximity X-ray Lithography (PXL), the technique is demonstrated that extends to 15 nm printed feature size with 2: 1 ratio of pitch to line width. "Demagnification by bias" of clear mask features is positively used in Fresnel diffraction together with rapid, multiple exposures of sharp peaks. Pitch is kept small by multiple, stepped exposures of the intense image followed by single development. The optical field is kept compact at the mask. Since the mask-wafer gap scales as the square of the mask feature size, mask feature sizes and mask-wafer gaps are comparatively large. A Critical Condition has been identified which is typically used for the highest resolution. Many devices, including batches of microprocessors, have been demonstrated previously by traditional 1X PXL which is the most mature of the Next Generation Lithographies and which is now further extended. Throughput and cost are conventional.

A scanning soft x-ray microscope has been built and operated at the National Synchrotron Light Source. It makes use of a mini-undulator as a bright source of 3.2-nm photons. An electron beam fabricated Fresnel zone plate focuses the beam onto the specimen, which is scanned under computer control. The scanning stage can be moved by both piezoelectric transducers and stepping motors, and the location is monitored by a high-speed laser interferometer. X rays transmitted through the specimen are detected using a flow proportional counter. Images of biological specimens and of artificial microstructures have been made with resolution in the 75–100-nm range. Acquisition time for 256×256-pixel images is about 5 min.