Prospects of nano-lithographic tools for the fabrication of surface-enhanced Raman spectroscopy (SERS) substrates

The previous decades have seen a massive increase in the research towards reproducible and optimized surface-enhanced Raman spectroscopy (SERS) substrates. While traditional colloidal synthesis methods have commonly been used for SERS substrate fabrication, they lack reproducibility hindering their usage for many applications. The need for reproducible nanostructures showing high orders of enhancement factors has brought about a shift in the methods one can use to fabricate SERS nanostructures. Lithographic techniques have thus piqued the interest of researchers as a viable option for SERS substrate fabrication. Not only do they offer high enhancement factors and reproducible nanostructures, they also provide the ability to fabricate nanostructures with many different geometries, shapes, sizes and periodicities. Some of the most established lithographic techniques include electron beam lithography, nanosphere lithography, laser interference lithography and many more. This review discusses established lithographic techniques, such as mentioned above, along with other upcoming lithographic techniques to understand the principles and the methodology behind them. A deep understanding of how various parameters can influence the nanostructure fabrication and thereby influence the SERS enhancement is developed. A detailed description of how these nanostructures can be fabricated is also provided for better insight. In addition, strengths and limitations of each method are discussed in detail. Lastly, we also discuss the applicability of SERS substrates for commercial applications comparing the performance of chemical synthesis routes and lithography for SERS substrate fabrication. This review serves as a base to understand the concept and application of SERS from a microfabrication perspective.