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The effect of Ar+ irradiation on grain growth and reducing reaction temperature in Pd-silicide formation
532
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 532-536 Nosh-Holl~d
The effect of Arf irradiation on grain growth and reducing reaction temperature in Pd-silicide formation R.Y. Lee ‘, B.S. Choi ‘, J.H. Song, K.W. Kim, S.T. Kang and C.N. Whang Department of Physics, Yansei University Seoul 120-749, Republic of Korea
R.J. Smith Department of Physics, Moniana Stare University, Bozeman, MT 59717, USA
Pd films deposited on Si(ll1) were irradiated with Ar+ Ions at room temperature. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Auger electron spectroscopy (AES) were employed to study the grain growth due to the Ar+ bombardment and the change in silicide formation temperature during the post-annealing treatment after Ar+ embedment. Compared with 8 nm for the as-deposited sample, the grain size was increased up to 25-30 nm after ion bombardment. Moreover, the PdaSi formation temperature after irradiation is appreciably decreased as compared with the as-deposited sample. These lower reaction temperatures for Pd-silicide formation are discussed in terms of the dispersions of interfacial contaminants such as oxides or impurities owing to the atomic mixing process.
1. Introduction Recently, the ion beam mixing technique has been intensively investigated for application in the formation of silicides because ion beam mixing induced silicides possess much smoother surfaces and interfaces and better electrical properties than the thermally annealed silicides [Q]. It is well known [3] that, for a given ion and a given substrate temperature during the ion beam mixing process, a critical dose is required for silicide formation. Along these lines, most ion beam mixing studies have
been primarily concerned about phase identification and phase transformation with higher doses than the critical one. Recently, we have reported [4,5] on the silicide formation in the Pd/Si system irradiated with 80 keV Art. We have found that dimetal silicide, Pd,Si, was formed at a dose of 1 X lOI Ar+/cm’ at room temperature. The present study was intended to investigate the phenomena involved in lower doses than the critical one. These phenomena are usually neglected in ion beam mixing studies. However, they may be important for understanding the interaction between ions and target atoms in the solid. We will discuss the gram growth associated with ArC irradiation, and also
’ Permanent address: Department of Materials Science and Engineering, Dankook Univ., Cheonahn 330-180, Korea. * Permanent address: Department of Physics, Jeonju Univ., Jeonju, Korea. 0168-583X/9~/$03.50
0 1991 - Else&r Science ~b~sbers
the change in silicide formation temperature upon post-annealing after Ar+ bombardment below the critical dose.
2. Experimental procedure Pd films of 450 A in thickness were vapor deposited on chemically cleaned Si(ll1) substrates. Mixing was carried out with a 150 keV accelerator at room temperature under the pressure of 2 X lo-’ Torr. The ArC was chosen to be 80 keV so as to give the projected range equal to the Pd thickness. Doses from 1 X 10” to 1.5 x 10” Ar’/cm’ were used for mixing the Pd/Si bilayer at a beam current density of 1 rA/cm2. An X-ray diffractometer (Cu K, ra~ation) was employed to identify the phases. For TEM observation, the sample was etched from the backside using ion milling. AES was employed to study the change in Pd-silicide formation temperature during post-annealing after Ar+ bombardment. The sample was heated indirectly under the base pressure of 5 X 10-t’ Torr. The primary electron energy and the modulation voltage were maintamed at 3 keV and 2.2 V peak-to-peak, respectively. Sputtering was carried out with a 3 keV Ar+ ion beam. 3. Results and discussion Prior to the TEM, XRD, and AES observation, Rutherford backscattering spectroscopy (RBS) was per-
B.V. (North~Ho~l~d)
R Y. Lee et al. / Ar + irradiation in Pd-silicide formation
Si(ll1)
28.36O
(a)
Pd(200) 46.740
.92"
Pdy
I
_(b)
Si(ll1)
28.300
Pd(lll)
39.26O Pd(2SlO) 45.86O
\
Cc)
PdzSi(228) 38.00” Pd&i(Pfi)
25
40
39-a”
60
70
Fig. 1. X-ray diffraction patterns of Pd/Si(lll) for (a) the as-deposited sample, and the samples irradiated by (b) 3 x 1015 Ar+/cm*, and (c) 1 X 1016Ar”/cm2.
formed to investigate the trends of intermixing and the silicide phase formation with increasing Ar+ dose. The RBS analysis was carried out using 1.4 MeV He+ obtained from a Van de Graaff accelerator (details of RBS measurements and results were presented in our previous paper [4,5]). The XRD patterns are shown in fig. 1. Since the deposited Pd layer is very thin, the substrate Si(ll1) peaks appeared. As compared with the 28 = 39.98” for the as-deposited sample, the corresponding 28 for
533
Pd(ll1) peak is decreased to 39.26”, and the full width at half maximum (FWHM) is increased from 0.45” to 0.89” after 3 x 1015 Ar’/cm’ bombardment. The XRD result for the sample irradiated with 5 X 1015 Ar+/cm’ shows the same trend as fig. lb. The XRD pattern of the sample irradiated with 1 X 1016 Ar+/cm’ exactly coincides with that of bulk Pd,Si [6], which is the same as reported in our previous papers [4,5] analyzed by RBS and TEM. The shift in Bragg peak and the broadening of FWHM with Ar+ irradiation are considered to have arisen from the formation of the nonuniform composition in the samples as indicated in RBS [4,5] at these lower doses of ions. Another possible explanation of the peak shift and FWHM broadening may be associated with the defects generation and the tensile strain due to Ar+ bombardment. Fig. 2 shows the bright-field images of TEM for the as-deposited and the irradiated Pd films with the dose of 3 x 1015 Ar’/cm’. The average grain size for the as-deposited sample is found to be 8 nm, but the grain size is increased to 25-30 nm at the dose of 3 x 1015 Ar’/cm’. And then the grain size does not show any further growth up to 1.5 x 1016 Ar’/cm2. These results are consistent with Hung and Mayer’s results [7]. Thus, we suggest that the tensile strain associated with Ar+ ion irradiation is thought to be the reason for the XRD peak shift to a lower angle, and that the broadening of the FWHM of Pd(ll1) XRD peak is caused by the dislocations and point defects induced by Ar+ bombardment. This tensile strain combined with the surface energy corresponding to grain boundaries increases the total free energy in the thin film. Therefore, it can be expected that the grain growth and the stress relaxation into the grain boundary occur in order to decrease the free energy. Moreover, the high atomic mobility [7] associated with ion irradiation will contribute to the grain growth which is observed in this study. It has recently been revealed [3,8] that, upon postannealing, the silicide formed by ion beam mixing can transform into another phase at temperatures somewhat lower than those required in direct heat treatment. For example, the PdSi phase has been obtained by annealing an ion implanted Pd/Si (or Pd,Si/Si) sample at 350~400°C [3], while temperatures as high as 750 OC are needed to form that phase by direct annealing. Therefore, the following study was performed to elucidate the change in ion beam induced silicide formation temperature upon post-annealing with varying ion dose. The samples of as-deposited and irradiated Pd/Si (3 X 1015 and 5 X 10” Ar+/cm2) were heated in the AES system. The Auger spectrum for an unirradiated specimen prior to heating, but after sputtering to remove the contaminants, is shown in fig. 3a. As expected, only the Auger peaks corresponding to Pd (43, 80, and 330 eV) appear. After annealing for 10 min at V. ION BEAM MIXING
534
R. Y. Lee et al. / Ar + irradiation in Pd-silicide formation
Fig. 2. TEM images of (a) the as-deposited sample and (b) the sample irradiated by 3 X lOI Ar’/cm’.
208’C, a Si LW single peak (92 eV) began to appear, while intensity of the Pd MNN AES peak (330 eV) decreased slightly compared with that of the as-deposited sample (fig. 3b). We could not obtain the split Si LW spectrum which has been already identified as due to the Pd,Si [9,10]. After only 1 mm of sputtering, the Si peak was removed (fig. 3~). The typical sputtering rate was estimated to be about 0.2 nm/min for Pd under the present experimental conditions. Thus it would appear that the Si LW single peak which evolved after annealing at 208’C is due to a shallow enriched Si layer associated with Si surface segregation. The sample was then sputtered for 5 min to remove the whole segregated region and then reheated and checked the AES spectrum at each temperature. The pure Si LW single peak appeared again at around 22O’C as shown in fig. 3d. After annealing at this temperature for 5 min, the pure Si LW AES peak still remained. When the annealed specimen was sputtered for 5 min, the silicide (Pd,Si) spectrum [lo] consisting of three peaks (85, 90, and 94 eV) appeared as shown in fig. 3e, and the intensity of Pd MNN AES peak increased slightly compared with that of fig. 3d. Thus we conclude that the temperature required for the as-deposited sample to form metal-rich silicide (Pd,Si) is at least 220°C. From the quantitative AES analysis using the sensitivity factors of Si and Pd, the stoichiometric ratio of Pd,_,Si, is found to be x = 0.36, which is close to the value of the Pd,Si phase (x = 0.33). For the specimens irradiated with a dose of 3 x 1015 and 5 X 1Ol5 Ar+/cm’, we found that the reaction temperature for Pd,Si formation to be 95 and 70°C, respectively. As can be seen in figs. 3f and 3g, a distinctive Si LW Auger peak splitting appears which shows the
formation of Pd,Si. The peak retained the same shape even after the sputtering for 14 min, which is the characteristic of Pd,Si. We could not observe the Si surface segregation phenomena in the ion beam mixed sample. At a higher fluence of 1 X 1016 Ar+/cm* (fig. 3h), it is found that Pd,Si is formed at room temperature by Ar + irradiation, which is consistent with our previous results [4,5] investigated by RBS and TEM. To study the effects of radiation damage on the formation of a silicide phase, the following experiments have been carried out. The Si substrates were predamaged with 5 keV Ar+ (1 x 1Ol6 Ar’/cm*), and then exposed to air. These predamaged samples were then cleaned chemically before deposition of Pd films. The Pd films were deposited in UHV chamber, and the deposited samples were subsequently annealed and analysed with AES. No difference in the silicide formation temperature was observed between the damaged and undamaged samples. This result suggests that the radiation damage in the substrate is not the primary factor responsible for the reduction of the silicide formation temperature. Another factor worth considerating is the Pd-Si interface. The Si substrate prepared by conventional chemical cleaning procedures contains at least one atomic layer of oxide and other impurities [ll], which may act as a barrier for silicide formation. When Ar+ ions bombard the Pd/Si bilayer, this barrier may be dispersed as a result of atomic mixing process. Once the barrier is destroyed, nucleation and growth of the silitide phase can be initiated even at low temperatures. This suggestion was confirmed by the following experimental results. Si layers of 10 nm thickness were deposited first on a Si wafer and then a Pd film was
RI’. L.eeetal.
(a)
/Ar
+ irradiation in Pd-silicide formation
535
the silicide formation requires a high temperature (22O’C) similar to that of the as-deposited sample in fig. 3. These results clearly indicate that the Ar+ bombardment leads to the dispersion of interfacial contaminations (oxides or impurities) owing to the atomic mixing processes, and then leads to the reduction of silicide formation temperatures.
7
4. Conclusion Evaporated Pd films on Si(ll1) substrates were irradiated by Ar+ ions at room temperature. The TEM images show that the grain size in the Pd film increase after ion bombardment. The average grain size for the unirradiated film is 8 nm, while the grain size is increased to 25-30 nm after Ar+ irradiation. It is believed that the combined effect of defects, such as vacancy formation, and the tensile strain in the Pd film association with ion bombardment contribute to the grain growth. AES experiments show that the temperature required to convert a Pd/Si bilayer to Pd,Si is appreciably decreased from 220°C for an as-deposited sample to 95, 70, and 25°C by Ar + ion irradiation with the dose of 3 x 1015, 5 x 1015 and 1 x 1016 Ar’/cm’, respectively. From the AES results for a sample prepared on a predamaged Si substrate and direct annealing of the in situ deposited sample, the effect of ion bombardment is believed to lead to the dispersion of interfacial contaminants owing to the atomic mixing process, and result in the reduction of silicide formation temperature.
(d)
(e)
(f)
(g)
(h)
-$Tq-90 1
Acknowledgement
Si(1.W)
a0 IZLJXC-1-N
ENERGY
Fig. 3. AES spectrum of (a) as-deposited after sputtering, (b) annealing at 208“C followed by (c) sputtering for 1 min, (d) reheating to 220°C, followed by (e) sputtering for 5 min. For the samples irradiated with (f) 3~10”~ Ar+/cm*, (g) 5 ~10’~ Ar+/cm*, and (h) 1 X lOi Ar+/cm2, a distinctive Si LW peak splitting appears at 95, 70 and 25’C, respectively.
deposited on the top without breaking the UHV condition, and then analysed with the AES system. The metal-Si interface in this sample is believed to be relatively free of contamination compared to that of the sample prepared on chemically cleaned Si wafer. Direct annealing of this in situ deposited Pd/Si sample results in the formation of silicide (Pd,Si) at low temperature (150°C), which is similar to that observed in the ion beam mixed sample. However, when the deposited Si film is exposed to the air before deposition of Pd film,
This study was supported in part by the Inter-University Semiconductor Research Center (IUSRC) at Seoul National University, Korea and the Korea Research Foundation.
References [l] H. Okabayashi, Nucl. Instr. and Meth. B39 (1989) 246. [2] E. Nagasawa, H. Okabayasi and M. Morimoto, Jpn. J. Appl. Phys. 22 (1983) L57. [3] B.Y. Tsaur, S.S. Lau and J.W. Mayer, Appl. Phys. Lett. 35 (1979) 225. [4] R.Y. Lee, C.N. Whang, H.K. Kim and R.J. Smith, Nucl. Instr. and Meth. B33 (1988) 661. [5] R.Y. Lee, C.N. Whang, H.K. Kim and R.J. Smith, J. Mater. Sci. 23 (1988) 2740. [6] Joint Committee on Powder Diffraction Standards, ASTM, Powder Diffraction File (1974). [7] L.S. Hung and J.W. Mayer, Thin Solid Films 123 (1985) 135. V. ION BEAM MIXING
536
R. Y. Lee et al. / Ar f irradiation in Pd-silicide formation
[8] B.Y. Tsaur and L.S. Hung, Appl. Phys. Lett. 37 (1980) 922. [9] J.A. Roth and C.W. Crowell, J. Vat. Sci. Technol. 15 (1978) 1317. [lo] K. Oura, S. Okada and T. Hauawa, Appl. Phys. Lett. 35 (1979) 705.
[ll] M. von Allmen et al. in: Thin Fii - Interfaces and Interactions, eds. J.E.E. Baglin and J.M. Poate (Electrochemical Society, Princeton, NJ, 1980) vol. 80-2, p. 195.
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 532-536 Nosh-Holl~d
The effect of Arf irradiation on grain growth and reducing reaction temperature in Pd-silicide formation R.Y. Lee ‘, B.S. Choi ‘, J.H. Song, K.W. Kim, S.T. Kang and C.N. Whang Department of Physics, Yansei University Seoul 120-749, Republic of Korea
R.J. Smith Department of Physics, Moniana Stare University, Bozeman, MT 59717, USA
Pd films deposited on Si(ll1) were irradiated with Ar+ Ions at room temperature. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Auger electron spectroscopy (AES) were employed to study the grain growth due to the Ar+ bombardment and the change in silicide formation temperature during the post-annealing treatment after Ar+ embedment. Compared with 8 nm for the as-deposited sample, the grain size was increased up to 25-30 nm after ion bombardment. Moreover, the PdaSi formation temperature after irradiation is appreciably decreased as compared with the as-deposited sample. These lower reaction temperatures for Pd-silicide formation are discussed in terms of the dispersions of interfacial contaminants such as oxides or impurities owing to the atomic mixing process.
1. Introduction Recently, the ion beam mixing technique has been intensively investigated for application in the formation of silicides because ion beam mixing induced silicides possess much smoother surfaces and interfaces and better electrical properties than the thermally annealed silicides [Q]. It is well known [3] that, for a given ion and a given substrate temperature during the ion beam mixing process, a critical dose is required for silicide formation. Along these lines, most ion beam mixing studies have
been primarily concerned about phase identification and phase transformation with higher doses than the critical one. Recently, we have reported [4,5] on the silicide formation in the Pd/Si system irradiated with 80 keV Art. We have found that dimetal silicide, Pd,Si, was formed at a dose of 1 X lOI Ar+/cm’ at room temperature. The present study was intended to investigate the phenomena involved in lower doses than the critical one. These phenomena are usually neglected in ion beam mixing studies. However, they may be important for understanding the interaction between ions and target atoms in the solid. We will discuss the gram growth associated with ArC irradiation, and also
’ Permanent address: Department of Materials Science and Engineering, Dankook Univ., Cheonahn 330-180, Korea. * Permanent address: Department of Physics, Jeonju Univ., Jeonju, Korea. 0168-583X/9~/$03.50
0 1991 - Else&r Science ~b~sbers
the change in silicide formation temperature upon post-annealing after Ar+ bombardment below the critical dose.
2. Experimental procedure Pd films of 450 A in thickness were vapor deposited on chemically cleaned Si(ll1) substrates. Mixing was carried out with a 150 keV accelerator at room temperature under the pressure of 2 X lo-’ Torr. The ArC was chosen to be 80 keV so as to give the projected range equal to the Pd thickness. Doses from 1 X 10” to 1.5 x 10” Ar’/cm’ were used for mixing the Pd/Si bilayer at a beam current density of 1 rA/cm2. An X-ray diffractometer (Cu K, ra~ation) was employed to identify the phases. For TEM observation, the sample was etched from the backside using ion milling. AES was employed to study the change in Pd-silicide formation temperature during post-annealing after Ar+ bombardment. The sample was heated indirectly under the base pressure of 5 X 10-t’ Torr. The primary electron energy and the modulation voltage were maintamed at 3 keV and 2.2 V peak-to-peak, respectively. Sputtering was carried out with a 3 keV Ar+ ion beam. 3. Results and discussion Prior to the TEM, XRD, and AES observation, Rutherford backscattering spectroscopy (RBS) was per-
B.V. (North~Ho~l~d)
R Y. Lee et al. / Ar + irradiation in Pd-silicide formation
Si(ll1)
28.36O
(a)
Pd(200) 46.740
.92"
Pdy
I
_(b)
Si(ll1)
28.300
Pd(lll)
39.26O Pd(2SlO) 45.86O
\
Cc)
PdzSi(228) 38.00” Pd&i(Pfi)
25
40
39-a”
60
70
Fig. 1. X-ray diffraction patterns of Pd/Si(lll) for (a) the as-deposited sample, and the samples irradiated by (b) 3 x 1015 Ar+/cm*, and (c) 1 X 1016Ar”/cm2.
formed to investigate the trends of intermixing and the silicide phase formation with increasing Ar+ dose. The RBS analysis was carried out using 1.4 MeV He+ obtained from a Van de Graaff accelerator (details of RBS measurements and results were presented in our previous paper [4,5]). The XRD patterns are shown in fig. 1. Since the deposited Pd layer is very thin, the substrate Si(ll1) peaks appeared. As compared with the 28 = 39.98” for the as-deposited sample, the corresponding 28 for
533
Pd(ll1) peak is decreased to 39.26”, and the full width at half maximum (FWHM) is increased from 0.45” to 0.89” after 3 x 1015 Ar’/cm’ bombardment. The XRD result for the sample irradiated with 5 X 1015 Ar+/cm’ shows the same trend as fig. lb. The XRD pattern of the sample irradiated with 1 X 1016 Ar+/cm’ exactly coincides with that of bulk Pd,Si [6], which is the same as reported in our previous papers [4,5] analyzed by RBS and TEM. The shift in Bragg peak and the broadening of FWHM with Ar+ irradiation are considered to have arisen from the formation of the nonuniform composition in the samples as indicated in RBS [4,5] at these lower doses of ions. Another possible explanation of the peak shift and FWHM broadening may be associated with the defects generation and the tensile strain due to Ar+ bombardment. Fig. 2 shows the bright-field images of TEM for the as-deposited and the irradiated Pd films with the dose of 3 x 1015 Ar’/cm’. The average grain size for the as-deposited sample is found to be 8 nm, but the grain size is increased to 25-30 nm at the dose of 3 x 1015 Ar’/cm’. And then the grain size does not show any further growth up to 1.5 x 1016 Ar’/cm2. These results are consistent with Hung and Mayer’s results [7]. Thus, we suggest that the tensile strain associated with Ar+ ion irradiation is thought to be the reason for the XRD peak shift to a lower angle, and that the broadening of the FWHM of Pd(ll1) XRD peak is caused by the dislocations and point defects induced by Ar+ bombardment. This tensile strain combined with the surface energy corresponding to grain boundaries increases the total free energy in the thin film. Therefore, it can be expected that the grain growth and the stress relaxation into the grain boundary occur in order to decrease the free energy. Moreover, the high atomic mobility [7] associated with ion irradiation will contribute to the grain growth which is observed in this study. It has recently been revealed [3,8] that, upon postannealing, the silicide formed by ion beam mixing can transform into another phase at temperatures somewhat lower than those required in direct heat treatment. For example, the PdSi phase has been obtained by annealing an ion implanted Pd/Si (or Pd,Si/Si) sample at 350~400°C [3], while temperatures as high as 750 OC are needed to form that phase by direct annealing. Therefore, the following study was performed to elucidate the change in ion beam induced silicide formation temperature upon post-annealing with varying ion dose. The samples of as-deposited and irradiated Pd/Si (3 X 1015 and 5 X 10” Ar+/cm2) were heated in the AES system. The Auger spectrum for an unirradiated specimen prior to heating, but after sputtering to remove the contaminants, is shown in fig. 3a. As expected, only the Auger peaks corresponding to Pd (43, 80, and 330 eV) appear. After annealing for 10 min at V. ION BEAM MIXING
534
R. Y. Lee et al. / Ar + irradiation in Pd-silicide formation
Fig. 2. TEM images of (a) the as-deposited sample and (b) the sample irradiated by 3 X lOI Ar’/cm’.
208’C, a Si LW single peak (92 eV) began to appear, while intensity of the Pd MNN AES peak (330 eV) decreased slightly compared with that of the as-deposited sample (fig. 3b). We could not obtain the split Si LW spectrum which has been already identified as due to the Pd,Si [9,10]. After only 1 mm of sputtering, the Si peak was removed (fig. 3~). The typical sputtering rate was estimated to be about 0.2 nm/min for Pd under the present experimental conditions. Thus it would appear that the Si LW single peak which evolved after annealing at 208’C is due to a shallow enriched Si layer associated with Si surface segregation. The sample was then sputtered for 5 min to remove the whole segregated region and then reheated and checked the AES spectrum at each temperature. The pure Si LW single peak appeared again at around 22O’C as shown in fig. 3d. After annealing at this temperature for 5 min, the pure Si LW AES peak still remained. When the annealed specimen was sputtered for 5 min, the silicide (Pd,Si) spectrum [lo] consisting of three peaks (85, 90, and 94 eV) appeared as shown in fig. 3e, and the intensity of Pd MNN AES peak increased slightly compared with that of fig. 3d. Thus we conclude that the temperature required for the as-deposited sample to form metal-rich silicide (Pd,Si) is at least 220°C. From the quantitative AES analysis using the sensitivity factors of Si and Pd, the stoichiometric ratio of Pd,_,Si, is found to be x = 0.36, which is close to the value of the Pd,Si phase (x = 0.33). For the specimens irradiated with a dose of 3 x 1015 and 5 X 1Ol5 Ar+/cm’, we found that the reaction temperature for Pd,Si formation to be 95 and 70°C, respectively. As can be seen in figs. 3f and 3g, a distinctive Si LW Auger peak splitting appears which shows the
formation of Pd,Si. The peak retained the same shape even after the sputtering for 14 min, which is the characteristic of Pd,Si. We could not observe the Si surface segregation phenomena in the ion beam mixed sample. At a higher fluence of 1 X 1016 Ar+/cm* (fig. 3h), it is found that Pd,Si is formed at room temperature by Ar + irradiation, which is consistent with our previous results [4,5] investigated by RBS and TEM. To study the effects of radiation damage on the formation of a silicide phase, the following experiments have been carried out. The Si substrates were predamaged with 5 keV Ar+ (1 x 1Ol6 Ar’/cm*), and then exposed to air. These predamaged samples were then cleaned chemically before deposition of Pd films. The Pd films were deposited in UHV chamber, and the deposited samples were subsequently annealed and analysed with AES. No difference in the silicide formation temperature was observed between the damaged and undamaged samples. This result suggests that the radiation damage in the substrate is not the primary factor responsible for the reduction of the silicide formation temperature. Another factor worth considerating is the Pd-Si interface. The Si substrate prepared by conventional chemical cleaning procedures contains at least one atomic layer of oxide and other impurities [ll], which may act as a barrier for silicide formation. When Ar+ ions bombard the Pd/Si bilayer, this barrier may be dispersed as a result of atomic mixing process. Once the barrier is destroyed, nucleation and growth of the silitide phase can be initiated even at low temperatures. This suggestion was confirmed by the following experimental results. Si layers of 10 nm thickness were deposited first on a Si wafer and then a Pd film was
RI’. L.eeetal.
(a)
/Ar
+ irradiation in Pd-silicide formation
535
the silicide formation requires a high temperature (22O’C) similar to that of the as-deposited sample in fig. 3. These results clearly indicate that the Ar+ bombardment leads to the dispersion of interfacial contaminations (oxides or impurities) owing to the atomic mixing processes, and then leads to the reduction of silicide formation temperatures.
7
4. Conclusion Evaporated Pd films on Si(ll1) substrates were irradiated by Ar+ ions at room temperature. The TEM images show that the grain size in the Pd film increase after ion bombardment. The average grain size for the unirradiated film is 8 nm, while the grain size is increased to 25-30 nm after Ar+ irradiation. It is believed that the combined effect of defects, such as vacancy formation, and the tensile strain in the Pd film association with ion bombardment contribute to the grain growth. AES experiments show that the temperature required to convert a Pd/Si bilayer to Pd,Si is appreciably decreased from 220°C for an as-deposited sample to 95, 70, and 25°C by Ar + ion irradiation with the dose of 3 x 1015, 5 x 1015 and 1 x 1016 Ar’/cm’, respectively. From the AES results for a sample prepared on a predamaged Si substrate and direct annealing of the in situ deposited sample, the effect of ion bombardment is believed to lead to the dispersion of interfacial contaminants owing to the atomic mixing process, and result in the reduction of silicide formation temperature.
(d)
(e)
(f)
(g)
(h)
-$Tq-90 1
Acknowledgement
Si(1.W)
a0 IZLJXC-1-N
ENERGY
Fig. 3. AES spectrum of (a) as-deposited after sputtering, (b) annealing at 208“C followed by (c) sputtering for 1 min, (d) reheating to 220°C, followed by (e) sputtering for 5 min. For the samples irradiated with (f) 3~10”~ Ar+/cm*, (g) 5 ~10’~ Ar+/cm*, and (h) 1 X lOi Ar+/cm2, a distinctive Si LW peak splitting appears at 95, 70 and 25’C, respectively.
deposited on the top without breaking the UHV condition, and then analysed with the AES system. The metal-Si interface in this sample is believed to be relatively free of contamination compared to that of the sample prepared on chemically cleaned Si wafer. Direct annealing of this in situ deposited Pd/Si sample results in the formation of silicide (Pd,Si) at low temperature (150°C), which is similar to that observed in the ion beam mixed sample. However, when the deposited Si film is exposed to the air before deposition of Pd film,
This study was supported in part by the Inter-University Semiconductor Research Center (IUSRC) at Seoul National University, Korea and the Korea Research Foundation.
References [l] H. Okabayashi, Nucl. Instr. and Meth. B39 (1989) 246. [2] E. Nagasawa, H. Okabayasi and M. Morimoto, Jpn. J. Appl. Phys. 22 (1983) L57. [3] B.Y. Tsaur, S.S. Lau and J.W. Mayer, Appl. Phys. Lett. 35 (1979) 225. [4] R.Y. Lee, C.N. Whang, H.K. Kim and R.J. Smith, Nucl. Instr. and Meth. B33 (1988) 661. [5] R.Y. Lee, C.N. Whang, H.K. Kim and R.J. Smith, J. Mater. Sci. 23 (1988) 2740. [6] Joint Committee on Powder Diffraction Standards, ASTM, Powder Diffraction File (1974). [7] L.S. Hung and J.W. Mayer, Thin Solid Films 123 (1985) 135. V. ION BEAM MIXING
536
R. Y. Lee et al. / Ar f irradiation in Pd-silicide formation
[8] B.Y. Tsaur and L.S. Hung, Appl. Phys. Lett. 37 (1980) 922. [9] J.A. Roth and C.W. Crowell, J. Vat. Sci. Technol. 15 (1978) 1317. [lo] K. Oura, S. Okada and T. Hauawa, Appl. Phys. Lett. 35 (1979) 705.
[ll] M. von Allmen et al. in: Thin Fii - Interfaces and Interactions, eds. J.E.E. Baglin and J.M. Poate (Electrochemical Society, Princeton, NJ, 1980) vol. 80-2, p. 195.