Soft X-ray absorption and fragmentation of tin-oxo cage photoresists

"Tin-oxo cage" organometallic compounds are considered as photoresists for extreme ultraviolet (EUV) photolithography. To gain insight into their electronic structure and reactivity to ionizing radiation, we trapped bare gas-phase n-butyltin-oxo cage dications [(BuSn)12O14(OH)6]2+ in an ion trap and investigated their fragmentation upon soft X-ray photoabsorption by means of mass spectrometry. In complementary experiments, the tin-oxo cages with hydroxide and trifluoroacetate counter-anions were cast in thin films and studied using X-ray transmission spectroscopy. Quantum-chemical calculations were used to interpret the observed spectra. At the carbon K-edge, a distinct pre-edge absorption band can be attributed to transitions in which electrons are promoted from C1s orbitals to the lowest unoccupied molecular orbitals, which are delocalized orbitals with strong antibonding (Sn-C σ*) character. At higher energies, the most prominent resonant transitions involve C-C and C-H σ* valence states and Rydberg (3s and 3p) states. In the solid state, the onset of continuum ionization is shifted by ∼5 eV to lower energy with respect to the gas phase, due to the electrostatic effect of the counterions. The O K-edge also shows a pre-edge absorption, but it is devoid of any specific features, because there are many transitions from the different O1s orbitals to a large number of vacant orbitals. In the gas phase, formation of the parent [(BuSn)12O14(OH)6]3+ radical ion is not observed at the C K-edge nor at the O K-edge, because the loss of a butyl group from this species is very efficient. We do observe a number of triply charged photofragment ions, some of which have lost up to 5 butyl groups. Structures of these species are proposed based on quantum-chemical calculations, and pathways of formation are discussed. Our results provide insight into the electronic structure of alkyltin-oxo cages, which is a prerequisite for understanding their response to EUV photons and their performance as EUV photoresists.

Correction: UV and VUV-induced fragmentation of tin-oxo cage ions

Correction for 'UV and VUV-induced fragmentation of tin-oxo cage ions' by Jarich Haitjema et al., Phys. Chem. Chem. Phys., 2021, 23, 20909-20918, https://doi.org/10.1039/D1CP03148A.

Fluorine-Rich Zinc Oxoclusters as Extreme Ultraviolet Photoresists: Chemical Reactions and Lithography Performance

The absorption of extreme ultraviolet (EUV) radiation by a photoresist strongly depends on its atomic composition. Consequently, elements with a high EUV absorption cross section can assist in meeting the demand for higher photon absorbance by the photoresist to improve the sensitivity and reduce the photon shot noise induced roughness. In this work, we enhanced the EUV absorption of the methacrylic acid ligands of Zn oxoclusters by introducing fluorine atoms. We evaluated the lithography performance of this fluorine-rich material as a negative tone EUV photoresist along with extensive spectroscopic and microscopic studies, providing deep insights into the underlying mechanism. UV-vis spectroscopy studies demonstrate that the presence of fluorine in the oxocluster enhances its stability in the thin films to the ambient atmosphere. However, the EUV photoresist sensitivity (D ₅₀) of the fluorine-rich oxocluster is decreased compared to its previously studied methacrylic acid analogue. Scanning transmission X-ray microscopy and in situ X-ray photoelectron spectroscopy in combination with FTIR and UV-vis spectroscopy were used to gain insights into the chemical changes in the material responsible for the solubility switch. The results support decarboxylation of the ligands and subsequent radical-induced polymerization reactions in the thin film upon EUV irradiation. The rupture of carbon-fluorine bonds via dissociative electron attachment offers a parallel way of generating radicals. The mechanistic insights obtained here will be applicable to other hybrid materials and potentially pave the way for the development of EUV materials with better performance.

Fluorescent Labeling to Investigate Nanopatterning Processes in Extreme Ultraviolet Lithography

Extreme ultraviolet (EUV) lithography uses 13.5 nm light to reach the sub-20 nm resolution. However, the process of pattern formation induced by this high-energy light is not well-understood. In this work, we provide an inorganic EUV photoresist with fluorescence properties by introducing a carbazole derivative as a ligand, and we study its effect on the patterning process. Using the fluorescence properties, changes in the emission of the material after EUV exposure could be tracked by means of spectroscopy and microscopy. The resist sensitivity was substantially reduced by the incorporation of the carbazole benzoate ligands, which is attributed to hole trapping and steric hindrance. After EUV irradiation of the resist films, infrared, UV-visible absorption, and fluorescence spectroscopies showed that the carbazole units were still mostly intact, although their fluorescence intensity was lowered. Our work shows that fluorescent labeling can provide relevant mechanistic insights in the patterning process of resists, potentially with a molecular resolution.

UV and VUV-induced fragmentation of tin-oxo cage ions

Photoresist materials are being optimized for the recently introduced Extreme Ultraviolet (EUV) photolithographic technology. Organometallic compounds are potential candidates for replacing the ubiquitous polymer-based chemically amplified resists. Tin (Sn) has a particularly large absorption cross section for EUV light (13.5 nm, 92 eV), which could lead to a lower required EUV dose for achieving the desired solubility change (improved sensitivity). However, the fundamental interaction between organometallic materials and higher energy photons is poorly understood. In this work, we exposed n-butyltin-oxo cage dications (M2+) in the gas phase to photons in the energy range 4-35 eV to explore their fundamental photoreactivity. Photoproducts were detected using mass spectrometry. Homolytic cleavage of tin-carbon bonds was observed for all photon energies above the onset of electronic absorption at ∼5 eV (∼250 nm), leading to photoproducts which have lost one or more of the attached butyl groups (Bu). Above 12 eV (<103 nm), dissociative photoionization occurred for the dication (M2+), competing with the neutral loss channels. The photoionization threshold is lowered by approximately 2 eV when one counterion (triflate, OTf- or tosylate, OTs-) is attached to the tin-oxo cage (MOTf+ and MOTs+). This threshold is expected to be even lower if each tin-oxo cage is attached to two counterions, as is the case in a solid film of tin-oxo cages. Addition of counterions also affected the fragmentation pathways; photoexcitation of (MX)+ (X = counterion, OTf or OTs) always led to formation of (MX-2Bu)+ rather than (MX-Bu)+. MOTs+ was much more reactive than MOTf+ in terms of reaction products per absorbed photon. A possible explanation for this is proposed, which involves the counterion reacting with the initially formed tin-based radical.

Bottom-Up Nanofabrication with Extreme-Ultraviolet Light: Metal-Organic Frameworks on Patterned Monolayers

The fabrication of integrated circuits with ever smaller (sub-10 nm) features poses fundamental challenges in chemistry and materials science. As smaller nanostructures are fabricated, thinner layers of materials are required, and surfaces and interfaces gain a more important role in the formation of nanopatterns. We present a new bottom-up approach in which we use the high optical resolution offered by extreme ultraviolet (EUV) lithography to print patterns on self-assembled monolayers (SAMs). Upon radiation, low-energy electrons induce chemical changes in the SAM so that the projected image is transferred to the substrate surface. We use the chemical differences between exposed and unexposed regions to promote a selective growth of hybrid structures that can act as an etch-resistant layer for further pattern transfer or can be used as functional nanostructures. The EUV doses required to promote selective growth on exposed areas are close to industrial requirements. Furthermore, this method allows for the independent tuning of different steps in the EUV lithography process (photo-induced chemistry, spatially resolved chemical contrast, and formation of nanopatterns), an advantage over current resists, in which the same material plays all roles.

Role of low-energy electrons in the solubility switch of Zn-based oxocluster photoresist for extreme ultraviolet lithography

The electron-induced chemistry of a resist material for extreme ultraviolet lithography (EUVL) consisting of Zn oxoclusters with methacrylate (MA) and trifluoroacetate (TFA) ligands (Zn(MA)(TFA)) has been studied. Electron energies of 80 eV and 20 eV mimic the effect of photoelectrons released by the absorption of EUV photons and low-energy secondary electrons (LESEs) produced by those photoelectrons. The chemical conversion of the resist is studied by mass spectrometry to monitor the volatile species that desorb during electron irradiation, combined with reflection absorption infrared spectra (RAIRS) measured before and after irradiation. The observed reactions are closely related to those initiated upon EUV absorption. Also, the conversion of the Zn(MA)(TFA) resist layer that is required in EUVL is achieved by a similar energy input upon electron irradiation. The dominant component of the desorbing gas is CO2, but CO detection also suggests Zn oxide formation during electron irradiation. In contrast, species deriving from the ligand side chains predominantly remain within the resist layer. RAIRS gives direct evidence that, during electron irradiation, C[double bond, length as m-dash]C bonds of the MA ligands are more rapidly consumed than the carboxylate groups. This supports that chain reactions occur and contribute to the solubility switch in the resist in EUVL. Remarkably, 20 eV electrons still evolve roughly 50% of the amount of the gas that is observed at 80 eV for the same electron dose. The present results thus provide complementary and new insight to the EUV-induced chemistry in the Zn(MA)(TFA) resist and point towards the important contribution of low-energy electrons therein.

Universal direct patterning of colloidal quantum dots by (extreme) ultraviolet and electron beam lithography

Colloidal quantum dots have found many applications and patterning them on micro- and nanoscale would open a new dimension of tunability for the creation of smaller scale (flexible) electronics or nanophotonic structures. Here we present a simple, general, one-step top-down patterning technique for colloidal quantum dots by means of direct optical or electron beam lithography. We find that both photons and electrons can induce a solubility switch of both PbS and CdSe quantum dot films. The solubility switch can be ascribed to cross-linking of the organic ligands, which we observe from exposure with deep-UV photons (5.5 eV) to extreme-UV photons (91.9 eV), and low-energy (3-70 eV) as well as highly energetic electrons (50 keV). The required doses for patterning are relatively low and feature sizes can be as small as tens of nanometers. The luminescence properties as well as carrier lifetimes remain similar after patterning.

Key Role of Very Low Energy Electrons in Tin-Based Molecular Resists for Extreme Ultraviolet Nanolithography

Extreme ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE microscopy (LEEM), electron energy loss spectroscopy (EELS), and atomic force microscopy (AFM) to study changes induced by electrons in the 0-40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM-EELS uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2-40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm³ per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV sensitivity of tin-oxo cages.

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials

Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Photoswitchable metal organic frameworks: turn on the lights and close the windows

Progress and challenges in the development of photo-responsive metal organic frameworks.

Red Organic Light-Emitting Radical Adducts of Carbazole and Tris(2,4,6-trichlorotriphenyl)methyl Radical That Exhibit High Thermal Stability and Electrochemical Amphotericity

Synthesis and characterization of new carbazolyl derivatives with a pendant stable radical of the TTM (tris-2,4,6-trichlorophenylmethyl radical) series are reported. The EPR spectra, electrochemical properties, absorption spectra, and luminescent properties of these radical adducts have been studied. All of them show electrochemical amphotericity being reduced and oxidized to their corresponding stable charged species. The luminescence properties of them cover the red spectral band of the emission. The luminescence of the electron-rich carbazole adducts shows the donor-acceptor nature of the excited state. On the other hand, the EPR parameters of these radical adducts show an imperceptible variation with the substituents in the carbazole.

Significance of an atrial septal aneurysm in the presence of an ostium secundum atrial septal defect. A transesophageal echocardiographic study

The aim of this study was to elucidate the mechanism by which an atrial septal aneurysm is formed in the presence of an atrial septal defect by characterizing the distinguishing echocardiographic features of atrial septal defects between patients with and without an atrial septal aneurysm. The transesophageal echocardiograms of 30 consecutive patients who underwent surgical closure of a secundum atrial septal defect were compared with those of 8 normal controls. In patients with secundum atrial septal defect, the maximal diameter (mean +/- SD) of the fossa ovalis was 21.73 +/- 3.43 compared to 11.43 +/- 1.00 mm in the control group (p < 0.01). In the 7 (23%) patients with atrial septal aneurysm, the mean maximal diameter of the fossa ovalis was 25.28 +/- 3.03 compared to 20.65 +/- 2.78 mm in those without an atrial septal aneurysm (p < 0.01). The atrial septal defect was smaller in patients with than in those without an atrial septal aneurysm. In 4 patients with atrial septal aneurysm who had a history of a cerebrovascular event, the interatrial communication was only detected by contrast echocardiography. In conclusion, in patients with atrial septal aneurysm, atrial septal defects tend to be smaller but the incidence of cerebrovascular events is greater.

Contrast echocardiographic shunts may persist after atrial septal defect repair

We performed contrast echocardiography on 19 subjects who were asymptomatic in the postoperative period after surgical repair of atrial septal defects. Eighteen of these subjects had adequate right heart echocardiographic contrast to assess the presence or absence of right-to-left shunting. Multiple M-mode and two-dimensional echocardiographic views were studied during several contrast injections with and without the Valsalva manoeuvre. Six patients had postoperative shunts and 12 patients had no postoperative shunts. The age of the six patients with postoperative shunts was 26 +/- 10 years (mean +/- s.d.) and that of the 12 patients without postoperative shunts was 39 +/- 14 years. Four out of six of the postoperative shunt group were males and of these three had patch repairs compared with two males out of 12 with patch repair in the no shunt group. There were no definite differences between the two groups in the following variables: type of atrial septal defect (primum v. secundum), preoperative shunt size, pre-operative peak right ventricular pressure, pre-operative New York Heart Association functional class, pre- or postoperative right ventricular or left ventricular dimensions, aortic and left atrial dimensions. Four of the six patients with postoperative contrast echo shunting underwent cardiac catheterization, showing no significant step-up in oxygen saturation in three, and a significant shunt in one patient who had patch dehiscence at re-operation. We conclude that right-to-left shunts as demonstrated by contrast echocardiography are common in the late postoperative period after atrial septal defect repair. They need not indicate unsuccessful repair or a haemodynamically important residual shunt.

Noninvasive study of early diastole in mitral stenosis

Cardiac events of early diastole were studied in 50 normal subjects and 46 patients with mitral stenosis (MS) by simultaneous recordings of the mitral valve echogram (MVE), phonocardiogram, and apexcardiogram (ACG). Left ventricular isovolumic relaxation time (IRT), measured between A2 and onset of the MVE opening motion, had almost the same values in normals 54 +/- 7 msec, and MS 51 +/- 16 msec. The interval between A2 and the ACG "O" point was approximately double that of IRT: 99 +/- 11 msec in normal subjects, 109 +/- 20 msec in MS. The normal MVE opening motion had a velocity 293 +/- 76 mm/sec and duration 45 +/- 6 msec, values significantly different (P less than 0.001) from 536 +/- 271 mm/sec, 23 +/- 7.5 msec found in MS patients. In atrial fibrillation the length of the cardiac cycle did not affect A2-O interval or mitral valve opening movement duration; however cycle length was clearly related to isovolumic relaxation time. This resulted in a variation in the interval between completion of the mitral valve opening (opening snap) and O point. This interval was longer after a short diastole and vice versa.