Nuclear Instruments and Methods in Physics Research Bh5 (1902) 129-138

N(~rth-Holland

Nuclear Instruments 86 Methods

in Physics Research s\ictMY B

Ion beam mixing of metakeramic interfaces

A. Perez, E. Abonneau, G, Fuchs and M. Treilleux Dt;l,urrrmeni r/e Pltpsiqtrc dcs Mat&ulcu, Llnitwsiti C’lartdu Bernurd - Lyon I, 64622 Vilhrhanne C’&X, Frurtw

Iron and copper thin metallic films ithickness 5 100 nmf deposited on sapphire substrates iru-Al,O,) were mixed at room

temperature by various rare gas ions (Ne. Ar, Kr and Xef in large energy and dosu ranges. Microstructu~~~ and chemical

characterization of the mixed interfaces were performed using three complementary techniques: high resolution transmission

electron microscopy (HRTEMJ, X-ray photoemission spectroscopy (XPSJ and conversion electron Miissbauer spectrometry

(CEMSJ in the particular case of Fe/Al,O, samples. The final atom distributions at the mixed interface are the result of the

ballistic phase of mixing and no long range diffusion of mixed elements in Al,O, has been observed. The result is a very narrow

and damaged interface (amorphous or not in the case of Fe/Al,O, or Cu/AI,O, respectively) containing a very high

concentration of metallic atom recoils. The composition and nature of this interfacial layer, which completely depends on the

energy deposition at the interface. which is controlled by the mixing parameters (mass of the incident ion, energy and fluence) are

the origin of the spectacular adhesion enhancement observed. In the particular case of the FejAiZO, interface. the formation of

iron recoils in AI@, in the Fe’+ state and the form&ion of the spine1 phase seem to be correlated with the quality of the

interfacial joint in terms of adhesion. Thermal annealings in a low temperature range f < 6WCf promote the spine1 phase

formati(~n and produce an addition~ll positive effect on adhesion.

1. Introduction

km beam mixing has been extensively used in the past fifteen years mainly to synthesize silicides and metallic alloys from metal-silicon and metal-metal bilaycr or n~ultjlayer sampies [I]. The studies of mclal-insu~at~~r systems are more recent, and they concern, for a large part, metal-oxide couptes [Z]. Much of the interest in ion beam mixing as applied to metal insulator systems arises from the observation that bombardment with an ion beam often increases the adhesion of the film to the substrate ]I%-S]. Mctat- ceramic bonding is nowadays an ~mp~)rtant problem because of the large increase of ceramic applications in various industrial sectors (aeronautical, armaments, space, automobile, electronic, . . .I. The properties of the metal-ceramic interfaces are of prime importance for the development of muitilaycr systems in micro- electronics as well as for th~rmomechani~al ceramics used as structural materiats. Many conventional tcch- niques tth~rmocompr~ssion, brazing, sintering, . . . ) [9,10] allow good adhesion to develop between these two different types of materials in terms of reliability and thermal and mechanical stability. In many cases,

the bonding of immiscible and nonreactive materials at room temperature very often requires a newly dcvcl- opcd technique such as ion beam mixing, This tcch- nique is based on the interaction of energetic ions at the interface between two materials [I]. The spccimcns

oftcn consists of thin films deposited on substrates. Energetic ions (of a few tens of kcV to a few MeW are passed through the thin film, causing changes in the interfa~iai properties. The nature of the process allows any clemcnts to be mixed in the interfacial region in a controlled and reproducible manner that is indepen- dent of most equilibrium constraints. Since the process is non~quilibrium in nature, ~~~rnpositj~~ns and struc- tures unattainahie by conventional methods may be produced in the interfacial zone.

On the basis of previous studies with metal-metal and metal-semiconductor systems, the initial models for mixing focussed on the dynamics of ion-solid intcr- actions and were based on direct recoil or ballistic effects, cascade mixing, and enhanced diffusion due to radiati(~n-produced defects [ 1,2]. The important rote of thermodynamic and chemical effects in the ion-in- duced collision cascades was also clearly emphasized [I]. However, in the application of ion mixing to a

tlths-SX3X/92/$OS.O0 G 1992 - Elsevier Science Publishers B.V. All rights reserved JJJ. MIXlN~/A~~JESJ~N

system where at least one component is an insulating compound, e.g., a ceramic, some additional considcra- tions of chemical bonding, crystal structure, defects and ternary multicomponent systems must bc discussed

PI. The interaction of cncrgctic ions with ceramics dif-

fers from metals in a number of significant ways. Cc- ramics contain two or more chemical spccics dis-

tributcd usually in a very ordcrcd manner. Hence an atom of one sublatticc is unlikely to bc found at another sublattice site. The atoms have different atomic masses and the displacement energy due to elastic collisions may be diffcrcnt. The influcncc of thcsc

factors on defect production can bc important [I l]. For cxamplc, in the particular cast of Al,O,, the displacc-

ment cncrgies measured for Al and 0 arc I8 and 76 cV. respectively [l2]. Thcrc is a range of chemical bonding types in the materials of interest: ionic to covalent to “metallic-like“. The type of bonding influ- cnces the kind of defects that arc produced during irradiation vvhich, in turn, may influence the mobility of various atomic species [2]. In ionic crystals, the introduction of a dcfcct (or impurity) must provide for local electrical neutrality and consequently chargcd- point defects or impurity-point defect complcxcs often

arc produced [13-IS]. In this context, for a bcttcr understanding of the ion

beam mixing effects at the metal-ceramic interfaces, experiments have been carried out [I61 using well cho- sen metal-oxide couples (Cu-AI,O, and Fc-AI,O,), mixing parameters (ions, energy, fluencc) and charac- terization techniques of the interfaces. The ultrahigh purity, crystalline and surface qualities of sapphire partially justify the choice of this material as substrates for our samples. In addition defect creation and nonequilibrium phase formation by Cu or Fc ion im- plantations have been cxtensivcly studied in this matc- rial. The Cu-AI,O, system. interesting for applications in electrical device interconnections, is currently treated by conventional techniques (i.c. thermocompression) [9, IO. 171; results of adhesion tests and interface charac- terizations are comparable to those obtained with ion beam mixed samples. Among the methods that have been used to study the states of implanted cations in insulators are conversion electron Miissbauer spec- troscopy (CEMS) in the particular case of iron c5’Fc isotope), X-ray photoemission spectroscopy (XPSI, cx- tended X-ray absorption fine structure (EXAFS). and X-ray absorption near-edge structure (XANES). The most definitive results have come from CEMS studies of Fe implanted into various oxides (MgO, AI,O,.

SiO,, TiO,, garnets,. . ) [14.15,18-201 and alkali halides [21] which justifies our choice of the Fe-AI,O, couple. From Miissbaucr spectra analysis one can cx- tract information about the electronic structure of Fe atoms introduced in the Al,0,3 matrix, near the inter-

fact, cvcn with quantities as low as those concerned with the mixing process. Miiasbaucr also yields infor- mation about the local symmetry sites, bondings even with surrounding oxygen atoms. and the nature of the precipitated phases.

Finally, in this paper WC summarize the results obtain& with Cu and Fe thin metallic films ( I 100 nm) deposited on sapphire substrates and mixed at room tcmpcraturc with various rare gas ions (NC. Ar, Kr and Xc) in large energy and dose ranges. The microstructural and chemical modifications induced at the intcrfaccs, characterized by high resolution trans- mission electron microscopy observations (HRTEM), CEMS. and XPS mcasurcmcnts. arc reported and dis- cussed with regard to the specific aspects of mixing process itself and the propertics of ceramics under ion

bombardment. Also, the results of mechanical tests arc presented that show strong cffccts due to the adhesion

enhancement at the metal-ceramic interface related to the mixing conditions or the subscqucnt thermal an- ncalings and the nature of the interfacial joint formed.

2. Experimental procedure

Ultrahigh purity sapphire (u-AIZO,l wafers (thick- ness 0.5 mm) used in microelectronic devices wcrc used for our cxperimcnts. The optical grade polished surface of these substrates had a (17021 crystallo- graphic orientation. These substrates were chemically cleaned by rinsing thoroughly in acetone ethanol and dicthyloxide and then anncalcd in air for two days at 1000°C to remove any surface carbon contaminant layer [l6,22]. Iron or copper metallic films wcrc then subsc- qucntly deposited in a vacuum chamber using an clec- tron gun evaporator system. The rate of deposition was about 0.2 rim/s in a vacuum of 10 ’ Pa and the total thicknesses of the films ranged from 60 to 100 nm. The thickncsscs depend on the incident ions to bc used in order to take into account the diffcrcnt sputtering rates.

Ion beam mixing was carried out at room tempcra- ture using NC, Ar, Kr and Xc energetic ions produced with a 2.5 MV Van dc Graaff accelerator or a 200 kV ion implantor. The energies of these incident ions were chosen from TRIM Monte Carlo Code [23] simula- tions. Two restrictions guided sclcction of the ion encr- gies: (i) all the energies allowed the incoming ions to penetrate deeply in the substrate in order to minimize the local concentration of the implanted rare gas atoms at the interface, (ii) a constant ratio of the nuclear stopping power to the electronic stopping power [(d E/dx),,,,,/(d E/dxl,,,,,] was maintained for every ion passing through the interface. Table 1 shows the paramctcrs obtained in the particular case of Fe-Al ?O, mixed samples. One should note that while the energy

loss ratio at the interface (dE/dx),,,,,/(dE;/dx),,,,, is maintained constant around a mean value of 1.34 for all the incident ions the individual values of each component of the energy deposition ((dE/dr),,,, and (dE/dx),,,,,) increase by a factor of IQ from Nc to Xc. While only nuclear energy loss is at the origin of dcfcct production in metals, electronic excitation processes lead to damage creation in some insulators such as alkali halides [24]. Even if this process cannot dircctty create Frenkel defects in refractory oxidch such as AI,O, or MgO, experiments of impurity state changes under ionizing radiations [25,26] emphasize the rtrlc of electronic interactions in metal-insulator systems.

constitutes 2.2% of natural iron, is detected by the Miissbaucr technique. Consequently. in order to in- crease the number of mixed intcrfaccs and thus the number of Fe atoms mixed in AI&, multilayers sam- ples wcrc specially prepared. Six layers of Fc of 9 nm thickness were deposited altcrnativcly with six layers of amorphous alumina of Ih nm thickness onto a glass suhstratc. Since the CEMS technique probes the sur- fact layer of the sample to a depth of about 150 nm, all the interfaces of the multilaycr were dctcctcd, allowing good sensitivity for mcasuremcnts of the mixing cffcct.

Three tcchniyucs were used for the microstructural and chemical charactcriz~ltions of the interfaces: HRTEM. XPS and CEMS. HRTEM observations wtrc performed with a Jeol 200 CX high resolution micro- scope operatill& at 200 kV ~~cceieratin~ voltage. Speci- mens were prepared in the cross section configuration to directly image the intcrfacc. They were first mcchan- ically thinned and then ion milted to electron trans- parency using 5 kcV argon h(~~~b;~rdrncnt at room tempcraturc.

Among various adhesion tests applicable to metallic film on ceramic substrate [27]. WC mainly used the mechanical pin pull test. Polished copper cylindrical pins of 2 mm di~~meter were attached perpendicularly to the motaltic film with an epoxy glue. A stress test machine was used to record the peak force required to pull the metallic layer off the sapphire surface.

XPS measurements were performed with Fe--AI,O1 samples using a CAMECA-Nanoscan SO Auger-XPS microprobe. Data were obtained with Al K,, radiation. Scqucnccs of sputtering (5 keV Ar ions) and data acquisition were repeated untit the sapphire surface was dctocted. A iow sputtering rate of about 2 rim/h alfowed a very precise depth anatysis.

CEMS spectra of Fe-AI,O, samples were mea- sured at room tempe~dture using a heijunl-t~o~v pro- portional counter in which the sample was placed in backsc~ttering geometry. A high activity (100 mCif “?Co M&sbauer source was mounted on a constant acceleration triangular-motion velocity transducer. The spectra were fitted with a computer least-squares pro- cedure with the assumption of Lorentzian shape of Mlissbauer lines to determine the nature and quantity of mixed species as a function of the mixing dose with Xe and Ne ions. All the isomer shifts iIS are given with respect to a metallic a-iron absorber at room temperature. Note that only the “Fe isotope, which

Also some prelimina~ measLIremcnts using the nan~~indentation technique have been performed at Oak Ridge National Laboratory. Details on this tcch- niquc arc given in ref. [28]. Fifteen indcnt~~ti(~ns wcrc performed at diffcrcnt points of each sample with a constant speed of the indentor during loading (1 rim/s)) and unloading (0.6 rim/s)) phases. Measurements at four penetration depths wcrc recorded at each point: t/4, 3/4, 5/4 and 2 times the thickness of the metallic film which corresponds to depths of 20, 60, 100 and 160 nm in the cast of the X0 nm Fc/AI?O, samples used. The hardness measurements are prcscnted rela- tively to the hardness of the sapphire substrates.

3. Micru~tructural and themicat analyses of the mixed interfaces

Cu/AI,O, and Fc/AI,O, interfaces mixed with 1.5 MeV Xe and 100 KeV Nc ions, respectively, to a dose of 1 X 10’” ions/cm’ have been observed by HRTEM and arc presenied ported in previous [8,22,29].

in fig. I. These results were rc- papers for both types of samples

Table 1 Ion beam mixing parameters calcntated for 80 nm Fe/At$& samples: fdE/dmf,,,,, and idE/dr&,,,,,: nuclear and efcctronic energy losses in iron; R, and AR,: projected range and mean range straggling

Ion Initial Energy at energy the interface

LkeVl IkeVl

(dE/dx),,,, at the interface

[eV/nm]

(dE/dx),,,,, at the interface

LeV/nml

(dE/dx-),,,l

CdE/dx),,,,, interfacial

In the case of Cu-At& samples (fig. Ia), the

ff”?fU)-type pianes of the sapphire sidxtratc were im-

aged and lattice fringes wcrc clcarl~ ?;ccn, indicating

that w amorphization of the substrate in the rcgian of

penetration of the incident Xc ions (clown to w 2000 A

from the intcrfacc) takes place at room tcmpcraturc

for the dose under consideration. This result is in good

agreement with reports of Burnett ut al. [.%I] whcr

dcccrmined B thresh&d dose of 600 +;I for nmorphiza-

tion ctf sapphire at rtmm temper;tture; this fhrcshofd

dose is for ~rn~~r~h~z~ti~il only by accumulatirm of

defects. 3%~ e&n&cd damage in the casi: ttf our

X~-irr~~dj~ted Cu/AL,O, sample is around 45 dpa, f;rr

from the thresh&d mcntioncd abtrve. At~~~3r~~~~~~~3~

ofsapphirc for ioiv ~rr~~~~~i~~n doscc has been observed

in some specific cases when ;I chemical effect due to

the implanted impurities takes place [2]. Such chemical

amcwphimtion Cannot bc involved in our Cu-A120,

samples bccausc the lattice fringes arc clearly seen

starting from an abrupt intcrfaec with a WXWW intcrf+

ciaX zone layer where fu atomS arcI mixed with clc-

mcnts of the substrate [8]. While the substrate surfrrcc

was very &it heforc mixing, the main diffcrcncc after

mixing in the cast of Cu/Af,03 systems is the prcs-

The main diffcrcncc observed in mixed Fc/AI,O,

compared to C’u/AI,O-, is the presence of ;I thin

amorphous layer (- 3 nml at the intcrfacc between

iron and sapphire [IS]. This layer, which is not prcscnt

bcforc mixing, can bc attribtztcd to a chemical effect

i~olvinp the f(~rrn~~t~~~n of a new phase at the intcrfacc

including the mixed atoms af both matcriaf?;.

W

Fig. 1. High resolution transmission electron microscopy observations of Cu/A12C13 (a) and Fr/AI,O, (II) interfaces mixed nt room

temperature with 1.5 MeV Xe inns and 0.1 McV Ne ions respectively. In both cases the ion fluence was I x IO'" ions/cm’.

by Xe or Ne incident ions [&I@ Widths of few nan~~meters are obtained for those profiles, in good agreement with HRTEM obs~~t~ons.

Another experimental confirmation of a localization trf the mixing effect in a very thin region at the inter- face was provided by XPS analysis pcrformcd on Fc/A120, samples mixed at room tcmperaturc with 1 X IO” NC ’ ions/cm’ at an energy of 100 kcV [lb]. From the cvotutions of the Is and 2p signals of 0 and

Fe, respectively, as a fun&m of the distance from the interface in the metallic film and in the substrate, we deduced a mixed layer with a total thickness of about 5 nm 116.281 in good agreement with HRTEM and TRIM

(b)

Velocity (mm/s)

Fig. 2. Cw~versicx~ electron MOsshauer spectra obtained with Fe/Al@, multilayer unmixed (a) and mixed at room temper- ature with 1.5 MeV Xe ions and doses of 0.6 (b), 1.2 Cc), 2.6 tdf, and 3.2~ 10 ” ions/cm* Cef. The solid line for each spectrum represents the best fit obtained with Fe’+ and Fe” (or Fe”+ 1 components, the parameters of which are

given in section 3.

1 06

1 04

I c2

1

1.06

104

1.02

1

1.06

7 I OS

t 1.02

r" i!! 1

0) 2 1 06

f 1 04

z $ 1.02

1

1.06

I.04

1.02

1

Cd)

(e)

WFa'+

or UFe3*

IFez*

I I I I I 1 FB met.

1 I , I I I I .6 -4 .2 0 2 4 6

Velocity (mm/s)

Fig. 3. Conversion electron Miissbauer spectra obtained with Fe/AI,O, muitil~~er unmixed (a) and mixed at room temper- ature &h 90 keV Ne ions and doses of 1 (bt, Z Cc), 6 (d), and iO-lC)‘h ions/cm’ Ce). The solid line for each spectrum repre- sents the best fit obtained with Fe’+ and Fe” (or Fe’+)

components, the parameters of which are given in section 3.

results. Unfortunately, it was not possible to determine unambiguously the nature of the phases formed in this layer from the XPS experiments because the cncrgy shifts of the peaks characteristic of various iron oxides (ferrous or ferric) are too smail i < 0.5 eVf to be resolved in our spectra with wide lines (FWtfM = 2 CV).

Due to the lack of information on the chemical nature of the mixed zone from XPS measurcmcnts, CEMS analyses of Fe/Al,O, multilayers mixed at room temperature with 500 keV Xe ions and 90 kcV Ne ions in the dose range from 1 x 10” to 1 x IO” ions/cm2 have been performed. For the case of multi- layers as mentioned above, the good CEMS sensitivity

III. MIXING/ADHESION

I 7 W ;;‘

B Fe3+ (Or Fe“‘)

a- . Fez* ) Ne-mrred

r”

al

I . Fe3+ (Or Fe4+)

2 . Fe** f Xe-mIxed

0 2 4 5 8 10 12

Ion ftuence (x 1016 ionsicmz)

2 2 unmixed samples the metallic iron scxtct reprcscnts

100% of the spectra (figs. 2a and 31) which confirms

the ahsencc of significant reaction at the intcrfaccs

between iron and alumina films consistent with

HRTEM &scrvations. Howcvcr. in the mixed samples,

as a function of increasing ion flucnces. the appcar-

ancc and growth of a paramagnetic signal is observed

in the ccntcr of the spectra due to iron atoms intro-

duccd in the alumina matrix. In the Xc-mixed samples

thcsc p~lr~~rn~gnctic c(~nlp~~ncnts arc dctcctablc from

ittn flucnces higher than 1 x 10” ions/cm’ while the

minimum dose is 1 x IO” ions/cm’ in NC-mixed sam-

ples. This cffcct is consistent with the ballistic mixing

efficiency of Xc which is higher than that of Nc.

Fig. 3. Dose dependence of the relative intensities of various comp[lnents, Fe metallic (a). Fe’ ’ and Fe” ’ (or Fe’ ’ ) (bt deduced from the computer fittings of the conversion electron

Miisshauer spectra presented in figs. 2 and 3.

and resolution of the components characteristic of

0” various iron statcs allowed the mixing process to he 9 4 -

followed from doses as low as I x IO’” ions/cm’. Figs.

+ “E 2 and 3 prcscnt the CEMS spectra obtained at room

0 ) “E z

tcmpcraturc with Xc and NC mixed samples. The main

3 0 component of the spectra is a magnetic scxtct (isomer

m ’ z shift IS = 0 mm/s. magnetic hyperfinc field HF = 331

I!! s ID kOc) due to the metallic iron films. The intensity ratio

z-, of lines 2 and I or 5 and 6 (A?;, or Ai,.,,) which is of

G the order of 1.33 is due to an in-plant magnetization og x cffcct gcncrally obscrvcd for iron thin films. In the

Computer fittings of the CEMS spectra wcrc pcr-

formed in order to determine the components prcscnt

in the p~i~m~gnctic signal in the ccntcr of the spectra

as well as the rclativc quantities of iron in the diffcrcnt

Fig. 5. Transmission electron microscopy observations on cross-sectioned Fe/AI,O, multilayer before mixing (a) and after mixing

at rc>c>rn temperature with 500 keV Xe ions (dose I x IO’” ions/cm’)(b).

states. These results arc reported in fig. 4 for Xc and

Nc mixed samples. For both types of samples, the

relative intensity of the metallic iron component (mag-

netic sextet) dccrcases linearly as a function of the ion dose (fig. 4a). Since we know the total number of Fe atoms prcscnt in the multilayers. WC can dcducc the numbcr of Fe atoms introduced in the alumina matrix, near the interface, by the mixing process, from the decrease of the metallic iron component. A scale giving these values is reported on the right hand side of fig. 4~. This allows us to obtain the average numbers of Fc atoms in sapphire per interface and per incident ion which are of the order of 7-H X IO- and h-7 x 10 ’

for NC and Xc, respectively. Note that thcsc values are about I5 to 20 times lower than the average numbers

of Fe-recoils in AI?Q, dcduccd from TRIM calcula- tions, but the order of magnitude of the ratio (numbcr of Fe-atoms in Xc-mixed interfacc)/(numbcr of Fc- atoms in Ne-mixed interface) = 8 is consistent with the relative ballistic mixing efficiency theoretically csti- mated from TRIM. In fact. the TEM observations of cross sectional multilayers hcforc and after mixing with 1 x 10’” Xc ions/cm’ (figs. Sa and 5) show a dra- matic dcform~tion of the films which could explain the difference between the n~c~~sure~i and calculated mix- ing efficiencies.

Concerning the identification of the phase formed at the interface which includes the Fe atoms responsi- hlc for the paramagnctic signal present in the CEMS spectra. two hypothcscs have been considcrcd: (i) the presence of Fe’+ (‘: r\omcr shift IS -- 0.4 mm/s and quadrupolc splitting QS = 0.8 mm/s) and Fe’ ’ (IS = I mm/s and QS = 2 mm/s) as rcportcd by Ogalc et al. 1331, or (ii) the prcsencc of FcJ ’ (-0.12 1 IS 2 -0.03 mm/s and 0.3 5 QS 5 0.6 mm/s) and Fe” (I I IS I 1.5 mm/s~~nd 1.2<QS<2 mm/s) as observed in our previous works on iron-in~pl~ntcd sapphire [ 151. With either hypothesis, WC obtain good fits of the spectra and thus cannot detcrminc whether Fc” or FcJ’ is present. Howcvcr, the quadrupolc douhlct rcprescnt- ing the Fe’ ’ component has a high velocity line easily visible around 2 mm/s and is unambiguously deter- mined. Since the metallic ion component (magnetic sextet) is also unambiguously fitted, we can finally dctcrmine correctly the relative quantities of Fe in various states. Thcsc results arc rcportcd in fig. 4b for both Ne- and Xc-mixed samples. In the cast of the No-mixed multilaycrs, a linear increase of Fc” and

Fe’.’ (or Fe’ ’ 1 compt~ncnts (correlated with the linear dccrcaso of the metallic iron c~)mp~~nent) is ohscrvcd with Fc” having a dominant role in the whole range investigated. A diffcrcnt bchaviour is observed in the Xe-mixed sample for which the intensity of the Fe’+ saturates while the Fe.” (or FcJ” ) component in- creases continuously. Finally, the large differences he- twecn Fe concentrations at the intcrfaccs mixed with

Nc and Xc ions as well as between compositions

(Fc”/Fe’ ’ (or Fe” )) determined from CEMS, and

microstructurcs determined by HRTEM emphasize

clearly the role of the energy deposition at the intcr- fact on the nature and consequently on the properties of the intcrfacial joint formed. This effect is clearly illustrated by the adhesion cnhanccment at the mixed interfaces prcscnted in the next section.

4. Effect on adhesion

The characteristic mixing effect observed in Cu or

Fe/AI,O, systems consists of constant and linear in- creases of adhesion as a function of ion dose. This is clearly illustrated in fig. 6, a plot of the peak forces vs ion fluencc measured by the pin pull test for 60 nm Fc/AI,O, samples mixed with various rare gas ion. In all cases a constant and linear increase of the stress at failure is observed but the slopes are different, indicat- ing some significant differences in the mixing cfficicn- tics on adhesion. This cffcct allows optimization of the adhesion cnhanccmcnt for the Ne mixed samples: an incrcasc by a factor of 5 of the adhesion force com- pared to the as dcpositcd sample is observed for a dose as low as 2 X IO” Nc-ions/cm’. Since the nature of the interfacial joint. in terms of microstructure and chemistry, depends strongly on the energy deposition during the ballistic mixing phase (see previous section), some interpretations of the characteristic effect oh- served in fig. 5 can be proposed.

The prcscncc and the differcnt evolutions of the Fe” component in the Nc- and Xc-mixed samples vs the ion tlucncc (fig. 4b) seem to be at the origin of the adhesion cnhancemcnt effect observed. In NC-mixed samples the intensity of the Fe?+ c~~rn~(~nent is two

0 1 2 3

Ion fluence (x 1016 lonslcm2)

Fig. 6. Evolution of the pull off force YS the ion thence for 60

nm Fe/AI,O, samples mixed at room temperature with Xe

t I.5 MeV). Kr (1.3 MeV). Ar (0.3 MeV) and Ne (0.1 Me!/)

ions.

III. MlXING/ADlIESION

times larger than the Fe’+ (or Fe”+) one in the whole dose range studied while it rapidly saturates in Xe- mixed samples lading to a dominance of Fe3’ (or

Fe’ ’ ). On the other hand, the maximum local conccn- trations (Fe/AI) of Fe in the mixed Al?O, layers dctermincd from TRIM calculations vary with ion dose maximum values of 60% and 120%. respcctivcly, in Nc- and Xc-mixed samples up at the highcst ion doses (I x 10” and 3.2 x IO ” ions/cm’. respectively). Con- scqucntly, it seems reasonably to describe the intcrfa-

cial joint in Nc-mixed samples as an Fe doped alumina. When the local iron concentration increases with the mixing efficiencies. the nature of the interfacial joint

evolves towards a11 iron oxide layer doped with alu- minum which is consistent wjith the CEMS results. In addition, it is intcrcsting to remark that in a Cu/AI,O, thcrmocompresscd interface [17], the precipitation of the spine1 phase CuAi,O, plays a positive role in adhesion. Taking into account the Miisshaucr paramc- tcrs of the Fe” component in our samples, a local cnvironmcnt comparable to those of iron in spine1 FeAI,O, compound [ 15.341 cannot bc ruled out.

Some other interesting effects deduced from the adhesion mcaauremcnts must bc mentioned. For cxani- plc. the role of a contaminant layer at the interface bctwccn the metal and the ceramic has been observed [31]. In the particular USC of Cu/A120, system mixed with I.5 MeV Xc ions in the dose range up to 3 X 10’” ions/cm’. the pin pull tests exhibit linear and parallel incrcascs of the pull off forces vs the ion flucncc for both contaminated ( - 5 nm of amorphous carbon due to the sample preparation with unanncalcd sapphire substrata) and clean (direct contact of Cu on AI,O, due to the sample preparation with annealed sapphire substrates) interfaces. However. the curve’ for contami- nated samples is shifted towards lower values of adhc- sion (50’6) with rcspcct to the curve’ of clean samples. Finally, a shift at the origin (unmixed samples) due to the contaminant layer is conserved after mixing.

Also an effect related to the nature of the metallic layer dcpositcd on sapphire has hcen observed from the comparison of Fc/AI,O, and Cu/AI,O, systems mixed in the same conditions (I.5 McV Xe ions in the dose range up to 3 X IO “’ ions/cm’). Both types of samples exhibit linear and parallel increases of the pull off force vs the ion dose but the curve for Fc is systematically shifted towards higher forces (40?+;1) with respect to the curve of Cu. Note that the ballistic mixing efficiencies are very similar for Fc and Cu due to the close masses of thcsc elements. Thus, as for the case of the contaminant layer mentioned above. the shift at the origin (unmixed samples) between Fc and Cu/AIIO, systems is conscrved after mixing. Such a shift can bc due to the differcncc between the intcrfa- cial cnergics of Fe and Cu/A120, systems lading to better bonding for Fc [IO].

0.1 ’ I 0 50 100 150 200

Depth (nm).

Fig. 7. Nanoindentation measurements performed for X0 nm

Fc/A120T samples unmixed (a) and mixed at room trmpera- turr with 100 keV Ne ions and doxs of 2~ IO” (b) and

2 X IO’” ions/cm (c).

It is also interesting to mention an effect related to the crystalline nature of the substrate. In particular, the slope of the linear incrcasc of the pull off force vs the ion dose measured in the case of the Cu/sapphire interface mixed with Xc-ions in the dose range up to 3 x IO” ions/cm’ is significantly higher than the slope mcasurcd for Cu deposited on sintercd a-Al,O, sub- stratc and mixed in the same conditions [32]. This cffcct cmphasizcs Ihe role of grain boundaries on the mechanical process of crack formation at the mctal- ceramic intcrfacc leading to lower pull off forces in the case of samples prepared on sintercd substrata.

The adhesion incrcasc reported using mainly the pin pull test have also been obscrvcd by other tcch- niques. For example u limited number of mcasurc- ments using the peeling test described by Baglin et al.

[4] have confirmed the adhesion increase as a function of the mixing ion dose [lh]. However, among various tests applicable to metal-ceramic systems, some inter- esting preliminary results have been obtained using the nanoindcntation tcchniquc developed at Oak Ridge National Laboratory [28]. With this tcchniquc some mechanical characteristics (e.g. hardness, elastic modu- lus) of various layers which compost the system (metallic film, interfacial region, substrate) are acccssi- blc which arc helpful for a mechanical approach to the adhesion mechanism. Fig. 7 shows the results obtained for 80 nm Fe/sapphire mixed with I00 kcV Ne-ions (doses 2 X lOI and 2 x lO’h ions/cm’). The relative hardnesses for the mixed samples incrcasc more strongly than those of the unmixed sample as a func- tion of the indentation depth. These curves arc charac- teristic of adherent and nonadhcrcnt systems. In the unmixed sample (nonndhcrcnt system), the interface is a bad transmitter of shear strcsscs and consequently the contribution of the substrate hardness is not as important to the recorded value. On the contrary in the mixed samples, the adhesion increases with the dose suggesting a better transmission of shear stresses

at the interface and a corresponding increase of the

relative hardness measured. Note that the influence of

the metallic film is still present in the mcasuremcnt

performed down to the higher indcnmtion depth (two

times the film thickness1 because, taking into account

the gcomctry of the indentor, the contact surface hc-

twcen the indentor and the film is not negligible. This

explains the measured values lower than that of hulk

Ai,03 cvcn at this depth.

5. Annealing effect

80 rtm Fc/AI,O, sample mixed with 1.5 MeV Xc-

ions at a dose of IO” ions/cm’ wcrc ~nnealcd in

v:tcuum ( < 10 .’ Torrf in order to limit the oxidation

of the Fc films. The pull off forces mcasurcd after

anncaliny steps for 1 h tit 30tf, 500 ;md 7WC are

reported in fig. 8. In the low tempcraturu annealing

smge ( i 3ftO’C). we ohscrve an increase of adhesion

followed by a saturation in the tcmpcrr\ture range

3OEL-500°C. The saturation value shows sn adhesion

incrcasc of a factor of 2 with rcspcct to the unanncaled

sample. From previous studies of damage recovery in

Fc implanted sapphire [IS], WC have shoun that an-

nealing in oxygen is characterized by the prcscncc of

two recovery stages and is accompimicd by complex

microstructural changes. Annealing in the tcmpcraturc

range vvherc the first recovery stage occws (below

XfWCI is characterized by the f~~rn~~ition of small

spin&type prccipitatcs and the formation of a mixed

oxide spinei. On the other hand, CEMS mcasurcmcnts

pcrformcd by OgaIe et at. [3.3] in Fc/Ai,C>, multitap-

crs mixed with krypton ions have shown that :mncaf-

ings in vacuum in the temperature range 300~600°C

Icad mainly to the oxidation of iron (Fc’ t ). Howovcr,

WC hive shown [IS] that the Fe.j+ component in im-

planted samples arises from its prcscncc in :I mixed

oxide that has a spin4 structure after the anneal. Since

the CEMS data indicated this Fe3 + component to be

present in octahedral rather than tetrahedral coordina-

tion, this spine1 phase must be a defect spine1 structure

similar to the y-Al,O, type. In iron-jmpl~ntecl sap-

phire at low tcmpcraturc (77 IQ, whcrc ~rn~~rphiz~iti(~n

of the matrix takes platc. the regrowth of the nmor-

phous Lone during ~tnncaling occurs in the scqucncc

amorphous --) y-A120, + wAI~O, [IS]. Finally in :ill

the c:rscs mcntioncd above the spin4 structure forma-

tion including Fc seems to he at the basis of the

recovery mechanisms which occur during the first an-

nealing stage in Fc-AI,O, systems. This result is con-

sistent with the adhesion enhanccmcnt ohscrvcd in our

mixed samples after thermal anncalings since the spincl

phase formahon iit the interface seems to have tl

f~vorablc effect on adhesion BS discussed in the previ-

ous section. Note that the Rutherford backscattcring-

~h~~nll~ling mc:tsurcmcnts pcrfot-mrd in iron implanted

samples fl?i] have shown that the r~~i.r~lngeIr~ents in

~ilun~inLlJ~1 and oxygen sublatticcs :tre significant :tt

tcmpcratures higher than - 500°C. Consequently the

phase prucipit:ition mechanisms mentioned ;~hove,

which can be considered to explain the thermal cvolu-

tion of obscrvcd adhesion. certainly take plrrcc in ;I

strongly dumagcd Fe/Al,O, intcrfacial joint,

6. Conclusion

C~~m~lernent~~~ rn~~r~~stru~tur~l and chemicai char-

actcrizution of Cu and Fc/AI,ff, mixed intcrf:iccs

allow :t preliminary ~~r~derst~~nd~ng of the spccihc cf-

fccts observed in mctol-ceramic systems under ion

bombardment. An important result concerns the mix-

ing mechanism at room tempcraturc which. in such

systems, occurs during the ballistic phase. The alrsencc

of radiation cnhrrnccd diffusion in the ceramic Icads to

a very murow :rnd damaged mixed interface containing

a very high concentration of metallic atom recoils. The

composition and nature of these interfacial layers which

complctcly depend on the energy deposition at the

intcrfacc which is controlled by the mixing parantctcrs

fmass of the incident ion. energy and flttcncef, is the

origin of the spectacular adhesion cnhancomont oh-

served. The rvlc of ~(~n~anl~n~~nt htpur and crystalline

nature of thu suhstratc on adhesion is also emphasized.

In the particular case of the Fc/Al,O, interface where

the CEMS tcchniquc has been applied, the formation

of iron recoils in Al,O, in the Fe” state and the

formation of the spincl phase seem to hc correlated

with the quality of the interiacial joint in term of

adhesion. Thermal annealing in the low tcmpcruturc

range (< Wt3”Cf which seems to promote the spine1

phase ft?rmatiorx, produces an additional positive cffcct

on adhesion. Since the role of nuclear elastic coliisions

during the b;illistic phase of mixing has been quantitrt-

tivcly approached in these works, the roic of electronic excitations in the insulating substrate is not elucidated

and further experiments in this contcxt would bc hclp-

ful.

Acknowledgements

Rcsuarch at Oak Ridge National Laboratory spon- sored by the Division of Materials Sciences and Assis-

tant Sccrctaty for ~[~ns~~~~~(~n and Rcncwablc En- crgy, Office of Tr~jlsp(~rt~ti~~~ Systems, as part of the Ceramic Technology for Advanced f-icat Engines Pro- ject of the Advanced Materials Dcvelopmcnt Program,

U.S. Dcpartmcnt of Energy, under contract DE-ACOS- 840R21400 with Martin Marietta Energy Systems, Inc.

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