Mat. Res. Soc. Symp. Proc. Vol. 784 2004 Materials Research Society C7.7.1 Fabrication of Ru/Bi 4-x La x Ti 3 O 12 /Ru Ferroelectric Capacitor Structure Using a Ru Film Deposited by Metalorganic Chemical Vapor Deposition Taisuke Furukawa 1, Takeharu Kuroiwa 1, Yoshihisa Fujisaki 1,2, Takehiko Sato 1 and Hiroshi Ishiwara 2 1 R&D Association for Future Electron Devices, 4259 Nagatsuta-cho Midori-ku, Yokohama 226-8503, JAPAN 2 Frontier Collaborative Research Center, Tokyo Institute of Technology, 4259 Nagatsuta-cho Midori-ku, Yokohama 226-8503, JAPAN ABSTRACT A Ru/ Bi 4-x La x Ti 3 O 12 /Ru (Ru/BLT/Ru) capacitor structure with Ru top electrodes deposited by metalorganic chemical vapor deposition (MOCVD) was fabricated. On a Ru film deposited by MOCVD, BLT film was formed by a sol-gel method and crystallized in vacuum. Depositing a conformal Ru film on a BLT/Ru structure by MOCVD, Ru/BLT/Ru stack with smooth and flat surface was successfully formed. Then, ferroelectric Ru/BLT/Ru capacitors were fabricated through a dry etching process. It exhibited both good ferroelectric properties (2P r =16 µc/cm 2 ) and low leakage current density (J=10-7 A/cm 2 ), suggesting that Ru film deposited by MOCVD showed sufficient properties for the tope electrode of Ru/BLT/Ru structures. INTRODUCTION Ruthenium has been paid much attention as a promising candidate for the electrode material of semiconductor devices. There have been many researches on the Ru film deposited by RF sputtering or metalorganic chemical vapor deposition (MOCVD). For the Ru film deposited by MOCVD, some advantages are expected such as good step coverage, low damage to the underlying structures and so on, which might be important properties for the fabrication of future ferroelectric random access memories (FeRAM). Therefore, we believe that realization of metal-ferroelectrics-metal capacitors using MOCVD-deposited Ru films will give important contribution to the future FeRAM. We have already investigated Pt/Bi 4-x La x Ti 3 O 12 /Ru (Pt/BLT/Ru) ferroelectric capacitor structures using Ru films deposited by MOCVD methods, and revealed that they show good ferroelectric properties. In this paper, we deposit a Ru film on a BLT/Ru structure to form Ru/BLT/Ru structure, and fabricate Ru/BLT/Ru capacitors through dry etching. It is shown that their hysteresis properties (2P r =16 µc/cm 2 ) are good and leakage current density (J=10-7 A/cm 2 ) is low.
C7.7.2 EXPERIMENTAL DETAILS A ferroelectric Ru/BLT/Ru capacitor structure was fabricated on a Si wafer covered with a 100 nm-thick SiO 2 film. Figure 1 shows the fabrication steps of the Ru/BLT/Ru capacitor structure. It is well known that Ru is easily oxidized if it is heated in O 2 ambient, and that oxidation of Ru leads to increase of the volume and degradation of the capacitor properties. Therefore, the process conditions were so tuned that oxidation of the Ru film became less significant. All Ru films used in this experiment were deposited by an MOCVD apparatus, JETSTAR 200ST (JIPELEC). The wafer was heated with infrared lamps and the temperature was measured and controlled with a pyrometer. A liquid-type source, Ru[C 2 H 5 C 5 H 4 ] 2 (Ru[EtCp] 2, Mitsubishi Materials Co.) was introduced into the vaporizer through a pulse-type injector, in SiO 2 (100nm)/Si Substrate Ru (60nm) Deposition by MOCVD Post Annealing 400 o C 30 min in Vacuum Spin Coating of BLT Sol-gel Solution Bi: La: Ti=3.35:0.75:3.00 Drying 240 o C 10 min in Air x 2 Baking 400 o C 5min in O 2 Crystallization Annealing 650 o C 30 min in Vacuum Ru (60nm) Deposition by MOCVD Patterning by Dry Etching Process Annealing 400 o C 30min in N 2 Figure 1. Fabrication steps and conditions of Ru/BLT/Ru capacitor structure.
C7.7.3 which the source flow was controlled by the opening time and frequency of the injector. Then, the source was vaporized and introduced into the growth chamber with N 2. Details of the system have been described elsewhere [1]. The Ru film for the bottom electrode was deposited at a temperature of 400 C without mixing such oxidant as O 2 to enhance the decomposition of Ru source. Under this condition, a Ru film oriented to the c-axis was obtained and the grains in the film exhibited columnar structures. Therefore, a Ru film with smooth and flat surface morphology was obtained [1], which is desirable for the Ru/BLT/Ru structure fabrication. After the deposition, the Ru film was annealed in vacuum at a temperature of 400 C for 30 min to decrease contaminants in the film. Some kinds of contaminants such as carbon and hydrogen are contained in the CVD-deposited films, and they might react with oxygen in the BLT film and make oxygen vacancies. Actually, post annealing of the Ru film in vacuum was effective for the suppression of leakage current density through the BLT films [2]. Next, a BLT film was formed on the CVD-deposited Ru film by spin-coating a sol-gel solution. The composition of the solution (Mitsubishi Materials Co.) was Bi: La: Ti = 3.35: 0.75: 3.00. The sample was dried at a temperature of 240 C, and baked in O 2 ambient at 400 C. These processes were repeated twice to obtain a 150-nm-thick BLT film. Then, annealing for crystallization was performed in vacuum to suppress the Ru oxidation and to keep the process margin wide [3]. The annealing temperature was 650 C. After that, a Ru film for the top electrode was deposited on the BLT/Ru structure under the same deposition condition. Then, the Ru film was patterned through a dry etching process. Finally annealing at 400 C for 30 min in N 2 ambient was performed The crystallographic orientation of the film was examined by x-ray diffraction (XRD) analysis. Surface morphology was measured by scanning electron microscopy (SEM). Polarization hysteresis loops were obtained with a ferroelectric test system (FCE-1), and leakage current density was measured with a semiconductor parameter analyzer (HP-4156C). RESULTS AND DISCUSSIONS Figure 2 shows an XRD pattern of the Ru/BLT/Ru structure with the BLT film crystallized in vacuum. As can be seen from the figure, strong BLT-related peaks were observed, which indicates that the BLT film formed on Ru was well crystallized with preferred orientation to c-axis. Figures 3(a) and 3(b) show SEM images of a Ru film deposited on the BLT/Ru structure. As can be seen from the figure, a Ru film with smooth and flat morphology was successfully deposited. Additionally, the interface between Ru for bottom electrode and BLT film was so
Ru(002) C7.7.4 BLT(0014) BLT(117) BLT(0010) BLT(008) Ru(101) 10 Figure 2. BLT(006) Intensity [a.u.] BLT(004) Si 20 30 2θ [degree] 40 An XRD pattern of a Ru/BLT/Ru structure. clear, indicating that no significant degradation was observed in Ru film during the crystallization of BLT film. To obtain the insulating and ferroelectric properties, the Ru/BLT/Ru structure was patterned through a dry etching process and Ru/BLT/Ru capacitors with the top electrode area of 2500 µm2 were fabricated. Figure 4(a) shows typical polarization hysteresis loops of a Ru/BLT/Ru capacitor. Remanent polarization of 2Pr=16 µc/cm2 (at 5V) was successfully obtained. Figure 4(b) shows the leakage current density of the same capacitor. As can be seen from the (a) (b) Ru BLT Ru 600 nm 300 nm Figure 3. SEM image of Ru film deposited on BLT/Ru structure: (a) surface image, cross sectional image. (b)
C7.7.5 20 10-2 Polarization [µc/cm 2 ] 10 0-10 Leakage Current Density [A/cm 2 ] 10-3 10-4 10-5 10-6 10-7 -20-4 -2 0 2 4 Applied Voltage [V] 10-8 -4-2 0 2 4 Applied Voltage [V] Figure 4. (a) Polarization hysteresis loops of a Ru/BLT/Ru capacitor. The area of the capacitor was 2500 µm 2. (b) Leakage current density of the same Ru/BLT/Ru capacitor as that in Fig. 4(a). figure, leakage current density as low as 10-7 A/cm 2 was achieved (at ±2V), suggesting that Ru film deposited by MOCVD showed sufficient properties for the tope electrode of Ru/BLT/Ru structures. In this sample, the annealing temperature for the damage recovery was kept below 400 C, in order to suppress the oxidation of Ru. Because of the low annealing temperature, some of the damages induced during the capacitor fabrication process seem to remain. Therefore, the insulating and ferroelectric properties will be much improved by suppressing the damages induced during the process. CONCLUSIONS Ru/BLT/Ru capacitors were fabricated using Ru films deposited by MOCVD method. A Ru/BLT/Ru structure with smooth and flat morphology was successfully obtained by depositing a Ru film on the BLT/Ru structure by MOCVD, and Ru/BLT/Ru capacitors were successfully fabricated through a dry etching process. They exhibited both good ferroelectric properties (2P r =16 µc/cm 2 ) and low leakage current density (J=10-7 A/cm 2 ). ACKNOWLEDGMENTS This work was performed under the auspices of the R&D Projects in Cooperation with Academic Institutions (Next-Generation Ferroelectric Memories), supported by NEDO (New
C7.7.6 Energy and Industrial Technology Development Organization in Japan) and managed by FED (R&D Association for Future Electron Devices). REFERENCES 1. T. Furukawa, T. Kuroiwa, Y. Fujisaki, T. Sato, and H. Ishiwara, Integrated Ferroelectrics 59 1437 (2003). 2. T. Furukawa, T. Kuroiwa, Y. Fujisaki, T. Sato, and H. Ishiwara, Integrated Ferroelectrics to be published (2003). 3. T. Sato, T. Kuroiwa, and H. Ishiwara, Extended Abstract of 1st International Meeting on Ferroelectric Random Access Memories 168 (2001).