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Raman studies of the thermal oxide of silicon
Solid State Communications, Vol. 37, pp. 719-723. Pergamon Press Ltd. 1981. Printed in Great Britain.
0038 - 1098/81/090719 ~ ) 5 $02.00/0
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON F. L. Galeener and J. C. Mikkelsen, Jr. Xerox Pato Alto Research Center, 3333 Coyote Hill Road, Pato Alto, California 94304, U.S.A. (Received 5 August 1980 by J. Tauc) We report the Raman spectra of silicon oxide films that have been formed by the wet oxidation of Si wafers at ~ 10 atmospheres of steam and at temperatures near 800 ° C. Apart from small features due to differing water content, the Raman spectra are identical to those of melt-quenched fused silica that has been annealed at the film growth temperature. We detect no growth anisotropy and conclude that the gross structures are the same, that numerous properties of steam oxide films can be safely inferred from measurements on appropriately annealed samples of bulk fused silica, and vice-versa. 1. INTRODUCTION THE PROPERTIES OF AMORPHOUS (a-)SiO2 are currently the subject of much interest [1 ]. Bulk vitreous silica (v-SiO2) is one of the best controlled and most studied glasses available, and may well be the "canonical" substance for improving our understanding of network glasses. Thin films of SiO2 are of great importance in technology: the most critical insulating layers in MOS electronic devices are fabricated by thermal oxidation of the silicon substrate in an atmosphere of oxygen (dry oxidation), or in steam (wet oxidation). Several of the physical properties of these films are difficult to measure because the layers are normally quite thin ( ~ 0.1/am) and are intimately in contact with the Si substrate. Examples of the X-ray diffraction and infrared (i.r.) absorption of such films are discussed in the book by Wong and Angell [2]. In this paper we report techniques for obtaining high signal-to-noise Raman spectra from films of steam oxidized Si and show that the network and Raman active defect structures of our films are indistinguishable from those of appropriately annealed bulk fused silica. In preliminary studies, it was found that backscattering (180 °) Raman spectra from dry thermal oxide films on Si substrates are dominated by second order Raman scattering from the supporting Si, so that even for 1/am thick wet oxide layers the main features of the SiO2 spectra are obscured [3]. Raman crosssections measured on bulk SiO2 enabled us to predict that satisfactory backscattering spectra can be obtained from unbacked SiO2 f'rims ~ 1 #m thick. Although our attempts to prepare edge supported 1 #m thick films by etching away a small area of the Si substrate were unsuccessful, we believe that improved preparation techniques and long data acquisition times will make the method useful, especially for studying dry oxide films
whose thickness is limited by practical oxidation times and temperatures to much less than 1/am. It is important to note that in case of such experiments from 1/am thick films, we predict a signal-to-noise ratio for equivalent spectrometric parameters that is about two orders of magnitude less than is obtained using the alternative procedures to be described in this paper. By utilizing high pressure steam oxidation of Si, we have produced oxide f'rims that are thick enough to be self-supporting and to also allow use of the optically more efficient 90 ° Raman scattering configuration. Our closed ampoule method for preparing thick steam oxide films is described in detail elsewhere [4]. The use of high pressure ( ~ 10 atm) steam oxidation to increase growth rates has recently attracted attention in the MOS electronics industry [5], and lower pressure ( ~ 1 atm) procedures have long been used [6]. As we shall indicate, there are scientific as well as technological reasons for studying steam oxidized material. 2. SPECTROSCOPIC TECHNIQUE The Raman spectra were obtained at room temperature using spectrometric equipment that is described adequately elsewhere [7]. From 1 to 2 watts of Ar ÷ ion laser radiation with wavelength X = 514.5 nm was directed at each sample, the scattered radiation was collected with f/1 optics and the monochromator spectral slit width was 5 cm - 1, or less. The relation of each sample to incident and scattered light beams is shown schematically in Fig. 1. The laser beam with diameter d -~ 2 mm was focused by a 10 power microscope objective having focal length f ~ 16 mm. The theoretical beam waist (spot diameter) Wo = (4/lr)X(f/d) was therefore about 5 p.m. The thinnest f-tim from which good spectra were obtained had thickness t -~ 6/am, and in this case it appeared that the spot
719
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RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
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\ \
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FILM THICKNESS t
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SCATTERED LIGHT TO BE DETECIED
Vol. 37, No. 9
the entrance and exit edges of the film. It is the 2 - 3 mm length of this path (compared to the film thickness) that accounts for the much greater signal to noise obtained in the 90 ° configuration. That is, the scattering volume is approximately spot diameter (or film thickness, whichever is smaller), times spot diameter, times 2 - 3 mm, while it is only spot diameter squared times film thickness in the backscattering configuration. 3. RAMAN SPECTRA OF THE STEAM OXIDE VERSUS BULK SILICA
/
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Fig. 1. Schematic diagram of the arrangement for obtaining 90 ° Raman scattering spectra from freestanding films of oxidized silicon with 6/am ~ t ~<40/am. diameter w0 was larger than t, the situation depicted in Fig. 1. We estimate that our effective spot diameter was on the order of 10-15/am, as judged by observation of the behaviour of laser light striking a screen placed above the samples, whose thickness ranged up to 40/am. (Samples were typically 5 mm square.) Careful alignment of the system in Fig. 1 was important. The apparatus was first positioned to obtain optimized Raman scattering from a polished bulk glass sample. This established the nominally correct relative positions of the incident laser beam and the scattered light detection system. The bulk sample was then replaced by a thin film sample that was manipulated into place so as to give a maximum Raman response, followed by touch-up adjustments of the collection optics. The thin film sample was attached to the end o f a small shaft that can be imagined to extend behind the sample into the plane of Fig. 1. This shaft could be rotated to make the plane of the film parallel to the laser beam, as is shown in Fig. 1. The rotator was mounted on an x - y - z micrometer stage allowing precise selection of the portion of the sample to be illuminated by the laser beam. These steps were guided by observation of the symmetry o f the pattern of laser light on the screen placed above the sample, and by the position of this pattern relative to the pattern obtained with the bulk sample. While somewhat tedious, the alignment procedure can be routinized, and it becomes easy for samples with t >/20/am. The best results were obtained for samples whose bottom edge (in Fig. 1) had been broken to produce a "square" edge. The entrance slits of the monochromator were parallel to the image of the path of laser light in the film, and were masked at top and bottom to exclude the intense laser light scattered from
A typical set of polarized Raman spectra is shown in Fig. 2. The solid line spectra were obtained from an 11/am thick film grown in 14 days at 750 ° C and 10.5 atm of steam. These are to be compared with the dashed line spectra obtained from bulk high purity watercontaining SiO2 (Suprasil-1) [8]. The bulk material was annealed to an equilibrium structure at 800 ° C, then quenched to room temperature; it was prepared in tiffs way because it has previously been shown that the Raman spectra [9-11 ] and certain other properties [12] of v-SiO2 are dependent on thermal history, in a way that can be controlled [ 10, 12]. Tire pair of spectra from the bulk samples were scaled (with one multiplicative factor) so that the HH intensity of the dominant Raman line coincides with that of the film at 420 cm - i.
3.1. Network Structure The strength and position of the peaks marked by solid vertical lines at 420 cm - 1,800 cm - 1, 1065 cm and 1200 c m - I are identical in steam oxide and bulk glass. These particular features are known to be due to the basic network of corner-connected SiO4 tetrahedra in v-SiQ [7, 13], and since their positions are quite sensitive to bond angles [13] we conclude that the basic network structure of the steam oxide is the same. In particular, since the dominant HH Raman peak occurs at the same place in both materials within 5 em -1, we calculate (cf. Table IV of [13]) that the most frequent S i - O - S i angle is the same within 0.5 °.
3.2. Raman Active Defect Structure The sharp features marked by dashed vertical lines at 495 cm -1 and 606 cm-1 have been ascribed to defects in the network [14], and have received appreciable attention in the literature [ 1 5 - 1 7 ] . They are not seen at all in the i.r. spectra of v-SiO2 [7]. The present authors [9, t 0] have made a detailed study of the dependence of the strength of these Raman active defect lines on thermal history, and have shown that the line strengths are independent of water content when samples are held for a sufficiently long time [18] at the normal fictive temperature (here TF = 800°C) before
Vol. 37, No. 9
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON RAMAN SPECTRA
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Fig. 2. A comparison of the polarized Raman spectra of bull fused silica (Suprasil-l) and an 11 #m thick film of the steam oxide of silicon. They are essentially identical, except for the two OH peak marked by vertical dotted lines. quenching to room temperature. In particular, we have shown that the equilibrated 606 cm -1 line strength varies rapidly with TF, according to exp (-- 0.44 eV/kTF) in both bulk and thermal oxide material [10]. Since the 495 cm -1 and 606 cm -l lines are virtually unchanged in Fig. 1, we conclude that the concentration of Raman active defects is the same in the steam oxide as in bulk material that has been annealed for an adequate period of time [18] at the rdm oxidation temperature.
3.3. Water content There are two Raman peaks indicating the presence of OH in the oxide film that appear to have the same position and width as in the bull material, but have about twice the amplitude. They are marked by dotted vertical lines at 970 cm -x and 3695 cm -1, and have been shown in bulk material to be due to Si-(OH) and O - H vibrations, respectively [11, 19]. As noted for Suprasil-1 in [19], the intensity at 420 cm -1 is essentially independent of OH content, and the OH concentration can be measured by the ratio of intensity at 3695 c m - 1 to that at 420 cm-1. Using data given in [11], we estimate that the film in Fig. 2 contains about 1600 ppm OH by weight. The spectra give no evidence
that water is incorporated differently in our steam oxide films than in bulk Suprasil-1. We shall discuss the OH content of steam oxide ffdms in more detail elsewhere [4, 20].
3. 4. Tyndall Scattering The Bose-Einstein peak at about 50 cm -1 in Fig. 2 is larger in the steam oxide. This may represent some small difference in the microscopic (atomic level)
structure of the materials, but is more likely due to a higher level of Tyndall scattering in the thin film. In some thin film spectra the effect was great enough to wash out the Bose-Einstein peak, but in these cases Tyndall scattering from inhomogeneities in the film interior or on its surfaces was obvious to the eye.
3.5. Anisotropy and Inhomogeneity We also looked for anisotropies that might occur as the oxide grows from the Si substrate. The film spectra illustrated in Fig. 2 were taken with the electric vector of the laser out of the plane o f Fig. 1, so that HH represents an analysis for scattered light vectors out of the plane of Fig. 1, and HV for scattered vectors in the plane. When the laser polarization was rotated into the plane of Fig. 1, normal to the film surface, both analyzer positions yielded a spectrum like the H V spectrum in Fig. 2. In another set of experiments on this film, the film itself was rotated so that its normal was perpendicular to the plane of Fig. 1, and the scattered light was collected from a (previously unused) film edge: For the standard laser polarization (out of the figure) the principal features of the HH and HV spectra were unchanged from Fig. 2 in position and relative strength. In other words, no differences were seen for laser polarization parallel or perpendicular to the plane of the film. We conclude that the microscopic
structure (the average geometry, topology and orientation o f the Si02 structural units) is isotropic. To the best of our present understanding, the isotropy of the Raman spectra does not rule out the existence of pinholes, oriented channels or oriented columnar structures, when the Si02 material involved is structurally isotropic, or nearly so. We f'md no
722
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
evidence for such morphological structures as the diffusion channels suggested by Revesz [21 ], but these may be better investigated by electron microscopy, or small-angle X-ray or neutron scattering. The coincidence of observed Raman frequencies suggests that the structure and density of the SiO2 material accompanying such structures (if they exist in our films) is essentially the same as that of bulk material.
4. CONCLUDING REMARKS We have shown that high SIN polarized Raman spectra can be obtained from the thermal steam oxide of Si, and that apart from differing water content the atomic level structure of the material is the same as that of bulk v-SiO2 appropriately annealed and quenched to room temperature. The basic networks are the same, including structural units, bond angles and bond distances. The well known Raman active defects are the same, and no new ones are seen. The ubiquitous OH impurity behaves approximately as it does in bulk glass, and no other impurity species are detected in the steam oxide. These statements are accurate to perhaps 1%; that is, most defects or impurities (except for low mass units like = S i - O ) - H and = S i - H ) would not have been observed if their concentration were below about 1%. For example, the bulk material is thought to contain as much as 500 ppm C1 which we estimate would give a polarized Raman line at ~ 350 cm -1 ; this line is not seen, presumably because it is too weak compared to the strong network scattering associated with the 420 cm -1 peak. With the exception of special (high frequency) cases like hydrogen, we estimate that non-resonance Raman scattering is unable to detect structural differences (defects, impurities) that occur in concentration ranges ~< 10 z° cm -3, the ranges that are usually considered in connection with electron and hole traps in MOS insulators and insulator-semiconductor interfaces. The techniques described here are also unlikely to detect certain morphological differences such as the "channels" proposed by Revesz [21 ]. It is with these restrictions in mind that we say the gross structures are the same, that thin films o f steam thermal oxide are structurally equivalent to water containing bulk v-SiO2. Wong and Angell [2] review X-ray and l.r. studies of films of the dry oxide which indicate that its structure is also quite similar to that of bulk v-SiO2, although no account is taken of the likely differences in Raman active defect structure associated with inequivalent thermal histories. (Such differences are implied by Raman scattering observations of greatly different structural relaxation times for dry and wet bulk v-SiO2 [9, 10].) These authors [2] also report significant
Vol. 37, No. 9
differences between the i.r. spectra of dry thermal and as-deposited CVD Si02-films. Because of their relative sharpness and sensitivity to changes in defect and network structure, it should be revealing to compare the polarized Raman spectra of Si02 films made by various techniques. 4.1. Study o f Modified Forms o f v-Si02
A clear advantage of the present techniques is to facilitate scientific study of various modified forms of v-SiO2. By preparing steam oxide films, one can circumvent the very high temperature equipment required to melt bulk SiO2 (m.p. ~ 1700 ° C), and one can greatly reduce the substantial quantities of starting material that are typically required. In the latter connection, e.g., one can use steam thermal oxidation to make isotopically enriched v-SiO2, beginning with Si wafers or Sifilms, which are then oxidized in H20 (steam) enriched with heavy hydrogen, or oxygen. The authors have already used H20 28 to prepare f'dms of v-SiO2 containing only O 28, and are using the results to test qualitative and quarLtitative predictions [13] concerning the role of oxygen motion in v-SiO: network dynamics [22]. Similarly, one can "dope" v-SiO2 by adding appropriate gases to the steam, e.g., to obtain v-SiO2 : C1 (or one can start with a doped substrate), tt may also be possible to study completely different vitreous oxides by steam oxidation of an appropriate substrate other than Si. Acknowledgements - We acknowledge helpful contributions from R. H. Geils, N. M. Johnson, and R. J. Nemanich, and are especially grateful to W. J. Mosby for acquiring most of the Raman spectra referred to in the paper.
REFERENCES 1.
2. 3. 4. 5.
6. 7. 8.
See, e.g., The Physics o f Si02 and lts Interfaces, (edited by S. T. Pantelides), Pergamon, New York, (1978); also The Physics o f MOS Insulators, (edited by G. Lucovsky, S. T. Pantelides & F. L. Galeener), Pergamon, New York, (1980). J. Wong & C. A. Angell, Glass Structure by Spec~ troscopy Marcel Dekker, New York (1976). F.L. Galeener & N. M. Johnson, (unpublished). J.C. Mikkelsen, Jr. & F. L Galeener, (submitted to Appl. Phys. Lett. (1980). See, e.g., L. E. Katz & L. C. Kimerling, J. Electrochem. Soc. 125, 1680 (1978); also L. E. Katz & B. F. Howells, Jr., J. Electrochem. Soc. 126, 1822 (1979). See, e.g., R. R. Razouk & B. E. Deal, J. Electrochem. Soc. 126, 1573 (1979). F.L. Galeener & G. Lucovsky, Phys. Rev. Lett. 37, 1474 (1976). Samples and specifications obtained from Amersit, Inc., 685 Ramsey Avenue, Hillside, New Jersey, 07205.
Vol. 39, No. 9 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
F.L. Galeener, J. C. Mikkelsen, Jr. & N. M. Johnson, in Ref. [l(a)], p. 284. J.C. Mikkelsen, Jr. & F. L. Galeener, J. NonOyst. Solids 37, 71 (1980). R.H. Stolen & G. E. Walrafen, J. Chem. Phys. 64, 2623 (1976). R. Bruckner, J. Non-Cryst. Solids 5, 123ff and 177ff (1970). F.L. Galeener, Phys. Rev. B 19, 4249 (1979). R.H. Stolen, J. T. Krause & C. R. Kurkain, Disc. Faraday Soc. 50, 103 (1970). J.B. Bates, R. W. Hendrick & L. B. Shaffer, J. Chem. Phys. 61,4163 (1974). G. lmcovsky, Phil. Mag. B39,531 (1979). F.L. Galeener,J. Non-Cryst. Solids 40,527 (1980). Adequate time periods can be estimated from data
19.
20.
21. 22.
723
given in [10], or determined by following procedures described therein. F.L. Galeener & R. H. Geils, The Structure of Non-Crystalline Materials, (edited by P. H. Gaskell), p. 223. Taylor and Francis, London (1977). J.C. Mikkelsen, Jr., F. L. Galeener & W. J. Mosby, Electronic Materials Conf., Ithaca, New York, June 1980. To be submitted to J. Electrochem. Soc. A.G. Revesz, J. Non-Crystall. Solids 4, 347 (1970);Phys. Stat. Sot. (a) 57,235 (1980); (a)57, 657 (1980); (a)58, 107 (1980). F.L. Galeener & J. C. Mikkelsen, Jr., (work in progress).
0038 - 1098/81/090719 ~ ) 5 $02.00/0
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON F. L. Galeener and J. C. Mikkelsen, Jr. Xerox Pato Alto Research Center, 3333 Coyote Hill Road, Pato Alto, California 94304, U.S.A. (Received 5 August 1980 by J. Tauc) We report the Raman spectra of silicon oxide films that have been formed by the wet oxidation of Si wafers at ~ 10 atmospheres of steam and at temperatures near 800 ° C. Apart from small features due to differing water content, the Raman spectra are identical to those of melt-quenched fused silica that has been annealed at the film growth temperature. We detect no growth anisotropy and conclude that the gross structures are the same, that numerous properties of steam oxide films can be safely inferred from measurements on appropriately annealed samples of bulk fused silica, and vice-versa. 1. INTRODUCTION THE PROPERTIES OF AMORPHOUS (a-)SiO2 are currently the subject of much interest [1 ]. Bulk vitreous silica (v-SiO2) is one of the best controlled and most studied glasses available, and may well be the "canonical" substance for improving our understanding of network glasses. Thin films of SiO2 are of great importance in technology: the most critical insulating layers in MOS electronic devices are fabricated by thermal oxidation of the silicon substrate in an atmosphere of oxygen (dry oxidation), or in steam (wet oxidation). Several of the physical properties of these films are difficult to measure because the layers are normally quite thin ( ~ 0.1/am) and are intimately in contact with the Si substrate. Examples of the X-ray diffraction and infrared (i.r.) absorption of such films are discussed in the book by Wong and Angell [2]. In this paper we report techniques for obtaining high signal-to-noise Raman spectra from films of steam oxidized Si and show that the network and Raman active defect structures of our films are indistinguishable from those of appropriately annealed bulk fused silica. In preliminary studies, it was found that backscattering (180 °) Raman spectra from dry thermal oxide films on Si substrates are dominated by second order Raman scattering from the supporting Si, so that even for 1/am thick wet oxide layers the main features of the SiO2 spectra are obscured [3]. Raman crosssections measured on bulk SiO2 enabled us to predict that satisfactory backscattering spectra can be obtained from unbacked SiO2 f'rims ~ 1 #m thick. Although our attempts to prepare edge supported 1 #m thick films by etching away a small area of the Si substrate were unsuccessful, we believe that improved preparation techniques and long data acquisition times will make the method useful, especially for studying dry oxide films
whose thickness is limited by practical oxidation times and temperatures to much less than 1/am. It is important to note that in case of such experiments from 1/am thick films, we predict a signal-to-noise ratio for equivalent spectrometric parameters that is about two orders of magnitude less than is obtained using the alternative procedures to be described in this paper. By utilizing high pressure steam oxidation of Si, we have produced oxide f'rims that are thick enough to be self-supporting and to also allow use of the optically more efficient 90 ° Raman scattering configuration. Our closed ampoule method for preparing thick steam oxide films is described in detail elsewhere [4]. The use of high pressure ( ~ 10 atm) steam oxidation to increase growth rates has recently attracted attention in the MOS electronics industry [5], and lower pressure ( ~ 1 atm) procedures have long been used [6]. As we shall indicate, there are scientific as well as technological reasons for studying steam oxidized material. 2. SPECTROSCOPIC TECHNIQUE The Raman spectra were obtained at room temperature using spectrometric equipment that is described adequately elsewhere [7]. From 1 to 2 watts of Ar ÷ ion laser radiation with wavelength X = 514.5 nm was directed at each sample, the scattered radiation was collected with f/1 optics and the monochromator spectral slit width was 5 cm - 1, or less. The relation of each sample to incident and scattered light beams is shown schematically in Fig. 1. The laser beam with diameter d -~ 2 mm was focused by a 10 power microscope objective having focal length f ~ 16 mm. The theoretical beam waist (spot diameter) Wo = (4/lr)X(f/d) was therefore about 5 p.m. The thinnest f-tim from which good spectra were obtained had thickness t -~ 6/am, and in this case it appeared that the spot
719
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RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
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Vol. 37, No. 9
the entrance and exit edges of the film. It is the 2 - 3 mm length of this path (compared to the film thickness) that accounts for the much greater signal to noise obtained in the 90 ° configuration. That is, the scattering volume is approximately spot diameter (or film thickness, whichever is smaller), times spot diameter, times 2 - 3 mm, while it is only spot diameter squared times film thickness in the backscattering configuration. 3. RAMAN SPECTRA OF THE STEAM OXIDE VERSUS BULK SILICA
/
INCIDENT &SER B E A M
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Fig. 1. Schematic diagram of the arrangement for obtaining 90 ° Raman scattering spectra from freestanding films of oxidized silicon with 6/am ~ t ~<40/am. diameter w0 was larger than t, the situation depicted in Fig. 1. We estimate that our effective spot diameter was on the order of 10-15/am, as judged by observation of the behaviour of laser light striking a screen placed above the samples, whose thickness ranged up to 40/am. (Samples were typically 5 mm square.) Careful alignment of the system in Fig. 1 was important. The apparatus was first positioned to obtain optimized Raman scattering from a polished bulk glass sample. This established the nominally correct relative positions of the incident laser beam and the scattered light detection system. The bulk sample was then replaced by a thin film sample that was manipulated into place so as to give a maximum Raman response, followed by touch-up adjustments of the collection optics. The thin film sample was attached to the end o f a small shaft that can be imagined to extend behind the sample into the plane of Fig. 1. This shaft could be rotated to make the plane of the film parallel to the laser beam, as is shown in Fig. 1. The rotator was mounted on an x - y - z micrometer stage allowing precise selection of the portion of the sample to be illuminated by the laser beam. These steps were guided by observation of the symmetry o f the pattern of laser light on the screen placed above the sample, and by the position of this pattern relative to the pattern obtained with the bulk sample. While somewhat tedious, the alignment procedure can be routinized, and it becomes easy for samples with t >/20/am. The best results were obtained for samples whose bottom edge (in Fig. 1) had been broken to produce a "square" edge. The entrance slits of the monochromator were parallel to the image of the path of laser light in the film, and were masked at top and bottom to exclude the intense laser light scattered from
A typical set of polarized Raman spectra is shown in Fig. 2. The solid line spectra were obtained from an 11/am thick film grown in 14 days at 750 ° C and 10.5 atm of steam. These are to be compared with the dashed line spectra obtained from bulk high purity watercontaining SiO2 (Suprasil-1) [8]. The bulk material was annealed to an equilibrium structure at 800 ° C, then quenched to room temperature; it was prepared in tiffs way because it has previously been shown that the Raman spectra [9-11 ] and certain other properties [12] of v-SiO2 are dependent on thermal history, in a way that can be controlled [ 10, 12]. Tire pair of spectra from the bulk samples were scaled (with one multiplicative factor) so that the HH intensity of the dominant Raman line coincides with that of the film at 420 cm - i.
3.1. Network Structure The strength and position of the peaks marked by solid vertical lines at 420 cm - 1,800 cm - 1, 1065 cm and 1200 c m - I are identical in steam oxide and bulk glass. These particular features are known to be due to the basic network of corner-connected SiO4 tetrahedra in v-SiQ [7, 13], and since their positions are quite sensitive to bond angles [13] we conclude that the basic network structure of the steam oxide is the same. In particular, since the dominant HH Raman peak occurs at the same place in both materials within 5 em -1, we calculate (cf. Table IV of [13]) that the most frequent S i - O - S i angle is the same within 0.5 °.
3.2. Raman Active Defect Structure The sharp features marked by dashed vertical lines at 495 cm -1 and 606 cm-1 have been ascribed to defects in the network [14], and have received appreciable attention in the literature [ 1 5 - 1 7 ] . They are not seen at all in the i.r. spectra of v-SiO2 [7]. The present authors [9, t 0] have made a detailed study of the dependence of the strength of these Raman active defect lines on thermal history, and have shown that the line strengths are independent of water content when samples are held for a sufficiently long time [18] at the normal fictive temperature (here TF = 800°C) before
Vol. 37, No. 9
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON RAMAN SPECTRA
420 I
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721
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Fig. 2. A comparison of the polarized Raman spectra of bull fused silica (Suprasil-l) and an 11 #m thick film of the steam oxide of silicon. They are essentially identical, except for the two OH peak marked by vertical dotted lines. quenching to room temperature. In particular, we have shown that the equilibrated 606 cm -1 line strength varies rapidly with TF, according to exp (-- 0.44 eV/kTF) in both bulk and thermal oxide material [10]. Since the 495 cm -1 and 606 cm -l lines are virtually unchanged in Fig. 1, we conclude that the concentration of Raman active defects is the same in the steam oxide as in bulk material that has been annealed for an adequate period of time [18] at the rdm oxidation temperature.
3.3. Water content There are two Raman peaks indicating the presence of OH in the oxide film that appear to have the same position and width as in the bull material, but have about twice the amplitude. They are marked by dotted vertical lines at 970 cm -x and 3695 cm -1, and have been shown in bulk material to be due to Si-(OH) and O - H vibrations, respectively [11, 19]. As noted for Suprasil-1 in [19], the intensity at 420 cm -1 is essentially independent of OH content, and the OH concentration can be measured by the ratio of intensity at 3695 c m - 1 to that at 420 cm-1. Using data given in [11], we estimate that the film in Fig. 2 contains about 1600 ppm OH by weight. The spectra give no evidence
that water is incorporated differently in our steam oxide films than in bulk Suprasil-1. We shall discuss the OH content of steam oxide ffdms in more detail elsewhere [4, 20].
3. 4. Tyndall Scattering The Bose-Einstein peak at about 50 cm -1 in Fig. 2 is larger in the steam oxide. This may represent some small difference in the microscopic (atomic level)
structure of the materials, but is more likely due to a higher level of Tyndall scattering in the thin film. In some thin film spectra the effect was great enough to wash out the Bose-Einstein peak, but in these cases Tyndall scattering from inhomogeneities in the film interior or on its surfaces was obvious to the eye.
3.5. Anisotropy and Inhomogeneity We also looked for anisotropies that might occur as the oxide grows from the Si substrate. The film spectra illustrated in Fig. 2 were taken with the electric vector of the laser out of the plane o f Fig. 1, so that HH represents an analysis for scattered light vectors out of the plane of Fig. 1, and HV for scattered vectors in the plane. When the laser polarization was rotated into the plane of Fig. 1, normal to the film surface, both analyzer positions yielded a spectrum like the H V spectrum in Fig. 2. In another set of experiments on this film, the film itself was rotated so that its normal was perpendicular to the plane of Fig. 1, and the scattered light was collected from a (previously unused) film edge: For the standard laser polarization (out of the figure) the principal features of the HH and HV spectra were unchanged from Fig. 2 in position and relative strength. In other words, no differences were seen for laser polarization parallel or perpendicular to the plane of the film. We conclude that the microscopic
structure (the average geometry, topology and orientation o f the Si02 structural units) is isotropic. To the best of our present understanding, the isotropy of the Raman spectra does not rule out the existence of pinholes, oriented channels or oriented columnar structures, when the Si02 material involved is structurally isotropic, or nearly so. We f'md no
722
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
evidence for such morphological structures as the diffusion channels suggested by Revesz [21 ], but these may be better investigated by electron microscopy, or small-angle X-ray or neutron scattering. The coincidence of observed Raman frequencies suggests that the structure and density of the SiO2 material accompanying such structures (if they exist in our films) is essentially the same as that of bulk material.
4. CONCLUDING REMARKS We have shown that high SIN polarized Raman spectra can be obtained from the thermal steam oxide of Si, and that apart from differing water content the atomic level structure of the material is the same as that of bulk v-SiO2 appropriately annealed and quenched to room temperature. The basic networks are the same, including structural units, bond angles and bond distances. The well known Raman active defects are the same, and no new ones are seen. The ubiquitous OH impurity behaves approximately as it does in bulk glass, and no other impurity species are detected in the steam oxide. These statements are accurate to perhaps 1%; that is, most defects or impurities (except for low mass units like = S i - O ) - H and = S i - H ) would not have been observed if their concentration were below about 1%. For example, the bulk material is thought to contain as much as 500 ppm C1 which we estimate would give a polarized Raman line at ~ 350 cm -1 ; this line is not seen, presumably because it is too weak compared to the strong network scattering associated with the 420 cm -1 peak. With the exception of special (high frequency) cases like hydrogen, we estimate that non-resonance Raman scattering is unable to detect structural differences (defects, impurities) that occur in concentration ranges ~< 10 z° cm -3, the ranges that are usually considered in connection with electron and hole traps in MOS insulators and insulator-semiconductor interfaces. The techniques described here are also unlikely to detect certain morphological differences such as the "channels" proposed by Revesz [21 ]. It is with these restrictions in mind that we say the gross structures are the same, that thin films o f steam thermal oxide are structurally equivalent to water containing bulk v-SiO2. Wong and Angell [2] review X-ray and l.r. studies of films of the dry oxide which indicate that its structure is also quite similar to that of bulk v-SiO2, although no account is taken of the likely differences in Raman active defect structure associated with inequivalent thermal histories. (Such differences are implied by Raman scattering observations of greatly different structural relaxation times for dry and wet bulk v-SiO2 [9, 10].) These authors [2] also report significant
Vol. 37, No. 9
differences between the i.r. spectra of dry thermal and as-deposited CVD Si02-films. Because of their relative sharpness and sensitivity to changes in defect and network structure, it should be revealing to compare the polarized Raman spectra of Si02 films made by various techniques. 4.1. Study o f Modified Forms o f v-Si02
A clear advantage of the present techniques is to facilitate scientific study of various modified forms of v-SiO2. By preparing steam oxide films, one can circumvent the very high temperature equipment required to melt bulk SiO2 (m.p. ~ 1700 ° C), and one can greatly reduce the substantial quantities of starting material that are typically required. In the latter connection, e.g., one can use steam thermal oxidation to make isotopically enriched v-SiO2, beginning with Si wafers or Sifilms, which are then oxidized in H20 (steam) enriched with heavy hydrogen, or oxygen. The authors have already used H20 28 to prepare f'dms of v-SiO2 containing only O 28, and are using the results to test qualitative and quarLtitative predictions [13] concerning the role of oxygen motion in v-SiO: network dynamics [22]. Similarly, one can "dope" v-SiO2 by adding appropriate gases to the steam, e.g., to obtain v-SiO2 : C1 (or one can start with a doped substrate), tt may also be possible to study completely different vitreous oxides by steam oxidation of an appropriate substrate other than Si. Acknowledgements - We acknowledge helpful contributions from R. H. Geils, N. M. Johnson, and R. J. Nemanich, and are especially grateful to W. J. Mosby for acquiring most of the Raman spectra referred to in the paper.
REFERENCES 1.
2. 3. 4. 5.
6. 7. 8.
See, e.g., The Physics o f Si02 and lts Interfaces, (edited by S. T. Pantelides), Pergamon, New York, (1978); also The Physics o f MOS Insulators, (edited by G. Lucovsky, S. T. Pantelides & F. L. Galeener), Pergamon, New York, (1980). J. Wong & C. A. Angell, Glass Structure by Spec~ troscopy Marcel Dekker, New York (1976). F.L. Galeener & N. M. Johnson, (unpublished). J.C. Mikkelsen, Jr. & F. L Galeener, (submitted to Appl. Phys. Lett. (1980). See, e.g., L. E. Katz & L. C. Kimerling, J. Electrochem. Soc. 125, 1680 (1978); also L. E. Katz & B. F. Howells, Jr., J. Electrochem. Soc. 126, 1822 (1979). See, e.g., R. R. Razouk & B. E. Deal, J. Electrochem. Soc. 126, 1573 (1979). F.L. Galeener & G. Lucovsky, Phys. Rev. Lett. 37, 1474 (1976). Samples and specifications obtained from Amersit, Inc., 685 Ramsey Avenue, Hillside, New Jersey, 07205.
Vol. 39, No. 9 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
RAMAN STUDIES OF THE THERMAL OXIDE OF SILICON
F.L. Galeener, J. C. Mikkelsen, Jr. & N. M. Johnson, in Ref. [l(a)], p. 284. J.C. Mikkelsen, Jr. & F. L. Galeener, J. NonOyst. Solids 37, 71 (1980). R.H. Stolen & G. E. Walrafen, J. Chem. Phys. 64, 2623 (1976). R. Bruckner, J. Non-Cryst. Solids 5, 123ff and 177ff (1970). F.L. Galeener, Phys. Rev. B 19, 4249 (1979). R.H. Stolen, J. T. Krause & C. R. Kurkain, Disc. Faraday Soc. 50, 103 (1970). J.B. Bates, R. W. Hendrick & L. B. Shaffer, J. Chem. Phys. 61,4163 (1974). G. lmcovsky, Phil. Mag. B39,531 (1979). F.L. Galeener,J. Non-Cryst. Solids 40,527 (1980). Adequate time periods can be estimated from data
19.
20.
21. 22.
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given in [10], or determined by following procedures described therein. F.L. Galeener & R. H. Geils, The Structure of Non-Crystalline Materials, (edited by P. H. Gaskell), p. 223. Taylor and Francis, London (1977). J.C. Mikkelsen, Jr., F. L. Galeener & W. J. Mosby, Electronic Materials Conf., Ithaca, New York, June 1980. To be submitted to J. Electrochem. Soc. A.G. Revesz, J. Non-Crystall. Solids 4, 347 (1970);Phys. Stat. Sot. (a) 57,235 (1980); (a)57, 657 (1980); (a)58, 107 (1980). F.L. Galeener & J. C. Mikkelsen, Jr., (work in progress).