Thermal Nanoimprint

On the other hand, the high temperature and pressure required for the nanoimprinting of thermal plastic materials limit the throughput and the application scope of the NIL technique. In addition, the thermal expansion mismatch between the mold and the substrate frequently incurred often presents an obstacle for pattern alignment over large substrates. Although various means have been attempted to make the thermoplastics imprintable at a temperature close to room... 

Nanoimprint lithography (NIL) is a relatively new technique designed to produce precisely defined nanoscale features in a parallel, efficient, and cost-effective manner [3,4]. In a typical NIL process, a prepatterned mold is used to press against a resist layer which can go through a thermal plastic transition at an elevated temperature or can be permanently... 

The nanoimprint was first developed by Chou and colleagues.Their process is called thermal nanoimprint, which is shown in Figure 4.1. The transfer layer on a Si wafer is thermal plasticity polymethylmethacrylate (PMMA) (Tg = 105°C). A mold of Si02 is produced by electron beam writing. 

It has been reported that with thermal nanoimprint it is possible to achieve resolutions of less than 10 nm in pattern transfer. It has been confirmed that no resolution limit exists in nanoimprint, and resolution is controlled by the accuracy of mold fabrication. It is possible to produce fine patterns similar to photolithography without high-cost equipment in nanoimprint. PMMA is superior for thermal plasticity polymers because of its stability in division. 

A novel liquid thermal polymerization resist is reported to be a superior polymer for thermal nanoimprint. The resist is a mixture of polymethylmethacrylate (PMMA), methylmethacrylate (MMA), n-butylacrylate (n-BA), methacrylic acid (MAA), and 2,2-azobisisobutylnitrile (AIBN). The resist can be imprinted at room temperature with a pressure of 1.2 MPa, and then cured at 95°C to obtain nanoscaled and microscaled patterns. Replications... 

Figure 15.15 All-polymer encapsulated organic microscope image of a laser (inset). (Reprinted semiconductor laser chips. (a1)-(a3) Scheme with permission from Ref [69]. Copyright 2010, ofthe main fabrication steps, including thermal Optical Society of America.) nanoimprint, (b) Photograph of two chips and a...

The nanoimprint process utilises a patterned, 3D mould (template or stamp) to define patterns by embossing a soft polymer or liquid material. Once the material has completely filled the template cavities, it is hardened, using either a thermal or photochemical process, and the template is removed. The hardened imprinted polymer is an inverse 3D replication of the template mould. NIL uses a stamp or template to imprint or emboss a pattern into a polymer. The 3D polymer structures themselves may be used to create the desired nanostructures or alternatively, the polymer structures may be used as a protective mask to selectively protect a substrate during a subsequent process e.g. etching or deposition. 

Fig. 3.16 Embossing or nanoimprint lithography procedure. A hard or soft mold is in contact with surface of a film (a) A pressure is accomplished on the hard mold or a conformal contact is realized for the soft one in order to transfer the features of the mold on the film surface (b) sometimes at this stage a thermal treatment is necessary in order to soften the film and allow abetter transfer of the patterning. Mold is removed (c) A final etching process can be performed to reduce the residual layers between the transferred structures (d)...

In an effort to realize the roller-based nanoimprint process. Tan et al. have used a solid rod to apply pressure to a piece of Si mold in a thermal nanoimprint... 

The first reported use of NIL was in 1995 by Chou et al. who fabricated 25 nm diameter dots with a 120 nm period by thermal nanoimprint lithography (TNIL). In their work, a thermoplastic material, poly(methyl methacrylate) (PMMA), was used as the moldable material followed by oxygen plasma reactive ion etch (RIE) process to expose the substrate. The deposition of titanium/gold via evaporation and lift-off to yield an array of individually isolated titanium/gold dots was achieved with success. Figure 2 shows the imprinted PMMA template and the titanium/gold dot array after metallization and lift-off processes. 

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