Chapter 18
High-Energy Processes in Organometallic Chemistry Downloaded from pubs.acs.org by TUFTS UNIV on 07/12/18. For personal use only.
Photon- and Ion-Beam-Induced Reactions in Metallo-organic Films: Microchemistry to Microelectronics 1
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M. E.Gross ,W. L.Brown ,J. Linnros , and H. Funsten 1
AT&T Bell Laboratories, Murray Hill, NJ 07974 School of Engineering and Applied Sciences, University of Virginia, Charlottesville, VA 22901
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The photothermal chemistry of thin palladium acetatefilmswhen irradiated with continuous wave Ar laser and the non-thermal chemistry induced by high energy ion irradiation (2 MeV He and Ne ions) have been studied. The photothermal decomposition induced by scanned laser irradiation leads to pure metal lines that, however, may exhibit pronounced periodic structure. This results from coupling of the rate of heating by absorption of the laser radiation with the rate of release of the heat of reaction, giving an "explosive" reaction front that propagates ahead of the laser beam. In contrast, He ion beam irradiation produces smooth metallic-looking features that contain up to 20% of the original carbon and 5% of the original oxygen content of the film. Films irradiated with Ne ions contained slightly lower amounts of carbon and oxygen residues, but thefilms'appearance varied with thickness. +
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The decomposition reactions of acetic acid and metal acetates in solution induced by various energetic sources have been a subject of considerable study. Electrolytic oxidation and reduction and alpha (He ) and gamma radiolysis provide interesting mechanistic comparisons of ionic and radical pathways (1,2). Most of the work in the solid state has focused on the thermal decomposition of metal acetates (3-6). Questions relating to formation of ions and radicals and the mobility of these species through the solid vis d vis further reactions or escape from the solid are but some of the issues of interest. Metallo-organic compounds in general are of current interest as precursors for metallization in microelectronics fabrication (7). Thus, studies of the reaction chemistries of metallo-organic compounds will contribute to the rational design of new molecular systems for these applications. Palladium acetate, [Pd0i-O CCH ) l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule's ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. +
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0097-6156/87/0333-0290$06.00/0 © 1987 American Chemical Society
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Palladium itself is of interest for its metallic conductivity as well as its ability to catalyze electroless plating of copper. A schematic of the solid state decomposition process that we use is shown in Figure 2. Note that the difference in density between metallo-organic precursor and resultant metal leads to considerable reduction in thickness of the irradiated area as organic matter is lost. This decrease in thickness occurs in both thermal and non-thermal decompositions. Previous work on laser-induced photothermal gold deposition from metalloorganicfilmsrevealed additional structure in the deposited metal that resulted from coupling of the energy absorbed from the laser with the exothermic heat of reaction to give various forms of periodic structure (9-11). In this paper we present results on the spatially localized thermal and non-thermal decomposition of palladium acetatefilmsinduced by continuous wave (cw) A r laser or ion beam irradiation, respectively. +
Experimental Palladium acetate, purchased from Aldrich Chemical Co., is spin-coated from chloroform solution onto quartz, silicon or polished beryllium substrates to thicknesses of 0.1-1.5 Mm depending on solution concentration and spin speed. After exposure to the laser or ion beam, films are developed in chloroform to remove unirradiated material. Samples should be developed promptly as degradation of the palladium acetate films may occur upon standing, giving rise to particulate residue as seen in the background of Figure 3. All analyses of laserexposed materials were carried out on developed samples. Analyses of ion beam-irradiated regions were performed on undeveloped samples for consistency with in situ measurements. In either case, adhesion of the irradiated area to the substrate was good. Localized photothermal decomposition of thefilmsis effected by translating the sample relative to a 20W cw A r laser irradiation focused to 0.8 μπι FWHM in single line mode (5145À) or ~50 Mm in multi-line mode (principally 5145 and 4880Â). All laser exposures were done in air. Ion beam irradiations were carried out in vacuum with 2 MeV He and Ne ions from a 3.75 MeV Van de Graaff accelerator at current densities of < 1 μΑ/cm using currents of 16 1
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Figure 8. Sheet resistance of 0.90 μπι palladium acetate film as a function of 2 MeV He* and N e ion dose. +
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a dose of ~~ 3 x l 0 ions/cm . It is important to remember that since we are dealing with a solid target, detection thresholds for species at the mass spectrometer reflect not only the sequence of chemical reactions in the film, but also the rate of diffusion of the resultant molecules and radicals through the film and into the vacuum. Table I. Evolved Gases From 2 MeV H e Irradiation of 0.9 μτη Palladium Acetate Film. Molecules arranged in order of decreasing abundance within each group +
I C0 , C H CO, C H H 0 2
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II (m/e 44) (m/e 28) (m/e 18)
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CH , C H H 3
C H C H 4
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(m/e 15,16) (m/e 2)
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CH3CHO CH CO CH C0 H 2
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The observed yields for some of the parent ion masses at m/e 44 and below contain significant contributions from fragments of higher mass species, making quantitative yield assignments difficult. Qualitatively, however, it is clear that C 0 is the most abundant species evolved. Overall, the proposed parent species, in order of decreasing yield are: C 0 , C H > CO, C H > H 0 > · CH , C H > H - C H C H O - C H C O > C H C 0 H — C 4 H > C Hg > C4H4. 2
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Many of the above products are also observed in a- and 7-radiolysis of glacial acetic acid (20,21). Proposed mechanisms involve ionization of the acid followed by cleavage of the C-C bond to produce methyl radicals and carbon dioxide as the main decomposition pathway. The methyl radicals can then combine with each other to form ethane, abstract hydrogen from a parent molecule to form methane or combine with oxygen-containing fragments to form acetone and methyl acetate. Water, hydrogen, carbon monoxide and acetaltehyde are also formed. Electrolytic oxidation and reduction of acetate ions in solution produce similar species CO. The most significant difference between our studies of the H e ion-induced decomposition of palladium acetate and the above experiments is our observation of species in the mass spectra that we assign to be saturated and unsaturated C and C hydrocarbons. We propose that palladium, either in its oxidized form as palladium acetate or as fine metal particles, catalyzes the coupling of smaller hydrocarbon fragments (22). The relatively high amount of residual carbon and oxygen in the fully exposed palladium acetate films is consistent with this mechanism, being most probably present in the form of higher molecular weight molecules whose formation is catalyzed by the active palladium centers. Thermal decomposition of palladium acetate, either by laser irradiation or conventional means, leads to complete volatilization of the organic components. The purity of the ion beamirradiated samples is significantly improved by heating the samples in hydrogen at 300 °C after removal of unirradiated palladium acetate. +
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The electronic stopping power of the 2 MeV N e ions in the palladium acetate films is much larger than that of 2 MeV H e ions. The most obvious difference between the effects of the two ions is in the appearance of the films at the high dose limit. A 0.90 μτα thick palladium acetate film exposed to 2 MeV N e ion irradiation until no further spectroscopic changes occur looks black, compared with the metallic silvery films produced in the H e ion irradiation. However +
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Figure 9. The quadrupole mass spectrometer signal for volatile species released from 0.90 μτη palladium acetatefilmas a function of 2 MeV He ion dose. Mass 15 is shown for both C H and C H because of overlap at m/e 16 with oxygen. Mass 31 is shown for C H ( C isotope) because of overlap at m/e 30 with major fragments of other parent ions. +
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Ne ion irradiation of a 0.13 μηι thick film produces a metallic silvery film. A plot of the infrared COO" vibrations as a function of fluence in Figure 10 shows that the intensity decreases with approximately the same functional dependence as in the He ion irradiation, but at a dose that is 17 times lower. In addition, a new band appears at 1616 cm" , peaking at a dose of — 1.7x10 ions/cm , then decreasing rapidly to the same level as the original acetate bands. This may represent the formation of some monodentate acetate species as the palladium acetate trimers are cleaved. In situ infrared spectra of the He* ion-irradiatedfilmsshow a similar band of much smaller relative intensity. RBS analysis of a palladium acetate film at intervals during irradiation reveals that loss of carbon and oxygen from the film, plotted in Figure 11, occurs at doses a factor of 50 lower than those required to achieve similar loss with He* ions. Significant loss of carbon and oxygen is already observed at the lowest doses that we could practically achieve. Note that the dose dependence of the loss of carbon and oxygen is more closely correlated with the decrease in intensity of the infrared acetate vibrations than was observed in the He ion irradiations. A limiting residue content of 0.6 and 0.2 atoms of carbon and oxygen per palladium atom, respectively, is achieved beyond a dose of 4xl0 ions/cm . Thus, although the final film appears black, it has a slightly lower carbon content than the silver-looking film produced by He ion irradiation. This suggests a difference in nucleation kinetics and particle growth relative to removal of organic residue. In situ measurements of sheet resistance as a function of Ne ion dose reveal a similar dose dependence relative to He* irradiation as the RBS data namely a factor of —50 lower dose for comparable resistivity values. The limiting resistivity value of 2x10 μ Ω-cm is the same as for the He* ion irradiation despite the difference in the film's final appearance. Assuming comparable decomposition chemistry in the He and Ne ion irradiations, the higher electronic stopping power of the Ne ions gives rise to a higher density of events along individual ion paths. We propose that this higher density of events produces a fluffy film with deposited metal particles that are responsible for the film's black appearance. The above analyses offer little information regarding the chemical nature of the organic residue. Preliminary X-ray photoelectron spectroscopy analyses reveal that the palladium particles formed by both He and Ne* ion irradiations are metallic and contain no palladium oxide. +
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Conclusion Comparison of the photothermal laser-induced and non-thermal ion beam-induced decompositions of palladium acetatefilmsprovides immediate contrasts. The thermal reaction produces pure palladium metal whose morphology negatively affects the feature definition. Periodic structure arising from the coupling of absorbed laser energy with the heat of reaction, and porosity of the deposited metal are responsible for the increased electrical resistivity of the deposits relative to bulk metal. In contrast, He ion irradiation produces silvery, well defined, smooth deposits. The appearance offilmsirradiated with Ne ions varies from black to silver, depending on starting film thickness. Unfortunately, all features contain significant carbon and oxygen residues that are responsible for the high electrical resistivities of these films. Mass spectrometry of the volatile products of the He ion-induced reaction reveal the formation of saturated and unsaturated C and C hydrocarbon molecules not previously observed in the aand 7-radiolysis of acetic acid. We propose that palladium is catalyzing the coupling of smaller hydrocarbon species to form not only the higher molecular weight volatile species observed, but larger non-volatile molecules that comprise the solid organic residue in which metallic palladium particles are dispersed. +
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Acknowledgments The authors thank A. Appelbaum for Auger analyses, J. M. Gibson for transmission electron microscopy, S. B. Dicenzo for X-ray photoelectron spectroscopy and G. K. Celler and L. E. Trimble for use of their laser.
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