Comparison of dielectric characteristics of Ta2O5 thin films on RuO2 and Ru bottom electrodes

Comparison of Dielectric Characteristics of Ta

O

Thin Films

on RuO

and Ru Bottom Electrodes

J. H. Huang, Yi-Sheng Lai, and J. S. Chen

*

Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan

Dielectric properties of Ta2O5thin films with RuO2and Ru as the bottom electrodes were investigated. The Ta2O5thin films were reactively sputtered on the bottom electrodes and then annealed in oxygen ambient at 700°C for 30 min. Using X-ray diffraction and Auger electron spectrometry, it has been found that the Ru bottom electrode was partially oxidized during annealing, while the RuO2electrode remained its structure. However, the annealed Ta2O5exhibited a higher dielectric constant, as well as a smaller leakage current, on the Ru electrode than on the RuO2electrode. Accordingly, the Ru bottom electrode is satisfactory for Ta2O5 storage capacitors, even in a high temperature, oxidizing environment. The divergent electrical performances of two electrodes are attributed to the different crystallinity of annealed Ta2O5on Ru and RuO2.

© 2001 The Electrochemical Society. 关DOI: 10.1149/1.1374218兴 All rights reserved.

Manuscript submitted January 5, 2001; revised manuscript received March 3, 2001. Available electronically May 29, 2001.

Due to the continuing dimensional shrinkage of integrated cir-cuits, high density dynamic random access memories共DRAMs兲 re-quire high capacitance per area to attenuate the noise effect.1 Tan-talum pentoxide (Ta2O5) is a promising material for charge storage capacitors because it has high dielectric constant, low leakage cur-rent, high breakdown strength, and low loss tangent.2,3However, the choice of bottom electrode and its oxidation behavior are of great importance because fabrication of Ta2O5often requires high tem-perature deposition and/or postdeposition annealing in an oxygen-containing environment. By using a polycrystalline Si electrode, de-crease of capacitance arises from the formation of a low dielectric constant interfacial silicon oxide layer.4 Therefore, a metal-insulator-metal 共MIM兲 capacitor is preferred in applications to high-density DRAMs. Transition metals, such as Pt,5Ru,5-8Ti,9,10 Ta,9-11and W,9,10,12have been considered as the candidates of metal electrodes. Among these metals, the Ru electrode has received much attention because its oxide, RuO2, has a low resistivity共35 ␮⍀ cm in bulk form13兲 and excellent etching properties.14

In the reported literature regarding the Ru electrodes,5-8the ther-mal treatments had mostly been carried out at low temperatures (400-450°C) or with rapid thermal annealing in nitrogen ambient to avoid the oxidation of Ru. However, since as-deposited Ta2O5is an oxygen-deficient oxide and the oxygen vacancies behave as the charge carriers,15annealing in the oxygen-contained ambient will be a more efficient approach to improve the dielectric characteristics of Ta2O5. In this study, we have investigated the dielectric perfor-mances of Ta2O5-based MIM capacitors with RuO2and Ru bottom electrodes. The Ta2O5films were prepared by reactive sputtering and they were annealed in oxygen ambient at 700°C for 30 min after deposition. Material and dielectric characteristics of Ta2O5/RuO2 and Ta2O5/Ru structures, before and after the 700°C annealing, were explored.

Experimental

In our experiments, n-type Si共100兲 wafers were used as the sub-strates. Ru and RuO2films were both sputtered from a Ru共99.9% purity兲 target, in a pure Ar ambient and 50% Ar ⫹ 50% O2 mix-ture, respectively. Thicknesses of Ru and RuO2films were both kept at 150 nm and the resistivities of Ru and RuO2were 90␮⍀ cm and 310␮⍀ cm, respectively.

Ta2O5 films were deposited by sputtering from a Ta 共99.95% purity兲 target in a 70% Ar ⫹ 30% O2mixture. The thickness of the Ta2O5 film is 45 nm. After deposition of Ta2O5, some of the Ta2O5/RuO2and Ta2O5/Ru samples were annealed in a tube

fur-nace with flowing oxygen at 700°C for 30 min. For all samples, the surface roughness and morphology was examined using atomic force microscopy关共AFM兲, Digital Instruments, Nano-Scope Series E兴. The characteristic phases were identified by ␪-2␪ X-ray diffrac-tion关共␪-2␪ XRD兲, Rigaku D-Max-IV兴 and glancing incident angle X-ray diffraction关共GIAXRD兲, MAC Science MXP18兴 at an incident angle of 5°. Compositional depth profile analysis was performed with Auger electron spectrometry关共AES兲, VG AES-310D兴.

For electrical characterization, aluminum top electrodes of 200 ␮m diam were fabricated by sputter deposition in combination with lift-off process to form the Al/Ta2O5/Ru共or RuO2兲 MIM capacitors. Capacitance-voltage 共C-V兲 measurements on the capacitors were performed with a HP 4284 LCR meter at a frequency of 100 kHz, and the current-voltage共I-V兲 curves were measured using HP 4156 semiconductor parameter analyzer.

Results and Discussion

The crystal structure of the as-deposited Ta2O5film on a blank Si wafer was amorphous when analyzed with either ␪-2␪ XRD or GIAXRD. Figure 1a shows the GIAXRD spectrum of an as-deposited Ta2O5film on a Si substrate. After oxygen furnace anneal-ing at 700°C for 30 min, it crystallized into the orthorhombic ␤-Ta2O5phase共JCPDS file 25-0922兲 and the ␪-2␪ XRD spectrum of the annealed Ta2O5on Si is shown in Fig. 1b. As for RuO2and Ru, the RuO2 film sputtered from the Ru target with 50% Ar ⫹ 50% O2revealed a polycrystalline structure of rutile RuO2phase 共JCPDS file 40-1290兲, with a predominant RuO2共101兲 orientation.16 The as-sputtered Ru film exhibited a polycrystalline structure of hexagonal Ru phase共JCPDS file 01-1256兲.

After depositing Ta2O5films on RuO2and Ru, the surfaces of samples, before and after oxygen furnace annealing at 700°C, were examined with AFM. The root-mean-square surface roughness (Rrms) values of these samples are listed in Table I. Table I shows that the surface of Ta2O5 film on Ru is very smooth (Rrms ⫽ 1.088 nm) while the Ta2O5 film on RuO2 is a little rougher (Rrms⫽ 2.972 nm). The higher roughness of Ta2O5on RuO2than on Ru shall be arisen from the rougher surface of as-deposited RuO2 than that of Ru. After annealing at 700°C in oxygen for 30 min, the AFM images of the samples are presented in Fig. 2. Figure 2 reveals that the surface of Ta2O5on RuO2is very granular, and the surface of Ta2O5on Ru is smooth but a little undulating. Figure 2 apparently indicates that the roughness of the Ta2O5on RuO2is still higher than that on Ru, which is also seen by the Rrmsvalues shown in Table I. However, the Rrms values of both types of samples increase after annealing. The increase of Rrmsvalues in the thin-film systems after heat-treatment is often observed because grain growth and/or inter-facial reactions may occur during annealing. These effects may in-*Electrochemical Society Active Member.

z_{E-mail: [email protected]}

Journal of The Electrochemical Society, 148共7兲 F133-F136 共2001兲

0013-4651/2001/148共7兲/F133/4/$7.00 © The Electrochemical Society, Inc.

F133

duce the volume change so that the surface becomes rough. Figure 3 shows the ␪-2␪ XRD spectra of the 700°C-annealed Ta2O5films on RuO2and Ru electrodes. Both spectra have exhibited diffraction peaks of ␤-Ta2O5 phase. However, more diffraction peaks and higher peak intensity of␤-Ta2O5phase are observed for the Ta2O5 film grown on Ru than on RuO2. The crystallinity of Ta2O5 on Ru is thus superior to that on RuO2. In addition, the 700°C-annealed Ta2O5/Ru spectrum shows Ru diffraction peaks, as well as the RuO2diffraction peaks. This indicates that the Ru elec-trode was partially oxidized during the furnace annealing in oxygen at 700°C.

To further clarify the oxidation of Ru during annealing, compo-sitional depth profiles of Ta2O5/Ru samples, as-deposited and after annealing at 700°C, were analyzed using AES and the spectra are shown in Fig. 4. Without adequate standard samples to calibrate the sensitivity factors of the involving elements, the values of atomic concentration shown in the AES spectra are qualitative, but not quantitative. From Fig. 4, it is apparent that after annealing at 700°C, oxygen atoms had diffused into Ru, through the Ta2O5layer. The Ru layer was not fully oxidized because the oxygen concentra-tion gradually drops down at the end of the Ru layer共Fig. 4b兲. This is consistent with the observation of diffraction peaks for both Ru and RuO2phases in the XRD spectrum. On the other hand, the AES spectra of the Ta2O5/RuO2samples, before and after annealing at 700°C, are shown in Fig. 5. The major characteristics of the two spectra are similar except that the oxygen concentration in the Ta2O5 and RuO2layers is slightly increased after annealing共Fig. 5b兲. As a result, the stoichiometry of Ta2O5 and RuO2 in the Ta2O5/RuO2

sample may be changed upon annealing. As compared with the oxy-gen profiles of the as-deposited Ta2O5/Ru, 700°C-annealed Ta2O5/Ru, and as-deposited Ta2O5/RuO2samples, the oxygen con-centration in 700°C-annealed Ta2O5/RuO2sample is relatively high. This high oxygen concentration may affect the dielectric leakage property of the annealed Ta2O5/RuO2sample and is discussed later. Capacitances and the relative dielectric constants of Ta2O5 ob-tained from the C-V measurements are listed in Table II. The rela-tive dielectric constants of Ta2O5 are about the same (␧Ta2O5 ⯝ 19) for the films as deposited on Ru and RuO2electrodes. After annealing at 700°C, the relative dielectric constant of Ta2O5with Ru electrode increased to 33 while that with RuO2electrode was only increased to 28. Crystallized Ta2O5generally have higher di-electric constants as compared with amorphous Ta2O5because the crystal grains will be more effectively polarized under an external electric field.17 Therefore, the dielectric constants of Ta2O5 in-creased after annealing for both cases. However, with a better crys-Figure 1. 共a兲 GIAXRD spectrum of as-deposited Ta2O5film on Si substrate,

and共b兲 ␪-2␪ X-ray diffraction spectrum of 700°C-annealed Ta2O5film on Si

substrate.

Figure 2. AFM images of the Ta2O5films as deposited on共a兲 RuO2and共b兲 Ru bottom electrodes, after annealing at 700°C in O2ambient for 30 min.

Table I. The root-mean-square surface roughness„Rrms… values of Ta2O5films on RuO2and Ru, before and after annealing at 700°C in O2ambient for 30 min.

Sample

Rrms共nm兲

As-deposited 700°C annealed

Ta2O5/RuO2 2.972 4.127

Ta2O5/Ru 1.088 3.734

Journal of The Electrochemical Society, 148共7兲 F133-F136 共2001兲

F134

tallinity, the Ta2O5film on a Ru electrode hence possesses a higher dielectric constant than that on an RuO2electrode.

With regard to the leakage current characteristics, the I-V curves shown in Fig. 6 reveal that for the Ta2O5films without receiving any heat-treatment, the leakage current of capacitor with a RuO2 elec-trode is slightly higher than that with a Ru elecelec-trode. It may be related with the slightly higher surface roughness of the RuO2 sample than that of the Ru sample共see Table I兲. Nonetheless, simi-lar to the capacitance measurement, the difference is very minor. It indicates that the electrical properties of as-sputtered Ta2O5are inert to the choice of bottom electrode materials between RuO2and Ru. After annealing in oxygen at 700°C for 30 min, the leakage current of the Ta2O5capacitor with the Ru electrode decreased after annealing while the leakage current of the capacitor with the RuO2 electrode increased dramatically. We have also carried out a parallel experiment using polycrystalline-Si as the bottom electrode and the result showed that the leakage current of annealed Ta2O5on Ru was Figure 3. ␪-2␪ X-ray diffraction spectra of 700°C-annealed Ta2O5films,

deposited on the共a兲 RuO2and共b兲 Ru bottom electrodes.

Figure 4. AES depth profiles of the Ta2O5/Ru films deposited on the Si

substrate,共a兲 before and 共b兲 after 700°C annealing in O2ambient for 30 min.

Figure 5. AES depth profiles of the Ta2O5/RuO2films deposited on the Si

substrate,共a兲 before and 共b兲 after 700°C annealing in O2ambient for 30 min.

Table II. Capacitances and relative dielectric constants of Ta2O5 „␧Ta2O5… for the MIM capacitors, using RuO2or Ru bottom elec-trode, before and after annealing at 700°C in O2ambient for 30 min.

Sample

Capacitance共pF兲 ␧Ta2O5

As-deposited Annealed As-deposited Annealed Al/Ta2O5/RuO2 115.5 173.2 18.7 28.0

Al/Ta2O5/Ru 119.6 206 19.4 33.3

Journal of The Electrochemical Society, 148共7兲 F133-F136 共2001兲 F135

as low as that of on polycrystalline Si. Therefore, a Ru bottom electrode is satisfactory for Ta2O5capacitors, even in a high tem-perature, oxidizing environment.

Although the relatively large surface roughness of the annealed Ta2O5/RuO2sample共see Table I兲 may be a cause for the high leak-age current, in the present case, surface roughness is not fully ap-propriate for explaining the different leakage behaviors of the an-nealed Ta2O5on RuO2and Ru electrodes. Because the Rrmsvalues of both samples increase after annealing and the difference is quite small共Table I兲. Analogous to the capacitance result, we believe the different behaviors in the leakage current of Ta2O5capacitors with different bottom electrodes can be attributed to the crystallinity of Ta2O5 after annealing. Heat-treatments may reduce the leakage of the LPCVD Ta2O5film via annihilation of electrical carriers, such as oxygen vacancies and/or impurity atoms 共C or H兲.18 Hence, this effect is not perceptible in the current case of sputtered Ta2O5. When the heat-treatment induces crystallization, the leakage current of Ta2O5film may increase due to the expedient paths for electric current flow provided by the grain boundaries.17 In the present study, the more evident Ta2O5 diffraction peaks of an annealed Ta2O5/Ru sample imply that Ta2O5 grains are large in size and better oriented on the Ru electrode. The better crystallinity may indicate less intergrain boundaries and less intragrain defects. On the contrary, the annealed Ta2O5film on a RuO2electrode is still in a nanocrystalline state. According to the Poole-Frenkel emission mechanism,19the leakage current was amplified because of the large electron emission from the numerous grain boundaries and/or de-fects in Ta2O5.

It is very interesting to see that instead of deteriorating the mutilayered structure, the oxidation of Ru at high temperature induces a well crystallized Ta2O5 film. Gibbs free energies of formation for Ta2O5 and RuO2 at 1000 K (⬃723°C) are ⌬Gf,Ta2O5⫽ ⫺1605 kJ/mol and ⌬Gf,RuO2⫽ ⫺138 kJ/mol, respectively.20According to this data, Ru will not reduce Ta2O5to form RuO2. Therefore, the source of oxygen atoms for oxidizing Ru is from the annealing ambient, but not from the Ta2O5film. How-ever, the oxygen atoms must diffuse across the Ta2O5film to oxidize Ru. With a negative⌬Gf,RuO2as the driving force, the flux of oxy-gen diffusing through the Ta2O5layer of Ta2O5/Ru must be larger than that of Ta2O5/RuO2. The massive diffusion of oxygen shall promote the mobility of atoms in the Ta2O5layer, so that the

kinet-ics of nucleation and growth of Ta2O5crystals are enhanced. There-fore, the Ta2O5film on Ru has a better crystallinity than that on RuO2.

In addition, it is also possible that the high leakage current of the annealed Ta2O5/RuO2 sample may be attributed to the relatively high oxygen concentration in the Ta2O5layer after annealing 共see Fig. 5b兲. The oxygen atoms in the annealing atmosphere can be ‘‘absorbed’’ by the Ru layer in the Ta2O5/Ru sample. However, the ambient oxygen atoms will accumulate in the Ta2O5layer and be-come excessive to the Ta2O5stoichiometry because the underneath RuO2layer may not accommodate too much additional oxygen. The excess oxygen atoms are perhaps unfavorable for the crystallization of Ta2O5, as well as the dielectric leakage. Consequently, the leak-age current of the annealed Ta2O5/RuO2 sample becomes high. Nonetheless, there is not enough evidence to prove this hypothesis at present since the compositions detected by AES are only qualita-tive. The detailed mechanism for the current leakage certainly needs further investigation.

Conclusions

In conclusion, by investigating the material and electrical prop-erties of sputtered Ta2O5films on RuO2and Ru bottom electrodes, we have observed that the as-sputtered amorphous Ta2O5films have similar dielectric properties on either electrode. However, after an-nealing at 700°C in oxygen ambient, the Ta2O5 film exhibited a higher dielectric constant, as well as a smaller leakage current, on the Ru electrode than on the RuO2electrode. The Ru bottom elec-trode is oxidized during oxygen annealing. This oxidation process, instead of deteriorating the capacitor structure, promotes the nucle-ation and growth kinetics of Ta2O5crystallization process. Conse-quently, the Ta2O5film on Ru has a better crystallinity and it exhib-its superior dielectric characteristics.

Acknowledgments

The authors gratefully appreciate the financial support from the National Science Council of Taiwan, R.O.C.共contract no. NSC 89-2216-E-006-076兲.

National Cheng Kung University assisted in meeting the publication costs of this article.

References

1. T. C. May and M. H. Woods, IEEE Trans. Electron Devices, ED-26, 2共1979兲. 2. A. Ishitani, P.-Y. Lesaicherre, S. Kamiyama, K. Ando, and H. Watanabe, IEICE

Trans. Electron., E76-C, 1564共1993兲.

3. C. Chaneliere, J. L. Autran, R. A. B. Devine, and B. Balland, Mater. Sci. Eng., R., R22, 269共1998兲.

4. Y. Nishoka, H. Shinriki, and K. Mukai, J. Appl. Phys., 61, 2335,共1987兲. 5. K. Kishiro, N. Inoue, S-C. Chen, and M. Yoshimaru, Jpn. J. Appl. Phys., 37, 1336

共1998兲.

6. T. Aoyama, S. Yamazaki, and K. Imai, J. Electrochem. Soc., 145, 2961共1998兲. 7. B. K. Moon, J. Aoyama, and K. Katori, Appl. Phys. Lett., 74, 824共1999兲. 8. J. Lin, N. Masaaki, A. Tsukune, and M. Yamada, Appl. Phys. Lett., 74, 2370

共1999兲.

9. J. P. Chang, M. L. Steigerwald, R. M. Fleming, R. L. Opila, and G. B. Alers, Appl. Phys. Lett., 74, 3705共1999兲.

10. H. Matsuhashi and S. Nishikawa, Jpn. J. Appl. Phys., Part 1, 33, 1293共1994兲. 11. S. Ezhilvalavan and T.-Y. Tseng, Appl. Phys. Lett., 74, 2477共1999兲. 12. B. C.-M. Lai and J. Y.-M. Lee, J. Electrochem. Soc., 146, 266共1999兲. 13. W. D. Ryden, A. W. Lawson, and C. C. Sartain, Phys. Rev. B, 1, 1494_共1970兲. 14. S. Saito and K. Kuramasu, Jpn. J. Appl. Phys., Part 1, 31, 135_共1992兲. 15. H. Sawada and K. Kawakami, J. Appl. Phys., 86, 956_共1999兲. 16. J. H. Huang and J. S. Chen, Thin Solid Films, 382, 139_共2001兲. 17. P. C. Joshi and M. W. Cole, J. Appl. Phys., 86, 871_共1999兲.

18. T. Aoyama, S. Saida, Y. Okayama, M. Fujisaki, K. Imai, and T. Arikado, J. Elec-trochem. Soc., 143, 977_共1996兲.

19. S. M. Sze, Physics of Semiconductor Devices, 2nd ed., p. 403, John Wiley & Sons, New York共1981兲.

20. I. Barin, Thermochemical Data of Pure Substances, 3rd ed., p. 1400 and 1606, VCH, New York共1995兲.

Figure 6. Leakage currents of the Ta2O5 capacitors using RuO2 and Ru

bottom electrodes, before and after annealing at 700°C in O2ambient for 30

min.

Journal of The Electrochemical Society, 148共7兲 F133-F136 共2001兲 F136