Secondary Electrons in EUV Lithography

Abstract

Secondary electrons play critical roles in several imaging technologies, including extreme ultraviolet (EUV) lithography. At longer wavelengths of light (e.g. 193 and 248 nm), the photons are directly involved in the photochemistry occurring during photolysis. EUV light (13.5 nm, 92 eV), however, first creates a photoelectron, and this electron, or its subsequent daughter electrons create most of the chemical changes that occur during exposure. Despite the importance of these electrons, the details surrounding the chemical events leading to acid production remain poorly understood. Previously reported experimental results using high PAG-loaded resists have demonstrated that up to five or six photoacids can be generated per incident photon. Until recently, only electron recombination events were thought to play a role in acid generation, requiring that at least as many secondary electrons are produced to yield a given number of acid molecules. However, the initial results we have obtained using a Monte Carlo-based modeling program, LESiS, demonstrate that only two to three secondary electrons are made per absorbed EUV photon. A more comprehensive understanding of EUVinduced acid generation is therefore needed for the development of higher performance resists.

Acid Generation Efficiency of EUV PAGs via Low Energy Electron Exposure

“…These interactions could result in a useful acid generation event and/or generate more secondary electrons at the cost of some energy. It is believed that 2-4 secondary electrons are generated from an EUV absorption event [2,5,11,12]. Electron cascades produced by EUV exposures allow for quantum yields greater than 1, whereas typical 193 nm photoresists have film quantum yields much less than 1 [13].…”

“…It is well known that when a photoresist is exposed to EUV photons that molecules will outgas [1,[4][5][6][7]. For select PAGs, some of these outgassing molecules correlate to an acid generation event (Fig.…”

“…To observe the acid generation efficiency as a function of PAG concentration for various PAG molecules, the following experiment was performed in our Electron Resist Interaction Chamber (ERIC), which is a vacuum chamber equipped with a Kimball Physics EFG-7 electron gun and an Extrel MAX300 quadrupole mass spectrometer [3][4][5][6][7]. In ERIC, a photoresist is exposed with a normal incident electron beam at 80 eV [6,7].…”

“…The PAG is a photon-or electron-sensitive molecule that is capable of producing an acid which catalyzes numerous chemical transformations in the surrounding resist [1][2][3][4][5][6][7]. Further improvement of chemically amplified photoresists for use with EUV is crucial to meet the lithographic requirements of the microelectronics industry, or an innovative nonchemically amplified resist must be developed to satisfy this need.…”

What We Don’t Know About EUV Exposure Mechanisms

“…The absorption of EUV photons causes molecular electrons to be ejected with ~75-82 eV energy, leaving a radical-cation "hole" in the molecule [1][2][3][4][5][6]. These liberated electrons have sufficient energy to cause further ionization or other reactions.…”