The shape and dimension of atomic force microscope (AFM) probes are essential considerations to acquire high resolution images at the nanoscale. In the cases of nanostructures with high aspect ratio (HAR) features, including dot and line arrays, commercially available standard silicon AFM probes fail to provide satisfactory results. This is due to the low aspect ratio pyramidal tip profile which cannot adequately follow the sample surface. Here a simple method is introduced to convert commercially available pyramidal probes to HAR probes by combining focused ion beam (FIB) and plasma etching techniques. A bilayer metal is deposited on standard AFM probes using e-beam evaporation, followed by a quick FIB milling process to pattern the very thin top metal layer. The FIB milled metal layer is used as the hard mask to transfer the pattern to the underlying metal layer and an HAR pillar on top of the tip apex was achieved via plasma etching. Compared to the traditional HAR tip fabrication, where the tips are individually manufactured at high cost and in a time-consuming fashion, our method significantly reduces the usage of FIB milling. Additionally, this method is suitable for batch fabrication via plasma etching once the simple FIB milling step is performed.

High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contact. It is also reported that a higher bias power leads to less undercut than a lower one, due to increased and more directional physical bombardment by ions.

Among various grating structure fabrication techniques, potassium hydroxide (KOH) wet anisotropic etching of Si(110) wafers offers low cost and impressive aspect ratio over large areas with high etch uniformity. The aspect ratio is ultimately limited by lateral etching that constantly widens the trenches. In this paper, the authors demonstrated a method to double the achievable aspect ratio using two-step KOH etching. After first KOH etching, the grating structure was grown with a thermal oxide; and after removing the oxide from the trench bottom using reactive ion etching, a second KOH etching was carried out with the original trench sidewall protected by the thermal oxide. The authors achieved the highest anisotropy [etching rate ratio of (110) and (111)] of 247 with 50 wt.% KOH at room temperature. Using the two-step KOH etching, it is possible to increase the aspect ratio by more than a factor of 2 while keeping the trench width almost unchanged.

A CubeSat is a type of miniaturized and modular satellite designed for space research or technology demonstration. By filling the unused capacity of major launch vehicles, CubeSats significantly lower the cost of entry to the space. To reduce the energy loss at the solar panel coverglass on CubeSats, the authors investigated the potential of the biomimetic moth-eye antireflection (MEAR) technique which features a gradient refractive index. A novel fabrication process combining nanosphere lithography, reactive-ion etching, and dry oxidation for patterning glass with moth-eye structures was reported that allows for the production of well-defined nanopillars with tunable sidewall profiles. The thermal annealing treatment of colloidal particles was introduced for making MEAR structures for which partial deformation of nanospheres was achieved. The broadband and omnidirectional antireflective performance of fabricated glasses was experimentally demonstrated by using a spectrophotometer and ellipsometer. A significant 10% decrease of reflectance throughout the measured spectral range was observed at a high incident angle of 75°.

We have studied the dependence of Schottky junction (I–V) characteristics on the metal contact size in metal–semiconductor (M–S) junctions using different metal nanoprobe sizes. The results show strong dependence of (I–V) characteristics on the nanoprobe size when it is in contact with a semiconductor substrate. The results show the evolution from sub-10 nm reversed Schottky diode behavior to the normal diode behavior at 100 nm. These results also indicate the direct correlation between the electric field at the M–S interface and the Schottky rectification behavior. The effect of the metal contact size on nano-Schottky diode structure is clearly demonstrated, which would help in designing a new type of nano-devices at sub-10 nm scale.

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.

One major problem for most commercial atomic force microscope (AFM) probes is the uncertainty of the tip location relative to its cantilever. In most scenarios, AFM probes have tips 5–25 μm away from the very end of the cantilever, and it is thus impossible to know where exactly the tip is because the camera in an AFM system shows only the backside of the AFM cantilever. This uncertainty of the tip location has raised some major problems, e.g., the initial scanning area must be set very large to ensure that the area of interest is within the scanning field. Here, the authors will show a straightforward fabrication method that can convert a wafer of regular pyramidal-shaped probes into direct positioning probes, for which the tip is located either at the very end of its cantilever or next to a through-cantilever hole that is visible when viewed from the backside of the cantilever. Our method involves angle evaporation of a hard mask layer onto the AFM probe, followed by dry etching of silicon that etches the area not covered by the metal layer, i.e., the shadow area of the pyramid-shaped tip. As an additional benefit, because our process etched away half of the tip pyramid, the resulting tip is sharper with a smaller half cone angle than the original one, leading to higher resolution imaging.

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.

Sharp tips are essential for high-resolution atomic force microscopy (AFM) imaging and high-performance electron emitters in vacuum microelectronic devices. Thermal oxidation at high temperature followed by oxide removal is widely used in the nanofabrication of sharp silicon AFM/emitter tips. This method relies on the fact that oxide grows slower on areas with a smaller radius of curvature. Thermal oxidation is commonly carried out in a dedicated oxidation furnace that is costly, and the tips or wafer of tips must be cleaned thoroughly using Radio Corporation of America (RCA) cleaning. Here, the authors report that oxidation sharpening can also be attained using a very low-cost generic box furnace in the atmospheric environment that does not require the tips to go through an RCA cleaning process. As is apparent, such cleaning is not convenient for millimeter-scale AFM probes. The minimum tip apex radius of 2.5 nm was obtained by oxidation at 950 °C in the atmospheric environment. The obvious application of this approach is the regeneration of sharp tips out of worn out and thus blunt AFM probes at very low cost.

Formation of metal-semiconductor (M-S) contacts at sub-20 nanometer range is a key requirement for down-scaling of semiconductor devices. However, electrical measurements of M-S contacts at this scale have exhibited dramatic change in the current-voltage (I-V) characteristics compared to that of conventional (or planar) Schottky contacts. This change is actually attributed to the limited metal contact region where the transferred charge from the semiconductor into the metal is confined to a small surface area, which in turn results in an enhanced electric field at the nano-M-S interface. We here present detailed theoretical models to analyze the nano-M-S junctions at 10 nm contact range and then implement this analysis on the experimental data we conducted under these conditions. Both theoretical and experimental results demonstrate a significant effect of the contact size on the electronic structure of the M-S junctions and thus on the I-V characteristics. This effect is rather prominent when the size of the metal contact is substantially smaller than the width of conventional depletion region of the relevant planar M-S contacts.

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.

In electron beam lithography, poor resist adhesion to a substrate may lead to resist structure detachment upon development. One popular method to promote resist adhesion is to modify the substrate surface. In this study, the authors will show that a poly(methylmethacrylate-co-methacrylic acid) [P(MMA-co-MAA)] monolayer "brush" can be grafted onto a silicon substrate using thermal annealing that leads to chemical bonding of the P(MMA-co-MAA) copolymer to the hydroxyl group-terminated substrate, followed by acetic acid wash to remove the bulk, unbonded copolymer. The monolayer brush has a thickness of 12 nm. The authors will show that it can greatly improve the adhesion of positive resist, the ZEP-520A, and negative resist polystyrene to bare silicon surfaces, which led to high resolution patterning without resist detachment upon development. The improvement was more dramatic when patterning dense sub-100nm period grating structures. But the improvement was negligible for an aluminum substrate, because, even without the brush layer, resist adhesion to aluminum is found already to be strong enough to prevent resist structure peeling off. The current simple and low cost method could be very useful when resist adhesion to the substrate for a given developer is weak. (C) 2015 American Vacuum Society.

Due to the lack of feedback, conventional electron beam lithography (EBL) is a ‘blind’ open-loop process where the exposed pattern is examined only after ex situ resist development, which is too late for any improvement. Here, we report that self-developing nitrocellulose resist, for which the pattern shows up right after exposure without ex situ development, can be used as in situ feedback on the e-beam distortion and enlargement. We first exposed identical test pattern in nitrocellulose at different locations within the writing field; then, we examined in situ at high magnification the exposed patterns and adjusted the beam (notably working distance) accordingly. The process was repeated until we achieved a relatively uniform shape/size distribution of the exposed pattern across the entire writing field. Once the beam was optimized using nitrocellulose resist, under the same optimal condition, we exposed the common resist PMMA. We achieved approximately 80-nm resolution across the entire writing field of 1 mm × 1 mm, as compared to 210 nm without the beam optimization process.

Lift-off and direct etch are the two most popular pattern transfer methods for electron beam lithography. For some applications negative resist would offer significantly less exposure time than positive one. Unfortunately, lift-off using negative resist is very challenging because the resist profile is typically positively tapered due to electron forward scattering, and upon exposure, negative resist is cross-linked and thus insoluble in solvents. Here, the authors will show that low energy exposure can circumvent both issues simultaneously, and the authors achieved liftoff of Cr with polystyrene resist using a solvent xylene. Moreover, low energy exposure offers proportionally higher resist sensitivity. Lastly, since low energy electrons are mostly stopped inside the resist layer, radiation damage to the sublayer is greatly reduced. Thus, the current method may be employed to fabricate metal nanostructures on top of an organic conducting layer.

There is an increasing interest in reducing the size of semiconductor devices to sub 20 nm scale for technical requirements, like low power consumption and high switching speed. Electronic devices based on nano Schottky junctions have the potential to address these issues. This is because nano metal-semiconductor contacts are expected to have narrower barriers compared to conventional Schottky diodes. Nano Schottky junctions have been investigated experimentally using gold (Au) coated AFM tips in contact with different silicon (Si) substrates. For nano-tips with an apex radius around 7 nm, the current-voltage (I-V) curves on low n-dope Si substrates have showed a reversed rectification diode behavior compared to the high n-dope Si samples. We have used a new theoretical model to study the electric field enhancement at the nano metal-semiconductor interface, and thus the enhancement of the tunneling current. We have found out that the tunneling current at the reverse bias is dominant on low dope substrates and very small on high dope substrates. This accounts for the reversed I-V rectification behavior on low dope Si Schottky contacts. The calculated I-V curves showed good agreement with the experimental results for both types of Si samples.

Polystyrene is a negative electron beam resist whose exposure properties can be tuned simply by using different molecular weights (Mw). Most previous studies have used monodisperse polystyrene with a polydispersity index (PDI) of less than 1.1 in order to avoid any uncertainties. Here we show that despite the fact that polystyrene's sensitivity is inversely proportional to its Mw, no noticeable effect of very broad molecular weight distribution on sensitivity, contrast and achievable resolution is observed. It is thus unnecessary to use the costly monodisperse polystyrene for electron beam lithography. Since the polydispersity is unknown for general purpose polystyrene, we simulated a high PDI polystyrene by mixing in a 1:1 weight ratio two polystyrene samples with Mw of 170 and 900 kg mol(-1) for the high Mw range, and 2.5 and 13 kg mol(-1) for the low Mw range. The exposure property of the mixture resembles that of a monodisperse polystyrene with similar number averaged molecular weight Mn, which indicates that it is Mn rather than Mw (weight averaged molecular weight) that dominates the exposure properties of polystyrene resist. This also implies that polystyrene of a certain molecular weight can be simulated by a mixture of two polystyrenes having different molecular weights.