9230
J. Phys. Chem. 1995,99, 9230-9235
Direct Evidence for Silver Film Deposition below Room Temperature on pt(ll1) from the ((CH3)3CCOCHCOC3F7)A@Et3 Precursor S. Serghini-Monim, Z. Yuan, K. Griffiths, P. R. Norton,* and R. J. Puddephatt Department of Chemistry, The University of Westem Ontario, Ontario, N6A 5B7, Canada Received: January 23, 1995; In Final Form: March 23, 1999
The study of the adsorption of 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (fodH), and (1,1,1,2,2,3,3hept~uoro-7,7-dimethyl-4,6-octanedionato)(~ethylphosphine)silver(I)([(fod)AgPEt3]) on Pt( 111) has been carried out by reflection absorption IR spectroscopy (R4IRS) and temperature-programmed desorption (TPD) techniques. Both molecules adsorb molecularly on Pt( 1 11) at 90 K with their COCHCO skeleton parallel to the surface at low coverage and with random orientation at high coverage. When the Pt( 111) sample is heated, the chemisorbed states of fodH and [(fod)AgPEt3] dissociate below room temperature. Molecular desorption of fodH and [(fod)AgPEt3] occurs at 275 and 340 K, respectively. Thermally induced dissociation of fodH leads to the generation and the desorption of a ketone species at 187 K, which we tentatively attribute to the C4HgCOCH3 molecule. The remaining intermediates dissociate and desorb in the form of the CF3 radical at 215 K and CO between 400 and 600 K. Larger and more stable intermediates are generated following fodH decomposition than those generated from hexafluoroacetoacetone (hfacH) dissociation. More importantly, this study showed that the [(fod)AgPEt3] precursor releases Ag atoms at very low temperature, the lowest yet reported. Thermal desorption of the deposited Ag atoms takes place at 1035 and 940 K from the monolayer and the multilayer, respectively. The main byproduct desorbing at low temperature upon precursor dissociation is fodH at 236 K. We also observed the thermal desorption of HF and CO at 236, 340, 616, and 948 K.
Introduction Metal-organic chemical vapor deposition (MOCVD) is a well-established technique for growing thin films and is used in a number of industrial sectors. Nevertheless, its application for growing films of all metals of interest is limited by the lack of suitable precursors. Many known precursors are air and moisture sensitive or possess a very low vapor pressure and so are not suitable for application in industry. During the last few years, considerable progress has been made in the synthesis of stable precursors. For example, (P-diketonato)copper(I)complexes and particularly (hexafluoroacetylacetonato)copper(I) complexes ([(hfac)Cu-L]; hfac = hexafluoroacetylacetonate, L = neutral ligand) were prepared for use in microelectronics technology, since copper possesses a lower resistivity than conventional metals (Al and W) used as interconnect materials in integrated circuits (IC's).'-3 Silver has strong potential for use in the ultra-large scale integrated (ULSI) circuits, since it possesses the lowest resistivity in the periodic table. However, the silver complexes analogous to the known copper precursors are much less volatile and are often thermally unstable at temperatures needed for volatilization, and some need a high deposition temperature! In a previous article we reported the low-pressure CVD results of a new silver precursor, (1,1,1,2,2,3,3-heptafluor0-7,7-dimethyl-4,6-octanedionato)(triethylphosphine)silver(I) ([(fod)AgPEt3]), on Ag( 11l).5 This complex has demonstrated the best thermal stability during the deposition process and the lowest dissociation temperature yet reported in the adsorbed phase. Temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) results of the adsorption of [(fod)AgPEt3] on Ag( 111) revealed that at low coverage the [(fod)AgPEt3] molecules adsorb with their COCHCO skeleton parallel to the surface whereas at high coverage a randomly oriented [(fod)AgPEt3]molecular state is populated. @
Abstract published in Advance ACS Abstmcrs, May 1, 1995.
Only molecules having their COCHCO skeleton parallel to the Ag( 111) surface undergo dissociation at low temperature. The randomly oriented molecules undergo a reorientation of their molecular plane with respect to the surface normal prior to their desorption near room temperature. In the present work we have investigated the adsorption of [(fod)AgPEts]on Pt( 111) using TPD and RAIRS techniques in order to probe the interaction of this precursor with materials having relatively high reactivity toward adsorbates. This reactivity is comparable to that of materials encountered in integrated circuits. In addition, the thermal desorption of deposited Ag atoms on Pt(111) is possible, because silver atoms do not diffuse into the bulk of platinum at elevated temperature but remain only in the first layer of Pt( 111).6 A knowledge of the mode of thermal decomposition of a precursor in the adsorbate phase is important, since the quality of the growing material depends strongly on the process of decomposition of the precursor and its ligands. Therefore, we also report the results of the adsorption of the 1,1,1,2,2,3,3-heptafluoro-7,7dimethyl-4,6-octanedioneligand (fodH) on Pt( 111).
Experimental Section The MOCVD experiments were performed in a turbomolecular-pumped (330 Us)ultrahigh vacuum (UHV) system with a base pressure 2 x Torr. This UHV system was equipped with a Hiden-601 mass spectrometer to monitor up to 16 desorbing species during temperature-programmed desorption (TPD) measurements. The controlled heating rate, p, was set to 2 K-s-'. A Mattson Cygnus-100 spectrometer was employed for reflection absorption infrared spectroscopy (RAIRS) measurements. A liquid nitrogen cooled, narrow band mercury-cadmium-tellerium (MCT) detector was used to collect the RAIRS data. A typical spectrum was obtained using an average of 500 scans at a resolution of 8 cm-'. The UHV chamber was also equipped with a sputtering gun for single-
0022-365419512099-9230$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 22, I995 9231
Silver Film Deposition on Pt( 111)
0.006 [
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Figure 1. RAIR spectra after exposing clean P t ( l l 1 ) at 90 K to 0.14 L (x5), 0.8 L, 1.7 L, and 2.5 L of fodH.
Figure 2. RAIR spectra of adsorbed fodH (2.5 L) on the P t ( l l 1 ) surface as a function of anneal temperature over the range 90-380 K.
crystal cleaning. The sample holder used in these experiments was identical to the cold finger described by Thiel.' The Pt(ll1) single crystal was 2 mm thick and 1 cm in diameter. This crystal was spot welded to Ta ribbon. The temperature was monitored with a chromel-alumel thermocouple which was spot welded on the back of the sample. The Pt( 111) crystal was routinely cleaned by repetitive cycles of argon sputtering and annealing in oxygen. The sputtering energy was 700 eV with an ion current of 12-15 PA. The sputtering angle was varied between 60" and 70" with respect to the surface normal. The sputtering was followed by an Torr and annealing in oxygen at a pressure of 5 x temperature of 573 K, and the sample was then flashed to 1400 K in vacuum.8 The precursor [(fod)AgPEt3] used in this work was prepared in our laboratories. The details of the preparation method, the physical parameters, and high-pressure CVD measurements on the glass substrate are given el~ewhere.~ Special care was taken during the precursor adsorption; the reservoir containing the precursor was pumped for several minutes while it was kept at 340 K. During the admission and for the first 5-10 s, the shutter located at the end of the home-made'O doser was closed to prevent surface contamination by any volatile species coming from the reservoir. The pressure during the adsorption was always below 1 x Torr. The distance between the substrate and the end of the doser was approximately 2 cm. The 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (fodH) was obtained from Aldrich Chemical Co. and was degassed by repetitive freeze-pump-thaw cycles.
shift implies that fodH molecules are more constrained in the adsorbed phase than in the liquid phase and differ from the chemisorbed layer. The major difference between low and high exposure, except for increases in the intensity of the C-H and C-F vibrational bands, lies in the emergence of a new peak around 1600 cm-I. The absence of bands in the 1600-1700 cm-I region and especially the absence of v(C=O) at low exposure are an indication that this vibrational mode was completely inactive. Several possible origins of this effect are reasonable. First, if we consider that the adsorption of fodH on Pt(ll1) at 90 K is a dissociative process, the absence of these vibrational modes could be attributed to the desorption of readily formed byproducts containing carbonyl groups. Generally the desorption of CO molecules or fragments containing CO groups occurs from Pt( 111) at relatively high temperature (above 150 K) and not at 90 K. The second explanation for the absence of these bands is that the v(C=O) and v(C=C) dipoles are parallel to the surface. According to RAIRS selection rulesi2 only active dipole moments possessing components perpendicular to the metallic substrate are observed. Thus, we propose that the first molecules of fodH adsorb with their COCHCO skeleton parallel to the Pt( 111) surface. The adoption of such a configuration was proposed by ParmeterI3 in the case of the adsorption of the hexafluoroacetylacetone (hfacH) on Pt( 111). However, the reactivity of platinum did not allow the thermal desorption of these species from the Pt(1 11) surface. More evidence for the presence of this adsorption state was obtained from the study of the fodWAg( 111) ~ y s t e m . ~ The thermal stability of this state on the Ag(ll1) substrate allows the desorption of this species (fodH molecules adsorbed with the COCHCO skeleton parallel to the substrate) at 250 K. In addition, the present study revealed that fodH molecules may exist in the adsorbed phase in the enol form rather than in the keto form of the keto-enol tautomer equilibrium. The absence of a doublet at 1720 cm-' (v(C=O)), which characterizes the keto form of P-diketone molecule^,'^ supports this proposition. At high exposure the broad feature near 1600 cm-' is due to the randomly oriented dipole moments of the fodH molecules in the enol form in the condensed phase. Thermally induced surface reactions have been followed by the RAIRS technique, and typical RAIR spectra are illustrated in Figure 2. These spectra were taken after heating the sample to the indicated temperature and allowing it to cool to 90 K. Dramatic changes of RAIR spectra occurred upon heating the Pt( 111) sample from 90 to 380 K. The decrease in intensity of vibrational bands over the whole spectrum upon substrate
Results and Discussion (A) Surface Chemistry and Adsorbed States of fodH on R(ll1)at 90 K. RAIRS Results of the Interaction of fodH with Clean Pt(ll1). RAIR spectra of the adsorption of fodH on clean Pt(ll1) at 90 K as a function of the exposure are shown in Figure 1. For low as well as for high exposure these spectra are dominated by the C-F vibrational band. The band position of v(C-F) appears at 1234 cm-' for lower fodH doses and shifts to 1246 cm-' with increasing exposure. The observed shift of the C-F, C-C, and C-H peak positions to higher frequencies is probably due to coupling between dipole moments and is not probably a consequence of a change in chemical environment. The most important reason for this conclusion is that this shift takes place only in the condensed phase, which is less perturbed by the substrate. Similar behavior was reported in the case of the adsorption of perfluorocarbons on Pt( 11l).I This
9232 J. Phys. Chem., Vol. 99, No. 22, 1995 5000 1
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Figure 3. TPD spectra obtained after exposing a clean Pt( 111) surface to 0.5 L of fodH at 90 K. Traces for masses 296 (fodH) (xlo), 69 (CF3) ( x l ) , 57 (C(CH&) (x0.25),and 31 (CHIOH) ( ~ 0 . 2 5 ) .
Figure 4. TPD spectra obtained after exposing a clean Pt(11 1) surface to 0.5 L of fodH at 90 K. Traces for masses 41, 29, 28, 59, and 45
heating to 190 K indicates that some species resulting from fodH dissociation are desorbing from the surface (the nature of these species is given in the TPD section). At 190 K the spectrum is still dominated by the C-F stretching mode, which is shifted back to 1234 cm-I. The shape of the RAIR spectrum at this temperature indicates that some of the fodH remains intact on the surface. However, at this temperature fodH molecules may coexist on the surface with other fodH decomposition byproducts. Between 250 and 380 K the RAIR spectrum undergoes important changes and it is no longer similar to that obtained with intact fodH. The broad peak extending from 1150 to 1400 cm-' and the appearance of a new vibrational peak at 1755 cm-' are an indication that a variety of fragments are present at 380 K. The latter band might be due to the formation of a new adsorbed species containing a C=O group such as a ketone or aldehyde. It should be pointed out here that this species is different from that generated following the dissociation of hfacH and the hfac-copper complex on Cu( 110) and Cu( 11l).3 Girolami et ale3reported in their studies of [Cu(hfac)(vtms)] (vtms = vinyltrimethylsilane) and [Cu(hfac)z] the formation of ketenylidene (MZC-C=O) intermediates on Cu( 110) and Cu(1 ll). These ketenylidene species were characterized by the presence of a vibrational band v(C-0) appearing in the range 2038-2095 cm-' depending on the nature of the substrate and had a decomposition temperature above 400 K. It was expected that the interaction of an unsymmetrical molecule like fodH with metal surfaces would lead to the formation of fragments which differ in nature from those generated by symmetrical molecules like acacH and hfacH. The combined effects of the two substituents C3F7 (electron withdrawing) and C(CH3)3 (electron releasing) might contribute to the weakening of the C-C bond of the COCHCO skeleton. Furthermore, the presence of such substituents might stabilize the moieties formed and lead to their desorption rather than dissociation, as observed in the case of the hfacH and acacH ligands. TDS Results of the Adsorbed States of fodH on Pt(ll1). Typical thermal desorption spectra of the species resulting from the interaction of fodH (0.5 L) with Pt( 111) are illustrated in Figures 3 and 4. Since the thermal desorption spectra of some fragments do not trace the parent ion spectrum (mass 296 amu), this indicates that some of the adsorbed fodH undergoes decomposition. The molecular desorption of fodH from the Pt(111) substrate takes place at 273 K. The corresponding activation energy of desorptionI5 of this state is 80 Idmo1-I assuming that the pre-exponential factor is l O I 3 s-', The molecular desorption state is confirmed by the detection of ions
resulting from fragmentation of the parent in the mass spectrometer. The relative abundance of selected ions resulting from fodH fragmentation in our mass spectrometer operating at 70 eV is (57 amu):CF3+ (69 amu):CF+ (31 amu):fodH+ (296 amu):C3F,+ (169 amu) = 100:6.1:3.0:0.14:1.5:0.3%. TPD measurementsI6 showed that desorption of byproducts occurs at 187 and 215 K. These temperatures coincide with the temperature at which decreases in RAIRS intensities are observed (Figure 2). The main desorbing fragments detected at 187 K are masses 29 (C=OH+), 31 (CF+ or CH2=OH+), 41 (C-C=OH+), 45 (H(C=OH)CH3+), 57 ((CH3)3C+), 59 (CH3(C=OH)CH3+), and 74 (CH3CH2CHOHCH3+) with the relative intensities (integrated peak areas) of 45.4,100, 21.7, 30.2, 5.4, 22.4, and 14.4%, respectively. Closer inspection of these fragments suggests that the desorbing species are ketones [(CH3)3CC(O)-CH,] rather than alcohols or aldehydes. Carbon-carbon bond rupture of the RIC(OH)=CHCOR~(RI = C3F7 and R2 = C4H9) skeleton led inevitably to the formation of RIC(OH)ads and R2COCHads or to RIC(OH)=CHads and Rztoads moieties. The formation of RzCOCH3 in the adsorbed phase may then proceed from R2COCHads fragment hydrogenation or from the recombination of R2COadsand CH3 fragments. The latter moieties may result from partial decomposition of adsorbed species and particularly of tert-butyl groups. In the homogeneous phase, the addition of base (OH-) to RC(O)CH*COR' leads to the formation of RCOCH3 and R'COO-. The desorption of ketone molecules from this substrate is favored over the desorption of alcohol molecules, since the adsorption of the latter yields dehydrogenation of a fraction of alcohol molecules to form ketones as the main desorbing byproducts whereas the remainder of the alcohol molecules decompose between 150 and 200 K.I8 It is well-known that the Pt(ll1) substrate leads to C-C bond scission, hydrogenation, and dehydrogenation of hydrocarbon molecules; however, C-C bond formation has never been reported to occur on this substrate. Therefore the most probable route to generate [(CH3)3CC(O)CH3] is the hydrogenation of the (CH3)3CCOCH,d, fragment. However, the recombinative reaction between CH3 groups and the possibly generated fragments R2-COads is not a negligible route for ketone formation/ desorption, since the presence of large fragments on the surface may inhibit the decomposition and dehydrogenation reactions and allow for C-C bond formation. It was reported that the coadsorption of CO stabilizes hydrofragments and leads to their recombination or hydr~genation.'~ On the other hand, molecular desorption of fodH at 273 K is due to the presence of species
amu.
J. Phys. Chem., Vol. 99, No. 22, 1995 9233
Silver Film Deposition on Pt( 111) 0.020
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Figure 5. Evolution of the RAIR spectra of adsorbed (fod)AgPEt3 (2.0 L) on clean Pt( 11 1) with annealing temperature over the range 90-233 K.
resulting from fodH dissociation. These fragments act as site blockers; a minimum number of free sites is required to allow the decomposition reaction to proceed. As mentioned earlier, the weakening of the C-C bond of the COCHCO skeleton is more probably due both to the difference in the electronegativity of the two subtituents (C3F7 and C4H9) and to the inductive effects of the substrate. The Lewis acidity of the Pt( 111) surface allows electron transfer from the adsorbate to the substrate. Ketone molecules bond weakly to the Pt( 111) surface via oxygen lone-pair electrons (monohapto, vi, configuration).20 The adoption of such a configuration leads to their desorption. The major desorbing byproducts at 215 K are composed of fluorocarbons, which of course constitute the remainder of the fragments resulting from fodH dissociation. The most intense fragment leaving the surface at this temperature is mass 69 amu (CF3), which is accompanied by less intense peaks of masses 50 (CF2) and 31 (CF). The integrated peak areas of masses 69, 50, and 31 are 100, 58.3, and 70.4%, respectively. These results indicate that the remaining moieties on the surface after desorption of byproducts at 187 K undergo further dissociation, which leads to the desorption of fluorocarbon species at 215 K. Comparison of the relative intensities of these species with published datal7 does not allow their assignment to any molecules like C3F7CHO or C3F8 or the C3F7’ radical. Since no other positively charged species were detected at this temperature (negatively charged particles could not be detected with our quadrupole mass spectrometer (QMS)), we tentatively attribute the detection of these fragments to the desorption of the readily formed CF3 radical. The adsorption of perfluorocarbons (hexane and heptane),” perfluoroethers, and perfluoroalcoholsls on Pt( 111) revealed that these molecules interact weakly with this substrate. However, the adsorption of CF31 on Ag( 111),21Pt( 111),**and R U ( O O ~leads ) ~ ~ to the desorption of CF3 radicals at 310, 623, and 705 K, respectively, with relative intensities CF2 > CF > CF3. It is conceivable that the desorption of CF3 radicals resulting from metal-CF3 bond dissociation and that resulting from C-CF3 bond rupture (obtained in our laboratory) have different electronic states and consequently give different cracking patterns. The desorption of CF3 radicals bonded to metal surfaces might not be in the ground-states. The RAIR spectrum obtained at 380 K shows that some C,F, fragments remain on the Pt( 111) surface (Figure 2) and supports C-CF3 bond rupture. Figure 4 shows that the thermal desorption of mass 28 a m u (CO’) occurs in a broad peak extending from 400 to 600 K. This spectrum is noisy because the backgound of this mass is
2500
2000 1500 Wavenumbers (cm-l)
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Figure 6. Thermal modification of RAIR spectra of adsorbed (fod)AgPEt3 (2.0 L) on clean Pt( 11 1) between 256 and 500 K.
TABLE .I: Observed Vibrational Frequencies and Band Assignments for (fod)AgPEtP ~
band assignment
(fod)AgPEt3 in KBr
(fod)AgPEt3 on Pt(ll1) at 90 K
va(CH3) va(CHd VS(CH3) vSCHZ) v(C=O) v(C=C) d(CH3) v(CC) d(CH2) v(CCF3) W33) va(CF2) va(CF3) vACF3) v(C-C) d(CH) e(CH3) e(CH2)
2957 s 2930 m 2901 m 2879 m 1631 vs 1580 vw 1535 m, 1493 vs nr 1395 vw, 1345 s 1345 s, 1316 vw 1267 m 1221 vs 1177 m 1155 m, 11 15 m 1046 w, 962 w 941 w
2971 w 2945 w 2921 w 2883 w 1636 vs 1594 vw 1535 vw, 1487 w 1462 w, 1439 vw 1395 vw, 1354 m 1354 m, 1316 w 1285 m 1243 vs 1203 m, 1192 m 1165m, 1131 m 1069 w, 969 w 934 w
+
+
All frequencies are given in cm-I. The assignments of these bands are based on reported data of species containing such groups.Z8 nr, not resolved; v, very; s, strong; m, medium; w, weak. (I
too high. The shape of this desorption peak is characteristic of a gradual decomposition process. This desorption is due to further decomposition of the species containing the C=O group responsible for the feature at 1755 cm-’ in the RAIR spectra. A simple scheme is proposed to summarize the reaction pathway of the interaction of fodH with the Pt( 111) substrate: C3F7C(OH) -CH -C O-C4H9(ads) CH,COC,H,(,) 4-C3F7COCHads4- C$Iv(ads)T = 187 K
-
CF3(,,
T = 215 K
(CHOF),(ad,,
C3F7C(OH)-CH-CO-C4H,(,,
-cog -
T = 273 K
T = 400-600 K
( c ; 0; F)x(ads)
T
’600 K
(B) Surface Reactivity of the [(fod)AgPEt3] Precursor on Pt(ll1). RAIRS Results of the Adsorption of (fod)AgPEtj on Clean P t ( l l 1 ) . The thermal evolution of the RAIR spectra of adsorbed [(fod)AgPEt3] (2.0 L) on clean Pt( 111) are displayed in Figures 5 and 6. The assignment of the vibrational modes of [(fod)AgPEts] molecules in a KJ3r pellet obtained by transmission and adsorbed on Pt(ll1) at 90 K obtained by reflection absorption IR is given in Table 1. As might be expected, the first [(fod)AgPEt3] molecules (low coverage)
9234 J. Phys. Chem., Vol. 99, No. 22, 1995
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Figure 7. TPD spectra of masses 107 (Io7Ag),109 (Io9Ag),520 (parent ion), and 118 (PEt3) ( x 5 ) following the adsorption of 0.4 L of (fod)AgPEt3 on Pt(ll1) at 90 K. Inset: TPD spectra of masses 107, 109, and 520 amu obtained following the adsorption of the precursor (0.3 L) at 330 K on Pt(ll1). adsorb with their molecular plane COCHCO parallel to the Pt(111) surface, as for the fodH/Ag( 11l), fodwpt( 11l), and [(fod)AgPEt3]/Ag( 111) systems. For low exposure as well as for high exposure the RAIR spectra are dominated by the Y(C-F) stretching mode at 1246 cm-I. However, upon heating of the Pt(ll1) crystal to 189 K, marked changes occur in the RAIR spectra. Between 180 and 200 K the intensity of the vibrational band at 1165 cm-' increases rapidly to reach its maximum around 190 K. The vibrational modes associated with this peak are the C-C stretching mode and the d(CH3) deformation mode. This modification is an indication that a part of adsorbed [(fod)AgPEt,] undergoes dissociation and the resulting fragments adopt a new orientation with respect to the surface normal.I2 The dissociation of [(fod)AgPEts]is due to the strong interaction between the substrate and the molecules laying parallel to the surface, which constitute the first adsorbed state (first adsorbed molecules). The decrease in the intensity of this feature (peak at 1165 cm-I) suggests that byproducts are desorbing between 195 and 240 K. The RAIR spectrum taken at 240 K indicates that the intact precursor may exist on the surface at this temperature. However, after the sample is heated above 280 K (Figure 6 ) , significant decomposition of the adsorbed intermediates occurs, as reflected by the broadening of the IR bands. These results are a direct indication that [(fod)AgPEt3] was dissociated below room temperature. It is important to underline that the interaction of [(fod)AgPEt3] and fodH with Pt( 111) does not lead to the generation of MEC-CEO species, as in the case of the interaction of hfacH and its copper complexes. TPD Results of the Adsorption of uod)AgPEt3 on Clean Pt(I1I ) . The adsorption of [(fod)AgPEt3] on Pt( 111) has been carried out at different substrate temperatures (90,220, and 330 K). Selected thermal desorption spectra following the adsorption of [(fod)AgPEt3] at 90 K are shown in Figures 7 and 8. These data show two interesting results: the molecular desorption of the precursor and the thermal desorption of deposited Ag atoms (Figure 7). Molecular desorption of the precursor at 340 K is confirmed by the detection of the parent ion, mass 520 amu, and ions resulting from the fragmentation of the parent ions in the mass spectrometer such as 118 (PEt3), 107 ('O'Ag), 109 ('@Ag), and 57 (C(CH3)3). In order to determine if silver atoms are desorbing from the surface, masses 107 and 109 were monitored during TPD cycles. The natural abundance of silver isotopes is 51.83 and 48.17% for masses 107 and 109,
Oe+O 200
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Temperature (K)
Figure 8. TPD spectra of masses 57 (C(CH3)3) (xO.l), 28 (CO),and 20 (HF) after dosing Pt( 111) at 90 K with 0.4 L of (fod)AgPEt3. re~pectively.~~ The molecular desorption of the precursor implies that the precursor can reach the substrate surface intact. This result is important, since autocatalytic decomposition (or cluster formation) may occur in the gas phase and therefore compete with reactions taking place in the adsorbed phase in selective metalization processes. The thermal desorption peak of this state exhibits a zero-order kinetic reaction with an activation energy of 99 kJmol-' assuming that the preexponential factor is l O I 3 s-'. The relative abundance (integrated TPD peak areas) of the most important ions resulting from the precursor fragmentation is 57 (loo%), 69 (0.7%),107 (OS%), 109 (0.4%), 520 (0.1%) and 169 (0.1%). Figure 7 shows that deposited silver atoms desorb in a large peak centered around 1035 K (which yields an approximate activation energy of desorption of 310 kJmol-I). The adsorption of [(fod)AgPEt3] on Pt( 111) at 330 K, which is above the temperature of the decomposition of the precursor, led to the deposition of more than the one monolayer, as shown in the inset of Figure 7. The temperature of desorption of the Ag multilayer is 940 K, which corresponds to an activation energy of desorption of 282 Hmol-'. The feature centered at 1100 K is due to a spurious effect of the manipulator. This feature corresponds to precursor sublimation from the manipulator parts (feedthroughs) during sample heating, since they are colder than the sample. These results indicate that more than a monolayer (ML) can be deposited near room temperature. A study of the Ag/Pt( 111) system25showed that, for a coverage below 1 ML, Ag desorbs in a single state at 1050 K whereas, for coverages above 1 ML, a second multilayer peak appears between 900 and 1000 K. The observed shift of the thermal desorption peak maximum to lower temperature is due to the heating rate, which is 2 K-s-' in our experiment compared to 30 and 8 K-s-' for the reported data.25 The TPD spectrum of mass 118 amu, PEt3, shows only one feature, which corresponds to the precursor fragmentation in the mass spectrometer. The absence of other desorption peaks implies that PEt3 molecules have been dissociated following Ag-PEt3 bond rupture at low temperature on Pt( 111). On the Ag( 111) surface two desorption peaks of the PEt3 molecule were observed in the temperature range 135-170 K following [(fod)AgPEt31 di~sociation.~ The study of the adsorption of P(CH3)3 on Pt( 111) showed that P-C bonds dissociate (demethylation) at low temperature with subsequent decomposition of adsorbed methyl groups.26 This may explain why we did not observe any other desorption states of PEt3 from Pt( 111). In order to obtain more information on the process of decomposition of the intermediates formed on the surface, a
Silver Film Deposition on Pt( 111)
J. Phys. Chem., Vol. 99, No. 22, 1995 9235
number of ions were monitored during TPD measurements. The TPD spectrum of mass 57 amu, which is the most abundant ion of the [(fod)AgPEt3]molecule, shows three desorption peaks at 236,340, and 948 K (Figure 8). The thermal desorption peak at 340 K corresponds to the molecular desorption state of the precursor, whereas the two other features are due to desorption of byproducts following precursor dissociation. The distribution of the relative intensities of the fragments detected at 236 K indicates that fodH is the desorbing species at this temperatureaZ7 The formation and desorption of this species are consistent with the changes in RAIR spectra between 180 and 240 K. The generation of fodH suggests the coadsoqkion of fod,d, and Hads. The presence of the latter is mainly due to the decomposition of a part of fod or PEt3. As shown in Figure 8, multiple desorption peaks of masses 20 (HF) and 28 (CO) at 236, 340, 616, and 948 K support the decomposition of intermediates. In addition, the desorption of masses 119 (C2F5), 69 (CF3), 58 (HC4H9), 57 (C4H9), and 28 (CO) around 948 K indicates that large fragments remain on the surface at high temperature. The shape of these peaks is characteristic of the pyrolysis of intermediates. RAIRS data (Figure 6) obtained at high temperature (above 350 K) show the presence of large vibrational bands, supporting the coexistence of a variety of species on the surface. Under these conditions (UHV conditions) the formation of clean and pure silver films is unlikely because Ag(1) complexes do not easily undergo a disproportionation reaction like copper. 2Cu(I)
-
Cu(0)
+ Cu(I1)
Clean silver films however can be obtained by coadsorbing hydrogen, H20, or methanol in order to reduce the fod ligand. In fact, under CVD conditions in the presence of hydrogen and water, clean silver films were ~ b t a i n e d . ~
Conclusions In this study we have demonstrated that fodH is a suitable molecule for preparing stable precursors. The molecular desorption of [(fod)AgPEt3] from Pt( 111) at 340 K indicates that this precursor reaches the substrate intact and the dissociation reaction takes place in the adsorbed phase. More importantly, this study showed that the decomposition reaction of this precursor [(fod)AgPEt3]occurs below room temperature, leading to the release of silver atoms which desorb at 1035 and 940 K from the monolayer and the multilayer, respectively. The study of the surface chemistry of fodH on the other hand has revealed that the thermally induced dissociation on a Pt(111) substrate generates ketone species which desorb at 185 K which we attribute to the CH3COC4H9 molecule. This study also showed that, above room temperature, large and stable fragments remain on the surface in comparison to the fragments generated following the interaction of hfacH with this substrate. This means that, in the presence of reducing agents, volatile species can easily be formed and clean Ag films can be obtained.
Acknowledgment. We thank the Natural Sciences and Engineering Council of Canada and the Ontario Centre for Materials Research for financial support.
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