A Survey of Nano-imaging Technology

Overview
In recent years, the phenomenon of cross-cutting, mutual penetration, and mutual integration between different disciplines and technologies has become evident. A large number of new disciplines and technologies have emerged from the surface. Nanoimprint Technology, which is one of the "soft printing technologies", is just a new technology. When it made its appearance on the stage, it was very compelling and it followed suit and was put into research and development.

This technology has absorbed the essence of many science and technology such as nanotechnology, fine processing technology, contact printing technology, interface science and new materials, and has been developed in-depth research. At first glance, this technology is very similar to the hot embossing technology used in the production of CDs and DVDs. However, after careful comparison, it is not difficult for people to see that there are obvious differences between the two. The former is only a micron-scale processing technology, and the latter shows in many aspects the formation of images through atomic and molecular level transfer (displacement); the former is a general sense of fine processing, and the latter is the use of nanomanufacturing. Principle, access to ultra-fine graphics processing technology. If we call the former the 20th century technology, then the latter is undoubtedly the emerging technology of the 21st century.

Nano-image printing technology is remarkable because it is different from traditional fine processing (such as photolithography, vacuum evaporation, electron beam processing, and chemical etching). It does not require harsh processing environment requirements. (such as ultra-clean workshops, earthquake-proof technical indicators, etc.), and without strict conditional control (such as constant temperature, constant humidity, high vacuum, etc.), various types of nano concave-convex patterns can be processed with high precision. And it can also produce such products in large quantities and at low cost. In this regard, Japan's Miyauchi et al., in the relevant articles published, in particular, compared the soft printing technology including nano-image printing technology with traditional arts and crafts.

As a highly practical application technology, nano-image printing technology will show its unique technical advantages in nanoelectronic devices, nano-optical devices, nano-biosensors, and other functional graphic productions with nanostructures.

Today, people have no doubt that nano-imaging technology will have a significant impact on the accelerated development of IT and microelectronics, biology and life sciences, and environmental and new energy technologies. The research and development of nano-imaging technology began in the mid-to-late 1990s. It was first created by Prof. Chou of the University of Princeton in the United States. After nearly 10 years of research, in-depth research, and technical support, the technology is now available. It is entering a preliminary practical stage. At the same time, Prof. Chou personally planned and organized (or planned) a series of related international conferences (Internoional Conference on Nanoimprint Nanoprint Technology) to promote this technology and expand it globally. The 1st and 2nd international conferences were held in December 2002 and December 2003 respectively in the United States and Boston. Only one year later, in December 2004, the third international conference was held in Vienna, Austria. Next, the planned fourth conference will be held in Nara, Japan in October 2005. In short, nano-image printing technology has taken a solid step from theory to practice, from research and development to preliminary practical application, and its influence has spread all over the world.

First, what is the nano-image printing technology
The so-called nano-image printing technology, in the final analysis is a new type of imprint transfer technology, it will be widely used in the processing of nano concave and convex patterns. The principle is shown in Figure 2. For ease of understanding, the current text is as follows. The nano-image printing technology is to use a mold with a nano concave-convex image as a “printing plate”, and use a silicon wafer or a glass plate coated with a polymer coating in advance as a substrate (printed material), in cooperation with the corresponding equipment and apparatus, After accurate stamping and shaping, the mold is separated from the substrate. At this time, people will find that the nano-convex image present on the surface of the mold is accurately transferred to the polymer film on the surface of the substrate. This transferred image is equal in size to the concave and convex pattern on the mold surface, and has the same depth. However, the shape is the opposite (an image of negative conversion), that is, the protrusion of the former is where the latter is recessed, and vice versa. We transfer this technique using a printing stamper to a technology with nano concave-convex patterns, called nano-image printing technology. With regard to the process of nano-image printing, we can summarize the text of Professor Chou's early experiments and describe them as follows.

1.Molding of the mold Making the mold is an important part of implementing nano-image printing, and it is also the "donor" of the image. The degree of fineness of the mold processing, in a sense, it will determine the level of transfer image quality bumps. Therefore, the production of molds is particularly important. First, Professor Chou applied a resin (polymethyl methacrylate) film called an electron resist on a template sheet (eg, metal, glass, silicon, silicon dioxide/silicon, etc.) and then, According to the pre-designed pattern, the mask is directly scanned on the film by the electron beam, and then on the basis of the mask, the corresponding bump image is etched on the surface of the template by the dry etching method. Thus, we The required mold will be completed. Experiments show that with this mold, it is possible to transfer uneven images with an accuracy of 10 nm or less. Prof. Chou believes that there are no restrictions on the level of image clarity in the transfer technology itself, but the quality of the transferred image quality is determined to a large extent by the precision of the processing of the mold.

2. The coated substrate of the substrate is the "acceptor" of the image in nanoimage printing. It consists of two parts: one is the substrate (silicon, silicon dioxide / silicon, glass and other sheet); the second is a polymer film. The early polymers used were thermoplastic resins such as polymethyl methacrylate, polystyrene, polycarbonate, and the like. At the time of film formation, people use a centrifugal method or other coating methods to uniformly coat the resin solution on the surface of the substrate to become a substrate for image transfer. 3. Transfer of Image As described above, the transfer of the image is imprinted on the coating film of the substrate by a die, and the two are separated after setting, and at this time, the image is transferred. It should be noted here that each type of thermoplastic resin has its own glass transition temperature (Tg). At this temperature, the new technical field 7 is screen printed at 2005.7. The resin becomes soft and full of plasticity. In contrast, at this temperature In the following, the resin becomes a brittle solid. Therefore, before imprinting, we must increase the temperature of the mold and the substrate above the glass transition temperature of the polymer before the image imprinting process can be performed. In contrast, when the mold is separated from the substrate, in order to ensure that the transferred image does not deform, it is also an indispensable condition to lower the temperature immediately below the glass transition temperature of the polymer before separation. The glass transition temperatures of several commonly used resins are listed in Table 1. Furthermore, if the mold we use is quartz glass with excellent transparency, and the polymer coated on the surface of the substrate is a photosensitive resin, both the imprinting and separation need not be heated, nor need to be cooled and cooled. The imprinted coating film was exposed to ultraviolet light, and the photosensitive film was immediately cured and set. Take out the mold, a piece of the image of the transferred bump is present in front of us. This method differs from the hot embossing process in that it is a new process at room temperature imprinting. In short, no matter which embossing method we use, we can get the convex and concave images with the accuracy of several nanometers to several hundred nanometers.

Second, nano-image printing process and its characteristics
Nano-imaging technology So far, a variety of imprinting and transfer processes have been developed. The respective methods and their features are described below.

1. Hot embossed nano-image printing process so-called hot imprinting nano-image printing process (referred to as hot embossing process). Since the polymer coated on the substrate is a thermoplastic resin (such as polymethyl methacrylate, etc.), when we raise the substrate temperature to 105 °C (Tg of polymethyl methacrylate), the resin It becomes something soft and full of plasticity. At this time, if the mold is imprinted on the resin film of the substrate, the soft and rich resin can be easily extruded and completely fill all gaps between the mold and the substrate. When the temperature cools and returns below its glass transition temperature (105°C), the extruded resin film cures. At this time, the mold was taken out and the nano concave-convex image was transferred onto the resin film of the substrate. We call this hot embossing and cold setting image transfer process a hot embossed nano image printing process. When we use this process to produce ultra-fine images, precise positioning of the mold and the substrate is essential. To ensure accurate positioning, people often rely on the power of optical microscopes. In addition, special tools are required for fixing the mold and the substrate.

In short, when people apply a thermal imprinting process to transfer images on a resin film of a substrate, not only can various nano concave-convex patterns such as single-layered bumps (columnar), lines, or grids be processed, but also they can be on the same substrate. In the above, nano-convex images of multi-layered structures were obtained by repeated coating and repeated imprinting.

Here, we can not forget that as a variant of the hot embossing process, the new technology of "rolling nano-image printing". This technique is to produce nano-convex image on the surface of the roller and use it as a mold. This mold can be imprinted on a strip-shaped substrate of any length coated with a thermoplastic resin, so that an arbitrary number of concavo-convex images can be continuously produced on the substrate. It is said that with this technology, nano-images can be automated and efficiently printed. It is particularly suitable for the production of multilayer nanostructure graphics.

In general, the single layer is cheaper than the multi-layer image, and the product price is also cheaper. The application prospects in the production of high-density memory disks and diffraction gratings are attractive. However, multilayer nano-images will show more attractive technological advantages in the application of photoetch traces.

Figure 5 shows a nano-convex image produced on a polymethyl methacrylate film by a hot embossing process. If we replat a nickel metal on the surface of the image, a high-density disk can be produced. Here, we should place special emphasis on the fact that if one uses functional (light, electrical, biological, etc.) macromolecules instead of ordinary polymers on a substrate, it can be found that the polymer is hot-pressed. In India, the original function remains unchanged. With this feature, people can very simply process and produce functional devices with various applications. Such as organic light emitting devices, optical waveguides for broadband optical components and so on.

Hot embossed nano-image printing process has the following major features:

(1) The process is simple and the products can be processed in large quantities;

(2) It is possible to produce uniform nano-convex images on various areas of large-area substrates (eg, 300-mm-diameter silicon wafers).

(3) The positioning accuracy of the mold and the substrate in the imprinting is not high enough. In addition, during the image transfer process, due to thermal expansion and cold shrinkage, the transfer accuracy of the pattern is not very satisfactory. Therefore, the image accuracy is often not To be applied in areas that are too demanding;

(4) Because of the time required for heating and cooling in image thermal transfer printing, it is difficult to transfer images with high efficiency;

(5) On the substrate, either a single layer or a multilayer nanostructure pattern can be fabricated.

2. Room temperature nano image printing process
Aiming at the disadvantages of the above-mentioned hot embossing process, such as poor accuracy, it was later explored that a nano-image was made under room temperature conditions.