Nanoimprint LithographyWei Wu, Hewlett-Packard Labs, Palo Alto, USA

Content1 Introduction 191

2 Mold 192

3 Resist 195

4 Press 196

5 Resolution Limit 197

6 3-D Nanoimprint 197

7 Applications 198

8 Summary – Challenges and Prospects 199

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9Nanoimprint Lithography

1 IntroductionNanoelectronics and other nanotechnologies require nanopatterning techniques as themost important process step in the fabrication sequence. A versatile nanopatterning tech-nique must (1) not be restricted by the diffraction limit of conventional optical photoli-thography, (2) be able to transfer arbitrarily designed pattern, (3) be applicable to allrelevant materials, and (4) be a low-cost high-throughput technique. The bottom-upapproach, such as self-assembly techniques (see Chapter 13), for example, easily fulfillrequirements (1) and (4). However, it can only generate patterns with low informationcontent with limited material systems. In order to realize sophisticated functionality,information has to be put into the nano-device or the nano-systems. That means most ofnano-devices and nano-systems cannot be built solely by self-assembly; top-downapproach (i.e. nano-lithography) has to be used to incorporate information into a system.Nanopatterning technologies, such as electron-beam direct write lithography (EBL) andX-ray lithography, fulfill requirements (1), (2) and (3), however, EBL lacks the through-put and X-ray lithography is prohibitively expensive. Therefore, high resolution highthroughput nanolithography is needed. Nanoimprint lithography (NIL) [1], [2] is one ofthe most promising nano-lithographies.

Similar to printing processes, NIL is based on mechanical contacts. While NIL wasinvented in 1995 [1], its predecessors can be traced over more than a thousand years. Theearliest dated printed book known, “Diamond Sutra”, was printed in China in 868. How-ever, it is suspected that book printing may have occurred long before this date. TheGutenberg press with movable letters was invented in 1440 and injection molding hasbeen used to make compact discs since 1970s.

NIL is a non-conventional high-resolution and high-throughput lithography. It isbased on the mechanical deformation of the resist rather than local chemical reaction byradiation, like other lithographies. The information content in the mask, or mold as con-ventionally called in NIL, is presented in the form of topography rather than light trans-mission function as in photolithography. During a NIL process, patterns in the mold areduplicated into the resist by pressing the mold onto the resist. There are mainly two typesof NIL, thermal NIL and UV NIL, depending on the resist used. Thermal NIL was theoriginal version invented in 1995. It uses a thermal plastic material as the resist. Afterspin-coating, the resist is heated 50–100 °C above its glass transition temperature (Tg) tomake it a viscous liquid. Then a mold is pressed into the resist. After the resist coolsbelow glass transition temperature and solidifies, the mold is removed. Finally, the resi-due layer of the resist in the compressed area is removed by anisotropic reactive ion etch-ing (RIE) and the whole process is finished. (Figure 1) The resist is ready to be used aseither a etching mask or lift-off material for the further processes.

UV NIL was invented in 1996 [3]. It uses UV curable liquid monomers, instead ofthermal plastic materials as the resist. The resist is already a low viscosity liquid andready to deform at room temperature. Therefore, no heating is needed. After imprintingand polymerization under UV exposure, the resist turns into solid. Then the mold isdetached and the patterns duplicated in the resist can be used for the following processes.While single-layer resist is commonly used in thermal NIL, both single-layer resist anddouble-layer resists are used in UV NIL. In the double-layer resist scheme, a transferlayer is spin-coated on the substrate before coating the nanoimprint resist layer (alsocalled imaging layer). There are mainly two ways to apply the imaging layer, dropping[4], [5], in which the resist is dispensed on top of the transfer layer as droplets, andspin-coating [6]–[9]. (Figure 2) While the double-layer resist scheme is only used insome special applications in thermal NIL [10], it is widely used in UV NIL partially dueto the difficulty of the removal of the cured resist layer by common solvent.

UV NIL offers several advantages at the cost of the constrain of UV-transparentmolds and more complex resist material systems. First, it is at room temperature, so ther-mal expansion issue is much easier to control. That makes UV NIL more suitable forhigh-precision multi-layer overlay. Second, UV resists normally have much lower vis-cosity (i.e. η0 ∼ 5–1000 mPa⋅s) [5], [11]–[16] than the thermal ones (i.e.η0 ∼ 103–107 Pa⋅s) [15]–[20], therefore, the imprint pressure is much less, and molds last

Figure 1: Schematic diagram of thermal nanoimprint lithography.

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IIlonger. Most UV NIL only needs a nanoimprint pressure less than 1 atm. [11], [21], [22],while most thermal NIL requires a pressure at least 100 psi. (about 7 atm.) [1], [23]–[25].

On the other hand, UV NIL is not the only way to solve the thermal expansion issueof thermal NIL. For example, if the mold uses the same material as the substrate, then themold and the substrate have no mismatch in thermal expansion. In another example, thethermal expansion issue was addressed by using excimer laser as the heating methods ofthermal NIL [26]. The heat cycle was only about 200 ns, and only the resist layer and thesurfaces in contact with the resist were heated [27]. The bulk of the whole systemremained at room temperature; therefore, thermal expansion had much less effect.

NIL is essentially a mechanical process, hence there is no diffraction limit, so NILhas high resolution; it is a parallel process, unlike electron-beam direct write (EBL),which is a serial process, so it has high throughput. For example, a full wafer nanoimprintcan be finished within a few seconds. NIL was first put onto the ITRS (InternationalTechnology Roadmap for Semiconductor) roadmap as an alternative lithography tool in2003, and it is listed as an alternative for 16 nm and below as of ITRS 2010 update. It ismainly used in research labs and universities, and getting used more and more in indus-trial applications, such as high density magnetic recording [24], [28], [29], and sub-wave-length optical elements [30]–[33].

In the following sections, we will first discuss the nanoimprint process itself, thatwill include the mold, the resist and the press. After that we will cover more advancedtopics: the resolution limit and 3-D nanoimprint. Then we will review some applicationexamples, and we will discuss the challenges and prospects in the end.

2 MoldThe topography on a mold carries the information to be transferred to the substratethrough the NIL process. The thermal NIL mold can be either transparent or not, on theother hand, the UV NIL mold has to be UV transparent. Various types of materials areused for nanoimprint molds, they can be categorized into hard and soft materials. Mosthigh resolution (i.e. smaller than 100 nm) NIL uses hard molds. In thermal NIL, Si basedmaterial mold are most common. For example, when NIL was first invited, The mold wasmade on a Si substrate with a layer of thermally grown SiO2 [1], [2]. Patterns were etchedinto the SiO2 layer using RIE with the Si as etching stop. The Si based molds, includingother variations, such as Silicon Nitride on Silicon [34] or Si only [35], are all widelyused. Other types of hard materials, for example, Quartz or metals based hard materials,such as Nickel [36]–[39] and metallic glass, are used in thermal NIL too. Generally, amold based on the same material as the substrate to be patterned is preferred in order tomatch the thermal expansions of the mold and the substrate. In UV NIL, hard molds aremainly made of Quartz or glass. In some setups a layer of ITO (Indium Tin Oxide) isdeposited on top of the quartz substrate to help dispense charges during the EBL and RIEetching steps of the mold making [40]. In another setup, the patterns are etched in a layerof a Silicon Nitride deposited on top of the Quartz substrate to create color contrast foreasier overlay alignment [41].

Soft materials, mainly polymer meterials, such as ethylene-tetrafluoroethylene(ETFE) [42] and Polydimethylsiloxane (PDMS) [43], are used as NIL mold as well.Compared with hard molds, soft molds are much more flexible, so that they form confor-mal contact with the substrate more easily at very low or without external pressure evenon non-flat surface during the nanoimprint process. Moreover, they normally cost less,and are more tolerable to dust trapped between the mold and the substrate. However, dueto the elasticity of the soft material, soft molds are mainly used in UV NIL, whichrequires less pressure, and were not commonly used in high resolution patterning (i.e.sub-100 nm). However, high resolution pattering of nanostructures on both flat surfaceand highly curved surface were achieved using a hybrid mold with a flexible PDMS sup-port and a thin hard crust layer [44]. Both soft molds and metal molds are used inroll-to-roll nanoimprint, because they are more flexible and can be mounted on a rollereasily [42], [45], [46].

During the mold and substrate separation (demolding), the resist patterns will be bro-ken or peeled-off from the substrate, if the sticking force between the mold and the resistis larger than the resist strength or the adhesion force between the resist and the substrate(Figure 3). This issue gets more serious when imprinted patterns get smaller, since theresist patterns are weaker. To reduce the separation force, a self-assemble monolayer, so

Figure 3: The nanoimprinted resist breaks if the sticking force between the mold and the resist is bigger than the resist strength.

Figure 2: Schematic diagram of UV nanoimprint lithography. The nanoimprint resist can be applied either by dropping (a) or by spin-coating (b).

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called mold releasing layer, is coated on the mold before the first imprint. The most com-monly used materials are Trichlorosilanes with fluorocarbon chains, such as Trideca-fluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (CF3-(CF2)5(CH2)2SiCl3. Best moldreleasing layers are achieved using a vapor-phase treatment, during which amono-molecular layer of the releasing agent is deposited on the surface of the mold bychemisorptions and, in a subsequent step, water vapor is used to trigger a polymerizationreaction between the chemisorbed molecules (Figure 4). As a result, a monolayer ofTEFLON®-like fluorocarbon chains is coated on the mold surface [47]. This layer signif-icantly reduces the sticking and friction of the surfaces.

The releasing layer loses a small amount of F atoms during each imprint and thatcauses wearing of the releasing layer [48], [49]. Therefore, either a re-coat of the releas-ing layer after a certain number of imprints (i.e. every a few hundred times) or a re-supplyof the mold releasing molecules during each imprint by adding a small amount of thereleasing molecules into the imprint resist are required [11]. Alternatively, diamond-likecarbon (DLC) was used as mold material and it showed good releasing properties evenwithout mold releasing layer [50]. Therefore, DLC, especially F doped DLC may have agreat potential as the mold material for mass production.

The state-of-art photolithography uses 4× masks. That means the patterns on themask are four times as large as the patterns to be exposed on the substrate. Shrunkenimages of the mask are projected onto the substrate through the optical system during theexposure. That relaxes the requirements on mask. However, nanoimprint mold is a 1×mask instead of 4× mask, hence lithography with the same or better resolution is neededin mold fabrication ideally. Normally the NIL is made by direct write electron beamlithography (EBL) and RIE etching. That raises two issues, first the resolution of NIL iseffectively limited by the resolution of EBL; second, EBL is a serial process so that themold itself can be both extremely expensive and time consuming to fabricate.

In order to go beyond the resolution limit of EBL, at least for specific pattern suchas parallel lines, several very smart mold fabrication processes were invented. In the firstexample, GaAs/AlxGa1−xAs periodic multi-layers were first deposited on a GaAs sub-strate using molecular beam epitaxy (MBE). Then the sample is cleaved and the cleavedfacet is etched by acid. A dense line array is created on the etched facet, becauseAlxGa1−xAs etches faster than GaAs. The etched facet can be used as a NIL mold withdense line patterns beyond EBL resolution. (Figure 5) In this way, nanoimprint of denselines with 7 nm half-pitch were achieved [51]. The second example used a Super-latticeNanowire Pattern Transfer (SNAP) process to fabricate NIL molds with dense lines(Figure 6). In this process, it also relies on MBE grown super-lattice multilayer to gener-ate dense lines. First, dense lines are created on a differentially etched facet of a cleavedsubstrate with MBE grown GaAs/AlxGa1−xAs periodic multi-layers the same way as inthe first example. Then Pt is deposited on the top of the lines by angled e-beam evapora-tion. After that, the facet is pressed onto an SOI wafer with epoxy coated on the top sur-face. By etching away the super-lattice sample, Pt dens lines are transferred onto the topof the SOI substrate. After transferring the patterns into the Si layer by RIE etching usingthe Pt lines as etching mask, and Pt removal, the mold is fabricated. Cross-bar circuitswith 17 nm half-pitch were fabricated using a mold fabricated by SNAP process [52]. Inthe third example, molds with smaller feature size and higher density from an EBL-fab-ricated mold through a spatial frequency doubling process (also called as spacer lithog-raphy) [53], [54] (Figure 7). During the process, each unit cell was transformed into twounit cell through a serial of etching and deposition processes. Molds of nanowire arrayswith 15 nm half-pitch were demonstrated (Figure 8) [54]. Molds made in all those exam-ples can be used in both thermal NIL and UV NIL.

Figure 4: Schematic diagram of mold releasing layer treatment. [45]

Figure 5: (a) Schematic diagram of a super-lattice mold

fabricated by differentially etching facet of a cleaved substrate with MBE grown GaAs/AlxGa1−xAs periodic multi-layers.

(b) SEM image of a gratings with 7 nm half-pitch imprinted on nanoimprint resist using the super-lattice mold.

[49] (Courtesy: S. Chou et. al. and Applied Physics Letters)

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With the progress of EBL, the resolution of EBL are catching up with those molds[55]. Therefore, complex patterns were successfully imprinted using molds defined byEBL [22]. For example, Figure 9 [22] shows nested Ls patterns on nanoimprint resistusing a UV NIL process. It shows isolated lines with line width less than 10 nm, denselines at 12 nm half-pitch and sharp corners.

To preserve the expensive master mold, ways to duplicate and preserve the mastermold in an economic and high fidelity fashion were developed. One example is aphoto-curable polymer process for replicating the master mold (Figure 10) [56]. The

Figure 6: (a) “SNAP” process to fabricate

nanoimprint mold. (b) Crossbar circuit at 17 nm half-pitch

fabricated using a mold by “SNAP” process.

[50] (Courtesy: Hewlett-Packard Co. and Nano Letters)

Figure 7: Process flow of mold fabrication using “spatial frequency doubling”. (a) Original nanowires were patterned on

Si-on-insulator (SOI) substrate using NIL and RIE etching.

(b) Ni is deposited on top of the nanowires by angled electron-beam evaporation.

(c) Cr deposition using electron-beam evaporation. (d) Etching Ni using acid and lifting-off Cr on top. (e) Transferring the patterns into buried oxide layer

by RIE using Cr and Si as etching mask. (f) Si and Cr removal. [52] (Courtesy: Hewlett-Packard Co. and Nanotechnology)

Figure 8: SEM images of the original nanoimprint mold (top) and the mold fabricated using “spatial frequency doubling” (bottom). [52] (Courtesy: Hewlett-Packard Co. and Nanotechnology)

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9cross-linked polymer mold was fabricated directly with high fidelity from a master byimprinting and photo-curing a low viscosity liquid pre-polymer film spun onto a sub-strate. The surface of the cross-linked polymer mold can be treated using an O2 plasma,and then vapor primed with a low surface energy mold release layer for repeatable nano-imprinting. Molds duplicated in this way are capable of both thermal NIL and UV NIL.

3 ResistThermal NIL and UV NIL use different resists. Thermal NIL uses thermal plastic mate-rials and UV NIL uses UV-curable materials. Figure 11 [15], [57] shows the dependenceof the viscosity on temperature of a typical thermal NIL resist. At room temperature, ther-mal NIL resist is in solid (glassy, brittle) state. The resist undergoes a transition from asolid to a rubbery state and its viscosity drops by several orders of magnitude around theglass transition temperature Tg. Further temperature increases make the resist into termi-nal flow region and drop the viscosity further. That is the temperature range resists areimprinted. Generally, lower viscosity is preferred, because it is faster for the resist to flowto fill the patterns under the press. Higher temperature means lower viscosity; however,the resist shows instable behaviors, if the temperature is too high [58]. Therefore, mostNIL is carried out at a temperature 50–100 °C above its Tg.

One advantage of thermal NIL is that there are a wide variety of thermal plasticmaterials available. Some thermal plastic materials, such as Poly(methyl methacrylate)(PMMA) and Polystyrene (PS), can be used as thermal NIL resist without modifications.Moreover, it is relatively easy to engineer resists to meet the specific needs, such as etch-ing properties, of each application. As a comparison, UV NIL resists are more complexmaterial systems. There is no monomer that can be used as UV resist alone, and all UVresist requires formulation by mixing various components together, sometimes synthesiz-ing of new molecules.

While thermal NIL resists are applied by spin-coating, the same way as photolithog-raphy, UV-curable resists are applied either by dropping or spin-coating. Applying resistby dropping methods enables resist dispensing based on the pattern filling ratio, henceuniform residue layer distribution even the pattern filling ration varies dramaticallyacross the whole die. On the other hand, this method requires very low-viscosity resist,which needs to flow globally (i.e. over a distance in the order of millimeters), andultra-flat mold and double polished wafer to form a uniform film. Applying resist by spincoating, in both thermal and UV NIL, creates a uniform thin film with better than 1 nmuniformity before the press [8]. It results in uniform residue layer cross the wholewafer/die, as long as the filling ratio is evenly distributed. Moreover, it has less stringentrequirement on viscosity so that there is more freedom in the resist formulation. As a con-sequence, UV resist designed for spin-coating tends to have better etching properties,which make the consequent pattern transfer easier. Both of the techniques are widelyused.

UV-curable nanoimprint resists are normally composed of photo curable monomers,cross linkers, photo-initiator and some additives. The resist is polymerized under UVexposure. Radical polymerization reaction (Figure 12) is most commonly used. First, thephoto-initiator generates radicals under UV exposure. Radicals open the double bonds inmonomers, and then the opened bond from each monomer finds other monomers andlinks to them. Therefore, those monomers link together into a polymer chain. With crosslinkers, which have multiple functional groups, branches are formed in the polymer chainhence polymer networks instead of simple chains are created. The network structureincreases the mechanical strength and chemical stability. Moreover, by adjusting thecomposition, the right network density can be tailored, so that the volume of the resistkeeps constant during the curing process. Those are critical for high-resolution pattern-ing.

There are more exotic materials used as NIL resist. For example thermal curablesol-gels were used as NIL resist to create silica-like glass nanostructure with a single ther-mal NIL step [34], [59]. It is effectively a thermal curable NIL. Nano-patterns were alsocreated by directly imprinting on metallic glass film or substrate [60]–[63]. In a sense,the metallic glass materials were used as thermal nanoimprint resist. In some extremecases, even Aluminum sheets were used as NIL resist. SiC and diamond molds were usedto imprint nanostructures directly onto Al sheets with very high imprint pressure [64].Various functional materials were used as nanoimprint resist to create functional struc-tures in a single nanoimprint step [65]–[67].

Figure 9: Nested “Ls” patterns on nanoimprint resist using a UV NIL process. It shows isolated lines with line width less than 10 nm, dense lines at 12 nm half-pitch and sharp corners. [22](Courtesy: Hewlett-Packard Co. and Nano Letters)

Figure 10: Mold duplication using a photo-curable polymerization process. [54] (Courtesy: Hewlett-Packard Co. and Nano Letters)

Figure 11: Typical dependence of the viscosity of a thermal NIL resist on temperature.

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4 PressNanoimprint process can be either single-field, step-and-repeat or roll-to-roll. In sin-gle-field NIL, the whole sample was patterned with one press. The sample area is as largeas the wafers used in the process, for example production size wafers (8 or 12 inch).Step-and-repeat NIL patterns the resist in a step-and-repeat fashion. (Figure 13) Eachimprint field size varies from 1 cm × 1 cm to 1 inch × 1 inch. Reducing the imprint fieldwill result in, first, better pressure and temperature uniformity, second, shorter path fortrapped air to escape, third, less differential thermal expansion between mold and sub-strate. Therefore, step-and-repeat NIL has better imprint quality, especially higher over-lay precision and lower defects density, at the cost of lower throughput than single-fieldnanoimprint. Step-and-repeat machines are also more complex than single-field machine.Therefore, step-and-repeat NIL is mainly used for applications that have stringentrequirements on quality and overlay; single-field NIL is mainly used for research andapplications in which high throughput is more critical than overlay precision such asbit-patterned magnetic media [68].

In both single-field and step-and-repeat setup, the nanoimprint pressure is appliedeither using solid parallel plate (Figure 14a) or air cushion (Figure 14b). Obviously, anair cushion press has more uniform pressure distribution (Figure 14c), is more immuneto topography variations on the back side of the substrate or mold and more tolerable totrapped dust particles [69]. However, solid plate press is more compatible with currentstep-and-repeat platform, so it dominates the step-and-repeat nanoimprint machines.

Roller nanoimprint [70], or roll-to-roll nanoimprint is based on the principle of largevolume paper printing machines. It is mainly used for high volume, large area patterningon both rigid [70] and flexible substrates [42], [45]. In the most common setup, the moldis mounted on a roller and patterns are imprinted onto the resist while the sample goesthrough the roller. (Figure 15) Both thermal [42], [70], [72] and UV [45] were imple-mented on roller nanoimprint.

Figure 12: Schematic of the UV-curing processes of the nanoimprint resist during UV nanoimprint.

Figure 13: Schematic of a step-and-repeat nanoimprint lithography.

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5 Resolution LimitIn a photolithography system, light is the information carrier and the optical systemworks as a spatial frequency low-pass filter (i.e. diffraction limit), and that limits the res-olution of photolithography. In NIL, the information on the mask is transferred to theresist through a mechanical press instead of an optical exposure system. Therefore NILhas much better resolution than photolithography, because there is no diffraction limit.For example, features with size down to 5 nm and dense lines with half-pitch down to6 nm have been successfully imprinted on resist [6]. So far the resolution demonstratedby NIL is still limited by the resolution of nanoimprint mold.

The ultimate resolution of NIL will be determined by the mechanical properties ofthe resist eventually, assuming there is enough resolution on the mold. For instance, whenthe feature size is too small, the mechanical strength of resist will be weaker than the sep-aration/friction force. Therefore, the patterns will be broken during the mold and sub-strate separation. (Figure 3) Normally the UV-cured NIL resist is much stronger than athermal NIL resist, because the UV cured resist forms heavily cross-linked polymer net-works. Therefore, most of the experimental results of the highest NIL resolution are alldemonstrated by UV NIL. On the other hand, the patterns in the resist have to be trans-ferred to fabricate devices. The critical dimension (CD) loss due to RIE pattern transferwill be the bottleneck before reaching the ultimate NIL resolution, unless RIE improvessignificantly.

6 3-D NanoimprintDuring a NIL process, the 3-D topography of the mold is duplicated onto the resist. Inother words, the 3-D information is incorporated into the resist, instead of only 2-D infor-mation as in other lithographies. That is a unique advantage of NIL. A lot of fabricationprocesses can be improved or simplified by taking advantage of the 3-D nature. Forexample, micron-sized dot-like arrowhead structures [72] and surface enhanced Ramanspectroscopy (SERS) sensors based on high aspect ratio nano-cones, which have heightsup to 2 µm and tip radii less than 10 nm, were made by NIL (Figure 16) [73]. Broad bandanti-reflective structures, that consisted of triangle cross-section pillar array (Figure 17),were fabricated by single nanoimprint and etching [32], [74]. Metal T-gates (Figure 18)were fabricated by single nanoimprint and lift-off [75].

Figure 15: Schematic of an exemplary roller nanoimprint setup.

Figure 16: SEM image of a SERS sensor. It consists of nanoimprinted polymer nano-cones coated with gold nanoparticles.

Figure 17: SEM image of a broad band anti-reflective structure made by NIL. The structure consists of needle shaped pillar array. [29] (Courtesy: S. Chou et. al. and JVST B)

Figure 14: Two ways to apply nanoimprint force: (a) using parallel plate press, (b) using air pressure. (c) The force distributions over 4-inch area

recorded by pressure paper. Colors on the paper

correlate to the pressure at each point. Air cushion press (right) has more uniform pressure distribution than parallel plate press (right).

[64] (Courtesy: S. Chou et. al. and Nano Letters)

Figure 18: An SEM image of a Metal T-gate fabricated by single nanoimprint and lift-off processs. [68] (Courtesy: S. Chou et. al. and Applied Physics Letters)

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One of the applications with the most potential commercial impact is the duel dam-ascene process based on NIL [76], [77]. A damascene process is used to pattern the Cuvias and wires in state of art ICs, because there are no effective plasma etch processes forCu. In order to pattern a Cu layer and the via layer, which connects to the underneathlayer, a serial of process steps are needed. Two lithography steps and two RIE etchingsteps are needed to define the via and the Cu wire trench patterns in the interlayer dielec-trics (ILD) of each layer respectively. After that, the patterns in ILD will be filled withCu by plating. By using a NIL mold with two different heights, where the higher patternsare used to define the vias and the lower patterns are used to define the trenches, both thevia and metal layers are patterned with single NIL and RIE etching (Figure 19) [78].Therefore, the processes are simplified greatly.

7 ApplicationsNIL has been used for many applications. Actually, it can be used for any applications thatrequire nano-patterning. In electronics, NIL has been used to fabricate both conventionalcircuits with smaller scale and devices based on new physics mechanisms, such as sin-gle-electron devices [79], [80], spintronics and memristor [81], [82]. In photonics, bothphotonic crystal based devices and optical meta-materials were fabricated using NIL. NILhas also been used to make patterned magnetic media, chemical sensors [73], lab-on-chip,organic opto- and electronics, and nano electro-mechanical systems (NEMS).

In this section, several examples of NIL applications are described in detail in orderto give a general impression.

The first example is in nanoelectronics. Nano cross bar memory circuits were fabri-cated by NIL. Figure 20 shows a cross bar memory circuit with density of 100 Gbits/cm2

(17 nm half-pitch). The three metal layers, bottom electrode, top electrode and addresslines, are all fabricated using NIL and metal lift-off processes. HP labs also demonstratedthe integration of nanoimprinted nano-crossbars on top of Si CMOS circuits fabricated ina real ASIC foundry. Those nanowires served at data routing network for the CMOS gates

Figure 19: The duel damascene process based on NIL. (a) The schematic of conventional

dual-damascene process.(b) The NIL based process. The process is

greatly simplified. (c) An SEM image of via and metal layer

patterns patterned by single NIL and RIE etching.

[71] (Courtesy: C. G. Wilson, Molecular Imprint Inc. and SPIE)

Figure 20: SEM images of crossbar memory circuits. The center square region (within red square) is the memory array at 17 nm half-pitch. The four areas within green squares are MUX/ DEMUX areas. The four groups of the wires connected to the MUX/DEMUX areas are address lines. The whole circuit was fabricated by NIL only. (Courtesy: Hewlett-Packard)

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9underneath to form a field programmable gate array (FPGA) type of circuits. Othernano-electronic devices, such as single electron transistor [83] and single electron mem-ory [79] were demonstrated by NIL too.

One example of nano-photonics application is optical negative index meta-materials(NIMs). NIMs are artificial materials with nanostructure patterns, which have both neg-ative permittivity and permeability in the same frequency range due to resonant proper-ties of the nanostructure, and hence exhibit a negative refractive index in this frequency.They were originally demonstrated in microwave range [84], and to make NIMs at opti-cal wavelength feature sizes must shrink accordingly. By using NIL, several types of neg-ative index meta-materials (NIMs) working at near-IR range (i.e. 1.55 µm wavelength)were fabricated [85], [86]. Figure 21 shows one example of NIMs. It consists of two Aglayers separated by a dielectric layer, and all three layers have a fishnet pattern. The min-imum “n” located at 1560 nm wavelength, which is only 10 nm off the design. Thatrequires about 1.6 nm fabrication repeatability [87], and it is still very challenging forother nano-lithographies, such as EBL.

A possible mass-production application of NIL is bit-patterned magnetic media(BPM). Conventional hard disks use a continuous thin film as a recording medium, andeach bit on the disk consists of multi-grains [88]. The ultimate recording density of suchmedia is limited by two aspects, firstly each grain has to be large enough to be thermallystable (superparamagnetism), secondly each bit has to have enough numbers (i.e. 100) ofgrains to minimize transition noise below a reasonable level [88], [89]. To go beyond thedensity limit of continuous thin film magnetic media (i.e. 1 Tbits/Inch2) patterned mag-netic media [68], [90] are needed. BPM consists of a discretely patterned single domainmagnetic island for each bit [91]–[93]. Each bit is effectively a small magnet. The prac-tical recording density of BPM is limited by the lithography resolution. Most importantlythe fabrication approach has to be cost-effective for mass production. Therefore, NIL issuitable for BPM manufacture.

Figure 22a shows the first large area BPM which was fabricated by NIL [24]. It hasNi pillars embedded in SiO2 layer. As shown in the magnetic force microscopy (MFM)image (Figure 22b) [24], either a magnetic pole point up, a bright spot, or point down, adark spot, at each Ni pillar location. That shows each Ni pillar is a single magneticdomain, and the two magnetic pole directions represent “1” or “0”.

NIL is also used in other applications, such as patterning the “fin” layer of flashmemory [94] and patterning quasi-photonic crystals to improve the light extraction effi-ciency in LED [95].

8 Summary – Challenges and ProspectsNIL demonstrated superb advantages on resolution, throughput and cost. Moreover, ithas been used in a wide range of applications. Great progresses have been achieved onissues such as overlay alignment [94], [96] and mold fabrication [94], [97], [98]. Variousflavors of NIL are developed to satisfy specific requirements of certain applications.However, NIL is based on mechanical deformation of the resist. It does not only givethose advantages to NIL, but also provides a set of new challenges. The same as contactphotolithography, the yield of NIL suffers from the direct mechanical contact of the moldto the resist. For example, the defect density is still about 30 times higher than IC industryrequirement [99]. Even so, NIL has been adopted as a fabrication tool for research anddevelopment purposes. Moreover, it is most likely that NIL will find its foot stands inhard drive, display and LED applications, where the requirement of defect density is lessstringent and cost-effective high resolution nano-patterning triumphs.

AcknowledgmentsThe editor is grateful for a critical review and symbol check by Matthias Meier andFlorian Lentz (Forschungszentrum Jülich).

Figure 21: (a) Schematic of a negative index

meta-material (NIM) structure for near-IR wavelengths. [76]

(b) An SEM image of such a NIM made by NIL. [74]

Figure 22: (a) An SEM image of a patterned magnetic

medium (b) A MFM image of the patterned magnetic

medium. It shows that each Ni pillar was a single magnetic domain.

[20] (Courtesy: JVST B)

Technology and Analysis

200

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