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

Abstract

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.

Water soluble and metal-containing electron beam resist poly(sodium 4-styrenesulfonate)

“…This is because, with low energy exposure, water can penetrate through the fully cross-linked upper layer and swell/weaken the linear or partly cross-linked lower layer at the resist/P3HT interface, which leads to a very weak adhesion. Similarly low resolution is also reported for low energy exposures of polystyrene resist [25].…”

“…Here we carried out electron beam lithography at a very low energy of 2 keV on sodium PSS coated on P3HT that is one of the most popular conducting polymer materials, followed by development and liftoff of 10 nm Cr, both with water. Here the low energy exposure is essential because: 1) it has a low penetration depth and thus does not fully cross-link the lower part of the resist, making liftoff using water possible; 2) the unexposed or under-exposed lower part can have quick lateral development, forming an under-cut profile ideal for liftoff (see figure 3(a) for schematic drawing and [25] for SEM images of undercut resist profile); 3) as most electrons are stopped in the resist layer, the sublayer is not significantly exposed, thus there is minimal radiation damage. In addition, since the resist sensitivity (μC/cm 2 ) is roughly proportional to the exposure energy (keV) as predicted by the Bethe equation for electron energy loss (E loss ∝ 1/E · log(αE) with α being constant), lower energy exposure offers faster exposure of the pattern.…”

Electron beam lithography on nonplanar and irregular surfaces

“…PS can be dissolved in various solvents, including acetone, tetrahydrofuran, chlorobenzene, cyclohexane, xylene, and anisole. However, crosslinking occurs when PS is exposed to an electron beam, rendering these solvents ineffective 41,42 . PS resists have many benefits: for instance, low-molecular-weight PS has an exceptionally high resolution 41 , while high-molecular-weight PS has a high sensitivity 40 .…”

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

“…6 For negative resists, lift-off is rarely used for pattern transfer because a slightly positively tapered profile, instead of an undercut profile, would usually result because of electron forward scattering. Nevertheless, lift-off using the negative resist such as polystyrene 7 or water soluble poly(sodium 4-styrenesulfonate) 8 can be realized when using very low energy exposure such that the resist bottom is inadequately exposed and thus dissolves fast in the developer. It is important to point out that, for lifting off some structures such as nanoscale trenches or holes, a negative resist is preferred as it offers far less exposure time than positive resists (yet a complicated double lift-off process using a positive resist can be employed to obtain such structures 9 ).…”