Si(100) Surfaces

Sep 26, 2011 - after deposition of thin manganese films on SiO2/Si(100) surfaces at different temperatures. In all traces, the main peaks are centered...
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Chemical Nature of the Thin Films that Form on SiO2/Si(100) Surfaces Upon Manganese Deposition Huaxing Sun, Xiangdong Qin, and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: There has been much interest in recent years in the use of manganese for the self-formation of diffusion barriers to prevent the electromigration of copper interconnects into the underlying silicon substrate during microelectronic fabrication. However, the films resulting from the deposition of manganese on silicon wafers, which are covered by a thin native silicon dioxide layer, are complex and have not been fully characterized to date. Here we report on X-ray photoelectron spectroscopy (XPS) studies on the nature of films prepared by chemical means using methylcyclopentadienylmanganese tricarbonyl as the precursor. It was found that at the temperatures typically used for deposition, between approximately 550 and 750 K, a manganese silicate layer grows first upon reaction with the top SiO2 surface, and a thin manganese silicide film develops latter at the SiO2/Si(100) interface. The performance of these structures, of the silicide in particular, in microelectronic applications is still to be determined. SECTION: Surfaces, Interfaces, Catalysis

anganese films may be useful in catalysis,1 as components in lithium2 5 and supercapacitor-based6 batteries, in memory devices,7,8 as sensors,8 and in flat panel displays.9 In microelectronics in particular, manganese films have been identified as potential candidates for the self-formation of barriers to prevent the diffusion of copper interconnects into the underlying silicon substrate.10 14 However, the nature of the final films in that case appears to be complex, and it is not fully understood. Here we summarize the results from our X-ray photoelectron spectroscopy (XPS) characterization of films made by exposing Si(100) wafers covered with their native silicon dioxide layer to methylcyclopentadienylmanganese tricarbonyl (MeCpMn(CO)3), a useful precursor for the chemical deposition of manganese.7,15,16 The left panel of Figure 1 displays the Mn 2p XPS data obtained after deposition of thin manganese films on SiO2/Si(100) surfaces at different temperatures. In all traces, the main peaks are centered at 641.8 and 653.9 eV, binding energies characteristic of Mn 2p3/2 and Mn 2p1/2 photoelectrons from oxidized manganese, respectively.17,18 The exact nature of the manganese in this case is not straightforward to identify, given that many manganese oxides display similar binding energies,17,18 but the additional appearance of satellite peaks at ∼645.9 and 659.4 eV strongly suggest the presence of manganese in a +2 oxidation state.19 21 The spectra obtained at low temperatures, below ∼550 K, are typical of MnO. The features for Mn2+ in the data obtained after deposition at higher temperatures are displaced slightly toward lower binding energies and are accompanied by less intense satellites, and are therefore likely to be due to manganese silicate instead (see below). A new pair of peaks develops in the Mn XPS traces in Figure 1 upon deposition at temperatures of 600 K or above, at 638.2 and 649.3 eV for the Mn 2p3/2 and Mn 2p1/2, photoelectrons,

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respectively. Such low binding energies are typically associated with reduced species, possibly metallic manganese, but are also consistent with manganese silicide.22 On the basis of the appearance of additional broad and weak peaks at ∼660 and 670 eV in some of the spectra (in particular for manganese deposited on sputtered silicon, see Figure 5 below), which have been reported to originate from plasmon losses in Mn silicide,23 we here favor the assignment of the 638.2 and 649.3 eV features in these films mainly to manganese silicide, although a small component from Mn(0) may also be present in some instances (see below). As summarized in the right panel of Figure 1, the signal for Mn silicide first grows in intensity with increasing temperature, peaks at about 625 650 K, and decreases again at higher temperatures. The signal intensities for the Mn2+ species decrease monotonically all throughout the temperature range discussed here, from 500 to 775 K. To better map out the distribution of the different manganese species within the films deposited at the intermediate temperatures where manganese silicide is formed, additional angleresolved spectra were obtained for a film grown at 625 K. This temperature was chosen because it was the temperature at which the maximum intensity was obtained for the low-binding-energy peak associated with the Mn silicide. Figure 2 displays the data for the Mn 2p (left) and Si 2p (right) XPS peaks obtained from these studies, respectively, and Figure 3 provides a summary of the signal intensities of the main features in those data. It should be noted that the temperature range where the signal for the Mn silicide is maximized is fairly sharp and somewhat difficult to Received: August 26, 2011 Accepted: September 14, 2011 Published: September 26, 2011 2525

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Figure 1. Left: Mn 2p X-ray photoelectron spectra (XPS) obtained after dosing methylcyclopentadienylmanganese tricarbonyl (MeCpMn (CO)3, 0.15 mTorr for 2000 s) on a Si(100) sample covered with its native thin (∼1 nm) SiO2 layer as a function of temperature. Three pairs of features are observed, assigned to manganese silicide (638.2 and 649.3 eV), MnO or manganese silicate (641.8 and 653.9 eV), and satellites of the oxidized manganese (645.9 and 659.4 eV). Right: Summary of peak intensities for the Mn species identified in the data on the right, converted to film thicknesses by using the procedure mentioned in the Experimental Methods section.

Figure 2. Mn 2p (left) and Si 2p (right) XPS as a function of detection (takeoff) angle, measured from the surface normal, from samples obtained by dosing 8  10 5 Torr of MeCpMn(CO)3 on SiO2/Si(100) at 625 K for 63 min. The results from deconvolution of the data are shown in the form of three pairs of Mn peaks corresponding to Mn silicide and Mn silicate (main and satellite features) for the Mn 2p XPS data, and of three features (each a combination of its 2p3/2 and 2p1/2 peaks) for Si(0), silicate, and SiO2 in the Si 2p XPS traces. A Si 2p XPS trace is also shown for the original SiO2/Si(100) substrate at the top of the right panel.

reproduce because of a kinetic effect by which Mn is first deposited on top and then diffuses into the film and forms the Mn silicate and Mn silicide (see discussion in connection with the data in Figure 5). The spectra for 625 K shown in Figure 2 do show a much larger contribution from the low-binding-energy peaks associated with the manganese silicide than those in

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Figure 3. Summary of XPS peak intensities obtained for the sample studied in Figure 2 as a function of detection angle (symbols with error bars). Data are provided for the each of the layers identified, including a topmost monolayer of adsorbed carbon-containing ligands (raw data not shown). A model of the resulting layered structure, proposed on the basis of the angular dependence displayed in this Figure, is depicted in the inset, and the angular dependence of each of the sets of data calculated using that model is provided as solid lines.

Figure 1 but were not added to the sequence in that Figure because the sample was prepared under slightly different conditions. This kinetic effect is less significant at the high- or lowtemperature ends of the range in Figure 1 and does not change the qualitative trends reported here. The deconvolution of the Mn 2p XPS data in Figure 2 (left) clearly shows pairs of peaks for each of the three components discussed above, namely, for Mn silicide and for the main and satellite features of the manganese silicate. The Si 2p XPS traces in Figure 2 (right) show two clear components at approximately 99.1 and 103.0 eV, corresponding to Si(0) and SiO2, respectively. In fact, each of those is composed of two (2p3/2 and 2p1/2) spinsplit peaks and is fitted accordingly with peak separations of 0.5 eV and fixed 2:1 area ratios. The goodness of the overall fits is illustrated by the results for the original SiO2/Si(100) surface, before any Mn deposition, shown in the top trace of Figure 2 (right); a SiO2 film thickness of ∼1 nm was estimated from those data. For the samples after Mn deposition, a third species becomes evident at ∼101.7 eV (for the Si 2p3/2 peak), which becomes more intense at glancing detection angles. This new binding energy matches that of metal silicates24 26 and is consistent with the discussion provided above in connection with the interpretation of the Mn 2p XPS data. The angular dependence of the intensities of the different Mn and Si XPS features, shown in Figure 3, clearly points to a layered structure of the final films. The intensities of some peaks, the one for carbon in particular, increase with increasing detection angle (measured from the surface normal), indicating that those atoms are placed close to the surface, whereas other peaks show the opposite trend, decreasing in intensity at glancing detection angles because they originate from species buried underneath other layers. The latter include the signal for Si(0) (the Si 2p peak at 99.1 eV), which is initially below the SiO2 native layer and 2526

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Figure 4. Left: Uptake Mn 2p XPS data from a SiO2/Si(100) surface dosed with 0.15 mTorr of MeCpMn(CO)3 at 675 K as a function of dosing time. Right: Summary of peak intensities for the Mn species identified in the data on the right, converted to film thicknesses by using the procedure mentioned in the Experimental Methods section. The features due to manganese silicide appear only after saturation of those for manganese silicate.

covered by all other films after manganese deposition, and also the signals from Mn silicide (Mn 2p3/2 peak at 638.2 eV), which follows a similar, albeit less pronounced, trend than Si(0), indicating that the Mn silicide film is formed right above the Si(0) substrate but below all other components. The signals associated with the manganese silicate (average of Si 2p3/2 peak at 101.7 eV and Mn 2p3/2 peak at 641.8 eV) and silicon dioxide (Si 2p peak at 103.0 eV) show an intermediate behavior, with a weak angular dependence (increasing somewhat with detection angle), indicating that those layers are present at intermediate depths in the overall manganese plus silica system. A schematic representation of the proposed order of the layers formed by deposition of Mn on SiO2/Si(100) at temperatures around 625 K is shown in the inset of Figure 3. All of these layers are relatively thin, corresponding to only a few monolayers. Calculations using reported photoelectron mean free paths27,28 and exponential attenuation of the signals29 yielded the following approximate thicknesses for the different layers: C = 0.23 ( 0.05 nm, Mn silicate = 0.32 ( 0.05 nm, SiO2 = 0.78 ( 0.05 nm, and Mn silicide ∼0.50 ( 0.15 nm. The angular dependence estimated by using this layered model and a simple exponential attenuation equation for the photoelectron signal as a function of thickness, provided as solid lines in Figure 3, are within experimental error of the data for all layers except the top carbon deposits, providing further support for the validity of our model, at least at a semiquantitative level; the use of any other ordering of the layers could not reproduce the angular trends with detection angle seen experimentally. Large deviations from the predictive values are seen for the carbon signal, but that is easily explained by the fact that this topmost layer is in fact a molecular monolayer of adsorbed methylcyclopentadienyl ligands, not a homogeneous carbon film. It should also be mentioned that our model assumes sharp and flat interfaces between each pair of films. Although we have no direct evidence of the layering proposed in our model, several arguments can be made in its favor. First, the original SiO2/Si films are fairly flat according to AFM images (data not shown). In addition, similar silicon manganese mixed oxide

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Figure 5. Mn 2p XPS traces from silicon surfaces dosed with ∼2  106 L of MeCpMn(CO)3 at 625 K (left) and after annealing at 725 K (right). Data are shown for deposition on sputter-clean silicon (top) and on thin (∼1 nm thick, native oxide, middle) and thick (300 nm, bottom) SiO2 films.

films made in other laboratories do show good monolayer-resolved interfaces.13,30,31 Such behavior is in fact typical of self-made layers produced by diffusion of one element into a solid film. The rather good agreement of our angular dependent data to the layered model is also consistent with this view. Of course, this is only a simplified model; some degree of roughness and other defects are expected in the real samples at these interfaces. However, it has been shown that the effect of roughness on the estimation of film thicknesses is negligible at detection angles of ∼40°.32 34 An interesting aspect of this manganese silicon mixed oxide system is the fact that although the manganese silicide layer is buried in the interface in between the SiO2 and Si(100) components, it grows only after saturation of the more superficial manganese silicate layer. This is indicated by the uptake data shown in Figure 4. It is clear from that Figure that the first Mn 2p XPS peaks to grow are those associated with the silicate, with the main Mn 2p3/2 feature centered at 641.8 eV; only after that signal reaches saturation do the Mn silicide signals (at lower binding energies) appear. Presumably, these manganese-containing layers form via diffusion of the Mn atoms through the top layers of the substrate, first reacting with the SiO2 film to make the silicate and later traveling through that layer to react with Si(0) and produce the Mn silicide film. One thing to notice in Figure 4 is the fact that the total Mn uptake at 675 K asymptotically reaches a saturation value at high exposures, in this case at the equivalent of ∼0.25 nm of manganese silicate and another 0.25 nm of manganese silicide. This self-limiting growth is a behavior highly desirable in ALD processes and may produce a thin diffusion barrier film, even after one single deposition cycle. It should be indicated, though, that the manganese silicide may keep growing slowly if the exposure of the surface to the MeCpMn(CO)3 precursor is continued. The sequence of events discussed above is supported by the data in Figure 5, which correspond to the Mn 2p XPS traces 2527

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The Journal of Physical Chemistry Letters obtained from films produced via Mn deposition on silicon substrates with silicon dioxide films of different thicknesses. The left panel shows the spectra recorded after deposition at 625 K on (from top to bottom): (1) a sputtered silica surface, with all SiO2 removed, (2) the ∼1 nm thick native SiO2 layer that forms on the Si(100) wafers, and (3) a Si(100) wafer covered with a 300 nm thick SiO2 layer. Only Mn silicide is made on the sputtered surface because there is no oxygen for further oxidation; the identification of the low binding energy peaks with Mn silicide is helped by the observation of the plasmon losses at 660 and 670 eV, as previously mentioned.23 Oxidized Mn is also seen on the surfaces covered with SiO2 films, but even on the substrate with the thick oxide, where the Si(0) is not visible in the Si 2p XPS traces (data not shown), a low binding energy peak is still observed in the Mn 2p XPS traces. That peak does become smaller upon annealing to 725 K, as shown in the right panel of Figure 5, the opposite of what is seen on the thin silicon dioxide film, suggesting that the Mn atoms may first be reduced upon adsorption on the surface: the low-binding-energy peak seen in the thick SiO2 surface may correspond to metallic manganese, which may diffuse into the oxide films at higher temperatures to make the silicate species (which grows in thickness at the expense of the SiO2 layer as the Mn diffuses in). On the thin (native) SiO2 film, a mixture of Mn(0) and Mn silicide may be seen instead. The results presented here were obtained by depositing manganese chemically using MeCpMn(CO)3 as a precursor, but we believe that the conclusions on the nature of the surface made by addition of manganese to SiO2/Si substrates are general. For one, we have recently reported some of the same behavior during the deposition of Mn using Mn2(CO)10 instead; in that case, the buried nature of the manganese silicide layer was proven by depth profiling XPS experiments.35 The formation of a silicate layer has also been inferred in chemical vapor deposition (CVD) studies using a manganese acetamidinate precursor,36 and mixed manganese silicate/manganese silicide films have been produced via Mn ion implantation on silica glass.22 A study where Cu Mn alloys were deposited by simultaneous sputtering of both elements on a SiO2 substrate lead to the observation of a silicate phase as well.10 13 It should be noted that the formation of manganese silicates from mixtures of MnO and SiO2 is thermodynamically quite favorable.37 39 On the other hand, to the best of our knowledge, no formation of manganese silicide films at the SiO2/Si(100) interface has been reported to date. A low binding energy peak in films deposited by CVD using (EtCp)2Mn was assigned to metallic manganese in one study, but we suspect that such feature may have corresponded to the same buried manganese silicide layer characterized in our study.31 Manganese silicides are thermodynamically favored over separate Mn and Si phases but are much less stable than the silicates,40 a fact that may explain why the silicide is only produce after saturation of the silicate. The combined manganese silicate/silicon dioxide/manganese silicide films reported here are only 1 to 2 nm in total thickness, and their growth by chemical means is self-limiting. The implication is that very thin self-forming diffusion barriers can be made this way, much thinner than the barriers used at present, typically metal nitrides, mixed carbide-nitrides, or late transition metals,41 44 and also thinner than the dimensions required for the current and near-future technology nodes in the microelectronic industry (∼22 nm at present).45 Deposition of these self-limiting Mn-based diffusion barriers by CVD (or by atomic layer deposition) also solves the problem of conformality often found with physical deposition

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methods when complex surface topologies are involved, as is the case with modern microelectronic circuits and devices; activation of the precursor is believed to be induced by reaction with OH surface groups, and that can occur after adsorption on any exposed silicon oxide surface regardless of orientation. The one remaining challenge with this approach is the need to develop a way to clean the top layer from the organic matter, the methylcyclopentadienyl ligand in this case, that is codeposited during the exposure of the surface to the MeCpMn(CO)3 precursor. In an atomic layer deposition (ALD) type process, this could potentially be achieved by using a cleaning reagent in the second half of the cycle, either a reducing agent such as hydrogen or an oxidizing compound such as ozone or N2O. We are at present exploring these alternatives to complete a chemical deposition procedure designed to deposit these self-forming diffusion barriers cleanly on SiO2 films. Further studies are also needed to characterize fully the electrical properties of these diffusion barriers, in particular, to evaluate the role that the manganese silicide layer may play in defining the electronic properties of the final devices. In more general terms, our results from the characterization of this system offers a cautionary story on the possible complexity of films prepared by simple deposition of one element on top of a solid layer, because a combination of kinetic and thermodynamic driving forces can lead to the diffusion of certain elements through, and reaction with, specific layers, leading to the development of multiple mixed phases.

’ EXPERIMENTAL METHODS All X-ray photoelectron spectroscopy (XPS) experiments reported here were obtained by using a two-chamber stainless-steel ultrahigh vacuum (UHV) apparatus described elsewhere.46,47 The main chamber is evacuated to a base pressure of ∼1  10 9 Torr by using a turbomolecular pump and is used for the acquisition of the XPS data. The auxiliary chamber, which is also evacuated by a (separate) turbomolecular pump, is mainly designed to facilitate the fast insertion of samples from the outside into the UHV XPS analysis chamber but in this study was also used as a preparation chamber to perform the exposure of the samples to the manganese precursor. The silicon substrate is mounted on a supporting rod, used for transferring between the two chambers without exposure to the outside atmosphere. The sample transferring rod can be rotated for alignment and for angle-resolved XPS studies, and is capable of cooling to ∼175 K and resistive heating to close to 1000 K. The temperature of the substrate is monitored with a K-type thermocouple inserted between the sample and the metallic clip that holds the sample in place. The XPS data are collected using a Leybold EA11 multichannel detection system and a dual Mg-Kα/Al-Kα anode X-ray excitation source; the data reported here were acquired using the aluminum anode (hν = 1486.6 eV). All XPS peak positions are reported in binding energies (BEs), calibrated relative to the reported value of the Si 2p XPS peak for the neutral silicon underneath the silicon oxide native layer of the substrate.48 The XPS spectra were analyzed by first subtracting a Shirley background and then fitting the data to Gaussian peaks, taking into account the appropriate spin-split contributions. An example of the results of such deconvolution is provided in Figure 2. The peak intensities were then used to calculate film thicknesses by using the layered model illustrated in Figure 3, exponential signal attenuation equations, and published photoelectron mean free 2528

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The Journal of Physical Chemistry Letters paths.27,28 The results from such calculations are reported in the right panels of Figures 1 and 4 for the films deposited at different temperatures and for the uptake at 675 K, respectively. The methylcyclopentadienylmanganese tricarbonyl (MeCpMn (CO)3) precursor used in these studies was purchased from Strem Chemicals (97% minimum purity), purified by a series of freeze pump thaw cycles in situ in our gas-handling manifold, and dosed by introducing its vapor into the chamber using a leak valve. The silicon samples were cut into square pieces from commercial Si(100) wafers (Si-Tech), and washed with water and acetone prior to introduction in the UHV chamber. Gas doses were set by fixing the gas pressure (followed continuously by using a nude ion gauge) and dosing times and are reported in units of Langmuirs (1 L = 1  10 6 Torr).

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Funding for this project was provided by Intel and by the University of California Discovery program. Additional funding was available from the U.S. Department of Energy. ’ REFERENCES (1) Kapteijn, F.; Vanlangeveld, A. D.; Moulijn, J. A.; Andreini, A.; Vuurman, M. A.; Turek, A. M.; Jehng, J. M.; Wachs, I. E. AluminaSupported Manganese Oxide Catalysts. I. Characterization: Effect of Precursor and Loading. J. Catal. 1994, 150, 94–104. (2) Thackeray, M. M. Manganese Oxides for Lithium Batteries. Prog. Solid State Chem. 1997, 25, 1–71. (3) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novak, P. Insertion Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 1998, 10, 725–763. (4) Shukla, A. K.; Prem Kumar, T. Materials for Next-Generation Lithium Batteries. Curr. Sci. 2008, 94, 314–331. (5) Belanger, D.; Brousse, T.; Long, J. W. Manganese Oxides: Battery Materials Make the Leap to Electrochemical Capacitors. Electrochem. Soc. Interface 2008, 17, 49–52. (6) Xu, C.; Kang, F.; Li, B.; Du, H. Recent Progress on Manganese Dioxide Based Supercapacitors. J. Mater. Res. 2010, 25, 1421–1432. (7) Wen-bin, S.; Durose, K.; Brinkman, A. W.; Tanner, B. K. Growth and Characterization of Magnetic Metal Mn Film by MOCVD. Mater. Chem. Phys. 1997, 47, 75–77. (8) Nakamura, T.; Tai, R.; Nishimura, T.; Tachibana, K. Spectroscopic Study on Metallorganic Chemical Vapor Deposition of Manganese Oxide Films. J. Electrochem. Soc. 2005, 152, C584–C587. (9) Topol, A. W.; Dunn, K. A.; Barth, K. W.; Nuesca, G. M.; Taylor, B. K.; Dovidenko, K.; Kaloyeros, A. E.; Tuenge, R. T.; King, C. N. Chemical Vapor Deposition of ZnS:Mn for Thin-Film Electroluminescent Display Applications. J. Mater. Res. 2004, 19, 697–706. (10) Koike, J.; Wada, M. Self-Forming Diffusion Barrier Layer in Cu-Mn Alloy Metallization. Appl. Phys. Lett. 2005, 87, 041911–041913. (11) Usui, T.; Nasu, H.; Takahashi, S.; Shimizu, N.; Nishikawa, T.; Yoshimaru, M.; Shibata, H.; Wada, M.; Koike, J. Highly Reliable Copper Dual-Damascene Interconnects with Self-Formed MnSixOy Barrier Layer. IEEE Trans. Electron Devices 2006, 53, 2492–2499. (12) Koike, J.; Haneda, M.; Iijima, J.; Otsuka, Y.; Sako, H.; Neishi, K. Growth Kinetics and Thermal Stability of a Self-Formed Barrier Layer at Cu-Mn/SiO2 Interface. J. Appl. Phys. 2007, 102, 043527–043527. (13) Haneda, M.; Iijima, J.; Koike, J. Growth Behavior of SelfFormed Barrier at Cu--Mn/SiO2 Interface at 250 °C. Appl. Phys. Lett. 2007, 90, 252107–252103.

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The Journal of Physical Chemistry Letters

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