Thin Solid Films 435(2003) 170–173

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0040-6090Ž03.00338-9

The distribution of Cu and resultant resistivity change in sputterdeposited Al–Cu film as a conductive layer

D.H. Lee*, D.M. Jeon, S.Y. Yoon, J.P. Lee, B.G. Kim, S.J. Suh

Department of Advanced Materials Engineering, and Advanced Materials and Process Research Center for IT, Sungkyunkwan University,Suwon, 440-746, South Korea

Abstract

As a conductive layer, sputter-deposited Al–Cu 1 at.% thin film was investigated, with particular focus on the effect of Cuprecipitates and film strain on the electric conductivity. The 400-nm-thick film was deposited on Si wafer and heat-treated at 200and 4808C. Cu precipitation was investigated using electron microscopy and film strain using XRD. As a result of heat treatment,changes in the electric resistivity showed the same trend as the film strain. This study concluded that the trends in resistivity andfilm stress could be related to Cu precipitation to some extent. In other words, the resistivity changes depending on whether theprecipitates are coherent with the Al matrix and on the distribution of the precipitates.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Resistivity; Metallization; Precipitation; Strain

1. Introduction

For application as conductive layers, copper is apromising material for next-generation high-density inte-gration. However, because it cannot be easily dry etched,copper-alloyed aluminum is currently widely used formetallization in semiconductor devices or as electrodesin electronic devices. Aluminum has low resistivity, andis easy to deposit and etch. Copper alloying in Alimproves the electromigration property. This improve-ment is attributed to Cu precipitation at grain boundariesw1x. However, alloying with Cu increases the resistivityof the Al film in generalw2x.

The resistivity of the Al film can be affected byseveral factors. Among these, the effect of the Cudistribution and the lattice strain can be considerable.Cu precipitates can be variously distributed with heattreatment: randomly distributed coherent or grain bound-ary-segregated incoherentw3x. The precipitates can alsocause lattice strain, depending on the coherence.In this study, changes in Cu distribution with heat

treatment were investigated, as well as the relationshipwith lattice strain and the resultant resistivity.

*Corresponding author. Tel.:q82-31-290-7373; fax:q82-31-290-5644.

E-mail address: duhyun@skku.edu(D.H. Lee).

2. Experimental details

Films of Al–Cu 1 at.% were deposited in a DCmagnetron sputtering system onto Si wafer. It is widelyknown that Al–Cu alloy shows optimum properties asa conductive layer at an approximate composition ofAl–Cu 0.43 at.%w4x. However, this composition doesnot contain enough Cu to observe its behavior. Sincethis study was mainly focused on observation of theeffects of Cu, the composition Al–Cu 1 at.% was used.The substrate temperature was approximately 408C andremained unchanged. The deposition conditions areshown in Table 1.The samples deposited were subjected to vacuum heat

treatment at 200 and 4808C for 30 min. The temperatureof 200 8C was chosen to obtain coherent precipitatesand 4808 for incoherent precipitatesw3x. The resistivityof the film was measured using the four-point probemethod. To determine the distribution of Cu precipitates,SEM and TEM observations were carried out. Samplepreparation for TEM observation involved peeling offthe film with plastic tape, except for the 4808C heat-treated sample, which was subjected to conventionalTEM sample preparation. A chemically etched samplewas used for better observation of the precipitates. EDSanalyses under SEM and TEM observations were carriedout to investigate the precipitates. The lattice strain was

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Table 1Sputter deposition conditions for Al–Cu 1at.% thin film

Ar pressure 4.5 mTorrBase pressure 1=10 Torry6

Target Al–Cu 1 at.%, 3 in.Target to substrate distance 5 cmInput power DC 200 WSubstrate temperature Room temperatureSubstrate Si(100)Film thickness 400 nm

Fig. 2. Lattice strain change with heat treatment. XRD peaks of Alwere analyzed using the Philips X’pert system.

Fig. 1. Resistivity change with heat treatment. A four-point probemethod was used for a 2-cm=2-cm sample.

Fig. 3. SEM observations for heat-treated samples:(a) as-deposited;(b) 200; and(c) 480 8C.

calculated by deconvolution of the(111), (200) and(220) Al XRD peaks. X-Rays incident at various angles(V) were used to obtain depth profiles.

3. Results and discussion

After the heat treatment, a graph of the resistivity vs.temperature was plotted(Fig. 1). A slight increase inresistivity was observed for the 2008C heat-treatedsample compared with the as-deposited sample, andvery low resistivity for the 4808C sample.The lattice strain profile(Fig. 2) obtained from XRD

shows a similar trend to Fig. 1. It is well known thatheat treatment usually relieves internal stress, but in thiscase it did not. The 2008C heat-treated sample hadslightly greater lattice strain than the as-deposited sam-ple, and a drastic decrease in lattice strain was observedfor the 4808C sample. These results can be explainedby the precipitation of Cu. If Cu forms precipitatescoherent with the Al matrix, these can give rise to anincrease in lattice strain. However, if Cu diffuses outfrom the Al matrix and forms incoherent precipitates,these cannot influence the lattice strain and the Cuconcentration in the Al matrix will be decreased. A

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Fig. 4. EDS line scan for Cu in Al–Cu 1 at.% film heat-treated at 4808C.

Table 2Cu concentration measured by EDS during TEM observation and thecrystallite size measured by XRD

Heat treatment wCux at the Al Crystallitetemperature(8C) matrix (at.%) size(nm)

As deposited 1.11 47.1200 0.73 46.8480 0 60.7

Fig. 6. TEM observation of the heat-treated samples:(a) as-deposited;(b) 200; and(c) 480 8C.

Fig. 5. SEM micrograph of the sample heat-treated 4808C and chem-ically etched.

temperature of 2008C can be sufficient for the formationof coherent precipitates, but not for incoherent. However,480 8C is a high enough temperature for Cu to diffuseout towards the grain boundaries of Al and to formcoarse, incoherent precipitates. In other words, the latticestrain profile on heat treatment can be attributed to theprecipitation behavior to some extent. Some relief inlattice strain can be caused by the rearrangement ofsputter-deposited matrix atomsw5x and this lattice strainprofile can influence the resistivity of the film.The microstructure of heat-treated samples was

observed. Conventional SEM observation results areshown in Fig. 3. As-deposited and 2008C heat-treated

samples show no apparent precipitates, but the 4808Csample shows uniformly distributed precipitates. Thiswas verified by EDS(Fig. 4). Open grain boundariesformed during deposition remained up to 2008C, and

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no apparent change in grain size was noted. However,a change in microstructure was noted for the 4808Cheat-treated sample. The grain size increased and theopen grain-boundary structure disappeared. The crystal-lite size obtained from XRD corresponded to SEMobservations, as shown in Table 2. According to SEMobservations, incoherent precipitates are evident in the480 8C sample. By chemically etching the Al matrix, itwas easy to observe incoherent precipitates that stillseemed to have partial coherence with the matrix(Fig.5); some coherent precipitates were also observed. Theseresults were confirmed by the TEM observations shownin Fig. 6. Incoherent precipitates were observed alongthe grain boundary for the 4808C sample. There are noprecipitates apparent for the 2008C sample.According to EDS observations in TEM and SEM,

the Cu content in the Al matrix decreased as thetemperature increased. This implies that Cu diffuses andagglomerates to form precipitates. The strain profile canbe attributed to strain caused by the coherence of theprecipitates for the 2008C sample and the strain reliefcaused by the formation of incoherent precipitates forthe 480 8C sample. This strain caused by the Cudistribution can affect the resistivity of the Al–Cu films.In other words, as the strain in the film increases, theresistivity of the Al–Cu film increases.Cu alloyed for the inhibition of elecromigration is

known to be effective when it is at the grain boundaryw1x. Thus, it is desirable for the sample to undergosufficient heat treatment to form uniformly distributedfine precipitates at the grain boundary.

4. Conclusion

As a conductive layer for semiconductor or electronicdevices, Al–Cu 1 at.% thin film was investigated. Theprecipitation behavior and accompanying lattice strainwere considered to improve the resistivity and theelectromigration property. This study confirmed that thecoherence influenced the lattice strain and, as a result,the resistivity of the film. However, there might be otherfactors that can influence the lattice strain and thesewere not considered in this study. Further studies mustbe carried out to investigate these factors.

Acknowledgments

This work was supported by the Advanced Materialsand Process Research Center for IT at SungkyunkwanUniversity (Grant No R12-2002-057-01001-0).

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