Radiation Sensitivity of Soluble Polysilane Derivatives - American

24 Radiation Sensitivity of Soluble Polysilane Derivatives Downloaded by TUFTS UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch024

Robert D. Miller Almaden Research Center, IBM Research Division, 650 Harry Road, San Jose, CA 95120-6099

Polysilane derivatives have very unusual electronic properties asso ciated with extensive σ delocalization along the polymer backbone. Strong electronic transitions that depend on the nature of substit uents, polymer molecular weight, and backbone conformation appear in the UV spectra. These materials are radiation sensitive and are degraded to lower molecular weight fragments upon exposure to light and ionizing radiation. This chapter is an overview of the nature of these radiation-induced processes and describes some potential ap plications, primarily in the field of microlithography.

THE FIRST DIARYL-SUBSTITUTED POLYSILANE

derivative was probably pre pared over 60 years ago by Kipping (1). The simplest dialkyl-substituted material, poly (dime thylsilane), was described in 1949 by Burkhard (2). These materials are highly insoluble and intractable and attracted little scientific interest until recently. The modern era in polysilane chemistry began about 10 years ago with the synthesis of a number of soluble homo- and copolymers (3-5). The current interest in substituted polysilanes has resulted in a rapidly expanding list of new potential applications, which include the use of polysilanes as: • thermal precursors to β-silicon carbide (6-8), • oxygen-insensitive photoinitiators for vinyl polymerizations (9), • polymeric charge conductors (10, 11), 0065-2393/90/0224-0413$12.50/0 © 1990 American Chemical Society

Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

414

SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

• radiation-sensitive materials for microlithographic applications (12, 13), and

Downloaded by TUFTS UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch024

• materials with interesting nonlinear optical properties (14-15). High-molecular-weight substituted polysilanes are usually prepared by a modified Wurtz coupling of the respective dichlorosilanes with sodium metal (16). Other procedures have been described recently (17-21), but these methods generally result in the production of lower molecular weight polymers or oligomers. With the modified Wurtz coupling, a large number of high-molecular-weight, soluble polysilane derivatives have been prepared (16, 22). The mechanism of this heterogeneous polymerization is quite com plex, and significant solvent effects have been reported (12, 16, 23).

Electronic Spectra One of the most interesting characteristics of the polysilanes is their unusual electronic spectra (16). Even though the backbone is fully σ bonded, all substituted polysilanes absorb strongly in the UV-visible region. Their ab sorption spectra depend to some extent on the nature of the substituents. Alkyl-substituted, atactic, amorphous materials absorb at 300-325 nm, with sterically bulky groups producing a shift to longer wavelengths (13, 24). Aryl substituents that are directly bonded to the silicon backbone result in sig nificant red shifts of 25-30 nm (24). The absorption spectra ot polysilane derivatives (25-31) depend also on the conformation of the silicon backbone. This polymer backbone effect results in a curious thermochromic behavior for many polysilane derivatives both in the solid state and in solution (32, 33). The planar zigzag conformation results in large spectral red shifts, and the position of the absorption max imum is sensitive to small changes in backbone conformation (31). Soluble diaryl-substituted polysilane derivatives exhibit the greatest red shifts, and it has been proposed that this effect is probably conformational in origin (31, 34). Recent light-scattering studies have provided some additional support for the hypothesis that diaryl-substituted polysilanes are significantly ex tended even in solution (35). The absorption spectra of polysilane derivatives also depend on the polymer molecular weight (24). The X m a x of the long-wavelength absorption, which moves progressively to longer wavelengths with increasing catenation for short oligomeric chains, rapidly approaches a limiting value for highmolecular-weight polymers at a degree of polymerization of —40-50 (Figure la). A similar limiting effect is observed for the molar extinction coefficients as a function of molecular weight (Figure lb).


24.

MILLER

Radiation Sensitivity of Soluble Polysilane

Derivatives

415

300 /

275

J

250


225

I I-

(a)

I

200'

50

100

150

200

1350

Chain Length, η

c

3

ω I

ω

Φ CL

CO

b χ

2

3

10

15

Chain Length (η χ 10~ ) 2

Figure 1. (a) Plots of UV absorption maxima versus chain length (n) for poly(alkylsilane)s. Key: · , poly(dimethylsilane); I , poly(n-dodecylmethylsilane). (b) Plots of absorptivity per Si-Si bond at Xmax versus chain length n. Key: · , poly(phenylmethylsilane); • , poly(n-dodecylmethylsilane).


416


Radiation Sensitivity of Polysilanes


The polysilanes constitute a new class of radiation-sensitive materials that are sensitive to UV light, as well as to various types of ionizing radiation (vide infra). The observed dependence of both the position of the absorption maximum and the molar extinction coefficients on polymer molecular weight suggests that any process that significantly reduces the molecular weight should result in spectral bleaching. This effect would have major lithographic consequences (vida infra). Spectral Bleaching. The response of a typical polysilane derivative to irradiation is shown in Figure 2. The strong bleaching suggests that the molecular weight of the polymer is reduced significantly upon exposure. The bleaching phenomenon is characteristic of both alkyl- and aryl-substituted polysilanes upon exposure to UV light and is quite general. Subsequent studies have confirmed that the polymer molecular weight is reduced sig nificantly upon exposure. This effect is demonstrated in Figure 3, which shows the decrease in the molecular weight of a typical polysilane, poly(ndodecylmethylsilane), in solution upon exposure to UV light. Similar effects are observed upon irradiation of solid polysilane films. Although spectral bleaching with molecular weight reduction occurs both in air and under high vacuum, some oxidation of the silicon backbone undoubtedly occurs in the presence of air. This reaction was first demon strated by Zeigler and co-workers (13), who reported the appearance of a strong IR band at 1020 cm" 1 , which is characteristic of the Si-O-Si func tionality, upon exposure of a poly(cyclohexylmethylsilane-co-dimethylsilane) film at 254 nm in the presence of air. Similar changes occur in poly(di-npentylsilane) films upon irradiation in the presence of air (Figures 4a and 4b). In addition to the appearance of the characteristic band at 1021 cm - 1 , weaker bands at 2080 and 3382 c m 1 , which are ascribed to S i - Η and Si-OH vibrations, also are apparent. Similar observations are described in reference 13. Effect of Air. Because most imaging processes involving polysilanes use the materials in thin-film form and are most often conducted in air, the effect of air on the photochemical degradation is important. In a study of spectral bleaching rates for a number of polysilanes in air relative to those observed in vacuum, we (36) noticed that the magnitude of the effect depends strongly on the structure of the polymer, although some acceleration is observed for almost all samples when irradiated in air. At this point, it is not clear whether the differences in the bleaching rates are a function of the nature of substituents or depend on the physical characteristics of the poly mer, such as molecular weight and glass transition temperature (Tg). The polymer properties could influence reactivity by changing the mobility of reactive sites, the rate of oxygen diffusion, etc.


24.

MILLER

Radiation Sensitivity of Soluble Polysilane Derivatives

417

g CO >-H CO


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to ι ^§

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to SX co

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Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989. -1

3000.0 2000.0 Wavenumber (cm )

Figure 4a. IR spectrum of an 815-nm poly(di-n-pentyhilane)

4000.0

0.05001 h

0.10010h

0.05001

0.20010h

1000.0 film.



2000.0 -1

Wavenumber (cm )

3000.0

1000.0

2

Figure 4b. IR spectrum of an 815-nm poly(di-n-pentybilane) film after exposure at 254 nm in air (exposure 400 m] I cm ).

4000.0

0.15001 h

0.20010Γ-


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MILLER


421

Derivatives


A detailed study of the effect of air has been reported recently by Ban and Sukegawa (37), who analyzed the siloxane content of irradiated films of poly(methylphenylsilane) (PMPS) and poly(n-propylmethylsilane) (PMPrS) by IR spectroscopy. On the basis of these experiments, the authors concluded that at 254 nm PMPS and PMPrS are almost completely oxygenated at saturation. The extent of oxygenation of both samples was considerably less at 330 nm, presumably because the spectral bleaching of the initial absorption band results in inefficient light absorption. In each case, the molecular weight of the oxygenated polymer was reduced. Subsequent comparative imaging studies on these materials irradiated in air and under vacuum suggested that photooxidation plays an important role in subsequent pattern development. Effect of Polyhalogenated Additives. We (36) have also observed that the rate of bleaching of solid polysilane films upon irradiation is considerably slower than that observed for solutions at comparable optical densities. Although this result is consistent with the observed decrease in the quantum yields for scission, (s), in going from solution to the solid state (24) (vide infra), this decreased sensitivity is inconvenient for imaging processes. For this reason, a search was made for compatible additives that might influence the bleaching rate of polysilane derivatives in the solid state. We have found that a certain number of polyhalogenated aromatic derivatives such as compounds 1-3 greatly accelerate the rate of bleaching of a number of polysilane derivatives in the solid state (38). This effect is dramatically demonstrated in Figure 5 for a PMPS film doped with —20% by weight of l,4-bis(trichloromethyl)benzene, 1. Similar results were obtained with substituted triazine sensitizers such as 2. In these cases, the polysilane is the primary absorber of the incident radiation. Interestingly, when compound 3, which absorbs at —400 nm, was incorporated into a PMPS film and the sample was irradiated at 404 nm, where only the sen-

0CH

3

3


422


1 (a)

I

10.0 9.0 8.0

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-

-e 5.o ο CO 2 < 4.0

0 mJ/cm

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^ _ ^ O y U - 200 mJ/cm \

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300 mJ/cm

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2

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2

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2.0

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350 400 Wavelength (nm)

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500

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0.20

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500

Figure 5. Irradiation of poly(phenylmethylsilane) at 313 nm in the presence or absence of polyhalogenated additives: (a) film containing no additive and (b) film containing approximately 20% by weight of 1,4-bis(trichloromethyl)benzene.


24.

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Derivatives

423


sitizer absorbs, both the polysilane and the sensitizer absorptions at 343 and 400 nm, respectively, were efficiently bleached. However, this accelerated bleaching in the solid state does not appear to be a general phenomenon and depends on the structure of the polysilane. For example, the incorporation of sensitizer 2 into a film of a typical dialkyl-substituted polysilane such as poly(cyclohexylmethylsilane) did not accelerate the bleaching of the polymer film. In fact, the presence of compound 2 actually seems to inhibit somewhat the photochemical bleaching process. Selectivity of Polyhalogenated-Additive Effect. The mechanism by which halogenated additives promote the bleaching of PMPS but inhibit the same process in poly(cyclohexylmethylsilane) is a source of some speculation and is currently under investigation. Although halogenated sensitizers such as compounds 1-3 could serve simply as halogen-atom-transfer reagents to inhibit the recombination of incipient reactive silicon chain fragments (e.g., silyl radicals), it seems unlikely that they would be selective only for fragments derived from PMPS, unless the radicals generated from PMPS are intrinsically longer lived and radical recombination is the predominant pathway for the fragments in the solid state in the absence of additives. The photochemistry of polysilane derivatives may occur also via the triplet state (13, 30), on the basis of the observation of a weak-structured phosphorescence characteristic of a localized excited state for a number of polysilane derivatives. In principle, the halogenated additives could promote intersystem crossing via an intermolecular heavy-atom effect (39). Again, however, why the two structurally similar polysilanes should respond so differently to the presence of the additive is unclear. One possible explanation for the selective accelerated decomposition of PMPS in the presence of the chlorinated additives is that photochemical bleaching occurs by electron transfer. If electron transfer is involved, the two polysilane derivatives may behave differently, because the peak oxidation potential of a thin film of PMPS is lower by —0.4 eV than that of typical dialkyl-substituted polysilane derivatives (40). In addition, the electrochemical oxidation of PMPS is highly irreversible, and the polymerfilmis removed from the electrode presumably as smaller silicon-containing fragments. Such a photochemically mediated electron-transfer reaction would also be facilitated by the high electron affinities of polyhalogenated aromatic compounds, which subsequently decompose by dissociative electron attachment (41). The instability of both the polysilane radical cations and the polyhalogenated radical anions should improve the overall efficiency of the process by inhibiting back electron transfer.

Photochemistry Despite the considerable interest in the radiation sensitivity of high-molecular-weight polysilanes, relatively few detailed studies on the nature of the Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

424



products and intermediates produced upon irradiation have been reported. Oligomeric polysilane derivatives, on the other hand, have been studied extensively (42). Because the results of these pioneering investigations form the basis for the interpretation of many subsequent photochemical studies on the high-molecular-weight polysilanes, this background will be described here. Reactive Intermediates. The simplest radiation-sensitive polysilane contains disilane structural units. Although simple alkyl-substituted disilanes absorb in the far- and vacuum-UV regions, aromatic substituents cause a red shift to the accessible UV region. The photochemical decomposition of disilanes leads predominantly to silicon-silicon bond homolysis and the pro duction of substituted silyl radicals. Once generated, silyl radicals may ab stract hydrogen or halogen atoms from suitable donors, add to vinyl and aromatic groups, react with alcohols by abstracting hydrogen atoms or alkoxy groups, or undergo other characteristic reactions (43). Because of the rela tively low energy of the S i - Η bond (81-88 kcal/mol) (44), the abstraction of hydrogen atoms from typical alkanes is normally endothermic, except for the most highly activated carbon-hydrogen bonds. Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silènes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the α position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. In another model study, Ishikawa and co-workers (42, 48) showed that for phenylpentamethyldisilane, the phenyl-substituted silyl radicals can undergo ortho radical addition to produce unstable silènes that can be subsequently trapped and identified (Scheme I). Recent spectroscopic studies have identified the silene as a reactive intermediate in this process (49). Si-Si Bond Homolysis. Ishikawa and co-workers (50, 51) have also studied the photolysis of a number of polymeric disilane derivatives. These workers have examined both derivatives with pendant silicon substituents (50) and materials in which the disilane moiety is incorporated into the polymeric backbone (51). For derivatives with pendant silicon substituents (Scheme II), irradiation destroys the characteristic phenyldisilane absorption



+

Scheme I. Disproportionation

+

and dimerization

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Scheme 11. Si-Si bond homolysis in siloxane polymers containing pendant silicon

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at 235 nm, and the polymer becomes insoluble through cross-linking. The IR spectrum of the photodegraded polymer shows a weak -SiH absorption at 2150 cm" 1 . Model studies on l,l-bis(trimethylsiloxy)-l-phenyltrimethyldisilane showed that Si-Si bond homolysis occurs, and products characteristic of silyl radical abstraction and substitution were isolated. On the basis of these model studies, the mechanism shown in Scheme II, which involves Si-Si bond homolysis followed by characteristic silyl radical reactions, was postulated. Furthermore, oxidation studies on the photodegraded polymer suggested that, unlike phenylpentamethyldisilane, the disiloxydisilane polymer shows little inclination toward trimethylsilyl radical addition at the ortho positions of the incipient phenylsilyl chain radical. Ishikawa and co-workers (50, 51) have also reported the photodegradation of backbone disilane polymers (Scheme III). In this case, as in the previous example with pendant silyl substituents, silicon-silicon bond homolysis to produce silyl radicals was the predominant process. When irradiated in toluene, the photodegraded polymer showed a substantial - S i H band at 2150 cm" 1 in the IR spectrum. Similarly, NMR spectroscopic examination of the irradiated polymer showed evidence of silyl radical substitution into the solvent toluene. Irradiation of the polymeric disilane in deuterated methanol produced no bands due to Si-D in the IR spectrum and resulted in the incorporation of the elements of methanol into the chain ends (as revealed by NMR spectroscopy). For the phenyl-substituted polymer, the NMR evidence indicated that -SiR 2 -SiR = CHR' + HSiR 2 R'CH = SiR-SiR 2 - - » -SiR 2 -CHR'-SiR

(4) (5a) (5b)

Radiation Chemistry of Polysilanes Whereas oxygen can play a role when polysilanes are photodegraded in air, photobleaching with attendant molecular weight reduction occurs even when polysilane derivatives are irradiated in an inert atmosphere or under vacuum.


452



This has prompted the investigation of the potential of substituted polysilane derivatives as electron beam resists. Figure 12 shows images created in a bilayer composed of a thin layer of a typical polysilane coated over a thick, hard-baked layer of a typical AZtype photoresist. After solvent development of the imaged layer, the pattern

Figure 12. Electron beam imaging of poly(di-n-pentylsilane) (0.14 μιη) coated over 2.0 \Lm of a hard-baked AZ-type photoresist exposed at 20 μθ/cm and wet developed. Pattern transfer was by 0 -RIE. 2

2



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MILLER

Radiation Sensitivity of Soluble Polysilane Derivatives

453

was transferred by 0 2 -RIE. The process described is unoptimized, but it demonstrates that polysilanes can be imaged in a positive mode even when the imaging step is conducted in vacuum with ionizing-radiation sources. To study the structural sensitivity of polysilanes to ionizing radiation, a number of samples were irradiated with a calibrated 6 0 Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). The data in Table IV show that polysilanes undergo predominantly scission during 7 radiolysis. In all cases, the G(s)/G(x) ratios were greater than 10. When aromatic substituents are directly attached to the aromatic ring, the G(s) value decreases significantly. This trend is consistent with the previous observation that G values for polystyrene derivatives are considerably lower than those observed for comparable saturated carbon-backbone polymers and suggests that aromatic substituents impart some radiation stability (83). The data in Table IV further suggest that for polysilanes this effect is limited to aromatic substituents that are directly attached to the polymer backbone, because the G(s) value for poly(phenethylmethylsilane) is among the highest that we have measured. These results raise the interesting question of whether remote aromatic substitution in carbon-based polymers will still impart the radiation stability characteristic of substituted polystyrene derivatives. These data (Table IV), although preliminary and somewhat incomplete, suggest that structural variations in high-molecular-weight polysilanes can result in significant differences in their sensitivity to ionizing radiation. These preliminary trends provide some insight for the development of new polysilanes for resist studies.

Conclusion Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.


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428.7

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Table IV. ""Co γ-Radiolysis Data for Substituted Polysilane Derivatives


is poly(methyl

—

30

35

18.6

24.6

10.5

—

18

G(s)/G(x)


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Derivatives

455

photolysis of high-molecular-weight polysilanes is wavelength dependent (84) suggest that the photodecomposition is more complex than was first indicated by initial exhaustive irradiation studies performed at 254 nm. Also, ESR studies on irradiated samples show that the persistent silicon-centered radicals generated are not those expected from simple chain homolysis. Additional studies will be necessary before the mechanism of persistent radical formation can be described with confidence. In addition, mechanistic studies of the photochemical reactions are necessary to determine whether similar processes occur in the solid state. Polymer chain scission is usually the predominant process in the solid state, although cross-linking reactions become more important in the presence of pendant unsaturation. However, little is known about the nature of the intermediates produced in the solid state. Information of this type is important, because most of the applications of polysilane derivatives require the materials as solid films. The usefulness of high-molecular-weight polysilane derivatives has been demonstrated for a variety of lithographic processes. Although the intense absorption of the polysilane backbone suggests that high-molecular-weight polysilanes are best suited for multilayer processes, the observed bleaching of the absorption upon irradiation, as well as the tendency of these polymers to photoablate with intense light sources, suggests that single-layer applications are not necessarily precluded. A better understanding of the radiation-induced processes, particularly the structure-reactivity relationships, is essential for the optimization of current lithographic procedures, as well as the development of new applications. In summary, polysilane derivatives constitute a new class of radiationsensitive materials for which a number of new applications have appeared. Much of the interest has centered around their unusual electronic properties and their sensitivity to various types of radiation. The nature of these radiation-induced processes is very complex, and although some progress has been made recently in understanding these processes, this area remains a fertile field for future study.

Acknowledgments I acknowledge the contributions of D. Hofer and D. LeVergne of IBM in the 7-radiolysis experiments. The GPC studies were performed by C. Cole of IBM. Finally, I gratefully acknowledge the partial financial support for the work performed at IBM by the Office of Naval Research.

References 1. Kipping, F. S. J. Chem. Soc. 1924, 125, 2291. 2. Burkhard, C . A. J. Am. Chem. Soc. 1949, 71, 963. 3. Trujillo, R. E. J. Organomet. Chem. 1980, 198, C27.



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RECEIVED for review May 27, 1988. ACCEPTED revised manuscript March 27, 1989.


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