The Influence of Thermal Treatments on the Adhesion

8/3/2019 The Influence of Thermal Treatments on the Adhesion

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T h e I n flu e n c e o f T h e r m a l T r e a tm e n t s o n th e A d h e s i o no f E l e c tr o le s s ly D e p o s i te d N i (P ) L a y e r s

o n A l u m i n a C e r a m i cJ . W . S e v e r i n , R . H o k k e , H . v a n d e r W e l , a n d G . d e w i t h

P h i l i p s R e s e a r c h L a b o r a t o r i e s , P r o f. H o l s t l a a n 4 , 56 5 6 A A E i n d h o v e n , T h e N e t h e r l a n d sABSTRACT

The adhesion of electrolessly deposited Ni(P) on 96 and 99.5% pure alumina was studied as a function of annealingtemperatu re, up to 580~ The adhesio n was measured with the direct pull-off test and the peel test. The interfa ce structurewas analyzed with cross-section transmission electron micrographs. Fracture surfaces were analyzed with scanning elec-tron microscop y/energy dispersive x-ra y analysis, static-se condar y ion mass spectroscopy, and x-r ay photoelec tron spec-troscopy. The optimum anne ali ng tempe ratu re was found to be 400~ at which an increase in peel energy and adhesio nstrength by a factor of two to three was measure d, with respect to the as-deposite d value. The weak bou nda ry layer, whichwas previously reported to be present in this system, is still present after a nneali ng and the fracture p ath remains throughthis interra cial layer of a few nanom eter s in thickness. Therefore, the adh esion impro veme nt is ascribed to strongercohesion of the material in the weak boundary layer.

Electroless metal lizat ion of oxidic surfaces is frequen tlyused for electronic applications. The thermal behavior ofthe metal-ce ramic interface is of great importance for theseapplications. Thermal shocks occur with soldering andtherm al cycling is a standa rd test procedure for most elec-tronic parts. Ret ention of strong adhesio n is require d sincedifferences in thermal expansion, for example betweenelectronic components and the printed-circ uit board or be-tween metal layers and substrates cause mechanicalstresses. Interra cial fracture may rapi dly lead to electronicfailure.Ni(P) denotes the amorphous, nonsto ichiom etric Ni andP con tain ing alloy which is formed by electroless deposi-tion from an Ni bath containing a hypophosphite reducingagent. Depend ing on the pH value of the bath, the P c onten tmay vary between 3 and 13 weight percent (w/o). In ourcase the P co ntent is 10 w/o. In Ref. 1 and 2 backgro unds ofthe electroless metal liza tion process and properties of thedeposits are described. Literature data of the adhesion ofelectrolessly deposited Ni(P) layers on alumina ceramichave b een reviewed elsewhere. 3Generally, the adhesion strength of metal layers on oxi-dic substrates increases with anneal ing temperature2 "5 Ananneal ing treatment after deposition might therefore be asimple method for impro ving the adhesion of electrolesslydeposited Ni(P) layers. However, for Ni(P) on 96% purealumin a diverging results on the effect of temperature u ponadhesion have been reported, as measured with the directpull-off (DPO) test. Honma and Mizushima6 found an in-crease in adhesion strength with anne aling time and an-neal ing temperature. The greatest effect, an increase of afactor of three to four, was found after a nne ali ng for 1 h at250~ in air. In contrast, in a later public ation than Ref. 6Honma and Kanemitsu7 did not measure significantchanges in the adhesion strengt h after ann eal ing at 250~in air for between 0.5 and 24 h, with respect to the as-de-posited value. In addition, Osaka e t a l . 8 did not find signif-icant differences between the adhesion strengths beforeand after a nneali ng for 1 h in vacuum at temperatures of300 and 500~ Since these lite ratur e data do not allow adefinitive conclusion to be drawn on the influence of ther-mal treatment s upon the adhesion, more insight into thismatter is required.On the basis of adhesion stren gth data only, as measuredby the direct pull-off (DPO) test in the references citedabove, it is very difficult to explain changes in the adhe-sion. According to the Gr iffit h-Irwi n theory, the adhes ionstrength is determined on the one hand by interracia l inter-actions on a molec ular scale (intrinsic adhesion) and on theother ha nd by the size of interracia l flaws due to, for exam-ple, pores or foreign particles. Strength is thus a hybrid8 1 6

quantity. See Ref. 9 an d 10 for a gene ral discussion on thismatter. For that reason not only adhesion strength mea-surements but also fracture energy measurements wereperformed i n this study, which provide inform atio n on theintrinsic adhesion. Moreover, various interfacial analyseswere carried out in order to obtain more information onchanges of the intrinsic adhesion with anne aling tempera-ture. The approach for both the mechanical and the inte rfa-cial analyses was si milar as described in Ref. 11.In previous investigations,11'12 cross- secti onal tra nsm is-sion electron microscopy (TEM) m icrographs revealed thepresence of an interfacial layer between the alu mina sub-strate and the Ni(P) layer for as-deposited samples. Thethickness of this interracial layer was a few nanometers.Static secondary ion mass spectroscopy (static-SIMS) andx-ray photoelectron spectroscopy (XPS) analyses of thefracture surfaces showed that this interracial layer mainlyconsisted of remai ning components of the metallization so-lution (Ni PO3x , Na C1 , an d orga nic co mpl ex ing agen ts

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J . E l e c t r o c h e m . S o c . , V o l . 1 4 1 , N o . 3 , M a r c h 1 9 9 4 9 T h e E l e c t r o c h e m i c a l S o c i e t y , I nc .


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J . E l ec t rochem. S oc . , Vol . 141, No. 3 , March 1994 9 The Electrochem ical Society, Inc. 817

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818 J . E l ec t rochem. S oc . , Vol . 141 , No. 3 , March 1994 9 The E l e c t r o c h e m i c a l S o c i e t y , Inc.

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F i g . 3 . D i r e c t p u l l - o f f a d h e s i o n s t r e n g t h s (~ T f)VS . a n n e a l in g t a m p e r -a t a r e f o r s a m p l e s w i t h 2 t~ m e le c t r a le s s N i (P ) o n l y . T h e s y m b o l s 9a n d r e p re s e n t s a m p l e s w i th s m o o t h - t y p e a n d r a u g h - t y p e s u b -s t r a t e s , r e s p e c t i v e l y .

surements with samples with smooth-type and rough-typesubstrates, adhesion improvement with temperature wasfound for this sample with the rough-type substrate. Theinitial decrease in peel energy was not observed for thissample with this ann eali ng procedure. For Fig. 2 the meas-urement of peel energy vs. anneali ng temperature for sam-ples with rough-type substrates, with square symbols inFig. 1, was repeated. Furthermore, the relative decreaseiG p is shown which was found after a drop of water hadbeen placed at the peel front. This relative decrease is sig-nificantly higher for samples annealed at tempreaturesabove 250~ tha n for samples anne ale d at lower tempe ra-tures. These results will be discussed in the subsecti on onmechanical behavior in the Discussion.Since plastic deformation may contribute to peel energyvalues, it was necessary to estimate rela tive changes in theyield strength of the metal layer due to the thermal treat-ments. This was done by hardness measureme nts using theVickers test. With an ind ent ati on load of 0.01N, Vickershardness values of 400, 130, and 100 MPa were measuredfor samples anne aled at 150,250, and 450~ respectively.This means that a decreasing trend in the hardness of themetal layer was observed with increasing temperature. Toeliminate the influence of changing mechanical propertiesof the electrodeposited nickel layer, the DPO tests wereperformed with samples with only electrolessly depositednickel.D P O t e s t s . - - S i n c e electrodeposited Ni had not been ap-plied for the DPO test samples, a muc h thicker electrotesslydeposited Ni(P) layer was applied wit h a layer thickn ess of2 _+ 0.4 ~m. The DPO str ength vs . anneal ing temperature isplotted in Fig. 3 for Samples with the rough-type and thesmooth-type substrates. The numb ers of test samples andthe standard deviations are given in Table I. The sampleswere annealed for 1 h in an argon atmosphere at the toptemperature indicated in the figure. Since pull-studs werebond ed with an epoxy adhesive at 150~ for the DPO test,DPO strengt h values of the as-de posited sample could not

be obtained. At temperatures of 250~ or lower no system-atic trend was observed in the adhesi on strength, but at 300and 400~ a two- to three-fold increase in the adhesionstrength was clearly seen. For both substrate types a re-marka ble decrease in the DPO strength was measured afteranneal ing at 500~The XRD pattern of the DPO samples with rough-typeand smooth-type substrates annealed at temperatureslower than 400~ only showed a broad ba nd due to anamorphous Ni(P) phase, apa rt from peaks o rigin ating fromthe alumina substrates. The samples annealed at 400~gave rise to a n Ni3P diffraction pattern. In addit ion, a smallcontribu tion from amorphous material was still observed.The XRD pattern s of samples anne aled at 500~ showedpeaks characte ristic of Ni3P and Ni phases. With these sam-ples an ind ication of the presence of an amorphous phasewas not visible in the XRD patterns anymore.

I n t e r fa c e a n d f r a c t u r e s u rf a c e s t r u c t u r e . - - S E M / E D X . - -In order to explain the large differences in peel energiesbetwe en the two subst rate types, cross sections were madeof the metal-ceramic interfaces. Optical micrographs ofthese cross sections, shown in Fig. 4, show the pen etra tionof the metal layer into the surface pores of the rough-typesubstrates. This type of roughness with narrow structuresand cavities is difficult to measure with, for example, astep-profiler. It is obviou s that this roughness gives rise toa much stronger adhesion due to mechanical interlockingthan on the smooth-type substrate surface shown inFig. 4B, where such inte rlock ing structures are n ot present.The SEM micrographs of rough-type al umina fracture sur-faces of samples anneale d at 150 and 450~ shown inFig. 5, reveal a larger d ensity of rema ini ng metal pa rticleson the sample annealed at the higher temperature. On thesmooth-type alumina fracture surfaces (not shown), suchremaining metal particles were not found for samples an-nea led at 150, 320, and 450~ With EDX, nevert heless, avery small Ni signal was observed for the sample annealeda t 320~ and a stronger Ni signal for that ann eale d at450~ When a relative ly large area of about 50 ~m diamwas scanned with the electron beam, the same intensitieswere found as when a small area of about i ~m on a smoothalumina grain surface was irradiated. This means that avery thin, Ni cont aining layer is present all over the alu-mina fracture surfaces of smooth-type samples annealed at320 and 450~ Ni was not detected with EDX on the sur-face of the sample anne aled at 150~C r o s s - s e c t i o n a l T E M . - - T h e TEM images shown in Fig. 6provide information on the material structure bo th at theinterface and in the bulk of the metal layers after anneali ngat 150 and 580~ in vacuum. The colu mnar structure of theas-deposited Ni(P) mater ial (Fig. 6A) has completely disap-peared after an nea lin g at 150~ (Fig. 6B) and 580~(Fig. 6C). Instead , m icrocrys talline particles are observed,and extensive microcracking has tak en place all over theNi(P) layer an d in a ll direc tion s (Fig. 6C). The size of thesemicrocrystats is too small to give rise to a cry stalli ne-ty peXRD patte rn. On top of the microcrysta lline Ni(P) layer, Nicrystals are visible from the e lectrodeposited Ni layer. Nocracks a long the metal -ceram ic interfa%e are observed. Theinterracial layer which is observed for the low-temperat uresample remain s present after anne ali ng (Fig. 6D). The con-

T a b l e I . M e a n a d h e s i o n s t re n g t h cr~ o f e le c t r o le s s N i ( P ) o n r o u g h - a n d s m o o t h - ty p e a l u m i n a c e r a m i c a s a f u n c t io n o f a n n e a l in gt e m p e r a t u r e T a s m e a s u r e d b y t h e D P O t e s t. N is th e n u m b e r o f te s t s a m p l e s a n d s x i s th e s t a n d a r d d e v i a ti o n i n t h e m e a n .Rough-type substrates Smooth-type substrates

O-f 8 x O-f 8 zT (~ N (MPa) (MPa) N (MPa) (MPa)150 21 16.4 0.9 20 28.2 3.3200 21 25.0 1.8 21 19.1 1.4250 19 20.2 1.6 19 19.2 2.2300 19 44.8 2.3 19 39.9 5.0400 21 51.6 2.3 21 53.1 2.9500 21 27.4 1.8 20 17.9 3.4


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F i g . 4 . O p t ic a l m i c ro g r a p h s o f c r o s s s e c t i o n s o f s a m p l e s w i t h r o u g h - t y p e s u b s l ra t e ( A , l e f t) a n d s m o o t h - t y p e s u b s t r a t e (B , r ig h t ) .

F i g . 5 . S E M m i c ro g r a p h s o f ro u g h - t y p e a l u m i n a f ra c t u r e s u r f a c e s f ro m s a m p l e s a n n e a l e d a t 1 5 0 ( A , l e f t ) a n d 4 50 ~ ( B, r ig h t ) . T h e s a m p l ea n n e a l e d a t t h e h i g h e r t e m p e r a t u re s h o w s m o r e r e m a i n i n g m e t a l p a r ti c le s o n t h e s u b s t ra t e s u r fa c e . S o m e o f t h e m e t a l p a r ti c le s a r e i n d i c a te dw i th a r ro w s .

trast between the interracial layer and the neighboringphases is much weaker for the annea led sample than for theas-deposited sample. This may be an indic ation that thedensity of the interracial layer increases upon annealing.XPS fracture surface analyses.--With XPS the fracturesurfaces were analyzed of samples with rough-type sub-strates, annea led at 150 and 450~ in vacuum. The peelenergy values of these samples are gi ven in Fig. 1 (squaresymbols). The SEM micrographs of the alumina fracturesurfaces of these samples are depicted in Fig. 5A and B. Forthe XPS analyses fresh fracture surfaces were prepared bypeeling a small par t of the film in a glove box filled with Nf,with less than 0.2 ppm O2 and H20. After peeling, the Ni(P)and alumina fracture surfaces were transferred in a vac-uum-ti ght vessel into the XPS apparatus. The surface com-positions of the Ni(P) and a lum ina fracture surfaces of bothsamples are listed i n Table II. The relative accuracy of theXPS relativ e coverages is wit hin 10%. The spot area d urin gthe XPS measureme nt was ca. 2 mm 2, which m eans t hat theresults are not influenced by inhomogeneities with the sizeof a few micrometers.All surfaces show a similar coverage with C, which isprobably at least partly due to organic contaminations inthe XPS ap paratus or during handling. For that reason, the

coverage with this element will not be discussed further.More rema rkab le is the relatively high coverage of the alu -mina fracture surfaces with Ni. For both anneal ing emper-atures the in tensity of the Ni signal is in the same range asthat from A1 from the substra te. After a nne ali ng at 150~the Ni/A1 ratio is 1.05 and this changes sligh tly to 1.25 uponann eal ing at 450~ The P coverage on alumin a is consider-ably higher after annea ling at the higher temperature. This

is also the case for the P coverage on the Ni(P) fracturesurface after ann ealing at the higher temperature. Thispoints to enrichment of the interface with P, originatingfrom the Ni(P) bulk. The oxygen content on the aluminasurface does not differ for the two temperatures and isprobably determined by the oxidic bulk. The nucleationmaterial remains almost entirely on the Ni(P) fracture sur-face a nd the coverage is lower after anne aling at the highertemperature . Apa rt from P, S also tends to segregate to theinterface at higher temperatures as observed on the Ni(P)fractu re surface. This element prob ably origin ates from theSO~- ions in the electroless metallization solution. Anotherinteresting observation is the significantly lower amount ofO after annealing at 450~ Al was not detected on the Ni(P)side, which means that few or no alumina gra ins are de-tached from the substrate surface during peeling.An assignment of the peak s of the elements listed inTable II to compounds, ions , or molecules with relativeamounts, obtained by multisean m easurements, is listed inTable III. The relative coverages are given in atom percent(a/o). Reference data are used from R ef. 13.Ni and P which are present on the alumina fracture sur-face after peeling are entirely (Ni) or for the greater part (P)in the oxidized state, for both annealing temperatures. Onthe Ni(P) fracture surface Ni and P are for a greater p art inthe metallic state after annealing at 450~ than after an-nealing at 150~ At the lower temperature the ratios Nin+/Ni~and pn+/p0 are 3.8 and 2.3 while at the h igher tem pera-ture the se ratios are 1.4 and 1.5, respectively. Nickel-carbon compounds were not formed at either temperature.

Static-SIMS measurements.--The alumina and nickelfracture surfaces of the annealed and as-prepared samples


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F i g . 6 . T E M c r o s s s e c t i o n i m a g e s o f s a m p l e s w i t h s m o o t h - t ~ e s u b s t r a le s . ( A , t o p l e f t) a s - d e p o s i l~ l ; ( B, t o p r i g h t ) a f t e r a n n e a l in g a t 1 50~( C, b o t t o m l e f t ) a f t e r a n n e a l i n g a t 58 0~ a n d ( D , b o t t o m r i g h t ) a s C.were analyzed with static-SIMS. Since peeling was doneinside the static-SIMS apparatus, only samples with lowpeel energy and therefore only samples with smooth-typesubstrates could be analyzed in this experiment. Thechange in relative intensities of various inorganic frag-ments in the static-SIMS spectra contained interesting in-formation on the change in the composition of the fracturesurfaces. An overview of the most imp orta nt results for thealu min a fracture surfaces is given in Table IV. The inten si-ties are normalized to the most intense peak from the sub-strate, which is A1 for the positive ion spectra and O- forthe negat ive ion spectra. The inten sity ratios listed inTable IV are obtai ned from two different position s on thefracture surfaces. For each measure ment the analyze d areais about 2000 ~m2. The spread in results represents thespread in surface composition. The accuracy of the relativestatic-SIMS int ensitie s is on the order of 10%.The positive and negative ion spectra of the aluminafracture surfaces of an as-prepared sample and a sampleann eal ed at 450~ are shown in Fig. 7. The ~SNi+/A] P O JO-, and PO JO - intensity ratios in Table IV show a stronglyincrea sing coverage of Ni and P con tain ing compound swith increasing anneal ing emperature. The nucleation ele-ments Sn, Ag, and Pd were also detected on the nickel frac-ture surface but the signal intensi ty of these elements wastoo weak for si gnific ant changes in relative coverages to beobserved. The relative coverage of F decreases with in-creasing temperature while increasing relative intensitiesare observed for C1 and Na. This may be associated eitherwith diffusion and segregati on or with a different fracturepath, a point which w ill be discussed in greater detail in thefollowing section.

D i s c u s s i o nMechanical behavior.--Energy balance.--During peel-ing, energy is consume d by fract ure (G~) and possibly by

bul k plas tic def orm ati on of the fi lm (Gd~f), while ener gy issupplied externally by peeling (Gp) and intern ally by relax-ation o f r e s i d u a l s t r e s s e s p r e s e n t i n t h e f i l m ( Ge ~ ). T h e r e f o r e ,t h e f o l l o w i n g e n e r g y b a l a n c e i s v a l i d f o r t h e p e e l t e st 11

G~ = Gp + Gde~ - Gd [1]All energy terms are per u nit area. The fractur e energy termG~ is made up of an intrinsic contribution, which repre-sents the energy required for b reaking interracial bonds,and a dissipation contribution which is due to crack-tipplasticity. ~4 The in fluen ce of e ach of these terms upon thepeel energy Gp will be con sidered in the discussion whichfollows.Residual strain energy.-- In a previous in vestig ation 1 it hasbeen shown that built-in elastic strain energy due to thedeposition process itself does not play a signif icant role inthe en ergy balance. As shown i n the d iscussion below, theinternal strain energy due to thermal effects are more im-portant. The amo unt of energy per uni t area Gel, stored inthe metal layer at elevated temperatures owing to the dif-ference in thermal expansion coefficients ha between thelayer and the substrate, can be estima ted with Eq. 2 11

Gel = DE (AcxAT)2 [2]

where D is the metal la yer thickness, E is the Young's mod-ulus of the layer an d AT is the tempe rature difference withthe deposition t emperature . G~I is a bout 11 J/m 2 for thecurre nt layer thickne ss of 7 ~m in the peel test wit h AT is550~ E is 200 GPa, 1 an d Ac~ is 6 9 0 -8 K -1. 1,15This means


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J . E l ec t rochem. S oc . , Vo l . 141 , No . 3 , Ma rch 1994 9 The Electrochem ical Society, Inc.T a b l e I I . R e l a t iv e a t o m c o n c e n l r a ti o n s (% ) o n t h e N i ( P ) a n d t h e A I 2 O a f ra c t u r e s u r f a c e o f s a m p l e s w i t hr o u g h - t y p e s u b s t r a t e s a f t e r a n n e a l i n g a t 1 5 0 a n d a t 4 50 ~

82 1

Surface T (~ C ls O ls Ni 2p A1 2p P 2p Sn 3d Ag 3d Pd 3d S 2pA12 O3 150 14.6 53.1 15.6 14.9 1.9 . . . .A12 O3 450 14.6 52.6 14.5 11.5 6.8 . . . .Ni(P) 150 14.0 4L5 40.7 - - 2.6 0.2 0.2


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822 J . E l ec t rochem. S oc . , Vol . 141 , No. 3 , March 1994 9 The Electrochem ical Society, Inc.T ab le I II . A ss ig n men t o f exact p e ak p o s i t io n s t o t h e ch emica l en v iro n me n t o f Ih e sam e sam p le as measured n T ab le Ih

ElementRelative amount (%)

Position (eV) 150~ 450~ EnvironmentA1203 surfaceC

OPNiA1

Ni(P) surfaceCOSSnAgPdPNi

284.8 90 90 -- C H286.5 10 10 -- C- -O531.0 100 100 A1203129.5 15 15 Ni(P)132.5 85 85 PO4856.2 100 100 Ni203, Ni(OH~), NiPO~73.8 100 100 A1203284.8 85 85 -- C- -H288.5 15 15 -- O- -C ~O531.5 100 100 PO4, Ni(OH)2 or Ni203162.1 100 100 NiS486.0 100 - - Sn oxide367.5 100 100 Ag oxide335.2 100 - - Pd metalli c133.5 70 60 -PO4129.5 30 40 NiP852.5 21 42 Metallic Ni, Ni(P)856.2 79 58 Ni(OH}2, NiPO4, Ni~O3

the mechanical interlocking remained constant during an-nealing, and therefore that the higher coverage of nickelpa/'ticles indicates a stronger intrinsic adhesion. Thishigher metal particle coverage does not become apparent ina higher XPS Ni coverage of the alumina surface of thesample annealed at 450~According to the XPS measurements, the Ni coverage ofthe alumina fracture surfaces is similar to the A1 coverage,although the SEM micrographs show a metal particle cov-erage of only a few percent at most, as visually estimated;see Fig. 5. This confirms the assignment of the Ni and PXPS coverage to the interfacial layer a few nanometersthick, observed with TEM. The crack proceeds through thisinterfacial layer, leaving behind an Ni containing surfacelayer all over the alumina fra cture surface, which layer isfa r too thin to be observed with SEM. Only the smallamount of P which is assigned to Ni(P) in the multiscanXPS measurements of the alumina surface can be ex-plained by the meta l particles. Hence, the fracture for thesamples with rough-type alumina proceeds mainly throughthe interracial layer and passes through the metal only atinterlocking sites. The crack does not enter the ceramic.Because of a stronger intrinsic adhesion for the samplesannealed at the higher te mperature, it is more difficult topull out metal from interlocking sites, and this may explai nthe higher density of metal particles.The changes in relative coverage of Ni and P containingspecies with annealing temperature, measured by static-SIMS on the alumina fracture surfaces of samples withsmooth-type substrates, are different from those measuredwith XPS on rough-type substrates as discussed above.With static-SIMS an increasing Ni and P coverage wasfound with increasing annealing temperature, whereaswith XP S this coverage was constant. The cause of this

Table IV. Relat ive intensit ies in stat ic-SIMS spectra f romalu min a f ract u re su r f aces as a f u n ct io n o f an n ea l in gt emp erat u re f o r samp les w i t h a s mo o t h - t yp e su b st ra t e .Temperature (~

Rel. Int. As prep. 200 450 580Ni+/A1+ 0.157 0.219 1.026 8.338Ni+/A1 0.162 0.2802 1.2417 4.095Na+/A1+ 0.0690 0.0626 1.014 14.389Na+/AF 0.0857 0.0521 1.161 4.762PO~/O- 0.095 0.0979 0.2018 0.489PO~/O- 0.0709 0.0852 0.166 - -PO~/O- 0.096 0.0788 0.1736 0.592PO~/O- 0.0775 0.0731 0.146 - -F- /O - 0.1814 0.0658 0.0406 0.035F-/O 0.0973 0.0568 0.0400' - -C1-/O 0.0166 0.0172 0.0746 0.109C1-/O- 0.0172 0.0153 0.0546 - -

d i f fe r e n ce b e t w e e n b o t h s u b s t r a t e t y p e s is n o t u n d e r s t o o d .T o a v o i d p o s s i b l e u n c e r t a i n t i e s i n t h e i n t e r p r e t a t i o n o f t h er e la t iv e s t a t i c - S I M S i nt en si ti e s, t h e s m o o t h - t y p e a l u m i n af r a c t ur e s u rf a c e s w e r e t h e r e f o re a l s o m e a s u r e d w i t h E D Xa n d t h e i n c r e a s i n g c o v e r a g e w a s c o n f i r m e d . F o r t h e s a m -p l es o n w h i c h N i w a s f o u n d w i t h E D X , i t p r o v e d t o b ep r e s e n t a ll o v e r t h e s m o o t h - t y p e s u b s t r a t es , n o t o n in t e r-l o c k i n g s i te s b e c a u s e s u c h s it es c o u l d n o t b e d i s c o v e r e d o nt h e s m o o t h - t y p e s u b st r a te s . A p o s s i b l e e x p l a n a t i o n m i g h th a v e b e e n t h a t m o r e N i p a r t i c le s r e m a i n e d o n t h e s m o o t h -t y p e a l u m i n a a t h i g h e r a n n e a l i n g t e m p e r a t u r e . S u c h p i e c e sw e r e n o t f o u n d w i t h S E M u p t o t h e h i g h e s t m a g n i f i c a t i o no f 4 0 , 0 0 0 t i m e s . I t i s t h e r e f o r e m o r e p r o b a b l e t h a t t h e r e -m a i n i n g N i a n d P o n t h e s m o o t h - t y p e s u b s t r at e s o r ig i n a t ef r o m t h e i n t e rf a c i a l l a y e r d i s c u s s e d b e f o r e . T h i s m e a n s t h a ta n i n c r e a s i n g f r a c t i o n o f t h e i n t e r r ac i a l l a y e r r e m a i n s o nt h e s e s u b s t r a t e w i t h i n c r e as i n g t e m p e r a t u r e . B e c a u s e o ft h e h i g h s u r f a c e s e n s i ti v it y o f t h e s t a t i c - S I M S t e c h n i q u e ,a n N i a n d P s u r f a c e l a y e r w i t h a m e a n t h i c k n e s s o f 1 o r2 n m c a n d o m i n a t e t h e s p e c t r u m .

F o r s a m p l e s w h i c h h a v e n o t b e e n a n n e a l e d , t h e l a rg e rpart of the nucleation material was detected on the metalside, too, as for the samples anneale d at 150~ as descri bedin more deta il in Ref. 11. In tha t case also, Ni and P ions andother compounds of the metallization bath were found onthe substrate fracture surface. Therefore, in this respect thesituation after annealing is similar to the one before an-nealing, implying that diffusion probably did not play arole here. We believe that a liquid film penetrate s under-neath the metal film as soon as the Ni(P) nuclei, includingthe nucleation material, have grown out and for m a contin-uous but still porous film. After drying the sample, the bat hcompounds remain present at the interface, located under-neath the metal film and thus underneath the nucleationmaterial. Only for the sample annealed at 450~ and ana-lyzed with XPS, evidence has been obtain ed for diffusion ofnucleation material into the metal bulk, due to the de-creased coverage. 1The stress which caused the extensive microiracture inthe Ni(P) layer which was observed with TEM, may havearisen as a consequence of lateral shrinkage of the Ni(P)material during crystallization. During annealing the ad-hesion of the Ni(P) layer both to the substrate and to theelectrodeposited layer was apparently stronger than thecohesion because cracks along the interface were not ob-served. Moreover, the brittleness of the Ni(P) material in-creases during crystallization thereby promoting micro-cracking. Additi onal stress is probably introduce d into theNi(P) layer during annealing owing to thermal-expansiondifferences between the electrodeposited Ni layer and thesubstrate. Despite the increased brittleness of the Ni(P)phase a nd despite the microcracks in this layer, the fracture


8/9

J . E l e c t r o c h e m . S o c . , V o l . 1 4 1 , N o . 3 , M a r c h 1 9 9 4 9 The E lec t rochem ica l Soc ie ty , I nc. 82 3

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F - 40 60 80 100 120/.... l~,. ,I ,h.I . . . . [. . . . . . . , , i . . . . . . , , , i . . . . . . . . . i i . . . . . . . . . i . . . .

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20 40 60 80 100 120 140m a s s ( a m u )

F i g . 7 . S t a t ic - S I M S s p e c t r a o f t h e a lu m i n a f r a d u r e s u r f a c e s o f s a m p l e s w i th r o u g h - t y p e s u b s t ra t e s a n d a c e t a te - ty p e N i ( P ) b e f o r e a n d a f t e ra n n e a l in g . P e e l i n g w a s d o n e i n t h e v a c u u m o f th e a n a l y z e r . A l in e a r in t e n s i ty s c a l e is u s e d . A , P o s i t iv e - io n s p e c t r u m o f a n a s - p r e p a r e d s a m p l e ;B, p o s i ti v e -i o n s p e c t r u m o f a s a m p l e a n n e a l e d a t 45 0~ C , n e g a t iv e - io n s p e c t r u m o f a n a s - p r e p a r e d s a m p l e ; a n d D , n e g a t iv e - io n s p e c ~ u mo f a s a m p l e a n n e a l e d a t 45 0 ~

i s f o u n d t o p r o c e e d t h r o u g h t h e i n t e r r a c i a l l a y e r a t t h em e t a l - c e r a m i c i n t er f a c e .Conclusion

A n i m p r o v e m e n t b y a f a c t o r o f t w o t o t h r e e i n th e a d h e -s i o n o f e l e c t r o l e s s N i ( P ) t o a l u m i n a i s o b s e r v e d w i t h b o t hp e e l t e s ts a n d D P O t e st s a f t e r a n n e a l i n g a t t e m p e r a t u r e sa b o v e 2 5 0 ~ F r a c t u r e s u r f a ce a n a l y se s w i t h S E M / E D X ,X P S , a n d s t a t i c - S I M S s h o w t h a t, i r r e sp e c t i v e o f t h e a n -n e a l i n g t r e a t m e n t , f r a c t u r e o c c u r s t h r o u g h a n i n t e r r a c i a ll a y e r o f a f e w n a n o m e t e r s t h i c k n e s s , o b s e r v e d w i t h c r o s s -s e c t i o n a l T E M . I t i s t h e r e f o r e c o n c l u d e d t h a t t h e a d h e s i o ni m p r o v e m e n t i s d u e t o s t r o n g e r c o h e s io n w i t h i n t h i s i n te r -f a c i a l l ay e r. W i t h T E M , i n d i c a t i o n s w e r e o b t a i n e d f o r d e n -s i f i c a t i o n o f t h e i n t e r r a c i a l l a y e r b y a n n e a l i n g . W i t h s t a t i c -S I M S a n d X P S , c h a n g e s w e r e o b s e r v e d i n t h e c h e m i c a lc o m p o s i t i o n o f t h e f r a c t u r e s u r f a ce s .T h e c o n t r i b u t i o n o f m e c h a n i c a l i n t e r l o c k i n g f o r t h er o u g h - t y p e s u b s t r a t e c an n o t b e c h a n g e d w i t h h e a t - t r e a t -m e n t o f t h e m e t a l l i z e d s a m p l es . T h e r e f o r e , th e a d h e s i o ni m p r o v e m e n t i s e n t i re l y a s c r ib e d t o a l a r g e r c o n t r i b u t i o nb y c h e m i c a l i n te r a c t i o n s . T h e s a m e h o l d s f o r t h e s m o o t h -t y p e s u b s t r at e s , f o r w h i c h e v i d e n c e f o r m e c h a n i c a l i n t e r-l o c k i n g w a s n o t o b t a i n e d a t a l l. T h e g r e a t e r e f f e c t o f w a t e ro n t h e f r a c t u r e e n e r g y o f s a m p l e s a n n e a l e d a t t h e h i g h e rt e m p e r a t u r e s i s c o n s i s t e n t w i t h t h i s e x p l a n a t i o n .S i n c e o n l y D P O r e s u l t s a r e r e p o r t e d i n th e l i t e r a t u r e , ac o m p a r i s o n b e t w e e n t h e p r e s e n t r es u l ts a n d l i t e r a tu r e d a t am u s t b e m a d e u s i n g t h e s t r e n g t h r e s u l t s o n l y . T h e s t r o n gd e p e n d e n c e o f t he D P O s t r e n g th o n t e m p e r a t u r e i n t h er a n g e b e t w e e n 2 0 0 a n d 3 0 0 ~ ( F i g . 3 ), m a y e x p l a i n t h e d i -v e r g e n t r e s u l t s r e p o r t e d i n t h e l i t e r a tu r e , a s s u m m a r i z e d i nt h e i n t r o d u c t i o n . T h u s , s m a l l c h a n g e s i n p r o c e s s i n g m a yl e a d to l a r g e c h a n g es i n D P O s t re n g t h , a t t e m p e r a t u r e s o fa b o u t 25 0 ~ T h e f a c t t h a t t h e s h a r p d e c r e a s e i n D P Os t r e n g t h o b s e r v e d a f t e r a n n e a l i n g a t 5 0 0 ~ i s a p p r o x i -

m a t e l y e q u a l t o t h o s e o b s e r v e d a f t e r a n n e a l i n g a t 1 0 0 t o2 0 0 ~ i s c o n s i s t e n t w i t h l i t e r a t u r e r e p o r t s . H o w e v e r , a ta b o u t 4 0 0 ~ w h e r e w e h a v e m e a s u r e d t t ie h i g h e s t D P Os t r e n g t h s, l i t e r a t u r e r e p o r t s h a v e n o t b e e n f o u n d .Acknowledgments

T h e a u t h o r s g r a t e f u l ly a c k n o w l e d g e e x p e r i m e n t a l a s s is -t a n c e f r o m M . v a n W e e r t, X P S m e a s u r e m e n t s f r o m F . v a nW e e g b e r g , X R D a n a l y s e s f r o m J . T i m m e r s , S E M m i c r o -g r a p h s f r o m C . G e e n e n a n d M . S l an g e n , c r o s s - s e c t i o n a l o p -t i c a l m i c r o g r a p h s f r o m W . G i j s b e r s a n d s t i m u l a t i n g d i s c u s -s i o n s w i th a n d t h e p r o v i s i o n o f m a t e r i a l f r o m J . J a n s s ena n d M . V o s s e n .M a n u s c r i p t s u b m i t t e d J u n e 1 7 , 1 9 93 ; re v i s e d m a n u s c r i p tr e c e i v e d N o v . 8 , 1 9 93 .Philips Research Laboratories assisted in meeting thepublication costs of thi s article.

R E F E R E N C E S1 . W . R i e d e l , i n Funktionelle Chemische Vernicklung,E . G . L e u z e V e r l a g, C h a p . 1 0 , S a u l g a u ( 1 9 8 9 ) .2 . W . H . S a f r a n e k , i n The Properties of ElectrodepositedMetals and Alloys, C h a p . 2 2 , E l se v i e r, N e w Y o r k(1974).3. J. W Sever in and G. de With,J. Adhesion Sci. Teehnol.,7, 115 (1993).4. H. E Fischmeister, G. E]ssner, B. Gibbesch, and WM a d e r , Materials Research Society InternationalMeeting on Advan ced Materials, V o l . 8 , p . 2 2 7 , M R S

( 1 9 8 9 ) .5 . T . S . O h , R . M . C a n n o n , a n d R . O . R i t c h i e , Mater. Res.Soc. Symp. Proc., 1 3 9 , 2 1 9 ( 1 9 8 9 ) .6 . H . H o n m a a n d S . M i z u s h i m a , J. Met. Finish. Soc. Jpn.,3 3 , 3 8 0 ( 1 9 8 2 ) .7 . H . H o n m a a n d K . K a n e m i t s u , Plat. Surf. Finish., 7 4 , 6 2( 1 9 8 7 ) .8 . T . O s a k a , E . N a k a j i m a , Y . T a m i y a , a n d I . K o i w a , J. Met.Finish. Soc. Jpn., 4 0 , 5 7 3 ( 1 9 8 9 ) .


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824 J . E l e c t r o c h e m . S o c . , Vol. 141, No. 3, March 1994 @ The Electrochem ical Society, Inc.9. D. Broek, in E l e m e n t a r y E n g i n e e r i n g F r a c t u r e M e -chan i c s , p. 4, 22, Kluw er Academic Publishers, Dor-drecht (1986).10. R. W. Davidge, in M e c h a n i c a l B e h a v i o u r o f C e ra m i c s,p. 32, Cambridge Unive rsity Press, Cambridge(1979).11. J.W. Severin, R. Hokke, H. van der Wel, and G. de With,To appear in J . A p p l . P h y s . , J. W. Severi n, Ph.D. Th e-sis, Chap. 4. (A copy of this thesis is available atPhilips Research Laboratories, Prof. Holstlaan 4,

5656 AA, Eindhoven , The Netherlands.)12. J.W. Severin , R. Hokke, H. van der Wel, and G. de With,

Thi s Jou r na l , 140, 682 (1993).13. K. S. Rajam, S. R. Rajagopalan, M. S. Hegde, and B.Viswanathan, M at er . Chem . P hys . , 27, 141 (1991); andNational Insti tute of Standards and Technology XPSDat a Base Version 1.0 (Oct. 1989), NIST, Gaithers-burg, MD.14. D. Brock, in E l e m e n t a r y E n g i n e e r i n g F r a c t u r e M e -chan i c s , p. 14, K]uwer Academic Pub., Dordrecht(1986).15. E. DSrre and H. Hiibner, in A l u m i n a , P r o c e s s i n g , P r o p -e r t i e s a n d A p p l i c a t i o n s , Springer Verlag, Berlin(1984).

Ch em ica l V a p o r D epo s ition of S ilicon f rom D is ilaneun der R educ ed Pressure in a C ircular Im pinging Jet Reactor

Simula tion and ExperimentsY . B . W a n g a n d F . T e y s s a n d ie r

C N R S , I n s t i t u t d e S c i e n c e e t d e G d n i e d es M a t d r i a u x e t P r o c dd E s , F - 6 6 8 6 0 P e r p i g n a n C e d e x , F r a n c e

J . S im o n a n d R . F e u re rE N S C T C N R S , L a b o r a t o i r e d e C r is t a l lo c h i m i e , R ~ a c t iv i td , P r o t e c t i o n d e s M a t d r i a u x , F - 3 1 0 7 7 T o u l o u s e C e d e x , F r a n c e

ABSTRACT9The chemical vapor deposition of silicon from disilane under reduced pressure in an impinging jet reactor has beenstudied experimentall y and simulated numericall y by a 2D model. The measured deposition rate and profile have beencompared to the results of the cal culatio ns performed with vario us hypotheses concernin g both ga s-phase and surfacereactions. The influence of the nu mb er of species considered, of the kine tic rate con stants, and of the models used for the

reactive sticki ng coefficient of silane and di silane were investigated. Among the 17 species that may be pre sent in the gasphase, a mechanism in cluding 8 silicon-carrier species has been determined to represent satisfactorily the deposition rateand profile experimentally observed.

Chemical vapor deposition (CVD) is a method widelyused to deposit a large dive rsity of materials such as pureelements, compounds, solid solutions, and composites. 1Further improvement and comprehension of the CVD pro-cessing technology is largely dependent on our ability todevelop reliable predictive modeling. These simulationmodels provide reliable prediction s of the growth rate pro-files. Recent publica tion s have shown relati vely successfulmodeling of carbon incorporation,2 microscopic step-cov-erage, 3 self-lim iting growth inhib ition ,4 and selectivity?Other properties such as microstruciure or surface mor-phology are studied experimentally. Any signifi cant prog-ress in this field seems far away.Here, we look at t he in fluence of both ho mogeneousand heterogeneous chemistry on the growth rate of silicondeposits.It is well known th at a depositi on process involves threebasic transport phenome na (i.e., motion, energy, and mass)governed by a set of coupled partia l differential equations,together with gas-phase and surface-reaction kinetics.Since Eversteyn et al . , 6 boun dary layer models were usedextensively to describe the flow field for a long time in theCVD process. 7-1~ In the ear ly 1980s, r ea l progress wasachieved in solving the complete set of governing equa-tions. After detailed analy sis of flow structures wi th 2D or3D models, the incre asing power of supercom puters allowsan increasingly accurate description of all coupled phe-nomena including chemical reactions.It is common to use simplifica tions for modelin g eitherthe chemical mechanisms involved or the flow fields. Aschematic c hemical model of only one species, one surfacereaction, and no gas-phase reaction was considered byWahl 11 for the 2D mod eling of a stagn atio n point flow reac-

tor. Such a model was used recently by Snyder et aI . forCdTe deposition.12In contrast, comp licated chemical reaction systems weretake n i nto account by Michaelidis and Pollard, 13 Rebenn eand Pollard ~415 approa ching the flow by the 1D simil arityequations, and Coltrin et aI . 16.17using a 2D flow model withbound ary layer simplifications, or the 1D similarity equa-tions applied to the rota ting disk flow. TMIt is on ly since 1984that real improvement in modeling CVD reactors have beenobserved. 2D and even 3D geometry now is treated to getherwith a realistic gas-phase and surface chemical mecha-nism. We may quote the works of Moffat and Jensen, ~'2~who have used a fully parabolic flow approximation in ax-ial directi on for a 3D horizon tal reactor. In a similar reactorgeometry, Gokoglu et a l . 21 have stud ied the silico n deposi-tion with a more detailed 3D flow treatment. In a con fined2D axisymmet rical reactor, Fotia dis et a l . 22 have carriedout a d etailed stud y of the velocity profiles for various re-actor shapes. Kleijn et al. 2~,~4 stu died sili con depos iti onwith a 2D model including multicompo nent diffusion.Axisym metric al reactors present at tractiv e features. It isthe only configuration that can be treated as two-d imen-sional. It can be assumed even one-dimensio nal in thevicinity of the axis by suitable tra nsformations. The as-sumptions required for such a 1D treatment have been dis-cussed for stagn ation point flow25 and for rotating disk26:flow field can be treated as 1D only when the bo rder effectscan be neglected and the bu oyancy is absent, otherwise 2Dmodels must be used.With better master of flow treatment, the main difficultyremains our ignorance of the chemical mechanism or thereaction kinetic rates. The Si-H system is one of the moststudied because of silicon applications to the electronics-

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