Al2O3 interface

Surface Science Letters 249 (1991) L327-L332 North-Holland

Surface Science Letters

Influence of the Al 203 (0001)surf ace reconstruction on the Cu/Al,O, M. Gautier, DSM/DPHG/SPAS,

interface

J.P. Duraud and L. Pham Van CEN/SACUY,

91191 Gif-SW-Yvette

Cedex, France

Received 13 November 1990; accepted for publication 26 February 1991

The initial states of the copper/alumina interface formation by copper vapor deposition are studied on two (0001) cy-alumina surfaces, with different crystallographic structures: primitive (1 X 1) and reconstructed (m X fi)R f 9 ‘. The surface phase transition corresponding to the latter surface structure leads to a difference in the growth of copper clusters: on both surfaces, clustering occurs even for the lowest copper coverages, but the size of the clusters is larger on the reconstructed surface than on the primitive one.

The metal/alumina interface is an important technological concern in several fields, such as metallization for microelectronic packaging applications and heterogeneous catalysis. As for as the copper/alumina interface is concerned, the experiments reported on monocrystalline alumina are scarce [l], owing to the difficulty of preparing clean and well defined monocrystalline Al,O, surfaces, exhibiting well structured diffraction patterns. Most experimental studies dealing with copper growth on alumina were indeed carried out on thermally oxidized aluminum [2-51. A recent study deals with the role of the alumina surface structure in the metal/alumina bonding [l]. However, if the role of the surface reconstructions is evocated, no clear distinction is made between those due to isolated ordered oxygen vacancies and those corresponding to a phase transition occurring in the first atomic layers. In order to understand the influence of such a phase transition on the Cu/Al,O, interface, we have studied the initial stages of its formation, by copper vapor deposition, on two (0001) cu-alumina crystallographic surfaces, exhibiting different structures: primitive (1 x 1) and commensurate (m x m)R f 9 O. This latter corresponds to a surface phase transition.

This experimental approach was based on LEED (low energy electron diffraction), XPS (Xray photoelectron spectroscopy, EELS (electron energy loss spectroscopy) and XANES (X-ray-absorption near edge structure) measurements. LEED, XPS and EELS experiments were carried out in a Meca 2000 UHV system (base pressure: 5 x 10-l’ mbar), the analysis chamber of which was fitted with an electrostatic electron gun (energy spread around 0.8 at 100 eV of primary energy), and a non monochromatized X-ray source equipped with an aluminum anode. The emitted electrons were energy filtered in an electrostatic analyzer (Mac 2-Riber). Detection was made by counting method; data could be stored and processed using a microcomputer. LEED was performed using a reverse view type device, with an incidence angle of 45 ‘, reducing then the charging effects on insulating samples. X-ray absorption measurements were performed at the CuK edge, on the D13 beam line of the synchrotron radiation facility at LURE (Orsay, France). Two detection modes were used: either the total electron yield measured with a channeltron, or the CuKcu fluorescence yield with a solid state detector. The samples were thin square slabs of mono-

0039-6028/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

M. Gautier et al. / Injlwnce

Fig. 1. (1

Fig. 2. (m

x 1)

x fl)

of the ~~~~~~~~

SWf ace reco~t~ction

on the CU/AI@~

LEED pattern of the (0001) wA1203 surface. EP = 150 e?’

LEED pattern of the (0001) cu-Alz03 surface. EP = 140 eV.

inietface

M. Gautier et al. / Influence of the A1~#3001) surface reconstruction on the Cu/Al&

crystalline Al@, (Verelec, France), with one face polished optically flat and oriented perpendicular to the c axis. They were first ultra-sonic cleaned in acetone before their loading in the preparation chamber, and then heated in situ thanks to an electron bombardment heating stage, allowing to reach temperatures higher than 1OOO’C with a residual pressure in the 10-a mbar range. Temperature was controlled using both an optical pyrometer and a W-Re thermocouple. The samples heated at 1OOO’C for 10 min under vacuum (lo-’ mbar) exhibited a fair (1 X 1) diffraction pattern, characteristic of the bulk hexagonal symmetry (fig. 1). Further heating at 1400°C for 10 min showed a more complex diffraction pattern, identified as the (v% x &i)R * 9O surface reconstruction (fig. 2). This surface reconstruction is characteristic of an oxygen-deficient surface [6,7], and was interpreted by the formation of a cubic Al-rich overlayer on the (0001) substrate with hexagonal symmetry [SJ. The XPS spectra recorded on both (1 X 1) and (m x m) surfaces showed that they were clean without any carbon or calcium contamination. The me~ur~ent of the ratios of the intensity of the phot~l~tron lines 02s/Al2p confirmed the Al enrichment of the (v% X fi) surface. In a previous paper, we showed that this surface structure modification was accompanied by a change in the electronic structure [7,9]. In the Meca 2000 UHV apparatus, copper deposition was carried out in the preparation chamber using a Knudsen effusion cell calibrated with the help of a quartzOoscillator, with a mean deposition rate of 0.1 A/min. During evaporation, the sample was kept at room temperature. After each evaporation, the experimental procedure was the following: (I x I) surface: the sample was heated at 1100 * C in situ in order to remove the copper deposit, and then at 800” C under air to restore the surface stoicbiometry. A further heating in the preparation chamber at 1000“ C was then performed to ensure cleanliness. (m x m)R f 9 o surface: the sample was heated at 1400 o C in the preparation chamber, in order to get rid of the copper deposit. Before the fo~o~g evaporation, the surface

A,,(eV)

interface

/

o(sji

x

m)RgO

Fig. 3. Evolution of the copper Auger parameter A,, as a function of the R ratio of the intensity of the photoelectron line CuZpSfl to the backgroundlevel. ln~~tion of the equivalent depositedcopper thicknesse is given(see text).

cleanliness was probed using XPS, and the surface structure was checked by LEED. This procedure allowed us to realize all copper evaporations on the same sample, respectively for the (1 x 1) and (m X m) surfaces. In order to study the interface formation as ‘a function of copper deposition, we followed the evolution of the Cu Auger parameter, defined by: A CU = &i,tCu~&W&~)

-

&.i,(Cu2~,/,)

+ hv,

E,,(Cu L23M45M45) being the energy of the Auger transition and -Ekin(Cu2~~,~) the kinetic energy of the Cu2p,,, photoelectron, hv the photon energy (1486.6 eV for the AlKa line). It has been shown that this parameter can be determined with a very good accuracy (0.1 eV) [lo], and can be used to follow small changes in the chemical environment of copper Ill]. In particular, it is possible to ~sc~ate between the different oxidation states of the copper atom. From an experimental point of view, this parameter is obtained from an energy difference measured on the same XPS spectrum, and can be assumed as insensitive to surface charging on insulating samples, provided that no differential charging occurs t121. Fig. 3 shows the evolution of A, as a function of the number of copper atoms at the surface, which was monitored, for the (1 X 1) and the ($% x m) surfaces respectively, by the ratio of

M. GauFier et al. / Influence

of theAl~~(~~~

surface reconsiruction on the Cu/A12ql

P

8950 bonding I

I

12 10

non bonding

E

8990

PHOTON

I

I

I

I

8

6

1

2

0 -

BINDING ENERGY

AB

8970

5

I

interface

9010

ENERGY

9030

9050

E (eV1

(eV)

Fig. 4. XPS valence band evolution as a function of the equivalent deposited thickness, for the (a X fi) surface (Al Ka source, hv = 1486.6 eV). Binding energies are referred to the Fermi level.

the intensity of the photoelectron line Cu2p,,,, to the background level. In an attempt to calibrate the deposition in terms of copper monolayers (ML), we used the equivalent deposited thickness on the quartz oscillator. The relationship between the R ratio and the equivalent deposited thickness is not linear, indicating a three-dimensional growth.

N (El E$lOOeV

8950

8970

8990

PHOTON

9010 ENERGY

9030

9050

IeV)

I

i

10 ENERGY

8

6

L

LOSS (eVf

2

I 0

Fig. 5. Band gap part of the electron energy loss spectrum, obtained in the reflexion mode, as a function of the equivalent deposited thickness, for the fi x v% surface ( Er, = 100 eV).

I! 8950

8970

8990

PHOTON

9010

ENERGY

9030

9050

E leV)

Fig. 6. CuK adsorption edge (XANES spectrum) obtained for (a) metallic copper (total electron yield detection), (b) 30% ML copper on stoichiometric ff-Al~O~(~l) (CuKn fluorescence yield detection), (c) 60% ML copper on stoichiometric aAi,O~(OOOl) (total electron yield detection).

M. Gautieret al. / Infi’uenceof the A1_@3(000i)surface reconstructionon the Cu/AI,O, interface

For both samples, the A,, evolution can be described by two stages; (1) Up to 0.25 ML (1 A), the value of A, corresponds to Cu,O. Copper atoms are dispersed onto the surface and bonded to oxygen atoms with an oxidation state + I. (2) Above 1 ML, the Cu Auger parameter exhibits the metal value. The valence band evolution reported in fig. 4 surface shows an increase of for the (m x m) the Cu3d photoelectron peak intensity with the copper deposition. In the early stages, the peak position corresponds to the Cu@ oxide and is located at 3 eV below the Fermi level. It shifts to 2.5 eV at higher coverages indicating the formation of metallic copper. These results are in good agreement with the previous experimental work [3,5] and the theoretical approach developed in ref. [5,13]. Fig. 5 displays the band gap part of the electron loss spectrum, recorded at 100 eV of primary energy, on the (m X m) surface. It can be noted that surface and bulk plasmon structures are observed for an equivalent deposited thickness of 1 ML (3.6 A). Up to this coverage, a fair LEED pattern is observed on both surfaces (1 X 1) and (&i x m), which indicates that the copper deposition does not induce any structure modification of the Al,O, surface. This LEED pattern observation does not permit us to choose between the following hypotheses: clusters or continuous film. The experimental evidence for cluster formation was obtained from XANES measurements at the CuK edge. Evaporation was performed in the UHV chamber at LURE, from small copper balls heated by a tungsten wire. Respectively 30% and 60% Cu monolayers (ML) were evaporated on stoichiometric cr-A.l,O, (0001) surfaces. The XANES spectra recorded at the CuK edge (8976 eV) on these surfaces are represented on figs. 6b and 6c respectively. The spectrum of fig. 6b resembles closely to the one obtained by Montano et al. [14] for 10 A-mean diameter Cu particles embedded in solid argon, indicating that even for the low coverage 30% ML, clusters are formed. The spectrum of fig. 6c is closer to that of metallic copper (fig. 6a), with the two ch~acte~stic structures A and B well defined.

These results are consistent with the VolmerWeber type of growth suggested by Di Castro et al. [2]. Then, the differences reported on fig. 3 concerning the Cu Auger parameter evolution for the two surface structures can be explained by a difference in the nucleation of copper clusters. For the (1 x 1) surface, A,, reaches the metal value for an equivalent deposited coverage of 2 ML, whereas for the (&i x m) surface, the Cu(0) oxidation state is reached for a much lower equivalent deposited coverage: 0.5 ML. This shows that for a given copper coverage the cluster size is larger for the r~onst~cted surface. The number of clusters per unit area is then smaller on the reconstructed surface, indicating that this latter presents less nucleation centers than the primitive (1 x 1) one. This would be contradictory with a surface reconstruction resulting from an ordered arrangement of oxygen vacancies acting as nucleation sites and strengthens the hypothesis of a surface transition leading to the formation of a new oxide in the surface. layers. As no diffraction pattern characteristics of the copper structure appears, it can be deduced that the size of the clusters is lower than the LEED coherence length (100 A). The A,, values intermediate between the one of Cu(1) and Cu(O), mainly observed on the stoichiometric (1 x 1) surface, could arise from a shift in the Cu electronic energy levels due to the small size of the clusters [2,15]. Considering the 0-Cu chemical bond formation [5,13], the earlier clustering observed on the (&% X @i) AL&l, surface may be related to the lower oxygen concentration at the surface, as reported above: when there are no more oxygen atoms available at the surface to form chemical bonds with the copper atoms, these latter begin to interact with each other so as to form clusters. But the eventual formation of AI-Cu bond on the aluminum-rich (&%i x &%i) surface can not be discarded on the basis of the present results. Indeed high resolution transmission electron microscopy showed that an AlCuO* compound was formed, at high temperature, at the copperalumina interface produced by thermal compression [15]. As no significant difference could be

observed in the Al Auger parameter upon copper deposition, this seems to indicate that such a compound is not formed at the Cu/Al,O, interface. Further experiments will confirm this point. We are grateful to D. Chandesris (LURE, Orsay), H. Magnan (DSM-DPHG-SPAS) and the staff of LURE for their help in the X-ray absorption experiments.

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