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Clustering of ultra-low-energy implanted boron in silicon during activation annealing
Materials Science and Engineering B71 (2000) 219 – 223 www.elsevier.com/locate/mseb
Clustering of ultra-low-energy implanted boron in silicon during activation annealing E. Schroer a,*, V. Privitera a, F. Priolo b, E. Napolitani c, A. Carnera c, S. Moffatt d a CNR-IMETEM, Stradale Primosole 50, 95121 Catania, Italy INFM and Dipartimento di Fisica, Uni6ersita` di Catania, Corso Italia 57, 95129 Catania, Italy c INFM and Dipartimento di Fisica, Uni6ersita´ di Pado6a, Via Marzolo 8, 35131 Pado6a, Italy d Applied Materials, 2727 Augustine Dri6e, Santa Clara, CA 95054, USA
b
Abstract The clustering kinetics and the electrical activation of boron during the post-implantation activation annealing of ultra-low-energy implanted boron ( B1 keV) in silicon has been investigated. By analyzing the boron concentration profiles obtained by means of secondary ion mass spectroscopy (SIMS) non-diffusing boron clusters dissolving with a temperature dependent time constant have been found which exhibit a thermal activation energy of 2.3 eV. The formation of the boron clusters shows at 900°C an incorporation efficiency of boron atoms into clusters of approximately 1. The incorporation efficiency is decreasing with increasing temperature and shows an activation energy of 0.9 eV. The depth profiles of the active boron concentration as measured by spreading resistance profiling was analyzed. The comparison of electrical activation and boron clustering shows that the boron clusters are determining the electrical activation. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Activation annealing; Boron
1. Introduction The ever decreasing sizes of CMOS devices makes it necessary to reduce the scaling of the source/drain junction not only in vertical also in lateral direction in order to avoid short channel effects [1]. For a gate length of 70 nm a junction depth of less than 30 nm is projected [2]. Doped layers are produced nowadays nearly exclusively by ion implantation and a subsequent thermal treatment in order to electrically activate the dopants. The reduction of junction depth is achieved, on the one hand, by reducing the ion energy used for the implantation below energies of 1 keV [1] and, on the other hand, by optimizing the thermal budged during activation annealing [3]. The junction depth reduction is in particular a challenge for boron which is the nearly exclusively used p-type dopant. The mechanism responsible for the profile broadening of dopants implanted with energies larger than 5
* Corresponding author. Tel.: + 39-95-591-212; fax: + 39-95-7139154. E-mail address: [email protected] (E. Schroer)
keV is quite well understood [4]. This is in particular true for the transient enhanced diffusion (TED) which has been correlated with the evolution of {311} self-interstitial-type defects [4,5]. However, the mechanism leading to TED of implanted boron with ultra-low-energies (B 1 keV) is still under investigation [6–9]. Since no extended defects are observed, the determination of the amount of excess self-interstitials introduced by the implantation is not as straightforward as in the high energy case [8]. Additionally, the capacity of the surface in absorbing self-interstitial atoms is presently not well understood. Beside the amount of self-interstitials, the amount of boron in substitutional sites determines the profile broadening. It has been reported that after ultra-lowenergy implantation a considerable amount of boron is located in an immobile clustered configuration [6]. Here it is reported on the analysis of the formation and dissolution kinetics of the clustered boron. Furthermore, the analysis of the electrical activation shows that the clustered boron is electrically inactive and that the release of the boron from the clusters plays a significant role for the electrical activation of the boron.
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2. Experimental A p-type (100) epitaxially grown silicon layer with a resistivity of 10 Vm has been used to implant boron (%%B) with an energy of 500 eV and a dose of 1014 or 1015 cm − 2. Implantation was performed with an Applied Materials xR LEAP™ [10] implanter at the Applied Materials Implant Division in Horsham (UK). The
Fig. 3. (a) To determine the boron incorporation efficiency the boron concentration after the shortest annealing cycle is plotted against the as-implanted boron concentration. (b) Arrhenius plot of the incorporation efficiency.
Fig. 1. Secondary ion mass spectroscopy (SIMS) profiles of ultra-lowenergy (0.5 keV) implanted boron before and after annealing at 900°C. In (a) the implantation dose is 1 × 1015 cm − 2 and in (b) 1 × 1014 cm − 2.
Fig. 2. Analysis of the release of boron from clusters in the temperature range from 900 to 1200°C. The release time constant shows an activation energy of 2.3 eV.
Fig. 4. Configuration energy diagram for the incorporation and the release of boron atoms from the clusters.
native oxide was stripped by HF dipping immediately before the implantation. Rapid thermal annealing was used to anneal the samples in the temperature range between 900 and 1200°C for times ranging from 1 to 600 s in a nitrogen atmosphere. Chemical concentration profiling of the boron was done by secondary ion mass spectroscopy (SIMS) profiling. A CAMECA IMS-4f with an O+ 2 primary low-energy ion beam of 1.5 keV was used. The low-energy primary beam was used to reduce the depth at which the equilibrium SIMS sensitivity is reached to approximately 5 nm. Details on the SIMS measurement can be found in Ref. [11]. Electrical activity of the boron profiles was measured by the spreading resistance profiling (SRP) technique. A
E. Schroer et al. / Materials Science and Engineering B71 (2000) 219–223
combination of advanced smooth bevel preparation and a probe load of 5 g were used to obtain reliable concentration profiles [12].
3. Boron clustering kinetics In Fig. 1 the SIMS profiles of the boron concentration are shown relative to as implanted and annealed samples at a temperature of 900°C for the indicated durations. The implantation energy is in both cases 500 eV. The implantation dose is: in (a) 1 × 1015 cm − 2 and in (b) 1 × 1014 cm − 2. In (a) the inset shows a magnified plot of the shallowest 10 nm of the high concentration region of the profile. It can be seen (Fig. 1(a)) that a so-called kink and tail profile shape develops. From this profile shape,
Fig. 5. Comparison of the chemical (secondary ion mass spectroscopy, SIMS) and the electrical (spreading resistance profiling, SRP) depth profiles after annealings at 900°C.
Fig. 6. The complementary of the electrical active fraction of boron and the normalized amount of boron in clusters in dependence of annealing time for the indicated temperatures.
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however, it can be concluded that the boron diffusivity is different in the high and in the low concentration region. In fact, the profile in the high concentration region (see inset in (a)) does not show any broadening. Only a reduction of the concentration with prolonged annealing time is visible. From this it can be concluded that the vast amount of boron in this region is not subject to diffusion as expected to be the case for substitutional boron in silicon. From this reasoning it can be concluded further that the vast amount of boron is located in clusters. The reduction of the boron concentration indicates that the boron is released from the clusters with prolonged annealing time. The boron clustering is much less pronounced for an implantation dose of 1× 1014 cm − 2 as can be seen in Fig. 1(b). Two possible reasons for this dose dependence are discussed here; first, in contrast to the lower dose implantation, the higher dose implantation produces an as-implanted boron concentration far above the solubility value (6.8× 1019 cm − 3 at 900°C [13]) for substitutional boron in silicon. Secondly, the damage in the silicon lattice introduced by the boron implantation has to be taken into account. As the implantation damage increases with increasing dose and at the same time also the clustering of boron gets more pronounced, it seems to be reasonable to assume that the implantation damage is another important factor. The development of a local minimum has been reported after annealing at 1100°C and above [15]. The boron concentration of the minimum has been found to be considerably lower than the solubility value of substitutional boron at the corresponding annealing temperature. From this observation it can be concluded that the transition of the boron atoms from the clusters into the substitutional site is the limiting step for the reduction of the concentration of boron atoms in clusters. Moreover, the diffusion of boron away from the clusters does not inhibit the release of boron atoms from the clusters. An analysis of the release kinetics of boron from the clusters has been performed based on the observation that in the peak region nearly all boron is clustered. Further, for this analysis all boron concentrations were taken at a depth of 8 nm. A normalization of the boron concentration after annealing was performed with respect to the boron concentration of the as-implanted sample. The dependence of the normalized boron concentration on the annealing time is shown in Fig. 2(a) in a semi-logarithmic plot for temperatures ranging from 900 to 1200°C. With prolonged annealing time the boron concentration is decreasing and the decrease is faster for higher temperatures. An exponential decay function as shown in Fig. 2(a) was fitted to the experimental data. From this fitting the release time constant trel for the boron from the clusters is derived. The temperature dependence of the release time constant trel
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is shown in Fig. 2(b) in an Arrhenius plot. It can be seen that the release time constant is varying from about 1000 s at 900°C to about 12 s at 1200°C. From the best linear fit in the Arrhenius plot an activation energy of 2.3 eV has been derived for the release time constant. The time constants reported for mechanisms which are important for the profile broadening at higher implantation energies (dissolution time constant of {311} defect, t{311} [4,5] cease time of TED, tTED [14] and dissolution time constant of higher energy implantation induced boron clusters, tBcl [16] are also reported in Fig. 2(b) in order to compare them with the release time constant of the ultra-low-energy implantation induced boron clusters. It can be seen that the time constant of the ultra-low-energy implantation induced boron clusters is longer and that the activation energy is lower than for the other processes. From this comparison it can be concluded that the release process from the ultra-low-energy implantation induced boron clusters is most likely governed by a different mechanism than the other processes. To determine the initial incorporation of boron into the clusters the boron concentration after the shortest annealing cycle for each temperature, in dependence of the boron concentration of the as-implanted sample has been plotted in Fig. 3(a). In this plot care was taken that the boron concentrations of the as-implanted and annealed samples correspond to the same depth. The dependence can be regarded as approximately linear and consequently a linear fit to the data is performed. The slopes obtained from these fits contain information about the amount of boron atoms in clustered configuration per boron atom initially present. As the annealing times shown in the plot are much smaller than the release time constants reported in Fig. 2, the slopes can be interpreted as the initial incorporation efficiencies into the clusters. Fig. 3(b) shows the temperature dependence of the incorporation efficiency in an Arrhenius plot. The incorporation efficiency attains its maximum value of one at a temperature of 900°C and is decreasing with increasing temperature and an activation energy of 0.9 eV. An additional experiment has been performed by pre-forming the boron clusters at 900°C with an incorporation efficiency of about 1 and subsequently annealing at 1200°C for 1 s. The comparison of the thus treated sample with a sample only annealed at 1200°C for 1 s shows that the incorporation efficiency values are comparable. Due to the pre-annealing, the boron has been incorporated into clusters with an efficiency of 1. The additionally clustered boron in the pre-annealed sample (with respect to the only 1200°C annealed sample) is released from the clusters within 1 s. This is much faster than the 12 s which are determined as the
release time constant at 1200°C in Fig. 2. From this comparison, however, it can be deduced that the part which is additionally incorporated into the boron clusters at 900°C (with respect to the 1200°C annealed sample) is bound less tightly to the clusters than the part at 1200°C. From these considerations it can furthermore be concluded that the activation energy of the release time constant, trel, is a mean value. The results so far presented have been merged in the scheme of a configuration energy diagram which is shown in Fig. 4. After implantation boron is incorporated into the clusters which requires an activation energy of 0.9 eV as determined from the incorporation efficiency (see Fig. 3(b)). The clustered configuration, however, is metastable and boron atoms are released from the clusters into substitutional lattice sites with a mean activation energy of 2.3 eV. Unfortunately, from the results reported so far it is not possible to extract a microscopical model for the clustered boron. But from the observation that the boron is bound differently tightly to the clusters it can be concluded that the boron does not occupy a unique site in the clusters.
4. Electrical activation The SRP depth profiles of the electrical active boron are shown in Fig. 5 for the high dose implanted boron and the annealing series performed at 900°C. For comparison the corresponding SIMS profiles are also included into the plot. From this comparison it can be seen that the electrical activity in the high concentration region is low. As already mentioned earlier the vast amount of boron in this region is clustered. Therefore, it can be concluded that the boron attached to clusters is electrically not active. Further, it can seen that the electrical activation attains the solubility concentration (indicated with a dotted line in the figure) at the annealing temperature after 300 s. For all temperatures the maximum measured electrical activation was either at or below the solubility value at the corresponding annealing temperature. This observation is in agreement with the assumption that the generation of an activation higher than the solubility by means of rapid thermal annealing is not achieved. Comparing the junction depth of the SRP profiles and the SIMS profiles in Fig. 5 it can be stated that the electrical junction depth is, for the shorter times, the same as the chemically determined junction depth. Only for the 600 s annealing case the electrical junction depth is slightly shallower than the chemical junction depth. In general, the electrical junction depth corresponds to the chemical junction depth or is slightly shallower. It is important to mention at this point that the SRP measurements of the samples implanted with a dose of 1× 1014 cm − 2 revealed that the maximum concentra-
E. Schroer et al. / Materials Science and Engineering B71 (2000) 219–223
tion of activated boron was about 1× 1019 cm − 3 at 900°C. This is considerably below the solubility concentration of boron. From this observation it can be deduced that implantation doses larger than 1 ×1014 cm − 2 are needed in order to produce high-conductive ultra-shallow junctions. To account for the electrically activated fraction, f, the integrated SRP profile is divided by the integrated SIMS profile. During the integration of the SIMS profile, care was taken that the shallowest 5 nm are not considered because in this region the SIMS signal is transient. The complementary of the electrical active fraction, 1−f, together with the boron clustering data (cf. Fig. 1(a)) are plotted against the annealing time in Fig. 6. The comparison shows that both sets of data have the same trend for annealings up to about 1100°C. In this temperature region both the complementary of the electrically active fractions and the concentration of boron in clusters are decreasing with increasing annealing time and the decrease is faster for higher temperatures. Different trends are observed at a temperature of 1200°C. While for the cluster dissolution the trend is still valid, the electrical active fraction attains approximately a constant value. The out-diffusion of the boron through the native oxide is one of the reasons for this behavior. From this comparison it seems to be reasonable to propose that the release of boron from clusters is an important mechanism for the electrical activation of ultra-low-energy implanted boron. Considering further that the initial incorporation efficiency is decreasing for higher temperatures it seems to be reasonable that the importance of the boron clustering is decreasing with increasing temperature.
5. Conclusion The investigation on the activation annealing of ultralow-energy implanted boron in silicon by means of chemical (SIMS) and electrical (SRP) profiling techniques has been presented. From the SIMS profiles the presence of immobile boron clusters has been deduced. An analysis of the formation and dissolution kinetics of the clusters is performed. The SRP measurements revealed that the boron in clusters is electrically inactive. Comparing the electrical activation and the cluster dissolution it is proposed that the boron clustering is determining the electrical activation.
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Acknowledgements This work was partially supported by the project ENDEASD funded by the European Union under the contract number FMRX-CT98-0208 and by the project 5% Microelettronica of Consiglio Nazionale delle Ricerche (CNR). References [1] E.C. Jones, E. Ishida, Mater. Sci. Eng. R24 (1998) 1. [2] National Technology Roadmap for Semiconductors, Semiconductor Industry Association, 1997 Edition, San Jose, CA. [3] A. Agarwal, A.T. Fiory, H.J.L. Gossmann, C.S. Rafferty, P. Frisella, Mater. Sci. Semicond. Process. 1 (1998) 237. [4] D.J. Eaglesham, P.A. Stolk, H.J. Gossmann, J.M. Poate, Appl. Phys. Lett. 65 (1994) 18. [5] P.A. Stolk, H.J. Gossmann, D.J. Eaglesham, D.C. Rafferty, G.H. Gilmer, M. Jaraiz, J.M. Poate, H.S. Luftman, T.E. Haynes, J. Appl. Phys. 81 (1997) 6031. [6] N.E.B. Cowern, E.J.H. Collart, J. Politiek, P.H.I. Bancken, J.G.M. van Berkum, K. Kyllesbech Larsen, P.A. Stolk, H.G.A. Huizing, P. Pichler, A. Burenkov, D.J. Gravesteijn, Mater. Res. Soc. Symp. Proc. 469 (1997) 265. [7] E. Napolitani, A. Camera, E. Schroer, V. Privitera, F. Priolo, S. Moffatt, Appl. Phys. Lett. (submitted). [8] A. Agarwal, H.-J. Gossmann, D.J. Eaglesham, L. Pelaz, S.B. Herner, D.C. Jacobson, T.E. Haynes, R. Simonton, Mater. Sci. Semicond. Process. 1 (1998) 17. [9] V. Privitera, E. Napolitani, F. Priolo, S. Moffatt, A. La Magna, G. Mannino, A. Camera, A. Picariello, Mater. Sci. Semicond. Process. 2 (1999) 35. [10] C. Lowrie, J. England, A. Hunter, D. Burgin, B. Harrison, In: E. Ishida, S. Banerjee, S. Mehta, T.C. Smith, M. Current, L. Larson, A. Tasch (Eds.), Proceedings of Ion Implantation Technology 1996, Austin, TX, June 16 – 21, 1996 edition, IEEE, New York, 1997, p. 447. [11] E. Napolitani, A. Carnera, R. Storti, V. Privitera, F. Priolo, G. Mannino, and S. Moffat, Fifth International Workshop on the Measurement, Characterisation and Modeling of UltraShallow doping Profiles in Semiconductors, Research Triangle Park, NC, 1999, Proceedings Vol. [12] W.L. Harrington, C.W. Magee, M. Pawlik, D.F. Downey, C.M. Osburn, S. Felch, J. Vac. Sci. Technol. B16 (1998) 296. [13] A. Armigliato, D. Nobili, P. Ostoja, M. Servidori, S. Solmi, in: H.R. Huff, E. Sirtl (Eds.), Semiconductor Silicon, Electrochemistry Society, Princeton, NJ, 1977, p. 638. [14] S. Solmi, F. Baruffaldi, R. Canteri, J. Appl. Phys. 69 (1991) 2135. [15] E. Schroer, V. Privitera, F. Priolo, E. Napolitani, A. Camera, Appl. Phys. Lett. (accepted). [16] M. Uematsu, J. Appl. Phys. 84 (1998) 4781.
Clustering of ultra-low-energy implanted boron in silicon during activation annealing E. Schroer a,*, V. Privitera a, F. Priolo b, E. Napolitani c, A. Carnera c, S. Moffatt d a CNR-IMETEM, Stradale Primosole 50, 95121 Catania, Italy INFM and Dipartimento di Fisica, Uni6ersita` di Catania, Corso Italia 57, 95129 Catania, Italy c INFM and Dipartimento di Fisica, Uni6ersita´ di Pado6a, Via Marzolo 8, 35131 Pado6a, Italy d Applied Materials, 2727 Augustine Dri6e, Santa Clara, CA 95054, USA
b
Abstract The clustering kinetics and the electrical activation of boron during the post-implantation activation annealing of ultra-low-energy implanted boron ( B1 keV) in silicon has been investigated. By analyzing the boron concentration profiles obtained by means of secondary ion mass spectroscopy (SIMS) non-diffusing boron clusters dissolving with a temperature dependent time constant have been found which exhibit a thermal activation energy of 2.3 eV. The formation of the boron clusters shows at 900°C an incorporation efficiency of boron atoms into clusters of approximately 1. The incorporation efficiency is decreasing with increasing temperature and shows an activation energy of 0.9 eV. The depth profiles of the active boron concentration as measured by spreading resistance profiling was analyzed. The comparison of electrical activation and boron clustering shows that the boron clusters are determining the electrical activation. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Activation annealing; Boron
1. Introduction The ever decreasing sizes of CMOS devices makes it necessary to reduce the scaling of the source/drain junction not only in vertical also in lateral direction in order to avoid short channel effects [1]. For a gate length of 70 nm a junction depth of less than 30 nm is projected [2]. Doped layers are produced nowadays nearly exclusively by ion implantation and a subsequent thermal treatment in order to electrically activate the dopants. The reduction of junction depth is achieved, on the one hand, by reducing the ion energy used for the implantation below energies of 1 keV [1] and, on the other hand, by optimizing the thermal budged during activation annealing [3]. The junction depth reduction is in particular a challenge for boron which is the nearly exclusively used p-type dopant. The mechanism responsible for the profile broadening of dopants implanted with energies larger than 5
* Corresponding author. Tel.: + 39-95-591-212; fax: + 39-95-7139154. E-mail address: [email protected] (E. Schroer)
keV is quite well understood [4]. This is in particular true for the transient enhanced diffusion (TED) which has been correlated with the evolution of {311} self-interstitial-type defects [4,5]. However, the mechanism leading to TED of implanted boron with ultra-low-energies (B 1 keV) is still under investigation [6–9]. Since no extended defects are observed, the determination of the amount of excess self-interstitials introduced by the implantation is not as straightforward as in the high energy case [8]. Additionally, the capacity of the surface in absorbing self-interstitial atoms is presently not well understood. Beside the amount of self-interstitials, the amount of boron in substitutional sites determines the profile broadening. It has been reported that after ultra-lowenergy implantation a considerable amount of boron is located in an immobile clustered configuration [6]. Here it is reported on the analysis of the formation and dissolution kinetics of the clustered boron. Furthermore, the analysis of the electrical activation shows that the clustered boron is electrically inactive and that the release of the boron from the clusters plays a significant role for the electrical activation of the boron.
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2. Experimental A p-type (100) epitaxially grown silicon layer with a resistivity of 10 Vm has been used to implant boron (%%B) with an energy of 500 eV and a dose of 1014 or 1015 cm − 2. Implantation was performed with an Applied Materials xR LEAP™ [10] implanter at the Applied Materials Implant Division in Horsham (UK). The
Fig. 3. (a) To determine the boron incorporation efficiency the boron concentration after the shortest annealing cycle is plotted against the as-implanted boron concentration. (b) Arrhenius plot of the incorporation efficiency.
Fig. 1. Secondary ion mass spectroscopy (SIMS) profiles of ultra-lowenergy (0.5 keV) implanted boron before and after annealing at 900°C. In (a) the implantation dose is 1 × 1015 cm − 2 and in (b) 1 × 1014 cm − 2.
Fig. 2. Analysis of the release of boron from clusters in the temperature range from 900 to 1200°C. The release time constant shows an activation energy of 2.3 eV.
Fig. 4. Configuration energy diagram for the incorporation and the release of boron atoms from the clusters.
native oxide was stripped by HF dipping immediately before the implantation. Rapid thermal annealing was used to anneal the samples in the temperature range between 900 and 1200°C for times ranging from 1 to 600 s in a nitrogen atmosphere. Chemical concentration profiling of the boron was done by secondary ion mass spectroscopy (SIMS) profiling. A CAMECA IMS-4f with an O+ 2 primary low-energy ion beam of 1.5 keV was used. The low-energy primary beam was used to reduce the depth at which the equilibrium SIMS sensitivity is reached to approximately 5 nm. Details on the SIMS measurement can be found in Ref. [11]. Electrical activity of the boron profiles was measured by the spreading resistance profiling (SRP) technique. A
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combination of advanced smooth bevel preparation and a probe load of 5 g were used to obtain reliable concentration profiles [12].
3. Boron clustering kinetics In Fig. 1 the SIMS profiles of the boron concentration are shown relative to as implanted and annealed samples at a temperature of 900°C for the indicated durations. The implantation energy is in both cases 500 eV. The implantation dose is: in (a) 1 × 1015 cm − 2 and in (b) 1 × 1014 cm − 2. In (a) the inset shows a magnified plot of the shallowest 10 nm of the high concentration region of the profile. It can be seen (Fig. 1(a)) that a so-called kink and tail profile shape develops. From this profile shape,
Fig. 5. Comparison of the chemical (secondary ion mass spectroscopy, SIMS) and the electrical (spreading resistance profiling, SRP) depth profiles after annealings at 900°C.
Fig. 6. The complementary of the electrical active fraction of boron and the normalized amount of boron in clusters in dependence of annealing time for the indicated temperatures.
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however, it can be concluded that the boron diffusivity is different in the high and in the low concentration region. In fact, the profile in the high concentration region (see inset in (a)) does not show any broadening. Only a reduction of the concentration with prolonged annealing time is visible. From this it can be concluded that the vast amount of boron in this region is not subject to diffusion as expected to be the case for substitutional boron in silicon. From this reasoning it can be concluded further that the vast amount of boron is located in clusters. The reduction of the boron concentration indicates that the boron is released from the clusters with prolonged annealing time. The boron clustering is much less pronounced for an implantation dose of 1× 1014 cm − 2 as can be seen in Fig. 1(b). Two possible reasons for this dose dependence are discussed here; first, in contrast to the lower dose implantation, the higher dose implantation produces an as-implanted boron concentration far above the solubility value (6.8× 1019 cm − 3 at 900°C [13]) for substitutional boron in silicon. Secondly, the damage in the silicon lattice introduced by the boron implantation has to be taken into account. As the implantation damage increases with increasing dose and at the same time also the clustering of boron gets more pronounced, it seems to be reasonable to assume that the implantation damage is another important factor. The development of a local minimum has been reported after annealing at 1100°C and above [15]. The boron concentration of the minimum has been found to be considerably lower than the solubility value of substitutional boron at the corresponding annealing temperature. From this observation it can be concluded that the transition of the boron atoms from the clusters into the substitutional site is the limiting step for the reduction of the concentration of boron atoms in clusters. Moreover, the diffusion of boron away from the clusters does not inhibit the release of boron atoms from the clusters. An analysis of the release kinetics of boron from the clusters has been performed based on the observation that in the peak region nearly all boron is clustered. Further, for this analysis all boron concentrations were taken at a depth of 8 nm. A normalization of the boron concentration after annealing was performed with respect to the boron concentration of the as-implanted sample. The dependence of the normalized boron concentration on the annealing time is shown in Fig. 2(a) in a semi-logarithmic plot for temperatures ranging from 900 to 1200°C. With prolonged annealing time the boron concentration is decreasing and the decrease is faster for higher temperatures. An exponential decay function as shown in Fig. 2(a) was fitted to the experimental data. From this fitting the release time constant trel for the boron from the clusters is derived. The temperature dependence of the release time constant trel
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is shown in Fig. 2(b) in an Arrhenius plot. It can be seen that the release time constant is varying from about 1000 s at 900°C to about 12 s at 1200°C. From the best linear fit in the Arrhenius plot an activation energy of 2.3 eV has been derived for the release time constant. The time constants reported for mechanisms which are important for the profile broadening at higher implantation energies (dissolution time constant of {311} defect, t{311} [4,5] cease time of TED, tTED [14] and dissolution time constant of higher energy implantation induced boron clusters, tBcl [16] are also reported in Fig. 2(b) in order to compare them with the release time constant of the ultra-low-energy implantation induced boron clusters. It can be seen that the time constant of the ultra-low-energy implantation induced boron clusters is longer and that the activation energy is lower than for the other processes. From this comparison it can be concluded that the release process from the ultra-low-energy implantation induced boron clusters is most likely governed by a different mechanism than the other processes. To determine the initial incorporation of boron into the clusters the boron concentration after the shortest annealing cycle for each temperature, in dependence of the boron concentration of the as-implanted sample has been plotted in Fig. 3(a). In this plot care was taken that the boron concentrations of the as-implanted and annealed samples correspond to the same depth. The dependence can be regarded as approximately linear and consequently a linear fit to the data is performed. The slopes obtained from these fits contain information about the amount of boron atoms in clustered configuration per boron atom initially present. As the annealing times shown in the plot are much smaller than the release time constants reported in Fig. 2, the slopes can be interpreted as the initial incorporation efficiencies into the clusters. Fig. 3(b) shows the temperature dependence of the incorporation efficiency in an Arrhenius plot. The incorporation efficiency attains its maximum value of one at a temperature of 900°C and is decreasing with increasing temperature and an activation energy of 0.9 eV. An additional experiment has been performed by pre-forming the boron clusters at 900°C with an incorporation efficiency of about 1 and subsequently annealing at 1200°C for 1 s. The comparison of the thus treated sample with a sample only annealed at 1200°C for 1 s shows that the incorporation efficiency values are comparable. Due to the pre-annealing, the boron has been incorporated into clusters with an efficiency of 1. The additionally clustered boron in the pre-annealed sample (with respect to the only 1200°C annealed sample) is released from the clusters within 1 s. This is much faster than the 12 s which are determined as the
release time constant at 1200°C in Fig. 2. From this comparison, however, it can be deduced that the part which is additionally incorporated into the boron clusters at 900°C (with respect to the 1200°C annealed sample) is bound less tightly to the clusters than the part at 1200°C. From these considerations it can furthermore be concluded that the activation energy of the release time constant, trel, is a mean value. The results so far presented have been merged in the scheme of a configuration energy diagram which is shown in Fig. 4. After implantation boron is incorporated into the clusters which requires an activation energy of 0.9 eV as determined from the incorporation efficiency (see Fig. 3(b)). The clustered configuration, however, is metastable and boron atoms are released from the clusters into substitutional lattice sites with a mean activation energy of 2.3 eV. Unfortunately, from the results reported so far it is not possible to extract a microscopical model for the clustered boron. But from the observation that the boron is bound differently tightly to the clusters it can be concluded that the boron does not occupy a unique site in the clusters.
4. Electrical activation The SRP depth profiles of the electrical active boron are shown in Fig. 5 for the high dose implanted boron and the annealing series performed at 900°C. For comparison the corresponding SIMS profiles are also included into the plot. From this comparison it can be seen that the electrical activity in the high concentration region is low. As already mentioned earlier the vast amount of boron in this region is clustered. Therefore, it can be concluded that the boron attached to clusters is electrically not active. Further, it can seen that the electrical activation attains the solubility concentration (indicated with a dotted line in the figure) at the annealing temperature after 300 s. For all temperatures the maximum measured electrical activation was either at or below the solubility value at the corresponding annealing temperature. This observation is in agreement with the assumption that the generation of an activation higher than the solubility by means of rapid thermal annealing is not achieved. Comparing the junction depth of the SRP profiles and the SIMS profiles in Fig. 5 it can be stated that the electrical junction depth is, for the shorter times, the same as the chemically determined junction depth. Only for the 600 s annealing case the electrical junction depth is slightly shallower than the chemical junction depth. In general, the electrical junction depth corresponds to the chemical junction depth or is slightly shallower. It is important to mention at this point that the SRP measurements of the samples implanted with a dose of 1× 1014 cm − 2 revealed that the maximum concentra-
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tion of activated boron was about 1× 1019 cm − 3 at 900°C. This is considerably below the solubility concentration of boron. From this observation it can be deduced that implantation doses larger than 1 ×1014 cm − 2 are needed in order to produce high-conductive ultra-shallow junctions. To account for the electrically activated fraction, f, the integrated SRP profile is divided by the integrated SIMS profile. During the integration of the SIMS profile, care was taken that the shallowest 5 nm are not considered because in this region the SIMS signal is transient. The complementary of the electrical active fraction, 1−f, together with the boron clustering data (cf. Fig. 1(a)) are plotted against the annealing time in Fig. 6. The comparison shows that both sets of data have the same trend for annealings up to about 1100°C. In this temperature region both the complementary of the electrically active fractions and the concentration of boron in clusters are decreasing with increasing annealing time and the decrease is faster for higher temperatures. Different trends are observed at a temperature of 1200°C. While for the cluster dissolution the trend is still valid, the electrical active fraction attains approximately a constant value. The out-diffusion of the boron through the native oxide is one of the reasons for this behavior. From this comparison it seems to be reasonable to propose that the release of boron from clusters is an important mechanism for the electrical activation of ultra-low-energy implanted boron. Considering further that the initial incorporation efficiency is decreasing for higher temperatures it seems to be reasonable that the importance of the boron clustering is decreasing with increasing temperature.
5. Conclusion The investigation on the activation annealing of ultralow-energy implanted boron in silicon by means of chemical (SIMS) and electrical (SRP) profiling techniques has been presented. From the SIMS profiles the presence of immobile boron clusters has been deduced. An analysis of the formation and dissolution kinetics of the clusters is performed. The SRP measurements revealed that the boron in clusters is electrically inactive. Comparing the electrical activation and the cluster dissolution it is proposed that the boron clustering is determining the electrical activation.
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Acknowledgements This work was partially supported by the project ENDEASD funded by the European Union under the contract number FMRX-CT98-0208 and by the project 5% Microelettronica of Consiglio Nazionale delle Ricerche (CNR). References [1] E.C. Jones, E. Ishida, Mater. Sci. Eng. R24 (1998) 1. [2] National Technology Roadmap for Semiconductors, Semiconductor Industry Association, 1997 Edition, San Jose, CA. [3] A. Agarwal, A.T. Fiory, H.J.L. Gossmann, C.S. Rafferty, P. Frisella, Mater. Sci. Semicond. Process. 1 (1998) 237. [4] D.J. Eaglesham, P.A. Stolk, H.J. Gossmann, J.M. Poate, Appl. Phys. Lett. 65 (1994) 18. [5] P.A. Stolk, H.J. Gossmann, D.J. Eaglesham, D.C. Rafferty, G.H. Gilmer, M. Jaraiz, J.M. Poate, H.S. Luftman, T.E. Haynes, J. Appl. Phys. 81 (1997) 6031. [6] N.E.B. Cowern, E.J.H. Collart, J. Politiek, P.H.I. Bancken, J.G.M. van Berkum, K. Kyllesbech Larsen, P.A. Stolk, H.G.A. Huizing, P. Pichler, A. Burenkov, D.J. Gravesteijn, Mater. Res. Soc. Symp. Proc. 469 (1997) 265. [7] E. Napolitani, A. Camera, E. Schroer, V. Privitera, F. Priolo, S. Moffatt, Appl. Phys. Lett. (submitted). [8] A. Agarwal, H.-J. Gossmann, D.J. Eaglesham, L. Pelaz, S.B. Herner, D.C. Jacobson, T.E. Haynes, R. Simonton, Mater. Sci. Semicond. Process. 1 (1998) 17. [9] V. Privitera, E. Napolitani, F. Priolo, S. Moffatt, A. La Magna, G. Mannino, A. Camera, A. Picariello, Mater. Sci. Semicond. Process. 2 (1999) 35. [10] C. Lowrie, J. England, A. Hunter, D. Burgin, B. Harrison, In: E. Ishida, S. Banerjee, S. Mehta, T.C. Smith, M. Current, L. Larson, A. Tasch (Eds.), Proceedings of Ion Implantation Technology 1996, Austin, TX, June 16 – 21, 1996 edition, IEEE, New York, 1997, p. 447. [11] E. Napolitani, A. Carnera, R. Storti, V. Privitera, F. Priolo, G. Mannino, and S. Moffat, Fifth International Workshop on the Measurement, Characterisation and Modeling of UltraShallow doping Profiles in Semiconductors, Research Triangle Park, NC, 1999, Proceedings Vol. [12] W.L. Harrington, C.W. Magee, M. Pawlik, D.F. Downey, C.M. Osburn, S. Felch, J. Vac. Sci. Technol. B16 (1998) 296. [13] A. Armigliato, D. Nobili, P. Ostoja, M. Servidori, S. Solmi, in: H.R. Huff, E. Sirtl (Eds.), Semiconductor Silicon, Electrochemistry Society, Princeton, NJ, 1977, p. 638. [14] S. Solmi, F. Baruffaldi, R. Canteri, J. Appl. Phys. 69 (1991) 2135. [15] E. Schroer, V. Privitera, F. Priolo, E. Napolitani, A. Camera, Appl. Phys. Lett. (accepted). [16] M. Uematsu, J. Appl. Phys. 84 (1998) 4781.