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Kinetics of selective epitaxial growth of Si and relaxed Ge by ultrahigh vacuum chemical vapor deposition in Si(0 0 1) windows
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 9 (2006) 460–464
Kinetics of selective epitaxial growth of Si and relaxed Ge by ultrahigh vacuum chemical vapor deposition in Si(0 0 1) windows M. Halbwaxa,b, Lam H. Nguyena,c, Fre´de´ric Fossarda,, X. Le Rouxa, V. Matheta, V. Yama, Dao Tran Caod, D. Bouchiera a
Institut d’Electronique Fondamentale (IEF), Centre National de la Recherche Scientifique (Unite´ Mixte de Recherche 8622), Baˆt 220, Universite´ Paris-Sud, 91405 Orsay Cedex, France b Lasers, Plasmas et Proce´de´s Photoniques (LP3) UMR 6182 CNRS—Universite´ de la Me´dite´ranne´e, Poˆle Scientifiqueet Technologique de Luminy, 163 Avenue de luminy—C. 917, 13288 Marseille Cedex 9, France c Institute of Engineering Physics, Hanoi University of Technology (HUT), 1 Dai Co Viet, Hanoi, Vietnam d Institute of Material Science, Vietnamese Academy of Science and Technology, 18, Hoang quoc viet, caugiay, Hanoi, Vietnam Available online 10 October 2006
Abstract Relaxed germanium was deposited following a low temperature–high temperature procedure by ultrahigh vacuum chemical vapor deposition in Si trenches opened through a SiO2 mask. The resulting growth is selective, the germanium fills the Si trenches and evolves towards a roof-shaped morphology limited by (0 0 1), {1 1 3} and {1 1 1} facets. The evolution in height of the Ge structure depends on the trench width, and can be understood by considering a growth rate in the /1 1 3S direction equal to 22% of that measured along the /0 0 1S axis. At last, a surprisingly strong Ge diffusion under the SiO2 mask was revealed by selective chemical etching. Such a phenomenon was unexpected because no diffusion through the Si/Ge interface was previously observed on plain wafer. r 2006 Elsevier Ltd. All rights reserved. PACS: 81.15.Gh; 68.47.Fg; 68.35.Fx Keywords: Selective epitaxial growth; Relaxed germanium; UHV–CVD
1. Introduction Optical interconnects processed on silicon-oninsulator [1] are considered as an attractive alternative to overcome the limitations of metallic interconnects in future microelectronic circuits [2]. In order to integrate fast and high responsivity Corresponding author. Tel.: +33 1 69 15 62 98; fax: +33 1 69 15 40 10. E-mail address: [email protected] (F. Fossard).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.08.040
photodetectors in the optical interconnects, germanium represents the best candidate because of its high absorption coefficient at 1300 nm (7500 cm1) and also because it can be deposited in trenches etched at the end of the Si microwaveguides, as shown in this paper. The epitaxial process must be selective and must fill the trenches in order to insure a perfect continuity from the 400 nm thick Si guide to the Ge detector. Due to a lattice mismatch of 4.17% between Si and Ge, the growth of thin and fully relaxed germanium films on silicon requires a
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
relevant approach [3] for locating the misfit dislocations at the very interface. This procedure preserves the most part of the active material from extended defects and has previously enabled the realization of efficient photodetectors [3,5,6]. 2. Experimental The epitaxial growth was carried out in an ultrahigh vacuum chemical vapor deposition (UHV–CVD) system using pure SiH4 and GeH4 diluted at 10% in H2 as gas sources. The substrates were p-type Si(0 0 1) wafers covered by a SiO2 layer, either thermally grown or deposited by high temperature CVD (HTO). Trenches with widths ranging from 0.3 to 100 mm were etched by reactive ion etching (RIE) through the SiO2 mask up to a depth of 180 nm. These trenches were oriented along the /1 1 0S direction. The samples treated by a modified Shiraki chemical cleaning [7] were desoxidized in vacuo by flashing at 1050 1C. In order to eliminate the remaining surface defects, a Si buffer layer was grown during 10 min at 600 1C. This results in smoothing the walls of the trenches and in the formation of {1 1 3} and {1 1 1} facets at their bottom edges. The remaining trench depth was equal to 130 nm. The Ge heteroepitaxy proceeded in two steps. In a first step, a Ge layer of 27 nm in thickness was deposited at 330 1C under a germane pressure of 0.18 Pa. As previously reported [4], the plastic relaxation is drastically enhanced at this low temperature value and can be observed by reflexion high energy electron diffraction (RHEED) during the deposition of the first monolayers. In a second step, a Ge layer was grown at 600 1C at a pressure of 1.3 Pa during 90 min [4,8]. Energy dispersive spectroscopy (EDS) was performed on an Hitachi 3600N scanning electronic microscope equipped with a ThermoNoran System SIX spectrometer.
461
(O2/3)) with the (0 0 1) plane. When considering trenches or windows oriented along the /1 1 0S directions, this behavior was observed previously for selectively grown Ge [9] and commonly reported in the case of the selective epitaxial growth of silicon [10] or silicon-germanium alloys [11]. As the surface energies do not vary significantly between these two different orientations, this faceting is attributed to a more or less drastic reduction of the precursor chemisorption rate on the {1 1 3} faces with respect to that observed on the (0 0 1) one [10]. Concerning the height of the deposit, two regimes can be distinguished as a function of the trench width. For all trenches broader than or equal to 2 mm, the deposit exhibits the shape shown in Fig. 1a and their height equals the thickness expected on a plain wafer, i.e. 520 nm for a deposition time of 90 min [4,8]. On the contrary, for the narrowest trenches, the height of the deposit depends on the trench width. For these widths ranging between 0.3 and 1 mm, the (0 0 1) top terrace has nearly disappeared and additional {1 1 1} facets can be observed, as seen in Fig. 1b. In addition, the
3. Results and discussion Fig. 1a shows a typical cross-sectional scanning electron microscopy (SEM) view of a Ge deposit grown in a broad Si trench for a deposition time of 90 min. The epitaxial growth from germane is selective: there is no Ge deposition on the SiO2 mask while the Si trench is filled by germanium. The section of the deposit is a trapezoid limited by a (0 0 1) top terrace and {1 1 3} facets starting nearly from the foot of the SiO2 walls. These facets are characterized by an angle close to 251 (i.e. arctan
Fig. 1. Cross-section SEM images of trenches of (a) 2 mm and (b) 0.3 mm in width after Ge epitaxy.
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
epitaxial film begins to wet the SiO2 walls. This behavior can be explained as following: in a first step (i) the Si trench is filled by germanium with the growth rate v/0 0 1S expected along the /0 0 1S direction. Then (ii), the Ge deposit increases in height above the Si surface at the same rate, i.e. E5.8 nm/min, and evolves towards a roof-shaped structure limited by {1 1 3} facets. In the narrowest trenches, the (0 0 1) top facet area tend to become negligible and the most part of the adsorbed germane molecules becomes available for the growth on the {1 1 3} and {1 1 1} facets. This means that (iii) the growth continues mostly along the /1 1 3S directions at a rate equal to v/1 1 3S. The related increase in height is given by v/1 1 3S/ cos(251). The contribution of the growth along the /1 1 3S axis can be evaluated by assuming firstly that v/1 1 3S is strictly nil: in this condition, the height above the Si surface should not exceed h(w) ¼ w 2/6, where w is the trench width. The height measured in excess of h(w) is attributed to the growth along /1 1 3S. According to this rough model, a growth rate v/1 1 3S close to 1.3 nm/min (i.e. 22% of the v/0 0 1S value) accounts for the data plotted in Fig. 2. Such a low growth rate enables the development of the observed {1 1 1} facets, which are assumed to evolve even more slowly. When examining more carefully the SEM image in Fig. 1b, one can remark that the gray area related to the Ge deposit seems to spread under the SiO2 layer. In order to check the possible presence of germanium under the oxide layer, concentration
profiles were performed on a 2 mm width trench by using EDS. X-rays created by the material exposed to the electron beam are collected by a detector in order to analyze the chemical composition. Combined with microscopy, we deduce the spatial distribution of chemical elements. Since silicon is present both in the mask and the substrate, it does not appear as a good marker for cross-section scans. However, since oxygen should be present only in the composition of the mask, it offers an excellent contrast with the germanium which remains in the apertures. The spatial resolution of the scan is deduced from the K-line signal of oxygen at 523 eV. Unfortunately, the Ge K-line was not observable, even for higher electron energies up to 25 keV. Therefore, we used the L-lines of Ge around 1.2 keV to analyze its location. In order to enhance the surface signal, the energy of the electron is limited at 10 keV. For higher energies, oxygen and germanium signals decrease rapidly. Fig. 3 shows the intensity of Ge L-line and O Kline as a function of the beam position along a 2 mm wide trench. The O spectrum (filled squares) presents several behaviors: far from the trench, the signal remains constant and at the edge of mask, the signal drops rapidly to 0. As mentioned above, this signal represents the cross-section map of the trench. The weak peak located at the middle of the trench is probably due to oxidized Ge since samples have been exposed to air prior to the EDS measurements. The L-line Ge spectrum (full circle) is peaked at the middle of the graph, i.e. the middle of the trench. The Ge signal decreases rapidly when reaching the edge of the trench but the curve exhibits two shoulders for positions located under silica, i.e. where the O signal is maximum. The Ge signal including these two shoulders presents a full
2.23 µm
intensity (a.u.)
1000
O (K) Ge (L)
100
1000
100
intensity (a.u.)
462
2.83 µm
10
10 0 Fig. 2. SEG Ge height as a function of the trench width after 90 min of deposition.
2 Distance (µm)
4
Fig. 3. EDS spectra of a Ge deposit in a 2 mm wide SiO2 trench.
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
463
Fig. 4. SEM image showing a trench initially filled by SEG relaxed Ge, after etching by the HF:H2O2:CH3COOH solution during 3 s.
width measured at half shoulder edges equals to 2.83 mm, to be compared to the 2.23 mm FWHM value observed for oxygen. As the trench was not laterally etched prior to the deposition of Ge, this suggests that some Ge has diffused by a length of at least 300 nm under the SiO2 mask. This result was confirmed by an experiment of chemical etching by using the solution HF:H2O2:CH3COOH (1:2:3) which etches very quickly the germanium (5200 nm/min), and very slowly the silicon (0.1 nm/min) [12]. A similar approach was proposed for evidencing the interdiffusion between Ge and Si in self-assembled quantum dots [13]. Fig. 4 shows the presence of etched regions under the SiO2 layer after 3 s of etching. This confirms unambiguously a strong lateral diffusion of Ge in Si by a length of more than 50 nm. The observed Ge diffusion under the oxide mask at 600 1C is a very surprising result, as no diffusion through the bottom Si/Ge interface was revealed within the in-depth resolution of Rutherford backscattering (i.e. 20 nm), even after a post deposition annealing at 720 1C during 2 h [8]. The possible development of tensile stress in Si under the SiO2 can favor the Ge diffusion. It is noteworthy that similar observations were made both for thermally grown SiO2 and HTO layers. The intrinsic stress in thermally grown oxide is compressive [14] and the thermal contribution resulting of differences in thermal expansion coefficients between SiO2 and
Si is also compressive [15]. Consequently the stress can be retained as a possible cause of the enhanced diffusion of Ge at the SiO2/Si interface. Acknowledgment This work was supported by the French Ministry of Research (‘‘Re´seau Micro et Nanotechnologie’’, RMNT) and by the Commission of European Communities (IP NanoCMOS, contract no. 507587). Chemical etchings and EDS spectra have been performed at the ‘‘Centrale de Technologie Universitaire’’ MINERVE facilities.
References [1] Lardenois S, Pascal D, Vivien L, Cassan E, Laval S, Orobtcouk R, et al. Opt Lett 2003;28:1150. [2] International Technology Roadmap for Semiconductors (ITRS), Interconnect Topic, 2003. [3] Colace L, Masinni G, Galluzzi F, Assento G, Capellini G, Di Gaspare L, et al. Appl Phys Lett 1998;72:3175. [4] Halbwax M, Bouchier D, Yam V, De´barre D, Nguyen LH, Zheng Y, et al. J Appl Phys 2005;97:064907. [5] Luan H-C, Wada K, Kimerling LC, Masinni G, Colace L, Assento G. Opt Mater 2001;17:71. [6] Rouviere M, Halbwax M, Cercus J-L, Cassan E, Vivien L, Pascal D, et al. Opt Eng 2005;44:1. [7] Nguyen LH, Le Thanh V, Debarre D, Yam V, Bouchier D. Mater Sci Eng B 2003;101:199. [8] Halbwax M, Rouviere M, Zheng Y, Debarre D, Nguyen LH, Cercus J-L, et al. Opt Mater 2005;27:822.
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M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
[9] Hartmann JM, Abbadie A, Papon AM, Holliger P, Rolland G, Billon T, et al. J Appl Phys 2004;95:5905. [10] Hirayama H, Hiroi M, Ide T. Phys Rev B 1993;48:17331. [11] Vescan L, Dieker C, Souifi A, Stoica T. J Appl Phys 1997;81:6709. [12] Carns TK, Tanner MO, Wang KL. J Electrochem Soc 1995;142:1260.
[13] Schmidt OG, Denker U, Christiansen S, Ernst F. Appl Phys Lett 2002;81:2614. [14] Tiller WA. J Electrochem Soc 1980;127:625. [15] (a) Jaccodine RJ, Schlegel WA. J Appl Phys 1966;37:2429; (b) Weitering HH. Appl Phys Lett 2003;83:5002.
Materials Science in Semiconductor Processing 9 (2006) 460–464
Kinetics of selective epitaxial growth of Si and relaxed Ge by ultrahigh vacuum chemical vapor deposition in Si(0 0 1) windows M. Halbwaxa,b, Lam H. Nguyena,c, Fre´de´ric Fossarda,, X. Le Rouxa, V. Matheta, V. Yama, Dao Tran Caod, D. Bouchiera a
Institut d’Electronique Fondamentale (IEF), Centre National de la Recherche Scientifique (Unite´ Mixte de Recherche 8622), Baˆt 220, Universite´ Paris-Sud, 91405 Orsay Cedex, France b Lasers, Plasmas et Proce´de´s Photoniques (LP3) UMR 6182 CNRS—Universite´ de la Me´dite´ranne´e, Poˆle Scientifiqueet Technologique de Luminy, 163 Avenue de luminy—C. 917, 13288 Marseille Cedex 9, France c Institute of Engineering Physics, Hanoi University of Technology (HUT), 1 Dai Co Viet, Hanoi, Vietnam d Institute of Material Science, Vietnamese Academy of Science and Technology, 18, Hoang quoc viet, caugiay, Hanoi, Vietnam Available online 10 October 2006
Abstract Relaxed germanium was deposited following a low temperature–high temperature procedure by ultrahigh vacuum chemical vapor deposition in Si trenches opened through a SiO2 mask. The resulting growth is selective, the germanium fills the Si trenches and evolves towards a roof-shaped morphology limited by (0 0 1), {1 1 3} and {1 1 1} facets. The evolution in height of the Ge structure depends on the trench width, and can be understood by considering a growth rate in the /1 1 3S direction equal to 22% of that measured along the /0 0 1S axis. At last, a surprisingly strong Ge diffusion under the SiO2 mask was revealed by selective chemical etching. Such a phenomenon was unexpected because no diffusion through the Si/Ge interface was previously observed on plain wafer. r 2006 Elsevier Ltd. All rights reserved. PACS: 81.15.Gh; 68.47.Fg; 68.35.Fx Keywords: Selective epitaxial growth; Relaxed germanium; UHV–CVD
1. Introduction Optical interconnects processed on silicon-oninsulator [1] are considered as an attractive alternative to overcome the limitations of metallic interconnects in future microelectronic circuits [2]. In order to integrate fast and high responsivity Corresponding author. Tel.: +33 1 69 15 62 98; fax: +33 1 69 15 40 10. E-mail address: [email protected] (F. Fossard).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.08.040
photodetectors in the optical interconnects, germanium represents the best candidate because of its high absorption coefficient at 1300 nm (7500 cm1) and also because it can be deposited in trenches etched at the end of the Si microwaveguides, as shown in this paper. The epitaxial process must be selective and must fill the trenches in order to insure a perfect continuity from the 400 nm thick Si guide to the Ge detector. Due to a lattice mismatch of 4.17% between Si and Ge, the growth of thin and fully relaxed germanium films on silicon requires a
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
relevant approach [3] for locating the misfit dislocations at the very interface. This procedure preserves the most part of the active material from extended defects and has previously enabled the realization of efficient photodetectors [3,5,6]. 2. Experimental The epitaxial growth was carried out in an ultrahigh vacuum chemical vapor deposition (UHV–CVD) system using pure SiH4 and GeH4 diluted at 10% in H2 as gas sources. The substrates were p-type Si(0 0 1) wafers covered by a SiO2 layer, either thermally grown or deposited by high temperature CVD (HTO). Trenches with widths ranging from 0.3 to 100 mm were etched by reactive ion etching (RIE) through the SiO2 mask up to a depth of 180 nm. These trenches were oriented along the /1 1 0S direction. The samples treated by a modified Shiraki chemical cleaning [7] were desoxidized in vacuo by flashing at 1050 1C. In order to eliminate the remaining surface defects, a Si buffer layer was grown during 10 min at 600 1C. This results in smoothing the walls of the trenches and in the formation of {1 1 3} and {1 1 1} facets at their bottom edges. The remaining trench depth was equal to 130 nm. The Ge heteroepitaxy proceeded in two steps. In a first step, a Ge layer of 27 nm in thickness was deposited at 330 1C under a germane pressure of 0.18 Pa. As previously reported [4], the plastic relaxation is drastically enhanced at this low temperature value and can be observed by reflexion high energy electron diffraction (RHEED) during the deposition of the first monolayers. In a second step, a Ge layer was grown at 600 1C at a pressure of 1.3 Pa during 90 min [4,8]. Energy dispersive spectroscopy (EDS) was performed on an Hitachi 3600N scanning electronic microscope equipped with a ThermoNoran System SIX spectrometer.
461
(O2/3)) with the (0 0 1) plane. When considering trenches or windows oriented along the /1 1 0S directions, this behavior was observed previously for selectively grown Ge [9] and commonly reported in the case of the selective epitaxial growth of silicon [10] or silicon-germanium alloys [11]. As the surface energies do not vary significantly between these two different orientations, this faceting is attributed to a more or less drastic reduction of the precursor chemisorption rate on the {1 1 3} faces with respect to that observed on the (0 0 1) one [10]. Concerning the height of the deposit, two regimes can be distinguished as a function of the trench width. For all trenches broader than or equal to 2 mm, the deposit exhibits the shape shown in Fig. 1a and their height equals the thickness expected on a plain wafer, i.e. 520 nm for a deposition time of 90 min [4,8]. On the contrary, for the narrowest trenches, the height of the deposit depends on the trench width. For these widths ranging between 0.3 and 1 mm, the (0 0 1) top terrace has nearly disappeared and additional {1 1 1} facets can be observed, as seen in Fig. 1b. In addition, the
3. Results and discussion Fig. 1a shows a typical cross-sectional scanning electron microscopy (SEM) view of a Ge deposit grown in a broad Si trench for a deposition time of 90 min. The epitaxial growth from germane is selective: there is no Ge deposition on the SiO2 mask while the Si trench is filled by germanium. The section of the deposit is a trapezoid limited by a (0 0 1) top terrace and {1 1 3} facets starting nearly from the foot of the SiO2 walls. These facets are characterized by an angle close to 251 (i.e. arctan
Fig. 1. Cross-section SEM images of trenches of (a) 2 mm and (b) 0.3 mm in width after Ge epitaxy.
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
epitaxial film begins to wet the SiO2 walls. This behavior can be explained as following: in a first step (i) the Si trench is filled by germanium with the growth rate v/0 0 1S expected along the /0 0 1S direction. Then (ii), the Ge deposit increases in height above the Si surface at the same rate, i.e. E5.8 nm/min, and evolves towards a roof-shaped structure limited by {1 1 3} facets. In the narrowest trenches, the (0 0 1) top facet area tend to become negligible and the most part of the adsorbed germane molecules becomes available for the growth on the {1 1 3} and {1 1 1} facets. This means that (iii) the growth continues mostly along the /1 1 3S directions at a rate equal to v/1 1 3S. The related increase in height is given by v/1 1 3S/ cos(251). The contribution of the growth along the /1 1 3S axis can be evaluated by assuming firstly that v/1 1 3S is strictly nil: in this condition, the height above the Si surface should not exceed h(w) ¼ w 2/6, where w is the trench width. The height measured in excess of h(w) is attributed to the growth along /1 1 3S. According to this rough model, a growth rate v/1 1 3S close to 1.3 nm/min (i.e. 22% of the v/0 0 1S value) accounts for the data plotted in Fig. 2. Such a low growth rate enables the development of the observed {1 1 1} facets, which are assumed to evolve even more slowly. When examining more carefully the SEM image in Fig. 1b, one can remark that the gray area related to the Ge deposit seems to spread under the SiO2 layer. In order to check the possible presence of germanium under the oxide layer, concentration
profiles were performed on a 2 mm width trench by using EDS. X-rays created by the material exposed to the electron beam are collected by a detector in order to analyze the chemical composition. Combined with microscopy, we deduce the spatial distribution of chemical elements. Since silicon is present both in the mask and the substrate, it does not appear as a good marker for cross-section scans. However, since oxygen should be present only in the composition of the mask, it offers an excellent contrast with the germanium which remains in the apertures. The spatial resolution of the scan is deduced from the K-line signal of oxygen at 523 eV. Unfortunately, the Ge K-line was not observable, even for higher electron energies up to 25 keV. Therefore, we used the L-lines of Ge around 1.2 keV to analyze its location. In order to enhance the surface signal, the energy of the electron is limited at 10 keV. For higher energies, oxygen and germanium signals decrease rapidly. Fig. 3 shows the intensity of Ge L-line and O Kline as a function of the beam position along a 2 mm wide trench. The O spectrum (filled squares) presents several behaviors: far from the trench, the signal remains constant and at the edge of mask, the signal drops rapidly to 0. As mentioned above, this signal represents the cross-section map of the trench. The weak peak located at the middle of the trench is probably due to oxidized Ge since samples have been exposed to air prior to the EDS measurements. The L-line Ge spectrum (full circle) is peaked at the middle of the graph, i.e. the middle of the trench. The Ge signal decreases rapidly when reaching the edge of the trench but the curve exhibits two shoulders for positions located under silica, i.e. where the O signal is maximum. The Ge signal including these two shoulders presents a full
2.23 µm
intensity (a.u.)
1000
O (K) Ge (L)
100
1000
100
intensity (a.u.)
462
2.83 µm
10
10 0 Fig. 2. SEG Ge height as a function of the trench width after 90 min of deposition.
2 Distance (µm)
4
Fig. 3. EDS spectra of a Ge deposit in a 2 mm wide SiO2 trench.
ARTICLE IN PRESS M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
463
Fig. 4. SEM image showing a trench initially filled by SEG relaxed Ge, after etching by the HF:H2O2:CH3COOH solution during 3 s.
width measured at half shoulder edges equals to 2.83 mm, to be compared to the 2.23 mm FWHM value observed for oxygen. As the trench was not laterally etched prior to the deposition of Ge, this suggests that some Ge has diffused by a length of at least 300 nm under the SiO2 mask. This result was confirmed by an experiment of chemical etching by using the solution HF:H2O2:CH3COOH (1:2:3) which etches very quickly the germanium (5200 nm/min), and very slowly the silicon (0.1 nm/min) [12]. A similar approach was proposed for evidencing the interdiffusion between Ge and Si in self-assembled quantum dots [13]. Fig. 4 shows the presence of etched regions under the SiO2 layer after 3 s of etching. This confirms unambiguously a strong lateral diffusion of Ge in Si by a length of more than 50 nm. The observed Ge diffusion under the oxide mask at 600 1C is a very surprising result, as no diffusion through the bottom Si/Ge interface was revealed within the in-depth resolution of Rutherford backscattering (i.e. 20 nm), even after a post deposition annealing at 720 1C during 2 h [8]. The possible development of tensile stress in Si under the SiO2 can favor the Ge diffusion. It is noteworthy that similar observations were made both for thermally grown SiO2 and HTO layers. The intrinsic stress in thermally grown oxide is compressive [14] and the thermal contribution resulting of differences in thermal expansion coefficients between SiO2 and
Si is also compressive [15]. Consequently the stress can be retained as a possible cause of the enhanced diffusion of Ge at the SiO2/Si interface. Acknowledgment This work was supported by the French Ministry of Research (‘‘Re´seau Micro et Nanotechnologie’’, RMNT) and by the Commission of European Communities (IP NanoCMOS, contract no. 507587). Chemical etchings and EDS spectra have been performed at the ‘‘Centrale de Technologie Universitaire’’ MINERVE facilities.
References [1] Lardenois S, Pascal D, Vivien L, Cassan E, Laval S, Orobtcouk R, et al. Opt Lett 2003;28:1150. [2] International Technology Roadmap for Semiconductors (ITRS), Interconnect Topic, 2003. [3] Colace L, Masinni G, Galluzzi F, Assento G, Capellini G, Di Gaspare L, et al. Appl Phys Lett 1998;72:3175. [4] Halbwax M, Bouchier D, Yam V, De´barre D, Nguyen LH, Zheng Y, et al. J Appl Phys 2005;97:064907. [5] Luan H-C, Wada K, Kimerling LC, Masinni G, Colace L, Assento G. Opt Mater 2001;17:71. [6] Rouviere M, Halbwax M, Cercus J-L, Cassan E, Vivien L, Pascal D, et al. Opt Eng 2005;44:1. [7] Nguyen LH, Le Thanh V, Debarre D, Yam V, Bouchier D. Mater Sci Eng B 2003;101:199. [8] Halbwax M, Rouviere M, Zheng Y, Debarre D, Nguyen LH, Cercus J-L, et al. Opt Mater 2005;27:822.
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M. Halbwax et al. / Materials Science in Semiconductor Processing 9 (2006) 460–464
[9] Hartmann JM, Abbadie A, Papon AM, Holliger P, Rolland G, Billon T, et al. J Appl Phys 2004;95:5905. [10] Hirayama H, Hiroi M, Ide T. Phys Rev B 1993;48:17331. [11] Vescan L, Dieker C, Souifi A, Stoica T. J Appl Phys 1997;81:6709. [12] Carns TK, Tanner MO, Wang KL. J Electrochem Soc 1995;142:1260.
[13] Schmidt OG, Denker U, Christiansen S, Ernst F. Appl Phys Lett 2002;81:2614. [14] Tiller WA. J Electrochem Soc 1980;127:625. [15] (a) Jaccodine RJ, Schlegel WA. J Appl Phys 1966;37:2429; (b) Weitering HH. Appl Phys Lett 2003;83:5002.