APPLIED PHYSICS LETTERS
VOLUME 75, NUMBER 18
1 NOVEMBER 1999
Electrical characteristics of Ta2O5 thin films deposited by electron beam gun evaporation V. Mikhelashvili and G. Eisensteina) Electrical Engineering Department, Technion, Haifa 32000 Israel
共Received 29 June 1999; accepted for publication 8 September 1999兲 We report electrical characteristics of Ta2O5 films deposited by a simple electron beam gun evaporator. We describe thickness-dependent characteristics for films with thicknesses of 7–130 nm. An equivalent SiO2 thickness of 3.5–4.5 nm for films whose leakage current density at an electric field of 106 V/cm is lower than 10⫺7 A/cm2 is demonstrated. © 1999 American Institute of Physics. 关S0003-6951共99兲03544-5兴
The use of high dielectric constant films as thin insulators for metal–insulator–semiconductor 共MIS兲 structures has been reported in numerous recent publications.1–12 Two materials stand out as holding the largest promise to potentially replace SiO2 in small scale MIS and metal–insulator–metal devices: TiO2 and Ta2O5. The best reported results for both use radio frequency sputtering and different chemical vapor deposition 共CVD兲 techniques such as: ultravioletphotoassisted, metal organic, and low pressure. Film deposition is always followed by an annealing procedure which ensures optimum film characteristics. This letter reports on the electrical characteristics of Ta2O5 thin films deposited by a simple electron beam gun evaporator which enables versatility in the choice of materials and avoids complications common in CVD systems. All films were deposited on P-type Si substrates at a pressure of 10⫺5 Torr and the deposition rate was 0.05 nm/s.13 Following deposition, the films were annealed in an O2 environment at 750 °C for 60 min. MIS capacitors were constructed with Al contacts. We describe measurements of the thicknessdependent relative dielectric constant and leakage current density for as-deposited and annealed films whose thicknesses range from 7 to 130 nm. For the thick films, we obtain from 1 MHz capacitance–voltage (C – V) measurements a dielectric constant of 28–31. Measured values are affected by an unavoidable SiO2 layer on the Si surface as well as by the accumulation layer capacitance of the Si substrate.4 For very thin films, the SiO2 series capacitor dominates and the extracted ⑀ reduces to 8–10. The leakage current density for a field strength of 106 V/cm is smaller than 10⫺7 A/cm2 in the thin 共up to 25 nm兲 films and increases significantly for thicker films. Figure 1 shows measured capacitance as a function voltage for two exemplary films having as-deposited thicknesses of 7.5 and 25 nm. Using ellipsometric measurements we observe that the film thicknesses were reduced to 7 and 20 nm, respectively, after annealing and their refractive index changed from 2.0 to 2.15, indicating an increase in the Ta2O5 density. Without annealing, the thinner film is leaky and does not exhibit a decent C – V plot while both annealed films and a兲
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the 25 nm nonannealed film exhibit a flat response yielding a well defined dielectric constant. The dielectric constant was extracted for films with eight thicknesses and the results are summarized in Fig. 2共a兲. The thick as-deposited films have an ⑀ of 31 which is almost unchanged following the annealing process. The extracted dielectric constant reduces monotonically for thinner films and reaches ⬃10 for the 7.5 nm case. Figure 2共a兲 also shows calculated values of the effective ⑀ for a series 1.5 and 3-nmthick SiO2 capacitor, respectively. For Ta2O5 films of up to 25 nm the calculations fit the experimental results well. For thicker than 25 nm Ta2O5 films, the effect of the SiO2 film is reduced and the only series capacitance affecting the C – V measurement is due to the Si accumulation layer. Figure 2共b兲 shows the thickness dependent flatband voltage, also extracted from the C – V measurements. Again, the thick as-deposited films show an almost constant V fb which reduces significantly for the thin films, while the values of V fb of the annealed film stay almost constant for all thicknesses. Figure 3 describes Terman analysis14 of the interface state density using a comparison between measured and calculated C – V curves. The film we analyzed had a thickness of 25 nm which reduced to 20 nm after the annealing process. The midgap densities in both cases were less than 1011 cm⫺2 eV⫺1. The left side of the annealed films shifts compared to the curve of the as-deposited case independent
FIG. 1. C – V characteristics Al–Ta2O5 –pSi–Al structures.
of
as-deposited
and
annealed
0003-6951/99/75(18)/2836/3/$15.00 2836 © 1999 American Institute of Physics Downloaded 08 Oct 2003 to 132.68.1.29. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Appl. Phys. Lett., Vol. 75, No. 18, 1 November 1999
FIG. 2. 共a兲 Thickness dependence of the relative dielectric constant of Ta2O5 films before and after annealing. 共b兲 Thickness dependence of the flatband voltage for structures Al–Ta2O5 –pSi–Al before and after annealing.
of thickness. This fact is caused by an increase of the total fixed positive charges in the annealed films and is correlated with V fb becoming more negative, as seen in Fig. 2共b兲. In Fig. 4共a兲 we describe current density versus electric field characteristics of the 7.5 and 25 nm films before and
V. Mikhelashvili and G. Eisenstein
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FIG. 4. 共a兲 Electric field versus current characteristics of as deposited and annealed Al–Ta2O5 –pSi–Al structures for as deposited and annealed films. 共b兲 Thickness dependence of the leakage current density measured at an electric field of 106 V/cm before and after annealing.
after annealing with the leakage showing a dramatic reduction due to the annealing process. Figure 4共b兲 summarizes the leakage current densities at an electric field of 106 V/cm for all thicknesses. For films which are thinner than 25 nm, the leakage current density is extremely small ⬃3 ⫻10⫺8 A/cm2 while it increases dramatically for thicker films. The main reason for this increase is related to an incomplete oxygen diffusion into the thicker than 25 nm Ta2O5 films during 60 min of the annealing process.3 The leakage current density in all cases is very high for the nonannealed films as is also seen in Fig. 4共b兲. To summarize, we have demonstrated high quality thin Ta2O5 films deposited by a simple electron beam gun evaporator and annealed in an oxygen atmosphere. For the films which are 7–25 nm thick, the leakage current density is below 10⫺7 A/cm2 at an electric field intensity of 106 V/cm. With the effective ⑀ values shown in Fig. 2共a兲 we calculate for the 7–20 nm annealed Ta2O5 films an equivalent SiO2 thickness of 3.5–4.5 nm, similar to the values obtained by CVD deposited Ta2O5 and TiO2 films.8,12 1
M. Matui, S. Oka, K. Yamagishi, K. Kuroiwa, and Y. Tarui, Jpn. J. Appl. Phys., Part 1 27, 506 共1988兲. 2 C. Hashimoto, H. Oikava, and N. Honma, IEEE Trans. Electron Devices 36, 14 共1989兲. FIG. 3. Terman spectra of the interface traps for Al–Ta2O5 –pSi–Al struc3 H. Shinriki and M. Nakata, IEEE Trans. Electron Devices 38, 455 共1991兲. tures before and after annealing. Downloaded 08 Oct 2003 to 132.68.1.29. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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J. Yan, D. C. Gilmer, S. A. Campbell, W. L. Gladfelter, and P. G. Schmid, J. Vac. Sci. Technol. B 14, 1706 共1996兲. 5 H. Kim, D. C. Gilmer, S. A. Campbell, and D. L. Polla, Appl. Phys. Lett. 69, 3860 共1996兲. 6 F. Chui, J. Wang, J. Y. Lee, and S. C. Wu, J. Appl. Phys. 81, 6911 共1997兲. 7 H. Kim, S. A. Campbell, and D. C. Gilmer, IEEE Electron Device Lett. 18, 465 共1997兲. 8 H.-J. Lee, R. Sinclair, M. Lee, and H.-D. Lee, J. Appl. Phys. 83, 139 共1998兲. 9 Q. Lu, D. Park, A. Kalnitsky, C. Chang, C. Cheng, S. P. Tay, T. King,
V. Mikhelashvili and G. Eisenstein and C. Hu, IEEE Electron Device Lett. 19, 341 共1998兲. B. K. Moon, C. Isobe, and J. Aoyama, J. Appl. Phys. 85, 1731 共1999兲. 11 J. Lin, N. Masaaki, A. Tsukune, and M. Yamada, Appl. Phys. Lett. 74, 2370 共1999兲. 12 H. Kim, S. A. Campbell, D. C. Gilmer, V. Kaushik, J. Conner, L. Prabhu, and A. Anderson, J. Appl. Phys. 85, 3278 共1999兲. 13 V. Mikhelashvili, Y. Betzer, I. Prudnikov, M. Orenstein, D. Ritter, and G. Eisenstein, J. Appl. Phys. 84, 6747 共1998兲. 14 E. H. Nicollian and J. R. Brews, Metal Oxide Semiconductor Physics and Technology 共Wiley, New York, 1982兲. 10
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