Since the 1990s, NIL has emerged as one of the most versatile methods for nanoscale patterning, capable of fabricating patterns from less than 10 nm to a few microns on a large area, at a low cost and with a high throughput. It is therefore considered on the current ITRS roadmap as an emerging lithography technique.

To perform NIL on a substrate, three basic components are required: (i) a stamp with suitable feature sizes; (ii) the material to be imprinted, usually a thin layer of a polymeric resist with a suitable glass transition temperature; and (iii) equipment for printing with adequate control over temperature, pressure and parallelism of the stamp and the substrate.

During thermal imprinting, also called hot embossing, the stamp is pressed at high temperature and pressure into the then fluid polymer layer, which leads to permanent structure replication upon cooling. This principle of NIL is quite simple and therefore the technology has found widespread use. However, the resist material is generally regarded as one of the challenges of this technique because the material has to fulfill several requirements such as low viscosity, low adhesion to the mold, good adhesion to the substrate and high etch resistance to allow pattern transfer into the substrate. In particular, a high etch resistance is required to create high-aspect-ratio structures. This is commonly achieved by changing the polymeric resist after imprinting, by additional metal evaporation and lift-off steps, into a metal mask that performs better in reactive ion etching than most organic polymer resists.

In a recent paper published in Nanotechnology, the MESA+ team has proposed a new type of organometallic resist – poly(ferrocenylsilane)s (PFSs). The material provides excellent etch contrast and does away with the need for metal lift off.

PFSs are composed of alternating ferrocene and silane units in the main chain, which adds a distinctive functionality to this material. These polymers were found to be effective resists in reactive ion etching processes due to the formation of an etch-resistant iron/silicon oxide layer in oxygen-containing plasmas, which was the motivation to use this polymer as a NIL resist.

PFS films obtained by spin coating were conveniently imprinted by thermal NIL. After removal of the residual layer, the patterns were directly transferred into silicon substrates by reactive ion etching with very high etch contrasts, showing the potential for the formation of high-aspect-ratio structures and a reduction in the number of processing steps required to achieve them.