Mean Free Path of Electrons in Organic Photoresists for Extreme Ultraviolet Lithography in the Kinetic Energy Range 20–450 eV

Abstract

The blur caused by the nonzero mean free path of electrons in photoresists exposed by extreme ultraviolet lithography has detrimental consequences on patterning resolution, but its effect is difficult to quantify experimentally. So far, most mean free path calculations use the dielectric formalism, which is an approximation valid in the optical limit and fails at low kinetic energy. In this work, we used a modified substrate-overlayer technique that exploited the attenuation of the Si 2p core level originating specifically from the native silicon dioxide to evaluate the attenuation of electrons traveling through 2 and 4 nm of photoresist overlayers to provide a close estimation of the inelastic mean free path relevant for photoresist lithography patterning and for electron microscopy. The photoemission spectra were collected in the proximity of the Si 2p edge (binding energy ∼101 eV) using synchrotron light of energy ℏω ranging between 120 and 550 eV. The photoresist films were prototypical chemically amplified resists based on organic copolymer of poly hydroxystyrene and tertbutyl methacrylate with and without triphenyl sulfonium perfluoro-1butanesufonate photoacid generator and trioctylamine quencher. The inelastic mean free path of electrons, in the range that is relevant for photoresist exposure in extreme ultraviolet lithography (20−92 eV), was found to be between 1 and 2 nm. At higher kinetic energy, the mean free path increased, consistently with the well-known behavior. The presence of the photoacid generator and quencher did not change the mean free path, within experimental error. Our results are discussed and compared with the existing literature on organic molecules measured via dielectric formalism and electron transmission experiments.

XUV Absorption Spectroscopy and Photoconversion of a Tin-Oxo Cage Photoresist

“…At the EUV energy of 92 eV the yields of valence and Sn­(4d) electrons measured using XPS are similar . Most photoelectrons do not escape from the thin film sample, as their mean free paths are small, at most a few nanometers. − In the present work, we focus on the chemical conversion and on the possible reaction mechanisms. The TinOAc molecule consists of a dicationic core structure, with two hydrogen bonded acetate counteranions.…”

The importance of less damaged and surface sensitive metrology to identify true EUV process monitoring

“…The main difference compared to the DUV era is that electron induced reactions in the chemicals become vital role rather than photon-induced reactions. Thus, understanding of electron scattering with a spectrum of energies (rather than just one well-defined energy) has become much more important because the PAGkey component of solubility changesdoes not directly absorb photon but is directly involving chemical reaction with low energy electrons [3,4,[6][7][8]. Once EUV light is irradiated on the EUV photoresist, it excites electron-hole pair in resist polymers and presumably 80eV energies of photoelectrons cause subsequent ionization events, while losing further energy through its energy loss cascade (also called as thermalization).…”

“…Once EUV light is irradiated on the EUV photoresist, it excites electron-hole pair in resist polymers and presumably 80eV energies of photoelectrons cause subsequent ionization events, while losing further energy through its energy loss cascade (also called as thermalization). The scattered/emitted electrons with excess energy lose their energy through the various interaction with surrounding molecules such as ionization, plasmonic damping, electron attachment, excitons, intra/inter-molecular vibrations, and phonons as shown in figure 1 [1,2,4,9,10]. Charged radical generated during ionization process react with a nearby unexposed polymer unit, creating a neutral radical and a cationic polymer group.…”

“…This suggests that the electron trajectory from the initial photoabsorption site (or photoelectron-created site) to the eventual thermalization point detrimentally impacts patterning resolution. Additional repercussions of electron blur encompass worsen sidewall roughness and critical dimension non-uniformity in photoresist patterns, impacting the functionality and defectivity of semiconductor devices [4]. On the photoresist reaction mechanism point of view, elastic scattering or vibration excitation might not as important as secondary electron cascade, propagation, and reaction.…”

“…EUV photons have shorter wavelengths (13.5 nm and 92eV) compared to the previous optical lithography techniques (for example, DUV light 193nm and 6.4 eV), which lead to indirect reaction mechanism in the resist materials via photoelectron creation and secondary electron cascade because of relatively higher energy irradiation on the lower HOMO (highest occupied molecular orbital) -LUMO (lowest unoccupied molecular orbital) gap of most small molecules. Another consequence of the shorter wavelength (higher energy) is a relatively low number of photons interacting with the resist (~14× fewer photons are needed), leading to statistical fluctuations in the energy deposition process [1][2][3][4][5]. The main adverse impact of stochastic effect is the creation of local variability in the patterns being printed, leading to defects and reduced yield.…”