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permeation from bubbles in silicon: A novel method of void production
Nuclear Instruments and Methods in Physics Research B27 (1987) 417-420 North-Holland, Amsterdam
417
Letter to the Editor HELIUM DESORPTION/PERMEATION FROM BUBBLES A NOVEL METHOD OF VOID PRODUCTION C.C. GRIFFIOEN,
IN SILICON:
J.H. EVANS *, P.C. DE JONG and A. VAN VEEN
Interuniversity Reactor Institute and De& University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands
Received 10 December 1986 and in revised form 5 March 1987
The annealing behaviour of helium bubbles formed by helium implantation into silicon has been studied using both helium desorption spectroscopy and transmission electron microscopy. The combination of these techniques has demonstrated that helium can permeate out of bubbles in silicon during annealing to leave behind empty cavities.
In contrast to the relatively large literature on the effects of implanting the heavier inert gas atoms into silicon, very few studies have involved helium implants. Elliman et al. [l] have reported the formation of helium bubbles in silicon after 80 keV helium ion irradiation while Paszti et al. [2] describe flaking effects induced by 1 MeV helium implants. These results appear to be similar to those generally observed for helium implants into metals. However, this may not hold for all aspects of helium behaviour. The main purpose of this letter is to report an interesting effect seen during the annealing of helium bubbles in silicon where results very different to those in metals have been found. The behaviour of interest concerns the stability of helium entrapped within bubbles: the weight of evidence from previous studies in metals is that helium dissociation from bubbles is effectively negligible and that bubble coarsening processes are dominated by the thermal migration of bubbles and subsequent coalescence [3]. For gas implants leading to near-surface bubble formation, release of helium occurs when bubbles meet the free surface. In the present work using thermal helium desorption spectroscopy (THDS) to measure the release of helium during thermal annealing, together with transmission electron microscopy (TEM), we show that the expected helium release processes are pre-empted in silicon by the fact that helium can unexpectedly permeate out of bubbles to leave behind empty cavities (voids). However, the observed first order helium release mechanism can be described in terms of pre-exponentials and activation enthalpies which fit closely the permeation results of Van Wieringen and Warmoltz obtained in 1956 [4]. The TEM studies were made on 3 mm diameter silicon samples prethinned by argon-ion milling and * Materials Development OX11 ORA.
Division,
Harwell
Laboratory,
Oxon.
UK.
0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
subsequently implanted at room temperature with a 10 keV mass-selected helium ion beam up to a fluence of 2 x 10” cm-‘. THDS measurements were made on similar samples implanted with helium but also on a larger silicon (111) crystal. Results from both techniques are included in fig. 1. As-implanted specimens always showed a very high density of small helium bubbles with diameters up to 3.5 nm, fig. la. typical of helium implants into metals. The response of the bubbles to thermal annealing was followed in several samples by utilising an electron microscope hot stage specimen holder allowing heating up to 1275 K. Up to about 1000 K the bubble population was always rather stable but around this temperature and above, cavity migration was observed together with coalescence events and the loss of cavities to the surface. These effects are seen in figs. lb-d where the same area in one sample was observed after step-wise anneals to 1100. 1155 and 1245 K respectively, with an average heating rate of approximately 0.05 K/s. The thermal desorption spectrum of helium from a sample similar to the TEM samples, but obtained with a heating rate of 2 K/s, is given in fig. le. Helium release occurs partly at low temperature from 600 to 900 K, where annealing has been shown to induce blister effects [5], and finally in a high temperature regime centred at about 1200 K. At first sight the combined results of TEM and THDS appear similar to those expected from typical bubble annealing in metals. However, when the difference in heating rates is taken into account, a very different picture emerges. The necessary adjustment (utilising more detailed THDS measurements on the desorption kinetics to be described below) is given as the TEM temperature scale in fig. le. On this basis. the results show that just above 1000 K in the TEM sample anneals, the helium will have permeated out of the bubbles to the specimen surface. The observed cavity migration and subsequent coalescence events were thus
418
C. C. Griffioen et al. / Helium bubbles in silicon and void production
TEM TEMPERATURE 6
8
6~10’~
10
12
/c, I
I
I
( 1OOK)
I
1
4
0
4
8
12
THDS TEMPERATURE
16
(1 OOK)
Fig. 1. Electron micrographs of cavities in silicon: (a) a high density of helium bubbles in a sample implanted with 2 x 10” cmd2 10 keV He ions; (b) to (d) after annealing at different temperatures to illustrate the migration and coalescence of cavities. In (e), the helium desorption spectrum (heating rate 2 K/s) taken on a similar sample is shown. The upper temperature scale is appropriate to the different heating rate in the TEM results. As marked by the arrow, all the helium is removed by just above 1000 K; the micrographs (b) to (d) thus refer to empty cavities.
taking place with empty cavities (or voids) rather than gas-filled bubbles. This picture was carefully checked by THDS measurements on the thicker Si(ll1) sample where lower helium doses were used together with heating rate variations. A desorption spectrum after an implant of 2 X 1Ol6 cm-2 2.5 keV helium ions, obtained at a heating rate of 10 K/s linear heating, is shown in curve (a) of fig. 2. The final anneal stage is centred between 1100 and 1200 K, with all the helium being released by 1300 K. To confirm that the sample still contained cavities at this point, additional helium implants and thermal release spectra were then made, using a small helium probe dose of 5 X 1013 cm- ’ of 2.5 keV ions. This probe dose of helium was sufficient to partially refill the empty
cavities, though trapping in the Si matrix also took place. The helium desorption spectrum for this situation (heating rate 5 K/s) is shown in curve (b) of fig. 2. The low temperature helium release is due to helium detrapping from defects created by the probe dose itself but of importance here is the clear evidence of a helium peak at 1150 K corresponding to desorption from the cavities still present in the sample. The experiment with the helium probe dose implants could be repeated many times. However, after higher temperature anneals into the regime where TEM observations had showed cavity coalescence and loss to the surface, the cavity peak started to disappear: a 10 min anneal at 1300 K reduced the peak content to 50%. This result taken together with the complete disap-
C. C. Griffioen et al. / Helium bubbles in silicon and void production
less than might be expected for a thermally activated process in the 1100 to 1300 K range. For example, for first order helium desorption from molybdenum, the pre-exponential would be about 1 X 1Ol3 s-l (i.e. the Debye frequency of the solid) and the activation energy about 3.5 eV [6]. Nevertheless, it emerged that the release rate found can be described rather well by a model based on the permeation of helium from bubbles to the sample surface. The helium release rate can be written
Sit1 11)
I)
dN/dt
dN/dt
:I 8
4 CRYSTAL
TEMPERATURE
12
16
(lOOK)
Fig. 2. Helium desorption spectra (heating rates 10 K/s in (a) and 5 K/s in (b)) after implantation of silicon with helium: (a) high dose implant, 2.5 keV He+ to a dose of 2 X 11)‘~ cmm2; (b) as for (a) but followed by an anneal to 1350 K to remove all helium and then implanted with a probe dose of 5 X lOI cme2 2.5 keV He+ ions. The shaded area is due to the desorption of this helium from cavities. Curve (c) is a calculated curve based on a first order helium release mechanism, see text.
pearence of high temperature helium release after krypton sputtering the surface layer containing the cavities, is strong additional evidence for the proposed model of helium desorption from cavities in silicon. An analysis of the THDS data was made by using peak fitting and the effect of heating rate variations. Both methods indicated a first order helium release mechanism given by dN/dt
= -N(3
+ 1) x lo6
Xexp[ -(1.70
f O.O5)/kT]
(s-l),
= - (3NP/Rr)
exp( -AH/kT)
(s-l),
(2)
where R is the average distance of the bubbles to the surface, r the bubble radius, P the permeation rate factor (pressure dependent), and AH is the activation enthalpy for permeation, i.e. the sum of the migration enthalpy and the heat of solution of helium. The 30 year old permeation data of Van Wieringen and Warmoltz [4] give values of AH = 1.7 eV and P = 2.6 X 10m6 cm*/s (pressure = 1 bar). Making appropriate corrections for changes in pressure, and substituting the value R = 30 nm for the average helium range for 2.5 keV helium ions [7] with r = 3 nm (this work), give a release rate
1)
0
419
(1)
where N is the number of helium atoms in the cavities. Curve (c) in fig. 2 has been calculated using eq. (1) and clearly corresponds to the cavity release peak curve (b). It is worth commenting that the activation enthalpy AH = 1.70 eV and the pre-exponential factor is rather
= -N
8.7
X
lo6 exp( -1.7eV/kT)
(s-l),
(3)
almost identical to eq. (1). A more detailed description and discussion of the model will be given elsewhere. Adopting first order desorption kinetics for helium release from bubbles, it is in principle a straightforward procedure to calculate adjustments of the temperature scale when different heating rates (p = dT/dt) have been used. The temperatures at which the release has evolved to the same stage in two experiments with different heating rates & and & respectively are found from f(r,)/f(T~)
= &/Pl.
(4)
where f( T,,,)
= i71”exp(
-A
H/kT)
dT.
The heating rate in the TEM epxeriment has been - 40 times lower than in the THDS experiments. Evaluation of 4 with AH = 1.70 eV led to the TEM temperature scale indicated in fig le. It should be noted that in the case of desorption from the tiny TEM specimen mounted on a Si substrate the temperature definition has not been as accurate as in the THDS measurements on a thick Si crystal. The temperature might have been lower than indicated (- 50 K at 1200 K) due to imperfect heat conduction from the substrate to TEM specimen. To summarise, we have established that at high temperature helium can be desorbed from helium bubbles in silicon, via a relatively low energy activated process, to leave empty cavities or voids. This mode of behaviour has not been demonstrated previously for
420
C. C. Griffioen et al. / Helium bubbles in silicon and void production
bubbles in other materials where the heats of solution or formation energy of helium are always found to be high [6]. The fact that voids can be produced relatively easily in silicon by this method makes the result important if studies on the thermodynamical behaviour of voids in silicon are required. In addition, the effect of helium pressure on permeation can be studied in detail. The experiments reported need not be restricted to helium. Preliminary measurements with hydrogen ions revealed that stable cavities can be made by high fluence 3 keV Hl-ion bombardment. The presence of the cavities could again be detected by probing with helium. This finding ties in with recent results reported by Collins et al. [8] for hydrogen irradiated silicon and by Slaoui et al. [9] on laser introduced defects. Their work indicates the presence of defects, presumably cavities, which are stable to temperatures higher than 675 K. We are indebted to K.R. Bijkerk and H.A. Filius for the THDS measurements on the TEM specimens; J. de Roode, T. Heyenga and C.D. de Haan (Materials Department, Delft University) for technical assistance. This work was supported by the EC stimulation programme
CODEST XII, contract STI-075J-C(CD), and the underlying research programme of the UKAEA.
References [l] R.G. Elliman, ST. Johnson, K.T. Short and J.S. Williams, Mater. Res. Sot. Symp. Proc. 27 (1983) 229. [2] F. Pa&i, Cs. Hajdu, A. Manuaba, N.T.My.E. Kotai, L. Pogany, G. Mezey, M. Fried, Gy. Virkelethy and J. Gyulai, Nucl. Instr. and Meth. B7/8 (1985) 371. [3] P.J. Goodhew and S.K. Tyler, Proc. R. Sot. (London) A377 (1981) 151. [4] A. van Wieringen and N. Warmoltz, Physica 22 (1956) 849. [5] C.C. Griffioen, unpublished results. [6] A. van Veen, J.H. Evans, W.Th.M. Buters and L.M. Caspers, Radiat. Eff. 78 (1983) 53. [7] H. Gnaser, H.L. Bay and W.O. Hofer, Nucl. Instr. and Meth. B15 (1986) 49. [8] R.W. Collins, B.G. Yacobi, K.M. Jones and Y.S. Tsuo, J. Vat. Sci. Technol. A4 (1986) 153. [9] A. Slaoui, A. Barhdadi, J.C. Muller and P. Siffert, Appl. Phys. A39 (1986) 159.
417
Letter to the Editor HELIUM DESORPTION/PERMEATION FROM BUBBLES A NOVEL METHOD OF VOID PRODUCTION C.C. GRIFFIOEN,
IN SILICON:
J.H. EVANS *, P.C. DE JONG and A. VAN VEEN
Interuniversity Reactor Institute and De& University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands
Received 10 December 1986 and in revised form 5 March 1987
The annealing behaviour of helium bubbles formed by helium implantation into silicon has been studied using both helium desorption spectroscopy and transmission electron microscopy. The combination of these techniques has demonstrated that helium can permeate out of bubbles in silicon during annealing to leave behind empty cavities.
In contrast to the relatively large literature on the effects of implanting the heavier inert gas atoms into silicon, very few studies have involved helium implants. Elliman et al. [l] have reported the formation of helium bubbles in silicon after 80 keV helium ion irradiation while Paszti et al. [2] describe flaking effects induced by 1 MeV helium implants. These results appear to be similar to those generally observed for helium implants into metals. However, this may not hold for all aspects of helium behaviour. The main purpose of this letter is to report an interesting effect seen during the annealing of helium bubbles in silicon where results very different to those in metals have been found. The behaviour of interest concerns the stability of helium entrapped within bubbles: the weight of evidence from previous studies in metals is that helium dissociation from bubbles is effectively negligible and that bubble coarsening processes are dominated by the thermal migration of bubbles and subsequent coalescence [3]. For gas implants leading to near-surface bubble formation, release of helium occurs when bubbles meet the free surface. In the present work using thermal helium desorption spectroscopy (THDS) to measure the release of helium during thermal annealing, together with transmission electron microscopy (TEM), we show that the expected helium release processes are pre-empted in silicon by the fact that helium can unexpectedly permeate out of bubbles to leave behind empty cavities (voids). However, the observed first order helium release mechanism can be described in terms of pre-exponentials and activation enthalpies which fit closely the permeation results of Van Wieringen and Warmoltz obtained in 1956 [4]. The TEM studies were made on 3 mm diameter silicon samples prethinned by argon-ion milling and * Materials Development OX11 ORA.
Division,
Harwell
Laboratory,
Oxon.
UK.
0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
subsequently implanted at room temperature with a 10 keV mass-selected helium ion beam up to a fluence of 2 x 10” cm-‘. THDS measurements were made on similar samples implanted with helium but also on a larger silicon (111) crystal. Results from both techniques are included in fig. 1. As-implanted specimens always showed a very high density of small helium bubbles with diameters up to 3.5 nm, fig. la. typical of helium implants into metals. The response of the bubbles to thermal annealing was followed in several samples by utilising an electron microscope hot stage specimen holder allowing heating up to 1275 K. Up to about 1000 K the bubble population was always rather stable but around this temperature and above, cavity migration was observed together with coalescence events and the loss of cavities to the surface. These effects are seen in figs. lb-d where the same area in one sample was observed after step-wise anneals to 1100. 1155 and 1245 K respectively, with an average heating rate of approximately 0.05 K/s. The thermal desorption spectrum of helium from a sample similar to the TEM samples, but obtained with a heating rate of 2 K/s, is given in fig. le. Helium release occurs partly at low temperature from 600 to 900 K, where annealing has been shown to induce blister effects [5], and finally in a high temperature regime centred at about 1200 K. At first sight the combined results of TEM and THDS appear similar to those expected from typical bubble annealing in metals. However, when the difference in heating rates is taken into account, a very different picture emerges. The necessary adjustment (utilising more detailed THDS measurements on the desorption kinetics to be described below) is given as the TEM temperature scale in fig. le. On this basis. the results show that just above 1000 K in the TEM sample anneals, the helium will have permeated out of the bubbles to the specimen surface. The observed cavity migration and subsequent coalescence events were thus
418
C. C. Griffioen et al. / Helium bubbles in silicon and void production
TEM TEMPERATURE 6
8
6~10’~
10
12
/c, I
I
I
( 1OOK)
I
1
4
0
4
8
12
THDS TEMPERATURE
16
(1 OOK)
Fig. 1. Electron micrographs of cavities in silicon: (a) a high density of helium bubbles in a sample implanted with 2 x 10” cmd2 10 keV He ions; (b) to (d) after annealing at different temperatures to illustrate the migration and coalescence of cavities. In (e), the helium desorption spectrum (heating rate 2 K/s) taken on a similar sample is shown. The upper temperature scale is appropriate to the different heating rate in the TEM results. As marked by the arrow, all the helium is removed by just above 1000 K; the micrographs (b) to (d) thus refer to empty cavities.
taking place with empty cavities (or voids) rather than gas-filled bubbles. This picture was carefully checked by THDS measurements on the thicker Si(ll1) sample where lower helium doses were used together with heating rate variations. A desorption spectrum after an implant of 2 X 1Ol6 cm-2 2.5 keV helium ions, obtained at a heating rate of 10 K/s linear heating, is shown in curve (a) of fig. 2. The final anneal stage is centred between 1100 and 1200 K, with all the helium being released by 1300 K. To confirm that the sample still contained cavities at this point, additional helium implants and thermal release spectra were then made, using a small helium probe dose of 5 X 1013 cm- ’ of 2.5 keV ions. This probe dose of helium was sufficient to partially refill the empty
cavities, though trapping in the Si matrix also took place. The helium desorption spectrum for this situation (heating rate 5 K/s) is shown in curve (b) of fig. 2. The low temperature helium release is due to helium detrapping from defects created by the probe dose itself but of importance here is the clear evidence of a helium peak at 1150 K corresponding to desorption from the cavities still present in the sample. The experiment with the helium probe dose implants could be repeated many times. However, after higher temperature anneals into the regime where TEM observations had showed cavity coalescence and loss to the surface, the cavity peak started to disappear: a 10 min anneal at 1300 K reduced the peak content to 50%. This result taken together with the complete disap-
C. C. Griffioen et al. / Helium bubbles in silicon and void production
less than might be expected for a thermally activated process in the 1100 to 1300 K range. For example, for first order helium desorption from molybdenum, the pre-exponential would be about 1 X 1Ol3 s-l (i.e. the Debye frequency of the solid) and the activation energy about 3.5 eV [6]. Nevertheless, it emerged that the release rate found can be described rather well by a model based on the permeation of helium from bubbles to the sample surface. The helium release rate can be written
Sit1 11)
I)
dN/dt
dN/dt
:I 8
4 CRYSTAL
TEMPERATURE
12
16
(lOOK)
Fig. 2. Helium desorption spectra (heating rates 10 K/s in (a) and 5 K/s in (b)) after implantation of silicon with helium: (a) high dose implant, 2.5 keV He+ to a dose of 2 X 11)‘~ cmm2; (b) as for (a) but followed by an anneal to 1350 K to remove all helium and then implanted with a probe dose of 5 X lOI cme2 2.5 keV He+ ions. The shaded area is due to the desorption of this helium from cavities. Curve (c) is a calculated curve based on a first order helium release mechanism, see text.
pearence of high temperature helium release after krypton sputtering the surface layer containing the cavities, is strong additional evidence for the proposed model of helium desorption from cavities in silicon. An analysis of the THDS data was made by using peak fitting and the effect of heating rate variations. Both methods indicated a first order helium release mechanism given by dN/dt
= -N(3
+ 1) x lo6
Xexp[ -(1.70
f O.O5)/kT]
(s-l),
= - (3NP/Rr)
exp( -AH/kT)
(s-l),
(2)
where R is the average distance of the bubbles to the surface, r the bubble radius, P the permeation rate factor (pressure dependent), and AH is the activation enthalpy for permeation, i.e. the sum of the migration enthalpy and the heat of solution of helium. The 30 year old permeation data of Van Wieringen and Warmoltz [4] give values of AH = 1.7 eV and P = 2.6 X 10m6 cm*/s (pressure = 1 bar). Making appropriate corrections for changes in pressure, and substituting the value R = 30 nm for the average helium range for 2.5 keV helium ions [7] with r = 3 nm (this work), give a release rate
1)
0
419
(1)
where N is the number of helium atoms in the cavities. Curve (c) in fig. 2 has been calculated using eq. (1) and clearly corresponds to the cavity release peak curve (b). It is worth commenting that the activation enthalpy AH = 1.70 eV and the pre-exponential factor is rather
= -N
8.7
X
lo6 exp( -1.7eV/kT)
(s-l),
(3)
almost identical to eq. (1). A more detailed description and discussion of the model will be given elsewhere. Adopting first order desorption kinetics for helium release from bubbles, it is in principle a straightforward procedure to calculate adjustments of the temperature scale when different heating rates (p = dT/dt) have been used. The temperatures at which the release has evolved to the same stage in two experiments with different heating rates & and & respectively are found from f(r,)/f(T~)
= &/Pl.
(4)
where f( T,,,)
= i71”exp(
-A
H/kT)
dT.
The heating rate in the TEM epxeriment has been - 40 times lower than in the THDS experiments. Evaluation of 4 with AH = 1.70 eV led to the TEM temperature scale indicated in fig le. It should be noted that in the case of desorption from the tiny TEM specimen mounted on a Si substrate the temperature definition has not been as accurate as in the THDS measurements on a thick Si crystal. The temperature might have been lower than indicated (- 50 K at 1200 K) due to imperfect heat conduction from the substrate to TEM specimen. To summarise, we have established that at high temperature helium can be desorbed from helium bubbles in silicon, via a relatively low energy activated process, to leave empty cavities or voids. This mode of behaviour has not been demonstrated previously for
420
C. C. Griffioen et al. / Helium bubbles in silicon and void production
bubbles in other materials where the heats of solution or formation energy of helium are always found to be high [6]. The fact that voids can be produced relatively easily in silicon by this method makes the result important if studies on the thermodynamical behaviour of voids in silicon are required. In addition, the effect of helium pressure on permeation can be studied in detail. The experiments reported need not be restricted to helium. Preliminary measurements with hydrogen ions revealed that stable cavities can be made by high fluence 3 keV Hl-ion bombardment. The presence of the cavities could again be detected by probing with helium. This finding ties in with recent results reported by Collins et al. [8] for hydrogen irradiated silicon and by Slaoui et al. [9] on laser introduced defects. Their work indicates the presence of defects, presumably cavities, which are stable to temperatures higher than 675 K. We are indebted to K.R. Bijkerk and H.A. Filius for the THDS measurements on the TEM specimens; J. de Roode, T. Heyenga and C.D. de Haan (Materials Department, Delft University) for technical assistance. This work was supported by the EC stimulation programme
CODEST XII, contract STI-075J-C(CD), and the underlying research programme of the UKAEA.
References [l] R.G. Elliman, ST. Johnson, K.T. Short and J.S. Williams, Mater. Res. Sot. Symp. Proc. 27 (1983) 229. [2] F. Pa&i, Cs. Hajdu, A. Manuaba, N.T.My.E. Kotai, L. Pogany, G. Mezey, M. Fried, Gy. Virkelethy and J. Gyulai, Nucl. Instr. and Meth. B7/8 (1985) 371. [3] P.J. Goodhew and S.K. Tyler, Proc. R. Sot. (London) A377 (1981) 151. [4] A. van Wieringen and N. Warmoltz, Physica 22 (1956) 849. [5] C.C. Griffioen, unpublished results. [6] A. van Veen, J.H. Evans, W.Th.M. Buters and L.M. Caspers, Radiat. Eff. 78 (1983) 53. [7] H. Gnaser, H.L. Bay and W.O. Hofer, Nucl. Instr. and Meth. B15 (1986) 49. [8] R.W. Collins, B.G. Yacobi, K.M. Jones and Y.S. Tsuo, J. Vat. Sci. Technol. A4 (1986) 153. [9] A. Slaoui, A. Barhdadi, J.C. Muller and P. Siffert, Appl. Phys. A39 (1986) 159.