- Email: [email protected]
composition interplay in thin SiGe layers on insulator processed by Ge condensation
Materials Science in Semiconductor Processing 42 (2016) 251–254
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Strain/composition interplay in thin SiGe layers on insulator processed by Ge condensation Victor Boureau a,b, Daniel Benoit b, Bénédicte Warot a, Martin Hÿtch a, Alain Claverie a,n a b
CEMES – CNRS and Université de Toulouse, 29 rue J. Marvig, 31055 Toulouse, France STMicroelectronics, 850 rue J. Monnet, 38920 Crolles, France
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2015 Received in revised form 9 July 2015 Accepted 13 July 2015 Available online 23 July 2015
We study the interplay between strain and composition during the elementary process steps which allow the fabrication of strained ultra-thin SiGe layers on insulators from a Silicon-On-Insulator (SOI) substrate by the Ge condensation technique. Strain maps with subnanometer resolution and high precision are obtained using the dark-field electron holography technique. We confirm that two basic mechanisms drive the final composition of the top layer, namely Ge injection during oxidation and Si/Ge interdiffusion, both thermally activated. We show that during this process the observed strain results from the out-of-plane relaxation of the stress generated by the substitution of Si by Ge atoms in the Si lattice, rigidly bounded to the underlying buried oxide. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Germanium condensation process Strain Electron interferometry
1. Introduction
2. Experimental and samples
Ultra-Thin Body SOI substrates (UTB–SOI) are starting materials for the fabrication of ultimate planar Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). The thin Si layer on top of the Buried Oxide (BOX) insulator is used as a fully depleted canal and allows a reduction of the short channel effects and of the current leakages, compared to planar MOSFETs fabricated on Si bulk wafers [1]. Ideally, the top Si film has to be transformed locally into a SiGe thin layer to obtain a better hole mobility as required for p-type MOSFETs areas. Additionally, some in-plane compression of this SiGe channel is desirable as it would enhance further the hole mobility [2]. In principle, the Ge condensation process [3] is a technique which permits this chemical conversion. However, little is known neither on the mechanical characteristics of the resulting layers nor on the applicability of the method to Si top layers of the order of 10 nm or less. The goal of this work is threefold, (i) demonstrate the adequateness of the Dark-Field Electron Holography (DFEH) technique to obtain strain and composition information from such thin layers, (ii) gain some insight on the interplay between strain and composition during the main process steps involved on the Ge condensation technique and (iii) demonstrate that strained-SiGe-OnInsulator (s-SGOI) layers suitable for the fabrication of p-MOS transistors can be locally fabricated using this process.
2.1. From SOI to SGOI wafers: the Ge condensation technique We start from a SOI wafer with a top 11 nm-thick Si layer on a 20 nm-thick BOX. Then, a 8.5 nm-thick Si0.75Ge0.25 layer is epitaxially grown on this wafer (Fig. 1), by Rapid Thermal Chemical Vapor Deposition (RTCVD) in an AMAT Centura Epi reactor. In this study, we focus on three successive elementary steps. A high temperature oxidation by Rapid Thermal Oxidation (RTO) at 1100 °C for 45 s in an AMAT Radiance chamber, consuming 6.5 nm of the top of the layer. Then a low temperature oxidation at 800 °C during 1 h 30 min, consuming 4 nm of the remaining crystalline top of the structure and finally a non-oxidizing Rapid Thermal Annealing (RTA) at 1050 °C during 30 min. Both oxidation processes, steps 1 and 2 in Fig. 1, refer to a Ge condensation process. During that process, the oxidation of the SiGe layers results in the preferential oxidation of the Si atoms, due to the lower Gibbs energy of formation of SiO2 compared to GeO2 [4]. Thus, the remaining Ge atoms are rejected into the underlying SiGe crystal, below the oxidation front. As the miscibility of Ge in SiO2 is very small, the Ge atoms remain trapped between the two oxide layers leading to an overall enrichment of the crystalline top layer [5]. 2.2. Dark-field electron holography for strain measurement
n
Corresponding author. E-mail address: [email protected] (A. Claverie).
http://dx.doi.org/10.1016/j.mssp.2015.07.034 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
The DFEH technique is an interferometry technique, performed
252
V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
Fig. 1. Sample structure and processes studied in this work.
an ultimate strain precision of 2.10 4 over fields of view of hundreds of nanometers. Samples were prepared using a Focused Ion Beam (FIB) system (FEI Helios 600i). For that, the chunk was extracted then mounted on a copper grid before thinning the sample at 30 kV then at 5 kV. 2.3. Strain/composition conversion
Fig. 2. Principle of the DFEH measurement (top) and hologram of a SGOI layer obtained using the 004 diffracted beam (bottom).
in a TEM [6]. Its aim is to measure the strain in a region-of-interest of a crystalline sample relatively to a reference (unstrained) part of the same sample (Fig. 2). For that, we force the diffracted beams from these two regions to overlap and interfere by biasing an electrostatic biprism. The interference pattern which forms in the image plane results from the phase shift between the two electron waves caused by the deformation of the region-of-interest. This phase difference is proportional to the atomic displacement field probed by this diffracted beams and can be retrieved by Fourier analysis [7]. Finally, repeating the experiment using two non-collinear diffracted beams, one obtains from simple formulas all the strain components in the 2D observation plane. For this work, the DFEH measurements have been performed on the I2TEM, a unique-in-the-world TEM (Hitachi HF-3300) designed for interferometry experiments and installed at CEMESCNRS in 2013 [8]. It offers a subnanometer spatial resolution and
Fig. 3 is an “in-plane” strain map obtained from a DFEH image of the SGOI sample obtained after RTO (step 1 in Fig. 1) and taken using a o220 4 diffraction vector. On this map, the top layer appears with the same color than the substrate. The quantitative analysis (profile in Fig. 3) confirms that the top SiGe layer is unstrained “in-plane” with respect to bulk Si and thus the SiGe crystal itself is under compression “in-plane”. This characteristic was observed on all the SGOI samples investigated in this study. This shows that while Ge is injected and diffuses in the layer during Ge condensation and or annealing, the material retains the initial lattice parameter of pure Si in the plane parallel to the surface. Finally, we do not observe any relaxation of the structure. As reported in literature, there is no plastic relaxation because the layer thicknesses are well below the critical thickness [9,1,0] and no elastic relaxation because no SOI opening nor trench offer this possibility [11]. We also confirm that the SiGe layers are defect free. In other words, the mechanical connection between the BOX and the top layer is rigid during the enrichment process whatever the Ge content of the top layer. Actually, as the a relaxed SiGe crystal has a lattice parameter larger than that of pure Si, the stress stored in the top Ge-enriched layer can only relax in the direction perpendicular to the surface through out-of-plane strain. This is shown in Fig. 4 where the red color of the top layer indicates that its lattice is elongated along the out-of-plane direction. If we assume that the Ge atoms are all in substitution sites in the SiGe crystal, at every depth in the top layer, the lattice parameter along the out-of-plane direction results from the Poisson reaction of the material to the in-plane compression. This writes,
(
)
a si . 1 + εzzDFEH = a SiGe − 2 (a si − a SiGe )
SiGe C12 SiGe C11
(1)
where a si is the Si lattice parameter, εzzDFEH the out-of-plane strain, the elastic constants of SiGe (using a Vegard interpolation C1SiGe X between the Si and the Ge ones) and 2 aSiGe = 5431 + 0200. xGe + 0027. xGe [Å] the lattice parameter of relaxed SiGe [12]. Thus, the local Ge content of the layer can be retrieved from the out-of-plane strain measured at every depth. Actually, when thinning the sample for TEM analysis, some relaxation may occur. The strain state of the bulk material can be slightly different from that measured by DFEH on a thin lamella. Another effect which we have taken into consideration is the possible influence of the dynamical scattering of electrons on the “weight” of the different slices of the material along the electron path [13]. For this reason, we have modeled the structure by Finite Element Modeling (FEM) using the Comsol Multiphysics software.
V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
253
Fig. 3. Bright field image of a SGOI after the first oxidation step (left). DFEH in-plane strain map (middle) and profile (right) measured across the sample.
DFEH, with results obtained by Electron Energy Loss Spectroscopy (EELS) and Time-Of-Flight Secondary Ion Mass Spectrometry (TOFSIMS) which probably have a lower spatial resolution but the required precision. The good agreement between these different results allows us to validate the method we have proposed above in this paper. It also confirms that our initial hypothesis stating that all the Ge atoms are in substitution sites in the Si lattice is correct. Indeed, Ge interstitials would dramatically increase stress in the layer, giving rise to some out-of-plane strain significantly larger than observed.
3. Results and discussion
Fig. 4. Left, bright field image of the SGOI sample after Ge condensation by RTO (step 1 in Fig. 1). Right, out-of-plane strain map of the same sample obtained by DFEH. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
We have thus retrieved the Ge composition profile which generates the strain profile experimentally obtained (Fig. 5(a)). Fig. 5(b) compares the Ge profile across the layer obtained by
Fig. 6 summarizes the different results we have obtained on the 3 different samples we have investigated (see Fig. 1). After the first RTO step at 1100 °C, we obtain a smooth but gradually decreasing Ge concentration profile across the 13 nm-thick remaining top crystalline layer. After conventional oxidation at 800 °C, we observe a large accumulation of Ge close to the top of the layer, i.e. close to the oxidation front. Finally, non-oxidizing annealing by RTA of this sample at 1050 °C homogenizes the Ge content across the SiGe layer. Through these 3 examples we evidence the two distinct mechanisms which drive the final distribution of Ge in the layer. On one hand, the oxidation of the SiGe injects Ge atoms in the sublayer giving rise to a characteristic snow-plow mechanism
Fig. 5. (a) Comparison between the out-of-plane strain profile across the SiGe layer measured by DFEH and obtained by FEM output. (b) Comparison between the Ge concentrations across the top layer obtained by DFEH, EELS and TOF-SIMS.
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V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
uncertainty of the method, from the DFEH measurements, the FEM fitting to the strain/composition conversion or to some process variability over the wafers as the samples may be not all the same on a given wafer. However, within this uncertainty, our result also confirms that there is no Ge loss during all these elementary processes and thus that Ge does not get oxidized neither injected into the BOX during condensation.
4. Conclusion
Fig. 6. Ge concentration across the the top s-SiGe layer, extracted from DFEH strain measurements, after high temperature oxidation, low temperature oxidation and after final annealing.
[14]. On the other hand, Ge diffusion occurs which tends to reduce the chemical gradient generated by the Ge injection. From the experiment discussed here, we also confirm that the “source”, i.e. the oxidation, can be largely activated at relatively low temperatures, at which the interdiffusion is only weakly activated. The gradually decreasing Ge concentration profile observed in the sample oxidized by RTO indicates that during this process the SiGe interdiffusion is not strong enough to homogenize all the Ge accumulated beneath the oxidation front across the layer. In other words, the Ge diffusion length l = Dt is not much larger than the layer thickness, what is confirmed by [15] from which we get l ¼8.6 nm in a fully strained Si0.75Ge0.25 crystal. However, since we do not observe some strong Ge accumulation beneath the oxidation front, we can infer that the oxidation rate at the top of the crystalline layer is smaller than the Ge diffusivity. From our results we get an oxidation rate of 0.14 nm s 1, indeed smaller than the diffusivity of Ge in fully strained Si0.75Ge0.25 crystal, estimated at 1.65 nm2 s 1 [15]. In contrast, after low temperature oxidation, the large Ge accumulation at the top of the layer suggests that the Ge diffusion length is much smaller than the thickness of the top layer what is again confirmed by [15], from which we get that l ¼0.35 nm in a fully strained Si0.75Ge0.25 crystal. In this case, the oxidation rate is 0.74 pm s 1, much larger than the diffusivity of Ge in fully strained Si0.75Ge0.25 crystal, estimated at 0.02 pm s 1 [15]. Thus, during oxidation Ge atoms are rejected beneath the oxidation front and accumulate over time at the top of the crystalline layer, as the Ge diffusivity is too small to drive them deeper into the layer. This is known as the snow-plow mechanism. High temperature annealing in a non-oxidizing ambient leads to the homogenization of the composition of the layer. For the conditions used here, we can estimate from [15] the diffusion length to be indeed of about 26 nm, again in a fully strained Si0.75Ge0.25 crystal. Finally we discuss the conservation of Ge during all these process steps. Initially, the sample consisted in a 8.5 nm-thick Si0.75Ge0.25 epitaxially grown on a SOI substrate. This initial amount of Ge can be expressed by 212.5% nm. To get the total amount of Ge composing the different layers after processing, we measure on Fig. 6 the integral below the Ge concentration profiles across the layer. After RTO, we find a Ge amount of 205% nm and of 220% nm after conventional oxidation. After the final RTA, we obtain 203% nm. These small differences (of the order of 5%) are not significant and must be either ascribed to the overall
In conclusion, we have studied by DFEH fully strained ultrathin SiGe layers on insulator fabricated by the Ge condensation method from UT-SOI substrates. We demonstrate that this experimental technique has no competitor to image strain with subnanometer resolution and high precision as mandatory to study such nanostructures. We show that during this process the observed strain results from the out-of-plane relaxation of the stress generated by the substitution of Si by Ge atoms in the Si lattice, rigidly bounded to the underlying BOX. The condensation process is ruled by two competitive mechanisms, namely Ge injection, at a rate depending of the SiGe oxidation rate, and Si/Ge interdiffusion. At low oxidation temperature, the Ge injection mechanism is dominant and one obtains a characteristic Ge pileup profile, resulting from the snow-plow mechanism. In contrast, at high oxidation temperature Si/Ge interdiffusion is dominant and allows evacuating the Ge rejected from the oxide beneath the oxidation front, resulting in smooth and gradually decreasing Ge profiles.
Acknowledgments The authors acknowledge the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2 and the “Conseil Regional Midi-Pyrenees” and the European FEDER for financial support within the CPER program. This work has been supported by the French National Research Agency, France under the “Investissement d’Avenir” program Reference no. ANR-I0EQPX-38-01.
References [1] T. Skotnicki, F. Arnaud, O. Faynot, Future Fab Int. 42 (2012) 72–79. [2] M.V. Fischetti, S.E. Laux, J. Appl. Phys. 80 (1996) 2234–2252. [3] T. Tezuka, N. Sugiyama, T. Mizuno, M. Suzuki, S. Takagi, Jpn. J. Appl. Phys. 40 (2001) 2866–2874. [4] Y. Maeda, Phys. Rev. B 51 (1995) 1658–1670. [5] K.H. Heinig, B. Schmidt, A. Markwitz, R. Grötzschel, M. Strobel, S. Oswald, Nucl. Instrum. Methods Phys. Res. Sect. B 148 (1999) 969–974. [6] M.J. Hÿtch, F. Houdellier, F. Hüe, E. Snoeck, Nature 453 (2008) 1086–1090. [7] M.J. Hÿtch, C. Gatel, K. Ishizuka, HoloDark Phase v1.3 (HREM Research), a plugin for DigitalMicrograph, 2010. [8] 2013 〈http://www.dailymotion.com/video/x1telvk_i2tem-more-than-amicroscope_tech〉. [9] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322–324. [10] T. Tezuka, S. Nakaharai, Y. Moriyama, N. Hirashita, E. Toyoda, T. Numata, T. Irisawa, K. Usuda, N. Sugiyama, T. Mizuno, S. Takagi, Semicond. Sci. Technol. 22 (2007) S93–S97. [11] T. Tezuka, N. Sugiyama, S. Takagi, J. Appl. Phys. 94 (2003) 7553–7559. [12] M.M. Rieger, P. Vogl, Phys. Rev. B 48 (1993) 14276–14287. [13] E. Javon, Ph.D thesis, Université de Toulouse, France, 2012. [14] F.K. LeGoues, R. Sosenberg, B.S. Meyerson, Appl. Phys. Lett. 54 (1989) 644–646. [15] R. Kube, H. Bracht, J.L. Hansen, A.N. Larsen, E.E. Haller, S. Paul, W. Lerch, J. Appl. Phys. 107 (2010) 073520.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Strain/composition interplay in thin SiGe layers on insulator processed by Ge condensation Victor Boureau a,b, Daniel Benoit b, Bénédicte Warot a, Martin Hÿtch a, Alain Claverie a,n a b
CEMES – CNRS and Université de Toulouse, 29 rue J. Marvig, 31055 Toulouse, France STMicroelectronics, 850 rue J. Monnet, 38920 Crolles, France
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2015 Received in revised form 9 July 2015 Accepted 13 July 2015 Available online 23 July 2015
We study the interplay between strain and composition during the elementary process steps which allow the fabrication of strained ultra-thin SiGe layers on insulators from a Silicon-On-Insulator (SOI) substrate by the Ge condensation technique. Strain maps with subnanometer resolution and high precision are obtained using the dark-field electron holography technique. We confirm that two basic mechanisms drive the final composition of the top layer, namely Ge injection during oxidation and Si/Ge interdiffusion, both thermally activated. We show that during this process the observed strain results from the out-of-plane relaxation of the stress generated by the substitution of Si by Ge atoms in the Si lattice, rigidly bounded to the underlying buried oxide. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Germanium condensation process Strain Electron interferometry
1. Introduction
2. Experimental and samples
Ultra-Thin Body SOI substrates (UTB–SOI) are starting materials for the fabrication of ultimate planar Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). The thin Si layer on top of the Buried Oxide (BOX) insulator is used as a fully depleted canal and allows a reduction of the short channel effects and of the current leakages, compared to planar MOSFETs fabricated on Si bulk wafers [1]. Ideally, the top Si film has to be transformed locally into a SiGe thin layer to obtain a better hole mobility as required for p-type MOSFETs areas. Additionally, some in-plane compression of this SiGe channel is desirable as it would enhance further the hole mobility [2]. In principle, the Ge condensation process [3] is a technique which permits this chemical conversion. However, little is known neither on the mechanical characteristics of the resulting layers nor on the applicability of the method to Si top layers of the order of 10 nm or less. The goal of this work is threefold, (i) demonstrate the adequateness of the Dark-Field Electron Holography (DFEH) technique to obtain strain and composition information from such thin layers, (ii) gain some insight on the interplay between strain and composition during the main process steps involved on the Ge condensation technique and (iii) demonstrate that strained-SiGe-OnInsulator (s-SGOI) layers suitable for the fabrication of p-MOS transistors can be locally fabricated using this process.
2.1. From SOI to SGOI wafers: the Ge condensation technique We start from a SOI wafer with a top 11 nm-thick Si layer on a 20 nm-thick BOX. Then, a 8.5 nm-thick Si0.75Ge0.25 layer is epitaxially grown on this wafer (Fig. 1), by Rapid Thermal Chemical Vapor Deposition (RTCVD) in an AMAT Centura Epi reactor. In this study, we focus on three successive elementary steps. A high temperature oxidation by Rapid Thermal Oxidation (RTO) at 1100 °C for 45 s in an AMAT Radiance chamber, consuming 6.5 nm of the top of the layer. Then a low temperature oxidation at 800 °C during 1 h 30 min, consuming 4 nm of the remaining crystalline top of the structure and finally a non-oxidizing Rapid Thermal Annealing (RTA) at 1050 °C during 30 min. Both oxidation processes, steps 1 and 2 in Fig. 1, refer to a Ge condensation process. During that process, the oxidation of the SiGe layers results in the preferential oxidation of the Si atoms, due to the lower Gibbs energy of formation of SiO2 compared to GeO2 [4]. Thus, the remaining Ge atoms are rejected into the underlying SiGe crystal, below the oxidation front. As the miscibility of Ge in SiO2 is very small, the Ge atoms remain trapped between the two oxide layers leading to an overall enrichment of the crystalline top layer [5]. 2.2. Dark-field electron holography for strain measurement
n
Corresponding author. E-mail address: [email protected] (A. Claverie).
http://dx.doi.org/10.1016/j.mssp.2015.07.034 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
The DFEH technique is an interferometry technique, performed
252
V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
Fig. 1. Sample structure and processes studied in this work.
an ultimate strain precision of 2.10 4 over fields of view of hundreds of nanometers. Samples were prepared using a Focused Ion Beam (FIB) system (FEI Helios 600i). For that, the chunk was extracted then mounted on a copper grid before thinning the sample at 30 kV then at 5 kV. 2.3. Strain/composition conversion
Fig. 2. Principle of the DFEH measurement (top) and hologram of a SGOI layer obtained using the 004 diffracted beam (bottom).
in a TEM [6]. Its aim is to measure the strain in a region-of-interest of a crystalline sample relatively to a reference (unstrained) part of the same sample (Fig. 2). For that, we force the diffracted beams from these two regions to overlap and interfere by biasing an electrostatic biprism. The interference pattern which forms in the image plane results from the phase shift between the two electron waves caused by the deformation of the region-of-interest. This phase difference is proportional to the atomic displacement field probed by this diffracted beams and can be retrieved by Fourier analysis [7]. Finally, repeating the experiment using two non-collinear diffracted beams, one obtains from simple formulas all the strain components in the 2D observation plane. For this work, the DFEH measurements have been performed on the I2TEM, a unique-in-the-world TEM (Hitachi HF-3300) designed for interferometry experiments and installed at CEMESCNRS in 2013 [8]. It offers a subnanometer spatial resolution and
Fig. 3 is an “in-plane” strain map obtained from a DFEH image of the SGOI sample obtained after RTO (step 1 in Fig. 1) and taken using a o220 4 diffraction vector. On this map, the top layer appears with the same color than the substrate. The quantitative analysis (profile in Fig. 3) confirms that the top SiGe layer is unstrained “in-plane” with respect to bulk Si and thus the SiGe crystal itself is under compression “in-plane”. This characteristic was observed on all the SGOI samples investigated in this study. This shows that while Ge is injected and diffuses in the layer during Ge condensation and or annealing, the material retains the initial lattice parameter of pure Si in the plane parallel to the surface. Finally, we do not observe any relaxation of the structure. As reported in literature, there is no plastic relaxation because the layer thicknesses are well below the critical thickness [9,1,0] and no elastic relaxation because no SOI opening nor trench offer this possibility [11]. We also confirm that the SiGe layers are defect free. In other words, the mechanical connection between the BOX and the top layer is rigid during the enrichment process whatever the Ge content of the top layer. Actually, as the a relaxed SiGe crystal has a lattice parameter larger than that of pure Si, the stress stored in the top Ge-enriched layer can only relax in the direction perpendicular to the surface through out-of-plane strain. This is shown in Fig. 4 where the red color of the top layer indicates that its lattice is elongated along the out-of-plane direction. If we assume that the Ge atoms are all in substitution sites in the SiGe crystal, at every depth in the top layer, the lattice parameter along the out-of-plane direction results from the Poisson reaction of the material to the in-plane compression. This writes,
(
)
a si . 1 + εzzDFEH = a SiGe − 2 (a si − a SiGe )
SiGe C12 SiGe C11
(1)
where a si is the Si lattice parameter, εzzDFEH the out-of-plane strain, the elastic constants of SiGe (using a Vegard interpolation C1SiGe X between the Si and the Ge ones) and 2 aSiGe = 5431 + 0200. xGe + 0027. xGe [Å] the lattice parameter of relaxed SiGe [12]. Thus, the local Ge content of the layer can be retrieved from the out-of-plane strain measured at every depth. Actually, when thinning the sample for TEM analysis, some relaxation may occur. The strain state of the bulk material can be slightly different from that measured by DFEH on a thin lamella. Another effect which we have taken into consideration is the possible influence of the dynamical scattering of electrons on the “weight” of the different slices of the material along the electron path [13]. For this reason, we have modeled the structure by Finite Element Modeling (FEM) using the Comsol Multiphysics software.
V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
253
Fig. 3. Bright field image of a SGOI after the first oxidation step (left). DFEH in-plane strain map (middle) and profile (right) measured across the sample.
DFEH, with results obtained by Electron Energy Loss Spectroscopy (EELS) and Time-Of-Flight Secondary Ion Mass Spectrometry (TOFSIMS) which probably have a lower spatial resolution but the required precision. The good agreement between these different results allows us to validate the method we have proposed above in this paper. It also confirms that our initial hypothesis stating that all the Ge atoms are in substitution sites in the Si lattice is correct. Indeed, Ge interstitials would dramatically increase stress in the layer, giving rise to some out-of-plane strain significantly larger than observed.
3. Results and discussion
Fig. 4. Left, bright field image of the SGOI sample after Ge condensation by RTO (step 1 in Fig. 1). Right, out-of-plane strain map of the same sample obtained by DFEH. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
We have thus retrieved the Ge composition profile which generates the strain profile experimentally obtained (Fig. 5(a)). Fig. 5(b) compares the Ge profile across the layer obtained by
Fig. 6 summarizes the different results we have obtained on the 3 different samples we have investigated (see Fig. 1). After the first RTO step at 1100 °C, we obtain a smooth but gradually decreasing Ge concentration profile across the 13 nm-thick remaining top crystalline layer. After conventional oxidation at 800 °C, we observe a large accumulation of Ge close to the top of the layer, i.e. close to the oxidation front. Finally, non-oxidizing annealing by RTA of this sample at 1050 °C homogenizes the Ge content across the SiGe layer. Through these 3 examples we evidence the two distinct mechanisms which drive the final distribution of Ge in the layer. On one hand, the oxidation of the SiGe injects Ge atoms in the sublayer giving rise to a characteristic snow-plow mechanism
Fig. 5. (a) Comparison between the out-of-plane strain profile across the SiGe layer measured by DFEH and obtained by FEM output. (b) Comparison between the Ge concentrations across the top layer obtained by DFEH, EELS and TOF-SIMS.
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V. Boureau et al. / Materials Science in Semiconductor Processing 42 (2016) 251–254
uncertainty of the method, from the DFEH measurements, the FEM fitting to the strain/composition conversion or to some process variability over the wafers as the samples may be not all the same on a given wafer. However, within this uncertainty, our result also confirms that there is no Ge loss during all these elementary processes and thus that Ge does not get oxidized neither injected into the BOX during condensation.
4. Conclusion
Fig. 6. Ge concentration across the the top s-SiGe layer, extracted from DFEH strain measurements, after high temperature oxidation, low temperature oxidation and after final annealing.
[14]. On the other hand, Ge diffusion occurs which tends to reduce the chemical gradient generated by the Ge injection. From the experiment discussed here, we also confirm that the “source”, i.e. the oxidation, can be largely activated at relatively low temperatures, at which the interdiffusion is only weakly activated. The gradually decreasing Ge concentration profile observed in the sample oxidized by RTO indicates that during this process the SiGe interdiffusion is not strong enough to homogenize all the Ge accumulated beneath the oxidation front across the layer. In other words, the Ge diffusion length l = Dt is not much larger than the layer thickness, what is confirmed by [15] from which we get l ¼8.6 nm in a fully strained Si0.75Ge0.25 crystal. However, since we do not observe some strong Ge accumulation beneath the oxidation front, we can infer that the oxidation rate at the top of the crystalline layer is smaller than the Ge diffusivity. From our results we get an oxidation rate of 0.14 nm s 1, indeed smaller than the diffusivity of Ge in fully strained Si0.75Ge0.25 crystal, estimated at 1.65 nm2 s 1 [15]. In contrast, after low temperature oxidation, the large Ge accumulation at the top of the layer suggests that the Ge diffusion length is much smaller than the thickness of the top layer what is again confirmed by [15], from which we get that l ¼0.35 nm in a fully strained Si0.75Ge0.25 crystal. In this case, the oxidation rate is 0.74 pm s 1, much larger than the diffusivity of Ge in fully strained Si0.75Ge0.25 crystal, estimated at 0.02 pm s 1 [15]. Thus, during oxidation Ge atoms are rejected beneath the oxidation front and accumulate over time at the top of the crystalline layer, as the Ge diffusivity is too small to drive them deeper into the layer. This is known as the snow-plow mechanism. High temperature annealing in a non-oxidizing ambient leads to the homogenization of the composition of the layer. For the conditions used here, we can estimate from [15] the diffusion length to be indeed of about 26 nm, again in a fully strained Si0.75Ge0.25 crystal. Finally we discuss the conservation of Ge during all these process steps. Initially, the sample consisted in a 8.5 nm-thick Si0.75Ge0.25 epitaxially grown on a SOI substrate. This initial amount of Ge can be expressed by 212.5% nm. To get the total amount of Ge composing the different layers after processing, we measure on Fig. 6 the integral below the Ge concentration profiles across the layer. After RTO, we find a Ge amount of 205% nm and of 220% nm after conventional oxidation. After the final RTA, we obtain 203% nm. These small differences (of the order of 5%) are not significant and must be either ascribed to the overall
In conclusion, we have studied by DFEH fully strained ultrathin SiGe layers on insulator fabricated by the Ge condensation method from UT-SOI substrates. We demonstrate that this experimental technique has no competitor to image strain with subnanometer resolution and high precision as mandatory to study such nanostructures. We show that during this process the observed strain results from the out-of-plane relaxation of the stress generated by the substitution of Si by Ge atoms in the Si lattice, rigidly bounded to the underlying BOX. The condensation process is ruled by two competitive mechanisms, namely Ge injection, at a rate depending of the SiGe oxidation rate, and Si/Ge interdiffusion. At low oxidation temperature, the Ge injection mechanism is dominant and one obtains a characteristic Ge pileup profile, resulting from the snow-plow mechanism. In contrast, at high oxidation temperature Si/Ge interdiffusion is dominant and allows evacuating the Ge rejected from the oxide beneath the oxidation front, resulting in smooth and gradually decreasing Ge profiles.
Acknowledgments The authors acknowledge the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2 and the “Conseil Regional Midi-Pyrenees” and the European FEDER for financial support within the CPER program. This work has been supported by the French National Research Agency, France under the “Investissement d’Avenir” program Reference no. ANR-I0EQPX-38-01.
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