Nanoimprint Lithography: Methods and Material Requirements

The ability to fabricate structures from the microto the nanoscale with high precision in a wide variety of materials is of crucial importance to the advancement of microand nanotechnology and the nanosciences. The semiconductor industry has been pushing high-precision nanoscale lithography to manufacture ever-smaller transistors and higher-density integrated circuits (ICs). Critical issues, such as resolution, reliability, speed, and overlay accuracy, all need to be addressed in order to develop new lithography methodologies for such demanding, industrially relevant processes. On the other hand, less stringent conditions are found in many other areas, for example, photonics, microand nanofluidics, chip-based sensors, and most biological applications. Several alternative approaches towards nanostructure fabrication have been exploited in the past 15 years, without resorting to expensive tools such as those used in deep-UV projection lithography and electron-beam lithography. These techniques include microcontact printing (or soft lithography), nanoimprint lithography (NIL), scanning-probe-based techniques (e.g., atomic force microscope lithography), and dip-pen lithography. A good overview of several of these techniques was presented in a recent paper. This article will focus on NIL technology and review the progress that has been made in this field in recent years. Nanoimprinting can not only create resist patterns, as in lithography, but can also imprint functional device structures in various polymers, which can lead to a wide range of applications in electronics, photonics, data storage, and biotechnology. Some of these applications have been discussed in a previous review. The principle of nanoimprinting is very simple. Figure 1a shows a schematic of the originally proposed NIL process. A hard mold that contains nanoscale surface-relief features is pressed into a polymeric material cast on a substrate at a controlled temperature and pressure, thereby creating a thickness contrast in the polymeric material. A thin residual layer of polymeric material is intentionally left underneath the mold protrusions, and acts as a soft cushioning layer that prevents direct impact of the hard mold on the substrate and effectively protects the delicate nanoscale features on the mold surface. For most applications, this residual layer needs to be removed by an anisotropic O2 plasma-etching process to complete the pattern definition. R EV EW