J . Phys. Chem. 1993,97, 11530-1 1541
11530
Copper CVD Chemistry on a Reactive Substrate: Cu(hfac)z and hfacH on P t ( l l 1 ) John E. Parmeter Department 11 26, Sandia National Laboratories, Albuquerque, New Mexico 871 85 Received: July 19, 1993’
The chemistry of the copper CVD precursor Cu(hfac)z and of the related hfacH molecule on the Pt( 1 1 1) surface has been investigated using vibrational (FTIR) spectroscopy, thermal desorption measurements, and Auger spectroscopy. At high coverages, Cu(hfac)z adsorption leads to the formation of a “standing-up” hfac [OC(CF3)CHC(CF,)O], adsorbed with its OCCCO skeleton essentially perpendicular to the surface. In the case of hfacH adsorption, and also for lower coverages of Cu(hfac)z, a different surface species is formed that is tentatively identified as “lying-down” hfac, i.e., hfac adsorbed with the O C C C O plane essentially parallel to the surface. The hfac species begin to decompose below 300 K. In the case of a saturated layer of hfacH, a large mass 69 (CF3+) thermal desorption signal is observed centered near 295 K, accompanied by the formation on the surface of both carbon monoxide and fluorine containing fragments. The carbon monoxide desorbs near 430 K, with nearly simultaneous HF desorption. Above this temperature, only two fluorocarbon species are present on the surface in significant concentration. The first (unidentified) fragment is characterized by an intense C F stretch near 1250 cm-l. Between 450 and 750 K it converts to adsorbed CFz, which desorbs or decomposes below 850 K. Decomposition of the “standing-up” hfac formed from Cu(hfac)z is similar in most respects. These results are compared to those obtained on copper surfaces, and the implications for the CVD of copper onto more reactive substrates are discussed.
I. Introduction
The chemical vapor deposition (CVD) of copper has been an area of much research interest recently. This interest results from the potential usefulness of copper in a variety of microelectronics applications, e.g., as an interconnect material in integrated circuitry. The desirable physical properties of copper for such applications include its very low resistivity (significantly lower than aluminum and much lower than tungsten)l and electromigrationresistance that is far superior to that of aluminum (though inferior to tungsten).2 These properties suggest that copper might be a superior alternative to either tungsten or aluminum in certain instances, provided that satisfactory copper CVD processes can be developed. Two of the most useful families of copper CVD precursors that have been used recently are the Cu(1) and Cu(I1) P-diketonates. The Cu(I1) precursors require the use of an external reducing agent such as hydrogen to deposit copper films that are largely free of impurities, while the Cu(1) precursors can deposit pure copper films without the use of an external reducing agent via a bimolecular disproportionation reaction that produces a Cu(11) P-diketonate as a volatile byproduct. The 6-diketonateligand most often present in these precursors is hexafluoroacetylacetonate or hfac [OC(CF3)CHC(CF,)O]. Studies of the surface chemistry of these copper precursors and of the associated ligands are of interest for several reasons. Fundamental studies can serve to isolate surface intermediates that may be involved in the overall deposition mechanism, thus adding to our understanding of how the overall process takes place. Experiments performed on different substrates can be helpful in understanding the differences in reactivity that these precursors exhibit on different surfaces. In situ analysis of the deposited films coupled with studies of ligand decomposition can demonstrate plausible mechanisms by which such ligand decomposition leads to impurity incorporation into the growing copper films and thus suggest means of minimizing such reactions. In drawing conclusions it must of course be remembered that the conditions of temperature and pressure prevailing in ultrahigh vacuum (UHV) surface science experiments are quite different e
Abstract published in Advance ACS Abstracts, October 1, 1993.
from those under which CVD is usually performed. However, if care is taken in interpreting the results, these experiments can provide information that is useful in seeking to optimize the CVD process as well as being of fundamental chemical interest. There have already been several significant studies of the surface chemistry of these copper precursors. The most extensive work has been performed by Dubois and co-workers,” who have studied the chemistry of hfacH, (hfac)Cu(VTMS) [VTMS = vinyltrimethylsilane, CH$HSi(CH3)3], and Cu(hfac)z on‘single crystalline copper surfaces. They utilized thermal desorption measurementsto determine the ultimate gas-phase decomposition products resulting from the adsorption of these species on copper, and infrared spectroscopy to identify adsorbed intermediates formed from each of the above three compounds on Cu(lOO).’94 More recently, Dubois has used infrared spectroscopy to study the adsorption of copper precursors on silica.5 Cohen et al.6~7 used electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) to study the interactions of both Cu(hfac)2 and (hfac)Cu(COD) [COD = 1,5-cyclooctadiene] with a silver film deposited on silicon. Hardcastle et a1.8 have used infrared spectroscopy to compare the chemistry of (hfac)Cu(P(CH3)s) and (hfac)Cu(COD) on silica, andadditional recent work has studied the chemistry of copper CVD precursors on TiN.9 In some microelectronics applications, it is desirable to deposit copper onto a more reactive metal which serves as a barrier material.IO This is necessary because the high diffusion coefficient of copper into silicon and Si02 makes direct deposition of copper onto these materials impractical. Tungsten and titanium are the barrier metals most commonly used in such applications, though platinum has also been used as a substrate for copper CVD in more fundamental experiments.l1 The chemistry of copper CVD precursors on surfaces of these more reactive metals is of importance in understanding the interfacial chemistry that occurs when CVD copper is deposited onto these substrates, especially in terms of impurity incorporation in the interfacial region. Platinum was chosen as a substrate to model copper CVD precursor chemistry on these more reactive surfaces. Experiments performed on tungsten or titanium in UHV would be somewhat problematic, since these surfaces will undoubtedly be far more
0022-365419312097-11530%04.00/0 0 1993 American Chemical Society
Cu(hfac)z and hfacH on Pt( 1 11) reactive under (clean) UHV conditions than under CVD conditions. In terms of overall surface reactivity, platinum probably represents a reasonable compromise. The molecules studied in this work are Cu(hfac)z and hfacH. The results obtained for each molecule are presented separately in section 111,while section IV discusses some significant aspects of the surface chemistry of both. In previous CVD studies, Cu( h f a ~ has ) ~ been by far the most widely studied of the Cu(I1) 8-diketonate precursors.12-20 It has a planar structure (apart from the CF3 groups), with two hfac ligands placed symmetrically about the copper atom. The two oxygen atoms of the hfac ligands are bonded to the copper atom, so that the compound consists of two six-membered rings having the copper atom as a common vertex. The hfacH molecule, OC(CF3)CH2C(CF3)0, is the simplest molecule containing the hfac ligand. It is noncyclic, containing two carbon-xygen double bonds and having a second hydrogen atom bonded to the central carbon. Comparing it to Cu(hfac)z allows some information to be obtained concerning the influence of copper atoms on the surface chemistry of the hfac ligand. The chemistry of both of these molecules as well as the chemistry of other copper precursors containing the hfac ligand is clearly intimately related. Thus the results presented in this paper are of relevance to understanding the surface chemistry of both Cu(1) and Cu(I1) 8-diketonates. Some experiments were alsoattempted with (hfac)Cu(VTMS),oneofthemorepromising Cu(1) precur~ors,21-~3 but the resultsobtained wereofquestionable valuedue to dissociation of a substantial fraction of thecompound before it reached the Pt(ll1) surface. We note that some preliminary results of the research reported here have been published elsewhere in a much shorter paper.24
11. Experimental Section The experiments discussed in this paper were performed in a newly constructed UHV chamber. The main chamber is octagonal in design with 8-in. square faces supporting eight inch flanges (in one case, a 6-in. flange). It is pumped by a 450 C/s Balzers turbomolecular pump and a Varian titanium sublimation pump. The base pressure is approximately 3 X 10-11 Torr. Various flanges of the octagon support a UTI lOOC quadrupole mass spectrometer for performing thermal desorption measurements and residual gas analysis, a PHI Auger spectrometer, and a PRI reverse view LEED optics. A Thermionics manipulator is mounted on center on the top of the octagon. A lower level of the UHV chamber contains an ion gauge and a Convectron gauge for pressure measurement, an LK Technologies NGI3000 ion gun for sputter cleaning of samples, a leakvalve for backfilling the UHV chamber, and a home-made doser for beam dosing of the samples used in those cases where backfilling the chamber is inappropriate. A small cell at the bottom of the chamber consisting of a modified 6-way cross allows samples to be positioned for Fourier transform infrared spectroscopy (FTIR) using a Mattson Instruments research series RS-10000 spectrometer. The thermal desorption measurements described herein were obtained by heating the Pt( 11 1) crystal resistively and recording the mass spectrometer signal for a desired mass on a chart recorder. Heating rates are given in the appropriate figure captions and were in the range of 3-10 K s-1. The heating rates were quite linear from about 150 to 700 K but above 700 K decreased considerably and were no longer linear. Thus peak desorption temperatures above 700 K should only be considered approximate. Care was taken to minimize the time that the surface was exposed to the electron flux from the mass spectrometer, in order to assure that electron induced decomposition of surface intermediates did not produce misleading results. In particular, when performing monolayer thermal desorption the multilayers were annealed away before exposing the Pt( 111) surface to the electron flux from the mass spectrometer. The FTIR measurements were performed in single reflection mode using an angle of incidence of 10 degrees from grazing. The
The Journal of Physical Chemistry, Vol. 97, No. 44, I993 11531 instrumental resolution was set a t 4 cm-1, and 1000 scans were signal averaged. It took approximately 2.5 min to collect a spectrum under these conditions, so that the scan rate was about 7 scans per s. The IR beam entered and exited the UHV chamber via KBr windows mounted on 2.75“conflat flanges, obtained from Solon Technologies. A narrow band MCT detector was used. In practice, this allowed vibrational features with frequencies as low as about 900 cm-1 to be observed in most monolayer FTIR spectra. The spectra presented were referenced against background spectra of the clean Pt( 111) surface so that only vibrational features due to adsorbed species are seen. Both background and sample spectra were obtained after cooling to approximately 100 K. After collection of the spectra, Mattson software baseline correction was used in some cases to remove a sloping background that was present in most spectra. This did not effect peak shapes, intensities, or positions. Theoneexception is that in some instances weak, inverted carbon monoxide stretching features that were occasionally observed between 2050 and 2 100cm-1 were removed. These inverted features resulted when there was more carbon monoxide (at a particular vibrational frequency) adsorbed from the chamber background on the “clean” surface used to obtain the background spectrum than on the surface with the adsorbate present. It should also be noted that due to the design of the UHV chamber, it was not possible to dose the surface following collection of a background spectrum without moving the sample a substantial distance. Because of this, the usual sequence of background and sample spectrum collection was often reversed: first a spectrum wasobtainedof the surface with adsorbate present, then thesamplewasflashedtoca. 1000Ktoremovetheadsorbate, and finally a spectrum of the resulting clean surface was collected. This procedure produced spectra of high quality, and better signalto-noise was achieved than if the crystal was moved between collection of the background and sample spectra. It did not produce any spurious features in the FTIR spectra because after flashing to 1000 K only residual carbon remained on the surface, and this carbon produced no observable vibrational features in the spectral region studied. Dosing of the various compounds onto the platinum crystal was performed by positioning the crystal a few millimeters in front of a home-made doser. In the early stages of this work, a 1/8” stainless steel tube about 10” long was used for the dosing. This was later replaced with a glass tube, in hopes of reducing the extent to which (hfac)Cu(VTMS) decomposed before reaching the sample. However, this alteration had no apparent effect on the experimental results. Calibration experiments with CO showed for both dosing arrangements that the flux at the crystal was enhanced by a factor of approximately seven compared to what one would estimate from the pressure measured at the ion gauge. The exposures given in the remainder of this paper take this enhancement factor into account. Exposures are reported in units of Langmuirs (1 Langmuir = 1 L = 10-6 Torr-s) and are uncorrected for the relative ion gauge sensitivities of the different gases. The platinum crystal was obtained from Atomergic Corp. and was cleaned by methods outlined in the literature.25 Briefly, this involved sputtering of the crystal with 2 kV Ar+ ions at a few microamps current for 15-20 min, annealing in 5 X 10-7 Torr oxygen at 750 K for about 20 min, and finally annealing to about 1050 K in vacuum. The sputtering was not always necessary following hfacH exposures. However, it was always done following exposures of Cu(hfac)z because above approximately 500 K diffusion of copper atoms into the platinum crystal can occur.25 After the above cleaning procedures were performed, no impurities could be detected using Auger spectroscopy other than occasional trace amounts of carbon, and the surface gave a hexagonal LEED pattern characteristic of a close-packed surface. Occasional thermal desorption experiments with carbon
11532 The Journal of Physical Chemistry, Vol. 97,No. 44, 1993
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Parmeter
hfacfl Multilayers /Pt(lI I ) Thermal Desorption
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200 Temperature (K)
300
FTIR
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9
80 L hfacH
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