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FIGURE 3.10. Schematic of the HEL process.

FIGURE 3.11. Schematic of a roller type of NIL.

schematics of the HEL process. Basically, a nano-structured mold (typically made by EBL) is used to imprint a thin polymer film (mostly PMMA) attached to a flat solid substrate (typically silicon wafer). The residual polymer layer is removed by RIE and followed by either a lift-off or direct etching process to produce the nano-structures into the substrate. For PMMA, an embossing temperature from 130 to 190 °C and vacuum are applied to ensure complete embossing. An ultra-thin teflon-like film is often deposited onto the mold to minimize the fictional force and avoid sticking dust particles. Structures with feature size ranging from 10 to 100 nm have been fabricated using PMMA [98-104] or cellulose acetate [105]. Figure 3.11 shows the schematics of both flat bed and roller type of NIL. Potential applications of these nano-structures include silicon quantum dots, wire and ring transistors, and a nano compact disk (CD) with 400 Gbits/in2 storage density (near three orders of magnitude higher than the current CD). A major challenge of nanoscale embossing/imprinting is how to produce uniform features over a large surface area. A step-and-repeat process developed by Chou [106] is able to form nano-sized structures over large surface area. Another approach is to use a 'soft' mold and pneumatic embossing force. A diaphragm type press is used to provide good contact between the mold surface and the substrate surface [107].

Several "LIGA press" type cyclic embossing machines are available in the market. Figure 3.12 shows a hot embossing machine system HEX 03 manufactured by Jenoptik Mikrotechnik GmbH. Figure 3.13 shows an EVG520HE system from AV Groups in Austria. Both have similar capability. Recently, Obducat Company in Sweden developed a nanoimprinting system based on the diaphragm type press as shown in Figure 3.14. Molecular Imprints, Inc. in the U.S. has a micro/nanoembossing system based on UV cure of liquid resins. Their Imrpio 100 machine is shown in Figure 3.15.

3.4.2.2. Sacrificial Template Nanoimprinting In nanoscale replication, de-molding is the most challenging step because the stress resulting from the large contact surface between the tool and the polymer substrate tends to damage the molded polymer structures and/or the tool features. Using the simple soft lithography technique, low-cost sacrificial templates replicating the 3D tool features can be formed from water-soluble polymers reinforced with nanoparticles. These nanocomposite templates are strong enough to serve as molds during processing and can be easily removed and recycled in water.

In our laboratory, we are developing simple NIL techniques that can be used to mass-produce well-defined nanoparticles and nanofluidic devices with well-defined nano-pores for drug delivery and bioseparation. Figure 3.16 shows an example of this method. A nano-indenter or an AFM can be used to produce a master (mold), by first coating a photoresist on a metal-coated silicon wafer or quartz. Then, an array of holes is punched through the cured photoresist. Next, casting is carried out to produce a silicone rubber master with an array (e.g. 100 x 100 in 1 mm2 area) of nano-sized probes. As an example, Figure 3.17 shows SEM photos of a silicone rubber master with a 5 x 5 array of pyramidal shape probes made by a Hysitron nano-indenter with a 90° diamond tip. The probe radius is less than 50 nm (the probe surface was covered by 20 nm-thick gold for SEM imaging), and can be made smaller by using different tips.

Figure 3.18 shows another example of using a group of differential etched optic fiber bundles (Sumitomo IGN-037/10) as the male master mold, followed with casting to make the poly(dimethyl siloxane) (PDMS) female daughter mold, re-casting in the polyvinyl alcohol (PVA)/water solution to make the male sacrificial template for nano-imprinting, and then the resulting nanoporous poly(lactic-co-glycite acid) (PLGA) layer [108]. Masters with an array of probes are fabricated on the distal faces of coherent fiber-optic bundles by differential wet etching [109]. This is a simple method without the need of a clean room facility [110-112]. By pulling the optic fibers, the fiber diameter can be reduced from 3 ^m to less than 200 nm [113, 114]. This allows for the formation of high-density, small-sized nanotip arrays. The angle of the probe depends on the differential etching rates of the fiber and the surrounding cladding material. The master can be used to produce PVA sacrificial templates through a female PDMS daughter mold (resin curing), and then an array of well-defined nano-sized nozzles on a PLGA (solution casting) layer through a series of

FIGURE 3.12. A hot embossing machine system HEX 03 (Jenoptik Mikrotechnik GmbH, Germany); Spec: press force: <200 kN; temperature inside chamber: <500 °C; max substrate size: f 150 mm with vacuum and cooling system.

nano-imprinting steps. The size of the small end of the nozzle depends on the tip shape and the imprinting depth. The size of the larger end of the nozzle depends on the angle of the tip and the layer thickness. Replication accuracy requires complete wetting between the mold and the casting fluid (i.e. between optic fiber and PDMS resin, or cured PDMS and PVA/water solution, or dry PVA and PLGA solution).

PUMA Substrate mlf, Sutwwori t™s sivt Soaces SEM kmgt tflm/xintM substrate tviij sucm/cran inos a fid

Jrapnnlorfon EW3520HE Ccurfesy Bl/SH tVuppertai spaces. toipwtf«/ cm evgs20he Courtesy HUGH WUppurfni

PUMA Substrate mlf, Sutwwori t™s sivt Soaces SEM kmgt tflm/xintM substrate tviij sucm/cran inos a fid

Jrapnnlorfon EW3520HE Ccurfesy Bl/SH tVuppertai spaces. toipwtf«/ cm evgs20he Courtesy HUGH WUppurfni

FIGURE 3.13. An EVG520HE system (AV Groups, Austria); Spec: press force: <40 kN; temperature inside chamber: <550 °C; max substrate size: f 205 mm with vacuum and cooling system.

3.4.3. Injection Molding at the Nanoscale

Nanostructures can also be replicated on large surfaces using injection molding of polymeric materials. A successful example is the production of high density CDs with structures down to 25 nm. A Si wafer was v-notched by EBL and subsequent dry etching,

and used as an injection molding stamp with two different PC materials [115-117]. Nano-injection molding was also used to convert collagen assembles into a very distinct fibrillar structure with a characteristic 65 nm periodic cross-striation [118]. With a single master, more than 600 structures have been replicated, thus proving that the procedure is suitable for the mass production of nanostructures. Such structures can serve as simple calibration scales for SPMs (Scanning Probe Microscopes) to calibrate the piezo scan mechanism before measurement. For this application a single master grating structure has to be produced with a precise period, from which a reference structure can be replicated, taking into account the

FIGURE 3.15. Imrpio 100 machine (Molecular Imprints, Inc., USA).

known shrinkage. The results also show that using the CD injection molding process, mass fabrication of storage media with even higher density is possible [119].

3.4.4. Other Technologies

The above mentioned fabrication techniques are capable of producing 2D, in some cases 3D, polymer nanostructures with complicated patterns. Most of them, however, require expensive tools that are substantially different from the macroscale polymer processing equipment. Consequently, their implementation to large-scale industrial production remains unproven. Although nanoscale injection molding is an extension of its macro- and

FIGURE 3.16. Schematic of master making by AFM (nano-indentation and soft lithography).
FIGURE 3.17. SEM photos of a silicone rubber master with a 5 x 5 array of pyramidal shape probes made by a Hysitron nano-indenter with a 90° diamond tip.

microscale counterparts, and nanoimprinting is, in principle, very similar to micro-embossing and macro-stamping, the conventional injection molding and embossing/ stamping machines and molds cannot be used directly for producing nanoscale structures. There are, however, several well-known mass-producing polymer processing methods that can easily be modified to produce certain polymer nanostructures or products containing nanostructures. They are briefly introduced in the following sections.

3.4.4.1. Polymer Membranes with Nanopores Track etching of nanoporous polymer membranes is a well-established industrial process since the 1970s [120]. There are two methods of producing latent tracks in the polymer films to be etched into porous membranes-the irradiation with fragments from the nuclear reactors, or the use of ion beams from accelerators. Most membrane production is based on the latter method. Pore formation is achieved through chemical etching to remove the damaged polymer in the latent track. Several centers in the world manufacture track-etch membranes on a commercial scale [120]. Polycarbonate (PC) and poly(ethylene terephthalate) (PET) are the primary materials used. Polypropylene (PP), poly(vinyldiene fluoride) (PVDF), polyimides (PI) and several other high temperature polymers were also tried in the track-etching process. Difficulties in etching, high cost of the product and the existence of more competitive membranes limit their commercial uses. Pore diameter from 10 nm to tens of microns, pore density from 1 to 1010 cm-2, and pore shape from cylindrical, conical to cigar-like can be produced for PC and PET. Figure 3.19

Dissolve the sacrificial template in water to release ^^ the membrane

Small pore end

200nm

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