Electrochemical X-ray Photolithography

Abstract

Recent progress in information technologies has made microand nanolithography key processes in modern industry. Today these methods are among the most rapidly developing areas in both science and engineering. In the last two decades, wellknown and widespread methods of photoand electron-beam lithography have been complemented by a number of novel approaches that include direct writing processes, such as scanning probe lithography and dip-pen nanolithography, mask-based techniques including soft-lithography, nanoimprint, nanostencil and nanosphere lithography, and unconventional wet lithographies. Progress in the development of these methods has allowed high throughput to be attained at an ultimate resolution of less than 10 nm and enabled massproduction of microelectronic components. Further improvements in this method were introduced by the addition of chemical transformations, which can be applied through the integration of lithography with chemical or electrochemical processes. Electrochemical processing possesses a number of important advantages, including room-temperature synthesis (thereby avoiding thermal expansion problems), ability to control deposition rates, increased deposition density, and enhanced versatility. Moreover, electrochemical processes are easy to control through manipulation of charge values and current transient shapes. Several electrochemical lithography methods have been reported that achieve resolution down to 10 nm. However, all of these methods require contact lithography processing, as electrochemical photolithographic pattern-transfer techniques have yet to be developed. Nevertheless, numerous papers have been published on light-induced electrochemical effects, which have the potential to be further utilized for the development of non-contact electrochemical lithography techniques. One drawback to this approach however, is that the wavelengths of visible and UV light restrict the resolution level. A possible solution to this problem is the use of X-rays, which have long been known to trigger surface charging and radiolysis in matter. It could be expected that these effects would provide the conditions necessary for the electrochemical equilibrium shift required for electrochemical etching or deposition. Despite this, there have been only a few reports on X-rayassisted electrochemical processing. While some chemical X-ray lithography methods use X-rays for induced chemical transformations, they are restricted to reactions where radiolysis products are formed within the solution volume. Some work discusses reactions driven by photoelectrons ejected from the substrate surface, but do not complete the electrochemical circuit to control Helmholtz layer. Lastly, we also failed to find any studies concerning direct electrochemical lithography under X-ray irradiation. Thus in the present study we have focused our attention on the development of an electrochemical X-ray photolithography approach for direct pattern transfer to a liquid– solid interface by coherent X-ray irradiation. The concept is based upon variation in electrochemical deposition/etching rates provided by local photoionization effects. Local reduction of electrolyte components by photoelectrons generated on the working electrode surface under X-rays was chosen to test the concept. The electrochemical X-ray photolithography method was investigated by pattern transfer onto electrochemically deposited nickel, which plays a pivotal role in industry, particularly in microelectronics. The experiment is shown in Figure 1. A parallel X-ray beam collimated by slits was passed through a 4 mm pitch silicon grating and guided to the