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`ELSEVIER
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`Volume 307, Numbers 1-; 10 October 1997
`
`Contents of Volume
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`
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`The structure and residual stress in Si containing diamond-like carbon coating
`W.-J. Wu and M.-H. I-Ion .
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`6
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`Transmission electron microscopy study of Si 6-doped GaAs/AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility
`transistor structures
`S.I. Molina and T. Walther .
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`10
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`The peroxide route of the successive ionic layer deposition procedure for synthesizing nanolayers of metal oxides, hydroxides and
`peroxides
`V.P. Tolstoy .
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`Investigation of the plasma polymer deposited from pyrrole
`J. Zhang, M.Z. Wu, T.S. Pu, Z.Y. Zhang, R.P. Jin. Z.S. Tong, D.Z. Zhu, D.X. Cao, F.Y. Zhu and J.Q. Cao .
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`14
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`21
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`Effect of sandblasting on adhesion strength of diamond coatings
`B. Zhang and L. Zhou.
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`Deposition of boron carbide by laser CVD: a comparison with thennodynamic predictions
`J.C. Oliveira and O. Conde .
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`Correlation between microstructure and the optical properties of Ti02 thin fihns prepared on different substrates
`Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche and J. Rivory .
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`29
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`38
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`43
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`A structural approach to gallium phosphate thin solid films
`F. Tourtiu, P. Armand, A. Ibanez, A. Manteghetti and E. Philippot .
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`Preparation and characterization of ZnO:Al films by pulsed laser deposition
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`Z.Y. Ning, S.H. Cheng, S.B. Ge, Y. Chao, Z.Q. Gang, Y.X. Zhang and Z.G. Liu .
`Characterization of C—N thin films deposited by reactive excimer laser ablation of graphite targets in nitrogen atmosphere
`A.P. Caricato, G. Leggieri. A. Luches, A. Perrone, E. Gyorgy, l.N. Mihailescu, M. Popescu, G. Barucca, P. Mengucci, J.
`Zemek and M. Trchova .
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`60
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`C—'I‘he synthesis of CeO2+,, - n H20 nanolayers on silicon and fused-quartz surfaces by the successive ionic layer deposition
`technique
`V.P. Tolstoy and A.G. Ehrlich .
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`Multivariate analysis of noise-corrupted PECVD data
`A. von Keudell, A. Annen and V. Dose .
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`65
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`71
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`A kinetic model for photochemical vapor deposition from germane and silane
`M. Tao .
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`Crystallographic and morphological characterization of reactively sputtered Ta, Ta—N and Ta—N—0 thin films
`M. Stavrev. D. Fischer, C. Wenzel, K. Drescher and N. Mattem .
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`79
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`89
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`96
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`Texture in evaporated Ag thin fihns and its evolution during encapsulation process
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`Y. Zeng, Y.L. Zou and T.L. Alford .
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`Nucleation and growth of Cu thin films on silicon wafers deposited by radio frequency sputtering
`J.-C. Lin and C. Lee .
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`Surface dilational behavior of docosanic acid monolayers spread on the surface of drops of polymer solutions
`R. Wiistneck, J. Reiche and S. Forster .
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`A model of oxide layer growth on Ag"' and Pt‘* ion implanted nickel anode in aqueous alkaline solution
`I.S. Tashlylcov .
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`Effects of polymer substrate surface energy on nucleation and growth of evaporated gold films
`R.L.W. Smithson, D.J. McClure and DP. Evans .
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`Volume 307, Numbers l—?.. 10 October 1997
`
`Contents of Volume
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`The structure and residual stress in Si containing diamond-like carbon coating
`W.—J. Wu and M.-H. Hon .
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`Transmission electron microscopy study of Si 8-doped GaAs/AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility
`transistor structures
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`1
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`S.l. Molina and T. Walther .
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`10
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`The peroxide route of the successive ionic layer deposition procedure for synthesizing nanolayers of metal oxides, hydroxides and
`peroxides
`V.P. Tolstoy .
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`Investigation of the plasma polymer deposited from pyrrole
`J. Zhang, M.Z. Wu. T.S. Pu, Z.Y. Zhang, R.P. Jin, Z.S. Tong, D.Z. Zhu, D.X. Cao, F.Y. Zhu and J.Q. Cao .
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`Effect of sandblasting on adhesion strength of diamond coatings
`B. Zhang and L. Zhou .
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`Deposition of boron carbide by laser CVD: a comparison with thermodynamic predictions
`J.C. Oliveira and O. Conde .
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`Correlation between microstructure and the optical properties of TiO2 thin films prepared on different substrates
`Y. Leprince—Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche and J. Rivory .
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`29
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`38
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`A structural approach to gallium phosphate thin solid films
`F. Touitin, P. Armand, A. lbanez. A. Manteghetti and E. Philippot .
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`Preparation and characterization of ZnO:Al films by pulsed laser deposition
`Z.Y. Ning. S.l-l. Cheng, S.B. Ge. Y. Chao, Z.Q. Gang, Y.X. Zhang and Z.G. Liu .
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`50
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`Characterization of C—N thin films deposited by reactive excimer laser ablation of graphite targets in nitrogen atmosphere
`A.P. Caricato, G. Leggieri, A. Luches. A. Peirone, E. Gyorgy. I.N. Mihailescu, M. Popescu, G. Barucca, P. Mengucci, J.
`Zemek and M. Trchova .
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`60
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`C—The synthesis of CeO3+“ - n H30 nanolayers on silicon and fused-quartz surfaces by the successive ionic layer deposition
`technique
`V.P. Tolstoy and A.G. Ehrlich .
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`Multivariate analysis of noise—corrupted PECVD data
`A. von Keudell, A. Annen and V. Dose .
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`65
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`7]
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`A kinetic model for photochemical vapor deposition from germane and silane
`M. Tao .
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`Crystallographic and morphological characterization of reactively sputtered Ta. Ta—N and Ta—N—O thin films
`M. Stavrev. D. Fischer‘. C. Wenzel. K. Drescher and N. Mattem .
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`Texture in evaporated Ag thin films and its evolution during encapsulation process
`Y. Zeng, Y.L. Zou and T.L. Alford .
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`79
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`89
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`Nucleation and growth of Cu thin films on silicon wafers deposited by radio frequency sputtering
`J.—C. Lin and C. Lee .
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`Surface dilational behavior of docosanic acid monolayers spread on the surface of drops of polymer solutions
`R. Wilstneck. J. Reiche and S. Forster .
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`A model of oxide layer growth on Ag+ and Ptl ion implanted nickel anode in aqueous alkaline solution
`l.S. Tashlykov .
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`Effects of polymer substrate surface energy on nucleation and growth of evaporated gold films
`R.L.W. Smithson, D.J. McClure and DF. Evans .
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`100
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`106
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`Elsevier Science SA.
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`Page 4 of 16
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`Comparison of adsorption characteristics of methyl orange and oz-naphthol orange molecules onto the cationic Langmuir-—Blodgett
`films
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`M. Takahashi, K. Kobayashi, K. Takaoka and K. Tajima .
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`274
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`vii
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`Self-assembled multilayer formation of an aromatic bifunctional molecule via selective ionic interaction
`V. Patil, K.S. Mayya and M. Sastry .
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`Photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy
`J.F. Sanchez-Royo, A. Segura, 0. Lang, C. Peuenkofer, W. Jaegermarm, A. Chevy and L. Roa .
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`Sol—ge1 prepared In2O3 thin films
`A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar and W. Gtipel
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`283
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`288
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`The study of the antigen—antibody reaction by fluorescence method in LB films for immunosensor
`G.K. Chudinova, A.V. Chudinov, V.V. Savransky and A.M. Prokhorov .
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`294
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`Structural properties of a-Si
`N,:H films grown by plasma enhanced chemical vapour deposition by SiH. + NI-[3 + H2 gas mixtures
`F. Giorgis. C.F. Pirri and E. Tresso .
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`Nucleation of strontium titanate films grown by PLD on silicon: a kinetic model
`R. Castro-Rodriguez, E. Vasco, F. Leccabue, B.E. Watts, M. Zapata-Torres and A.l. Oliva .
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`298
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`306
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`311
`313
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`Author Index of Volume 307 .
`Subject Index of Volume 307 .
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`The table of contents of Thin Solid Films is included in ESTOC—Elsevier Science Tables of Contents service—which can be
`accessed on the World Wide Web at the following URL addresses:
`http://www.elsevier.nl/locate/estoc or http://www.elsevier.com/locate/estoc
`
`describing our requirements is available from the publisher upon request.
`
`The publisher encourages the submission of articles in electronic form thus saving time and avoiding rekeying errors. A leaflet
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`Page 6 of 16
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`Page 6 of 16
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`
`Thin Solid Films 307 (1997) 79-88
`
`the
`
`fl
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`rid]
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`Crystallographic and morphological characterization of reactively
`sputtered Ta, Ta—N and Ta—N—O thin films
`
`Momtchil Stavrev "°' , Dirk Fischer ", Christian Wenzel ", Kurt Drescher 3, Norbert Mattem "
`
`' Dresden University of Technology, Semiconductor and Micrcsystems Technology laboratory, Dresden 01062, Germany
`b Institute of Solid State and Materials Research Helmholtzstr. 20, Dresden 01069, Germany
`
`Received 29 January 1997; accepted 21 May 1997
`
`
`
`Abstract
`
`This paper concentrates on the deposition of Ta, Ta—N and Ta—N—O thin films by r.f. magnetron sputtering in Ar/N,/O2 gas
`mixtures. The film properties and their suitability as diffusion barriers and protective coatings in silicon devices were characterized using
`four-point probe measurements, Auger electron spectroscopy, Rutherford backscaxtering, glancing angle X-ray diffractometry, atomic
`force microscopy and scanning electron microscopy. With the addition of N, to the gas mixture a transition from tetragonal Ta to
`b.c.c.-Ta(N) was detected,
`leading to the nanocrystalline metastable b.c.c.-Ta(N) phase with approximately 20 at.% interstitially
`incorporated nitrogen. Increasing the nitrogen flow above a critical value, an abmpt transition between metal-sputtering to nitride-sputter-
`ing mode was observed, resulting in a sharp increase in the N:Ta atomic ratio slightly above the stoichiometric value for the TaN phase.
`which was found to exhibit f.c.c. structure. With the addition of oxygen at fixed nitrogen flow the films tend to grow in an amorphous
`state. Due to the lack of short-circuit diffusion paths, the as-deposited amorphous Ta(N,O) films are considered as excellent candidates for
`ultra-thin diffusion barriers and protection layers in future Cu-metalliaed ULSI devices. ©1997 Elsevier Science S.A.
`
`Keywords: Tantalum; PVD; Amorphous materials; Diffusion barrier
`
`1. Introduction
`
`In the past, Ta and Ta-based compounds have been
`investigated as thin film resistor materials with low tem-
`perature coefficient of resistivity [1] and as stable contact
`materials to Si [2]. More recently, the suitability as diffu-
`sion or drift barriers between metal layers (Al, Cu. etc.)
`and semiconductors (Si, GaAs) or dielectrics (SiO2, poly-
`imide) has been examined and a superior thermal stability
`has been reported [3—12].
`Concerning the future Cu-metallized interconnection
`systems, as the line widths diminish, the barrier layer can
`significantly increase the net line resistance. Hence, the
`barrier has to be very thin to avoid increasing net line
`resistance for a given cross-sectional area of the conductor,
`but it has to be stable enough to provide acceptable barrier
`properties [4—7]. The use of amorphous or amorphous-like
`diffusion barriers, free of extended defects, can signifi-
`
`‘ Corresponding author. Fax: +49 351 4637172;
`stavrev@ehmgw1.et.tu-dresden.de
`
`e-mail:
`
`0040-6090/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved.
`PII S0040-6090(97)00319-2
`Page 7 of 16
`
`cantly improve the thermal stability and reliability of the
`Cu-based contact and interconnect systems [3].
`Sputter-deposited Ta—N alloys have been known as an
`attractive class of materials because of their high chemical
`and mechanical stability and good conductivity. As has
`been reported earlier,
`the electrical properties,
`the stoi-
`chiometry, the crystallographic structure and the morphol-
`ogy of these metallic compounds depend on the sputtering
`conditions, substrate type, pre-treatment and film thick-
`ness. In order to optimize the Ta—N film properties many
`researchers have investigated the role of the reactive gas
`component, either as nitrogen partial pressure or nitrogen
`partial flow rate [l,4,13-18]. The reported sequence of the
`phases produced by increasing the nitrogen content [l,l3-
`15] is consistent with the equilibrium binary phase diagram
`[19] and can be summarized as follows: (i) equilibrium
`body-centered cubic at-Ta phase or metastable tetragonal
`[3-Ta, which are known to have a low solubility for N (5 S
`at.%) at room temperature [I9]; (ii) h.c.p.-Ta,N existing
`over a range between 22 and 35 at.% N [1,13,15,19]; (iii)
`f.c.c.-TaN phase [l,l3,l4]; and (iv) various nitrogen-rich
`compounds (Ta5N5, Ta4N,, Ta3N5), at which the nitrogen
`concentration levels off [l3,l5,19].
`
`Page 7 of 16
`
`

`

`80
`
`M. Sravrev er a1. / Thin Solid Films 307 (1997) 79-88
`
`On the contrary, other researchers have observed in the
`initial stage of N2 addition a partial or complete transfor-
`mation of B-Ta to oz-Ta phase prior to the Ta2N or TaN
`formation [16—18]. Aita et al. [18] observed that after such
`transformation, a change from b.c.c. Ta(N) to f.c.c.-TaN
`occurred abruptly without deposition of Ta,N, when the
`reactive gas component was increased by only 0.3%. But
`no sufficient fundamental research on the deposition condi-
`tions leading to this transfomration and on the film compo-
`sition and microstructure is present in the literature.
`In the Ta—O binary system, up to approximately 30
`at.% of oxygen could be interstitially dissolved in the
`b.c.c.-Ta lattice prior to conversion into amorphous or
`polycrystalline Ta20, phase [1,l6]. As an impurity in
`polycrystalline Ta films, 0 is believed to increase the
`effectiveness of the diffusion barriers by decorating the
`extended defects such as grain boundaries, thereby block-
`ing the active paths for grain boundary diffusion [12].
`Furthermore, with the growth of tantalum nitrides and
`oxides a reduction in grain size was observed, leading to
`nanocrystalline or amorphous-like structure [l,5,6,l3].
`To clarify the influence of N and 0 addition on the
`diffusion barrier behaviour,
`it is necessary not only to
`determine the composition and crystalline structure of the
`thin films but also the film morphology in terms of grain
`size, grain orientation, degree of amorphization, etc. Since
`there are some discrepancies in the literature on how the
`nitrogen and oxygen influence the film properties,
`this
`paper contributes to the general understanding of the con-
`ditions for formation of different Ta, Ta—N and Ta—N—O
`
`phases. Furthermore, it concentrates on the reactive sput-
`tering of conductive diffusion barriers with a special em-
`phasis on the crystallography and the existence of
`nanocrystalline or amorphous phases in the Ta—N binary
`system and partially in the Ta—N—O ternary system.
`
`2. Experimental details
`
`Both substrate cleaning and film deposition were per-
`fonned on a five-chamber-cluster-tool including load-lock,
`dealer, Ta-PVD module and inductively coupled plasma
`(ICP) soft etch module. The base pressure in the PVD
`chamber was 3 X 10" Pa. The Ar, N2 and 02 gas flows
`were controlled within 10.1 sccm by mass flow con-
`trollers, which guarantee reproducible deposition condi-
`tions. The gas purity was 99.9999%. The base vacuum and
`the Ar/N2/O2 gas mixture were analyzed by quadrupole
`mass spectrometer, mounted in a separate throttled UHV
`chamber, which was differentially pumped down to 10"
`Pa. With this configuration a reduction in total and partial
`pressure in the spectrometer chamber occurs. This enables
`gas monitoring during sputtering and guarantees high sig-
`nal-to-noise ratio.
`
`For this study, 100 mm Q Si(100) wafers were used. A
`standard RCA clean was performed prior to loading them
`
`Page 8 of 16
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`into the load-lock. After soft-etching at 200 W in Ar
`plasma and without breaking vacuum, 10 to 100 nm thin
`Ta-based films were deposited by r.f. magnetron sputtering
`at 1 kW forward power from a 332 m E Ta target
`(99.9S%). The target-to-substrate distance was 50 mm.
`While the Ar flow was kept at 5 sccm, the nitrogen flow
`<15": and oxygen flow (D02 were varied between 0-5 sccm
`and 0-10 sccm respectively, resulting in process pressures
`between 0.22 and 0.3 Pa. For determination of the deposi-
`tion rates and thickness non-uniforrnities the films were
`
`patterned by lift-off technique and measured by surface
`profilometry. For characterization of the film microstruc-
`ture 1.5 pm thick multilayers consisting of several single
`films were sputtered under simultaneous variation of the
`nitrogen flow from 0 to 5 sccm with the rate of 0.1
`sccm/rnin.
`The Ta, Ta—N and Ta—N—O films were analyzed by
`four-point probe sheet resistance measurements and laser
`profilometry for characterizing the resistivity and intrinsic
`film stress, respectively. The fihn composition was deter-
`mined by Auger electron spectroscopy (ABS) in combina-
`tion with Ar sputter etching using elemental sensitivity
`values from Ref. [20]. Additionally, the film composition
`was
`characterized by 3.34 MeV ‘I-le"' Rutherford
`backscattering spectrometry (RBS). The RBS measure-
`ments were quantified by RUMP simulation. For determi-
`nation of the nitrogen content a calibration using the
`procedure described by Herring [21] was performed. X-ray
`diffraction (XRD) patterns of 100 nm Ta, Ta—N and
`Ta—N—O films were recorded at a HZG4 diffractometer
`
`equipped with Siemens rotating Cu anode, thin film sup-
`plement and secondary graphite monochromator. To en-
`hance the sensitivity for X-rays, the measurements were
`performed in parallel beam geometry at a constant incident
`angle of a:= 2°. The registered angle range was 20=
`20.0—l00.0° with a step size A2@= 0.05° and a measur-
`ing time of 20 s per step. To analyze the phase composi-
`tion of the thick Ta—N multilayers, XRD patterns at differ-
`
`3a.
`
`5..
`
`-0...
`
`[Pa] 3
`PartialPressurepm»
`
`
`J.
`
`'
`
`high-nu
`maul-sputtering
`mode
`
`0.0
`0.5
`1.0
`1,5
`2.0
`2.5
`3.0
`3.5
`4.0
`4.5
`5.0
`
`Nitrogen Flow Rate (DN: [sccm]
`
`Fifi. l. Nitrogen partial pressure p"; in the quadrupole chamber VS-
`“i“'°8¢|1 flow <15": Parameters: Ar flow £15,“: 5 sccm; forward power! 1
`kW; total pressure: 0.22. . .0.3 Pa.
`
`Page 8 of 16
`
`RED
`ACT
`ED
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`

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`Page 9 of 16
`
`Page 9 of 16
`
`

`

`82
`
`M. Staurev er a1. / Thin Solid Films 307 (1997) 79-88
`
`Resistivityp[|,LQcm] 10’
`
`2
`
`3
`
`4
`
`5
`
`e
`
`1
`
`o
`
`1
`
`Oxygen Flow 00: [sccm]
`
`Fig. 2. Resistivity of Ta—N—O thin films vs. oxygen flow 4502 at fixed
`nitrogen flow CDNZ.
`
`ent angles of incidence from a = 1° to 20° were measured.
`The penetration depth, t of the X-rays perpendicular to the
`film surface is determined by the absorption coefficient ].L
`(for Ta—N: p. z 2500 cm“) and the angle of incidence a.
`t is given as:
`
`t
`
`sina
`P-
`
`(1)
`
`The XRD represents therefore only the upper 70 nm of the
`film in the case of a = 1°, whereas in the case of a = 20°
`
`with t= 1.5 pm the contribution comes from the whole
`multilayer.
`For analyzing the surface topography, measurements
`were performed on a D1 Nanoscope III atomic force
`microscope (AFM) in contact mode. Both height and
`friction type data were collected. Root mean square (RMS)
`roughness values were calculated from the 500 X 500 um’
`images by use of the Nanoscope HI software. Due to the
`limitations of the conventional AFM technique in deter-
`mining smaller features [22] and gathering in-depth infor-
`mation on the film structure, only a qualitative survey of
`the thin film morphology was possible. Additionally,
`cleaved multilayers were characterized by scanning elec-
`tron microscopy (SEM) using a Zeiss DSM 962 micro-
`scope.
`
`3. Results
`
`3.1. Deposition of Ta, Ta—N and Ta—N—O films
`
`Results from reactive sputtering of Ta in Ar/N2 mix-
`ture are presented in Fig. 1. This figure shows a typical
`hysteresis change in nitrogen partial pressure with increas-
`ing nitrogen flow 451.2. The arrows indicate whether the
`measurements are taken during increasing or decreasing
`QM. In order to operate in the high-rate metal-sputtering
`mode, two critical threshold N2 flows have to be taken
`into account, <D,,',1=2.6 sccm and ¢15,,‘,2' =2 sccm for
`
`Page 10 of 16
`
`increasing and decreasing the flow, respectively. Due to
`the differential pumping of the quadrupole mass spectrom-
`eter the measured partial pressure values are reduced by a
`factor of approximately 5000.
`Concurrently,
`the deposition rate of Ta—N films de-
`creases and the resistivity increases by increasing the
`nitrogen flow as listed in Table l with only a very slight
`drop in both below <1>,;1. Without nitrogen, the film resis-
`tivity is very close to the resistivity of B-Ta (165 p.() cm).
`For nitrogen flow above the threshold value 45,;
`the
`resistivity dramatically changes up to 4 m0, cm, due to
`sputtering of reaction products from the poisoned target
`and formation of Ta—N compounds on the substrate. Nev-
`ertheless, for 45,,‘ S 3.5 sccm Ta—N films with resistivity
`below 1 mil cm have been achieved. Resistivity in this
`range is reported as acceptable for diffusion barriers [23].
`The intrinsic stress,
`(7
`in the films was found to be
`compressive, with oz -0.9 j; 0.2 GPa for <I>N1 5 (N1
`and oz -1.5 ;t 0.3 GPa for <15NZ > <15,.‘,z.
`The addition of oxygen above a specific oxygen flow
`4502 at fixed <15”: results always in oxide target poisoning
`and leads to drastic decrease of Ta—N—O deposition rate
`and increase in Ta—N—O resistivity (see Fig. 2). In this
`study, a parameter set (QM: tI>Nz:<.‘l5oz = 5:2.5:2) for deposi-
`tion of Ta-N-O thin films was chosen, which guarantees
`high deposition rates and resistivities about 250 /1.0 cm.
`
`(3) 1oo
`
`0
`
`3
`
`.
`
`(b) 100
`
`15
`12
`9
`Sputter Time [min]
`
`18 “ 24
`
` Atomic
`
`Fraction[Va]
`0|0
`
`
`
`
`
`Atomicfraction[96]
`
`0
`
`3
`
`e
`
`1
`12
`I
`9
`Sputter time [min]
`
`15
`
`24
`
`Fig. 3. M38 depth profiles of Ta-N films sputtered at (a) 415": = 2.5 80¢!“
`and ‘par ‘ 5 50cm: (b) 45": — 3.5 sccm and 45,, = 5 sccm.
`
`RED
`ACT
`ED
`
`REDACTED
`
`Page 10 of 16
`
`

`

`M. Staurev et al. / Thin Solid Films 307 (1997) 79-88
`
`83
`
` Nitrogen Flow
`
`5 seem
`
`out
`
`3.5 SOCIT1
`
`3 seem
`
`2.5 seem
`
`2 seem
`
`1.5 SCCITI
`
`1 seem
`
`0 seem
`
`
`
`Intensity(a.u.)
`
`550
`
`500
`
`450
`
`400
`
`0) 01O
`
`008
`
`250
`
`200
`
`150
`
`100
`
`50
`
`0
`
`o
`
`10
`
`20
`
`so 40 so so 70
`
`so so 1oo
`
`Fig. 4. XRD patterns of 100 nm Ta—N thin films sputtered in Ar/N, gas mixture.
`
`28 [deg]
`
`Table 1 gives a summaiy of the main properties of the
`investigated Ta—N—O films.
`
`3.2. Composition of the Ta, Ta—N and Ta—N-0 thin films
`
`The nitrogen-to-tantalum (N:Ta) atomic ratio in the
`films as determined by RBS and AES is presented in Table
`1. As measured by both techniques, there is only a slight
`increase in the nitrogen concentration between 0 and 2.5
`sccm, resulting in about 20 at.% of N for ID": = 2.5 sccm.
`By further addition of nitrogen, an abrupt increase of the
`N:Ta-ratio is observed. Neither by RBS nor by ABS,
`N:Ta—ratios typical for the fonnation of the Ta2N phase
`were found. According to ABS, for <15", =3.5 sccm a
`nearly stoichiometric N:Ta-ratio was detemiined, suggest-
`ing the deposition of a TaN phase. Above 3.5 sccm N2
`flow a saturation of the N:Ta atomic ratio at 1.1 occurs.
`
`On the other hand, using RBS a much higher N:Ta-ratio of
`about 1.6 is estimated. In order to exclude non-uniformity
`effects, ABS depth profiling was performed. The results,
`plotted in Fig. 3 for 45": = 2.5 sccm and 47”, - 3.5 sccm,
`show uniform distribution of the nitrogen throughout both
`
`films. As further impurities small amounts of 0, C and Ar
`(total impurity concentration 5 5 at.%) were found in the
`Ta—N films.
`
`In the case of Ta—N—O films, sputtered at <PA,:<15N2:¢oz
`= 5:2.5:2, the predominant impurity dissolved in the films
`was found to be nitrogen (about 17 at.%). Furthennore,
`approximately 3 at.% oxygen were measured by AES.
`
`Table 2
`
`Summary of the XRD results on Ta and Ta—N thin films
`
`Nitrogen
`flow, <17"1
`(sccm)
`
`Crystalline
`structure/
`phase
`
`0
`1
`
`1.5
`2
`2.5
`3
`3.5
`5
`
`predominantly tetragonal Ta
`tetragonal Ta +
`b.c.c.-Ta
`b.c.c.-Ta(N)
`b.c.c.-Ta(N)
`b.c.c.-Ta(N)
`f.c.c.-TaN
`f.c.c.-TaN
`f.e.c.-TaN + amorphous
`
`Lattice
`constants.
`no (nm)/
`co (nm)
`
`0.53/ 1 .006
`0.53/ 1.006
`0.334
`0.335
`0.337
`0.342
`0.436
`0.435
`0.435
`
`Mean
`crystallite
`size,
`D (nm)
`
`90
`80
`10
`5.5
`4
`3
`6
`5
`5
`
`Page 11 of 16
`
`Page 11 of 16
`
`

`

`M. Slaureu et al. / Thin Solid Film: 307 (1997) 79-88
`
`3.3. Crystallographic and morphological characterization
`of thin Ta—based films
`
`Fig. 4 shows the X-ray diffraction patterns of 100 nm
`Ta—N films grown in various Ar/N, mixtures on Si(100)
`
`substrates. The variations in position, intensity and shape
`of the reflections indicate changes in the phase composi-
`tion of the Ta—N films. The analysis of the angle depen-
`dence of the width at half maximum points to grain size
`effects. The calculation of the mean crystallite size, D
`
`
`
`
`.....,,......n»_e.....esm.m.m..:.....ami.at.§..._._eE3....e.wa..%eh...L....._....mrw....K$.LE.“
`
`
`
`
`..4...l.....,3»)
`
`
`
`
`0.‘3...............2...fl..»..5.....a.u.....:
`
`Fig. 5. AFM images of 100 nm Ta-N thin films sputtered in Ar ‘-
`/N gum’:
`scan size: 500 X 500 nmz; Note an differences
`Z ranlgetflt On
`seem: (0 3.5 socm
`
`Page 12 of 16
`
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`Page 12 of 16
`
`
`

`

`M. Staureu er al. / Thin Solid Films 307 (I997) 79-88
`
`85
`
`500
`
`‘5°
`400
`
`350
`
`amorphous Ta(N,O)
`
`300
`
`250
`
`
`
`100 50
`
`from the full width at half maximum (FWHM) after
`correction for instrumental contribution using the Scherrer
`equation [24]:
`A
`D FWHM - cos0
`
`(2)
`
`with A, X-ray wave length and 9, diffraction angle, gives
`values in the nanometer range. These values along with the
`determined phase composition and the calculated lattice
`constants are given in Table 2.
`Pure Ta films sputtered in Ar plasma grow predomi-
`nantly in the tetragonal B-phase and are apparently poly-
`crystalline as determined by XRD (Fig. 4). In Fig. 5a, an
`AFM image of the film surface is plotted, showing grains
`with circular cross-section and low surface roughness
`(RMS '4 0.54 nm).
`The addition of only 1 sccm N, to the sputtering gas
`results in phase mixture of tetragonal B-Ta and b.c.c.-Ta.
`According to the AFM image (Fig. 5b) this film consists
`not only of circular grains like the [%-Ta film, but also of
`long grains parallel to the surface.
`At 4?": = 1.5 sccm, a b.c.c. phase is formed with lattice
`constant ac, = 0.335 nm, which is slightly higher than the
`value reported for the b.c.c.-Ta phase [JCPDS-ICDD 4-
`788]. As shown in Fig. 5c, this films consist predominantly
`of long. randomly oriented grains attributable to the b.c.c.
`phase. The calculated mean crystallite size is about 6 nm
`and the RMS roughness about 0.75 nm.
`By further addition of N, up to the threshold flow <1>,';2,
`the b.c.c. crystalline structure is preserved. Additionally, a
`peak broadening and a slight shift in peak position towards
`smaller 20-angles become obvious,
`the latter revealing
`further increase of the lattice constant an.