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Reaction of Ni and Si0.8Ge0.2: phase formation and thermal stability
Microelectronic Engineering 76 (2004) 297–302 www.elsevier.com/locate/mee
Reaction of Ni and Si0.8Ge0.2: phase formation and thermal stability O. Chamirian
a,b,*
, A. Lauwers a, J.A. Kittl c, M. Van Dal d, M. De Potter a, R. Lindsay a, K. Maex a,b a
IMEC, Kapeldreef 75, 3001 Leuven, Belgium b E.E. Department, K.U. Leuven, Belgium c IMEC, Texas Instruments, Leuven, Belgium d Philips Research Leuven, Belgium Available online 8 August 2004
Abstract The reaction of Ni films with a Si0.8Ge0.2 alloy is studied in the temperature range of 200–750 C from the point of view of process window, morphology of the resulting silicide, and mechanisms of degradation at high temperatures. The effect on the reaction of substrate crystallinity (single crystal and polycrystalline Si0.8Ge0.2), dopants (arsenic and boron) and process parameters is investigated. Sheet resistance measurements, X-ray diffraction and scanning electron microscopy were used for film characterization. Ni-rich germanosilicide is formed at low (<350 C) temperatures, the Ni monogermanosilicide phase is then formed, which is stable up to 550–600 C. At temperatures above 600 C, segregation of Ge and morphological changes result in an islanded structure of germanosilicide, with at least partial expulsion of Ge from Ni(SiGe) grains. Formation of the low resistivity Ni monogermanosilicide phase is delayed by the addition of Ge in comparison to the pure NiSi case. Ni(SiGe) films have lower thermal stability than pure NiSi films. Activation energies of degradation of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for Ni on poly-SiGe were determined, which are significantly lower than those measured for pure NiSi films. These results suggest that segregation of Ge play a significant role in the mechanism of degradation of Ni(SiGe) films. The process window for a low resistivity film is narrower for the reaction of Ni with Si–Ge in comparison to the reaction of Ni with pure Si. 2004 Published by Elsevier B.V.
1. Introduction
*
Corresponding author. Tel.: +32 16 288 203; fax: +32 16 281 214. E-mail address: [email protected] (O. Chamirian). 0167-9317/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.mee.2004.07.033
Nowadays, Si1 xGex alloys are of great interest for the fabrication of high-performance electronic devices. There are a number of possibilities for the use of Si1 xGex in MOSFETs, including
298
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
source/drain areas and gates [1]. For the successful implementation of Si1 xGex into conventional CMOS devices, reliable ohmic contacts to Si1 xGex have to be formed, preferably using a process flow similar to the SALICIDE process. Therefore, reactions of metals with Si1 xGex alloys have been studied for many metals including Ti, Co, Pt and Ni [2–5]. Currently, Ni is the most promising candidate for use in technologies incorporating Si1 xGex. However, the presence of Ge alloying can affect the process window for the formation of the desired low-resistivity phase. In this work, the reaction of Ni films with a Si0.8Ge0.2 alloy is studied in a wide range of temperatures (200–750 C). The effect on the reaction of substrate crystallinity (single crystal and polycrystalline Si0.8Ge0.2), dopants (arsenic and boron) and process parameters is investigated. Mechanisms of thermal degradation are studied using sheet resistance measurements, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Activation energies for the process of morphological degradation (agglomeration) are determined for NiSi on As- and B-doped single- crystalline and polycrystalline SiGe.
2. Experimental Blanket 8 in. silicon wafers were used for the experiments. After initial cleaning, 100 nm epitaxial SiGe 20% Ge films were grown or 100 nm of polycrystalline SiGe with the same Ge content were deposited (in the latter case 15 nm of silicon oxide was deposited before the SiGe deposition). Fabricated SiGe was implanted with either arsenic (4 · 1015 atoms/cm2, 20 keV) or boron (2 · 1015 atoms/cm2, 2 keV). A spike junction activation anneal was done at 1100 C. Wafers were then dipped in diluted HF for 20 s followed by an argon pre-clean (removing an equivalent oxide thickness of 2 nm). Finally, 10 nm thick Ni films were deposited by PVD. After metal deposition wafers were cut into samples and annealed in nitrogen ambient in a heatpulse rapid thermal processing (RTP) tool. For the phase formation study, after RTP the unreacted metal was selectively removed using a
wet etch. To study the thermal degradation behavior, Ni(SiGe) was formed at 450 C 30 s and subsequently annealed at 500–700 C. Samples with same RTP conditions (temperature and time) were annealed simultaneously. Samples were characterized by four-point probe sheet resistance (Rsh) measurements, by XRD for phase identification, and by SEM for determination of film morphology.
3. Results and discussion 3.1. Phase formation Phase formation was studied for a one-step silicidation process in the temperature range of 200– 750 C. Transformation curves for the reaction of 10 nm Ni with As- or B-doped SiGe are shown in Fig. 1(a) – for single-crystalline SiGe, and (b) – for poly-SiGe. Corresponding curves for the reaction of Ni with pure silicon are shown for comparison. Sheet resistance was measured after RTP and after selective removal of the unreacted metal. The characteristics of the transformation curves are similar for both Si and SiGe substrates. At low temperatures Ni-rich phases are formed (predominantly Ni2Si or Ni2(SiGe)), then the plateau of low sheet resistance values indicates the formation of Ni-monosilicide or Ni-mono-germanosilicide. Finally, at elevated temperatures morphological degradation of (germano)silicide causes a sharp increase in Rsh. However, formation of Ni(SiGe) is delayed to higher temperatures by the presence of Ge (around 350 versus 300 C for pure NiSi), and thermal degradation of germanosilicide films occur at lower temperatures than for pure NiSi films (Fig. 1). Measured sheet resistance of mono-germanosilicide is slightly higher compared to pure NiSi for both single-crystalline and poly-SiGe. Presence of Ni2(SiGe) at low temperatures and Ni mono-germanosilicide at higher temperatures was verified by XRD. In Fig. 2 h/2h scans are shown for As-doped samples after RTP at 325, 450, and 700 C and selective etch. Spectra for Ni germanosilicide on B-doped substrates are similar and not shown. Only peaks corresponding to Ni2(SiGe) were found at 325 C, and only
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302 100
299 Si (400)
Si (200)
As 10nm Ni c-Siafter RTP As 10nm Ni c-Si after SE B 10 nm Ni c-Siafter RTP
Ni(SiGe)
As 10nm Ni c-SiGe after RTP
Intensity (a.u.)
Sheet resistance ( Ohm/sq )
B 10 nm Ni c-Si after SE
80
As 10nm Ni c-SiGe after SE B 10 nm Ni c-SiGe after RTP B 10 nm Ni c-SiGe after SE
60
40
700C
Ni(SiGe)
450C Ni (SiGe) 2 Ni (SiGe) 2
20 Ni (SiGe) 2
325C
0 200
300
400
500
600
700
100
40
50
60
70
θ/2θ (deg)
(a)
o
Temperature ( C)
(a)
30
800
As 10nm Ni poly-Si after RTp As 10 nm Ni poly-Si after SE B 10 nm Ni poly-Si after RTP
Sheet resistance ( Ohm/sq )
B 10 nm Ni poly-Si after SE
80
Ni on SiGe 700C
As 10 nm Ni poly-SiGe after RTP As 10nm Ni pol;y-SiGe after SE B 10 nm Ni poly-SiGe after RTP
Ni on SiGe 450C
B 10 nm Ni poly-SiGe after SE
60
Ni on c-Si 500C
40
50
(b) 20
0 200
(b)
300
400
500
600
700
800
o
Temperature ( C)
Fig. 1. Transformation curves for (germano)silicidation of 10 nm Ni on (a) c-SiGe and (b) poly-SiGe. RTP for 30 s in N2 ambient. Data for the silicidation of Ni on pure Si are shown for the comparison.
Ni(SiGe) peaks were observed for 450 and 700 C. The Ge content in the monogermanosilicide (formed at 450 C), estimated from the peak shift assuming VegardÕs law is similar to the Ge concentration in the SiGe, i.e. around 20% (Fig. 2(b)). 3.2. Thermal stability The effects of both anneal temperature and time on sheet resistance were studied for Ni(SiGe) on c-
52
54
56
58
θ/2θ (deg)
Fig. 2. XRD spectra for Ni on As-doped single-crystalline SiGe: (a) h–2h scans; (b) shift of the Ni(SiGe) peak from the pure NiSi (germanosilicide formed at 450 C and at 700 C, NiSi at 500 C).
SiGe and poly-SiGe. The germanosilicide films were first formed by RTP (450 C 30 s) and the initial sheet resistance value (Rsh0) was measured. Since all the metal was consumed in the reaction, no selective etch was needed after this initial RTP. Further RTP steps at varying temperatures and times were performed subsequently, and followed by sheet resistance measurements. On both n+ and p+ c-SiGe and poly-SiGe, the sheet resistance of Ni(SiGe) increases abruptly above a given temperature (600–650 C) and keeps increasing with temperature to large values. An earlier onset of degradation was observed for Ni (SiGe) films compared to pure NiSi films. From thermodynamical considerations, the nucleation of NiSi2 from Ni(SiGe) is more difficult
300
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
Table 1 Thermal degradation times for Ni(SiGe) films on c-SiGe and poly-SiGe RTP temperature (C)
Times to 20% increase in sheet resistance (s) c-SiGe
500 550 600
poly-SiGe
As-doped
B-doped
As-doped
B-doped
195 55 22
260 75 32
105 19 8
160 31 16
compared to pure NiSi, i.e. the presence of Ge inhibits the transformation of mono(germano)silicide into the di(germano)silicide and Ni(SiGe) phase is stable up to the 800–850 C [1]. Therefore, within the temperature range studied, no disilicide
formation is expected. The fact that the sheet resistance keeps increasing and does not saturate in this range is consistent with a mechanism of morphological degradation. Similarly to the pure NiSi case, Ni-germanosilicide is more prone to
Fig. 3. Plan-view and cross-section SEM micrographs of Ni(SiGe) films on As-doped (left) and B-doped (right) single-crystalline SiGe. Samples were annealed for 30 s at 450, 600, and 700 C.
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
agglomeration when formed on As-doped SiGe compared to B-doped SiGe. The polycrystalline substrate also is a factor promoting the morphological changes in germanosilicide film. A ‘‘degradation time’’ s(T) was defined as the time at a given temperature corresponding to a 20% increase in sheet resistance (from the initial Ni(SiGe) sheet resistance). Degradation times are shown in Table 1. To check whether the sharp increase in sheet resistance with temperature is indeed caused by morphological changes, SEM inspections were done on films annealed at 450–700 C. The planview and cross-sectional SEM images are shown
301
in Figs. 3 and 4 for c-SiGe and poly-SiGe substrates respectively. Few pinholes present in the films formed at 450 C are probably caused by the selective etch. After anneals at higher temperatures, a continuous film evolves into a partially agglomerated silicide and finally to almost disconnected islands. These islands protrude into the substrate while the surface of the film remains quite smooth and lined up with SiGe surface (Figs. 3 and 4). As both Si and Ge are available from the SiGe substrate and driving force for the germanosilicide formation is higher than for Ge segregation, the initial Ge content of the Ni(SiGe) films is close to the Ge content in the substrate.
Fig. 4. Plan-view and cross-section SEM micrographs of Ni(SiGe) films on As-doped (left) and B-doped (right) polycrystalline SiGe. Samples were annealed for 30 s at 450, 600, and 700 C.
302
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302 7 6
ln τ
5 4 3 Ni/c-SiGe As
2 Ni/c-SiGe B
1 0 10
Ni/c-Si As
11
12
13
14
15
16
sistent with the presence of NiSi grains with very small Ge content (less than 1%) while some grains would still conserve a high Ge content (Fig. 2(b)). Finally, activation energies were estimated for the process of thermal degradation. Arrhenius plots of s for c-SiGe and poly-SiGe substrates are shown in Fig. 5(a) and (b) respectively. Values obtained were of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for Ni on poly-SiGe. These values are quite smaller than those for Ni on pure silicon (around 2.5 eV) [6,7]. This indicates a different mechanism for the films with Ge, and is consistent with Ge segregation playing a role in the degradation mechanism.
1/ kT (1/eV)
4. Conclusions
7 6 5
ln τ
4 3 2 Ni/poly-SiGe As
1
Ni/poly-SiGe B
0 Ni/c-Si As
-1 10
11
12
13
14
15
16
1/ kT (1/eV) Fig. 5. Arrhenius plot of degradation times (s corresponds to 20% increase in sheet resistance) for NiSi films on singlecrystalline SiGe (a) and polycrystalline SiGe (b). Calculated activation energies for morphological degradation are of 1.65 ± 0.2 eV for c-SiGe and of 1.56 ± 0.2 eV for poly-SiGe.
However, the Ge content in the ternary phase can be reduced after annealing at elevated temperatures. The shape of the Ni(SiGe) peaks in the XRD spectra of a Ni(SiGe) sample annealed at 700 C suggests that there are grains within the film with different Ge content, and would be con-
The reaction of Ni films with a Si0.8Ge0.2 alloy is studied in the temperature range of 200–750 C. The presence of Ge results in a delay in the formation of NiSi and lower thermal stability compared to pure NiSi. Ni-rich germanosilicide is formed at low (<350 C) temperatures, then monogermanosilicide phase is formed and stable up to 550–600 C. At temperatures above 600 C, segregation of Ge and morphological changes result in an islanded structure of germanosilicide, with at least partial expulsion of Ge form Ni(SiGe) grains. Activation energies of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for poly-SiGe were determined for the process of morphological degradation.
References [1] S.-L. Zhang, Microelectron. Eng. 70 (2003) 174. [2] O. Thomas, F.M. dÕHeurle, S. Delage, J. Mater. Res. 5 (1990) 1453. [3] Q.Z. Hong, J.W. Mayer, J. Appl. Phys. 66 (1989) 611. [4] B.I. Boyanov, et al., J. Appl. Phys. 86 (1999) 1355. [5] K.L. Pey, et al., J. Vac. Sci. Technol. 20 (2002) 1903. [6] J.A. Kittl, et al., Microelectron. Eng. 70 (2003) 158. [7] O. Chamirian, et al., Microelectron. Eng. 70 (2003) 201.
Reaction of Ni and Si0.8Ge0.2: phase formation and thermal stability O. Chamirian
a,b,*
, A. Lauwers a, J.A. Kittl c, M. Van Dal d, M. De Potter a, R. Lindsay a, K. Maex a,b a
IMEC, Kapeldreef 75, 3001 Leuven, Belgium b E.E. Department, K.U. Leuven, Belgium c IMEC, Texas Instruments, Leuven, Belgium d Philips Research Leuven, Belgium Available online 8 August 2004
Abstract The reaction of Ni films with a Si0.8Ge0.2 alloy is studied in the temperature range of 200–750 C from the point of view of process window, morphology of the resulting silicide, and mechanisms of degradation at high temperatures. The effect on the reaction of substrate crystallinity (single crystal and polycrystalline Si0.8Ge0.2), dopants (arsenic and boron) and process parameters is investigated. Sheet resistance measurements, X-ray diffraction and scanning electron microscopy were used for film characterization. Ni-rich germanosilicide is formed at low (<350 C) temperatures, the Ni monogermanosilicide phase is then formed, which is stable up to 550–600 C. At temperatures above 600 C, segregation of Ge and morphological changes result in an islanded structure of germanosilicide, with at least partial expulsion of Ge from Ni(SiGe) grains. Formation of the low resistivity Ni monogermanosilicide phase is delayed by the addition of Ge in comparison to the pure NiSi case. Ni(SiGe) films have lower thermal stability than pure NiSi films. Activation energies of degradation of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for Ni on poly-SiGe were determined, which are significantly lower than those measured for pure NiSi films. These results suggest that segregation of Ge play a significant role in the mechanism of degradation of Ni(SiGe) films. The process window for a low resistivity film is narrower for the reaction of Ni with Si–Ge in comparison to the reaction of Ni with pure Si. 2004 Published by Elsevier B.V.
1. Introduction
*
Corresponding author. Tel.: +32 16 288 203; fax: +32 16 281 214. E-mail address: [email protected] (O. Chamirian). 0167-9317/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.mee.2004.07.033
Nowadays, Si1 xGex alloys are of great interest for the fabrication of high-performance electronic devices. There are a number of possibilities for the use of Si1 xGex in MOSFETs, including
298
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
source/drain areas and gates [1]. For the successful implementation of Si1 xGex into conventional CMOS devices, reliable ohmic contacts to Si1 xGex have to be formed, preferably using a process flow similar to the SALICIDE process. Therefore, reactions of metals with Si1 xGex alloys have been studied for many metals including Ti, Co, Pt and Ni [2–5]. Currently, Ni is the most promising candidate for use in technologies incorporating Si1 xGex. However, the presence of Ge alloying can affect the process window for the formation of the desired low-resistivity phase. In this work, the reaction of Ni films with a Si0.8Ge0.2 alloy is studied in a wide range of temperatures (200–750 C). The effect on the reaction of substrate crystallinity (single crystal and polycrystalline Si0.8Ge0.2), dopants (arsenic and boron) and process parameters is investigated. Mechanisms of thermal degradation are studied using sheet resistance measurements, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Activation energies for the process of morphological degradation (agglomeration) are determined for NiSi on As- and B-doped single- crystalline and polycrystalline SiGe.
2. Experimental Blanket 8 in. silicon wafers were used for the experiments. After initial cleaning, 100 nm epitaxial SiGe 20% Ge films were grown or 100 nm of polycrystalline SiGe with the same Ge content were deposited (in the latter case 15 nm of silicon oxide was deposited before the SiGe deposition). Fabricated SiGe was implanted with either arsenic (4 · 1015 atoms/cm2, 20 keV) or boron (2 · 1015 atoms/cm2, 2 keV). A spike junction activation anneal was done at 1100 C. Wafers were then dipped in diluted HF for 20 s followed by an argon pre-clean (removing an equivalent oxide thickness of 2 nm). Finally, 10 nm thick Ni films were deposited by PVD. After metal deposition wafers were cut into samples and annealed in nitrogen ambient in a heatpulse rapid thermal processing (RTP) tool. For the phase formation study, after RTP the unreacted metal was selectively removed using a
wet etch. To study the thermal degradation behavior, Ni(SiGe) was formed at 450 C 30 s and subsequently annealed at 500–700 C. Samples with same RTP conditions (temperature and time) were annealed simultaneously. Samples were characterized by four-point probe sheet resistance (Rsh) measurements, by XRD for phase identification, and by SEM for determination of film morphology.
3. Results and discussion 3.1. Phase formation Phase formation was studied for a one-step silicidation process in the temperature range of 200– 750 C. Transformation curves for the reaction of 10 nm Ni with As- or B-doped SiGe are shown in Fig. 1(a) – for single-crystalline SiGe, and (b) – for poly-SiGe. Corresponding curves for the reaction of Ni with pure silicon are shown for comparison. Sheet resistance was measured after RTP and after selective removal of the unreacted metal. The characteristics of the transformation curves are similar for both Si and SiGe substrates. At low temperatures Ni-rich phases are formed (predominantly Ni2Si or Ni2(SiGe)), then the plateau of low sheet resistance values indicates the formation of Ni-monosilicide or Ni-mono-germanosilicide. Finally, at elevated temperatures morphological degradation of (germano)silicide causes a sharp increase in Rsh. However, formation of Ni(SiGe) is delayed to higher temperatures by the presence of Ge (around 350 versus 300 C for pure NiSi), and thermal degradation of germanosilicide films occur at lower temperatures than for pure NiSi films (Fig. 1). Measured sheet resistance of mono-germanosilicide is slightly higher compared to pure NiSi for both single-crystalline and poly-SiGe. Presence of Ni2(SiGe) at low temperatures and Ni mono-germanosilicide at higher temperatures was verified by XRD. In Fig. 2 h/2h scans are shown for As-doped samples after RTP at 325, 450, and 700 C and selective etch. Spectra for Ni germanosilicide on B-doped substrates are similar and not shown. Only peaks corresponding to Ni2(SiGe) were found at 325 C, and only
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302 100
299 Si (400)
Si (200)
As 10nm Ni c-Siafter RTP As 10nm Ni c-Si after SE B 10 nm Ni c-Siafter RTP
Ni(SiGe)
As 10nm Ni c-SiGe after RTP
Intensity (a.u.)
Sheet resistance ( Ohm/sq )
B 10 nm Ni c-Si after SE
80
As 10nm Ni c-SiGe after SE B 10 nm Ni c-SiGe after RTP B 10 nm Ni c-SiGe after SE
60
40
700C
Ni(SiGe)
450C Ni (SiGe) 2 Ni (SiGe) 2
20 Ni (SiGe) 2
325C
0 200
300
400
500
600
700
100
40
50
60
70
θ/2θ (deg)
(a)
o
Temperature ( C)
(a)
30
800
As 10nm Ni poly-Si after RTp As 10 nm Ni poly-Si after SE B 10 nm Ni poly-Si after RTP
Sheet resistance ( Ohm/sq )
B 10 nm Ni poly-Si after SE
80
Ni on SiGe 700C
As 10 nm Ni poly-SiGe after RTP As 10nm Ni pol;y-SiGe after SE B 10 nm Ni poly-SiGe after RTP
Ni on SiGe 450C
B 10 nm Ni poly-SiGe after SE
60
Ni on c-Si 500C
40
50
(b) 20
0 200
(b)
300
400
500
600
700
800
o
Temperature ( C)
Fig. 1. Transformation curves for (germano)silicidation of 10 nm Ni on (a) c-SiGe and (b) poly-SiGe. RTP for 30 s in N2 ambient. Data for the silicidation of Ni on pure Si are shown for the comparison.
Ni(SiGe) peaks were observed for 450 and 700 C. The Ge content in the monogermanosilicide (formed at 450 C), estimated from the peak shift assuming VegardÕs law is similar to the Ge concentration in the SiGe, i.e. around 20% (Fig. 2(b)). 3.2. Thermal stability The effects of both anneal temperature and time on sheet resistance were studied for Ni(SiGe) on c-
52
54
56
58
θ/2θ (deg)
Fig. 2. XRD spectra for Ni on As-doped single-crystalline SiGe: (a) h–2h scans; (b) shift of the Ni(SiGe) peak from the pure NiSi (germanosilicide formed at 450 C and at 700 C, NiSi at 500 C).
SiGe and poly-SiGe. The germanosilicide films were first formed by RTP (450 C 30 s) and the initial sheet resistance value (Rsh0) was measured. Since all the metal was consumed in the reaction, no selective etch was needed after this initial RTP. Further RTP steps at varying temperatures and times were performed subsequently, and followed by sheet resistance measurements. On both n+ and p+ c-SiGe and poly-SiGe, the sheet resistance of Ni(SiGe) increases abruptly above a given temperature (600–650 C) and keeps increasing with temperature to large values. An earlier onset of degradation was observed for Ni (SiGe) films compared to pure NiSi films. From thermodynamical considerations, the nucleation of NiSi2 from Ni(SiGe) is more difficult
300
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
Table 1 Thermal degradation times for Ni(SiGe) films on c-SiGe and poly-SiGe RTP temperature (C)
Times to 20% increase in sheet resistance (s) c-SiGe
500 550 600
poly-SiGe
As-doped
B-doped
As-doped
B-doped
195 55 22
260 75 32
105 19 8
160 31 16
compared to pure NiSi, i.e. the presence of Ge inhibits the transformation of mono(germano)silicide into the di(germano)silicide and Ni(SiGe) phase is stable up to the 800–850 C [1]. Therefore, within the temperature range studied, no disilicide
formation is expected. The fact that the sheet resistance keeps increasing and does not saturate in this range is consistent with a mechanism of morphological degradation. Similarly to the pure NiSi case, Ni-germanosilicide is more prone to
Fig. 3. Plan-view and cross-section SEM micrographs of Ni(SiGe) films on As-doped (left) and B-doped (right) single-crystalline SiGe. Samples were annealed for 30 s at 450, 600, and 700 C.
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302
agglomeration when formed on As-doped SiGe compared to B-doped SiGe. The polycrystalline substrate also is a factor promoting the morphological changes in germanosilicide film. A ‘‘degradation time’’ s(T) was defined as the time at a given temperature corresponding to a 20% increase in sheet resistance (from the initial Ni(SiGe) sheet resistance). Degradation times are shown in Table 1. To check whether the sharp increase in sheet resistance with temperature is indeed caused by morphological changes, SEM inspections were done on films annealed at 450–700 C. The planview and cross-sectional SEM images are shown
301
in Figs. 3 and 4 for c-SiGe and poly-SiGe substrates respectively. Few pinholes present in the films formed at 450 C are probably caused by the selective etch. After anneals at higher temperatures, a continuous film evolves into a partially agglomerated silicide and finally to almost disconnected islands. These islands protrude into the substrate while the surface of the film remains quite smooth and lined up with SiGe surface (Figs. 3 and 4). As both Si and Ge are available from the SiGe substrate and driving force for the germanosilicide formation is higher than for Ge segregation, the initial Ge content of the Ni(SiGe) films is close to the Ge content in the substrate.
Fig. 4. Plan-view and cross-section SEM micrographs of Ni(SiGe) films on As-doped (left) and B-doped (right) polycrystalline SiGe. Samples were annealed for 30 s at 450, 600, and 700 C.
302
O. Chamirian et al. / Microelectronic Engineering 76 (2004) 297–302 7 6
ln τ
5 4 3 Ni/c-SiGe As
2 Ni/c-SiGe B
1 0 10
Ni/c-Si As
11
12
13
14
15
16
sistent with the presence of NiSi grains with very small Ge content (less than 1%) while some grains would still conserve a high Ge content (Fig. 2(b)). Finally, activation energies were estimated for the process of thermal degradation. Arrhenius plots of s for c-SiGe and poly-SiGe substrates are shown in Fig. 5(a) and (b) respectively. Values obtained were of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for Ni on poly-SiGe. These values are quite smaller than those for Ni on pure silicon (around 2.5 eV) [6,7]. This indicates a different mechanism for the films with Ge, and is consistent with Ge segregation playing a role in the degradation mechanism.
1/ kT (1/eV)
4. Conclusions
7 6 5
ln τ
4 3 2 Ni/poly-SiGe As
1
Ni/poly-SiGe B
0 Ni/c-Si As
-1 10
11
12
13
14
15
16
1/ kT (1/eV) Fig. 5. Arrhenius plot of degradation times (s corresponds to 20% increase in sheet resistance) for NiSi films on singlecrystalline SiGe (a) and polycrystalline SiGe (b). Calculated activation energies for morphological degradation are of 1.65 ± 0.2 eV for c-SiGe and of 1.56 ± 0.2 eV for poly-SiGe.
However, the Ge content in the ternary phase can be reduced after annealing at elevated temperatures. The shape of the Ni(SiGe) peaks in the XRD spectra of a Ni(SiGe) sample annealed at 700 C suggests that there are grains within the film with different Ge content, and would be con-
The reaction of Ni films with a Si0.8Ge0.2 alloy is studied in the temperature range of 200–750 C. The presence of Ge results in a delay in the formation of NiSi and lower thermal stability compared to pure NiSi. Ni-rich germanosilicide is formed at low (<350 C) temperatures, then monogermanosilicide phase is formed and stable up to 550–600 C. At temperatures above 600 C, segregation of Ge and morphological changes result in an islanded structure of germanosilicide, with at least partial expulsion of Ge form Ni(SiGe) grains. Activation energies of 1.65 ± 0.2 eV for Ni on c-SiGe and of 1.56 ± 0.2 eV for poly-SiGe were determined for the process of morphological degradation.
References [1] S.-L. Zhang, Microelectron. Eng. 70 (2003) 174. [2] O. Thomas, F.M. dÕHeurle, S. Delage, J. Mater. Res. 5 (1990) 1453. [3] Q.Z. Hong, J.W. Mayer, J. Appl. Phys. 66 (1989) 611. [4] B.I. Boyanov, et al., J. Appl. Phys. 86 (1999) 1355. [5] K.L. Pey, et al., J. Vac. Sci. Technol. 20 (2002) 1903. [6] J.A. Kittl, et al., Microelectron. Eng. 70 (2003) 158. [7] O. Chamirian, et al., Microelectron. Eng. 70 (2003) 201.