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Aspects of TiN and Ti deposition in an ECR plasma enhanced CVD process
.....~:~::~i~:'~.'.-i::::'. ::~::;..~.3.i~:~ .~::-:,.-... #..:-:~~t.~~:~ ~. ~:~:-.';:.-:.:." <~. .~:..4.~:~::~::. . . . :.~$,.~.~i,~i~i~.: ":'
applied surface science ELSEVIER
Applied Surface Science 91 (1995) 314-320
Aspects of TiN and Ti deposition in an ECR plasma enhanced CVD process A. Weber a, *, R. Nikulski a, C.-P. Klages a, M.E. Gross b, R.M. Charatan b, R.L. Opilan b, W.L. Brown b a Fraunhofer-lnstitutefor Surface Engineering and Thin Films, Bienroder Weg 54 E, D-38108 Braunschweig, Germany b AT& T Bell Laboratories, Murray Hill, NJ 07974, USA
Received 19 March 1995; accepted for publication 25 April 1995
Abstract Tetrakis(dimethylamido) titanium (TDMAT) was used to deposit pure TiN at temperatures < 300°C by introducing it into the downstream region of an electron cyclotron resonance (ECR) plasma using nitrogen as plasma gas. The mechanism of TiN formation from TDMAT was elucidated with labeled nitrogen as plasma gas. Titanium was deposited on silicon at 500°C using titanium tetrachloride (TIC14) and a hydrogen ECR downstream plasma. The formation of titanium disilicide was confirmed by X-ray photoelectron spectroscopy (XPS) after annealing the Ti film on silicon at 800°C. After silicide formation, a TiN cap was deposited from TiC14 and a nitrogen/hydrogen plasma gas mixture. The chlorine content of the film was less than 1 at%. Thus, the combination of the TiC14 and TDMAT process is a possible approach for contact level and upper level metallization.
1. I n t r o d u c t i o n TiN is needed as a diffusion barrier to prevent the interaction o f A1 or W with the Si in ultra-large scale integrated (ULSI) devices. Ti is deposited prior to the T i N and annealed to form the low resistance material C54 TiSi 2 at the contact level. Both materials are currently deposited by sputtering methods. This line-of-sight physical vapor deposition does not achieve adequate bottom coverage in the high aspect
* Corresponding author. Tel.: +49 531 2155525; Fax: +49 531 2155901.
contact windows found in U L S I devices. Chemical vapor deposition (CVD) and plasma enhanced (PECVD) processes provide conformal coverages and are objectives o f the current research. Aside from TiC14 [1,2], metalorganic titanium compounds have been used as precursors to avoid the chlorine problem and to lower the deposition temperature for TiN. The most widely used metalorganic precursor is tetrakis(dimethylamido) titanium (TDMAT). T D M A T was employed in various M O C V D processes [ 3 - 5 ] and downstream plasma C V D processes [6,7]. The deposition o f a titaniumrich layer was reported by Akahori et al. [8] using TiC14 as precursor in a downstream E C R p l a s m a
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00137-9
A. Weberet al. /Applied Surface Science 91 (1995) 314-320 process. The successful deposition of Ti was recently reported by Ameen et al. [9] using TiCI 4 in a conventional radio frequency (rf) plasma process. Recently, we reported the successful deposition of high quality TiN using TDMAT in an electron cyclotron resonance (ECR) plasma process at substrate temperatures as low as 100°C [10]. In this paper, we describe details on the mechanism of TiN formation based on experiments with labeled nitrogen (15N2). The isotopic ratio of the N in the deposited TiN was determined by Rutherford backscattering spectrometry (RBS) and secondary ionization mass spectrometry (SIMS). The composition of the gas phase was investigated by in situ chemical ionization mass spectrometry (CIMS). Furthermore, we report preliminary results of Ti and TiN deposition employing TiC14 in a downstream ECR plasma process.
2. Experimental details Depositions were carried out using an ASTeX H P M / M ECR source mounted on a reaction vessel equipped with a heatable substrate holder. A detailed description of the experimental set up was published previously [10]. The deposition chamber was pumped by a turbomolecular pump backed by a mechanical pump to a base pressure of 1 . 0 × 10 -4 Pa. The substrates, Si (111), and thermally oxidized Si (0.5 /xm SiO2), were placed 4 inches below the ECR source flange. 2.1. Elucidation of the mechanism of TiN formation using TDMAT A flow of 15 sccm [standard cm 3 min -1] 15N2, regulated by a mass flow controller, was introduced through the ECR cavity while TDMAT vapor was introduced through a gas ring in the downstream region between the plasma cavity and the susceptor. The TDMAT was kept in a double glass wall container to allow precise thermostatic temperature control; at 25°C we obtained a flow of 2.5 sccm. The microwave power was set to 400 W and the suhstrate temperature to 300°C. The process pressure measured by a baratron capacitance manometer was 0.1 Pa.
315
TiN films deposited on Si were analyzed by secondary ion mass spectrometry (SIMS) using a Physical Electronics 6300 spectrometer with quadrupole mass filter. A 5 keV Cs + primary ion beam was used, rastered over a 600 /xm square region at a sputter rate of 0.5 n m / s . The secondary molecular ion adducts of CCs +, 14NCs+, ~5NCs+, and 48TiCs+ were monitored. The experimental details of the Rutherford backscattering spectrometry (RBS) were previously described [10]. Gas phase compositions during depositions were analyzed by CIMS [11] using Kr and Xe ions with energies of 13.99 and 12.13 eV, respectively. The incoming gas from the reactor vessel is reacting in an interaction region with the Kr or Xe ions which are created in a soft ionization unit. The mass selection takes place in quadrupole mass filter with a sensitive counting unit covering a molecular range of 500 amu. The quadrupole mass spectrometer system in the CIMS equipment is separately pumped down to 10 - 9 mbar by a turbomolecular pump. The low ionization energy compared to conventional mass spectrometry avoids the fragmentation of the parent molecules and allows therefore a safe identification of the gas species. This technique can only detect stable gas species. Due to the high ionization potential (IP) of N 2 of 14.53 eV, it does not interfere with detection of other species at m / e 28 such as HC15N, with an IP of 13.58 eV.
2.2. Deposition of Ti and TiN using
TiCl 4
Ti deposition was carried out at 500°C using 2 sccm titanium tetrachloride (TIC14) and 15 s c c m H 2 at a microwave power of 1000 W. After the Ti deposition, the sample Was annealed in situ under vacuum at 800°C for 20 min to form the titanium silicide. The susceptor temperature was lowered to 500°C and a TiN cap was deposited using 2 sccm TiC14 and adding 2 sccm N 2 to the 15 sccm H 2 flowing through the plasma cavity. The process pressure for all experiments was 0.1 Pa. The deposits were characterized by four point probe resistivity measurements on films deposited on oxidized Si wafers. The film composition was investigated by Rutherford backscattering spectrometry (RBS), secondary ion mass spectrometry (SIMS),
316
A. Weber et al. /Applied Surface Science 91 (1995) 314-320 counts/sec.
X-ray photo-electron spectroscopy (XPS), and electron probe rnicroanalysis (EPMA).
106 105
Ti
3. Results and discussion
104 103 ~ 102
3.1. Elucidation of the mechanism of TiN formation using TDMAT
C 14N
SIMS analysis of the TiN films deposited from TDMAT and ]SN2 confirms that the majority of the nitrogen in TiN originates from the plasma activated nitrogen gas and not from the dimethylamido group of the TDMAT. The strong ]SN signal in the SIMS s~Nectrum of the TiN film (Fig. 1) corresponds to a content of 93%. This agrees, within experimen-
101 r I I I ~ I I I r I I I
0
200
400 600 Depth (A)
800
1000
Fig. I. SIMS spectrum of TiN film deposited at 200°C using 15N2 as plasma gas.
background
,L
~
14N-nitrogen
=
15N-nitrogen
I
¢R
==
15NH3
14N(CH3)2
~//H
1 14NH Y
HC15N
~'//
14N(CH3)3
¢1
10 12 14 16 18 20 22 24 26 28 ao 32 34 36 38 40 42 44 46 48 s0 52 54 56 58 60
mass ~'~"~,~
[m/• ] H-C
14N. H
H-C
14N. H
m e-
91
93
95
97
99
101 103 105 107 109 111
mass
113 115 117 119 121
[m/e]
Fig. 2. CIMS spectra of the gas phase during TiN deposition using 15 sccm 14N2 or15N2 and 2.5 sccm TDMAT. Microwave power 400 W, susceptor temperature 100°C.
A. Weber et al. /Applied Surface Science 91 (1995) 314-320 ECR plasma Ti[14N(CH3)2]4 + 15N 2
~
7r-bonded ligand. The elimination of H 2 from intermediate (I) leads to the detected metallacycle (HI). A recent gas phase infrared spectroscopy study of the thermal decomposition of TDMAT in vacuum also identified the formation of three-membered T i - N - C metallacycles [12]. The attack of plasma generated nitrogen and hydrogen radicals on complex (HI) may lead to the detected gas species HC14N, HC15N,14NH3 and H~4N(CH3). The formation of 15NH3 can be explained by the reaction of 15N2 with hydrogen that originates from the conversion of I to HI in reaction Scheme 2. A related plasma CVD system involving ligand substitution whose mechanism has been explored using labeling experiments is the deposition of SiO z from an 02 plasma and tetraethoxysilane (TEOS). The reaction of TEOS (5 sccm) with a downstream 1802 (100 sccm) plasma produced a SiO 2 film with an 18O / 16 O ratio of 0.68 [13]. This indicates that the ethoxy ligand is only partially substituted by the 1So 2 and is an important source of S i - O bonds for the growing film despite starting with an 180/160 feed gas ratio of 10: 1. The retention of a60 in the SiO 2 films without significant incorporation of C suggest that cleavage of the O - C 2 H 5 bond occurs cleanly. This contrasts with our experiments, in which a starting 15N/InN feed gas ratio of 3:1 (15 sccm 15N2/2.5 sccm TDMAT) leads to a 15N/14N ratio of > 90 in the TiN film as shown by the RBS and SIMS results. That means that a plasma induced cleavage of the N - C H 3 bonds in Ti[14N(CH3)2]4 leading to Til4N plays only a minor role, as supported by the absence of 15N-containing amines in the CIMS analyses. Thus, the substitution reaction of
~ TilSN÷ [HxI4N(CH3)3.x] +
[Ti(I-ICI4N)(HI4N=CH2]+ HCI4N +HcL.~N+ 14NH3 + ISNH3 + H2 Scheme 1. Reaction products of TDMAT with ECR plasma activated labeled nitrogen as derived from RBS and CIMS measurements.
tal error (-t-5%), with the RBS measurements that showed a 15N content of 99% [10]. Although the nitrogen species (N2*, N f , or N radicals) responsible for the substitution of the dimethylamido group and the formation of TiN are not yet known, in situ CIMS using 15N2 provides some insight into the reaction mechanism. The spectra in Fig. 2 show that only NH 3 and HCN contain 15N while no 15N is detectable in the alkyl amines, H2N(CH 3) (m/e 30 = M - 1 ) , HN(CH3) z (m/e 42, 44, 46 = M - 3, M " 1, M + 1) and N(CH3) 3 (m/e 58 = M - 1). We assign mass 103 to the intermediate [Ti(HC14N)Q4NH=CH2)]. The appearance of this species along with mass 58, related to N(CH3) 3, after the ignition of the ECR nitrogen plasma may explain the formation of H 2, HCN and NH 3. The results of these analyses are summarized without considering the stoichiometry in reaction Scheme 1. Due to its high ionization poten. tial, hydrogen (15.43 eV) was not detectable by CIMS but was clearly detected by a conventional quadrupole mass spectrometer after pumping down the system to a suitable pressure range. As illustrated in reaction Scheme 2, the N 2 plasma-TDMAT interaction can lead to the fragmentation of two N(CH3) 3 (II) molecules by formation of a three-membered metallacycle (I) that is in resonance with a side-on
.CH., XN14 14~,~ o H3C" ~Ti("'CH3 H3C\N14 1~N/CH3
H3C
H:/
"C.~
317
•
H14N 14N-H
H14N 14N H + 2 14N(CH3)3
H2
H2C
CH2
Jll:[M-1- S8] I
"H2~ H-~\ 14N-H
H-C 14N-H N
CHZ
J,,,: t.-, -,o 1 I Scheme 2. Proposed fragmentation of TDMAT in the downstream ECR plasma.
318
A. Weber et al. / Applied Surface Science 91 (1995) 314-320
tion agent for the dimethylamido ligands at low temperatures as NH 3 does in the thermal M O C V D process [14]. 3.2. Formation o f " T i "
and TiN using titanium
tetrachlo ride
RBS analysis of highly reflective, silver-looking films deposited using TiC14 and a H 2 plasma at 500°C showed a high impurity incorporation, with a representative stoichiom etry of Tii.0Oo.20N0.40C0.12C10.o02. An AFM scan of a 100 nm film, shown in Fig. 3, gives a root mean square roughness of 8 ~,. The C, N, and O can be traced to the pyrolytic graphite susceptor and high background levels of N 2 and O 2 in the elastomer-sealed multipurpose C V D chamber. The low C1 content is indicative of effective reduction of the TiC14 and is comparable to results in radio frequency (13.56 MHz) H e plasma depositions with TiC14 by Ameen et al. [9].
e
Fig. 3. AFM image (800 × 800 nm2) of "Ti" layer, thickness 100 nm, RMS surface roughness 8 ,~. the dimethylamido ligands by plasma activated N 2 is strongly preferred. Our experiments clearly demonstrate that plasma activated N 2 reacts as a substituECRPACVD
TI
TI JDi
in situ anneal
Si (100)
TISix
20800°Cmin
ECR-
S (100)
TiN
TI[""-~N
[
Si (100)
I
g
Sample a)
ioo
~
I
i
,
,
l
t
'
..:............ ~l'~. . . . . t ........
/ BO .
-
"< 40-~
0
/
/ ............./ "
~
.
/
1/01,
.
60"
rd
TiSi 2
TiN cap
.__
~!
Ill
..... ~..
si~../ /
~ n ' ~ - - - "/.... " x Iri'j
J.O
20
30
nl
~
40
:""
50
Sputter T i m e (min) b)
Ti 2p region 2Pl/2
2P3/2
5
BINDING ENERGY
eV
I Ti oxide
2 TiN
Fig. 4. (a) XPS sputter depth profile of TiN/TiSix/Si multilayer. (b) Ti2p XPS spectra recorded during sputter depth profile of TiN/TiSix/Si multilayer.
A. Weber et aL /Appfied Surface Science 91 (1995) 314-320
The low deposition rate of 1.5 n m / m i n provides ample opportunity for impurity incorporation as highly reactive Ti is formed. The thermal reaction of TiC14 and H 2 to form Ti metal is thermodynamically unfavorable under conditions that are compatible with integrated circuit processing [15]. Therefore, a CVD Ti process for this application will require plasma activation to open non-equilibrium reaction pathways. The resistivity of the film of 120/x12. cm and the ability to form TiSi 2 upon annealing, as described below, suggest a viable process for contact window applications. We have also deposited TiN films at 500°C using TiCI 4 by adding N 2 to the H 2 plasma gas stream. A representative stoichiometry of Til.oNo.85Co.10Clo.0ol, as determined by RBS analysis, again reveals extremely low C1 levels. Films deposited from TiC14 and NH 3 at these temperatures have C1 contents of 5 - 8 at% [16]. Combining these processes, a " T i " film was deposited at 500°C and annealed in situ to 800°C for 20 min, and a capping film of TiN was deposited again at 500°C without breaking vacuum. An XPS depth profile of this multilayer (Fig. 4a) clearly shows formation of TiSi 2 closest to the Si substrate, a mixed TiSixOy intermediate layer, and the TiN layer on top. Looking more closely at the Ti2p region, Fig. 4b, the Ti 2p3/2 peak at 453.2 eV corresponds to the TiSi 2 layer. Thus, despite the non-ideal experimental conditions a titanium rich layer with active Ti for TiSi 2 formation could be deposited.
4. Conclusions Experiments with labeled nitrogen (15N2 ) demonstrate that the N in TiN deposited using TDMAT is mainly originated from the plasma gas and not from the dimethylamido ligand of the TDMAT. The deposition is driven by a substitution reaction with plasma activated nitrogen that leads to the deposition of high quality TiN at low substrate temperatures. Despite the highly activated species created in the ECR plasma the substitution of the dimethylamido ligands by plasma activated nitrogen occurs with a remarkable selectivity. Deposition of Ti from TiC14 in a thermal process is not possible under conditions
319
compatible with integrated circuit fabrication. Introducing TiC14 into the downstream region of a hydrogen ECR plasma leads to the formation of Ti rich films at a substrate temperature of 500°C. TiSi 2 was formed by annealing the " T i " films on Si at 800°C, and a cap of TiN could be deposited at 500°C by adding N 2 to the H 2 plasma gas. A combination of TiC14 and TDMAT processes may offer a complete CVD solution for contact level and upper level metallization, respectively. The objectives of current research are the enhancement of the deposition rates, investigations of the step coverage and film properties.
Acknowledgements The authors are indebted to K. Schiffmann (FhGIST) for AFM images. The work in Germany was supported by the Bundesministerium f'tir Bildung und Forschung (BMBF) under Grant No. 13N5849/3.
References [1] R. Kurtz and R.G. Gordon, Thin Solid Films 140 (1986) 277. [2] M.J. Buiting, A.F. Otterloo and A.H. Montree, J. Electrochem. Soe. 138 (1991) 500. [3] R.M. Fix, R.G. Gordon and D.M. Hoffmann, Chem. Mater. 2 (1990) 235. [4] A. Katz, A. Feingold, S.J. Pearton, S. Nakahara, M. Ellington, U.K. Chakrabarti, M. Geva and E. Lane, J. Appl. Phys. 70 (1991) 3666. [5] G.S. Sandhu, S.G. Meikle and T.T. Doan, Appl. Phys. Lett. 62 (1993) 240. [6] A. Weber, R. Nikulski and C.-P. Klages, Appl. Phys. Lett. 63 (1993) 325. [7] A. Intemann, H. Koerner and F. Koch, J. Electrochem. Soc. 140 (1993) 3215. [8] T. Akahori, T. Murakami and Y. Morioka, VMIC Conference Proc. 1993, p. 405. [9] M.S. Ameen, J.T. Hillman, J. Faguet, R.F. Foster, C. Arena and F. Martin, Proc. of Advanced Metallization for ULSI Applications in 1994, Ed. R. Blumenthel, 1995, accepted for publication. [10] A. Weber, R. Nikulski, C.-P. Klages, M.E. Gross, W.L. Brown, E. Dons and R.M. Charatan, J. Electrochem. Soc. t41 (1994) 849. [11] U. Tegtmeier, H.-P. Weiss and R. SchliSgl, Fresenius J. Anal. Chem. 347 (1993) 263. [12] L.H. Dubois, B.R. Zegarski and G.S. Girolami, J. Electrochem. Soc. 139 (1992) 3603.
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A. Weber et al. /Applied Surface Science 91 (1995) 314-320
[13] N. Selamoglu, J.A. Mucha, D.E. Ibbotson and D.L. Flamm, J. Vac. Sci. Technol. B 7 (1989) 1345. [14] J.M. Prybyla, C.-M. Chiang and L.H. Dubois, J. Electrochem. Soc. 140 (1993) 2695. [15] Calculations with software equiTherm Version 2.1, M.
Zeitler, B. Wittig and W. Schmidt, VCH Wissenschaftliche Software. [16] R.I. Hedge, R.W. Fiordalice, E.O. Travis and P.J. Tobin, J. Vac. Sci. Technol. B 11 (1993) 1287.
applied surface science ELSEVIER
Applied Surface Science 91 (1995) 314-320
Aspects of TiN and Ti deposition in an ECR plasma enhanced CVD process A. Weber a, *, R. Nikulski a, C.-P. Klages a, M.E. Gross b, R.M. Charatan b, R.L. Opilan b, W.L. Brown b a Fraunhofer-lnstitutefor Surface Engineering and Thin Films, Bienroder Weg 54 E, D-38108 Braunschweig, Germany b AT& T Bell Laboratories, Murray Hill, NJ 07974, USA
Received 19 March 1995; accepted for publication 25 April 1995
Abstract Tetrakis(dimethylamido) titanium (TDMAT) was used to deposit pure TiN at temperatures < 300°C by introducing it into the downstream region of an electron cyclotron resonance (ECR) plasma using nitrogen as plasma gas. The mechanism of TiN formation from TDMAT was elucidated with labeled nitrogen as plasma gas. Titanium was deposited on silicon at 500°C using titanium tetrachloride (TIC14) and a hydrogen ECR downstream plasma. The formation of titanium disilicide was confirmed by X-ray photoelectron spectroscopy (XPS) after annealing the Ti film on silicon at 800°C. After silicide formation, a TiN cap was deposited from TiC14 and a nitrogen/hydrogen plasma gas mixture. The chlorine content of the film was less than 1 at%. Thus, the combination of the TiC14 and TDMAT process is a possible approach for contact level and upper level metallization.
1. I n t r o d u c t i o n TiN is needed as a diffusion barrier to prevent the interaction o f A1 or W with the Si in ultra-large scale integrated (ULSI) devices. Ti is deposited prior to the T i N and annealed to form the low resistance material C54 TiSi 2 at the contact level. Both materials are currently deposited by sputtering methods. This line-of-sight physical vapor deposition does not achieve adequate bottom coverage in the high aspect
* Corresponding author. Tel.: +49 531 2155525; Fax: +49 531 2155901.
contact windows found in U L S I devices. Chemical vapor deposition (CVD) and plasma enhanced (PECVD) processes provide conformal coverages and are objectives o f the current research. Aside from TiC14 [1,2], metalorganic titanium compounds have been used as precursors to avoid the chlorine problem and to lower the deposition temperature for TiN. The most widely used metalorganic precursor is tetrakis(dimethylamido) titanium (TDMAT). T D M A T was employed in various M O C V D processes [ 3 - 5 ] and downstream plasma C V D processes [6,7]. The deposition o f a titaniumrich layer was reported by Akahori et al. [8] using TiC14 as precursor in a downstream E C R p l a s m a
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00137-9
A. Weberet al. /Applied Surface Science 91 (1995) 314-320 process. The successful deposition of Ti was recently reported by Ameen et al. [9] using TiCI 4 in a conventional radio frequency (rf) plasma process. Recently, we reported the successful deposition of high quality TiN using TDMAT in an electron cyclotron resonance (ECR) plasma process at substrate temperatures as low as 100°C [10]. In this paper, we describe details on the mechanism of TiN formation based on experiments with labeled nitrogen (15N2). The isotopic ratio of the N in the deposited TiN was determined by Rutherford backscattering spectrometry (RBS) and secondary ionization mass spectrometry (SIMS). The composition of the gas phase was investigated by in situ chemical ionization mass spectrometry (CIMS). Furthermore, we report preliminary results of Ti and TiN deposition employing TiC14 in a downstream ECR plasma process.
2. Experimental details Depositions were carried out using an ASTeX H P M / M ECR source mounted on a reaction vessel equipped with a heatable substrate holder. A detailed description of the experimental set up was published previously [10]. The deposition chamber was pumped by a turbomolecular pump backed by a mechanical pump to a base pressure of 1 . 0 × 10 -4 Pa. The substrates, Si (111), and thermally oxidized Si (0.5 /xm SiO2), were placed 4 inches below the ECR source flange. 2.1. Elucidation of the mechanism of TiN formation using TDMAT A flow of 15 sccm [standard cm 3 min -1] 15N2, regulated by a mass flow controller, was introduced through the ECR cavity while TDMAT vapor was introduced through a gas ring in the downstream region between the plasma cavity and the susceptor. The TDMAT was kept in a double glass wall container to allow precise thermostatic temperature control; at 25°C we obtained a flow of 2.5 sccm. The microwave power was set to 400 W and the suhstrate temperature to 300°C. The process pressure measured by a baratron capacitance manometer was 0.1 Pa.
315
TiN films deposited on Si were analyzed by secondary ion mass spectrometry (SIMS) using a Physical Electronics 6300 spectrometer with quadrupole mass filter. A 5 keV Cs + primary ion beam was used, rastered over a 600 /xm square region at a sputter rate of 0.5 n m / s . The secondary molecular ion adducts of CCs +, 14NCs+, ~5NCs+, and 48TiCs+ were monitored. The experimental details of the Rutherford backscattering spectrometry (RBS) were previously described [10]. Gas phase compositions during depositions were analyzed by CIMS [11] using Kr and Xe ions with energies of 13.99 and 12.13 eV, respectively. The incoming gas from the reactor vessel is reacting in an interaction region with the Kr or Xe ions which are created in a soft ionization unit. The mass selection takes place in quadrupole mass filter with a sensitive counting unit covering a molecular range of 500 amu. The quadrupole mass spectrometer system in the CIMS equipment is separately pumped down to 10 - 9 mbar by a turbomolecular pump. The low ionization energy compared to conventional mass spectrometry avoids the fragmentation of the parent molecules and allows therefore a safe identification of the gas species. This technique can only detect stable gas species. Due to the high ionization potential (IP) of N 2 of 14.53 eV, it does not interfere with detection of other species at m / e 28 such as HC15N, with an IP of 13.58 eV.
2.2. Deposition of Ti and TiN using
TiCl 4
Ti deposition was carried out at 500°C using 2 sccm titanium tetrachloride (TIC14) and 15 s c c m H 2 at a microwave power of 1000 W. After the Ti deposition, the sample Was annealed in situ under vacuum at 800°C for 20 min to form the titanium silicide. The susceptor temperature was lowered to 500°C and a TiN cap was deposited using 2 sccm TiC14 and adding 2 sccm N 2 to the 15 sccm H 2 flowing through the plasma cavity. The process pressure for all experiments was 0.1 Pa. The deposits were characterized by four point probe resistivity measurements on films deposited on oxidized Si wafers. The film composition was investigated by Rutherford backscattering spectrometry (RBS), secondary ion mass spectrometry (SIMS),
316
A. Weber et al. /Applied Surface Science 91 (1995) 314-320 counts/sec.
X-ray photo-electron spectroscopy (XPS), and electron probe rnicroanalysis (EPMA).
106 105
Ti
3. Results and discussion
104 103 ~ 102
3.1. Elucidation of the mechanism of TiN formation using TDMAT
C 14N
SIMS analysis of the TiN films deposited from TDMAT and ]SN2 confirms that the majority of the nitrogen in TiN originates from the plasma activated nitrogen gas and not from the dimethylamido group of the TDMAT. The strong ]SN signal in the SIMS s~Nectrum of the TiN film (Fig. 1) corresponds to a content of 93%. This agrees, within experimen-
101 r I I I ~ I I I r I I I
0
200
400 600 Depth (A)
800
1000
Fig. I. SIMS spectrum of TiN film deposited at 200°C using 15N2 as plasma gas.
background
,L
~
14N-nitrogen
=
15N-nitrogen
I
¢R
==
15NH3
14N(CH3)2
~//H
1 14NH Y
HC15N
~'//
14N(CH3)3
¢1
10 12 14 16 18 20 22 24 26 28 ao 32 34 36 38 40 42 44 46 48 s0 52 54 56 58 60
mass ~'~"~,~
[m/• ] H-C
14N. H
H-C
14N. H
m e-
91
93
95
97
99
101 103 105 107 109 111
mass
113 115 117 119 121
[m/e]
Fig. 2. CIMS spectra of the gas phase during TiN deposition using 15 sccm 14N2 or15N2 and 2.5 sccm TDMAT. Microwave power 400 W, susceptor temperature 100°C.
A. Weber et al. /Applied Surface Science 91 (1995) 314-320 ECR plasma Ti[14N(CH3)2]4 + 15N 2
~
7r-bonded ligand. The elimination of H 2 from intermediate (I) leads to the detected metallacycle (HI). A recent gas phase infrared spectroscopy study of the thermal decomposition of TDMAT in vacuum also identified the formation of three-membered T i - N - C metallacycles [12]. The attack of plasma generated nitrogen and hydrogen radicals on complex (HI) may lead to the detected gas species HC14N, HC15N,14NH3 and H~4N(CH3). The formation of 15NH3 can be explained by the reaction of 15N2 with hydrogen that originates from the conversion of I to HI in reaction Scheme 2. A related plasma CVD system involving ligand substitution whose mechanism has been explored using labeling experiments is the deposition of SiO z from an 02 plasma and tetraethoxysilane (TEOS). The reaction of TEOS (5 sccm) with a downstream 1802 (100 sccm) plasma produced a SiO 2 film with an 18O / 16 O ratio of 0.68 [13]. This indicates that the ethoxy ligand is only partially substituted by the 1So 2 and is an important source of S i - O bonds for the growing film despite starting with an 180/160 feed gas ratio of 10: 1. The retention of a60 in the SiO 2 films without significant incorporation of C suggest that cleavage of the O - C 2 H 5 bond occurs cleanly. This contrasts with our experiments, in which a starting 15N/InN feed gas ratio of 3:1 (15 sccm 15N2/2.5 sccm TDMAT) leads to a 15N/14N ratio of > 90 in the TiN film as shown by the RBS and SIMS results. That means that a plasma induced cleavage of the N - C H 3 bonds in Ti[14N(CH3)2]4 leading to Til4N plays only a minor role, as supported by the absence of 15N-containing amines in the CIMS analyses. Thus, the substitution reaction of
~ TilSN÷ [HxI4N(CH3)3.x] +
[Ti(I-ICI4N)(HI4N=CH2]+ HCI4N +HcL.~N+ 14NH3 + ISNH3 + H2 Scheme 1. Reaction products of TDMAT with ECR plasma activated labeled nitrogen as derived from RBS and CIMS measurements.
tal error (-t-5%), with the RBS measurements that showed a 15N content of 99% [10]. Although the nitrogen species (N2*, N f , or N radicals) responsible for the substitution of the dimethylamido group and the formation of TiN are not yet known, in situ CIMS using 15N2 provides some insight into the reaction mechanism. The spectra in Fig. 2 show that only NH 3 and HCN contain 15N while no 15N is detectable in the alkyl amines, H2N(CH 3) (m/e 30 = M - 1 ) , HN(CH3) z (m/e 42, 44, 46 = M - 3, M " 1, M + 1) and N(CH3) 3 (m/e 58 = M - 1). We assign mass 103 to the intermediate [Ti(HC14N)Q4NH=CH2)]. The appearance of this species along with mass 58, related to N(CH3) 3, after the ignition of the ECR nitrogen plasma may explain the formation of H 2, HCN and NH 3. The results of these analyses are summarized without considering the stoichiometry in reaction Scheme 1. Due to its high ionization poten. tial, hydrogen (15.43 eV) was not detectable by CIMS but was clearly detected by a conventional quadrupole mass spectrometer after pumping down the system to a suitable pressure range. As illustrated in reaction Scheme 2, the N 2 plasma-TDMAT interaction can lead to the fragmentation of two N(CH3) 3 (II) molecules by formation of a three-membered metallacycle (I) that is in resonance with a side-on
.CH., XN14 14~,~ o H3C" ~Ti("'CH3 H3C\N14 1~N/CH3
H3C
H:/
"C.~
317
•
H14N 14N-H
H14N 14N H + 2 14N(CH3)3
H2
H2C
CH2
Jll:[M-1- S8] I
"H2~ H-~\ 14N-H
H-C 14N-H N
CHZ
J,,,: t.-, -,o 1 I Scheme 2. Proposed fragmentation of TDMAT in the downstream ECR plasma.
318
A. Weber et al. / Applied Surface Science 91 (1995) 314-320
tion agent for the dimethylamido ligands at low temperatures as NH 3 does in the thermal M O C V D process [14]. 3.2. Formation o f " T i "
and TiN using titanium
tetrachlo ride
RBS analysis of highly reflective, silver-looking films deposited using TiC14 and a H 2 plasma at 500°C showed a high impurity incorporation, with a representative stoichiom etry of Tii.0Oo.20N0.40C0.12C10.o02. An AFM scan of a 100 nm film, shown in Fig. 3, gives a root mean square roughness of 8 ~,. The C, N, and O can be traced to the pyrolytic graphite susceptor and high background levels of N 2 and O 2 in the elastomer-sealed multipurpose C V D chamber. The low C1 content is indicative of effective reduction of the TiC14 and is comparable to results in radio frequency (13.56 MHz) H e plasma depositions with TiC14 by Ameen et al. [9].
e
Fig. 3. AFM image (800 × 800 nm2) of "Ti" layer, thickness 100 nm, RMS surface roughness 8 ,~. the dimethylamido ligands by plasma activated N 2 is strongly preferred. Our experiments clearly demonstrate that plasma activated N 2 reacts as a substituECRPACVD
TI
TI JDi
in situ anneal
Si (100)
TISix
20800°Cmin
ECR-
S (100)
TiN
TI[""-~N
[
Si (100)
I
g
Sample a)
ioo
~
I
i
,
,
l
t
'
..:............ ~l'~. . . . . t ........
/ BO .
-
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0
/
/ ............./ "
~
.
/
1/01,
.
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rd
TiSi 2
TiN cap
.__
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Ill
..... ~..
si~../ /
~ n ' ~ - - - "/.... " x Iri'j
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20
30
nl
~
40
:""
50
Sputter T i m e (min) b)
Ti 2p region 2Pl/2
2P3/2
5
BINDING ENERGY
eV
I Ti oxide
2 TiN
Fig. 4. (a) XPS sputter depth profile of TiN/TiSix/Si multilayer. (b) Ti2p XPS spectra recorded during sputter depth profile of TiN/TiSix/Si multilayer.
A. Weber et aL /Appfied Surface Science 91 (1995) 314-320
The low deposition rate of 1.5 n m / m i n provides ample opportunity for impurity incorporation as highly reactive Ti is formed. The thermal reaction of TiC14 and H 2 to form Ti metal is thermodynamically unfavorable under conditions that are compatible with integrated circuit processing [15]. Therefore, a CVD Ti process for this application will require plasma activation to open non-equilibrium reaction pathways. The resistivity of the film of 120/x12. cm and the ability to form TiSi 2 upon annealing, as described below, suggest a viable process for contact window applications. We have also deposited TiN films at 500°C using TiCI 4 by adding N 2 to the H 2 plasma gas stream. A representative stoichiometry of Til.oNo.85Co.10Clo.0ol, as determined by RBS analysis, again reveals extremely low C1 levels. Films deposited from TiC14 and NH 3 at these temperatures have C1 contents of 5 - 8 at% [16]. Combining these processes, a " T i " film was deposited at 500°C and annealed in situ to 800°C for 20 min, and a capping film of TiN was deposited again at 500°C without breaking vacuum. An XPS depth profile of this multilayer (Fig. 4a) clearly shows formation of TiSi 2 closest to the Si substrate, a mixed TiSixOy intermediate layer, and the TiN layer on top. Looking more closely at the Ti2p region, Fig. 4b, the Ti 2p3/2 peak at 453.2 eV corresponds to the TiSi 2 layer. Thus, despite the non-ideal experimental conditions a titanium rich layer with active Ti for TiSi 2 formation could be deposited.
4. Conclusions Experiments with labeled nitrogen (15N2 ) demonstrate that the N in TiN deposited using TDMAT is mainly originated from the plasma gas and not from the dimethylamido ligand of the TDMAT. The deposition is driven by a substitution reaction with plasma activated nitrogen that leads to the deposition of high quality TiN at low substrate temperatures. Despite the highly activated species created in the ECR plasma the substitution of the dimethylamido ligands by plasma activated nitrogen occurs with a remarkable selectivity. Deposition of Ti from TiC14 in a thermal process is not possible under conditions
319
compatible with integrated circuit fabrication. Introducing TiC14 into the downstream region of a hydrogen ECR plasma leads to the formation of Ti rich films at a substrate temperature of 500°C. TiSi 2 was formed by annealing the " T i " films on Si at 800°C, and a cap of TiN could be deposited at 500°C by adding N 2 to the H 2 plasma gas. A combination of TiC14 and TDMAT processes may offer a complete CVD solution for contact level and upper level metallization, respectively. The objectives of current research are the enhancement of the deposition rates, investigations of the step coverage and film properties.
Acknowledgements The authors are indebted to K. Schiffmann (FhGIST) for AFM images. The work in Germany was supported by the Bundesministerium f'tir Bildung und Forschung (BMBF) under Grant No. 13N5849/3.
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[13] N. Selamoglu, J.A. Mucha, D.E. Ibbotson and D.L. Flamm, J. Vac. Sci. Technol. B 7 (1989) 1345. [14] J.M. Prybyla, C.-M. Chiang and L.H. Dubois, J. Electrochem. Soc. 140 (1993) 2695. [15] Calculations with software equiTherm Version 2.1, M.
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