Thin Solid Films 445(2003) 112–117

0040-6090/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.PII: S0040-6090Ž03.01237-9

Titanium nitride diffusion barrier for copper metallization on galliumarsenide

H.C. Chen , B.H. Tseng , M.P. Houng *, Y.H. Wanga b a, a

Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan,a

ROCInstitute of Materials Science and Engineering, National Sun Yat-sen University, 70 Lien-hai Road, Kaohsiung 804, Taiwan, ROCb

Received 16 November 2002; received in revised form 28 July 2003; accepted 25 August 2003

Abstract

The diffusion properties of Cu, Cuytitanium nitride(TiN) and CuyTiNyTi metallization on GaAs, including as-deposited filmand others annealed at 350–5508C, were investigated and compared. Data obtained from X-ray diffractometry, resistivitymeasurements, scanning electron microscopy, energy dispersive spectrometer and Auger electron spectroscopy indicated that inthe as-deposited CuyGaAs structure, copper diffused into GaAs substrate, and a diffusion barrier was required to block the fastdiffusion. For the CuyTiNyGaAs structure, the columnar grain structure of TiN films provided paths for diffusion at highertemperatures above 4508C. The CuyTiNyTi films on GaAs substrate were very stable up to 5508C without any interfacialinteraction. These results show that a TiNyTi composite film forms a good diffusion barrier for copper metallization with GaAs.� 2003 Elsevier B.V. All rights reserved.

Keywords: Titanium nitride; Diffusion barrier; Copper diffusion; Interfaces

1. Introduction

Copper metallization has been widely used in ultra-large-scale-integrated(ULSI) technologies with featuressmaller than 0.18mm. For example, IBM announcedthat copper metallization was successfully applied in thevery-large-scale-integrated processes of siliconw1x. Cumetallization for Si ULSI devices has advantages overAl-based materials such as lower resistivity and higherelectromigration resistance, but Cu metallization requiresa barrier layer to prevent fast diffusion of Cu into Siand dielectric materialsw2x. Although copper metalliza-tion has been popularly used in Si technology, the useof Cu metallization for GaAs devices is rare. Reports ofbarrier layers for CuyGaAs interface are, to our knowl-edge, confined to one report presenting tantalum nitride(TaN) as a diffusion barrier for backside Cu metalliza-tion of GaAs MEFETsw3x. Traditionally, GaAs devicesuse Au or Au-based alloys(AuGeNi, AuBe, etc.) for

*Corresponding author. Tel.:q886-6-275-7577; fax:q886-6-234-5482.

E-mail address: mphoung@eembox.ncku.edu.tw(M.P. Houng).

metallization. The advantages of copper over gold forinterconnect and background plane metallization arelower resistivity, higher thermal conductivity and lowercost w3x. As in the Si case, copper diffuses very rapidlyinto GaAs when it is in contact with the substratewithout any diffusion barrierw4x.The titanium nitride(TiN) has been widely used in

Si devices as a diffusion barrier material for Al-basedw5–9x and Cuw10–16x metallization because of its highmelting point, good thermal and chemical stability, highhardness and low diffusivity. However, the columnargrain structure of TiN filmsw17x provides paths fordiffusion at high temperatures.In previous work on Si devices, to improve TiN

barrier performance and adhesion, a thin Ti interlayer isused between TiN and the substrate. In the present study,this technique is applied to GaAs substrate. The diffu-sion properties of CuyGaAs, CuyTiNyGaAs and CuyTiNyTiyGaAs before and after annealing at 350–5508C are studied and compared. As will be shown, the Tilayer reacts with GaAs to form a Ti–As compound layerthat helps TiN adhere to the GaAs substrate and alsoenhances barrier reliability.

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Fig. 1. XRD spectra of CuyGaAs samples, including as-deposited sample and others annealed at 350–5508C for 30 min in N ambient.2

2. Experiments

Ti and TiN were sputter-deposited onto cleaned andoxide-stripped (1 0 0)-oriented n-type (Si doped at1=10 ycm ) GaAs substrates in an RF sputtering18 3

system. RF plasma power was fixed at 110 W and thesubstrate temperature was set at 3008C to grow TiNand Ti films. Copper films were grown at room temper-ature in an MBE system with a base pressure of4.6=10 Torr before growth. The pressure went up toy9

1.0=10 Torr during film deposition with a fusion celly7

temperature of 11008C. The thicknesses of the Ti, TiNand Cu films were 15, 50 and 150 nm. The sampleswere heated in a rapid thermal annealing system toenhance thermal diffusion so an investigation of thediffusion barriers in various samples can be conducted.Annealing temperatures ranged from 350 to 5508C andannealing duration was fixed at 30 min. All annealingprocesses were carried out in a nitrogen atmosphere.The modified Transmission Line Model(TLM) theory

was used for accessing the quality of electrical propertiesof the Cu metallization layers. The TLM pattern con-sisted of a rectangular mesa and three rectangular contactpads. The mesa dimension was 4=3 mm ; the metal2

pads (Al) were 3=0.4 mm (corresponding to width2

and length, respectively); and the pad spacing was 0.5and 1 mm. HP 4156A semiconductor parameter analyzerwas used to supply a constant current of 50 mA andthen measured the voltage drop between two pads.Consequently, the total resistance between the pads andthe resistivity of the Cu metallization layers could beobtained. Compositional depth profiles were character-ized by Auger electron spectroscopy(AES) on a VGAES-310D spectrometer. A scanning electron micro-scope(SEM), JEOL JXA-840, was used to examine thesurface morphology. An energy dispersive spectrometer(EDS) on a LINKS AN10000y85S was used to identifyand analyze the surface composition. X-ray diffraction(XRD) data were collected by using a SIEMENS D5000diffractometer with Cu Ka radiation with a scan rate of68ymin.

3. Results and discussion

Figs. 1–3, respectively show the XRD spectra of CuyGaAs, CuyTiNyGaAs and CuyTiNyTiyGaAs samples.In each series, it contains one from as-deposited sampleand three others that were annealed at 350, 350 and 5508C for 30 min. All spectra in Fig. 1 contain peaks forCuGa , Cu As, Cu Ga and Cu As indicating that the2 6 9 4 3

114 H.C. Chen et al. / Thin Solid Films 445 (2003) 112–117

Fig. 2. XRD spectra of CuyTiNyGaAs samples, as-deposited and after annealing at 350–5508C for 30 min in N ambient.2

Fig. 3. XRD spectra of CuyTiNyTiyGaAs samples, as-deposited and after annealing at 350–5508C for 30 min in N ambient.2

CuyGaAs structure is very unstable. Copper has diffusedinto the GaAs substrate even in the as-deposited sample.Fig. 2 shows the XRD spectra for the CuyTiNyGaAsstructure. The basic peak structure remains unchangeduntil 450 8C, indicating that the CuyTiNyGaAs structureis relatively stable. At 5508C, the intensity of Cu

(1 1 1) decreases, accompanied by the appearance ofCuGa and Ga Ti phases. The reaction products2 3 2

between the substrate and the Cu metallization layersclearly indicate diffusion across the TiN barrier. Fig. 3presents XRD results for the CuyTiNyTiyGaAs sample.The Cu (1 1 1) and Cu (2 0 0) peak structure remain

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Fig. 4. Resistivity of CuyGaAs, CuyTiNyGaAs and CuyTiNyTiyGaAsstructures as a function of annealing temperature.

unchanged until 5508C, at which a new TiAs peak2

forms. Clearly, the CuyTiNyTiyGaAs structure is quitestable up to 5508C. Thus, Ti acts as a good diffusionbarrier material for the TiN grain boundary.The resistivity of all samples as a function of anneal-

ing temperature is displayed in Fig. 4. As the annealingtemperature increases, the resistivity of the CuyGaAsstructure gradually increases due to the formation ofhighly resistive Cu compounds as Cu diffuses and reactswith Ga or As throughout the sample. The resistivity ofthe CuyTiNyGaAs sample remains approximately con-stant until 4508C, at which point resistivity increasessharply. This presumably results from the diffusion ofGa or As into the Cu layer through the TiN film, asindicated by the formation of CuGa and Ga Ti as2 3 2

shown in the XRD results. On the other hand, theresistivity of the CuyTiNyTiyGaAs sample was approx-imately constant throughout the range of test tempera-tures, showing that a Ti interlayer between TiN and theGaAs substrate greatly improved the ability of TiN filmas a diffusion barrier.Fig. 5a and b show SEM micrographs of the surface

of the as-deposited CuyGaAs sample at two differentmagnifications. As seen in Fig. 5b, there are largeparticles scattered on the surface. EDS analysis of thelarge bright particle in the center of Fig. 5b, shown inFig. 5c, shows that the particles contain Cu, Ga and As,indicating that intermixing of Cu and GaAs may haveoccurred during sample fabrication.The SEM micrographs of the surface of the CuyTiNy

GaAs sample after annealing at 5508C are shown inFig. 6a and b. Particles are present at the surface, butthe particles are considerably smaller than those shownin Fig. 5a. Unlike Fig. 5a, small open pinholes areobserved(Fig. 6a). EDS analyses of Fig. 6b is shownin Fig. 6c. It is seen that the particles contain Cu, Ga

and As, but the ratio of Ga and As to Cu is muchsmaller than in Fig. 5c. These results again show that aTiN layer helps prevent diffusive intermixing of Cu andGaAs. The SEM micrograph of the surface of the CuyTiNyTiyGaAs sample after annealing at 5508C is shownin Fig. 7a. Fig. 7b shows the EDS analysis of theparticle as marked by a circle in Fig. 7a, it is seen thatthe particle only contains Cu, and it indicates thatintermixing of Cu and GaAs has not occurred. Theseresults again show that a Ti interlayer between TiN andthe GaAs substrate significantly improves the ability ofTiN film as a diffusion barrier.Fig. 8a and b show AES depth profiles for the as-

deposited CuyGaAs sample and the 5508C-annealedCuyTiNyTiyGaAs sample. The trailing edges of the Cu,Ga and As signals are much higher in Fig. 8a than inFig. 8b. It shows that severe CuyGaAs interdiffusionhas taken place during fabrication without the diffusionbarrier layers. In Fig. 8b, the oxygen profile shows athin oxide layer at the CuyTiN interface and at the TiNyTi interface. These oxide layers are considered as aresult of oxidation during the brief transfer in normalambient atmosphere outside the deposition chamberduring the fabrication process. Very little oxygen contentis observed in the Cu, TiN and Ti layers. Most impor-tantly, no copper is seen in the GaAs substrate, showingthat CuyGaAs interdiffusion did not take place in theCuyTiNyTiyGaAs sample, even after it was annealed at550 8C. Thus, it is confirmed that Ti can be successfullyemployed as a diffusion-preventative barrier for CuMetallization on GaAs.

4. Conclusion

XRD, SEM, EDS, AES and resistivity were used toinvestigate the properties of CuyGaAs, CuyTiNyGaAsand CuyTiNyTiyGaAs samples. For as-deposited CuyGaAs, interfacial mixing of Cu with the GaAs substrateoccurred, thus necessitating a diffusion barrier to preventthe fast diffusion of copper into the substrate. Diffusionin the CuyGaAs sample resulted in the formation ofCuGa , Cu As, Cu Ga and Cu As phases. For Cuy2 6 9 4 3

TiNyGaAs, the columnar grain structure of the TiN filmprovided diffusion paths for Cu at higher temperatures,as witnessed by the presence of CuGa and Ga Ti2 3 2

phases found after 5508C annealing. The CuyTiNyTiyGaAs samples were found very stable without formingdiffusion products up to 5508C. From these studies, itis concluded that the deposition of a Ti layer betweenTiN and GaAs significantly improves ability of TiN asa diffusion barrier much like that used in the AlyTiNyTiySi system, presumably by interrupting the routes ofCu andyor GaAs diffusion provided by macro-defectsin the columnar TiN film.

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Fig. 5. SEM micrographs of(a) as-deposited CuyGaAs structure;(b) at different magnification and(c) shows EDS data of(b).

Fig. 6. SEM micrographs of CuyTiNyGaAs structure(a) annealed at 5508C; (b) at different magnification and(c) EDS data of(b).

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Fig. 7. SEM micrographs of CuyTiNyTiyGaAs structure(a) annealedat 5508C and(b) EDS data of(a).

Fig. 8. AES depth profiles of:(a) as-deposited CuyGaAs sample and(b) 550 8C-annealed CuyTiNyTiyGaAs sample.

Acknowledgments

This work is supported by the National ScienceCouncil of the Republic of China under Contract No.NSC 90-2215-E006-018.

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