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...

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.

Athena will be the largest space-based x-ray telescope to be flown by the European Space Agency: its large 2.6 m diameter lens will use a revolutionary new modular technology, Silicon Pore Optics (SPO). The lens will consist of several hundreds of smaller x-ray lenslets, called mirror modules, which each consist of about 70 mirror pairs. Those mirror modules are arranged in circles in a large optics structure and will focus x-ray photons with an energy of 0.5 to 10 keV at a distance of 12 m onto the detectors of Athena. The point-spread function (PSF) of the optic shall achieve a halfenergy width (HEW) of 5" at an energy of 1 keV, with an effective area of about 1.4 m 2 , corresponding to several hundred m 2 of super-polished mirrors with a roughness of about 0.3 nm and a thickness of only 150 μm. SPO using the highest grade double-side polished 300 mm wafers commercially available, have been invented to enable such telescopes. SPO allows the cost-effective production of high-resolution, large area, x-ray optics, by using all the advantages that mono-crystalline silicon and the mass production processes of the semi-conductor industry provide. SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research. This paper will present the status of the technology and of the mass production capabilities, show latest performance results and discuss the next steps in the development.