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TiN adhesion bilayers: mechanical properties
COATING$ ELSEVIER
Surface and Coatings Technology99 (1998) 274-280
An experimental study of chemical vapour deposition of tungsten on Ti/TiN adhesion bilayers: mechanical properties A. M o u r o u x
~'*, S . - L . Z h a n g a, C.S. P e t e r s s o n
a
J. K e i n o n e n
R. Palmans b K. Maex
b T.
A h l g r e n °,
°
Royal Institute of Technology-Electrum, Deparnnent oJ'Electronics, Solid State Electronics, Box 229, S-164 40 Kista, Swe&'n b IMEC, Kapelreef'75. B-3001 Leut~en, Belgium Universio, of Helsinki, Accelerator Laboratory, Hiimeentie 100, SF-O0550 Hels#dci, FTnlamt
Received 7 July 1997; accepted 23 September 1997
--
Abstract
The mechanical properties of chemical vapour deposited tungsten (W-CVD) on Ti/TiN adhesion bilayers have been studied. Rapid thermal annealing (RTA) of the Ti/TiN bilayers has been found to strongly affect subsequentIy deposited W films. The stress in W is reduced as a consequence of the RTA treatment of Ti/TiN, and the stress reduction is especially pronounced for thin W films. However. the stress in the Ti/TiN bilayers deposited at 300 or 550 "C increases substantially after the RTA treatment at 650 °C, leading to appreciable increase in the total stress of the whole Ti/TiN/W stack. The nucleation deposition of W on the annealed Ti/TiN is somewhat hindered. The retardation of W growth on annealed Ti/TiN is discussed in terms of thermodynamics and classical nucleation theory in conjunction with the reduction of the interracial impurities (i.e. fluorine and oxygen) as a result of the RTA of Ti/TiN [A. Mouroux, R. Palmans, J. Keinonen, S.-L. Zhang, K. Maex, S. Petersson, in: Materials Research Society, MRS, San Francisco, CA, 1996], as well as the evolution of the W film texture with the W '~hickness. © 1998 Elsevier Science S.A. Keywords: Ti/TiN CVD-W; Stress: Impurity: Nucleation
1. Introduction
As the dimensions of integrated circuits are scaIed down, the requirements of metallisation materials become more stringent. For contact via filling and interconnection applications in very large scale integration (VLSI) technology, the chemical vapour deposition of tungsten (W-CVD) is an attractive solution [2], W-CVD by the reduction of WF6 provides three key features: (1) high-purity, (2) even step coverage, and (3) selective deposition. However, problems such as adhesion, void formation, high stress and impurities at interfaces can arise. Several papers have been presented on this subject with the aim of improving the W-CVD process [3-5]. In order to prevent W-CVD peeling from an oxide surface, an adhesion layer is needed. Among the different possibilities, TiN is a good alternative [6, 7]. TiN is a fairly good conductor with a resistivity of * Corresponding author. Tel.: +46 8 7521408: Fax: +46 8 7527782 e-mail: [email protected] 0257-8972/98/$19.00 ~D1998ElsevierScienceB.V. All rights reserved. PII S0257-8972(97)00567-7
about 50 f2. cm. It is also thermally stable, which makes it good diffusion barrier. W-CVD is typically deposited by the reduction of WE6 by H:: or Sill 4. The chemical process of W deposition has been widely studied [8-10] and the properties of the W deposited have been found to depend strongly on the preparation of the TiN adhesion layer. The properties of TiN determine not only the mechanical properties of W. but also the amounts of impurities present at the T i N - W interface. Proper heat treatment of TiN has been shown to improve its electrical properties [11]. Our recent results [1,12] demonstrate that rapid thermal annealing (RTA) of Ti/TiN adhesion bilayers prior to W deposition t~as a strong positive impact on W-CVD with regard to the reduction of stress in W and of impurities at the interface between W-CVD and TiN. However, the stress in the Ti/TiN bilayers increases as a result of annealing [ 1,12]. It appears that there exists an optimum anneal temperature at which the increase in the stress in the Ti/TiN bilayers is still under control but both the stress in W-CVD and the content of the interfadN1 impurities can be significantly reduced.
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As a complementary study to Refs. [ 1,12], the emphasis of the present work will mainly be on the role of such an RTA treatment of Ti/TiN adhesion bilayers on the nucleation and growth of W-CVD. The most stringent experimental conditions (i.e. the highest annealing temperature used in our previous experiments) were chosen so as to show to what extent the mechanical properties of W films deposited subsequently can vary as a result of annealing the bilayers. The results are discussed in terms of thermodynamics and classical nucleation theory. The nucleation of W has an important bearing on the mechanical properties of W films, particularly for thin (30-200 nm) W films, which correspond to the amount of W which is used for via-filling of deep sub-tam contact windows. Hence. W films of various thicknesses were deposited.
2. Experiment and analysis The experiments were carried out with Si wafers 150 m m in diameter and of <100> orientation. A 250 nm thick plasma-enhanced CVD (PECVD) silicon dioxide film was deposited at 400 :C. Before W-CVD, Ti and TiN layers were sputter-deposited using a d.c. magnetron technique in a Balzers CLC9000 cluster tool without breaking the vacuum between the two deposition steps. The base pressure was 10-s Tort, whereas the deposition pressure was between 4 x 1 0 3 and 5 × 1 0 - 3 T o r r . The distance between the target and the wafer was 7 cm. Two types of adhesion bilayers were used: 100 nm of TiN on top of 20 nm of Ti deposited at 300 C (denoted type 1) and 8 0 n m of TiN on top of 3 0 n m of Ti deposited at 550 :C (denoted type 2). The temperature given for Ti/TiN deposition is the wafer susceptor temperature. The wafers were divided into three groups: one was kept as reference without post heat treatment, whereas the two others were submitted to an RTA at 650 "C in NH3 o r N 2 prior to W deposition. The RTA system was an A and G Heatpulse apparatus. W deposition was performed in an Applied Materials PS000 cold-wail CVD reactor at a temperature between 434 and 438 =C (wafer susceptor temperature). " N u c l e a t i o n deposition" was performed at a chamber pressure of 4.5 Torr using Sill4 reduction of WF<,, with H2 as the carrier gas. During this first deposition step, the W thickness varied from 0 to about 30 nm. Then, the reaction gas mixture and the pressure were modified for the second deposition step ("bulk deposition"). The latter was performed at 80 Torr with He reduction of W F o. Tungsten films with different thicknesses were deposited from about 30 nm (nucleation layer) up to 1 I-tin. The amount of W deposited was controlled by the deposition time. Different analyses were used to characterise the W films as well as the impact of the RTA treatment of the adhesion bilayers on subsequent
W deposition. Mechanical stress was studied by wafer curvature measurement on an Eichborn and Hausmann MX203 wafer geometry gauge. For the W films, the thickness was measured by weight difference. Details of the various analyses can be found elsewhere [1,9, i2, 13].
3. Results Altogether, four different values of stress were measured: ( I ) the substrate stress, which is the total stress after the PECVD silicon dioxide deposition, (2) the stress of the Ti/TiN bilayers measured after sputter deposition and post-thermal annealing, (3) the stress in W measured after W deposition, and (4) the total stress of the T i / T i N / W stack measured after W deposition. The substrate stress was taken as a reference for the measurement of the total stress and of the Ti/TiN bilayer stress, and it was assumed to be the same for all wafers. The Ti/TiN bilayer stress was taken as a reference to measure the W stress. 3.1. Stress in T i / T i N adhesion bilayers
Table i gives the mean stress value of the adhesion bilayers. A large variation in the stress in the bilayers is found after the RTA treatment. The change is more pronounced with type I than with type 2 bilayers, most probably as a result of the lower deposition temperature used for the type 1 adhesion bilayers. As the deposition temperature for W-CVD is higher than that for the deposition of type 1 bilayers, an annealing effect of the reference bilayers during W CVD is anticipated. According to our previous experimental results [1], the stress of type 1 bilayers changed from - 0 . 5 7 G P a (compressive) to +0.15 GPa (tensile, the value given in parentheses in Table 1) after RTA at 450 °C. Hence, for further calculations of the W stress, the reference value for type I bilayers was assumed to be 0.15 GPa. For type 2 bilayers, the stress did not change after the same RTA, as can be expected since the temperature for the sputter deposition was 550 °C, about 100 °C higher than that for W-CVD.
Table I Impact of the deposition temperature and post heat treatment at 650 :C on the mechanical stress of the Ti/TiN adhesion bilayers Mechanical stress of the bilayers (GPa) No RTA
Type I Type 2
-0.57 (0.15) 0.71
Atmosphere of RTA
N2
NHa
2.34 1.62
2.30 1.72
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A. Mouroux et al. / Smface and Coatings Technology 99 (1998) 274-280
---
140
3.2. Stress and texture of W
:
The W films are found to be under high tension and strongly oriented toward the (110) direction, although this was less pronounced with W deposited on type 2 bilayers than on type 1 bilayers (Figs. 1 and 2). Further, the stress is lower and the film is less oriented for thicker W films. These results are in agreement with the findings reported in Ref. [13]. Post heat treatment of the Ti/TiN bilayers does not seem to alter these trends for the evolution of the stress or the texture of W-CVD films.
~
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3.2.1. Stress and texture of W o n type l bilayers Figs. la and 2a show the evolution of the stress in W and the orientation factor for the W ( l l 0 ) diffraction peak versus the W thickness for W deposited on type 1 bilayers. The orientation factor was calculated following the standard procedure: the integrated intensities of the diffraction peaks were normalised to the respective intensity values of a standard powder sample. Thin film correction was then done to account for the absorption factor [ 14]. On the reference bilayer, both the W stress and the
(a)
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Fig. 2. Variation of the orientation factor relative to the W ( I ~ diffraction peak versus W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH3 and N>
1.5 1
0.5 0
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!~(b) '
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Table 2 Thickness of the W nucleation layers deposited on different Ti,q'iN bilayers
0.7 r~
0.6 0.5
I
I
~
I
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(b)
orientation factor for W ( l i 0 ) decrease monotonically with increasing W thickness. On the annealed bilayers, the W stress is considerably lower as compared to that on the reference bilayer. The stress reduction due to the RTA of the bilayers is most significant for thin W fihns (40-50%). The orientation factor for W (110), however, first increases until the W thickness reaches about 150 nm and then decreases monotonically beyond this thickness. W nucleation deposition on the annealed bilayers seems to be retarded, and thus the W layers are thinner after identical deposition (see Table 2).
?
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T
I
I
I
I
600
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I
800
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Thickness of the nucleation layer (nm)
f
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No RTA
W thickness (nm)
Fig. 1. Evolution of W stress versus W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH3 and N>
Type I Type 2
34.02 34.77
Atmosphere of RTA Na
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23.18 27.29 _
277
,4. Mom'ott.x er al. / SuJjhc'e and CoeztiHgs Technology 99 (1998) 2 7 4 - 2 8 0
3.2.2. Stress a m / t e x t u r e o/" IV otz O'pe 2 bilayers Figs. lb and 2b show the thickness dependence of the stress in W and the orientation factor for the W(I10) peak for W films grown on type 2 bilayers. In general, both the stress and the orientation factor of W deposited on the reference bilayer have a similar thickness dependence as W deposited on the type 1 reference bilayer. However, the stress in W is lower and the film is less oriented for W deposited on type 2 bilayers than on type 1 bilayer, as noted above. On annealed bilayers, different characteristics are observed. The W stress first increases from low values of 0.5 to 0.8 GPa with W thickness to a maximum value of about 1 GPa. Them it slowly decreases when the W is greater than about 150-200 nm thick. Although still textured, the orientation factor for W ( l l 0 ) is several times smaller as compared to the case of W deposited on type 1 bilayers. The evolution of the orientation factor follows a similar pattern for all W films deposited on type 2 bilayers: it decreases monotonically with the W thickness, and there is essentially no maximum except for some local variations. W nucleation deposition on the annealed bilayers is again retarded (Table 2). The W stress is also somewhat lower when the bilayers are annealed ( Fig. 1b).
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3.3. Toted stress in Ti/TiN/~t" and stmarjbrce oJ" I4~
Figs. 3a and 3b show the wtriation of the total stress of the Ti/TiN/W stack with W thickness for W films deposited on type I and type 2 bilayers. In all cases, the stress tends to approach a constant value of around 1 GPa as the W thickness increases. This convergence behaviour is independent of the type of Ti/'TiN adhesion bilayer used, and is insensitive to the RTA treatment of the bilayers prior to W deposition. For both types of bilayer, annealing alters the total stress in such a way that it decreases monotonically with W thickness, in contrast to the monotonic increase in the total stress with increasing W thickness for W films deposited on the reference (unannealed) bilayers. The effect of annealing on the total stress is more sensitive for small W thicknesses, and only a minor difference is observed between annealing in NH3 and N 2. The shear force is defined as the product of the fihn stress and the film thickness. Fig. 4a and 4b show the shear force of W deposited on type 1 and type 2 bilayers versus W thickness. The variation of the shear force of W with W thickness seems to be in agreement with the results reported in Ref. [15]. The shear force of W deposited on type 2 bilayers is quite insensitive to bilayer annealing. This is not true for W deposited on type 1 bilayers: bilayer annealing results in an appreciably lower shear force, but the atmosphere (i.e. N2 or N H 3) has a minor effect.
2.5
0 (b)
200
400 W thickness
600
800
1000
(nm)
Fig. 3. Dependence of the total stress on W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH 3 and N>
4. Discussion The nucleation and growth of W can be regarded as being governed energetically by the dynamic balance between the strain energy (Estrain) and the surface energy (£~rf). During nucleation deposition, the surface energy Esurf controls the process. Tungsten (body-centred cubic) starts to grow with a preferential crystallographic orientation which corresponds to the surfaces with the lowest surface energy (i.e. the (110) orientation) [16]. As the W grows thicker, the surface energy term becomes less influential, and simultaneously the strain energy E~train gradually increases. Thus, other crystallographic orientations begin to compete with the (110) orientation in order to reduce /ZTstrain [17], leading to less oriented W films with increasing film thickness. On annealed type 1 bilayers, the W texture initially develops further in the (110) orientation with increasing W thickness (Fig. 2a) while the W stress still remains at a low level (Fig, la). In contrast, on annealed type 2 bilayers, the orientation factor for W (I 10) continues to decrease (Fig. 2b) while
A. Mouroux et aL / Sur[ace and Coatings Technology 99 (1998) 274-280
278
~a
1200 1000 800 600 400
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200
. I ' ~N" -
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-
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'-
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of the new phase [19]. As a consequence, nucleation deposition of W-CVD on annealed bilayers becomes more difficult and W growth becomes slower (Table 2). During annealing of the bilayers in N2, oxidation of the TiN surface can take place as an increased amount of interfaciaI oxygen is found at the T i N - W interface [1,12]. This increased interracial oxygen could explain why the W nucleation layer is thinnest on N2-annealed bilayers (Table 2), as the chemical driving force for the reaction between WF6 and TiO, is even smaller than that for the reaction between WFo and TiN [18]. (2) Annealing causes the stress in the Ti/TiN bilayers to increase. After RTA. the stress in [ype i bilayers i s higher than in type 2 bilayers (Table 1 ). As can be seen in Fig. 5, type 2 bilayers are generally more randomly oriented than type 1 bilayers. "Therefore, W deposited on type 2 bilayers is also more randomly oriented, with a lower stress. Moreover. annealing does not have the same effect on these two types of bilayers: annealing makes type l bilayers even more oriented towards the <111 > orientation, and in contrast makes type 2 bilayers less oriented (see Fig. 5). As a direct consequence, W --~ Reference ---4t== RTA of Ti/TiN in N2 - - ~ , - - RTA of Ti/TiN in NIl3
140 0
120 200
(b)
400
600
800
1000
I00
W thickness (nm)
80
Fig. 4. Shear force associated with W films for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH 3 and N,.
60 40 *"*
the W stress increases with W thickness for W thicknesses below 150-200nm (Fig. lb). These seemingly controversial trends of development will be discussed below. The stress and texture of the W-CVD are dependent on the stress and texture of the underlying adhesion layers [1,9,12,13]. As the mechanical and chemical properties of the two types of adhesion bilayer can be different, their effects on W are also expected to be different. Annealing the adhesion bilayers in N2 or NH3 has at least three effects. (1) Annealing converts free Ti atoms in the sputterdeposited TiN into titanium nitride, thereby making the TiN surface more resistant to chemical attack by WF6. According to thermodynamics [ 18], the chemical driving force for the interaction between WF6 and Ti is considerably greater than that for the reaction between WFo and TiN. The amounts of the interfacial fluorine at the T i N - W interface have been shown to be substantially reduced as a result of the RTA of Ti/TiN bilayers [1]. Further, the nucleation rate ~br the growth of a new phase is known to be exponentially proportional to the square of the chemical driving force for the formation
~,~
20 36
38
(a)
,fi t,,*
42
44
2,0 (deg.) 70
.=.
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----I~-~ Reference ---it--- RTA of TiffiN in N2 - - - ~ - RTA of TiffiN in NH3
~"
4O
20 10 0
(h)
I , 36
38
40 2.0
42
44
(deg.)
Fig. 5. Comparison of the XRD spectra of (a) type [ and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NF[-3 and N 2, showing the strong impact of annealing on the texture of Ti/TiN.
A. Molo'oux c't al,
Sutjace and CoaHngs Technology 99 (1998) 274-280
deposited on annealed type 2 bilayers shows a much weaker film texture (Fig. 2). Annealing also has a less pronounced impact on type 2 bilayers than on type 1 bilayers, so that the stress in the bilayers increases much less for the type 2 bilayers (Table 1 ). Therefore, annealing type 2 bilayers has less effect on the W films deposited subsequently: W nucleation deposition is relatively less affected, especially after NH 3 annealing (Table 2), and the orientation factor for W ( l I 0 ) is not much influenced by bilayer annealing and thus evolves in a similar fashion (Fig. 2b). (3) Annealing increases the content of nitrogen in the Ti/TiN bilayers. This can be seen in the XRD spectra in Fig. 5: the two peaks assigned to ~-Ti shift towards lower angles after RTA of Ti/TiN as a result of more nitrogen incorporation into the underlying ~-Ti layer [1]. During annealing, additional nitrogen atoms could be pumped into the upper (stoichiometric) TiN layer in order to bring the T i N - N : (or NH 3) system to equilibrium with respect to the chemical potential of nitrogen. During subsequent W deposition, a certain amount of nitrogen can then diffuse into the W by a similar driving force to bring the T i N - W system to equilibrium. It is highly probable that nitrogen is present ill the W lattice as an interstitial impurity, resulting in an increased lattice parameter for W and reduced tensile stress in W [20]. The impact of nitrogen on W stress is anticipated to be more pronounced for thinner W films, since nitrogen, similar to fluorine and oxygen [1.9, I2,13], is likely to be concentrated at the T i N - W interface. The contrasting trends of stress and film texture development noted above can now be explained in terms of nitrogen incorporation into the W lattice. On annealed type 1 bilayers, the W stress is considerably reduced, mainly by nitrogen incorporation (Fig. Ia), because the film texture is comparable at each W fihn thickness throughout the entire thickness range (Fig. 2a) and the thermal 2.5
+ •
"~
2
.2
1.5
41.I nm 95.3 nm 169.3 nm
1 0.5
0
im,
j llt-111,4t-X~e'~l~a.~,.~
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loo
150
200
250
300
,i., - -
350
4oo
279
stress in W should be independent of bilayer annealing. 1::o1" W deposited on type 2 bilayers, the results shown in Figs. 2a and 2b can be discussed in similar terms. As high-temperature TiN is expected to be more structurally perfect, with a lower density of defects [15], it can be understood that bilayer annealing has a less pronounced impact on W deposited on type 2 bilayers than W on type 1 bilayers. The stress value which the total stress of the whole Ti/TiN/W stack approaches at large W thicknesses (i.e. 1 GPa: Figs. 3a and 3b) is most probably determined by the stress in W (both intrinsic and extrinsic). The extrinsic thermal stress in W is calculated to be about 0.3 GPa, and the intrinsic stress should include structural and impurity contributions. Apart from the known facts that fluorine increases the resistivity of W and is responsible for the formation of bubble-like features [21 ], how the presence of fluorine at the T i N - W interface directly affects stress in the Ti/TiN/W system is not clear. Fluorine is present mainly at the T i N - W interface and remains there during W growth at the temperature used here (440 ~C ), according to the nuclear resonance broadening (NRB) results shown in Fig. 6. It is conceivable that interracial fluorine can reduce the adhesion of W on TiN. Moreover, fluorine is not desirable in this system as it may also lead to outgassing during subsequent process steps at temperatures higher than that used for W-CVD.
5. Conclusions From the experimental results presented in this work as well as in Refs. [1,9,12, I3], it is clear that the temperature used for the deposition of Ti/TiN adhesion bilayers plays a crucial role with regard to control of the film stress and impurities in the Ti/TiN/W system. A high deposition temperature is preferred in order to reduce both tile stress in W and the impurity content at the T i N - W interface. However, the total stress can increase as a consequence of the increased stress in the Ti/TiN adhesion bilayers resulting from the high deposition temperature used. Post-RTA treatment of the deposited bilayers in NH 3 02" N2 can be employed to increase the chemical stability of the bilayers by nitridation of the unreacted Ti in TiN as well as the uncovered Ti in the bilayer system. As the stress increase due to post-RTA treatment of Ti/TiN can become a potential reliability issue in contact via-filling and interconnection applications, the temperature used %1" RTA has to be chosen carefully.
Depth (nm) Fig. 6. Nuclear resonance broadening (NRB) data showing that interfacial fluorine remains at the TiN-W interface during W growth from 41.I to 95.3 nm and to 169.3 nrn on type 1 bilayers sputter-deposited at 250-C.
Acknowledgement The authors are grateful to Professor F. d'Heurle for enlightening discussions. A.M. is a grant holder of
280
A. Mouroux et al, / Su(/ace and Coatings Tectmology 99 (1998) 274-280
the E u r o p e a n program "Training and Mobility of Researchers" (TMR) (Contract No. ERBFMB I C T 9 6 0 8 8 6 ) . T h i s w o r k was p a r t l y s u p p o r t e d by the S w e d i s h B o a r d for I n d u s t r i a l a n d T e c h n i c a l D e v e l o p m e n t ( N U T E K ) a n d by the F l e m i s h I n s t i t u t e for the A d v a n c e m e n t of Scientific-Technological R e s e a r c h in I n d u s t r y ( I W T ) .
References [I] A. Mouroux. R. Palmans, J. Keinonen, S,-L. Zhang, K. Maex, S. Peterssom in: K.N. Tu, J.W, Mayer, J.M. Poate, L.J. Chen (Eds.), Advanced Metallization for ULS. voi. 427, MRS, San Francisco, CA, 1996, p. 365. [2] N.E. Miller, I, Beinglass, Solid State Technol. 25 (12) (1982) 85. [3] J, Schmitz, S. Kang, R. Wolters, K.v.d. Aker, J. Electrochem. Soc. 141 (1994) 843. [4] S. Sivaram, M.L.A. Dass, C.S. Wei, B. Tracy, R. Shukla, J. Vac. Sci, Technol, A 11 11993) 87. [5] D.A. Bell, C.M. McConica, K.L. Backer, J. Electrochem. Soc. 143 (1996) 296. [6] E.K. Broadbent, J. Vac. Sci. Technol. B 5 11987) 166I. [7] V.V.S. Rana, J.A. Taylor, L.H. Holschwandner, N.S. Tsai, in: E.K. Broadbent (Ed.), Tunsgten and Other Refractory Metals ['or VLSI Applications, vol. [I, MRS, Palo Alto, CA, 1987, p. t87.
[8] Y.J. Lee, C.,O. Park, D.-W. Kim, J.S. Chun, J. Electron. Mater. 23 (1994) 1075. [9] S.-L. Zhang, R. Palmans, J. Keinonen, C.S, Petersson, K. Maex, AppI. Phys. Lett. 67 11995)2998. [10] D,A. Webb, J. Hitlman, F. Foster, in: R.C. Ellwanger, S.-Q. Wang (Eds.), Advanced Metallization and interconnect Systems for VLSI applications, MRS, Portland, OR, 1995. [1I] S.C. Sun, M.H. Tsai, Appl. Phys. Left. 68 (1996) 670. [12] R. Pahnans, A. Mom'oux, S,-L. Zhang, S. Petersson, K. Maex, in: R.C. Elhvanger, S.-Q. Wang {Eds,), Advanced Metallization and Interconnect Systems for VLSI Applications, MRS, Portland, OR, 1995, p~ 555. [I3] S.-L. Zhang, R. Pahnans, C.S. Petersson, K. Maex, J. Appl. Phys. 78 (1995) 7313. [14] P. Joubert, B. Loisel, Y. Chouen, I_. Haji, J. Electrochem. Sac. 134 11987) 2541. [15] S. Sivaram, S, Chen, D. Liao, R. Shukla, D. Fraser, in: V.A. Wells ( Ed.L Tungsten and Other Refractory Metals for VLSI Applications, vol. II, MRS, New York, 1988, p. 407. [16] D. Wolf, J. Appl. Phys. 69 (19911 185. [t7] U.C. Oh, J.H. Je, J. Appl. Phys. 74 11993) 1692. = [18] I. Barin, O. Knacke, O. Kubaschewski, Thermochemieal Properties of Inorganic Substances, suppl.~ Springer, Berlin, 1977, ___ [19] F.M. d'Heurle, J. Mater. Res. 3 11988) 162. [20] H.J. Goldschmidt, Interstitial Alloys, Butterworth, London, 1967, p. 226, = [2l~ T. Eriksson, J.-O. Carlsson, E. Niemi, M. Ostling, C.S. Petersson, in: G.W. Cullen (Ed.), Proceedings of the Tenth international Conference on Chemical Vapor Deposition, Electrochemical Society, Princeton, N J, 1987, p. 736.
Surface and Coatings Technology99 (1998) 274-280
An experimental study of chemical vapour deposition of tungsten on Ti/TiN adhesion bilayers: mechanical properties A. M o u r o u x
~'*, S . - L . Z h a n g a, C.S. P e t e r s s o n
a
J. K e i n o n e n
R. Palmans b K. Maex
b T.
A h l g r e n °,
°
Royal Institute of Technology-Electrum, Deparnnent oJ'Electronics, Solid State Electronics, Box 229, S-164 40 Kista, Swe&'n b IMEC, Kapelreef'75. B-3001 Leut~en, Belgium Universio, of Helsinki, Accelerator Laboratory, Hiimeentie 100, SF-O0550 Hels#dci, FTnlamt
Received 7 July 1997; accepted 23 September 1997
--
Abstract
The mechanical properties of chemical vapour deposited tungsten (W-CVD) on Ti/TiN adhesion bilayers have been studied. Rapid thermal annealing (RTA) of the Ti/TiN bilayers has been found to strongly affect subsequentIy deposited W films. The stress in W is reduced as a consequence of the RTA treatment of Ti/TiN, and the stress reduction is especially pronounced for thin W films. However. the stress in the Ti/TiN bilayers deposited at 300 or 550 "C increases substantially after the RTA treatment at 650 °C, leading to appreciable increase in the total stress of the whole Ti/TiN/W stack. The nucleation deposition of W on the annealed Ti/TiN is somewhat hindered. The retardation of W growth on annealed Ti/TiN is discussed in terms of thermodynamics and classical nucleation theory in conjunction with the reduction of the interracial impurities (i.e. fluorine and oxygen) as a result of the RTA of Ti/TiN [A. Mouroux, R. Palmans, J. Keinonen, S.-L. Zhang, K. Maex, S. Petersson, in: Materials Research Society, MRS, San Francisco, CA, 1996], as well as the evolution of the W film texture with the W '~hickness. © 1998 Elsevier Science S.A. Keywords: Ti/TiN CVD-W; Stress: Impurity: Nucleation
1. Introduction
As the dimensions of integrated circuits are scaIed down, the requirements of metallisation materials become more stringent. For contact via filling and interconnection applications in very large scale integration (VLSI) technology, the chemical vapour deposition of tungsten (W-CVD) is an attractive solution [2], W-CVD by the reduction of WF6 provides three key features: (1) high-purity, (2) even step coverage, and (3) selective deposition. However, problems such as adhesion, void formation, high stress and impurities at interfaces can arise. Several papers have been presented on this subject with the aim of improving the W-CVD process [3-5]. In order to prevent W-CVD peeling from an oxide surface, an adhesion layer is needed. Among the different possibilities, TiN is a good alternative [6, 7]. TiN is a fairly good conductor with a resistivity of * Corresponding author. Tel.: +46 8 7521408: Fax: +46 8 7527782 e-mail: [email protected] 0257-8972/98/$19.00 ~D1998ElsevierScienceB.V. All rights reserved. PII S0257-8972(97)00567-7
about 50 f2. cm. It is also thermally stable, which makes it good diffusion barrier. W-CVD is typically deposited by the reduction of WE6 by H:: or Sill 4. The chemical process of W deposition has been widely studied [8-10] and the properties of the W deposited have been found to depend strongly on the preparation of the TiN adhesion layer. The properties of TiN determine not only the mechanical properties of W. but also the amounts of impurities present at the T i N - W interface. Proper heat treatment of TiN has been shown to improve its electrical properties [11]. Our recent results [1,12] demonstrate that rapid thermal annealing (RTA) of Ti/TiN adhesion bilayers prior to W deposition t~as a strong positive impact on W-CVD with regard to the reduction of stress in W and of impurities at the interface between W-CVD and TiN. However, the stress in the Ti/TiN bilayers increases as a result of annealing [ 1,12]. It appears that there exists an optimum anneal temperature at which the increase in the stress in the Ti/TiN bilayers is still under control but both the stress in W-CVD and the content of the interfadN1 impurities can be significantly reduced.
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As a complementary study to Refs. [ 1,12], the emphasis of the present work will mainly be on the role of such an RTA treatment of Ti/TiN adhesion bilayers on the nucleation and growth of W-CVD. The most stringent experimental conditions (i.e. the highest annealing temperature used in our previous experiments) were chosen so as to show to what extent the mechanical properties of W films deposited subsequently can vary as a result of annealing the bilayers. The results are discussed in terms of thermodynamics and classical nucleation theory. The nucleation of W has an important bearing on the mechanical properties of W films, particularly for thin (30-200 nm) W films, which correspond to the amount of W which is used for via-filling of deep sub-tam contact windows. Hence. W films of various thicknesses were deposited.
2. Experiment and analysis The experiments were carried out with Si wafers 150 m m in diameter and of <100> orientation. A 250 nm thick plasma-enhanced CVD (PECVD) silicon dioxide film was deposited at 400 :C. Before W-CVD, Ti and TiN layers were sputter-deposited using a d.c. magnetron technique in a Balzers CLC9000 cluster tool without breaking the vacuum between the two deposition steps. The base pressure was 10-s Tort, whereas the deposition pressure was between 4 x 1 0 3 and 5 × 1 0 - 3 T o r r . The distance between the target and the wafer was 7 cm. Two types of adhesion bilayers were used: 100 nm of TiN on top of 20 nm of Ti deposited at 300 C (denoted type 1) and 8 0 n m of TiN on top of 3 0 n m of Ti deposited at 550 :C (denoted type 2). The temperature given for Ti/TiN deposition is the wafer susceptor temperature. The wafers were divided into three groups: one was kept as reference without post heat treatment, whereas the two others were submitted to an RTA at 650 "C in NH3 o r N 2 prior to W deposition. The RTA system was an A and G Heatpulse apparatus. W deposition was performed in an Applied Materials PS000 cold-wail CVD reactor at a temperature between 434 and 438 =C (wafer susceptor temperature). " N u c l e a t i o n deposition" was performed at a chamber pressure of 4.5 Torr using Sill4 reduction of WF<,, with H2 as the carrier gas. During this first deposition step, the W thickness varied from 0 to about 30 nm. Then, the reaction gas mixture and the pressure were modified for the second deposition step ("bulk deposition"). The latter was performed at 80 Torr with He reduction of W F o. Tungsten films with different thicknesses were deposited from about 30 nm (nucleation layer) up to 1 I-tin. The amount of W deposited was controlled by the deposition time. Different analyses were used to characterise the W films as well as the impact of the RTA treatment of the adhesion bilayers on subsequent
W deposition. Mechanical stress was studied by wafer curvature measurement on an Eichborn and Hausmann MX203 wafer geometry gauge. For the W films, the thickness was measured by weight difference. Details of the various analyses can be found elsewhere [1,9, i2, 13].
3. Results Altogether, four different values of stress were measured: ( I ) the substrate stress, which is the total stress after the PECVD silicon dioxide deposition, (2) the stress of the Ti/TiN bilayers measured after sputter deposition and post-thermal annealing, (3) the stress in W measured after W deposition, and (4) the total stress of the T i / T i N / W stack measured after W deposition. The substrate stress was taken as a reference for the measurement of the total stress and of the Ti/TiN bilayer stress, and it was assumed to be the same for all wafers. The Ti/TiN bilayer stress was taken as a reference to measure the W stress. 3.1. Stress in T i / T i N adhesion bilayers
Table i gives the mean stress value of the adhesion bilayers. A large variation in the stress in the bilayers is found after the RTA treatment. The change is more pronounced with type I than with type 2 bilayers, most probably as a result of the lower deposition temperature used for the type 1 adhesion bilayers. As the deposition temperature for W-CVD is higher than that for the deposition of type 1 bilayers, an annealing effect of the reference bilayers during W CVD is anticipated. According to our previous experimental results [1], the stress of type 1 bilayers changed from - 0 . 5 7 G P a (compressive) to +0.15 GPa (tensile, the value given in parentheses in Table 1) after RTA at 450 °C. Hence, for further calculations of the W stress, the reference value for type I bilayers was assumed to be 0.15 GPa. For type 2 bilayers, the stress did not change after the same RTA, as can be expected since the temperature for the sputter deposition was 550 °C, about 100 °C higher than that for W-CVD.
Table I Impact of the deposition temperature and post heat treatment at 650 :C on the mechanical stress of the Ti/TiN adhesion bilayers Mechanical stress of the bilayers (GPa) No RTA
Type I Type 2
-0.57 (0.15) 0.71
Atmosphere of RTA
N2
NHa
2.34 1.62
2.30 1.72
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A. Mouroux et al. / Smface and Coatings Technology 99 (1998) 274-280
---
140
3.2. Stress and texture of W
:
The W films are found to be under high tension and strongly oriented toward the (110) direction, although this was less pronounced with W deposited on type 2 bilayers than on type 1 bilayers (Figs. 1 and 2). Further, the stress is lower and the film is less oriented for thicker W films. These results are in agreement with the findings reported in Ref. [13]. Post heat treatment of the Ti/TiN bilayers does not seem to alter these trends for the evolution of the stress or the texture of W-CVD films.
~
120 L ~ 100
g
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.~"
60
• - - Reference
,K C
40
20
200
0
3.2.1. Stress and texture of W o n type l bilayers Figs. la and 2a show the evolution of the stress in W and the orientation factor for the W ( l l 0 ) diffraction peak versus the W thickness for W deposited on type 1 bilayers. The orientation factor was calculated following the standard procedure: the integrated intensities of the diffraction peaks were normalised to the respective intensity values of a standard powder sample. Thin film correction was then done to account for the absorption factor [ 14]. On the reference bilayer, both the W stress and the
(a)
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lOffO
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_ _ =
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18 ~
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.~.
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W thickness (nm)
, ~
!
• - - RTA of TiYTiN in N2 A - RTA °f Ti/TiN in NH3
(b)
2
600
400 W
thickness
800
1000
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Fig. 2. Variation of the orientation factor relative to the W ( I ~ diffraction peak versus W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH3 and N>
1.5 1
0.5 0
200
400
(a)
600
800
1000
W thickness (nm) 1.3 1.2
!~(b) '
• • A
Reference RTA of Ti/TiN in N2 RTA of Ti/TiN in NH3
1.l t 0.9 0.8
Table 2 Thickness of the W nucleation layers deposited on different Ti,q'iN bilayers
0.7 r~
0.6 0.5
I
I
~
I
200
(b)
orientation factor for W ( l i 0 ) decrease monotonically with increasing W thickness. On the annealed bilayers, the W stress is considerably lower as compared to that on the reference bilayer. The stress reduction due to the RTA of the bilayers is most significant for thin W fihns (40-50%). The orientation factor for W (110), however, first increases until the W thickness reaches about 150 nm and then decreases monotonically beyond this thickness. W nucleation deposition on the annealed bilayers seems to be retarded, and thus the W layers are thinner after identical deposition (see Table 2).
?
I
~
]
400
T
I
I
I
I
600
I
I
800
I
I
Thickness of the nucleation layer (nm)
f
1000
No RTA
W thickness (nm)
Fig. 1. Evolution of W stress versus W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH3 and N>
Type I Type 2
34.02 34.77
Atmosphere of RTA Na
NH3
22.43 23.92
23.18 27.29 _
277
,4. Mom'ott.x er al. / SuJjhc'e and CoeztiHgs Technology 99 (1998) 2 7 4 - 2 8 0
3.2.2. Stress a m / t e x t u r e o/" IV otz O'pe 2 bilayers Figs. lb and 2b show the thickness dependence of the stress in W and the orientation factor for the W(I10) peak for W films grown on type 2 bilayers. In general, both the stress and the orientation factor of W deposited on the reference bilayer have a similar thickness dependence as W deposited on the type 1 reference bilayer. However, the stress in W is lower and the film is less oriented for W deposited on type 2 bilayers than on type 1 bilayer, as noted above. On annealed bilayers, different characteristics are observed. The W stress first increases from low values of 0.5 to 0.8 GPa with W thickness to a maximum value of about 1 GPa. Them it slowly decreases when the W is greater than about 150-200 nm thick. Although still textured, the orientation factor for W ( l l 0 ) is several times smaller as compared to the case of W deposited on type 1 bilayers. The evolution of the orientation factor follows a similar pattern for all W films deposited on type 2 bilayers: it decreases monotonically with the W thickness, and there is essentially no maximum except for some local variations. W nucleation deposition on the annealed bilayers is again retarded (Table 2). The W stress is also somewhat lower when the bilayers are annealed ( Fig. 1b).
Z
• •
I [" "~,
Reference RTA of T.i]TiN !n N2
2 1.5
1
0.5
,
0
,
r
I
200
(a)
,
~
,
I
,
,
,
400
I
,
,
600
~
I
~
,
800
1000
W thickness (nm) 1.8 • •
!
¢~
1.4
~
1.9
Reference RTA of T.i/T!N !n N2
1
~"
0.8 0.6
3.3. Toted stress in Ti/TiN/~t" and stmarjbrce oJ" I4~
Figs. 3a and 3b show the wtriation of the total stress of the Ti/TiN/W stack with W thickness for W films deposited on type I and type 2 bilayers. In all cases, the stress tends to approach a constant value of around 1 GPa as the W thickness increases. This convergence behaviour is independent of the type of Ti/'TiN adhesion bilayer used, and is insensitive to the RTA treatment of the bilayers prior to W deposition. For both types of bilayer, annealing alters the total stress in such a way that it decreases monotonically with W thickness, in contrast to the monotonic increase in the total stress with increasing W thickness for W films deposited on the reference (unannealed) bilayers. The effect of annealing on the total stress is more sensitive for small W thicknesses, and only a minor difference is observed between annealing in NH3 and N 2. The shear force is defined as the product of the fihn stress and the film thickness. Fig. 4a and 4b show the shear force of W deposited on type 1 and type 2 bilayers versus W thickness. The variation of the shear force of W with W thickness seems to be in agreement with the results reported in Ref. [15]. The shear force of W deposited on type 2 bilayers is quite insensitive to bilayer annealing. This is not true for W deposited on type 1 bilayers: bilayer annealing results in an appreciably lower shear force, but the atmosphere (i.e. N2 or N H 3) has a minor effect.
2.5
0 (b)
200
400 W thickness
600
800
1000
(nm)
Fig. 3. Dependence of the total stress on W thickness for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH 3 and N>
4. Discussion The nucleation and growth of W can be regarded as being governed energetically by the dynamic balance between the strain energy (Estrain) and the surface energy (£~rf). During nucleation deposition, the surface energy Esurf controls the process. Tungsten (body-centred cubic) starts to grow with a preferential crystallographic orientation which corresponds to the surfaces with the lowest surface energy (i.e. the (110) orientation) [16]. As the W grows thicker, the surface energy term becomes less influential, and simultaneously the strain energy E~train gradually increases. Thus, other crystallographic orientations begin to compete with the (110) orientation in order to reduce /ZTstrain [17], leading to less oriented W films with increasing film thickness. On annealed type 1 bilayers, the W texture initially develops further in the (110) orientation with increasing W thickness (Fig. 2a) while the W stress still remains at a low level (Fig, la). In contrast, on annealed type 2 bilayers, the orientation factor for W (I 10) continues to decrease (Fig. 2b) while
A. Mouroux et aL / Sur[ace and Coatings Technology 99 (1998) 274-280
278
~a
1200 1000 800 600 400
t..
200
. I ' ~N" -
2
-
-• ~.
rd~
-RTA-of TifFiN in RTA of TiffiN in NH3
0 200
(a)
400
600
800
1000
W thickness (nm) 1000 800
= ~N
6o0
<~ ~
400
'-
200
of the new phase [19]. As a consequence, nucleation deposition of W-CVD on annealed bilayers becomes more difficult and W growth becomes slower (Table 2). During annealing of the bilayers in N2, oxidation of the TiN surface can take place as an increased amount of interfaciaI oxygen is found at the T i N - W interface [1,12]. This increased interracial oxygen could explain why the W nucleation layer is thinnest on N2-annealed bilayers (Table 2), as the chemical driving force for the reaction between WF6 and TiO, is even smaller than that for the reaction between WFo and TiN [18]. (2) Annealing causes the stress in the Ti/TiN bilayers to increase. After RTA. the stress in [ype i bilayers i s higher than in type 2 bilayers (Table 1 ). As can be seen in Fig. 5, type 2 bilayers are generally more randomly oriented than type 1 bilayers. "Therefore, W deposited on type 2 bilayers is also more randomly oriented, with a lower stress. Moreover. annealing does not have the same effect on these two types of bilayers: annealing makes type l bilayers even more oriented towards the <111 > orientation, and in contrast makes type 2 bilayers less oriented (see Fig. 5). As a direct consequence, W --~ Reference ---4t== RTA of Ti/TiN in N2 - - ~ , - - RTA of Ti/TiN in NIl3
140 0
120 200
(b)
400
600
800
1000
I00
W thickness (nm)
80
Fig. 4. Shear force associated with W films for W deposited on (a) type 1 and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NH 3 and N,.
60 40 *"*
the W stress increases with W thickness for W thicknesses below 150-200nm (Fig. lb). These seemingly controversial trends of development will be discussed below. The stress and texture of the W-CVD are dependent on the stress and texture of the underlying adhesion layers [1,9,12,13]. As the mechanical and chemical properties of the two types of adhesion bilayer can be different, their effects on W are also expected to be different. Annealing the adhesion bilayers in N2 or NH3 has at least three effects. (1) Annealing converts free Ti atoms in the sputterdeposited TiN into titanium nitride, thereby making the TiN surface more resistant to chemical attack by WF6. According to thermodynamics [ 18], the chemical driving force for the interaction between WF6 and Ti is considerably greater than that for the reaction between WFo and TiN. The amounts of the interfacial fluorine at the T i N - W interface have been shown to be substantially reduced as a result of the RTA of Ti/TiN bilayers [1]. Further, the nucleation rate ~br the growth of a new phase is known to be exponentially proportional to the square of the chemical driving force for the formation
~,~
20 36
38
(a)
,fi t,,*
42
44
2,0 (deg.) 70
.=.
40
60
----I~-~ Reference ---it--- RTA of TiffiN in N2 - - - ~ - RTA of TiffiN in NH3
~"
4O
20 10 0
(h)
I , 36
38
40 2.0
42
44
(deg.)
Fig. 5. Comparison of the XRD spectra of (a) type [ and (b) type 2 Ti/TiN adhesion bilayers with and without RTA treatment in NF[-3 and N 2, showing the strong impact of annealing on the texture of Ti/TiN.
A. Molo'oux c't al,
Sutjace and CoaHngs Technology 99 (1998) 274-280
deposited on annealed type 2 bilayers shows a much weaker film texture (Fig. 2). Annealing also has a less pronounced impact on type 2 bilayers than on type 1 bilayers, so that the stress in the bilayers increases much less for the type 2 bilayers (Table 1 ). Therefore, annealing type 2 bilayers has less effect on the W films deposited subsequently: W nucleation deposition is relatively less affected, especially after NH 3 annealing (Table 2), and the orientation factor for W ( l I 0 ) is not much influenced by bilayer annealing and thus evolves in a similar fashion (Fig. 2b). (3) Annealing increases the content of nitrogen in the Ti/TiN bilayers. This can be seen in the XRD spectra in Fig. 5: the two peaks assigned to ~-Ti shift towards lower angles after RTA of Ti/TiN as a result of more nitrogen incorporation into the underlying ~-Ti layer [1]. During annealing, additional nitrogen atoms could be pumped into the upper (stoichiometric) TiN layer in order to bring the T i N - N : (or NH 3) system to equilibrium with respect to the chemical potential of nitrogen. During subsequent W deposition, a certain amount of nitrogen can then diffuse into the W by a similar driving force to bring the T i N - W system to equilibrium. It is highly probable that nitrogen is present ill the W lattice as an interstitial impurity, resulting in an increased lattice parameter for W and reduced tensile stress in W [20]. The impact of nitrogen on W stress is anticipated to be more pronounced for thinner W films, since nitrogen, similar to fluorine and oxygen [1.9, I2,13], is likely to be concentrated at the T i N - W interface. The contrasting trends of stress and film texture development noted above can now be explained in terms of nitrogen incorporation into the W lattice. On annealed type 1 bilayers, the W stress is considerably reduced, mainly by nitrogen incorporation (Fig. Ia), because the film texture is comparable at each W fihn thickness throughout the entire thickness range (Fig. 2a) and the thermal 2.5
+ •
"~
2
.2
1.5
41.I nm 95.3 nm 169.3 nm
1 0.5
0
im,
j llt-111,4t-X~e'~l~a.~,.~
50
loo
150
200
250
300
,i., - -
350
4oo
279
stress in W should be independent of bilayer annealing. 1::o1" W deposited on type 2 bilayers, the results shown in Figs. 2a and 2b can be discussed in similar terms. As high-temperature TiN is expected to be more structurally perfect, with a lower density of defects [15], it can be understood that bilayer annealing has a less pronounced impact on W deposited on type 2 bilayers than W on type 1 bilayers. The stress value which the total stress of the whole Ti/TiN/W stack approaches at large W thicknesses (i.e. 1 GPa: Figs. 3a and 3b) is most probably determined by the stress in W (both intrinsic and extrinsic). The extrinsic thermal stress in W is calculated to be about 0.3 GPa, and the intrinsic stress should include structural and impurity contributions. Apart from the known facts that fluorine increases the resistivity of W and is responsible for the formation of bubble-like features [21 ], how the presence of fluorine at the T i N - W interface directly affects stress in the Ti/TiN/W system is not clear. Fluorine is present mainly at the T i N - W interface and remains there during W growth at the temperature used here (440 ~C ), according to the nuclear resonance broadening (NRB) results shown in Fig. 6. It is conceivable that interracial fluorine can reduce the adhesion of W on TiN. Moreover, fluorine is not desirable in this system as it may also lead to outgassing during subsequent process steps at temperatures higher than that used for W-CVD.
5. Conclusions From the experimental results presented in this work as well as in Refs. [1,9,12, I3], it is clear that the temperature used for the deposition of Ti/TiN adhesion bilayers plays a crucial role with regard to control of the film stress and impurities in the Ti/TiN/W system. A high deposition temperature is preferred in order to reduce both tile stress in W and the impurity content at the T i N - W interface. However, the total stress can increase as a consequence of the increased stress in the Ti/TiN adhesion bilayers resulting from the high deposition temperature used. Post-RTA treatment of the deposited bilayers in NH 3 02" N2 can be employed to increase the chemical stability of the bilayers by nitridation of the unreacted Ti in TiN as well as the uncovered Ti in the bilayer system. As the stress increase due to post-RTA treatment of Ti/TiN can become a potential reliability issue in contact via-filling and interconnection applications, the temperature used %1" RTA has to be chosen carefully.
Depth (nm) Fig. 6. Nuclear resonance broadening (NRB) data showing that interfacial fluorine remains at the TiN-W interface during W growth from 41.I to 95.3 nm and to 169.3 nrn on type 1 bilayers sputter-deposited at 250-C.
Acknowledgement The authors are grateful to Professor F. d'Heurle for enlightening discussions. A.M. is a grant holder of
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A. Mouroux et al, / Su(/ace and Coatings Tectmology 99 (1998) 274-280
the E u r o p e a n program "Training and Mobility of Researchers" (TMR) (Contract No. ERBFMB I C T 9 6 0 8 8 6 ) . T h i s w o r k was p a r t l y s u p p o r t e d by the S w e d i s h B o a r d for I n d u s t r i a l a n d T e c h n i c a l D e v e l o p m e n t ( N U T E K ) a n d by the F l e m i s h I n s t i t u t e for the A d v a n c e m e n t of Scientific-Technological R e s e a r c h in I n d u s t r y ( I W T ) .
References [I] A. Mouroux. R. Palmans, J. Keinonen, S,-L. Zhang, K. Maex, S. Peterssom in: K.N. Tu, J.W, Mayer, J.M. Poate, L.J. Chen (Eds.), Advanced Metallization for ULS. voi. 427, MRS, San Francisco, CA, 1996, p. 365. [2] N.E. Miller, I, Beinglass, Solid State Technol. 25 (12) (1982) 85. [3] J, Schmitz, S. Kang, R. Wolters, K.v.d. Aker, J. Electrochem. Soc. 141 (1994) 843. [4] S. Sivaram, M.L.A. Dass, C.S. Wei, B. Tracy, R. Shukla, J. Vac. Sci, Technol, A 11 11993) 87. [5] D.A. Bell, C.M. McConica, K.L. Backer, J. Electrochem. Soc. 143 (1996) 296. [6] E.K. Broadbent, J. Vac. Sci. Technol. B 5 11987) 166I. [7] V.V.S. Rana, J.A. Taylor, L.H. Holschwandner, N.S. Tsai, in: E.K. Broadbent (Ed.), Tunsgten and Other Refractory Metals ['or VLSI Applications, vol. [I, MRS, Palo Alto, CA, 1987, p. t87.
[8] Y.J. Lee, C.,O. Park, D.-W. Kim, J.S. Chun, J. Electron. Mater. 23 (1994) 1075. [9] S.-L. Zhang, R. Palmans, J. Keinonen, C.S, Petersson, K. Maex, AppI. Phys. Lett. 67 11995)2998. [10] D,A. Webb, J. Hitlman, F. Foster, in: R.C. Ellwanger, S.-Q. Wang (Eds.), Advanced Metallization and interconnect Systems for VLSI applications, MRS, Portland, OR, 1995. [1I] S.C. Sun, M.H. Tsai, Appl. Phys. Left. 68 (1996) 670. [12] R. Pahnans, A. Mom'oux, S,-L. Zhang, S. Petersson, K. Maex, in: R.C. Elhvanger, S.-Q. Wang {Eds,), Advanced Metallization and Interconnect Systems for VLSI Applications, MRS, Portland, OR, 1995, p~ 555. [I3] S.-L. Zhang, R. Pahnans, C.S. Petersson, K. Maex, J. Appl. Phys. 78 (1995) 7313. [14] P. Joubert, B. Loisel, Y. Chouen, I_. Haji, J. Electrochem. Sac. 134 11987) 2541. [15] S. Sivaram, S, Chen, D. Liao, R. Shukla, D. Fraser, in: V.A. Wells ( Ed.L Tungsten and Other Refractory Metals for VLSI Applications, vol. II, MRS, New York, 1988, p. 407. [16] D. Wolf, J. Appl. Phys. 69 (19911 185. [t7] U.C. Oh, J.H. Je, J. Appl. Phys. 74 11993) 1692. = [18] I. Barin, O. Knacke, O. Kubaschewski, Thermochemieal Properties of Inorganic Substances, suppl.~ Springer, Berlin, 1977, ___ [19] F.M. d'Heurle, J. Mater. Res. 3 11988) 162. [20] H.J. Goldschmidt, Interstitial Alloys, Butterworth, London, 1967, p. 226, = [2l~ T. Eriksson, J.-O. Carlsson, E. Niemi, M. Ostling, C.S. Petersson, in: G.W. Cullen (Ed.), Proceedings of the Tenth international Conference on Chemical Vapor Deposition, Electrochemical Society, Princeton, N J, 1987, p. 736.