A proposal for maskless, zone-plate-array nanolithography

Abstract
We propose a novel form of x‐ray projection lithography that: (1) requires no mask, and hence can be considered an x‐ray pattern generator; (2) is, in principle, capable of reaching the limits of the lithographic process. The new scheme utilizes an array of Fresnel zone plates, and matrix‐addressed micromechanical shutters to turn individual x‐ray beamlets on or off in response to commands from a control computer. Zone plate resolution is approximately equal to the minimum zone width, which can approach 10 nm. Zone plates are narrow‐band lensing elements: For a diffraction limited focus, the source bandwidth Δλ/λ should be less than or equal to the reciprocal of the number of zones N. An undulator having N u magnetic sections emits collimated radiation in a bandwidth Δλ/λ=1/N u . N u is usually in the range 35–100. We present a system design based on 25 nm lithographic resolution using λ=4.5 nm. For N=100 the zone‐plate diameter is 10 μm. Each zone plate of the array would be responsible only for exposure within its ‘‘unit cell.’’ To fill in a full pattern, the stage holding the sample would be scanned in X and Y while the beamlets are multiplexed on and off. A microundulator designed for installation on a commercial compact synchrotron can provide 87 mW within a 2% bandwidth around 4.5 nm in a divergence cone of 0.28 mrad. The calculated efficiency of first‐order focus for a zone plate operating at 4.5 nm is 31%, using 130 nm of spent U as the absorber/phase shifter. An exposure rate of ∼1 cm2/s at 25 nm resolution appears feasible.

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