The study of high dielectric constant gate-used (1-x) La 2 O 3 -x SiO 2 ceramics

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1 The study of high dielectric constant gate-used (1-) La 2 O 3 - SiO 2 ceramics Ho-Hua Chung 1, Kuan-Huei Lu 2, Cheng-Fu Yang *3, Ping-Shou Cheng 2 1 Dept. Automation Eng., K.Y.I.T., Kaohsiung, Taiwan, R.O.C. 2 Dept. Electronic Eng., N.K.U.A.S., Kaohsiung, Taiwan, R.O.C. 3 Dept. Chemical and Material Eng., N.U.K., Kaohsiung, Taiwan, R.O.C. Corresponding author. cfyang@nuk.edu.tw Abstract: In order to find the FET gate-used ceramic materials with dielectric constant higher than that of SiO 2, La 2 O 3 and SiO 2 are mied to format the (1-) La 2 O 3 - SiO 2 compositions. The miing (1-) La 2 O 3 - SiO 2 powders, =0.6, 0.7, and 0.8, are calcined at 1100 o C. After the (1-) La 2 O 3 - SiO 2 ceramics are sintered at 1400 o C~1550 o C for 2h, X-ray patterns are used to find the crystal phases, SEM is used to observe the sintered surface, and the dielectric characteristics are also developed. It is found that the dielectric constants of (1-) La 2 O 3 - SiO 2 ceramics increase with the increase of SiO 2 content and sintering temperature, but change of the dielectric loss is independent of La 2 O 3 content and sintering temperature. Keywords: high dielectric constant, (1-) La 2 O 3 - SiO 2, gate thin film 1. Introduction At 1999, the International Technology Roadmap for Semiconductors predicted the thickness of FET SiO 2 gate would be shortened from 2.5 nano-meter at 1999 to 1.0 nano-meter at So far, 1.5 nano-meter SiO 2 gate for 0.1~0.12µm FET used has been well developed, but the problem for the fabrication of SiO 2 gate below 1.5 nano-meter has not been dissolved [1-3]. -1-

2 Recently, the thickness of FET gate has been lowered down. For that, the dielectric constant of gate-used materials must be increased. The development of not pure SiO 2 gate materials will be the important study topic. But high dielectric materials have higher unstability than original SiO 2 in contact with silicon substrates. Thermodynamic stable data was investigated [4]. Many materials systems are currently under consideration as potential replacements for SiO 2 as the gate dielectric material for metal-oide-semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e)compatibility with the current or epected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability [5]. Many dielectric appear favorable in some of these areas. For eample: ZrO 2, TiO 2, La 2 O 3, etc.[6-9] In this study, solid state reaction method was used, SiO 2 and La 2 O 3 were used as source materials to format (1-) La 2 O 3 - SiO 2 compositions, where =0.6, 0.7, and 0.8. The influences of sintering temperature and compositions on the sintering and dielectric characteristics La 2 O 3 - SiO 2 ceramics were developed, in order to find the optimum compositions would be used as the target materials of sputtering method. 2. Eperimental Procedures The ceramic materials (1-) La 2 O 3 - SiO 2 were prepared by the mied oide method. Reagent-grade raw materials of La 2 O 3 and SiO 2 with higher than 99.5% purity were used as starting materials, mied according to the composition (1-) La 2 O 3 - SiO 2, where =0.6, 0.7, and 0.8, respectively, and ball-milled for 5h -2-

3 with deionized water. After drying and grinding, then the powders were calcined at 1100 o C for 2h. After calcinations, the powders were ground again, then polyvinylalcohol (PVA) was added as a binder. The calcining powders were uniaially pressed into pellets in a steel die with the size of 1mm in thick and 12mm in diameter. After debindering, sintering of these pellets was carried out from 1400 o C to 1550 o C for 2h. The crystal structures of (1-) La 2 O 3 - SiO 2 ceramics were investigated using XRD patterns. The morphologies of the sintered specimens were observed by using scanning electronic micrograph (SEM). The sintered ceramics were painted with Ag-Pd paste and sintered at 700 o C for 15min. Dielectric characteristics were measured at 1MHz with an oscillating amplitude (50mV) by an HP4194 impedance analyzer. 3. Results and Discussion Fig.1 shows the diffraction patterns of (1-) La 2 O 3 - SiO 2 ceramics, as a function of sintering temperature and SiO 2 content. As Fig.1 shows, the crystalline phases of (1-) La 2 O 3 - SiO 2 ceramics are different and dependent on the SiO 2 content, but independent of sintering temperature. For =0.6, the La 9.31 (Si 1.04 O 4 ) 6 O 2 and La 2 Si 2 O 7 phases are coeisted; For =0.7, the only crystalline phase is La 2 Si 2 O 7, and no other crystal phases are observed; And for =0.8, the La 2 Si 2 O 7 phase is coeisted with the SiO 2 phase. It is believed that the crystalline phases will influence the dielectric characteristics of (1-) La 2 O 3 - SiO 2 ceramics. Fig.2 shows the micrographs of (1-) La 2 O 3 - SiO 2 ceramics, as a function of sintering temperature and SiO 2 content. For =0.6 and 0.7 and sintered at 1450 o C (Fig.1(a) and 1(c)), the pores are easily observed; Sintered at 1550 o C (Fig.1(b) and 1(d)), the pores are decreased but still eisted and grain size increases apparently. As the results in Fig.1 are compared, the pores decrease -3-

4 with the increase of SiO 2 content. This result suggests that the needed sintering temperature of (1-) La 2 O 3 - SiO 2 ceramics decreases with the increase of SiO 2 content. For =0.8 and sintered at 1450 o C and 1500 o C, almost no pores are observed; Sintered at 1550 o C, the square-typed grains are revealed, but the pores increase apparently. Fig.3 shows the dielectric constant and dielectric loss of (1-) La 2 O 3 - SiO 2 ceramics, sintered at 1450 o C~1550 o C and the measured frequency is 1MHz. The dielectric constant increases with the increase of SiO 2 content, independent on the sintering temperature. For =0.6 and 0.7, the dielectric constant increases with the increase of sintering temperature, the decrease of pores will cause this result. For =0.8, the dielectric constant increase and then decrease in the 1550 o C-sintered ceramics, the increase of pores will cause this result. For all (1-) La 2 O 3 - SiO 2 ceramics, the dielectric loss decreases with the increase of sintering temperature, ecept the 1550 o C-sintered 0.2 La 2 O SiO 2 ceramic. It is believed that the increase in pores, as Fig.2(f) shows, will also cause this result. 4. Conclusions The main topic of this study is to find an optimum (1-) La 2 O 3 - SiO 2 composition, which can be used as the sputtering target materials to fabricate high dielectric FET gate oide. In the (1-) La 2 O 3 - SiO 2 ceramics, the ceramics have the higher dielectric constant than that of SiO 2 ceramics. In the (1-) La 2 O 3 - SiO 2 composition, the 0.2 La 2 O SiO 2 ceramic has the lowest sintering temperature and highest dielectric constant, it would be the optimum chose to be the sputter target source. -4-

5 ACKNOWLEDGMENTS Acknowledgments: The authors will also acknowledge to the financial support of the National Science Council of the Republic of China (contract NSC E ). REFERENCES [1] G.Lucovsky, Y.Wu, H.Niimi, V.Misra, and J.C. Phillips, Appl.Phys.Lett. 174 (1999) [2] T.P.Ma, IEEE Trans. Electron Dev. 45 (1998) 680. [3] V.Misra, G.P.Heuss, and H.Zhong, Appl.Phys.Lett. 78 (2001) [4] K.J. Hubbard, D. G. Schlom, J. Mater. Res. 11 (1996) [5] G. D. Wilk, R. M. Wallace, J. M. Anthony, J. Appl. Phys. 89 (2001) [6] Robert Beyers, J. Appl. Phys. 56 (1984)147. [7] M. Houssa, M. M. Heyns, Applied Physics Letters 77 (2000) [8] M. M. Frank, Y. J. Chabal, and G. D. Wilk, Appl.Phys.Lett., 182 (2003) [9] J. Robertson, Mater. Sci. Bul. 27 (2002) 217. (a) X La 9.31 (Si 1.04 O 4 ) 6 O 2 La 1550 O C 2 Si 2 O 7 (b) La 2 Si 2 O O C Intensity 1500 O C 1450 O C Intensity 1500 O C 1450 O C 1400 O C 1400 O C θ value θ value -5-

6 (c) La 2 Si 2 O 7 SiO O C Intensity 1500 O C 1450 O C 1400 O C θ value Fig.1 The X-ray diffraction patterns of (1-) La 2 O 3 - SiO 2 ceramics, as a function of sintering temperature and SiO 2 content. (a) (b) (c) (d) -6-

7 (e) (f) Fig.2: The SEM micrographs of (1-) La 2 O 3 - SiO 2 ceramics. For =0.6 sintered at (a) 1450 o C and (b) 1550 o C. For =0.7 sintered at (c) 1450 o C and (d) 1550 o C. For =0.8 sintered at (e) 1450 o C and (g) 1550 o C. tanδ (10-2 ) ε r =0.6 =0.7 = Sintering temperature ( o C) Fig.3 The dielectric constant and dielectric loss of (1-) La 2 O 3 - SiO 2 ceramics, as a function of sintering temperature and SiO 2 content. -7-