Stretching the Horizon of Nanosphere Lithography

Abstract

Periodic nano-structured superlattice with symmetry breaking along the surface as well as thickness is of high importance in plasmonic research due to its immense potential in bio-medical and environmental sensor applications. The structures are rich with electromagnetic hotspots and anisotropic optical properties contributing to the sensitivity of the device. In nanotechnology, nanosphere lithography (NSL) is a promising lithography technique that is in the limelight of research for the last decade due to its simplicity, scalability, and cost-effectiveness. Despite many advantages, the technique is limited in its versatility when it comes to real application. Conventional NSL offers less scope for structural variation with the most common six-fold symmetric structure as the nanosphere mask self-assembles in a hexagonal close-pack fashion due to the very nature of this process. Research efforts have been made to overcome the bottleneck. The conventional NSL approach is modified in combination with other techniques. In this chapter, we discuss the evolution of the NSL technique to achieve maturity. The chapter emphasizes modern NSL formalism associated with techniques like multistep shadow mask deposition and anisotropic etching to achieve 2D and 3D nanostructures with controlled shape, size, inter-structural gap, aspect ratio, sharpness, and special and vertical symmetry. The chapter will serve as an effective knowledge base for further research development in plasmonics, magnonics, and spintronics.

1. Introduction

Nanosphere lithography (NSL) is a popular and cost-effective technique used to fabricate periodic patterns of nanoscale structures on a substrate using monodisperse polystyrene nanospheres as mask. In this technique, one or more layers of nanospheres are deposited onto a substrate followed by a thin monolayer or multilayer coating of target material such as metals and polymers. The material is deposited only in the spaces between the nanospheres. Later, the nanospheres are removed, leaving a periodic array of nanostructures on the substrate.

The NSL technique has been the research highlight for several decades due to its unbeatable advantages over other lithography techniques. In nanotechnology, conventional lithography (CL) is a popular tool used to fabricate various nanostructures [1, 2, 3]. However, in the case of photolithography, it is tricky to obtain nicely resolved structures in the sub-micron range. E-beam lithography has higher resolutions, but the technique fails when it comes to large-area patterning. Moreover, CL is not the right candidate for patterning 3D nanostructures (i.e., structures having asymmetry along thickness). Such a 3D structure can be patterned using a focused ion beam, but again it is limited to small-area patterning. In short, these techniques are expensive, and sensitive to cleanroom conditioning, hence, not suitable for large-scale industrial applications. On the contrary, NSL [4, 5, 6, 7, 8] is a package with all the qualities in it. It offers a simple alternative to realize large-area patterning of 2D and 3D nanostructures in a cost-effective manner and does not necessarily require special cleanroom conditioning. In fact, the entire process can be carried out using standard laboratory equipment and does not require much time to perform. NSL can produce high-resolution patterns with feature sizes down to a few nanometers, which is comparable to the electron beam lithography (EBL). It is a versatile technique and does not involve the use of harsh chemicals, making it a greener alternative compared to its peers.

Despite the advantages, the technique has some bottlenecks which are limiting the realization of this technique in modern technology to its full potential. The uniformity of nanosphere mask assembly is the key to successful patterning. However, the self-assembly process is stochastic and sensitive to various environmental parameters such as temperature, humidity, hydrophilicity, and pH, which can affect the quality and reproducibility of the patterns. Defects and dislocations present in the nanosphere mask are carried forward to the patterned structure. NSL is typically limited to flat substrates, such as glass or silicon wafers, which can be problematic for applications that require patterned features on non-planar or curved surfaces. Material compatibility of the substrate for NSL is an also issue. Another concern to the NSL is its limited control of structures. In other words, the technique cannot produce a wide range of nanostructures. Most conventionally, it is used to create triangular crystals with hexagonal symmetry as the technique is based on nanosphere masks which self assembles in hexagonal close-packed (HCP) order [8, 9].

Research efforts have been made to overcome these challenges. Recently, significant development has been made over the conventional NSL approach to achieve complex 2D/3D nanostructures over a large area. For instance, a symmetry breaking along the thickness through non-uniform etching has been observed by Darvill et al. [10]. Myint et al. [11] observed crystal structures from three-fold azimuthal symmetry deposition. The development enhances the appeal and relevance of the NSL technique in plasmonic sensors, magnonic memory, optoelectronics, and biomedical applications. This futuristic evolution of the NSL technique is called “stretching the horizon” in the title of this chapter. The chapter emphasizes modern NSL formalism associated with techniques like multistep shadow mask deposition and anisotropic etching in order to achieve 2D and 3D nanostructures with controlled shape, size, inter-structural gap, aspect ratio, sharpness, and spacial and vertical symmetry.

2. History of nanosphere lithography

The journey of NSL started back in early 1983. Deckman and Dunsmuir [12] first published the fundamental concept of NSL which they called “natural lithography” by using monolayers of self-assembled ordered or random polystyrene spheres as a mask for patterning a substrate with nanoscale features. Later in 1995, Hulteen and Duyne [8] demonstrated the concept in detail for periodic arrays of particles creating triangular patterns with hexagonal symmetry connected or separated from each other through vertical material deposition. From that time the technique is famously known as “nanosphere lithography”. In 1996, Winzer et al. [9] applied this technique to fabricate nano-dot and nano-ring arrays. The next year, periodic nanoholes with tunable diameter were demonstrated by Haginoya et al. [13] using reactive ion etching (RIE) assisted reduction of nanosphere mask followed by material deposition. Later in 1999, [14] size-tunable nanoparticles and nano-cluster arrays are created by varying the size of the nanosphere and the thickness of the deposited material. The concept of a bi-layer mask NSL is also introduced in this paper. In a venture to create ultra-flat nanostructures through NSL, Frey et al. [15] combined this technique with ultra-flat template stripping [16] in the year 2000. In this technique, a nanosphere mask is deposited on an ultra-flat mica substrate followed by material deposition. The nanosphere is removed leaving the bi-layer pattern on the mica similar to conventional NSL. In the next step, the pattern was covered by the deposition of a thin film adhesive layer and the subsequent application of epoxy on the top. Finally, mica is stripped off from underneath exposing an ultra-flat material pattern embedded in the matrix of the adhesive layer on the epoxy substrate. Further in the journey, NSL has evolved and experimented in association with other techniques. In 2004, more complex periodic structures are obtained using shadow NSL which is a union of shadow mask deposition and NSL [17]. In this study, structures are created by the controlled rotation of nanosphere shadow by tilting and rotating the substrate in the collimated shower of depositing material atoms. In 2005 arrays of rings, dots, and rods are demonstrated by shadow NSL with a deformed nanosphere mask obtained through annealing treatment [18]. In 2006, high aspect ratio nanopillars are created by NSL coupled with deep reactive ion etching (DRIE) [19]. The approach is based on the reduction of the size of the nanosphere mask followed by the DRIE of the substrate in the presence of the reduced mask. The technique is improvised and extended to create 3D hierarchical nanostructures like arrays of hollow cylinders, and step-like (solid or hollow) cylinders with decreasing diameter by performing multistep etching assisted NSL [20]. In 2007, Nano crescent [21] structures are created using nano template lithography (NTL) a simple extension of NSL. In this technique, instead of a large number of assembled nanospheres, individual nanospheres act as a template for angled deposition. The technique is further investigated for antenna application [22, 23, 24]. In 2015, a new variant of NSL called Moiré NSL (M-NSL) is developed which introduced in-plane rotation of neighboring monolayers creating meta-surfaces [25]. In the following years, the capability of M-NSL is expanded to create various superlattices with different materials aimed at diverse applications [26, 27, 28, 29]. The concept of NSL is utilized to create ordered arrays of hollow nanospherical particle elements [30]. Self-assembled polystyrene nanospheres template is coated with fluorinated tin-oxide solution by slow drop casting. The liquid is dried, and the sample undergoes several steps of annealing to obtain the structure. One of the recent works on NSL finds a technique to pattern bridge networks between nanostructured arrays called nano-bridged NSL (NB-NSL) [31]. Further, step-wise formation of patterns using shadow mask deposition-based NSL is developed to precisely control the shape, size, gap between features, and overall symmetry of the complex 2D and 3D nanostructures [11, 32] using the azimuthal and polar angle of the shadow mask as a control parameter. As further extension of this work angular acceleration of a continuously rotating substrate has been introduced as functional control to the vertically asymmetric unique 3D structure [32]. Darvill et al. [10] described a scheme of etching-assisted NSL for structural symmetry breaking along the thickness. The concept is based on anisotropic etching induced by the temperature gradient in the etching chamber.

The scope of NSL is not limited to the creation of a library of nanoarray structures. The knowledge base developed in this journey finds numerous opportunities for application-oriented research in various fields including plasmonics [4, 33, 34, 35, 36], photonics [37], magnonics [38, 39, 40, 41, 42], optoelectronics [43], and biomedicine [44, 45, 46, 47, 48] which will be discussed in the application section.

3. Methods

In the last section, we have seen that the NSL technique is employed in various configurations in combination with several other techniques. In that sense, there is no unique recipe for NSL. However, the most standard form of NSL involves material deposition in the presence of a self-assembled nanosphere mask. Another form of NSL involves the etching of the nanosphere mask and the substrate. The process involves the following steps which are schematically presented in Figure 1.

Figure 1.

Schematic diagram of the standard and etching-assisted NSL process along with an example of nanosphere lithographed structures (labeled as 1–6) at the bottom row [10, 11, 20, 21, 32]. (Ref 10—Creative Commons Attribution 3.0 Unported License, Ref 11—Creative Commons Attribution 4.0 license, Ref. 20, 21—Incorporated with permission and Ref 32—license under CCBY 4.0).

In the first step, the substrate for instance Si(/SiO₂) wafer is cleaned using acetone and isopropyl alcohol (IPA) in an ultrasound bath, then washed with distilled water and dried by N₂ gas flow. Then the wafer undergoes Ar/O₂ plasma cleaning to eliminate organic impurities from the substrate and enhance its hydrophilicity. This treatment substantially affects the mobility of nanoparticle mask units, enabling them to distribute uniformly over the large surface of Si. In the second step, a solution of micro/nanobeads and ethanol is drop-casted on the substrate and allowed to be self-assembled in an HCP structure. Finally, the material is deposited usually by a sputterer or evaporator. Following the deposition, the sample undergoes ultrasonication in IPA to lift off the nanosphere masks and any material that has been deposited on them. The deposited material grains that can penetrate the gaps of the nanosphere assembly are left on the substrate as a lithographic structure as shown in the orange-shaded region of step 3 in Figure 1.

For etching-assisted NSL, the initial two steps remain unchanged. However, in the third step, the sample undergoes a two-step etching process instead of deposition as shown in the blue-shaded region of step 3 in Figure 1. Firstly, the nanospherical masks are treated with oxygen plasma in the RIE chamber. The polystyrene is etched by the oxygen plasma while the silicon substrate remains unaffected, causing the diameter of the beads to decrease. The diameter of the beads and their separation are determined by the etching duration. Subsequently, the exposed silicon is etched in the RIE/DRIE chamber while the silicon masked by the polystyrene beads remains unetched, resulting in a periodic nanostructured surface profile. Finally, the loosely bound polystyrene mask is removed from the substrate by immersing the sample in IPA and subjecting it to ultrasonic vibrations for 5 min. Figure 1 displays a series of lithographed structures produced with NSL that are identified as 1–6 in the bottom row.

The hexagonal symmetry is an intrinsic property of the conventional which originates from the close packing of the nanospheres. When the nanospheres are deposited onto the substrate, they self-assemble into a HCP arrangement due to the minimization of the free energy of the system. This arrangement provides the highest packing density with the least amount of void space between the spheres. The self-assembly process is driven by the intermolecular force factors between the nanospheres and the interface, such as van der Waals force, Marangoni force (originating from the surface tension difference at the interface), zeta potential (ζ) electrostatic interaction forces. These forces cause the nanospheres to aggregate and form a well-defined pattern on the substrate [49, 50]. The dynamics of self-assembly is a stochastic process having partial control over external parameters. Large area monolayer assembly of nanosphere can be achieved by optimizing a few control parameters. For instance, the temperature of the substrate can be optimized to control the evaporation rate of the solution drop-casted on it. Further, a rotating substrate with suitable angular speed applies centrifugal force on the nanospheres just sufficient for the close-pack monolayer formation. The self-assembly process is significantly influenced by the molar ratio of the solvent and the substrate’s surface area. The solvent allows the micro/nanobeads to move until it evaporates, after which the particles become stationary. To ensure complete coverage of the surface with a monolayer particle mask, the volume of the drop-casted solution and the surface area of the substrate are meticulously selected to attain an appropriate particle density per unit square area. A higher concentration of micro/nanobeads leads to the formation of multilayers, while a lower density results in the creation of uncovered surface areas or islands. Apart from multilayer and island formation self-assembled nanospheres exhibit various lattice defects. Line defect is one of the most common defects which looks like a missing line in a matrix of nanospheres. Domain formation is another common form of defect where small domains of hexagonal lattices are formed with relative angle to each other. Apart from that, point defects (i.e., missing particles in a matrix array), random agglomeration, dislocations, and the uneven gap between particles are also observed.

4. Shadow-mask deposition assisted NSL

The deposition of a shadow mask is a powerful method that allows for the creation of a limitless range of structural patterns with distinct features such as size, shape, inter-structural gaps, and symmetry. The deposition angle in relation to the mask is a crucial factor in governing the direction and shape of the shadow, which ultimately determines the resulting structural pattern. In this technique, both azimuthal (θ) and polar (φ) angles are used as control parameters. The scanning electron microscope (SEM) image of a nanosphere mask arranged in HCP order is presented in Figure 2a, with the inset displaying the image at a higher magnification. Figure 2b explains the scheme of shadow mask deposition. The substrate on which the nanosphere is placed has a slope φ with respect to the horizontal plane. The red line indicates normal to the substrate and the vertical black line indicates the deposition vector. Hence, the red and black lines make the same angle φ between each other indicating the polar rotation of the substrate. Figure 2c illustrates the shadowing effect of a single nanosphere where green corresponds to the deposition of the substrate whereas red corresponds to the deposition of the nanosphere creating a shadow on the substrate.

Figure 2.

(a) SEM image of nanosphere mask placed on a Si substrate, with the scale bars of the image and inset measuring 10 and 2 μm, respectively; (b) schematic diagram of the shadow mask deposition setup; and (c) Shadow of a single nanosphere [32]. (Ref. license under CCBY 4.0).

The shadow structures can be simulated by various tools like Matlab or the light tracing software, Pov-Ray (The Persistence of Vision Ray-tracer). In this chapter, we use Matlab to simulate the structures. A program has been developed to solve equations related to straight lines and a group of spheres on a planar surface. The broken straight lines (Figure 2c) signify the shower of metal atoms with certain θ and φ, whereas the spheres symbolize the micro or nanobeads mask unit, and the plane corresponds to the substrate. When there is a real solution between a sphere and a straight line, it means that the evaporated beam is hitting the nanosphere (red), while an imaginary solution indicates that there is no contact between them (green). The first scenario corresponds to shadowing, while the latter implies that deposition on the substrate has occurred. In the latter case, the straight line and substrate plane are further solved to determine the coordinates of the deposition.

4.1 Single-step NSL

Figure 3a shows a matrix of nanostructures with systematically varying θ and φ. The largest pattern is observed for the column φ = 0 having surface exposure of 7.59% of the substrate area. The patterns become smaller with increasing φ as we move from the left to the right of the row for each value of θ. For instance, at φ = 40 degrees and θ = 0 degree the exposure reduces to 0.62%. When the value of φ is increased, the deposition vector becomes more slanted in relation to the substrate. As a result, a greater portion of the substrate is shaded by the nanosphere, resulting in a smaller pattern size. The curves in Figure 3b show the variation in the exposure % as a function of φ for θ = 0 and 90 degrees. The latter (θ = 90 degrees) has a larger exposure over the range of φ. The theoretical value of the minimum tilt angle for which the nanosphere completely covers the substrate is defined as the extinction angle (φ). For the two perpendicular θ (=0 and 90 degrees) φ_e is calculated as 46 and 51 degrees respectively. The importance of this parameter is that for shadow mask deposition-assisted NSL the magnitude of φ must be lower than φ_e. The pattern is hexagonal symmetric for φ = 0 (i.e., normal incidence) as shown by the white lines in Figure 3. The pattern remains exactly the same for any angle θ meaning the configuration is azimuthally symmetric. The symmetry breaks for a nonzero value of φ. The elements are shifted such that some neighboring structures get closer, and some get farther from each other giving rise to a polarized pattern as indicated by the red arrows in Figure 3.

Figure 3.

(a) Simulated structure matrix with systematically varying θ and φ. (b) Exposured area (%) with varying φ at θ = 0 degree and θ = 90 degrees.

4.2 NSL with varying φ

Here we show 3D nanostructures with step-like shapes that are achieved by varying the angle φ while keeping θ constant, as illustrated in panels 1 and 2 of Figure 4. The inset in each figure shows a magnified image of the lattice. Panel 1 of Figure 4 displays SEM images of the four steps of deposition, in which φ is varied as 10, 15, 20, and 25 degrees at constant θ (Figure 4a 20 degrees, 4b 0 degree and 4c 45 degrees). The resulting structure consists of four curved arm triangles of different sizes and aspect ratios, which are superposed with a gradual shift. Each deposition step serves as a building block to create a vertically asymmetric 3D nanostructure. Panel 2 of Figure 4 presents the simulated result, which agrees with the experimental observation. Different colors are used to indicate the four depositions (D1–D4) in this figure. In contrast, when the deposition is carried out by continuously varying φ, rather than in steps, unique 3D structures with smoothly varying surface topology are formed, as shown in panel 3 of Figure 4. These structures were simulated for constant θ, with φ varying from 0 to 30 degrees in 1 degree steps at a constant angular velocity. A further variation of shape can be obtained by introducing angular acceleration to the rotating substrate.

Figure 4.

The deposition of shadows in four steps, with different values of φ (10, 15, 20, and 25 degrees), keeping θ constant at (a) 20 degrees, (b) 0 degree and (c) 45 degrees. The scale bars in the figure and in the insets in panel 1 correspond to 2 μm and 500 nm, respectively. Panel 2 and 3 show the simulation result for four steps of φ angle and continuously varying φ angle deposition [32]. (Ref. license under CCBY 4.0).

4.3 Two-step NSL with varying θ

Lattice structures obtained from two-fold azimuthal symmetry are shown in Figure 5. Unlike the previous case, this configuration performs a two-step deposition with φ as constant and θ varying as θ_i and θ_i + 180 degrees, where θ_i stands for the θ of the first deposition. Panel 1 and 2 correspond to the experimental and simulated results which are consistent with each other. The patterns break the conventional hexagonal symmetry with the rearrangement of elements rather explained by linear symmetry. The values of θ and φ are mentioned for each figure. Figure 5a shows pairs of triangles arranged in a zigzag pattern around regularly spaced line gaps. In Figure 5b and c, the unit cell structures are made up of four triangles combined. Figure 5d displays square structures that are intersected by line gaps. Figure 5e exhibits a mixture of square and triangle shapes, while Figure 5f is composed of a combination of star and triangular arrays.

Figure 5.

Two-step deposition for varying θ (θ_i and θ_i + 180 degrees) at constant φ. The φ value for figure (a–c) is 15 degrees, and for figure (d–f) is 25 degrees. Panels 1 and 2 show the SEM and simulation results respectively. In panel 1, the scale bars displayed in the figures and insets are 2 μm and 500 nm, respectively [32]. (Ref. license under CCBY 4.0).

4.4 Complex structures with varying θ and φ

The possibilities for generating diverse lattices through shadow mask deposition-aided NSL are practically endless. In this section, examples of some complex lattice structures are shown with uncommon shapes and symmetry. In Figure 6a and b, the nanostructures are produced with four-fold symmetry of θ. It is a quantitative addition of two times of two-fold symmetry perpendicular to each other configured as θ = θi + (0, 90, 180, and 270 degrees). Figure 6a and b are the SEM images for two different θi (15 and 355 degrees respectively) at a constant φ of 25 degrees. A bunch of eight triangles reorient and restructure themselves to form a unit cell as shown in the insets. Further, the structure in Figure 6c is obtained with φ = 25 degrees and θ = 0 and 90 degrees. The deposition is neither two-fold nor four-fold symmetric to θ rather can be termed as incomplete (or semi) four-fold symmetric. Figure 6d is obtained by a four-step deposition of simultaneously varying θ and φ as θ = 130 degrees + (0, 90, 180, and 270 degrees) and φ = 15, 18, 22and 29 degrees. In Figure 6e–h, simulated structures are obtained by integrating over the full rotation of θ. The simulation involves varying θ from 0 to 359 degrees in increments of 1 degree. In Figure 6e–h, the value of φ is kept constant at 15, 25, and 30 degrees respectively, while in Figure 6h, φ is continuously increasing from 15 to 30 degrees at a constant angular velocity over the entire span of θ. Figure 6e–h shows a series of unique 3D patterns that are interconnected in a hexagonal framework. Among these patterns, the nano spiral shape illustrated in Figure 6h is particularly important and could be employed in chiral spectroscopy.

Figure 6.

(a–d) SEM images of complex lattice structures obtained by multistep deposition using various combinations of θ and φ. The scale bars in the figure and the insets correspond to 2 μm and 500 nm, respectively. Simulated structures upon complete rotation of θ for (e-g) constant φ and (h) continuously varying φ [32]. (Ref. license under CCBY 4.0).

5. Etching-assisted NSL

Figure 7 presents the summary of the SEM results obtained from etching-assisted NSL [4]. The image at the top of Figure 7a displays a self-assembled nanosphere mask following size reduction. Etching of the nanosphere mask is achieved using oxygen plasma in the RIE chamber. This process reduces the diameter of the nanospheres and evenly spaces them apart. The spacing between nanospheres increases as the size of the nanospheres reduces with a longer exposure time to the oxygen plasma. The image at the bottom offers a closer view of the region marked by the red rectangle. Figure 7b showcases SEM side view images of a Si nanopillar array after undergoing Si etching in the presence of a size-reduced nanosphere mask. In this process, the exposed Si is etched from the top while the Si underneath the nanosphere beads remains protected, resulting in a cylindrical nanopillar array pattern. The process involves inductively coupled plasma reactive ion etching (ICP-RIE). Alternatively, the time-multiplexed silicon etching, commonly referred to as “Bosch” can be employed in a Deep RIE chamber to achieve high aspect ratio pillars with minimal etching of the side walls [19]. In this technique, Si is etched with SF₆, followed by passivation with a C₄F₈ polymer in repeated cycles. The top and bottom images of Figure 7b correspond to different time exposure to oxygen plasma (4 and 5 min, respectively). The spherical particles seen on top of each pillar represent the nanosphere masks. Ideally, the diameter of the cylindrical pillars should be the same as the size of the nanosphere. However, in this case, it appears slightly smaller due to nonuniformity in the etching process. Comparing the top and bottom figures, it is evident that the pillars are narrower in the bottom image due to reduced beads size. Thus, the size of the nanopillars can be controlled by the etching time. Finally, the top and bottom images of Figure 7c present side and slanted view images of the pillars after the removal of the nanosphere beads.

Figure 7.

SEM images of the sample at different stages of etching-assisted NSL. (a) Top: nanospheres on Si substrate after oxygen etching. Bottom: Magnified image of the red square marked region. White lines in the top and bottom images correspond to 2 μm and 500 nm respectively. (b) Side view of nanopillars array with etched nanospheres on the top of each pillar. The top and bottom images correspond to 4 and 5 min of oxygen etching respectively. (c) Side and slanted view images of nanopillars array after removal of nanosphere mask. Scale bars in figures (b) and (c) correspond to 1 μm [4]. (Ref. license under CCBY 4.0).

The conventional etching-assisted NSL technique has been improved to create a hollow cylindrical pillar array through multiple-patterning nanosphere lithography (MP-NSL) [20], as shown in Figure 8a. The first three steps of MP-NSL are the same as the standard etching-assisted NSL, where nanospheres on a Si substrate are treated with oxygen plasma to reduce their diameter, followed by DRIE to create solid cylindrical arrays. The diameter of the reduced nanosphere will be the same as the outer diameter of the resulting hollow cylinder. In step 4, the nanospheres are treated with oxygen plasma again, further reducing their diameter, which will become the inner diameter of the cylinder. The reduction of diameter in this step determines the thickness of the hollow cylinder. Next, Ni is deposited from the top, and the nanosphere is removed, leaving a ring-shaped mask at the top of the solid cylinder along its outer diameter (steps 4 and 5). Finally, in steps 6 and 7, the inner hole is created through DRIE, and the Ni ring is removed to obtain a hollow cylindrical array of Si. In Figure 8b SEM images labeled 1–6 are shown corresponding to the lithography steps. Label 1 shows closely packed monolayer nanospheres of diameter 1 μm on Si substrate (top view). In label 2 nanospheres are shown with the first reduction of diameter (top view). In label 3, a tilted view (tilt angle = 30 degrees) of solid pillars of Si with nanospheres on top is shown. Label 4 shows the size of the nanosphere on top of Si pillars is reduced further (tilted view). The top view after Ni coating is shown in label 5. Finally, in label 6 tilted views of hollow nanosphere are shown. In the tilted view images, the tilt angle is kept at 30 degrees. The scale bars correspond to 1 μm.

Figure 8.

(a) Schematic illustration of the MP-NSL process. (b) 1. Nanosphere mask on a Si wafer; 2. nanospheres after the first size reduction; 3. nanospheres on top of Si nanopillar arrays; 4. second size reduction of nanospheres; 5. Ni nanorings on top of nanopillars and 6. etching of the inner regions by DRIE forming Si nanotubes. Scale bars are 1 μm [20]. (c) Scheme of vertical temperature gradient induced anisotropic etching. (d) Morphology of nano mushroom. (e) SEM image of gold-coated nano mushroom arrays [10]. (Ref. 10—Creative Commons Attribution 3.0 Unported License, Ref. 20—Incorporated with permission).

In the previous discussion we have seen etching to be isotropic, however, controlled anisotropy can be introduced in the etching process by creating a temperature gradient inside the etching chamber, resulting in broken symmetry along the thickness of a structure. The process of anisotropic plasma etching involves an isotropic oxygen plasma etching along with a vertical temperature gradient that is applied from the bottom to the top of the nanospheres. The rate of etching is dependent on the chamber temperature, with higher temperatures resulting in a faster rate of etching. A uniform change in chamber temperature does not change the shape of the structure, only the speed of the etching process. By introducing the temperature gradient, the etching rate can be engineered along the thickness, resulting in a highly anisotropic etching process perpendicular to the substrate [10]. Figure 8c depicts the setup of the etching chamber, while Figure 8c and d schematically illustrate how a nanosphere transforms into a mushroom-shaped structure, with the lower hemisphere which is in contact with the substrate eroding faster due to heat accumulation from the higher temperature. Finally, Figure 8e shows a slanted view SEM image of the resulting nano mushroom array coated with a thin film of gold.

6. Application

From the discussion above we found NSL to be a powerful tool to create ordered arrays of nanoscale features with controlled size, shape, and spacing. Nanostructures having nanodomains that extend down to a few nm are crucial for plasmonics, particularly in applications such as surface-enhanced Raman spectroscopy (SERS), chiral spectroscopy, and biosensors. The applications rely on surface plasmons creating anisotropic optical behavior to manipulate light at the nanoscale. These structures from NSL can concentrate the electric field at an extremely small apex (radius of curvature < 10 nm) or vertices, resulting in amplified optical signals. It is widely used to create SERS substrates, which can enhance the Raman scattering of molecules from ultra-low concentration with high sensitivity and specificity. SERS has applications in the detection of molecules, proteins, and cells for chemical detection, environmental monitoring, and medical diagnosis. For example, a recent report [51] in 2020, demonstrates early detection of liver cancer (hepatocellular carcinoma) using the SERS technique. The study uses a special bridged knobby nanostructured template fabricated by NSL for the detection of low-concentration cancer bio-markers [52]. NSL patterning can be used for high-resolution imaging of a live cell or a single molecule using the localized surface plasmon resonance (LSPR) technique [53]. The research conducted centers around distinct metallic nanostructures that possess varying symmetries, inter-structural gaps, and chirality. These structures are studied in order to regulate electromagnetic hotspots and adjust optical characteristics based on the alignment of their constituent elements [54, 55, 56, 57, 58, 59]. Farther more, the effectiveness of magnetic memory applications such as data storage, data transfer, and logic devices are reliant on the regulated transmission of spin waves through magnonic crystals via exchange interactions. Over the last decade, there has been considerable research conducted in controlling the chiral magnetic domains [60, 61] as well as spin wave properties like frequency modes and damping [62, 63] by modifying the thickness and shape anisotropy of nanostructures [64, 65]. Hence, nanostructures, rich in various spin-wave modes, are of high importance [66, 67, 68, 69, 70]. NSL has also been used to fabricate plasmonic color filters [71, 72], waveguides [73, 74], and antennas [74, 75, 76] with high efficiency and tunability [71]. Moreover, smart window application employs a lithographed pattern of thermochromic material by NSL to control the transmittance of the window material [77]. NSL can be used to create periodic texture on the surface of solar cells, which can increase light absorption and thereby improve the efficiency of the solar cells [78, 79]. It can also be used for fabricating antibacterial surfaces [80]. Arrays of nanoneedles fabricated by NSL are demonstrated for intracellular delivery of nanoparticles with an attached microfluidic system for cellular immunotherapies [81].

7. Conclusion

To summarize, NSL is a micro/nanofabrication technique that is inexpensive and straightforward, offering numerous benefits over other lithography techniques such as scalability, versatility, time efficiency, and eco-friendliness. Consequently, it has become a widely popular technique, with substantial research efforts invested over the last few decades. As the technique has developed over time, it has progressed from simple hexagonal patterns of triangular elements to more complex 2D/3D structure fabrication with controlled shape, symmetry, and inter-structural spacing. To create various nanostructures for different applications, the NSL method has been modified and associated with various other techniques. The NSL method can be broadly categorized into two parts: deposition-assisted NSL and etching-assisted NSL. The former involves depositing material on closely packed nanosphere masks to create a shadow pattern on the substrate, while the latter involves patterning the substrate by etching it through the gaps of the nanosphere. NSL can also be performed with a complex rotation (either continuous or discrete) of the substrate, producing unique patterns such as star and spiral patterns. Additionally, a vertical temperature gradient during etching induces symmetry breaking along the thickness, which is required for engineering 3D structures. The NSL technique finds significant applications in the field of plasmonics, magnonics, and biomedicine.