12 Figure 1.4: Three plasmonic approaches to enhance light absorption a Metal nanoparticles at the front surface of a solar cell.. 17 Figure 1.7: Fabrication process of plasmonic nanostr
OVERVIEW OF PLASMONIC SUBSTRATE
Plasmonic effect
Surface plasmon (SP) is the collective oscillation of conduction electrons stimulated by light at the interface between two media with opposite dielectric constants, such as dielectric and metal, or strongly mixed semiconductor materials Light can couple with
SP when satisfying the conditions of energy and momentum conservation, and the electric field near the interface of metal-dielectric is enhanced [1] Surface plasmons are divided into two types: localized surface plasmons (LSPs) and propagating surface plasmons (PSPs) [2]
Localized surface plasmons (LSPs) are a phenomenon that occurs at the nanoscale, primarily on the surfaces of metal nanoparticles or nanostructures When incident light, usually in the visible or near-infrared range, interacts with these structures, it induces a coherent oscillation of the free electrons in the metal (Fig 1.1) This collective oscillation of electrons results in a resonant response, characterized by strong absorption and scattering of light The resonant frequency of this oscillation is highly dependent on the size, shape, and material properties of the nanoparticle, as well as the surrounding dielectric environment This tunability makes LSPs a valuable tool for controlling and manipulating light at the nanoscale [3]
The electric field associated with the LSPs is highly localized to the vicinity of the nanoparticle, typically within a few nanometers, hence the term "localized." This localization allows for a dramatic enhancement of the electromagnetic field intensity, which is exploited in various applications [4].
Applications of plasmonic substrate
Plasmonic substrates, which typically involve surfaces or structures that support plasmonic excitations, have a wide range of applications across various fields due to their unique optical properties
1.2.1 Surface-Enhanced Raman Spectroscopy (SERS)
Surface-enhanced Raman Spectroscopy (SERS) is a powerful analytical technique that enhances the Raman scattering signal by several orders of magnitude, enabling the detection and characterization of molecules at extremely low concentrations This makes SERS a valuable tool for various applications, including chemical sensing, biomedical imaging, and environmental monitoring.
11 magnitude through the interaction of molecules with plasmonic nanostructures or substrates [5]
Figure 1.2: Schema of the surface-enhanced Raman scattering (SERS) effect originated by the localized plasmon resonance of metallic inter-nanoparticles (NPs)
When light is incident on the substrate containing metal nanoparticles of suitable dimensions, plasmonic effects will occur When molecules (analytes) are in close proximity to or adsorbed onto the surface of these plasmonic nanoparticles, their electromagnetic fields are significantly enhanced (fig 1.2) This enhancement arises from the intense electromagnetic field produced by the collective oscillation of electrons (plasmons) at the nanoparticle's surface The electromagnetic field is highly localized in the immediate vicinity of the nanoparticle, leading to enormous enhancements in the Raman scattering signal of the adjacent molecules [6]
The enhancement in SERS is typically described in terms of enhancement factors (EF), which quantitatively represent the increase in Raman scattering intensity due to the presence of plasmonic substrates Enhancement factors can range from 10 6 to 10 14 [7], making SERS an incredibly sensitive technique for molecular detection
The shape, size (fig 1.3), composition, and arrangement of the plasmonic nanostructures significantly influence the enhancement in SERS[8-12] Nanoparticles with sharp edges or tips exhibit higher enhancement due to stronger electric field localization Aggregates of nanoparticles can also create hotspots with exceptionally high electric field intensities, further amplifying the enhancement Plasmonic substrates used in SERS are typically composed of metal nanoparticles or nanostructures, often made of silver, gold, or other noble metals
The SERS substrate is used to detect low-concentration biological molecules, thus enabling the detection of proteins in bodily fluids Early detection of pancreatic cancer biomarkers is possible using immunological testing methods A multi-channel protein biomarker detection device based on SERS on a conductive nanopore chip is also utilized to predict diseases and crucial biological indicators, enhancing diagnostic opportunities between diseases with similar biological markers like PC, OVC, and pancreatitis This technology has been employed to detect urea in human serum and may become the next- generation technology in screening and detecting cancers
Figure 1.3: Normalized extinction cross-sectional area for spherical gold nanoparticles of diameters ranging from 20–100 nm[12]
With the ability to analyze the components of molecular mixtures at the nano- molecular level, the use of SERS substrates holds significant potential and benefits in analyzing hazardous substances in the environment, pharmaceutical analysis, materials science, art and archaeology research, forensic science, drug and explosive detection, food quality analysis, and detecting single-celled algae Plasmon-sensitive SERS substrates can be employed to quantitatively analyze small molecules in human biofluid waste with high precision, detecting and quantifying biological molecule interactions, and studying oxidative processes at the single-molecule level [13-15]
Plasmonic sensing is a highly sensitive and versatile analytical technique that utilizes the unique optical properties of plasmonic nanostructures to detect and analyze changes in the local refractive index or molecular interactions occurring at the surface of these structures This technique has gained significant attention in various fields due to its ability to achieve label-free, real-time, and ultrasensitive detection of analytes [16] Plasmonic nanostructures exhibit high sensitivity to changes in the refractive index of the surrounding medium Even small alterations in the local refractive index, such as those caused by molecular adsorption, induce noticeable shifts in the LSP wavelength or intensity This sensitivity is harnessed for the detection of analytes in various sensing applications The shift in the LSP wavelength or the change in the LSP intensity can be measured and correlated with the concentration or presence of analytes Molecular binding or changes in the local environment around the plasmonic nanostructure cause variations in these optical properties, allowing for qualitative and quantitative analysis
13 Plasmonic nanostructures, including nanoparticles [17], nanorods [18], nanoshells [19], and more, are designed to have specific shapes, sizes, and surface properties to achieve optimal LSPR responses and enhance sensitivity to analytes These nanostructures can be functionalized or coated to selectively interact with target molecules
The principles of plasmonic sensing are applied in various fields, including biosensing (detecting biomolecules), environmental monitoring (detecting pollutants), food safety (detecting contaminants), healthcare (disease diagnosis), drug development (molecular interaction studies), and beyond [20]
1.2.3 Plasmonic Photovoltaics and Plasmonic Solar Cells
Plasmonic photovoltaics and plasmonic solar cells are fields that leverage plasmonic effects to enhance the efficiency and performance of solar cells Plasmonics involves the study and manipulation of plasmons, which are collective oscillations of free electrons in a material when stimulated by light Plasmonic nanostructures, such as metallic nanoparticles, can concentrate and manipulate light at the nanoscale, enhancing light absorption and trapping, making them promising for improving solar cell technologies
Figure 1.4: Three plasmonic approaches to enhance light absorption a) Metal nanoparticles at the front surface of a solar cell b) Metal nanoparticles embedded inside the cell c) Metal corrugations at the back surface of a cell [21]
The plasmonic structure can be integrated into thin-film solar cells with various configurations to confine light and reduce the thickness of the absorptive layer while maintaining the optical thickness In the first configuration, metal nanoparticles serve as scattering elements with sizes smaller than the wavelength, performing "folding" of light into the absorptive layer (strong scattering of light into the guided modes of the substrate) These particles are located on the surface of the solar cell, scattering and trapping light into the semiconductor thin film due to large-angle and multiple scattering, increasing the effective optical path within the solar cell In the second configuration, metal nanoparticles function as optical antennas, immersed in the semiconductor material (transition layer), generating a plasmonic near-field, enhancing absorption in the vicinity of the particles, and inducing localized surface plasmon resonance The near-field of the particles stimulates the creation of electron-hole pairs within the semiconductor In the third configuration, a nanostructured metal film on the rear surface of the solar cell can couple light into surface plasmon polariton (SPP) modes These modes propagate along the metal/semiconductor interface, confining or trapping light along the boundary to transfer energy to the photogenerated carriers
Plasmonic enhancement is based on the interaction between incident light and plasmonic nanostructures, typically metallic nanoparticles, at the nanoscale The key principle involves the excitation of localized surface plasmons (LSPs) when these nanostructures are illuminated LSPs are collective oscillations of the free electrons at the surface of the nanoparticles, induced by the incoming light The resonant frequency at which these LSPs occur depends on the material, size, shape, and surrounding dielectric environment of the nanoparticles[21]
Enhanced Electric Field: When illuminated at the LSPR frequency, the plasmonic nanostructures generate highly localized and enhanced electric fields at their surfaces
This enhanced electric field extends into the surrounding medium and is significantly stronger than the incident electromagnetic field The concentration of this electric field amplifies light-matter interactions at the nanoscale
Field Localization and Intensity: The electric field generated by the LSPs is tightly confined to the immediate vicinity of the nanoparticles, creating "hotspots" where the electric field intensity is exceptionally high This localization leads to a significant enhancement of light absorption and scattering in the regions around the nanoparticles
1.2.3.2 Enhanced Light Absorption and Trapping
Material and structure of plasmonic substrate
Gold is a cornerstone material in plasmonics, offering unique properties that are harnessed for a variety of applications Its plasmonic resonance occurs predominantly in the visible and near-infrared regions, making it ideal for bioimaging, biosensing, and photothermal therapy Gold nanoparticles, depending on their size, shape, and surrounding environment, exhibit tunable localized surface plasmon resonances (LSPRs), enabling precise control over their interactions with light [23] The exceptional stability and biocompatibility of gold make it a preferred choice for biomedical applications, where it finds use in targeted drug delivery, cancer therapy, and bioassays Gold's ability to enhance electromagnetic fields near its surface has led to advancements in surface- enhanced Raman spectroscopy (SERS) and surface plasmon-enhanced fluorescence, revolutionizing chemical and biological sensing Its easy synthesis and functionalization further contribute to its widespread adoption in plasmonic devices and systems
Silver, renowned for its high electrical conductivity and strong plasmonic effects, plays a pivotal role in plasmonics, especially in the visible and ultraviolet regions [24] The plasmonic resonance of silver nanoparticles can be precisely tailored by adjusting their size, shape, and dielectric environment, offering a rich toolbox for optimizing applications such as sensing, imaging, and surface-enhanced spectroscopies The ability of silver to support surface plasmons with minimal damping has led to ultra-sensitive biosensors for detecting biomolecules at very low concentrations, making it a key material in the field of biosensing Silver nanoparticles, particularly nanorods and nanostars, possess versatile plasmonic properties that facilitate multiple resonances, enhancing applications like photothermal therapy and nonlinear optics The unique interplay of size and shape effects on plasmonic behavior in silver nanoparticles continues to drive research toward innovations and broader applications in the realm of plasmonics
Semiconductor materials such as indium tin oxide (ITO) have garnered attention for their ability to exhibit plasmonic effects in the near-infrared region [25] These materials find applications in optoelectronic devices, transparent conductive coatings, and sensing technologies Graphene, a two-dimensional carbon allotrope, has emerged as a promising material in plasmonics due to its unique electronic and optical properties [26] Graphene plasmons offer tunability in the mid-infrared and terahertz ranges, making them valuable for applications like photodetectors, modulators, and biosensors Dielectric materials, although not traditional plasmonic metals, can also support localized surface phonon polaritons, analogous to plasmons Dielectric resonators and metasurfaces based on materials like silicon and silicon nitride are being engineered to manipulate and enhance light-matter interactions, broadening the range of applications in plasmonics to include meta devices, waveguides, and integrated optics By exploring a diverse range of materials, researchers strive to unlock new possibilities and tailor plasmonic properties to meet specific application requirements, pushing the boundaries of plasmonic technology
1.3.2 Type of structure of plasmonic substrate
Until the present moment, a wide array of substrates featuring diverse nano plasmonic structures have been introduced The structure of these substrates may exhibit either an ordered or disordered pattern, depending on the method of fabrication In the realm of chemical processes, metallic nanoparticles are amalgamated into various forms, contingent on the procedure, precursor, and parameters regulating the synthesis For example, nanostructures encompass spherical, cubic, pyramidal, octahedral, floral nanosphere, necklaces, nanobars, nanocubes, nanoprism bipyramids, nanostars, nanowires [27-34]
Figure 1.6: Representative images of electron microscopy of synthesized Ag nanostructures, demonstrating that diverse sizes and morphologies are made possible by controlling the reaction chemistry (A) Silver nanosphere [27], (B) Silver necklaces [28], (C) Silver nanobars [29], (D) Silver nanocubes [30], (E) Silver nanoprism [31], (F) Silver bipyramids [32], (G) Silver nanostar [33], (H) Silver nanowire [27], (I) Silver nanoparticle embedded silica particle [34]
The dimensions and shapes of these metal nano-particles significantly impact the augmentation of the electric field intensity, consequently affecting absorption and scattering rates [35] Each specific particle shape possesses an optimal size and surface thickness for achieving the most effective field enhancement [36] Excessively large particles allow for multipolar excitation, resulting in non-radiative scattering This is because only dipole shifts lead to Raman scattering Higher-order shifts reduce the overall enhancement efficiency Conversely, particles that are overly minute lose their conductive properties and fail to enhance the field As the size of the particles decreases to a few atoms, the coalescent plasmon effect diminishes since a substantial assembly of electrons is necessary to oscillate in unison [37] The ideal plasmonic substrate ought to display uniformity and a high field enhancement This can be achieved on either a large or minuscule scale, easily observable under high-resolution microscopy [38]
Nanostructures such as nanotips [39], nanopyramids [40], and nanoinverted pyramids [41] are essential components in the realm of plasmonics Nanotips, distinguished by their sharp apexes and high curvature, concentrate electric fields, making them pivotal in applications like Tip-Enhanced Raman Spectroscopy (TERS) and field electron emission On the other hand, nanopyramids with their pyramidal shape exhibit localized plasmon resonances and find utility in Surface-Enhanced Raman Spectroscopy (SERS) and plasmonic sensing by enhancing electromagnetic field confinement at their apexes and edges Conversely, nano inverted pyramids, characterized by their inverted pyramidal geometry, excel in light trapping and scattering, making them valuable in photovoltaics to increase light absorption and in photodetectors to enhance light sensitivity These nanostructured substrates, meticulously engineered and tailored for specific purposes, continue to fuel advancements in sensing technologies, photonics, and renewable energy applications, showcasing their immense potential in shaping the future of plasmonic-based devices and systems.
Manufacturing methods
The wet chemistry processes for reducing precursors to metals have been demonstrated to be effective for large-scale synthesis with low cost, high productivity, and reproducibility to obtain prominent plasmonic properties In the wet chemistry method, the formation of metal nanostructures with various morphologies and sizes can be controlled by manipulating parameters of the reaction process, such as trapping agents, catalysts, reaction solution, time, and temperature For example, the size of silver nanorods can be adjusted from 30 nm to 70 nm by introducing a small amount of sulfide or hydrosulfide into the reaction solution or by modifying the silver precursor [42] The length of nano-rods changes from 120 nm to 250 nm by adjusting the precursor concentration or reaction time [43] The diameter of nanowires can be tuned from 100 nm to 300 nm with an increase in the initial precursor concentration [44] Chemically synthesized metal nanostructures can be utilized for surface plasmon resonance (SPR) or surface plasmon polariton (SPP) effects, and the plasmonic properties are dependent on the morphology of the structures The chemical synthesis method can control the physical parameters of nanostructures, enabling control over their plasmonic properties for various applications
However, these methods may not precisely position the plasmonic structures as desired, limiting their effectiveness in certain applications
Electron beam lithography (e-beam lithography) is a technique that can be used to fabricate nanostructures with the desired high resolution, which may not be easily achievable using other methods With its ability to precisely fabricate high-resolution plasmonic structures, electron beam lithography allows the study of fundamental properties of the enhanced electric field phenomenon created by metallic nanostructures through simulated design The principle of electron beam lithography involves using a high-energy electron beam directed at an electron-sensitive polymer layer Under the influence of the electron beam, the polymer layer can either dissolve in a developer solution or remain unaffected The dissolved polymer layer is referred to as a positive- tone polymer, and common polymers in this form include PMMA (Poly Methylmethacrylate), EBR-9, PBS (Poly Butene-1 Sulphone), and ZEP (a copper polymer of a-chloromethacrylate and a-methylstyrene)
Figure 1.7: Fabrication process of plasmonic nanostructures using electron beam lithography: (a) Substrate, (b) Electron-sensitive polymer coating, (c) Etching and visualization, (d) Metal coating and (e ) Remove the polymer layer
In contrast to traditional photolithography, electron beam lithography utilizes an electron beam and does not require a physical mask to create the pattern The electron beam is directly projected onto the sample surface, and coils are used to scan the electron beam to draw the desired details The electron beam's wavelength ranges from a few nanometers to several hundred nanometers, providing high resolution The fundamental steps of the electron beam lithography technique in manufacturing bowtie plasmonic antennas are illustrated in Fig 1.7, including the etching process, metal deposition, and lift-off To create the desired plasmonic nanostructures, the first step involves patterning a polymer coating by exposing it to the electron beam, as shown in (a)-(c) After structuring the pattern, a metal such as gold, used in plasmonic applications, is deposited (Figure (d)) Finally, the polymer and metal layers on the surface are removed, leaving the desired plasmonic nanostructure as shown in (e) [45]
Focused Ion Beam (FIB) is a technique commonly used to fabricate highly precise structures based on a focused beam of ions accelerated at high energy and controlled to converge onto a small spot using electrostatic or electromagnetic lenses [46] In principle, FIB has a structure similar to a scanning electron microscope Modern focused ion beam devices consist of two beams: an ion beam for fabrication operations and a narrow electron beam used for imaging and direct observation of the working process Gallium ions (Ga) are often used because Ga is easily vaporized and ionized from liquid Ga metal [47] The Ga ions are heated to vaporize and ionize, then accelerated and converged into a narrow ion beam using electromagnetic lenses (or electrostatic lenses) The common acceleration voltage used ranges from 10 to 50 kV, and the ion beam can be converted into a small beam with an area ranging from a few nanometers The electron beam functions like a scanning electron beam in an electron microscope, scanning the surface details to capture images through recording secondary electron signals
Figure 1.8: The process of fabricating plamonic structure using FIB
FIB devices operate based on principles similar to sputtering When a focused ion beam with high energy scans the surface, the kinetic energy of the ions causes the atoms of the solid material on the surface to be instantly ejected The depth and width of the ejected solid material depend on the acceleration voltage and the intensity of the ion beam The current intensity of the ion beam can vary from tens of picoamperes to tens of nanoamperes To shape the details, the ion beam is controlled and scanned (similar to scanning an electron beam to create images on a screen or in lithography) To protect the fabricated details from destruction by the ion beam, a layer of platinum or tungsten (often mixed with carbon for easier vaporization) can be coated These layers can form the shapes of the required components and details through the control of the lens system Figure 1.8 illustrates the process of fabricating plamonic structure using FIB [45]
Although physical methods such as e-beam lithography or focused ion beam can easily create plasmonic structures at specific positions and allow control over the size and shape of the structures, both methods are relatively costly Therefore, scientists have explored an alternative method, known as imprinting method
Imprinting technology, an ancient method for reproducing writings on suitable surfaces, has evolved significantly since the 1990s [48,49] Injection molding, a form of imprinting, has been employed in the production of compact disks (CDs) Recently, the semiconductor industry has shown interest in imprint-related techniques due to the necessity for mass-producing future microelectronic circuits with critical dimensions potentially as small as a few nanometers Traditional photolithography is expected to face limitations at this deep nanometer scale due to optical diffraction and material constraints Currently, integrated circuits (ICs) already have a minimum feature size of less than 50 nm, and existing manufacturing systems are highly sophisticated and costly Consequently, the semiconductor industry is exploring alternative patterning methods to align with Moore's law, predicting technological evolution
The history of imprint technology as a lithography method for pattern replication dates back to the 1970s, with significant advancements attributed to the research group led by S Chou in the 1990s [50] Since then, nanoimprint lithography has gained popularity and garnered substantial interest from both the research and industrial sectors Various innovative approaches have emerged alongside mainstream scientific advancements Chou et al.'s initial proposal focused on mass-producing high-density
21 magnetic storage media and demonstrated the feasibility of fine structure patterning at a nanometer-scale resolution, now known as nanoimprint lithography
Nanoimprint lithography involves surface structuring using a template with topographic patterns Post-imprinting, the patterns need to be transferred to achieve various functionalities Nanoimprint, as a lithography method, is fully compatible with standard micro-fabrication techniques, encompassing different transfer processes like etching, lift-off, selective re-growth, or diffusion Typically, a thin resist layer is deposited on the substrate and then imprinted, resulting in a contrast in thickness
In the imprinting process, a polymer sheet or spin-on polymer is heated above its glass transition temperature and pressed with significant force using a stamp (mold) The choice of stamp material depends on the desired feature sizes and the materials involved The top and bottom heaters are gradually heated above the glass transition temperature of the polymer Typically, the imprint temperature exceeds the glass transition temperature (Tg) of the polymer by 20-50°C Only thermoplastic polymers can be imprinted, as they can be dissolved in suitable solvents for spin coating and can deform under applied forces During the heating phase, the hot embossing chamber is evacuated to about 5 mbar Once the polymer reaches the imprint temperature, a contact force is applied The magnitude of this force depends on factors like stamp area, polymer type, and feature geometry The contact force is maintained until the heaters reach the de-embossing temperature, which allows the stamp to be separated from the polymer cleanly and reliably The total cycle time for a hot embossing process varies greatly based on the equipment's heating and cooling capabilities, typically ranging from 3 to 20 minutes [51]
Figure 1.9: The process of the nanoimprinting method[51]
The quickest imprinting processes are isothermal, where the imprint and de- embossing temperatures of the substrate are identical In this case, the heaters maintain a constant temperature throughout The polymer is in a sufficiently fluid state to enable rapid imprinting upon contact with the template and application of contact force De- embossing occurs outside the chamber Demonstrations have achieved cycle times as short as 2 minutes, yielding high-resolution features as small as 50 nm on a 200 mm Si substrate using an EVG520HE hot embossing equipment.
Microactuator for controlling the stamp
Microactuators are the unsung heroes of modern technology, silently powering numerous devices and systems that impact our daily lives Their small size and precision make them invaluable in applications ranging from consumer electronics to aerospace, healthcare, and beyond These miniature workhorses enable intricate movements and fine adjustments that are often taken for granted but are integral to the functioning of many devices
At their core, microactuators are designed to convert various energy sources into mechanical motion or force This conversion is achieved through a range of mechanisms, each suited to different applications and requirements Some of the common types of microactuators include piezoelectric actuators, electrostatic actuators, thermal actuators, shape memory alloy (SMA) actuators, microfluidic actuators, magnetic actuators, and electromagnetic actuators [60] Piezoelectric actuators, for instance, rely on piezoelectric materials that deform when subjected to an electric field These materials change shape at the atomic level, allowing for precise and rapid motion, often in the micrometer or nanometer range These actuators are widely used in applications where high precision and responsiveness are crucial, such as in scanning probe microscopy and optical devices like adaptive optics
The design and structure of microactuators are tailored to their intended use Key components include a substrate, actuation mechanism, microstructures The substrate serves as the foundation, providing support and stability for the actuator It is often made from materials such as silicon, glass, or various semiconductor materials, depending on the application
The actuation mechanism is the heart of the microactuator, responsible for generating motion or force The choice of mechanism depends on the specific actuator type and its requirements Microstructures, such as microcantilevers, microbridges, microvalves, micromirrors, microgrippers, and microlevers, directly respond to the actuation mechanism, allowing for the desired mechanical motion
The actuation mechanism is the heart and soul of any microactuator It determines how the energy input is converted into mechanical motion or force, defining the fundamental operation of the device The choice of actuation mechanism is pivotal and is made based on the specific requirements of the application, such as the need for precision, speed, force, or reliability
23 Piezoelectric materials, such as lead zirconate titanate (PZT), are the foundation of piezoelectric actuators When an electric field is applied to these materials, the individual crystal structures within them change shape at the atomic level This atomic rearrangement results in macroscopic expansion or contraction of the material [61] Conversely, when mechanical stress is applied to a piezoelectric material, it generates an electric potential difference, creating an electrical charge This intrinsic behavior makes piezoelectric materials ideal for precise and rapid actuation Piezoelectric actuators typically consist of piezoelectric elements arranged in specific configurations, such as stacks, benders, or tubes The application of voltage to these elements leads to deformation or bending, depending on their geometry and design
Figure 1.11: Principle of the piezoelectric actuators [61]
Piezoelectric Stack Actuators:These actuators consist of multiple layers of piezoelectric material stacked on top of each other When a voltage is applied, the stack expands or contracts along its length, producing linear motion Stack actuators are commonly used in applications where precision and high force are required, such as in nanopositioning stages and scanning probe microscopes
Piezoelectric Benders: Bending piezoelectric actuators are designed as elongated strips of piezoelectric material When a voltage is applied, they bend, producing angular displacement Benders are often used in micro-optomechanical systems (MOMS) for tilting mirrors and adjusting optical components
Piezoelectric Tubes: These actuators take the form of cylindrical tubes, which expand or contract radially when voltage is applied Piezoelectric tubes are used in applications like fluidic control, where they control the flow of liquids within microdevices
Piezoelectric Ring Actuators: Piezoelectric rings exhibit radial expansion or contraction when subjected to an electric field They find applications in areas like microvalves and micropumps, where precise fluid control is necessary
The piezoelectric actuators are indispensable components in the realm of micro actuation, known for their precision, responsiveness, and versatility These microactuators play a pivotal role in an array of applications, from microscopy and optical systems to microfluidics and vibration control However, the fabrication and integration of PZT materials into microstructures are challenging and not practical under real-world conditions in Vietnam Therefore, this type of actuation mechanism was not employed within the scope of my thesis
Thermal actuators operate on the fundamental principle that materials expand when heated and contract when cooled This thermal expansion or contraction can be harnessed to create mechanical motion [62]
Common materials used in thermal actuators include shape memory alloys (SMAs), bimetallic strips, and thermally responsive polymers Shape memory alloys, in particular, are often employed due to their ability to return to a predetermined shape when subjected to a specific temperature change This property, known as the shape memory effect, allows for precise and repeatable actuation.Thermal actuators consist of these materials in various configurations, such as wires, beams, or cantilevers When a specific component within the actuator is heated or cooled, it undergoes thermal expansion or contraction, causing the actuator to move or deform
Shape Memory Alloy (SMA) Actuators: These actuators utilize SMAs, such as nickel-titanium (NiTi) alloys, which change shape when exposed to a specific temperature range SMAs are commonly used in applications that require precise and repeatable motion, such as medical devices, where they control the movement of surgical instruments, or in micro-robotics, where they provide reliable actuation for miniature robotic systems
Bimetallic Actuators: Bimetallic strips consist of two different metals bonded together These strips bend when heated due to the different thermal expansion coefficients of the two metals Bimetallic actuators are employed in applications like thermostats and automotive temperature control systems
Thermally Responsive Polymers: These actuators use polymers that change shape in response to temperature changes When heated, these polymers can expand or contract, producing motion Thermally responsive polymers are used in various applications, including microvalves for controlling fluid flow
The research situation in Viet Nam and the objectives of the thesis
With many novel optical properties, nanometal structures are receiving active research interest from scientists both domestically and internationally They can be applied for various purposes, ranging from enhancing the efficiency of solar energy conversion to applications in biomedical sensors, environmental monitoring, and food safety The physical basis of these applications relies on the surface plasmon resonance effect in metal nanostructures Furthermore, modifying the structure and materials of plasmonic nanostructures to control their optical properties has opened up possibilities for manufacturing customized components and enhancing applications in nano science and technology
In Vietnam in recent years, research and fabrication of plasmonic nanostructures have been actively pursued in various universities and research institutes nationwide Metal nanostructures or nano composites containing plasmonic metals have been successfully fabricated in colloidal form using chemical synthesis methods and applied in fields such as biomedicine, environmental science, and catalytic enhancement for hydrogen production A prominent example is the core-shell nano particle structure, with different shapes and material combinations, successfully fabricated by the NanoBioPhotonic group at the Institute of Physics, Vietnam Academy of Science and Technology The thermal optical properties of these particles, based on the stimulated plasmon effect, were investigated to determine their application potential in diagnostics, cancer treatment, and enhanced Raman scattering
Regarding Surface-Enhanced Raman Spectroscopy (SERS) applications, the efficiency, stability, and reproducibility of plasmonic structures in solution are typically not high The current trend involves depositing plasmonic nanostructures on flat substrates or substrates containing nanostructures The research group led by Prof Dr Dao Tran Cao at the Institute of Materials Science, Vietnam Academy of Science and Technology, has successfully fabricated SERS substrates in the form of flower and leaf- shaped silver nanostructures or silver nanoparticles on porous silicon substrates [52,53] These substrates exhibit high enhancement factors and the ability to detect pesticides, plant protection chemicals, and harmful substances in water at very low concentrations, showcasing the diverse potential of this field Additionally, the research group led by Assoc Prof Dr Ngo Quang Minh at the Vietnam Academy of Science and Technology has also studied the ordered arrangement of metal nanostructures, creating a resonant surface plasmon effect for applications in smart plasmonic components and sensors using silver plasmonic structures fabricated by direct laser ablation combined with sputtering [54]
Research on plasmonics is also a key focus of the Laboratory for Micro- nano systems technology and Nanophotonics at the Department of Electronic Materials and Devices Under the guidance of Prof Dr Chu Manh Hoang, both surface plasmon phenomena, propagating plasmons applied in waveguides and localized plasmons applied in nano-optical antennas and plasmonic substrates, have been studied, with simulation results and fabrication achieved using microfabrication technology
Following the group's previous contributions, the objective of the thesis is to design and simulate in order to determine optimal parameters for both the microactuator and the plasmonic nanostructure when employed in the mold imprinting process The goal is to develop a successful manufacturing process for plasmonic structures with nanoscale dimensions The resulting plasmonic structures should exhibit good resonance effects, as demonstrated by their ability to enhance Raman signals Successfully accomplishing this topic will open up numerous applications of plasmonic substrates in solar cells and material analysis
DESIGN AND SIMULATION OF A MICROACTUATOR FOR
Microactuator for controlling the mold
As presented in Chapter 1, the microactuator structure is designed for controlling the mold composed of a serpentine spring system suspending a mold and the actuation structure (Figure 2.1) The actuation structure here uses the electrostatic capacitor type actuation structure In this section, the operation characteristics of the spring system will be simulated, and the electrical properties of the capacitor-type actuation system will be calculated in the following section
Figure 2.1: The model of the proposed microactuator
2.1.2 Operation characteristics of a serpentine spring system
2.1.2.1 Simulation of the serpentine spring structure
For an electrostatic actuation system, the frequency of the microstructure's configuration significantly impacts the performance of the microactuator Operating the electrostatic actuation system at or near its resonant frequency can substantially enhance its performance It allows for larger displacements at the low input voltage Additionally, the electrostatic actuation system may have multiple resonance modes (directions of structural oscillation), each associated with a different natural oscillation frequency Therefore, investigating and understanding the resonance frequencies and mode coupling of the structure is of utmost importance In this section, Three microstructure configurations will be simulated, consisting of 2, 4, and 6 serpentine springs with varying dimensions, to determine the optimal parameters for the microactuator (Figure 2.2)
Figure 2.2: The microstructure with: 2 (a); 4 (b); 6 serpentine springs (c)
To conduct a theoretical analysis of mechanical properties or surface plasmon resonance, various methods have been developed, such as the finite-difference time- domain (FDTD) method, boundary element method (BEM), discrete dipole approximation (DDA), multipole method (MMP), and finite element method (FEM) In this thesis, FEM simulation is employed due to several advantages over other methods FEM simulation can utilize tetrahedral elements or even curved elements, avoiding staircase artifacts typical in FDTD These artifacts pose a significant issue for FDTD simulations For accurate FDTD simulations, especially in the case of plasmonic structures, a fine mesh of 0.5 nm or less is required, demanding substantial memory and computation time.BEM and MMP methods encounter challenges at boundaries where more than two domains intersect Therefore, simulating multi-layered models or the presence of substrates becomes difficult with BEM and MMP, whereas this is easily accomplished with FEM
The Finite Element Method (FEM) is a numerical computational technique employed to approximate solutions to integral and differential equations in various engineering fields, such as structural analysis, fluid mechanics, heat transfer, biological modeling, electromagnetic field interactions, and various other physical phenomena The fundamental idea of FEM, like other numerical methods such as Finite Difference Method (FDM), begins with discretization, which involves dividing the computational domain into smaller elements and transforming the differential equations into approximately equivalent algebraic equations that can be solved on a computer
FEM addresses problems in two main steps:
Step 1: Discretization of the model by dividing the structure into smaller elements Step 2: FEM utilizes various approximation techniques, such as the Rayleigh-Ritz method, weighted residual methods, Galerkin method, least squares method, and variational methods These techniques are applied to each element, and the results are then aggregated over all elements to form a system of linear equations
31 COMSOL Multiphysics is a finite element analysis, solver, and simulation software package for various physics and engineering applications It allows engineers and scientists to simulate physical phenomena and study the behavior of different systems Especially, it is designed to handle multiphysics simulations, which means it can model and simulate coupled physics phenomena This is crucial for understanding real-world scenarios where multiple physical processes interact
The physical properties of the structures will be simulated using COMSOL software as part of this thesis
2.1.2.3 The boundary conditions and grid partitioning
The boundary conditions employed in this simulation consist of two conditions:
Free boundary condition: u(t,0) = 0 (The leading end of the structure is free to move, experiencing no external forces or constraints.)
𝑑𝑡(𝑡 0) = 0 (The leading end of the structure is fixed, with no free motion and no free forces acting on it.)
The three structures differ only in the number of beams, disc parameters, and specific beams as shown in the following table:
Rp 70 àm Radius of the disc g 20 àm Length of the conecting beam w 2-20 àm Width of the beam t 2-20 àm Thickness of the beam α 5°-50° opening angle
R1 R + g Radius of the first beam
R2 R1+g+w Radius of the second beam
R3 R2+g+w Radius of the third beam
Figure 2.3: Parameters of the structure
The structure is meshed with a minimum element size of 0.2 àm and a maximum element size of 10 àm The total number of elements for the 2-beam structure is 5000, for the 4-beam structure is 9000, and for the 6-beam structure is 13000
Figure 2.4: The structure comprises four beams after grid division
As previously explained, three structures will be simulated with parameters w, t, and α varying, while keeping the other two parameters constant
Figure 2.5: The simulated images of the behavior in the z-mode and the closest mode of the following structures: 2 springs (a-b); 4 springs (c-d); 6 springs (e-f)
Figure 2.5 depicts the results of simulating the behavior of the spring structures in mode z and the nearest neighboring mode It can be observed that in structures 4 and 6, the nearest neighboring mode of the spring is not a tilting mode in one direction, but rather a mode involving a deviation from the surface axis (Figure 2.5 d,f)
For convenience in processing and analyzing the results, I uses the following notation: f 2 , f 4 , f 6 represent the oscillation frequencies in mode z of springs with 2, 4, and
6 symmetric curved beams, respectively; Δf 2 , Δf 4 , Δf 6 represent the deviation between the frequency of mode z and the nearest neighboring mode of the springs with 2, 4, and 6 symmetric curved beams This deviation is calculated as a percentage, according to the following formula: Δfi = (fm – fi) / fm, where fm is the frequency of the nearest neighboring mode, and fi (i = 2, 4, 6) is the frequency in mode z, as defined earlier
The figure 2.6 illustrates the results of simulating the frequency response in the z- mode of three structures In each case, the trend in frequency variation of all three
33 structures is the same The frequency of the 2-spring structure is always the lowest, and the 6-spring structure always has the highest frequency This is because, when increasing the number of springs (in the case of springs connected in series), both the stiffness and the mass of the spring increase However, in this case, the increase in stiffness predominates over the increase in mass, leading to an increase in frequency
Figure 2.6: The operation frequency of three spring types with: w from 2-20 μm, in which t and α are fixed at 10 μm and 50°, respectively (a); t from 2- 20 μm, in which w and α are fixed at 10 μm and 50°, respectively (b); α from 5° - 50°, in which w and t are both 10 àm
When the width w increases from 2-20 μm (thickness t = 10 μm; α = 45), the frequencies of all three spring systems increase and peak at 12 μm (f 2 = 140 kHz, f 4 = 190 kHz, and f 6 = 230 kHz), and then exhibit a slow decrease (figure 2.6 a) This is because changing the width of the spring is equivalent to changing its mass distribution and stiffness The natural frequency of the spring is determined by its mass and stiffness characteristics When the width of the spring increases, the stiffness increases, which can compensate for the effect of increased mass Initially, stiffness becomes dominant, and the natural frequency begins to rise This is because a stiffer structure responds more quickly to applied forces, resulting in a higher natural frequency Then, when the width approaches a limit where the increase in stiffness will dominate the effect of increased mass, the highest natural frequency for that specific vibrational mode is reached, and at this point, the mass becomes dominant, causing the frequency to start decreasing A similar scenario occurs when the thickness t increases, and the frequencies of all three springs increase significantly and then decrease slowly, reaching the highest frequency when t = 20 μm (f 2 = 200 kHz, f 4 = 270 kHz, f 6 = 320 kHz) (figure 2.6 b) In the final case, when the angle alpha increases from 5-50, the frequencies of all three structures decrease (at α = 50, f 2 = 140 kHz, f 4 = 190 kHz, and f 6 = 220 kHz) This is because an increase in alpha corresponds to an increase in the length of the spring, and the length of the beam represents the lever arm (the distance from the fixed support to the point where the force is applied or where vibration occurs) A longer lever arm results in a larger lever arm for the applied force, leading to a decrease in the beam's stiffness As the length increases, the beam's ability to resist bending and deformation decreases Additionally, the frequency in the structure is directly proportional to its stiffness A longer spring is less stiff than a shorter one, and this decrease in stiffness leads to lower natural oscillation frequencies
The operating frequencies of the z-mode and the nearest mode vary with structural parameters, including width (w), thickness (t), and angle (α) For a fixed w and α, Figure 2.7a illustrates that t significantly affects the frequency difference, with larger t resulting in a smaller difference In contrast, for fixed t and α, Figure 2.7b shows that w has a negligible impact on the frequency difference Furthermore, Figure 2.7c demonstrates that α also plays a role, with larger α values resulting in a larger frequency difference.
The degree of frequency splitting in all three structures is illustrated in Figure 2.7
In all three cases, when varying the parameters: (a) for w, (b) for t, and (c) for α, we observe that Δf 4 and Δf 6 exhibit fairly close values, while Δf2 has significantly lower values When w ranges from 2-20 μm, all Δfi values increase until they reach a certain saturation point Δf 4 and Δf 6 provide values greater than 30% when w is in the range of 6-
20 μm, while Δf 2 only reaches values over 30% at w = 2 μm, 4 μm, and 20 μm When t increases from 2-20 μm, all Δf i values tend to decrease For t = 2-10 μm, both Δf 4 and Δf 6 have values greater than 40%, whereas for t = 2-20 μm, Δf 2 consistently yields values below 21% As α increases, all three values tend to decrease until reaching a specific threshold, after which they increase Δf 2 reaches its threshold faster (at α = 10°), while Δf 4 and Δf 6 reach theirs at α = 45° In the case of α = 5°-50°, both Δf 4 and Δf 6 consistently provide values greater than 38%, while Δf 2 yields values less than 21%
2.1.3 Calculation of capacitor type actuator
A model of plasmonic nanostructure based on the imprinting process
With the microactuator structure proposed in the above section, the imprinting process can generate the plasmonic substrate in the form of the hole as shown in Figure 2.10
Figure 2.10: The plasmonic substrate in the form of hole
As presented in Chapter 1, following the imprinting process, when nanostructures are formed on the polymer substrate, the metal sputtering process will be applied to create a complete plasmonic structure With this plasmonic structure, changes in shape, size, and material will result in corresponding variations in the resonance effects
PDMS (Polydimethylsiloxane) is commonly used in imprinting processes due to its unique properties It is a flexible and transparent silicone elastomer that can easily conform to the surface patterns of a mold Its low surface energy and non-stick characteristics make it an excellent material for creating replica molds in soft lithography and microfabrication techniques Silver is a commonly used material in investigating the resonant properties of structures in visible light due to its unique characteristics, as elucidated in Chapter 1
Therefore, in this section, I will investigate the resonance properties of a structure comprising a PDMS base with a nanostructured hole coated with silver (Figure 2.11)
Figure 2.11: Images of 2D and 3D structures
2.2.2 Operation characteristics based on FEM simulation
2.2.2.1 Differential equations, parameters, and mesh division
To complete the construction of the boundary value problem, in addition to the differential equation, boundary conditions must be provided Moreover, the Wave Optics module also offers various boundary conditions to reduce the computational domain For instance, periodic boundary conditions can be applied, utilizing only a single unit cell of the periodic structure Alternatively, symmetric boundary conditions, such as perfect electric conductor (PEC) or perfect magnetic conductor (PMC) boundaries, constrain the electric and magnetic fields perpendicular to the boundary
For perfect electric conductors or perfect magnetic conductors (infinite conductivity), the corresponding electric or magnetic field inside will be zero In this case, the boundary condition equations can be simplified to:
The Perfect Electric Conductor (PEC) boundary condition can be employed to approximately describe an object as a metal, with the advantage of avoiding the penetration of fields into the interior regions of the object Additionally, the PMC (Perfect Magnetic Conductor) boundary conditions are also used to represent planes of symmetry for electric or magnetic fields
The parameters of the structure are presented in the table below:
Symbol Value Definition λ 633 nm Wave length d0 500 nm Diameter of the air column hc 200 nm Thickness of the hole α 10°-110° Bottom-hole angle d 2*hc*tan(α/2) Diameter of the hole t 15 nm Thickness of silver layer
39 hair λ Height of the air hsub λ Height of DPMS theta 0 Elevation angle
The grid size plays a crucial role in determining the accuracy of the simulation In this simulation, the minimum grid size is 20nm, and the maximum grid size is 300nm The void structure utilizes a grid with smaller and thicker elements than the air columns and the PDMS base The total number of grid elements is 85,070
In this section, I investigate the collimation properties of the hole structure at various opening angles ranging from α = 10°- 110° The images depicting the electric field intensity at corresponding angles are illustrated in Figure 2.13
The electric field intensity within the hole increases gradually as the angle alpha (α) increases, reaching a maximum at α = 90° Subsequently, it sharply decreases as alpha continues to increase The resonance positions also vary with changes in the α When α °, there are two small resonance points at the intersection angle between the plane and the leg of the hole (Fig 2.13a) As the α increases, additional resonance positions emerge, gradually shifting towards the bottom of the hole The electric field intensity at these resonance points also increases, peaking at α = 90° (Fig 2.13e) When α = 110°, the resonance points virtually disappear (Figure 2.13f)
Figure 2.13: Distribution of electric field intensity with a plasmonic substrate of 200nm height at various opening angles α: 10° (a); 30° (b); 50° (c); 70° (d); 90° (e); and 110° (f) at a wavelength of 600nm
To provide a more detailed analysis of the electric field intensity, I measured the electric field intensity from the PDMS base to a location on the hole surface (as indicated by the red line in Fig 2.14)
Figure 2.14: Determination of Electric Field Intensity Location
41 Figure 2.15 illustrates the electric field intensity at the selected position in various structures at different α When α increases from 10° to 90°, the electric field intensity within the hole increases gradually from 35MV/m to 135MV/m
Figure 2.15: The electric field intensity at the selected location in structures at angles α: 10°
The distribution of the electric field within the hole also changes, with the position of the electric field starting to shift from the middle of the hole towards the Ag tip At α
= 90°, the electric field intensity begins to rise immediately from the Ag tip and reaches its maximum near the hole opening Conversely, α = 110° the electric field intensity within the hole sharply decreases, reaching only 49 MV/m
This can be explained by the fact that increasing α leads to an enlargement of the surface radius of the hole Initially, as the surface radius increases, the surface area and volume of the hole also increase, enhancing the ability to support plasmon resonance However, when the surface radius becomes excessively large, this effect may diminish significantly due to inefficient energy transmission during plasmon propagation Reflection from the surface structure can also contribute to energy loss, reducing the efficiency of plasmon resonance This can prevent the structure from achieving optimal plasmon resonance conditions, resulting in a decrease in plasmon resonance effects once the surface radius exceeds a certain threshold Therefore, it can be observed that with a 15nm thick silver layer and a pit depth of 200nm, the optimal alpha angle is 90° to generate a hotspot.
Conclusion
In this chapter, a microactuator model is proposed and simulated for application in the imprinting process, along with the simulation of the plasmonic structure The simulation results indicate:
• The operational frequency range of the microstructure is quite broad, spanning from 25 kHz to 632 kHz as the parameters vary Structures with 4 and 6 springs also exhibit notable anti-coupling properties, with anti- coupling exceeding 30% across most of the investigated parameter range The cylindrical capacitor structure is also calculated, revealing relatively low operation voltage as the structure displaces from 100 nm to 1 μm
• The plasmonic substrate in the form of a hole with a specific thickness and size will yield a hotspot Specifically, with a silver layer thickness of 15 nm and a pit depth of 200 nm, a pit angle of 90° will generate a hotspot This structure has the potential for developing plasmonic substrates.
STUDY AND FABRICATION OF PLASMONIC SUBSTRATES
Process of the imprinting method
The imprinting process consists of two crucial steps The first one is to form an imprinting mold, which is used as a template to generate structures with similar size and shape in a substrate In this thesis, the imprinting mold is an array of the silicon nanotips
In the second step, the imprinting mold is used as a stamp to create hole structures for sputtering the Ag metal layer to form plasmonic substrate This section will elaborate on the methods, procedures, and outcomes of fabricating the imprinting mold
3.1.1 Process of fabricating imprinting mold
As previously explained, the photolithography method is employed, in which two sequential processes are used: photolithography and anisotropic wet chemical etching (Fig 3.1)
Figure 3.1: Fabrication process of imprinting mold using photolithography combined with anisotropic wet chemical etching: (a) oxidation; (b) photolithography; (c) SiO 2 etching; (d) remove resist; (e) Silicon etching; (f) remove SiO 2
The photolithography process consists of 8 fundamental steps: primer spin coating, photoresist spin coating, soft bake, exposure, post-exposure bake, pattern development, post-bake, and inspection ( Fig 3.2)[55]
In the following, the specific procedures will be presented:
In the photolithography process, the mask is an indispensable component that crucially determines the shape of the photoresist mask and the silicon dioxide mask subsequently
In this thesis, masks of various shapes are designed and manufactured to optimize tip formation These masks are designed to match the dimensions of the UV beam as well as the frame that holds the mask in the optical engraving machine Autocad 2010 and Correl 6.0 software are employed for mask design Circular masks range in size from 20 μm to 70 μm in diameter (Fig 3.3)
Figure 3.3: Design of mask Preparation for photolithography
Photolithography requires that the surface of the wafer be exceptionally clean to prevent defects and reduce photolithography efficiency In this process, the wafer needs to be completely free of any organic impurities and metal ions to ensure the quality
45 First, the wafer is immersed in a solution of H2SO4 (98%) and H2O2 (in a 1:1 ratio) This solution helps to remove organic impurities from the wafer's surface When preparing this solution, a significant amount of heat is generated Therefore, the wafer is immersed in this solution until it reached room temperature Subsequently, the wafer is thoroughly rinsed with deionized water and dried using a centrifuge At this stage, the wafer is ready for photolithography
Coating with Adhesive and Photoresist Material
After treatment, the wafer is placed in a spin coating machine to apply a luminescent material to its surface The photoresist material is a polymer sensitive to ultraviolet radiation and is coated onto the surface of the oxide layer on a silicon substrate within the spin coating equipment During the coating process, the substrate is held on the support of the spin coating equipment under vacuum conditions The photoresist material serves as a protective layer for the sample's surface to prevent it from the effects of chemical solutions
First, the wafer is spin-coated with an adhesive using a spin coating machine at a spin speed of v = 3000 revolutions per minute (rpm) for a time of t = 30 seconds to enhance the adhesion of the substrate and the photoresist material Subsequently, the wafer is spin-coated with a positive photoresist, at a spin speed of v = 3000 rpm, for a time of t = 30 seconds
After the coating process, the photoresist layer often retains residual stress and solvent with a content of up to 15% To eliminate residual stress or solvent in the photoresist layer while enhancing its adhesion to the substrate surface, in this thesis, the wafer is soft-dried at a temperature of T = 90°C for a duration of t = 90 seconds
After initial annealing, the sample is subjected to exposure During the exposure process, photolithography machine PEM-800 with a wavelength of 365 nm and a resolution of 1 μm is utilized The exposure time is t = 60 seconds
The wafer is subjected to heating at a temperature of 90°C for 5 minutes after exposure to enhance the adhesion properties of the photoresist before the development process
After illumination, the process of revealing hidden images within the photosensitive layer formed during the exposure step results in the formation of a positive image In this process, developer PD 238 is used
After the formation process, the sample is dried at a temperature of T = 120°C for a duration of t = 20 minutes to eliminate any unnecessary residual photoactive substances
After the lithography process, the wafer is inspected using a microscope
As previously outlined, following the photolithography process, the sample is etched to create the silicon nano-tip structure This procedure consists of two fundamental steps: the formation of a Silicon dioxide mask and the creation of the nanotip structure
After the photolithography process, the wafer's surface is patterned with photoresist regions The research group uses these regions as masks for etching SiO2 The wafers are then wet-etched in a BHF solution (5:1) with an etching rate of v = 100 nm/minute [56], for a duration of t = 3 minutes
Subsequently, the wafers are rinsed successively in acetone, ethanol, and DI water for a period of t = 10 minutes to remove the photoresist residue
The final step in creating silicon nanotips is to etch the silicon wafer anisotropically The wafer is affixed to a Teflon tube and etched in a 30% KOH solution at a temperature of T = 70°C for an etching time of t = 20-40 minutes
3.1.2 Process of fabricating plasmonic substrate
As previously described in Chapter 1, the plasmonic substrate component in the nanoimprinting process is fabricated by spin-coating polymer onto a pre-existing substrate In this thesis, I employ two types of polymers, namely PDMS and MAP 1215 to create two distinct plasmonic substrate variations Below is the procedure for each type (Fig 3.4)
Figure 3.4: Imprinting process: (a) coating polymer; (b) mold-substrate contacted; (c) removing mold; (d) sputtering silver
PDMS is mixed with a curing agent at a corresponding ratio of 10:1, and the polymer mixture is allowed to rest for 30 minutes at room temperature to remove air bubbles
Once the preparation is complete, the polymer mixture is applied onto a previously prepared substrate To ensure uniformity of the membrane, the spin coating method is
47 employed The substrate is coated with the polymer mixture at a spin rate of v = 1300 revolutions per minute for a duration of t = 20 seconds To increase the thickness of the PDMS membrane, please repeat the spin coating step once Subsequently, the PDMS membrane is left to rest for 1 hour at room temperature to remove any remaining air bubbles
The substrate coated with PDMS is heat-treated at a temperature of T = 80 °C for t
= 10 minutes to temporarily stabilize the membrane
Results and Discussion
3.2.1 Results of the fabricating mold
After the design, the mask is printed on film, and the results are shown in Figure 3.5 The designed mask has dimensions ranging from 20-70 μm (a-f) However, only the
30 μm mask has a circular shape, similar to the designed mask In the other size ranges, most masks have a hexagonal shape This is believed to be due to limitations of the printing technology, and the 30 μm size represents the minimum resolution of the printing device
Figure 3.5: Mask printed with diameter: 20 àm (a), 30 àm (b), 40 àm (c), 50 àm (d), 60 àm
A 30-micrometer mask has been chosen for use in the subsequent experimental steps
After the photolithography process, a photoresist mask is created (Fig 3.6a) It can be observed that the photoresist structures (pink color) have dimensions ranging from 29-
31 àm and have a similar shape to the printed mask structures The green background is composed of silicon dioxide, with a thickness of approximately 300 nm, and the black dots are a result of the quality of the printed film
As previously explained, following the photolithography process, the silicon dioxide layer is selectively etched in a BHF solution (5NH4F: 1HF) to create the mask before etching the silicon Figure 3.6b illustrates the structure of the silicon dioxide mask formed These structures (green color) have a size and shape similar to that of the photoresist mask The background of the wafer has turned pale yellow, indicating that it has undergone etching into the silicon layer This also demonstrates that silicon dioxide masks have been successfully created
Figure 3.6: The substrate after photolithography process (a); The substrate etched in solution
This silicon dioxide mask is utilized for the subsequent etching process
After creating the SiO2 mask, the substrate is etched to produce silicon nanotips in 30% KOH solution at a temperature of 70°C
Figure 3.7: Substrate after 30 mins (a); 35 mins (b) and 40 mins etching in KOH solution
At etching time t = 30 minutes (Fig 3.7a), the silicon dioxide mask layer (indicated by the faint circle) is still present At t = 35 minutes (Fig 3.7b), most of the silicon dioxide mask has been removed, and the nano-tip structures have formed (shown in black) At t
= 40 minutes (Fig 3.7c), all the structures have lost the SiO2 mask layer
Figure 3.8: The SEM image of the substrate after 30 mins (a); 35 mins (b) and 40 mins etching in KOH solution
The SEM images of the tip undergoing corrosion for time durations of 30, 35, and
40 minutes are depicted in Figure 3.8 (a), (b), and (c) In the period at t = 30 minutes (a), the tip structure still retains the SiO2 layer on top, indicating that the tip apex remains relatively large At t = 35 minutes (b), the tip structure has lost the SiO2 mask, and the silicon tip apex becomes sharp, with a well-defined shape By t = 40 minutes (c), the SiO2 mask is completely removed, and the tip structure has collapsed.
Figure 3.9: Silicon nanotip after 35mins etching,
Figure 3.9 illustrates the SEM image of a silicon tip after 35 minutes of etching in a KOH solution The substrate of the tip measures approximately 20 μm, while the tip's dimensions are around 263 nm
51 These results align with the experimental findings on the etching rate of silicon in a KOH solution conducted by Ummikalsom Abidin and colleagues Initially, the structure undergoes etching, forming channel-like hole as shown in Figure 3.10 Subsequently, the structure continues to etch horizontally, reducing the tip's apex until it reaches a size small enough for the KOH solution's capillary forces (assisted by mechanical stirring during the etching) to remove the SiO2 mask However, prolonged etching in KOH leads to further erosion of the tip's apex and overall tip collapse
Figure 3.10: Shape of the etch profiles of a oriented silicon substrate after immersion in an anisotropic wet etchant solution
From the above results, the corroding nano-tip structure within 35 minutes is utilized as the mold for imprinting process
3.2.2 Result of fabricating plasmonic substrate
After the mold pressing process, surface structure images are captured using the Kruss 3000 optical microscope
Figure 3.11 illustrates plasmonic substrate-coated PDMS after the imprinting process with contact times of 20 minutes (Fig 3.11 a), 30 minutes (Fig 3.11 b), and 40 minutes (Fig 3.11 c), respectively
The outcomes reveal that at a 20-minute contact time (Fig 3.11 a), the holes have not been distinctly formed, and some hole perimeters are fragmented, attributed to the incomplete solidification of the polymer blend At a 30-minute contact time (Fig 3.11 b), the holes are clear and well-defined, and their dimensions correspond to the base size of the silicon tip array At a 40-minute contact time (Fig 3 11 c), although the hole perimeters are bolder, they exhibit blurriness, and certain hole positions fail to maintain their shapes This is attributed to the excessively prolonged solidification time, during which the separation of the mold from the plasmonic substrates could have disrupted the tip array structure
Consequently, exposing the Pyrex substrate coated with the polymer blend to the silicon wafer for 30 minutes obtains the best results This substrate was subsequently silver-coated to examine the Localized Surface Plasmon Resonance (LSPR) effect
Figure 3.11: Pyrex substrates coated with a polymer blend after the imprinting process with contact times of 20 minutes (a), 30 minutes (b), and 40 minutes (c), respectively
The image of the plasmonic substrate using PDMS after silver sputtering is depicted in Figure 3.12 It can be observed that the structure after sputtering maintains the sharpness of the mold-pressed structure, with a hole size ranging from 5-30 àm The color of the substrate exhibits a noticeable change, indicating the successful deposition of the silver layer onto the surface of the plasmonic substrate
Figure 3.12: The surface of the plasmonic substrate use PDMS
The thickness of the PDMS layer and the silver plating layer is measured using a DektakXT Stylus Profiler device The PDMS layer (black) has a thickness of approximately 7.5 μm (3.13b), while the silver layer (white) has a thickness of about 80 nm (3.13c)
Figure 3.13: The plasmonic substrate after sputtering (a); the thickness of PDMS layer (b); the thickness of Ag layer (c)
Figure 3.14: The depth of the hole structure after sputtering
The depth of the hole structures after sputtering are depicted in Figure 3.14 These hole have depths ranging from 300 nm to 1.2 μm Based on the obtained results, it can be asserted that through the combined process of imprinting and plating, silver hole structures with a thickness of 80 nm, depths ranging from 300 nm to 1.2 μm, and widths from 5 μm to 30 μm have been successfully fabricated
When using photoresist MAP-1215, the process outlined in section 2.2.1.2 is nearly optimized Figure 3.15a illustrates the surface of the substrate after silver sputtering The hole structures are quite sharp and range in size from 2.5 àm to 20 àm The thickness of the MAP-1215 layer and the silver layer is depicted in (Figure 3.15b) and (Figure 3.15c) The thickness of the MAP-1215 layer is approximately 2.2 àm, while the silver layer is around 80 nm thick
Figure 3.15: The substrate after sputtering (a); the thickness of MAP- 1215 layer (b); the thickness of Ag layer (c)
The thickness of the hole structures after silver sputtering on the MAP-1215-based plasmonic strucstures ranges from 200 nm to 890 nm The results indicate that by using MAP-1215 after the imprinting process, along with silver sputtering, it is possible to fabricate silver structures with a thickness of 80 nm, depths ranging from 200 nm to 890 nm, and widths between 2àm and 20àm
Figure 3.16: The depth of the hole structure after sputtering
The plasmon resonance properties of the fabricated plasmonic substrate (structures with identical silver layer thickness, depth, and bottom size) are determined through Raman scattering spectra using a à-Raman device (Renishaw inVia micro-Raman) The condition for enhanced Raman scattering effect is that the wavelength of the excitation laser falls within the plasmonic resonance range of the plasmonic substrate Absorbed and scattered photons generate photons with different energies from the incident photons, and the enhancement effect is further intensified when the scattered photons also have the capability to excite plasmon resonance
In this section, both types of plasmonic substrates formed from a PDMS layer with a sputtered Ag layer and a MAP-1215 layer with a sputtered Ag layer are investigated Therefore, for convenience in presenting the results, the plasmonic substrate formed from a PDMS layer with a sputtered Ag layer is abbreviated as substrate 1, and the plasmonic substrate formed from a MAP-1215 layer with a sputtered Ag layer is abbreviated as substrate 2
Figure 3.16 illustrates the Raman scattering spectra of Silicon substrates, substrate
Conclusion
In this chapter, the experimental procedure for fabricating both the mold and plasmonic substrate are presented The mold structure was successfully fabricated with a tip size of approximately 200nm and a bottom size of about 20 àm
The plasmonic substrate formed from the hole-patterned PDMS or MAP-1215 layer with a 80 nm sputtering Ag layer were also successfully fabricated, featuring silver hole structures with depths ranging from 100-1.2 μm, widths from 2-30 μm
The silver hole structures exhibited excellent plasmonic enhancement properties, with the ability to enhance intensity up to 10 5 times compared to conventional silicon substrates The plasmonic substrate formed from the hole-patterned MAP-1215 layer demonstrated superior enhancement compared to the plasmonic substrate formed from PDMS, attributed to its smaller thickness leading to lower photon absorption
Thesis "Study and fabrication of plasmonic nanostructures based on nano imprinting" has fundamentally achieved its initial objectives The following are the results that the thesis has attained during the implementation process:
• An overview of plasmonic effects, common nano-plasmonic structures, and typical fabrication methods, including wet chemical, electron beam lithography, and imprinting, is presented Subsequently, research directions for the fabrication of nano-plasmonic structures using the imprinting method are derived
• Microactuator structures are designed and simulated using electrostatically actuated honeycomb patterns, which are characterized by high noise immunity and applied to enhance the precision of imprinting processes A hole-patterned plasmonic substrate is proposed, the operating characteristics of the plasmonic substrate at a wavelength of 633 nm are simulated, and an optimal set of parameters is determined to generate high plasmonic resonance
"hotspots" for applications in enhanced Raman scattering
• A fabrication process for a nano-plasmonic structured substrate based on the nanoimprint lithography process is proposed The results demonstrate that the hole-patterned plasmonic substrate produced exhibits high plasmonic resonance characteristics, enhancing Raman signals up to 10 5 times compared to the Silicon substrate
The thesis has successfully completed the initial stage of the plasmonic substrate fabrication process using the imprinting method The next step will involve the fabrication of microactuator structures based on simulation results This research direction is significant as it has the potential to support various important areas of life, such as energy harvesting, environmental sensing, and the biomedical field
1 Nguyen Quoc Chien, Nguyen Van Duong, Vu Ngoc Hung, Chu Manh
Hoang“SpringArrangement Effect on Mode Coupling to Out- Of-Plane Oscillation electrochemical detection of bacterial pathogens” , The 12th NationalConference on Solid State Physics & Materials Science, 2021, 527-530
2 Nguyen Quoc Chien, Nguyen Van Duong, Nguyen Trung Dung, Tran
Trong An, Chu Manh Hoang* “Operation mode order in microactuators using serpentine spring”, International Conference on Advanced Materials and Nanotechnology, 2022, 52-
3 Nguyen Quoc Chien, Nguyen Van Duong, Chu Manh Hoang*,“Investigating the effect of mode coupling in microactuators using serpentine spring”, The 7th International Conference on Applied and Engineering Physics, 2021
4 Nguyen Quoc Chien, Nguyen Van Duong, Luu Thi Lan Anh, Vu Ngoc
Hung, Chu Manh Hoang*, ‘Fabrication of plasmonic nanostructures based on imprinting process’, The 12th NationalConference on Solid State Physics & Materials Science, 2023
5 Nguyen Van Duong, Nguyen Quoc Chien, Dang Van Hieu, Chu Manh Hoang*, “Performance Analysis of Serpentine Springs Compliant to Out-Of-Plane Oscillation” Journal of Theoretical and Applied Mechanics, 60, 1, 2022, 91-101
6 Nguyen Van Duong, Nguyen Quoc Chien, Dang Van Hieu, Chu Manh Hoang* “Design and performance analysis of a mechanically coupled spring compliant to out-of-plane oscillation” Archive of Mechanical Engineering, 69, 4, 2022, 629–643
Nguyen Quoc Chien, Chu Manh Hoang* “Three-dimensional microdisplacement, micro/nano pattern etching and printing system” Intellectual Property Office of Viet Nam, Vietnam Ministry of Science and Technology, 2022, 19763w/QĐ-SHTT
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