High-volume nanoscale imprinting

The ability to pattern materials at the nanoscale enables a variety of applications ranging from high-density data storage, displays, photonic devices, and CMOS integrated circuits to emerging applications in the biomedical and energy sectors. These applications require varying levels of pattern control, short- and long-range order, and have different cost tolerances.

Extremely large-area roll-to-roll manufacturing on flexible substrates is ubiquitous for applications such as paper and plastic processing.^1–3 It combines the benefits of high speed and inexpensive substrates to deliver an affordable commodity product. The challenge is to extend this approach to the realm of nanopatterning and realize similar benefits. The cost of manufacturing is typically driven by speed (or throughput), tool complexity, price of consumables (materials used, mold or master, and so forth) and substrates, and the downstream processing required (e.g., annealing, deposition, and etching). To make nanopatterning practical, it is imperative to move toward high-speed imprinting, less complex tools, near-zero waste of consumables, and low-cost substrates.

Unlike common imprint technologies that rely on heat embossing to melt thermoplastic solids, Jet and Flash Imprint Lithography (J-FIL™) uses inkjet dispensing of UV-curable resists to assist high-resolution patterning for subsequent dry etch pattern transfer.^4–6 The technology is actively being used to develop solutions for memory markets, including flash memory and patterned media for hard disk drives.

We have developed a roll-based J-FIL process to fabricate flexible bilayer ‘wire grid’ polarizers (WGPs) and high-performance WGPs on rigid glass substrates, and demonstrated it with a prototype we have named the LithoFlex 100: see Figure 1. The LithoFlex 100 has allowed us to explore the features of an inkjet-resist-driven nanoimprinter. The system uses a 150mm patterned fused silica wafer as a template, and imprints the pattern onto a flexible roll material such as polyethylene terephthalate (PET) or polycarbonate. The width of the film is 100mm.

Figure 1. Left: Our ‘technology demonstrator,’ LithoFlex100, being used to develop roll-to-roll inkjet-resist-driven nanoimprinting. Right: The imprint tool enclosed within an environment control unit.

We tested several pattern types to ensure that imprint non-fill and separation-induced defects were addressed. We chose an initial test pattern of curvilinear 120nm features on a 300nm pitch to understand if there were any separation issues resulting from pattern direction. All fields were cleanly imprinted: see Figure 2(a), which shows a 10m roll (over 100 imprints) printed with this pattern. We also ran a second longevity experiment using a 50nm grating with a 20 × 20mm field, and found no pattern degradation after more than 1000 consecutive imprinted fields: see Figure 2(b and c). Once the process was established, we tested the resolution by imprinting three different patterns: 100nm dense pillars, 50nm half-pitch lines, and dense 25nm holes. All patterns were faithfully resolved. We also demonstrated aspect ratios of up to 3:1 for 50nm lines. Further details on patterning can be found elsewhere.⁷

Figure 2. (a) Imprinted pattern covering 10 meters of the polycarbonate substrate. (b) First and (c) 1012th consecutive imprints, showing no reduction in quality.

WGPs, which typically consist of small pitch arrays of aluminum lines, are already used in digital projectors. Their combination of performance and temperature durability makes them an attractive choice for this market. Until now, the difficulty of scaling up WGPs to the areas required has limited their use for larger displays, including mobile phones, tablets, monitors, and TVs. A roll-based process enables printing over substantially larger areas and simultaneously raises performance and lowers cost.

Using a 65nm half-pitch grating template, we fabricated a flexible film polarizer with a measured diagonal of 5.7 inches, which we placed on top of an iPad display and rotated by 90° to demonstrate the performance of both the transverse magnetic (TM) and transverse electric (TE) modes: see Figure 3. Note that the same tooling that produced a flexible film polarizer can also be used to pattern rigid substrates (such as glass or silicon) by using the patterned roll film as an imprint template. Polarizer performance can also easily be enhanced by decreasing the half-pitch to 50nm or below.

Figure 3. Optical performance of a 5.7 inch flexible wire grid polarizer in both transverse magnetic (TM) and transverse electric (TE) modes.

Figure 4. (a) 300mm template. (b) 350mm prototype test platform.

Now that the prototype tool is performing efficiently and providing repeatable results, the next step is to scale the tool and process to address industry requirements for both area and throughput. Both a new template infrastructure and an imprinting scheme with a parallel processing configuration are required. Because the Lithoflex 100's process steps are sequential, it is expected that its throughput will be limited in comparison with parallel processes. Current throughput is approximately 180 printed fields per hour. We are developing a next-generation tool for parallel processing, and templates with a diameter of 300mm are now readily available: see Figure 4(a). In addition, we are working on a prototype module that will enable imprinting at substrate widths as large as 350mm: see Figure 4(b). We are continuing to develop and test this new prototype.

The authors would like to thank both the University of Texas and the College of Nanoscale Science and Engineering for their contributions. This work was funded in part by the US Department of Defense Small Business Technology Transfer (STTR) Program, contract FA9550-11-C-0046.