Polarization Dependence in the Carbon K-edge Photofragmentation of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Polarization Dependence in the Carbon K-edge Photofragmentation of MAPDST Photoresist: an Experimental and Theoretical Study Cleverson A. S. Moura, Guilherme K. Belmonte, Maximiliano Segala, Kenneth E. Gonsalves, and Daniel Eduardo Weibel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07288 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

Polarization Dependence in the Carbon K-edge Photofragmentation of MAPDST Photoresist: an Experimental and Theoretical Study Cleverson A. S. Moura†, Guilherme K. Belmonte†, Maximiliano Segala†, Kenneth E. Gonsalves§ and Daniel E. Weibel*,†

†Institute

of Chemistry, Universidade Federal do Rio Grande do Sul-UFRGS, Avenida Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil.

§

School of Basic Sciences, Indian Institute of Technology Mandi, Mandi − 175001, Himachal Pradesh, India.

*Corresponding author: Tel: +55-5133086204. Fax: +55-5133087304. E-mail: [email protected]

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ABSTRACT The use of tunable soft X-rays from synchrotron radiation (SR) opens the possibility of inducing selective chemical bond scission due to its high localization in a chemical bond. The selective fragmentation

of

a

potential

extreme

ultraviolet

(EUV)

resist,

poly(4-(methacryloyloxy)

phenyldimethylsulfoniumtriflate (MAPDST), was examined using inner shell polarized SR excitation. Selective bond dissociation processes were studied using a combination of carbon K-edge excitation, angle-resolved irradiation, and NEXAFS spectroscopy. Detailed theoretical calculations carried out with the FEFF9 modeling program allowed the interpretation of all the observed experimental features. NEXAFS results indicated that the aromatic group of the polymer lies parallel to the substrate surface. FEFF9 theoretical calculations confirmed the origin of the splitting of the main C 1s →π*C=C resonances observed. The transition C1s → πα*C=C (285.3 eV) can be associated with the four internal carbons of the aromatic ring. The transition C1s → πβ*C=C (286.9 eV) was assigned to the carbon atoms attached to the oxygen and sulfur atoms. According to the theoretical calculations, the origin of the splitting is due to the different absolute energy of C1s. The results showed a strong selective dissociation effect when the excitation energy was tuned to C1s → πα*C=C transition and the electric field vector of the photon was perpendicular to the substrate plane (grazing angle). On the contrary, other transitions were in general less affected. When the SR irradiation angle changed from grazing to normal incidence, the intensity of the C1s → π*C=C transitions was almost unaffected by 285.3 eV photons. The experimental results suggest that site-specific core excitation combined with the direction of the electric field vector of the incidence SR, can efficiently control the localization of the photon energy to produce selective bond dissociation in MAPDST thin films. The results presented here can also be useful to guide new processing lithographic methods for EUVL using the polarization properties of light in ordered polymeric thin films.


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1. Introduction Polymers were important objects of study using synchrotron radiation (SR) to understand X-ray radiation damage and the changes in their surface properties after irradiation. For example, poly(tetrafluoroethylene) (PTFE),1,2 poly(vinylidene fluoride) (PVDF),3 poly(butylene terephthalate) (PBT),8 poly(ethylene terephthalate) (PET)4,5 and many other polymers were investigated in the past. Coffey, T. et al., have carried out a systematic study of radiation damage to know the physical and chemical effects of soft X-ray irradiation, just above the C 1s binding energy, in several polymers.6 The excellent properties of SR have encouraged researchers in the past to also look for selective bond dissociation in molecules and polymers. In particular, Photon Stimulated Ion Desorption (PSID) studies combined with Time-of-flight Mass Spectrometry (TOF-MS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to search for highly specific chemical bond scission using core-level photoexcitation.7-13 The so-called ‘molecular knife or molecular scissor’ effect was proposed where the localized energy in a particular core excitation would lead to selective molecular bond breaking.11,14 SR selective photofragmentation was induced in the past using Vacuum Ultraviolet (VUV)15,16 as well as inner shell excitations.17-20 In several of those investigations, it was observed that some ions were selectively dissociated, but others were rapidly neutralized before fragmentation and desorption.17,20 Recently Nagaoka, S. et al.,21 in an experimental and computational study have shown that for an effective molecular dissociation it is necessary to satisfy two conditions: the atomic sites to be distinguished should be connected through a chain of saturated bonds and located far from each other. In this way, the electron migration would be reduced. De Castilho, R. et al.,22 have also studied the SR excitation and ionic dissociation of the carvone molecule (C10H14O) around the O 1s edge, using NEXAFS and TOF-MS techniques. They found evidence that the O 1s excitation leads to ACS Paragon Plus Environment

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the production of high yields of H+ ion desorption. The desorption of singly and doubly charged oxygen ions was also an indication of site-selective fragmentation following the excitation of O 1s, according to the authors. In spite of several investigations in the past in relation to selective bond-breaking using SR, only a few works have explored the possibility to combine selective core dissociation with the direction of the electric field vector (𝐸) of the incidence of light, i.e., the polarization of the SR.12,23-27 In general, those works were focused on condensed molecular systems25-27 and very few on polymers.23 For example, the ion polarization dependence observed in the methyl ester terminated self-assembled monolayer in ((CH3OCO(CH2)15S)/Au was strongly related to a direct site-specific dissociation and indirect processes.25 In another investigation using a complex rotatable TOF-MS, photoabsorption spectra and ion desorption from multilayer benzene films in the C 1s region have been measured with various polarization angles of incident SR.26 The ions yields results showed the random orientation of the adsorbed benzene molecules in relation to the substrate plane. However, results obtained at grazing angles of less than 10o showed enhanced C-D dissociation rates which corresponded to in-plane  transitions. Sekiguchi, T. et al., have studied PSID using condensed formamide24 and local bonding states of C-H and C-D bonds in graphite.12 In the first case, the authors found that the PSID yields of D+ ions were enhanced by the N 1s →*(N-D) transition but no polarization dependence was observed. Their interpretation was that the N-D bonds were not aligned with the surface-oriented CNO molecular plane of HCOND2. In the second study, they found polarization dependence in the PSID of the H+ and D+ ions on the angle of incidence of the SR. Those results indicated that the angle between the C-H and C-D bonds and the surface normal was 42o on average. Because this angle was somewhat smaller than 54.7o (magic angle28), which corresponds to the averaged angle of random orientation, it was suggested that the C-H and C-D bonds were tilted to some extent in the upward direction. Wada, S. et al., have investigated core excitation using the PSID of several systems, such as poly-methylmethacrylate thin film and self-assembled partially deuterated monolayers.23 They measured the site-specific sigma excitations C-C and O-C as a function of the excitation angle of the SR. It was found that the change in ACS Paragon Plus Environment

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the SR incidence angle enhanced the CH3+ ion desorption yield by about three times between the grazing and normal incidences. On the contrary, C2H3+ ions, for example, did not show any dependence on the 𝐸 of the SR. Together with X-ray absorption experiments, theoretical simulations data has been extremely useful to assign the spectrum resonances and to also understand changes in surface properties due to different treatments. In general, the theoretical methods used for NEXAFS analyses can be classified into the following groups: molecular orbital (MO) method, density functional theory (DFT), and multiple-scattering method.29 In spite that the MO routine is widely used, the use of DFT calculations in the simulation of NEXAFS spectra has increased in recent times. For example, by using the StoBe quantum-chemical program package, the adsorption of the symmetric terephthalic acid molecule on a graphene single layer on Ni(111) was studied.30 The experimental results and the comparison with the simulations have shown in detail the partial contributions of the four symmetric carbon atoms to the total NEXAFS spectrum. Wang et al.,31 have used the StoBe with the DFT method to simulate the C 1s XPS and NEXAFS spectra of three C56 fullerene isomers and their chlorinated derivatives clarifying the origins of the main features in the total experimental spectra. NEXAFS spectra of fluorinated organic semiconductors were also investigated using the StoBe, trying to clarify the origin of the F 1s resonance observed in the NEXAFS spectra and the dichroism observed.32 The multiple-scattering method shows interesting properties to simulate diverse spectroscopy techniques, such as X-ray absorption, X-ray Raman, inelastic X-ray scattering (non-resonant), and even electron energy-loss spectroscopy.33 The use of the multiple-scattering method is also increasing in recent years with applications in polymers,34 oxides and nanoparticles,29,33,35-37 and complex materials.38,39 The comparison of the different theoretical approaches to simulate NEXAFS spectra is out of the scope of the present work, which actually is to help in the interpretation of the experimentally obtained results. Rehr, J. J. et al.,33 have recently improved the FEFF code increasing the capabilities, features of analysis, algorithms, and computing power of several calculations including NEXAFS. The new FEFF9 code also gained a new graphical


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user interface and improved memory management. For those reasons, the FEFF9 code was chosen for the present investigation. In recent years we have studied the photochemistry and oxidation mechanisms of several resists to be used in extreme ultraviolet lithography (EUVL) for the next generation of integrated circuits.40-44 Those investigations were an attempt to understand the complex photofragmentation mechanisms after EUV or inner shell excitation using SR. In one of our studies, a selective bond dissociation process was observed when the excitation energy was tuned to the C 1s→π*C═C transition (285.3 eV).42 A high rate of defluorination and a loss of sulfonated groups was measured mainly at 285.3 eV of excitation energy. However, similar excitations to C 1s→π*C═O or C 1s→σ*C–F did not produce important fragmentation, leading to an efficient preservation of the original chemical structures of the resists. Based on our previous

studies

on

inner

shell

excitation

of

thin

poly(4-(methacryloyloxy)

phenyldimethylsulfoniumtriflate (MAPDST) films,44,45 here NEXAFS spectroscopy and theoretic calculations are used to prove the polarization dependence of the photofragmentation process on the 𝐸 of the SR on thin MAPDST film resists. The selective bond breaking processes were investigated using a combination of selective inner shell excitation and angle-resolved irradiation. The theoretical simulation carried out with the FEFF9 modeling program30,34,46 allowed a complete assignment of the transitions observed in the NEXAFS spectra. The experimental results showed a selective dissociation effect when the excitation energy was tuned to the C1s → π*C═C transition, and the 𝐸 was perpendicular to the substrate plane. To our best knowledge, such polarization-fragmentation studies using SR in polymers and its comparison with theoretical simulations have never been reported so far. In addition, the current work may provide a future guidance for the processing of high performance resist materials using the polarization properties of light.


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2. Experimental Materials The synthesis of poly(4-(methacryloyloxy) phenyldimethylsulfoniumtriflate (MAPDST) and characterization was previously described in detail.44 Methanol (99.9%) was obtained from Synth, Brazil. Oxygen 99.99% and Nitrogen 99.999% were purchased from White Martins PRAXAIR INC and used as received.

Film preparation Thin MAPDST films of about ∼100 nm thickness were prepared using the spin-coating technique dissolving 30 mg of material in 1 mL of Methanol inside a glove box containing a Nitrogen atmosphere without UV exposure. The films were cast on ∼5 × 10 mm Si(100) wafers rotating at about 2000 rpm for 60 s. Then the thin films were heated at 110o Celsius for 90 s.

Inner-shell irradiation study Synchrotron radiation (SR) experiments were conducted at the SGM (Spherical Grating Monochromator) beamline, for VUV and soft X-ray spectroscopy (250–1000 eV) at the Brazilian Synchrotron Light Source (LNLS), Campinas, Brazil. The spectral resolution (E/E) of the SGM beamline was better than 2,000. Additional experiments were also carried out in the PGM (Planar Grating Monochromator) beam line for EUV, VUV, and soft x-ray spectroscopy of the LNLS with a E/E between 1,000 and 25,000. Near-edge X-ray absorption fine structure (NEXAFS) measurements were carried out at room temperature under ultra-high vacuum (UHV) conditions with a base pressure of 10-7 Pa. The experimental set up included a computer-controlled XYZ sample manipulator housed in the UHV. The Si wafers substrates were directly attached to the sample holder using conducting double-sided tape, and the SR beam was slightly defocused. No sample charging was observed throughout the experiments. ACS Paragon Plus Environment

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Samples outside the UHV chamber were always manipulated in an inert atmosphere, and ultraviolet light exposition was avoided. NEXAFS spectra were obtained by measuring the total electron yield (electron current at the sample) simultaneously with a photon flux monitor (Au grid). The final data was normalized using this flux spectrum to correct for fluctuations in beam intensity. NEXAFS data was analyzed following the standard procedures where all spectra were processed using a constant-value pre-edge subtraction and post-edge normalization.47,48 The software package ATHENA was used for the final analysis of X-ray absorption spectroscopy.49 Absolute photon energy calibration was carried out using the photoemission signal of the Au 4f7/2 and then by plotting the second derivative of the C 1s  *C=C signal (label  in the NEXAFS spectra). By this way the zero crossing of the second derivative was found and the spectra were corrected introducing the value of the reference point of 285.3 eV. Irradiation by SR and surface oxidation of the thin resist surfaces followed the methodology already described.40-44,50 Briefly, specific transitions were selected, and for a fixed period of time, the polymer was irradiated under UHV conditions. After irradiation, the samples were transferred to a preparation chamber where they were exposed to pure oxygen at a pressure of 10-3 Pa for 30 min to neutralize and oxidize the remaining radicals on the film surfaces. Each irradiation was carried out on pristine films to allow comparison with nonirradiated data. NEXAFS spectra were obtained before and after irradiation. The precise positions of the irradiated areas were easily confirmed by moving the films approximately 1-1.5 mm up-down or right-left with the XYZ sample manipulator. Pristine spectra of the films were recorded outside the irradiated areas. Samples were mounted to allow rotation on the vertical axis of the XYZ sample manipulator, and in this way, the angle between the sample surface and the incident X-ray beam was changed. The NEXAFS angle was defined as the angle of the incident X-ray beam in relation to the sample surface. The incident beam normal to the surface was defined as 90° while a grazing incident beam was generally 20°. When the angle was 90°, the 𝐸 of the linearly polarized SR lies parallel to the surface, and at 20° the 𝐸 was nearly perpendicular to the substrate surface. ACS Paragon Plus Environment

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Computational methods Geometric optimization was carried out using Gaussion0951 with the Hartree-Fock theoretical level method and the 6-311+G(d,p) basis set.52 Guided by the dipole moment of the [MAPDST]+ and [CF3SO3]- previously optimized, some conformations between them were estimated, which were, by trial and error, checked for the most stable positions. After that, the neutral MAPDST - CF3SO3 was fully optimized (Figure 1). All final geometries were characterized as a minimum by the absence of the imaginary frequencies. NEXAFS spectra using self-consistent spherical muffin-tin potentials and a real space full multiple-scattering formalism were calculated using the computer software program FEFF9.30,33 The FEFF9 yields scattering amplitudes and phases used in many modern X-ray absorption fine structure (XAFS) analysis codes, as well as various other properties. With minimal input, consisting only of atomic species and coordinates within a cluster and no adjustable physical parameters, FEFF9 calculates the scattering contributions to the NEXAFS spectrum to all orders within the cluster. By projecting the full scattering matrix onto the direction of a polarization vector, FEFF9 calculates the orientation dependence of the NEXAFS spectrum, thus resolving spectral resonances.30,34 Figure 1 shows an illustrative image of the MAPDST monomer unit used for the calculation with the indication of each absorber atom. The complete set of Cartesian coordinates and configurations can be seen in Figure S1 (supporting information, SI).


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Figure 1. Structural representation of the spatial distribution of the monomer unit used for the FEFF9 calculations.

3. Results and Discussion 3.1 Experimental and Theoretical characterization of the NEXAFS spectra Figure 2 shows the NEXAFS carbon K-edge spectra of MAPDST homopolymer thin films before treatment. It is possible to attribute four main features in the C 1s absorption spectra as follows:3,53,54 1: C1s  *C=C (285.3 eV); 2: C1s  *C–S (286.9 eV) or C1s  *C=C (286.8 eV); 3: C1s  *C=O (288.5 eV) and 4: C1s  *C–F (295 eV). This last wide signal overlaps with a typical C1s  *C–C transition.3,55 The C1s  *C–S (286.9 eV) transition is very well characterized in previous works,56-58 but based on our experimental and theoretical results, the 286.9 eV transition was finally assigned mainly to an additional C1s  *C=C absorption (see text on the following pages).


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Figure 2. Carbon K-edge NEXAFS spectra of pristine MAPDST homopolymer thin film obtained at 45o. Signal assignments: 1: C1s *C=C (285.4 eV); 2: additional C1s  *C=C (286.8 eV; see text for details); 3: C1s *C=O (288.5 eV) and 4: C1s *C–F (295 eV). Inset: molecular representation of the MAPDST monomer unit.

It is well known that NEXAFS is highly sensitive to molecular orientation59 and dependence of the NEXAFS spectra on the angle of the SR radiation with respect to the surface can be observed in molecules31,58,60 and polymers samples.53,61-63 Angle resolved (AR)-NEXAFS spectra were obtained at different angles of the SR with respect to the surface, and the results are shown in Figure 3. When the angle of incidence changed from 90o to 20o, the perpendicular 𝐸 has a greater component along the axis of pz orbitals (out of plane), which mainly comprise the p bands of the aromatic groups and thus the intensity of the * resonance is successively increased in accordance with the dipole selection rules.47 In spite a thin film of MAPDST resist is coating the Si(100) substrate, the results presented in Figure 3 suggest that the molecular aromatic groups of the MAPDST pristine films are oriented preferentially parallel to the substrate surface. Signal 1, assigned to C1s *C=C, in Figure 2 followed what would be expected for a * resonance if the plane of the aromatic group lies parallel to the substrate surface. Interestingly, signal 2 also has a strong dependence on the SR angle of incidence. Signal labels 2 in Figure 2 could be assigned to a C1s *C–S (286.9 eV),56-58 but in this case, no dependence of signal 2 with the decrease in the incidence angle (grazing angle) would be expected. It is necessary to mention that there is also the possible presence of *C–H resonances59 in this energy region of the spectrum ACS Paragon Plus Environment

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overlapping with signal 2 in Figure 2. Because signal 1 and 2 have a strong dependence on the incidence angle of excitation, they would be both correlated to * resonances. The inset of Figure 3 shows the difference NEXAFS spectra between grazing (20o) and a normal angle (90o) of incidence. Both C1s  *C=C remained after the subtraction, and the C1s  * strongly decreased showing the statistical orientation of the monomer units in the thin film. AR-NEXAFS spectrum with polarization control probes depth profiling and relative orientation. To reduce the effect of polarization excitation and confirm the angle dependence measured, AR-NEXAFS spectra were performed including the magic angle (~55o).64,65 The results shown in Figure S2 (SI) confirmed the remarkable difference between the NEXAFS spectra of the films measured at the magic angle and 90o. On the contrary, signals 3 and 4 in Figure 2 assigned to a C1s  *C=O and C1s  *C–F ; *C–C transitions, respectively, almost showed no dependence on the excitation angle. The conclusion is that the ester and CF3 groups are not preferentially oriented to the film surface plane. Because the NEXAFS technique is a surface sensitive technique the above conclusions in relation to film orientation may not indicate similar configurations in

o

Intensity (a.u.): 20 - 90

o

the whole sample thickness.

Normalized Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


284 286 288 290 292 294 296 298 300

Photon Energy (eV)

o

20 o 45 o 90 285

290

295

300

305

310

Photon Energy (eV)

Figure 3. Angle-resolved partial electron yield of the carbon K-edge NEXAFS spectra of untreated MAPDST thin films for different beam incidence angles relative to the surface plane. The geometry of the experiment is schematized for the 20° case. Inset: the difference between grazing (I20°) and normal (I90°) incidence spectra. ACS Paragon Plus Environment

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Trying to get insight into the origin of the electronic excitations, theoretical simulations using the FEFF9 modeling program30,33 were carried out. A series of FEFF9 calculations were performed and the results are shown in Figures 4 and 5 together with the pristine experimental NEXAFS MAPDST spectra. Six absorbing atoms were chosen in the calculations as representatives of the main features observed in the experimental NEXAFS spectrum: C12, C14, and C15 belonging to the aromatic ring but linked to oxygen, carbon, and sulfur atoms, respectively (Figure 4). Actually, the selection of the C14 absorbing atom was at some point arbitrary, because it should be expected that the C14-C10 and C11C13 spectroscopic features would be different. But due to the resolution of the SGM beamline, it was not enough to resolve those transitions. The C14 absorbing atom was selected as representative of those four carbon atoms. Additionally, C7, C8, and C19 belonging to the ester, methyl linked to the sulfur atom and the sulfonium triflate moiety, CF3SO3-, respectively, were also chosen as absorbing atoms (Figure 5). In each of the situations shown in Figures 4 and 5, the 𝐸 was applied in several directions of the Cartesian system, which are symbolized by the coordinates of unit vectors. The way the MAPDST monomer was oriented to each absorbing atom is also shown on the right of Figures 4 and 5. The qualitative theoretical results show that the two first transitions corresponded to C1s  *C=C excitations that were labeled as * and * in Figure 4. As can be seen the intensity of these * transitions is at its maximum when the 𝐸 of the SR is perpendicular to the aromatic molecular plane (coordinates X, Y, Z: 0, 0, 1 in Figure 4). The FEFF9 calculations confirmed the existence of the split of the main * transitions in the MAPDST monomer which agreed with the experimental NEXAFS spectrum shown in Figure 3. In addition, the FEFF9 calculation showed that the C1s  *

C=C

corresponds to the four central carbon atoms of the aromatic ring and the more electronegative environment of the C12 and C15 linked to an oxygen and sulfur atoms, respectively, gave origin to the C1s  * C=C absorption (coordinates X, Y, Z: 0, 0, 1 in Figure 4). Therefore signals 1 and 2 in Figure 2 were originated by two C1s  *C=C excitations.


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Figure 4. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right, the absorbing atoms C12, C14, and C15 belonging to the aromatic ring are shown (see also Figure 1). Coordinates X, Y, Z in C15 are the same that C14.

The FEFF9 calculations of the C7 absorbing atom clearly show the presence of the C1s  *C=O excitation in Figure 5 that matched the experimental carbonyl signal at 288.5 eV (coordinates X, Y, Z: 0, 0, 1 and 0, 1, 1 in Figure 5). The C1s *C–S (286.9 eV) transition was simulated using the C8 absorbing atom, and it is shown also in Figure 5. This signal is partially overlapping with the C1s  *

C=C

transition. Due to the strong angle dependence observed in the signal 2 in Figure 2, it was

assumed that the contribution of the *C–S transition is much weaker compared to the C1s  * C=C excitation, in particular at grazing angles of irradiation.


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Figure 5. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right, the absorbing atoms C7, C8, and C19 belonging to the ester, methyl linked to the sulfur atom and the sulfonium triflate moiety, respectively (see also Figure 1). In the C7 absorbing atom, the intensity output of the calculations for E (0, 0, 1) and E (0, 1, 1) were divided by two for better presentation. Coordinates X, Y, Z in C19 are the same that C8.

Previous reports on carbon K-shell NEXAFS investigations of benzene derivatives have shown that different types of molecules, such as benzene, aniline, phenol or fluorobenzenes presented a clear splitting of the * resonances.59,66,67 Those resonances were assigned to transitions from different C ls levels into several * molecular orbitals. If the benzene molecule has a heteroatom linked to one of the carbon atoms, the spectra clearly showed the splitting of the * resonance due to the presence of, for example, the -NH, and -OH groups in aniline and phenol, respectively.66 Therefore, comparing with benzene, there are additional C ls  * transitions for aniline and phenol. Since the MAPDST monomer unit contains the six-member aromatic ring and two different heteroatoms linked to carbon atoms in para-positions (oxygen and sulfur), more possibilities for C ls  * transitions should be


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expected. Additionally, the absolute level of the C 1s core-shell also changes with the proximity of the heteroatom.66 In spite the FEFF9 calculations produced a good correlation with the experimental values, the relative theoretical information on the energy difference between the several carbons atoms of the MAPDST monomer unit was calculated using the Gaussion09.51 As can be seen in the results presented in Figure 6 the energy differences between the C 1s core energy of the atoms show the tendency in the calculated energy difference between C19 (CF3) and the carbons C4 – C5 – C6 (CH3-CH-CH3). Table 1 summarizes the experimental and theoretical results obtained by the FEFF9 program. As a first glance, a very good correlation between the experimental data and FEFF9 calculations can be observed, which is better than 0.5%. Previous theoretical results of benzene derivatives obtained using the self-consistent field-molecular orbital (SCF-MO) code have shown that NEXAFS measurements are not expected to agree with the SCF-MO calculations. This is due to the fact that the final state in NEXAFS is a hole (C 1s) with one electron temporally located in an unoccupied orbital, such as a * orbital, for example. SCF-MO calculations correspond to a negative ion state without a core hole. But this is not the case for the FEFF9 formalism where the photon absorption is necessary, and an empty final state is required.33 In this way, the multiple-scattering method intrinsically calculates excitation spectra and electronic structures using ab initio self-consistent real space multiple scattering approach which includes different phenomena, such as core-hole effects, local field corrections, and polarization dependence. Table 1 does not only show the tendency in the absorption energy measured, but also the absolute energy of the experimental NEXAFS spectra and the excellent correlation with the FEFF9 energy calculated. C12


17


C1

C1

C1

1

0C1

C4

2

C8

C9

C1 C7

C5

9

5

C1 C6

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C1

4

3

-E

= +E

ΔET E1

~6.63eV E2

E3

E4

E5

E6

E7

Figure 6. Carbon 1s level orbital calculation of the MAPDST monomer unit using Gaussion09.51 The color differences in blue indicate the depth of the energy level of each carbon atom, showing the dependence of the near close atoms on the energy of a particular carbon 1s orbital.

From the previous experimental evidence of benzene derivatives and from the theoretical calculations summarized in Table 1, matching the experimental results, it is possible to affirm with a great degree of confidence that the absorption at 285.3 eV can be assigned to the C 1s  *C=C transition originated from the internal aromatic carbon atoms: C10 – C11 – C13 – C14 (see Figure 6). The experimental AR-NEXAFS evidence confirmed the typical characteristic of a final * orbital lying nearly parallel to the substrate plane. The following excitation also showed the angle dependence in the NEXAFS spectra (signal 2 in Figure 2) and can be assigned to the C 1s  *C=C transition originated from the internal aromatic carbon atoms: C15 and C12. The relative energy of the carbon atoms C15 and C12 is lower than the C10 – C11 – C13 – C14 (see Figure 6) and therefore the absorption energy will be higher if the same final state is assumed. The C 1s  *C=O transition can be clearly originated from the C7 atom (see Figure 6) corresponding to the carbon belonging to the ester group. The additional transitions shown in Table 1 dominate at normal incidence (see Figure 2) and corresponded to sigma final states involving several carbons atoms of the MAPDST monomer unit.


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Table 1. Calculated transitions energies from the C ls core for the MAPDST monomer unit and their comparison with the experimental values.

C10 – C11 – C13 – C14

Transitions from C 1s *C=C

FEFF9 (eV) 285.5

Experimental (eV) 285.3

C15

*C=C

286.8

286.8

C12

*C=C

287.4

C7

*C=O

288.5

*C–C

293.4

*C–F

293.6

*C–O

294.4

*C–O

298.0

*C–C

299.1

Carbons

C12 – C14 – C15 – C19 – C7

C7 + Aromatics

288.5

295

299.8

Finally, assuming that the aromatic rings are planar in relation to the silicon substrate, new calculations were carried out optimizing the planar position of the monomer in relation to the Si(100) substrate and changing the polarization of the excitation, i. e., the direction of the 𝐸. Figure 7 shows the simulation results for the angles 20, 45, and 90 degrees. In the simulations, the 𝐸 was set at 70o, 45o, and a parallel position in relation to the XY plane. These angles correspond to the experimental ARNEXAFS data shown in Figure 2. The simulations accompanied the changes observed in the experimental spectrum, which is also presented in Figure 7 for each angle of irradiation. Additionally, Figure S3 (SI) shows the normalized changes in the experimental and the theoretical calculated intensities of the C 1s  *, transitions on the angle of irradiation. The results of Figure S3 show clearly the effect of the orientation on the experimental and simulated data. The theoretical calculations were originated from a perfect planar configuration of the monomer unit in relation to the surface plane. In this perfect configuration, the dependence of the simulated intensities for each carbon atom on the ACS Paragon Plus Environment

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angle of the SR incidence (electric field vector) is higher than the experimental data due to a non perfect planar orientation of the MAPDST film. Figure 7 also shows that the C10 – C11 – C13 – C14 are the main atoms that originate the first transition and its angle dependence observed, i. e., the C1s  * C=C

excitation (signal 1 in Figure 2). Comparison of the experimental C1s  *C=O AR-NEXAFS data

with the simulation shows that the optimized position of the MAPDST monomer unit did not match the experimental data. The simulations optimized the ester functional group with the C=O group in an interplanar configuration where the C–O was nearly perpendicular to the substrate plane. Therefore the optimized configuration favors the absorption of the photon when the SR 𝐸 is perpendicular to the Si substrate. The experimental data showed a weak dependence of the C1s  *C=O transition with the angle of irradiation of the SR incidence.

Figure 7. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right the absorbing atoms C7, C8, and C19 belonging to the ester, methyl linked to the sulfur atom (yellow) and CF3 functional groups, respectively (see Figure 1). Red: oxygen atoms. Blue light: Fluor atoms.

3.2 Angle-Resolved irradiation at 285.3 eV Taking into account the experimental results of AR-NEXAFS and the detailed theoretical simulation presented in section 3.1, a series of experiments were carried out to test the possibility of ACS Paragon Plus Environment

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using monochromatized SR for selective bond dissociation. For these experiments, the irradiation angle of the SR in relation to the substrate was fixed at 20o, 45o, and 85o. The NEXAFS spectra of the irradiations at 285.3 eV were obtained at 20o and 90o. The small difference between irradiation at 85o and analyze at 90o was due to geometrical configurations. Figure 8 shows the NEXAFS results obtained when the untreated and treated films were analyzed at a grazing angle (20o). The thin films were irradiated at the C1s *C=C transition (285.3 eV) for 30 min at 20o, 45o, and 85o. Due to the untreated MAPDST film being analyzed at a grazing angle, the * resonances labeled as * and * in Figure 4 (signals 1 and 2 in Figure 2) are more intense than the C1s *C=O transition. When the films were irradiated for 30 min at several angles, the results obtained showed that the transition to orbitals * and * almost disappeared at a grazing angle of excitation. When the angles of irradiation were 45o and 85o, minimal effects were observed on transitions to orbitals * and *. These results also show univocally that both signals correlate with *C=C resonances. On the contrary, the C1s *C=O transition with an absorption centered at 288.5 eV was not affected to a large extent indicating a non preferential orientation of the ester group in the films and therefore no selectively in the photofragmentation. However, changes in the Full Width at High Maximum (FWHM) can be seen in Figure 8 for the C 1s → *C=O excitation. As after irradiation the samples were oxidized in the preparation chamber, it cannot be ruled out that new ester and carbonyl functionalities would be formed with the generation of new C 1s → *C=O transitions. Finally, important changes in intensity and shapes of the spectra for the C1s  *C–F and C1s  *C–C in the energy region 291-298 eV were observed (see inset of Figure 8). In particular, changes observed at 45o and 90o may be correlated to atom reorganization and partially random orientation of those bonds.


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

 C=C





  C=C

Normalized ntensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

 C=O

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294

296

298

300

302

304

Photon Energy (eV)

Without treatment SR irradiation angle: o 20 o 45 o 85

285

286

287

288

289

290

291

292

Photon Energy (eV)

Figure 8. Angle-resolved carbon K-edge NEXAFS spectra of untreated and irradiated MAPDST thin films at different beam incidence angles relative to the surface plane. All the NEXAFS spectra were obtained at a grazing angle (20o). The films were irradiated at the C1s *C=C (285.3 eV) transition for 30 min at 20, 45 and 90o. The geometry of the experiment can be seen in Figure 3 for 20°. Inset: NEXAFS absorption region for the * transitions.

The same experimental methodology that led to the results shown in Figure 8 was carried out for an additional series of experiments, but in this new series, the analysis angle was fixed at 90o. The results are presented in Figure 9. NEXAFS spectra of the untreated film showed the unfavorable orientation of the 𝐸 in relation with the aromatic section of the MAPDST thin film, and the decrease in the * resonances intensity is observed as expected. After irradiation at 20o, the * resonances intensities analyzed at 90o strongly decreased showing again the high efficiency of the selective photofragmentation process. The resonance of the *C=O transition was almost not modified under irradiations at 20o and 45o, but the intensity of this transition decreased at 85o of irradiation in comparison with the untreated film. This effect may indicate some degree of preferencial orientation of the carbonyl groups. The optimized simulations showed the ester functional group with the C=O functionality in an interplanar configuration where the C–O was nearly perpendicular to the substrate


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plane (see Figure 7). Therefore the optimized configuration favors the absorption of the photon by the C=O group if the SR is perpendicular to the Si substrate. The 𝐸 will be in this case parallel to the Si substrate. The results shown in Figure 9 for the *C=O transition may prove that the C–O bonds have a preferential configuration perpendicular to the substrate plane favoring the C 1s *C=O excitation at 90o of irradiation. This configuration finally led to a decrease in the *C=O signal intensity due to partially loss of the carbon-oxygen double bonds. Nevertheless, it is necessary to mention that the energy of 285.3 eV does not correspond to the C 1s *C=O excitation (288.5 eV). Because in the present work the films were not irradiated at 288.5 eV it cannot be ruled out the presence of others contributions than can lead to a decrease in the *C=O signal intensity, such as overlapping transitions or efficient energy transfer within the films. Additionally, important changes in intensity and shapes of the spectra are observed for the C1s  *C–F and C1s  *C–C transitions in the energy region 291-298 eV (see inset of Figure 9).




 C=O 


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


  C=C



 C=C

294

296

298

300

302

304

Photon Energy (eV)

Without treatment SR irradiation angle: o 20 o 45 o 85

285

286

287

288

289

290

291

292

Photon Energy (eV)

Figure 9. Angle-resolved carbon K-edge NEXAFS spectra of untreated and irradiated MAPDST thin films at different beam incidence angles relative to the surface plane. All the NEXAFS spectra were obtained at 90o. The films were irradiated at the C1s *C=C (285.3 eV) transition for 30 min at 20, 45 and 90o. The geometry of the experiment can be seen in Figure 3 for 20°. The spectrum at 85o was multiplied by a 1.2 factor for a better comparison. Inset: NEXAFS absorption region for the * transitions.


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The results presented in Figures 8 and 9 showed that it is possible to use the high selectivity of the monochromatic SR excitation together with its high polarization to induce selective bond dissociations in an ordered thin film. The theoretical simulation carried out allowed a detailed understanding of the transitions involved at the several excitation energies. In the present investigation, only the C1s  *C=C excitation was used to induce selective bond breaking in the MAPDST films. The results also showed a high-intensity decrease in the transition corresponding to the C1s  * C=C excitation and a much lower effect on the C1s  *C=O transitions. The results presented here are very interesting taking into account our very recent work carried out using EUV excitation at 103.5 eV.40,41 Absorption of EUV photons is a not resonant process, and a high stability of the aromatic section of the resists was found in spite that the studied resists contained heavy metal atoms to enhance the absorption of EUV photons. In particular, in the EUV irradiation of a polyarylene – sulfonium resist (PAS),41 the aromatic sections of the polymer remained almost intact even after 15 min of continuous irradiation at 105.3 eV. In those experiments, the sizing dose used was more than 60 times higher than is usually used in EUV lithography. Additionally, the PAS experiments were carried out in the PGM beamline, and the present investigation of the MAPDST photofragmentation was done in the SGM beamline. The number of photon s-1 of the PGM line at 103.5 eV compared to the SGM beam line at 285.3 eV is more than 103 times higher.68 Therefore the present work showed the high efficiency to combine highly monochromatic light with polarization for the selective bond dissociation in thin MAPDST films.

4. Conclusions Polarization-angle-dependent irradiation at 285.3 eV of MADST thin films was investigated using NEXAFS spectroscopy combined with theoretical calculations of Carbon K-edge NEXAFS spectra ACS Paragon Plus Environment

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using the FEFF9 program. NEXAFS spectroscopy showed that the aromatic groups lay parallel with the silicon (100) substrate. The theoretical simulation of the NEXAFS spectra of a MAPDST monomer unit allowed the assignments of the experimental transitions with less than a 0.5% of error in the absolute energy. The transition C1s → πα*C=C (285.3 eV) was associated with the four internal absorbing carbons of the aromatic ring. The transition C1s → πβ* (286.9 eV) was assigned to the carbon atoms in the aromatic group attached to the oxygen and sulfur atoms. It was also shown by the Gaussion09 calculations that the origin of the splitting is mainly due to the difference in the C 1s absolute energy. A high selective photofragmentation process was found by the selection of the polarization of the SR. Excitation at 285.3 eV strongly decreased the C1s → π*α-β intensity signals only when the SR was set at a grazing angle, i. e., the 𝐸 was nearly perpendicular to the substrate plane. Other transitions, such as C1s → π*C=O were less affected. Nevertheless the decrease in the intensity of the C1s → π*C=O transition when the irradiation angle was 85o showed a preferential configuration of the C–O bond perpendicular to the substrate plane favoring the C 1s *C=O excitation at 90o of irradiation. This may lead to the decrease in the *C=O signal intensity observed due to loss of the carbon-oxygen double bond by photodissociation. FEFF9 simulations gave also support to the experimental results showing that the ester functional groups responsible for the C1s → π*C=O transitions were in an interplanar configuration where the C–O was nearly perpendicular to the substrate plane. ACS Paragon Plus Environment

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The experimental results suggest that site-specific inner shell excitation at the C 1s core combined with the direction of the electric field vector of the incidence SR can actively control the localization of the absorbed photon energy to induce a high selective bond dissociation event in MAPDST thin films. We hope that the experimental and simulation results present here may also open new horizons toward the processing of high performance resist materials for EUVL using the polarization properties of light.

Supporting Information Available: available free of charge via the Internet at http://pubs.acs.org.: Simulation parameters and cartesian coordinates of the monomer unit used for the FEFF9 calculations. Angle-resolved partial electron yield of the carbon K-edge NEXAFS spectra of untreated MAPDST thin films for different beam incidence angles including the magic angle.

Acknowledgment. This work was partially supported by the by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), process No 402360/2016-8, CAPES, LNLS and the National Supercomputing Center (CESUP), UFRGS, Brazil. The authors would also like to strongly acknowledge the technical assistance of the Accelerator Group, especially the VUV and Soft X-ray Spectroscopy Group. The authors C.A.S.M and G.K.B. acknowledge receipt of CNPq fellowships for financial support. We are grateful to Prof. Fernando D. Vila, University of Washington for helpful discussions on the computer software program FEFF9. ACS Paragon Plus Environment

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Figure 1. Structural representation of the spatial distribution of the monomer unit used for the FEFF9 calculations.


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Figure 2. Carbon K-edge NEXAFS spectra of pristine MAPDST homopolymer thin films obtained at 45o



Figure 3. Angle-resolved partial electron yield of the carbon K-edge NEXAFS spectra of untreated MAPDST thin films for different beam incidence angles relative to the surface plane. 59x45mm (300 x 300 DPI)


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Figure 4. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right, the absorbing atoms C12, C14, and C15 belonging to the benzyl ring are shown (see also Fig. 1). Coordinates X, Y, Z in C15 are the same that C14.



Figure 5. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right, the absorbing atoms C7, C8, and C19 belonging to the ester, methyl linked to the sulfur atom and triflate functional groups, respectively (see also Fig. 1).


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Figure 6. Carbon 1s level orbital calculation of the MAPDST monomer unit using Gaussion09.



Figure 7. FEFF9 calculations of the carbon K-edge NEXAFS spectra of MAPDST monomer unit. The experimental NEXAFS spectrum of the MAPDST thin film is over imposed in all the simulations for comparison. On the right the absorbing atoms C7, C8, and C19 belonging to the ester, methyl linked to the sulfur atom (yellow) and CF3 functional groups, respectively (see Figure 1). Red: oxygen atoms. Blue light: Fluor atoms.


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Figure 8. Angle-resolved carbon K-edge NEXAFS spectra of untreated and irradiated MAPDST thin films at different beam incidence angles relative to the surface plane. All the NEXAFS spectra were obtained at a grazing angle (20o). The films were irradiated at the C1s pi*C=C (285.3 eV) transition for 30 min at 20, 45 and 90o. The geometry of the experiment can be seen in Figure 3 for 20°. Inset: NEXAFS absorption region for the sigma* transitions. 74x59mm (300 x 300 DPI)



Figure 9. Angle-resolved carbon K-edge NEXAFS spectra of untreated and irradiated MAPDST thin films at different beam incidence angles relative to the surface plane. All the NEXAFS spectra were obtained at 90o. The films were irradiated at the C1s pi*C=C (285.3 eV) transition for 30 min at 20, 45 and 90o. The geometry of the experiment can be seen in Figure 3 for 20°. The spectrum at 85o was multiplied by a 1.2 factor for a better comparison. Inset: NEXAFS absorption region for the sigma* transitions. 75x59mm (300 x 300 DPI)


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Polarization Dependence in the Carbon K-edge Photofragmentation of

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