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Channeled ion beam synthesis of HfSi2
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 909±912
www.elsevier.nl/locate/nimb
Channeled ion beam synthesis of HfSi2 A.R. Ramos a
a,b
, J.G. Marques
a,b
, M.R. da Silva a,b, O. Conde c, M.F. da Silva J.C. Soares a,b,*
a,b
,
Centro de Fõsica Nuclear, Universidade de Lisboa, Av. Prof. Gama Pinto 2, P-1649-003 Lisboa, Portugal b Instituto Tecnol ogico e Nuclear, P-2686-953 Sacav em, Portugal c Dep. Fõsica, Univ. Lisboa, P-1749-016 Lisboa, Portugal
Abstract Channeled implantations were carried out on Si(1 0 0) wafers kept at 600°C with a 300 keV Hf2 beam incident along the á1 0 0ñ direction. Samples implanted with two ¯uences (2:3 1017 and 1:5 1017 Hf/cm2 ) were characterised by RBS/Channeling, Glancing Incidence X-Ray Diraction and Perturbed Angular Correlations. In the higher ¯uence thick polycrystalline HfSi2 surface layer was formed after implantation. A multi-step sample a stoichiometric 1660 A annealing procedure (800°C/900°C/1000°C) was found to favour layer stability as compared to a simpler two-step anneal (700°C/1000°C). Minimum resistivity values of 60 lXcm were obtained after the 1000°C annealings. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 82.80.Yc; 82.80.Ej; 81.40.Rs Keywords: Ion implantation; Hafnium silicide
1. Introduction Hafnium disilicide (HfSi2 ) has one of the lowest resistivities and the highest melting point among refractory silicides [1]. It has been formed by sputtering [2] and by electron-gun evaporation [3] of thin Hf ®lms on silicon wafers with heat treatments in the 700±1100°C range. Ion implantation has been shown to promote the formation of HfSi2 at lower temperatures but continuous stoichiometric layers could not be prepared due to *
Corresponding author. Tel.: +351-1-7904985; fax: +351-17954288. E-mail address: [email protected] (J.C. Soares).
the high sputtering yield of Hf [4]. In this work stoichiometric HfSi2 layers, stable up to 1000°C were prepared by channeled implantation. The samples were characterised by RBS/Channeling, glancing incidence X-ray diraction (GIXRD) and perturbed angular correlations (PAC).
2. Experimental details The Hf2 implantations were performed using the DanFysik 9020 implanter of ITN, Sacavem. Si(1 0 0) n-type, 1 Xcm substrates kept at 600°C were implanted with 300 keV Hf2 ions to ¯uences of 2:3 1017 Hf/cm2 (henceforth abbreviated as
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 9 1 - 6
910
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
HI for `higher' ¯uence) and 1:5 1017 Hf/cm2 (LO). The current density was 4.4 lA/cm2 and the beam was directed within 1:2° of the á1 0 0ñ axis. Channeled implantations were easily achieved since the critical angle is 4° [5]. The samples were annealed under vacuum (10ÿ7 mbar) in the 700± 1000°C range. RBS analyses were carried out at the 3.1 MV Van de Graa accelerator of ITN using a 1.6 MeV He beam. Surface barrier detectors in the IBM geometry at angles of 140° and 180° with 14 and 18 keV resolutions were used. For enhanced depth resolution detectors in the Cornell geometry positioned at scattering angles of 160° and 180° with resolutions of 12 and 18 keV were also used. Resistivity measurements were done after implantation and after each annealing using a four-point probe system. GIXRD measurements were performed using a Siemens D-5000 spectrometer. The diraction patterns were recorded using Cu-Ka radiation incident at an angle of 1° to the sampleÕs surface. The phase analysis was done using the JCPDS database cards [6]. The 181 Hf isotope for the PAC measurements was obtained via the 180 Hf(n,c)181 Hf reaction irradiating the HI sample in the Portuguese Research Reactor, to a thermal neutron ¯uence of 5 1017 n/cm2 . Measurements were done with a standard four-detector spectrometer after irradiation and after annealing at 600°C under vacuum for 1 h. The experimental time dierential anisotropy, R t, is described by the product A22 G22 (t), where A22 is de®ned by the cascadeÕs nuclear parameters and G22 (t) is a perturbation function which describes the modulation of the angular correlation. For the case of 181 Hf three frequencies xn Cn gmQ are observed, giving [7] G22 t
3 X
3. Results and discussion Figs. 1(a) and (b) show the RBS/Channeling results obtained after implantation for the HI and LO samples, respectively. A RUMP [8] simulation in the HI sample reveals a 1660 A surface layer with a 31 at.% Hf concentration. The minimum yield of Si surface is 1.0 and there is extensive dechanneling due to the suface-implanted layer and the defects caused by silicon recoils. The LO sample shows a gaussian-like Hf pro®le, projected range, 1000 A width and with a 1350 A 25 at.% Hf concentration. The tail of the gaussian pro®le extends to the surface where the Hf concentration is low (<1 at.%). The silicon surface did not amorphise completely (minimum yield of 0.72). Fig. 2(a) shows the XRD spectrum of sample HI after implantation and Fig. 2(b) shows the corresponding spectrum for a sample implanted
S2n g cos xn t:
n0
The quadrupole coupling constant mQ eQVzz =h and the asymmetry parameter g Vxx ÿ Vyy =Vzz , obtained from the frequency factors Cn (g), contain information about the magnitude of the principal component Vzz and the asymmetry of the electric ®eld gradient. The S2n coecients, which give the amplitudes for each frequency, can be calculated for polycrystalline and single crystalline samples.
Fig. 1. RBS/Channeling results for the as-implanted samples: (a) 2:3 1017 Hf/cm2 and (b) 1:5 1017 Hf/cm2 .
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
Fig. 2. Glancing incidence X-ray diraction for the as-implanted samples: (a) 2:3 1017 Hf/cm2 and (b) 0:6 1017 Hf/ cm2 .
with a ¯uence of 1/4 of HI. The HfSi2 peaks are comparatively stronger in the HI sample. In this sample the intensity ratios dier from the expected. The most striking features are the intensity inversion between the (1 3 1) and (1 1 0) peaks and the absence of the (0 2 1) peak, which should be the second most intense. Such facts indicate a textured layer. In the case of the LO sample the inversion is not clear, but again the (0 2 1) peak is absent. The broad Si(3 1 1) peak is probably due to Si crystallites in and below the implanted layer; this peak is not visible in the HI sample as 95% of the X-ray beam diracts from the ®rst 500 nm, where the silicide dominates. The identi®cation of the peak marked `*' is not clear. It does not belong to any Hf silicide phases or oxides. It could not also be the (1 0 1) Hf peak, the strongest peak in the Hf diraction pattern, since none of its companion peaks are visible.
911
The samples were subjected to two annealing procedures in vacuum (10ÿ7 mbar): a two-step anneal (700°C, 1 h + 1000°C, 1/2 h) and a multistep anneal (800°C, 1 h + 900°C, 1 h + 1000°C, 1/2 h). Fig. 3 shows glancing incidence spectra taken after the last step for HI samples. The twostep procedure resulted in a broader pro®le (1900 with a lower maximum Hf conversus 1650 A) centration (26% versus 30%). The resistivity decreased after each annealing step, with both procedures, reaching similar minima of 60 lXcm after the 1000°C anneal. This value compares well with the best given in the literature [9]. No channeling eects for Hf were seen in the aligned spectra taken after the last annealing. The two procedures resulted in similar Hf diffused pro®les for the LO sample. In the multi-step procedure, evidence of channeling in the Hf pro®le was seen immediately after the 800°C step and the minimum yield reached its lowest value of 0.76 after the 900°C step. This corresponds to a much lower temperature than reported earlier (1100°C) for the onset of epitaxy [10]. The width and height of the random Hf pro®le remained unchanged until the 1000°C step, where Hf diusion began. A minimum resistivity of 190 lXcm was obtained after the 900°C anneal, followed by a small increase after the last annealing step for both procedures.
Fig. 3. Glancing incidence RBS spectra of the 2:3 1017 Hf/cm2 implanted sample taken with the 160° detector in Cornell geometry with a 60° tilt.
912
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
The PAC measurements made after the additional annealing at 600°C con®rmed that sample HI had texture, since the amplitudes of the observable frequencies varied with the orientation of the substrate. After the initial measurements the sample was crushed. Figs. 4(a) and (b) show the anisotropy ratio then obtained and its Fourier analysis. The spectrum is well described by mQ 152 2 MHz and g 0:50 0:05 using polycrystalline S2n coecients. There is evidence of a faster frequency, mQ 1200 MHz, which does not correspond to any known Hf compound [11]. We assign it to neutron induced defects, as its intensity decreased signi®cantly with the annealing at 600°C. Fig. 4(b) also shows the Fourier analysis of the spectrum previously obtained with the
á0 0 1ñ axis of the Si substrate perpendicular to the detectorÕs plane. In this case there was a signi®cant decrease in the amplitude of x2 , compatible with a fraction of HfSi2 domains oriented along the substrateÕs á0 0 1ñ axis. A ®t with the NNFIT code [12] yielded a corresponding fraction of 10 5%. 4. Conclusions thick, essentially A stoichiometric 1660 A polycrystalline, HfSi2 surface layer was formed by channeled implantation of Hf2 to a ¯uence of 2:3 1017 at./cm2 at 600°C. A multi-step annealing procedure (800°C/900°C/1000°C) was found to favour layer stability compared to a two-step anneal (700°C/1000°C). Minimum resistivity values of 60 lXcm were obtained after the 1000°C annealings. Acknowledgements This work was supported by FCT, Portugal, through project FIS/348/94 and a Ph.D. grant BD/ 9242/96 (A.R.R.). References
Fig. 4. Anisotropy ratio obtained in a crushed sample implanted with 2:3 1017 Hf/cm2 (a) and its Fourier analysis (b).
[1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, London, 1983. [2] C.J. Kircher, J.W. Mayer, K.N. Tu, J.F. Ziegler, Appl. Phys. Lett. 22 (1973) 81. [3] C.S. Chang, C.W. Nieh, L.C. Chen, J. Appl. Phys. 61 (1987) 2393. [4] M.R. da Silva et al., Mat. Res. Soc. Symp. Proc. 402 (1996) 593. [5] D.S. Gemmell, Rev. Mod. Phys. 46 (1974) 129. [6] Joint Committee on Powder Diraction Standards, Powder Diraction File, ASTM, Philadelphia, 1992. [7] G. Schatz, A. Weidinger, Nuclear Condensed Matter Physics, Wiley, Chichester, 1996. [8] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [9] A. Borghesi, F. Marabelli, G. Guizzetti, M. Michelini, F. Nava, J. Appl. Phys. 69 (1991) 7645. [10] C.S. Chang, C.W. Nieh, L.J. Chen, J. Appl. Phys. 61 (1987) 2393. [11] A. Lerf, T. Butz, Hyp. Int. 36 (1987) 275. [12] N.P. Barradas, M. Rots, A.A. Melo, J.C. Soares, Phys. Rev. B 47 (1993) 8763.
www.elsevier.nl/locate/nimb
Channeled ion beam synthesis of HfSi2 A.R. Ramos a
a,b
, J.G. Marques
a,b
, M.R. da Silva a,b, O. Conde c, M.F. da Silva J.C. Soares a,b,*
a,b
,
Centro de Fõsica Nuclear, Universidade de Lisboa, Av. Prof. Gama Pinto 2, P-1649-003 Lisboa, Portugal b Instituto Tecnol ogico e Nuclear, P-2686-953 Sacav em, Portugal c Dep. Fõsica, Univ. Lisboa, P-1749-016 Lisboa, Portugal
Abstract Channeled implantations were carried out on Si(1 0 0) wafers kept at 600°C with a 300 keV Hf2 beam incident along the á1 0 0ñ direction. Samples implanted with two ¯uences (2:3 1017 and 1:5 1017 Hf/cm2 ) were characterised by RBS/Channeling, Glancing Incidence X-Ray Diraction and Perturbed Angular Correlations. In the higher ¯uence thick polycrystalline HfSi2 surface layer was formed after implantation. A multi-step sample a stoichiometric 1660 A annealing procedure (800°C/900°C/1000°C) was found to favour layer stability as compared to a simpler two-step anneal (700°C/1000°C). Minimum resistivity values of 60 lXcm were obtained after the 1000°C annealings. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 82.80.Yc; 82.80.Ej; 81.40.Rs Keywords: Ion implantation; Hafnium silicide
1. Introduction Hafnium disilicide (HfSi2 ) has one of the lowest resistivities and the highest melting point among refractory silicides [1]. It has been formed by sputtering [2] and by electron-gun evaporation [3] of thin Hf ®lms on silicon wafers with heat treatments in the 700±1100°C range. Ion implantation has been shown to promote the formation of HfSi2 at lower temperatures but continuous stoichiometric layers could not be prepared due to *
Corresponding author. Tel.: +351-1-7904985; fax: +351-17954288. E-mail address: [email protected] (J.C. Soares).
the high sputtering yield of Hf [4]. In this work stoichiometric HfSi2 layers, stable up to 1000°C were prepared by channeled implantation. The samples were characterised by RBS/Channeling, glancing incidence X-ray diraction (GIXRD) and perturbed angular correlations (PAC).
2. Experimental details The Hf2 implantations were performed using the DanFysik 9020 implanter of ITN, Sacavem. Si(1 0 0) n-type, 1 Xcm substrates kept at 600°C were implanted with 300 keV Hf2 ions to ¯uences of 2:3 1017 Hf/cm2 (henceforth abbreviated as
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 9 1 - 6
910
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
HI for `higher' ¯uence) and 1:5 1017 Hf/cm2 (LO). The current density was 4.4 lA/cm2 and the beam was directed within 1:2° of the á1 0 0ñ axis. Channeled implantations were easily achieved since the critical angle is 4° [5]. The samples were annealed under vacuum (10ÿ7 mbar) in the 700± 1000°C range. RBS analyses were carried out at the 3.1 MV Van de Graa accelerator of ITN using a 1.6 MeV He beam. Surface barrier detectors in the IBM geometry at angles of 140° and 180° with 14 and 18 keV resolutions were used. For enhanced depth resolution detectors in the Cornell geometry positioned at scattering angles of 160° and 180° with resolutions of 12 and 18 keV were also used. Resistivity measurements were done after implantation and after each annealing using a four-point probe system. GIXRD measurements were performed using a Siemens D-5000 spectrometer. The diraction patterns were recorded using Cu-Ka radiation incident at an angle of 1° to the sampleÕs surface. The phase analysis was done using the JCPDS database cards [6]. The 181 Hf isotope for the PAC measurements was obtained via the 180 Hf(n,c)181 Hf reaction irradiating the HI sample in the Portuguese Research Reactor, to a thermal neutron ¯uence of 5 1017 n/cm2 . Measurements were done with a standard four-detector spectrometer after irradiation and after annealing at 600°C under vacuum for 1 h. The experimental time dierential anisotropy, R t, is described by the product A22 G22 (t), where A22 is de®ned by the cascadeÕs nuclear parameters and G22 (t) is a perturbation function which describes the modulation of the angular correlation. For the case of 181 Hf three frequencies xn Cn gmQ are observed, giving [7] G22 t
3 X
3. Results and discussion Figs. 1(a) and (b) show the RBS/Channeling results obtained after implantation for the HI and LO samples, respectively. A RUMP [8] simulation in the HI sample reveals a 1660 A surface layer with a 31 at.% Hf concentration. The minimum yield of Si surface is 1.0 and there is extensive dechanneling due to the suface-implanted layer and the defects caused by silicon recoils. The LO sample shows a gaussian-like Hf pro®le, projected range, 1000 A width and with a 1350 A 25 at.% Hf concentration. The tail of the gaussian pro®le extends to the surface where the Hf concentration is low (<1 at.%). The silicon surface did not amorphise completely (minimum yield of 0.72). Fig. 2(a) shows the XRD spectrum of sample HI after implantation and Fig. 2(b) shows the corresponding spectrum for a sample implanted
S2n g cos xn t:
n0
The quadrupole coupling constant mQ eQVzz =h and the asymmetry parameter g Vxx ÿ Vyy =Vzz , obtained from the frequency factors Cn (g), contain information about the magnitude of the principal component Vzz and the asymmetry of the electric ®eld gradient. The S2n coecients, which give the amplitudes for each frequency, can be calculated for polycrystalline and single crystalline samples.
Fig. 1. RBS/Channeling results for the as-implanted samples: (a) 2:3 1017 Hf/cm2 and (b) 1:5 1017 Hf/cm2 .
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
Fig. 2. Glancing incidence X-ray diraction for the as-implanted samples: (a) 2:3 1017 Hf/cm2 and (b) 0:6 1017 Hf/ cm2 .
with a ¯uence of 1/4 of HI. The HfSi2 peaks are comparatively stronger in the HI sample. In this sample the intensity ratios dier from the expected. The most striking features are the intensity inversion between the (1 3 1) and (1 1 0) peaks and the absence of the (0 2 1) peak, which should be the second most intense. Such facts indicate a textured layer. In the case of the LO sample the inversion is not clear, but again the (0 2 1) peak is absent. The broad Si(3 1 1) peak is probably due to Si crystallites in and below the implanted layer; this peak is not visible in the HI sample as 95% of the X-ray beam diracts from the ®rst 500 nm, where the silicide dominates. The identi®cation of the peak marked `*' is not clear. It does not belong to any Hf silicide phases or oxides. It could not also be the (1 0 1) Hf peak, the strongest peak in the Hf diraction pattern, since none of its companion peaks are visible.
911
The samples were subjected to two annealing procedures in vacuum (10ÿ7 mbar): a two-step anneal (700°C, 1 h + 1000°C, 1/2 h) and a multistep anneal (800°C, 1 h + 900°C, 1 h + 1000°C, 1/2 h). Fig. 3 shows glancing incidence spectra taken after the last step for HI samples. The twostep procedure resulted in a broader pro®le (1900 with a lower maximum Hf conversus 1650 A) centration (26% versus 30%). The resistivity decreased after each annealing step, with both procedures, reaching similar minima of 60 lXcm after the 1000°C anneal. This value compares well with the best given in the literature [9]. No channeling eects for Hf were seen in the aligned spectra taken after the last annealing. The two procedures resulted in similar Hf diffused pro®les for the LO sample. In the multi-step procedure, evidence of channeling in the Hf pro®le was seen immediately after the 800°C step and the minimum yield reached its lowest value of 0.76 after the 900°C step. This corresponds to a much lower temperature than reported earlier (1100°C) for the onset of epitaxy [10]. The width and height of the random Hf pro®le remained unchanged until the 1000°C step, where Hf diusion began. A minimum resistivity of 190 lXcm was obtained after the 900°C anneal, followed by a small increase after the last annealing step for both procedures.
Fig. 3. Glancing incidence RBS spectra of the 2:3 1017 Hf/cm2 implanted sample taken with the 160° detector in Cornell geometry with a 60° tilt.
912
A.R. Ramos et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 909±912
The PAC measurements made after the additional annealing at 600°C con®rmed that sample HI had texture, since the amplitudes of the observable frequencies varied with the orientation of the substrate. After the initial measurements the sample was crushed. Figs. 4(a) and (b) show the anisotropy ratio then obtained and its Fourier analysis. The spectrum is well described by mQ 152 2 MHz and g 0:50 0:05 using polycrystalline S2n coecients. There is evidence of a faster frequency, mQ 1200 MHz, which does not correspond to any known Hf compound [11]. We assign it to neutron induced defects, as its intensity decreased signi®cantly with the annealing at 600°C. Fig. 4(b) also shows the Fourier analysis of the spectrum previously obtained with the
á0 0 1ñ axis of the Si substrate perpendicular to the detectorÕs plane. In this case there was a signi®cant decrease in the amplitude of x2 , compatible with a fraction of HfSi2 domains oriented along the substrateÕs á0 0 1ñ axis. A ®t with the NNFIT code [12] yielded a corresponding fraction of 10 5%. 4. Conclusions thick, essentially A stoichiometric 1660 A polycrystalline, HfSi2 surface layer was formed by channeled implantation of Hf2 to a ¯uence of 2:3 1017 at./cm2 at 600°C. A multi-step annealing procedure (800°C/900°C/1000°C) was found to favour layer stability compared to a two-step anneal (700°C/1000°C). Minimum resistivity values of 60 lXcm were obtained after the 1000°C annealings. Acknowledgements This work was supported by FCT, Portugal, through project FIS/348/94 and a Ph.D. grant BD/ 9242/96 (A.R.R.). References
Fig. 4. Anisotropy ratio obtained in a crushed sample implanted with 2:3 1017 Hf/cm2 (a) and its Fourier analysis (b).
[1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, London, 1983. [2] C.J. Kircher, J.W. Mayer, K.N. Tu, J.F. Ziegler, Appl. Phys. Lett. 22 (1973) 81. [3] C.S. Chang, C.W. Nieh, L.C. Chen, J. Appl. Phys. 61 (1987) 2393. [4] M.R. da Silva et al., Mat. Res. Soc. Symp. Proc. 402 (1996) 593. [5] D.S. Gemmell, Rev. Mod. Phys. 46 (1974) 129. [6] Joint Committee on Powder Diraction Standards, Powder Diraction File, ASTM, Philadelphia, 1992. [7] G. Schatz, A. Weidinger, Nuclear Condensed Matter Physics, Wiley, Chichester, 1996. [8] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [9] A. Borghesi, F. Marabelli, G. Guizzetti, M. Michelini, F. Nava, J. Appl. Phys. 69 (1991) 7645. [10] C.S. Chang, C.W. Nieh, L.J. Chen, J. Appl. Phys. 61 (1987) 2393. [11] A. Lerf, T. Butz, Hyp. Int. 36 (1987) 275. [12] N.P. Barradas, M. Rots, A.A. Melo, J.C. Soares, Phys. Rev. B 47 (1993) 8763.