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Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application – A review

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Abstract

Tantalum pentoxide (Ta2O5) thin films have rapidly evolved into an important field of research/development for both basic and applied science with the promise of creating a new generation of advanced micro devices for electronics applications. This paper provides a broad review of the current status and future trends of Ta2O5 thin films as a highly promising storage dielectric material for high-density dynamic random access memory applications. Various methods of thin film material processing are briefly reviewed. The physical, electrical and dielectric characteristics of Ta2O5 thin films with specific examples from recent literature and the associated conduction mechanisms are summarized and discussed. Some suggestions to improve the electrical properties of the films are finally included. ©1998Kluwer Academic Publishers

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References

  1. M. Koyanagi, H. Sunami, N. Hashimoto and M. Ashikawa, IEDM Tech.Dig. 348 (1978).

  2. H. Sunami, T. Kure, N. Hashimoto, K. Itoh, T. Toyabe and S. Asai, IEEE Trans.Electron Devices 31 (1984) 746.

    Google Scholar 

  3. K. Minegishi, S. Nakajima, K. Miura, K. Harada and T. Shibata, IEDM Tech.Dig. 319 (1983).

  4. T. Y. Tseng, IEDMS 89 (1996).

  5. B. Gnade, “Science and Technology of Electroceramic Thin Films: NATO ASI Series”, Vol. 284, edited by O. Auciello and R. Waser (Kluwer Academic Publishers, London, 1995) p. 373.

    Google Scholar 

  6. P. C. Fazan, Integr.Ferroelectr. 247 (1994).

  7. H. Shinriki, T. Kisu, S. I. Kimura, Y. Nishioka, Y. Kawamoto and K. Mukai, IEEE Trans.Electron Devices 37 (1990) 1939.

    Google Scholar 

  8. P. Y. Lesaicherre, S. Yamamichi, H. Yamaguchi, K. Takemura, H. Watanabe, K. Tokashiki, K. Satoh, T. Sakura, M. Yoshida, S. Ohnishi, K. Nakijima, K. Shibahara, Y. Miyasaka and H. Ono, IEDM Tech.Dig. 831 (1994).

  9. Y. Ohji, Y. Matsui, T. Itoga, M. Hirayama, Y. Sugawara, K. Torii, H. Miki, M. Nakata, I. Asano, S. Iijima and Y. Kawamoto, IEEE Trans. Electron Devices 111 (1995).

  10. T. Kaga, Y. Sudoh, H. Goto, K. Shoji, T. Kisu, H. Yamashita, R. Nagai, S. Iijima, M. Ohkura, F. Murai, T. Tanaka, Y. Goto, N. Yokoyama, M. Horiguchi, M. Isoda, T. NIShida and E. Takeda, IEEE Trans. Electron Devices 927 (1994).

  11. H. O. Pierson, “Handbook of Chemical Vapor Deposition: Principles, Technology and Applications” (Noyes Publications, Park Ridge, New Jersy, USA, 1992).

    Google Scholar 

  12. R. J. Cava, W. F. Peck Jr. and J. J. Krajewski, Nature 377 (1995) 215.

    Google Scholar 

  13. S. W. Park, Y. K. Baek, I. Y. Lee, C. O. Park and H. B. Im, J. Electron. Mater. 21 (1992) 635.

    Google Scholar 

  14. T. F. Chen, PhD Thesis, National Chiao Tung University, Hsinchu, Taiwan, (1997).

  15. S. Kamiyama, T. Saeki, H. Mori and Y. Numasawa, IEDM Tech. Dig. 827 (1991).

  16. H. K. Kang, K. H. Kim, Y. G. Shin, I. S. Park, K. M. Ko, C. G. Kim, K. Y. Oh, S. E. Kim, C. G. Hong, K. W. Kwon, J. Y. Yoo, Y. G. Kim, C. G. Lee, W. S. Paick, D. I. Suh, C. J. Park, S. I. Lee, S. T. Ahn, C. G. Hwang and M. Y. Lee, Ibid. (1994) 635.

  17. A. C. Adams, “Dielectric and Polysilicon Film Deposition”, in VLSI Technology edited by S. M. Sze, Ch. 3, McGraw-Hill, New York, 1993 p. 93.

    Google Scholar 

  18. W. Kern, 1985 Semicond. Int. 8 (1985) 122.

    Google Scholar 

  19. S. Wolf and R. N. Tauber, “Silicon Processing for the VLSI Era”, Vol. 1, (Lattice Pres, CA, USA 1986).

    Google Scholar 

  20. W. Kern and G. L. Schnable, IEEE Trans. Electron Devices 26 (1979) 647.

    Google Scholar 

  21. P. Singer, Semicond. Int. 5 (1984) 72.

    Google Scholar 

  22. R. S. Rosler, Solid State Technol. 20 (1977) 63.

    Google Scholar 

  23. A. C. Adams, Solid State Technol. 4 (1983) 135.

    Google Scholar 

  24. B. Gorowitz, T. B. Gorczyca and R. J. Saia, Solid State Technol. 4 (1985) 197.

    Google Scholar 

  25. J. Y. Chen and R. Henderson, J. Electrochem. Soc. 131 (1984) 2147.

    Google Scholar 

  26. R. Solanki, C. Moore and G. Collins, Solid State Technol. 6 (1985) 220.

    Google Scholar 

  27. A. Feingold and A. Katz, Mater. Sci. Eng. R 13 (1994) 56.

    Google Scholar 

  28. W. Kern and V. S. Ban, “Thin Film Processes”, edited by J. L. Vossen and W. Kern, (Academic Press, New York, 1978).

    Google Scholar 

  29. P. C. Van Buskirk, S. M. Bilodeau, J. F. Roeder and P. S. Kirlin, Jpn. J. Appl. Phys. 35 (1996) 2520.

    Google Scholar 

  30. J. B. Wachtman and R. A. Haber, “Ceramic Thin Films and Coatings” (Noyes Publications, New Jersey, USA, 1993) p. 208.

    Google Scholar 

  31. T. M. Graettinger, S. H. Rou, M. S. Ameen, O. Auciello and A. I. Kingon, Appl. Phys. Lett. 58 (1991) 1964.

    Google Scholar 

  32. A. I. Kingon, O. Auciello, M. S. Ameen, S. H. Rou and A. R. Krauss, Appl. Phys. Lett. 55 (1989) 301.

    Google Scholar 

  33. R. Ramesh, H. Gilchrist, T. Stands, V. G. Keramidas, R. Haakenaasen and D. K. Fork, Appl. Phys. Lett. 63 (1993) 3592.

    Google Scholar 

  34. R. Dat, J. K. Lee, O. Auciello and A. I. Kingon, Appl. Phys. Lett. 67 (1995) 572.

    Google Scholar 

  35. N. Taga, H. Odaka, Y. Shigesato, I. Yasui and T. E. Haynes, J. Appl. Phys. 80 (1996) 978.

    Google Scholar 

  36. J. M. Albella, I. Montero, J. M. Martinezduart and V. Parkhutik, J. Mater. Sci. 26 (1991) 3422.

    Google Scholar 

  37. J. J. Randall, W. J. Bernard and R. R. Wilkinson, Electrochim. Acta 10 (1965) 183.

    Google Scholar 

  38. I. Montero, J. M. Albella, J. M. Martinezduart and L. Soriano, J. Mater. Sci. 22 (1987) 1785.

    Google Scholar 

  39. J. J. Randall, Electrochim. Acta. 20 (1975) 663.

    Google Scholar 

  40. C. J. Dell'oca and L. Young, J. Electrochem. Soc. 117 (1970) 1545.

    Google Scholar 

  41. A. J. Schrijner and A. Middlehoek, J. Electrochem. Soc. 111 (1964) 1167.

    Google Scholar 

  42. D. J. Smi th and L. Young, Thin Solid Films 101 (1983) 11.

    Google Scholar 

  43. C. D. Lakeman and D. A. Payne, Mater. Chem. Phys. 38 (1994) 305.

    Google Scholar 

  44. D. L. Pulfrey, P. S. Wilcox and L. Young, J. Appl. Phys. 40 (1969) 3891.

    Google Scholar 

  45. R. L. Angle and H. E. Talley, IEEE Trans. Electron Devices 25 (1978) 1277.

    Google Scholar 

  46. G. S. Oehrlein and A. Reisman, J. Appl. Phys. 54 (1983) 6502.

    Google Scholar 

  47. J. G. Hwu and S. T. Lin, IEEE Proc. 137 (1990) 390.

    Google Scholar 

  48. S. I. Kimura, Y. Nishioka, A. Shintani and K. Ukai, J. Electrochem. Soc. 130 (1983) 2414.

    Google Scholar 

  49. Y. Nishioka, S. I. Kimura, H. Shinriki and K. Mukai, Ibid. 134 (1987) 410.

    Google Scholar 

  50. M. Saitoh, T. Mori and T. Tamura, IEDM Tech. Dig. 680 (1986).

  51. S. Zaima, T. Furuta and Y. Yasuda, J. Electrochem. Soc. 137 (1990) 1297.

    Google Scholar 

  52. P. A. Murawala, M. Sawai, T. Tatsuta, O. Tsuii, S. Fujita and S. Fujita, Jpn. J. Appl. Phys. 32 (1993) 368.

    Google Scholar 

  53. H. S. Moon, J. S. Lee, S. W. Kan, J. W. Park, J. H. Lee, S. K. Yang and H. H. Park, J. Mater. Sci. 29 (1994) 1545.

    Google Scholar 

  54. S. O. Kim, J. S. Byun and H. J. Kim, Thin Solid Films 206 (1991) 102.

    Google Scholar 

  55. J. G. Hwu and M. J. Jeng, J. Electrochem. Soc. 135 (1988) 2808.

    Google Scholar 

  56. K. Chen, M. Nielsen, G. R. Yang, E. J. Rymaszewski and T. M. lu, J. Electron. Mater. 26 (1997) 397.

    Google Scholar 

  57. S. Schiller, U. Heisig, K. Steinefelder and J. Strumpfel, Thin Solid Films 63 (1979) 369.

    Google Scholar 

  58. T. M. Reith and P. J. Ficalora, J. Vac. Sci. Technol. A 1 (1983) 1362.

    Google Scholar 

  59. G. V. samsonov, Ed., “The Oxide Hand Book” (IFI/Plenum, New York, 1973) p. 316.

    Google Scholar 

  60. F. C. Chiu, J. J. Wang, J. Y. Lee and S. C. Wu, J. Appl. Phys. 81 (1997) 6911.

    Google Scholar 

  61. R. A. B. Devine, Appl. Phys. Leti. 68 (1996) 1924.

    Google Scholar 

  62. Y. Chen, R. Singh and J. Narayan, J. Electron. Mater. 26 (1997) 350.

    Google Scholar 

  63. R. Singh, J. Appl. Phys. 63 (1988) R59.

    Google Scholar 

  64. S. Kamiyama, T. Saeki, H. Mori and Y. Numasawa, IEDM Tech. Dig. (1991) 827.

  65. S. Kamiyama, P. Y. Lesaicherre, A. Ishitani, A. Sakai, A. Tanikawa and I. Nishiyama, Ext. Abst. Int. Solid State Devices and Materials (1992) 521.

  66. P. C. Fazan, V. K. Mathews, R. L. Maddox, A. Ditali, N. Sandler and K. L. Kwong,Ext. Abst. Int. Solid State Devices and Materials (1992) 697.

  67. S. Kamiyama, P. Y. Lesaicherre, H. Suzuki, A. Sakai, I. Nishiyama and A. Ishitani, J. Electrochem. Soc. 140 (1997) 1617.

    Google Scholar 

  68. S. C. Sun and T. F. Chen, IEEE Electron Device Lett. 17 (1996) 355.

    Google Scholar 

  69. S. C. Sun and T. F. Chen, IEDM Tech. Dig. (1996) 687.

  70. W. S. Lau, P. W. Qian, N. P. Sandler, K. A. Mckinley and P. K. Chu, Jpn. J. Appl. Phys. 36 (1997) 661.

    Google Scholar 

  71. W. S. Lau, K. K. Khaw, P. W. Qian, N. P. Sandler and P. K. Chu, Jpn. J. Appl. Phys. 35 (1996) 2599.

    Google Scholar 

  72. C. Y. Ting and M. Wittmer, J. Appl. Phys. 54 (1983) 937.

    Google Scholar 

  73. L. Krusin-Elbaum, M. Wittmer and D. S. Yee, Appl. Phys. Lett. 50 (1987) 1879.

    Google Scholar 

  74. M. Wittmer,J. Vac. Sci. Technol. A 2 (1984) 273.

    Google Scholar 

  75. L. Krusin-Elbaum, M. Wittmer, C. Y. Ting and I. J. Cuomo, Thin Solid Films 104 (1983) 81.

    Google Scholar 

  76. R. G. Vadimsky, R. P. Frankenthal and D. E. Thompson, J. Electrochem. Soc. 126 (1979) 2017.

    Google Scholar 

  77. P. H. Holloway and C. C. Nelson, Thin Solid Films 35 (1976) L13.

    Google Scholar 

  78. M. L. Green, M. E. Gross, L. E. Papa, K. J. Schnoes and D. Brasen, J. Electrochem. Soc. 132 (1985) 2677.

    Google Scholar 

  79. H. Matsuhashi and S. Nishikawa, Jpn. J. Appl. Phys. 33 (1994) 1293.

    Google Scholar 

  80. I. Kim, J. S. Chun and W. J. Lee, Mater. Chem. Phys. 44 (1996) 288.

    Google Scholar 

  81. J. Harvey and H. W ILMAN, Acta. Crystallogr. 14 (1961) 1278.

    Google Scholar 

  82. D. M. Smyth, G. A. Shirn and T. B. Tripp, J. Electrochem. Soc. 130 (1983) 2414.

    Google Scholar 

  83. T. Aoyama, S. Saidu, Y. Okayama, M. Fujisaki, K. Imai and T. Arikado, J. Electrochem. Soc. 143 (1996) 977.

    Google Scholar 

  84. F. C. Chiu, J. J. Wang, J. Y. Lee and S. C. Wu, J. Appl. Phys. 81 (1997) 6911.

    Google Scholar 

  85. Y. Nishioka, N. Homma, H. Shinriki, K. Mukai, K. Yamaguchi, A. Uchida, K. Higeta and K. Ogiue, IEEE Trans. Electron Divices 34 (1987) 1957.

    Google Scholar 

  86. S. Roberts, J. Ryan and L. Neshi t, J. Electrochem. Soc. 133 (1986) 1405.

    Google Scholar 

  87. A. G. Revesz and T. D. Kirkendall, J. Electrochem. Soc. 123 (1976) 1514.

    Google Scholar 

  88. S. Ezhilvalavan and T. Y. Tseng, J. Am. Ceram. Soc. (1998) (in print).

  89. A. S. Pavlovic, J. Chem. Phys. 40 (1964) 951.

    Google Scholar 

  90. A. Reisman, F. Holtzburg, M. Berkenblit and M. Berry, J. Am. Chem. Soc. 78 (1956) 4514.

    Google Scholar 

  91. H. Hu, C. J. Peng and S. B. Krupanidhi, Thin Solid Films 223 (1993) 327.

    Google Scholar 

  92. A. Pignolet, G. Mohanrao and Krupanidhi, Thin Solid Films 258 (1995) 230.

    Google Scholar 

  93. B. W. Shen, I. C. Shen, S. Banerjee, R. A. Brown, J. Bohlman, P. H. Chang and R. R. Doering, IEDM Tech. Dig. 582 (1987).

  94. G. S. Oehrlein, F. M. D'heurle and A. Reisman, J. Appl. Phys. 55 (1984) 3715.

    Google Scholar 

  95. C. Isobe and M. Saitoh, Appl. Phys. Lett. 56 (1990) 907.

    Google Scholar 

  96. H. Shinriki and M. Nakata, IEEE Trans. Electron Divices 38 (1991) 455.

    Google Scholar 

  97. S. G. Byeon and Y. Tzeng, IEEE Trans. Electron Divices 37 (1990) 972.

    Google Scholar 

  98. S. W. Park and H. B. Im, Thin Solid Films 207 (1992) 258.

    Google Scholar 

  99. H. Demiryont, J. R. Sites and K. Geib, Appl. Opt. 24 (1985) 490.

    Google Scholar 

  100. S. C. Sun and T. F. Chen, Ext. Abst. Electrochem. Soc., Fall Meeting (1995) pp. 870.

  101. S. Kamiyama, H. Suzuki and H. Watanabe, J. Electrochem. Soc. 141 (1994) 1246.

    Google Scholar 

  102. Y. Matsui, K. Torii, M. Hirayama, Y. Fuj Isaki, S. Iijima and Y. Ohji, IEEE Electron Device Lett. 17 (1996) 431.

    Google Scholar 

  103. C. M. Osburn and D. W. Ormond, J. Electrochem. Soc. 109 (1972) 591.

    Google Scholar 

  104. K. Sayyah, “The Physics and Chemistry of SiO2 and Si-SiO2 Interface” (Plenum Press, New York, 1988) p. 129.

    Google Scholar 

  105. S. SekI, T. Unagami and O. Kogure, J. Electrochem. Soc. 131 (1984) 2457.

    Google Scholar 

  106. S. Seki, T. Unagami, O. Kogure and B. Tsu Jiyama, J. Vac. Sci. Technol. A5 (1987) 1771.

    Google Scholar 

  107. M. MatsuI, S. Oka, K. Yamagishi, K. Kuroiwa and Y. Tarui, Jpn. J. Appl. Phys. 27 (1988) 506.

    Google Scholar 

  108. S. Tanimoto, M. Matsui, K. Kamisako, K. Kuroiwa and Y. Tarui, J. Electrochem. Soc. 139 (1992) 320.

    Google Scholar 

  109. C. Hashimoto, H. Oikawa and N. Honma, IEEE Trans. Electron Devices 36 (1989) 14.

    Google Scholar 

  110. S. Tanimoto, Y. Shichi, K. Kuroiwa and Y. Tarui, Extended Abstracts of 19th Conference on Solid State Devices and Materials, (1993) p. 859.

  111. S. C. Sun and T. F. Chen, IEDM Tech. Dig. 333 (1994).

  112. X. M. Wu, S. R. Soss, E. J. Rymaszewski and T. M. Lu, Mater. Chem. Phys. 38 (1994) 297.

    Google Scholar 

  113. H. Treichel, A. Mitwalsky, N. P. Sandler, D. Tribula, W. Kern and A. P. Lane, Adv. Mater. Opt. Electron 1 (1992) 299.

    Google Scholar 

  114. S. G. Byeon and Y. Tzeng, J. Electrochem. Soc. 135 (1988) 2452.

    Google Scholar 

  115. G. S. Oehrlein, J. Appl. Phys. 59 (1986) 1587.

    Google Scholar 

  116. M. A. Mohammed and D. Morgan, Thin Solid Films 176 (1989) 45.

    Google Scholar 

  117. J. J. Lee, C. L. Thio and S. B. Desu, J. Appl. Phys. 78 (1995) 5073.

    Google Scholar 

  118. S. M. Sze, “Physics of Semiconductor Devices” (JohnWiley & Sons, New York, 2nd Edn 1981) p. 251.

    Google Scholar 

  119. G. S. Oehrlein, Thin Solid Films 156 (1988) 207.

    Google Scholar 

  120. A. G. Revesz and J. F. Allison, IEEE Trans. Electron. Devices 23 (1976) 527.

    Google Scholar 

  121. S. Kamiyama, H. Suzuki and H. Watanabe, A. Sakai, M. Oshida, T. Tatsumi, T. Tanigawa, N. Kasai and A. Ishitani, IEDM Tech. Dig. 49 (1993).

  122. H. Shinriki, M. Nakata, Y. Nishioka and K. Mukai, IEEE Trans. Electron Device Lett. 10 (1989) 514.

    Google Scholar 

  123. S. C. Sun and T. F. Chen, IEEE Trans. Electron Device Lett. 17 (1996) 355.

    Google Scholar 

  124. P. J. Tobin, Y. Okada, V. Lahkotia, S. A. Ajuria, W. A. Feil and R. I. Hedge, in VLSI Tech. Symp. (1993) p. 51.

  125. H. Kimura, J. Mizuki, S. Kamiyama and H. Suzuki, Appl. Phys. Lett. 66 (1995) 2209.

    Google Scholar 

  126. J. G. Simmons, J. Phys. D. Appl. Phys. 4 (1971) 613.

    Google Scholar 

  127. N. Schwartz and M. Gresh, J. Electrochem. Soc. 112 (1965) 295.

    Google Scholar 

  128. D. A. Vermilyea, J. Appl. Phys. 36 (1965) 3663.

    Google Scholar 

  129. N. N. Axelrod and N. Schwartz, J. Electrochem. Soc. 116 (1969) 460.

    Google Scholar 

  130. J. G. SImmons, Phys. Rev. 155 (1967) 657.

    Google Scholar 

  131. P. Mark and T. E. Hartman, J. Appl. Phys. 39 (1968) 2163.

    Google Scholar 

  132. D. L. Pulfrey, A. H. M. Shousha and L. Young,J. Appl. Phys. 41 (1970) 2838.

    Google Scholar 

  133. C. A. Mead, Phys. Rev. 128 (1962) 2088.

    Google Scholar 

  134. G. D. O'clock, Jr., Appl. Phys. Lett. 19 (1971) 403.

    Google Scholar 

  135. T. W. Hickmott, J. Electrochem. Soc. 113 (1966) 1223.

    Google Scholar 

  136. C. L. Standley and L. I. Maissel, J. Appl. Phys. 35 (1964) 1530.

    Google Scholar 

  137. W. D. Westwood, N. Waterhouse and P. S. Wilcox, “Tantalum Thin Films” (Academic Press, New York, 1975) p. 333.

    Google Scholar 

  138. G. Korinec, Electro. Technol 6 (1968) 108.

    Google Scholar 

  139. G. P. Klein and N. I. Jaeger, J. Electrochem. Soc. 117 (1970) 1483.

    Google Scholar 

  140. P. L. Young, J. Appl. Phys. 46 (1975) 2794.

    Google Scholar 

  141. P. L. Young, J. Appl. Phys. 47 (1976) 235.

    Google Scholar 

  142. P. L. Young, J. Appl. Phys. 47 (1976) 242.

    Google Scholar 

  143. J. L. Hartke, J. Appl. Phys. 39 (1968) 4871.

    Google Scholar 

  144. E. Kaplan, M. Balog and D. Frohmanbentchkowsky, J. Electrochem. Soc. 123 (1976) 1570.

    Google Scholar 

  145. S. M. Sze, J. Appl. Phys. 38 (1967) 2951.

    Google Scholar 

  146. M. Metikos-Hukovie and M. Cheraj-Ceric, Thin Solid Films 145 (1986) 39.

    Google Scholar 

  147. S. Banerjee, B. Shen, I. Chen, J. Bohlman, G. Brown and R. Doering, J. Appl. Phys. 65 (1989) 1140.

    Google Scholar 

  148. J. O'dwyer, “Theory of Electrical Conduction and Breakdown in Slolid Dielectrics” (Clarendon, Oxford, UK, 1973).

    Google Scholar 

  149. H. Seki, Phys. Rev. B 2 (1970) 4877.

    Google Scholar 

  150. S. Zaima, T. Furuta, Y. Koide and Y. Yasuda, J. Electrochem. Soc. 137 (1990) 2876.

    Google Scholar 

  151. S. Ezhilvalavan and T. Y. Tseng, J. Appl. Phys. 83 (1989) 4797.

    Google Scholar 

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  1. Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu, 30050, Taiwan

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Ezhilvalavan, S., Tseng, T.Y. Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application – A review. Journal of Materials Science: Materials in Electronics 10, 9–31 (1999). https://doi.org/10.1023/A:1008970922635

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