SiO2 multilayers

Thin Solid Films 379 Ž2000. 308᎐312

Thermal stability of MorSiO2 multilayers Feng-Ping Wang a,b , Li-Sen Cheng a , Pei-Xuan Wang b , Kun-Quan Lua,U a

Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China b Uni¨ ersity of Science and Technology Beijing, Beijing 100083, PR China

Received 19 April 2000; received in revised form 25 July 2000; accepted 2 August 2000

Abstract The thermal stability of magnetron sputtering deposited MorSiO2 multilayered films was investigated by isothermal annealing, cross-sectional high resolution electron microscopy and Auger electron spectroscopy. No observable structural variation was visualized at the interface between Mo and SiO 2 after annealing at 400⬚C for 2 h. At 600⬚C, a small amount of as-deposited amorphous Mo began to crystallize at the top surface of the sample. Further increase in the annealing temperature resulted in the formation of large Mo crystalline grains in the film and destruction of the periodically distributed MorSiO2 multilayer structure. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Auger electron spectroscopy ŽAES.; Interfaces; Transmission electron microscopy ŽTEM.

1. Introduction It has been demonstrated in sputter-deposited multilayers ŽMLs., that single layers are generally amorphous or polycrystalline. The formation of an amorphous structure results from rapid condensation of the target materials. Thus, the amorphous layers formed under this kind of non-equilibrium condition are metastable. They are sensitive to some energy-providing processes, such as thermal annealing and irradiation, which will cause the amorphous layers to crystallize. It is obvious that crystallization of amorphous layers will give rise to variations in the surface and interface structures of the MLs, which in turn leads to changes in their physical properties. As an insert device in the beamline of synchrotron radiation, MLs are used as pre-monochromator in order to relieve the thermal load of other monochromators Žbi-crystal and tri-

U

Corresponding author.

crystal. for reflection andror dispersion. Due to the enormous flux of X-rays in the synchrotron radiation, the temperature of monochromators may sometimes be elevated up to 500⬚C. Under these circumstances, thermal stability of the MLs used as a monochromator becomes extremely important. Extensive studies of the behavior of MLs after annealing have been made on systems such as WrSi, WrC, WrBN, WrB 4 C, MorSi, MorBN, and MorB 4 C w1᎐5x in order to improve their performance as monochromators. MorSiO2 MLs are found to exhibit excellent optical properties when used as a soft X-ray monochromator in the so-called ‘water window’ wavelength w6x. However, as far as we know, their thermal stability has not been studied up to now. In addition, the study of the thermal stability of metalrSiO 2 interfaces is of great significance, not only for soft X-ray applications, but also for semiconductor and superconductor devices. In this paper, the thermal stability of MorSiO2 MLs is investigated using high-resolution electron microscopy ŽHREM. and Auger electron spectroscopy ŽAES..

0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 4 1 9 - X

F. Wang et al. r Thin Solid Films 379 (2000) 308᎐312

309

2. Experimental Samples were prepared by magnetron sputtering in an argon atmosphere w7x. The molybdenum target was DC sputtered, whereas the SiO 2 target was RF sputtered. The purity of Mo and SiO 2 was better than 99.99%. The base pressure was approximately 10y4 Pa. SiŽ100. single crystal wafers of 50 mm in diameter and with the surface roughness of approximately 0.3 nm were used as substrates. A buffer layer of SiO 2 approximately 30 nm thick was deposited onto the surface of the substrates prior to the deposition of regular MorSiO2 MLs. HREM is a powerful tool in analyzing material structures in a very small area on an atomic scale. In this experiment, HREM observation was performed on a Philips CM 200 FEG electron microscope, which operated at a high tension of 200 kV and a point-to-point resolution of 0.192 nm. TEM samples were prepared by the standard technique. First, the cross-sectional specimen, which was prepared by gluing the as-deposited and annealed samples face-to-face, was ground to a thickness of approximately 20 ␮m, then it was dimpled to approximately 10 ␮m, and was finally ion milled to the thickness which is transparent to the electron beam. A PHI610 Auger electron spectrometer was used for the composition depth profiling of MLs. The energy of the primary electron beam was 3 keV. The argon ion beam for etching had an energy of 3 keV, and the sputtering rate calibrated for a SiO 2 standard was approximately 30 nmrmin. Isothermal annealing was carried out in a vacuum furnace. The pressure was kept lower than 10y3 Pa during annealing. The temperature ranged from 400 to 800⬚C at an interval of 200⬚C. It was kept constant within an error of "5⬚C for 2 h. The parameters of the as-deposited samples are summarized in Table 1, where N is the number of MorSiO2 bilayers; d, d Mo and dSiO2 are the calculated thickness of MorSiO2 bilayers and Mo and SiO 2 single layers, respectively. These values were obtained from low-angle X-ray diffraction w7x. 3. Results and discussion A cross-sectional HREM image of the as-deposited sample 噛 1 Žwithout thermal annealing. is shown in Fig. Table 1 The experimental and simulation parameters of MorSiO2 multilayers Samples N dŽnm. dMoŽnm. dSiO2 Žnm.

噛

1

10 7.10 3.90 3.20

噛

2

15 7.35 3.85 3.50

噛

3

7 7.70 4.40 3.30

Fig. 1. Cross-sectional HREM image of the as-deposited sample 噛 1 ŽMo layers in dark contrast and SiO 2 in bright contrast..

1, which shows similar cross-sectional morphology to that reported by Rosen on a MorSi system w8x. The multilayered structure can be clearly seen from the contrast shown in the image. The dark layers correspond to Mo ones, because Mo has a much larger atomic number and thus a much stronger electron scattering factor than Si and O in SiO 2 . This can also be demonstrated by the bright contrast of the SiO 2 buffer layer, which is located just on top of the Si substrate. Careful inspection of the specimen indicated that no crystalline features could be observed in the as-deposited film. Selected-area electron diffraction ŽSAD. patterns recorded from different areas of the film showed only diffuse rings, indicating the amorphous state of the as-grown films. Sample 噛 2, which was prepared under the same growth conditions as those used for sample 噛 1 but with five more MorSiO2 bilayers, was annealed at 400⬚C for 2 h. A typical HREM image of this treated specimen is presented in Fig. 2. Close inspection of this image indicated that crystallization occurred slightly in the Mo layers near the top surface of the film. However, abrupt interfaces between the Mo and SiO 2 layers were still present, implying that no chemical reaction or inter-diffusion between the Mo and SiO 2 layers occurred during the thermal annealing process. This is quite different from the result reported by Jiang et al. w5x. In their study, low-angle X-ray diffraction and Raman spectroscopy were used. Chemical reaction between Mo and Si was demonstrated to occur, and molybdenum silicide was identified at the interface between theMo and Si layers after annealing at 400⬚C for 0.5 h. It is obvious that the improvement in thermal stability of MorSiO2 MLs is mainly due to the chemi-

310

F. Wang et al. r Thin Solid Films 379 (2000) 308᎐312

increase in roughness means an increase in interface area, inter-diffusion can occur relatively easily at rough interfaces, in our case at those in the area close to the top surface. As for the explanation for the preferential

Fig. 2. Cross-sectional HREM image of the sample nealed..

噛

2 Ž400⬚C an-

cal inactivity of SiO 2 at high temperatures Žcompared with Si.. In addition, it seems that the use of SiO 2 instead of Si as the space material is beneficial to the reduction of inter-diffusion at the interface of the bilayer. It was also noted that the thickness of the SiO 2 layers shown in Fig. 2 seems to increase slightly from the bottom to the top surface. This is probably caused by the diffraction imaging phenomenon and the thickness effect of the specimen w9x. The annealing temperature was increased to investigate what changes would occur. A HREM image of sample 噛 2, which was annealed at 600⬚C is shown in Fig. 3. Fig. 3a shows the low magnification cross-section morphology of the as-treated ML films. Fig. 3b gives the HREM image recorded near the top surface, and Fig. 3c is the HREM image taken in the vicinity of the substrate. It can be seen from Fig. 3a that the periodical distribution of the MorSiO2 bilayer as shown in Fig. 2 still remains. By comparison of Fig. 3b with Fig. 3c, it is evident that the interfaces between Mo and SiO 2 in the top surface area become blurred, indicating that inter-diffusion between SiO 2 and Mo layers takes place in this region. Furthermore, lattice fringes in the Mo layers are clearly seen in Fig. 3b, showing that Mo crystallization occurred preferentially in the top surface area. However, no observable crystallization of Mo was evident in those layers near the interface between the film and substrate ŽFig. 3c.. The reason why inter-diffusion between Mo and SiO 2 occurs preferentially in the top surface region of the film may be related to the difference in the interface roughness between layers located near the top surface and those close to the substrate. According to the theory of roughness propagation suggested by Stearns w10x, the interfacial roughness will increase as the ML deposition progresses. Thus, the interfacial roughness of the upper layers may be larger than that of the bottom ones. Since the

Fig. 3. Cross-sectional HREM images of the sample 噛 2 Ž600⬚C annealed.: Ža. the whole MLs; Žb. near the top surface of MLs; and Žc. near the substrate.

F. Wang et al. r Thin Solid Films 379 (2000) 308᎐312

Fig. 4. Cross-sectional HREM image of the sample nealed..

噛

2 Ž800⬚C an-

crystallization of Mo near the top surface area, this could be related to the smaller nucleation barrier or larger driving force for crystallization at this position. Of course this is still open to further investigation. Moreover, it should be pointed out that the uneven thickness of the HREM specimen might enhance the difference in image contrast between Fig. 3b and c. Because the thickness of the ion-thinned cross-sectional specimen increases gradually from the free surface of the film to the substrate, for those layers near the substrate the specimen may be too thick to show crystallization of Mo in the HREM image. Therefore, further study with thinner TEM specimens is needed to clarify this point. With the elevation of annealing temperature, the crystallization becomes more and more severe. After annealing at 800⬚C, the crystalline grain boundaries even go across several bi-layers, and thus the periodicity of the MorSiO2 bilayers in the film was completely destroyed Žsee Fig. 4.. According to the above description, it can be concluded that in the MorSiO2 system, the mechanism for the thermal stability degradation is different from that in MorSi MLs. In MorSi MLs, thermal stability is limited by the chemical reaction between Si and Mo. However, the stability of MorSiO2 MLs is limited by the crystallization of Mo. The AES depth profile of as-deposited sample 噛 3, which was deposited under the same conditions as sample 噛 1 and 噛 2, is presented in Fig. 5, where the at.% of elements is plotted versus the sputtering time. The oscillation of Mo, Si, and O signals can be seen clearly. It is obvious that Si and Mo signals are opposite in phase, and so are the O and Mo signals. This implies that good periodic distribution of MorSiO2 bilayers was formed in the film, and the diffusion of Si orrand O into Mo layers is not obvious. Nevertheless, slight diffusion of O from the SiO 2 layer into the Mo layer has been identified when elemental mapping was employed Žsee w7x.. It is known that the resolution of

311

Fig. 5. AES depth profiles of the as-deposited sample 噛 3.

AES decreases as the depth increases. This explains why the amplitude of the oscillations reduced as the sputtering depth went from the top layers of the MLs to the interior, in spite of the similarity of the interfaces. Fig. 6 shows the AES depth profile from the second to the fourth MorSiO2 bilayer of sample 噛 3 after annealing at 400⬚C. Significant oscillations of the Mo, Si, and O signals can still be observed, and the opposite phases between Si and Mo, and those between O and Mo, are clear. Comparison between Fig. 6 and Fig. 5 shows that no obvious diffusion occurs between the Mo and SiO 2 layers, which is in agreement with the results obtained from the cross-sectional HREM images. The same AES depth profiles obtained for sample 噛 3 after thermal annealing at 600 and 800⬚C are shown in Fig. 7 and Fig. 8, respectively. It can be seen that the composition profile of Fig. 7 is similar to Fig. 6. This suggests that no intensive diffusion occurred between the SiO 2 and Mo layers. However, the structure difference between samples annealed at 400 and 600⬚C has been demonstrated by TEM images, as mentioned above. This implies TEM is much powerful than AES in analyzing the interface structures. In Fig. 8, it is apparent that the oscillations of the Mo, Si, and

Fig. 6. AES depth profiles of the sample 噛 3 annealed at 400⬚C.

312

F. Wang et al. r Thin Solid Films 379 (2000) 308᎐312

4. Conclusions

Fig. 7. AES depth profiles of the sample 噛 3 annealed at 600⬚C.

It was demonstrated that MorSiO2 MLs deposited by magnetron sputtering have better thermal stability than MorSi MLs. HREM images indicated that the as-deposited Mo and SiO 2 layers are amorphous. Thermal annealing experiments showed that the MorSiO2 bilayer is stable up to 400⬚C. The mechanism by which degradation of the thermal stability occurred was the crystallization of Mo. At 600⬚C, Mo starts to crystallize in the films and the MorSiO2 bilayer is completely destroyed if the temperature is elevated to 800⬚C.

References

Fig. 8. AES depth profiles of the sample 噛 3 annealed at 800⬚C.

O signals disappeared after the sample was annealed at 800⬚C, indicating the destruction of MorSiO2 bilayers in the film. This is consistent with the result obtained from cross-sectional HREM as described above.

w1x H. Okada, K. Mayama, Y. Goto, I. Kusunoki, M. Yanagihara, Appl. Opt. 33 Ž1994. 4219. w2x T.W. BarbeeJr, J.C. Rife, W.R. Hunter, M.P. Kowalski, R.G. Cruddace, J.F. Seely, Appl. Opt. 32 Ž1993. 4852. w3x Z. Jiang, B. Vidal, J. Appl. Phys. 74 Ž1993. 249. w4x M. Cai, D.D. Allredd, A. Reyes-Mena, J. Vac. Sci. Technol. A 12 Ž1994. 1535. w5x Z.M. Jiang, X.M. Jinag, W.H. Liu, Z.Q. Wu, J. Appl. Phys. 65 Ž1989. 196. w6x F.P. Wang, Ph.D. Thesis, Beijing University of Science and Technology, PR China, 1997. w7x F.P. Wang, P.X. Wang, K.Q. Lu, Z.Z. Fang, M. Gao, X.F. Duan, M.Q. Cui, H.J. Ma, X.M. Jiang, J. Appl. Phys. 85 Ž1999. 3175. w8x R.S. Rosen, D.G. Stearns, M.A. Viliardos, M.E. Kassner, S.P. Vernon, Y. Cheng, Appl. Opt. 32 Ž1993. 6975. w9x P. Donovan, B. George, A. Bruson, C. Dufour, G. Marchal, P. Mangin, J. Appl. Phys. 69 Ž1991. 1371. w10x D.G. Stearns, J. Appl. Phys. 71 Ž1992. 4268.