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Electronic Structure of Metal-Ceramic Interfaces

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Electronic Structure of Metal-Ceramic Interfaces ISIJ International, Vol. 30 (1990), No. 12, pp. 1059-l065 Electronic Structure of Metal-Ceramic Interfaces Fumio S. OHUCHIand Qian ZHONG1) Central Research and DevelopmentDepartment, E. l. DuPontde Nemoursand Company,Experimental Sation, Wilmington, DE19880- 0356, U.S A. 1)Department of Materials Science and Engineering, University of Pennsylvania, Piladelphia, PA19104-6272, U.S.A. (Received on March 1, 1990, accepted in the final form on May18. 1990) Increasing technological applications of metal-ceramic systems have demandeda fundamental understanding of the properties of interfaces In this paper, wedescribe our approach to the study of electronib structure of metal-ceramic intertaces. Electron spectroscopies have been used as primary techniques to investigate the interaction betweenmetal overlayers and ceramic substrates under various experimental conditions. Thesedata are further elucidated by theoretical calculations, from which the electronic structures of the interface have beendeduced Atemperature dependenceof the band structures of Al203 is first discussed, then the evolution of the electronic structure and bonding of Cuand Ni to Al203 is studied. The relationship between electronic structures and interfacial properties are also addressed. KEYWORDS:metal-ceramic interface; alumina; copper; nickel; electron spectroscopy; electronic structure; UPS; XPS; LEELS. Introduction have the capability of resolving the distinct nature of 1. evolving chemical and electronic states of both metal Increasing technological applications of metal-ce- and ceramic components. Lastly, the experiments ramic systems, such as structural and electronic mate- need to be free from artifact, particularly contamina- rials, have generated great interest in a variety of' sci- tions, thus an ultra high yacuum(UHV) condition is entific issues concerning interfacial properties. Me- required so as to control the environmcnt. chanical and structural aspects of the interfaces, and With these features highlighted, wedescribe a tech- their relationship have been most frequently studied nique representing a comprehensive methodology for in the past, because these macroscopic properties are the study of' electronic structures of rnetal-ceramic directly relevant to the mechanical strength of the interfaces. As the uses of Al203 expand into new interfaces. While electronic structure of the inter- and more demanding_applications, we first study the faces can be the basis for the macroscopic properties, surf~ce electronic structure of A1203, then discuss its this has not been experimentally investigated until interface interactions with Cu and Ni, in particu]ar, recently. This paper deals with the experirncntal focusing on the electronic structures at early stages of methodologyused to study the electronic structures of the interface formation. The kno\4'1edge thus ob- tained will assist in overall metal-ceramic interfaces. - understanding the prop- The study of metallic monolayers on well charac- erties, and more importantly, provide a basis for terized material surfaces pro_vides a fruitful approach controlling the structure and compo_sition of the inter- to the clarification of the physics and chemistry of face. adsorbate-adsorbate and adsorbate-substrate inter- characteristics. action Major advances have been 2. Experimental Approach achieved through applications of modernsurlace sci- ence techniques, which provide information about the Our experimental approach to metal-ceramic in- electronic and structural properties of such systems. terface studies involves the preparation of the inter- Wehave adopted similar concepts to the investigation faces via formation of metal layers onto well charac- ol' the intrinsic bonding and clectronic properties at terized ceramic surfaces in UHVunder controlled the metal-cerarnic interfaces. To demonstrate these, conditions. This process is schematically illustrated however, several criteria must be met in designing in Fig. l. Wehave designed the experimental ap- and performing_ the experiments. First, the interface paratus, as seen in Fig. 2, such that we are able to must be fabricated so that variation of the chemical control thc init.ial condition of ceramic surfaces, the or electronic state of each componentcan be probed rate of interface formation, and the incorporation of during formatio_ n of' the interi~Lce. Secondly, the con- desired dopants. Details of the experimental method figuration of the experimental apparatus is such that have been discussed elsewhere.1) Electron spectro- is the measurement compatible with the interface scopies have been used as primary tools for the ad- formation in terms of their geometry. Thirdly, the vantage of their sensitivity to different electronic experirnental techniques that have been chosen must states and a range of probing depths. C1990 ISI J l059 ISIJ International. Vol. 30 (1990). No. 12 metal (TOp View] (Side View] vapor ceramic surface ~~ e ' e . LEED e e' Metal Sources MS Molecular Doser ~• X-ray source metal-ceramic interface Sample / ~~~ lon Gun / ~ F ' _+~ CMA \ ) f Optica[ Microscope ~ Sample I Heater "'~ " Al-Ka Fig. 1. Concept diagram for the Energy' Metal source Analyze\_r -- Monochromatized with metal-ceramic interface x_ray Diff. Pump Energy Ana[yzer lon b'lolecu[ar Doser study. Gun Metal Source Fig. 2. Schematic diagram for thc apparatus for the metal-ceramic interface studies. Ultraviolet photoelectron spectroscopy (UPS) is a mid 10-11 torr range, while the pressure maybe in- technique that is used to characterize the surface creased to the 10-ro torr during the deposition. The electronic structure or ~ensity of states (DOS)of' clean metal source is held at a fixed temperature, thus or adsorbate covered solid surfaces.2) In this tech- yields a constant vapor pressure, from which the vapor nique a collimated beamof monoenergetic ultraviolet flux at the specimensurface maybe calculated. The photons is directed at the surface of interest. The metal deposition rate is typically maintained in the ~nergy ~istribution curve (EDC) of electrons that are range of O.1-1.0A/min. This means the time re- photoemitted is then determined and the occupied quired to form one equivalent monolayer is on the electron DOScan be inferred from the EDC. Be- order of 3-30 min. The time dependent changes of cause the escape depth for photoelectrons excited by the signals are monitored during metal deposition. typical photons is short, this technique samples only If necessary, the local atmosphere can be chan~"ed by the surface energy levels. Thus, the experimental delivering gas from a molecular doser located near DOScurve can be comparedwith theoretical surface the specimen. DOScalculated by appropriate methods. Also, in- In order to ensure the stoichiometry and the crys- formation about surface bonding and surface reac- tallographic structure of the ceramic surfaces, the tivity can be obtained. substrate routinely underwent the following proce- ~~-ray photoelectron ~Pectroscopy (XPS) is a simi- dures before any run of experiment: in case of Al203, lar technique but uses photons with muchhigher ex- an Ar ion-sputtering (5 x l0-5 torr, 1-3 kV, 25 mA) for followed in citation energies, and as a result, both EDCofvalence about 10 min, by annealing UHVat 10-5 electrons and core electrons are measured. There- l OOO'Cfor 5-lO min, and in oxygen (Po,=5X fore, XPSis used to determined the electronic struc- torr) at I OOO'Cfor 5min. ture oi' solid surfaces as well as to chemically identify surface components. Low-energyelectron energy loss 3. High Temperature Electronic Structure of spectroscopy (LEEI.S) gives information about transi- Al203 l tions between initial and final states, in particular, excitations from valence or relatively shallow core Al203 rs a mixed ionic and electronic conductor at states into both bulk and surface-related final states. high temperatures. The high temperature electrical Combining LEELSand photoemission spectroscopies conductivity behavior has been studied extensively in (UPSand XPS)during the course of' metal-ceramic the past to elucidate the defect chemistry with a goal interface formation, information can be obtained of' understanding the mass transport associated with .joints, l about the jo_ int density oi' both filled and emptystate; sintering process.3) In metal-ceramic high from this an energy level schemefor the interface of temperatures are usually involved in the process, interest can be deduced. While the primary empha- therefore, a clear understanding of the electronic sis for the techniques described here is on the study structure of Al203 at high temperatures is important. of the electronic structure, additional information Measurementsof absorption edges and reflectivity about interface chemistry and structure, as well as, peaks of Al203 with increasing temperature using the mechanismsof' interface formation can be ob- yacuumI~ltra-yiolet (VUV) spectroscopy have been tained. madeonly recently by French.4) This is a first di- The interface is fabricated at the surface by de- rect measurementof this kind. The conclusion made positing metal atoms from a differentially pumped frorn this study was that the temperature depen- is electronic leads decrease metal source. The base pressure maintained in the dence of the structure to a l060 ISIJ International, Vol. 30 (1990), No. 12 in the optical band gap energy from 8.8eV, at teracted by a broadening of the valence band caused 23'C, to 7.2eV, at 1490'C with a linear rate of by electron-phonon interaction.
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