Amorphous Ta-Si-N thin-film alloys as diffusion barrier in AI/Si ... - Core

Amorphous Ta-Si-N thin-film alloys as diffusion barrier in AI/Si metallizations E. Kolawa, J. M. Molarius,a> C. W. Nieh, and M.-A. Nicolet CalifornialnstituteojTechnology, Pasadena, California 91125

(Received 26 October 1989; accepted 18 December 1989) Amorphous Ta-Si-N thin films of a wide range of compositions were prepared by r~ ~eactive sputtering of a Ta5 Si 3 target in a N 2 / Ar plasma. The relatio~shi~ betwee~ films' compos1t10n an~ resistivity is reported. All obtained films were tested as dtffuston barners between ~1 and S1. Backscattering spectrometry combined with cross-sectional transmis_sion electron m1eroscopy were used to determine the barrier effectiveness. It was found that alummum can be melted on ~op of the Si!Ta-Si-N structure ( 675 oc for 30 min) without any evidence of metallurgtcal interactions between the layers. crystallization temperature of amorphous Ta-Si-N alloy, I. INTRODUCTION films were deposited on ( 100) Al 2 0 3 substrates. Prior to The fabrication of stable, reproducible, and uniform conloading into the deposition chamber, the silicon wafers were tacts is essential for successful device performance. In silicon etched with diluted HF. The AI 2 0 3 substrates were cleaned integrated circuits, aluminum is commonly used for contacts ultrasonically with trichloroethanol ( TCE), acetone, and and interconnections, but a severe degradation of contacts is methanol, and then blown dry with nitrogen. All depositions caused by intermixing and reactions between aluminum and were performed in an rf sputtering system equipped with a silicon. To minimize such deleterious interactions, diffusion cryopump and a cryogenic baffle. The sputtering chamber barriers are used in very large scale integrated (VLSI) conwas evacuated to a base presssure of about 5 X 10 7 Torr tact technologies. 1- 3 Various interstitial alloys such as nibefore deposition. A magnetron-type circular cathode, 7.5 trides, 4- 9 carbides, 10- 11 borides, 12 conductive oxides, !3-zo em in diameter was used as the sputtering source. The suband amorphous alloys 21 - 27 have been investigated as diffustrate holder was placed about 7 em below the target and was sion barriers in contact metallizations. Because they lack neither cooled nor heated externally. grain boundaries that can act as fast diffusion paths, amorTheTa-Si-N films were deposited by reactive sputtering phous alloys are an attractive alternative to polycrystalline of a Ta 5 Si 3 target in an Ar/N 2 gas mixture. The flow ratios of thin films. Until now, the stability of these amorphous alloy Ar to N 2 and total gas pressure were adjusted by mass flow films, when in contact with the substrate or metal layers, has controllers and monitored with capacitive manometer in a been limited by the tendency of these alloys to react with the feedback loop. All Ta-Si-N films were sputtered with 10 metal overlayers or with the substrate rather than by the mTorr total gas pressure and 300 W forward sputtering "~ T h us, t h e c h o1ee . crystallization of the amorphous films.~''O--power. The ratio of nitrogen to argon gas flow was in the of elements which form an amorphous diffusion barrier is range of 0% to 5.3%. Negative de bias was applied to the very important. For example, binary amorphous alloys substrate holder during some depositions. The aluminun formed by two transition metals are not effective diffusion overlayer was sputter-deposited in pure argon (5 mTorr barriers between AI and Si. For these amorphous alloys, the pressure) on top of the Si-Ta-N films without breaking formation of compounds between one or both of the barrier vacuum. elements and Al or Si far below their crystallization temThe composition of the Ta-Si-N films as well as the 21 25 peratures is the primary failure mechanism. - Interstitial amount of impurities was derived from backscattering specalloys in amorphous form are another type of diffusion bartra of these films deposited on carbon. X-ray diffraction torier8·9·14 tested previously. In general, their chemical stabilgether with electron transmission microscopy were used to ity with Al and Si is superior, but they usually lose their determine a microstructure and crystallographic structure effectiveness due to interdiffusion of AI and Si which occurs of the films. The electrical resistivities of the films were obthrough localized weak spots that are always present. Even tained from sheet resistance measurements with a four-point though it seems logical that amorphous alloys should be probe. Backscattering spectrometry and XTEM were used more effective diffusion barriers than their polycrystalline to evaluate the diffusion barrier capability of the Ta-Si-N counterparts, clear evidence in support of this idea still does films metallization. Annealing of the samples was carried not exist in literature. out in a vacuum of better than 1 X 106 Torr in the temperaWe report on properties of amorphous Ta-Si-N alloys as ture range of 500-675 oc for different annealing durations. diffusion barriers between Al and Si. Backscattering specTo prepare samples for XTEM of the contact structures, trometry and cross-sectional transmission electron microthe samples were first glued together face to face, followed by graphs (XTEM) have been used to determine the stability of mechanical thinning to 10 tJm. Finally, argon ion milling at amorphous Ta-Si-N films interposed between a (Si) subliquid nitrogen temperature was used to thin the specimen to strate and an AI overlayer (Si/Ta-Si-N/AI). electron transparency. II. EXPERIMENTAL PROCEDURE Ill. RESULTS AND DISCUSSION Substrates of ( 111) oriented n-type Si of0.005 n em resistivity were used throughout this experiment. To study the 3006

J. Vac. Sci. Techno!. A 8 (3), May/Jun 1990

The atomic composition of the Ta-Si-N films was determined by backscattering spectrometry of the samples depos-

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@ 1990 American Vacuum Society

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Kolawa eta!.: Amorphous Ta-Si-N thin-film alloys

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FIG. 3. The resistivity ofT a--Si-N films is a function of the N 2 /Ar flow ratio. FIG. 1. 2 MeV 4 He backscattering spectrum of the C/Ta16Si 14 N"' sample.

ited on carbon. Figure 1 shows the backscattering spectrum of a C/Ta36Si 14N 50 sample about 120 nm thick. There is about 3 at. % of argon and 3 at. % of oxygen incorporated into these films as impurities. This impurity level is typical for films of all compositions. Figure 2 shows the composition ofTa-Si-N films as a function of the nitrogen-to-argon flow ratio in the sputtering chamber. It is immediately noticeable that increased nitrogen levels can be incorporated in the films when the nitrogen/argon flow ratio rises. The nitrogen content in the films increases continuously from 0% to 64% as the N 2/ Ar flow ratio changes from 0% to 11%. There is no sharp transition in the nitrogen concentration of theTaSi-N films over the whole range of the N 2 / Ar flow ratios investigated. Figure 3 shows the resistivity ofTa-Si-N films as a function of their composition. Resistivity of a pure 120 nm thick Ta74 Si26 film is about 265 !1 em and rises slowly to 625 !l em as the composition of the film reaches Ta 36 Si 14 N 50 • Further increase in nitrogen concentration causes a rapid

increase in the resistivity. A Ta 26Si 10 N 64 film has about 4100 n em. All films reported in this paper including Ta74Si26 were amorphous as deposited. Figure 4 shows the microstructure and diffraction patterns of the Ta74 Si26 and

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