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The effects of collimation on intrinsic stress in sputter-deposited metallic thin films
ELSEVIER
Thin Solid Films 253 (1994) 372 376
The effects of collimation on intrinsic stress in sputter-deposited metallic thin films T. Janacek a, D. Liu b, S. K. Dew b, M. J. Brett b, T. J. Smy c ~Alberta Microelectronic Centre, No. 318, 11315-87 Avenue, Edmonton, AIta. T6G 2T9, Canada ~'Department of Electrical Engineering, Univesity of Alberta, Edmonton, Alta. T6G 2G7, Canada CDepartment of Electronics, Carleton University, Ottawa, Ont. K I S 5B6, Canada
Abstract
An experimental study is presented on the effects of collimation on intrinsic stress in sputter-deposited titanium thin films. A stainless steel square grid collimator with an aspect ratio of 1.0 was used, with a collimator wall spacing of 0.5 cm. Thin titanium films were deposited at 0.26, 0.53, 0.80, 1.07 and 1.33 Pa (2, 4, 6, 8 and 10mTorr) pressure with and without collimation under otherwise identical deposition conditions. The Monte Carlo simulation software package SIMSPUD was used in order to simulate some of the changes caused by the introduction of the collimator into the vacuum chamber. The overall level of the intrinsic stress was measured for each deposited film. Films deposited with a collimator consistently exhibited an increased level of accumulated compressive stress when compared with the films deposited by conventional (uncollimated) sputtering. This feature of collimated sputtering may be employed to tailor the stress level in thin film coatings.
Keywords: Computer simulation; Physical vapour deposition; Stress; Titanium
1. Introduction
The constant increase in aspect ratios of contacts and vias employed in very-large-scale integration (VLSI) and ultra large-scale integration (ULSI) devices creates a number of serious problems. Specifically, the issue of insufficient film coverage on the bottom and side walls of the contact or via hole poses a serious challenge to sputter deposition techniques used for the deposition of thin metal films. In the case of very thin (40-100 nm) titanium films, which are frequently used in VLSI and ULSI multilevel metallization schemes, it is very important to achieve conformal step coverage. This goal, however, is becoming impossible to attain via conventional sputtering techniques because of the increasing aspect ratio of contact holes. To improve the film coverage, a simple technique of collimated sputtering has been proposed [1]. This technique employs a physical collimator to control the angular divergence of the flux of the sputtered particles onto the substrate and retains almost all the advantages of conventional planar magnetron sputter deposition. The basic principle of collimation is thought to be the removal of particles whose trajectory intersects a collimator. If properly placed, a collimator can be used to 0040-6090/94/$7.00 1994 Elsevier Science S.A. All rights reserved SSDI 0040-6090( 94)04607-8
engineer the angular distribution of the flux arriving at the wafer. However, a collimator also changes the conditions in a vacuum chamber, which may have a significant impact on the properties of deposited films. For example, it has been demonstrated that the overall film uniformity is very sensitive to the collimator position inside a vacuum chamber as well as the collimator dimensions [2]. In previous work [3, 4] we have summarized the basic properties of films deposited by collimated sputtering, such as film uniformity, step coverage and crystallinity. SIMBAD (simulation by ballistic deposition), a Monte Carlo computer model developed earlier by this group, has been used successfully for the simulation of thin film physical vapor deposition and chemical vapor deposition, as well as plama- and ion-assisted etching [5, 6]. Another Monte Carlo simulator, SIMSPUD (simulation of sputter distributions), which simulates the vapor transport of the sputtered flux, was developed as an extension of SIMBAD [7]. Later, this model was extended to simulate collimated sputtering [3]. The results from our previous work confirmed collimator-induced changes that may be inituitively expected, such as a decrease in the deposition rate and the increase in the bottom coverage of VLSI and U L S I contact holes.
72 Janacek et al. / Thin Solid Films, 253 (1994) 372-376
However, preliminary data also showed an unexpected result that the introduction of a collimator into a vacuum chamber would also affect stress in thin metal films. This work reports in detail the stress transition with total pressure in thin titanium films deposited by collimated as well as conventional (uncollimated) sputtering and presents Monte Carlo simulations supporting our explanation of this effect.
2. Experiment To deposit films for this study, titanium films were d.c. sputter deposited from a 99.95% pure, 12.5 cm x 20 cm Ti target using a planar magnetron source at 500 W constant power with nominally 400 V applied, and argon pressures of 0.26, 0.53, 0.80, 1.07 and 1.33 Pa (2, 4, 6, 8 and 10 mTorr). The base pressure for the vacuum system prior to deposition was about 6.7 x 10 -5 Pa (5 × 10 -7 Torr). The substrate temperature was not controlled during the deposition. The square grid collimators with 0.5 cm pitch were fabricated from stainless steel 0.1 mm thick. The depth of the collimators were 0.5 cm, corresponding to an aspect ratio of 1.0. The distance between the bottom of the collimator and the substrate was 1.3 cm. The distance between the target and the substrate was 7.6 cm. The films were deposited onto a substrate featuring an overhang structure. To create this structure, we overetched the silicon under 5000 A of silicon dioxide using a solution of hydrofluoric acid, nitric acid and acetic acid (2:100:3 by volume). The undercut is about 4/am deep and 4/am wide. In addition to an overhang structure, long silicon dioxide cantilever beams were fabricated and mounted on each sample holder. These beams are commonly used in acceleration sensors manufactured by the Alberta Microelectronic Centre, and they were 80-260/am long, 80 gm wide and 2.2/am thick. The overhang was bent up (or down) in response to tensile (or compressive) intrinsic stress. A more precise value of the stress in titanium films was determined using the resultant deflection of the beam structure on the silicon wafer. This deflection of the silicon dioxide beams was measured utilizing the depth of the field of locus of a standard optical microscope. At least four beams from each chip were measured and the value for total accumulated stress for each chip was then taken as an average value of those measurements. After sputter deposition, the overhang samples were immersed into liquid N2 for 60 s in order to facilitate the consequent sample cleaving to produce cross-sections and scanning electron microscopy (SEM) was used to measure the film thickness. In order to deposit films of comparable thickness, the deposition time for films deposited by collimated sputtering was increased by a factor of 4, compared with films deposited by
373
conventional sputtering. This value was determined from the evaluation of deposition rates of films deposited by conventional and collimated sputtering. All other parameters were identical in order to ensure reproducibility of deposition conditions.
3. Simulation Simulation of sputtered flux transport through a collimator was accomplished by a vapor transport model, called SIMSPUD, which simulates the threedimensional macroscopic path of the individual atom from its emission from the target, during its travel through the working gas until its arrival at the substrate plane. The position, angle and energy of each atom are tracked in order to collect statistics on the corresponding sputter distributions. To account for gas sputtering, the model employs an energy-dependent collision crosssection with a hard-sphere scattering model. Atoms whose trajectories intersect the collimator vanes are removed from the simulation. For each particular set of the sputtering parameters, such as the aspect ratio of the collimator, target material, target erosion profile and deposition pressure, SIMSPUD can generate the corresponding angular distribution of the sputtered flux arrival at the substrate. This angular distribution may be used in the second-stage model, SIMBAD, to predict the film profile and microstructure over topography [5].
4. Results and analysis There are two main components of total accumulated film stress: thermal stress and intrinsic stress [8]. Thermal stress is caused by a mismatch of thermal expansion coefficients of the substrate and the film. Its importance largely depends on the ration of T / T m of substrate temperature during deposition to the film melting point. While thermal stress dominates for films deposited at large T/Tm ratios, it is usually the intrinsic stress which is prevalent at low T/Tm ratios. Since titanium is a refractory metal with high Tm (1943 K) and the substrate was not intentionally heated, we can assume that the thermal stress is minimal. Intrinsic stress is a result of the non-equilibrium nature of the film growth [9] and as such it is very sensitive to the deposition conditions. At high T / T m , diffusion processes are enhanced and relieve intrinsic stress. Many evaporated and sputter-deposited films are in the state of tensile stress. This is commonly explained as a consequence of film shrinkage due to atomic relaxation which occurs during and after the film growth [10]. Compressive stresses are believed to be caused by the presence of impurities in the lattice [11] and by atomic peening from energetic ions or neutrals.
T. Janacek et al. / Thin Solid Films, 253 (1994) 372-376
374
(a)
(b)
Fig. 1. SEM photographs of titanium films deposited onto overhang structure by (a) conventional sputtering at 1.3 Pa (10 mTorr) and (b) collimated sputtering at 1.3 Pa (10 mTorr).
The following equation was used for stress evaluation from the beam deflection [12]: o"
6
3L2tf(tf+
/--//Eftf3
ts)U--~',-+
Ests3 1 -
,'s/
(1)
where E, t and v are Young's modulus of the layer, the layer thickness and Poisson's ratio respectively, with the subscript f denoting the parameters of the Ti film layer and the subscript s denoting the parameters of the SiO2 layer. ,~ is the displacement at the end of the beam and L is length of the beam. Fig. 1 shows the SEM photographs of sputtered Ti films over the overhang structure without a collimator and with a collimator of aspect ratio 1.0 at 1.3 Pa (10 mTorr). The SiO2 overhang structure was verified to be flat before the deposition. Fig. l(a) shows that the overhang is bending up after conventional sputter deposition of a Ti film at 1.3 Pa (10 mTorr), indicating that the film stress is tensile. However, Fig. l(b), which captures the Ti film deposited by collimated sputtering under otherwise identical conditions, shows a film which is clearly under a compressive stress. The SEM photographs clearly demonstrate that the introduction of a collimator into the sputter process had a strong impact on the overall value of intrinsic film stress. Tables 1 and 2 show data obtained from the measurement of the deflection of long SiO2 beams at 0.5 Pa (4 mTorr) without and with a collimator. This set of data was obtained for each combination of deposition conditions (pressure and collimation). The data were used for the calculation of the overall value of intrinsic stress using the Eq. (l). The values of Youngs modulus and Poisson's ratio were Er= 100 GPa (1 × 10J2dync m 2) and vr=0.33 for titanum and E~= 55GPa ( 0 . 5 5 x 1012dyncm 2) and v~=0.16 for SiO2. The thickness of a SiO2 beam was t~ = 2.2 ~tm. Tables 3 and 4 show the numerical calculations of the transition curve for stress in thin titanium films de-
Table l Measurements of the SiO 2 beam deflections and stress calculations; titanium film deposited at 0.53 Pa (4 mTorr) argon pressure without collimation Sample
1 2 3 4
6
L
f)
(iJm)
(~tm)
(gm)
Stress cs ( x I00 MPa (109 dyn cm 2))
0.55 0.55 0.55 0.55
190 200 225 247.5
127.5 95 115 127.5
3.63 3.78 3,61 3,31
Result
3.58 + 0.17
Table 2 Measurements of the SiO 2 beam deflections and stress calculations; titanium film deposited at 0.53 Pa (4 mTorr) argon pressure without collimation Sample
q. (lam)
L (~am)
~ (gm)
Stress a ( × 100 MPa ( 109 dyn cm 2))
1 2 3 4
0.69 0.69 0.69 0.69
90 142.5 175 262.5
0 10 10 2.5
0 0.61 0.41 0.05
Result
0.27 4- 0.24
Table 3 Stress transition in titanium films deposited without collimator Pressure (Pa)
Stress a ( × 100MPa (10'~ dyn cm 2))
1.33 1.07 0.80 0.53 0.26
4.73 5.81 4.92 3.58 3.30
_4-0.33 + 0.83 _+ 0.34 ± 0.17 ± 0.38
375
T. Janacek et al. / Thin Solid Films, 253 (1994) 372-376
Table 4 Stress transition in titanium films deposited with collimator Pressure (Pa)
Stress ( x IOMPa (109dyncm 2))
1.33 1.07 0.80 0.53 0.26
5.51 + 0.33 5.02 _+o. 10 3.83 _+0.32 0.27 + 0.24 -2.17 +0.21
0.08
0.06
0.04
0.02
posited without and with a collimator for the pressure range involved. The results were calculated using Eq. (1) and data obtained from each respective set of experiments. An example of such a set of input data can be seen in Tables 1 and 2. Tensile stress is considered to have a positive sign, while compressive stress has a negative sign. Fig. 2 summarizes the numerical results from Tables 3 and 4. The graph convincingly shows the stress transition curve, i.e the increase in the compressive stress associated with the decrease in the argon pressure. This behaviour is well known and attributed to the atomic peening effect [8] wherein high energy neutral Ar and high energy neutral Ti atoms more readily reach the substrate at low pressures. Films deposited at low pressures are in the T zone of the Thornton structure zone model. This pattern of increasing compressive stress is identical for both conventional and collimated sputter deposition. However, films deposited by collimated sputtering consistently exhibit a higher degree of compressive stress than do the conventionally deposited films. To explain these results, it is useful to consider Fig. 3. This figure shows a SIMSPUD simulation of the shape of the incident angular distribution of sputtered titanium flux arriving at the substrate, for cases without and with collimation and at an argon pressure at 0.8Pa ( 6 m T o r r ) . The collimator narrows the distribution curve by trapping atoms traveling in more oblique path 8.0
0.00 -100.0
-50.0
0.0
50.0
100.0
Incident angle (deg)
Fig. 3. Simulationof the angular distribution of the titanium sputtered flux at 0.8 Pa (6 mTorr) for collimated and uncollimated sputtering. (with respect to the target-substrate axis). More importantly, the collimator also reduces the net deposition rate by a factor of 4 (in our experimental conditions) and the collimator will tend to filter out scattered (and hence lower energy) sputtered titanium atoms. However, high energy neutral Ar atoms reflected from the target will still reach the substrate at a similar flux for both the collimated and the uncollimated cases. The net result for the case of collimated sputtering is an increase in the ratio of high energy neutral bombardment rate of Ti deposition rate, and thus an increase in compressive stress. Fig. 4 shows the simulated shape of the development of the angular distribution of the titanium sputtered flux for various argon pressures. It is obvious that the distribution becomes wider and broader with increasing pressure as a result of scattering. The increase width of the angular distribution makes it more difficult to achieve reasonable bottom and side wall coverage in case of vias with higher aspect ratios. Fig. 4(a) corresponds to the angular distributions for the case of uncollimated sputtering, while Fig. 4(b) shows the angular distributions for the case of collimated sputtering. Angular distributions in Fig. 3(b) are narrower as a result of collimation. This angular distribution could significantly improve the bottom coverage for higher aspect ratio contacts.
6.0
5. Conclusions
4.0 ~
20
"~
0.0 -2.0
~~[,
-4 °0.o
21o
i 4.0
~ uxollL~ed film 61o 81o ,;.o Argon pressure (mTorr)
12.0
Fig. 2. Stresstransition curve for titanium films depositedby conventional and collimated sputtering.
In this paper we presented an experimental study of the stress in thin titanium films deposited by collimated as well as conventional (uncollimated) planar magnetron sputtering. Films deposited by collimated sputtering exhibit increased level of accumulated compressive stress compared with the films deposited by conventional (uncollimated) sputtering. Thus, although collimated sputtering may be beneficial for improving step coverage over VLSI topography, one must also beware of changes in film stress. Conceivably, other
376
T. Janacek et al. / Thin Solid Films, 253 (1994) 372 376 1.0
1.0
2 mTorr 6 mTorr
-
0.8 -
-
- 0.8
/
10 m T o r r
2 mTorr
...... 6 mTorr 10 mTorr
/ /
/ 0.6
/
/
0.6
/
/
', \
/
~- 0.4
/
/
/ /
~wO.4 /
/ I
/
/
'/ ¸ '
/
0.2
/
/
f
0.2
/" / /"
/
/ . 0.0
/:
'
°90.0
-45.0
0.0
45.0
90.0
0.0
(a)
~:
-90.0
Incident angle (deg)
-45.0
0.0
Incident angle
(b)
(deg)
45,0
90.0
Fig. 4. Simulation of the angular distribution of the titanium sputtered flux at various Ar pressures: (a) uncollimated flux; (b) collimated flux.
film properties related to microstructure will also be altered as a result o f the introduction o f a collimator. It is also possible that collimated sputtering m a y be employed to tailor the stress level in thin film coatings.
Acknowledgements This research was sponsored by the Alberta Microelectronic Centre, Varian Associates (Palo Alto, CA), the Micronet Centres o f Excellence, and the N a t u ral Sciences and Engineering Research C o u n c i l o f Canada. The authors w o u l d like to thank Dr. R. N. Tait and Mr. X. Gui for their helpful discussions and to Mr. G. Braybrook for the very high quality scanning electron microscope.
References [ 1] S. M. Rossnagel, D. Mikalsen, H. Kinoshita and J. J. Cuomo, J. Vac. Sci. Technol. A, 9 (2) (1991) 261.
[2] S. K. Dew, D. Liu, M. J. Brett and T. Smy, J. Vac. Sci. Technol. B, 11(4)(1993) 1281. [3] T. Janacek, D. Liu, S. K. Dew, M. J. Brett, T. Smy, W. Tsai, Proc. lOth VLS1 Multilevel lnterconnection Conf., Santa Clara, CA, 1993, IEEE, New York, 1993, p. 387.
[4] D. Liu, T, Janacek, S. K. Dew, M. J. Brett, T. Smy and W. Tsai, Thin Solid Films, 236 (1993) 267. [5] M. J. Brett, J. Vac. Sci. Technol. A, 6(1988) 1749. [6] S. K. Dew, T. Stay and M. J. Bren. J. Vac. Sci. Technol. A, 10 (2) (1992) 618. [7] S. K. Dew, Processes and simulation for advanced integrated circuit metallization, Ph.D. Thesis, University of Alberta, Edmonton, Alta., Canada, 1992. [8] J. A. Thornton and D. W. Hoffman, Thin Solid Films, 171 (1989) 5. [9] D. S. Campbell, in L. I. Maissel and R. Glang (eds.), 1landbook of Thin Flilm Technology, McGraw-Hill, New York, 1970, p. 12 3. [10] H. Oikawa and Y. Nakajima, J. Vac. Sci. Technol., 14 (1977) 1153. [11] P. M. Alexander and R. W. Hoffman, J. Vac. Sci. Technol., 13 (1976) 96. [12] X. Gui, W. C. Wu and G. B. Gao, Acta Electron. SinL, 14 (3) (1986) 123.
Thin Solid Films 253 (1994) 372 376
The effects of collimation on intrinsic stress in sputter-deposited metallic thin films T. Janacek a, D. Liu b, S. K. Dew b, M. J. Brett b, T. J. Smy c ~Alberta Microelectronic Centre, No. 318, 11315-87 Avenue, Edmonton, AIta. T6G 2T9, Canada ~'Department of Electrical Engineering, Univesity of Alberta, Edmonton, Alta. T6G 2G7, Canada CDepartment of Electronics, Carleton University, Ottawa, Ont. K I S 5B6, Canada
Abstract
An experimental study is presented on the effects of collimation on intrinsic stress in sputter-deposited titanium thin films. A stainless steel square grid collimator with an aspect ratio of 1.0 was used, with a collimator wall spacing of 0.5 cm. Thin titanium films were deposited at 0.26, 0.53, 0.80, 1.07 and 1.33 Pa (2, 4, 6, 8 and 10mTorr) pressure with and without collimation under otherwise identical deposition conditions. The Monte Carlo simulation software package SIMSPUD was used in order to simulate some of the changes caused by the introduction of the collimator into the vacuum chamber. The overall level of the intrinsic stress was measured for each deposited film. Films deposited with a collimator consistently exhibited an increased level of accumulated compressive stress when compared with the films deposited by conventional (uncollimated) sputtering. This feature of collimated sputtering may be employed to tailor the stress level in thin film coatings.
Keywords: Computer simulation; Physical vapour deposition; Stress; Titanium
1. Introduction
The constant increase in aspect ratios of contacts and vias employed in very-large-scale integration (VLSI) and ultra large-scale integration (ULSI) devices creates a number of serious problems. Specifically, the issue of insufficient film coverage on the bottom and side walls of the contact or via hole poses a serious challenge to sputter deposition techniques used for the deposition of thin metal films. In the case of very thin (40-100 nm) titanium films, which are frequently used in VLSI and ULSI multilevel metallization schemes, it is very important to achieve conformal step coverage. This goal, however, is becoming impossible to attain via conventional sputtering techniques because of the increasing aspect ratio of contact holes. To improve the film coverage, a simple technique of collimated sputtering has been proposed [1]. This technique employs a physical collimator to control the angular divergence of the flux of the sputtered particles onto the substrate and retains almost all the advantages of conventional planar magnetron sputter deposition. The basic principle of collimation is thought to be the removal of particles whose trajectory intersects a collimator. If properly placed, a collimator can be used to 0040-6090/94/$7.00 1994 Elsevier Science S.A. All rights reserved SSDI 0040-6090( 94)04607-8
engineer the angular distribution of the flux arriving at the wafer. However, a collimator also changes the conditions in a vacuum chamber, which may have a significant impact on the properties of deposited films. For example, it has been demonstrated that the overall film uniformity is very sensitive to the collimator position inside a vacuum chamber as well as the collimator dimensions [2]. In previous work [3, 4] we have summarized the basic properties of films deposited by collimated sputtering, such as film uniformity, step coverage and crystallinity. SIMBAD (simulation by ballistic deposition), a Monte Carlo computer model developed earlier by this group, has been used successfully for the simulation of thin film physical vapor deposition and chemical vapor deposition, as well as plama- and ion-assisted etching [5, 6]. Another Monte Carlo simulator, SIMSPUD (simulation of sputter distributions), which simulates the vapor transport of the sputtered flux, was developed as an extension of SIMBAD [7]. Later, this model was extended to simulate collimated sputtering [3]. The results from our previous work confirmed collimator-induced changes that may be inituitively expected, such as a decrease in the deposition rate and the increase in the bottom coverage of VLSI and U L S I contact holes.
72 Janacek et al. / Thin Solid Films, 253 (1994) 372-376
However, preliminary data also showed an unexpected result that the introduction of a collimator into a vacuum chamber would also affect stress in thin metal films. This work reports in detail the stress transition with total pressure in thin titanium films deposited by collimated as well as conventional (uncollimated) sputtering and presents Monte Carlo simulations supporting our explanation of this effect.
2. Experiment To deposit films for this study, titanium films were d.c. sputter deposited from a 99.95% pure, 12.5 cm x 20 cm Ti target using a planar magnetron source at 500 W constant power with nominally 400 V applied, and argon pressures of 0.26, 0.53, 0.80, 1.07 and 1.33 Pa (2, 4, 6, 8 and 10 mTorr). The base pressure for the vacuum system prior to deposition was about 6.7 x 10 -5 Pa (5 × 10 -7 Torr). The substrate temperature was not controlled during the deposition. The square grid collimators with 0.5 cm pitch were fabricated from stainless steel 0.1 mm thick. The depth of the collimators were 0.5 cm, corresponding to an aspect ratio of 1.0. The distance between the bottom of the collimator and the substrate was 1.3 cm. The distance between the target and the substrate was 7.6 cm. The films were deposited onto a substrate featuring an overhang structure. To create this structure, we overetched the silicon under 5000 A of silicon dioxide using a solution of hydrofluoric acid, nitric acid and acetic acid (2:100:3 by volume). The undercut is about 4/am deep and 4/am wide. In addition to an overhang structure, long silicon dioxide cantilever beams were fabricated and mounted on each sample holder. These beams are commonly used in acceleration sensors manufactured by the Alberta Microelectronic Centre, and they were 80-260/am long, 80 gm wide and 2.2/am thick. The overhang was bent up (or down) in response to tensile (or compressive) intrinsic stress. A more precise value of the stress in titanium films was determined using the resultant deflection of the beam structure on the silicon wafer. This deflection of the silicon dioxide beams was measured utilizing the depth of the field of locus of a standard optical microscope. At least four beams from each chip were measured and the value for total accumulated stress for each chip was then taken as an average value of those measurements. After sputter deposition, the overhang samples were immersed into liquid N2 for 60 s in order to facilitate the consequent sample cleaving to produce cross-sections and scanning electron microscopy (SEM) was used to measure the film thickness. In order to deposit films of comparable thickness, the deposition time for films deposited by collimated sputtering was increased by a factor of 4, compared with films deposited by
373
conventional sputtering. This value was determined from the evaluation of deposition rates of films deposited by conventional and collimated sputtering. All other parameters were identical in order to ensure reproducibility of deposition conditions.
3. Simulation Simulation of sputtered flux transport through a collimator was accomplished by a vapor transport model, called SIMSPUD, which simulates the threedimensional macroscopic path of the individual atom from its emission from the target, during its travel through the working gas until its arrival at the substrate plane. The position, angle and energy of each atom are tracked in order to collect statistics on the corresponding sputter distributions. To account for gas sputtering, the model employs an energy-dependent collision crosssection with a hard-sphere scattering model. Atoms whose trajectories intersect the collimator vanes are removed from the simulation. For each particular set of the sputtering parameters, such as the aspect ratio of the collimator, target material, target erosion profile and deposition pressure, SIMSPUD can generate the corresponding angular distribution of the sputtered flux arrival at the substrate. This angular distribution may be used in the second-stage model, SIMBAD, to predict the film profile and microstructure over topography [5].
4. Results and analysis There are two main components of total accumulated film stress: thermal stress and intrinsic stress [8]. Thermal stress is caused by a mismatch of thermal expansion coefficients of the substrate and the film. Its importance largely depends on the ration of T / T m of substrate temperature during deposition to the film melting point. While thermal stress dominates for films deposited at large T/Tm ratios, it is usually the intrinsic stress which is prevalent at low T/Tm ratios. Since titanium is a refractory metal with high Tm (1943 K) and the substrate was not intentionally heated, we can assume that the thermal stress is minimal. Intrinsic stress is a result of the non-equilibrium nature of the film growth [9] and as such it is very sensitive to the deposition conditions. At high T / T m , diffusion processes are enhanced and relieve intrinsic stress. Many evaporated and sputter-deposited films are in the state of tensile stress. This is commonly explained as a consequence of film shrinkage due to atomic relaxation which occurs during and after the film growth [10]. Compressive stresses are believed to be caused by the presence of impurities in the lattice [11] and by atomic peening from energetic ions or neutrals.
T. Janacek et al. / Thin Solid Films, 253 (1994) 372-376
374
(a)
(b)
Fig. 1. SEM photographs of titanium films deposited onto overhang structure by (a) conventional sputtering at 1.3 Pa (10 mTorr) and (b) collimated sputtering at 1.3 Pa (10 mTorr).
The following equation was used for stress evaluation from the beam deflection [12]: o"
6
3L2tf(tf+
/--//Eftf3
ts)U--~',-+
Ests3 1 -
,'s/
(1)
where E, t and v are Young's modulus of the layer, the layer thickness and Poisson's ratio respectively, with the subscript f denoting the parameters of the Ti film layer and the subscript s denoting the parameters of the SiO2 layer. ,~ is the displacement at the end of the beam and L is length of the beam. Fig. 1 shows the SEM photographs of sputtered Ti films over the overhang structure without a collimator and with a collimator of aspect ratio 1.0 at 1.3 Pa (10 mTorr). The SiO2 overhang structure was verified to be flat before the deposition. Fig. l(a) shows that the overhang is bending up after conventional sputter deposition of a Ti film at 1.3 Pa (10 mTorr), indicating that the film stress is tensile. However, Fig. l(b), which captures the Ti film deposited by collimated sputtering under otherwise identical conditions, shows a film which is clearly under a compressive stress. The SEM photographs clearly demonstrate that the introduction of a collimator into the sputter process had a strong impact on the overall value of intrinsic film stress. Tables 1 and 2 show data obtained from the measurement of the deflection of long SiO2 beams at 0.5 Pa (4 mTorr) without and with a collimator. This set of data was obtained for each combination of deposition conditions (pressure and collimation). The data were used for the calculation of the overall value of intrinsic stress using the Eq. (l). The values of Youngs modulus and Poisson's ratio were Er= 100 GPa (1 × 10J2dync m 2) and vr=0.33 for titanum and E~= 55GPa ( 0 . 5 5 x 1012dyncm 2) and v~=0.16 for SiO2. The thickness of a SiO2 beam was t~ = 2.2 ~tm. Tables 3 and 4 show the numerical calculations of the transition curve for stress in thin titanium films de-
Table l Measurements of the SiO 2 beam deflections and stress calculations; titanium film deposited at 0.53 Pa (4 mTorr) argon pressure without collimation Sample
1 2 3 4
6
L
f)
(iJm)
(~tm)
(gm)
Stress cs ( x I00 MPa (109 dyn cm 2))
0.55 0.55 0.55 0.55
190 200 225 247.5
127.5 95 115 127.5
3.63 3.78 3,61 3,31
Result
3.58 + 0.17
Table 2 Measurements of the SiO 2 beam deflections and stress calculations; titanium film deposited at 0.53 Pa (4 mTorr) argon pressure without collimation Sample
q. (lam)
L (~am)
~ (gm)
Stress a ( × 100 MPa ( 109 dyn cm 2))
1 2 3 4
0.69 0.69 0.69 0.69
90 142.5 175 262.5
0 10 10 2.5
0 0.61 0.41 0.05
Result
0.27 4- 0.24
Table 3 Stress transition in titanium films deposited without collimator Pressure (Pa)
Stress a ( × 100MPa (10'~ dyn cm 2))
1.33 1.07 0.80 0.53 0.26
4.73 5.81 4.92 3.58 3.30
_4-0.33 + 0.83 _+ 0.34 ± 0.17 ± 0.38
375
T. Janacek et al. / Thin Solid Films, 253 (1994) 372-376
Table 4 Stress transition in titanium films deposited with collimator Pressure (Pa)
Stress ( x IOMPa (109dyncm 2))
1.33 1.07 0.80 0.53 0.26
5.51 + 0.33 5.02 _+o. 10 3.83 _+0.32 0.27 + 0.24 -2.17 +0.21
0.08
0.06
0.04
0.02
posited without and with a collimator for the pressure range involved. The results were calculated using Eq. (1) and data obtained from each respective set of experiments. An example of such a set of input data can be seen in Tables 1 and 2. Tensile stress is considered to have a positive sign, while compressive stress has a negative sign. Fig. 2 summarizes the numerical results from Tables 3 and 4. The graph convincingly shows the stress transition curve, i.e the increase in the compressive stress associated with the decrease in the argon pressure. This behaviour is well known and attributed to the atomic peening effect [8] wherein high energy neutral Ar and high energy neutral Ti atoms more readily reach the substrate at low pressures. Films deposited at low pressures are in the T zone of the Thornton structure zone model. This pattern of increasing compressive stress is identical for both conventional and collimated sputter deposition. However, films deposited by collimated sputtering consistently exhibit a higher degree of compressive stress than do the conventionally deposited films. To explain these results, it is useful to consider Fig. 3. This figure shows a SIMSPUD simulation of the shape of the incident angular distribution of sputtered titanium flux arriving at the substrate, for cases without and with collimation and at an argon pressure at 0.8Pa ( 6 m T o r r ) . The collimator narrows the distribution curve by trapping atoms traveling in more oblique path 8.0
0.00 -100.0
-50.0
0.0
50.0
100.0
Incident angle (deg)
Fig. 3. Simulationof the angular distribution of the titanium sputtered flux at 0.8 Pa (6 mTorr) for collimated and uncollimated sputtering. (with respect to the target-substrate axis). More importantly, the collimator also reduces the net deposition rate by a factor of 4 (in our experimental conditions) and the collimator will tend to filter out scattered (and hence lower energy) sputtered titanium atoms. However, high energy neutral Ar atoms reflected from the target will still reach the substrate at a similar flux for both the collimated and the uncollimated cases. The net result for the case of collimated sputtering is an increase in the ratio of high energy neutral bombardment rate of Ti deposition rate, and thus an increase in compressive stress. Fig. 4 shows the simulated shape of the development of the angular distribution of the titanium sputtered flux for various argon pressures. It is obvious that the distribution becomes wider and broader with increasing pressure as a result of scattering. The increase width of the angular distribution makes it more difficult to achieve reasonable bottom and side wall coverage in case of vias with higher aspect ratios. Fig. 4(a) corresponds to the angular distributions for the case of uncollimated sputtering, while Fig. 4(b) shows the angular distributions for the case of collimated sputtering. Angular distributions in Fig. 3(b) are narrower as a result of collimation. This angular distribution could significantly improve the bottom coverage for higher aspect ratio contacts.
6.0
5. Conclusions
4.0 ~
20
"~
0.0 -2.0
~~[,
-4 °0.o
21o
i 4.0
~ uxollL~ed film 61o 81o ,;.o Argon pressure (mTorr)
12.0
Fig. 2. Stresstransition curve for titanium films depositedby conventional and collimated sputtering.
In this paper we presented an experimental study of the stress in thin titanium films deposited by collimated as well as conventional (uncollimated) planar magnetron sputtering. Films deposited by collimated sputtering exhibit increased level of accumulated compressive stress compared with the films deposited by conventional (uncollimated) sputtering. Thus, although collimated sputtering may be beneficial for improving step coverage over VLSI topography, one must also beware of changes in film stress. Conceivably, other
376
T. Janacek et al. / Thin Solid Films, 253 (1994) 372 376 1.0
1.0
2 mTorr 6 mTorr
-
0.8 -
-
- 0.8
/
10 m T o r r
2 mTorr
...... 6 mTorr 10 mTorr
/ /
/ 0.6
/
/
0.6
/
/
', \
/
~- 0.4
/
/
/ /
~wO.4 /
/ I
/
/
'/ ¸ '
/
0.2
/
/
f
0.2
/" / /"
/
/ . 0.0
/:
'
°90.0
-45.0
0.0
45.0
90.0
0.0
(a)
~:
-90.0
Incident angle (deg)
-45.0
0.0
Incident angle
(b)
(deg)
45,0
90.0
Fig. 4. Simulation of the angular distribution of the titanium sputtered flux at various Ar pressures: (a) uncollimated flux; (b) collimated flux.
film properties related to microstructure will also be altered as a result o f the introduction o f a collimator. It is also possible that collimated sputtering m a y be employed to tailor the stress level in thin film coatings.
Acknowledgements This research was sponsored by the Alberta Microelectronic Centre, Varian Associates (Palo Alto, CA), the Micronet Centres o f Excellence, and the N a t u ral Sciences and Engineering Research C o u n c i l o f Canada. The authors w o u l d like to thank Dr. R. N. Tait and Mr. X. Gui for their helpful discussions and to Mr. G. Braybrook for the very high quality scanning electron microscope.
References [ 1] S. M. Rossnagel, D. Mikalsen, H. Kinoshita and J. J. Cuomo, J. Vac. Sci. Technol. A, 9 (2) (1991) 261.
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[4] D. Liu, T, Janacek, S. K. Dew, M. J. Brett, T. Smy and W. Tsai, Thin Solid Films, 236 (1993) 267. [5] M. J. Brett, J. Vac. Sci. Technol. A, 6(1988) 1749. [6] S. K. Dew, T. Stay and M. J. Bren. J. Vac. Sci. Technol. A, 10 (2) (1992) 618. [7] S. K. Dew, Processes and simulation for advanced integrated circuit metallization, Ph.D. Thesis, University of Alberta, Edmonton, Alta., Canada, 1992. [8] J. A. Thornton and D. W. Hoffman, Thin Solid Films, 171 (1989) 5. [9] D. S. Campbell, in L. I. Maissel and R. Glang (eds.), 1landbook of Thin Flilm Technology, McGraw-Hill, New York, 1970, p. 12 3. [10] H. Oikawa and Y. Nakajima, J. Vac. Sci. Technol., 14 (1977) 1153. [11] P. M. Alexander and R. W. Hoffman, J. Vac. Sci. Technol., 13 (1976) 96. [12] X. Gui, W. C. Wu and G. B. Gao, Acta Electron. SinL, 14 (3) (1986) 123.