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A comparative study of Hillock formation in aluminum films
Thin Solid Films 271 ( 1995) 64-68
A comparative study of hillock formation in aluminum films B. Cao Martin a, C.J. Tracy a, J.W. Mayer b, L.E. Hendrickson b ’ Materials Technology Center, Materials Research and Strategic Technologies, Motorola, Inc.. 2200 W. Broadway Rd, Mesa, AZ 85202, USA b Department of Chemical, Bio, and Materials Engineering, Arizona State University, Box 876006, Tempe, AZ 85287, USA
Received2 December 1994; accepted 12 July 1995
Abstract Studies were conducted to evaluate the performance of a variety of aluminum alloys with respect to hillock formation. Specifically, the effects of film composition, deposition temperature, and underlayer on hillock formation were investigated. The film compositions included pure Al, Al with 1.5 wt.% Cu (or AlCu), AlCu with 0.2 wt.% W, and AlCu with 0.4 wt.% W. These films were sputter deposited at 300 “C and 450 “C on either oxide- or Ti-W-coated Si substrates. Following deposition, the films were annealed in air for 20 mitt at 450 “C. For all film compositions, the deposition temperature had the most significant effect on the hillock density. For a given deposition temperature, the addition of Cu to pure Al drastically reduced the hillock density, but the addition of W to AlCu increased the hillock density. The effect of the underlayer on the hillock density was negligible. Cross-sections of the hillocks showed that they were solid from the bottom interface to the top surface, and no voids were found near the hillocks. The grain size did not have a significant effect on the hillock density. However, a correlation between increasing ( 111) film texture and decreasing hillock density was observed. Keywords: Alloys; Aluminium; Stress
1. Introduction
In recent years, attempts to improve the performance and reliability of aluminum interconnect materials have motivated studies of a variety of alloying elements [ l-71. The performance of a new alloy material is often measured in terms of its resistance to stress-induced void formation and electromigration failure. Another commonly encountered problem in metallization is hillock formation. Hillocks can lead to excessive surface roughness and interfere with photolithography steps. They can also crack the dielectric film isolating the metal lines and result in interlevel electrical short circuits [ 83. Additionally, hillocks can protrude from the sides of metal lines and cause intralevel short circuits among adjacent metal lines [ 91. Hillocks form because of large thermal stresses in the metal films, arising from the large difference in the coefficients of thermal expansion (CTEs) of the metal and the underlayer. The CTE of Al is approximately 10 times larger than the CTE of Si; and consequently, when an Al film is deposited on Si and heated, a compressive stress is produced in the Al film. Hillock formation is one mechanism whereby this thermallyinduced compressive stress is relieved. A number of mechanisms have been proposed for the formation of hillocks in thin metal films, including grain bound0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)06941-O
ary diffusion, diffusional creep, and interfacial diffusion [ 10-251. Because hillock formation is a stress relaxation mechanism which occurs in localized regions in a polycrystalline metal film, one might expect inhomogeneities in the metal microstructure, such as the mean grain size and grain size distribution, and the grain orientations and their distribution about a reference direction, to play dominant roles. Experimental data will be presented to show the effects of the film composition, deposition temperature, and underlayer on the metal film microstructure, and the effects of the microstructure on hillock formation.
2. Experimental details Two full 23 factorial experiments were run to determine the contributions due to each of the three independent variables (film composition, deposition temperature, and underlayer). Two-variable interactions were also examined. Control sets comprising pure Al films and commonly used Al with 1.5 wt.% Cu (AlCu) films were included for comparison. The two levels of film compositions in the factorial experiments were AlCu with 0.2 wt.% W, and AlCu with 0.4 wt.% W (or 0.2 W and 0.4 W films, respectively). Metal deposition was carried out at 300 “C and 450 “C. The two types of underlayers were Si (100) wafers coated with Ti-
B.C. Martin etal. /Thin Solid Films 271(1995)
W or plasma-enhanced silicon oxide prepared from a tetraethylorthosilicate (TEOS) precursor. Four-inch Si ( 100) wafers were first coated with either TEOS or Ti-W. The TEOS films were approximately 300 nm thick, and the Ti-W, 150 nm. Following underlayer deposition, metal films of varying compositions were sputter deposited to a nominal thickness of 750 nm. The AlCuW and pure Al films were deposited in the Varian 3 180 sputter deposition system in the Materials Technology Center (Motorola, Mesa) at a rate of 18.5 nm s - *. The AlCuW films were prepared from homogeneous targets. Argon was the sputtering gas in all depositions. The chamber base pressure was typically lo-’ Pa, and during deposition, the chamber was backfilled with Ar to approximately 0.9 Pa. Substrate heating during sputter deposition was accomplished by Ar-gas-coupled backside heating. The AlCu films were sputter deposited in an MRC Eclipse sputter deposition system in the Advanced Technologies Center (Motorola, Mesa). Following blanket metal film deposition, the wafers were annealed in air in a Flexus F2300 thin film stress measuring apparatus. All isothermal anneals were performed at 450 “C for 20 min. Wafers were brought from room temperature to 450 “C in 35 min, held at 450 “C for 20 min, then cooled to room temperature. Following anneal, the wafers were characterized by seven different methods to determine the hillock density, hillock size distribution, hillock morphology, mean grain size and grain size distribution, grain and hillock orientations, and compositional variations. Results will be presented from scanning electron microscopy, transmission electron microscopy, profilometry, Auger electron spectroscopy, focused ion beam cross-sectioning, reflection high-energy electron diffraction, and X-ray/pole figure analysis.
3. Results 3.1. Compositional
analysis
A 0.2 W film was analyzed using Auger electron spectroscopy (AES) to compare the elemental compositions of the matrix and hillocking regions. The film was deposited at 300 “C on TEOS then annealed at 450 “C for 20 min prior to analysis by AES. Spectra were acquired on regions inside grains, on grain boundaries, and on hillocks. The Auger beam diameter was approximately < 25 nm (much smaller than the typical hillock diameter of 1 000-2 000 nm), and the detection limit for W and Cu were approximately 1 at.%. Tungsten was not detected in any of the areas analyzed either before or after a slight sputtering to remove approximately 30 nm of the surface material. Copper was detected in some grain boundaries, suggesting the presence of Cu-rich Al,Cu precipitates in the grain boundaries. A Cu distribution map using the most intense Cu LMM Auger signal confirmed the presence of Al,Cu precipitates in many grain boundaries.
64-68
65
The small concentration of W in the analyzed sample, 0.2 wt.%, is below the detection limit of AES (0. l-l at.%, depending on the element). Segregation of W to the grain boundaries or hillocks may increase the local concentration of W enough to be detected by AES. However, additional analysis on the hillocks using long acquisition times did not detect W (nor Cu), and it was concluded that there was no significant compositional difference between the matrix and the hillocking regions in the 0.2 W film. 3.2. Microstructural analysis Plan-view transmission electron microscopy (PTEM) was used to determine the mean grain sizes and size distributions of as-deposited and annealed metal films. Several trends were noted. The addition of W to AlCu alloy films decreased the mean grain size. Films deposited on Ti-W had smaller mean grain sizes than compared with films deposited on TEOS, for a given composition and deposition temperature. The grain sizes follow a log-normal distribution, and the plots of log of grain size versus cumulative probability fall along straight lines. The morphologies of hillocks in annealed samples of different compositions were examined in a Cambridge scanning electron microscope without any sample preparation. The various hillock morphologies found on annealed samples of different compositions can be seen in the SEM micrographs in Figs. l-3. In some cases, a moat can be seen around the outer edges of the hillocks. These moats probably formed during the cooling cycle, when the films were stressed in tension above their yield strengths. Terraces can also be seen on a number of hillocks, suggesting slip and/or that the hillocks formed in incremental steps. Cross-sections of hillocks were obtained by using a focused beam of Ga+ ions to gradually sputter away the material. The analysis was performed in a Micrion 908 focused ion beam (FIB) system in the Materials Characterization Laboratory (Motorola, Mesa). Fig. 4 and Fig. 5 are
Fig. 1. SEM micrograph appearance.
of a hillock with a relatively
smooth,
rounded
66
B.C. Martin et al. /Thin Solid Films 271(1995)
Fig. 2. SEM micrograph of a spire-like hillock with significant protrusion out of the film surface.
Fig. 3. SEM micrograph of a hillock with a flat top and striations/terraces on the left side.
Fig. 4. FIB cross-section of a hillock formedin an annealed 0.4 W film which was deposited at 300 “C on TBOS.
secondary electron micrographs of two different hillocks from the same annealed 0.4 W, 300 “C film. A low-temperature PECVD oxide cap was deposited on the metal surface
64-68
Fig. 5. FIB cross-section of a second hillock formed in an annealed 0.4 W film which was deposited at 300 “C on TEOS.
to prevent erosion of surface features during sputtering. After cross-sectioning in the FIB system, the hillocks were delineated in a mixture of 4:l NH,FHF for 5 s prior to SEM inspection. In all cases, the hillocks were solid from the bottom interface to the top surface, and no voids were seen near the hillocks. Grain boundaries can be seen, sometimes near or at the center of a hillock, but more often 1 00&2 000 nm from the center of the hillock. Because grain growth occurred during the 20 min anneal at 450 “C it is not possible to determine the original positions of the hillocks with respect to the grain boundaries. However, it cannot be excluded that a hillock has formed inside a grain. Reflection high energy electron diffraction (RHEED) was used to determine the orientations of 8 hillocks in a 0.4 W film which was deposited at 300 “C on TEOS then annealed at 450 “C for 20 min in air. The diffraction patterns were generated using a 200 kV incident beam. The RHEED beam diameter was approximately 10 nm, much less than the mean hillock distance and the mean hillock diameter. Analysis of the diffraction patterns (not shown) generated from the reflection of the beam from the hillocks’ surfaces showed that all the hillocks were randomly oriented and deviated from all major crystal axes, such as (lOO), (1 lo), and (111). Additionally, the orientations of 9 matrix grains surrounding one hillock were analyzed in the standard electron diffraction mode (transmission through a thinned specimen). It was found that the orientations of the matrix grains were very similar to each other, but they deviated significantly from the orientation of the hillock. The ( 111) film texture was characterized as a function of deposition temperature, film composition, and anneal treatment by means of X-ray/pole figure analysis. Because the ( 111) peak intensities were a function of both the degree of texture and the film thickness, corrections were made to account for differences in the film thickness. Several important trends were noted. 1. For a given film composition and deposition temperature, annealing increased the amount of ( 111) texture. How-
B.C. Martin et al. /Thin Solid Films 271 (1995) 64-68
67
1.410
\A
(111)
grainorientation
noW(l11)
angulardistdbuiion
/A (111)
graintientatDn non-(111)
*
angulsrdisttibution
Fig. 6. Schematic representation of changes in the grains’ orientations as a result of annealing and increased deposition temperature.
ever, the ( 111) angular distributions (the diameter of the pole diagram) remained unchanged. 2. Increasing the deposition temperature increased the ( 111) texture but exerted no significant effect on the ( 111) distribution. 3. Pure Al films were more textured in ( 111) than the 0.4 W films deposited at the same temperature. 4. There was no evidence of secondary texturing (no film texture other than ( 111) ) . The physical interpretation of the observed change in ( 111) intensity and lack of change in ( 111) distribution as a result of annealing and increased deposition temperature is believed to be as follows. The distribution of grain orientations in these as-deposited metal films is believed to be mainly ( 111) grains with a small population of non-( 111) grains. However, not all of the ( 111) grains were oriented perfectly parallel to the ( 111) pole, and there existed a finite distribution about the ( 111) pole. Upon annealing at 450 “C for 20 min, grain growth occurred, and the ( 111) grains grew at the expense of the non-( 111) grains. The driving force for this process is the minimization of surface energy. However, the grains which deviated only slightly from the ( 111) pole remained unaffected, perhaps because the energy difference was not great enough to drive the grain growth process described above (see the schematic representation in Fig. 6). 3.3. Hillock density All annealed metal films were characterized by profilometry to obtain the hillock densities and size distributions. To account for film surface roughness, only protrusions larger than 20 nm were counted as hillocks. For a given film composition, the deposition temperature had the most significant effect on the hillock density (Fig. 7). In AlCuW films, increasing the W concentration increased the hillock density, but the effect due to W was much smaller than that due to deposition temperature. Differences in the hillock densities due to the substrate material (TEOS vs. Ti-W) were statistically insignificant. Two-variable interactions (for instance, the interaction between film composition and deposition temperature) were also insignificant. The hillock size distribution was a function of the deposition temperature only and was similar across all film compositions and substrate materials. For the 300 “C deposition, approximately 30 to 50% of the total hillock population were
Wodinn
temperature (c)
Fig. 7. Hillock densities as a function of deposition temperature for pure Al, 0.2 W. and 0.4 W films. Increasing the deposition temperature resulted in significantly lower hillock densities.
at least 100 nm in height; approximately 10% were at least 200 nm; and less than 1% were equal to or greater than 500 nm. For the 450 “C deposition, virtually all hillocks were less than 100 nm in height. The addition of Cu to pure Al significantly decreased the hillock density, but the addition of W to AlCu films increased the hillock density. Indeed, no hillocks were observed in AlCu films. However, there was an abundance of AI&u precipitates on the annealed AlCu wafers. For this reason, the hillock densities of AlCu films are not reported.
4. Discussion Film texture appeared to have the most significant effect on the hillock density, under the conditions studied in this work. Review of the pole figure and hillock density data revealed a correlation between increasing ( 111) film texture and decreasing hillock density (see Fig. 8). AlCu films had virtually no hillocks and the highest ( 111) texture. Additionally, for all film compositions, increasing the deposition temperature resulted in an increase in ( 111) texture and a decrease in hillock density (Fig. 7 and Fig. 8). Pure Al films were more strongly textured in ( 111) than 0.4 W films deposited at the same temperature, and the hillock densities in the .0.4w3mC
a6 s
1.2 ld-
$
91o4-
.0.4w45gC
rFwmA13mC
.0.2W3DX
s f
510':
I 31d-
I 500
I 1000
.02W4!%C
1
1500
I 2000
APweAl45oC
I
2500
I
3000
Normalized(111) Counts Fig. 8. Hillock density plotted as a function of ( 111) texture for pure Al, 0.2 W, and 0.4 W films. Films with greater ( 111) texture formed fewer hillocks.
68
B.C. Martin et al. /Thin Solid Films 271 (1995) 64-68
pure Al films were lower (they were comparable with the 0.2 W densities). Surprisingly, the film grain size did not have a significant effect on the hillock density. Unannealed films deposited on Ti-W consistently had smaller mean grain sizes than compared with those deposited on TEOS, but the difference in hillock densities was negligible. Of the three process variables studied, the deposition temperature had the most significant effect on the hillock density and size distribution. From a processing standpoint, a few but very large hillocks can potentially cause more problems than a large number of hillocks of size -20 nm steps. For the compositions studied, increasing the deposition temperature from 300 “C to 450 “C eliminated all hillocks in the 500 nm range in height. This difference in size distribution may be due to different stress relaxation mechanisms which are operative in the 300 “C and the 450 “C films. Schwarzer, Gerth and coworkers [ 14,16-l 81 have studied the effect of grain orientation on hillock formation. They used electron diffraction to determine the orientations of over 1 100 matrix grains and 80 hillocks in Al-l% Si films and found that while the matrix grains were strongly textured in ( 1 1 1), the orientations of the hillocks were quite different from that of the matrix grains. Although the RHEED analysis in our study was much less extensive, we found results similar to those reported above. We were able to determine that the orientations of nine grains surrounding a hillock were very similar, but they differed significantly from that of the hillocks. We also analyzed the orientations of eight hillocks and found them to be random.
5. Conclusions In conclusion, for Al-based alloys, process conditions which enhance ( 111) texture formation, such as higher deposition temperature, are expected to suppress hillock formation. In addition, a temperature excursion is necessary to produce the compressive thermal strain/stress leading to hillock formation. Therefore, minimizing the temperature difference between the metal deposition step and subsequent high-temperature processing steps is also expected to be advantageous. Finally, characterization of the film texture by X-ray/pole figure has been shown to be a useful analytical method for predicting the susceptibility of a material to hillock formation.
Acknowledgements
The authors are grateful to the Materials Characterization Laboratory (Motorola, Mesa) for performing the various materials analyses and to Dan Sullivan, Materials Technology Center (Motorola, Mesa), for depositing the films. The X-ray/pole figure data were acquired by Dr. Anne Yates, Department of Chemistry (Arizona State University, Tempe) .
References [ I] S. Ogawaand H. Nishimura, IEEEElectron. Dev. Mater., (1991) 277. [2] H. Onoda, E. Takahashi, S. Makadoro, H. Fukuyo and S. Sawada, IEEE Symp. VLSI Technology, IEEE, New York, 1990, p. 57. [ 31 F.M. d’ Heurle and A. Gangulee, in H. Hu (ed.), Nature ana’ Behavior of Grain Boundaries, Plenum Press, 1972, p. 339. [4] F. Fisher and F. Neppi, IEEEIIRPS, ( 1984) 190. [5] D.K. Sadana, J.M. Towner, M.H. Norcott and R.C. Ellwanger, IEEW IRPS, (1986) 38. [6] H.J. Bhan,Appl. Phys. ht., 19 (1971) 30. [7] F.M. d’ Heurle and R. Rosenberg, in G. Hass and R. Hoffman (eds.), Physics of Thin Films, Vol. 7, Academic Press, New York, 1973, p. 257. [8] S. Wolf, Silicon Processing for the VLSI Era, Vol. 2, Lattice Press, Sunset Beach, CA, 1990, p. 270. [9] S. Wolf, Silicon Processing for the VLSI Era, Vol. 2. Lattice Press, Sunset Beach, CA, 1990, p. 270. [lo] P. Chaudhari, J. Appl. Phys., 45 ( 1974) 4339. [ 111 M.S. Jackson and C. Li, Acta Metall., 30 (1982) 1993. [ 121 M. Mumkami, Thin Solid Films, 55 (1978) 101. [ 131 P.H. Townsend and H.A. Vander Plas, Marer. Res. Sot. Symp. Proc., 47 (1985) 121. [ 141 R.A. Schwarzer and D. Getth, J. Electron. Mater., 22 (1993) 607. [ 151 F.M. D’ Heurle, fnt. Mater. Rev., 34 (1989) 53. [ 161 D. Getth and R.A. Schwarzer, Mater. Sci. Forum, 113-115 (1993) 625. [17] D. Gerth, D. Katzer and R. Schwarzer, Mater. Sci. Forum, 94-96 (1992) 557. [ 181 D. Gerth, D. Katzer and M. Krohn. Thin Solid Films, 208 ( 1992) 67. [ 191 F. Ericson, N. Kristensen and J.A. Schweitz, J. Vat. Sci. Technol., B9 (1991) 58. [20] S. Aceto, C.Y. Chang and R.W. Vook, Thin Solid Films, 219 ( 1992) 80. 1211 C.A. Pica and T.D. Bonifield,i. Mater. Res., 8 (1993) 1010. [22] J.E. Sanchez, Jr. and E. Arzt, Scripta Metall. Mater., 27 (1992) 285. [23] S. Bader, P.A. Flinn. E. Arzt and W.D. Nix, J. Mater. Res., 9 (1994)
318. [24] A.K. Sinha and T.T. Sheng, Thin Solid Films, 48 (1978) 117. [25] D.S. Herman, M.A. Schuster and R.M. Gerber, J. Vat. Sci. Technol., 9 (1972) 515.
A comparative study of hillock formation in aluminum films B. Cao Martin a, C.J. Tracy a, J.W. Mayer b, L.E. Hendrickson b ’ Materials Technology Center, Materials Research and Strategic Technologies, Motorola, Inc.. 2200 W. Broadway Rd, Mesa, AZ 85202, USA b Department of Chemical, Bio, and Materials Engineering, Arizona State University, Box 876006, Tempe, AZ 85287, USA
Received2 December 1994; accepted 12 July 1995
Abstract Studies were conducted to evaluate the performance of a variety of aluminum alloys with respect to hillock formation. Specifically, the effects of film composition, deposition temperature, and underlayer on hillock formation were investigated. The film compositions included pure Al, Al with 1.5 wt.% Cu (or AlCu), AlCu with 0.2 wt.% W, and AlCu with 0.4 wt.% W. These films were sputter deposited at 300 “C and 450 “C on either oxide- or Ti-W-coated Si substrates. Following deposition, the films were annealed in air for 20 mitt at 450 “C. For all film compositions, the deposition temperature had the most significant effect on the hillock density. For a given deposition temperature, the addition of Cu to pure Al drastically reduced the hillock density, but the addition of W to AlCu increased the hillock density. The effect of the underlayer on the hillock density was negligible. Cross-sections of the hillocks showed that they were solid from the bottom interface to the top surface, and no voids were found near the hillocks. The grain size did not have a significant effect on the hillock density. However, a correlation between increasing ( 111) film texture and decreasing hillock density was observed. Keywords: Alloys; Aluminium; Stress
1. Introduction
In recent years, attempts to improve the performance and reliability of aluminum interconnect materials have motivated studies of a variety of alloying elements [ l-71. The performance of a new alloy material is often measured in terms of its resistance to stress-induced void formation and electromigration failure. Another commonly encountered problem in metallization is hillock formation. Hillocks can lead to excessive surface roughness and interfere with photolithography steps. They can also crack the dielectric film isolating the metal lines and result in interlevel electrical short circuits [ 83. Additionally, hillocks can protrude from the sides of metal lines and cause intralevel short circuits among adjacent metal lines [ 91. Hillocks form because of large thermal stresses in the metal films, arising from the large difference in the coefficients of thermal expansion (CTEs) of the metal and the underlayer. The CTE of Al is approximately 10 times larger than the CTE of Si; and consequently, when an Al film is deposited on Si and heated, a compressive stress is produced in the Al film. Hillock formation is one mechanism whereby this thermallyinduced compressive stress is relieved. A number of mechanisms have been proposed for the formation of hillocks in thin metal films, including grain bound0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)06941-O
ary diffusion, diffusional creep, and interfacial diffusion [ 10-251. Because hillock formation is a stress relaxation mechanism which occurs in localized regions in a polycrystalline metal film, one might expect inhomogeneities in the metal microstructure, such as the mean grain size and grain size distribution, and the grain orientations and their distribution about a reference direction, to play dominant roles. Experimental data will be presented to show the effects of the film composition, deposition temperature, and underlayer on the metal film microstructure, and the effects of the microstructure on hillock formation.
2. Experimental details Two full 23 factorial experiments were run to determine the contributions due to each of the three independent variables (film composition, deposition temperature, and underlayer). Two-variable interactions were also examined. Control sets comprising pure Al films and commonly used Al with 1.5 wt.% Cu (AlCu) films were included for comparison. The two levels of film compositions in the factorial experiments were AlCu with 0.2 wt.% W, and AlCu with 0.4 wt.% W (or 0.2 W and 0.4 W films, respectively). Metal deposition was carried out at 300 “C and 450 “C. The two types of underlayers were Si (100) wafers coated with Ti-
B.C. Martin etal. /Thin Solid Films 271(1995)
W or plasma-enhanced silicon oxide prepared from a tetraethylorthosilicate (TEOS) precursor. Four-inch Si ( 100) wafers were first coated with either TEOS or Ti-W. The TEOS films were approximately 300 nm thick, and the Ti-W, 150 nm. Following underlayer deposition, metal films of varying compositions were sputter deposited to a nominal thickness of 750 nm. The AlCuW and pure Al films were deposited in the Varian 3 180 sputter deposition system in the Materials Technology Center (Motorola, Mesa) at a rate of 18.5 nm s - *. The AlCuW films were prepared from homogeneous targets. Argon was the sputtering gas in all depositions. The chamber base pressure was typically lo-’ Pa, and during deposition, the chamber was backfilled with Ar to approximately 0.9 Pa. Substrate heating during sputter deposition was accomplished by Ar-gas-coupled backside heating. The AlCu films were sputter deposited in an MRC Eclipse sputter deposition system in the Advanced Technologies Center (Motorola, Mesa). Following blanket metal film deposition, the wafers were annealed in air in a Flexus F2300 thin film stress measuring apparatus. All isothermal anneals were performed at 450 “C for 20 min. Wafers were brought from room temperature to 450 “C in 35 min, held at 450 “C for 20 min, then cooled to room temperature. Following anneal, the wafers were characterized by seven different methods to determine the hillock density, hillock size distribution, hillock morphology, mean grain size and grain size distribution, grain and hillock orientations, and compositional variations. Results will be presented from scanning electron microscopy, transmission electron microscopy, profilometry, Auger electron spectroscopy, focused ion beam cross-sectioning, reflection high-energy electron diffraction, and X-ray/pole figure analysis.
3. Results 3.1. Compositional
analysis
A 0.2 W film was analyzed using Auger electron spectroscopy (AES) to compare the elemental compositions of the matrix and hillocking regions. The film was deposited at 300 “C on TEOS then annealed at 450 “C for 20 min prior to analysis by AES. Spectra were acquired on regions inside grains, on grain boundaries, and on hillocks. The Auger beam diameter was approximately < 25 nm (much smaller than the typical hillock diameter of 1 000-2 000 nm), and the detection limit for W and Cu were approximately 1 at.%. Tungsten was not detected in any of the areas analyzed either before or after a slight sputtering to remove approximately 30 nm of the surface material. Copper was detected in some grain boundaries, suggesting the presence of Cu-rich Al,Cu precipitates in the grain boundaries. A Cu distribution map using the most intense Cu LMM Auger signal confirmed the presence of Al,Cu precipitates in many grain boundaries.
64-68
65
The small concentration of W in the analyzed sample, 0.2 wt.%, is below the detection limit of AES (0. l-l at.%, depending on the element). Segregation of W to the grain boundaries or hillocks may increase the local concentration of W enough to be detected by AES. However, additional analysis on the hillocks using long acquisition times did not detect W (nor Cu), and it was concluded that there was no significant compositional difference between the matrix and the hillocking regions in the 0.2 W film. 3.2. Microstructural analysis Plan-view transmission electron microscopy (PTEM) was used to determine the mean grain sizes and size distributions of as-deposited and annealed metal films. Several trends were noted. The addition of W to AlCu alloy films decreased the mean grain size. Films deposited on Ti-W had smaller mean grain sizes than compared with films deposited on TEOS, for a given composition and deposition temperature. The grain sizes follow a log-normal distribution, and the plots of log of grain size versus cumulative probability fall along straight lines. The morphologies of hillocks in annealed samples of different compositions were examined in a Cambridge scanning electron microscope without any sample preparation. The various hillock morphologies found on annealed samples of different compositions can be seen in the SEM micrographs in Figs. l-3. In some cases, a moat can be seen around the outer edges of the hillocks. These moats probably formed during the cooling cycle, when the films were stressed in tension above their yield strengths. Terraces can also be seen on a number of hillocks, suggesting slip and/or that the hillocks formed in incremental steps. Cross-sections of hillocks were obtained by using a focused beam of Ga+ ions to gradually sputter away the material. The analysis was performed in a Micrion 908 focused ion beam (FIB) system in the Materials Characterization Laboratory (Motorola, Mesa). Fig. 4 and Fig. 5 are
Fig. 1. SEM micrograph appearance.
of a hillock with a relatively
smooth,
rounded
66
B.C. Martin et al. /Thin Solid Films 271(1995)
Fig. 2. SEM micrograph of a spire-like hillock with significant protrusion out of the film surface.
Fig. 3. SEM micrograph of a hillock with a flat top and striations/terraces on the left side.
Fig. 4. FIB cross-section of a hillock formedin an annealed 0.4 W film which was deposited at 300 “C on TBOS.
secondary electron micrographs of two different hillocks from the same annealed 0.4 W, 300 “C film. A low-temperature PECVD oxide cap was deposited on the metal surface
64-68
Fig. 5. FIB cross-section of a second hillock formed in an annealed 0.4 W film which was deposited at 300 “C on TEOS.
to prevent erosion of surface features during sputtering. After cross-sectioning in the FIB system, the hillocks were delineated in a mixture of 4:l NH,FHF for 5 s prior to SEM inspection. In all cases, the hillocks were solid from the bottom interface to the top surface, and no voids were seen near the hillocks. Grain boundaries can be seen, sometimes near or at the center of a hillock, but more often 1 00&2 000 nm from the center of the hillock. Because grain growth occurred during the 20 min anneal at 450 “C it is not possible to determine the original positions of the hillocks with respect to the grain boundaries. However, it cannot be excluded that a hillock has formed inside a grain. Reflection high energy electron diffraction (RHEED) was used to determine the orientations of 8 hillocks in a 0.4 W film which was deposited at 300 “C on TEOS then annealed at 450 “C for 20 min in air. The diffraction patterns were generated using a 200 kV incident beam. The RHEED beam diameter was approximately 10 nm, much less than the mean hillock distance and the mean hillock diameter. Analysis of the diffraction patterns (not shown) generated from the reflection of the beam from the hillocks’ surfaces showed that all the hillocks were randomly oriented and deviated from all major crystal axes, such as (lOO), (1 lo), and (111). Additionally, the orientations of 9 matrix grains surrounding one hillock were analyzed in the standard electron diffraction mode (transmission through a thinned specimen). It was found that the orientations of the matrix grains were very similar to each other, but they deviated significantly from the orientation of the hillock. The ( 111) film texture was characterized as a function of deposition temperature, film composition, and anneal treatment by means of X-ray/pole figure analysis. Because the ( 111) peak intensities were a function of both the degree of texture and the film thickness, corrections were made to account for differences in the film thickness. Several important trends were noted. 1. For a given film composition and deposition temperature, annealing increased the amount of ( 111) texture. How-
B.C. Martin et al. /Thin Solid Films 271 (1995) 64-68
67
1.410
\A
(111)
grainorientation
noW(l11)
angulardistdbuiion
/A (111)
graintientatDn non-(111)
*
angulsrdisttibution
Fig. 6. Schematic representation of changes in the grains’ orientations as a result of annealing and increased deposition temperature.
ever, the ( 111) angular distributions (the diameter of the pole diagram) remained unchanged. 2. Increasing the deposition temperature increased the ( 111) texture but exerted no significant effect on the ( 111) distribution. 3. Pure Al films were more textured in ( 111) than the 0.4 W films deposited at the same temperature. 4. There was no evidence of secondary texturing (no film texture other than ( 111) ) . The physical interpretation of the observed change in ( 111) intensity and lack of change in ( 111) distribution as a result of annealing and increased deposition temperature is believed to be as follows. The distribution of grain orientations in these as-deposited metal films is believed to be mainly ( 111) grains with a small population of non-( 111) grains. However, not all of the ( 111) grains were oriented perfectly parallel to the ( 111) pole, and there existed a finite distribution about the ( 111) pole. Upon annealing at 450 “C for 20 min, grain growth occurred, and the ( 111) grains grew at the expense of the non-( 111) grains. The driving force for this process is the minimization of surface energy. However, the grains which deviated only slightly from the ( 111) pole remained unaffected, perhaps because the energy difference was not great enough to drive the grain growth process described above (see the schematic representation in Fig. 6). 3.3. Hillock density All annealed metal films were characterized by profilometry to obtain the hillock densities and size distributions. To account for film surface roughness, only protrusions larger than 20 nm were counted as hillocks. For a given film composition, the deposition temperature had the most significant effect on the hillock density (Fig. 7). In AlCuW films, increasing the W concentration increased the hillock density, but the effect due to W was much smaller than that due to deposition temperature. Differences in the hillock densities due to the substrate material (TEOS vs. Ti-W) were statistically insignificant. Two-variable interactions (for instance, the interaction between film composition and deposition temperature) were also insignificant. The hillock size distribution was a function of the deposition temperature only and was similar across all film compositions and substrate materials. For the 300 “C deposition, approximately 30 to 50% of the total hillock population were
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Fig. 7. Hillock densities as a function of deposition temperature for pure Al, 0.2 W. and 0.4 W films. Increasing the deposition temperature resulted in significantly lower hillock densities.
at least 100 nm in height; approximately 10% were at least 200 nm; and less than 1% were equal to or greater than 500 nm. For the 450 “C deposition, virtually all hillocks were less than 100 nm in height. The addition of Cu to pure Al significantly decreased the hillock density, but the addition of W to AlCu films increased the hillock density. Indeed, no hillocks were observed in AlCu films. However, there was an abundance of AI&u precipitates on the annealed AlCu wafers. For this reason, the hillock densities of AlCu films are not reported.
4. Discussion Film texture appeared to have the most significant effect on the hillock density, under the conditions studied in this work. Review of the pole figure and hillock density data revealed a correlation between increasing ( 111) film texture and decreasing hillock density (see Fig. 8). AlCu films had virtually no hillocks and the highest ( 111) texture. Additionally, for all film compositions, increasing the deposition temperature resulted in an increase in ( 111) texture and a decrease in hillock density (Fig. 7 and Fig. 8). Pure Al films were more strongly textured in ( 111) than 0.4 W films deposited at the same temperature, and the hillock densities in the .0.4w3mC
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Normalized(111) Counts Fig. 8. Hillock density plotted as a function of ( 111) texture for pure Al, 0.2 W, and 0.4 W films. Films with greater ( 111) texture formed fewer hillocks.
68
B.C. Martin et al. /Thin Solid Films 271 (1995) 64-68
pure Al films were lower (they were comparable with the 0.2 W densities). Surprisingly, the film grain size did not have a significant effect on the hillock density. Unannealed films deposited on Ti-W consistently had smaller mean grain sizes than compared with those deposited on TEOS, but the difference in hillock densities was negligible. Of the three process variables studied, the deposition temperature had the most significant effect on the hillock density and size distribution. From a processing standpoint, a few but very large hillocks can potentially cause more problems than a large number of hillocks of size -20 nm steps. For the compositions studied, increasing the deposition temperature from 300 “C to 450 “C eliminated all hillocks in the 500 nm range in height. This difference in size distribution may be due to different stress relaxation mechanisms which are operative in the 300 “C and the 450 “C films. Schwarzer, Gerth and coworkers [ 14,16-l 81 have studied the effect of grain orientation on hillock formation. They used electron diffraction to determine the orientations of over 1 100 matrix grains and 80 hillocks in Al-l% Si films and found that while the matrix grains were strongly textured in ( 1 1 1), the orientations of the hillocks were quite different from that of the matrix grains. Although the RHEED analysis in our study was much less extensive, we found results similar to those reported above. We were able to determine that the orientations of nine grains surrounding a hillock were very similar, but they differed significantly from that of the hillocks. We also analyzed the orientations of eight hillocks and found them to be random.
5. Conclusions In conclusion, for Al-based alloys, process conditions which enhance ( 111) texture formation, such as higher deposition temperature, are expected to suppress hillock formation. In addition, a temperature excursion is necessary to produce the compressive thermal strain/stress leading to hillock formation. Therefore, minimizing the temperature difference between the metal deposition step and subsequent high-temperature processing steps is also expected to be advantageous. Finally, characterization of the film texture by X-ray/pole figure has been shown to be a useful analytical method for predicting the susceptibility of a material to hillock formation.
Acknowledgements
The authors are grateful to the Materials Characterization Laboratory (Motorola, Mesa) for performing the various materials analyses and to Dan Sullivan, Materials Technology Center (Motorola, Mesa), for depositing the films. The X-ray/pole figure data were acquired by Dr. Anne Yates, Department of Chemistry (Arizona State University, Tempe) .
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