Langmuir 1995,11, 341-355
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Physical and Spectroscopic Studies of the Nucleation and Growth of Copper Thin Films on Polyimide Surfaces by Chemical Vapor Deposition Noo Li Jeont and Ralph G. Nuzzo*>t9* Department of Materials Science & Engineering, School of Chemical Sciences, and The Frederick Seitz Materials Research Laboratory, University of Illinois a t Urbana-Champaign, Urbana, Illinois 61801 Received August 15, 1994. I n Final Form: October 4, 1994@ The chemical vapor deposition (CVD)of copper from (hexafluoroacetylacetonate)(vinyltrimethylsilane)copper(1)[Cul(hfac)(vtms)land the thermal evaporation of copper on pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide have been studied with a variety of techniques including reflection absorption infrared spectroscopy (RAIRS), ellipsometry, X-ray photoelectron spectroscopy (XPS),scanning electron microscopy (SEM),and atomic force microscopy (AFM). Our studies reveal that the nucleation and growth of Cu by CVD occurs by the preferential reaction of surface carbonyl groups, (C=O),, of PMDA-ODA with the CVD reagent. Preferential trapping of thermally deposited metal atoms also has been seen, although the nucleation processes appear to be less chemically specificthan is seen in CVD growth on this substrate. Carbonyl groups at the surface of the polyimide react with the precursor molecules at 300 K, although the reactive sticking probabilities appear to be low The facility of nucleation on the polyimide surface depends on both the number and orientation of the carbonyl groups on the polymer surface which, in turn, depends sensitivelyon the thickness ofthe film. The nucleationof Cu growth from Cul(hfac)(vtms) is found to proceed from surface reactions mediated by these surface groups. Chemical-vapor deposited thin films grow on the PMDA-ODA surface by the Volmer-Weber mode. The grain size is found to correlate, at least qualitatively, with the number density of the complexes formed between the surface carbonyl groups and CuI(hfac)(vtms);these complexes, as inferred from infrared spectroscopic data, are formed by the displacement of vtms to give a Cul(hfac)(C=O),adduct.
I. Introduction Metallization has become increasingly important in advanced integrated circuit manufacturing. As the design rules of devices shrink below the 0.5pm level, their speed and reliability will increasingly become limited by the interconnect metallization material~.l-~ Aluminum (and its alloys) is extensively used as an interconnect material in very large scale integrated (VLSI) device^.^)^ Extension of these materials to the finer scale ultra large scale integrated (ULSI) architecture is complicated by their low electromigration resistance and sensitivity to stressinduced voiding. As a result, alternative metallization materials are being extensively studied as potential replacement for aluminum. Copper has received much attention in this regard in large part due to its low resistivity (1.67 pQ cm for Cu versus 2.65 pQ cm for Al) and high electromigration resistance.' The topographies of ULSI devices will be the most complex structures yet encountered in electronics manufacturing. Metallization schemes will involve complex hierarchical multilayer interconnections, and the deposition methods will have
* Author to whom correspondence should be addressed. t
Department of Materials Science & Engineering.
* School of Chemical Sciencesand The FrederickSeitz Materials
Research Laboratory. Abstract published in Advance ACS Abstracts, December 1, 1994. (1)Larrabee, G.; Chatterjee, P. Semicond. Int. 1991, 14, 84. (2) Holton, W. C. Adu. Tech. Integr. Circuit Process. 1990, SPZE Vol. @
1392, 27. (3) Shumay, W. C. J. Adu. Mater. Processes 1989, 135, 43. (4) Seidel, T. Inproceedings of thesematech Fourth Annual Sematech Centers of Excellence (SCOE) Technology Transfer Meeting; 1992; pp
121.
(5) VLSI Technology; 2nd ed.; Sze, S. M., Ed.; McGraw-Hill: New York, 1988. (6) Mayer, J. W.; Lau, S. S.Electronic Materials Science forlntegrated Circuits in Si and Gculs; Macmilan Publishing: New York, 1990. (7) Shingubara, S.; Nakasaki, Y.; Kaneko, H.AppZ. Phys. Lett 1991, 58, 42.
0743-7463/95/2411-0341$09.00/0
to fill high aspect ratio vias as well as conformally cover the complex fine line structures of the devicea8Chemical deposition methods perform very well in this regards-l0 and are therefore likely to play a very important role in ULSI device fabrication. Significant engineering problems can attend with these methods, and their introduction into a processing environment is often complex and expensive.'l To replace AI with Cu in ULSI devices will require that significant progress be made in our ability to control the deposition and subsequent processing of thin films of this metal by chemical methods.*-ll This paper addresses a specific issue which lies at the heart of nearly all chemical deposition technologies, namely the reaction dependent nucleation of crystal growth. Chemical depositions can be thought of as proceeding by a mechanism characterized by two distinct regimes. The easiest ofthese to understand, and one that has been studied extensively, is growth in the steadystate limit.9J2 This stage comprises most ofthe deposition process and involves the activation and subsequent reaction of molecular precursors at the growth surface. During this period, the chemical composition of the surface remains largely unchanged and each metal atom can be incorporated with a similar rate profile. The initial stage of the deposition is usually more complex. Metals are frequently deposited on nonnative substrate materials and the kinetics seen often reflect the activated (i.e., reaction dependent) nucleation of the deposit on the dissimilar material. These latter events are poorly understood, strongly coverage dependent, and, therefore, kinetically (8)Kaloyeros, A. E.; Fury,M. A.MRS Bull. 1993,18, 22-29. (9) Sherman, A. Chemical Vapor Deposition for Microelectronics; Noyes Publications: Park Ridge, 1987. (10) Vossen, J. L.; Kern, W. Thin Film Processes II; Academic Press:
New York, 1991. (11)Murarka, S. P.; Gutmann, R. J.; Kaloyeros, A. E.; Lanford, W. A. Thin Solid Films 1993, 257-266. (12) Jensen, K.F.; Kern, W. In Thin Film Processes II; Vossen, J. L., Kern, W., Eds.; Academic Press: New York, 1991; pp 283-368.
0 1995 American Chemical Society
342 Langmuir, Vol. 11, No. 1, 1995
Jeon and Nuzzo
complex and hard to control. It is for this reason that surface activation schemes figure centrally in chemical deposition t e c h n o l ~ g i e s . ~ ~ItJ ~is- most ~ ~ significant to note that the selectivity seen and often desiredin chemical depositions originates as a consequence of differentiated activated nucleation rates. The chemical vapor deposition (CVD)ofcopper has been investigated extensively in recent years and several precursors have proven useful in depositing copper thin films.16-20 One of the most promising classes of these reagents are the Lewis base adducts of copper(1)/3-diket o n a t e ~ such l ~ ~ ~as~ (hexafluoroacetylacetonate)(vinyltrimethylsilane)copper(I) [Cu'(hfac)(vtms)l. Copper(1) compounds of this structure have been reported to deposit copper films under a variety of conditions via a mechanism involving a surface-mediated disproportionation reaction shown in Scheme l.19320
Scheme 1 2Cu1(/3-diketonate)L(,,
-
+
CUO(~) Cu"(P-diketonate),(,,
+ 2Le,
Depending on the need for volatility and ease of reaction, the structure of the copper ligands can be varied widely.8,20 When L is the labile Lewis base vinyltrimethylsilane (vtms) and the P-diketonate is hexafluoroacetylacetonate (hfac), deposition of pure copper films can be carried out a t low temperatures (160-250 oC).21 Growth rates in excess of 500 k m i n have been rep0rted,2~and the influence of the substrate materials has been examined. In a cold wall CVD reactor, Cu'(hfac)(vtms) is a relatively nonselective reagent; efficient deposition is seen on a broad range of substrate materials including polymers, metals, glasses, and oxides.8J7,20-28 Polymedmetal interfaces are of great importance in both integrated circuit manufacturing as well as in microelectronics packaging. For electronic applications, polymers such as the polyimides are widely used as the dielectric layer on which metallizations are carried The nature of these interfaces is therefore an area of great interest in current research. Good adhesion between the metal and the polyimide is imperative for practical applications. ~
~~~~~
(13) Viehbeck, A,; Kovac, C. A.; Buchwalter, S. L.; Goldberg, M. M.; Tisdale, S. L. In Metallization ofPolymers;American Chemical Society: 1990; pp 394-414. (14) Baum, T. H. J . Electrochem. SOC.1990, 137, 252-255. (15) Baum, T. H.; Miller, D. C.; O'Toole,T. R. In Metallized Plastics 3; Mittal, K. L., Ed.; Plenum Press: New York, 1992; pp 9-17. (16) Gross, M. E. J . Electrochem. SOC.1991, 138, 2422. (17) Hampden-Smith,M. J.; Kodas, T. T.; Paffett, M. F.; Farr, J. D.; Shin, H.-K. Chem. Mater. 1990,2, 636. (18) Jeffries, P. M.; Girolami, G. S. Chem. Mater. 1989, 1, 8. (19) Norman, J. A. T.; Muratore, B. A.; Dyer, P. N.; Hochberg, D. A. J. Phys. N 1991, C2-263. (20) Shin, H.-K.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.; Farr, J. D.; Paffett, M. F. Adv. Mater. 1991, 3, 246. (21) Dubois,L. H.; Jeffries, P. M.;Girolami,G. S. In Proc.M u . Metal. ULSZAppl.; Rana V. V. S., Joshi, R. V., Ohdomari,Eds.; 1992; pp 375. (22) Norman, J. A. T.; Roberts, D. A.; Hochberg,A. K. Mat. Res. SOC. Symp. Proc. 1993,282, 347. (23) Chiang, C. M.;Dubois, H. Mat. Res. Soc. Symp. Proc. 1993,282, 341. (24) Dubois, L. H.; Zegarski, B. R. J . Electrochem. SOC.1992, 139, 3295-3299. .~.. (25) Shin, H. K.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.;
Paffet, M. F.; Farr, J. D. In Conf. Proc. ULSZ-VZt MRS: Pittsburgh, 1992; pp 403-411. (26) Girolami, G. S.; Jeffries, P. M.; Dubois, L. H. J . Am. Chem. SOC.
1993,115, 1015-1024. (27) Jain, A.; Chi, K. M.; Koda, T. T.; Hampden-Smith, M. J. J . Electrochem. SOC.1993, 140, 1434-1439. (28) Parmeter, J. E. 2. Phys. Chem. 1993,97, 11530-11541. (29) Polyimides-Synthesis,Characterization and Applications;Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vols. 1 and 2.
The need to understand the bonding and optimize the adhesive strength at polymer/metal interfaces has stimulated perhaps the most extensive research. Most studies on polymedmetal interfaces have concentrated on the effect of metals deposited on the surfaces by physical methods (i.e., thermal or sputter d e p o s i t i o r ~ ) . ~Deposi~-~~ tion via chemical vapor deposition has received much less attention. In this paper, we present the first results of detailed studies being carried out in our laboratory which seek to improve the understanding of CVD-derived metal/polymer interfaces. The focus of this paper is on the kinetic, chemical, and structural sensitivities exhibited in the chemical vapor deposition of Cu on a prototypical polyimide. This paper describes detailed spectroscopicstudies which establish the molecular characteristics of the activated nucleation of CVD growth on this substrate. We also present what we believe to be the first direct correlation between the characteristics of the deposition (in particular the nucleation kinetics) and the detailed molecular structure (in terms of functional group concentrations and orientations) of the polymeric substrates. We also present data which demonstrate the differing substrate sensitivities which operate in the initial nucleation of copper growth during chemical vapor (CVD) and physical vapor (PVD) depositions. We present our data in two main sections. The first of these describes the preparation and structural characterization of the thin-film polyimide substrates used in this study. The second section describes the results of PVD and CVD studies, the latter using Cu'(hfac)(vtms) as the precursor. The thin polyimide substrates used in the present work are of a novel character. We utilized thin film substrates of varying mass coverages (less than 380 A) supported on a highly reflective Al substrates. In this manner, we are able to characterize the reagent or metal atom interactions a t the ambient polymer surface directly by reflection absorption infrared spectroscopy (RAIRS). We find that the supporting substrate and mass coverage very strongly influence the molecular orientations of the segments present in the organic film. Through the use of the two differing polyimide materials shown in Scheme 2 [pyromellitic dianhydride-4,4'-oxydianiline (PMDAODA) and a closely related polymer benzophenone tetracarboxylic dianhydride -p-oxydianiline-m-phenylenediamine (BTDA-ODA-MPD)], we demonstrate the role played by the substrate in influencing the molecular orientations of the polymer main chain segments present a t the ambient interface. These studies also reveal a significant influence of the metal substrate-polymer interaction on the curing of the polyimide films from the amic acid precursor. Taken together, the data suggest a complex, though characterizable, dependence of the (30) Chou, N. J.; Tang, C. H. J . Vm. Sci. Technol. A 1984,2, 751755. (31) Hahn, P. 0.; Rubloff,G. W.; Bartha, J. W. Mat. Res. SOC.Symp. Proc. 1985,40, 251. (32) Haight, R.;White, R. C.; Silverman, B. D.; Ho, P. S. J . Vac. Sci. Technol. A 1988, 6, 2188-2199. (33) Ho, P. S.; Hahn, P. 0.; Bartha, J.; Rubloff, G. W.; LeGoues, F. K.; Silverman, B. D. J . Vac. Sci. Technol. A 1985,3, 739-745. (34) Silverman,B. D.:Bartha, J. W.: Clabes,.J. G.:. Ho,. P. S . J . Polvm. Sci. A 1986,24,3325-3333. (35) Rossi, A. R.; Sanda, P. N.; Silverman, B. D.; Ho, P. S. Organometallics 1987, 6, 580-585. (36) Stewart, W. C.;Leu, J.;Jensen, K. F. Mat. Res. SOC.Symp. Proc. 1989,154, 329. (37) Strunkus, T.; Hahn, C.; Frankel, D.; Grunze, M. J . Vac. Sci. Technol. A 1991,9, 1272-1277. (38) Grunze, M.;Killinger,A.; Thummler, C.;Hahn, C.; Strunskus, T. InMetallized Plastics2; Mittal, K. L., Ed.;Plenum: NewYork, 1991; pp 165-177.
Studies of Cu Thin Films on Polyimide Surfaces Scheme 2
PMDA-ODA
BTDA-ODA-MPD
molecular orientations of the main chain segments at the ambient polymer surface on important processing parameters. The most significant of these, for the PMDAODA system, is film thickness. In the second section, we describe how the nucleation and CVD growth processes are influenced by the structural and molecular features present at the ambient polymer surface. The data unambiguously demonstrate the intermediacy of a surface complex, Cu'(hfac)(C=O),, in the activated nucleation of Cu growth by CVD. We further describe how these features appear to be reflected in the ultimate character of the film deposited by CVD. Finally, we conclude this paper with a discussion of how the present work relates more generally to studies of interactions and reactions occurring at polymer surfaces. We describe, in the context of deposition chemistries, a more broadly applicablemethodology for studying polymer/ metal interfaces. 11. Experimental Section The aluminum substrates used in this study were prepared by DC sputtering; a 2000 A thick aluminum film was deposited on a Si(100)substrate. The Al target used was of 99.995%purity. Argon of 99.999% purity was used as the working gas in the deposition chamber. The deposition chamber was evacuated to 8 x Torr, back filled with 5 mTorr of Ar, and the film deposited a t a rate of 5 &s. Thin films ofPMDA-ODAand BTDA-ODA-MPD were made from DuPont Pyralin 2545 and *alin 2555 poly(amic acid) precursors, respectively. Poly(amic acid) films were deposited by spin casting on the Al metal substrates. Film thickness was controlled by varying the spinning speed and diluting the precursor with additional N-methylpyrolidone (NMP). Stock solutions of DuPont Pyralin 2545 were diluted with NMP in different proportions from 2 to 10% to give appropriate thicknesses (-20-380 A mass coverage). The BTDA-ODA-MPD thin films were similarly prepared using DuPont Pyralin 2555 diluted t o 2-5%. The films were obtained by flooding the substrate with the diluted poly(amic acid) solutions and spinning them a t 4000 rpm for 30 s. The spun films were dried by baking in nitrogen environment at 90 "C for 30 min and then curing a t 250 "C for 2 h. The thicknesses of the films were measured with a Gaertner Scientific Model L116C ellipsometer employing a He-Ne laser. The refractive indices of PMDA-ODA and BTDA-ODA-MPD were fured a t 1.73.39 Thicknesses were determined from the average of 10 measurements taken randomly across the sample. A reflection absorption infrared spectroscopy set-up was used where the IR beam from a conventional Fourier transform infrared spectrometer (Bio-Rad FTS-BO) was incident on polyimide-coated Al mirror a t a grazing angle of -84". The instrumental resolution used was 2 cm-l. The reflection optics were optimized a t 9712, and the incident radiation used was (39)Thomas, R. R.;Buchwalter, S. L.; Buchwalter, L. P.; Chao, T. H.Macromolecules 1992,25,4559-4568.
(40)In Situ FT-IRAnalysis ofPolyimide Curing;Snyder, R. W., Ed.; Elsevier Science Publishers: Amsterdam, 1989; pp 363-369. (41) Dubois,L. H.; Zegarski, B. R.; Nuzzo,R.G .J.Chem.Phys. 1993, 98,678-688.
Langmuir, Vol. 11, No. 1, 1995 343 p-polarized. The signal was detected by a liquid-nitrogen cooled, narrow band MCT detector. The RAIRS experiments were carried out in a high vacuum Z3/4in. conflat metal-seal 6-way cross (equipped with a heatable/ coolable sample holder, gas inlet system, differentially pumped KBr windows) pumped by a turbo molecular pump. The base pressure in this apparatus was Torr for 3 min at 500 K. The absolute intensities seen in these spectra cannot be quantitatively compared because of their significant sensitivity to sample positioning errors. either the +1 or 0 valent states. The spectra shown in Figure 12b shed further light on the Cu species present, especially in regards to the ligands to which they are coordinated. The lower trace shows the F 1s core level spectrum obtained after a saturation exposure a t 300 K. The single peak seen is the one expected for an intact hfac moiety. The upper trace shows the spectra obtained from a sample heated briefly to 500 K in the presence of CUI(hfac)(vtms) flux. These data show F 1s core level peak consistent with the presence of both inorganic fluorides (687 eV) and CF3 (690 eV) groups, respectively. We conclude from these spectra that the initial stages of the growth proceed with some significant degree of decomposition of the hfac ligands. The nucleation steps therefore must include in some manner reactions other than the simple disproportionation of Cu'(hfac1, species. At steady state, growth proceeds cleanly and thus must eventually come to be dominated by the latter process.
Figure 13. In situ RAIR spectra taken during the CVD of Cu from Cu'(hfac)(vtms) on PMDA-ODA (430 A thick) at 500 K at various precursor pressures. The pressure was increased Torr from (a) 1x 10-5T,(b) 1x and (c) and (d)at 5 x to greater than 1 x Torr for (e) and (0.
We have found that dosing the PMDA-ODA surface at room temperature with Cu'(hfac)(vtms) yields dissociatively chemisorbed Cu'(hfac), species in analogy with the results obtained for a variety of metal surfaces26and 'lW.59 When the PMDA-ODA substrate is heated to 500 K, exposure to Cu'(hfac)(vtms) at pressures higher than -5 x Torr leads to macroscopic film growth. This deposition process can be monitored spectroscopically, as is illustrated by the RAIR spectra shown in Figure 13.In these spectra, PMDA-ODA substrate was held at 500 K and exposed to Cu'(hfac)(vtms) a t several pressures all greater than Torr. The spectra shown in the Figure 13 are further divided into four groups, ones defined by the precursor pressure present in the cell at the time the Torr for (a); 1 x Torr spectra were taken (5 x Torr for (c) and (d); and P > Torr for for (b); 5 x (e) and (0). Approximately 7 min passed between each acquisition;the spectra were obtained on a single substrate which was stepped sequentially through each (increasing) pressure regime. During the initial stage of copper nucleation and growth Torr, two intense using a precursor pressure of 5 x C. Adsorption and Decomposition of Cu(hfac)peaks (1740 and -1230 cm-l) are seen in the difference (vtms) on PMDA-ODA at 600 K. It has been shown spectrum (Figure 13a). The negative intensity band at by Girolami et aLZ6and ParmeterZsthat pure copper films 1740 cm-' is due to the perturbation ofthe carbonyl groups can be grown using a continuous flux of Cu*(hfac)(vtms) of the PMDA-ODA substrate by the surface layer; the at pressures as low as Torr on single crystal and positive absorbance a t -1230 cm-l (with shoulder a t 1200 other metal surfaces a t 500 K. This deposition reaction cm-') is due to the CF stretching vibrations of CuYhfac), was not observed under UHV conditions suggesting a flux species. The bands seen in the latter region are signifisensitivity consistent with a bimolecular p r o c e s ~These . ~ ~ ~ ~ ~cantly different from those seen at 300 K (where three studies argue persuasively that the growth of pure copper complex bands were observed). films proceeds by a disproportionation reaction mechanism I t is interesting to note that the RAIR difference (Scheme 1). spectrum of a PMDA-ODA substrate exposed to Cul(hfac)(vtms) at 500 Kis very similarto a Cu(ll1)surface exposed (59) Donnelly, V.M.; Gross, M. E.J. Vac. Sei. Technol. A 1993,II, 66-77. to P d ( h f a ~ I 2and ~ ~ C u ( h f a ~ ) 2a ~ t 500 ~ K except for the
Studies of Cu Thin Films on Polyimide Surfaces contributions made to the spectra by polymer (most notably the band at 1740 cm-l). The data suggest that the exposure a t this temperature has led to the nucleation and subsequent growth of small islands of copper. These islands appear to have their surfaces terminated by hfac ligands oriented predominantly perpendicular to the larger substrate surface although other orientations must contribute as well. The growth of these islands is suggested by the continuing changes seen in the difference spectra, most notably the intensity changes seen at -1740 cm-'. A uniform copper metal overlayer would completely screen the PMDA-ODA substrate from the incident beam. In such an event, the spectra measured would, with the exception of the hfac bands, be that ofthe bulk PMDA-ODA film, only inverted. Indeed, when the depositions are carried out for long periods of time, that is precisely what we see. It is also clear from the data (Figure 13a-f) that not all the bands expected for the PMDA-ODA film grow in at the same rate; significant changes are seen initially mostly in the carbonyl region and only at later times do the other expected modes appear (e.g., at 1500 and 1380 cm-l). As the precursor pressure is raised (Figure 13b,c),the growth rate increases and the spectra suggest significant changes in the growth surfaces. This is seen most clearly by comparing Figure 13b to Figure 13c,e. At pressures of the order of Torr (Figure 13e), the growth surface must be complex as evidenced by both the structure and multiplicities seen in the hfac bands. The spectra are sufficiently complex that species such as CF3(s,may also contribute here as well. These results are quite unlike those obtained for depositions carried out on planar metal substrates. The spectra strongly suggest that, at a minimum, multiple orientations are adopted by the hfac ligands, ones which project differently on the normal direction to the substrate. We defer further comment on this point to later in the paper. The nucleation density achieved in this experiment can be estimated crudely from the data. If we assume that the absorption bands due to the hfac ligands originate from terraces of small metal crystallites and that the orientation adopted by these groups is perpendicular to the plane of these terraces, we can use the intensities of the bands in Figure 13a as a measure of their surface density. Again using the intensities seen on a Cu(ll1) sample as a calibration (see above),we estimate that there resides on this surface -3 x lo1' hfac/cm2. This is avalue which is -3 orders of magnitude less than the surface densities seen on a continuous planar Cu metal surface and thus establishes a crude qualitative boundary for the upper limit of the nucleation density in this system. This number is also roughly of the same order of magnitude estimated for the site density of the Cu'(hfac), species formed by dosing at 300 K. It should be noted that bands expected for the PMDAODA polymer appear anisotropically in the difference spectra; the one polarized component seen in the carbonyl region (1740 cm-l) grows in faster than does either its higher frequency component (1780 cm-l) or the other modes of the polymer. The characteristic vibrational bands of PMDA-ODA do not show substantial intensity in the difference spectra until relatively late in the experiment; for example, Figure 13a-c shows that the band at 1740 cm-l is uncharacteristically intense as compared to barely discernible modes at 1500 and 1380 cm-'. In the bulk spectrum of PMDA-ODA, these bands have an intensity ratio of -3:l:l (see Figure 5). This preferential attenuation of selected PMDA-ODA bands suggests the film nucleates favorably at surface carbonyl groups and that the island growth proceeds in a way
Langmuir, Vol. 11,No.1, 1995 351 I - '
!
I
I
'
'
I
k.005
1800
a
1600
1400
1200
1000
Wavenumbers (cm.')
Figure 14. In situ RAIR spectra taken during the evaporation of Cu on a PMDA-ODA substrate at 300 K. The spectra were taken in -2-min intervals from the start of deposition.
consistent with their selective monitoring in the RAIR difference spectra early in the deposition. D. Comparison between Evaporated and CVD Copper Thin Films As Investigated by in-Situ RAIRS. In-situ RAIR difference spectra were taken which allow us to compare the changes that occur on the PMDAODA surface during the chemical and physical nucleation of metal film growth. Figure 14 shows the RAIR spectra taken during a continuousdeposition of copper by thermal evaporation onto a 430 A thick PMDA-ODA substrate. The spectra were collected at -2 min intervals, and a very slow, albeit unknown, metal evaporation rate was used (the dimensions of the IR cell preclude the use of a thickness monitor). For clarity of discussion, only data from the early stages of nucleation and growth are shown; high coverage metal overlayers yield the expected inverted spectrum of the bulk film. The first spectrum shows a small (negative intensity) peak at -1740 cm-l. We note that at this point, the spectrum is comprised solely of bands due to PMDAODA. A difference band due to out-of-phase stretching vibration of the C-0 ofthe PMDA segment of the polymer appears first. Bands correspondingto ODA as well as the C-N-C linkage of the polymer are evident in the second spectrum. The intensity ratio of these peaks relative to the 1740 cm-' is less than that found in the bulk PMDAODA film (for example, I (1500/1740 cm-l) for the bulk is 4 3 as compared to the 4 5 for the data in Figure 14b). As more metal is deposited on the surface, the intensity ratios become similar to those seen in the bulk spectra (Figure 14c-e). The last spectrum shown, Figure 14e, closely resembles that of a bulk polyimide film (albeit inverted). The close resemblance of the data to the spectrum of the bulk film suggests that the interaction between the metal and the polymer is weak. Any strong chemical interaction should affect the spectra acutely. The data thus suggest that chemical reactions at the interface are relatively unimportant in this deposition system. The mode at 1740 cm-' appears before any other, thereby suggesting that the initial interaction between the metal and polymer involves the carbonyls of the PMDA segment. This conclusion agrees well with the observations reported by D ~ n as n well ~ ~as Ho et al.33Dunn found that a selective reaction occurs between the metal atoms and the PMDA segments. Ho et al., using in-situ XPS, found that the carbonyl C 1s peak showed a more significant attenuation than did other C 1s components at the earliest stages of the copper deposition. They suggest that the interactions occurring between thermally deposited copper and PMDA-ODA are predominantly
Jeon and Nuzzo
352 Langmuir, VoE. 11, No.1, 1995
’ ’ ’
1 - 1
1 1 0.005
’
’
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,
,
,
To.005
1
a
4
0 a,
m
e
w
9
1800
1400
1600
1200
1000
Wavenumbers (cm”)
Figure 16. In situ RAIR spectra taken during the CVD of Cu from Cul(hfac)(vtms)on a PMDA-ODA substrate at 450 K at 5 x Torr. The spectra were taken in -7-min intervals from the start of the deposition.
,
I
I
I
I
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I
n
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1800 1780
1780
1740 1720 1700
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Figure 17. Carbonyl region of the PMDA-ODA from Figures 14 and 15 for evaporated (a-e) and CVD (f-j) copper thin films, respectively.
1800
1600
1400 1200 Wavenumbers (cm”)
1000
Figure 16. Selected RAIR spectra from CVD and evaporated thin films of Cu at low coverage on PMDA-ODA substrates.
Assignments are described in the text.
physical in nature and that this interaction does not alter the chemical nature ofthe surface. This weakinteraction has been used to rationalize the observation that the thin film growth of copper by physical deposition on polyimide follows the Volmer-Weber mode.60 Figure 15shows the F N R difference spectra that were taken during chemical vapor deposition of Cu on a 430 A thick PMDA-ODA using Cu’(hfac)(vtms). The experiment was carried out using a constant precursor pressure Torr at a temperature of 450 K. The spectra of 5 x were taken in -7 min intervals. These spectra are similar to those shown in Figure 14 except for the presence of the hfac bands in the 1150-1300 and -1600 cm-l regions. The negative bands that correspond to PMDA-ODA are very similar to the ones observed for the evaporated copper case. The peak positions and shapes are also nearly identical, suggesting that for the CVD process, the interaction between the metal and PMDA-ODA is also weak and does not involve extensive degradation of the polymer. Though the overall interactions occurring between the metal and PMDA-ODA surface appear to be similar for the evaporated and the CVD methods, there are some important differences. These differences are best illustrated by comparing the spectra shown in Figure 16. Figure 16 shows spectra corresponding to roughly equivalent coverages of Cu deposited by CVD and thermal evaporation. Note that the intensity of the ODA ring modes and C-N-C linkage stretching modes (1500 and (60)Wetzel, J. T.; Smith, D. A,; Appleby-Mougham, G. Proc. Mat. Res. SOC.Symp. D 1984,37,271.
1380 cm-’) are very different in these two spectra. The very significant differences evident in these spectra suggest that the nucleation events occur much more selectively in the CVD process (see below). The higher selectivity of the nucleation events occuring in the CVD process can be demonstrated by examining the data presented in Figures 14 and 15 in finer detail. The spectra in Figure 17a-e correspond to those shown previouslyin Figure 14;spectra in Figure 17f-j correspond to spectra from Figure 15. First, we note that the C-0 difference bands seen with evaporated Cu are consistently -5 cm-’ higher in frequency than bands of comparable intensity due to a CVD deposit. The former’s line width and peak maxima changes with coverage (-5 cm-l), and a significant and broader asymmetry is observed a t all coverages on the lower frequency side of the band. We conclude from this that the chemical deposition selects a more homogeneous population of surface groups to interact with. It seems likely that steric effects underlay this selectivity. Nucleation in the chemical deposition apparently involves to a significant degree the Cu*(hfac), surface complexes. The formation of such moieties necessitates the displacement of the vtms ligands bound to the precursor. This process appears to happen most readily at a reasonably homogeneous population of surface sites. Thermal depositions, in which the impinging flux is zero-valent metal atoms, appear to be much less discriminating. The islands which nucleate in this latter case appear to interact with a much less homogeneous population of polymer groups. It is important to note that, even with this proviso, the nucleation processes for the latter still appear to involve a preferred trapping of the metal atom flux at polar, heteroatom containing moieties. It is important to note that the temperature of the substrate is very different in these two processes, making selectivity in the higher temperature CVD process all the more remarkable (see below). Finally, we note that the selective interaction between the variously deposited metal atoms and the polar surface functionality of the polymer is most important at the earliest stages of the deposition. As the depositions proceed, both processes fill in the regions between the islands. When the latter coalesce, any optical anisotropy
Langmuir, Vol. 11, No. 1, 1995 353
Studies of Cu Thin Films on Polyimide Surfaces
Scheme 4
7
\
Figure 18. SEM micrographs of thin copper films deposited on PMDA-ODA substrates (380 A)by (a) CVD at 500 K and 1 x lop4Torr for 30 min and (b) thermal evaporation after 12 min at -20 W. a
Figure 19. SEM micrographs of thick copper films deposited on PMDA-ODA substrate surfaces by (a) CVD at 450 K and Torr for 20 min a n (b) thermal evaporation after 5 min 5x at -66 W.
is lost and a normal (but inverted) spectrum of the PMDAODA thin film is obtained. E. Characterization of Cu Thin Film Deposits on PMDA-ODA by Scanning Electron Microscopy. Figure 18 shows SEM micrographs of very thin Cu films deposited on a 380 A thick PMDA-ODA thin-film substrate by (a) CVD at 500 K and (b) evaporation at 300 K. The data of Figure 18 are most revealing in their demonstration of the structure and surface density of the small metal clusters which result from nucleation and growth of Cu on the polymer surface. The mass coverages of the two metal films are low; the metal resides on the surface as small clusters of roughly 200-300 A in size for the CVD film and about 150-200 A for the evaporated film. This small size difference is altogether remarkable when one considersthe significant temperature differences in these deposition processes. The surface densities of clusters are -3.5 x 1O1O and -4.0 x 1O1O nuclei/cm2 for the CVD and evaporated films, respectively. We note that the difference in these values is within the experimental error of the measurement and, perhaps coincidentally, correlateswell with the number of nuclei (-3 x loll nuclei/ cm2)estimated by RAIRS (see above). One very important difference is noted in these two films, namely that the CVD film forms clusters which coalesce and show greater contrast in the SEM, while the evaporated film seems to cover the surface much more uniformly. This type of difference in microstructure is expected given that the CVD films were grown at a higher substrate temperature. SEM investigation of thicker copper deposits shows that films of both types are continuous (Figure 19). In comparing Figure 19a to Figure 18a, it is clear that the CVD film now exhibits a larger grain size as well as clusters that are merging side by side to give a continuous film.
b
C
The films shown in Figure 19b exhibit similar differences as compared to the corresponding lower mass coverage evaporated film (Figure 18b). Several points are evident in the data shown here. First, the PMDA-ODA film appears to be continuous and unaffected by the metal overlayer. Second, the grain density seen here (for very thin samples) is similar to the surface density of the clusters estimated spectroscopically. This strongly suggests that the Cul(hfac)species may be implicated in the formation of those clusters which in turn lead directly to the grain structure seen in the final film. The data also suggest that similar events may control nucleation in physical depositions. In this latter instance, thermal accommodation of atoms at specific sites rather than the formation of discrete complexes is envisioned to be the most likely mechanism.
IV. Concluding Remarks The CVD of Cu from CuI(hfac)(vtms)on a PMDA-ODA substrate reveals a number of interesting features. In the discussion that follows we will summarize several of the more important insights gained from this study. We will concern ourselves first with the nature of the polymer substrates used and then discuss the nature of the mechanisms involved in the nucleation and subsequent growth of Cu by CVD on this prototypical substrate material. This study employed PMDA-ODA films of several mass coverages as substrates. To study these materials by RAIRS, a highly reflectiveAl substrate was used to support them. The optics involved in this experiment deserve explicit discussion. We imagine that the investigation of Cu CVD by RAIRS will show varying experimental sensitivities in a t least three formal regimes related to coverage;these are shown in Scheme 4. The data show that the deposition proceeds by a cluster growth (Volmer-Weber) mode. At the earliest stages (finite-flux, low temperature), CuI(hfac)(vtms>reacts on the PMDA-ODA substrate and binds to the surface as a CuI(hfac), complex (a). These species in the presence of an adsorbate flux react a t higher temperatures to give clusters which ultimately grow and coalesce into the final thin film microstructure. The optics of the RAIRS experiment are actually quite complex in the one regime (b) shown in the scheme. At the outset, we expect spectra typical of a simple reflection geometry. The beam is incident on a thin film supported on a high quality mirror (the Al substrate). The spectra obtained are unexcep-
Jeon and Nuzzo
354 Langmuir, Vol. 11, No. 1, 1995 tional, comprising the responses of either the polymer, the Cul(hfac),adsorbates, or the differencebands described earlier. The incident beam is p-polarized, so in all cases we only see features whose transition moments project onto the surface normal direction. We assume, to a good approximation, that all the data reported above are due to a single reflection (except as noted below). During growth, as the sample passes between regimes (a)and (b),the analysis ofthe spectra becomes much more problematic. The metal clusters must a t some point become truly metallic, and this, in turn, must influence the optical response ofthe sample. We have not analyzed the optics of this regime in detail but note our qualitative observations here. It is clear that the fractional surface coverage of the metal, in terms of the area occupied by the Cu clusters, does not correlate simply with the intensities of the difference spectra. The modes seen in these spectra reflect perturbations of the sample due both to the surface bonding as well as the screening effects of the metal overlayer. It is our experience that the latter are "slow)) t o develop. The clusters are apparently quite large before significant intensity due solely to this latter effect is seen. This implies that efficient coupling of the incident radiation with the film occurs even in the presence of these metal islands. The end point, (c), reflects the completion of the transition between optical regimes. The sample is completely covered and the difference spectra largely reflect the spectrum of the polymer film which is now screened from the beam. In practice, we found that this latter regime grew in very rapidly; the "bulk" spectrum ofthe polymer would quickly"turn-on'). With the exception of bands due t o adsorbates present on the Cu surface, the spectra in (c) should be nearly identical (but inverted) to those of the original. Weak features due to presence of a new metal-polymer interface should not be visible;these might be visible in the intermediate regime however. It is also possible to directly record the spectra of the thin-film polymer substrates using a separate clean aluminum reference. This procedure is precisely that which was used to collect the data shown in Figures 5-7. We now turn to a discussion ofthe structure ofthe polymer substrates used in this study. It is evident from the data that cast films of the amic acid precursor of PMDA-ODA strongly wet the Al substrate and that this conformal coating is maintained during the cure. The topographies seen by AFM are simple and become increasingly planar as the mass coverage increases. It is unclear from the presently available data how coherent the thinnest coverage studied (-30 A) is, although it is likely that this film is discontinuous. The thicker films, as judged by both AFM and XPS, appear to completely cover the Al substrate. The metal substrate on which the polymer is supported presents an interesting issue for consideration. Aluminum, as used here, is coated by a native oxide which is highly sorbtive toward polar organic compounds. Carboxylic acids, for example, adsorb strongly in the form of a carboxylate moiety formed as a result of a proton transfer reaction (Scheme 5).4s
Scheme 5
The presence of such surface species is strongly suggested by the spectroscopic data shown in Figures 6 and 7. The spectra of the uncured polymer films are quite complex and the assignment ofthe weak features at -1600 and 1430 cm-l to surface carboxylates is qualitative, at best, given the conflicting overlap of other bands. The
assignment of both a n antisymmetric and symmetric stretching vibration of a surface carboxylate moiety can be made with greater certainty in the spectra of the cured films of both polyimide types, however. Most telling in this regard is the coverage dependent spectra of the BTDA-ODA-MPD thin films (Figure 6). I t is well-known that the cyclization reaction leading to the imide functional group will be greatly slowed, if not completely inhibited were the carboxylic acid moiety present in an unprotonated form (Scheme 6ha Scheme 6
k, cc k,
The spectroscopic signature of such an inhibition of the cure would be the retention of specific modes in the spectra, namely the strong amide I and amide I1 bands of the conserved amide group, as well as by bands due to the symmetric and antisymmetric stretching vibrations of the carboxylate anion moiety. The intensities of all these bands would be weighted by their surface coverage and would not necessarily track the mass coverage of the film. In both polyimide types, strong evidence of such coverage insensitive bands are seen for all the modes expected on the basis of this analysis. We therefore assign these "surface-bonding" modes (Figure 5-7) to the segments bound strongly to the substrate in the form of carboxylate groups and the amide functions which must necessarily accompany them. From previous studies of self-assembled monolayers of fatty acids on aluminum,44,61we can conclude that the site densities of the carboxylates found here are low. If we adopt the very crude approximation that the band intensities in the present data can be directly compared to the intensities of the carboxylate bands seen in the spectra of the fatty acid monolayers, we can then estimate their coverage by scaling it to the known coverages in the latter monolayers; we infer a surface coverage of carboxylates in the cured film of no more than one forth that found in the "Zisman-like" fatty acid monolayers ( -= 1COz-/ -100 A?. This value, though qualitative, is of the order of the imide segment density in an extended conformation of a polymer monolayer (see above). It is important to note that the site density of the carboxylate groups found here may not be reflective of those present at the Al surface in the uncured a m . Simple acid-base chemistries should be reversible, and in principle the cure should drive this system completely to the imide form. The retention of carboxylate groups argues strongly that other features of this system must, in part, prevent this. One of the more interesting points of consideration in the present study is the degree to which the substrate perturbs the polymer surface viz. the nucleation and growth of Cu by CVD. The data presented in Figures 8 clearly show that the surface of the various PMDA-ODA films bond the nascent Cul(hfac), complexes very differently. We note for example that the thinnest films exhibit severe heterogeneous broadening in the difference bands. The XPS data of Figure 4 also confirm the more complex (61)Allara, D.L.;Nuzzo, R. G.Langmuir 1985,1, 52-66.
Studies of Cu Thin Films on Polyimide Surfaces
chemical environments which must exist in the thinnest samples. We expect that some of the differences seen in the C 1s line shape must be due to a weighted contribution by the surface bound segments, one which diminishes in importance as the coverage increases. We note that the heterogeneousbroadeningnoted in the infrared difference spectra does not track the changes seen by XPS. In a 138 A thick PMDA-ODA film, the spectra obtained are essentially the same as that found in the bulk; from the vantage of its reactivity toward Cu’(hfac)(vtms),it is very much unlike a thicker sample, however. It seems inescapable that the underlying substrate constitutes a large perturbation of the organizational state of the polymer thin film, even at mass coverages exceeding 138 A. PMDA-ODA is known to exhibit bond-orientational ordering at a solid ~ u r f a c e . We ~ ~ expect , ~ ~ at equilibrium, therefore, for the chains to align along the surface. It seems likely that the finite corrugation of this surface must frustrate this in-plane ordering. It is most striking in this regard that we see optical anisotropies in RAIRS data which extend to distances (i.e.,mass coverages) that are of the order of the substrate roughness. There are several important results pertaining to film growth that emerge from this study. The most significant of these is the clear demonstration of the importance of coordination chemistry concepts in defining the earliest stages of the film growth process. We note, for example, the complex sensitivity of the reactive sticking probability (S,) on the mass coverage of the PMDA-ODA film. It is also clear that, in the present system, the value of S, is very low at any film thickness. These results clearly show that the nucleation density in this system will be low and the coarse grain texture which results can be directly related to the sparse interaction of the precursor at specific strong binding sites. The data further suggests that these strong bonding sites are carbonyl groups, ones whose steric environments are suitable for the formation of CuI(hfac), species. We also note that the disproportionation of these species is slow. Annealing the dosed sample at 500 K, a temperature where the steady-state deposition rate is fast, does not lead to the complete decomposition of the Cu’(hfac), moieties. In the presence of an adsorbate flux, however, a very different result obtains. Decomposition is rapid and large metallic clusters quickly come to replace the discrete Cu’(hfac), complexes. The character of these clusters depends very sensitively on flux and, to a lesser degree, on temperature as well. The island growth mechanism appears to propagate flat terraces under certain conditions (low flux < Torr and temperature (450 K), but this specificity is easily lost by changing the conditions as judged by the infrared spectra of the growth surface; for Torr lead to complex hfac example, pressures ’5 x adsorbate geometries, ones presumably reflective of a complex topography of the metal film. In any event, the most elementary process, namely the heterogeneous nucleation of the film, appears to proceed via a discrete molecular precursor. We take this as being a strong confirmation of the importance of molecular surface chemistries in thin film nucleation and growth by CVD. There also appears to be some relevance of molecular structure to thin film nucleation even when the depositions are affected using thermal atom sources (albeit at much lower substrate temperatures). We have demonstrated this in the past using chemically modified (monolayer) substrates (A1203 and Si02).62,63The same trends are (62)Allara, D. L.;Nuzzo, R. G.;Hebard, A. F.;Padden, F. J.; Falcone,
D.R. J . VUC.Sei. Technol. A 1983,1,316-382.
Langmuir, Vol. 11, No. 1, 1995 355
evidenced on the polymer surface, albeit more weakly than was found for CVD growth. Carbonyl groups at the surface appear t o be an efficient trap for atoms diffusing at the surface. From the pattern of the difference bands, the clusters which form at the earliest stages of the deposition appear to very strongly perturb polymer segments associated with the carbonyl groups. One of the more intriguing molecular concepts in the present context is that associated with the organization of the free polymer surface. The infrared spectra shown above strongly argue that the mass coverage of the film must strongly influence this larger structural organization. In the Cu’(hfac)(vtms)uptake studies, the difference spectra reveal the character of surface on which the CuYhfac), complex resides. At all coverages, the very heavy weighting of the out-of-phase mode demonstrates a not all too surprising preferred orientation of the chain axis in a plane parallel to the surface. We presume this orientation allows the best chance for aligning a carbonyl moiety along the surface normal direction, a feature necessary for its observance by RAIRS. The interpretations of the difference spectra thus strongly relate to characteristics of the molecular orientations present at the free polymer surface. From an examination of the exposure dependence (2’ = 300 K), it can be seen that the increasing intensity of the hfac bands correlates directly with the changes seen in the carbonyl band intensity. We therefore conclude that, at least with this reagent, we are following a specific chemical activation step. It is not clear whether this also involves a specific orientation of the carbonyl group (RAIRS only reveals an average). The significant heterogeneousbroadening seen in the two thinner polymer samples argues very strongly that many environments, and presumably orientations, contribute. Finally, we conclude by noting that the ‘reagent” also contributes very powerfully to the character of the RAIRS data. Whether as a metal atom or discrete molecule, adsorption at the polymer surface produces a perturbation of the relevant optical functions, one which can be monitored conveniently by RAIRS. This argues for a more general utility of these techniques in studies of molecular processes occurring at polymer surfaces. We will report on further applications along this line in future publications.
Acknowledgment. This work was supported by the National Science Foundation (CHE930095) and the Federation of Advanced Materials Industries (FAMI), an industrial consortium at the University of Illinois. XPS and SEM studies were carried out in the Center for Microanalysis of Materials, University of Illinois, which is supported by the Department of Energy under the Contract DEFG02-91ER45439. We would like to thank Mrs. V. Petrov, Dr. Helen Farrell, Dr. Rick Haasch, Dr. Michael Hostetler, Dr. Wenbin Lin, and Mr. Shrikant Lohokare for their help. We would also like to thank Dr. Richard Alkire and Mr. Wolfgang Schmidt for their help with AFM. We are particularly indebted to Dr. John Norman of the Schumacher company for the generous gift of CuYhfac)(vtms). LA940639E (63)Green, M. L.; Levy, R. A.; Nuzzo, R. G. Thin Solid Films 1984, 367-317.
