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

Abstract

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.

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

“…The sensitivity is similar to that of low molecular weight polystyrene [21], and may be improved drastically by using higher molecular weight sodium PSS, as for chain cross-linking polymer resist the sensitivity (μC/cm 2 ) is expected to be roughly inversely proportional to its molecular weight (kg mol −1 ) [22]. The polydispersity of our sodium PSS is unknown, but it is not expected to have a significant effect on its exposure property [23]. As for the resist contrast, one way to enhance it is to use lower molecular weight, but at the cost of reduced sensitivity.…”

Dry thermal development of negative electron beam resist polystyrene

“…It is not possible to derive a meaningful resist contrast from this curve, although clearly the contrast is very low. The low contrast is not a result of the resist's broad molecular weight distribution, as it has been shown that the polydispersity (defined as Mw/Mn, the ratio of weight averaged molecular weight and number averaged molecular weight) had an insignificant effect on polystyrene's exposure properties (Dey and Cui 2013 Fig. 1(b)…”

Chromium oxide as a hard mask material better than metallic chromium

“…PS is a very versatile negative resist offering tunable sensitivity and contrast simply by varying its molecular weight. 15,16 PS has a dry etching rate very close to that of ZEP-520A (positive) resist, and is two to three times slower than PMMA because PMMA is a linear polymer with higher oxygen content and higher radiation yield of chain scission reactions, whereas ZEP-520A has a stable phenyl group in its chemical structure. [17][18][19] PS film of 400 nm thickness was coated on silicon wafer using spin coating method.…”