Large area sub-100 nm direct nanoimprinting of palladium nanostructures

Abstract

Direct imprinting of metals is predominantly achieved by using polydimethylsiloxane (PDMS) molds to pattern metal nanoparticles and subsequently melting them to form continuous structures. Although such a combination can successfully imprint metals, the yield and reproducibility are usually low when sub-100 nm features over large areas are desired. In this work, we demonstrate a simple method involving the addition of a cross-linker ethylene glycol dimethacrylate (EDMA) to a palladium metal precursor, and its in situ free radical polymerization during imprinting, which not only dramatically increases the yield to ∼100% but also enables high reproducibility. Palladium mercaptide resist was formed by dissolving acetoxy(benzylthio)palladium, EDMA and azobis-(isobutyronitrile) in an organic solvent mixture. The resist underwent polymerization when imprinted using a silicon mold at 120 °C with pressures as low as 30 bar. Polymerization rigidly shapes the imprinted patterns, traps the metal atoms, reduces the surface energy and strengthens the structures, thereby giving ∼100% yield after demolding. Heat-treatment of the imprinted structures at 330 °C resulted in the loss of organics and their subsequent shrinkage without the loss of integrity or aspect ratio and converted them to palladium nanostructures as small as ∼35 nm wide, over areas >1 cm × 1 cm. With suitable precursors, our technique can potentially be extended to pattern noble metals such as platinum, gold and silver.

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Introduction

Palladium (Pd) is one of the most important precious metals, and finds numerous applications, including several industrially important catalytic reactions such as hydrogenation, photocatalysis, Suzuki coupling, Heck coupling and Sonogashira coupling.¹ In addition, palladium is also known for its gas sensing ability, particularly for hydrogen.² Recently, there has been huge interest in palladium-based nanomaterials as they possess very high surface areas that can lead to high-performance catalysts and devices.³ Owing to the high surface energy, nanomaterials are typically anchored onto a support or fabricated into nanostructures to enable easy handling and stability.^4–8 This gave the impetus for researchers to develop novel strategies and synthetic approaches for palladium nanopatterning. Several techniques such as photolithography,⁹ soft lithography,¹⁰ electron beam lithography,¹¹ parallel dip-pen nanolithography¹² and nanoimprint lithography¹³ have been utilized toward this end.

Nanoimprint lithography is emerging as a prominent patterning technique among many that enables fabrication and modification of functional materials in two- or three-dimensions with sub-100 nm resolution.^14–16 Its major advantages are low-cost and high-throughput production of nanostructures. Nanoimprinting involves pressing of a mold possessing nanostructures to deform and shape a thin film of material deposited on a substrate, and is, therefore, capable of achieving resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional light-based techniques. Successful nanoimprinting involves the material to have appropriate flow properties in order to enable complete mold pattern replication at reasonable processing conditions of pressure and temperature. Therefore, it is not surprising that the materials with low viscosities such as thermoplastics, thermoset polymers,¹⁷ UV-curable monomeric precursors¹⁸ and other deformable materials^19–23 are extensively used in nanoimprinting. These materials are either liquid at room temperature or turn viscoelastic on heating.

Whilst nanoimprint lithography is a versatile and well-established technique for patterning organic materials, it has been less explored for direct patterning of metals. This is because, unlike organic materials, metals are hard and do not have a low melting point. Conventionally, nanoimprinting of metals has been an indirect process where a polymer is first patterned and then used either as a part of metal lift-off process¹⁸ or as a mask for etching pre-deposited metal film. Evidently, this is a multistep and an expensive process. Due to higher melting point of metals, and as an alternative to their direct imprinting, a few solid-state embossing methods based on plastic deformation of metal films have been introduced. These approaches involved either high pressure deformation of metal film²⁴ or deformation of a metal thin film/polymer multilayer under lower pressure using sharp mold geometries.^25–27 Apart from lower mold life, these processes also suffer from disadvantages such as the inability to fabricate isolated, arbitrary features, and leave undesirable residual polymer layers. Alternatively, micromolding of metal nanoparticles or thermally unstable metal precursors has been demonstrated using PDMS soft molds.^28–32 Several metal candidates such as Au, Ag, Pt and Pd have been patterned by this approach.^33,34 Although such combinations can successfully imprint the above mentioned metals, the yield and reproducibility may suffer, especially when sub-100 nm scale features are desired. This is because PDMS molds are amenable to deformation at this length scale. Nanotransfer printing is another technique that has gained significant attraction among the researchers due to its simplicity in fabricating a variety of functional nanostructures, including metals.³⁵ This technique relies on the selective transfer of an ink from the raised regions of a stamp (typically elastomeric) onto a substrate. In addition to yielding isolated nanostructures, this technique is capable of printing two- and three-dimensional functional materials onto flat as well as curved substrates.^36,37 A robust silicon mold-based three-dimensional hierarchical palladium nanoimprinting has been demonstrated that utilizes a palladium precursor possessing hysteric melting behavior.¹³ This approach was also extended to fabricate palladium nanopatterns on flexible polymeric substrate for hydrogen sensing.⁸

Recently, we reported a polymerizable sol–gel approach to fabricate metal oxide nanostructures using thermal as well as step-and-flash nanoimprint lithography (S-FIL).^38–42 This approach is based on triggering in situ polymerization of metal-containing polymerizable precursor during nanoimprinting, which shapes the polymerized matrix as dictated by the features present in the mold. Upon demolding and subsequent calcination, the organics in the polymer matrix are burnt off to yield metal oxide nanostructures. This method not only harnesses the benefits of a rigid mold and liquid precursor to achieve very high resolution but also uses the reduction of surface energy of the imprinted structure to achieve dramatic improvement of yields close to 100%. High-throughput patternability of this approach was proven by its successful application using S-FIL technique.^40,43 However, this approach, hitherto, has not been applied for direct nanoimprinting of metals. Successful augmentation of this approach for metal patterning would not only allow high-throughput direct fabrication of metals, but would also open up possibilities for various metal–metal oxide hybrid nanoimprinting. In this work, we utilize thermally polymerizable liquid precursor containing a mixture of palladium mercaptide and EDMA using silicon molds for nanoimprinting. This paper describes the preparation and characterization of the polymerizable palladium mercaptide resist, its imprinting, and heat-treatment to give ordered arrays of ∼35 nm wide palladium structures over areas >1 cm × 1 cm.

Experimental details

Materials

Acetoxy(benzylthio)palladium (in short, ‘palladium mercaptide’) was prepared by reacting palladium acetate, Pd(OAc)₂ (Alfa Aesar, Pd ∼45.9–48.4%), and benzyl mercaptan (Fluka, 99%) in an equimolar ratio (1 mmol each) for 15 hours in 15 mL of toluene (Sigma Aldrich, 99%). After completion of reaction, the color of the solution turned deep orange. The unreacted Pd(OAc)₂ was removed by centrifugation and the supernatant liquid was subjected to low pressure distillation in a rotary evaporator to remove toluene. The obtained mercaptide was washed with anhydrous acetonitrile (Sigma Aldrich, 99.8%) to remove unreacted mercaptan and dried in a vacuum oven for 15 hours. A Nicolet™ 6700 Fourier transform infrared (FTIR) spectroscope was used to analyze the structure of the compound.

Nanoimprint lithography

Silicon substrates and molds were cleaned with the piranha solution at 150 °C for 2 hours (Caution: Piranha solution reacts violently with most organic materials and must be handled with extreme care!), followed by rinsing with deionized water, blow drying using a nitrogen gun, and left in an oven at 100 °C for 2 hours to remove any traces of moisture on the surface. The substrates were kept in a dry area while the silicon molds were treated with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (Alfa Aesar) in a desiccator for 5 hours to reduce their surface energy in order to facilitate a clean demolding after imprinting. Three types of molds, viz., 250 nm and 100 nm gratings with equal lines and spaces and a dimple mold with 200 nm holes which were 200 nm deep, were used for imprinting. The aspect ratio of features on each of these molds was 1.

The solution for nanoimprinting was prepared by dissolving 0.2 g of palladium mercaptide in chloroform or chloroform : toluene mixture. Subsequently, 0.3 mL of EDMA, a cross-linker, and azobis-(isobutyronitrile) (AIBN) weighing 6 mg were added to this solution. We term this mixture as ‘palladium mercaptide resist’. The amount of solvent used to prepare the resist and the spin-coating speed employed to coat the pre-cleaned wafers for imprinting using different molds are as follows: (a) 250 nm line gratings: 2 mL solvent (chloroform); spin speed – 3000 rpm. (b) 100 nm line gratings: 2 mL solvent (chloroform); spin speed – 6000 rpm. (c) 200 nm dimples: 4 mL solvent (chloroform : toluene = 1 : 3); spin speed – 5000 rpm. Imprint lithography was carried out in the Obducat imprinter (Obducat, Sweden).

Characterization of imprinted patterns

High resolution images of as-imprinted and heat-treated structures were acquired using a JEOL JSM6700F field-emission scanning electron microscope (FE-SEM). A NanoScope IV Multimode Atomic Force Microscope (AFM) was used to study the topography of imprinted patterns. For X-ray diffraction (XRD) analysis of heat-treated thin films of palladium, Bruker D8 General Area Detector Diffraction System (GADDS) equipped with a Cu-K_α source was used. X-ray photoelectron spectroscopy (XPS) was done using a VG ESCALAB – 220i XL machine to determine the chemical composition of palladium films. The conditions of XPS study were – Al monochromated source (spot size: 700 μm); operating pressure – 1 × 10⁻⁹ Torr; ion gun setting – 3 kV; sputter area – 4 mm × 4 mm.

Results and discussion

Successful direct nanoimprinting of a metal primarily depends upon a resist having metal precursor which does not crystallize during the evaporation of the solvent and at the same time forms a smooth and flowable thin film. For patterning of palladium, Pd(OAc)₂ was chosen as the starting material. Although Pd(OAc)₂ is slightly soluble in organic solvents such as toluene, it has the tendency to crystallize when spin-coated on a substrate. The inclination to crystallization can be suppressed when one of the acetate groups in Pd(OAc)₂ is replaced with benzyl mercaptan to give acetoxy(benzylthio)palladium (in short, ‘palladium mercaptide’). The structure of palladium mercaptide was confirmed using FTIR [Fig. 1(a)]. It was observed that palladium mercaptide has a better solubility in organic solvents (such as toluene and chloroform) and monomer (EDMA). Palladium mercaptide was mixed with EDMA and AIBN to form palladium mercaptide resist.

Fig. 1 (a) FTIR spectrum showing the evidence of formation of palladium mercaptide. The principal peaks are identified in Table 1. Note the presence of phenyl group in addition to the –C=O group in the mercaptide. (b) Time-resolved FTIR data of palladium mercaptide resist heat-treated at 120 °C. It is seen that peaks corresponding to the polymerizable –C=C group of EDMA decrease in intensity. The peaks associated with EDMA are indicated by ‘*’.

Characteristics of palladium mercaptide resist

Palladium mercaptide resist contains unsaturated –C=C bonds of EDMA monomer that can be polymerized to form a cross-linked polymeric network in the presence of AIBN, which acts as a free radical thermal initiator. Time-resolved FTIR spectra of palladium mercaptide resist coated on a pre-cleaned silicon substrate at 120 °C showed that the intensity of monomeric peak [small nu, Greek, macron](C=C) at 1640 cm⁻¹ of EDMA decreased continuously and eventually almost disappeared [Fig. 1(b)]. Furthermore, other peaks corresponding to EDMA at 1153 cm⁻¹, 1296 cm⁻¹ and 1723 cm⁻¹ (all marked with ‘*’) also decreased significantly in intensity as the acrylate units were depleted with furtherance of the reaction.³⁸ The two broad bands at 1538 cm⁻¹ [[small nu, Greek, macron]_a(C=O)] and 1416 cm⁻¹ [[small nu, Greek, macron]_s(C=O)] are still visible even after the reaction was complete, suggesting the entrapment of palladium mercaptide inside the polymeric structure.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies of palladium mercaptide resist were done in order to understand its mass loss behavior and to identify the polymerization exotherm, respectively. The TGA shows two distinct regions of mass loss – a major loss between 25–110 °C due to the evaporation of the solvent, and a minor loss between 290–400 °C resulting from the loss of organic material from the resist [Fig. 2(a)]. On the other hand, the DSC scan showed a sharp exothermic spike at 100 °C [Fig. 2(b)]. This was due to the free radical polymerization of EDMA in palladium mercaptide resist.

Fig. 2 (a) TGA, (b) DSC, and (c) isothermal heat-treatment of palladium mercaptide resist at 330 °C showing residual mass at 50 and 60 minutes.

The TGA analysis of palladium mercaptide resist suggests that at around 400 °C, the remnant mass should be palladium metal [Fig. 2(a)]. However, a better picture of decomposition can be obtained by studying the TGA analysis of as-prepared palladium mercaptide. It is seen that the mercaptide decomposes to 35.9% of the original mass at 363 °C [see ESI†]. This corresponds well with the theoretical palladium content of 36.9% in acetoxy(benzylthio)palladium. However, heating above 363 °C results in a steady increase of mass due to the formation of palladium oxide. In order to avoid the surface oxidation of palladium film, the decomposition of palladium mercaptide resist was conducted under isothermal conditions and at a reduced temperature of 330 °C [Fig. 2(c)]. The samples were placed inside a furnace at room temperature (27 °C) and heated at the rate of 10 °C min⁻¹. It took ∼30 minutes to reach 330 °C and the samples were soaked for 30 minutes at this temperature. Therefore, the total time the samples spent in the furnace was 60 minutes. At the end of isothermal heat-treatment, the samples were immediately removed from the furnace. The films heat-treated at this condition were analyzed using XRD and they showed the presence of only palladium metal peaks [Fig. 3]. SEM imaging of the film suggests the presence of sub-100 nm grain morphology [Fig. 3, inset]. The chemical composition of the palladium films was studied using the XPS. Fig. 4 shows the XPS survey scan and the binding energy peaks associated with palladium, oxygen, sulphur and carbon. Although, there is a noticeable presence of carbon, elements like oxygen and sulphur are to be found only in trace amounts. Since the binding energies of O(1s) and Pd(3p) peaks are very close, the presence of oxygen is best detected by shift in the Pd(3d) peaks to higher values. For the as-received Pd thin film after calcination, the XPS scan revealed the characteristic Pd(3d_5/2) and Pd(3d_3/2) peaks at 335.9 and 341.1 eV, respectively.³ These peaks were observed to be broad with shoulders appearing at 337.4 and 342.7 eV, that can be attributed to the oxidized Pd(3d_5/2) and Pd(3d_3/2), respectively [see Fig. 3s in the ESI†]. However, after 30 seconds of Ar sputtering, almost all the peaks in the survey scan not only appear sharper, but there is also a significant reduction in the amount of carbon (2.63 at%) and an almost complete absence of oxygen and sulphur [Fig. 4(a, d, e) and ESI†]. This suggests that carbon was most likely present as a surface contaminant and that the origin of oxygen may be due to a very slight oxidation of palladium surface.

Fig. 3 X-ray diffraction study of heat-treated palladium mercaptide resist showing the appearance of palladium metal film (JCPDS Card no. 46-1043). Inside the furnace, the sample took ∼30 minutes to reach 330 °C and it was isothermally held at this temperature for further 30 minutes. The total time of heat-treatment was 60 minutes. Inset: SEM image showing the nanometer-grain morphology of the palladium film. The grain size appears to be <100 nm.

Fig. 4 XPS study of the palladium film formed after heat-treatment of palladium mercaptide resist at 330 °C for 60 minutes. Notice the dramatic reduction of the carbon peaks after 30 seconds Ar sputter. The XPS data is not normalized with respect to film thickness.

Nanoimprint lithography of palladium mercaptide resist

Since polymerized palladium mercaptide resist is thermally unstable, it can be heat-treated in air to produce metallic palladium. A schematic diagram of direct nanoimprint lithography of palladium is shown in Fig. 5. A two-step nanoimprint lithography process was implemented to pattern palladium mercaptide resist. Firstly, at room temperature (27 °C), a pressure of 30 bar was applied for 5 minutes to ensure a complete filling of the mold with resist. Secondly, while holding the pressure constant, the coated substrate and mold were heated to 120 °C and held for 15 minutes at this temperature in order to induce in situ free radical polymerization of EDMA in the mercaptide resist. Although polymerization of EDMA is a fast reaction, taking only a few seconds, a longer polymerization time was provided in order to ensure the completion of the reaction as well as to remove any entrapped solvent. This resulted in strengthening of the imprinted features to achieve very high yield after demolding. After imprinting, the assembly was cooled down to 65 °C before releasing the pressure and this was followed by a careful and clean demolding, giving ∼100% yield over ∼2 cm × 1 cm area [Fig. 5 (inset)]. SEM studies indicate that the width of imprinted features was slightly smaller than the actual dimensions of the mold. This is most likely due to the polymerization-induced shrinkage of the patterns [Fig. 6].³⁸ For example, 250 nm and 100 nm grating molds gave 210 nm and 90 nm pattern widths, respectively, whilst, 200 nm dimple mold gave ∼120 nm wide pillars [Fig. 6 and Table 2].

Fig. 5 Schematic diagram showing the reaction to form palladium mercaptide precursor, the direct imprinting of precursor-cross-linker mixture and finally heat-treatment to produce palladium nanostructures. The inset shows uniformly imprinted palladium mercaptide resist lines over an area ∼2 cm × 1 cm using a 250 nm grating mold.

Fig. 6 Composite SEM images of various as-imprinted and heat-treated structures of palladium using molds with different features. The insets show the structures at higher magnification. The AFM line traces of the corresponding heat-treated imprinted structures are shown on the right.

Table 1 Characteristic infrared absorption peaks of palladium mercaptide showing principal peaks. Notice that the presence of peaks associated with –C=O and phenyl groups in the compound

Absorption peak (cm^−1)	Assignment, compound	Peak number
1610	ν(C=O)	1
1496	Phenyl	2

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Table 2 Summary of the approximate feature size reduction at every step of the palladium metal patterning using imprint lithography

Mold shape/size	Feature size of the imprint after free radical polymerization		Metal feature size after the heat-treatment of imprinted structures		Total feature size reduction with respect to mold feature size (%)
	Width of imprint (nm)	Feature size reduction (%)	Width of the metal feature (nm)	Feature size reduction (%)
Dimples, 200 nm	120	40%	75	38%	63%
Lines, 250 nm	210	16%	75	64%	70%
Lines, 100 nm	90	10%	35	61%	65%

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Heat-treatment of the imprinted features was carried out according to the conditions described in the earlier section. Briefly, the imprinted structures were heat-treated, as described earlier, by heating to and holding at 330 °C for 30 minutes each in air to remove the organics and convert them to palladium metal. The topography and morphology of the heat-treated imprinted structures were analyzed by SEM and AFM. Fig. 6 shows that heat-treatment resulted in shrinkage of the imprinted grating patterns to approximately 40–60% of the original size while their aspect ratio was found to be slightly greater than 1. The AFM profile corroborates a uniform loss of organic components during heat-treatment over the entire imprinted structure. After heat-treatment, ∼75 nm palladium gratings can be achieved by using a 250 nm grating mold, whilst 100 nm gratings gave ∼35 nm wide palladium patterns. The imprinted pillar structures, however, showed slightly less shrinkage than gratings. For example, ∼73 nm palladium pillars were obtained using a 200 nm dimple mold [Fig. 6 and Table 2].

The three crucial parameters for successful imprinting of palladium nanostructures are (a) the amount of solvent in the palladium mercaptide resist, (b) the spin-coating speed, and (c) the heat-treatment time. Striking the right balance among these three parameters appears to be crucial to not only achieve good imprint quality and yield but also the integrity of palladium structures after heat-treatment. The amount of solvent and spin-coating speed determines the film thickness to be imprinted while the heat-treatment time determines grain growth and dewetting of the imprinted metal. It was found that residual layer thickness slightly less than 100 nm was ideal to achieve palladium lines of good integrity after heat-treatment. On the other hand, for obtaining palladium dots, it appears that thinner residual layer thickness of ∼50 nm is preferable. Palladium does not adhere well to silicon surface, and hence it has the proclivity to dewet when heated. Furthermore, considering the fact that decomposition of the mercaptide resist during heat-treatment results in sub-100 nm grain morphology [Fig. 3, inset], palladium also has the tendency to undergo grain growth when soaked at elevated temperatures for longer times [see ESI†]. In order to suppress dewetting as well as grain growth whilst maintaining the integrity of the palladium nanostructures, the heat-treatment time becomes very crucial. In all our heat-treatments, the time was strictly set to 60 minutes in the furnace to achieve imprinted palladium structures of good integrity.

From the above discussion, it is clear that there are several advantages of adding cross-linker EDMA to a metal precursor for nanoimprinting. Firstly, the in situ free radical polymerization of EDMA during imprinting not only rigidly shapes the imprint, but also traps palladium atoms. Furthermore, polymerization shrinks the pattern features, reduces the surface energy and strengthens the imprinted structures, the result of which is ∼100% yield after demolding. Secondly, the lower surface energy of the polymerized palladium mercaptide resist does not require a dedicated mold release system. Conventional mold release fluorinated surfactants on a silicon mold suffice for direct imprinting of metals. Thirdly, the use of conventional silicon molds in conjunction with liquid mercaptide resist results in achievement of very high resolution imprinting over large areas, which is not possible when a PDMS soft mold is used.

Our approach has shown that selectively polymerizing cross-linker in a mixture containing the metal precursor and cross-linker is suitable to directly imprint metal. This opens up the possibility of directly imprinting other noble metals such as platinum, gold and silver, by employing their appropriate precursor mixed with a suitable cross-linker. In addition, our technique of direct imprinting can potentially be extended to other metals as well via the hydrogen reduction of directly nanoimprinted metal oxides. Recently, Nedelcu et al. have shown that fabrication of sub-10 nm metallic lines of low line-width roughness is possible by hydrogen reduction of electron beam-patterned metal–organic materials.⁴⁴ The Ellingham diagram suggests that the metal oxides residing above the water formation line (i.e., H₂ + ½O₂ = H₂O line) are amenable to reduction to their respective metals by hydrogen. Thus imprinted patterns containing metal oxides that are thermodynamically less stable than water (such as NiO, CuO, PbO, CoO, etc.) can be reduced by hydrogen to their respective metals.⁴⁵

Conclusions

In conclusion, we have demonstrated large area direct imprinting of ∼35 nm features of palladium using nanoimprint lithography of palladium mercaptide resist. This resist consists of a mixture containing acetoxy(benzylthio)palladium, EDMA and AIBN. In situ thermal free radical polymerization adds rigidity to the imprinted patterns, traps the metal atoms, reduces the surface energy and strengthens the patterned structures, thereby increasing the yield. Heat-treatment of the polymerized imprints at 330 °C resulted in the loss of organics and their subsequent shrinkage (∼60–70%), which yielded palladium nanopatterns without the loss of integrity or aspect ratio. Thus, a pattern made from 250 nm grating gave ∼75 nm lines. Likewise, ∼35 nm palladium lines were obtained from a 100 nm grating mold over areas >1 cm × 1 cm. Such nanoscale imprinted palladium may find potential applications such as solid-supported green catalysts and hydrogen gas sensors. In the presence of suitable precursors, our technique can also be potentially extended to pattern noble metals like platinum, gold and silver.

Acknowledgements

One of the authors (MSMS) would like to thank Professor Giridhar Kulkarni and Dr B. Radha of the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore (India), and Professor Ghim Wei Ho of the National University of Singapore, for scientific discussions. The authors would like to thank Lim Poh Chong and Daniel Li of Institute of Materials Research and Engineering for their assistance in XRD and experiments. Assistance of Ms Shreya Kundu in SEM studies and Mr Jie Yong Chan for palladium mercaptide synthesis is gratefully acknowledged. This work was partially supported by the A*STAR Nanoimprint Foundry Project No. IMRE/13-2B0278. RG thanks the Department of Science & Technology, India, for the financial aid (SERB/F/4864/2013-14).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00234j

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