- Email: [email protected]
Sublimation and diffusion of arsenic implanted into silicon at rapid electron beam annealing
Nuclear Instruments North-Holland
and Methods
in Physics
Research
573
B55 (1991) 573-575
Sublimation and diffusion of arsenic implanted into silicon at rapid electron beam annealing R. Gr(itzsche1 Central Institute
of Nuclear Research, Rossendorf,
V.A. Kagadey Institute
Germany
and N.I. Lebedeva
of Semiconductor
Deuices, Tomsk, USSR
D.I. Proskurovsky Institute of High Current Electronics, Tomsk, USSR
Properties of silicon layers implanted with As + ions (100 keV, lOI cm-2) through 30 nm SiO, film and annealed in vacuum by means of a 15 s duration electron beam were studied using Rutherford backscattering plus channeling and electrical ,measurements. The influence of the irradiation conditions on As activation, diffusion and sublimation, as well as on the crystal structure perfection of the samples are discussed. Results have been obtained which testify to As sublimation and diffusion in connection with its deactivation in the area of realization of an electrically active As nonequilibrium concentration.
1. Introduction It is established [l-4], that the annealing of implanted layers of Si(As+) using pulse annealing (10 ns-1 PLS)or certain conditions of rapid thermal annealing (RTA) result in the realization of a nonequilibrium concentration of electrically active As. However, further thermal treatments (as well as RTA duration increase) result in an impurity deactivation and the obtaining of an equilibrium concentration value (3 X 102’ cme3 at 1370 K). When the As concentration, as implanted, in a distribution maximum exceeds N,,,, = 4 x 10” cm-3, an enhanced As diffusion into the Si bulk is observed during a photon annealing process (PA), but no As sublimation occurs usually [3]. Whereas, using an electron beam annealing (EBA), one does not observe considerable As atom diffusion into the bulk, as a rule, and a certain part of these atoms sublimate [4]. This report is devoted to elucidating the reasons for the aforesaid peculiarities in the As atom behaviour in Si at EBA, as well as to investigate possible connections between the deactivation, sublimation and impurity diffusion processes.
2. Experimental (100) Si was implanted with As+ ions (E = 100 keV, %+= 1 x 1016 cmm2, Nmax= 1.5 X 1021 cme3) through an 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
SiO, film (30 nm). The annealing of samples was made by a 15 s duration electron beam. The temperature of the samples reached T,,, = 1400 or 1490 K. The dwell time at these temperatures was 5 s. Two sample pairs were irradiated simultaneously. The film was previously removed from the surface of one sample pair and preserved on the other one to capsulate surfaces at the annealing. One sample from each pair was irradiated from the implanted layer (front to beam) and the other from the back side (back to beam). The samples were investigated by the Van der Pauw method and Rutherford backscattering spectroscopy (RBS) plus channeling of He’ ions (1.7 MeV).
3. Results The analysis of RBS spectra yielded the values of xmin - 0.04 at T,, = 1400 K, and 0.03 at T,,, = 1490 K. These values of xmin show the high quality of the crystal structure in the recrystallized layer. Besides, the quality of the crystal structure does not depend on the direction of irradiation as well as the presence of an SiOZ cap layer, being only determined by the annealing temperature. After the annealing at T_ = 1400 K only 50-60s of the As atoms being in substituting positions are electrically active, so the decrease in electrical activation at this temperature can be partially associated with the
B.V. (North:Holland)
VI. MATERIALS
SCIENCE
574
R Griitzschel et al. / Subfiwz&tionand diffiion
of As in Si
2_ 1
A
d
9
11
2
”
e-7
f
;1
3 4
A 0
400
cH4NNEL NO.
350 C-
NO.
Fig. 1. RBS and channeling spectra of As implanted Si irradiated from the implanted layer: (1,3) with Siq, (2,4) without SiO,; (1,2) random, (3,4) aligned. T,, = 1490 K.
formation of either impurity-defect complexes [l] or coherent precipitates [2]. Moreover, the area density of the interstitial As in the samples after the annealing at Tmax = 1400 K under the SiO, cap (4 x 1015 cm-‘) considerably exceeds ‘the corresponding As concentration in noncapsulated samples (0.45 X 1015 cm-2). These data coincide with our previous data [4] and they confirm the fact that As+ deactivation connected with As displacement into the interstices is also realized. If the SiO, film impedes the following sublimation of the As atoms from Si, their amount in channels is increased, RBS and He+ ion channeling testify to that fact. We also think that no As atoms from the Si near-surface region go out onto the surface and sublimate, but only those from the interstices. The RBS spectra characterizing the depth distribution of As atoms in Si wafer are depicted in figs. 1 and
Fig. 2. RBS and channeling spectra of As implanted Si irradiated from the back wafer side: (1.3) with Siq, (2,4) without SiO*; (1,2) random, (3,4) aligned. 7”,, = 1490 K.
2. The results obtained from the analysis of these spectra are presented in table 1. Comparison of the values of N, and Nr show that after the annealing at T_ = 1490 K all the As atoms in substituting positions are electrically active. So at this temperature the deactivation mechanism connected with the As atom displacement into the interstices may be considered to be dominating. In this case, after the annealing under the SiO, cap layers, as electrical activity reaches about 90% of the implanted As dose, while at the annealing without SiO,, it decreases to 45% with 55% implanted ions sublimating. The sublimation intensity is considerably decreased by the SiO, film capsulation. Moreover the intensity of sublimation is affected ‘by the radiation direction: the amount of sublimated As atoms is smaller for samples irradiated from the implanted layer side. This effect is more noticeable for the SiO, capsulated samples. This can be explained by the formation of a thin film of
Table 1 Data of RBS and channeling spectra and Van der Pauw measurements of As implanted Si (T,,, =1490 K). N, and N, are the random and aligned area density of As atoms, respectively; f = (1- x,&/(1 - xmin) is the fraction of As atoms on lattice sites; Nf = N,f is the density of As atoms on lattice sites; NsUblis the sublimated density of As atoms; N, is the sheet carrier concentration. Irradiation conditions With SiO,
front to beam back to beam
f
N, ’ [x10’5cm-2]
N,
9.4 8.1
1.3 1.0
0.89 0.91
4.1 3.9
0.1 0.4
0.86 0.92
[*X 1015cm-‘]
N sub1
N,
[X10’5cm-2]
[X10’5cm-2]
8.8 7.6
0.6 1.9
8.4 9.1
3.7 3.6
5.9 6.1
4.5 4.9
Nf [X
101’ cme2]
’
Without SiO,
front to beam back to beam
R. Grb’tzschel et al. / Sublimation and diffusion of As in Si
polymerized oil from the residual atmosphere on the surface irradiated by the electron beam. Perhaps this film covers the defects in the SiO, cap layer formed due to the As+ implantation, thus resulting in As loss minimization from 19 to 6% of the implanted As dose. The diffusion of As is also dependent on the conditions on the implanted layer surface. At the presence of the SiO, film at T,, = 1490 K, an enhanced As diffusion in the Si bulk is observed, the diffusion coefficient being g= 3.6 X lo-” cm2/s. This 9 value is within the range of 9 = (2.4-12) X lo-l2 cm2/s, determined in experiments on the photon annealing of similar samples [S]. At the same time the diffusion of As into the bulk is much weaker at the annealing of uncapsulated samples (g= 1.3 X lOpI2 cm2/s).
4. Discussion There are some different models [6-81 and their modifications explaining the mechanism of the enhanced impurity diffusion at RTA. The results we have obtained by RTA are in favour of the interstitial mechanism of diffusion, whereby substitutional dopant atoms are displaced from site to site by passing interstitials [8] (relay-race mechanism). The spectra analysis of RBS and channeling of He+ ions show that in Si(As+) samples, being in a thermodynamical equilibrium state, the ratio of the As concentration in the interstitial state to that in the substituting state (at xmin < 0.04) is within the range of 0.01-0.1. The deactivation of the impurity in the samples implanted with a large dose of As+ results in the appearance of a non-equilibrium As excess in the interstices, which has a tendency of being “dispersed”. If sublimation is possible, the above-mentioned ratio at the surface is sharply diminished and there occurs a replenishment of the near-surface region by the interstitial atoms using the relay-race-mechanism. As a result
515
the flux of excessive interstitial As atoms moves toward the surface. If the surface is capsulated this ratio is not diminished there by the sublimation and, with the As reactivation at the surface being excluded due to the As active concentration relaxed to the equilibrium value, it is thermodynamically more efficient for nonequilibrium interstitial As atoms appearing at deactivation to diffuse into the Si bulk. It is in this case that we observe the enhanced As diffusion with the diffusivity exceeding many times the equilibrium value (defined by a vacancy mechanism) for these temperatures. The necessity of the realization of the above-mentioned ratio results in the interstitial As atoms reactivating in the Si bulk. This process increases the As electrical activity in the SiO, capsulated samples. Thus the peculiarities in the behaviour of As atoms in Si(As’) at the PA and EBA in similar regimes are defined by different conditions, realized on the implanted layer surface, as far as at the photon annealing, even when the samples are not previously capsulated, a SiO, film of - 10 nm is formed on the Si surface, which prevents As sublimation.
References
Ill A. Lietoila, J. Gibbons
and T.W. Sigmon, Appl. Phys. Lett. 36 (1980) 765. B.P. Kashnikov and A.I. PI V.P. Popov, A.V. Dvurechenskii, Popov, Phys. Status Solidi A94 (1986) 569. I31 A. Nylandsted, 0. Larsen and V.E. Borisenko, Appl. Phys. A33 (1984) 51. [41 V.S. Budishevsky, R. Gr8tzschel. V.A. Kagadei, N.I. Lebedeva, D.I. Proskurovsky and E.B. Yankelevich, Phys. Res. 8 (1988) 262. 151 J. Narayan and O.W. Holland, J. Appl. Phys. 56 (1984) 2913. 161 M. Servidori and S. Solmi, J. Appl. Phys. 65 (1989) 98. [71 R. Kalish, T.O. Sedgwick and S. Maden, Appl. Phys. Lett. 44 (1984) 107. PI S.J. Pennycock, J. Narayan and O.W. Holland, J. Electrothem. Sot. 132 (1985) 1962.
VI. MATERIALS
SCIENCE
and Methods
in Physics
Research
573
B55 (1991) 573-575
Sublimation and diffusion of arsenic implanted into silicon at rapid electron beam annealing R. Gr(itzsche1 Central Institute
of Nuclear Research, Rossendorf,
V.A. Kagadey Institute
Germany
and N.I. Lebedeva
of Semiconductor
Deuices, Tomsk, USSR
D.I. Proskurovsky Institute of High Current Electronics, Tomsk, USSR
Properties of silicon layers implanted with As + ions (100 keV, lOI cm-2) through 30 nm SiO, film and annealed in vacuum by means of a 15 s duration electron beam were studied using Rutherford backscattering plus channeling and electrical ,measurements. The influence of the irradiation conditions on As activation, diffusion and sublimation, as well as on the crystal structure perfection of the samples are discussed. Results have been obtained which testify to As sublimation and diffusion in connection with its deactivation in the area of realization of an electrically active As nonequilibrium concentration.
1. Introduction It is established [l-4], that the annealing of implanted layers of Si(As+) using pulse annealing (10 ns-1 PLS)or certain conditions of rapid thermal annealing (RTA) result in the realization of a nonequilibrium concentration of electrically active As. However, further thermal treatments (as well as RTA duration increase) result in an impurity deactivation and the obtaining of an equilibrium concentration value (3 X 102’ cme3 at 1370 K). When the As concentration, as implanted, in a distribution maximum exceeds N,,,, = 4 x 10” cm-3, an enhanced As diffusion into the Si bulk is observed during a photon annealing process (PA), but no As sublimation occurs usually [3]. Whereas, using an electron beam annealing (EBA), one does not observe considerable As atom diffusion into the bulk, as a rule, and a certain part of these atoms sublimate [4]. This report is devoted to elucidating the reasons for the aforesaid peculiarities in the As atom behaviour in Si at EBA, as well as to investigate possible connections between the deactivation, sublimation and impurity diffusion processes.
2. Experimental (100) Si was implanted with As+ ions (E = 100 keV, %+= 1 x 1016 cmm2, Nmax= 1.5 X 1021 cme3) through an 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
SiO, film (30 nm). The annealing of samples was made by a 15 s duration electron beam. The temperature of the samples reached T,,, = 1400 or 1490 K. The dwell time at these temperatures was 5 s. Two sample pairs were irradiated simultaneously. The film was previously removed from the surface of one sample pair and preserved on the other one to capsulate surfaces at the annealing. One sample from each pair was irradiated from the implanted layer (front to beam) and the other from the back side (back to beam). The samples were investigated by the Van der Pauw method and Rutherford backscattering spectroscopy (RBS) plus channeling of He’ ions (1.7 MeV).
3. Results The analysis of RBS spectra yielded the values of xmin - 0.04 at T,, = 1400 K, and 0.03 at T,,, = 1490 K. These values of xmin show the high quality of the crystal structure in the recrystallized layer. Besides, the quality of the crystal structure does not depend on the direction of irradiation as well as the presence of an SiOZ cap layer, being only determined by the annealing temperature. After the annealing at T_ = 1400 K only 50-60s of the As atoms being in substituting positions are electrically active, so the decrease in electrical activation at this temperature can be partially associated with the
B.V. (North:Holland)
VI. MATERIALS
SCIENCE
574
R Griitzschel et al. / Subfiwz&tionand diffiion
of As in Si
2_ 1
A
d
9
11
2
”
e-7
f
;1
3 4
A 0
400
cH4NNEL NO.
350 C-
NO.
Fig. 1. RBS and channeling spectra of As implanted Si irradiated from the implanted layer: (1,3) with Siq, (2,4) without SiO,; (1,2) random, (3,4) aligned. T,, = 1490 K.
formation of either impurity-defect complexes [l] or coherent precipitates [2]. Moreover, the area density of the interstitial As in the samples after the annealing at Tmax = 1400 K under the SiO, cap (4 x 1015 cm-‘) considerably exceeds ‘the corresponding As concentration in noncapsulated samples (0.45 X 1015 cm-2). These data coincide with our previous data [4] and they confirm the fact that As+ deactivation connected with As displacement into the interstices is also realized. If the SiO, film impedes the following sublimation of the As atoms from Si, their amount in channels is increased, RBS and He+ ion channeling testify to that fact. We also think that no As atoms from the Si near-surface region go out onto the surface and sublimate, but only those from the interstices. The RBS spectra characterizing the depth distribution of As atoms in Si wafer are depicted in figs. 1 and
Fig. 2. RBS and channeling spectra of As implanted Si irradiated from the back wafer side: (1.3) with Siq, (2,4) without SiO*; (1,2) random, (3,4) aligned. 7”,, = 1490 K.
2. The results obtained from the analysis of these spectra are presented in table 1. Comparison of the values of N, and Nr show that after the annealing at T_ = 1490 K all the As atoms in substituting positions are electrically active. So at this temperature the deactivation mechanism connected with the As atom displacement into the interstices may be considered to be dominating. In this case, after the annealing under the SiO, cap layers, as electrical activity reaches about 90% of the implanted As dose, while at the annealing without SiO,, it decreases to 45% with 55% implanted ions sublimating. The sublimation intensity is considerably decreased by the SiO, film capsulation. Moreover the intensity of sublimation is affected ‘by the radiation direction: the amount of sublimated As atoms is smaller for samples irradiated from the implanted layer side. This effect is more noticeable for the SiO, capsulated samples. This can be explained by the formation of a thin film of
Table 1 Data of RBS and channeling spectra and Van der Pauw measurements of As implanted Si (T,,, =1490 K). N, and N, are the random and aligned area density of As atoms, respectively; f = (1- x,&/(1 - xmin) is the fraction of As atoms on lattice sites; Nf = N,f is the density of As atoms on lattice sites; NsUblis the sublimated density of As atoms; N, is the sheet carrier concentration. Irradiation conditions With SiO,
front to beam back to beam
f
N, ’ [x10’5cm-2]
N,
9.4 8.1
1.3 1.0
0.89 0.91
4.1 3.9
0.1 0.4
0.86 0.92
[*X 1015cm-‘]
N sub1
N,
[X10’5cm-2]
[X10’5cm-2]
8.8 7.6
0.6 1.9
8.4 9.1
3.7 3.6
5.9 6.1
4.5 4.9
Nf [X
101’ cme2]
’
Without SiO,
front to beam back to beam
R. Grb’tzschel et al. / Sublimation and diffusion of As in Si
polymerized oil from the residual atmosphere on the surface irradiated by the electron beam. Perhaps this film covers the defects in the SiO, cap layer formed due to the As+ implantation, thus resulting in As loss minimization from 19 to 6% of the implanted As dose. The diffusion of As is also dependent on the conditions on the implanted layer surface. At the presence of the SiO, film at T,, = 1490 K, an enhanced As diffusion in the Si bulk is observed, the diffusion coefficient being g= 3.6 X lo-” cm2/s. This 9 value is within the range of 9 = (2.4-12) X lo-l2 cm2/s, determined in experiments on the photon annealing of similar samples [S]. At the same time the diffusion of As into the bulk is much weaker at the annealing of uncapsulated samples (g= 1.3 X lOpI2 cm2/s).
4. Discussion There are some different models [6-81 and their modifications explaining the mechanism of the enhanced impurity diffusion at RTA. The results we have obtained by RTA are in favour of the interstitial mechanism of diffusion, whereby substitutional dopant atoms are displaced from site to site by passing interstitials [8] (relay-race mechanism). The spectra analysis of RBS and channeling of He+ ions show that in Si(As+) samples, being in a thermodynamical equilibrium state, the ratio of the As concentration in the interstitial state to that in the substituting state (at xmin < 0.04) is within the range of 0.01-0.1. The deactivation of the impurity in the samples implanted with a large dose of As+ results in the appearance of a non-equilibrium As excess in the interstices, which has a tendency of being “dispersed”. If sublimation is possible, the above-mentioned ratio at the surface is sharply diminished and there occurs a replenishment of the near-surface region by the interstitial atoms using the relay-race-mechanism. As a result
515
the flux of excessive interstitial As atoms moves toward the surface. If the surface is capsulated this ratio is not diminished there by the sublimation and, with the As reactivation at the surface being excluded due to the As active concentration relaxed to the equilibrium value, it is thermodynamically more efficient for nonequilibrium interstitial As atoms appearing at deactivation to diffuse into the Si bulk. It is in this case that we observe the enhanced As diffusion with the diffusivity exceeding many times the equilibrium value (defined by a vacancy mechanism) for these temperatures. The necessity of the realization of the above-mentioned ratio results in the interstitial As atoms reactivating in the Si bulk. This process increases the As electrical activity in the SiO, capsulated samples. Thus the peculiarities in the behaviour of As atoms in Si(As’) at the PA and EBA in similar regimes are defined by different conditions, realized on the implanted layer surface, as far as at the photon annealing, even when the samples are not previously capsulated, a SiO, film of - 10 nm is formed on the Si surface, which prevents As sublimation.
References
Ill A. Lietoila, J. Gibbons
and T.W. Sigmon, Appl. Phys. Lett. 36 (1980) 765. B.P. Kashnikov and A.I. PI V.P. Popov, A.V. Dvurechenskii, Popov, Phys. Status Solidi A94 (1986) 569. I31 A. Nylandsted, 0. Larsen and V.E. Borisenko, Appl. Phys. A33 (1984) 51. [41 V.S. Budishevsky, R. Gr8tzschel. V.A. Kagadei, N.I. Lebedeva, D.I. Proskurovsky and E.B. Yankelevich, Phys. Res. 8 (1988) 262. 151 J. Narayan and O.W. Holland, J. Appl. Phys. 56 (1984) 2913. 161 M. Servidori and S. Solmi, J. Appl. Phys. 65 (1989) 98. [71 R. Kalish, T.O. Sedgwick and S. Maden, Appl. Phys. Lett. 44 (1984) 107. PI S.J. Pennycock, J. Narayan and O.W. Holland, J. Electrothem. Sot. 132 (1985) 1962.
VI. MATERIALS
SCIENCE