Large-Area Nanopatterning Based on Field Alignment by

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Large-Area Nanopatterning Based on Field Alignment by Microscale Metal Mask for Etching Process Jihye Lee, Jun-Young Lee, and Jong-Souk Yeo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09730 • Publication Date (Web): 07 Sep 2019 Downloaded from pubs.acs.org on September 7, 2019

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ACS Applied Materials & Interfaces

Large-Area Nanopatterning Based on Field Alignment by Microscale Metal Mask for Etching Process Jihye Leea,b, Jun-Young Leea,b and Jong-Souk Yeoa,b,* a

School of Integrated Technology, Yonsei University, Incheon, 406-840, Rep. of Korea,

b

Yonsei Institute of Convergence Technology, Yonsei University, Incheon, 406-840,

Rep. of Korea * Correspondence:

[email protected]

KEYWORDS Plasma etching, Metal mask, Nanopatterning, Electric field-induced bowing effect, Silicon etching, Non-Bosch process

ABSTRACT

ACS Paragon Plus Environment

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Recently, researchers have dedicated efforts toward producing large-area nanostructures using advanced lithography techniques and state-of-the-art etching methods. However, these processes involve challenges such as the diffraction limit and an unintended etching profile. In this work, we demonstrate large-area nanopatterning on a silicon substrate using a microscale metal mask by meticulous optimization of the etching process. Around the vertex of a microscale metal mask, a locally induced electric field is generated by a bias voltage on a silicon mold. We utilize this field to change the trajectory of reactive ions and their effect flux, thus providing a controllable bowing effect. The results are analyzed by both numerical simulations and experiments. Based on this field alignment by metal mask for etching (FAME) process, we demonstrate the fabrication of 378-nm size nanostructure patterns which translates to a size reduction of 63% from 1μm size mask patterns on a wafer by optimization of the processes. This is much higher than the undercut (~37%) usually achieved by a typical non-Bosch process under similar etching conditions. The optimized nanostructure is used as a mold for the transfer printing of nanostructure arrays on a flexible substrate to demonstrate enabling the functionality of FAME-processed nanostructures.


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1.

INTRODUCTION

In nanotechnology, designing new processes is necessary for achieving highly integrated micro electromechanical systems (MEMS), nanoelectromechanical systems (NEMS), and optoelectronic devices.1-2 One key factor to consider when designing new processes for high-density device fabrication is the feature size because small feature sizes, particularly nanometer-scale ones, allow us to obtain a reduced device size and weight, fast and precise operation, and higher energy efficiency benefiting from high heat dissipation. Advanced lithography techniques and etching processes have been developed to introduce all of these advantages at the nanometer scale. Frequently used lithography techniques in the industry include use of the ArF excimer laser (193nm wavelength) with the immersion technique using a high-refractive-index liquid, use of extreme ultraviolet (EUV) with 13.5-nm wavelength with a high numerical aperture (NA), nanoimprint lithography, and directed self-assembly (DSA). These next-generation lithography techniques allow high-resolution pattern, but the short wavelengths used induce stray light known as “flares” by scattering and diffraction of light arising from defects on the lens materials and coating surfaces of tools.3-6 This can result in contrast degradation. In order to prevent such a phenomenon, the lithography is followed by a highly developed etching step. Etching processes are classified as wet or dry techniques.7 Wet etching, which uses chemical solutions or etchants, can remove etching materials in both isotropic and anisotropic shapes. Most wet-etching methods produce isotropic shapes on the target materials. However, when we used potassium hydroxide (KOH), ethylenediamine pyrocatechol (EDP), tetramethylammonium hydroxide (TMAH), or metal-assisted chemical etching, anisotropic shapes could be formed.8-11 For anisotropic etching, a dry-etching process is usually preferred.12-17 It avoids the use of


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hazardous and toxic solutions that may be required in wet etching. Dry etching can also be used to change the surface properties such as the hydrophilicity and surface profile (shape and roughness) by controlling the pressure, type of etching gas, and voltage and power of the bias in the plasma electrode.18-22 However, undercutting, profile tilting, notching, bowing effects, loading effects, and charging effects during surface treatment or surface etching must be overcome to produce a purposed nanostructure while maintaining their anisotropic shape and purposed surface profile.2326

In this study, we utilized one of the challenges in the plasma etching process — the bowing effect — to our advantage in order to fabricate a nanoscale structure from a microscale metal mask without resorting to advanced lithography. The bowing effect occurs when a facet of the structure deviates the ion trajectory from the original pathway such that the ions undertake lateral etching (local bowing) and undercut the target materials.27-28 Both the facet of the structure and the local electric field distribution on the metallic mask affect the angle and trajectory of the reactive ion. Many researchers generally use polymer and insulator-type photomasks in lithography and plasma-etching processes but these materials produce charging effects when interacting with plasma.29-32 However, when we used the metal mask in this experiment, there was no charging effect due to the conductive metal mask. Meanwhile, a local electric field occurred around the vertex due to the bias voltage applied on the silicon mold.33 This local electric field can induce ion deflection such that the side underneath metal mask is etched as a result. By uncovering the relationship between the metal mask and its effect on the field alignment of ions in plasma etching, we demonstrated the large area fabrication of nanoscale structures in a controlled manner using microscale mask patterns.


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2.

EXPERIMANTAL DETAILS

2.1 Materials Acetone

(99.5%),

isopropyl

alcohol

(IPA)

(99.7%),

and

trichloro(1H,1H,2H,2H-

perfluorooctyl)silane (FOTS) were purchased from Sigma-Aldrich. AZ®300 MIF DEVELOPER was purchased from MERCK. AZ® 5214 E Photoresist and the gold etchant, which was a mixture of HCl/HNO3 (3:1), were purchased from MicroChemicals. 2.2 Preparation of microscale metal mask by photolithography The fabrication process was achieved by photolithography as shown in Figure 1(a) (NanoSystem Solutions, Inc. D-LIGHT DL-1000). This process utilizes a locally induced electric field around the mask vertex. We call this technique the field alignment by metal mask for etching (FAME) process. To form a nanostructure based on FAME, the silicon substrate (1 0 0) was cleaned by IPA, acetone, and distilled water for five minutes, in that order. Then, the silicon was coated with the 1.4-μm-thick AZ5214 photoresist. Soft bake of the photoresist was then undertaken at 110 ℃ for 50 s on a hotplate. This photoresist was illuminated with I-line light (405-nm wavelength) at 30 mJ/cm2. Next, hard bake of the photoresist was conducted at 120 ℃ for 2 min. To achieve image inversion of the photoresist from positive to negative, the substrate was illuminated with UV light at 200 mJ/cm2 for 200 s and then was immersed in a MIF 300 DEVELOPER solution so that a microscale photoresist pattern was finally formed on the silicon substrate. This process ensured the removal of any soluble photoresist on the silicon. Next, gold was deposited on the microscale photoresist pattern by thermal evaporation (5-10 nm thickness) and sputtering (over 10 nm thickness) and the resist pattern was removed by acetone in a lift-off process. 2.3 Plasma-etching process and characteristics of the silicon mold


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The dry-etching process was followed with inductively coupled plasma-reactive ion etching (ICPRIE, 13.56 MHz, SNTEK in Rep. of Korea), which is frequently used in conventional microfabrication, as shown in Figure 1(a). The RF source power for RIE was 200 W, which can affect the ion flux and radical density. The DC bias power of 50 W accelerated the ions. The flow rates of the etching gases were 20 sccm for O2, 100 sccm for SF6, 20 sccm for N2, and 1,000 sccm for He. The role of O2 increases the amount of F to induce a high etch rate. The SF6 is a main etching gas in order to induce the chemical reaction between reactive F ion and Si thus making a volatile SiF4.34-35 The He and N2 increase plasma stabilization and etch anisotropy with enhanced silicon etch rate.36 Additionally, the He gas has a high thermal conductivity, which helps heat transfer between wafer and the electrode to be cooled. The etching pressure of 10 mTorr, which is suitable for radical reactions to be dominant. We used non-Bosch etching without a passivation layer on the silicon mold, which resulted in etching of the sidewall of the silicon specimen. Sidewall etching is a critical drawback of dry etching, but we take this drawback to our advantage in order to produce a nano-size silicon structure. By controlling the thickness and pitch of the mask structure and the etch time, a silicon nanostructure was thus formed from a microscale gold mask. The microscale gold layer was removed using the gold etchant solution at room temperature (a 1:3 mixture of nitric acid and hydrochloric acid). Figure 1 (b) shows the scanning electron microscopy (SEM) images. The size of the micropattern with the photoresist was 1 μm and the pitch of the structure was 3 μm, as shown in Figure 1 (b-1). Figure 1 (b-2) shows the micropattern with sputtered gold and Figure 1 (b-3) shows its etching profile. Using the FAME process, we achieved a 378-nm-size nanostructure from the 1-μm-size microscale gold mask (63% reduction) utilizing the bowing effect caused by the locally enhanced electric field around the microscale gold mask. The size of the silicon obtained after the FAME


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process is smaller than that using the conventional etching process (~37%) which utilized the similar etching condition.37 The FAME process is illustrated in detail in Figure 2, along with the numerical modeling results. 2.4

Condition of the FOTS coating on silicon mold

The FOTS layer plays a role of anti-adhesion layer for releasing and transferring the metal to the flexible substrate. To make a FOTS layer, the 30 μl FOTS was prepared in the desiccator. When the desiccator chamber was changed to the vacuum state, the FOTS was vaporized and deposited on the silicon mold for 2 hours. This layer was prepared with a self-assembled monolayer (SAM) with 1-2 nm thickness as shown in Transmission electron microscopy (TEM) image in Supporting Information while SAM layer was accumulated to 10 nm thickness when the mold was reused several times. After transfer printing process, this FOTS layer is cleaned by O2 plasma treatment (Pressure 400 mTorr, O2 40 sccm, Power 200 W, Time 300 seconds) and additional Aceton, isopropyl alcohol (IPA), deionized (DI) water cleaning. 2.5

Simulation settings

To clarify the underlying electric field distribution around the gold mask, a set of numerical simulation was carried out by a Lumerical DEVICE 7.2.1703 software. The material conditions are set first according to material library from Lumerical software. The DC permittivity, the work function, and band gap of silicon are 11.7 and 4.59 eV, and 1.11452 eV, respectively. The ni (1/cm3), carrier concentration, is 1.05494×1010. The work function of gold is 5.1 eV. The air-relative dielectric permittivity is set by 1. In a CHARGE solver, boundary conditions are given to simulate our situation: top contact 0V, bottom contact 10 V, steady state bc mode, and single type sweep. In a CHARGE simulation, conditions for temperature, mesh


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condition, and monitor are given as follows: temperature - isothermal temperature dependence, 300 K simulation temperature, global mesh constraints - min edge length 0.001 μm, max edge length 0.1 μm, max refine steps 100000 (auto refinement settings), results monitor - electric field (V/m) and electrostatic potential (V). A bias condition of 10 V is applied to the gold on silicon substrate while 0V is applied to the top electrode to simulate the field distribution. In the middle, an air gap with a length of 5 μm between the top and bottom electrode is assumed to provide a sufficient separation compared to the thickness of gold mask. Substrate is approximated by placing a silicon with 500 nm thickness underneath the the gold mask (thickness: 10, 50, 100, 150, 200 nm, width: 1 μm). 2.6 Characterization Characterizations were performed on surface images using field emission scanning electron microscopy (FE-SEM) (JEOL, JSM-7100F) and on cross-sectional images using high-resolution transmission electron microscopy (HRTEM) with spherical aberration (CS) corrected scanning transmission electron microscopy (JEOL, JEM-ARM200F). For our application in a transfer printing of nanoscale structures, we functionalized the surface of the etched silicon mold using FOTS by vaporization in glass desiccator and then Au layer by thermal evaporation. These layers also served to protect the silicon nanostructure during TEM analysis. 3.

RESULTS AND DISCUSSIONS

Metal (such as silver, copper, chromium, and aluminum) or metal oxides are conventionally used as mask materials for silicon etching since they can withstand high DC bias voltage because of their low corrosiveness and offer infinite selectivity with low physical sputtering. In our experiment, we chose a gold layer to produce the microscale mask pattern. Gold is soft and


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malleable and is not generally used as an etching mask since it causes re-sputtering issues.34 In addition, it is highly conductive, which can generate unwanted local electric fields around the metal mask owing to the bias voltage on the substrate. However, the gold film as a mask can be removed cleanly using an etchant solution. Furthermore, the high conductivity can be used to control the trajectory of the reactive ions, particularly when using a locally induced electric field around the metal mask. Using these interesting properties of the noble metal, we obtained nanostructures from the microscale pattern and optimized the process by changing the thickness and pitch of the gold mask and the etch time. The primary principle of side etching on silicon owing to the bowing effect is illustrated in Figure 2 (a). The bowing effect originally occurs when we use a mask with specific facets bearing a certain side slope that deviates ions from their original trajectory and causes lateral etching or undercut at the contact sidewall.27-28 However, the facet-induced bowing effect cannot be controlled owing to the randomness of the facet shape produced during the lithography and metal deposition. Another widely known reason for the bowing effect is charging of the insulating mask material, which needs to be controlled.29 Compared to these known bowing effects, local electric field around the metal mask can also give rise to the electric field-induced bowing effect. A strong electric field is locally formed around the conductive gold materials so that the reactive ions can change their trajectory and attack the underside of the gold mask. To characterize the effect of this local electric field, we first changed the microscale gold mask thickness and analyzed the local electric field distribution around the gold. Based on how reactive ions can interact with metal masks, three different regimes can be identified for the ranges of gold mask thickness: regimes I, II, and III (Figure 2 (a)), which correspond to the thickness range of 5–30 nm, 40–100 nm, and 150 nm, respectively.


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The distributed intensity of the electric field along the gold surface and around the gold in regime I are minimal compared to the intensities observed for the other regimes (Figure 2 (b)). A small local electric field results in a low deflection angle for the etching ions directed toward the surface (Figure 2 (c), regime I); therefore, side etching is not effective for this condition. Furthermore, this thickness is insufficient for resisting the reactive ions. Based on the simulation results obtained for regime I, this range of mask thickness is not considered sufficient to achieve controllable side etching on silicon. The local electric field around the gold mask is stronger for regime II than for regime I. As the thickness of the mask increases, the local field around the vertex of the gold mask also increases, as demonstrated by numerical modeling. Due to this strong local field around the vertex, the angle of deflection is large and the silicon underneath the gold mask is etched deeply. Consequently, these fields around the mask gradually change the trajectories of reactive ions and thus lead to controllable side etching of the silicon (Figure 2 (b)–(c), regime II). Regime III shows the highest electric field distribution around the gold mask (Figure 2 (b), Regime III). Since this is the strongest local field obtained at the vertex of the mask, the deflection angle of the reactive ion is the highest of those obtained for all the conditions tested. However, the thickness of the mask is also largest, which lead to the collision of the ions with the gold mask, thereby changing the shape of the metal mask itself (Figure 2 (c)–(d) Regime III). The quantitative simulation results are shown in Figure 2 (d) and (e). The variation in the strength of the electric field according to the thickness of the mask is plotted in Figure 2 (d). As the thickness of the mask increases, the intensity of the electric field at the top of the edge increases


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linearly and consequently the deflection angle of the reactive ion used in the etching process increases as well. Since the rate of etching is determined by the flux of reactive ions directed to the sidewall, it is useful to introduce a concept of threshold electric field effectively deflecting the ions that can contribute to the side etching. In our simulation, the distance between top and bottom electrodes is set as 5 μm long and biased with 10 V and 0 V, respectively. In our modeling geometry, the electric field at the bottom electrode (gold mask) is 2 × 106 V/m. This can be defined as a threshold electric field since the local field from the mask needs to be larger than this electric field between the electrodes (green color in the field map) to provide an effective local bowing of reactive ions for side etching as shown in Figure 2 (e). A length of the region where the field is larger than 2 × 106 V/m is termed the effective field length, which we represent as “l” here. The thickness of the mask is denoted as “t”. Then, the l/t ratio indicates how far the field can affect the ion flux compared to the thickness of the mask and it decreases as the thickness of the mask increases. The effective length of the electric field becomes shorter for a thicker mask, and as a result, the effective ion flux at the sidewall should decrease drastically. Since sidewall etching is determined by the combination of the effective ion flux and their bowing due to the local electric field for the given mask thickness, their comprehensive relationship can be presented in Figure 2 (f) by multiplying the quantities shown in Figure 2 (d) and Figure 2 (e). The strength and effective length of the electric field depend on the thickness of the metal mask so the three different regimes of I - III depending on the thickness of metal mask are shown in Figure 2 (f) to illustrate how the degree of sidewall etching changes according to the schematics explained in Figure 2 (c).


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Regime I intensity shows the highest value but the mask was too thin to withstand the etching process. In Regime II the thicknesses of 70 and 100 nm shows the highest effective field intensity. This range enables maximum side etching on the silicon. In Regime III, an effective field intensity decreases with the mask thickness, which means that the electric field around the mask does not achieve side etching effectively. The experimental results shown in Figure 3 support the overall trend from the simulation results. Figure 3 (a)–(b) show the SEM image and trend of the etching profile (Red square: regime I; Green square: regime II; Blue square: regime III). In regime I, as seen in Figure 3 (a), when the thickness of the mask increased, the size after etching increased. The size after etching for 5-nm thick gold mask was about 560 nm, which was obtained from the 1-μm-size microscale pattern (Figure 3 (b)). When we used thicknesses of 10, 20, and 30 nm for the gold mask, the sizes of the nanostructures were reduced to 640, 700, and 716 nm, respectively. The nanostructures obtained in this regime were not sufficiently etched for the given condition indicating that the ions were not effectively controlled for side etching. Hence, the conditions were deemed unsuitable to fabricate desired nanoscale structures. For the masks with the thickness in the range of 40–100 nm and the fixed pitch of 3-μm, the size after etching gradually decreased as the mask thickness increased. This regime showed a linear trend: the average sizes of the nanostructures were 691, 617, 522, and 378 nm after etching for the mask thicknesses of 40, 50, 70, and 100 nm, respectively. Based on the experimental results, regime II was considered suitable for controlled nanofabrication. Regime III shows that the size after etching was 702 nm, which resulted from the 150-nm-thick gold mask. The size after etching did not decrease dramatically as in regime II because the


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deflection angle of the ions was too large for the thicker mask to appropriately etch underneath the gold mask. This can be further explained by the decrease of the effective field strength as shown in Figure 2 (e). Thus, we found that the mask thickness in the range of 70–100 nm in Regime II is optimal to achieve more effective nanofabrication compared to the conventional etching process.37 In order to show the possibility to fabricate various shapes by using the optimized gold mask thickness, we have tried a circular and T shaped pattern and the results were shown in Supporting Information Figure S1. The patterns’ evolution from microsize photoresist, microsize gold mask, and etched pattern before lift-off of the gold mask to etched pattern after lift-off of the gold mask were provided with normal and tilted SEM images. The results from the FAME process demonstrate the versatility of our etching process and its potential applications. Regarding the smoothness of the pattern’s edge corresponding to the thickness of the gold mask layer, the gold mask layers after FAME process were shown in Supporting Information Figure S2. Figure 3 (c) and (d) show that the experimental results were consistent with the simulation results and the results can be further optimized by changing other parameters. Based on the comparison, the strength of the locally induced electric field and the affected ion flux along with appropriate mask thickness are important factors for the controlled fabrication of nanostructures employing the FAME process. Since the process only uses microscale metal mask patterns, the FAME process can naturally be extended to a larger scale etching, thus enabling large-area nanopatterning without having to rely on advanced nanolithographic techniques. We further investigated the fabrication of nanostructures by changing the pitches of the mask pattern: we used values of 3, 5, 10, and 15 μm. As shown in Figure 4 (a), microscale pitches do not affect the size after etching. According to previous reports, the nanoscale pitch limits the number of reactive ions that reach the bottom of the substrate, thus changing the etching efficiency,


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as shown in Figure 4 (b) right.24 This is explained as loading effect. In the range of microscale pitches shown on the left in Figure 4 (b), the reactive ions fully saturated the bottom of the silicon sample and therefore uniformly affected the etching profile. Consequently, we stably obtained silicon nanostructures from the microscale pitches. The optical microscopy images shown in Figure 4 (c)–(f) (left) provide information about the size of the gold mask (1 μm) and pitches (3, 5, 10, and 15 μm); the SEM images shown in Figure 4 (c)–(f) (right) provide the sizes after etching at different pitches. The average sizes after etching were 378, 400, 377, and 400 nm, for pitches of 3, 5, 10, and 15 μm, respectively. Each structure was obtained from a 1-μm-size and 100-nm-thick mask. Based on the results, we can conclude that the microscale pitch offers a saturated ion flux and an effective ion bowing effect, which is sufficient for preparing nanoscale structures in a nonBosch etching process. The last condition considered for the FAME process was etching time. Typically, a long etch time produces deep trenches on the silicon substrate at a constant etch rate. However, in our experiment, some interesting results were obtained for lateral and vertical aspects. The vertical etch rate was almost the same for different etching times (Figure 5 (a), red). This phenomenon is generally understood: when we increase the time of exposure, there is an increased opportunity for interaction between the bottom of the silicon substrate and the reactive ions. Thus, the average etch rate in the vertical direction remains constant. The lateral etch rate, however, decreased when we increased the etching time (Figure 5 (a), black). This means that the exposure time was inversely correlated with the bowing effect. Based on the simulation results in Figure 2, the deflection angle of the reactive ion was determined by the thickness of the mask, not the time. As we prolonged the etching time, the rate of size change decreased and reached a saturation. This means that the gold mask in our experiment was gradually etched away owing to a reaction and


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was redeposited when we prolonged the etching time, thus reducing the lateral etch rate. Consequently, selecting an optimal etching time is important. In order to compare the isotropic etching and FAME process, an etching profile was illustrated in Figure 5 (b). The pitch of the mask is denoted as “p” and the size of the nanostructure after etching process is denoted as “w”. The red dotted line represents the isotropic etching that has a (p-w)/2 length both in vertical and lateral direction thus showing a rounded profile. In contrast, the FAME process has a different etch rate between lateral and vertical direction and exhibits improved sidewall etching as indicated by the green dotted line. This enhanced sidewall etching compared to the conventional isotropic etching process provides a large-area nanopattening by using a microsize mask. In order to analyze the profile of the etched silicon, we investigated the roughness and shape of the top and the sidewall of the silicon mold as shown in Figure 5 (c)-(d). The cross-sectional image obtained by TEM is shown in Figure 5 (c). The top surface of the etched silicon was fairly smooth owing to the protection offered by the gold mask. The sidewall of silicon had an almost anisotropic shape while the bottom was isotropic. Since the pressure used was quite low (=10 mTorr) and the bias power was larger (=70 mW) than that for the undercutting condition,37 the ion had a larger mean free path and high directionality, which realized the anisotropic shape for the upper sidewall. To fully illustrate the etching profile of the silicon at an atomic level, the high-resolution TEM image is shown in Figure 5 (d). As shown in the figure, the maximum roughness of the etched silicon layer was 1.46 nm. This mold with an anti-adhesion layer of FOTS could be reused 5~6 times for transfer printing which is shown in Figure 6. Traditionally, the anti-stick layer on silicon master could be formed by a self-assembled layer (SAM) which has 1-2 nm thickness shown in


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HRTEM image of Figure S3 (a) in Supporting Information. However, when the silicon mold is used repeatedly, the residue of the SAM polymer is not completely removed even with the several cleaning processes, thus 5-10 nm thick residual layer is shown in Figure S3 (b) and Figure 5 (d). In order to demonstrate a practical application of our FAME process for the fabrication of silicon nanostructures, we have used a contact transfer printing that allows the transfer of metallic nanostructures on a flexible substrate of polyethyleneterephthalate (PET). This can be used as a sensing surface for biochemical application or as a flexible optoelectronic device for an Internet of Things (IoT) application.38-39 As shown in Figure 6 (a), an anti-adhesion layer of FOTS was deposited on the silicon nanostructure, and then a gold and adhesive layer was deposited on the FOTS layer.39-40 Heat and pressure were applied during the transfer printing process, the result of which was investigated using SEM images. Figure 6 (b) shows the silicon mold with gold before printing. It has a FOTS layer and sputtered gold. Figure 6 (c) shows the silicon mold without the gold layer after printing. Both SEM images showed roughness along the sidewall of the silicon mold. This roughness resulted from deposited gold that had not been transferred to the PET substrate during the printing process. As shown in Figure 6 (d), the ~378–400 nm structure was stably transferred to the PET substrate. As a result, the size reducing etch (non-transfer etch) enabled the transfer of nanostructures to the target substrate along with the target material, demonstrating that the FAME-processed silicon mold can be used in practical applications. 4. CONCLUSIONS Here, we report silicon nanostructure arrays on a 4 cm2 substrate obtained using the FAME


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technique. In this process, we controlled etching parameters such as the thickness and pitch of the mask and the etch time to optimize these conditions for ion bowing. The optimized conditions in our FAME process were a thickness of 100 nm, an etch time of 125 s, a pitch of 3 μm, and a mask size of 1 μm for the gas condition used in our experiments. Using these conditions, we demonstrate the fabrication of 378-nm-size silicon nanostructure showing a 63% size reduction from a microscale mask. The etching conditions may further be optimized by changing the pressure, type, and amount of gas and by using other side effects of the plasma etching process. This large-area nanopatterning technique on a silicon substrate can not only be applied to imprinting or contact transfer printing by fabricating a nanostructure mold as demonstrated in our experiment, but can also be used for fabricating sub-wavelength structures applicable in optoelectronic devices for an IoT system.41-43


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Figure 1 (a) Schematic of the field alignment by metal mask for etching (FAME) process for producing nanostructures from a microscale mold. The microsize photoresist (PR) was formed on silicon using maskless photolithography, and the PR was developed in solution. Subsequently, a microsize gold mask structure was achieved by sputtering gold and removing the PR. By changing the thickness and pitch of the gold mask pattern and the duration of etching, the nanostructures were obtained by FAME process, the non-conventional etching process. Compared to conventional etching process under similar etching condition37, the FAME process achieved 63% reduction of silicon. (b) Scanning electron microscopy (SEM) images for micropattern developed in solution (b-1), microsize gold mask structure obtained by sputtering gold and removing the PR (b-2), and nanosize silicon structure fabricated using FAME process (b-3).


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Figure 2 Numerical modeling results for microscale gold mask to investigate the locally enhanced field distribution. (a) Schematics of FAME process undertaken for different thicknesses of the gold mask (b) Electric field distribution around gold mask on silicon substrate. The thicknesses for regimes I, II, and III were 5–30 nm (very thin mask), 40–100 nm (optimized thickness), and 150 nm (thick metal mask), respectively. (c) Schematics showing the changes in ion trajectory for different electric field distribution and thickness of metal mask. (d) Dependence of electric field strength at edge on the thickness of the mask. The strength of locally enhanced field is important as it is related to the bowing angle. (e) The ratio of l/t wherein ‘l’ is the effective length of the electric field from a vertex of the gold mask and ‘t’ is the thickness of the gold mask. This ratio determines the amount of reactive ion flux to the sidewall. (f) Dependence of effective electric field strength on the thickness of the gold mask, calculated by multiplying (d) and (e). This equation represents the total amount of reactive ions that effectively deflected to the sidewalls underneath the mask, correlating to the degree of sidewall etching.


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Figure 3 Size reducing etch by FAME process and comparison between experimental and simulation results. (a) SEM images of final nanostructures after FAME process for mask thicknesses of (a-1) 5 nm, (a-2) 10 nm, (a-3) 20 nm, (a-4) 30 nm, (a-5) 40 nm, (a-6) 50 nm, (a-7) 70 nm, (a-8) 100 nm, and (a-9) 150 nm. The etching time for all nanostructures was 125 s and the initial mask size was 1 μm. (b) Variation in the size after etch process according to the mask thickness. Regime I (red) involves mask thicknesses of 5–30 nm, regime II (green) represents 40– 100 nm, and regime III (blue) denotes a thickness of 150 nm. The gray-shaded regime explains the undercut that can be achieved from a typical non-Bosch process. This range is calculated from the result obtained under similar etching conditions. The undercut ranges from 50 to 376 nm. The extent of etching depends on the thickness of the mask as demonstrated by (c) experiments and (d) simulations. In regime II, the degree of etching increased linearly, while it decreased drastically in regime III. The tendencies observed for extents of etching in the experiments and simulations agreed well.


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Figure 4 Fabrication of nanostructure by changing pitch of mask pattern. (a) Dependence of size after etching on the pitch of the gold mask for an optimal thickness of 100 nm. (b) Schematic illustrating the dependence of loading effect on the pitch of the gold mask: microscale (left) nanoscale (right) pitches. Optical microscopy images of gold mask structure before FAME process (c, d, e, f; left) and SEM images of the nanostructure after FAME process (c, d, e, f; right). The pitches of the nanostructure were (c) 3 μm, (d) 5 μm, (e) 10 μm, and (f) 15 μm. The etching time for all nanostructures was 125 s. Since the ion flux was saturated in the micro-range pitch and the loading effect was not observed for our etching conditions, all sizes obtained after etching were the same.


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Figure 5 (a) Lateral and vertical etch rates in FAME process. The vertical etch rate was almost the same at 125 s, 250 s, and 500 s. The lateral etch rate shows a tendency to be inversely proportional to the increase in the etching time. As the etching time increased, redeposition of the gold mask increased, thus reducing the lateral etch rate. (b) Cross-sectional transmission electron microscopic (TEM) image to compare the shape and size between isotropic etching and FAME process. Isotropic etching should have same vertical and lateral etch rate while our FAME process provides the enhanced lateral etch rate compared to the vertical etch, thus showing a higher undercut percentage (63%) than achieved by the typical isotropic etching (37%). (c) TEM image to investigate the cross-sectional shape and size after etching for 125 s. The height and size after etching for 125 s were 650 nm and 378 nm, respectively. All images were obtained after passivation by a FOTS and a gold layer. By using a non-Bosch-based FAME process, the shape obtained for the top after etching was anisotropic and that for the bottom was isotropic. (d) Highresolution (HR) TEM image of the surface of silicon after etching. FOTS and gold layers used for the application in contact transfer printing also served as passivation layers for TEM analysis.


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Figure 6 Transfer printing of nanostructure on flexible PET substrate using a silicon mold fabricated in the FAME process. (a) Schematic of transfer printing. The FOTS, gold, and selfassembled monolayer were deposited on the silicon mold. The flexible PET substrate was introduced on top of the silicon mold under heat and pressure. Transfer printing of nanostructure was achieved on the PET substrate. SEM images showing (b) silicon mold with gold layer before transfer printing and (c) silicon mold without gold layer after transfer printing. The roughness of the sidewall was due to the remaining gold that had not been transferred to the PET. (d) SEM image of printed gold nanostructure on PET substrate.


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TABLE OF CONTENTS (TOC)


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ASSOCIATED CONTENT Supporting Information This supporting Information is available free of charge on the ACS Publications website. Fabrication of various shapes and size of nanostructures, Edge profile after etching process depending on the Regime I, II, and III, High-resolution TEM images of a single and multi FOTS layer on etched silicon mold AUTHOR INFORMATION

Corresponding Author

*Corresponding Author E-mail: [email protected]

ORCID

Jihye Lee 0000-0003-4970-9881

Jun-Young Lee 0000-0002-7449-4950

Jong-Souk Yeo 0000-0001-6452-2404

Author Contributions


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Jihye Lee designed, performed the experiments and analyzed the data; Jun-Young Lee analyzed the TEM data and discussed the results; Jong-Souk Yeo supervised the project. All authors contributed to the writing of the manuscript and approved the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This research was supported by the Ministry of Trade, Industry & Energy (MOTIE, project number 10080625) and Korea Semiconductor Research Consortium (KSRC) support program for the development of future semiconductor devices and also under the “Midcareer Researcher Program” (NRF-2016R1A2B2014612) funded by the National Research Foundation (NRF).


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Field Alignment by Microscale Metal Mask for Etching (FAME) process for the fabrication of nanostructure 379x198mm (96 x 96 DPI)


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Large-Area Nanopatterning Based on Field Alignment by

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