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

A. Thete, D. Geelen, S. J. van der Molen, and R. M. Tromp

Phys. Rev. Lett. 119, 266803 – Published 28 December 2017

Abstract

The effects of exposure to ionizing radiation are central in many areas of science and technology, including medicine and biology. Absorption of UV and soft-x-ray photons releases photoelectrons, followed by a cascade of lower energy secondary electrons with energies down to 0 eV. While these low energy electrons give rise to most chemical and physical changes, their interactions with soft materials are not well studied or understood. Here, we use a low energy electron microscope to expose thin organic resist films to electrons in the range 0–50 eV, and to analyze the energy distribution of electrons returned to the vacuum. We observe surface charging that depends strongly and nonlinearly on electron energy and electron beam current, abruptly switching sign during exposure. Charging can even be sufficiently severe to induce dielectric breakdown across the film. We provide a simple but comprehensive theoretical description of these phenomena, identifying the presence of a cusp catastrophe to explain the sudden switching phenomena seen in the experiments. Surprisingly, the films undergo changes at all incident electron energies, starting at $\sim 0 eV$ .

Received 20 February 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.266803

Physics Subject Headings (PhySH)

Capacitance Charge Dielectric properties Electrical conductivity Electrical properties Insulators Radiation damage

Polymers

Electron microscopy Lithography

Polymers & Soft MatterGeneral PhysicsInterdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Thete^1,2, D. Geelen¹, S. J. van der Molen¹, and R. M. Tromp^1,3

¹Leiden University, Huygens-Kamerlingh Onnes Laboratory, P.O. Box 9504, 2300 RA Leiden, The Netherlands
²Advanced Research Center for Nanolithography, Science Park 102, 1098 XG Amsterdam, The Netherlands
³IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, USA

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Issue

Vol. 119, Iss. 26 — 29 December 2017

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Images

Figure 1
(a) PMMA exposures as a function of electron current, energy, and dose. At each current (0.05, 1.6, and 2.0 nA) we find an exposure threshold which does not depend on dose. PMMA thickness $20 \pm 4 nm$ , spin-coated onto a Si substrate. (b) An electron beam with current density $I_{0}$ impinges on PMMA of thickness $d$ . $E_{0}$ is the electron energy relative to $V_{substrate}$ . The surface charges to a potential $V_{surface}$ . The charging voltage $V$ is defined as $V = V_{substrate} - V_{surface}$ . (c) Schematic of elementary processes, including electrical breakdown (time $t_{1}$ ), increasing trap creation (white dots at $t_{2}$ and $t_{3}$ ), decreasing SEE and increasing conductance during exposure, $V$ switching sign between $t_{2}$ and $t_{3}$ . These processes depend on experimental parameters that change over time.
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Figure 2
(a)–(e) Electron energy spectra during exposure for $E_{0} = 14$ , 15, 20, 25, and 30 eV. The energy scale is a loss scale, with elastic electrons at zero. Thus, the highest energy at which signal is observed (i.e. the cut-off of the secondary electrons) is a direct measure of $E_{land}$ . In (a) and (e) this corresponds at $t = 0$ to $E_{land} \approx 0$ , and 55 eV, respectively. Red lines are fits based on Eqs. (5), with $g_{0}$ and $E_{1}$ at $t = 0$ , and time derivatives $g_{0}^{'}$ and $E_{1}^{'}$ given in the figures.
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Figure 3
(a) Blue lines: Normalized electron current vs $V$ without secondary electrons for $E_{0} = 10$ , 15, and 20 eV. Black dash: left-hand side of Eq. (3) with $g_{0} = 0.0045$ . Equation (3) is satisfied at the intersections of the blue and black lines (arrows). (b) Blue line: as in (a) with $E_{0} = 15 eV$ . Black lines: as in (a) with $g_{0} = 0.01$ (1), 0.0045 (2), and 0.0025 (3). Charging increases with decreasing $g_{0}$ , i.e., increasing $I_{0}$ . (c) Blue lines: Normalized electron current vs $V$ with secondary electrons [right-hand side of Eq. (5b)] for $E_{0} = 10$ , 15, and 20 eV, $α = 0.5$ , and $E_{1} = 25 eV$ . Black dashed curve: left-hand side of Eq. (3) with $g_{0} = 0.0045$ . For $E_{0} = 10 eV$ , $V_{eq}$ has not changed relative to (a), but for $E_{0} = 15$ and 20 eV it has decreased markedly (arrows). (d) Blue line: as in (c) with $E_{0} = 15 eV$ . Black lines: as in (c) with $g_{0} = 0.01$ (1), 0.0045 (2), and 0.0025 (3). For the highest beam current (3) $V_{eq}$ has changed little compared to (b). For (1) and (2) $V_{eq}$ has decreased markedly.
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Figure 4
(a) Black dashed curve: left-hand side of Eq. (5b), $g_{0} = 0.0025$ . Blue lines: right-hand side of Eq. (5b) for different values of $E_{1}$ . Arrows indicate solutions of Eq. (5b). (b) Solutions to Eq. (5b) vs $E_{0}$ for different values of $E_{1}$ . $g_{0} = 0.0025$ . Green dashed line: no secondary electrons (c) $E_{land}$ vs $E_{1}$ at $E_{0} = 25 eV$ . As $E_{1}$ increases (solid lines), $E_{land}$ drops abruptly at a critical value. Reducing $E_{1}$ , the landing energy jumps back up at a much lower value (dashed lines). This hysteresis, characteristic of a cusp catastrophe, closes as $g_{0}$ increases (blue lines). (d) Solutions to Eq. (5b) vs $E_{0}$ for different values of $g_{0}$ . $E_{1} = 25 eV$ . This is typical of a cusp catastrophe with parameters $E_{0}$ and $g_{0}$ . $α = 0.5$ throughout.
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