Ripon Kumar Dey

Effect of molecular weight distribution on e-beam exposure properties of polystyrene.

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 polystyrenes 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.

Theoretical and experimental investigations of nano-Schottky contacts

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...

Characterization of nano Schottky junctions for a new structure of nano-electronic devices

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.

Stitching error reduction in electron beam lithography with in-situ feedback using self-developing resist

In electron beam lithography (EBL), a large area pattern is divided into smaller writing fields, which are then stitched together by stage movement to generate the large area pattern. Precise stage movement is essential to minimize the stitching error, and this can be achieved by using laser interferometer-controlled stage. In addition, electron beam deflection must be adjusted to match the stage movement, which is referred to as “writing field alignment.” To expose large area nanostructures, a large writing field must be used; otherwise, the stage movement time would be impractically long. However, writing field alignment accuracy decreases with a larger writing field owing to its low magnification. Here, the authors report that self-developing resist (for which the pattern shows up immediately after exposure, thus eliminating the need for ex-situ development) can provide in-situ feedback for writing field alignment accuracy, which in turn can be used to optimize the alignment. After several iterations u...

Lift-off with solvent for negative resist using low energy electron beam exposure

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.

Grafted Polystyrene Monolayer Brush as Both Negative and Positive Tone Electron Beam Resist

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.

Mixture of ZEP and PMMA with varying ratios for tunable sensitivity as a lift-off resist with controllable undercut

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...

Enhanced adhesion of electron beam resist by grafted monolayer poly(methylmethacrylate-co-methacrylic acid) brush

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-100 nm period grating structures. But the improvement was negligible for an aluminum substrate, because, ev...

The dependence of Schottky junction (I–V) characteristics on the metal probe size in nano metal–semiconductor contacts

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.

Single Layer Surface-Grafted PMMA as a Negative-Tone e-Beam Resist

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.