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METAL-CERAMIC BONDING BY PULSED LASERPROCESSINGA.J. Pedraza aa Department of Materials Science and Engineering , The University of Tennessee , Knoxville,Tennessee, 37996-2200Published online: 27 Mar 2007.

To cite this article: A.J. Pedraza (1993) METAL-CERAMIC BONDING BY PULSED LASER PROCESSING, Materials and ManufacturingProcesses, 8:2, 239-257, DOI: 10.1080/10426919308934827

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MATERIALS & MANUFACTURING PROCESSES, 8(2), 239-257 (1993)

METAL-CERAMIC BONDING BYPULSED LASER PROCESSING

AJ. PedrazaDepartment of Materials Science and Engineering

The University of TennesseeKnoxville, Tennessee 37996-2200

Excimer lasera are able 10 deliver an Intense and localized enorgy to the workpiece.Materials processing w~h this type of laser can be performed at high speed, excellentreproduclbil~ and process control, and w~hout the need of high vacuum. A XeCI exclmerlaser was used to treat copper films sputter-depcslted onto sapphire and fused silicasubstrates and nickel films deposlted onto sapphire substrates. For these metal-ceramicsystems the film-substrate bond is very weak In the as-depos~ed condlflcn. It was found thatlaser processing strongly enhances the adhesion of the matallic films to the ceramicsubstrates. Melting of the film and a thin substrate layer, as well as Inlxlng of both liquids,take place during laser processing. Rapid solidffication of unmixed and the mixed liquidstake place after the laser pulse. Due to the rapid quenching (10· ·C/s) an intermediatecompound forms in the mixed region. This intermediate layer bonds film to substrate.

INTRODUCTION

The beam energy per pulse, per unit area normal to the beam (energy density)generated by an excimer laser is in the range of hundreds of millijoules. Modernlasers have microprocessors for controlling the main functions of the laser beam,including constant energy density within 5%, and can generate hundreds of pulses persecond. The initial laser energy density can be increased by focusing the pulse withan appropriate system of lenses. Energy densities of more than 10 J/cm2 can beconcentrated in small regions. The pulse duration is in the range of tens ofnanoseconds and the power density can reach hundreds of megawatts. Large areascan be scanned if the workpiece is mounted on an x-y translational stage. Excimerlasers operate in the ultraviolet range (UV) and the wavelength of the laser radiationdepends on the gas that fills the laser cavity. Table 1 gives the different UV pulsed-

239

Copyright 0 1993 by Marcel Dekker, Inc.

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240 PEDRAZA

lasers available together with the wavelength of the radiation, the pulse duration andthe energy densities that can be attained (1).

When an electromagnetic wave encounters a boundary between two media thathave different optical properties, part of it is reflected and part is transmitted. Thetransmitted part can be partially or totally absorbed depending upon the opticalproperties of the second medium. The power density (P) absorbed at time t anddepth x of a normally incident electromagnetic radiation follows the law,

P(x,t) = 1o(t) (I-R) a e-ax (1)

where 10 is the power intensity incident at time t, R is the reflectivity and a is theabsorption coefficient.

TABLE 1UV Lasers Specifications

Laser type Wavelength Energy Pulse Repetition(nm) Density Duration Rate

(mJ/cm-~ (ns) (Hz)

Fluoride 157 10 - 30 6 - 10 10 - 100

Argon 193 15 - 700 10-60 2 • 2500Fluoride

Krypton 222 20 • 200 5 - 20 10 - 140Chloride

Krypton 248 0.02 - 900 13 - 29 2 - 2500Fluoride

Xenon 308 0.02 - 800 8 - 250 2 - 2500Chloride

Nitrogen 337 0.07 - 4 0.3 - 12 20 - 100

Xenon 351 0.02 - 650 12 - 30 10 - 2500Fluoride

For laser processing the first medium is normally air or an inert gas, and bothreflectivity and the absorption coefficient are a function of the complex index ofrefraction (n') of the material target alone,

n' = n - ik (2)

(3)

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METAL·CERAMIC BONDING BY PULSED LASER PROCESSING

a = 4Jl:k/A.

241

(4)

The complex index of refraction is strongly dependent upon the nature of the targetand the wavelength, A., of the laser radiation.

The nature of the interaction between target material and radiation is differentwhether the material is a dielectric, a semiconductor or a conductor. The intenseelectromagnetic field of the laser beam interacts with dielectric materials producingeither oscillations of the electronic cloud surrounding the ions or oscillations of theions, or both (2). The laser energy is absorbed in insulators because these oscillationsare dumped by interactions with other ions or with phonons. This type of interactionis generally weak. A strong coupling between insulators and radiation occurs whenthe photon energy is higher than the band gap (3). Electrons can then be pumpedup into the conduction band.

In semiconductors two main processes can take place: (i) electrons can be excitedfrom the valence to the conduction band by photon absorption if the photon energyis larger than the bandgap energy; and (ii) electron in the conduction band, whichare equivalent to free electrons in metals, can further increase their energy by photonabsorption (4,5).

In the case of metals the electromagnetic field produces oscillations of freeelectrons that transfer their energy to the lattice by electron- phonon collisions in avery short time (10,11 to 10,12 s) (5).

Silicon has an absorption coefficient in the UV range similar to that of metalswhile the reflectivity has a maximum at around 300 nm. Photons in the UV rangehave an energy large enough to pump electrons into the conduction band. These freeelectrons are responsible for both the increase in reflectivity and in absorptioncoefficient. Excimer lasers are used as a source of local heating for processingsemiconductors materials (6).

The absorption coefficient of crystalline silica, shown in Fig.l (3), has low valuesfor wavelengths between - ISO nm and - 1250 nm. Insulators have, in general, a verylow coefficient of absorption in the visible. A pronounced increase of this coefficientis observed at a given wavelength within the UV range. For instance, natural (cubic)diamond type IIa has an absorption edge at 222 nm. If nitrogen is present as animpurity the absorption edge of diamond moves to 291 nm (7). Some insulators aretransparent to the near UV radiation and, therefore, cannot be heat treated withexcimer lasers.

The reflectivities and the absorption coefficients of aluminum and copper areplotted in Fig.2 (8,9). The value of a for metals is - HJ'i implying that 63% of theradiation is absorbed in the first 10 nm. The reflectivity of metals is very high bothin the infrared and in the visible part of the spectrum, but decreases drasticallytoward the UV range. When a solid metallic surface is irradiated with UV light upto 40 per cent of the pulse can be reflected. The remainder is absorbed almost

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242

0.8

PEDRAZA

"..-..,

J

10 sEu

'---"

10 4-+-'CQ)

10 3U'+-'+-Q)

10 20U

c10 0

-+-'.0

L.

1 0(f)

10 -0«

+-R

1Wovelenqth (microns)

0.00.1

'I-

~0.2

0.6»-,

+-'

>~0.4uQ)

Figure 1. Absorption coefficient and reflectivityof quartz at room temperature.

0'

10' i'0::..o

10. 0c:.2Ci...o

10' 21oct

10'1

10 '101

Wave length ( J:1 m)

1

IX (AI)

-~

a(eu)

R(AI) r

R(Cu)

0.2.1

0.4

II: 1.0

'::'>U 0.8..:;:..II: 0.6

Figure 2. Absorption coefficient and reflectivityof aluminum and copper at roomtemperature.

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METAL-CERAMIC BONDING BY PULSED LASER PROCESSING 243

entirely in a near-surface layer, of thickness - 2xlQ-6 em. As can be seen in Table I,power densities of the order of tens of megawatts/em!are generated byexcimer lasersand deposited in this near-surface region. The heat produced in this way must bequickly removed to avoid damage and explosive vaporization of the metal. All theheat is removed by conduction through the bulk or through a substrate, except whenvaporization or ablation take place. Heat dissipation by radiation is negligible duringthe duration of the processing.

THERMAL ANALYSIS OF THE PROCESS

Excimer laser heating can be mathematically treated as a one dimensional heatconduction problem because the irradiated area is of the order of several millimeters,while the heat-affected region is only a few micrometers thick, The heat generatedby the laser P(x,t) is included as a source term into the Fourier equation,

er a( a~pc_ - _ K_ + P(x,t)at ax ax

where p is the density, c is the specific heat and K is the thermal conductivity.

(5)

For films deposited on ceramic substrates there is, first, heat conduction acrossthe film, then heat transfer at the film/substrate interface and finally heat removalby conduction across the substrate. A computer program has been developed recentlyto calculate these heat conduction processes (10). A variable grid size was employedfor the substrate that reduces greatly the computational time. The program allowsfor phase changes (melting of the film and substrate) and also indicates whenvaporization of the film starts. The enthalpy model is used for simulating and keeptrack of the phase transformations taking place during laser processing. A Newtonianboundary condition is used for simulating the heat transfer across the metal/ceramicinterface. If the interfacial thermal resistance is set to a very low value, say 10'7(J/cmZ.·C.srl (no thermal barrier), the extrapolated temperatures at both sides of theinterface are the same within half a degree or less.

The crystalline perfection of a ceramic material in the near surface region canbe strongly perturbed by the deformation produced by mechanical polishing. Thishighly defective surface layer has a lower thermal conductivity than the undamagedbulk. Surface contamination also introduces an intermediate layer that can affect thethermal transport. These effects have been modeled byinterposing a layer of materialbetween the metallic film and the substrate. This layer may have different thermaland thermodynamic properties from either the substrate or the metallic film.

The heat generated in the film by laser energy absorption, the heat transfer fromthe film to the substrate, and the enthalpy changes of films and substrate arecalculated. Integrated values of enthalpies and generated heal.are computed at everytime-step in order to monitor the accuracy of the calculations. The increase of theenthalpy must equal the heat generated by the laser, because this is practically an

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244 PEDRAZA

adiabatic process. The enthalpy increase of the substrate must equal the heattransferred from the film to the substrate.

The time required for complete melting during laser processing of films ofcopper and stainless steel deposited on sapphire is plotted as a function of filmthickness in Fig.3. The energy density used in these calculations was 0.5 J/cm2and thewavelength of the laser radiation was 308 nm (XeCl). The liquid-SOlid interfacevelocity can be seen to tend to constant values for thicker films. For copper thesaturation interface velocity is very close to 10 mls and for stainless steel, 7 m/s.

The conductivity of 316 stainless steel at room temperature is almost 16 timessmaller than that of copper. The difference in interface velocity, however, is notcommensurate with the large difference in thermal conductivity of the two materials.The reason is that heat removal from thin films is not only limited by the thermalconductivity of the film, but also by the heat transport across the substrate. The heatextraction is thus partly controlled by the thermal conductivity of sapphire which ismuch lower than that of copper.

Thermal gradients of the order of Hf K!cm, produced during laser processing,generate high thermal stresses which tend to separate film from substrate. Thistendency arises because the metal-ceramic bonding, initially weak, might notwithstand the thermal stresses. If the film starts separating during the initial stagesof the processing the mechanical and thermal contacts with the substrate are lost,thus damaging and ultimately destroying the film. It is then very important to reducethe time required to melt the film because the thermal stresses can only act while thefilm is solid. As can be seem from Fig.3, the time required to melt the film can bereduced if the thickness of the film to be processed is reduced.

Another way of reducing the time for melting is by increasing the laser energydensity. In Fig.4 melting times are plotted as a function of film thickness for threeenergy densities. The calculations were performed for copper films deposited onsapphire and irradiated with a XeCllaser. Ten nanoseconds are required at an energydensity of 0.5 J/cm2 to melt an 80 nm-thick film, while a 200 nm-thick film can bemelted in the same time if an energy density of 1.0 J/cm2 is used.

One of the characteristics of nanoseconds laser treatments is the extremely hightemperatures that the film can attain. Using relatively low laser energy densities themetallic films can reach the boiling point and accumulate enough enthalpy so thatevaporation takes place. Fig.5 gives the laser energy density threshold for evaporationas a function of the thermal diffusivity of a film for two different thicknesses. Thelaser energy threshold is defined as the energy for evaporating 10 nm of the filmthickness. For the purpose of these calculations the thermal diffusivity of the filmwas maintained constant for the solid and the melt. The values shown in the abscissaof Fig.S refer to the solid; the thermal diffusivity in the melt was taken to be half the'corresponding diffusivity in the solid. The substrate was considered to be sapphire.For values of the film thermal diffusivity close to the thermal diffusivity of thesubstrate the laser energy evaporation threshold is essentially independent of the film

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METAL·CERAMIC BONDING BY PULSED LASER PROCESSING 245

40

0130

c:

L

o'+-

.......... Copper on Sapphire

..... 316 Stainless Steelon Sapphire

O-h-~~TT1rrr~~.",rrrrTT~"TT"'rrrrrT·rr/o 10 20 30 40

Film Thickness (*10nm)

Figure 3. Time required for complete melting of films deposited on sapphire,during laser processing as a function of film thickness. Thermal andthermodynamic constants are taken for sapphire from Ref.12, for copperfrom references 3 and 11, and for stainless steel from references 13 and14.

J ' ,I,cm,

J/,cm,J/cm

-0.50OQ~€> 0.75••••• 1.00

QJ 10

Ef=

Lo

'+-

~40

Ulc:

'-../

o -r-r-r-r-r11.,1., j"" 'i" i' iii"1 i •• "', Ii i"" iii i ".

10 20 30 40 50 60

Film Thickness (*1Onm)

Figure 4. Time required for complete melting of copper films deposited onsapphire during laser processing at three laser energy densities, as afunction of film thickness, For thermal and thermodynamic constant orcopper and sapphire see references 3, 11, and 12, respectively.

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246

0.79

---­N

Eo

':::;-0.69'-'>,.....UlC

~0.59

>,CJlC-Q)

t.5 0.49

••••• 160 nm-thick filmQ.QQQ9 80 nm-thick film

1.00

PEDRAZA

Figure 5. Laser energy density threshold for evaporation as a function of thethermal diffusivity of films deposited on sapphire for two differentthicknesses. For thermal and thermodynamic constant of sapphire seeRef.12.

Figure 6. Morphology of an 80 nm-thick copper filmdeposited on a mechanicallypolished sapphire substrate and laser irradiated in an argon-4%hydrogen atmosphere with an energy density of 0.5 J/cm2

•

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METAL-CERAMIC BONDING BY PULSED LASER PROCESSING 247

thickness. For higher values of film thermal diffusivity higher energy densities arerequired to start evaporating the thicker films.

Film evaporation should be avoided, in general, because the explosive gasevolution can damage or destroy the film. This condition limits the maximum energydensity that can be used for non-destructive laser processing of films. In addition, thiscondition and the production of high thermal stresses discussed previously stronglylimit the maximum initial film thickness that should be processed by excimer laser.We have found that once the initial metal-ceramic bonding has been enhanced fora thin film, a thicker film can be built by metal deposition followed by sequentiallaser irradiations (15).

Once a strong metal-ceramic bond is produced, thicker films can be built fromthe initial thin film by physical or chemical deposition, ego in-situ sputtering,electrolytic or electroless deposition.

METAL-CERAMIC PROCESSING

1\vo features of the excimer laser-solid interaction discussed above are relevantto the metal-ceramic bonding process. First, thin layers can be melted in areproducible and controllable manner. Second, due to the very short pulse durationthe heating and quenching cycle has a very short duration as well and, as aconsequence, thermally activated process in the solid state (e.g, diffusion and phasetransformations) are inhibited.

A copper film in the liquid state is not stable when deposited on a sapphiresubstrate. A surface tension analysis indicates that copper should break into smalldroplets having a contact angle with the substrate of 100·(16). However, theprocessing time during laser irradiation is so short that the break-up of the liquidfilm can be avoided. The following example illustrates this point.

An 80 nm-thick copper film was sputter-deposited on a mechanically polishedsapphire substrate and then laser treated with a XeCI excimer laser using an energydensity of 0.5 J/cmz. This and all the other sputter-deposited films were uniform,mimicking the substrate surface. Except for substrate scratches the surface of the as­deposited films was smooth. Fig.6 shows the morphology of the film after laserirradiation. It can be seen that the originally uniform film has been collected intosmall particles or islands. Fig.7 is a profilometry of a region in the same sample. Thethickness-to-width ratio of these particles is 0.1. Assuming an equilibrium wettingangle of 1000 the height of a particle should be larger than its radius. The calculatedthickness-to-width ratio is close to 0.6, which is significantly larger than the averagemeasured ratio of 0.1. This result is consistent with the far-from-equilibriumconditions that exist during laser processing, which limit melt motion. By sequentialdeposition and laser irradiation the islands grow laterally and a uniform film can bebuilt.

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248

200.-------------------,

PEDRAZA

t60

"..... t20§'-'-.c:!lIl'Qj 80:x:

40

toDistance (urn)

20

Figure 7. Profilometry of a region of the film shown in Figure 6.

•

•

Figure 8. Morphology of an 80 nm-thick copper film deposited on an annealedsapphire substrate and laser irradiated in an argon-4% hydrogenatmosphere with an energy density of 0.5 J/cm2

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METAL-CERAMIC BONDING BY PULSED LASER PROCESSING 249

Fig.8 is an optical micrograph from an 80 nm-thick film deposited on anannealed sapphire substrate and laser irradiated at an energy density of 0.6 J/cm2•

The substrate was annealed at 13500C for two days. The film remains intact, havingthe same appearance as as-deposited, if irradiated at an energy density not higherthan 0.7 J/cm'l. as shown in the micrograph. The dramatic difference between theprevious experiment and that described above is due to different substrate conditions.Channeling, using Rutherford back scattering (RBS) and scanning electronmicroscopy, was performed on annealed and mechanically polished sapphire. It wasfound that the mechanically polished substrates have a heavilydamaged region thatpartially recovers after prolonged annealing. This region, which is RBS-amorphous,extends to a depth of 100 nm after which a region still damaged follows (17).

ADHESION ENHANCEMENT

Copper films deposited on sapphire and fused silica were pulse-laser irradiatedand tested for adhesion. Also nickel films deposited on sapphire were laser treatedand tested for adhesion. These metal-ceramic samples were selected because of thevery weak bond between film and substrate. Tape, scratch and pull tests were usedfor measuring the increase in bond strength.

All the as-deposited metallic films studied were easily removed from theirsubstrates by the tape test. After suitable laser irradiations the films remained intactand attached to the ceramic, even after many tape test passes. A much more severetest was done with a microhardness tester. Fig.9a shows a 300 nm-thick as-depositedfilm of copper on sapphire with a scratch performed using a Vicker stylus under a10 g load on a manual stage. The same load was used to scratch a 480 rim-thicklaser-treated copper film on sapphire (Fig.9b). This 480 nm-thick film was grown byalternating sputter deposition of copper with laser treatment in air in a sequentialmanner (15). The initial laser energy density was 0.5 J/cm2

, and towards the end ofthe sequence it was 1.1 J/cm2

• The as-deposited film is completely detached from thesubstrate in the region were the sharp stylus passed, as can be seen in Fig.9a. Thephotographs were taken in both reflection and transmission modes. In regions wherethe film is completely separated from the substrate the transmitted light goes throughproducing the bright spots seen in Fig.lOa. On the other hand, as can be seen inFig.9b, the laser treated copper film shows no peeling at all (15).

A scratch tester that has a balanced lever arm with a diamond stylus of 0.2 mmradius and a load platform above the stylus was used for adhesion testing. During thetest, the stage with the specimen had a translational motion at a constant speed of27 /Jm/s. A strain gage rigidly attached to the load platform measured the appliedtangential forces. A profilometer was used to measure the scratch profile. Table 2shows the normal forces measured in an as-deposited 300 nm-thick copper film andin laser treated 300 nm-thick and 1080 nrn-thick copper films (IS). All these filmswere deposited on sapphire substrates. The shear stresses calculated using the modeldeveloped by Benjamin and Weaver (18), are also shown in Table 2. After applyingtwice the normal force required for de-adhesion of the as-received films there is noobservable detachment of those that were laser treated.

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250 PEDRAZA

Figure 9. (a) Micrograph of a scratch performed using a microhardness tester anda Vicker stylus under a 10 g load, on an as-deposited 300 nm-thickcopper film on sapphire, (b) Micrograph of a scratch performed underidentical conditions as in a) on a 480 nm-thick copper film on sapphirelaser-treated in air. The photographs were taken with the microscopeillumination in transmission and reflection modes. From Ref.l5.

TABLE 2Normal Forces During Scratch Tests of Copper Films on Sapphire

Film Thickness Normal Shear Comments(nm) Force (N) Stress

(GPa)

LT I 1080 3.92" 0.38 No de-adhesion

LT2 300 3.92" 0.65 No de-adhesion

As-deposited 300 0.98 0.21 No de-adhesion1.96 0.38 De-adhesion

" Higher Loads are beyond the limits of the tester.I Processed in air; 2 Processed in an argon-4% hydrogen atmosphere

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METAL·CERAMIC BONDING BY PULSED LASER PROCESSING 251

Figure 10. Photograph of a fused silica substrate where rectangular regions of laserirradiated copper remain. The original 80 nm-thick copper filmdeposited on fused silica and irradiated with energy densities rangingfrom 0.1 to 2.6 J/cm2 was submitted to a 10 minutes grinding operation.The atmosphere used was argon-4% hydrogen.

The following example illustrates the strong bonding that develops after lasertreatment of copper films deposited on fused silica. An 80 nm-thick copper film wasdeposited on a fused silica plate and laser irradiated in different regions using energydensities ranging from 0.1 to 2.6 J/cm2

• The film was next faced to a paper towel andtenaciously rubbed against it for more than 10 minutes, using acetone for moreeffective removal of the film. Fig.10 is a photograph of the substrate; the squareregions where copper remains, are laser-irradiated areas. The unirradiated parts ofthe film were easily removed, while for energy densities of 0.1 J/cm2 to 0.4 J/cm2 thecopper was partially or completely removed. Grinding is certainly not a very orthodoxadhesion test but judging from the force exerted during the 10 minutes operation, theadhesion of properly laser-irradiated copper to fused silica is quite strong.

Figure lla shows a series of scratches made on an as-deposited 300 nm-thicknickel film deposited on sapphire (19). The same set of scratches were performed ona laser treated 300 nm-thick nickel film deposited on sapphire (Fig.llb) (19). Ascratch test apparatus with normal loads in the range from 100 g to 400 g was used

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252 PEDRAZA

Figure 11. (a) Micrographs of scratches made with a scratch test apparatus with100, 200, 300 and 400 g loads respectivelyon as deposited 300 nm-thicknickel film on sapphire, (b) Same series of scratches performed on a 300nm-thick nickel film deposited on sapphire and laser-treated in air.From Ref.19.

1.0(a)

0.5E:i.

0.0

- O. 5 f=1==;==;===;==;===;====;:====!,

o 20 40~m

60 20 40~m

60

Figure 12. (a) 400 g scratch profile on an as deposited 300 nm-thick nickel film onsapphire, (b) 400 g scratch profile on a laser treated (in air) 300 nm­thick nickel film on sapphire. From Ref.19.

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METAL·CERAMIC BONDING BY PULSED LASER PROCESSING 253

for these experiments. A pronounced rippling is detected in the as-deposited film forall four scratches. This effect is almost not observed in the irradiated samples. Fig.12shows the profiles of the 400 g scratches in the as-deposited and in the laser treatednickel films (19). The separation of the film from the substrate in the case of as­deposited films is detected in the profile as large bulging regions at both sides of thegroove. By contrast, in the laser irradiated films the material has been displaced byplastic deformation.

MECHANISMS OF LASER-ENHANCED ADHESION

In analyzing the nature of bonding between substrate and metal atoms, adistinction must be made between true chemical bonding and other mechanisms thatdo not involve the transfer or sharing of electrons between individual atoms. Theexperimental signature of a chemical bond is a change in the electronic structure ofthe atoms at the metal-ceramic interface.

Ohuchi, French and Kasowski (20) and Ohuchi (21) used x-ray photoelectronspectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) to study theinterfacial reactions produced when copper is deposited on aluminum oxide andaluminum nitride. The aluminum oxide and the aluminum nitride were produced inthe deposition chamber. UPS was used to study the shift of the Cu (3d) orbital peakwith respect to that of the bulk copper, as a function of substrate coverage. In thecase of an AlZ0 3substrate, that shift was small and quickly tended toward the valuefor bulk copper. By contrast, for aluminum nitride the copper peak was shifted by2 eV, indicating a strong binding of copper to AlN at submonolayer coverage.

Johnson and Pepper (22) performed calculations using a molecular orbital modelof the interaction between an alumina cluster and metallic atoms (Fe, Ni, Cu andAg) to represent the direct metal-Al.O, bonding. It was found that a chemical bondcould be established at the metal-sapphire interface. Bonding and antibondingmolecular orbitals between the metal (d) states and the oxygen (p) states, as well asa metal-to-oxygen transfer, were found as indications of true chemical bonds. Theyalso found an increase in the occupancy of the metal-sapphire antibonding orbitalsthrough the series Fe, Ni, Cu and Ag. The occupancy of the antibonding orbitalstends to cancel the effect of bonding orbital occupancy, thus decreasing the metal­sapphire bond strength. The chemical bonding between Cu and sapphire was thusfound to be very weak due to a small charge transfer, in agreement with theexperimental results described above (20,21).

The adhesion of films to substrates can be enhanced by ion (23) or electron (24)irradiation. Baglin has comprehensively reviewed the effects of ion beams on thinfilms adhesion (25), but the mechanisms of adhesion enhancement by panicleirradiation are not completely understood. Extensive mixing in a collision cascadeseems not to take place in immiscible systems such as Cu on AlP3 (26). Ionbombardment, however, seems to create chemical bonds and produce local atomic

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254 PEDRAZA

rearrangements at the interface level by generating a more-or-less uniforminterlocking of the two crystalline structures (26).

Godbole, Pedraza, Lowndes and Kenik (27) studied the copper-sapphire interfacein a copper film deposited on sapphire and laser irradiated next. During laserprocessing not only the film but also a thin layer of sapphire melted. Epitaxialregrowth of the alumina with misoriented crystallites occurred in a region adjacentto the unmodified substrate. At the metal-film interface alumina reacted with thecopper to form a compound. The nature of that intermediate compound was foundto depend on the atmosphere used during laser treatment. Laser processing in airproduces a compound that has a trirutile-Iike crystal structure which is either oxygenor copper deficient. In argon-4% hydrogen atmosphere the compound has anhexagonal structure, very close to that of sapphire. Structure factor calculationsshowed that in this structure some aluminum had been replaced by copper, either atrandom or layer by layer. The presence of copper in this sapphire-like intermediatecompound has also been detected by energy dispersive x-rayspectrometry. It was thusconcluded that in copper-sapphire couples laser-enhanced adhesion is mediated byan intermediate compound.

Excimer laser irradiation, as ion bombardment, is an extremely non-equilibriumprocess. Each ion when moving through the lattice generates a cascade of atomicdisplacements in a very localized region where the lattice is highly excited. The timerequired to transfer the energy deposited in the cascade to the surrounding crystal,is of the order of 10.12 to 10,11 s (28). By contrast, each laser pulse produces a heatpulse that lasts several tens of nanoseconds. Due to the longer duration of theinteraction of the beam with the crystal, laser irradiation promotes chemical bondingon a wider spatial scale than ion bombardment, the latter being restricted to theinterface. The formation of an intermediate compound in the case of laser irradiationcan be understood on the basis of the duration of the laser-material interaction aswell as the type of interaction. Mixing of liquid copper and liquid sapphire takesplace during laser processing. Immediately after irradiation a very rapid quenchingproduces the solidification of the mixed liquid into the intermediate compound.

CONCLUSIONS

1. The adhesion of weakly bonded copper films to sapphire and fused silica and ofnickel films to sapphire can be strongly enhanced by excimer laser irradiation.Adhesion enhancement of copper films on sapphire was measured whether laserprocessing was done in air or in an argon-4% hydrogen atmosphere.

2. Excimer laser processing offers the following advantages:i) High processing speeds. Modern lasers allow to scan large areas in a very

short time, eg, 25 em by 25 cm per second.ii) Atmosphere. Vacuum is not required. An inert atmosphere is enough for

maintaining a bright metallic surface during processing.

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METAL·CERAMIC BONDING BY PULSED LASER PROCESSING 255

iii) Very localized treatments. Regions in the range of microns can be treatedby focusing the beam with lenses or by masking the surface to be treated.

iv) Excellent reproducibility and process control. The laser energy density canbe maintained constant within a few percents.

v) No heat affected region. Diffusion in the solid state is negligible and phasetransformations in the solid state during laser processing do not take place.

vi) Amenable to computer control.

3. The process has the following requirements:i) Film thickness. Only thin films can be initially bonded to the substrate.ii) Process control. The laser parameters need to be carefully determined and

closely controlled.

4. Adhesion is due to the presence of an intermediate compound. During laserprocessing, melting of the film and of a thin layer of the substrate as well asmixing of the two melts occur. Rapid solidification of the mixed liquids, at thefast quenching rate immediately after the laser pulse is over, produces thisintermediate layer that bonds film to substrate.

ACKNOWLEDGEMENTS

The author wishes to thank Dr. M.J. Godbole, University of Tennessee, for hisefforts in the present research project. The continuing collaboration of Dr. D.H.Lowndes, Oak Ridge National Laboratory, is gratefully acknowledged.

REFERENCES

1. Laser Focus World. The Buyers Guide, PennWeJl Publishing Co., Tulsa, p.98,(1991).

2. Saifi, M. A, in Lasers in Industry, edited by S.S. Charschan, Van NostrandReinhold Co., New York, p.1H, (1972)

3. von Allmen, M. E, in Physical Processes in Laser-Materials Interactions, editedby M. Bertolotti, Plenun Press, New York, p.57, (1983).

4. von Allmen, M.E, in Laser and Electron Beam Processing of Materials, editedby C.W. White, and P.O. Peercy, Academic Press, New York, p.6, (1980).

5. Brown, W.L., in Laser and Electron Beam Processing of Materials, edited byC.W. White, and P.O. Peercy, Academic Press, New York, p.20, (1980).

Dow

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ded

by [

Thu

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rsity

& L

ande

sbib

lioth

ek]

at 1

2:41

13

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embe

r 20

14

256 PEDRAZA

6. Lowndes, D.H., J.W. Cleland, W.H. Christie, RE. Eby, G.E. Jellison,Jr., J.Narayan, RD. Westbrook and RF. Wood, Materials Research SymposiumProceedings, Vo1.13, p.407 (1983).

7. Edwards, D.E, and H.R Philipp, in Handbook of Optical Constants of Solids,edited by E.D. Palik, Academic Press, p.665, (1985).

8. American Institute of Physics Handbook, 3rd ed. McGraw-Hill p.6-1, (1972).

9. Smith, D. Y., E. Shiles, and M. Inokuti, in Handbook of Optical Constants ofSolids, edited by E.D. Palik, Academic Press, p.369, (1985).

10. Pedraza, AJ., in Laser Materials Processing III, ed by J. Mazumder, and K.N.Mukherjee, 183, (1989).

11. Coates, RB., and J.W. Andrews, Journal of Physics E: Metals Physics, Vol.8,No.2, (1978).

12. Wefers, K., and G.M. Bell, Technical Paper W19, Alcoa Research Laboratory,(1972).

13. Metals Handbook, Ninth Edition, Vol.3, American Society for Metals, (1979).

14. Touloukian, Y.S., and D.P. DeWitt, Thermal Radiative Properties. MetallicElements and Alloys, in Thermophysical Properties of Matter, Vol.7, (1970).

15. Pedraza, A J., M.J. Godbole, D.H. Lowndes, and J.R Thompson, Journal ofVacuum Science and Technology A, Vol.6, No.3, p.1763, (1988).

16. Kingery, W. D., Journal of the American Ceramics Society, Vol.37, p.18, (1954).

17. Pedraza, AJ., M. J. Godbole, Symposium on Interfaces in Materials, 1991 FallMeeting of the Materials Research Society (in press), (1992).

18. Benjamin, P., and C. Weaver, Proceedings of the Royal Society (London), A254,p.163, (1960).

19. Godbole, M.J., Adhesion Enhancement of Metal-Ceramic Couples using PulsedExcimer Laser, Ph.D. Thesis, The University of Tennessee, Knoxville, (1989).

20. Ohuchi, ES., RH. French, and R.V. Kasowski, Journal of Vacuum Science andTechnology, Vol.A5, No.4, p.1175, (1987).

21. Ohuchi, E S., Journal de Physique, Colloque 10, Sup.lO, p.783, (1988).

22. Johnson, K.H., and S.V. Pepper, Journal of Applied Physics, Vol.53, No.10,p.6634, (1982).

Dow

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at 1

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23. TombrelIo, T./\., Materials Research Society Symposium Proceedings, Vo1.25,p.173, (1984).

24. Michell, LV.,J.8. Williams, P. Smith, and R.G. Elliman, Applied Physics Letters,p.193, (1984).

25. Baglin, J.E.E., in Ion Beam Modification of Insulators, ed, P. Mazzoldi, andG.W. Arnold, Elsevier, p.585, (1987).

26. Baglin, J.E.E., AG. Schrott, R.D. Thompson, K.N. Tu, and A SegmulIer,Nuclear Instruments and Methods, VoI.B19/20, p.782, (1987).

27. Godbole, M.J., AJ. Pedraza, D.H. Lowndes, and E.A Kenik, Journal ofMaterials Research, Vol.4, p.1202, (1989).

28. Doran, D.G., and J.O. Schiffgens, Proceedings of the Workshop on Correlationof Neutron and Charged Particle Damage, in ORNL Report CONF-760673, OakRidge National Laboratory, Oak Ridge, TN, p.3, (1976).

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