Sapphire nanophotonics: Fabrication challenges and optical properties

Geometrically complex, high aspect ratio microstructures have been successfully formed in Bulk Metallic Glass (BMG) via superplastic forming against silicon dies [1–3]. Although nanoscale features have been created in a similar fashion, there exists a demand to develop these metallic nanofeatures into high aspect ratio nanostructures with controlled geometries. In past research a process model was created to predict the achievable nanoscale feature sizes and aspect ratios through a flow model [4]. The flow model assumes force equilibrium with a viscous term to account for the required force to produce flow and a capillary pressure term required to overcome surface effects which are significant at the nanoscale. In this paper, a thin film model to predict the pressure distribution across the BMG during the forming process when it is in the supercooled liquid state is presented. Silicon molds with various nanofeatures were produced using Deep Reactive Ion Etching to achieve high aspect ratio dies over a relatively large area in order to validate these models.

Channel fabrication technology has become increasingly important for microfluidic and nanofluidic devices. In particular, glass channels have high chemical and physical stability, high optical transparency, and ease of surface modification, so that there is increasing interest in glass microfluidic devices for chemical experiments in microfluidics and nanofluidics. For the fabrication of glass channels, especially those with a high aspect ratio (depth/width), lithography using a metal resist and dry etching have mainly been used. However, there are still issues involving the surface roughness of the etched channel and the low etching selectivity. In this study, a microchannel fabrication method with high etching selectivity that produces a smooth etched surface was developed. First, interference during dry etching by remaining Cr particles after the photolithography and Cr etching processes was assumed as the cause of the rough etched surface. Three different dry etching processes were introduced to verify this. In process 1 without removal of the Cr particles, the etched surface was not flat and had a 1 μm scale roughness. In process 2 where a cleaning process was included and high power etching was conducted, a smooth surface with a 1 nm scale roughness and a faster etching rate of 0.3 μm min−1 were obtained. For this high-power etching condition, the etching selectivity (fused silica/Cr) was relatively low at approximately 39–43. In process 3 with a cleaning process and low-power etching, although the etching rate was relatively low at 0.1 μm min−1, a smooth surface with 1 nm scale roughness (10 nm scale roughness deeper than 40 μm in the depth region) and a much higher etching selectivity of approximately 79–84 were obtained. The dry etching method presented in this study represents a significant contribution to microfluidics/nanofluidics for microchannel/nanochannel fabrication.

High aspect ratio nanostructuring by high energy electrons and electroplating

Nano structures on organic solar material are expected to improve power generation efficiency instead of bulk hetero junctions. In this study, we fabricated high aspect ratio nano structures using poly-3-hexylthiophene (P3HT) by a novel nano-imprint process. Fine structures 70-nm wide and 550-nm high were successful obtained. However, the pattern defects in the de-molding process were increased by basing on the increment of the pattern′s aspect ratio. We also investigated the relation between aspect ratio and defect yield.

At present, perforation of high aspect ratio through glass vias (TGVs) is one of the primary factors limiting the development of glass interposers. Herein, it is based on a large number of experiments, combined with mechanistic analysis, to realize the perforation of ultrahigh/high aspect ratio fused silica TGVs using selective laser‐induced etching. First, the effect of the number of pulses as well as other laser parameters on the etching selectivity is systematically investigated. It is found that the formation of nanochannels in fused silica is crucial in determining the high selectivity. Subsequently, a model based on diffusion theory is proposed to mathematically explain the reasons for the high selectivity generation. Finally, the perforations of TGVs with diameters of 5.4–26.4 μm, aspect ratios of 11.7–61.3, and vertical sidewalls are achieved, at a laser treatment speed of ≈10 000 TGVs per minute. This research can serve as an essential reference for the manufacturing of TGVs with high aspect ratios and glass interposers with high interconnection density.

Simulation of Neuroelectrodes Characteristics with Integrated High Aspect Ratio Nano Structures

3D structures made out of glass can be used in various fields starting from optical components or microfluidic devices to micromechanics. Selective laser etching (SLE) is a unique technology that allows the production of low surface roughness (around 200 nm RMS) and a high aspect ratio (around 1000) 3D structures. However, some advanced applications as biomedical microfluidics and optical devices require to increase these aspects. In this work, we present SLE technological improvements towards better surface roughness and higher aspect ratio structures. Chemical etching process improvements enable to increase selectivity up to 3000 which ameliorates the accuracy and possible aspect ratio of the structures.

A novel mask technique, combining high selectivity silicon dioxide patterns over high aspect-ratio photoresist (PR) patterns has been exploited to perform mesa etching for device delineation and electrical isolation of HgCdTe third-generation infrared focal plane arrays (IRFPAs). High-density silicon dioxide film covering high aspect-ratio PR patterns was deposited at the temperature of 80°C and silicon dioxide film patterns over high aspect-ratio PR patterns of HgCdTe etching samples was developed by standard photolithography and wet chemical etch. Scanning electron microscopy (SEM) shows that the surfaces of inductively coupled plasma (ICP) etched samples are quite clean and smooth. The etching selectivity between the novel mask and HgCdTe of the samples is increased to above 32: 1 while the side-wall impact of etching plasma is suppressed by the high aspect ratio patterns. These results show that the combined patterning of silicon dioxide film and thick PR film is a readily available and promising masking technique for HgCdTe mesa etching.

Selective thermal emitters with a metallic 2D photonic crystal structure, such as shown in Fig. 1a), have been demonstrated to control the thermal emission spectrum of selective emitters for thermophotovoltaic applications. Traditionally, tungsten has been the metal of choice for thermal emitters due to their natural thermal emission selectivity to wavelengths > 2μm and high melting temperatures. However, due to the unweldable nature of tungsten, system integration of tungsten is difficult. As a result, weldable tantalum nanophotonic cavities have been demonstrated as effective thermal emitters with long wavelength emissivity below that of tungsten, allowing for improved selectivity performance compared to tungsten. However, the aspect ratios (AR) of the tantalum cavities have been demonstrated with a depth to radius aspect ratio of 11.2. The high aspect ratios in relation to tungsten (AR = 2.9) are due to the smaller material absorption at optical wavelengths around 2 μm, which causes Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">abs</sub> per unit length to be reduced thus affecting the Q matching condition. The aspect ratio is determined based on Q-matching, where the radiative Q (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rad</sub> ) and absorption Q (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">abs</sub> ) of the cavity must be equal for maximum absorption. The high cavity aspect ratio significantly hinders the fabrication methods available to manufacture the tantalum devices. For example, because of the high aspect ratio cavities, low-cost and scalable fabrication methods such as surface deposition of tantalum via sputtering or evaporation are not possible due to poor step coverage. Thus, previous methods used expensive and difficult to prepare 2" diameter tantalum wafers with custom etching methods. Accordingly, low aspect ratio cavities are key for a low-cost and scalable deployment of thermophotovoltaic devices.

Silicon nanostructures with high aspect-ratio (AR) features have played an important role in many fields. In this study, we report the fabrication of high AR silicon nanostructures using an inductively coupled plasma reactive ion etching (ICP-RIE) process by controlling the voltage bias at the substrate. The results show that by reducing the radio frequency (RF) bias power to 10 W, the etch selectivity of silicon to photoresist can be enhanced up to 36 times. Using the photoresist as a mask, this process can fabricate 300 nm-period one-dimensional (1D) grating structures with a height up to 807 nm, an improvement of 3.75-fold compared with structures fabricated by normal bias power. Furthermore, the analysis of the etch rate shows that the etch rate decreases over time in 1D gratings but remains constant in 2D pillar arrays, which can be attributed to the removal of the sidewall passivation. By including an O2 ICP-RIE step to remove the remaining polymer mask, the highest AR of 2D pillar structures that can be achieved is 8.8. The optical characterization of the fabricated structures demonstrates effective antireflection properties, where the measurements show that the reflectivity can be suppressed from 35% to 0.01% near normal incidence and 35% to 2.4% at 65° incident angle. The demonstrated low-RF power ICP-RIE process can create high AR nanostructures without the need for an inorganic mask and can find applications in integrated circuits, photonics, and functional nanostructures.

As the design rule of device shrinks below O.14j.tm, the higher resolution is required for real device application.With smaller feature size below 0. 14.tm, the lower coating thickness of resist is essential because of the patterncollapse issue at the high aspect ratio. However, the lower resist thickness induces the problem of etch selectivitydue to the limited etch resistance of resist. In this study, the method of electron beam stabilization has beenapplied for improving the etch selectivity ofresist patterns having an aspect ratio less than 3 : 1. With applying theelectron beam stabilization, the Deep-UV photoresists based on the chemical structures of Acetal (AS1O6) andEscap (UV82) types have been evaluated in the respect of etch selectivity as the functions of an electron beamdose and etch condition. The metal etch rate reductions of 20 percent and 26 percent have been occurred for theresists of Acetal and Escap type, respectively, at 2OOOtC/cm2. And the thermal and chemical properties werecharacterized before and after electron beam stabilization using DSC, TGA, and FT-JR. The cross-sectionalviews of resist pattern after electron beam processing were also investigated to know the chemical stability ofresist during the electron beam process. Based on the experimental results, the application possibility of electronbeam stabilization for real device fabrication below O.l4prn has been presented in this paper.Key Words : Electron Beam Stabilization, Deep-UV Photoresist, Acetal Type, Escap Type, Aspect Ratio

Inductively coupled plasma etching of silicon nanostructures for metalens applications using a continuous, multi-step C4F8/SF6 plasma was investigated to achieve high aspect ratio (HAR) features down to tens of nanometers with smooth sidewalls. In the process, the ion bombardment and the free radical transport significantly change among HAR nanostructures as the etching progresses, posing challenges to profile control. With a fixed gas ratio, a change in the profile angle occurs at a depth of approximately 400 nm, transitioning from a positive taper to a negative one. Additionally, a wave-like pillar profile is produced when using three separate (i.e., plasma turned off after each step) etching processes with varying gas ratios. To optimize passivation and etching, we adopt a three-step C4F8/SF6 plasma etching process with varying gas ratios at different etching depths. By keeping the plasma on after each step, the continuous, three-step process provides more flexibility for tuning the etching of HAR nanostructures with smooth and vertical profiles. Metalens nanostructures with 71 nm diameter and 1 μm height were created using the appropriate gas ratio. The feature size variation is less than 10 nm. This proposed continuous, multi-step process improves the controllability of silicon etching in C4F8/SF6 plasma, facilitating the nanofabrication of silicon metalens and other nanodevices.

Glass-ceramics play an important role in todays science and industry as it can withstand immense heat, mechanical and other hazards. Consequently, there is a need to find ever-new ways to acquire more sophisticated free-form 3D ceramic and glass structures. Recently, stereo-lithographic 3D printing of hybrid organic-inorganic photopolymer and subsequent pyrolysis was demonstrated to be capable of providing true 3D ceramic and glass structures. However, such approach was limited to (sub-)millimeter scale, while one of the aims in the field is to acquire functional 3D glass-like structures in micro- or even nano-dimensions. In this paper, we explore a possibility to apply ultrafast 3D laser nanolithography in conjunction with pyrolysis to acquire glass-ceramic 3D structures in micro- and nano-scale. Laser fabrication allows production of initial 3D structures with relatively small (hundreds of nm) feature sizes out of hybrid organic-inorganic material SZ2080. Then, a post-fabrication heating at different temperatures up to 1000°C in Ar , air or O2 atmospheres decomposes organic part of the material leaving only the glass-ceramic component of the hybrid. As we show, this can be done to 3D woodpiles and bulk objects. We uncover that the shrinkage during sintering can reach up to 40%, while the aspect ratio of single features as well as filling ratio of the whole object remains the same. This hints at homogeneous reduction in size that can be easily accounted for and pre-compensated before manufacturing. Additionally, the structures prove to be relatively resilient to focused ion beam (FIB) milling, hinting at increased rigidity. Finally, thermal gravimetric analysis (TGA) and Fourier transform infrared micro-spectroscopy measurements are performed in order to uncover undergoing chemical and physical phenomena during pyrolysis and composition of the remnant material. The proposed post-processing approach offers a straightforward way to downscale true 3D micro-/nanostructures for applications in nanophotonics, microoptics and mechanic devices with improved performance while being highly resilient to harsh surrounding conditions.

Polymeric nanostructures with high aspect ratios, so-called nanopillars, are of interest for a wide range of applications. However, it remains a challenge to fabricate high-density, polymeric nanopillars using soft lithography when the feature size is decreased to hundreds of nanometers and the structures are close to each other. Here, we investigate the fidelity of replica molding technique to fabricate polymer nanopillar arrays with diameters ranging from 300 nm to 1 mum, and we compare the experimental results to the theoretical prediction to understand the nature of the instability of nanopillars. Nanopillars molded from soft materials, poly(dimethylsiloxane) (PDMS), mainly ground collapse due to the adhesive force when the aspect ratio is above 6, whereas those from stiffer materials, polyurethane and epoxy, collapse laterally at a much higher aspect ratio (>/=12), of which the critical value is dependent on the nanopillar's feature size, spacing, height, and shape. Further, we attempt to restore the collapsed high-aspect-ratio nanopillars using supercritical CO(2) drying.

This work investigates the fabrication process to achieve high aspect ratio nanostructures on quartz substrates using electron beam lithography (EBL) patterning and fluorinated plasma etching processes. A PMMA bilayer resist was coated on a quartz substrate and then e-beam exposed with the designed pattern of sub-100 nm feature sizes, using the Raith-150 EBL patterning tool. Additive pattern transfer was employed by depositing a 40 nm nichrome layer on the resist pattern using a metal evaporator and later lifted off by soaking in the acetone. The etching was performed on quartz substrates with nichrome pattern masks using the conventional Oxford Plasmalab 80Plus reactive ion etcher. The etching process was carried out in a gas mixture of CHF3/Ar with a flow rate ratio of 50/30 sccm, pressure of 20 mTorr, RF power of 200 W and at room temperature. This etching process setting was found to achieve a 10 nm/min etch rate and tall vertical side walls profile. An aspect ratio of 10:1 has been achieved on a 60 nm feature size structures.