Nanoimprint Lithography: An Enabling Process for Nanofabrication

Patterning technology is crucial in micro/nanofabrication. Development of photolithography roadmap is agreeable with Moore’s law, which claims that the number of transistors per square inch may double approximately every 18 months. However, due to exposure wavelength diffraction limit, the technical complexity and manufacturing costs have been increased dramatically for the nanometer-scale line-width manufacturing. In such a context, next-generation lithography (NGL) has been proposed to replace conventional photolithography.

Nanoimprint lithography is a replication technique in essence, which can copy a master geometry. The master, also known as the template, the stamp, or the mold, is different from a mask for photolithography. The template material and geometrical parameters directly affect the template deformation and the quality of transfer pattern. Alignment accuracy is determined by the mark pattern on the master; also, the pattern transfer resolution is affected by the master geometry. Therefore, the production of high-quality, high-precision imprint template is a key issue. Besides, evaluation and repair for nanoimprint mold are increasingly becoming a focus. The aforementioned issue is a bottleneck for nanoimprint lithography process. Therefore, research on the stamp must be distinctly elucidated. There are three types of molds: hard mold, soft mold, and rigiflex mold. Silicon, quartz, or metals are used for hard mold, whereas polymers are typically used for soft and rigiflex molds. There are many methods for stamp fabrication, which have conventional and unconventional techniques. In the chapter, stamp materials, stamp fabrication, and evaluation of mold have been introduced.

Nanoimprint lithography is a simple process. In short, a substrate is coated with resin, and the stamp is pressed into the resin by mechanical deformation of imprint resist. The imprint resist is cured by heat or UV light during the imprinting process. Then, the mold is released from the substrate, referred to as demolding. Subsequent processes, such as plasma etching and lift-off, are applied for the pattern transferring [1]. The schematic of the nanoimprint lithography processes is given in Fig. 4.1.

Resist is a mixture of a polymer or its precursor and other small molecules that the solubility or viscosity is changed by UV light or electron beam or ion beam or X-ray and used in the fabrication of IC field. Resists used during photolithography are called photoresists. It can be functioned as an etching resistant material to protect the substrate. Photoresists can be classified into two groups: the positive and the negative, according to the polarity of pattern structures after photon exposure. A positive resist is a type of photoresist in which the long-chain molecules can be broken by exposure into short chains. The exposed portion becomes soluble to the photoresist developer, and an unexposed portion of the photoresists is insoluble to the photoresist developer. A negative resist is a type of photoresist in which the short-chain molecules can form together to long chains. The exposed portion becomes insoluble to the photoresist developer, and an exposed portion of the photoresists is soluble to the photoresist developer [1]. Figure 5.1 shows the basic microlithography process of photoresists.

It is well known now that a nanoimprint lithography process generally consists of stamp modification, spin coating of resist, imprinting, and then etching for pattern transfer. The stamp modification has been already demonstrated in Chap.​ 4. Before imprinting, the substrate surface is coated by a thin film for imprinting; how to create a uniform imprinted film is a crucial issue. The spin coating is generally a common method to control film thickness. There are three main nanoimprint lithography techniques: hot embossing (HE), UV-based nanoimprint lithography (UV-NIL), and soft lithography. Various soft lithography techniques have been proposed such as microcontact printing (μCP), replica molding (REM), microtransfer molding (μTM), micromolding in capillaries (MIMIC), and solvent-assisted micromolding (SAMIM). They can be widely used in various fields. In nanoimprint lithography process, the controlling of pattern defect, alignment, and full area imprinted pattern are among the hot topics. Recently, a great deal of attention has been paid to soft UV nanoimprint because of the full area conformal contact with the substrate.

Nanoimprint lithography is a method for fabricating nanoscale patterns by pressing stamp into viscous materials. To enhance a better understanding of nanoimprint lithography process, NIL simulation provides an efficient and accurate use of nanoimprint lithography. By simulation, we can know how to design the stamp, control the process for nanoimprint lithography, and decrease the defect of imprinted patterns.

Light-emitting diode (LED) is a very popular semiconductor diode available today. In the background of the world energy crisis, it has been proved to be highly efficient and energy saving and have a long lifetime. Compared to some conventional lamps, LED has given birth to the new light technology (solid-state lighting). Among them, GaN-LED is widely considered as one of the most promising next-generation light sources due to its reliability, durability, and efficiency. GaN-LED has been widely used in displays, traffic signals, and backlights [1–5]. However, the external quantum efficiency of the GaN-LED is generally much lower than the theoretical expectations due to total internal reflection effect on its surface. Consequently, one of the current essential research interests in this area is to find out methods of enhancing efficiency of GaN-LED. How to enhance efficiency of GaN-LED is a key subject in solid-state light.

Semiconductor memories together with logic devices have played critical roles in the IC industry in terms of providing cost-effective solutions for ever-increasing need for data storage [1]. Among these nonvolatile products, flash memory is very popular especially in portable application fields. To enhance its density and high programming throughput, flash devices must be continued scaling to lower dimension. However, the fabrication techniques are difficult to meet the needs of scaling. Researchers are trying to develop the next-generation nonvolatile memory. Some of those emerging memory technologies, such as ferroelectric, magnetic, and chalcogenide, are under development. These memories are evolutional, but some challenges must be overcome to realize small cell sizes, high density, and low power consumption.

The problems of environmental and energy crisis in the world are serious and pressing, which cause many countries to spend billions of dollars on research to produce clean energy. Among these novel energies, solar energy is a kind of clean energy and a solution to the energy crisis. The International Energy Agency said that “the development of affordable, inexhaustible and clean solar energy technologies will have huge long-term benefits. It will increase countries’ energy security reliance on an indigenous, inexhaustible and mostly import-independent resource” [1]. Many nations have put forward proposals to use solar energy. For example, the so-called Solar Roofs Plan in the USA and Germany began in 1996. Japan especially paid attention to the real applications of solar energy. “New Sunshine Project” started in 1993, and the incentive program “Residential PV System Dissemination Program” began in 1994 [2]. China has also taken effective measures to promote the development of solar cell.