Today lithography techniques are among the most rapidly developing areas in both science and engineering. In the last two decades, well-known and widespread methods of photo- and e-beam lithography have been complemented by a number of novel approaches that include direct writing processes, mask-based techniques, and unconventional wet lithographies [1]. Further improvements in this method can be introduced by the integration of lithography with chemical or electrochemical processes. Electrochemical processing possesses a number of important advantages, including room-temperature synthesis, the ability to control deposit morphology, and enhanced versatility. The idea of combining electrochemistry with X-ray photolithography originated from experimental observation of the variation in electrochemical deposition/etching rates at the working electrode under illumination.

Generally, absorption of a high-energy photon by the electrode surface leads to numerous secondary events with an avalanche-type creation of low-energy excited electrons. In some cases, the energy of the electron exceeds the Volta potential difference, permitting it to escape from the surface to the electrolyte volume. In the case of cathodic polarisation, low-energy electrons emitted from the electrode surface can further reduce cations in the Helmholtz layer, while high-energy electrons pass through the electric double layer to the electrolyte volume (Figure 115). Thus, emitted photoelectrons generally produce an additional electric current at the electrode-electrolyte interface that involves electrochemical transformation. Unlike the radiolysis products, these electrons have no oxidising pairs, which obviously shifts the electrochemical equilibrium and enhances deposition rates on the illuminated areas of the electrode. The holes on core levels generated by X-rays move away from the electrode surface under the applied potential and annihilate effectively in the electrode volume. The circuit completes through electrochemical reactions on the anode.

Indeed, in nickel electrochemical deposition experiments, the thickness of the deposited metal film was found to increase substantially on the illuminated area of the electrode as compared to the non-irradiated surface. Moreover, an extensive increase in current was observed on switching on X-ray illumination.

As X-rays provide opportunities for optical imaging, the X-ray assisted electrochemical approach can be successfully utilised for a direct pattern transfer to a liquid–solid interface by coherent X-ray irradiation. Moreover, the small (<<10 nm) inelastic mean free path of low-energy secondary electrons brings hope to proceed with lithographic imaging with the resolution of X-ray optics. Proof-of-concept pattern transfer experiments were carried out through electrochemical deposition of nickel under coherent X-ray illumination guided through a lithographic mask with 4 micrometre pitch at the micro-optics test bench of beamline ID06. A silicon grating illuminated with collimated coherent light (12 keV, total flux 2×1013 photons/s) was used to produce a pattern in the transmitted light. To create a periodic intensity distribution on the substrate, the mask-to-working electrode distance was adjusted to half of the Talbot length. Electrodeposition was carried out from a Watts nickel plating bath in potentiostatic mode at room temperature.

Fig. 115: Illustration of the charge transfer during electrochemical X-ray processing.

Figure 116 shows a SEM image of nickel electrodeposited onto an X-ray-illuminated Au/Si electrode. The resulting structure corresponds well to the image of the diffraction grating; the periodicity of the fabricated Ni pattern (4 μm) is identical to that of the mask. The height of the islands in the resulting structure (inset in Figure 116) is clearly dependent on the contrast provided by the self-image of the grating. The EDX mapping of the resulting structure indicates a metal electrodeposition enhancement of up to 150% in the illuminated areas compared to the unilluminated regions.

Fig. 116: Top-view SEM image of the nickel electrodeposited onto an Au/Si electrode under periodic X-ray illumination.

The proposed electrochemical X-ray photolithography method thus results in a direct non-contact pattern transfer onto an electrodeposited metal film. The rather short lifetime of radiolysis products, in combination with a small spur radii and electron inelastic mean free paths in condensed matter, provide a possible means to rapidly improve the ultimate resolution of the proposed method to tens of nanometres with further development of the X-ray optics. Moreover, regulation of the working electrode potential represents an effective pathway to control deposition rates through variation of ion concentration in the Helmholtz layer. We believe that this electrochemical X-ray photolithography technique presents no limitations for electrochemical processing and can be further expanded to various technical applications including electrodeposition of metals, semiconductors, and complex structures, and X-ray assisted anisotropic electrochemical etching.

 

Principal publication and authors

A.A. Eliseev (a), N.A. Sapoletova (a), I. Snigireva (b), A. Snigirev (b) and K.S. Napolskii (a,c), Angewandte Chemie International Edition 51, 11602–11605 (2012).

a) Department of Materials Science, Lomonosov Moscow State University (Russia)

(b) ESRF

(c) Department of Chemistry, Lomonosov Moscow State University (Russia)

 

References

[1] F.C. Simeone, C. Albonetti and M. Cavallini, J. Phys. Chem. C 113, 18987-18994 (2009).