Forming gas annealing on physical characteristics and electrical properties of Sr 0.8 Bi
2 Ta 2 O 9 / Al 2 O 3 / Si capacitors
Bang Chiang Lan, Jung-Jui Hsu, San-Yuan Chen, and Jong-Shing Bow
Citation: Journal of Applied Physics 94, 1877 (2003); doi: 10.1063/1.1588362
View online: http://dx.doi.org/10.1063/1.1588362
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/94/3?ver=pdfcov Published by the AIP Publishing
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0.8 2 2 9 共SBT兲 ferroelectric film constructed on Al2 3
insulator–semiconductor 共MFIS兲 was prepared to study the degradation behavior of SBT films under forming gas annealing 共FGA兲. Although the diffusion of hydrogen ions has been detected during FGA treatment, no significant differences in the microstructure and crystalline phase are observed for the SBT film compared to that without FGA treatment. However, the diffusion of hydrogen ions leads to the rapid decrease in the dielectric constant but shows no apparent influence on the memory window of Sr0.8Bi2Ta2O9/Al2O3/Si capacitors. In addition, it was found that FGA shows a positive effect on the leakage current of MFIS in contrast to that of metal/ferroelectric/metal structure. The leakage current density of MFIS dramatically decreases as much as two orders of magnitude after FGA at 500 °C compared to that without FGA treatment that was attributed to the reduced defects at the interface of Al2O3/Si. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1588362兴
I. INTRODUCTION
Ferroelectric thin films have been widely investigated for nonvolatile random access memory applications.1 SrBi2Ta2O9 共SBT兲 thin film has superior ferroelectric
prop-erties in retention endurance, fatigue, and leakage current and therefore appears to be one of the most favorable candi-dates for ferroelectric materials in ferroelectric memory device.2 However, there are still several problems that im-pede the incorporation of the ferroelectric thin films in mi-croelectronic devices. One of the problems is the degradation of ferroelectric properties after forming gas annealing共FGA兲 treatment because this FGA process is required in microchip fabrication to passivate the device and eliminate defects in the field effect transistor.3 Several reports in literature show that the treatment of FGA leads to the decrease of switch charge and fatigue resistance, and the increase of leakage current density.4 – 6It was reported that the degradation on the ferroelectric properties of SBT thin films in metal/ ferroelectric/metal共MFM兲 structure is related to the dissocia-tive adsorption of the hydrogen molecule 共e.g., H2⫹2M
→2H⫺M) and the generation of the hydrogen atoms by the
catalytic activity of Pt共e.g., 2H⫺M→2H⫹2M).7However, Kwon et al.8 supposed that the H⫹ ions and electrons pro-duced by the dissociative adsorption of hydrogen molecules on the Pt surface are responsible for the suppressed Pr due to the pinning of the domains. With the increase of annealing temperature, the H⫹ions and electrons may diffuse into the SBT and accumulate around grain boundaries to form space charge and prohibit the domain motion, and thus result in the
ferroelectric degradation in MFM structure. Wang et al.9 studied the fatigue behavior of FGA-SBT samples and sup-posed that more weak pinning center of domain will be formed on SBT thin film after FGA process. This will cause the increase in both leakage current and fatigue degradation.9 These pinning centers of domain in the SBT films could be unlocked after oxygen recovery at 750 °C and the fatigue property of SBT thin films could be recovered to the original state before FGA.10In addition, it was also reported that the forming gas atmosphere and deposited top electrode show a strong influence on the degradation behavior of the MFM structure.8,11
In comparison with the existing MFM structure, metal– ferroelectric–semiconductor ferroelectric memory has at-tracted much attention due to the unique performance and advantages including obeying the scaling rule, high switch speed, and nondestructive reading.12 Furthermore, the metal–ferroelectric–insulator–semiconductor 共MFIS兲 ca-pacitor has been widely studied because it has not only the same small 1 T cell structure as flash memory but also the merits of ferroelectric random access memory mentioned herein.13 Recently, we have reported that free-fatigue Bi3.25La0.75Ti3O12ferroelectric films could be integrated in a MFIS capacitor with Al2O3 as the insulator and excellent
ferroelectric properties can be obtained, such as smaller leak-age current and larger memory window.14,15However, there have been few studies focused on the structural variation of the MFIS under FGA treatment. Therefore, in this work, the effect of forming gas annealing on the physical characteris-tics and electrical properties of SBT/Al2O3/Si will be
inves-tigated. The role of top electrode in the passivation and elec-trical characterization in a MFIS structure is further
a兲Author to whom correspondence should be addressed; electronic mail:
sychen@cc.nctu.edu.tw
1877
0021-8979/2003/94(3)/1877/5/$20.00 © 2003 American Institute of Physics
discussed. For comparison, the MIM structure is also pre-pared.
II. EXPERIMENT
4-in.-Si p-type wafers were used in this study. A HF-vapor passivation was used to suppress the native oxide for-mation before other treatment. The Al layer was thermally evaporated on wafers, oxidized at a temperature of 400 °C for 2 h to form 5-nm-thick Al2O3, and finally annealed
at 900 °C for 30 min in a nitrogen ambient. Stron-tium 2-ethylhexanotate 关Sr(C8H15O2)2兴, bismuth
2-ethylhexanoate 关Bi(C8H15O2)2兴, tantalum ethoxide
关Ta(OC2H5)5兴 were used as the metalorganic precursors for
the SBT formula solution synthesis and xylene was used as the solvent for adjusting the concentration of the solution following our previous works.16 Sr0.8Bi2Ta2O9 composition
was selected because a large memory window can be ob-tained with a smaller applied voltage in the MFIS structure according to our previous work and a related report in literature.17 The solutions were spin coated on the gate di-electrics and then followed by subsequent drying. This pro-cedure was repeated for several times to obtain the desired film thickness about 300 nm. Between each coating, the wet films were pyrolyzed at 400 °C for several minutes. After that, the as-deposited films were annealed at 800 °C for 30 min in the oxygen ambient. Pt top and back side electrodes were formed by sputtering. The hard mask size used for the fabrication of MFIS capacitor is 3.14⫻10⫺4cm2. In the work, the FGA is carried out at the temperature ranging from 200 to 500 °C for 15 min under an atmosphere of 5% H2/95% N2. The crystal structure of the SBT films was
analyzed by an x-ray diffractometer 共MAC Science M18XHF兲 with Cu K␣ radiation and the thickness of SBT films was measured using Alpha-stepper共Sloan Dektak兲. The capacitance–voltage (C⫺V) and current–voltage (I⫺V) were characterized and measured using HP4284 and HP4155B, respectively. A secondary ion mass spectrometer 关共SIMS兲 Cameca IMS-4f兴 was used for depth profiling of each element in the capacitors.
III. RESULTS AND DISCUSSION A. Crystal phase and microstructure
Figure 1 shows the x-ray diffraction patterns of SBT thin films annealed at 800 °C and then treated under FGA at 200 to 500 °C for 15 min using 5% H2/95% N2 for both samples
without and with Pt top electrode coated. It revealed that neither major changes in the film crystallinity nor any notice-able second-phase formation after FGA were observed for SBT films on Al2O3/Si substrates.16,18 This indicated that the ferroelectric perovskite phase remains unchanged even after FGA treatment. In addition, the scanning electron mi-croscopy共SEM兲 images of the SBT film reveal no significant differences on the surface morphology of the films before and after 500 °C annealing as shown in Fig. 2. The micro-structure of both SBT films present similar features with round and smaller grains. Figure 3共a兲 demonstrates the cross-section transmission electron microscopy 共TEM兲 micro-graphs of the Pt/SBT/Al2O3/Si structure without a top
elec-trode after FGA process, respectively. In comparison with the sample with a top electrode, the cross section of TEM of the sample without a top electrode shows no apparent
differ-FIG. 1. X-ray diffraction patterns for SBT thin films共a兲 without top elec-trode and共b兲 with Pt top electrode after FGA at 200–500 °C.
FIG. 2. Surface morphology of SBT thin films共a兲 before FGA and 共b兲 after FGA at 500 °C.
1878 J. Appl. Phys., Vol. 94, No. 3, 1 August 2003 Lanet al.
ences on TEM morphology the SBT grains also become smaller near the interface of SBT/Al2O. However, both
im-ages give a different contrast, especially around the grain boundary and near the interface of SBT/Al2O3, indicating that some H⫹ has diffused into the SBT structure for the sample with FGA treatment.
Figure 4 shows the measurement of SIMS for the SBT thin films annealed at 800 °C in an oxygen ambient and then treated with a FGA process at 400 °C for 15 min. While the sample was treated with FGA, hydrogen atoms highly exist in a SBT thin film as shown in Fig. 4共a兲. In addition, Sr is also detected on the film surface along with a great amount of Bi. It was believed that the hydrogen ion would cause the reduction of bismuth oxide, and accelerates the volatilization of Bi and the formation of Bi vacancy. The formation of bismuth vacancies may cause the accumulation of Sr because both Sr⫹2 and Bi⫹3 have a similar ion size and easily
ex-change during heat treatment. On the other hand, after the sample was deposited with a Pt top electrode and then treated under FGA, it was found that only Bi accumulated on the film surface as shown in Fig. 4共b兲. In other words, little Sr was detected on the film surface. Furthermore, the H in-tensity at the film surface became weaker compared to the sample without Pt top electrode coating. That indicates the importance of a Pt top electrode in preventing the H⫹ accu-mulation and Sr outdiffusion in comparison with that without Pt coating.
B. Capacitance–voltage and memory window characteristics
Some research in literature focused on the FGA degra-dation of a MFM structure with different top electrodes and found that H2 is easily catalyzed into H ions with a Pt
elec-trode and the electrical properties would deteriorate.8,19 Spe-cifically, while the H ions penetrate into the ferroelectric thin film, the electric properties will become worse. However, the FGA treatment for the MFIS field-effect transistors has been reported. In this work, however, it was found that when the Pt/SBT/Al2O3/Si共MFIS兲 was postannealed at various FGA temperatures, good ferroelectric characteristics were ob-tained. The Pt/SBT/Al2O3/Si shows the clockwise hyster-esis loops of C⫺V curves as observed in Fig. 5, indicating the switching of the ferroelectric polarization. In other words, the FGA temperature does not have a significant in-fluence on memory window properties. However, the FGA can cause the shift of C⫺V to a negative direction, indicat-ing that positive charge has been inserted into SBT thin film. Figure 5 also shows that a rapid decrease in the dielectric constant was detected for the sample after FGA treatment. FIG. 3. Cross-sectional TEM micrograph of SBT/Al2O3/Si structure
with-out Pt top electrode after FGA process.
FIG. 4. SIMS spectra of SBT/Al2O3/Si capacitors共a兲 without top electrode and共b兲 with Pt top electrode after FGA at 400 °C.
FIG. 5. C⫺V characteristics of SBT/Al2O3/Si capacitors共a兲 without top electrode and共b兲 with Pt top electrode after FGA at 200–500 °C.
For the sample without a Pt electrode, a remarkable degra-dation of capacitance for the SBT thin films is observed in Fig. 5共a兲 and the capacitance decreases from 65 pF to 32 pF at 200 °C. Above that temperature, no further degradation is detected with increasing forming gas temperature. The de-crease in the dielectric constant may be attributed to the dif-fusion of H⫹ ions into the SBT thin films and change the dipole motion during FGA process. That is because the H⫹ ions are probably bonded with a dangling bond in the SBT/Al2O3/Si interface. In addition, the H ion tends to form
a space charge around the grain boundaries, which will lead to the reduction of the dipole moment and the rapid drop of the dielectric constant.
On the other hand, Fig. 5共b兲 shows the C⫺V curve of the Pt/SBT/Al2O3/Si 共MFIS兲 structure with a Pt top
elec-trode under FGA treatment. No significant variation on the memory windows is discerned compared to that 关Fig. 5共a兲兴 of the sample without a Pt top electrode. However, the dec-rement of capacitance in Fig. 5共b兲 is strongly dependent on the annealing temperature of FGA, which is completely dif-ferent from that in Fig. 5共a兲 where the capacitance rapidly drops to the lowest value as the annealing temperature reached 200 °C. Apparently, the coated Pt electrode plays a passivation role in buffering the diffusion of H⫹and reduces the degradation of a SBT thin film caused by FGA. The effect of FGA temperature on the dielectric constant of both samples can be summarized in Fig. 6. It was found that with increasing annealing temperature, a minimum dielectric con-stant in the SBT films appears at around 300 °C, especially for the sample with a Pt top electrode.20 That is because while the sample was treated with FGA below the Curie temperature (TC), the accumulated H⫹ ions and electrons around the grain boundaries of the SBT film tend to induce an internal electric field and thus pin the domain motion. Therefore, a rapid decrease in the dielectric constant was obtained. On the other hand, as the FGA temperature was above TC, the high-temperature effect may cause the disper-sion of both hydrogen atoms and electrons more homo-genously. In addition, the induced local electrical field will become weak due to the disappearance of spontaneous po-larization. The two reasons are believed to be responsible for the increase of dielectric constant over 300 °C. However, by further raising the FGA temperature, the amount of hydrogen atoms diffusing into the SBT film keep increasing that will
lead to the reduction in the dielectric constant again. As com-pared with the result on the MFM structure in which no ferroelectric properties were detected for the SBT thin film after FGA,8 this work demonstrates that the SBT film on MFIS still exhibits the ferroelectric properties in spite of the reduced capacitance. It implies that both the distribution of hydrogen ions and electrons play different roles in the MFM and MFIS structures, which further reflect different physical characteristics and electrical properties in these two struc-tures.
C. Leakage current
Figure 7 shows the leakage current characteristics of SBT films on a MFIS structure at different FGA tempera-tures. As shown in Fig. 7共a兲, for the sample without a Pt top electrode, the leakage-current density of the SBT films de-creases. The leakage current density dropped directly from 1⫻10⫺6A/cm2 to 1⫻10⫺8A/cm2 at 2 V. On the other
hand, as the SBT films were coated with a Pt top electrode, Fig. 6共b兲 shows that the leakage current is gradually reduced with the increase of FGA temperature, completely different from that in the MFM structures.21,22The leakage current in a MFM structure is highly increased after the FGA and that was attributed to the disappearance of the Schottky barrier caused by the neutralization of the depletion layer due to the production of the space charge at the interface of SBT/ electrode. However, it was believed that the decreased leak-age current of the SBT film in the MFIS structure is strongly correlated with the interaction of indiffused H⫹ion with the interface defects of SBT/Al2O3/Si.
FIG. 6. Capacitance of SBT/Al2O3/Si with respect to the original value as a function of FGA temperatures.
FIG. 7. Current density–voltage characteristics of SBT/Al2O3/Si capacitors 共a兲 without top electrode and 共b兲 with Pt top electrode after FGA at
200–500 °C.
1880 J. Appl. Phys., Vol. 94, No. 3, 1 August 2003 Lanet al.
In order to clarify the effect of the SBT/Al2O3/Si
inter-face on the leakage current of a MFIS structure, Al2O3/Si
sample was prepared and annealed at the similar condition 共800 °C in O2). The leakage current of Al2O3/Si without
FGA was compared with the sample under FGA treatment and shown in Fig. 8. It was found that, after FGA at 500 °C, a decreased leakage current was observed for the Al2O3/Si
sample under FGA treatment. Although a systematic study of FGA dependence of the reduced leakage current in the Sr0.8Bi2Ta2O9/Al2O3/Si remains to be further conducted,
the result in Fig. 8 indicates that the reduced defects 共or dangling bonds兲 in the Al2O3/Si interface are probably
re-sponsible for the decreased leakage current of SBT-based MFIS structure after FGA.
IV. CONCLUSION
In summary, we have investigated the effect of FGA treatment on the ferroelectric properties of SBT ferroelectric film constructed on Al2O3/Si. After FGA treatment, X-ray
diffraction shows only perovskite SBT phase but TEM re-veals the probable diffusion of H⫹ ion into the SBT film. Due to the diffusion of hydrogen ions, a rapid decrease in the dielectric constant was observed but the memory window size of Sr0.8Bi2Ta2O9/Al2O3/Si capacitors was kept
un-changed. In addition, it was found that FGA shows a positive effect on the leakage current of MFIS in contrast to that of a MFM structure. A rapid decrease as much as two orders of magnitude was obtained for SBT-based MFIS capacitors
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