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Out-diffusion of hydrogen from hydrogen plasma-processed oxygen-implanted silicon
Applied Surface Science 260 (2012) 54–58
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Out-diffusion of hydrogen from hydrogen plasma-processed oxygen-implanted silicon A. Misiuk a , J. Bak-Misiuk b,∗ , A. Barcz b , P. Romanowski b , I. Tyschenko c , A. Ulyashin d , M. Prujszczyk a a
Institute of Electron Technology, al. Lotnikow 46, 02-668 Warsaw, Poland Institute of Physics, PAS, al. Lotnikow 32/46, 02-668 Warsaw, Poland c Institute of Semiconductor Physics RAS, Novosibirsk, 63009, Russia d SINTEF, P.O. Box 124 Blindern, Norway b
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 7 March 2012 Accepted 9 March 2012 Available online 28 March 2012 Keywords: Si:O Hydrogen gettering Out-diffusion High-pressure annealing SIMS X-ray diffraction Defect structure
a b s t r a c t Hydrogen gettering and its out-diffusion from implantation-disturbed buried layers formed in oxygenimplanted silicon, annealed and subsequently treated in hydrogen plasma, have been investigated. Energy and doses of implanted oxygen ions were 200 keV and 1 × 1017 cm−2 , respectively. After implantation Si:O samples were annealed at up to 1573 K, also under enhanced hydrostatic pressure, up to 12.3 kbar. Depending on processing conditions, disturbed buried layers, containing vacancy-like and other defects, SiO2−x clusters and/or precipitates, were formed. To produce hydrogen-enriched silicon structures, Si:O,H, with hydrogen accumulated within implantation-disturbed buried layers, Si:O samples were treated in hydrogen plasma. Out-diffusion of hydrogen from Si:O,H samples was investigated after annealing at 723 K and 973 K under atmospheric pressure. Depth profiles of oxygen and hydrogen were determined using secondary ion mass spectroscopy; X-ray reciprocal space mapping was applied for defect structure determination. Part of hydrogen remains to be present at surface and, especially, within implantationdisturbed areas even after annealing of Si:O,H at 973 K. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Direct synthesis of buried insulating silicon dioxide in Si by oxygen implantation is one of the most important approaches to form silicon-on-insulator structure (SOI). SOI is usually produced by O2 + implantation into Czochralski grown silicon (Cz-Si), typically at doses, D ≥ 2 × 1017 cm−2 , with subsequent annealing of resulting Si:O at about 1600 K (HT) [1]. Microstructure of the SOI-like samples depends, among other factors, on implanted oxygen ion energy (E), dose (D), temperature (HT) and time of processing (t). Enhanced hydrostatic pressure (HP) applied at HT results in the formation of specific Si:O structures with usually improved quality of the oxygen-rich layer and of the SiOx /Si interface [1,2]. In the case of self-implanted silicon (Si:Si), buried defect layers have been reported to getter hydrogen from hydrogen plasma [3]. Hydrogen accumulation and its release in result of subsequent annealing of the Si:O,H samples at up to 973 K are investigated for the SOI-like structures of different microstructure prepared by Si:O processing at different HT, HP and t conditions. Oxygen
∗ Corresponding author. E-mail address: [email protected] (J. Bak-Misiuk). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.038
was introduced into silicon by O2 + implantation into Cz-Si at D = 1 × 1017 cm−2 and hydrogen – by the subsequent treatment of Si:O in hydrogen plasma. Our investigations are also taking in view of possible application of hydrogen implantation for recognition of disturbed areas in the SOI-like and similar structures.
2. Experimental The (0 0 1) oriented Cz-Si wafers with concentration of interstitial oxygen, co = 1 × 1018 cm−3 were implanted at room temperature with O2 + ions at energy, E = 200 keV to a dose, D = 1 × 1017 cm−2 , (projected range of implanted ions, Rp = 400 nm). Implantation produced the Gaussian-like oxygen concentration depth profile with the peak oxygen concentration near Rp . The Si:O samples were processed in purified Ar atmosphere at 723–1573 K for t = 5–10 h, both under atmospheric pressure and HP up to 12.3 kbar. To produce Si:O,H, the subsequent direct hydrogen plasma treatment of the HT–HP processed Si:O samples was carried at 530 K for 2 h in a plasma enhanced chemical vapor deposition reactor. The plasma frequency and power were 110 MHz and 50 W, respectively [3]. After the plasma treatment the Si:O,H samples were annealed in N2 ambient for 1 h under atmospheric pressure (1 bar), at first at
A. Misiuk et al. / Applied Surface Science 260 (2012) 54–58
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Table 1 Annealing conditions at preparation of Si:O and maxima of hydrogen concentration in plasma-treated Si:O,H before (cp ) and after annealing for 1 h at 723 K (c723 K ) and at 973 K (c973 K ). Sample no.
Processing conditions at preparation of Si:O, HT, HP and t
After plasma treatment cp (cm−3 )
After annealing at 723 K c723 K (cm−3 )
After final annealing at 973 K c973 K (cm−3 )
1 2 3 4 5 6 7
As-implanted sample (no processing) 923 K, 1 bar, 10 h 923 K, 11 kbar, 5 h 1233 K, 1 kbar, 5 h 1233 K, 10 kbar, 5 h 1573 K, 100 bar, 5 h 1573 K, 12.3 kbar, 5 h
1 × 1020 7 × 1018 5 × 1020 1 × 1020 6 × 1019 4 × 1018 3 × 1018
5 × 1019 7 × 1018 4 × 1019 3 × 1019 1 × 1019 2.5 × 1018 2 × 1018
3.5 × 1018 4 × 1018 1.5 × 1019 6 × 1018 5 × 1018 6 × 1018 2.5 × 1018
723 K and next at 973 K. The hydrogen and oxygen depth profiles in Si:O,H were determined by secondary ion mass spectroscopy (SIMS) using a Cs+ ion source. The processing parameters of Si:O as well as the peak concentrations of hydrogen in Si:O,H are presented in Table 1. The reference as-implanted sample was also investigated (sample no. 1, Table 1). To determine microstructure of the samples, X-ray measurements were performed using a high resolution Philips Material Research Diffractometer (MRD). X-ray reciprocal space maps (RSM) for the 004 reflection were recorded. 3. Results and discussion The Si:O,H samples were prepared by the RF plasma treatment from three kinds of the Si:O samples: (a) The as-implanted (reference) one (no. 1), (b) Si:O processed at 923–1233 K, in which SiO2−x clusters (precipitates) and numerous point-like defects were formed and (c) Si:O processed at 1573 K in which most point like defects were out-annealed [1,2,4]. Specific processing of Si:O was applied in this study to produce the samples with different microstructures of the buried implantation-related damaged layer known to be strongly dependent on the HT, HP and t conditions [1–4]. Typical results obtained for the Si:O,H samples, finally annealed for 1 h at 973 K under 1 bar are presented in Figs. 1–4. The hydrogen and oxygen depth profiles obtained by SIMS for the Si:O,H sample nos. 3, 5 and 4 are shown in Fig. 1; the profiles for the no. 6 and 7 samples are presented in Fig. 2. Similar profiles (not shown)
were observed for other samples. The RSM patterns for the Si:O,H samples are presented in Figs. 3 and 4. The hydrogen concentration maxima detected for investigated samples, just after the plasma treatment, after annealing at 723 K and, subsequently, at 973 K under 1 bar (to cause out-diffusion of hydrogen introduced at the plasma treatment) are shown in Table 1. The shape of oxygen depth profile is related to the HT, HP and t conditions at HT–HP processing of Si:O while practically not on the further plasma treatment and subsequent annealing at 723 K and 973 K. Oxygen distribution depends first of all on post-implantation annealing temperature of Si:O while less on pressure applied during the HT–HP treatment (compare [1,2]). In the case of no. 1, 2 and 3 samples, the oxygen and hydrogen depth profiles are of similar shape (compare [4]) while the values of cp , c723 K and of c973 K differ markedly (Table 1). It is important to note that the Si:O structures form implantation-disturbed buried layer in which, in effect of the HT–HP processing, numerous, mostly oxygen-related defects, are formed. For the oxygen dose applied (D = 1 × 1017 cm−2 , calculated for oxygen atoms) no continuous buried SiO2 layer was created in Si:O. The oxygen-related defects are composed mostly of substoichiometric SiO2−x clusters and precipitates [1,2,4]. The oxygen concentration peak maximum in Si:O, as-implanted and processed at lower temperatures, corresponds to Rp (Fig. 1a). In the case of Si:O,H samples prepared from Si:O processed at higher temperatures, implanted oxygen and plasma-introduced hydrogen atoms form the bimodal profiles (Fig. 1b and c). One of the peaks is close to the top silicon surface, while the second peak position corresponds approximately to Rp . As seen, oxygen and hydrogen atoms are accumulated in part also near the top Si surface. This effect can be considered as an artefact specific for SIMS measurements as well as related to surface gettering; in what follows it will be not discussed.
Fig. 1. Oxygen and hydrogen depth profiles measured by SIMS for samples 3 (a), 5 (b), and 4 (c) subjected to final annealing at 973 K (compare Table 1).
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Fig. 2. Oxygen and hydrogen depth profiles measured by SIMS for samples 6 (a) and 7 (b) (Table 1) subjected to final annealing at 973 K.
Fig. 3. RSM’s for samples no. 3 (a), 5 (b), and 4 (c) (compare Fig. 1 and Table 1) after annealing at 973 K.
At a depth of about 1 m and deeper the concentration of oxygen is below co (co = 1018 cm−3 ) evidencing strong oxygen diffusion toward implantation-disturbed layer. As post-implantation annealing temperature of Si:O increased to 1233 K, the formation of the double-peaked oxygen profiles took place (compare [4]). An increase of post-implantation annealing temperature of Si:O to 1573 K is also accompanied with oxygen diffusion from the wings of its distribution toward the buried oxide layer at a depth of about 400 nm (Fig. 2). The oxygen peak of lower intensity is observed at a depth of about 300 nm below the top Si surface, its position
corresponds to the maximum of elastic losses of the O2 + energy in Si. It is also seen that, as HP during post-implantation processing increases, the buried silicon oxide layer becomes a little wider and oxygen concentration near Rp drops slightly. The shape of hydrogen profiles reminds that of oxygen. The values of peak hydrogen concentration (cp ) in the Si:O,H samples just after the hydrogen plasma treatment are presented in Table 1. The hydrogen concentrations were high also in Si:O,H prepared from as-implanted Si:O and from Si:O processed at 923 K–11 kbar and at 1233 K–1 kbar (compare [4]), being evidently dependent on the
Fig. 4. RSM’s for samples 6 (a) and 7 (b) after annealing at 973 K (compare Fig. 2 and Table 1).
A. Misiuk et al. / Applied Surface Science 260 (2012) 54–58
microstructure of buried oxygen-containing SiO2−x layer formed in effect of oxygen implantation and subsequent HT–HP treatment. The dominant hydrogen peak position also corresponds to the range of O2 + ions in silicon (Rp ≈ 400 nm). In the case of no. 4–7 samples implanted hydrogen forms the broad profiles at a depth from 200 to 500 nm. Subsequent annealing of Si:O,H at 723 K for 1 h under atmospheric pressure leads to the non-changed oxygen concentration profiles (compare [4]) while the peak concentration of hydrogen decreased for up to above ten times (Table 1). This reduction of hydrogen content is caused mainly by its out-diffusion to ambient. Negligible out-diffusion of hydrogen in the case of no. 2 sample can be considered as an evidence of the strongest bonding of hydrogen to defects formed within the implantation-disturbed area just in this sample. Contrary to that, strong reduction of the hydrogen content in the case of sample 3 (hydrogen content reduction for about 90%) and of sample 5 (reduction for about 80%) evidences relatively weak bonding of hydrogen to defects in the areas near Rp . Subsequent annealing at 973 K of the nos. 1–5 Si:O,H samples (annealed at first at 723 K) leads to further decrease of hydrogen content (Table 1, Fig. 1). The shapes of oxygen and hydrogen depth profiles in the no. 1 and 2 samples (not shown), as hydrogen plasma treated and annealed at 723 K and 973 K, are practically the same as these for the sample no. 3 (Fig. 1 a). Annealing at 723 K leads to hydrogen content decreasing (Table 1) for about 90% (sample no. 1), 60% (sample no. 3) or 40% (sample no. 2). In the case of samples prepared from Si:O processed at 1233 K, the peak hydrogen concentration remains to be still at the 6 × 1018 cm−3 level after final annealing at 973 K for t = 1 h; the shapes of hydrogen concentration profiles remain similar, almost not dependent on HP applied at processing of Si:O (Fig. 1b and c). Contrary to the case of as-plasma treated Si:O,H and of that prepared from Si:O processed at 923 K, the peak hydrogen concentrations in Si:O,H prepared from Si:O processed at 1233 K are, after final annealing at 973 K, at the similar level, within 5–6 × 1018 cm−3 . The peak concentration of hydrogen within the buried disturbed layer remains to be relatively high, of about (3–6) × 1018 cm−3 , also in the case of no. 6 and 7 samples prepared from Si:O processed at 1573 K. Moreover, the peak concentration of hydrogen in the no. 6 and 7 samples increases after final annealing at 973 K (Table 1, Fig. 2). This effect can be explained assuming redistribution of hydrogen accumulated within the disturbed layer to the area indicating especially high affinity to hydrogen. The peak concentration of hydrogen accumulated within the disturbed layer of Si:O,H is dependent on HP; it decreased for about 2 times as HP at processing of Si:O increased from 100 bar to 12.3 kbar (Fig. 2 and Table 1). In order to study the reason for hydrogen accumulation and its correlation with the oxygen depth profiles (Figs. 1 and 2), defect structure of the Si:O,H samples was investigated by X-ray reciprocal space mapping, RSM. Analysis of diffuse scattering near the Bragg reflection is a powerful method to study point defects or their clusters, inducing lattice distortion in the crystal [5,6]. The distribution of diffuse scattering along the radial direction of RSM is sensitive to the sign of deformation. The defects causing a decrease of the lattice parameters (e.g. vacancies) lead to increased intensity of X-ray diffuse scattering at the lower angles while the defects resulting in the increased lattice parameter (e.g. the interstitial-type ones) – to the increased intensity of X-ray diffuse scattering at the higher angles [5]. In the case of Si:O,H prepared from the as-implanted Si:O (no. 1), due to considerable amorphization of the implanted buried layer near Rp , one X-ray diffraction peak connected with the silicon substrate was observed only (not presented here).In the case of Si:O,H
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from Si:O prepared by processing at relatively low temperature, 923 K (Fig. 3a), two diffraction maxima originating from the substrate and from the re-crystallized material of initially amorphous disturbed buried implanted layer (at annealing the amorphous layer is subjected to so called solid phase epitaxial re-growth – SPER) are detected. RSM of this sample is similar to that obtained for the no. 1 and 2 samples (not shown). For the samples prepared from Si:O processed at 1233 K (Fig. 3b and c), a difference between the lattice parameter values of the layer and substrate materials disappears and therefore only one reciprocal lattice point on RSM’s is observed. Diffuse scattering observed on RSM’s can be related to the presence of point defects but also to some larger precipitates. It is known that processing of Si:O at above 1500 K leads to a creation of SiO2−x clusters and precipitates [1,4]. An increase of post-implantation annealing temperature of Si:O to 1573 K results in asymmetric distributed X-ray diffuse scattering along the radial direction observed for Si:O,H annealed finally at 973 K (Fig. 4). The observed asymmetry in Fig. 4 may suggest, in this case, the presence of strain at the boundary between SiO2−x precipitates and Si matrix leading to decreased interplanar distanced in the [0 0 1] direction. Just such strain can effect in marked hydrogen gettering. Any marked effect of hydrostatic pressure, applied at processing of Si:O, on diffuse scattering was detected in this case (compare Fig. 4a and b). Still, similarity of the hydrogen and oxygen depth profiles suggests that H and O atoms are components of the same defect centers (oxygen in such structures forms mostly SiO2−x clusters and precipitates [1,2,4]). One possible mechanism to produce such centers, especially in the nos. 1–5 samples, is the formation of oxygen–hydrogen bonding with vacancy clusters. The vacancy clusters and vacancy–oxygen complexes were reported earlier for the low-dose SIMOX wafers studied by slow positron beams [7]. Their distribution is close to the damage profiles peaking at 70–80% of Rp for implanted oxygen ions. Both for the oxygen and hydrogen distributions, one of these peaks is placed at this depth (Figs. 1 and 2). However, according to results reported earlier [8], annealing at above 1470 K is accompanied with decreased number of oxygen atoms coupled with the vacancy-type defect. On the other hand, the presence of hydrogen within the ion-implanted silicon bulk can stimulate the formation of hydrogen–vacancy–oxygen clusters providing their higher thermal stability. The X-ray results obtained for the Si:O,H samples prepared from Si:O processed at 1573 K (Fig. 4) demand, however, other explanation because an existence of such clusters is doubtful in the case of Si:O processed at so high temperature. The obtained oxygen profiles are in good agreement with the results observed earlier for the low-energy ion-beam synthesized SOI structures [8,9]. Generally, the low-dose ion-beam synthesis can result in the formation of amorphous buried silicon oxide layer, which contains some silicon islands covered by a relatively high quality single crystal Si film [8]. However, in our case, the stoichiometric (in respect of SiO2 ) oxygen concentration was not achieved. It means that the formation either of the non-stoichiometric SiO2−x layer or of the SiO2−x islands can be expected within the ion implanted region in effect of the HT–HP processing of Si:O. It suggests the presence of the Si/SiO2−x interfaces within the ionimplanted and high-temperature processed region. The Si/SiO2−x interface can contain dangling Si bonds known as the Pb centers [10]. The presence of hydrogen is very efficient tool to produce the high-quality Si/SiO2−x interface by passivating these dangling bonds. As a result, the Si/SiO2−x interface may be a preferable sink for hydrogen atoms. So, the presence of dangling bonds at the interfaces between SiO2−x precipitates and the surrounding Si matrix is the possible reason for hydrogen accumulation near the range of implanted oxygen (Rp ).
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Among others, in this work we stated the following: - Oxygen-implanted Cz-Si:O samples subjected to the different high temperature–pressure processing and so of different microstructure, getter hydrogen from hydrogen plasma at so low temperature as 530 K. The concentration of accumulated hydrogen is related to different kinds of defects in Si:O, in particular to these ones containing oxygen. - Processing of Si:O,H at 723 K under 1 bar for 1 h results in the substantial decrease of hydrogen concentration while the oxygen distribution is practically not affected by such processing. - Subsequent processing of Si:O,H at 973 K leads to the hydrogen depth profiles very similar to these observed for oxygen. Not only out-diffusion of hydrogen but also its redistribution toward the oxygen-containing areas is clearly detectable in the samples finally annealed at 973 K, especially in the case of Si:O,H prepared from Si:O processed at the highest temperature applied (1573 K). Just these samples contained a lot of SiO2−x precipitates while most of point defects were removed at temperature of preparation [11]. One can speculate that hydrogen removed from Si:O,H in effect of annealing at 723 K under 1 bar for 1 h was originally bonded mostly to the point-like and similar defects, while hydrogen still detectable after annealing at 973 K was bonded predominantly to some oxygen-related defects at the SiO2−x /Si interface and so almost perfectly reproduced the oxygen depth profile. This last observation suggests a possibility to consider hydrogen introduction into Si:O and similar structures [12] followed by its subsequent out-diffusion at annealing the sample as a useful tool to discriminate the point like defects from the oxygen-containing lager ones related to the presence of oxygen. 4. Conclusions An amount and distribution of hydrogen introduced into Si:O by the hydrogen plasma treatment are highly sensitive to the defect structure of Si:O related in turn to the parameters of its processing. Specific character of hydrogen interaction with defects and oxygen has been stated for the processed Si-based SOI-like structures prepared by oxygen implantation followed by the high temperature–pressure treatment. Our results confirm that small vacancy clusters and SiO2−x clusters/precipitates can be the preferable form of residual defects in the HT–HP-processed oxygen implanted silicon, also after the subsequent hydrogen plasma treatment followed by final low-temperature annealing.
Hydrogen gettered within the Si:O,H samples remains to be detectable even after annealing at 973 K. This suggests dominating chemical interaction of hydrogen with oxygen (with that probably in the form of sub-stoichiometric SiO2−x ) while some part of hydrogen may be bonded to other point like defects (such as Si dangling bonds, etc.). This suggests also a possibility to use the hydrogen plasma treatment for detection/profiling of defects and of the depth profile/state of oxygen in the SOI-like and similar systems. More complete understanding of the effects reported in this study demands further research, in particular on the defect structure of investigated samples. References [1] A. Misiuk, A. Barcz, J. Ratajczak, L. Bryja, Effect of high hydrostatic pressure during annealing on silicon implanted with oxygen, Journal of Materials Science: Materials in Electronics 14 (2003) 295–298. [2] I.V. Antonova, A. Misiuk, C.A. Londos, Electrical properties of multiplelayer structures formed by implantation of nitrogen or oxygen and annealed under high pressure, Journal of Applied Physics 99 (2006) 0335061–0335066. [3] A.G. Ulyashin, J.S. Christensen, B.G. Svensson, R. Kogler, W. Skorupa, Hydrogen gettering at buried defect layers in ion-implanted silicon by plasma hydrogenation and annealing, Nuclear Instruments and Methods in Physics Research B 253 (2006) 126–129. [4] A. Misiuk, A. Barcz, A. Ulyashin, I.V. Antonova, M. Prujszczyk, Hydrogen gettering in annealed oxygen-implanted silicon, Semiconductor Physics, Quantum Electronics & Optoelectronics 13 (2010) 161–165. [5] M. Moreno, B. Jenichen, V.M. Kaganer, W. Braun, L.A. Trampert, L. Daweritz, K.H. Ploog, MnAs nanoclusters embedded in GaAs studied by X-ray diffuse and coherent scattering, Physical Review B 67 (2003) 2352061–2352067. [6] C.A. Londos, I.V. Antonova, M. Potsidou, A. Misiuk, J. Bak-Misiuk, A.K. Gutakovskii, Study of the conversion of VO to the VO2 defect in silicon heat treated under uniform stress conditions, Journal of Applied Physics 91 (2002) 1198–1200. [7] A.G. Ulyashin, A.I. Ivanov, R. Job, A.V. Frantskevich, F.F. Komarov, A.C. Kamyshan, The hydrogen gettering at post-implantation hydrogen plasma treatments of helium and hydrogen implanted Czochralski silicon, Materials Science and Engineering B 73 (2000) 64–68. [8] Z.Q. Chen, A. Uedono, A. Ogura, H. Ono, R. Suzuki, T. Ohdairac, T. Mikado, Oxygen-related defects and their annealing behavior in low-dose separationby-implanted oxygen (SIMOX) wafers studied by slow positron beams, Applied Surface Science 194 (2002) 112–115. [9] Y. Li, J.A. Kilner, P.L.F. Hemment, A.K. Robinson, J.P. Zhang, K.J. Reeson, C.D. Marsh, G.R. Booker, Study of the microstructure of low energy (70 keV) oxygen implanted silicon, Applied Physics Letters 59 (1991) 3130–3132. [10] P.J. Caplan, E.H. Poindexter, B.E. Deal, R.R. Razouk, ESR centers interface states, and oxide fixed charge in thermally oxidized silicon wafers, Journal of Applied Physics 50 (1979) 5847–5854. [11] A. Misiuk, J. Bak-Misiuk, B. Surma, Effect of high temperature–pressure on buried silicon dioxide in SIMOX and SOI structures, Radiation Effects & Defects in Solids 158 (2003) 407–410. [12] A. Misiuk, A.G. Ulyashin, A. Barcz, P. Formanek, Accumulation of hydrogen within implantation-damaged areas in processed Si:N and Si:O, Solid State Phenomena 156–158 (2010) 319–324.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Out-diffusion of hydrogen from hydrogen plasma-processed oxygen-implanted silicon A. Misiuk a , J. Bak-Misiuk b,∗ , A. Barcz b , P. Romanowski b , I. Tyschenko c , A. Ulyashin d , M. Prujszczyk a a
Institute of Electron Technology, al. Lotnikow 46, 02-668 Warsaw, Poland Institute of Physics, PAS, al. Lotnikow 32/46, 02-668 Warsaw, Poland c Institute of Semiconductor Physics RAS, Novosibirsk, 63009, Russia d SINTEF, P.O. Box 124 Blindern, Norway b
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 7 March 2012 Accepted 9 March 2012 Available online 28 March 2012 Keywords: Si:O Hydrogen gettering Out-diffusion High-pressure annealing SIMS X-ray diffraction Defect structure
a b s t r a c t Hydrogen gettering and its out-diffusion from implantation-disturbed buried layers formed in oxygenimplanted silicon, annealed and subsequently treated in hydrogen plasma, have been investigated. Energy and doses of implanted oxygen ions were 200 keV and 1 × 1017 cm−2 , respectively. After implantation Si:O samples were annealed at up to 1573 K, also under enhanced hydrostatic pressure, up to 12.3 kbar. Depending on processing conditions, disturbed buried layers, containing vacancy-like and other defects, SiO2−x clusters and/or precipitates, were formed. To produce hydrogen-enriched silicon structures, Si:O,H, with hydrogen accumulated within implantation-disturbed buried layers, Si:O samples were treated in hydrogen plasma. Out-diffusion of hydrogen from Si:O,H samples was investigated after annealing at 723 K and 973 K under atmospheric pressure. Depth profiles of oxygen and hydrogen were determined using secondary ion mass spectroscopy; X-ray reciprocal space mapping was applied for defect structure determination. Part of hydrogen remains to be present at surface and, especially, within implantationdisturbed areas even after annealing of Si:O,H at 973 K. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Direct synthesis of buried insulating silicon dioxide in Si by oxygen implantation is one of the most important approaches to form silicon-on-insulator structure (SOI). SOI is usually produced by O2 + implantation into Czochralski grown silicon (Cz-Si), typically at doses, D ≥ 2 × 1017 cm−2 , with subsequent annealing of resulting Si:O at about 1600 K (HT) [1]. Microstructure of the SOI-like samples depends, among other factors, on implanted oxygen ion energy (E), dose (D), temperature (HT) and time of processing (t). Enhanced hydrostatic pressure (HP) applied at HT results in the formation of specific Si:O structures with usually improved quality of the oxygen-rich layer and of the SiOx /Si interface [1,2]. In the case of self-implanted silicon (Si:Si), buried defect layers have been reported to getter hydrogen from hydrogen plasma [3]. Hydrogen accumulation and its release in result of subsequent annealing of the Si:O,H samples at up to 973 K are investigated for the SOI-like structures of different microstructure prepared by Si:O processing at different HT, HP and t conditions. Oxygen
∗ Corresponding author. E-mail address: [email protected] (J. Bak-Misiuk). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.038
was introduced into silicon by O2 + implantation into Cz-Si at D = 1 × 1017 cm−2 and hydrogen – by the subsequent treatment of Si:O in hydrogen plasma. Our investigations are also taking in view of possible application of hydrogen implantation for recognition of disturbed areas in the SOI-like and similar structures.
2. Experimental The (0 0 1) oriented Cz-Si wafers with concentration of interstitial oxygen, co = 1 × 1018 cm−3 were implanted at room temperature with O2 + ions at energy, E = 200 keV to a dose, D = 1 × 1017 cm−2 , (projected range of implanted ions, Rp = 400 nm). Implantation produced the Gaussian-like oxygen concentration depth profile with the peak oxygen concentration near Rp . The Si:O samples were processed in purified Ar atmosphere at 723–1573 K for t = 5–10 h, both under atmospheric pressure and HP up to 12.3 kbar. To produce Si:O,H, the subsequent direct hydrogen plasma treatment of the HT–HP processed Si:O samples was carried at 530 K for 2 h in a plasma enhanced chemical vapor deposition reactor. The plasma frequency and power were 110 MHz and 50 W, respectively [3]. After the plasma treatment the Si:O,H samples were annealed in N2 ambient for 1 h under atmospheric pressure (1 bar), at first at
A. Misiuk et al. / Applied Surface Science 260 (2012) 54–58
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Table 1 Annealing conditions at preparation of Si:O and maxima of hydrogen concentration in plasma-treated Si:O,H before (cp ) and after annealing for 1 h at 723 K (c723 K ) and at 973 K (c973 K ). Sample no.
Processing conditions at preparation of Si:O, HT, HP and t
After plasma treatment cp (cm−3 )
After annealing at 723 K c723 K (cm−3 )
After final annealing at 973 K c973 K (cm−3 )
1 2 3 4 5 6 7
As-implanted sample (no processing) 923 K, 1 bar, 10 h 923 K, 11 kbar, 5 h 1233 K, 1 kbar, 5 h 1233 K, 10 kbar, 5 h 1573 K, 100 bar, 5 h 1573 K, 12.3 kbar, 5 h
1 × 1020 7 × 1018 5 × 1020 1 × 1020 6 × 1019 4 × 1018 3 × 1018
5 × 1019 7 × 1018 4 × 1019 3 × 1019 1 × 1019 2.5 × 1018 2 × 1018
3.5 × 1018 4 × 1018 1.5 × 1019 6 × 1018 5 × 1018 6 × 1018 2.5 × 1018
723 K and next at 973 K. The hydrogen and oxygen depth profiles in Si:O,H were determined by secondary ion mass spectroscopy (SIMS) using a Cs+ ion source. The processing parameters of Si:O as well as the peak concentrations of hydrogen in Si:O,H are presented in Table 1. The reference as-implanted sample was also investigated (sample no. 1, Table 1). To determine microstructure of the samples, X-ray measurements were performed using a high resolution Philips Material Research Diffractometer (MRD). X-ray reciprocal space maps (RSM) for the 004 reflection were recorded. 3. Results and discussion The Si:O,H samples were prepared by the RF plasma treatment from three kinds of the Si:O samples: (a) The as-implanted (reference) one (no. 1), (b) Si:O processed at 923–1233 K, in which SiO2−x clusters (precipitates) and numerous point-like defects were formed and (c) Si:O processed at 1573 K in which most point like defects were out-annealed [1,2,4]. Specific processing of Si:O was applied in this study to produce the samples with different microstructures of the buried implantation-related damaged layer known to be strongly dependent on the HT, HP and t conditions [1–4]. Typical results obtained for the Si:O,H samples, finally annealed for 1 h at 973 K under 1 bar are presented in Figs. 1–4. The hydrogen and oxygen depth profiles obtained by SIMS for the Si:O,H sample nos. 3, 5 and 4 are shown in Fig. 1; the profiles for the no. 6 and 7 samples are presented in Fig. 2. Similar profiles (not shown)
were observed for other samples. The RSM patterns for the Si:O,H samples are presented in Figs. 3 and 4. The hydrogen concentration maxima detected for investigated samples, just after the plasma treatment, after annealing at 723 K and, subsequently, at 973 K under 1 bar (to cause out-diffusion of hydrogen introduced at the plasma treatment) are shown in Table 1. The shape of oxygen depth profile is related to the HT, HP and t conditions at HT–HP processing of Si:O while practically not on the further plasma treatment and subsequent annealing at 723 K and 973 K. Oxygen distribution depends first of all on post-implantation annealing temperature of Si:O while less on pressure applied during the HT–HP treatment (compare [1,2]). In the case of no. 1, 2 and 3 samples, the oxygen and hydrogen depth profiles are of similar shape (compare [4]) while the values of cp , c723 K and of c973 K differ markedly (Table 1). It is important to note that the Si:O structures form implantation-disturbed buried layer in which, in effect of the HT–HP processing, numerous, mostly oxygen-related defects, are formed. For the oxygen dose applied (D = 1 × 1017 cm−2 , calculated for oxygen atoms) no continuous buried SiO2 layer was created in Si:O. The oxygen-related defects are composed mostly of substoichiometric SiO2−x clusters and precipitates [1,2,4]. The oxygen concentration peak maximum in Si:O, as-implanted and processed at lower temperatures, corresponds to Rp (Fig. 1a). In the case of Si:O,H samples prepared from Si:O processed at higher temperatures, implanted oxygen and plasma-introduced hydrogen atoms form the bimodal profiles (Fig. 1b and c). One of the peaks is close to the top silicon surface, while the second peak position corresponds approximately to Rp . As seen, oxygen and hydrogen atoms are accumulated in part also near the top Si surface. This effect can be considered as an artefact specific for SIMS measurements as well as related to surface gettering; in what follows it will be not discussed.
Fig. 1. Oxygen and hydrogen depth profiles measured by SIMS for samples 3 (a), 5 (b), and 4 (c) subjected to final annealing at 973 K (compare Table 1).
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Fig. 2. Oxygen and hydrogen depth profiles measured by SIMS for samples 6 (a) and 7 (b) (Table 1) subjected to final annealing at 973 K.
Fig. 3. RSM’s for samples no. 3 (a), 5 (b), and 4 (c) (compare Fig. 1 and Table 1) after annealing at 973 K.
At a depth of about 1 m and deeper the concentration of oxygen is below co (co = 1018 cm−3 ) evidencing strong oxygen diffusion toward implantation-disturbed layer. As post-implantation annealing temperature of Si:O increased to 1233 K, the formation of the double-peaked oxygen profiles took place (compare [4]). An increase of post-implantation annealing temperature of Si:O to 1573 K is also accompanied with oxygen diffusion from the wings of its distribution toward the buried oxide layer at a depth of about 400 nm (Fig. 2). The oxygen peak of lower intensity is observed at a depth of about 300 nm below the top Si surface, its position
corresponds to the maximum of elastic losses of the O2 + energy in Si. It is also seen that, as HP during post-implantation processing increases, the buried silicon oxide layer becomes a little wider and oxygen concentration near Rp drops slightly. The shape of hydrogen profiles reminds that of oxygen. The values of peak hydrogen concentration (cp ) in the Si:O,H samples just after the hydrogen plasma treatment are presented in Table 1. The hydrogen concentrations were high also in Si:O,H prepared from as-implanted Si:O and from Si:O processed at 923 K–11 kbar and at 1233 K–1 kbar (compare [4]), being evidently dependent on the
Fig. 4. RSM’s for samples 6 (a) and 7 (b) after annealing at 973 K (compare Fig. 2 and Table 1).
A. Misiuk et al. / Applied Surface Science 260 (2012) 54–58
microstructure of buried oxygen-containing SiO2−x layer formed in effect of oxygen implantation and subsequent HT–HP treatment. The dominant hydrogen peak position also corresponds to the range of O2 + ions in silicon (Rp ≈ 400 nm). In the case of no. 4–7 samples implanted hydrogen forms the broad profiles at a depth from 200 to 500 nm. Subsequent annealing of Si:O,H at 723 K for 1 h under atmospheric pressure leads to the non-changed oxygen concentration profiles (compare [4]) while the peak concentration of hydrogen decreased for up to above ten times (Table 1). This reduction of hydrogen content is caused mainly by its out-diffusion to ambient. Negligible out-diffusion of hydrogen in the case of no. 2 sample can be considered as an evidence of the strongest bonding of hydrogen to defects formed within the implantation-disturbed area just in this sample. Contrary to that, strong reduction of the hydrogen content in the case of sample 3 (hydrogen content reduction for about 90%) and of sample 5 (reduction for about 80%) evidences relatively weak bonding of hydrogen to defects in the areas near Rp . Subsequent annealing at 973 K of the nos. 1–5 Si:O,H samples (annealed at first at 723 K) leads to further decrease of hydrogen content (Table 1, Fig. 1). The shapes of oxygen and hydrogen depth profiles in the no. 1 and 2 samples (not shown), as hydrogen plasma treated and annealed at 723 K and 973 K, are practically the same as these for the sample no. 3 (Fig. 1 a). Annealing at 723 K leads to hydrogen content decreasing (Table 1) for about 90% (sample no. 1), 60% (sample no. 3) or 40% (sample no. 2). In the case of samples prepared from Si:O processed at 1233 K, the peak hydrogen concentration remains to be still at the 6 × 1018 cm−3 level after final annealing at 973 K for t = 1 h; the shapes of hydrogen concentration profiles remain similar, almost not dependent on HP applied at processing of Si:O (Fig. 1b and c). Contrary to the case of as-plasma treated Si:O,H and of that prepared from Si:O processed at 923 K, the peak hydrogen concentrations in Si:O,H prepared from Si:O processed at 1233 K are, after final annealing at 973 K, at the similar level, within 5–6 × 1018 cm−3 . The peak concentration of hydrogen within the buried disturbed layer remains to be relatively high, of about (3–6) × 1018 cm−3 , also in the case of no. 6 and 7 samples prepared from Si:O processed at 1573 K. Moreover, the peak concentration of hydrogen in the no. 6 and 7 samples increases after final annealing at 973 K (Table 1, Fig. 2). This effect can be explained assuming redistribution of hydrogen accumulated within the disturbed layer to the area indicating especially high affinity to hydrogen. The peak concentration of hydrogen accumulated within the disturbed layer of Si:O,H is dependent on HP; it decreased for about 2 times as HP at processing of Si:O increased from 100 bar to 12.3 kbar (Fig. 2 and Table 1). In order to study the reason for hydrogen accumulation and its correlation with the oxygen depth profiles (Figs. 1 and 2), defect structure of the Si:O,H samples was investigated by X-ray reciprocal space mapping, RSM. Analysis of diffuse scattering near the Bragg reflection is a powerful method to study point defects or their clusters, inducing lattice distortion in the crystal [5,6]. The distribution of diffuse scattering along the radial direction of RSM is sensitive to the sign of deformation. The defects causing a decrease of the lattice parameters (e.g. vacancies) lead to increased intensity of X-ray diffuse scattering at the lower angles while the defects resulting in the increased lattice parameter (e.g. the interstitial-type ones) – to the increased intensity of X-ray diffuse scattering at the higher angles [5]. In the case of Si:O,H prepared from the as-implanted Si:O (no. 1), due to considerable amorphization of the implanted buried layer near Rp , one X-ray diffraction peak connected with the silicon substrate was observed only (not presented here).In the case of Si:O,H
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from Si:O prepared by processing at relatively low temperature, 923 K (Fig. 3a), two diffraction maxima originating from the substrate and from the re-crystallized material of initially amorphous disturbed buried implanted layer (at annealing the amorphous layer is subjected to so called solid phase epitaxial re-growth – SPER) are detected. RSM of this sample is similar to that obtained for the no. 1 and 2 samples (not shown). For the samples prepared from Si:O processed at 1233 K (Fig. 3b and c), a difference between the lattice parameter values of the layer and substrate materials disappears and therefore only one reciprocal lattice point on RSM’s is observed. Diffuse scattering observed on RSM’s can be related to the presence of point defects but also to some larger precipitates. It is known that processing of Si:O at above 1500 K leads to a creation of SiO2−x clusters and precipitates [1,4]. An increase of post-implantation annealing temperature of Si:O to 1573 K results in asymmetric distributed X-ray diffuse scattering along the radial direction observed for Si:O,H annealed finally at 973 K (Fig. 4). The observed asymmetry in Fig. 4 may suggest, in this case, the presence of strain at the boundary between SiO2−x precipitates and Si matrix leading to decreased interplanar distanced in the [0 0 1] direction. Just such strain can effect in marked hydrogen gettering. Any marked effect of hydrostatic pressure, applied at processing of Si:O, on diffuse scattering was detected in this case (compare Fig. 4a and b). Still, similarity of the hydrogen and oxygen depth profiles suggests that H and O atoms are components of the same defect centers (oxygen in such structures forms mostly SiO2−x clusters and precipitates [1,2,4]). One possible mechanism to produce such centers, especially in the nos. 1–5 samples, is the formation of oxygen–hydrogen bonding with vacancy clusters. The vacancy clusters and vacancy–oxygen complexes were reported earlier for the low-dose SIMOX wafers studied by slow positron beams [7]. Their distribution is close to the damage profiles peaking at 70–80% of Rp for implanted oxygen ions. Both for the oxygen and hydrogen distributions, one of these peaks is placed at this depth (Figs. 1 and 2). However, according to results reported earlier [8], annealing at above 1470 K is accompanied with decreased number of oxygen atoms coupled with the vacancy-type defect. On the other hand, the presence of hydrogen within the ion-implanted silicon bulk can stimulate the formation of hydrogen–vacancy–oxygen clusters providing their higher thermal stability. The X-ray results obtained for the Si:O,H samples prepared from Si:O processed at 1573 K (Fig. 4) demand, however, other explanation because an existence of such clusters is doubtful in the case of Si:O processed at so high temperature. The obtained oxygen profiles are in good agreement with the results observed earlier for the low-energy ion-beam synthesized SOI structures [8,9]. Generally, the low-dose ion-beam synthesis can result in the formation of amorphous buried silicon oxide layer, which contains some silicon islands covered by a relatively high quality single crystal Si film [8]. However, in our case, the stoichiometric (in respect of SiO2 ) oxygen concentration was not achieved. It means that the formation either of the non-stoichiometric SiO2−x layer or of the SiO2−x islands can be expected within the ion implanted region in effect of the HT–HP processing of Si:O. It suggests the presence of the Si/SiO2−x interfaces within the ionimplanted and high-temperature processed region. The Si/SiO2−x interface can contain dangling Si bonds known as the Pb centers [10]. The presence of hydrogen is very efficient tool to produce the high-quality Si/SiO2−x interface by passivating these dangling bonds. As a result, the Si/SiO2−x interface may be a preferable sink for hydrogen atoms. So, the presence of dangling bonds at the interfaces between SiO2−x precipitates and the surrounding Si matrix is the possible reason for hydrogen accumulation near the range of implanted oxygen (Rp ).
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Among others, in this work we stated the following: - Oxygen-implanted Cz-Si:O samples subjected to the different high temperature–pressure processing and so of different microstructure, getter hydrogen from hydrogen plasma at so low temperature as 530 K. The concentration of accumulated hydrogen is related to different kinds of defects in Si:O, in particular to these ones containing oxygen. - Processing of Si:O,H at 723 K under 1 bar for 1 h results in the substantial decrease of hydrogen concentration while the oxygen distribution is practically not affected by such processing. - Subsequent processing of Si:O,H at 973 K leads to the hydrogen depth profiles very similar to these observed for oxygen. Not only out-diffusion of hydrogen but also its redistribution toward the oxygen-containing areas is clearly detectable in the samples finally annealed at 973 K, especially in the case of Si:O,H prepared from Si:O processed at the highest temperature applied (1573 K). Just these samples contained a lot of SiO2−x precipitates while most of point defects were removed at temperature of preparation [11]. One can speculate that hydrogen removed from Si:O,H in effect of annealing at 723 K under 1 bar for 1 h was originally bonded mostly to the point-like and similar defects, while hydrogen still detectable after annealing at 973 K was bonded predominantly to some oxygen-related defects at the SiO2−x /Si interface and so almost perfectly reproduced the oxygen depth profile. This last observation suggests a possibility to consider hydrogen introduction into Si:O and similar structures [12] followed by its subsequent out-diffusion at annealing the sample as a useful tool to discriminate the point like defects from the oxygen-containing lager ones related to the presence of oxygen. 4. Conclusions An amount and distribution of hydrogen introduced into Si:O by the hydrogen plasma treatment are highly sensitive to the defect structure of Si:O related in turn to the parameters of its processing. Specific character of hydrogen interaction with defects and oxygen has been stated for the processed Si-based SOI-like structures prepared by oxygen implantation followed by the high temperature–pressure treatment. Our results confirm that small vacancy clusters and SiO2−x clusters/precipitates can be the preferable form of residual defects in the HT–HP-processed oxygen implanted silicon, also after the subsequent hydrogen plasma treatment followed by final low-temperature annealing.
Hydrogen gettered within the Si:O,H samples remains to be detectable even after annealing at 973 K. This suggests dominating chemical interaction of hydrogen with oxygen (with that probably in the form of sub-stoichiometric SiO2−x ) while some part of hydrogen may be bonded to other point like defects (such as Si dangling bonds, etc.). This suggests also a possibility to use the hydrogen plasma treatment for detection/profiling of defects and of the depth profile/state of oxygen in the SOI-like and similar systems. More complete understanding of the effects reported in this study demands further research, in particular on the defect structure of investigated samples. References [1] A. Misiuk, A. Barcz, J. Ratajczak, L. Bryja, Effect of high hydrostatic pressure during annealing on silicon implanted with oxygen, Journal of Materials Science: Materials in Electronics 14 (2003) 295–298. [2] I.V. Antonova, A. Misiuk, C.A. Londos, Electrical properties of multiplelayer structures formed by implantation of nitrogen or oxygen and annealed under high pressure, Journal of Applied Physics 99 (2006) 0335061–0335066. [3] A.G. Ulyashin, J.S. Christensen, B.G. Svensson, R. Kogler, W. Skorupa, Hydrogen gettering at buried defect layers in ion-implanted silicon by plasma hydrogenation and annealing, Nuclear Instruments and Methods in Physics Research B 253 (2006) 126–129. [4] A. Misiuk, A. Barcz, A. Ulyashin, I.V. Antonova, M. Prujszczyk, Hydrogen gettering in annealed oxygen-implanted silicon, Semiconductor Physics, Quantum Electronics & Optoelectronics 13 (2010) 161–165. [5] M. Moreno, B. Jenichen, V.M. Kaganer, W. Braun, L.A. Trampert, L. Daweritz, K.H. Ploog, MnAs nanoclusters embedded in GaAs studied by X-ray diffuse and coherent scattering, Physical Review B 67 (2003) 2352061–2352067. [6] C.A. Londos, I.V. Antonova, M. Potsidou, A. Misiuk, J. Bak-Misiuk, A.K. Gutakovskii, Study of the conversion of VO to the VO2 defect in silicon heat treated under uniform stress conditions, Journal of Applied Physics 91 (2002) 1198–1200. [7] A.G. Ulyashin, A.I. Ivanov, R. Job, A.V. Frantskevich, F.F. Komarov, A.C. Kamyshan, The hydrogen gettering at post-implantation hydrogen plasma treatments of helium and hydrogen implanted Czochralski silicon, Materials Science and Engineering B 73 (2000) 64–68. [8] Z.Q. Chen, A. Uedono, A. Ogura, H. Ono, R. Suzuki, T. Ohdairac, T. Mikado, Oxygen-related defects and their annealing behavior in low-dose separationby-implanted oxygen (SIMOX) wafers studied by slow positron beams, Applied Surface Science 194 (2002) 112–115. [9] Y. Li, J.A. Kilner, P.L.F. Hemment, A.K. Robinson, J.P. Zhang, K.J. Reeson, C.D. Marsh, G.R. Booker, Study of the microstructure of low energy (70 keV) oxygen implanted silicon, Applied Physics Letters 59 (1991) 3130–3132. [10] P.J. Caplan, E.H. Poindexter, B.E. Deal, R.R. Razouk, ESR centers interface states, and oxide fixed charge in thermally oxidized silicon wafers, Journal of Applied Physics 50 (1979) 5847–5854. [11] A. Misiuk, J. Bak-Misiuk, B. Surma, Effect of high temperature–pressure on buried silicon dioxide in SIMOX and SOI structures, Radiation Effects & Defects in Solids 158 (2003) 407–410. [12] A. Misiuk, A.G. Ulyashin, A. Barcz, P. Formanek, Accumulation of hydrogen within implantation-damaged areas in processed Si:N and Si:O, Solid State Phenomena 156–158 (2010) 319–324.