Resists for sub-20-nm electron beam lithography with a focus on HSQ: state of theart

Abstract

In the past decade, the feature size in ultra large-scale integration (ULSI) has beencontinuously decreasing, leading to nanostructure fabrication. Nowadays, variouslithographic techniques ranging from conventional methods (e.g. photolithography, x-rays)to unconventional ones (e.g. nanoimprint lithography, self-assembled monolayers) are usedto create small features. Among all these, resist-based electron beam lithography(EBL) seems to be the most suitable technique when nanostructures are desired.The achievement of sub-20-nm structures using EBL is a very sensitive processdetermined by various factors, starting with the choice of resist material and endingwith the development process. After a short introduction to nanolithography, aframework for the nanofabrication process is presented. To obtain finer patterns,improvements of the material properties of the resist are very important. Thepresent review gives an overview of the best resolution obtained with severaltypes of both organic and inorganic resists. For each resist, the advantages anddisadvantages are presented. Although very small features (2–5 nm) have beenobtained with PMMA and inorganic metal halides, for the former resist the low etchresistance and instability of the pattern, and for the latter the delicate handling ofthe samples and the difficulties encountered in the spinning session, prevent thewider use of these e-beam resists in nanostructure fabrication. A relatively newe-beam resist, hydrogen silsesquioxane (HSQ), is very suitable when aiming forsub-20-nm resolution. The changes that this resist undergoes before, during and afterelectron beam exposure are discussed and the influence of various parameters(e.g. pre-baking, exposure dose, writing strategy, development process) on theresolution is presented. In general, high resolution can be obtained using ultrathinresist layers and when the exposure is performed at high acceleration voltages.Usually, one of the properties of the resist material is improved to the detriment ofanother. It has been demonstrated that aging, baking at low temperature, immediateexposure after spin coating, the use of a weak developer and development at a lowtemperature increase the sensitivity but decrease the contrast. The surface roughnessis more pronounced at low exposure doses (high sensitivity) and high bakingtemperatures. A delay between exposure and development seems to increase bothcontrast and the sensitivity of samples which are stored in a vacuum after exposure,compared to those stored in air. Due to its relative novelty, the capabilities of HSQhave not been completely explored, hence there is still room for improvement.Applications of this electron beam resist in lithographic techniques other than EBL are alsodiscussed. Finally, conclusions and an outlook are presented.