Nanoimprint Lithography (NIL) is a surface patterning technique that, in the recent literature, has been shown to provide resist linewidth resolutions anywhere from several hundred microns down to about 5 nm. NIL can be broken down into three main mechanisms: Hot embossing, thermal curing NIL and UV-NIL. Hot embossing utilizes elevated temperature and pressure to drive polymer resist from a spin-coated film into the cavities of a hard mold, as shown in Figure 1. After the mold is released, the replicated resist patterns often go through a plasma etching step to remove the residual layer, which is always present after the imprinting step. Thermal curing NIL obtains resist patterns in a somewhat different fashion by driving the flow of a liquid precursor resin into the mold cavities by pressure alone and then heating the resin above its curing temperature. UV-NIL operates similarly to thermal curing NIL except the entire process is carried out at room temperature and the resin is cured by UV exposure through a transparent mold.
A very basic NIL hot embossing process is shown in Figure 1. Essentially the surface pattern of a hard mold is replicated into a thermoplastic resist material at elevated pressure when the temperature is raised above the glass transition temperature of the resist.
Figure 1: Basic NIL hot embossing process
Compared to other nanofabrication techniques such as e-beam lithography, x-ray lithography, and immersion-assisted photolithography, NIL benefits from higher throughput and lower capital equipment costs. In addition, NIL can define features without the use of solvent developers and wet etches commonly used in photolithography. This represents significant cost savings and environmental benefits from reduced chemical usage and disposal. Furthermore, the NIL process is very flexible. It can accommodate a large variety of polymeric materials and commercially available resists. It is also compatible with either stiff or flexible substrate materials particularly silicon, glass, metallic sheets and films as well as plastics. The many advantages of NIL make it well-disposed towards the fabrication of a long list of potential applications, including semiconductors, micro- and nano-electro-mechanical devices, optical components and biological or chemical templates. In addition, new applications for NIL are constantly being developed at breathtaking speed.
A typical process flow of NIL can be summarized in Figure 2