Electron beam lithography: resolution limits and applications

Abstract

We report on the resolution limits of Electron Beam Lithography (EBL) in the conventional polymethylmethacrylate (PMMA) organic resist. We show that resolution can be pushed below 10 nm for isolated features and how dense arrays of periodic structures can be fabricated at a pitch of 30 nm, leading to a density close to 700 Gbit/in². We show that intrinsic resolution of the writing in the resist is as small as 3 to 5 nm at high incident electron energy, and that practical resolution is limited by the development of the resist after exposure and by pattern transfer. We present the results of our optimized process for reproducible fabrication of sub-10 nm lines by lift-off and 30-nm pitch pillar arrays by lift-off and reactive ion etching (RIE). We also present some applications of these nanostructures for the fabrication of very high density molds for nano-imprint lithography (NIL) and for the fabrication of Multiple Tunnel Junction devices that can be used for single electron device applications or for the connection of small molecules.

Introduction

Many applications of nanofabrication techniques, like single electron devices [1], electrical connection of individual molecules [2] or ultra-high density storage media [3], now require the production of sub-10 nm structures. In this context, Electron-Beam Lithography (EBL) could be a tool of best quality, combining a reasonable writing speed with a possible high level of integration of complex devices, if reliable sub-10 nm nanofabrication procedures could be demonstrated. This paper addresses the important issue of EBL on the conventional polymethylmethacrylate (PMMA) organic resist associated with pattern transfer techniques like lift-off and reactive ion etching (RIE). Our objective is to demonstrate that full nanofabrication procedures including transfer technologies can be achieved at the sub-10 nm length scale.

The problem of the ultimate resolution of the conventional positive organic resist for EBL: PMMA, has been addressed by Chen and Ahmed [4] who demonstrated that the 10-nm limit, previously established [5], could be surpassed using an ultrasonic (US) agitation during the development of the resist. These authors propose that when the linewidth decreases to below 10 nm, the intermolecular forces prevent exposed resist molecules to be dissolved into the developer solution unless US agitation is used to assist the dissolution process. Lines of 5–7 nm have been written in PMMA using this optimized development technique [4], making a real breakthrough in the domain. On the other hand, a recent study on EBL using PMMA resist has demonstrated the feasibility of 25-nm pitch dot arrays and 40-nm pitch line arrays using pure IsoPropylic Alcohol (IPA) development [6]. In this work, the advantage of using pure IPA as a developer for fabricating very dense arrays was demonstrated, but transfer techniques like lift-off and etching were not successful at this length scale.

In this paper, we investigate the resolution limits of EBL on PMMA for isolated features and for very dense arrays of nanostructures. We insist on the optimization of resist development, which is the key process for reaching the sub-10 nm level. In particular, we investigate the effects of the combination of both US agitation during development and pure IPA as a developer for PMMA. We present lift-off experiments at a sub-10 nm level opening interesting perspectives for single electron devices or single molecule connection. We also report on a complete process including writing in the resist, lift-off and subsequent RIE for making dot and line arrays at pitches and periods down to 30 and 40 nm, respectively. These gratings can form suitable high-density molds for Nano-Imprint Lithography (NIL) applications [7]. Our technique allows us to fabricate pillar arrays of ∼700 Gbit/in² density (100 Gbit/cm²) and line arrays of 20-nm line and space etched in silicon dioxide with a nickel mask obtained by lift-off.