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`Volume 307, Numbers 1-2, 10 October 1997
`ELSEVIER
`eerc“OFOFCONGRst
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`1998
`Contents of Volume 30%LeventOfow &
`
`
`
`Thestructure and residual stress in Si containing diamond-like carbon coating
`W.-J. Wu and M.-H. Hon. 2... 0ee eee ee ee
`Transmission electron microscopy study of Si 6-doped GaAs/AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility
`transistor structures
`S.L. Molina and T. Walther... 0...ee ee ee ee ee
`The peroxide route of the successive ionic layer deposition procedure for synthesizing nanolayers of metal oxides, hydroxides and
`peroxides
`V.P. Tolstoy 2 ee ee en EE Re eee EE EE Er ee
`Investigation of the plasma polymer deposited from pyrrole
`J. Zhang, M.Z. Wu,T.S. Pu, Z.Y. Zhang, RP. Jin, Z.S. Tong, D.Z. Zhu, D.X. Cao, F.Y. Zhu and J.Q. Cao... - see ees
`Effect of sandblasting on adhesion strength of diamond coatings
`B. Zhang and L. Zhou...eeee Ee es
`Deposition of boron carbide by laser CVD: a comparison with thermodynamic predictions
`J.C. Oliveira and O. Conde. 2... ee ee ee EEE Ee ES
`Correlation between microstructure and the optical properties of TiO, thin films prepared on different substrates
`Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche and J. Rivory. ...- 6-0 e eee teens
`A structural approach to gallium phosphate thin solid films
`F. Tourtin, P. Armand, A. Ibanez, A. Manteghetti and E. Philippot. 2... 6. eee ee ee eee eee
`Preparation and characterization of ZnO:Al films by pulsed laser deposition
`Z.Y. Ning, S.H. Cheng, S.B. Ge, Y. Chao, Z.Q. Gang, Y.X. Zhang and Z.G. Liu... ee ee ee ee
`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, I.N. Mihailescu, M. Popescu, G. Barucca, P. Mengucci, J.
`Zemek and M. Trchova.
`.1...ee
`C—Thesynthesis of CeO,,,,° H,O nanolayers onsilicon and fused-quartz surfaces by the successive ionic layer deposition
`technique
`V.P. Tolstoy and A.G. Ehrlich. 2... ee ee ee nn eee
`Multivariate analysis of noise-corrupted PECVD data
`A. von Keudell, A. Annenand V. Dose... 6.ee ee ee ee Ee eee
`A kinetic model for photochemical vapor deposition from germane and silane
`M. TaO. cccee EE ee ee eee
`Crystallographic and morphological characterization of reactively sputtered Ta, Ta—N and Ta-—N-Othin films
`M.Stavrev, D. Fischer, C. Wenzel, K. Drescher and N. Mattern 6... ee eee ee ee ee eee
`Texture in evaporated Ag thin films andits evolution during encapsulation process
`Y. Zeng, Y.L. Zou and T.L. Alford. 2... eee eee ee
`Nucleation and growth of Cu thin films onsilicon wafers deposited by radio frequency sputtering
`T.-C. Linand C. Lee...ee ee ee Ee ee ee eee
`Surface dilational behavior of docosanic acid monolayers spread on the surface of drops of polymer solutions
`R. Wiistneck, J. Reiche and S. Forster... ee ee ee ee ee ee eee
`A modelof oxide layer growth on Ag* and Pt* ion implanted nickel anode in aqueous alkaline solution
`LS. Tashlykovoe ne ee ees
`Effects of polymer substrate surface energy on nucleation and growth of evaporated gold films
`R.L.W.Smithson, D.J. McClure and D.F. Evans...ee ee ee eee ees
`
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`106
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`110
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`Elsevier Science S.A.
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`
`
`
`
`
`
`ELSEVIER
`
`
`
`
`
`Volume 307, Numbers 1—2, 10 October 1997
`
`Contents of Volume
`
`eTOI
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`any OF CONGR
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`JAN 2
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`The structure and residual stress in Si containing diamond-like carbon coating
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`Transmission electron microscopy study of Si 6-doped GaAs/AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility
`transistor structures
`SUL Molinaland Ie Waltheta.. .s¢s so clee ce see ee es ete eee ete ele oc) oy eeelialme aide pra ieyegioile els
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`Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche and J. Rivory..... 6... eee ee eet ete tees
`A structural approach to gallium phosphate thin solid films
`F. Tourtin, P. Armand, A. Ibanez, A. Manteghetti and E. Philippot...... 1... 11s ee ee ee ee ee ee 43
`Preparation and characterization of ZnO:Alfilms by pulsed laser deposition
`Z.Y. Ning, S.H. Cheng, S.B. Ge, Y. Chao, Z.Q. Gang, Y.X. Zhang and Z.G. Liu... 1...) - ee ee ee 50
`Characterization of C—N thin films deposited by reactive excimerlaser ablation of graphite targets in nitrogen atmosphere
`A.P. Caricato, G. Leggieri, A. Luches, A. Perrone, E. Gyorgy, I.N. Mihailescu, M. Popescu, G. Barucca, P. Mengucci, J.
`Zemek and M. Trchova. ... 2... ee eee ee ee ee ee ee ee ee ee
`C—Thesynthesis of CeO,,,,:n H,O nanolayers on silicon and fused-quartz surfaces by the successive ionic layer deposition
`technique
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`Crystallographic and morphological characterization of reactively sputtered Ta, Ta—N and Ta—N-Othin films
`M. Stavrev, D. Fischer, C. Wenzel, K. Drescher and N. Mattern... 1. ee ee ee ee eee
`Texture in evaporated Ag thin films and its evolution during encapsulation process
`¥. Zeng, Ya. Zowand VL. Alford... Ne Oe ee ee ile 2 + ls ecco 89
`Nucleation and growth of Cuthin filmson silicon wafers deposited by radio frequency sputtering
`tts cagee Me ociisikedoa
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`Surface dilational behavior of docosanic acid monolayers spread on the surface of drops of polymersolutions
`R. Wiistneck, J. Reiche and S. Forster...et ee tt te te tt ee ee ee eee 100
`A model ofoxide layer growth on Ag* and Pt* ion implanted nickel anode in aqueous alkaline solution
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`Effects of polymersubstrate surface energy on nucleation and growth of evaporated gold films
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`MCh e RETR SS 106
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`Elsevier Science S.A.
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`Vii
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`Comparison of adsorption characteristics of methyl orange and a-naphthol orange molecules onto the cationic Langmuir—Blodgett
`films
`
`M.Takahashi, K. Kobayashi, K. Takaoka and K. Tajima... 6... eee eee eens 274
`Self-assembled multilayer formation of an aromatic bifunctional molecule via selective ionic interaction
`V. Patil, K.S. Mayya and M. Sastry 2.0. ee ee eee ee ee eee eee ee ee eee 280
`Photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy
`/
`JF. Sénchez-Royo, A. Segura, O. Lang, C. Pettenkofer, W. Jaegermann, A. Chevy and L. Roa... -.-. +--+ 2+ s eer 283
`Sol—gel prepared In,O, thin films
`.
`A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar and W. Gipel
`. 6. 6 ee ee ee eee ees 288
`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... 6... 2s eee eee eee eee ees
`Structural properties of a-Si
`N,:H films grown by plasma enhanced chemical vapour deposition by SiH, + NH, +H, gas mixtures
`F. Giorgis, C.F. Pirri and E. Tress0. 6...ee eee eee eee ee ees 298
`Nucleation of strontium titanate films grown by PLD onsilicon: a kinetic model
`R. Castro-Rodriguez, E. Vasco, F. Leccabue, B.E. Watts, M. Zapata-Torres and A.J. Oliva 2... 6. eee eee ee ees 306
`
`294
`
`Author Index of Volume 307 1... 0. ce ee ee eee EE ee 311
`Subject Index of Volume 307 .. 0... eee eee eet eee tee ee ete tees 313
`
`describing our requirements is available from the publisher upon request.
`
`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 Webatthe following URL addresses:
`http: //www.elsevier.nl/locate/estoc or http: //www.elsevier.com/locate/estoc
`The publisher encourages the submission ofarticles in electronic form thus saving time and avoiding rekeying errors. A leaflet
`
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` ell
`LSEVIER
`
`Thin Solid Films 307 (1997) 79-88
`
`
`
`thi.
`
`fun
`
`
`
`Crystallographic and morphological characterization of reactively
`sputtered Ta, Ta—N and Ta—N-—Othin films
`
`Momtchil Stavrev **, Dirk Fischer *, Christian Wenzel *, Kurt Drescher *, Norbert Mattern °
`* Dresden University of Technology, Semiconductor and Micrcsystems Technology Laboratory, Dresden 01062, Germany
`> Institute ofSolid State and Materials Research, Helmhoitzstr. 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,/O, 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 backscattering, glancing angle X-ray diffractometry, atomic
`
`force microscopy and scanning electron microscopy. With the addition of N, to the gas mixtureatransition 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 abovea critical value, an abrupt transition between metal-sputtering to nitride-sputter-
`ing mode wasobserved,resulting in a sharp increase in the N:Taatomicratio 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 ofshort-circuit diffusion paths, the as-deposited amorphous Ta(N,O)filmsare considered as excellent candidates for
`ultra-thin diffusion barriers and protection layers in future Cu-metallized 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 (SiO,, 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,
`butit 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
`stavrev @ehmgw L.et.tu-dresden.de
`
`4637172;
`
`e-mail:
`
`0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved.
`PII $0040-6090(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 knownas 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 [1,4,13—18]. The reported sequence of the
`phases produced by increasing the nitrogen content[1,13-
`15] is consistent with the equilibrium binary phase diagram
`[19] and can be summarized as follows: (i) equilibrium
`body-centered cubic a-Ta phase or metastable tetragonal
`B-Ta, which are knownto have a low solubility for N (< 5
`at.%) at room temperature [19]; (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 [1,13,14]; and (iv) various nitrogen-rich
`compounds (Ta,N,, Ta,N;, Ta;N;), at which the nitrogen
`concentration levels off [13,15,19].
`
`Page 7 of 16
`
`

`

`80
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`M.Stavrev etal. / Thin Solid Films 307 (1997) 79-88
`
`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. magnetronsputtering
`at
`1 kW forward power from a 332 mm &@ Tatarget
`(99.95%). The target-to-substrate distance was 50 mm.
`While the Ar flow was kept at 5 sccm, the nitrogen flow
`x, and oxygen flow ®p, were varied between 0-5 sccm
`and0-10 sccm respectively, resulting in process pressures
`between 0.22 and 0.3 Pa. For determination of the deposi-
`tion rates and thickness non-uniformities the films were
`patterned by lift-off technique and measured by surface
`profilometry. For characterization of the film microstruc-
`ture 1.5 ym 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/min.
`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 film composition was deter-
`mined by Auger electron spectroscopy (AES) in combina-
`tion with Ar sputter etching using elemental sensitivity
`values from Ref. [20]. Additionally, the film composition
`was
`characterized by 3.34 MeV “He* Rutherford
`backscattering spectrometry (RBS). The RBS measure-
`ments were quantified by RUMPsimulation. 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
`performedin parallel beam geometry at a constant incident
`angle of a= 2°. The registered angle range was 20=
`20.0—100.0° 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-
`
`low-rate
`compound-sputtering
`mode
`
`high-rate
`metal-sputtering
`mode
`
`Onthe contrary, other researchers have observedin the
`initial stage of N, addition a partial or complete transfor-
`mation of f-Ta to a-Ta phase prior to the Ta,N 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 transformation and on the film compo-
`sition and microstructure is presentin 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 Ta,O,; phase [1,16]. As an impurity in
`polycrystalline Ta films, O 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 [1,5,6,13].
`To clarify the influence of N and O 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-—Oternary system.
`
`2. Experimental details
`
`Both substrate cleaning and film deposition were per-
`formed 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, N, and O, gas flows
`were controlled within +0.1 sccm by mass flow con-
`trollers, which guarantee reproducible deposition condi-
`tions. The gas purity was 99.9999%,. The base vacuum and
`the Ar/N,/O, gas mixture were analyzed by quadrupole
`mass spectrometer, mounted in a separate throttled UHV
`chamber, which wasdifferentially pumped down to 107°
`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 @ Si(100) wafers were used. A
`standard RCA clean was performed prior to loading them
`
`Page 8 of 16
`
`00
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`40
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`[Pa]
`
`PartialPressurePy,"
`
`35
`30
`25
`20
`#15
`Nitrogen Flow Rate Oy, {sccm]
`Fig. 1. Nitrogen partial pressure Py in the quadrupole chamber vs.
`nitrogen flow Py, Parameters: Ar flow ®,,: 5 sccm; forward power:
`1
`kW; total pressure: 0.22...0.3 Pa.
`
`40
`
`45
`
`50
`
`Page 8 of 16
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`M.Stavrevet al. / Thin Solid Films 307 (1997) 79-88
`
`82
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`Resistivityp[11Qcm]
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`Oxygen Flow ®, [sccm]
`
`Fig. 2. Resistivity of Ta-N-O thin films vs. oxygen flow ®o, atfixed
`nitrogen flow Py,.
`
`t=
`
`ent angles of incidence from @ = 1° to 20° were measured.
`The penetration depth, t of the X-rays perpendicular to the
`film surface is determined by the absorption coefficient w
`(for Ta—N: yt ~ 2500 cm~') and the angle of incidence a.
`t is given as:
`
`sina
`
`(1)
`h
`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 ym the contribution comes from the whole
`multilayer.
`For analyzing the surface topography, measurements
`were performed on a DI Nanoscope III atomic force
`microscope (AFM) in contact mode. Both height and
`friction type data were collected. Root mean square (RMS)
`roughnessvalues were calculated from the 500 x 500 nm?
`images by use of the Nanoscope III 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/N, mix-
`ture are presented in Fig. 1. This figure showsa typical
`hysteresis change in nitrogen partial pressure with increas-
`ing nitrogen flow ®y,. The arrows indicate whether the
`measurements are taken during increasing or decreasing
`®,,. In order to operate in the high-rate metal-sputtering
`mode,
`twocritical threshold N, flows have to be taken
`into account, Py, =2.6 sccm and Px.* =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 1 with only a very slight
`drop in both below ®,x.. Without nitrogen, the film resis-
`tivity is very closeto the resistivity of B-Ta (165 w© cm).
`For nitrogen flow above the threshold value x the
`resistivity dramatically changes up to 4 mQ cm,due to
`sputtering of reaction products from the poisoned target
`and formation of Ta-N compounds on the substrate. Nev-
`ertheless, for Py, < 3.5 sccm Ta-N films with resistivity
`below 1 mQ cm have been achieved. Resistivity in this
`range is reported as acceptable for diffusion barriers [23].
`The intrinsic stress, o in the films was found to be
`compressive, with o~ —0.9+0.2 GPa for Py, < Px,
`and = —1.5 + 0.3 GPa for Py, > Py,.
`The addition of oxygen above a specific oxygen flow
`®,, at fixed Py, 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 parameterset (®,,: Dy, :Dy, = 5:2.5:2) for deposi-
`tion of Ta—N-O thin films was chosen, which guarantees
`high deposition rates and resistivities about 250 wp cm.
`
`= °So
`
`>
`
`~~
`
`-
`
`
`
`AtomicFraction
`
`(b)
`
`Sputter Time [min]
`
`[%]
`[(%] 0
`
`Atomicfraction
`
`3
`
`6
`
`9
`
`42
`
`15
`
`1821
`
`24
`
`Sputter time [min]
`
`Fig. 3. AES depth profiles of Ta—N filmssputtered at (a) ®y, = 2.5 scom
`and @,, = 5 scem; (b) Py = 3.5 scom and @, = 5 sccm.
`
`RED
`ACT
`ED
`
`REDACTED
`
`Page 10 of 16
`
`

`

`M.Stavrev et al. / Thin Solid Films 307 (1997) 79-88
`
`83
`
` Nitrogen Flow
`
`Pn,
`
`5 sccm
`
`3.5 sccm
`
`3 sccm
`
`2.5 sccm
`
`2sccm
`
`1.5 sccm
`
`1 sccm
`
`O sccm
`
`550
`
`500
`
`450
`
`400
`
`350
`
`}-f.c.c.-TaN
`
`f.c.c.-TaN
`
`
`
`Intensity(a.u.)
`
`300 b.c.c.-Ta(N),
`
`250 6.C.-Ta(N)
`
`b.¢.c.-Ta(N)
`
`200
`
`150
`
`tetr. Ta &
`400 b.c.c.-Ta(N)
`
`50
`
`0
`
`tetragonal Ta
`
`0
`
`10
`
`20
`
`30
`
`40
`
`50
`
`60
`
`70
`
`80
`
`90 100
`
`20 [deg]
`
`Fig. 4. XRD patterns of 100 nm Ta-N thinfilms sputtered in Ar/N, gas mixture.
`
`Table 1 gives a summary of the main properties of the
`investigated Ta—N—O films.
`
`3.2. Composition of the Ta, Ta—N and Ta—N-Othin films
`
`The nitrogen-to-tantalum (N:Ta) atomic ratio in the
`films as determined by RBS and AESis 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 Py, = 2.5 sccm.
`By further addition of nitrogen, an abrupt increase of the
`N:Ta-ratio is observed. Neither by RBS nor by AES,
`N:Ta-ratios typical for the formation of the Ta,N phase
`were found. According to AES, for ®y,=3.5 sccm a
`nearly stoichiometric N:Ta-ratio was determined, suggest-
`ing the deposition of a TaN phase. Above 3.5 sccm N,
`flow a saturation of the N:Ta atomic ratio at 1.1 occurs.
`Onthe other hand, using RBS a much higher N:Ta-ratio of
`about 1.6 is estimated. In order to exclude non-uniformity
`effects, AES depth profiling was performed. The results,
`plotted in Fig. 3 for Dy, = 2.5 sccm and Py. = 3.5 sccm,
`show uniform distribution of the nitrogen throughout both
`
`films. As further impurities small amounts of O, C and Ar
`(total impurity concentration < 5 at.%) were found in the
`Ta-N films.
`In the case of Ta~N—O films, sputtered at ®,,:Py:Po,
`= 5:2.5:2, the predominant impurity dissolved in the films
`was found to be nitrogen (about 17 at.%). Furthermore,
`approximately 3 at.% oxygen were measured by AES.
`
`Table 2
`
`Summary of the XRD results on Ta and Ta—N thin films
`
`Nitrogen
`flow, Py,
`(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.c.c.-TaN + amorphous
`
`Lattice
`constants,
`Qo (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
`
`

`

`84
`
`M.Stavrev et al. / Thin Solid Films 307 (1997) 79-88
`
`3.3. Crystallographic and morphological characterization
`-based films
`of thin Ta
`
`-ray diffraction patterns of 100 nm
`Fig. 4 shows the X
`mixtures on Si(100)
`Ta-N films grown in various Ar/N,
`
`substrates. The variations in position, intensity and shape
`of the reflections indicate changes in the phase composi-
`-N films. The analysis of the angle depen-
`tion of the Ta
`dence of the width at half maximum points to grain size
`effects. The calculation of the mean crystallite size, D
`
`vypopeOetT AA
`
`eytyVy
`ri
`SENTMIEI“ret
`ope
`
`3
`
`{Favsial
`
`BeaTS
`=<fi;
`tea
`Fig. 5. AFM images of 100 nm Ta-Nthin films sputtered in Ar/N.
`2 gas mixture on Si(100). (a) 0 sccm; (b) 1 sccm; (c) 1.5 sccm; (d) 2.5 seem; (e) 3
`scem; (f) 3.5 sccm. Scan size: 500 X 500 nm?;
`; Note the differences in Z range.
`
`a,s
`
`Page 12 of 16
`
`Page 12 of 16
`
`

`

`M. Stavrevet al. / Thin Solid Films 307 (1997) 79-88
`
`85
`
`500
`
`te
`400
`
`350
`
`amorphousTa(N,O)
`
`100 50
`
`300
`
`250
`
`
`
`D
`
`from the full width at half maximum (FWHM)after
`correction for instrumental contribution using the Scherrer
`equation [24]:
`xr
`(2)
`~ FWHM- cos@
`with A, X-ray wave length and @, 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. Sa, an
`AFM image of the film surface is plotted, showing grains
`with circular cross-section and low surface roughness
`(RMS = 0.54 nm).
`to the sputtering gas
`The addition of only 1 sccm N,
`results in phase mixture of tetragonal £-Ta and b.c.c.-Ta.
`According to the AFM image (Fig. 5b) this film consists
`not only of circular grains like the B-Ta film, but also of
`long grains parallel to the surface.
`At By, = 1.5 scem,a b.c.c. phase is formed withlattice
`constant a) = 0.335 nm, whichis slightly higher than the
`value reported for the b.c.c.-Ta phase [JCPDS-ICDD 4-
`788]. As shown in Fig. Sc, this films consist predominantly
`of long, randomly oriented grainsattributable to the b.c.c.
`phase. The calculated meancrystallite size is about 6 nm
`and the RMSroughness about 0.75 nm.
`By further addition of N. up to the threshold flow ®y,,
`the b.c.c. crystalline structure is preserved. Additionally, a
`peak broadening anda slight shift in peak position towards
`smaller 2@-angles become obvious,
`the latter revealing
`further increase of the lattice constant a). The calculated
`lattice constant for the phase sputtered at By, = 2.5 sccm
`is 4% higher than that of pure b.c.c.-Ta and suggests the
`formation of a metastable b.c.c.-Ta(N) phase. This lattice
`constant dy = 0.342 nm is even higher than the lattice
`constant of
`the b.c.c.-TaNy,-phase (a) = 0.337 nm)
`[JCPDS-ICDD 25-1278]. According to the calculations,
`this film is nanocrystalline with crystallites about 3 nm in
`size. The surface, examined by AFM (Fig. 5d), appears to
`be nearly structureless and very smooth (RMS ~ 0.3 nm).
`Above the critical value @y,, at Py, =3 sccm the
`crystallographic structure changes abruptly to f.c.c.-TaN
`according to the XRD measurements. The AFM image
`(Fig. 5e) shows larger grains as compared to B or b.c.c.-Ta,
`accompanied by an increase in the surface roughness
`(RMS = 1.29 nm). This trend is even much more pro-
`nounced for the f.c.c.-TaN film sputtered at Dy, = 3.5
`sccm (see Fig. 5f). The further addition of nitrogen up to 5
`sccm leads to the formation of a phase mixture consisting
`of f.c.c.-TaN and amorphous phase, evidenced by the
`changed shapeofthe first reflection. The peak consists of
`a superposition of the (111)-TaN-reflection and a diffuse
`maximum(sce Fig. 4).
`The addition of O,
`
`to the Ar/N, gas mixture at the
`
`Page 13 of 16
`
`Intensity(a.u.) 200
`
`150
`
`20
`
`30
`
`40
`
`50
`
`60
`
`70
`
`80
`
`90
`
`100
`
`20 [deg]
`
`Fig. 6. XRD pattern of 100 nm thin b.c.c.-Ta(N,O) film sputtered at
`Py, PyPo, = 5:2.5:2.
`
`ratio By:Py:Po,=5:2.5:2 produces an XRD pattern
`(Fig. 6) with only two very broad peaks comparable to the
`(110) and (211) peaks of the b.c.c. Ta(N) phase at Py, =
`2.5 sccm. The peak shape and the disappearing of the
`(200) and (220) peaks indicate lattice distortion and/or
`growth of an amorphous Ta(N,O) thin film. The shift in
`peak position towards smaller 2@-angles reveals a further
`increase of the interatomic distances. AFM measurements
`showed that the surface of the 100 nm thin amorphous
`Ta(N,O) films was very smooth (RMS = 0.28 nm).
`
`3.4. Investigation of thick Ta /Ta—N multilayers
`
`As shown above the phase composition of the Ta—N
`films changes from predominantly tetragonal
`to b.c.c.-
`Ta(N) and finally to f.c.c-TaN by variation of the nitrogen
`flow. This offers the possibility to prepare gradual multi-
`layers with a defined sequence under variation of the
`nitrogen flow from 0 to 5 sccm. Fig. 7 shows the cross-
`section of an 1.5 wm thick multilayer sputtered under
`variation of the ®,,. Three different zones could be
`distinguished. In the first Zone A, the Ta film consists of
`vertically oriented close-packed columns. With further ad-
`dition of nitrogen to the gas mixture the formation of a
`
` oP
`
`Acta2anaes
`peta)
`
`Fig. 7. Cross-sectional SEM image of cleaved Ta/Ta-N multilayer.
`
`Page 13 of 16
`
`

`

`86
`
`Intensity
`
`(a.u.)
`
`20 [deg]
`
`Fig. 8. XRD pattems of 1.5 thick multilayer at different angles of
`incidence a from 1 to 20°.
`
`second Zone B becomes obvious. In this zone, the film
`fracture results in tilted patterns, which could be associated
`with the elongated grains of the b.c.c.-Ta(N) phase. The
`density of these patterns increases with increasing film
`thickness and hence nitrogen flow. At a thickness of
`approximately 1 4m and ®y, = Px. the abrupt formation
`of a third Zone C can be observed. The Ta—N layers in
`Zone C exhibit an apparent polycrystalline microstructure,
`i.e., vertical columns with mean diameter about 50 nm.
`This is consistent with the grain size estimated by AFM on
`the 100 nm f.c.c.-TaN films. The conformation of the
`phase sequenceis given in Fig. 8, where the XRD patterns
`obtained at different incident angles are plotted. For a= 1°
`only the reflections of the f.c.c.-TaN phase are visible. At
`a@=1.5° (t= 100 nm), small contributions of the b.c.c-
`Ta(N) are additionally observed, which become enhanced
`with increasing incident angle. As in the case of single
`Ta-N films the formation of the Ta,N is not observed. At
`the penetration depth of t+ 0.5 wm (a= 6°), the multi-
`layer consists of several hundred nanometers surface layer
`of f.c.c.-TaN with b.c.c.-Ta(N) underneath. The further
`increase of a (a= 10°) shows additional reflections of
`tetragonal Ta in the XRD pattern, which indicates the
`existence o