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 O 2 and Cl 2 or CCl 4 environment to form the volatile etching product CrO 2 Cl 2 , but addition of O 2 gas leads to fast resist etching. In this work, the authors show that Cr 2 O 3 can be etched readily in a Cl 2 /O 2 gas mixture with less oxygen than needed for Cr etching, because Cr 2 O 3 contains oxygen by itself. Thus it is easier to transfer the resist pattern into Cr 2 O 3 than into Cr. For the subsequent pattern transferring into the substrate here silicon using nonswitching pseudo-Bosch inductively coupled plasma-reactive ion etching with SF 6 /C 4 F 8 gas and Cr or Cr 2 O 3 as mask, it was found that the two materials have the same etching resistance and selectivity of 100:1 over silicon. Therefore, Cr 2 O 3 is a more suitable hard mask material than Cr for pattern transferring using dry plasma etching.

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.

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 C 4 F 8 and SF 6 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 C 4 F 8 /SF 6 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 lm diameters. V

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.

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.