Introduction to nanoimprint lithography

Micro- and nanopatterning techniques are key processes in the fabrication of modern optics or microelectronics and are becoming increasingly important for biological applications.^[95–97] One of the most prominent examples is photolithography, a common technique to produce integrated circuits. This technique, however, is facing technical and economic limits with decreasing pattern sizes. A problem, that needs to be overcome by technological innovations or by the identification of alternative patterning techniques. Nanoimprint lithography (NIL) is a method with the potential to solve these issues. Figure 25 gives an overview of the main nanoimprint lithography techniques.

Figure 25: Overview: Main nanoimprint lithography methods.^[98]

NIL, an alternative to well-established lithographic processes, is a cost-effective high-throughput micro- and nanopatterning technique whose basic principle was developed by Chou et al. in 1995.^[99] The NIL process consists of two basic steps (see Figure 26). At first, a stamp (mold) with features in the nanometer regime is pressed into a resist, e.g. a polymethyl methacrylate (PMMA).^[99] The pressure applied and the temperature needed for the process depends on the resist material. In this case, the temperature for the imprint process was adjusted to 200 °C and the pressure to 131 bar. As a consequence, the material flows into the cavities of the mold. After cooling the resist below its Tg of 105 °C, the stamp is lifted-off (released) and the negative-type pattern of the mold is imprinted into the PMMA film. Pillars with a diameter of 25 nm were successfully transferred from the mold to the resist. In a second step, the residual layer on the patterned silicon wafer was etched using oxygen gas in a reactive ion etching (RIE) process. In 1997, Chou et al. even

1 Parts of this chapter have been published in the Journal of Advanced Materials.

C. Probst, C. Meichner, K. Kreger, L. Kador, C. Neuber, H.-W. Schmidt, Adv. Mater. 2016, 28, 2624.^[94]

Nanoimprint

presented patterns with a structure size down to 10 nm. Since heat is necessary, this technique is often referred to as thermal NIL (t-NIL). Among the main advantages of this technique, the low cost and the large area that can be patterned are the most outstanding ones. Major drawbacks are the processing time needed and thermal expansion and shrinkage during heating and cooling cycles leading to distortions of the involved resist and mold materials.^[100]

Figure 26: Left: Schematic of the nanoimprint lithography process: (1.) pressing the mold into the PMMA film to create a thickness contrast in the resist; release of the mold, and (2.) reactive ion etching of the resist to transfer the pattern from the template into the resist. Middle: SEM micrograph the mold used for the imprinting process. Right: SEM image of the replicated structures of the mold in a PMMA film.^[101]

Ultraviolet nanoimprint lithography (UV-NIL) allows for imprinting of nanopatterns at room temperature. In this case, a low viscous UV curable fluid is placed onto a substrate by dispensing or spin-coating. After pressing the UV-transparent stamp onto the thin film, the resist is crosslinked by exposure to UV light (see Figure 27). The applied pressure is much lower than in case of t-NIL (between 0-5 bar) and no heating of the substrate is necessary.^[100] While in t-NIL rigid stamps (quartz, Si, nickel, etc.) are common, flexible stamps (e.g. PDMS or fluorinated polymers) can be used in UV-NIL processes. Nanoimprinting methods using flexible or soft stamps are embraced by the term soft lithography. Since soft stamps simplify the release procedure due to their flexibility and low surface energy, defects are reduced compared to methods working with rigid stamps.^[96]

Another advantage of UV-NIL towards t-NIL is the significantly shortened cycle time. Since no heating or cooling is needed, the average duration for the imprinting process is below 1 min.^[100]

However, one of the drawbacks of this technique is the material shrinkage occurring during the crosslinking step in which the material solidifies. This curing shrinkage is most pronounced in curing systems utilizing radical initiators.^[102]

Figure 27: Left: Schematic of the UV-NIL process: the resist is coated on a substrate and aligned; the stamp is imprinted into the resist and cured by exposure to UV light; Separation of stamp and substrate. Middle and Right: SEM images of UV-NIL imprinted samples. a) top view; b) cross-sectional view along line A-B in a).^[102,103]

In 2001 Hong H. Lee et al. developed an interesting patterning technique that combines the main features of conventional t-NIL with the advantages of soft lithography methods.^[104] In the so-called capillary force lithography (CFL), a spin-coated polymer film (e.g. PS or SBS) is heated above its Tg

while an elastomeric mold is placed onto it (see Figure 28). During the whole imprinting process, no pressure is applied to the mold. A prerequisite for CFL to function is that the free energy of the system is lowered by the polymer, which wets the walls of the PDMS mold. If the effect of gravity is neglected, the viscosity of the polymer melt η and its surface tension γpolymer/air as well as the size of the capillary determine the rate of the flow of the polymer into the cavities of the mold. The time t, to fill the mold is given by:

𝑡 =

𝑅𝛾𝑝𝑜𝑙𝑦𝑚𝑒𝑟/𝑎𝑖𝑟𝑐𝑜𝑠𝜃 (4.1)

Where z is the length of the capillary to be filled, R the hydraulic radius (ratio of the volume of the liquid like polymer in the capillary to the solid/polymer interface) and θ the contact angle at the resist/mold interface. In the experiments presented, the time for the SBS triblock copolymer to completely fill cavities with a height of 600 nm and a width of 400 nm was about 30 minutes at a temperature of 100°C. Since no pressure is applied to the highly-viscous polymer film (viscosity ≈ 10^6 Pa s), the time needed for the imprinting process becomes extremely high. The long time period needed for the process is the biggest drawback of this technique.

Figure 28: Left: Schematic representation of the function principle of CFL. Right: SEM micrographs of imprinted microstructures via CFL patterning technique. The arrow indicates the residual layer.^[104]