SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 98È102 (1999)

Study on Interfacial Reaction of Ti/AlN by SIMS,RBS and XRD

Ruifeng Yue,*,1,2 Yan Wang,3 Youxiang Wang2 and Chunhua Chen21 Institute of Microelectronics, Tsinghua University, Beijing 100084, China2 State Key Laboratory of Surface Physics, Chinese Academy of Sciences, PO Box 912, Beijing 100084, China3 Department of Electronic Engineering, XiÏan University of Technology, XiÏan 710048, China

A 200 nm Ti Ðlm was deposited on a polished AlN ceramic substrate at 200 ÄC by electron beam evaporation andthen annealed under high vacuum conditions. The MCs‘-SIMS technique (detecting MCs‘ secondary ions underCs‘ primary ion bombardment, where M is the element to be analysed), RBS and x-ray di†raction (XRD)measurements were employed to probe the solid interfacial reaction between Ti and AlN from 200 ÄC to 850 ÄC,and the variation of interfacial composition distribution with annealing temperature and time was given. Ternaryaluminides were discovered and the formation and development of the aluminides were observed in the interfacialregion. The results indicate that the MCs‘-SIMS technique is an e†ective method to study the interfacial reactionbetween metal and ceramic. Copyright 1999 John Wiley & Sons, Ltd.(

KEYWORDS: Ti ; AlN; SIMS; RBS; interfacial reaction

INTRODUCTION

In recent years, AlN ceramic has attracted much atten-tion because of its much higher thermal conductivitythan that of and matching thermal expansionAl2O3coefficient with Si, and it has been regarded as the mostpromising substrate material in the electronic packageindustry.1 The metallization process of the surface of aceramic substrate is of utmost importance because ithas a direct e†ect on the quality and reliability of theintegrated circuits. The adhesive strength betweenmetal and ceramic depends on many factors, amongwhich the most important is the interfacial di†usion andreaction. Metal/ceramic interfacial reactions are ofprimary importance in the surface science and materialscience Ðelds. Titanium is a commonly used active metaland can adhere to many kinds of ceramics properly, soit is usually the Ðrst metal layer on a substrate acting asan adhesion layer in a multilayer metallization tech-nique. Thus, to probe the interfacial reaction of Ti/AlNhas great scientiÐc meaning and practical value.

There is some research work on the interfacial reac-tion between Ti and AlN,2h7 but much of it concen-trates on the interfacial products at a certaintemperature, rather than on how the interfacial pro-ducts vary with annealing temperature and time. Thereare also some di†erences among these works on inter-facial products, especially in connection with aluminidesand what they are. In this paper, the MCs`-SIMS tech-nique (detecting MCs` secondary ions under Cs`primary ion bombardment, where M is the element to

* Correspondence to : F. Novel Devices Research Division,R. Yue,Institute of Microelectronics, Tsinghua University, Beijing 100084,China. E-mail : yueruifeng=263.net

be analysed), RBS and x-ray di†raction (XRD) wereemployed to analyse the solid interfacial reactionbetween Ti and AlN from 200 ¡C to 850 ¡C.

EXPERIMENT

The AlN substrate was sintered from high-purity AlNpowder with 3.5% additives of and CaO atDy2O31850 ¡C for 4 h. The substrate was polished mechani-cally with powder and then diamond paste to aAl2O3Ðnal polish of D0.1 lm, and cleaned ultrasonically inacetone, ethanol and deionised water. A 200 nm Ti Ðlmwas deposited on AlN substrate at 200 ¡C by using aBalzers UTT400 ultrahigh vacuum electron beamevaporator. The purity of the Ti source was 99.99%.The vacuum background was 1.4] 10~6 Pa and thedeposition rate was 0.2 nm s~1. The sample was thenannealed in a constant-temperature furnace with aresidual pressure of 1.6 ] 10~4 Pa at the followingtemperature/time combinations : 450 ¡C/1 h, 650 ¡C/1 h,850 ¡C/1 h and 805 ¡C/4 h. Secondary ion mass spec-trometry analysis was conducted on a Riber MIQ-156with Cs` used as the primary ion with a beam currentof 0.12 lA, an ion beam energy of 10 keV and an inci-dence angle of 45¡. The beam was rastered over a0.48] 0.29 mm2 area and the secondary ions weredetected from the centre 5% area of the scanningregion. A ]16 V bias voltage was exerted on thesample to improve the detecting sensitivity and anACE576N electron gun was employed to reduce thecharging e†ect of the insulating substrate under Ðlamentcurrent regulation mode. Helium ions of incident energy2.023 MeV were used in RBS analysis, with a back-scattering angle of 165 ¡C; Cu Ka radiation was used inXRD with a scanning speed of 4¡ min~1.

CCC 0142È2421/99/020098È05 $17.50 Received 22 July 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 9 November 1998

INTERFACIAL REACTION OF Ti/AlN 99

EXPERIMENTAL RESULTS

The MCs‘-SIMS depth analysis

In SIMS depth analysis the traverse axis is the sputter-ing time, which represents the sputtering depth of thematerial, and the vertical axis is the intensity of the sec-ondary ion signals collected by the mass analyser, whichcould be considered to be proportional to the corre-sponding composition concentrations. In order toachieve the composition distribution of elements Ti, N,Al and O, the TiCs`, NCs`, AlCs` and OCs` second-ary ions produced by Cs` bombardment were detected ;at the same time, TiNCs` was also detected. Detectionof MCs` secondary ions (where M is the element to be

analysed) is a well-known ion microprobe approach inthe SIMS Ðeld,8 because this technique can reduce oreven diminish the matrix e†ect and is suitable for quan-titative analysis of both matrix composition and traceelement simultaneously and also for interface analysis.

For the as-deposited sample [Fig. 1(a)], the signalintensities (except for Ti) are very low in the Ti Ðlm andcan be regarded as background noise, indicating thatthe Ti Ðlm is of high purity. On the other hand, inter-facial di†usion begins and the TiNCs` peak appears,but Ti, Al, N and O signals fall or rise sharply, indicat-ing a very narrow interfacial region. After the sample isannealed at 450 ¡C for 1 h, as shown in Fig. 1(b), theintensity of the Al signal improves to 5] 102, theTiNCs` peak in the interface broadens a little and theTiCs` signal moves to the substrate slightly, but there isno obvious overall change.

Figure 1. The MCs½-SIMS depth profiles of Ti/AlN samples : (a) as-deposited; (b) 450 ¡C/1 h; (c) 650 ¡C/1 h; (d) 850 ¡C/1 h; (e)850 ¡C/4 h.

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 98È102 (1999)

100 R. F. YUE ET AL .

Figure 2. The RBS spectra of Ti/AlN samples.

After the sample is annealed at 650 ¡C for 1 h, there isa considerable change in the interface region [Fig. 1(c)] :the intensity of AlCs` in the Ti Ðlm has reached8 ] 103 and the NCs`, OCs` and TiNCs` signals havealso increased, showing that Al, N and O in the AlNsubstrate have di†used to the Ti Ðlm; also, the TiNCs`peak broadens further in the interface and the TiCs`signal advances to the substrate even more. Theseresults indicate that there is a stronger interfacial reac-tion between Ti and AlN.

When the annealing temperature reaches 850 ¡C for 1h [Fig. 1(d)], a violent interfacial reaction occurs : in theTi Ðlm, the intensities of the AlCs` and TiNCs` signalsincrease remarkably, the NCs` and OCs` signalsincrease signiÐcantly and a valley of NCs` appears ;moreover, the intensity of the TiNCs` peak in the inter-face increases and the TiCs` signal moves furthertowards the substrate. When the annealing time is pro-longed to 4 h, as shown in Fig. 1(e), an AlCs` signalplateau with an intensity of 7 ] 104 emerges in theinterface near the Ti Ðlm side, which illustrates thatsome aluminides with Ðxed stoichiometry are probablyformed. The NCs` signal at the central part in the Alplateau increases greatly, so the signal valley becomesless distinct ; at the same time, the OCs` peak and theTiNCs` valley move to almost the same front part ofthe Al plateau.

RBS analysis

Figure 2 shows the RBS spectra of Ti/AlN samples.Under the annealing conditions of 650 ¡C/1 h, the

channel number of the rear edge in the Ti spectrumreduces from 331 to 324, the whole Ti spectrumbecomes shorter and wider and the interface movestowards the substrate ; on the other hand, the forwardedges of the Al and N spectra move to a high channelnumber, showing that an obvious interfacial di†usionhas taken place between Ti and the AlN substrate. Afterthe sample has been annealed at 850 ¡C for 1 h, thechannel number of the rear edge in the Ti spectrumdecreases to 318, the Ti spectrum becomes even shorterand wider, the forward edge yield of the Al spectrumincreases further and the forward edges of the N and Ospectra have reached the surface, showing that the inter-facial reaction is becoming stronger. After the samplehas been annealed at 850 ¡C for 4 h, the channelnumber of the rear edge in the Ti spectrum drops to 315and the Ti spectrum becomes a plateau ; also, a terraceappears in the forward edge of the Al spectrum. Theseresults indicate that the Ti Ðlm and the AlN substratereact quite thoughly in the interface and aluminideswith Ðxed stoichiometry are produced in the forwardedge of the Al spectrum. According to the theory ofbackscattering,9 the Ti/Al atomic ratio is 2.75 : 1 in theposition of the Al terrace, but we cannot conclude thatthe aluminide is because it also contains N andTi2.75AlO atoms. Comparing the SIMS and RBS analyses, weÐnd that the forming processes of the AlCs` plateauand the Al terrace are very similar, but the detectingsensitivity and the depth resolution of SIMS are muchbetter than those of RBS.

XRD analysis

Besides the main AlN crystalline phase in the AlNceramic substrate, there is also an phase.AlDyO3Because conventional XRD was employed for analysisand the Ti Ðlm is too thin, strong background signalsinterfere with the determination of the reaction pro-ducts. For the as-deposited Ti Ðlm on the AlN sub-strate, there is a strong (002) texture that overlaps alittle with the AlN(101) di†ractive peak. After thesample was annealed at 450 ¡C for 1 h, the Ti(002) peakbecomes stronger and narrower, showing that crys-tallinity improves with the rising annealing temperature.After the sample was annealed at 650 ¡C for 1 h, theintensity of the Ti(002) peak decreases and weak

and peaks appear, indicatingTi3Al(400) TiN0.3(002)

Figure 3. The XRD spectra of Ti/AlN samples : (a) 850 ¡C/1 h; (b) 850 ¡C/4 h.

Surf. Interface Anal. 27, 98È102 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

INTERFACIAL REACTION OF Ti/AlN 101

that a distinct interfacial reaction has occurred. Underthe annealing conditions of 850 ¡C/1 h, as shown in Fig.3(a), the Ti(002) peak disappears completely and the

TiN andTiN0.3 , Ti3Al2N2 , Ti2AlN, Ti3Al, TiAl3 , Ti2Npeaks reported in the JCPDS Ðles emerge, thepeak being the strongest. When the anneal-TiN0.3(002)

ing time reaches 4 h [Fig. 3(b)], the peakTiN0.3(002)becomes much weaker and the peakTi2AlN(002)increases signiÐcantly. Besides the above products, thereare also peaks. Among all the probable reactionTi2Oproducts, only the peak at 2h \ 13.05¡ belongs to thetypical peak of it is also the secondTi2AlN(002) ;strongest peak of in Ðle JCPDS18-70.Ti2AlN

DISCUSSION

According to thermodynamics, if a reaction occursspontaneously, the change in the Gibbs free energy offormation (*G) between reactants and products must beless than zero. Based on the thermodynamic data,10 thefollowing reactions may go on even at room tem-perature :

Ti] AlN] Al] TiN

*G\ [22.158 kJ mol~1 (1)

4Ti] 3AlN] TiAl3] 3TiN

*G\ [205.997 kJ mol~1 (2)

2Ti] AlN] TiAl] TiN

*G\ [95.462 kJ mol~1 (3)

Reaction (2) is the most probable thermodynamicallyaccording to Gibbs free energy data and is conÐrmed bythe TiÈAlÈN ternary phase diagram11 shown in Fig. 4.The three stable tie lines (STL) are AlNÈTiAl3 ,

and TiNÈAlN. At an arbitrary Ðxed tem-TiAl3ÈTiNperature and pressure, the Gibbs phase rule predicts amaximum number of three phases to be in equilibriumin a three-component system. An STL cannot intersect

Figure 4. The Ti–Al–N ternary phase diagram at 600 ¡C proposedby Beyers et al .11

another STL for the simple reason that at the point oftheir intersection there would be four phases in equi-librium. There is no STL between Ti and AlN, so whenTi meets AlN at 600 ¡C the system becomes unstableand they react with each other to produce andTiAl3TiN. In a region where the AlN/Ti atomic ratio is[60%, AlN will exist in equilibrium with TiN and

otherwise whether STL exists in theTiAl3 ,region will decide the species in equi-TiÈTiNÈTiAl3librium, but that does not include AlN. If there is no

STL in that region, Ti will coexist with TiN and TiAl3 ,as described in reaction (2). Therefore, this ternaryphase diagram provides us with a theoretical basis tostudy the interfacial reaction and determine the reactionproducts between Ti and AlN. Now, most reports onthe interfacial reaction between Ti and AlN incline tothe view that the products are TiÈN and TiÈAl binarycompounds, and the possibility of ternary compounds isneglected, so some experimental results are difficult toexplain.

In fact, when the TiÈAlÈN ternary phase diagram wasÐrst proposed, Beyers et al.11 stressed the possibility ofits imperfection and indicated that whether there is anySTL in the region needs to be proved byTiÈTiNÈTiAl3experiment. In 1990, Bhansali et al.12 supplemented thisphase diagram, as shown in Fig. 5. We Ðnd that STLs inthe region are close and numerous andTiÈTiNÈTiAl3that there are also three stable ternary compounds :

and this means that bothTi3Al2N2 , Ti2AlN Ti3AlN;the interfacial reaction process and the productsbetween Ti and AlN are very complicated. It is gener-ally acknowledged that the interfacial reactions betweenTi and AlN include the following steps : AlN is reducedunder the e†ect of Ti ; the dissolved Al and N atomsdi†use to the Ti Ðlm; and they react with each other toproduce compounds. The radii of N, Ti and Al atomsare 0.07, 0.1448 and 0.1431 nm,13 respectively, so Natoms di†use mainly interstitially in the Ti Ðlm andshould have a higher di†usion velocity than that of Alatoms which (di†use mainly by substitution). Di†er-ences in di†usion velocities between Al and N atomswill result in a di†erent gradient distribution in the dif-fusion direction. In the process when the interfacial

Figure 5. The Ti–Al–N ternary phase diagram at 900 K proposedby Bhansali et al .12

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 98È102 (1999)

102 R. F. YUE ET AL .

reaction is not in equilibrium, all the compounds in theregion may be produced.TiÈTiNÈTiAl3From the above experimental results, we can under-

stand that with increasing annealing temperature andtime the interfacial reaction between Ti and AlN hasundergone a process from unstable to equilibrium.During this reaction, the component distribution in theTi Ðlm changes violently and the AlCs` plateau in theinterface comes into being gradually. The AlCs`plateau related to aluminides should consist of TiÈAlbinary and TiÈAlÈN ternary compounds ; after thesample was annealed at 850 ¡C for 4 h the Ti/Al atomicratio is 2.75 : 1 in the Al terrace region in Fig. 2 ;together with the high di†ractive peak in Fig.Ti2AlN3(b), these facts prove that constitutes a rela-Ti2AlNtively large proportion in the aluminides. In Fig. 1(e),the OCs` peak and the TiNCs` valley appear almostsimultaneously in the front part of the Al plateau, the

peak decreases and the peaks emerge,TiN0.3(002) Ti2Oso we deduce that after the sample is annealed at 850 ¡Cfor 4 h may exist in the OCs` peak positionTi2O

whereas TiN may be present at the TiNCs` peak posi-tion in the interface.

CONCLUSION

In this paper, the formation and development processesof aluminides are observed. The aluminides producedby interfacial reaction are conÐrmed to be TiÈAl binaryand TiÈAlÈN ternary compounds by experimentalresults and thermodynamic analysis. When the anneal-ing temperature is below 450 ¡C, the interfacial reactionspeed is slow and the interfacial region is narrow, butwhen the temperature reaches 650 ¡C, the reactionaccelerates ; when the annealing temperature isincreased to 850 ¡C, a strong interfacial reaction occurs.All these facts also prove that the MCs`-SIMS tech-nique is an e†ective method to study the interfacial dif-fusion and reaction between metal and ceramic.

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Surf. Interface Anal. 27, 98È102 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.