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 Nu magnetic sections emits collimated radiation in a bandwidth Δλ/λ=1/Nu. Nu 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.