Steven Grzeskowiak

Studying secondary electron behavior in EUV resists using experimentation and modeling

EUV photons expose photoresists by complex interactions starting with photoionization that create primary electrons (~80 eV), followed by ionization steps that create secondary electrons (10-60 eV). Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. An electron exposure chamber was built to probe the behavior of electrons within photoresists. Upon exposure and development of a photoresist to an electron gun, ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses first-principles based Monte Carlo model called “LESiS”, to track secondary electron production, penetration depth, and reaction mechanisms within materials-defined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.

Cross sections of photoacid generators at low electron energies

Optimizing the photochemistry in extreme ultraviolet (EUV) photoresists due to EUV exposures may enable faster, more efficient resists, leading to a greater throughput in manufacturing. Since the fundamental reaction mechanisms in EUV resists are believed to be due to electron interactions after incident 92 eV photons (13.5 nm) generate photoelectrons during ionization events, understanding how these photoelectrons interact with resist components is critical for optimizing the performance of EUV resists and EUV lithography as a whole. The authors will present an experimental method to measure the cross section of incident electron induced decomposition of three different photoacid generators (PAGs). To study the photoelectrons generated by the EUV absorption and measure their effect within resists, photoresists were exposed to electron beams at electron energies between 80 and 250 eV. The reactions between PAG molecules and electrons were measured by using a mass spectrometer to monitor the levels of smal...

Studying thickness loss in extreme ultraviolet resists due to electron beam exposure using experiment and modeling

Abstract. Extreme ultraviolet (EUV) photons expose photoresists by complex interactions starting with photoionization that create primary electrons (∼80  eV), followed by ionization steps that create secondary electrons (10 to 60 eV). Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. A resist exposure chamber was built to probe the behavior of electrons within photoresists. Resists were exposed under electron beam and then developed; ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses a first principles-based Monte Carlo model called low-energy electron scattering in solids (LESiS) to track secondary electron production, penetration depth, and reaction mechanisms within materials-defined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.

Cross sections of EUV PAGs: influence of concentration, electron energy, and structure

Optimizing the photochemistry of extreme ultraviolet (EUV) photoresists should provide faster, more efficient resists which would lead to greater throughput in manufacturing. The fundamental reaction mechanisms in EUV resists are believed to be due to interactions with energetic electrons liberated by ionization. Identifying the likelihood (or cross section) of how these photoelectrons interact with resist components is critical to optimizing the performance of EUV resists. Chemically amplified resists utilize photoacid generators (PAGs) to improve sensitivity; measuring the cross section of electron induced decomposition of different PAGs will provide insight into developing new resist materials. To study the interactions of photoelectrons generated by EUV absorption, photoresists were exposed to electron beams at energies between 80 and 250 eV. The reactions between PAG molecules and electrons were measured using a mass spectrometer to monitor the levels of small molecules produced by PAG decomposition that outgassed from the film. Comparing the cross sections of a variety of PAG molecules can provide insight into the relationship between chemical structure and reactivity to the electrons in their environments. This research is a part of a larger SEMATECH research program to understand the fundamentals of resist exposures to help in the development of new, better performing EUV resists.

Analytical techniques for mechanistic characterization of EUV photoresists

Extreme ultraviolet (EUV, ~13.5 nm) lithography is the prospective technology for high volume manufacturing by the microelectronics industry. Significant strides towards achieving adequate EUV source power and availability have been made recently, but a limited rate of improvement in photoresist performance still delays the implementation of EUV. Many fundamental questions remain to be answered about the exposure mechanisms of even the relatively well understood chemically amplified EUV photoresists. Moreover, several groups around the world are developing revolutionary metal-based resists whose EUV exposure mechanisms are even less understood. Here, we describe several evaluation techniques to help elucidate mechanistic details of EUV exposure mechanisms of chemically amplified and metal-based resists. EUV absorption coefficients are determined experimentally by measuring the transmission through a resist coated on a silicon nitride membrane. Photochemistry can be evaluated by monitoring small outgassing reaction products to provide insight into photoacid generator or metal-based resist reactivity. Spectroscopic techniques such as thin-film Fourier transform infrared (FTIR) spectroscopy can measure the chemical state of a photoresist system pre- and post-EUV exposure. Additionally, electrolysis can be used to study the interaction between photoresist components and low energy electrons. Collectively, these techniques improve our current understanding of photomechanisms for several EUV photoresist systems, which is needed to develop new, better performing materials needed for high volume manufacturing.

Reactivity of metal-oxalate EUV resists as a function of the central metal

the microelectronics industry. Traditional EUV photoresists have been composed of organic compounds which are moderately transparent to EUV. Resist stochastics and sensitivity can be improved by increasing the number of photons absorbed. Molecular organometallic resists are a type of metal containing resist aimed at improving EUV absorption. This work focuses on studying the role of the metal center (Metal = Co, Fe, Cr) in an oxalate complex by comparing the number of absorbed photons and the photoelectron reactivity in each compound. In the study presented here, the EUV absorption coefficients are determined experimentally by measuring the transmission through a resist coated on a silicon nitride membrane using an Energetiq EQ-10M xenon plasma EUV source. Additionally, the photochemistry is evaluated by monitoring outgassing reaction products. This particular resist platform eliminates oxalate ligands when exposed to electrons or EUV photons resulting in a solubility difference between the exposed and unexposed regions. In the process, carbon dioxide is produced and is monitored using mass spectrometry, where quantitative values are obtained using a calibration technique. For the metal oxalate complexes studied, the absorption of EUV changed minimally due to the low concentrations of metal atoms. However, EUV and electron reactivity greatly changed between the three compounds likely due to the reducibility of the metal center. A correlation is shown between Esize and the reducibility of each photoresist.

Mechanisms of EUV exposure: electrons and holes

In extreme ultraviolet (EUV) lithography, 92 eV photons are used to expose photoresists. Current EUV photoresists are composed of photoacid generators (PAGs) in polymer matrices. Secondary electrons (2 - 80 eV) created in resists during EUV exposure play large role in acid-production. There are several proposed mechanisms for electron-resist interactions: internal excitation, electron trapping, and hole-initiated chemistry. Here, we will address two central questions in EUV resist research: (1) How many electrons are generated per EUV photon absorption? (2) By which mechanisms do these electrons interact and react with molecules in the resist? We will use this framework to evaluate the contributions of electron trapping and hole initiated chemistry to acid production in chemically amplified photoresists, with specific emphasis on the interdependence of these mechanisms. We will show measurements of acid yield from direct bulk electrolysis of PAGs and EUV exposures of PAGs in phenolic and nonphenolic polymers to narrow down the mechanistic possibilities in chemically amplified resists.

Studying electron-PAG interactions using electron-induced fluorescence

In extreme ultraviolet (EUV) lithography, 92 eV photons are used to expose photoresists. Typical EUV resists are organic-based and chemically amplified using photoacid generators (PAGs). Upon exposure, PAGs produce acids which catalyze reactions that result in changes in solubility. In EUV lithography, photo- and secondary electrons (energies of 10- 80 eV) play a large role in PAG acid-production. Several mechanisms for electron-PAG interactions (e.g. electron trapping, and hole-initiated chemistry) have been proposed. The aim of this study is to explore another mechanism – internal excitation – in which a bound PAG electron can be excited by receiving energy from another energetic electron, causing a reaction that produces acid. This paper explores the mechanism of internal excitation through the analogous process of electron-induced fluorescence, in which an electron loses energy by transferring that energy to a molecule and that molecule emits a photon rather than decomposing. We will show and quantify electron-induced fluorescence of several fluorophores in polymer films to mimic resist materials, and use this information to refine our proposed mechanism. Relationships between the molecular structure of fluorophores and fluorescent quantum yield may aid in the development of novel PAGs for EUV lithography.

Mechanisms of photodecomposition of metal-containing EUV photoresists: isotopic labelling studies

We have investigated the mechanism of photodecomposition of antimony carboxylate complexes of the type Ph3Sb(O2CR′)2 by means of EUV outgassing in combination with isotopic labelling. A series of photoresists were examined to determine the mechanistic pathways by which volatile photoproducts are generated during EUV exposure. A primary volatile photoproduct from triphenylantimony complexes is benzene. However, the source of hydrogen needed to convert the phenyl groups to benzene (Ph-H) is not obvious. We concluded that the primary source of hydrogen to create benzene is external to the film. Additionally, we have prepared isotopically-labelled versions of Ph3Sb(O2CCH(CH3)2)2 in which the hydrogens in the isobutyrate ligand were replaced with 0, 1, 6 and 7 deuteriums, to provide information about the relative reactivity of these protons during EUV exposure as analyzed by mass spectrometry. High reaction selectivity was identified within the carboxylate dictated by hydrogen location relative to the carbonyl for both benzene and phenol generation. Lastly, the results of these studies were used to propose a series of reaction pathways to generate the aforementioned reaction byproducts.

Molecular organometallic resists for EUV (MORE): Reactivity as a function of metal center (Bi, Sb, Te and Sn)

This paper describes the photoreactivity of six organometallic complexes of the type PhnMX2 containing bismuth, antimony and tellurium, where n = 3 for bismuth and antimony and n = 2 for tellurium, and where X = acetate (O2CCH3) or pivalate (O2CC(CH3)3). These compounds were exposed to EUV light to monitor photodecomposition via in situ mass spectral analysis of the primary outgassing products of CO2, benzene and phenol. This paper explores the effect of metal center and carboxylate ligand on the EUV reactivity of these EUV photoresists.