In dry plasma silicon etching, it is desired to have a high etching rate, a high etching selectivity to mask material, a vertical or controllable sidewall profile, and a smooth sidewall. Since the standard Bosch process (switching between SF6 and C4F8 gases) leads to a wavy/rough sidewall profile, the nonswitching pseudo-Bosch process is developed to give a smooth sidewall needed for nanostructure fabrication. In the process, SF6 and C4F8 gases are introduced to the chamber simultaneously. Here, the authors show that by introducing a periodic oxygen (O2) plasma cleaning step, that is, switching between SF6/C4F8 etching and O2 cleaning, the silicon etching rate can be significantly improved (by up to ∼55%, from 139 to 216 nm/min) without any adverse effect. This is mainly because O2 plasma can remove the fluorocarbon polymer passivation layer at the surface. The etching and cleaning step durations were varied from 5 s to 40 min and from 0 to 60 s, respectively. The fastest etching rates were obtained when the cleaning step takes roughly 10% of the total etching time.

Hydrogen silsesquioxane (HSQ) is arguably the most popular negative e-beam resist for academic research. One of the most significant advantages of HSQ is its ultrahigh resolution. However, it has a short shelf life, which increases its cost. As an alternative, a new type of dry powder HSQ resist resin (Applied Quantum Materials, referred to as AQM) was introduced here, the shelf life of which can be considered as infinitely long. A small amount of the powder can be dissolved in a solvent as needed right before exposure. Furthermore, this powder HSQ resist has a similar resolution and sensitivity parameters. By using the high-contrast development process with a salty developer, a 7.5 nm half-pitch nested "L" shape structure is achieved. Published by the AVS.

An electron beam resist is usually coated by conventional coating methods such as spin‐coating, which cannot be reliably applied on irregular surfaces. Here, it is demonstrated that a monolayer resist can be grafted on nonflat surface to enable nanofabrication on it. As a proof‐of‐concept of patterning on irregular surfaces, poly(methyl methacrylate) (contains 1.6% methacrylic acid that has the carboxyl group needed for grafting) is chosen and is grafted on irregular surfaces by thermal treatment which induces a chemical reaction of the carboxyl group with the hydroxyl group on substrate. Subsequently, nanostructures are patterned by electron beam lithography on this monolayer resist grafted on nonflat surface such as atomic force mocroscopy (AFM) cantilevers, and then the patterns are transferred to the layer underneath. A high resolution of 30 nm line width is achieved using this monolayer resist. Nanofabrication on irregular surfaces may have applications in the fields of tip‐enhanced Raman spectroscopy for chemical analysis and lab‐on‐fiber technology for sensor applications.

In nanofabrication, use of thin resist is required to achieve very high resolution features. But thin resist makes pattern transferring by dry etching difficult because typical resist has poor resistance to plasma etching. One widely employed strategy is to use an intermediate hard mask layer, with the pattern first transferred into this layer, then into the substrate or sublayer. Cr is one of the most popular hard etching mask materials because of its high resistance to plasma etching. Cr etching is carried out in O2 and Cl2 or CCl4 environment to form the volatile etching product CrO2Cl2, but addition of O2 gas leads to fast resist etching. In this work, the authors show that Cr2O3 can be etched readily in a Cl2/O2 gas mixture with less oxygen than needed for Cr etching, because Cr2O3 contains oxygen by itself. Thus it is easier to transfer the resist pattern into Cr2O3 than into Cr. For the subsequent pattern transferring into the substrate here silicon using nonswitching pseudo-Bosch inductively coupled plasma-reactive ion etching with SF6/C4F8 gas and Cr or Cr2O3 as mask, it was found that the two materials have the same etching resistance and selectivity of 100:1 over silicon. Therefore, Cr2O3 is a more suitable hard mask material than Cr for pattern transferring using dry plasma etching.

One of the important challenges in electron beam lithography is nanofabrication on nonflat or irregular surfaces. Although spin coating is the most popular technique for resist coating, it is not suitable for nonflat, irregular substrates because a uniform film cannot be achieved on those surfaces. Here, it is demonstrated that single layer surface-grafted PMMA can be used as a negative-tone e-beam resist, and it can be applied to nonflat, irregular surfaces as well as flat, conventional surfaces. Although it is well known that heavily exposed PMMA undergoes cross-linking and works as a negative-tone e-beam resist when developed by solvent, solvent does not work as a developer for negative-tone single-layer surface-grafted PMMA. Instead, thermal treatment at 360 °C for 1 min is used to develop PMMA.

Although spin coating is the most widely used electron-beam resist coating technique in nanolithography, it cannot typically be applied for nonflat or irregular surfaces. Here, we demonstrate that monolayer polystyrene brush can be grafted on substrates and used as both positive and negative electron-beam resist, which can be applied for such unconventional surfaces. Polystyrene is a popular negative resist when using solvent developer but solvent cannot be used for grafted polystyrene brush that is firmly bonded to the substrate. Instead, we employed two unconventional development methods to lead polystyrene brush to positive or negative tone behavior. Negative tone was achieved by thermal development at 300 °C because exposed thus cross-linked polystyrene brush is more thermally stable against vaporization than unexposed linear one. Surprisingly, positive tone behavior occurred when the brush was grafted onto an aluminum (Al) layer and the film stack was developed using diluted hydrofluoric acid (HF) that etched the underlying Al layer. By transferring the patterns into the silicon (Si) substrates using the thin Al layer as a sacrificial hard mask for dry etch, well-defined structures in Si were obtained in two different electron-beam resist tones as well as in nonflat surfaces.

Dry etching of silicon has been extensively studied, mostly with a goal of obtaining perfectly vertical sidewalls with high aspect ratio. Yet, sloped sidewall with a negative taper angle (i.e., diameter/width decreases linearly with depth) may find various applications. However, the systematic study on the etching process development to obtain such a profile is rather scarce. In this work, the authors present a controlled and reproducible fabrication process to achieve silicon nanostructures with negatively tapered sidewall profile using inductively coupled plasma-reactive ion etching with C4F8 and SF6 gas. The plasma etching parameters have been thoroughly optimized in order to avoid the undercut or curved reentrant profile due to isotropic etching, so as to achieve a negatively tapered profile. The influence of the plasma etching parameters, especially the radio freguency power and C4F8/SF6 gas flow ratio, on the etching rate and the sidewall taper angle has been analyzed. With an optimal etching recipe, the silicon nanostructures with an unprecedented large 10° negative taper angle were achieved. These results were demonstrated on different structure sizes of 500 nm, 700 nm, and 1.2 μm diameters.

A lift-off process is a popular method to pattern metals, especially for the noble metals that are hard to dry-etch. For a “clean” lift-off process, an undercut profile is critical and is commonly achieved by using a bilayer resist stack. A resist with tunable sensitivity is apparently the most desirable, since it offers a controlled amount of undercut when used as the bottom layer, with the top layer being a less sensitive resist. In this study, the authors show that a simple mixture of poly (methyl methacrylate) (PMMA) and ZEP can offer tunable sensitivity by adjusting the ratio of the two resists dissolved in anisole. Higher sensitivity was attained by increasing the ZEP content in the mixture since ZEP is about 3× more sensitive than PMMA. However, the relationship is not a linear one, and the contrast curve for a mixture containing more PMMA (e.g., PMMA:ZEP ratio of 2:1) is closer to that of pure ZEP than to PMMA. For dense line array patterns with a periodicity of 200 and 500 nm, a moderate undercut obtained by using a low ZEP concentration (PMMA:ZEP = 2:1 as the bottom layer, PMMA as the top layer) gave the result for lift-off of 100 nm Cr. While using pure ZEP as the bottom layer, the undercut was often too large that the resist lines collapsed because of capillary force or even completely detached when the adjacent undercut merged together.

For fabrication of nanostructures that do not need long range ordering and precise placement, such as antireflective structure for photovoltaic and display applications and super-hydrophobic surface for lab-on-chip applications, bottom-up fabrication techniques are more preferable than top-down techniques due to their low cost. Here, the authors report a simple process to obtain nanostructures using low-cost spin-coating method and pattern transfer. They first dissolved metal salt and polymer in a solvent. After spin-coating to form a thin film, the authors annealed the film to attain a phase separation. Next, the nanocomposite film was etched with oxygen plasma to remove the polymer matrix, leaving behind nanoscale metal salt islands that can be used as a hard mask for dry etching the substrate or sublayer. With optimal metal salt (nickel nitrate hexahydrate) and polymer (polymethylmethacrylate) weight ratio, the authors achieved wafer-scale high resolution (down to 20 nm) pillar structures etched in silicon with 100 nm height and ∼100 nm spacing.

Different metal-insulator-metal diodes using the fixedbottom electrode with different top electrodesandmulti-insulator layers were investigated. Chromium was usedfor the bottom metal electrode andTi, Cr, Al, and Pt were used asthe top metal electrode. The effects of tunneling junction and its effects in the diode performance were also studied. The high asymmetry value of 40, 16 and 6 were obtainedon applied voltage in the range of 0.5-3V respectively on using the Ti, Cr and Pt as the second metal electrode in the Metal-4 insulator layers-Metal diodes, while a combination of Cr-insulator layers-Al was shown relatively weak asymmetry curve.