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Ferroelectric characteristic of group IV elements added SrBi2Ta2O9 thin films
Microelectronics Reliability 47 (2007) 830–833 www.elsevier.com/locate/microrel
Ferroelectric characteristic of group IV elements added SrBi2Ta2O9 thin films Susumu Tamura *, Yasuhisa Omura High-Technology Research Center, Faculty of Engineering, Kansai University, 3-3-35 Yamate-chou, Suita, Osaka 564-8680, Japan
Abstract The SrBi2Ta2O9 (SBT) thin film added IV group elements was fabricated on the Pt/Ti/SiO2/Si substrate by the metal–organic decomposition (MOD) method, the Pt electrode was deposited on the SBT thin film by the DC spattering method. The electric properties of the ferroelectric capacitor were measured. The remanent polarization and the relative dielectric constant of the SBT thin film have decreased, according to the amount of IV group elements addition. The IV group elements added SBT thin film with low relative dielectric constant and low remanent polarization is suitable for the application of the ferroelectric gate FET type memory. 2007 Published by Elsevier Ltd.
1. Introduction The development of the non-volatile memory using the ferroelectric thin film is actively advanced [1,2]. The ferroelectric memory using the ferroelectric capacitor (1T1C) has been put to practical use. However, because preparing the ferroelectric capacitor minute is difficult, a high-density memory has not been achieved. While, the ferroelectric gate FET (FeFET) memory [3–6] to which one memory can be formed with one transistor is expected as a highdensity memory, because it obeys the scaling rule. However, the FeFET has not been put to practical use because its retention time is not so long. The ferroelectric thin film with a big remanent polarization is preferable for the 1T1C memory as is well known. However, the big remanent polarization is not necessary for the FeFET memory. In the FeFET memory of the metal/ferroelectric/insulator/semiconductor (MFIS) structure, the big remanent polarization generates a big voltage in the capacitor of the insulator film, the voltage is applied in the direction in which the polarization of the ferroelectric thin film decreases when the memory is maintaining a
*
Corresponding author. Tel.: +81 6 6368 1121; fax: +81 6 6330 3770. E-mail address: [email protected] (S. Tamura).
0026-2714/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.microrel.2007.01.050
memory state, and it shortens the retention time of the MFIS type FeFET memory. Moreover, the gate voltage which impresses to reverse the polarization of the ferroelectric is divided in the series capacitor: the ferroelectric capacitor and the insulator capacitor. The majority of the gate voltage should be applied to the ferroelectric capacitor to operate a low voltage. Thus, the low capacity of the ferroelectric capacitor is preferable and the ferroelectric thin film with small relative dielectric constant is necessary. The development of the ferroelectric thin film with low remanent polarization and low relative dielectric constant is important to practical use of the MFIS type FeFET memory. We have reported that the remanent polarization and the relative dielectric constant of the silicon added SrBi2Ta2O9 (SBT) thin film decreases with the amount of silicon addition [7]. In this report, we report that the similar decrease effect is caused by germanium and tin addition to SBT thin films. 2. Experimental The metal–organic decomposition method (MOD) was adopted for the fabrication of the ferroelectric thin film because it could transmute the composition of the SBT thin film very easily. The SBT solution used for the MOD was Y1 type O (Sr:Bi:Ta = 0.9:2.2:2.0) manufactured by Kojundo
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
3. Results and discussion Figs. 1 and 2 show the dependence of remanent polarization of SBT thin films on amount of germanium and tin addition, respectively. The remanent polarization of the SBT thin films decreased with the addition amount of germanium or tin, just like a silicon addition to the SBT thin films [7]. Figs. 3 and 4 show the dependence of relative dielectric constant of SBT thin films on amount of germanium and tin, respectively. The relative dielectric constant of SBT thin films decreased with the addition amount of germanium or tin, just like a silicon addition to the SBT thin films [7]. Therefore, the effect of the decrease of remanent polarization and the relative dielectric constant by the silicon addition to the SBT thin film are common pheno-
Remanent polarization (μC/cm 2)
12
10
700 8
750 800
6
4
2
0 0
0.1
0.2
0.3
Ge mol fraction Fig. 1. Addition amount dependency of remanent polarization of germanium addition SBT thin films.
15
Remanent polarization ( μ C/cm 2 )
Chemical Lab. Co. Ltd. SYM-DE03 solution or SYMSN05 solution manufactured by Symetrix Co. were added to the SBT solution (Y-1 type O) to add the germanium or tin, respectively. The amount of addition of the germanium and tin had been changed from 0 mol to 0.3 mol to 1 strontium mole in the SBT solution. Ferroelectric capacitors were fabricated and the ferroelectric characteristic of the IV group elements added SBT thin films was evaluated. Pt (100 nm)/Ti (50 nm) bottom electrodes were deposited on SiO2/Si (1 0 0) wafers by DC magnetron sputtering. The SBT solutions were spincoated on the Pt/Ti/SiO2/Si structure at 500 rpm for 3 s and subsequently rotated at 1500 rpm for 30 s. Then, the films were dried at 120 C for 2 min, and then at 250 C for 5 min in air on hot plates and then were subsequently annealed in O2 at crystallization temperatures for 10 min. This process was repeated four times. Then, the SBT thin films were annealed in O2 at crystallization temperatures for 60 min to achieve crystallization. The thickness of the SBT thin films was approximately 180 nm. Pt top electrodes were deposited by DC magnetron sputtering onto the SBT thin films. The SBT capacitors were sintered in O2 at crystallization temperatures for 30 min. The crystallization temperature of the SBT thin films was changed from 700 C to 800 C. The MFIS structure using silicon addition SBT thin film were prepared for the application of IV group elements added SBT thin films to the FeFET memory. TiO2 was used for the insulating layer. The TiO2 thin film was prepared by the RF magnetron spatter method on the silicon substrate removing the natural oxidation layer by the HF processing. The TiO2 thin film was annealed at 650 C in O2 for 1 h. The film thickness of the TiO2 thin film was approximately 12 nm. Silicon addition SBT thin films were prepared on the TiO2 thin films at 650 C. The amount of the silicon addition has changed with 0 mol, 0.025 mol, 0.05 mol, and 0.075 mol. The thickness of the SBT thin films was approximately 180 nm. Pt top electrodes were deposited by DC magnetron sputtering onto the SBT thin films. The MFIS structure were sintered in O2 at 650 C for 30 min.
831
10
5
0 0
0.1
0.2
0.3
Sn mol fraction Fig. 2. Addition amount dependency of remanent polarization of tin addition SBT thin films.
mena generated by the addition of IV group elements to the SBT thin films. The leakage currents of germanium or tin added SBT thin films prepared at 800 C are shown in Figs. 5 and 6, respectively. The leakage currents of SBT thin films negligibly changed with amount of germanium and tin addition and are approximately, 1 · 10 8 A/cm2. The result of the leakage current indicates that the added tin elements exist in the SBT crystal grains. If the added tin exists in the grain boundary, tin becomes oxidized and tin oxide layer encloses the SBT grains. The leakage current of the SBT thin films must increase with the
832
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
1.E-07
400
70 0
1.E-08
75 0
Current Density (A/cm 2 )
Dielectric constant
300
80 0
200
100
0 0
0.1 0.2 Ge mol fraction
0.3
1.E-09
Ge mol fraction 0 mol 0.1 mol
1.E-10
0.2 mol 0.3 mol
Fig. 3. Addition amount dependency of relative dielectric constant of germanium addition SBT thin films.
1.E-11
400
0
1
2
3
4
5
Voltage (V) Fig. 5. Addition amount dependency of leakage current of germanium addition SBT thin films prepared at 800 C.
Dielectric constant
300
1.E-07 200
1.E-08
0 0
0.1
0.2
0.3
Sn mol fraction Fig. 4. Addition amount dependency of relative dielectric constant of tin addition SBT thin films.
Current Density (A/cm 2)
100
1.E-09
Sn mole fraction
0 mol 0.1 mol
1.E-10
amount of tin addition because tin oxide is conductor. The added silicon and germanium must exist in the SBT crystal grain because the atomic radius of silicon and germanium are smaller than that of tin. The XRD pattern of the group IV element added SBT thin film hardly changed by group IV element addition, just like a silicon addition to the SBT thin films [7], and indicates the existence of perovskite structure and those SBT thin films are non-orientated films. Because the structure of crystal is not changed even if the added group IV elements exist in the SBT crystal grain, it is seem that the added group IV elements substitute a composition element of the SBT thin films. Thus, the effect of the group IV ele-
0.2 mol 0.3 mol 1.E-11 0
1
2
3
4
5
Voltage (V) Fig. 6. Addition amount dependency of leakage current of tin addition SBT thin films prepared at 800 C.
ment addition to SBT thin films is caused by this substitution for a composition element of the SBT thin film.
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
stant for the application of the MFIS type FeFET non-volatile memory device. When IV group elements is added to the SBT thin film, IV group elements substitutes a composition element of the SBT thin film. The remanent polarization and the relative dielectric constant of the SBT thin film are simultaneously reduced by this IV group elements substitution. This IV group elements-added SBT thin film is suitable to be applied as the ferroelectric material for the FeFET memory of the MFIS structure, because of their low remanent polarization and low relative dielectric constant.
120 100 0 mol
Capacitance (pF)
0.025 mol
80
833
0.05 mol 0.075 mol
60 40 20
Acknowledgements
0 -4
-2
0
2
4
Voltage (V) Fig. 7. C–V characteristic of the MFIS structure using silicon-added SBT thin films prepared at 650 C and TiO2 thin film.
Fig. 7 shows the C–V characteristic of the MFIS structure using the silicon added SBT thin film and the TiO2 layer. The capacitance of the MFIS structure decrease with the amount of the silicon addition, and the memory window becomes large. This result depends on the large voltage applied to the ferroelectric layer by the capacitance of the ferroelectric layer decreasing so that the relative dielectric constant of the ferroelectric thin film may decrease by the silicon addition. 4. Conclusions We attempted to fabricate a new ferroelectric film with low remanent polarization and low relative dielectric con-
This research was partially supported by the Kansai University Special Research Fund, 2005, and the Kansai University HRC project. References [1] Haratake K, Shigemitsu N, Nishijima M, Yoshimura T, Fujimura N. Jpn J Appl Phys 2005;44:6977. [2] Sakai S, Ilangovan R, Takahashi M. Jpn J Appl Phys 2005;44:7876. [3] Moll JL, Tarui Y. IEEE Trans Electr Dev 1963;ED-10:338. [4] Okuyama M, Ueda T, Hamakawa Y. Jpn J Appl Phys 1985;24(Suppl. 24-2, Pe.1):619. [5] Matsubara S, Miura S, Miyasaka Y, Shohata N. J Appl Phys 1989;66: 12. [6] Hirai T, Teramoto K, Nishi T, Goto T, Tarui Y. Jpn J Appl Phys 1994;33:5219. [7] Tamura S, Omura Y, Nakahara S. Jpn J Appl Phys 2004;43:7871.
Ferroelectric characteristic of group IV elements added SrBi2Ta2O9 thin films Susumu Tamura *, Yasuhisa Omura High-Technology Research Center, Faculty of Engineering, Kansai University, 3-3-35 Yamate-chou, Suita, Osaka 564-8680, Japan
Abstract The SrBi2Ta2O9 (SBT) thin film added IV group elements was fabricated on the Pt/Ti/SiO2/Si substrate by the metal–organic decomposition (MOD) method, the Pt electrode was deposited on the SBT thin film by the DC spattering method. The electric properties of the ferroelectric capacitor were measured. The remanent polarization and the relative dielectric constant of the SBT thin film have decreased, according to the amount of IV group elements addition. The IV group elements added SBT thin film with low relative dielectric constant and low remanent polarization is suitable for the application of the ferroelectric gate FET type memory. 2007 Published by Elsevier Ltd.
1. Introduction The development of the non-volatile memory using the ferroelectric thin film is actively advanced [1,2]. The ferroelectric memory using the ferroelectric capacitor (1T1C) has been put to practical use. However, because preparing the ferroelectric capacitor minute is difficult, a high-density memory has not been achieved. While, the ferroelectric gate FET (FeFET) memory [3–6] to which one memory can be formed with one transistor is expected as a highdensity memory, because it obeys the scaling rule. However, the FeFET has not been put to practical use because its retention time is not so long. The ferroelectric thin film with a big remanent polarization is preferable for the 1T1C memory as is well known. However, the big remanent polarization is not necessary for the FeFET memory. In the FeFET memory of the metal/ferroelectric/insulator/semiconductor (MFIS) structure, the big remanent polarization generates a big voltage in the capacitor of the insulator film, the voltage is applied in the direction in which the polarization of the ferroelectric thin film decreases when the memory is maintaining a
*
Corresponding author. Tel.: +81 6 6368 1121; fax: +81 6 6330 3770. E-mail address: [email protected] (S. Tamura).
0026-2714/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.microrel.2007.01.050
memory state, and it shortens the retention time of the MFIS type FeFET memory. Moreover, the gate voltage which impresses to reverse the polarization of the ferroelectric is divided in the series capacitor: the ferroelectric capacitor and the insulator capacitor. The majority of the gate voltage should be applied to the ferroelectric capacitor to operate a low voltage. Thus, the low capacity of the ferroelectric capacitor is preferable and the ferroelectric thin film with small relative dielectric constant is necessary. The development of the ferroelectric thin film with low remanent polarization and low relative dielectric constant is important to practical use of the MFIS type FeFET memory. We have reported that the remanent polarization and the relative dielectric constant of the silicon added SrBi2Ta2O9 (SBT) thin film decreases with the amount of silicon addition [7]. In this report, we report that the similar decrease effect is caused by germanium and tin addition to SBT thin films. 2. Experimental The metal–organic decomposition method (MOD) was adopted for the fabrication of the ferroelectric thin film because it could transmute the composition of the SBT thin film very easily. The SBT solution used for the MOD was Y1 type O (Sr:Bi:Ta = 0.9:2.2:2.0) manufactured by Kojundo
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
3. Results and discussion Figs. 1 and 2 show the dependence of remanent polarization of SBT thin films on amount of germanium and tin addition, respectively. The remanent polarization of the SBT thin films decreased with the addition amount of germanium or tin, just like a silicon addition to the SBT thin films [7]. Figs. 3 and 4 show the dependence of relative dielectric constant of SBT thin films on amount of germanium and tin, respectively. The relative dielectric constant of SBT thin films decreased with the addition amount of germanium or tin, just like a silicon addition to the SBT thin films [7]. Therefore, the effect of the decrease of remanent polarization and the relative dielectric constant by the silicon addition to the SBT thin film are common pheno-
Remanent polarization (μC/cm 2)
12
10
700 8
750 800
6
4
2
0 0
0.1
0.2
0.3
Ge mol fraction Fig. 1. Addition amount dependency of remanent polarization of germanium addition SBT thin films.
15
Remanent polarization ( μ C/cm 2 )
Chemical Lab. Co. Ltd. SYM-DE03 solution or SYMSN05 solution manufactured by Symetrix Co. were added to the SBT solution (Y-1 type O) to add the germanium or tin, respectively. The amount of addition of the germanium and tin had been changed from 0 mol to 0.3 mol to 1 strontium mole in the SBT solution. Ferroelectric capacitors were fabricated and the ferroelectric characteristic of the IV group elements added SBT thin films was evaluated. Pt (100 nm)/Ti (50 nm) bottom electrodes were deposited on SiO2/Si (1 0 0) wafers by DC magnetron sputtering. The SBT solutions were spincoated on the Pt/Ti/SiO2/Si structure at 500 rpm for 3 s and subsequently rotated at 1500 rpm for 30 s. Then, the films were dried at 120 C for 2 min, and then at 250 C for 5 min in air on hot plates and then were subsequently annealed in O2 at crystallization temperatures for 10 min. This process was repeated four times. Then, the SBT thin films were annealed in O2 at crystallization temperatures for 60 min to achieve crystallization. The thickness of the SBT thin films was approximately 180 nm. Pt top electrodes were deposited by DC magnetron sputtering onto the SBT thin films. The SBT capacitors were sintered in O2 at crystallization temperatures for 30 min. The crystallization temperature of the SBT thin films was changed from 700 C to 800 C. The MFIS structure using silicon addition SBT thin film were prepared for the application of IV group elements added SBT thin films to the FeFET memory. TiO2 was used for the insulating layer. The TiO2 thin film was prepared by the RF magnetron spatter method on the silicon substrate removing the natural oxidation layer by the HF processing. The TiO2 thin film was annealed at 650 C in O2 for 1 h. The film thickness of the TiO2 thin film was approximately 12 nm. Silicon addition SBT thin films were prepared on the TiO2 thin films at 650 C. The amount of the silicon addition has changed with 0 mol, 0.025 mol, 0.05 mol, and 0.075 mol. The thickness of the SBT thin films was approximately 180 nm. Pt top electrodes were deposited by DC magnetron sputtering onto the SBT thin films. The MFIS structure were sintered in O2 at 650 C for 30 min.
831
10
5
0 0
0.1
0.2
0.3
Sn mol fraction Fig. 2. Addition amount dependency of remanent polarization of tin addition SBT thin films.
mena generated by the addition of IV group elements to the SBT thin films. The leakage currents of germanium or tin added SBT thin films prepared at 800 C are shown in Figs. 5 and 6, respectively. The leakage currents of SBT thin films negligibly changed with amount of germanium and tin addition and are approximately, 1 · 10 8 A/cm2. The result of the leakage current indicates that the added tin elements exist in the SBT crystal grains. If the added tin exists in the grain boundary, tin becomes oxidized and tin oxide layer encloses the SBT grains. The leakage current of the SBT thin films must increase with the
832
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
1.E-07
400
70 0
1.E-08
75 0
Current Density (A/cm 2 )
Dielectric constant
300
80 0
200
100
0 0
0.1 0.2 Ge mol fraction
0.3
1.E-09
Ge mol fraction 0 mol 0.1 mol
1.E-10
0.2 mol 0.3 mol
Fig. 3. Addition amount dependency of relative dielectric constant of germanium addition SBT thin films.
1.E-11
400
0
1
2
3
4
5
Voltage (V) Fig. 5. Addition amount dependency of leakage current of germanium addition SBT thin films prepared at 800 C.
Dielectric constant
300
1.E-07 200
1.E-08
0 0
0.1
0.2
0.3
Sn mol fraction Fig. 4. Addition amount dependency of relative dielectric constant of tin addition SBT thin films.
Current Density (A/cm 2)
100
1.E-09
Sn mole fraction
0 mol 0.1 mol
1.E-10
amount of tin addition because tin oxide is conductor. The added silicon and germanium must exist in the SBT crystal grain because the atomic radius of silicon and germanium are smaller than that of tin. The XRD pattern of the group IV element added SBT thin film hardly changed by group IV element addition, just like a silicon addition to the SBT thin films [7], and indicates the existence of perovskite structure and those SBT thin films are non-orientated films. Because the structure of crystal is not changed even if the added group IV elements exist in the SBT crystal grain, it is seem that the added group IV elements substitute a composition element of the SBT thin films. Thus, the effect of the group IV ele-
0.2 mol 0.3 mol 1.E-11 0
1
2
3
4
5
Voltage (V) Fig. 6. Addition amount dependency of leakage current of tin addition SBT thin films prepared at 800 C.
ment addition to SBT thin films is caused by this substitution for a composition element of the SBT thin film.
S. Tamura, Y. Omura / Microelectronics Reliability 47 (2007) 830–833
stant for the application of the MFIS type FeFET non-volatile memory device. When IV group elements is added to the SBT thin film, IV group elements substitutes a composition element of the SBT thin film. The remanent polarization and the relative dielectric constant of the SBT thin film are simultaneously reduced by this IV group elements substitution. This IV group elements-added SBT thin film is suitable to be applied as the ferroelectric material for the FeFET memory of the MFIS structure, because of their low remanent polarization and low relative dielectric constant.
120 100 0 mol
Capacitance (pF)
0.025 mol
80
833
0.05 mol 0.075 mol
60 40 20
Acknowledgements
0 -4
-2
0
2
4
Voltage (V) Fig. 7. C–V characteristic of the MFIS structure using silicon-added SBT thin films prepared at 650 C and TiO2 thin film.
Fig. 7 shows the C–V characteristic of the MFIS structure using the silicon added SBT thin film and the TiO2 layer. The capacitance of the MFIS structure decrease with the amount of the silicon addition, and the memory window becomes large. This result depends on the large voltage applied to the ferroelectric layer by the capacitance of the ferroelectric layer decreasing so that the relative dielectric constant of the ferroelectric thin film may decrease by the silicon addition. 4. Conclusions We attempted to fabricate a new ferroelectric film with low remanent polarization and low relative dielectric con-
This research was partially supported by the Kansai University Special Research Fund, 2005, and the Kansai University HRC project. References [1] Haratake K, Shigemitsu N, Nishijima M, Yoshimura T, Fujimura N. Jpn J Appl Phys 2005;44:6977. [2] Sakai S, Ilangovan R, Takahashi M. Jpn J Appl Phys 2005;44:7876. [3] Moll JL, Tarui Y. IEEE Trans Electr Dev 1963;ED-10:338. [4] Okuyama M, Ueda T, Hamakawa Y. Jpn J Appl Phys 1985;24(Suppl. 24-2, Pe.1):619. [5] Matsubara S, Miura S, Miyasaka Y, Shohata N. J Appl Phys 1989;66: 12. [6] Hirai T, Teramoto K, Nishi T, Goto T, Tarui Y. Jpn J Appl Phys 1994;33:5219. [7] Tamura S, Omura Y, Nakahara S. Jpn J Appl Phys 2004;43:7871.