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Proximity X-ray lithography (XRL), using wavelengths between 0.8 and 1.5 nm, provides a near-ideal match to the “system problem” of lithography for feature sizes from 500 to 30 nm, by virtue of “absorption without scattering” and recently developed mask technology. The effects of photoelectrons, at one time thought to be problematic, are now understood not to limit resolution. With experiments and simulations via Maxwell’s equations, we show that useful resolution is not limited by diffraction until linewidths are below 50 nm. It is critically important to optimize the source spatial incoherence to eliminate the deleterious effects of high spatial frequencies. Mask architecture and patterning methods are presented which we believe are compatible with manufacturing at linewidths from 500 to 30 nm. Distortion due to mask frame flexing and absorber stress can now be eliminated. Elimination of distortion at the pattern generation stage remains the problem of greatest concern. We discuss a proposed method of spatial-phase-locked electron-beam lithography which could solve this problem. Our new interferometric alignment scheme has achieved 18-nm alignment at 3σ. We assert that projection XRL using multilayer mirrors at 13 nm can never match the present performance of proximity XRL. Applications of sub-100-nm XRL, including MOS, quantum-effect, and optoelectronic devices are discussed which illustrate the benefits of high resolution, process robustness, low distortion, low damage, and high throughput.
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