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silicon oxide multilayer with smooth interfaces produced by in situ controlled plasma-enhanced MOCVD
Thin Solid Films 358 (2000) 90±93 www.elsevier.com/locate/tsf
Metal oxide/silicon oxide multilayer with smooth interfaces produced by in situ controlled plasma-enhanced MOCVD F. Hamelmann a,*, G. Haindl a, J. Schmalhorst a, A. Aschentrup a, E. Majkova a, U. Kleineberg a, U. Heinzmann a, A. Klipp b, P. Jutzi b, A. Anopchenko c, M. Jergel c, S. Luby c a b
FakultaÈt fuÈr Physik, UniversitaÈt Bielefeld, UniversitaÈtsstrasse 25, 33615 Bielefeld, Germany FakultaÈt fuÈr Chemie, UniversitaÈt Bielefeld, UniversitaÈtsstrasse 25, 33615 Bielefeld, Germany c Institute of Physics, Slovak Acad. Sci., 842 28 Bratislava, Slovakia Received 26 May 1999; received in revised form 5 August 1999; accepted 6 August 1999
Abstract Molybdenum oxide/silicon oxide and tungsten oxide/silicon oxide multilayer with 24 periods and a period thickness of 9.2 nm were fabricated with plasma-enhanced MOCVD. The layer thickness was controlled by an in situ soft X-ray re¯ectivity measurement. For the deposition of the SiO2 layers, a new silicon organic precursor, pentamethylcyclopentadienyldisilane (Me5C5Si2H5) was used in an O2 remote plasma process. The high quality of multilayer interfaces was observed by cross-section transmission electron microscopy (TEM), the interface toughness was measured by hard X-ray re¯ectivity and diffuse scattering at grazing incidence experiments to be about 0.1 nm. Auger electron spectroscopy (AES) gives the information, that the silicon oxide is practically carbon free, and the carbon content of the metal oxides is low (,5%). q 2000 Elsevier Science S.A. All rights reserved. Keywords: Multilayers; Oxides; Plasma processing and deposition
1. Introduction Alternating multilayers of two materials with different complex refraction indices and a layer thickness of a few nanometers can be used as mirrors for soft X-rays at nongrazing incidence angles [1]. A lot of studies have been made on this ®eld, the common methods for the deposition of such multilayers are physical vapor deposition (PVD), such as electron beam evaporation [2,3] and sputtering [4]. However, little has been published about the deposition of such multilayers with chemical vapor deposition (CVD) [5]. Although this method is well established for a broad variety of industrial and experimental applications, and it has potentially some advantages to PVD methods (deposition of complex compounds, uniform deposition on shaped substrates...), it is a challenge to deposit multilayers with suf®ciently smooth interfaces with CVD. The main dif®culties are the lack of suitable precursor molecules, the high substrate temperatures needed in most CVD processes and the missing layer thickness control on a nanometer scale. While Mo/Si is a good material combination for high * Corresponding author. Tel.: 149-521-106-5465; fax: 149-521-1066001. E-mail address: [email protected] (F. Hamelmann)
re¯ectivity at wavelengths above the silicon L-edge (l . 12:4 nm) [4], the oxides of these materials are, due to their optical properties at these wavelengths, not a good choice for mirrors at 13 nm wavelength. However, for the `water-window' (between 2.4 and 4.4 nm) the absorption of oxygen is negligible, furthermore, oxide multilayers can provide generally very stable interfaces with no interdiffusion of the layers [6]. Since smooth interfaces are, because of the necessary small period thickness and high number of doublelayers, extremely important for multilayers in the `water-window', oxide multilayers with smooth interfaces may be useful for optical applications at these wavelengths. 2. Deposition procedure We produced the multilayers in a downstream plasma reactor. The plasma source is mounted above the substrate, inductively driven by a 13.54 MHz r.f. generator and a gas ¯ow of 4 sccm O2. The precursor is, along with N2 carriergas, introduced to the reactor through a ring-shaped nozzle near the silicon substrate. This setup provides a so-called `remote' plasma [7], because the plasma zone is separated from the substrate. The precursor is not activated in the plasma, but by collisions with excited O atoms or excited
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0069 5-1
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
O2 molecules near the substrate [8]. Slightly heating the substrate to 1608C has proven to be helpful for the formation of smooth layers. The total pressure was 1 Pa during the deposition, using a molecular drag pump. The depositions were made at a plasma power of just 2 W, suf®cient for deposition rates of 4 nm/min for molybdenum oxide (from Mo(CO)6), 1 nm/min for tungsten oxide (from W(CO)6) and 5 nm/min for silicon oxide (from Me5C5Si2H5). It has to be noted that the Me5C5Si2H5 precursor was cooled down to 08C to reduce the deposition rate (.10 nm/min at room temperature). This was necessary for a precise control of the layer thickness with the in situ soft X-ray re¯ectivity measurement. The use of organic precursors provides the advantage of an easy handling, compared to common CVD precursors such as silane (SiH4). The in situ soft X-ray re¯ectivity measurement consists of a XPS X-ray source, which is emitting 4.4 nm radiation of the carbon K-line. The X-ray source is separated by a thin foil from the MOCVD reactor, because of the pressure requirements (better 10 25 Pa). The source is mounted at an angle of 208 to the substrate surface, a proportional counter detects the re¯ected radiation under the same angle of incidence. The values of the wavelength l and angle of incidence a determine the period thickness d of the multilayer, as given by Bragg's law [9]: l 2dcosa 1 2 2d 2 d2 =cos2 a1=2 . In this equation l is the wavelength, d is the period thickness, a is the angle of incidence and d is the difference of the real part of the multilayer's refraction index from 1 (n 1 2 d). The use of this in situ thickness control is limited to processes with pressures lower than 10 Pa, at higher pressures too much of the 4.4 nm radiation is absorbed. The advantage of this thickness control is, that it is directly measuring the re¯ectivity of the growing multilayer stack in the soft X-ray region. The thickness of each single layer is optimized for highest possible re¯ectivity of the stack, minor errors in the thickness of a single layer will be corrected with the thickness of the next layer.
Fig. 1. In situ re¯ectivity of a 24 period WOx/SiOy multilayer.
oscillation amplitude remains constant. This behavior shows, that the interface roughness, which would result in a decreasing re¯ectivity with reduced amplitude, remains constant through the multilayer stack. Cross-section transmission electron microscopy (TEM) con®rms the conclusion from the in situ re¯ectivity measurement. Fig. 2 shows the cross-section image of the WOx/SiOy multilayer mentioned above. The periodic structure on the bottom of the image is the crystal structure of the silicon substrate. On top of the substrate, the native oxide layer of the silicon substrate can be seen in light gray. The following dark layers are the tungsten oxide, while the light layers are the deposited silicon oxide layers. In the deposited layers, no periodic structure can be observed, all deposited oxide layers are amorphous. In Fig. 3 the cross-section TEM image of a 24 period molybdenum oxide/silicon oxide multilayer, produced under identical conditions as the
3. Results Fig. 1 shows the re¯ectivity of a 24 period tungsten oxide/ silicon oxide multilayer during deposition. Starting with the re¯ectivity of the silicon substrate, the re¯ectivity is increasing during tungsten oxide deposition until it reaches a maximum. At this point, the best layer thickness is reached, shutting down the precursor ¯ow stops the deposition. The following deposition of a silicon oxide layer results in a decreasing re¯ectivity. In the minimum, the best thickness for this layer is reached. Due to the increasing number of re¯ecting interfaces, an overall increase of the re¯ectivity can be observed during the following ten periods (Fig. 1). Because of the absorption in the material, the re¯ectivity is not increasing with a further increasing number of periods, but remains constant until the last (24th) period. Also, the
91
Fig. 2. TEM image of a 24 period WOx/SiOy multilayer.
92
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
Fig. 3. TEM image of a 24 period MoOx/SiOy multilayer.
Fig. 4. (a) Cu Ka re¯ectivity (R) and detector (D) scans (dots), the later one with the sample set to the 1st Bragg order and their simulations (full lines) of the MoOx/SiOy sample, (b) Sample scans of the MoOx/SiOy sample (dots) with the detector set to 1st and 3rd Bragg maxima and their simulation (full lines).
WOx/SiOy sample, is displayed. Due to the different optical constants, the relation of the layer thickness is different, but the interface quality is equally good. The detailed study of the interface roughness was performed for the sample MoOx/SiOx by using hard X-ray re¯ectivity (XRR) and diffuse scattering at the grazing incidence (DSGI) measurements. The Cu Ka radiation (l 0:154 nm) was used. The hard XRR spectra were simulated with Fresnel computational code [10]. The interface roughness was incorporated at each interface by Debye-Waller-like attenuation factor. The evaluation of non-specular intensity due to the diffuse scattering was simulated using semikinematical approach to the distorted wave Born approximation [11]. Some results are seen in Fig. 4a,b. In the specular XRR spectrum shown in Fig. 4a, the 1st, 3rd and 5th Bragg maxima can be seen, while the Kiessig maxima coming from the ®nite size of the sample are not clearly observable due to high number of multilayer periods. The absence of 2nd Bragg maximum points at the same MoOx and SiOy layer thickness [10]. In the non-specular detector scan a strong specular maximum together with additional maxima at the positions corresponding the different Bragg orders are seen. Such maxima provide a direct experimental evidence for the existence of vertical interface correlation. The non-specular sample scans taken with the detector ®xed at the specular position for the 1st and 3rd Bragg order are shown in Fig. 4b. Except for the sharp specular maximum one can observe a broad distribution of the diffuse intensity enhanced by resonant diffuse scattering which originates from constructive interference of the X-rays scattered diffusely on different rough interfaces with replicated morphology [12]. For simulation of the DSGI data the frequency independent model of the vertical interface roughness correlation (details are in Ref. [11]) was used. The increasing roughness starting from the substrate as sj 2 sN 2 1 N 2 jDs2 was assumed. Here, s j is the rms roughness of j-th interface and s N is the rms roughness of the substrate. The multilayer period, individual layer thickness, interface rms roughness and its lateral and vertical correlation lengths obtained from the simulations of hard XRR and DSGI spectra are summarized in Table 1. The fractal parameter (h) of the interfaces is equal to unity, i.e. no fractal behavior of the interfaces was observed. The simulation used a refractive index of d 20:5 £ 1026 for the molybdenum oxide, which is between the values for MoO2 (18:8 £ 1026 ) and Mo3O (22:6 £ 1026 ) Table 1 The parameters of the MoOx/SiOx multilayer obtained from the specular and non-specular simulations; d d Mo 1 dSi is multilayer period, dMo and dSi are the individual layer thicknesses, s is the interface rms roughness, j is the lateral and Lvert vertical correlation length. Sample
d (nm)
dMo (nm) dSi (nm) s (nm)
j (nm) Lvert (nm)
MoOx/SiOy
9.2
4.6
30
4.6
0.1
40
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
93
the MoOx layers mainly contents MoO3. The carbon content of these layers is as low as 5%. The same effect of incomplete oxidation can be observed inside the SiOy layers: at the peak position of the silicon, the silicon to oxygen ratio is 1:1.3, which is most likely the result of a 2:1 mixture of SiO2 and Si. At the surface, the silicon to oxygen ratio is 1:1.6. The carbon concentration is about 2% in the silicon oxide layers, which is near the detection limit of this method. 4. Conclusions Fig. 5. Sputter AES depth pro®le of a MoOx/SiOy multilayer.
[13] at a wavelength of 0.154 nm. For the silicon oxide a refractive index of d 7:1 £ 1026 was used, close to SiO2 (7:13 £ 1026 [13]). In accordance to the TEM cross-section image, the layer thickness for the simulation was 4.6 nm for both silicon oxide and molybdenum oxide, with a thickness variation of about 8%. The low rms roughness of about 0.1 nm of the multilayer interfaces corresponds to the cross-section TEM analyses (we estimate the accuracy of our simulation procedure as ^ 0.1 nm). The amorphous structure of the individual layers promotes formation of smooth interfaces. According to our calculations, there is no substantial difference for MoOx -on SiOy and SiOy -on MoOx interface roughness. Within the accuracy of our simulation the same value of interface rms roughness can be expected in the whole multilayer stack. As follows from the value of vertical correlation length, which stretches approximately over four periods, the interfaces are to some extent correlated. Summarizing the results above, using the MOCVD technique it is possible to prepare multilayers with high number of interfaces of very low interface roughness. This can be explained by the isotropic nature of the MOCVD process, resulting in the suppression of shadowing effects and interface roughness replication. However, the interface quality of this MoOx/SiOy multilayer is comparable to titanium/carbon multilayers produced by electron-beam evaporation with ion-beam polishing [11]. A sputter Auger Electron Spectroscopy (AES) depth pro®le of the uppermost 10 periods of a MoOx/SiOy multilayer is shown in Fig. 5. The molybdenum oxide layers show a metal to oxygen ratio of 1:1.2 (at the peak of the molybdenum concentration, in the middle of the MoOx layer), the oxidation is incomplete. This is consistent with the results of the hard X-ray analysis. The oxygen content is much higher on the upper side of the MoOx layers, which is the result of a further oxidation by the O2 plasma after the layer was deposited and the precursor ¯ow was turned off. A higher oxygen gas ¯ow, or a reduced deposition rate by a reduced precursor ¯ow, should lead to a higher state of oxidation in the ®lm. The metal to oxygen ratio is 1:2.7 at the peak position of the oxygen concentration. This means the surface of
In summary, we have demonstrated that an oxygen plasma enhanced MOCVD process, with Me5C5Si2H5 as silicon precursor, is suitable for the deposition of very smooth metal oxide/silicon oxide multilayer with a single layer thickness of a few nanometer, controlled in situ by a soft X-ray re¯ectivity measurement. TEM images show amorphous multilayers with very smooth interfaces. Hard X-ray re¯ectivity and diffuse scattering at grazing incidence experiments con®rm the high quality of interfaces. The carbon content of the metal oxide layers is low (,5%), the silicon oxide layers are practically carbon free. With the high quality of the interfaces, this deposition method promises to be useful for the production of `water-window' soft X-ray mirrors. Acknowledgements Authors from the University Bielefeld acknowledge the support by the German Research Society DFG (Forschergruppe Nanometerschichtsysteme). S.L. acknowledges the support by Alexander von Humboldt Foundation, Bonn. Authors from Slovak Acad. Sci. acknowledge the support of this work by Scienti®c Grant Agency VEGA, Bratislava, under the grant 5083/98. References [1] E. Spiller, Appl. Phys. Lett. 20 (1972) 365. [2] E. Spiller, A. SegmuÈller, J. Rife, R.-P. Haelbich, Appl. Phys. Lett. 37 (1980) 1048. [3] H.-J. Stock, U. Kleineberg, B. Heidemann, et al., Appl. Phys. A 58 (1994) 371. [4] T.W. Barbee, S. Mrowka, M.C. Hettrick, Appl. Opt. 24 (1985) 883. [5] Y. Yamada, S. Takeyama, T. Miyata, Rev. Sci. Instrum. 66 (1995) 4501. [6] H. Kumagai, K. Toyoda, K. Kobayashi, M. Obara, Y. Iimura, Appl. Phys. Lett. 70 (1997) 2338. [7] L.G. Meiners, J. Vac. Sci. Technol. A 7 (1982) 655. [8] D.V. Tsu, G.N. Parsons, G. Lucovsky, M.W. Watkins, J. Vac. Sci. Technol. A 7 (1989) 1115. [9] T.W. Barbee, Proc. SPIE 563 (1985) 2. [10] J.H. Underwood, T.W. Barbee, Conf. Proc. 75 (1981) 170. [11] M. Jergel, V. Holy, E. Majkova, et al., Physica B 253 (1998) 28. [12] V. Holy, T. Baumbach, Phys. Rev. B 49 (1994) 10668. [13] B.L. Henke, E.M. Gullikson, J.C. Davis, Atomic Data and Nuclear Data Tables 54 (1993) 181.
Metal oxide/silicon oxide multilayer with smooth interfaces produced by in situ controlled plasma-enhanced MOCVD F. Hamelmann a,*, G. Haindl a, J. Schmalhorst a, A. Aschentrup a, E. Majkova a, U. Kleineberg a, U. Heinzmann a, A. Klipp b, P. Jutzi b, A. Anopchenko c, M. Jergel c, S. Luby c a b
FakultaÈt fuÈr Physik, UniversitaÈt Bielefeld, UniversitaÈtsstrasse 25, 33615 Bielefeld, Germany FakultaÈt fuÈr Chemie, UniversitaÈt Bielefeld, UniversitaÈtsstrasse 25, 33615 Bielefeld, Germany c Institute of Physics, Slovak Acad. Sci., 842 28 Bratislava, Slovakia Received 26 May 1999; received in revised form 5 August 1999; accepted 6 August 1999
Abstract Molybdenum oxide/silicon oxide and tungsten oxide/silicon oxide multilayer with 24 periods and a period thickness of 9.2 nm were fabricated with plasma-enhanced MOCVD. The layer thickness was controlled by an in situ soft X-ray re¯ectivity measurement. For the deposition of the SiO2 layers, a new silicon organic precursor, pentamethylcyclopentadienyldisilane (Me5C5Si2H5) was used in an O2 remote plasma process. The high quality of multilayer interfaces was observed by cross-section transmission electron microscopy (TEM), the interface toughness was measured by hard X-ray re¯ectivity and diffuse scattering at grazing incidence experiments to be about 0.1 nm. Auger electron spectroscopy (AES) gives the information, that the silicon oxide is practically carbon free, and the carbon content of the metal oxides is low (,5%). q 2000 Elsevier Science S.A. All rights reserved. Keywords: Multilayers; Oxides; Plasma processing and deposition
1. Introduction Alternating multilayers of two materials with different complex refraction indices and a layer thickness of a few nanometers can be used as mirrors for soft X-rays at nongrazing incidence angles [1]. A lot of studies have been made on this ®eld, the common methods for the deposition of such multilayers are physical vapor deposition (PVD), such as electron beam evaporation [2,3] and sputtering [4]. However, little has been published about the deposition of such multilayers with chemical vapor deposition (CVD) [5]. Although this method is well established for a broad variety of industrial and experimental applications, and it has potentially some advantages to PVD methods (deposition of complex compounds, uniform deposition on shaped substrates...), it is a challenge to deposit multilayers with suf®ciently smooth interfaces with CVD. The main dif®culties are the lack of suitable precursor molecules, the high substrate temperatures needed in most CVD processes and the missing layer thickness control on a nanometer scale. While Mo/Si is a good material combination for high * Corresponding author. Tel.: 149-521-106-5465; fax: 149-521-1066001. E-mail address: [email protected] (F. Hamelmann)
re¯ectivity at wavelengths above the silicon L-edge (l . 12:4 nm) [4], the oxides of these materials are, due to their optical properties at these wavelengths, not a good choice for mirrors at 13 nm wavelength. However, for the `water-window' (between 2.4 and 4.4 nm) the absorption of oxygen is negligible, furthermore, oxide multilayers can provide generally very stable interfaces with no interdiffusion of the layers [6]. Since smooth interfaces are, because of the necessary small period thickness and high number of doublelayers, extremely important for multilayers in the `water-window', oxide multilayers with smooth interfaces may be useful for optical applications at these wavelengths. 2. Deposition procedure We produced the multilayers in a downstream plasma reactor. The plasma source is mounted above the substrate, inductively driven by a 13.54 MHz r.f. generator and a gas ¯ow of 4 sccm O2. The precursor is, along with N2 carriergas, introduced to the reactor through a ring-shaped nozzle near the silicon substrate. This setup provides a so-called `remote' plasma [7], because the plasma zone is separated from the substrate. The precursor is not activated in the plasma, but by collisions with excited O atoms or excited
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0069 5-1
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
O2 molecules near the substrate [8]. Slightly heating the substrate to 1608C has proven to be helpful for the formation of smooth layers. The total pressure was 1 Pa during the deposition, using a molecular drag pump. The depositions were made at a plasma power of just 2 W, suf®cient for deposition rates of 4 nm/min for molybdenum oxide (from Mo(CO)6), 1 nm/min for tungsten oxide (from W(CO)6) and 5 nm/min for silicon oxide (from Me5C5Si2H5). It has to be noted that the Me5C5Si2H5 precursor was cooled down to 08C to reduce the deposition rate (.10 nm/min at room temperature). This was necessary for a precise control of the layer thickness with the in situ soft X-ray re¯ectivity measurement. The use of organic precursors provides the advantage of an easy handling, compared to common CVD precursors such as silane (SiH4). The in situ soft X-ray re¯ectivity measurement consists of a XPS X-ray source, which is emitting 4.4 nm radiation of the carbon K-line. The X-ray source is separated by a thin foil from the MOCVD reactor, because of the pressure requirements (better 10 25 Pa). The source is mounted at an angle of 208 to the substrate surface, a proportional counter detects the re¯ected radiation under the same angle of incidence. The values of the wavelength l and angle of incidence a determine the period thickness d of the multilayer, as given by Bragg's law [9]: l 2dcosa 1 2 2d 2 d2 =cos2 a1=2 . In this equation l is the wavelength, d is the period thickness, a is the angle of incidence and d is the difference of the real part of the multilayer's refraction index from 1 (n 1 2 d). The use of this in situ thickness control is limited to processes with pressures lower than 10 Pa, at higher pressures too much of the 4.4 nm radiation is absorbed. The advantage of this thickness control is, that it is directly measuring the re¯ectivity of the growing multilayer stack in the soft X-ray region. The thickness of each single layer is optimized for highest possible re¯ectivity of the stack, minor errors in the thickness of a single layer will be corrected with the thickness of the next layer.
Fig. 1. In situ re¯ectivity of a 24 period WOx/SiOy multilayer.
oscillation amplitude remains constant. This behavior shows, that the interface roughness, which would result in a decreasing re¯ectivity with reduced amplitude, remains constant through the multilayer stack. Cross-section transmission electron microscopy (TEM) con®rms the conclusion from the in situ re¯ectivity measurement. Fig. 2 shows the cross-section image of the WOx/SiOy multilayer mentioned above. The periodic structure on the bottom of the image is the crystal structure of the silicon substrate. On top of the substrate, the native oxide layer of the silicon substrate can be seen in light gray. The following dark layers are the tungsten oxide, while the light layers are the deposited silicon oxide layers. In the deposited layers, no periodic structure can be observed, all deposited oxide layers are amorphous. In Fig. 3 the cross-section TEM image of a 24 period molybdenum oxide/silicon oxide multilayer, produced under identical conditions as the
3. Results Fig. 1 shows the re¯ectivity of a 24 period tungsten oxide/ silicon oxide multilayer during deposition. Starting with the re¯ectivity of the silicon substrate, the re¯ectivity is increasing during tungsten oxide deposition until it reaches a maximum. At this point, the best layer thickness is reached, shutting down the precursor ¯ow stops the deposition. The following deposition of a silicon oxide layer results in a decreasing re¯ectivity. In the minimum, the best thickness for this layer is reached. Due to the increasing number of re¯ecting interfaces, an overall increase of the re¯ectivity can be observed during the following ten periods (Fig. 1). Because of the absorption in the material, the re¯ectivity is not increasing with a further increasing number of periods, but remains constant until the last (24th) period. Also, the
91
Fig. 2. TEM image of a 24 period WOx/SiOy multilayer.
92
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
Fig. 3. TEM image of a 24 period MoOx/SiOy multilayer.
Fig. 4. (a) Cu Ka re¯ectivity (R) and detector (D) scans (dots), the later one with the sample set to the 1st Bragg order and their simulations (full lines) of the MoOx/SiOy sample, (b) Sample scans of the MoOx/SiOy sample (dots) with the detector set to 1st and 3rd Bragg maxima and their simulation (full lines).
WOx/SiOy sample, is displayed. Due to the different optical constants, the relation of the layer thickness is different, but the interface quality is equally good. The detailed study of the interface roughness was performed for the sample MoOx/SiOx by using hard X-ray re¯ectivity (XRR) and diffuse scattering at the grazing incidence (DSGI) measurements. The Cu Ka radiation (l 0:154 nm) was used. The hard XRR spectra were simulated with Fresnel computational code [10]. The interface roughness was incorporated at each interface by Debye-Waller-like attenuation factor. The evaluation of non-specular intensity due to the diffuse scattering was simulated using semikinematical approach to the distorted wave Born approximation [11]. Some results are seen in Fig. 4a,b. In the specular XRR spectrum shown in Fig. 4a, the 1st, 3rd and 5th Bragg maxima can be seen, while the Kiessig maxima coming from the ®nite size of the sample are not clearly observable due to high number of multilayer periods. The absence of 2nd Bragg maximum points at the same MoOx and SiOy layer thickness [10]. In the non-specular detector scan a strong specular maximum together with additional maxima at the positions corresponding the different Bragg orders are seen. Such maxima provide a direct experimental evidence for the existence of vertical interface correlation. The non-specular sample scans taken with the detector ®xed at the specular position for the 1st and 3rd Bragg order are shown in Fig. 4b. Except for the sharp specular maximum one can observe a broad distribution of the diffuse intensity enhanced by resonant diffuse scattering which originates from constructive interference of the X-rays scattered diffusely on different rough interfaces with replicated morphology [12]. For simulation of the DSGI data the frequency independent model of the vertical interface roughness correlation (details are in Ref. [11]) was used. The increasing roughness starting from the substrate as sj 2 sN 2 1 N 2 jDs2 was assumed. Here, s j is the rms roughness of j-th interface and s N is the rms roughness of the substrate. The multilayer period, individual layer thickness, interface rms roughness and its lateral and vertical correlation lengths obtained from the simulations of hard XRR and DSGI spectra are summarized in Table 1. The fractal parameter (h) of the interfaces is equal to unity, i.e. no fractal behavior of the interfaces was observed. The simulation used a refractive index of d 20:5 £ 1026 for the molybdenum oxide, which is between the values for MoO2 (18:8 £ 1026 ) and Mo3O (22:6 £ 1026 ) Table 1 The parameters of the MoOx/SiOx multilayer obtained from the specular and non-specular simulations; d d Mo 1 dSi is multilayer period, dMo and dSi are the individual layer thicknesses, s is the interface rms roughness, j is the lateral and Lvert vertical correlation length. Sample
d (nm)
dMo (nm) dSi (nm) s (nm)
j (nm) Lvert (nm)
MoOx/SiOy
9.2
4.6
30
4.6
0.1
40
F. Hamelmann et al. / Thin Solid Films 358 (2000) 90±93
93
the MoOx layers mainly contents MoO3. The carbon content of these layers is as low as 5%. The same effect of incomplete oxidation can be observed inside the SiOy layers: at the peak position of the silicon, the silicon to oxygen ratio is 1:1.3, which is most likely the result of a 2:1 mixture of SiO2 and Si. At the surface, the silicon to oxygen ratio is 1:1.6. The carbon concentration is about 2% in the silicon oxide layers, which is near the detection limit of this method. 4. Conclusions Fig. 5. Sputter AES depth pro®le of a MoOx/SiOy multilayer.
[13] at a wavelength of 0.154 nm. For the silicon oxide a refractive index of d 7:1 £ 1026 was used, close to SiO2 (7:13 £ 1026 [13]). In accordance to the TEM cross-section image, the layer thickness for the simulation was 4.6 nm for both silicon oxide and molybdenum oxide, with a thickness variation of about 8%. The low rms roughness of about 0.1 nm of the multilayer interfaces corresponds to the cross-section TEM analyses (we estimate the accuracy of our simulation procedure as ^ 0.1 nm). The amorphous structure of the individual layers promotes formation of smooth interfaces. According to our calculations, there is no substantial difference for MoOx -on SiOy and SiOy -on MoOx interface roughness. Within the accuracy of our simulation the same value of interface rms roughness can be expected in the whole multilayer stack. As follows from the value of vertical correlation length, which stretches approximately over four periods, the interfaces are to some extent correlated. Summarizing the results above, using the MOCVD technique it is possible to prepare multilayers with high number of interfaces of very low interface roughness. This can be explained by the isotropic nature of the MOCVD process, resulting in the suppression of shadowing effects and interface roughness replication. However, the interface quality of this MoOx/SiOy multilayer is comparable to titanium/carbon multilayers produced by electron-beam evaporation with ion-beam polishing [11]. A sputter Auger Electron Spectroscopy (AES) depth pro®le of the uppermost 10 periods of a MoOx/SiOy multilayer is shown in Fig. 5. The molybdenum oxide layers show a metal to oxygen ratio of 1:1.2 (at the peak of the molybdenum concentration, in the middle of the MoOx layer), the oxidation is incomplete. This is consistent with the results of the hard X-ray analysis. The oxygen content is much higher on the upper side of the MoOx layers, which is the result of a further oxidation by the O2 plasma after the layer was deposited and the precursor ¯ow was turned off. A higher oxygen gas ¯ow, or a reduced deposition rate by a reduced precursor ¯ow, should lead to a higher state of oxidation in the ®lm. The metal to oxygen ratio is 1:2.7 at the peak position of the oxygen concentration. This means the surface of
In summary, we have demonstrated that an oxygen plasma enhanced MOCVD process, with Me5C5Si2H5 as silicon precursor, is suitable for the deposition of very smooth metal oxide/silicon oxide multilayer with a single layer thickness of a few nanometer, controlled in situ by a soft X-ray re¯ectivity measurement. TEM images show amorphous multilayers with very smooth interfaces. Hard X-ray re¯ectivity and diffuse scattering at grazing incidence experiments con®rm the high quality of interfaces. The carbon content of the metal oxide layers is low (,5%), the silicon oxide layers are practically carbon free. With the high quality of the interfaces, this deposition method promises to be useful for the production of `water-window' soft X-ray mirrors. Acknowledgements Authors from the University Bielefeld acknowledge the support by the German Research Society DFG (Forschergruppe Nanometerschichtsysteme). S.L. acknowledges the support by Alexander von Humboldt Foundation, Bonn. Authors from Slovak Acad. Sci. acknowledge the support of this work by Scienti®c Grant Agency VEGA, Bratislava, under the grant 5083/98. References [1] E. Spiller, Appl. Phys. Lett. 20 (1972) 365. [2] E. Spiller, A. SegmuÈller, J. Rife, R.-P. Haelbich, Appl. Phys. Lett. 37 (1980) 1048. [3] H.-J. Stock, U. Kleineberg, B. Heidemann, et al., Appl. Phys. A 58 (1994) 371. [4] T.W. Barbee, S. Mrowka, M.C. Hettrick, Appl. Opt. 24 (1985) 883. [5] Y. Yamada, S. Takeyama, T. Miyata, Rev. Sci. Instrum. 66 (1995) 4501. [6] H. Kumagai, K. Toyoda, K. Kobayashi, M. Obara, Y. Iimura, Appl. Phys. Lett. 70 (1997) 2338. [7] L.G. Meiners, J. Vac. Sci. Technol. A 7 (1982) 655. [8] D.V. Tsu, G.N. Parsons, G. Lucovsky, M.W. Watkins, J. Vac. Sci. Technol. A 7 (1989) 1115. [9] T.W. Barbee, Proc. SPIE 563 (1985) 2. [10] J.H. Underwood, T.W. Barbee, Conf. Proc. 75 (1981) 170. [11] M. Jergel, V. Holy, E. Majkova, et al., Physica B 253 (1998) 28. [12] V. Holy, T. Baumbach, Phys. Rev. B 49 (1994) 10668. [13] B.L. Henke, E.M. Gullikson, J.C. Davis, Atomic Data and Nuclear Data Tables 54 (1993) 181.