Low-energy electron (0-100eV) interaction with resists using LEEM

Abstract

Extreme Ultra Violet (EUV) lithography is a next generation lithographic technique using 13.5 nm wavelength light (91.7eV photon energy) to define sub-20 nm features. This high energy radiation generates lower energy electrons (LEEs) after being absorbed. The mean free path of LEEs increases rapidly below ca. 30 eV allowing them to migrate several nanometers from their point of origin. As LEEs can still have sufficient energy to react with the surrounding resist, this may give rise to pattern blurring, posing a challenge for sub 10 nm features. Here, we introduce Low Energy Electron Microscopy (LEEM) as an extremely useful technique to investigate the interactions of LEEs with EUV resists. Using LEEM we can expose the resist with precise electron energies and doses. We also report the initial results of LEE exposures on poly(methyl methacrylate) PMMA. We have studied the LEE-PMMA interaction depth as a function of electron energy; a distinct exposure threshold is found at ~15 eV, below which the resist responds only very weakly to electron exposure. Figure 1 (A) Line edge blur caused by secondary electrons generated upon 91.7 eV photon absorption. The inelastic mean free path of the electrons increases rapidly at lower energies as shown in (B). This can influence the blur as well as the electron penetration depth during EUV exposure.

Charge Catastrophe and Dielectric Breakdown During Exposure of Organic Thin Films to Low-Energy Electron Radiation

“…While these results are for PMMA, experiments on exploratory EUV resists containing Sn(oxo) cages [26,27] display the same effects: sensitivity for near-zero eV electrons and strong charging instabilities. As the same basic mechanisms apply, the experimental and theoretical methodology developed here will make it possible to study such EUV resists more fully, and to contribute directly to their characterization and optimization.…”

“…The sample is immersed in an electrostatic field of ∼100 kV=cm, slowing the 15 keV electrons produced by the gun to tunable 0-100 eV incident energy, E 0 . Secondary electrons leaving the sample are extracted by this field, and can never return [2][3][4]. The experiment is schematically shown in Fig.…”

“…Figure 1(a) shows a 20 nm PMMA film exposed to varying electron energies, currents, and doses [4]. Each bright spot represents a single exposure with ∼5 μm ∅.…”

“…Experiments were performed in the ESCHER LEEM facility [4] at Leiden University. The sample is immersed in an electrostatic field of ∼100 kV=cm, slowing the 15 keV electrons produced by the gun to tunable 0-100 eV incident energy, E 0 .…”

“…We use LEEM to expose thin PMMA films, monitoring changes both after and during exposure [4]. The radiation chemistry of PMMA and related materials has been well studied, and there is consensus that irradiation causes scission of the main chains and removal of side groups [5][6][7][8][9][10].…”