Final Master Project
Low Cost SERS Substrates based on mirror like patterned PMMA Foils prepared by Thermal Nanoimprinting Lithography (NIL)
Author
Daniel Santiago Tobón Vélez
Supervisors
Dra. María Pilar Pina Iritia Dr. Sergio Gutiérrez Rodrigo
Master in Nanostructured Materials for Nanotechnological Applications Instituto de Nanociencia y Materiales de Aragón (INMA)
Universidad de Zaragoza
Master Edition 2019-2020
i INDEX
Abstract
Resumen en español para público no científico List of Abbreviations
1. Introduction and Objectives...
1.1. Motivation and Context
1.2. Nanoimprint Lithography (NIL) Technologies 1.2.1. General Introduction
1.2.2. Current Status and Future Trends 1.2.3. Thermal NIL as Fabrication Tool
1.3. Surface Enhanced Raman Spectroscopy (SERS) 1.3.1. Electromagnetic Mechanism of SERS
1.3.2. Mirror-like 3D Periodic Microstructures for SERS 1.4. Objectives
2. Materials & Methods……….………...
2.1. Starting Materials and Equipment
2.2. General Process Layout to fabricate metallic microstructures on PMMA 2.3. Finite-Difference Time-Domain (FDTD) simulation
2.3.1. Lumerical Software 2.3.2. Simulated Structures 2.3.3. Materials properties
3. Results and discussion ...
3.1. DSC Analysis of PMMA
3.2. Morphological Characterization 3.3. FDTD Simulation Results
4. Conclusions ...
5. Bibliography ...
ii Abstract
In this final master project, conditions for fabrication of 3D periodic micropyramids by Thermal nanoimprinting lithography (T-NIL) are preliminary explored on PMMA (Poly methyl methacrylate)foils. Micropyramids are proposedly coated by a thin layer of gold (Au), copper (Cu) or aluminium (Al), in order to study its potential use as a substrate for detection of chemical threats by Surface Enhancement Raman Spectroscopy (SERS).
Different sizes of the metal-coated pyramids are investigated by FDTD simulations.
In order to explore T-NIL, several nanoimprinting approaches were attempted, by using Silicon (Si) moulds with different patterns. Nanoimprinting was made directly on PMMA substrate from the previously mentioned moulds as well as from an intermediate mould fabricated by casting with PDMS (Poly dimethyl siloxane). The goodness of the T-NIL process was mainly assessed by profilometry measurements on the patterned features.
For potential use as a SERS substrate, the PMMA micropyramids were conformally coated with a thin layer (40 nm) of metal on top. The influence of the characteristic length of the pyramid and the metal was analysed by FDTD module of Lumerical software in order to evaluate the amplification of the Electromagnetic field (EM) in the surface of the metal.
Finally, FDTD simulations allowed us to determine which is the better option of size and material (metal) of the micropyramids, among the tested; and to guide in the subsequent manufacturing of low-cost substrates for SERS applications.
iii Resumen en español
Este trabajo fin de máster explora, por primera vez en el grupo de investigación, las condiciones de fabricación de micropirámides periódicas 3D, mediante litografía por nanoimpresión asistida por temperatura (T-NIL) en láminas comerciales de PMMA (polimetil metacrilato). Con el fin de estudiar su uso potencial como sustrato para la detección de amenazas químicas mediante Espectroscopía Raman Amplificada en Superficie (SERS), propone el recubrimiento de las puntas de las micropirámides con una capa de oro (Au), cobre (Cu) o aluminio (Al) de espesor nanométrico y dimensiones controladas.
Para explorar T-NIL, se llevaron a cabo varios procesos de nanoimpresión, utilizando moldes de Silicio (Si) con diferentes patrones. La nanoimpresión se realizó directamente con los moldes mencionados anteriormente, así como a partir de un sello intermedio de PDMS (Polidimetil siloxano) preparado mediante réplica por llenado y desmoldeo. La calidad de la nanoimpresión se evaluó mediante perfilometría.
Para su uso potencial como sustrato SERS, se evaluó la amplificación del campo electromagnético (EM) en la superficie metálica de las micropiramides en función de las dimensiones de las mismas y de la naturaleza del metal. La herramienta de cálculo utilizado para ello fue el módulo FDTD del software de Lumerical
Las simulaciones FDTD nos permitieron determinar cuál es la mejor opción de tamaño y material (metal) de las micropirámides, de entre las estudiadas. Estos resultados preliminares van a servir de guía para orientar los ensayos futuros de fabricación mediante NIL y de validación de su comportamiento en aplicaciones SERS.
iv List of Abbreviations
AFM: Atomic Force Microscopy
DSC: Differential Scanning Calorimetry EF: Enhancement Factor
EM: Electromagnetic Field EPP: Edge Plasmon Polariton
ERSis: Effective Response to Potential Chemical Threats by Combining SERS &
Catalysis
FDTD: Finite Difference Time Domain HE: Hot Embossing
IFAFRI: International Forum to Advance First Responder Innovation LSPP: Localized Surface Plasmon Polariton
MICINN: Ministerio de Ciencia e Innovación NIL: Nanoimprint Lithography
NP: Nanoparticle
OPCW: Organization for the Prohibition of Chemical Weapons T-NIL: Thermal Nanoimprint Lithography
PMMA: Polymethyl Methacrylate PML: Perfectly Matched Layer
PSPP: Propagating Surface Plasmon Polariton SEM: Scanning Electron Microscopy
SERS: Surface Enhanced Raman Spectroscopy SPR: Surface Plasmon Resonance
SSIL: Step and Stamp Imprinting Lithography RS: Raman Spectroscopy
Tg: Glass Transition Temperature UV: Ultraviolet
1 1. Introduction and Objectives
1.1. Motivation and Context
The present final master thesis is framed in the Spanish MICINN 2019-2022 project
“ERSis: Effective Response to Potential Chemical Threats by Combining SERS &
Catalysis” which aims to develop:
i) novel instrumentation for near-real-time or on-demand detection/identification of chemical threats by Surface Enhanced Raman Spectroscopy (SERS) to aid responders and incident commanders in hazard assessment and decision-making.
ii) on-site decontamination technologies based on heterogeneous catalysis for sustainable recovery.
In this sense, the work herein carried out is part of the study of novel instrumentation for detection and identification of chemical threats by SERS.
In recent years, even though there is ban on the use of chemical weapons, many countries have registered attacks with chemical warfare agents [1]. A chemical weapon is defined as a chemical substance used with the aim of causing death or harm through its toxic properties, according with the Organization for the Prohibition of Chemical Weapons (OPCW) [2]. Currently, scientific community keeps working on the development of effective instrumentations for the detection of chemical agents used as chemical weapons.
The International Forum to Advance First Responder Innovation (IFAFRI) identifies up to ten areas in which advances on technology could greatly enhance the safety, effectiveness and efficiency of the world’s first responder [3]. Particularly, the real-time detection, monitoring and analysis of threats and hazards, and the rapid identification of hazardous agents and contaminants are those motivating this work.
This final master project came up after research findings from both supervisors in which non-coated 3D periodic Ag mirror-like pyramidal microstructures on silicon were used as SERS substrates, to detect organophosphorus pesticides [4]. In contrast, this work proposes the use of a soft material, i.e. polymethyl methacrylate (PMMA) for microstructuring and spatial-controlled metallization with the aim to explore more cost- effective substrates in SERS detection. Preliminary characterization of pristine and
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metallized commercial PMMA foils are attempted to assess on how feasible the fabrication approach by nanoimprint lithography (NIL) is. 3D simulations are performed using the finite-difference time-domain (FDTD) method in order to obtain information about the excited plasmonic modes in the structure, thus supporting the possible application of metallized microstructures on PMMA foils as SERS substrates.
1.2. Nanoimprint Lithography (NIL) Technologies
1.2.1. General Introduction
The Nanoimprint Lithography (NIL) is a novel method of fabricating micro/nanometer scale patterns with low-cost, high-throughput and high resolution [5]. Unlike traditional optical lithographic approaches, it consists of transfer a pattern from a stamp/mould/master/template to a polymer film (substrate) by direct mechanical deformation. Thus complex, and expensive optics are not required anymore, and the common resolution limitations imposed by light diffraction or beam scattering are overcome.
NIL historically refers to a hot embossing (HE) lithography process and was also used as a synonym for thermal NIL (T-NIL) due to the system temperature is increased above the glass transition temperature of the polymer (Tg). Besides temperature, pressure is increased too; that is why the term thermocompression NIL is also used [6]. These conditions allow the viscoelastic material to flow around the reliefs of the stamp surface.
The substrate/mould interface has to be considered throughout the entire process, both from topographical, chemical, and mechanical points of view. Thus, the filling behaviour of the polymer and demould capabilities arise as the key factors which can strongly influenced pattern quality and throughput. If the pattern replication is done under optimized process a uniform and large area with complex morphologies can be achieved [7], [8], [9].
In both technologies, thermal NIL and HE, the stamp is pressed against the polymer (resist) located onto the substrate. In fact, the terms T-NIL and HE are used indistinctively by the scientific community and also in this work. However, the company NILT Technologies (https://www.nilt.com/), in its compendium of open documents, states
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clearly the difference. In HE, the polymer film is thicker than the height of the stamp structures, therefore, the reliefs are only small alterations in proportion to the polymer thickness. In contrast, for T-NIL the thickness of the polymer used is comparable with the high of the stamp structure (see figure 1 a) and the relief might decrease the thickness of the polymer layer. Whereas in HE the cavities in the stamp are filled completely, in T- NIL there are two options. The first one is to fill completely the cavities, which is called complete imprint. Similarly, the second one is to do incomplete imprints (sometimes is advantageous) in which the cavities are not filled with the resist, it is called under filling imprints [10].
Figure 1. Two main process for NIL a) Thermal NIL, b) UV-NIL. Retrieved and modified from Nanoimprint Lithography [5].
There is another fundamental type of NIL, ultraviolet based NIL (UV-NIL), which uses a UV-curable liquid photopolymer instead of a thermoplastic (see figure 1 b). The resist is applied to the substrate and the mould is normally made of transparent material to enable the resin cross-linking and curing in the polymer by UV light exposure [11]. This process offers several advantages due to the absence of high imprint pressures and thermal heating cycles.
1.2.2. Current Status and Future Trends
NIL technologies nowadays have 3 outstanding characteristics: complex patterning capabilities, high resolution manufacturing tool, and cost-effective process. The latest
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report published by the business group Yole Développement announces a promising future for NIL technologies in the fields of photonics, biochips, and front-end memories (3D NAND) [12]. Currently, these devices are requiring tighter nanoscale resolutions, complex shapes, and better cost-effective ratio. Despite semiconductors devices are usually fabricated by photolithography technology, NIL is an excellent option that meet the new requirements and the growing need of the market. That is way it is expected that for 2024 these technologies reach the 24% of the semiconductor business, making the market worth US$ 146 million (see figure 2).
Figure 2. Current status and future trends for NIL technologies in Semiconductors market. Retrieved from “NIL Technology-Trends for Semiconductors Applications” [12].
In addition to the above, NIL technologies have also been employed, the latest years, for nanofabrication of solar cells, memory devices, nanoscale cells, hydrophobic surfaces, and biosensors. Some forecasts of possible applications of NIL are in artificial organs, diagnostic system, and fundamental research in cell biology [13].
1.2.3. Thermal NIL as Fabrication Tool
Since 1995, when the first report was made, NIL technique has had a huge progress becoming an important tool in the fabrication of devices with different applications on the market. NIL and in this specific case T-NIL, provides facilities that has allowed many researchers to pattern with, instead of the common photolithography methods, chemical, physical, and structural shapes by NIL, for multiple applications with affordable
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operating cost and high resolution. [14] This is due to the simplicity of the technique, the accuracy, and the complexes geometries that allows to imprint. Other characteristics that made of this technique so useful is that there is no need of expensive optics, excimer lasers, electron beams or ultra-high-vacuum chambers.
However, this technique presents some challenges which have been trying to solve. One of these issues is that, as for NIL it is needed that the stamp comes into contact with the substrate, after several cycles, it is needed that the stamp be cleaned. Another critical factor is the flatness of the substrate and the stamp since, if there are any protrusions or topographic features in the surface, those can affect the uniform contact between the stamp and the substrate causing defects in the impression. Summarizing, the main identified challenges in manufacturing by using this technique are air entrapment between the substrate and the stamp, particulate matter, stamp contamination and the throughput of the whole process.
For manufacturing, NIL can be used as a large-area lithography by means of parallel or sequential imprinting, respectively. In parallel imprinting, it has been reported that patterned wafers (stamp) with large areas up to 150 mm can be imprinted on a substrate with similar dimensions in a single stage. For sequential imprinting, also called Step and Stamp Imprinting Lithography (SSIL), a small stamp is moved along the substrate in a stepping mode [15] by following a step-and-repeat configuration. So, every time the pattern is transferred to the substrate, the stamp takes a step to the side to imprint in another place of the substrate [6].
Accordingly, the use of NIL is proposed in this work for the fabrication of SERS substrates.
1.3. Surface Enhanced Raman Spectroscopy (SERS)
Surface-Enhanced Raman Spectroscopy (SERS) is based on Raman Spectroscopy (RS), which in turns is based in the principle of inelastic scattering of light by the molecule. In 1928 Sir C.V Raman demonstrated this effect, later called Raman Effect. This consists of the frequency shift into either lower or higher value, of the scattered photons, in comparison with the frequency of the incident photon of the visible light after interacting
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with the sample. In figure 3, it can be seen that those photons providing energy for molecular vibrational modes produce the Stokes signals. In the opposite way, the photons that absorb energy from the molecule vibration generate anti-Stokes signals [16]. Thus, Raman is a vibrational spectroscopic technique that allows a deeper molecular insight via the characteristic vibrational modes specific to the structure of the tested material.
However, the widespread analytical use of Raman spectroscopy is hampered by the small fraction of the inelastically scattered photons (approximately 1 in 10 million) which renders in week Raman signals.
Figure 3. Energy level diagram, representing Raman Scattering process. Retrieved from [17]
SERS has been extensively studied since the Raman scattering enhancement of adsorbed molecules on rough metal surfaces was discovered by Fleischmann in 1974 [18]. The enhancement in SERS is characterized by two main mechanisms: long-range electromagnetic effect and short-range chemical (charge-transfer) effect. The Electromagnetic mechanism (see section below) is based on the surface plasmon resonances excitation of metallic nanostructures, which in turns induces an electric dipole moment of the adsorbed molecules over the enhancing metal surface. Thus, the amplification of Raman signal is obtained by an increase of the local electric field and also the polarizability of the adsorbed molecules. The chemical effect, less studied, is related to chemisorption of the analyte in the metal surface. This generates a charge transfer complex of electrons between metal and adsorbate, that when excited generates an amplification of the Raman signal [16].
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Recently, there has been an increasing attention in SERS due to its analytical capabilities for non-destructive and efficient detection of molecules. However, fabrication of SERS substrates for commercial use, has been an important challenge, since it is pursued that they have large-scale, strong and uniform distribution of “hot spots” [19].
1.3.1. Electromagnetic Mechanism of SERS
Raman signal is proportional to the fourth power of the electric field intensity (~E4), for negligible Raman shifts [19]. Therefore an efficient way to enhance Raman signal is figuring out material environments that naturally increases the local EM field seen by the molecules. Metal based platforms have been mostly chosen since the beginning for its strong SERS response and ease of fabrication of metallic surfaces.
Figure 4. Illustration of a) Propagating Surface Plasmon Resonance on metal surface and b) Localized Surface Plasmon Resonance on metal nanoparticles. Retrieved from [20]
Metals support electromagnetic (EM) modes called surface plasmons, which allow extreme concentrations of light on nanometer scales [21]. From a mathematical point of view, a confined mode is a solution that exponentially decays far from the defined boundaries, so the molecules, within regions of high EM field (high photon density), and, due to the excitation of plasmons, might suffer a large increase in the effective Raman section. This allows, for example, the detection of chemical compounds at very low concentration. From the point of view of plasmons, they can propagate long distances (compared to their wavelength) in continuous metallic systems, like in a flat metallic surface (figure 4,a), which are called surface plasmon-polaritons (SPPs); or can be
a) b)
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localized, as it happens for example in metallic nanoparticles (NP) [22] (figure 4, b), which are called localized-SPPs (LSPPs).
LSPP resonances appearing in structures with features in the nanometer size scale (e.g.
nanoparticles) strongly depend on the geometry, not just the dielectric constant. For instance colloidal solutions of metallic nanoparticles with different sizes feature different colours because LSPP excited absorption resonances modify the spectral content of light passing through them [22].
The optical properties of SPPs on a flat metal surface only depend on the dielectric constant. From this and the SPP dispersion relation result that these EM modes cannot be excited directly by an external light source (e.g. a laser) because frequency and momentum conservation cannot be satisfied simultaneously [21]. For a flat metallic surface their characteristic properties are given by propagation length (𝐿𝑎𝑏𝑠) (Equation 1) and skin-depth (Equation 2) and the both quantities give us information about the behaviour of SPPs: they are confined to the metal surface in a sub-wavelength scale and they strongly decay as they propagate along the surface.
For the present work we are interested in the propagation length and skin-depth of SPPs because the EM modes important for our study are mostly propagating waves. In propagation length formula, 𝜀𝑚′ (m stands for metal) is the real part of the dielectric constant and 𝜀𝑚′′ the Imaginary part; 𝜆0 represent the wavelength in vacuum.
𝐿𝑎𝑏𝑠 = 𝜆0(𝜀𝑚′ )2
2𝜋𝜀𝑚′′ (1)
The range in which this formula applies is when |𝑅𝑒(𝜀𝑚)| ≫ 1 (typically in the red and infrared part of the optical spectrum) in metal-air SPPs since 𝜀 = 1 in air.
Skin-depth formula specify penetration of the SPP fields into each medium:
𝛿𝑚 ≈ 𝜆 2𝜋√|𝜀𝑚′ |
(2) 𝛿𝜀 ≈ √|𝜀𝑚′ |𝜆
2𝜋𝜀
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Where parameter 𝛿𝜀 refers to the dielectric half-space, and 𝛿𝑚 refers to the metal, which it is called skin [23].
Both quantities (propagation length and skin-depth) give us information about the behaviour of SPPs: they are confined to the metal surface in a sub-wavelength scale and they strongly decay as they propagate along the surface.
Metal properties (heat and electrical conductivity, optical response) originate from the dynamics of free conduction electrons, which are not noting but a plasma. The physics of a plasma is well described by the Drude model that explain the optical response of many metals from the visible range to lower energies of the optical spectrum. The dielectric constant of the Drude model is described by the following mathematical expression:
𝜀(𝜔) = 𝜀𝑟− 𝜔𝑝2
𝜔(𝜔+𝜄𝛾) (3)
Where 𝜀𝑟 provides the optical response at the range of high frequencies, while 𝛾 is related to the Joule’s effect, it is the loss of energy by heating, and 𝜔𝑝 is the plasma frequency [23].
It is worth to mention that the metallic behaviour of metals arises due to the fact that the real part of the dielectric constant is negative. The imaginary part is related with absorption.
Still Drude model provides a very good description of the optical properties of some metals (e.g. silver), there exist additional contributions to the dielectric constant from other electronic transitions, like inter-band excitations of electrons in the case of gold and copper. In our study metals will include additional terms to Equations 1 from the so-called Lorentz model. In this model the optical response of an electron is described by a bound restoring force, which define a resonant frequency ω0.
Propagating plasmons exist in many structures, which can be seen as modifications of the flat metal surface. SPP-like modes have been observed in channels [24], [25], wedges [26] and edges, like the ones of the structures investigated in this master thesis. In all
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cases the optical properties are geometry dependent, however, will see that expressions for the skin depth and the propagation length of a SPP will have qualitative validity.
Thus, the amplification of the Raman signal of molecules mainly occurs when they are located in the proximity of the “hot” electric field spots on the topographic nanofeatures or protrusions on the metallic surface. Therefore, molecules trapped in the vicinity of these “hot” zones would experience a strong field, thus emitting amplified Raman intensities as can be seen in figure 5, in which Raman signal, according to the surface, whether is flat or has a G0 or G1 (G stands for generation and refers to the levels of constructions) structure can increase or decrease in detection of the same molecule .
Figure 5. Comparison between Raman signal between flat surface, G0 and G1 structures of the SERS target analyte 4-nitrobenzenethiol (4-NBT). Retrieved from [4].
The used parameter to quantify the enhancement of the Raman Signal is the enhancement factor (EF). It is described as the intensity of the ratio between SERS and normal Raman signal for an analyte normalized by the number of molecules probed [27]. The performance of a SERS substrate depends on several parameters, including the composition, size, morphology, topology, surface distribution and dielectric environment of the metallic components of the SERS substrate.
1.3.2. Mirror-like 3D Periodic Microstructures for SERS
Previous studies in our group have demonstrated the beneficial effects in terms of Raman signal enhancement in mirror-like surfaces with micrometric 3D structures. These structures are shown in figure 6 and have been labelled as G0, G1, G2 and G3, corresponding to panels a) to d). The SERS substrates based on these configurations can
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increase significantly SERS signal, in comparison with the one generated by a flat metallic surface [28] due to the 3D microstructures can support different plasmonic modes.
In this final master project, the G0 type pyramidal structure with a preferential metallization on the tip is mainly discussed [29]. This periodically replicated arrangement of 3 cm2 array, consist of pyramids with individual base diagonal of 2 µm and 9.9 µm pitch (measured from head to head of the pyramids).
Figure 6. Illustration of different type of structure. a) G0, b) G1, c) G2, d) G3. Retrieved from [29].
1.4. Objectives
The main objective of the present work is to explore G0 microstructures on PMMA imprinted by NIL and coated with a thin layer of metals (Au, Cu or Al) for a possible application in SERS detection. The final aim is to provide with a lower cost option, in comparison with more sophisticated Si based periodic micro-nanostructures fabricated using singular facilities, for the consecution of SERS active substrates.
In order to achieve the general objective, the following specific objectives have been proposed:
• Explore conditions for PMMA patterning using NIL.
• Propose a fabrication process for mirror-like micropyramidal structures on PMMA foils
• Study of the electromagnetic field enhancement of Au/Cu/Al coated PMMA micropyramids by simulation with Finite-Difference Time-Domain (FDTD) Method.
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• Propose specific structures (dimensions and coatings) to fabricate and measure in Raman SERS
2. Materials & Methods
2.1. Starting Materials and Equipment
Commercial foil of PMMA with a thickness of 250 µm was provided by Goodfellow Cambridge Limited. The glass transition temperature of the polymer was evaluated from Differential Scanning Calorimetry Analyses performed in a Metter Toledo AG-DSC 823e.
Two Si Stamps were used for the preliminary study of the imprinting conditions. At first the NILT Stamp, from NIL Technology ApS, was used to transfer the line patterning, 270 nm in height, directly to the commercial PMMA film by T-NIL. This was done to familiarize with the NIL process, and to start with the optimization of the settings (temperature, pression and time) for later use in micropyramids array replication on PMMA.
The second Si master was the Ion Milling Stamp, available in the clean-room and fabricated in house by ion milling. This mould has different features in the micrometer size scale distributed along the wafer (see annex figure A.I.1), but all of them 5 µm height.
This master was used to transfer the micropatterns to the Polydimethylsiloxane (PDMS) stamp. The PDMS stamp was prepared by pouring the PDMS silicone elastomer kit, Sylgard® 184 provided by Dow Chemical Co. (Midland, MI) on the Si mould. The so obtained pyramids must be negative, like a well and not protruding from the surface in positive like those in the Ion Milling master. Transfer of inverted pyramids on the PMMA foil was done by T-NIL.
Thermal NIL was carried out by using a Compact Nanoimprint (CNI) v2.0, from NIL Technology, allocated in the clean room class 10.000 of the Laboratory of Advanced
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Microscopies (see figure 7). Process and parameters for T-NIL are described in section 2.2.
Figure 7. Images of a) NIL Imprinting chamber, b) control module for T-NIL. Retrieved from [30].
The proposed metals for the coating of the 3D pyramids nanoimprinted in PMMA foil were gold (Au), aluminium (Al) and copper (Cu); with special emphasis on Cu due to their optical properties in the VIS range at the expense of less chemical stability compared to Au counterparts. Whereas all of them have been studied by FDTD analyses, only thermal stability of the PMMA foils during deposition of gold thin layers has been preliminary assessed. As a result, Au evaporation by electron beam (Edwards auto-500 BOC Edw) at 30 mA, 3 10-7 mbar and 5.3 kV were discarded. Instead, Au deposition by sputtering at 30 mA for 82 seconds was made in a high vacuum sputter coating unit (EM SCD500) from Leica available at Servicio de Microscopía Electrónica (SAI-Unizar). This equipment is commonly used for samples preparation for SEM observation.
The characterization of the Si masters, casted PDMS stamp and imprinted PMMA foils was carried out by scanning electron microscopy (CSEM-FEG INSPECT) and profilometry (Profilometer P-6 Stylus Profiler, from KLA Tencor with a 10 µm radius diamond tip).
2.2. General Process Layout to fabricate metallic microstructures on PMMA
The proposed fabrication process would involve three main steps: i) the fabrication of ad- hoc Si mould with inverted micropyramids (described in figure 7 left column); ii) the micropatterning of metal coated PPMA foils (described in figure 7 central/right column);
and, iii) the imprinting of micropyramids on the metal coated PMMA foils by T-NIL.
a) b)
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Figure 7. Schematic illustration of the proposed T-NIL for fabrication of microstructured Metal-PMMA substrates.
Briefly, the Si master with G0 microstructures, inverted micropyramids, is fabricated by anisotropic wet etching of silicon. As a starting point, a silicon (Si) wafer with SiO2
thermally grown is patterned by CF4/O2 plasma etching using a resist mask with a regular pattern of circular openings (2 µm diameter and 5 µm periodicity). The unmasked silicon is anisotropically etched in potassium hydroxide (KOH) in order to produce inverted pyramidal-shaped pits. Subsequently, the resist and oxide mask are stripped in O2 and CF4/O2 plasma etching, respectively. The height of the inverted micropyramids is circa 1.4 microns, i.e. 0.69 x 2 µm of feature diameter, due to the characteristic anisotropic etching of Silicon leading to sidewalls forming a 54.7º angle with the surface.
For the micropatterning of metal coated PMMA films, two different approaches are envisioned because there is no previous experience in the group and potential issues related to i) chemical compatibility of PMMA and photoresists, or ii) metal adhesion on PMMA foils could arise. The first one relies on Au micropatterning by lift-off process
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(described in figure 7 central). On the contrary, in the second approach, the thin metal layer on the PMMA foil is patterned by wet etching of Au using the resist mask with the regular pattern of circles 2 µm diameter and 5 µm periodicity (described in figure 7 right).
In T-NIL, the Si hard mould is placed in the NIL chamber laying on the ceramic stamp carrier where the heating cartridge is enclosed. The mould and the thermoplastic PMMA substrate are put in intimate contact with each other as well as with the ceramic stamp carrier in order to get optimal thermal contact to the imprinting polymer. A Teflon sheet (100 µm thick) is placed between the top of the PMMA foil and the membrane located in the upper part of the NIL chamber. Thus, the membrane is protected from potential PMMA melting and sticking.
To determine parameters for T-NIL process several papers were consulted (see in annex 1, table A.I.1). The imprinting parameters used in this work are 140 °C and 6 bar. In order to promptly reach this temperature, no controlled heating rate is selected and the equipment heats up as soon as it cans, reaching in 2 min approximately the set-point.
These conditions are kept for 10 minutes and then the temperature starts to go down. The release temperature is 80 °C (see figure 8) [31].
Figure 8. Recipe used for PMMA foils were imprinting temperature, pression, time and release temperature were set for the process.
The operating pressure herein used is the maximum available in the lab, which is limited by the pressure of the main compressor-air line fed to the system. The working
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temperature has been selected on the basis of the PMMA glass transition temperature, which is a range in which the polymer changes from a rigid material to a soft material [32]. Under these conditions, there will a viscous flow of the PMMA due to the pressure from the protrusions, and PMMA will flow into the cavities of the Si stamp.
Due to the lock-down period and lab access restrictions, the proposed fabrication layout in figure 7 has not been experimentally tested. Only, some preliminary T-NIL trials with pre-existing moulds on non-metallized PMMA foils has been performed.
2.3. Finite-Difference Time-Domain (FDTD) Simulation
The FDTD method has been applied to a wide variety of engineering and physical problems since it was proposed in 1966 by K. Yee. The FDTD method has been developed over the years [33], being one of the most widespread methods in computational electromagnetism worldwide. In short, the optical properties of a given system (a living cell, an airplane, a nanostructure ...) are obtained through calculations of the EM field, which propagates in a discretized space in discretized time steps, according to Maxwell's equations and the relationships between its components (which indicate how materials respond to the EM field). The FDTD method thus provides the full evolution of the EM field of a given system. This information is subsequently processed to obtain the EM response of the system under consideration.
The FDTD has been extensively used in Nanophotonics, with the goal in understanding optical phenomena at nanoscale, beyond the scope of conventional optics and the diffraction limit of light.
The FDTD method most relevant characteristics is the Yee’s cell, which gives raise to the most common algorithm. In Yee's algorithm the components of the electric field are located at the apex of a cubic cell and the component of the magnetic field is in the center of the cell faces [23].
The resulting finite-difference equations are solved in the so-called “leapfrog” algorithm:
the electric field vector components in a volume of space are solved at a specified instant
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in time; then the magnetic field vector components in the same spatial volume are solved at the next instant in time. This process is repeated over and over again until the desired transient or steady-state EM behaviour is fully evolved [23].
This time-domain method requires a meshing in the whole working region, with a fine enough discretization that allows to resolve the smallest details of the geometry, which considerably increases the simulation time [29]. The FDTD method has important advantages. It is faster and cheaper than prototyping a large number of designs and also allows to verify experimental results. Furthermore, it has important features worth to mentioning [23] as follow:
• Flexible: can simulate arbitrary geometries and different materials like dielectrics, metals, non-linear substances, etc.
• A single simulation in the time domain is enough to get the whole frequency response.
• Several illuminating sources can be used, like plane waves, dipole sources, Gaussian beams, etc.
• Optical properties which describe the physical response, can be retrieved, for instance: points at dispersion relation, curves transmission and reflection coefficients, the frequency domain and field maps.
• Fast method that does not consume excessive computer resources, comparing with other numerical methods
2.3.1. Lumerical Software
FDTD Simulation of different metal coated PMMA substrates with a periodic distribution of mirror-like coated 3D micropyramids was performed using Lumerical Software from Lumerical Solutions. It is a System Suite of system-level simulation products, that offer a width set of capabilities of analysis, which allows to design and optimize the performance of photonic components, circuits, and systems. Some of its common applications are in CMOS (Complementary Metal-Oxide Sensor) image sensor, diffractive optics and metalenses, integrated photonic components and circuits, LED / OLED’s, metamaterials and plasmonics, photovoltaics, photonic inverse design, lasers.
18 2.3.2. Simulated Structures
We used this program based on the FDTD method to calculate the EM field in the micropyramids investigated. The mesh step was varied from 2 nm to a minimum of 0.25 nm in the metallic regions where using a fine mesh is crucial to properly represent the plasmonic modes. The outer boundaries were Perfectly Matched Layers (PMLs). This is because PML absorbs light without create any reflexion. The excitation source was a Gaussian wave-packet containing the relevant spectral weights in the frequency range of interest, in our case for wavelengths ranging from 575 to 825 nm. The electric field of the incident wave was set parallel to the X axis (see figure 9) [29].
The studied microstructure is a dielectric (PMMA) micropyramid, coated with a thin metal layer (Au/Cu/Al) in the upper part. This unit is periodically replicated in an arrangement of pyramids whose individual base diagonal is 2 µm, along 3 cm2 array with 9.9 µm pitch (measured from head to head of the pyramids). The quadrangular 3D coated pyramid used in simulation is illustrated in figure 9.
Figure 9. Metal coated PMMA micropyramid, a) XYZ view of the simulation and different elements like simulation limits, laser source, discretization meshes and monitors, b) XY view of the pyramid showing
the used monitors.
In figure 9 a, it is shown different components needed for the simulation, like simulation limits, laser source, discretization meshes and monitors. In order to take out the results for analysis, monitors were placed in different parts of the pyramid as can be seen in
a) b)
19
figure 9 b (monitors are called as appear in the figure). These monitors represent different planes. The data obtained from every monitor is an average of the EM in that plane for a given wavelength.
The carried-out simulations were named starting with the letter S, followed by a number from 1 to 18. This number is associated to the material and the base size of the metallic pyramidal skin, varying from 500 nm to 1000 nm, that covers the PMMA microstructure.
In all the simulations the thickness of the metal layer is 40 nm. This is shown in table 1.
Table 1. Simulation Matrix
2.3.3. Metal Properties
Figures 10 a) and b) show the experimental dielectric constants of the materials used in this study, that is, Au, Cu and Al. The data were obtained from the Handbook of Materials Science by Lynch [34] , included in the database of material properties of Lumerical.
Figure 10. Dielectric constant of Gold (Au), Copper (Cu) and Aluminium (Al), with their a) real part and b) imaginary part.
a) b)
20 3. Results and discussion
3.1. DSC Analysis of PMMA
The DSC analysis was performed in order to determine the working temperature for the nanoimprinting over the PMMA foil. For this, a running of 3 cycles were made. The heating ramp was 20 °C/min, from 30 to 200 °C, cooling down was at maximum speed for the equipment, same for each cycle. An inert atmosphere of N2 was used. It is commonly stablished that the working temperature be a value between 20 and 50°C above the Tg of the material [35]. DSC Results are shown in figure 11. The event registered in the first cycle at around 75°C, is attributed to water desorption in accordance with the hygroscopic character of PMMA. This endothermic curve only appears during the first heating (cycle 1) and disappear completely in the next two cycles (see figure 11 a).
Figure 11. PMMA DSC analysis: a) standard, b) Derivative curve and magnification in proximity of Tg.
The derivative curve of PMMA DSC test is shown in figure 11 b). The glass temperature (Tg) of PMMA was found to be 113 °C, which is similar to the PMMA Tg found in literature, i.e. 110 °C [31].
3.2. Morphological Characterization
NILT and Ion Milling Si masters
a) b)
21
NILT mould has parallel linear patterns of 3, 5, 10 and 20 µm, with spacing ratios of 1:1, 1:2, 1:3 and height of 0.27 µm [30] (see figure 12 a and b). The Ion-milling Si master contains rectangular pyramids, circa 3 m base size, 5 m height and 9.9 µm pitch approximately (see figure 12 c and d).
Figure 12. SEM images of a) and b) NILT mould (lines of 20 µm with spacing ratio of 1:1). Retrieved from [30]. c) and d) Ion Milling mould.
In order to evaluate the goodness of the pattern replication by T-NIL, tests were performed onto the imprinted substrates.
Profilometry was made over different materials as it can be seen in figure 13. Since
“negative” or inverted pyramidal pattern was needed for T-NIL, an intermediate PDMS stamp (see figure 13 b) was prepared, by casting and release from the Si stamp (see figure 13 a), obtaining a negative well-like shape. The crosslinking process of the PDMS stamp was at 80 °C for 4 hours. Finally, the PDMS stamp were used to transfer the pyramidal pattern to the PMMA foil (see figure 12 c). Based on the profilometry results, it is concluded that the imprinting was not successful, since the height of the PMMA
a) b)
c) d)
22
protrusions (see figure 13 c) is 3 µm, i.e. 60 % of the well depth in PMDS (see figure 12 b). A similar effect is observed on PMMA samples directly imprinted with NILT master (see figure 14a). The thickness and roughness of planar Au coated PMMA foils are shown in figure 14 b.
It is also worthy to mention that all the measurements were performed with a profilometer tip of 5 µm radius. Such dimension, higher than the features size, explain the poor definition of the micropyramids along the scan lines (see figure 13 d) where only the upper part of the pyramid and the interspacing can be inferred. According to figure 13 d, PMMA, and PDMS features seems totally aligned, with corresponding pitch values of around 10 µm.
Figure 13. Profilometry of a) the Ion-Milling Si master, b) the PDMS stamp prepared by casting and release, c) the imprinting PMMA substrate, d) the inverted micropyramids in PDMS and those transferred
to PMMA foil.
Accounting from the results shown, larger imprinting cycles and higher temperatures and pressure settings (11 bar is the maximum affordable with the CNI tool) are required for
a) b)
c) d)
23
the softening and deformation of PMMA to fill completely the cavities of the mould. In addition, direct imprinting of hard Silicon master against PMMA substrate is preferred (see Figure 7) due to the mechanical deformation of the PDMS stamp during the imprinting process. In literature can be find that NIL of PMMA was performed at 170 °C, 40 bar and 8.3 minutes [36] and in another process it was heated up to 200 °C and then held until the temperature decayed below the PMMA's glass transition temperature, at 130 Bar [37]. A list of previous publications dealing with patterning of PMMA by NIL is included in Table AI.1 of Annex I.
Figure 14. Profilometry of a) NILT mould and b) Optical image of metallized PMMA.
3.3. FDTD Simulation Results
Several simulations were performed by Lumerical software with FDTD module as was shown above in table 1. In the present section the results of simulations are presented and discussed.
Figure 15 shows the average electric field intensity (normalized to vacuum) along the metallic edges of the base of the pyramids and for different base sizes and as a function of the wavelength. Strong resonances clearly appear at the edges, and we will see that can be attributed to Edge Plasmon Polaritons (EPP). The resonances in gold are much stronger than in copper and aluminium. From a qualitative viewpoint this behaviour can be easily understood from the optical properties of a SPP. Note however that EPP modes might behave differently than a SPP, being more confined and absorbed the first.
a) b)
24
Figure 15. Averaged electric field intensity (normalized to vacuum) along the metallic edges of the base of the pyramids and for different base sizes and as a function of the wavelength , for: a) Gold (Au), b) Copper (Cu), c) Aluminium (Al). Pyramid bases size in nm, are as follow: red=1000nm; blue=900nm;
green=800nm; gray=700nm; black=600nm; purple=500nm.
a)
b)
c)
25
The propagation length (Equation 1) is higher for gold that for the other two metals in almost the entire range of energies of interest (the propagation constant and skin-depth
are depicted in figure 16).
Thus, plasmons are expected to keep propagating on the edge much more time in the case of gold for wavelengths greater than 650 nm, allowing the formation of a standing wave.
At shorter wavelengths plasmons in Al would be expected to be much stronger, however it is not the case. Here is important to note that in the standing wave formation there are other mechanisms that take place which are controlled by the particular dispersion relation (relation between frequency and wavevector) of EPPs. Also important are the scattering properties of the corners (from which the EPP reflects). However, a complete study of the optical properties of EPPs is out of the scope of this master thesis.
In figure 16 b, it is possible to observe that the plasmon is more confined in Au and Cu, than in Al, for all the wavelengths. Thus, Al is less resonant than Au and Cu and can explain partially the reason why between 500 and 600 nm, being the propagation of Al greater than in Au and Cu, no notable resonances are formed for Al unlike for the others two.
Figure 16. a) Plasmon Absorption Length of Au, Cu, and Al. b) Plasmon Skin-depth of Au, Cu and Al, in air.
We attributed the spectral resonances of figure 15 to the excitation of EPPs. This is further confirmed by Figure 17, where the same data set shown in figure 15 (a) for Au coated micropyramids is depicted. Additionally, near field maps of the electric field for a crosscut passing through the pyramid base are shown, one for each plasmonic resonance.
The standing wave nature of the modes is clearly seen (a sequence of maxima and nodes
a) b)
26
on the electric field) for all the structures. In this regard, Cu behaviour is similar to Au.
Aluminium coated pyramids did not show any standing wave effect, there was EM indeed, but it was weak. Similar representations for Cu and Al are depicted in figure A.II.1 in Annex I. In figure A.II.2, the maps of the amplification of the EM in different points of the pyramid are shown. This analysis was made for S1 to S18 at a λ=633 nm and λ=785 nm, to determine which monitor give better information about resonance in different points of pyramid’s surface.
Figure 17. Resonance. Simulations from S1 to S6 with gold (Au). Pyramids bases size in nm, are as follow: red=1000nm; blue=900nm; green=800nm; gray=700nm; black=600nm; purple=500nm.
Figure 18 shows the spectral location of main resonances for gold and copper. There are two series of resonances clearly related (linked by solid lines) if we observe the near-field maps of Figure 17. They share the same near-field patterns at the edges, showing that the resonant wavelength (r) of each of these modes is a function of the metal pyramid base size (b). If EPPs would behave as simple standing waves r ~ b, but it is not the case. A
27
more comprehensive study would provide the exact dependence; but this aim is out of scope of the work.
Figure 18. Spectral location of main resonances for gold and copper coated PMMA micropyramids as a function of the base size of the metal cover. Lines are as follows: Gold=yellow (full circle-main peak, empty
circle-minor peak); Cupper=orange (full triangle-main peak, empty triangle-minor peak).
Accounting from the simulations, we can infer simple rules of design for 3D metal coated PMMA micropyramids. By considering the intersection of the line lasers available in the confocal Alpha300 Raman spectrophotometer from WITec and commonly used in SERS, i.e. 633 and 785 nm, with the spectral position of main resonances of the EPP modes, the estimation of the optimal base size of the coated micro-pyramids would be circa 800-900 nm range. For these substrate, one or even two resonances could be excited simultaneously upon Raman interrogation
4. Conclusions
In this work, the use of thermal NIL (PMMA) for microstructuring a soft material, i.e.
polymethyl methacrylate is explored as alternative to fabricate more cost-effective substrates in SERS detection. The combination of thermal NIL with the spatial-controlled metallization of pristine PMMA foils by standard lift off, could lead to periodic 3D metal coated PMMA micropyramidal substrates. From the preliminary T-NIL tests, it can be concluded that the use of hard mould against PMMA is preferable due to the mechanical deformation of PDMS stamps at the imprinting conditions. The use of more severe imprinting conditions on PMMA substrates, i.e. temperatures above 140ºC, time above 10 min and pressures near the maximum (i.e. 11 bar), are required to ensure better replication of patterns.
28
The information provided by FDTD simulations about the excited plasmonic modes (EPPs) in the 3D metal coated PMMA micropyramidal supports the application of metallized microstructures on PMMA foils as SERS substrates. Such results have revealed a design rule for metal coated PMMA micropyramids. Thus, it could be determined the metal and the ideal dimensions of the base of metal pyramid for an effective coupling with the Raman laser lines. Thus, Cu PMMA micropyramids with an individual base diagonal of 2 µm, 40 nm of conformal Cu coating on the top and a Cu base size of 800 nm, turns out to be a very good option in order to fabricate low-cost SERS substrates.
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31 Annex I.
Table A.I.1. Consulted papers for T-NIL process.
Ion Milling Stamp.
Distribution of the Si stamp Ion Milling. The part of the Milling used to transfer the patterns is the one that says Square-based Pyramids (See figure A.I.1)
Figure A.I.1. Distribution of different pattern arrays of Ion Milling Master.
Annex II.
32
Spectral result of simulations shows that there is standing wave effect in copper, contrasting with aluminium where there is not a clear standing wave effect (see figure A.II.1).
33
Figure A.II.1. Resonance. Simulations a) from S7 to S12 with copper (Cu) and b) from S13 to S18 with Aluminium (Al). Pyramids bases size in nm, are as follow: red=1000; blue=900; green=800; gray=700;
black=600; purple=500.
In figure A.II.2 it is presented the EM amplification of Simulation 1 (S1) in different points of the pyramid.
Figure A.II.2. EM amplification of simulation S1 (gold, base=500 nm) for λ=633 nm and. λ=785 nm.
This same analysis was made to every simulation from S1 to S18 in both λ=633 nm and λ=785 nm, in order to determine which monitor give better information about resonance in different points of pyramid’s surface.