Technical Information

H-1 High-Precision Nano-Imprinting Technology

H-1 High-Precision Nano-Imprinting Technology

We explain the overview of the nano-imprinting technology which is the low-priced microstructure fabricating technology, and the development situation of the roll-to-roll type imprinting machine which is attracting a great deal of users' attention as an approach to larger area and high throughput, in addition to the direct press type imprinting machine. Also, we introduce the examples imprinting and relevant devices of the both machines, then the current situation and future prospect.

[Platform]
H. Nano-imprinting technology
[Applications]
Nano-imprinting machines

[Technical points]

  • ST50: Designed for both thermal imprinting and UV imprinting.
  • Both the vacuum chamber and XY stage can cope with the options.
  • Step & repeat imprinting type.
  • Roll-to-roll type nano-imprinting machine CMT-400U: Very effective for imprinting patterns on a few meter or more wide resin sheet.
  • Supply of total technology including master and replica molds.
  • Intermittent coating in which UV resin is coated on the pattern area alone.

1. Introduction

The nano-imprinting technology is summarized as the patterning of various resin surfaces by pressing micro patterns of a few tens nanometer to a few hundred micron. As the patterning to the order of nanometer can be realized at a low cost and without using an expensive process equipment such as beam lithography and electron beam writing equipment, this technology has been widely noticed recently. In this report, we introduce the outline of the nano-imprinting process and the nano-imprinting machine.

2. Nano-imprinting

2.1 Nano-imprinting technology

Fig.1 Comparison of thermal, UV and room temperature imprinting processes

Fig.1 Comparison of thermal, UV and room temperature imprinting processes

The nano-imprinting technology is classified into the thermal imprinting and UV (Ultraviolet) imprinting according to the energy supplied at the time of imprinting. Recently, the room temperature imprinting, using spin-on-glass (SOG) material, is available in addition to them. This room temperature imprinting is one of the techniques widely noticed now due to the simple imprinting mechanism and characteristic of distinctive material. (Fig. 1) We call this room temperature imprinting as the "COLD IMPRINT".
The nano-imprinting technology is also grouped into the direct press type imprinting, roller press type imprinting and roll-to-roll type imprinting according to the pressing method. (Fig. 2)
In the following paragraph, we deal with the direct press type (batch press type and step & repeat type imprinting) and roll-to-roll type nano-imprinting machines and the results of imprinting, using these machines.

Fig. 2 Nano-imprinting techniques

Fig. 2 Nano-imprinting techniques

2.2 Nano-imprinting machine and results of imprinting

2.2.1 Direct press type (ST series)

Fig. 3 Direct press type nano-imprinting machine ST50

Fig. 3 Direct press type nano-imprinting machine ST50

The batch press type imprinting (see Fig. 2) is the most general technique which imprints patterns on the mold to the substrate in one process by locating the mold and substrate face-to-face in parallel, pressurizing and holding them in a batch. Basically, this is the more general-purposed technique because if you provide a mold with large area and press the mold, you can imprint at once the patterns on the larger area of the substrate. To imprint a large area (8 inch-diameter. wafer, for instance) with this technique, however, you have to take various measures on the machine. To be more specific, you have to provide a large pressing force (a few ten kN to a few hundred kN) press mechanism, a parallelism adjusting mechanism between the mold and substrate, a system for maintaining constant the temperature inside the mold and substrate (for thermal imprinting) and equalizing the pressure between contact surfaces, and a mold releasing mechanism (mold releasing force increases with increase in area).
The step & repeat type imprinting (see Fig. 2) is the technique which imprints patterns on the entire surface of substrate by repeating in turn the batch press type imprinting process described above. In the first pattern imprinting, the same process as in the batch press type imprinting is used, and the pattern size imprinted here is about a few ten mm × a few ten mm. After the first pattern imprinting has finished, relative positions of substrate and mold are shifted to imprint a pattern at the next position.
By repeating this process, patterns are imprinted in turn on a desired area. To select this technique, you have to provide a moving table (XY stage) for sifting in turn the relative positions of substrate and mold. When the travel range of this moving table is extended, imprinting of patterns on a larger area of substrate is possible. To imprint patters at predetermined positions, an alignment mechanism for detecting the relative positions of mold and substrate is mounted to locate them precisely.
The nano-imprinting machine ST50 developed by Toshiba Machine has maximum press force of 50kN to cope with the thermal imprinting and UV imprinting, fully considering expandability of the machine. (Fig. 3)
To cope with diversified molds, material profiles and heating systems, we individually design the mold and material mounting units and heating system for the requests of specific customer who selects the thermal imprinting. Likewise, when the customer chooses the UV imprinting, we individually design the material mounting unit for the requests. The control system is as developed by Toshiba Machine, which is capable of controlling the AC servo motor with desired press force, press speed and pressing sequence, and allows multiple data setting of heating temperature, speed and sequence for the thermal imprinting, and UV beam intensity and curing sequence for the UV imprinting.

Fig. 4 ST head<sup>&reg;</sup>: Example of slight angle adjusting mechanism

Fig. 4 ST head®: Example of slight angle adjusting mechanism

The ST50 machine can be equipped optionally with the vacuum chamber serving for the purpose of eliminating such abnormalities as worsened pattern imprinting ability and catch of air bubbles, the XY stage designed for the step & repeat type imprinting, the ST head® (Fig. 4) and ST stage® which are driven to adjust the slight angle after the mold surface is pressed against the material surface so that they can be brought into close contact with each other, and the like.

Fig. 5 Example of UV imprinting: L&S of 50nm width

Fig. 5 Example of UV imprinting: L&S of 50nm width

Next, we introduce the results of UV imprinting, using the ST50 machine. Fig. 5 shows the example imprinting of a hyperfine structure, that is, an example of line & space (hereinafter called the "L&S") patterns whose line width is 50nm level, using the UV nano-imprinting technology. A mold was fabricated from quartz glass by EB (electron beam) writing and etching, and patterns were imprinted with the UV nano-imprinting technique. As a result, hyperfine patterns of 50nm level could be formed.

Fig. 6 Example of UV imprinting: Batch press type imprinting to 8 inch-diameter area

Fig. 6 Example of UV imprinting: Batch press type imprinting to 8 inch-diameter area

Fig. 6 shows an example of UV imprinting. For the mold, an 8 inch-diameter. quartz glass wafer having L&S patterns of 90nm (180nm-pitch) in the range of 120mm × 120mm was used with UV curable resin coated on an 8 inch-diameter. quartz glass wafer like the mold, and UV imprinting was executed. As a result, favorable pattern profiles could be obtained.

2.2.2 Roll-to-roll type

Fig. 7 Major components of roll-to-roll type UV nano-imprinting machine

Fig. 7 Major components of roll-to-roll type UV nano-imprinting machine

Both the batch press type and step & repeat type imprinting are the nano-imprinting process in which a pattern or patterns are imprinted on a substrate of a few ten mm to a few hundred mm size, and this process is proceeded on each single wafer (each single substrate). Meanwhile, continuous imprinting of patterns on a successive resin sheet is called the "roll-to-roll type imprinting" (see Fig. 2). In this technique, a roll on which micro-patterning was directly processed beforehand is used as the mold ([1] patterned roll mold), or a relatively thin foiled plane sheet stamper is wound around the roll and fixed, which is used as the mold ([2] sheet mold). When the former mold ([1] patterned roll mold) is used, patterns can be imprinted on the resin sheet continuously without a break by repeated rotation of the roll, which is very effective for imprinting a pattern on a few meter or more wide resin sheet.
In this roll-to-roll technique, contact and release of the mold and film differ from those of the batch press type and step & repeat type imprinting techniques. Even if a vacuum environment is not prevailing, air bubbles are relatively hard to enter and release of the mold and film becomes easier. This technique allows nano-imprinting of comparatively large area with high efficiency, which is considered to be one of the mainstream techniques when elements applying diversified nano-imprinting processes are to be put to practical use and mass-produced in the future.
Fig. 7 and Fig. 8 show the major components and external view of roll-to-roll type nano-imprinting machine CMT-400U developed by Toshiba Machine, respectively.

Fig. 8 External view of roll-to-roll type UV nano-imprinting machine CMT-400U

Fig. 8 External view of roll-to-roll type UV nano-imprinting machine CMT-400U

This machine applies the roll technology of the extruding machine or printing machine, and UV curable resin coated on the base film is formed by means of the gravure roll as shown in Fig. 7. In addition to addressing the large area, improvement of productivity and throughput are considered, and extensive applications covering optical sheets for FPD (flat panel display), biotechnology, solar cells, electronic papers, wire grid polarizers, etc are also expected.

Fig. 9 Situation in which roll-to-roll type UV imprinting is under way, and gravure roll

Fig. 9 Situation in which roll-to-roll type UV imprinting is under way, and gravure roll

Fig. 9 shows the situation in which imprinting test is being performed with the machined gravure roll.

Fig. 10 Mold for roll-to-roll type UV imprinting

Fig. 10 Mold for roll-to-roll type UV imprinting

Now, Fig. 10 shows an example of mounting the thin sheet stamper on the cylindrical roll ([2] sheet mold). In many occasions, direct patterning on the roll surface is rather difficult due to the pattern size, degree of freedom of patterning, etc. As a result, the latter technique ([2] sheet mold) is used very frequently.

Fig. 11 Principle of intermittent imprinting

Fig. 11 Principle of intermittent imprinting

When the above sheet mold is used, patterns are imprinted intermittently on the resin sheet. If UV curable resin is coated on the resin sheet surface consecutively, the UV curable resin partially becomes wasteful. Additionally, as the peripheral of the sheet mold will include clearance and step, the UV curable resin will enter them, resulting in deterioration of imprinting accuracy and generation of mechanical abnormality.

Fig. 12 Situation where intermittent imprinting is under way

Fig. 12 Situation where intermittent imprinting is under way

To settle this problem, we are now developing the "intermittent coating" technology which coats the UV curable resin only to the inside pattern area of the sheet mold. Fig. 11 shows the principle of the intermittent imprinting using the intermittent coating technique, and Fig. 12 the situation where intermittent imprinting is going on.

Fig. 13 Imprinted products fabricated by the roll-to-roll type imprinting machine

Fig. 13 Imprinted products fabricated by the roll-to-roll type imprinting machine

When this intermittent coating technique is employed for the roll-to-roll type imprinting with sheet molds, wasteful UV curable resin can be avoided and residual sediment of UV curable resin is not left around the sheet mold mounting part, thus favorable imprinting can be assured. Concerning the roll-to-roll type imprinting machine, we are continuously developing it, taking "constant sheet feed at high speed", "high-accuracy UV curable resin coating that allows uniform thickness and forming of thin film", "easy mounting of mold", "mold releasing" as the keywords. Next, we introduce the results of imprinting, using the CMT-400U machine. Fig. 13 shows the imprinted products fabricated, using the machined gravure roll (Fig. 9).

Fig.14 Imprinted products (cell profile) by the roll-to-roll type imprinting machine

Fig.14 Imprinted products (cell profile) by the roll-to-roll type imprinting machine

On the part of the sphere whose radius is 100μm (SR100μm), convex lens profile of 7.5μm-high and quadrangular pyramid profile of 25μm-high with 50μm base could be imprinted. Also, results of imprinting cell profile (100μm-pitch), using the gravure roll where the Ni electroforming mold of 0.3mm-thick was wound around the roll (the mold shown in Fig. 10 was used) are shown in Fig. 14.

3. Conclusion

The nano-imprinting process is a very fascinating technology which allows imprinting of precise patterns in a simple process. It is expected to be applied in many fields including the electronic information communication devices and biomedical equipment in the years ahead. We are sure that this technology will contribute largely to the compact size, high performance and low cost in various fields.
To live up to such expectations, incessant improvement of the nano-imprinting machine for fabricating elements is mandatory at the same time with uninterrupted enhancement of the technologies for molds, resin material, process engineering and inspection which are required in a series of fabrication processes. Supply of these technologies all together as the total engineering rather than separately is very important to popularize the nano-imprinting technology.