- Email: [email protected]
SiO2 interfaces
Physica B 283 (2000) 103}107
Grazing incidence X-ray studies of twist-bonded Si/Si and Si/SiO interfaces D. Buttard *, J. Eymery , F. Rieutord , F. Fournel , D. LuK bbert, T. Baumbach, H. Moriceau
CEA/Grenoble, De& partement de Recherche Fondamentale sur la Matie% re Condense& e, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France Fraunhofer Institut Zersto( rungsfreie Pru( fverfahren, Kru( gerstr 22 D-01326 Dresden, Germany CEA/Grenoble, LETI, De& partement de Microtechnologies, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France
Abstract Twist-bonded Si/Si (0 0 1) and Si/SiO interfaces have been investigated by grazing incidence X-ray scattering methods. For Si/Si (0 0 1) bonding, conventional X-ray re#ectivity reveals the good quality of the interfaces in terms of #atness and roughness. In-plane grazing incidence di!raction measurements around the (2 2 0) re#ection show satellite peaks close to the substrate and the layer di!raction peaks. These sharp satellites are produced by a periodic displacement resulting from a very regular buried dislocation network. The Si/SiO bonding has been studied with X-ray re#ectivity within a transmission geometry. The analysis of the data shows the high quality of both bonded Si/SiO and thermal oxide SiO /Si interfaces. 2000 Elsevier Science B.V. All rights reserved. PACS: 68.35.ct; 61.10.Eq; 61.10.Kw Keywords: Silicon; Wafer bonding; Grazing incidence di!raction; X-ray re#ectivity
1. Introduction Quantum dots of semiconductor materials are of great interest for applications in microelectronics. Electron con"nement in low-dimensional boxes allow to control new electronic properties used for example in memory devices single electron transistor [1,2]. However, due to the nanometric scale, homogeneous quantum dot network is di$cult to obtain using conventional processes, so that new substrates are required. Recently, the direct wafer bonding of an ultra-thin silicon layer on an Si (0 0 1) substrate has been successfully realised [3]. The twist of the layer with respect to the substrate leads to a square network of screw dislocations at the Si/Si interface, while the miscut angle of the substrate induces edgeor 603-dislocations [4,5]. Recent calculations using
* Corresponding author. Tel.: #33-4-76889188. E-mail address: [email protected] (D. Buttard)
continuum elasticity theory [6,7] have shown that these dislocation lattices induce a periodic strain "eld, close to the surface if the layer is ultra thin (&100 As ). This could be used to a self-organised growth of quantum dots. Direct wafer bonding is also a basic process in several silicon-on-insulator (SOI) technologies dedicated to low-voltage and high-speed devices, for which Si/SiO /Si structures are now employed. Structural investigations of the silicon interface are of great interest to understand the bonding mechanisms. However, the experimental observation is a tricky problem due to the low contrast density between both sides of the interface. Transmission electron microscopy (TEM) observations of such interfaces have been performed, but the small analysed volume of this technique is a serious limitation [8]. X-ray scattering measurements which are sensitive to the electron distribution, are an alternative way to get quantitative results on a large sample area. These methods, which are a powerful nondestructive tool for the structural investigation, are also very sensitive to a weak electron density
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 9 0 0 - 6
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D. Buttard et al. / Physica B 283 (2000) 103}107
contrast and to the surface properties when used in grazing incidence geometry. The aim of this paper is to report our recent X-ray studies performed on two kinds of silicon bonded materials (hydrophobic bonding Si/Si and hydrophilic bonding Si/SiO ), with X-ray re#ectivity (XRR) and grazing inci dence di!raction (GID). After a short introduction about the sample preparation, preliminary results obtained at the european synchrotron research facility (E.S.R.F.) Xray source will be shown.
2. Experimental results and discussion
Fig. 1. Experimental (symbols) and calculated (solid line) SXR pro"les obtained from samples A and B for two di!erent twist angles.
2.1. Hydrophobic bonding Si/Si 4-in standard SOI (0 0 1) wafers were bonded to standard Si (0 0 1) substrates with a twist angle *h between their [1 1 0] crystalline axis. Before the direct wafer hydrophobic bonding, the two wafers were cleaned in an HF acid solution in order to remove the native oxide present at the top surface. After the bonding and a hightemperature annealing, the substrate of the SOI wafer was thinned by grinding using the initial buried oxide as stop etch layer. SOI layer is then thinned by thermal oxidation in order to get an accurate thickness. The samples result in an ultra thin Si (0 0 1) layer (90}250 As ) on a 500 lm thick silicon (0 0 1) substrate with a clean, polish and #at surface [9]. Four samples have been investigated with di!erent twist angles (see Table 1). Specular X-ray re#ectivity (SXR) was preliminary performed in our laboratory on a conventional 3-circle goniometer with a rotating anode [10]. It has been shown that the twisted samples have very small thickness #uctuations and no extended defects. Fig. 1 shows SXR results obtained at the ESRF BM32 (CRG-IF) beamline for samples A and B. The selected wavelength k was 1.500 As (8.3 keV) and the out-of-plane (in-plane) divergence was about 0.0073 (0.0603). After the normalisation with the incident beam intensity I , the re#ected intensity I is represented in a semi-logarithmic scale versus the momentum transfer q "(4p/j)sin a where a is the inciX dence angle. The re#ectivity pro"le can be analysed following the structure described in the inset of Fig. 1. After Table 1 Characteristics of the studied Si/Si samples. We have reported the nominal expected values Sample
Si thickness (As )
Twist angle *h (deg.)
Tilt angle (deg.)
A B C D
250 250 140 90
5 10 1 3
(0.4 (0.4 +0.4 *
the critical angle a of silicon, oscillations with two peri ods are observed. The small period fringes are well contrasted, this is due to the density contrast between the bonded interfacial layer (IL) of thickness t and of refrac'* tive index n , and the Si layer (t ,n ). This periodicity is '* 1 1 related to t . There is only a weak attenuation of the 1 fringes with large q which gives evidence of the very X good quality of the bonding interface. It can be shown from a kinematical model [10] that the disappearance of the fringes amplitude is related to t . The SXR results '* also reveal large period oscillations coming from a thin additional native oxide SiO surface layer (t , n ). In 1 - 1 order to get more quantitative results, these data were treated using a standard dynamical model [10]. The roughness p at the Substrate/IL, IL/Si and Si/SiO inter faces can be set to zero due to the weak density contrast between the layers. So that only "ve free parameters are considered: t , n , t , t and p . The IL introduces 1 '* '* 1 1 a change of phase between the two waves re#ected, respectively, by the silicon layer and the substrate. It has been shown [10] that *o "o !o increases '* 1 '* with *h (*o /o +1.3$0.2% for *h +53 and '* 1 *o /o +1.9$0.2% for *h +103). This evolution '* 1 can be attributed to the increase of the screw dislocation density and/or to the increase of quantity of interfacial defects (mainly SiO precipitates). The thickness estima tion of the IL from damping of the small period fringes at large q , has been found to be very low (t +(8.5$1) As X '* for *h +53, and t +(7$1) As for *h +103). '* Within the accuracy of the measurement it is roughly constant with *h . This means that the defects are mainly located in the interface plane, as con"rmed by TEM measurements [11]. The Si "lm thickness, t " 1 (234.5$1) As for *h "53 and t "(252.5$1) As for 1 *h "103, is very close to the nominal expected value given in Table 1. This con"rms the good reproducibility of the process in spite of the very small thickness. From the large period fringes we estimated t "(17$1) As . 1 The surface rms roughness p , which is related to the 1 -
D. Buttard et al. / Physica B 283 (2000) 103}107
overall re#ectivity decreases with q , is of about 4.5 As for X *h "53 and 103. This value of p has also been 1 con"rmed by atomic force microscopy. GID measurements were performed at the ESRF ID10 A (TromK ka) beamline around the (2 2 0) re#ections. The selected wavelength j was 0.930 As (13.3 keV) with a very small divergence (0.0063). The geometry of the experimental set-up is described in Fig. 2. We "rst used an NaI scintillation detector. For Sample C, Fig. 3(a) shows a transversal h-scan for a grazing incidence angle a close G to the critical angle (a "0.1343). This pro"le of di!rac tion is composed of two main di!raction peaks, one coming from the substrate (S) and the other from the bonding layer (L). As the sample is rotated in its plane,
Fig. 2. GID Geometry for (2 2 0) re#ection at the ID 10 A beam line.
Fig. 3. Transversal h-scan around the (2 2 0) re#ection from the low twisted (+1.43) sample C, with two sets of satellites around both substrate (S) and layer (L) peaks. (b) Reciprocal space mapping around the (2 2 0) re#ection of sample C, showing the dislocations truncation rods. The X scale corresponds to h scans (see Fig. 2) while the Y one corresponds to the surface normal q . X
105
the angular mis"t *h between (S) and (L) is directly related to the twist angle. We measure *h +1.43 close to the nominal expected value of 1.03. Around both substrate and layer (2 2 0) di!raction peaks, we observe well contrasted narrow satellites due to the periodic dislocations strain "eld. These peaks can be attributed to two di!erent sets of equidistant satellites. One is assigned to the substrate and the other one to the thin layer. The sharpness of the peaks (limited by the experimental resolution) gives evidence of a very good long-range periodic strain "eld along the [1 1 0] direction. Both re#ection sets have nearly the same period. The angle between two satellites of one set *h is related to the inverse of an average distance between dislocations. Indeed, let us consider the present di!raction geometry around (2 2 0) re#ection, with the transfer momentum q and q , respecV W tively, in the direction normal and parallel to the planes (2 2 0). The position of the detector is set to the 2h value, while the transversal h-scans of the sample around h is sensitive to material properties in the q direction. W By simple geometric considerations, variations along q are de"ned by *q "(4p/j)sin(h )*h . On the W W other hand, *h "2p/L, where L is a characteristic peri od in this direction. We then have the formula: L"k/ (2sin h *h ). For the substrate satellites group we calculate L "195 As , whereas for the layer set we estimate a close L value of 193 As . This average dis* tance may be predicted using the Frank's formula: L"b/(2sin (*h /2)), where b is the Burgers vector of the dislocations. From the measurements of *h value of 1.43, the pure screw dislocations (b"a[1 1 0]/2) should be spaced by 157 As . The di!erence between the Franck formula and the scan measurements could be due to an additional term in the Burgers vector, for example the edge component resulting from the tilt. Furthermore, by a rotation of the sample in its plane, we have successively measured the (2 2 0), (2 2 0) and (2 2 0) re#ections. All transverse h-scans reveal the two sets of sharp satellites. This give evidence of the presence of a square array of dislocations and it is consistent with previous electron microscopy results [3]. In order to measure the crystal truncation rods of these dislocation peaks, complementary measurements were performed using a position sensitive detector (PSD). Fig. 3(b) shows the di!raction map around the (2 2 0) re#ection obtained for sample C of Fig. 3(a). The two groups of sharp truncation rods are identi"ed. The truncation rods will be analysed to deduce the strain "eld normal to the surface. Around the Yoneda-like wings, one observed additional clouds of di!use scattering, indicating #uctuations in the strain "eld. Similar experiments were performed for di!erent samples with varying the twist angle. For example, Fig. 4 shows a pro"le of di!raction obtained with a scintillation detector from sample D. We observe the two main di!raction peaks of the substrate and of the layer with
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Fig. 4. (2 2 0) pro"le of di!raction along a transversal h-scan from the twisted (+33) sample D. This feature is di!erent from the di!raction pro"le of Fig. 3(a). We observe the two main peaks coming from the substrate and the layer, but the satellite peaks are broad, indicating #uctuations in the sample.
a measure separation angle of about 3.53. It is close to the expected nominal value of the twist angle (33). However, the full di!raction feature is very di!erent compared to Fig. 3(a) (sample C). The dislocation peaks are broad indicating strong #uctuations in the sample. This behaviour is consistent with re#ectivity data [10], with electron microscopy results performed on the same sample [11] and with recent di!use scattering experiments performed on the ESRF BM2 (D2AM) beam line showing that the density of oxides precipitates increases with the twist angle. 2.2. Hydrophilic bonding Si/SiO2 /Si We report now the results obtained on a hydrophilic bonding Si/SiO /Si. The sample is composed of a 4-inch 500 As thermal oxide grown on an Si (0 0 1) wafer hydrophilic bonded to a standard native oxide silicon (0 0 1) substrate. After a high-temperature annealing, we get the Si(500 lm)/SiO (500 As )/Si(500 lm) structure (in set of Fig. 5). In conventional X-ray re#ectivity, the signal coming from the buried interfaces is mixed with re#ection from outer surface, moreover absorption e!ect is important for thick samples. A particular geometry to avoid these problems consists in using an X-ray beam coming in and out the cleaved sample edges, with a grazing incidence angle with regard to the interface (inset of Fig. 5). The X-ray energy must be high enough to limit the absorption of the beam through the sample volume. So that the following results were obtained for j"0.459 As (27.0 keV), whereas the out-of-plane divergence was about 0.0023. Fig. 5 shows non-conventional XRR geometry results obtained on a 4 mm long sample. On the central part of the "gure, we observe a peak corresponding to the re#ection of the X-ray beam on both interfaces at an incidence angle close to zero (no critical angle in this case n 'n ). With small positive or negative 1 1 angle, we observe a slight asymmetry of the pro"le, due for example to a di!erent roughness of the bonding
Fig. 5. Experimental XRR pro"le (dotted curve) in transmission geometry obtained on sample Si/SiO /Si with a high-energy beam (the beam width is 50 lm). On both part of the central region (q "0) the oscillations reveal the good quality of either (Si/SiO ) or (SiO /Si) interface. The solid line corresponds to a "t using a dynamical model.
(Si/SiO ) and thermal oxide (SiO /Si) interface. The well contrasted Kiessig fringes give evidence of the good quality of each interface whereas the very slow damping is indicative of a low roughness. Experimental results were "tted using a dynamical model. From the fringes period, we estimate a silicon oxide thickness t of 1 492.9 As close to the nominal expected value of 500 As . The fringes amplitude, which is related to the electron density contrast *o/o between Si and SiO , has been "t to 1 a value of about 2.9%. With the density of silicon o "2.33 g/cm, we "nd o "2.26 g/cm, close to the 1 1 usual value of 2.24 g/cm for a thermal oxide. This technique is very powerful as it is a zero background technique, i.e. it can be used to follow directly the interfacial structure between two layers and to overcome the interaction e!ect of an X-ray beam and the surface.
3. Conclusion This work is devoted to the grazing incidence X-ray investigation of bonded interfaces Si/Si and Si/SiO . For Si/Si bonding the data analysis, shows that the density contrast between the interfacial layer and the bulk Si wafer is increasing with the twist angle, whereas the thickness is nearly constant. GID observation around the (2 2 0) re#ection reveals sharp satellites coming from a periodic displacement due to dislocations. The study of Si/SiO bonding using high energy transmission XRR measurements, shows oscillations produced by #at and clean interfaces. The density as well as the thickness of the buried SiO layer have been estimated. Acknowledgements The authors would like to thank J.L. Rouvie`re and K. Rousseau for T.E.M. measurements as well as N. MagneH a
D. Buttard et al. / Physica B 283 (2000) 103}107
and B. Aspar for fruitful discussions. They also gratefully acknowledge all the beamline sta!, in particular F. Zontone, J. L. Hodeau and J. F. BeH rar as well as A. VieuxChampagne for their technical help. References [1] K. Eberl, P.M. Petro!, P. Demeester (Eds.), Low Dimensional Structures Prepared by Epitaxial Growth or Regrowth on Patterned Substrates, Vol 298, NATO ASI Series E, Applied Science, Kluwer Academic Publishers, Dordrecht, 1995. [2] D. Leonard, K. Pond, P.M. Petro!, Phys. Rev. B 50 (1994) 11 687. [3] F. Fournel, H. Moriceau, N. MagneH a, J. Eymery, J.L. Rouvie`re, K. Rousseau, B. Aspar, E-MRS spring Meeting, Strasbourg June 1999.
107
[4] M. Benamara, A. Rocher, L. Laa( nab, A. Claverie, A. Laporte, G. Sarrabayrouse, L. Lescouze`re, A. PeyreLavigne, C.R. Acad. Sci. Paris 318 (2) (1994) 1459. [5] P.B. Howes, M. Benamara, F. Grey, R. Feidenhansl, M. Nielsen, F.B. Rasmussen, J. Baker, Physica B 248 (1998) 74. [6] A. Bourret, Surf. Sci. 432 (1999) 37. [7] A.E. Romanov, P.M. Petro!, J.S. Speck, Appl. Phys. Lett. 74 (1999) 2280. [8] M. Benamara, Ph. D. Thesis UniversiteH Paul Sabatier, Toulouse, France, 1996. [9] C. Maleville, B. Aspar, T. Poumeyrol, H. Moriceau, M. Bruel, A.J. Auberton-HerveH , T. Barge, Mater. Sci. Eng. B 46 (1997) 14. [10] J. Eymery, F. Fournel, F. Rieutord, D. Buttard, B. Aspar, Appl. Phys. Lett. 75 (1999) 3509. [11] J.L. Rouvie`re, K. Rousseau, F. Fournel, H. Moriceau, Appl. Phys. Lett., submitted.
Grazing incidence X-ray studies of twist-bonded Si/Si and Si/SiO interfaces D. Buttard *, J. Eymery , F. Rieutord , F. Fournel , D. LuK bbert, T. Baumbach, H. Moriceau
CEA/Grenoble, De& partement de Recherche Fondamentale sur la Matie% re Condense& e, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France Fraunhofer Institut Zersto( rungsfreie Pru( fverfahren, Kru( gerstr 22 D-01326 Dresden, Germany CEA/Grenoble, LETI, De& partement de Microtechnologies, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France
Abstract Twist-bonded Si/Si (0 0 1) and Si/SiO interfaces have been investigated by grazing incidence X-ray scattering methods. For Si/Si (0 0 1) bonding, conventional X-ray re#ectivity reveals the good quality of the interfaces in terms of #atness and roughness. In-plane grazing incidence di!raction measurements around the (2 2 0) re#ection show satellite peaks close to the substrate and the layer di!raction peaks. These sharp satellites are produced by a periodic displacement resulting from a very regular buried dislocation network. The Si/SiO bonding has been studied with X-ray re#ectivity within a transmission geometry. The analysis of the data shows the high quality of both bonded Si/SiO and thermal oxide SiO /Si interfaces. 2000 Elsevier Science B.V. All rights reserved. PACS: 68.35.ct; 61.10.Eq; 61.10.Kw Keywords: Silicon; Wafer bonding; Grazing incidence di!raction; X-ray re#ectivity
1. Introduction Quantum dots of semiconductor materials are of great interest for applications in microelectronics. Electron con"nement in low-dimensional boxes allow to control new electronic properties used for example in memory devices single electron transistor [1,2]. However, due to the nanometric scale, homogeneous quantum dot network is di$cult to obtain using conventional processes, so that new substrates are required. Recently, the direct wafer bonding of an ultra-thin silicon layer on an Si (0 0 1) substrate has been successfully realised [3]. The twist of the layer with respect to the substrate leads to a square network of screw dislocations at the Si/Si interface, while the miscut angle of the substrate induces edgeor 603-dislocations [4,5]. Recent calculations using
* Corresponding author. Tel.: #33-4-76889188. E-mail address: [email protected] (D. Buttard)
continuum elasticity theory [6,7] have shown that these dislocation lattices induce a periodic strain "eld, close to the surface if the layer is ultra thin (&100 As ). This could be used to a self-organised growth of quantum dots. Direct wafer bonding is also a basic process in several silicon-on-insulator (SOI) technologies dedicated to low-voltage and high-speed devices, for which Si/SiO /Si structures are now employed. Structural investigations of the silicon interface are of great interest to understand the bonding mechanisms. However, the experimental observation is a tricky problem due to the low contrast density between both sides of the interface. Transmission electron microscopy (TEM) observations of such interfaces have been performed, but the small analysed volume of this technique is a serious limitation [8]. X-ray scattering measurements which are sensitive to the electron distribution, are an alternative way to get quantitative results on a large sample area. These methods, which are a powerful nondestructive tool for the structural investigation, are also very sensitive to a weak electron density
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 9 0 0 - 6
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D. Buttard et al. / Physica B 283 (2000) 103}107
contrast and to the surface properties when used in grazing incidence geometry. The aim of this paper is to report our recent X-ray studies performed on two kinds of silicon bonded materials (hydrophobic bonding Si/Si and hydrophilic bonding Si/SiO ), with X-ray re#ectivity (XRR) and grazing inci dence di!raction (GID). After a short introduction about the sample preparation, preliminary results obtained at the european synchrotron research facility (E.S.R.F.) Xray source will be shown.
2. Experimental results and discussion
Fig. 1. Experimental (symbols) and calculated (solid line) SXR pro"les obtained from samples A and B for two di!erent twist angles.
2.1. Hydrophobic bonding Si/Si 4-in standard SOI (0 0 1) wafers were bonded to standard Si (0 0 1) substrates with a twist angle *h between their [1 1 0] crystalline axis. Before the direct wafer hydrophobic bonding, the two wafers were cleaned in an HF acid solution in order to remove the native oxide present at the top surface. After the bonding and a hightemperature annealing, the substrate of the SOI wafer was thinned by grinding using the initial buried oxide as stop etch layer. SOI layer is then thinned by thermal oxidation in order to get an accurate thickness. The samples result in an ultra thin Si (0 0 1) layer (90}250 As ) on a 500 lm thick silicon (0 0 1) substrate with a clean, polish and #at surface [9]. Four samples have been investigated with di!erent twist angles (see Table 1). Specular X-ray re#ectivity (SXR) was preliminary performed in our laboratory on a conventional 3-circle goniometer with a rotating anode [10]. It has been shown that the twisted samples have very small thickness #uctuations and no extended defects. Fig. 1 shows SXR results obtained at the ESRF BM32 (CRG-IF) beamline for samples A and B. The selected wavelength k was 1.500 As (8.3 keV) and the out-of-plane (in-plane) divergence was about 0.0073 (0.0603). After the normalisation with the incident beam intensity I , the re#ected intensity I is represented in a semi-logarithmic scale versus the momentum transfer q "(4p/j)sin a where a is the inciX dence angle. The re#ectivity pro"le can be analysed following the structure described in the inset of Fig. 1. After Table 1 Characteristics of the studied Si/Si samples. We have reported the nominal expected values Sample
Si thickness (As )
Twist angle *h (deg.)
Tilt angle (deg.)
A B C D
250 250 140 90
5 10 1 3
(0.4 (0.4 +0.4 *
the critical angle a of silicon, oscillations with two peri ods are observed. The small period fringes are well contrasted, this is due to the density contrast between the bonded interfacial layer (IL) of thickness t and of refrac'* tive index n , and the Si layer (t ,n ). This periodicity is '* 1 1 related to t . There is only a weak attenuation of the 1 fringes with large q which gives evidence of the very X good quality of the bonding interface. It can be shown from a kinematical model [10] that the disappearance of the fringes amplitude is related to t . The SXR results '* also reveal large period oscillations coming from a thin additional native oxide SiO surface layer (t , n ). In 1 - 1 order to get more quantitative results, these data were treated using a standard dynamical model [10]. The roughness p at the Substrate/IL, IL/Si and Si/SiO inter faces can be set to zero due to the weak density contrast between the layers. So that only "ve free parameters are considered: t , n , t , t and p . The IL introduces 1 '* '* 1 1 a change of phase between the two waves re#ected, respectively, by the silicon layer and the substrate. It has been shown [10] that *o "o !o increases '* 1 '* with *h (*o /o +1.3$0.2% for *h +53 and '* 1 *o /o +1.9$0.2% for *h +103). This evolution '* 1 can be attributed to the increase of the screw dislocation density and/or to the increase of quantity of interfacial defects (mainly SiO precipitates). The thickness estima tion of the IL from damping of the small period fringes at large q , has been found to be very low (t +(8.5$1) As X '* for *h +53, and t +(7$1) As for *h +103). '* Within the accuracy of the measurement it is roughly constant with *h . This means that the defects are mainly located in the interface plane, as con"rmed by TEM measurements [11]. The Si "lm thickness, t " 1 (234.5$1) As for *h "53 and t "(252.5$1) As for 1 *h "103, is very close to the nominal expected value given in Table 1. This con"rms the good reproducibility of the process in spite of the very small thickness. From the large period fringes we estimated t "(17$1) As . 1 The surface rms roughness p , which is related to the 1 -
D. Buttard et al. / Physica B 283 (2000) 103}107
overall re#ectivity decreases with q , is of about 4.5 As for X *h "53 and 103. This value of p has also been 1 con"rmed by atomic force microscopy. GID measurements were performed at the ESRF ID10 A (TromK ka) beamline around the (2 2 0) re#ections. The selected wavelength j was 0.930 As (13.3 keV) with a very small divergence (0.0063). The geometry of the experimental set-up is described in Fig. 2. We "rst used an NaI scintillation detector. For Sample C, Fig. 3(a) shows a transversal h-scan for a grazing incidence angle a close G to the critical angle (a "0.1343). This pro"le of di!rac tion is composed of two main di!raction peaks, one coming from the substrate (S) and the other from the bonding layer (L). As the sample is rotated in its plane,
Fig. 2. GID Geometry for (2 2 0) re#ection at the ID 10 A beam line.
Fig. 3. Transversal h-scan around the (2 2 0) re#ection from the low twisted (+1.43) sample C, with two sets of satellites around both substrate (S) and layer (L) peaks. (b) Reciprocal space mapping around the (2 2 0) re#ection of sample C, showing the dislocations truncation rods. The X scale corresponds to h scans (see Fig. 2) while the Y one corresponds to the surface normal q . X
105
the angular mis"t *h between (S) and (L) is directly related to the twist angle. We measure *h +1.43 close to the nominal expected value of 1.03. Around both substrate and layer (2 2 0) di!raction peaks, we observe well contrasted narrow satellites due to the periodic dislocations strain "eld. These peaks can be attributed to two di!erent sets of equidistant satellites. One is assigned to the substrate and the other one to the thin layer. The sharpness of the peaks (limited by the experimental resolution) gives evidence of a very good long-range periodic strain "eld along the [1 1 0] direction. Both re#ection sets have nearly the same period. The angle between two satellites of one set *h is related to the inverse of an average distance between dislocations. Indeed, let us consider the present di!raction geometry around (2 2 0) re#ection, with the transfer momentum q and q , respecV W tively, in the direction normal and parallel to the planes (2 2 0). The position of the detector is set to the 2h value, while the transversal h-scans of the sample around h is sensitive to material properties in the q direction. W By simple geometric considerations, variations along q are de"ned by *q "(4p/j)sin(h )*h . On the W W other hand, *h "2p/L, where L is a characteristic peri od in this direction. We then have the formula: L"k/ (2sin h *h ). For the substrate satellites group we calculate L "195 As , whereas for the layer set we estimate a close L value of 193 As . This average dis* tance may be predicted using the Frank's formula: L"b/(2sin (*h /2)), where b is the Burgers vector of the dislocations. From the measurements of *h value of 1.43, the pure screw dislocations (b"a[1 1 0]/2) should be spaced by 157 As . The di!erence between the Franck formula and the scan measurements could be due to an additional term in the Burgers vector, for example the edge component resulting from the tilt. Furthermore, by a rotation of the sample in its plane, we have successively measured the (2 2 0), (2 2 0) and (2 2 0) re#ections. All transverse h-scans reveal the two sets of sharp satellites. This give evidence of the presence of a square array of dislocations and it is consistent with previous electron microscopy results [3]. In order to measure the crystal truncation rods of these dislocation peaks, complementary measurements were performed using a position sensitive detector (PSD). Fig. 3(b) shows the di!raction map around the (2 2 0) re#ection obtained for sample C of Fig. 3(a). The two groups of sharp truncation rods are identi"ed. The truncation rods will be analysed to deduce the strain "eld normal to the surface. Around the Yoneda-like wings, one observed additional clouds of di!use scattering, indicating #uctuations in the strain "eld. Similar experiments were performed for di!erent samples with varying the twist angle. For example, Fig. 4 shows a pro"le of di!raction obtained with a scintillation detector from sample D. We observe the two main di!raction peaks of the substrate and of the layer with
106
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Fig. 4. (2 2 0) pro"le of di!raction along a transversal h-scan from the twisted (+33) sample D. This feature is di!erent from the di!raction pro"le of Fig. 3(a). We observe the two main peaks coming from the substrate and the layer, but the satellite peaks are broad, indicating #uctuations in the sample.
a measure separation angle of about 3.53. It is close to the expected nominal value of the twist angle (33). However, the full di!raction feature is very di!erent compared to Fig. 3(a) (sample C). The dislocation peaks are broad indicating strong #uctuations in the sample. This behaviour is consistent with re#ectivity data [10], with electron microscopy results performed on the same sample [11] and with recent di!use scattering experiments performed on the ESRF BM2 (D2AM) beam line showing that the density of oxides precipitates increases with the twist angle. 2.2. Hydrophilic bonding Si/SiO2 /Si We report now the results obtained on a hydrophilic bonding Si/SiO /Si. The sample is composed of a 4-inch 500 As thermal oxide grown on an Si (0 0 1) wafer hydrophilic bonded to a standard native oxide silicon (0 0 1) substrate. After a high-temperature annealing, we get the Si(500 lm)/SiO (500 As )/Si(500 lm) structure (in set of Fig. 5). In conventional X-ray re#ectivity, the signal coming from the buried interfaces is mixed with re#ection from outer surface, moreover absorption e!ect is important for thick samples. A particular geometry to avoid these problems consists in using an X-ray beam coming in and out the cleaved sample edges, with a grazing incidence angle with regard to the interface (inset of Fig. 5). The X-ray energy must be high enough to limit the absorption of the beam through the sample volume. So that the following results were obtained for j"0.459 As (27.0 keV), whereas the out-of-plane divergence was about 0.0023. Fig. 5 shows non-conventional XRR geometry results obtained on a 4 mm long sample. On the central part of the "gure, we observe a peak corresponding to the re#ection of the X-ray beam on both interfaces at an incidence angle close to zero (no critical angle in this case n 'n ). With small positive or negative 1 1 angle, we observe a slight asymmetry of the pro"le, due for example to a di!erent roughness of the bonding
Fig. 5. Experimental XRR pro"le (dotted curve) in transmission geometry obtained on sample Si/SiO /Si with a high-energy beam (the beam width is 50 lm). On both part of the central region (q "0) the oscillations reveal the good quality of either (Si/SiO ) or (SiO /Si) interface. The solid line corresponds to a "t using a dynamical model.
(Si/SiO ) and thermal oxide (SiO /Si) interface. The well contrasted Kiessig fringes give evidence of the good quality of each interface whereas the very slow damping is indicative of a low roughness. Experimental results were "tted using a dynamical model. From the fringes period, we estimate a silicon oxide thickness t of 1 492.9 As close to the nominal expected value of 500 As . The fringes amplitude, which is related to the electron density contrast *o/o between Si and SiO , has been "t to 1 a value of about 2.9%. With the density of silicon o "2.33 g/cm, we "nd o "2.26 g/cm, close to the 1 1 usual value of 2.24 g/cm for a thermal oxide. This technique is very powerful as it is a zero background technique, i.e. it can be used to follow directly the interfacial structure between two layers and to overcome the interaction e!ect of an X-ray beam and the surface.
3. Conclusion This work is devoted to the grazing incidence X-ray investigation of bonded interfaces Si/Si and Si/SiO . For Si/Si bonding the data analysis, shows that the density contrast between the interfacial layer and the bulk Si wafer is increasing with the twist angle, whereas the thickness is nearly constant. GID observation around the (2 2 0) re#ection reveals sharp satellites coming from a periodic displacement due to dislocations. The study of Si/SiO bonding using high energy transmission XRR measurements, shows oscillations produced by #at and clean interfaces. The density as well as the thickness of the buried SiO layer have been estimated. Acknowledgements The authors would like to thank J.L. Rouvie`re and K. Rousseau for T.E.M. measurements as well as N. MagneH a
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and B. Aspar for fruitful discussions. They also gratefully acknowledge all the beamline sta!, in particular F. Zontone, J. L. Hodeau and J. F. BeH rar as well as A. VieuxChampagne for their technical help. References [1] K. Eberl, P.M. Petro!, P. Demeester (Eds.), Low Dimensional Structures Prepared by Epitaxial Growth or Regrowth on Patterned Substrates, Vol 298, NATO ASI Series E, Applied Science, Kluwer Academic Publishers, Dordrecht, 1995. [2] D. Leonard, K. Pond, P.M. Petro!, Phys. Rev. B 50 (1994) 11 687. [3] F. Fournel, H. Moriceau, N. MagneH a, J. Eymery, J.L. Rouvie`re, K. Rousseau, B. Aspar, E-MRS spring Meeting, Strasbourg June 1999.
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[4] M. Benamara, A. Rocher, L. Laa( nab, A. Claverie, A. Laporte, G. Sarrabayrouse, L. Lescouze`re, A. PeyreLavigne, C.R. Acad. Sci. Paris 318 (2) (1994) 1459. [5] P.B. Howes, M. Benamara, F. Grey, R. Feidenhansl, M. Nielsen, F.B. Rasmussen, J. Baker, Physica B 248 (1998) 74. [6] A. Bourret, Surf. Sci. 432 (1999) 37. [7] A.E. Romanov, P.M. Petro!, J.S. Speck, Appl. Phys. Lett. 74 (1999) 2280. [8] M. Benamara, Ph. D. Thesis UniversiteH Paul Sabatier, Toulouse, France, 1996. [9] C. Maleville, B. Aspar, T. Poumeyrol, H. Moriceau, M. Bruel, A.J. Auberton-HerveH , T. Barge, Mater. Sci. Eng. B 46 (1997) 14. [10] J. Eymery, F. Fournel, F. Rieutord, D. Buttard, B. Aspar, Appl. Phys. Lett. 75 (1999) 3509. [11] J.L. Rouvie`re, K. Rousseau, F. Fournel, H. Moriceau, Appl. Phys. Lett., submitted.