**5. Soft UV NIL for plasmonic biosensing: a real case application**

Surface plasmon resonance (SPR) based sensors are a well established technology utilized for label-free bio-chemical sensing in different applications, from immunoassay and medical diagnostics, to environmental monitoring and food safety. Localized Surface Plasmon Resonance (LSPR) based sensors have different advantages and constitute a promising alternative to the SPR sensor (Stuart et al. (2005)). Because LSPR based biosensors are by design very sensitive to changes in the characteristics of nanoparticles (uniformity in nanoparticle size, shape and composition) the standard approach for the fabrication of LSPR based sensor is Electron Beam Lithography (EBL) which provide an extreme control over nanoscale size and shape of the nanoparticles thus improving sensitivity and reliability of the sensor. On the other side this is an expensive and time-consuming technique, consequently not suitable for mass production.

We have successfully realized a LSPR biosensor based on *λ*3/1000 plasmonic nanocavities fabricated by Soft UV Nanoimprint Lithography on large surfaces (0,5 - 1 cm2). These structures present nearly perfect omnidirectional absorption in the infra-red regime independently of the incident angle and light polarization and outstanding biochemical sensing performances with high refractive index sensitivity and figure of merit 10 times higher than conventional LSPR based biosensor (Cattoni et al. (2011)).

14 Will-be-set-by-IN-TECH

Fig. 9. SEM images of: (a) imprinted structures into AMONIL resist, the structure depth is 170 nm and the residual layer is 36 nm. Reprinted with permission from (Ji et al. (2010)). Copyright 2007 Elsevier. (b) imprinted structures into AMONIL resist, the structure depth is 160 nm and the residual layer is 25 nm. (c-d) High resolution replication at the 50 nm and at

Surface plasmon resonance (SPR) based sensors are a well established technology utilized for label-free bio-chemical sensing in different applications, from immunoassay and medical diagnostics, to environmental monitoring and food safety. Localized Surface Plasmon Resonance (LSPR) based sensors have different advantages and constitute a promising alternative to the SPR sensor (Stuart et al. (2005)). Because LSPR based biosensors are by design very sensitive to changes in the characteristics of nanoparticles (uniformity in nanoparticle size, shape and composition) the standard approach for the fabrication of LSPR based sensor is Electron Beam Lithography (EBL) which provide an extreme control over nanoscale size and shape of the nanoparticles thus improving sensitivity and reliability of the sensor. On the other side this is an expensive and time-consuming technique, consequently

We have successfully realized a LSPR biosensor based on *λ*3/1000 plasmonic nanocavities fabricated by Soft UV Nanoimprint Lithography on large surfaces (0,5 - 1 cm2). These structures present nearly perfect omnidirectional absorption in the infra-red regime independently of the incident angle and light polarization and outstanding biochemical sensing performances with high refractive index sensitivity and figure of merit 10 times higher

**5. Soft UV NIL for plasmonic biosensing: a real case application**

than conventional LSPR based biosensor (Cattoni et al. (2011)).

25 nm

160 nm

Amonil S4

(c) (d)

(a) (b)

b)

the 20 nm range in AMONIL resist.

not suitable for mass production.

Fig. 10. (a) Schematic of the tri-layer Soft UV NIL process, (b) SEM image of square gold nanoparticle (size = 200 nm, pitch 400 nm) realized with this method and lift-off, (c) the LSPR biosensor fabricated by Soft UV NIL, integrated in a microfluidic channel with its original Silicon master mold fabricated by EBL.

The basic element of the nanocavities array (not shown) is composed by lower thick gold film acting on a glass substrate, a thin dielectric layer forming the gap of the optical antenna and an upper gold nanoparticle realized by "tri-layer" Soft UV imprint lithography and standard lift-off. In figure (a) is shown the concept of the tri-layer system on a generic substrate: typical UV NIL resists (like Amonil) are not soluble in solvents and a lift-off process is possible by using a PMMA layer under the UV NIL resist. In our process a further 10 nm Ge layer is insert between the thick PMMA layer and the thin UV NIL resist (Amonil) to improve the selectivity of the former one over the PMMA layer. After imprinting with a UV light and separation, the top layer structure is transferred into the bottom layer by a sequential reactive ion etching. The high aspect ratio tri-layer so obtained can be used directly as etching mask or for the lift-off of metals. In Figure 10 (b) is shown a SEM image of square gold nanoparticle (size = 200 nm, pitch 400 nm) realized by "tri-layer" Soft UV imprint lithography. Figure 10 (c) shows the LSPR biosensor based on *λ*3/1000 plasmonic nanocavities fabricated by Soft UV Nanoimprint Lithography and integrated in a glass/PDMS/glass fluidic chamber for the

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optical index sensing experiments together with its original Silicon master mold fabricated by EBL.

#### **6. Conclusion**

As we have shown, the replication process of nanostructures by Soft UV NIL it is composed by three separate processes: the fabrication of the master mold, the replication of the soft polymeric stamp from this template, and the imprinting process by using this stamp. All these steps contribute equally at the quality of the final result, in terms of resolution and line edge roughness of the nanostructures. In this manuscript we have presented a detailed master mold fabrication process based on EBL. The performances of two EBL resists (PMMA and HSQ) for the replication of high density patterns (pitch=30 nm) have been discussed for ultra high resolution at the 15 nm scale. In addition a process combining T-NIL and etching has also been proposed for the cheap and fast replication of master molds. We have then detailed the replication of the polymeric stamp, based on a composite hard-PDMS/PDMS bilayer. Finally for the last third critical imprinting step, we have demonstrated the ability of Soft UV NIL to replicate nanostructures at the 20 nm scale with high uniformity at the whole pattern area. We do believe that Soft UV NIL will have in the near future an important role as a powerful and versatile tool for nanofabrication. To validate this statement, we conclude by presenting an example of a real case application, i.e. the fabrication by Soft UV NIL on large area (1 cm2) of gold plasmonic nanostructures with outstanding optical and biosensing performances.

#### **7. References**


16 Will-be-set-by-IN-TECH

optical index sensing experiments together with its original Silicon master mold fabricated by

As we have shown, the replication process of nanostructures by Soft UV NIL it is composed by three separate processes: the fabrication of the master mold, the replication of the soft polymeric stamp from this template, and the imprinting process by using this stamp. All these steps contribute equally at the quality of the final result, in terms of resolution and line edge roughness of the nanostructures. In this manuscript we have presented a detailed master mold fabrication process based on EBL. The performances of two EBL resists (PMMA and HSQ) for the replication of high density patterns (pitch=30 nm) have been discussed for ultra high resolution at the 15 nm scale. In addition a process combining T-NIL and etching has also been proposed for the cheap and fast replication of master molds. We have then detailed the replication of the polymeric stamp, based on a composite hard-PDMS/PDMS bilayer. Finally for the last third critical imprinting step, we have demonstrated the ability of Soft UV NIL to replicate nanostructures at the 20 nm scale with high uniformity at the whole pattern area. We do believe that Soft UV NIL will have in the near future an important role as a powerful and versatile tool for nanofabrication. To validate this statement, we conclude by presenting an example of a real case application, i.e. the fabrication by Soft UV NIL on large area (1 cm2) of gold plasmonic nanostructures with outstanding optical and biosensing performances.

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EBL.

**6. Conclusion**

**7. References**


**8** 

*Japan* 

**Repairing Nanoimprint Mold Defects by** 

Makoto Okada and Shinji Matsui

*University of Hyogo* 

**Focused-Ion-Beam Etching and Deposition** 

Nanoimprint lithography (NIL) has been attracting attention from many industries because of its potential use in producing various nanostructure applications through a simple, lowcost, and high-throughput process. There are three primary types of NIL: thermal (T-NIL), UV-NIL, and room-temperature (RT-NIL). T-NIL has a heating and cooling process because thermoset or thermoplastic resins are usually used as T-NIL resins. When a thermoset resin is used, the mold is pressed on a substrate coated with the resin at room-temperature. During pressing, the mold and substrate are heated to harden the thermoset resin, and after cooling, the mold is separated from the substrate. It is slightly different with thermoplastic resin: in this case, the mold is pressed on a substrate coated with the resin at the resin's glass-transition temperature (Tg). The mold and substrate temperatures are then decreased and the mold is removed from the substrate. Si, SiO2/Si, and Ni molds are usually used as T-NIL molds. UV-NIL is a room-temperature process because UV-curable resins are used as UV-NIL resins. The UV-NIL mold is pressed on the substrate coated with UV-curable resin and then the substrate is irradiated with 365-nm UV through the mold. After this irradiation, the mold is separated from the substrate. This means that UV transmissive material must be used as UV-NIL mold material. Generally, a quartz mold is used as a hard mold, and a polydimethylsiloxane (PDMS) mold is used as a soft mold. RT-NIL can be performed without heating, cooling, or UV irradiation. In this process, the sol-gel materials, such as hydrogen silsesquioxane (HSQ), spin-on-glass (SOG), and sol-gel indium tin oxide (ITO), are used as RT-NIL resins. This process requires high pressure, so Si or SiO2/Si molds

What type of mold to use is one of the most important factors in nanoimprint lithography because the mold must come into direct contact with the replication material and the imprinted pattern resolution depends on the mold pattern resolution. The pattern is therefore typically fabricated by electron beam (EB) lithography to obtain a high resolution pattern, thus necessitating a mold repair process with high resolution. In photolithography mask repair, focused ion beam (FIB) etching is used to remove Cr opaque defects, and FIB chemical vapour deposition (CVD), using hydrocarbon precursor gas, is used to repair clear defects. Two types of defect occur in NIL molds protrusion and hollow defects which correspond to the opaque and clear defects in photomasks. However, unlike photomask patterns, the NIL relief-structure patterns are formed on a substrate surface. Therefore, we

**1. Introduction** 

are usually used.

