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Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si
Thin Solid Films 424 (2003) 33–39
Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si S.L. Cheng*, H.M. Lo, L.W. Cheng, S.M. Chang, L.J. Chen Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
Abstract The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi2 and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metalySi interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted. 䊚 2002 Published by Elsevier Science B.V. Keywords: Stress; Amorphous interlayer; Auto-correlation function; Silicides
1. Introduction As the device dimensions scale down to sub-quarter micron regime with multilayer structures, issues related to stress become more pronounced in semiconductor device fabrication. Stress can degrade gate oxides in metal-oxide-semiconductor field effect transistors and cause cracks in interconnect lines. A recent finite element calculation showed that the substrate regions at the oxide edge are compressively stressed to a significant extent w1x. The stress level near the silicon substrate surface inside oxide openings is one of major factors to affect the formation of metal silicides w2,3x. In addition, previous studies also showed strong correlation between the size of miniature oxide openings and the growth kinetics of silicides w4,5x. For the self-aligned silicidation process, titanium silicide is currently the most widely used silicide for ULSI circuits. Typically, titanium disilicide exists in two crystallographic structures: the high-resistivity C49-TiSi2 phase (60–300 mV cm) forms first followed by the low-resistivity thermodynamically stable C54-TiSi2 phase (13–20 mV cm) w6x. However, as the linewidth of a device structure is steadily scaled down, it has been *Corresponding author.
found that the C49- to C54-TiSi2 conversion becomes increasingly difficult, the fact attributed to the lack of C54 nucleation sites at the C49 phase grain boundaries w7x. Recently, the occurrence of an amorphous interlayer (a-interlayer) between Ti metal thin films and Si substrate is speculated to act as a direct nucleation source for C54-TiSi2, which enhances the phase transformation of C49- to C54-TiSi2 w8,9x. Therefore, it is of much interest to investigate further the effects of stresses on the formation of the a-interlayer. Among metal silicides, low-resistivity nickel monosilicide (NiSi) is currently the most promising silicide to replace C54-TiSi2 for the self-aligned technology of ULSI circuits, because it processes the low resistivity (14–20 mV cm), low Si consumption (silicon consumption (nm)y(nm) of metal ;1.83), low processing temperature and has no phase transformation problem associated with miniature oxide openings w6x. In this paper, results from an investigation on the interfacial reactions of Ti and Ni metal thin films on stressed (0 0 1)Si are reported. 2. Experimental procedures Single crystal, 2–5 V cm, (0 0 1)Si wafers were used in the present study. Some of the Si wafers were polished
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to approximately 200 mm in thickness in order to promote the sensitivity when measuring the stresses. To induce stress on silicon substrate, thin films of SiO2, Si3N4 and CoSi2 were formed on the backside of silicon substrate. Following a standard cleaning procedure, SiO2 films were deposited by plasma enhanced chemical vapor deposition (PECVD) or thermally grown in wet oxygen at 1000 8C. Following the deposition of SiO2 films, the oxide films on the front side of the wafers were subsequently removed. For some of the samples, Si3N4 films were deposited by rf-biased PECVD using a SiH4 –NH3 –N2 mixture onto the back side of (0 0 1)Si wafers. Similarly, a 200-nm-thick Co film was deposited onto the back side of some of the blank (0 0 1)Si wafers first, then annealed in a rapid thermal annealing (RTA) apparatus at 800–900 8C for 120 s to form CoSi2 layer. The stresses of the SiO2, Si3N4 and CoSi2 films were determined by a scanning laser beam reflection technique. The radius of curvature of each of the silicon wafers was measured prior to the deposition of the film and following the deposition. All of the stressed samples were cleaned by a standard process and then dipped in a dilute HF solution before loading into an electron gun evaporation chamber. Thirty-nanometer-thick Ti or Ni thin films were then deposited onto the front side of the blank and stressed (0 0 1)Si wafers at room temperature. The base pressure is better than 5=10y7 Torr. Heat treatments were carried out in the RTA apparatus at 300–1000 8C for 30–60 s in N2 ambient. Transmission electron microscopy (TEM) and grazing incidence X-ray diffractometry (GIXRD) were carried out for phases identification and structures examination. The incident angle of X-ray was fixed at 0.58. Sheet resistance measurements were performed using a standard four-point probe. For high-resolution TEM (HRTEM) observation, a JEOL 4000EX TEM operating at 400 kV with a point-to-point resolution of 0.18 nm was used. Auto-correlation function (ACF) analysis has been used to determine the formation of nanocrystalline nuclei and short-range order in the a-interlayer from the scanned images of HRTEM micrographs w8,10–14x. ACF analysis was carried out on templates of 0.997=0.997 nm2 cut along the a-interlayer in the HRTEM micrographs. 3. Results and discussion 3.1. Stress measurement The laser beam method for substrate curvature measurement was used to measure the stress induced by the film at the backside of the sample. The method allows the stress of the film to be calculated using Stoney’s equation, which relates the curvature of the sample to the stress of the film w15x. The results of stress meas-
Fig. 1. The schematic diagrams of (a) compressively and (b) tensily stressed samples. (T, tensile; C, compressive.)
urement at room temperature showed that compressive stresses of 100–550 MPa were generated by SiO2 films. Tensile stresses of 250 and 690–870 MPa were present in Si3N4 and CoSi2 films, respectively. Therefore, tensile, compressive and tensile stresses were induced by the Si3N4, SiO2 and the CoSi2 film on the front sides of silicon substrate, respectively. Fig. 1 shows the schematic diagrams of stressed-Si samples. Fig. 2 shows the in situ measured stress as a function of annealing temperature during two heating and cooling cycles for (a) SiO2; (b) Si3N4 and (c) CoSi2 backcoated samples. The ramping-up rate was 10 8Cymin. The stress–temperature curves showed that the contribution of stresses on the films from the backside of the sample was diminishing with increasing temperature. In the present study, two points need to be clarified. First, the actual stress status of the samples for this silicidation study consists of the stresses contributed from the backside films and the stresses associated with the front side films. Secondly, the stress behavior from the backside film as a function of temperature measured with furnace annealing could be different from that of RTA owing to the difference in stress relaxation. In the following sections, the samples were labeled according to their stress statuses at room temperature prior to the deposition of Ti or Ni thin films instead of actual stress status of the samples. 3.2. TiyStressed (0 0 1)Si samples The samples were classified into five groups: Sample A: Ti (30 nm)y(0 0 1)SiySiO2 (y550 MPa) Sample B: Ti (30 nm)y(0 0 1)SiySiO2 (y100 MPa)
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Fig. 3. Sheet resistance data of samples A, B, C, D and E after 30 s RTA at various temperatures.
Fig. 2. Stress as a function of temperature (in situ measurements) during two heating and cooling cycles of (a) thermally grown SiO2 film; (b) Si3N4 film and (c) CoSi2 film.
Sample C: Ti (30 nm)yblank-(0 0 1)Si Sample D: Ti (30 nm)y(0 0 1)SiyCoSi2 (q690 MPa) Sample E: Ti (30 nm)y(0 0 1)SiyCoSi2 (q870 MPa) Following the convention, we denote the compressive stress to be negative and the tensile stress to be positive, respectively. After different heat treatments, the variations of sheet resistance as a function of annealing temperature for samples A, B, C, D and E are shown in Fig. 3. The sheet resistance data clearly demonstrate the impact of stress on the C49- to C54-TiSi2 phase transformation. A sharp drop in sheet resistance was found for samples E annealed at 700 8C. On the other hand, for samples A, B, C and D, as the annealing temperature was increased to 800, 750, 750 and 750 8C, respectively, a sharp drop in sheet resistance was found, indicating that appreciable C49- to C54-TiSi2 transformation occurred. From GIXRD and TEM analysis, the lowresistivity C54-TiSi2 was observed to be the only silicide phase formed in 700 8C annealed samples E and 750 8C annealed samples B and C. For samples D after annealing at 700 8C, C54-TiSi2 was found to coexist with the dominant polycrystalline C49-TiSi2 phase. On the other hand, even after annealing at 750 8C, both C49- and C54-TiSi2 were found to form in samples A. The increase in sheet resistances for Ti on blank(0 0 1)Si and stressed samples after annealing at high temperatures were correlated to the formation of island
structure of C54-TiSi2. The phases formed in different samples after annealing at various temperatures are listed in Table 1. Based on the results of sheet resistance data, GIXRD and TEM analysis, tensile and compressive stresses present in the silicon substrate were found to enhance and retard significantly the transformation of TiSi2 from C49 to C54 phase, respectively. The phase transformation temperature of C49- to C54-TiSi2 was previously found to be lower for small-grained C49-TiSi2 films than that of the large-grained films w7–9x. It is thought that with a higher density of nanocrystallites in the ainterlayer, more nucleation sites are available for the formation of C49-TiSi2. The larger area of C49-TiSi2 grain boundaries can supply more nucleation sites for the transformation of C49- to C54-TiSi2. From the HRTEM observation, the thickness of a-interlayer at the TiySi interface was found to be thicker in tensily stressed samples and thinner in compressively stressed samples. The thicknesses of a-interlayers for as-deposited and annealed samples are shown in Fig. 4. In order to investigate the effects of stress on the formation of the a-interlayer further, all the samples were annealed at 450 8C for 60 s to obtain thicker a-interlayer. Periodic structures were observed in the ACF images of outlined regions shown in Fig. 5a of sample E. The ACF-processed images corresponding to the regions outlined in Fig. 5a are shown in Fig. 5b. The crossTable 1 Phases formed in Ti thin films (30 nm) on different samples after annealing at various temperatures for 30 s Samples
650 8C
700 8C
750 8C
800 8C
(A) SiO2—550 MPa (B) SiO2—100 MPa (C) Blank-Si (D) CoSi2—690 MPa (E) CoSi2—870 MPa
C49 C49 C49 C49 C49
C49 C49 C49 C49qC54 C54
C49qC54 C54 C54 C54 C54
C54 C54 C54 C54 C54
C49, C49-TiSi2; C54, C54-TiSi2.
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Fig. 4. The thicknesses of a-interlayers for as-deposited and annealed samples A, C and E.
hatch patterns were found to correspond to Ti5Si3 or Ti5Si4 phases from both interplanar spacing and angular measurements. In addition, embedded nanocrystallites of Ti5Si3 and Ti5Si4 phases were also found from ACF analysis in samples A and C. However, the amount and extent of nanocrystallites in tensily stressed samples are higher than those in blank-Si and compressively stressed samples. The HRTEM image and corresponding ACFprocessed images of an A sample are shown in Fig. 6a and b. From planview TEM observation, the average grain size of C49-TiSi2 in tensily stressed sample is much smaller than that in blank and compressively
Fig. 6. (a) Cross-sectional HRTEM image, 450 8C, 60 s of Ti (30 nm) on SiO2—550 MPa compressively stressed-Si sample. (b) ACFprocessed images of the outlined regions shown in (a).
stressed samples. Examples are shown in Fig. 7. As a result, the tensile stress applied to the silicon substrate was correlated to the enhanced growth of the a-interlayer and increase in the density of embedded nanocrystallites in the a-interlayer. With a higher density of nanocrystallites in the a-interlayer, the grain size of C49-TiSi2 was reduced since more nucleation sites are available for the formation of C49-TiSi2. The grain boundaries were increased with the reduction of C49-TiSi2 grain size, which leads to the enhancement of the C49- to C54-TiSi2 transition. These results are corroborated with the sheet resistance data as well as results from GIXRD and TEM analysis described in previous paragraphs. Fig. 8 shows the thicknesses of TiSi2 films vs. annealing temperature curves for blank-Si and stressedSi samples. From Fig. 8, it is clearly seen that the thicknesses of C54-TiSi2 in samples D and E are thicker than those in samples A, B and C. In addition, the thicknesses of TiSi2 films were found to decrease with the compressive stress level and increase with the tensile stress level. These results are correlated with the differences in sheet resistance of TiSi2 among samples A, B, C, D and E in Fig. 3. 3.3. Niystressed (0 0 1)Si samples
Fig. 5. (a) Cross-sectional HRTEM image, 450 8C, 60 s of Ti (30 nm) on CoSi2—870 MPa tensily stressed-Si sample. (b) ACF-processed images of the outlined regions shown in (a).
The samples were also classified into five groups: Sample A-1: Ni (30 nm)y(0 0 1)SiySiO2 (y380 MPa) Sample B-1: Ni (30 nm)y(0 0 1)SiySiO2 (y100 MPa) Sample C-1: Ni (30 nm)yblank-(0 0 1)Si Sample D-1: Ni (30 nm)y(0 0 1)SiySi3 N4 (q250 MPa) Sample E-1: Ni (30 nm)y(0 0 1)SiyCoSi2 (q870 MPa)
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Fig. 8. The thicknesses of TiSi2 films vs. annealing temperature curves for samples A, B, C, D and E.
present. For samples annealed at 700 8C, the abrupt increase in sheet resistance was found for tensily stressed samples (samples D-1 and E-1). The data signaled that the transformation from low-resistivity NiSi phase to high-resistivity NiSi2 phase occurred. As the annealing temperature was increased to 750 8C, the sheet resistance of samples A-1 was found to be lower than those of samples B-1 and C-1. An abrupt increase in sheet resistance was found for samples A-1 annealed at 800 8C. From GIXRD and planview TEM analysis, Ni2Si appeared after annealing at 300 8C for samples B-1, C1, D-1 and E-1. However, no silicide phase was found to form in samples A-1 until annealing at 350 8C. Lowresistivity NiSi was observed to be the only silicide phase formed in 400 8C annealed samples C-1, D-1 and E-1. On the other hand, for samples A-1 and B-1 after annealing at the same temperature, Ni2Si was found to coexist with the dominant polycrystalline NiSi phase. As the annealing temperature was increased to 700 8C, epitaxial NiSi2 was found to form in samples D-1 and Fig. 7. Planview TEM images, 650 8C, 30 s of Ti on (a) SiO2—550 MPa stressed-Si; (b) blank-Si and (c) CoSi2—870 MPa stressed-Si samples.
Previous studies showed that Ni metal thin films react with silicon substrate to form three silicide phases: Ni2Si (200–350 8C), NiSi (350–750 8C) and NiSi2 (0750 8C). In addition, the resistivities of Ni2Si, NiSi and NiSi2 are approximately 24, 14–20 and 35–50 mV cm, respectively w6x. Fig. 9 shows the sheet resistance data of Ni metal thin films (30 nm) on blank-(0 0 1)Si and stressed-Si samples after annealing at different temperatures for 60 s. From the sheet resistance data, a sharp drop in sheet resistance occurred at 400 8C for blank and stressed-Si samples. The sheet resistance maintained the same low level for all samples annealed at 450–650 8C, indicating that only low-resistivity NiSi phase was
Fig. 9. Sheet resistance of nickel silicides versus annealing temperature curves for samples A-1, B-1, C-1, D-1 and E-1.
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
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Fig. 11. The thickness of Ni2Si and NiSi in Samples A-1, B-1, C-1, D-1 and E-1 after annealing at 450 8C for 15 s.
Fig. 10. Planview TEM images and corresponding diffraction patterns of (a) polycrystalline NiSi and (b) epitaxial NiSi2 in sample A-1 annealed at 750 8C for 60 s.
E-1. However, only polycrystalline NiSi was observed in 700 8C annealed samples A-1, B-1 and C-1. At 750 8C, complete transformation of epitaxial NiSi2 phase was observed in blank-Si samples. At the same temperature, a mixture of polycrystalline NiSi and epitaxial NiSi2 was found in compressively stressed samples (samples A-1 and B-1). Examples are shown in Fig. 10a and b. For both samples A-1 and B-1, complete transformation from NiSi to NiSi2 was found to occur at 800 8C. The results indicated that the phase transformation from NiSi to NiSi2 was more advanced in tensily stressed-Si samples (samples D-1 and E-1) than those in blank-Si and compressively stressed-Si samples (sam-
ples A-1, B-1 and C-1) annealed at the same temperature. The silicide phases formation data of GIXRD and TEM analysis are listed in Table 2. From XTEM observation, a two-layer structure was observed in all samples after annealing at 450 8C for 15 s. The top and bottom layers were identified to be Ni2Si and NiSi, respectively. Top Ni2Si layers in samples D-1 and E-1 are thinner than those in other samples and were discontinuous. In addition, the thickness ratio of NiSi to Ni2Si was found to increase and decrease with the tensile and compressive stress level, respectively. The thickness of Ni2Si and NiSi films for different samples after annealing at 450 8C for 15 s is shown in Fig. 11. Based on the sheet resistance measurement, GIXRD, TEM, HRTEM and ACF analysis, compressive stress present in the silicon substrate was found to retard significantly the interfacial reactions of Ti or Ni metal thin films on Si substrate. In contrast, the tensile stress present in the silicon substrate was found to promote the phase transformation of metal silicides. In addition, the thicknesses of silicide films were found to decrease and increase with the compressive and tensile stress level, respectively, for both TiySi and NiySi systems. For solid-state diffusion, the effect of stress on the diffusion coefficient D for vacancy diffusion in a crystalline solid can be described as w16x: w
x y
≠lnŽDya2n. z ≠P
1 | sy RT ŽV qV f
~T
.sy
m
Va RT
(1)
Table 2 Phase formation data for Ni (30 nm) on different samples annealed at various temperatures for 60 s by RTA Samples (A-1) SiO2—380 MPa (B-1) SiO2—100 MPa (C-1) Blank-Si (D-1) Si3N4—250 MPa (E-1) CoSi2—870 MPa Epi-NiSi2, epitaxial NiSi2. a Dominant phase.
300 8C Ni NiaqNi2Si NiaqNi2Si NiaqNi2Si NiaqNi2Si
350 8C
400 8C a
NiqNi2Si Ni2SiaqNiSi Ni2SiaqNiSi Ni2SiaqNiSi Ni2SiaqNiSi
a
Ni2SiqNiSi Ni2SiqNiSia NiSi NiSi NiSi
450–650 8C
700 8C
750 8C
800 8C
NiSi NiSi NiSi NiSi NiSi
NiSi NiSi NiSi Epi-NiSi2 Epi-NiSi2
NiSiqEpi-NiSi2 NiSiqEpi-NiSi2a Epi-NiSi2 Epi-NiSi2 Epi-NiSi2
Epi-NiSi2 Epi-NiSi2 Epi-NiSi2 Epi-NiSi2 Epi-NiSi2
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
where P is the pressure (compressive stress), a is the lattice parameter, and n is the mean vibrational frequency of the atom at equilibrium. Vf and Vm are the partial molar volumes of the vacancies and of the activated complexes. The sum VfqVmsVa is called activation volume. For the growth of TiSi2, Si is known to be the dominant diffusing species. The diffusivity of Si into the TiSi2 layer shall be the dominant factor in determining the growth rate. Physically, the removal of vacancies decreases the volume of the specimen. For diffusion in a crystalline solid, if the compressive stress (pressure) is increased, the specimen will lose vacancies in an effort to relieve the increase in compressive stress. According to Eq. (1), the decrease in the concentration of vacancies will in turn decrease D. As a result, for samples under compressive stress the diffusion of atoms is retarded. In contrast, if a tension is present in Si substrate, one can expect to enhance the atomic diffusion. For the formation of nickel silicides, Ni is known to be the dominant diffusing species. Although Ni was reported to diffuse by the kick-out mechanism in silicon w17x, it is expected that the same trend in stress– diffusion relationship holds. From thermodynamic consideration, the results could therefore be attributed to the hindrance of the migration of atoms through the metalySi interface by the presence of compressive stress, so that the formation of metal silicide films was retarded. On the other hand, tensile stress facilitates the diffusion of atoms and promotes the growth of metal silicides. 4. Summary and conclusions Effects of stress on the interfacial reactions of Ti and Ni thin films on (0 0 1)Si have been investigated by in situ stress measurement, sheet resistance measurement, GIXRD analysis, TEM observation and HRTEM in conjunction with ACF analysis. The stress was found to exert significant influence on the formation of Ti and Ni silicides. The C49- to C54-TiSi2 phase transformation temperature in tensily stressed-Si samples was found to lower by approximately 100 8C than that in compressively stressed-Si samples. From HRTEM observation and ACF analysis, the tensile stress present in the silicon substrate promotes the formation of a-interlayer and decreases the grain size of C49-TiSi2, which increases the nucleation density of the C54-TiSi2 phase. Consequently, the transformation of C49- to C54-TiSi2 phase is enhanced. In addition, the thicknesses of TiSi2 films
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were found to decrease and increase with the compressive and tensile stress level, respectively. The effects of stress on the formation of nickel silicides are similar to those of titanium silicides. The NiSi phase can be stabilized by the presence of compressive stress in the silicon substrate. For Ni on blank and stressed (0 0 1)Si substrates after RTA at 450 8C for 15 s, the thickness of NiSi film was found to increase with the tensile stress level. In the mean time, the thickness of Ni2Si became thinner and discontinuous. Based on thermodynamic consideration, the compressive stress hinders the interdiffusion of atoms through the metalySi interface, so that the formation of silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted. Acknowledgments The research was supported by the National Science Council through a Grant No. NSC 89-2218-E007-067. References w1x Y.L. Shen, S. Suresh, I.A. Blench, J. Appl. Phys. 80 (1996) 1388. w2x J.Y. Yew, L.J. Chen, W.F. Wu, J. Vac. Sci. Technol. B 17 (1999) 939. w3x J.Y. Yew, L.J. Chen, K. Nakamura, Appl. Phys. Lett. 69 (1996) 999. w4x H.F. Hsu, L.J. Chen, J.J. Chu, J. Appl. Phys. 69 (1991) 4282. w5x C.S. Chang, C.W. Nieh, L.J. Chen, Appl. Phys. Lett. 50 (1987) 259. w6x E.G. Colgan, J.P. Gambino, Q.Z. Hong, Mater. Sci. Eng. R 16 (1996) 43. w7x J.B. Lasky, J.S. Nakos, O.J. Cain, P.J. Geiss, IEEE Trans. Electron Devices ED-38 (1991) 262. w8x S.M. Chang, H.Y. Huang, H.Y. Yang, L.J. Chen, Appl. Phys. Lett. 74 (1999) 224. w9x R.T. Tung, K. Fujii, K. Kikuta, S. Chikaki, T. Kikkawa, Appl. Phys. Lett. 70 (1997) 2386. w10x J.M. Liang, L.J. Chen, Appl. Phys. Lett. 64 (1994) 1224. w11x J.M. Liang, L.J. Chen, J. Appl. Phys. 79 (1996) 4072. w12x L.J. Chen, J.H. Lin, T.L. Lee, C.H. Luo, W.Y. Hsieh, J.M. Liang, M.H. Wang, Micros. Res. Tech. 40 (1998) 136. w13x J.C. Chen, G.H. Shen, L.J. Chen, J. Appl. Phys. 83 (1998) 7653. w14x L.J. Chen, Mater. Sci. Eng. R 29 (2000) 115. w15x S.P. Murarka, Metallization: Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, Boston, MA, 1993, p. 74. w16x P.G. Shewmon, Diffusion in Solid, second ed., The Metallurgical Society, Warrendale, PA, 1989, p. 84. w17x T.Y. Tan, U. Goesele, Appl. Phys. A 31 (1985) 1.
Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si S.L. Cheng*, H.M. Lo, L.W. Cheng, S.M. Chang, L.J. Chen Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
Abstract The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi2 and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metalySi interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted. 䊚 2002 Published by Elsevier Science B.V. Keywords: Stress; Amorphous interlayer; Auto-correlation function; Silicides
1. Introduction As the device dimensions scale down to sub-quarter micron regime with multilayer structures, issues related to stress become more pronounced in semiconductor device fabrication. Stress can degrade gate oxides in metal-oxide-semiconductor field effect transistors and cause cracks in interconnect lines. A recent finite element calculation showed that the substrate regions at the oxide edge are compressively stressed to a significant extent w1x. The stress level near the silicon substrate surface inside oxide openings is one of major factors to affect the formation of metal silicides w2,3x. In addition, previous studies also showed strong correlation between the size of miniature oxide openings and the growth kinetics of silicides w4,5x. For the self-aligned silicidation process, titanium silicide is currently the most widely used silicide for ULSI circuits. Typically, titanium disilicide exists in two crystallographic structures: the high-resistivity C49-TiSi2 phase (60–300 mV cm) forms first followed by the low-resistivity thermodynamically stable C54-TiSi2 phase (13–20 mV cm) w6x. However, as the linewidth of a device structure is steadily scaled down, it has been *Corresponding author.
found that the C49- to C54-TiSi2 conversion becomes increasingly difficult, the fact attributed to the lack of C54 nucleation sites at the C49 phase grain boundaries w7x. Recently, the occurrence of an amorphous interlayer (a-interlayer) between Ti metal thin films and Si substrate is speculated to act as a direct nucleation source for C54-TiSi2, which enhances the phase transformation of C49- to C54-TiSi2 w8,9x. Therefore, it is of much interest to investigate further the effects of stresses on the formation of the a-interlayer. Among metal silicides, low-resistivity nickel monosilicide (NiSi) is currently the most promising silicide to replace C54-TiSi2 for the self-aligned technology of ULSI circuits, because it processes the low resistivity (14–20 mV cm), low Si consumption (silicon consumption (nm)y(nm) of metal ;1.83), low processing temperature and has no phase transformation problem associated with miniature oxide openings w6x. In this paper, results from an investigation on the interfacial reactions of Ti and Ni metal thin films on stressed (0 0 1)Si are reported. 2. Experimental procedures Single crystal, 2–5 V cm, (0 0 1)Si wafers were used in the present study. Some of the Si wafers were polished
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to approximately 200 mm in thickness in order to promote the sensitivity when measuring the stresses. To induce stress on silicon substrate, thin films of SiO2, Si3N4 and CoSi2 were formed on the backside of silicon substrate. Following a standard cleaning procedure, SiO2 films were deposited by plasma enhanced chemical vapor deposition (PECVD) or thermally grown in wet oxygen at 1000 8C. Following the deposition of SiO2 films, the oxide films on the front side of the wafers were subsequently removed. For some of the samples, Si3N4 films were deposited by rf-biased PECVD using a SiH4 –NH3 –N2 mixture onto the back side of (0 0 1)Si wafers. Similarly, a 200-nm-thick Co film was deposited onto the back side of some of the blank (0 0 1)Si wafers first, then annealed in a rapid thermal annealing (RTA) apparatus at 800–900 8C for 120 s to form CoSi2 layer. The stresses of the SiO2, Si3N4 and CoSi2 films were determined by a scanning laser beam reflection technique. The radius of curvature of each of the silicon wafers was measured prior to the deposition of the film and following the deposition. All of the stressed samples were cleaned by a standard process and then dipped in a dilute HF solution before loading into an electron gun evaporation chamber. Thirty-nanometer-thick Ti or Ni thin films were then deposited onto the front side of the blank and stressed (0 0 1)Si wafers at room temperature. The base pressure is better than 5=10y7 Torr. Heat treatments were carried out in the RTA apparatus at 300–1000 8C for 30–60 s in N2 ambient. Transmission electron microscopy (TEM) and grazing incidence X-ray diffractometry (GIXRD) were carried out for phases identification and structures examination. The incident angle of X-ray was fixed at 0.58. Sheet resistance measurements were performed using a standard four-point probe. For high-resolution TEM (HRTEM) observation, a JEOL 4000EX TEM operating at 400 kV with a point-to-point resolution of 0.18 nm was used. Auto-correlation function (ACF) analysis has been used to determine the formation of nanocrystalline nuclei and short-range order in the a-interlayer from the scanned images of HRTEM micrographs w8,10–14x. ACF analysis was carried out on templates of 0.997=0.997 nm2 cut along the a-interlayer in the HRTEM micrographs. 3. Results and discussion 3.1. Stress measurement The laser beam method for substrate curvature measurement was used to measure the stress induced by the film at the backside of the sample. The method allows the stress of the film to be calculated using Stoney’s equation, which relates the curvature of the sample to the stress of the film w15x. The results of stress meas-
Fig. 1. The schematic diagrams of (a) compressively and (b) tensily stressed samples. (T, tensile; C, compressive.)
urement at room temperature showed that compressive stresses of 100–550 MPa were generated by SiO2 films. Tensile stresses of 250 and 690–870 MPa were present in Si3N4 and CoSi2 films, respectively. Therefore, tensile, compressive and tensile stresses were induced by the Si3N4, SiO2 and the CoSi2 film on the front sides of silicon substrate, respectively. Fig. 1 shows the schematic diagrams of stressed-Si samples. Fig. 2 shows the in situ measured stress as a function of annealing temperature during two heating and cooling cycles for (a) SiO2; (b) Si3N4 and (c) CoSi2 backcoated samples. The ramping-up rate was 10 8Cymin. The stress–temperature curves showed that the contribution of stresses on the films from the backside of the sample was diminishing with increasing temperature. In the present study, two points need to be clarified. First, the actual stress status of the samples for this silicidation study consists of the stresses contributed from the backside films and the stresses associated with the front side films. Secondly, the stress behavior from the backside film as a function of temperature measured with furnace annealing could be different from that of RTA owing to the difference in stress relaxation. In the following sections, the samples were labeled according to their stress statuses at room temperature prior to the deposition of Ti or Ni thin films instead of actual stress status of the samples. 3.2. TiyStressed (0 0 1)Si samples The samples were classified into five groups: Sample A: Ti (30 nm)y(0 0 1)SiySiO2 (y550 MPa) Sample B: Ti (30 nm)y(0 0 1)SiySiO2 (y100 MPa)
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
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Fig. 3. Sheet resistance data of samples A, B, C, D and E after 30 s RTA at various temperatures.
Fig. 2. Stress as a function of temperature (in situ measurements) during two heating and cooling cycles of (a) thermally grown SiO2 film; (b) Si3N4 film and (c) CoSi2 film.
Sample C: Ti (30 nm)yblank-(0 0 1)Si Sample D: Ti (30 nm)y(0 0 1)SiyCoSi2 (q690 MPa) Sample E: Ti (30 nm)y(0 0 1)SiyCoSi2 (q870 MPa) Following the convention, we denote the compressive stress to be negative and the tensile stress to be positive, respectively. After different heat treatments, the variations of sheet resistance as a function of annealing temperature for samples A, B, C, D and E are shown in Fig. 3. The sheet resistance data clearly demonstrate the impact of stress on the C49- to C54-TiSi2 phase transformation. A sharp drop in sheet resistance was found for samples E annealed at 700 8C. On the other hand, for samples A, B, C and D, as the annealing temperature was increased to 800, 750, 750 and 750 8C, respectively, a sharp drop in sheet resistance was found, indicating that appreciable C49- to C54-TiSi2 transformation occurred. From GIXRD and TEM analysis, the lowresistivity C54-TiSi2 was observed to be the only silicide phase formed in 700 8C annealed samples E and 750 8C annealed samples B and C. For samples D after annealing at 700 8C, C54-TiSi2 was found to coexist with the dominant polycrystalline C49-TiSi2 phase. On the other hand, even after annealing at 750 8C, both C49- and C54-TiSi2 were found to form in samples A. The increase in sheet resistances for Ti on blank(0 0 1)Si and stressed samples after annealing at high temperatures were correlated to the formation of island
structure of C54-TiSi2. The phases formed in different samples after annealing at various temperatures are listed in Table 1. Based on the results of sheet resistance data, GIXRD and TEM analysis, tensile and compressive stresses present in the silicon substrate were found to enhance and retard significantly the transformation of TiSi2 from C49 to C54 phase, respectively. The phase transformation temperature of C49- to C54-TiSi2 was previously found to be lower for small-grained C49-TiSi2 films than that of the large-grained films w7–9x. It is thought that with a higher density of nanocrystallites in the ainterlayer, more nucleation sites are available for the formation of C49-TiSi2. The larger area of C49-TiSi2 grain boundaries can supply more nucleation sites for the transformation of C49- to C54-TiSi2. From the HRTEM observation, the thickness of a-interlayer at the TiySi interface was found to be thicker in tensily stressed samples and thinner in compressively stressed samples. The thicknesses of a-interlayers for as-deposited and annealed samples are shown in Fig. 4. In order to investigate the effects of stress on the formation of the a-interlayer further, all the samples were annealed at 450 8C for 60 s to obtain thicker a-interlayer. Periodic structures were observed in the ACF images of outlined regions shown in Fig. 5a of sample E. The ACF-processed images corresponding to the regions outlined in Fig. 5a are shown in Fig. 5b. The crossTable 1 Phases formed in Ti thin films (30 nm) on different samples after annealing at various temperatures for 30 s Samples
650 8C
700 8C
750 8C
800 8C
(A) SiO2—550 MPa (B) SiO2—100 MPa (C) Blank-Si (D) CoSi2—690 MPa (E) CoSi2—870 MPa
C49 C49 C49 C49 C49
C49 C49 C49 C49qC54 C54
C49qC54 C54 C54 C54 C54
C54 C54 C54 C54 C54
C49, C49-TiSi2; C54, C54-TiSi2.
36
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
Fig. 4. The thicknesses of a-interlayers for as-deposited and annealed samples A, C and E.
hatch patterns were found to correspond to Ti5Si3 or Ti5Si4 phases from both interplanar spacing and angular measurements. In addition, embedded nanocrystallites of Ti5Si3 and Ti5Si4 phases were also found from ACF analysis in samples A and C. However, the amount and extent of nanocrystallites in tensily stressed samples are higher than those in blank-Si and compressively stressed samples. The HRTEM image and corresponding ACFprocessed images of an A sample are shown in Fig. 6a and b. From planview TEM observation, the average grain size of C49-TiSi2 in tensily stressed sample is much smaller than that in blank and compressively
Fig. 6. (a) Cross-sectional HRTEM image, 450 8C, 60 s of Ti (30 nm) on SiO2—550 MPa compressively stressed-Si sample. (b) ACFprocessed images of the outlined regions shown in (a).
stressed samples. Examples are shown in Fig. 7. As a result, the tensile stress applied to the silicon substrate was correlated to the enhanced growth of the a-interlayer and increase in the density of embedded nanocrystallites in the a-interlayer. With a higher density of nanocrystallites in the a-interlayer, the grain size of C49-TiSi2 was reduced since more nucleation sites are available for the formation of C49-TiSi2. The grain boundaries were increased with the reduction of C49-TiSi2 grain size, which leads to the enhancement of the C49- to C54-TiSi2 transition. These results are corroborated with the sheet resistance data as well as results from GIXRD and TEM analysis described in previous paragraphs. Fig. 8 shows the thicknesses of TiSi2 films vs. annealing temperature curves for blank-Si and stressedSi samples. From Fig. 8, it is clearly seen that the thicknesses of C54-TiSi2 in samples D and E are thicker than those in samples A, B and C. In addition, the thicknesses of TiSi2 films were found to decrease with the compressive stress level and increase with the tensile stress level. These results are correlated with the differences in sheet resistance of TiSi2 among samples A, B, C, D and E in Fig. 3. 3.3. Niystressed (0 0 1)Si samples
Fig. 5. (a) Cross-sectional HRTEM image, 450 8C, 60 s of Ti (30 nm) on CoSi2—870 MPa tensily stressed-Si sample. (b) ACF-processed images of the outlined regions shown in (a).
The samples were also classified into five groups: Sample A-1: Ni (30 nm)y(0 0 1)SiySiO2 (y380 MPa) Sample B-1: Ni (30 nm)y(0 0 1)SiySiO2 (y100 MPa) Sample C-1: Ni (30 nm)yblank-(0 0 1)Si Sample D-1: Ni (30 nm)y(0 0 1)SiySi3 N4 (q250 MPa) Sample E-1: Ni (30 nm)y(0 0 1)SiyCoSi2 (q870 MPa)
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
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Fig. 8. The thicknesses of TiSi2 films vs. annealing temperature curves for samples A, B, C, D and E.
present. For samples annealed at 700 8C, the abrupt increase in sheet resistance was found for tensily stressed samples (samples D-1 and E-1). The data signaled that the transformation from low-resistivity NiSi phase to high-resistivity NiSi2 phase occurred. As the annealing temperature was increased to 750 8C, the sheet resistance of samples A-1 was found to be lower than those of samples B-1 and C-1. An abrupt increase in sheet resistance was found for samples A-1 annealed at 800 8C. From GIXRD and planview TEM analysis, Ni2Si appeared after annealing at 300 8C for samples B-1, C1, D-1 and E-1. However, no silicide phase was found to form in samples A-1 until annealing at 350 8C. Lowresistivity NiSi was observed to be the only silicide phase formed in 400 8C annealed samples C-1, D-1 and E-1. On the other hand, for samples A-1 and B-1 after annealing at the same temperature, Ni2Si was found to coexist with the dominant polycrystalline NiSi phase. As the annealing temperature was increased to 700 8C, epitaxial NiSi2 was found to form in samples D-1 and Fig. 7. Planview TEM images, 650 8C, 30 s of Ti on (a) SiO2—550 MPa stressed-Si; (b) blank-Si and (c) CoSi2—870 MPa stressed-Si samples.
Previous studies showed that Ni metal thin films react with silicon substrate to form three silicide phases: Ni2Si (200–350 8C), NiSi (350–750 8C) and NiSi2 (0750 8C). In addition, the resistivities of Ni2Si, NiSi and NiSi2 are approximately 24, 14–20 and 35–50 mV cm, respectively w6x. Fig. 9 shows the sheet resistance data of Ni metal thin films (30 nm) on blank-(0 0 1)Si and stressed-Si samples after annealing at different temperatures for 60 s. From the sheet resistance data, a sharp drop in sheet resistance occurred at 400 8C for blank and stressed-Si samples. The sheet resistance maintained the same low level for all samples annealed at 450–650 8C, indicating that only low-resistivity NiSi phase was
Fig. 9. Sheet resistance of nickel silicides versus annealing temperature curves for samples A-1, B-1, C-1, D-1 and E-1.
S.L. Cheng et al. / Thin Solid Films 424 (2003) 33–39
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Fig. 11. The thickness of Ni2Si and NiSi in Samples A-1, B-1, C-1, D-1 and E-1 after annealing at 450 8C for 15 s.
Fig. 10. Planview TEM images and corresponding diffraction patterns of (a) polycrystalline NiSi and (b) epitaxial NiSi2 in sample A-1 annealed at 750 8C for 60 s.
E-1. However, only polycrystalline NiSi was observed in 700 8C annealed samples A-1, B-1 and C-1. At 750 8C, complete transformation of epitaxial NiSi2 phase was observed in blank-Si samples. At the same temperature, a mixture of polycrystalline NiSi and epitaxial NiSi2 was found in compressively stressed samples (samples A-1 and B-1). Examples are shown in Fig. 10a and b. For both samples A-1 and B-1, complete transformation from NiSi to NiSi2 was found to occur at 800 8C. The results indicated that the phase transformation from NiSi to NiSi2 was more advanced in tensily stressed-Si samples (samples D-1 and E-1) than those in blank-Si and compressively stressed-Si samples (sam-
ples A-1, B-1 and C-1) annealed at the same temperature. The silicide phases formation data of GIXRD and TEM analysis are listed in Table 2. From XTEM observation, a two-layer structure was observed in all samples after annealing at 450 8C for 15 s. The top and bottom layers were identified to be Ni2Si and NiSi, respectively. Top Ni2Si layers in samples D-1 and E-1 are thinner than those in other samples and were discontinuous. In addition, the thickness ratio of NiSi to Ni2Si was found to increase and decrease with the tensile and compressive stress level, respectively. The thickness of Ni2Si and NiSi films for different samples after annealing at 450 8C for 15 s is shown in Fig. 11. Based on the sheet resistance measurement, GIXRD, TEM, HRTEM and ACF analysis, compressive stress present in the silicon substrate was found to retard significantly the interfacial reactions of Ti or Ni metal thin films on Si substrate. In contrast, the tensile stress present in the silicon substrate was found to promote the phase transformation of metal silicides. In addition, the thicknesses of silicide films were found to decrease and increase with the compressive and tensile stress level, respectively, for both TiySi and NiySi systems. For solid-state diffusion, the effect of stress on the diffusion coefficient D for vacancy diffusion in a crystalline solid can be described as w16x: w
x y
≠lnŽDya2n. z ≠P
1 | sy RT ŽV qV f
~T
.sy
m
Va RT
(1)
Table 2 Phase formation data for Ni (30 nm) on different samples annealed at various temperatures for 60 s by RTA Samples (A-1) SiO2—380 MPa (B-1) SiO2—100 MPa (C-1) Blank-Si (D-1) Si3N4—250 MPa (E-1) CoSi2—870 MPa Epi-NiSi2, epitaxial NiSi2. a Dominant phase.
300 8C Ni NiaqNi2Si NiaqNi2Si NiaqNi2Si NiaqNi2Si
350 8C
400 8C a
NiqNi2Si Ni2SiaqNiSi Ni2SiaqNiSi Ni2SiaqNiSi Ni2SiaqNiSi
a
Ni2SiqNiSi Ni2SiqNiSia NiSi NiSi NiSi
450–650 8C
700 8C
750 8C
800 8C
NiSi NiSi NiSi NiSi NiSi
NiSi NiSi NiSi Epi-NiSi2 Epi-NiSi2
NiSiqEpi-NiSi2 NiSiqEpi-NiSi2a Epi-NiSi2 Epi-NiSi2 Epi-NiSi2
Epi-NiSi2 Epi-NiSi2 Epi-NiSi2 Epi-NiSi2 Epi-NiSi2
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where P is the pressure (compressive stress), a is the lattice parameter, and n is the mean vibrational frequency of the atom at equilibrium. Vf and Vm are the partial molar volumes of the vacancies and of the activated complexes. The sum VfqVmsVa is called activation volume. For the growth of TiSi2, Si is known to be the dominant diffusing species. The diffusivity of Si into the TiSi2 layer shall be the dominant factor in determining the growth rate. Physically, the removal of vacancies decreases the volume of the specimen. For diffusion in a crystalline solid, if the compressive stress (pressure) is increased, the specimen will lose vacancies in an effort to relieve the increase in compressive stress. According to Eq. (1), the decrease in the concentration of vacancies will in turn decrease D. As a result, for samples under compressive stress the diffusion of atoms is retarded. In contrast, if a tension is present in Si substrate, one can expect to enhance the atomic diffusion. For the formation of nickel silicides, Ni is known to be the dominant diffusing species. Although Ni was reported to diffuse by the kick-out mechanism in silicon w17x, it is expected that the same trend in stress– diffusion relationship holds. From thermodynamic consideration, the results could therefore be attributed to the hindrance of the migration of atoms through the metalySi interface by the presence of compressive stress, so that the formation of metal silicide films was retarded. On the other hand, tensile stress facilitates the diffusion of atoms and promotes the growth of metal silicides. 4. Summary and conclusions Effects of stress on the interfacial reactions of Ti and Ni thin films on (0 0 1)Si have been investigated by in situ stress measurement, sheet resistance measurement, GIXRD analysis, TEM observation and HRTEM in conjunction with ACF analysis. The stress was found to exert significant influence on the formation of Ti and Ni silicides. The C49- to C54-TiSi2 phase transformation temperature in tensily stressed-Si samples was found to lower by approximately 100 8C than that in compressively stressed-Si samples. From HRTEM observation and ACF analysis, the tensile stress present in the silicon substrate promotes the formation of a-interlayer and decreases the grain size of C49-TiSi2, which increases the nucleation density of the C54-TiSi2 phase. Consequently, the transformation of C49- to C54-TiSi2 phase is enhanced. In addition, the thicknesses of TiSi2 films
39
were found to decrease and increase with the compressive and tensile stress level, respectively. The effects of stress on the formation of nickel silicides are similar to those of titanium silicides. The NiSi phase can be stabilized by the presence of compressive stress in the silicon substrate. For Ni on blank and stressed (0 0 1)Si substrates after RTA at 450 8C for 15 s, the thickness of NiSi film was found to increase with the tensile stress level. In the mean time, the thickness of Ni2Si became thinner and discontinuous. Based on thermodynamic consideration, the compressive stress hinders the interdiffusion of atoms through the metalySi interface, so that the formation of silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted. Acknowledgments The research was supported by the National Science Council through a Grant No. NSC 89-2218-E007-067. References w1x Y.L. Shen, S. Suresh, I.A. Blench, J. Appl. Phys. 80 (1996) 1388. w2x J.Y. Yew, L.J. Chen, W.F. Wu, J. Vac. Sci. Technol. B 17 (1999) 939. w3x J.Y. Yew, L.J. Chen, K. Nakamura, Appl. Phys. Lett. 69 (1996) 999. w4x H.F. Hsu, L.J. Chen, J.J. Chu, J. Appl. Phys. 69 (1991) 4282. w5x C.S. Chang, C.W. Nieh, L.J. Chen, Appl. Phys. Lett. 50 (1987) 259. w6x E.G. Colgan, J.P. Gambino, Q.Z. Hong, Mater. Sci. Eng. R 16 (1996) 43. w7x J.B. Lasky, J.S. Nakos, O.J. Cain, P.J. Geiss, IEEE Trans. Electron Devices ED-38 (1991) 262. w8x S.M. Chang, H.Y. Huang, H.Y. Yang, L.J. Chen, Appl. Phys. Lett. 74 (1999) 224. w9x R.T. Tung, K. Fujii, K. Kikuta, S. Chikaki, T. Kikkawa, Appl. Phys. Lett. 70 (1997) 2386. w10x J.M. Liang, L.J. Chen, Appl. Phys. Lett. 64 (1994) 1224. w11x J.M. Liang, L.J. Chen, J. Appl. Phys. 79 (1996) 4072. w12x L.J. Chen, J.H. Lin, T.L. Lee, C.H. Luo, W.Y. Hsieh, J.M. Liang, M.H. Wang, Micros. Res. Tech. 40 (1998) 136. w13x J.C. Chen, G.H. Shen, L.J. Chen, J. Appl. Phys. 83 (1998) 7653. w14x L.J. Chen, Mater. Sci. Eng. R 29 (2000) 115. w15x S.P. Murarka, Metallization: Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, Boston, MA, 1993, p. 74. w16x P.G. Shewmon, Diffusion in Solid, second ed., The Metallurgical Society, Warrendale, PA, 1989, p. 84. w17x T.Y. Tan, U. Goesele, Appl. Phys. A 31 (1985) 1.