**Lithography-Free Nanostructure Fabrication Techniques Utilizing Thin-Film Edges**

Hideo Kaiju1,2, Kenji Kondo1 and Akira Ishibashi1 *1Research Institute for Electronic Science, Hokkaido University 2PRESTO, Japan Science and Technology Agency Japan* 

#### **1. Introduction**

568 Recent Advances in Nanofabrication Techniques and Applications

Park, J.U., Lee, H. L., Paik, U., Lu, L., & Rogers, J. A. (2008). *Nanoscale Patterns of* 

*Spectrometry*. Journal Micromechanics and Microengineering, 12, 682–687

Cloupeau, M. & Foch B. P., (1994). *Electrohydrodynamic Spraying Functioning Modes: A Critical* 

Hohman, M. M., Shin, M. Rutledge, G. & Brenner, M. P. (2001). *Electrospinning and electrically* 

Chen, C. H., Kelder, E. M., Van-Der-Put J.J.M. & Schoonman, J. (1996). *Morphology Control of* 

Stachewicz U., Yurteri C.U., Marijnissen J.C.M., & Dijksman, J.F., (2009). *Stability Regime of Pulse Frequency for Single Event Electrospraying*. Applied Physics Letters 95, 224105 Li, J.L., & Zhang, P., (2009). *Formation and Droplet Size of EHD Drippings Induced by* 

Kim, J. H., Oh, H. C., & Kim, S. S. (2008). *Electrohydrodynamic Drop-on-Demand Patterning in Pulsed Cone-Jet Mode at Various Frequencies*. Journal of Aerosol Science, 39, 819-825 Rahman, K., Khan, A., Nam, N. M., Choi, K. H. & Kim D. S. (2011). *Study of Drop-on-Demand* 

Kim D. S., Khan, A., Rahman,, K., Khan, S., Kim, H. C. & Choi, K. H. (2010). *Drop-on-Demand* 

Materials and Manufacturing Processes (doi: 10.1080/10426914.2011.551956) Lee J.S., Kim S.Y., Kim Y.J., Park J., Kim Y., Hawng. J., & Kim Y.J. (2008). *Design and* 

Muhammad N. M., Sundharam, S., Dang H. W., Lee, A., Ryud B. H. & Choi K. H. (2010). *CIS* 

Niklas C. Schirmer, N. C., Kullmann, C., Schmid, M. S., Burg, B. R., Schwamb, T., &

*Biosensing and Nanomaterials Assembly*. Nano Letters, 8 (12), 4210-4216 Griss, P., Melin, J., Sjodahl, J., Roeraade, J., Stemme, G., (2002). *Development of Micromachine* 

Poon, H. F. (2202). *Electrohydrodynamic Printing.* PhD thesis, Princeton University

*forced jets. I. Stability theory*. Physics of Fluid, 13 (8), 2201-2220

*Review*. Journal of Aerosol Science, 24 (6), 1021-1036

Journal of Materials Chemistry, 6, 765-771

Engineering and Manufacturing, 12 (4)

567

6386-0)

4705

Physics, 11 (1), S68-S75

*Oligonucleotides Formed by Electrohydrodynamic Jet Printing with Applications in* 

*Hollow Tips for Protein Analysis Based on Nanoelectrospray Ionization Mass* 

*Thin LiCoO2 Films Fabricated Using the Electrostatic Spray Deposition (ESD) technique*.

*Superimposing an Electric Pulse to Background Voltage*. Journal of Electrostatic, 67, 562-

*Printing Through Multi-step Pulse Voltage*. International Journal of Precision

*Direct Printing of Colloidal Copper Nanoparticles by Electrohydrodynamic Atomization*.

*Evaluation of a Silicon Based Multi-Nozzle for Addressable Jetting Using a Controlled Flow Rate in Electrohydrodynamic Jet Printing*. Applied Physics Letter, 93, 243114 Khan A., Rahman K., Hyun M.T., Kim D.S., & Choi K.H. (2011). *Multi-Nozzle* 

*Electrohydrodynamic Inkjet Printing of Silver Colloidal Solution for the Fabrication of Electrically Functional Microstructure*. Applied Physics A (doi:10.1007/s00339-011-

*Layer deposition through electrospray process for solar cell fabrication*. Current Applied

Poulikakos D. (2010). *On Ejecting Colloids Against Capillarity from Submicrometer Openings: On-Demand Dielectrophoretic Nanoprinting*. Advanced Materials, 22, 4701Fabricating nanoscale patterns with sub-10 nm feature size has been an important research target for potential applications in next-generation memories, microprocessors, logic circuits and other novel functional devices. Typically, according to the International Technology Roadmap for Semiconductor (ITRS) from 2009, an 8.9 nm node device is targeted for the year 2024. To achieve this milestone, liquid immersion lithography and extreme ultraviolet (EUV) lithography can be expected to be among the most commonly used techniques for the fabrication of nanopatterns. With liquid immersion lithography using a wavelength of 193 nm and a high numerical aperture (NA), it has been demonstrated that 32 nm features can be patterned (Finders et al., 2008; Sewell et al., 2009). EUV lithography using a short wavelength of 13.5 nm and 0.3-NA exposure tool has also enabled the printing of 22 nm half-pitch lines (Naulleau et al., 2009).

On the other hand, attractive patterning techniques, such as a superlattice nanowire pattern transfer (SNAP) method (Melosh et al., 2003; Green et al., 2007), a mold-to-mold cross imprint (MTMCI) process (Kwon et al., 2005) and a surface sol-gel process combined with photolithography (Fujikawa et al., 2006), are currently proposed and pursued actively. The SNAP method, which is based on translating thin film growth thickness control into planar wire arrays, has enabled the production of molecular memories consisting of 16 nm wide titanium/silicon nanowires. The MTMCI process using silicon nanowires formed by spacer lithography, in which nanoscale line features are defined by the residual part of a conformal film on the edges of a support structure with the linewidth controlled by the film thickness, has been used to produce a large array of 30 nm wide silicon nanopillars. With the surface sol-gel process combined with photolithography, where the linewidth is determined by the thickness of coating silica layer on the resist pattern, the size reduction and the large area of sub-20 nm silica walls have been achieved.

Recently, we have proposed a double nano-baumkuchen (DNB) structure, in which two thin slices of alternating metal/insulator nano-baumkuchen are attached so that the metal/insulator stripes cross each other, as part of a lithography-free nanostructure fabrication technology (Ishibashi, 2003 & 2004; Kaiju et al., 2008; Kondo et al., 2008). The schematic illustration of the fabrication procedure is shown in Fig. 1. First, the metal/insulator spiral heterostructure is fabricated using a vacuum evaporator including a film-rolled-up system. Then, two thin slices of the metal/insulator nano-baumkuchen

Lithography-Free Nanostructure Fabrication Techniques Utilizing Thin-Film Edges 571

Fig. 1. Fabrication procedure of DNB structures and a schematic illustration of QC devices.

The fabrication method of QC devices is shown in Fig. 2. First, Ni thin films were thermally evaporated on PEN organic substrates (2 mm width, 10 mm length and 25 m thickness) in a high vacuum chamber at a base pressure of ~10-8 torr. PEN organic substrates of TEONEX Q65 were supplied by Teijin DuPont Japan and cut down from 5 to 2 mm width using a slitter in a clean environment. A boron nitride crucible N-1, made by DENKA, and a tungsten filament, made by CRAFT, were used for the thermal evaporation of Ni thin films. A heat-block stainless plate with a hole was inserted between the Ni vapor source and the PEN substrate. The length of the crucible and the aperture size in the stainless plate were designed using a geometrical simulation to evaporate uniform Ni films in-plane to PEN substrates. The temperature near PEN substrates was less than 62 °C, which was lower than the glass transition temperature *T*g of 120 °C for PEN substrates. The pressure during the evaporation was 10-5 torr and the growth rate was 1.0-1.5 nm/min at an evaporation power of 280-350 W. Then, fabricated Ni/PEN films were sandwiched between two polymethyl methacrylate (PMMA) resins using epoxy. The volume of the PMMA resin was 6 × 3 × 3 mm3. The edge of PMMA/Ni/PEN/PMMA structure was polished by chemical mechanical polishing (CMP) methods using alumina (Al2O3) slurries with particle diameters of 0.1, 0.3 and 1.0 m. The polishing pressure was 6.5 psi and the platen rotation speed was 75 rpm. Finally, two sets of polished PMMA/Ni/PEN/PMMA structures were prepared and attached together with their edges crossed in a highly clean environment of ISO class minus 1. The attachment pressure was 0.54 MPa and no glue was used. This is a basic process in fabricating QC devices. When

**2. Fabrication and evaluation method of QC devices** 

**2.1 Fabrication method of QC devices** 

are cut out from the metal/insulator spiral heterostructure. Finally, the two thin slices are attached together face to face so that each stripe crosses in a highly clean environment (Ishibashi et al., 2005; Kaiju et al., 2005; Rahaman et al., 2008). Utilizing this DNB structure, we can expect to realize high density memory devices, the crossing point of which can be scaled down to ultimate feature sizes of a few nanometers thanks to their atomic-scale resolution of the film thickness determined by the rate of metal deposition, ranging from 0.01 to 1 nm/s. This DNB structure also gives a huge potential impact and importance of uniting bottom-up structures with top-down systems (Ishibashi, 2003). One element of the DNB structure is called a quantum cross (QC) device, which consists of two metal thin films (nanoribbons) having the edge-to-edge configuration as shown in Fig. 1 (Ishibashi, 2004; Kondo et al., 2006; Kaiju et al., 2008). In this QC device, the area of the crossed section is determined by the film thickness, in other words 1-20 nm thick films can produce 1×1-20×20 nm2 nanoscale junctions. Since the vacuum evaporation has good spatial resolution of one atomic layer thickness, the junction size of QC devices could ultimately be as small as a few ångströms square (10-2nm2). This method offers a way to overcome the feature size limit of conventional optical lithography. When molecularbased self-assembled monolayers (SAMs), such as rotaxanes (Chen et al., 2003), catenanes (Balzani et al., 2000) and pseudorotaxanes (Pease et al., 2001), are sandwiched between the two thin metal films, QC devices can serve as novel non-volatile memory devices and switching devices. Moreover, when magnetic materials, such as Fe, Co and Ni, are used for the two thin metal films, QC devices can work as nanoscale spin injectors and tunneling magnetoresistance (TMR) devices. Among these devices, the resistance of the electrodes (thin metal films) can be reduced down to ~k since the width of films can be easily controlled to the one as long as ~mm. This makes it possible to realize a highly sensitive detection for a junction resistance and to apply QC devices to high-frequency devices.

In this chapter, we present structural and electrical properties of Ni/polyethylene naphthalate (PEN) films used as electrodes in QC devices (Kaiju et al., 2009 & 2010) and current-voltage (*I-V*) characteristics for three types of QC devices. The three types of QC devices are as follows: (i) Ni/Ni QC devices (17 nm linewidth, 17×17 nm2 junction area), in which two Ni thin films are directly contacted with their edges crossed (Kondo et al., 2009; Kaiju et al., 2010), (ii) Ni/NiO/Ni QC devices (24 nm linewidth, 24×24 nm2 junction area), in which NiO thin insulators are sandwiched between two Ni thin-film edges (Kaiju et al., 2010) and (iii) Ni/poly-3-hexylthiophene (P3HT): 6, 6-phenyl C61 butyric acid methyl ester (PCBM)/Ni QC devices (16 nm linewidth, 16×16 nm2 junction area), in which P3HT:PCBM organic molecules are sandwiched between two Ni thin-film edges (Kaiju et al., 2010; Kondo et al., 2010). In our study, we have successfully fabricated various types of QC devices with nano-linewidth and nano-junctions, which have been obtained without the use of electron-beam or optical lithography. Our method will open up new opportunities for the creation of nanoscale patterns and can also be expected as novel technique beyond conventional lithography. Furthermore, we present the calculation results of the electronic transport in Ni/organic-molecule/Ni QC devices and discussed their possibility for switching devices. According to our calculation, a high switching ratio in excess of 100000:1 can be obtained under weak coupling condition. These results indicate that QC devices fabricated using thin-film edges can be expected to have potential application in next-generation switching devices with ultrahigh on-off ratios.

Fig. 1. Fabrication procedure of DNB structures and a schematic illustration of QC devices.
