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

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 ∼0 eV. DOI: 10.1103/PhysRevLett.119.266803 The interaction of ionizing radiation with matter is of vast scientific and technological (including biological and medical) importance. The interaction of UV and x-ray photons with matter is mediated by photoelectrons, as well as secondary electrons with a broad energy distribution that induce chemical changes in the material, be it a polymer, organic or inorganic hybrid, biological tissue, or even DNA. But these complex processes are hard to disentangle, as photon illumination sets the entire electron cascade in motion at once, without the possibility of discerning the role of electrons with different energies. As a result, the interaction of low energy electrons (LEEs) with soft matter is not well understood. Here, we focus primarily on the interaction of low energy electrons with polymethylmethacrylate (PMMA) and related resist materials as used in extreme ultraviolet (EUV) lithography [1] to obtain a new understanding of key processes at low electron energies.In a low energy electron microscope [2] (LEEM) a sample is illuminated with electrons with adjustable 0-100 eV energy [3]. 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]. Here, we identify key physical processes largely ignored in the literature: resist charging, exposure-induced changes in conductivity and secondary electron emission, and dielectric breakdown. We present a simple quantitative theory describing our data, identifying a cusp catastrophe [11] causing the instabilities seen during exposure. Even electrons with near-zero energy change the resist, suggestive of dissociative electron attachment processes [12] commonly neg...

Low-Energy Electron Irradiation Damage in Few-Monolayer Pentacene Films

“…The new reactive state of the molecule can be a result of scission of the carbon–hydrogen bond and the subsequent removal of a hydrogen atom from the molecule, charge reorganization after electron attachment, or an energetically excited state after detachment of the electron. The increased resistance toward damage at very low electron energies is in contrast to results from similar LEEM experiments on PMMA and the molecular organometallic EUV resist known as tin-oxo cage; , for both, it was found that electrons with energies all the way down to 0 eV (with respect to vacuum energy) can cause chemical changes in the resist layer. Lower susceptibility of aromatic organic molecules toward irradiation damage, compared to their derivatives which contain also single bonds, such as the case of pentacene and bis­(triisopropylsilylethynyl) pentacene, , or compared to saturated molecules, has already been reported in various studies.…”

“…Knowledge of the role of secondary electrons, which generally have low energies, is also of great importance to understand and improve extreme ultraviolet (EUV) lithography, generally considered the key lithographic technology for the next decade(s). 21 , 22 …”

“…Knowledge of the role of secondary electrons, which generally have low energies, is also of great importance to understand and improve extreme ultraviolet (EUV) lithography, generally considered the key lithographic technology for the next decade(s). 21,22 In this work, we use low-energy electron microscopy (LEEM) to study the interaction of LEEs with crystalline layers of pentacene grown on a silicon (111) substrate. In LEEM, the energy of the electrons interacting with the sample can be tuned in the range 0−100 eV (with respect to the vacuum energy) with 0.25 eV energy spread.…”

“…A detailed understanding of the interaction of low-energy (secondary) electrons with the resist is essential for control over the properties of the exposed area, as well as sharpness of the written patterns and overall quality of the lithography process. Knowledge of the role of secondary electrons, which generally have low energies, is also of great importance to understand and improve extreme ultraviolet (EUV) lithography, generally considered the key lithographic technology for the next decade(s). , …”

Unraveling the role of photons and electrons upon their chemical interaction with photoresist during EUV exposure

“…In this way chemical yield can be obtained as function of the electron energy. Since the low energy electrons are injected in the resist from outside, the interpretation of electron-matter interaction and the ongoing reactions is much more complex especially for very low energies 1-10eV [17], however, this kind of measurements seem in particular interesting to determine the electron activation energy for a reaction, i.e. the minimum electron energy required to observe a significant reaction response [15].…”