Characteristics of Amorphous Bi2Ti2O7 Thin Films Grown by Atomic ...

Journal of The Electrochemical Society, 153 共1兲 F20-F26 共2006兲

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0013-4651/2005/153共1兲/F20/7/$20.00 © The Electrochemical Society, Inc.

Characteristics of Amorphous Bi2Ti2O7 Thin Films Grown by Atomic Layer Deposition for Memory Capacitor Applications Gyu Weon Hwang,a Wan Don Kim,a Yo-Sep Min,a,b Young Jin Cho,a,b and Cheol Seong Hwanga,*,z a

School of Materials Science and Engineering and Interuniversity Semiconductor Research Center, Seoul National University, Seoul 151-744, Korea Nano-fabrication Center, Samsung Advanced Institute of Technology, Yongin 449-712, Korea

b

Bi2Ti2O7 films were deposited on sputtered Ru/SiO2 /Si substrates by atomic layer deposition using tris共1-methoxy-2-methyl-2propoxy兲bismuth 关Bi共mmp兲3兴, titanium tetraisopropoxide as Bi and Ti precursors, and H2O as oxidant, respectively. The wafer temperature was varied from 225 to 300°C. The growth rates decreased from 0.075 nm/cycle at 225°C to 0.055 nm/cycle at 300°C. The Bi concentration in the film decreased with increasing growth temperature and it could be controlled within a certain range by changing the Bi feeding at a given temperature. The as-grown Bi2Ti2O7 films were amorphous but contained metallic Bi at high growth temperatures and high Bi concentrations. Bi and Ti piled-up on the film surface and the Ru interface, respectively. The electrical leakage property of the Bi2Ti2O7 thin film became worse with increasing deposition temperature and Bi concentration due to the metallic Bi formation. The dielectric constant at the optimum deposition condition 共225°C兲 was ⬃40, and the leakage current level was ⬍10−7 A/cm2 at 1.0 MV/cm at a film thickness of ⬃15 nm. Reasonable conformal film thickness and cation composition over contact holes with a diameter of 0.15 ␮m and a depth of 1.1 ␮m were obtained. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.2133713兴 All rights reserved. Manuscript submitted June 8, 2005; revised manuscript received August 29, 2005. Available electronically December 2, 2005.

As the density of dynamic random access memory 共DRAM兲 quadruples every three years, the fabrication of storage capacitors having a cell capacitance high enough to satisfy the refresh requirement becomes difficult. It is expected that the required capacitance of a storage capacitor will be still ⬃25 fF even for DRAMs with a design rule of ⬃50 nm.1 In order to achieve this capacitance, a dielectric film with an equivalent oxide thickness 共toxeq.兲 of ⬃0.5 nm is necessary for a simple stack-type capacitor. In order to achieve a toxeq. of ⬃0.5 nm, noble metal top and bottom electrodes have to be used even for high-dielectric-constant thin films like 共Ba,Sr兲TiO3 共BST兲 to avoid interfacial low-dielectric-layer formation. The most probable metal electrode is Ru for the metal-insulatormetal 共MIM兲 capacitor fabrication because the chemical vapor deposition 共CVD兲 process is already matured for thin-film Ru deposition. Conformal deposition and easy etching for fine patterning of Ru are possible. In addition, the oxidation potential of Ru is quite low compared to most of the high-dielectric materials and, if formed, Ru-oxide is a conducting oxide which minimizes the adverse interface effects.2,3 Recently, increasing focus lay on the atomic layer deposition 共ALD兲 of Ru films due to a better conformity than obtained by CVD. There have been numerous reports on high-k dielectric films for DRAM capacitors.4-8 The most promising material for high-k thin films is BST because of its superior dielectric constant 共⬎300 at a thickness of ⬃30 nm兲 compared to other binary metal oxide films.9 However, the adoption of BST capacitors to DRAMs has been hindered by many issues.10 One of the most serious difficulties was the limited success of the metallorganic CVD of the complex BST thin films. The difficulty primarily originates from the low volatility and instability of group II element 共Ba, Sr兲 precursors due to the low charge-to-radius ratios of Ba and Sr ions,11 which make the precursor delivery in the metallorganic chemical vapor deposition 共MOCVD兲 process less reliable. In addition to the precursor issues, nonuniform cation stoichiometry over the contact hole structured capacitors was observed when the hole diameter was ⬍150 nm.12 Therefore, it is necessary to develop a new but simpler capacitor dielectric material that can be more easily deposited by either MOCVD or ALD. In addition, the process temperature, including deposition and postannealing, if necessary, should be low enough 共⬍500°C兲 not to degrade the Ru bottom electrode. An amorphous

* Electrochemical Society Active Member. z

E-mail: [email protected]

high-dielectric film is preferred over the crystalline counterpart considering the more isotropic and uniform nature of amorphous material. In this regard, the authors developed a new amorphous highdielectric Bi2共Ti,Si兲2O7 thin film using a liquid-delivery ALD process.13,14 The optimized material showed quite promising properties as DRAM capacitor dielectric 共dielectric constant ⬃60, and low enough leakage current density at a film thickness ca.⬍13 nm兲. However, if the Si concentration in the film was too high 共⬎20%兲 the dielectric constant degraded largely without improving the leakage property that much. Therefore, in this study the characteristics of a simpler material, Bi2Ti2O7, are studied for a better understanding of this new dielectric material system. Bi2Ti2O7 appears to be a good candidate as high-k material because in crystallized state its dielectric constant is as high as 110–160.15-18 Furthermore, Bi has various available precursors with a high enough vapor pressure for CVD or ALD processes, such as alkyl, alkoxide, and halide.19 The ferroelectric Bi4Ti3O12 that has a perovskite structure is a well-known system in Bi–Ti–O. Bi2Ti2O7 has a pyrochlore structure with no ferroelectricity. Speranskaya et al.20 reported the existence of two additional phases, Bi8TiO14 and Bi2Ti4O11, with pyrochlore-type structure. However, pyrochloretype bismuth titanates are usually bismuth-deficient so that it is difficult to form stoichiometric bismuth titanate films.16 Schuisky et al. recently reported Bi2Ti2O7 共BTO兲 film growth by ALD using Bi共C6H5兲3 and Ti关OCH共CH3兲2兴4 共TTIP兲 as Bi- and Tiprecursor, respectively, and H2O as oxygen source.19 They reported that Bi2O3 could not be deposited using Bi共C6H5兲3 even with stronger H2O2 oxidant. However, the ternary oxide could be grown due to the catalytic effect of TiO2 on the Bi-oxide incorporation. The growth rate was about 0.02 nm/metal pulse. The film cation composition was controlled by using different Bi/Ti precursor pulse ratios, and at high ratios metallic Bi was included in the film. At a given Bi/Ti pulse ratio, metallic Bi was also included at higher deposition temperatures. ALD is characterized as having a unique self-limiting deposition mechanism which can produce a good step coverage over a contact hole even with an aspect ratio of 1:40.21 The problem related with the relatively slow growth rate of an ALD process may not seriously limit its application to the DRAM capacitor process considering the small required thickness 共⬃10 nm兲. It is believed that the ALD process should be the process of choice for the deposition of DRAM capacitor dielectrics considering the extreme three-dimensional geometry of the capacitor with a design rule of 50 nm. The expected


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aspect ratio of the capacitor stack is ⬎1:15, and it is believed that MOCVD can hardly offer sufficient conformity of the dielectric film for this kind of geometry. Therefore, in this study amorphous Bi2Ti2O7 films were grown by ALD using a different Bi precursor which was expected to facilitate Bi incorporation into the film. Experimental Amorphous Bi2Ti2O7 films were grown on a bare Si共100兲 substrate and a sputtered Ru 共50 nm兲/CVD SiO2 /Si substrate at wafer temperatures ranging from 225 to 300°C using a 4-in. wafer scale, traveling wave-type ALD reactor 共Ever-tek Co., Plus-100兲. The native oxide layer on the Si wafer was not removed prior to the film growth. The wafer temperature was calibrated using a thermocouple wafer, and the estimated temperature difference between the bare Si and Ru-sputtered wafer was negligible in this temperature region. Bi关OC共CH3兲2CH2OCH3兴3 关Bi共mmp兲3兴 and TTIP were used as Bi and Ti precursor, respectively. The metal precursors were evaporated at 130 and 65°C, respectively, and no carrier gas was used to deliver the metallorganic precursors. Water was used as the oxygen source and was evaporated at 14°C. No carrier gas was used for the water delivery either. Bi共mmp兲3 already has Bi–O bondings in the precursor molecules, so it was expected that a Bi-oxide film growth with this precursor might be facilitated. Bi共mmp兲3 has a relatively high vapor pressure and a similar thermal decomposition temperature to TTIP. The other commonly used Ti precursor, TiCl4, may result in Cl contamination in the grown films. In addition, the reaction byproducts from the ALD using H2O as oxidant, mainly HCl, corrode the hardware. TTIP has a high enough vapor pressure and a relatively high thermal decomposition temperature 共⬃280°C兲 compared to other Ti metal alkoxides, such as methoxide, ethoxide, and butoxide. The ternary oxide film was deposited by successive deposition of binary oxide layers. This was achieved by exposing the substrate surface to water vapor after each metal precursor pulse, thus forming a partial metal oxide layer. One cycle of binary oxide deposition consisted of the metal precursor pulse, the first Ar purge pulse, the water pulse, and the second Ar purge pulse. The lengths of the Bi共mmp兲3 and TTIP pulses were 2 and 1 s, respectively, and the H2O pulse was 0.3 s. The lengths of the Ar purging pulses were 30, 5, and 15 s after the Bi and Ti precursor pulses and H2O pulses, respectively. The film thickness was measured by a single-wavelength ellipsometer which was calibrated by spectroscopic ellipsometry and cross-sectional high-resolution transmission electron microscopy 共HRTEM兲. Because the Si substrates are covered by native oxide, the thickness of the layer of each substrate was measured just before the film growth, and this value was extracted from the total thickness. The cation ratio in the deposited ternary oxide film was defined using X-ray fluorescence spectroscopy 共XRF兲 which was calibrated by inductively coupled plasma mass spectroscopy 共ICP–MS兲. The chemical composition in depth direction, the crystalline structure of the film, and the chemical bonding state of atoms was investigated by Auger electron spectroscopy 共AES兲, X-ray diffraction 共XRD兲, and X-ray photoelectron spectroscopy 共XPS兲. The thickness and composition step coverage on a contact hole 共0.15-␮m diameter兲 of which the aspect ratio was 8 were confirmed by cross-sectional transmission electron microscopy 共TEM兲 and energy dispersion spectroscopy 共EDS兲. For the electrical measurements, a Pt 共100 nm兲/Bi2Ti2O7 / Ru 共50 nm兲 planar capacitor structure was used. The top Pt electrode was deposited by electron-beam evaporation using a shadow mask 共circular electrode diameter of 250 ␮m兲. The top electrodes were postannealed at 400°C under air for 30 min. The dielectric and leakage current properties were measured using a Hewlett-Packard 4194 impedance analyzer at 10 kHz and a 4140B picoammeter, respectively.

Figure 1. Variation in the growth rate and nonuniformity of BTO films on Si 共a兲 with Bi pulse time, while Tg, the TTIP pulse time, and the H2O pulse time were fixed to 225°C, 0.5 s, and 0.3 s; 共b兲 with Ti pulse time while Tg, the Bi共mmp兲3 pulse time, and the H2O pulse time were fixed to 225°C, 2 s, and 0.3 s, respectively.

Results and Discussion Figure 1a shows the variation of the growth rate and thickness nonuniformity, which is defined as 共maximum thickness − minimum thickness兲/共2 ⫻ average thickness兲 where the film thickness was measured from top, center, bottom, left, and right positions of the 4-in. wafer, of a BTO film as a function of the Bi共mmp兲3 pulse time on a Si wafer, when Tg and the TTIP and H2O pulse periods were 225°C, 0.5 s, and 0.3 s, respectively. The film growth rate saturated after 1 s of Bi共mmp兲3 pulse time, suggesting that the film growth was self-limited by ALD. However, the Bi precursor pulse time should be at least 2 s for the confirmed BTO film composition control, as shown in Fig. 2. Figure 1b shows the variation of the growth rate as a function of the TTIP pulse time on a Si wafer when Tg and the Bi共mmp兲3 and H2O pulse periods were 225°C, 2 s, and 0.3 s, respectively. The film growth rate saturated after 1 s of TTIP pulse time so that the TTIP pulse time was set to 1 s. Figure 2 shows the variation in the film cation composition ratio as a function of the TTIP feeding time for two different Bi共mmp兲3 feeding times 共1 and 2 s兲. For the case of a Bi共mmp兲3 feeding time of 1 s, the Bi concentration in the films decreases with increasing TTIP pulse time. This suggests that the surface reaction sites of the growing film are not saturated by Bi precursor after the Bi precursor pulse for 1 s. In this case, a larger number of reaction sites would be occupied by TTIP during the subsequent TTIP pulse step with a longer TTIP pulse time. However, in the case of a Bi共mmp兲3 feeding time of 2 s, the film cation composition was independent of the TTIP pulse time. This suggests that surface reaction sites were fully occupied with the Bi precursor molecules during the Bi pulse cycle and stable cation composition control can be achieved.

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Figure 2. Variation of the Bi concentration in the BTO films with Bi pulse time and Ti pulse time, while the H2O pulse time was fixed to 0.3 s.

In general, a proper Tg region should be determined for a successful ALD using MO precursors. A too-high Tg results in a thermal decomposition of the MO precursor so that a cyclic CVD process is obtained. Meanwhile, a too-low Tg produces films with a high impurity concentration, such as residual carbon and water, a low growth rate in some cases, and a high defect density. Figure 3 shows the variation in growth rate of the BTO films 共physical thickness increases per cycle兲 with the variation in Tg, while the Bi共mmp兲3, TTIP, and H2O pulse times were 2, 1, and 0.3 s, respectively. Here, the growth rates were decided from the slopes of the graphs that reveal the film thickness variation with the number of the process cycle at each Tg. Therefore, the data shown in Fig. 1 are free from any potential influence from the presence of interfacial layers 共for a Si substrate兲 or incubation periods 共for a Ru substrate兲. It can be observed that the growth rate of the BTO film decreases from 0.075 at a Tg of 225°C to 0.055 nm/cycle at a Tg of 300°C. This is due to the increasing volatility of Bi atoms with increasing Tg, which is confirmed by Fig. 4. Figure 4 shows the variation in the Bi/共Bi + Ti兲 composition ratio of the BTO film as a function of Tg

Figure 3. Growth rate of BTO films dependent on Tg on Ru and Si.

Figure 4. Variation of the cation ratio, Bi/共Bi + Ti兲, in the BTO films as a function of Tg.

which was measured by XRF. Under the given process conditions of precursor and oxidant inputs, the Bi composition monotonically decreased with increasing Tg due to the increased Bi desorption from the growing surface. However, it was found that the Bi concentration hardly changed after postdeposition annealing at 600°C 共data not shown兲. Figure 5a and b shows the AES depth profile results of the BTO films grown on Ru substrates at a Tg of 225 and 275°C, respectively. These films were deposited for 100 cycles, which corresponds to a thickness of 15 nm. The average carbon impurity concentrations in the film were 1.0 and 0.7% at a Tg of 225 and 275°C, respectively. It can also be confirmed by AES analysis that the Bi composition in the films decreases with increasing Tg. The cation composition along the film depth direction was not uniform; Bi atoms piled-up on the film surface resulting in a Ti atom pileup near the film/substrate interface. Similarly, a Bi pileup on the surface of Bi-containing titanates deposited by MOCVD has been reported by Lee et al.22 This might be related to the less active oxidation and much higher volatility of the Bi atoms compared to Ti atoms. In order to achieve a uniform cation composition along the film depth direction, a two-step deposition process was performed where the first half of the film growth was performed with a pulse sequence of Bi共mmp兲3–H2O–TTIP–H2O–Bi共mmp兲3–H2O and the second half of the film growth was performed with a pulse sequence of TTIP–H2O–Bi共mmp兲3–H2O–TTIP–H2O. In this case, the composition distribution along the depth direction became more uniform at a Tg of 225°C, as shown in Fig. 5c. However, this change did not improve the other characteristics, such as crystallinity, dielectric constant, and leakage current 共data not shown兲. Therefore, the twostep process was not used in this experiment anymore. Figure 6a shows the XRD patterns of the BTO films grown on Ru at 225, 250, 275, and 300°C, respectively. There are no peaks in the XRD patterns corresponding to a crystalline BTO phase. The common strong peaks are due to the Ru substrate. The 共210兲 peak of Bi metal evolves as Tg increases above 275°C, which coincides with an earlier report.19 HRTEM of the film grown at 250°C also confirmed that the film has an amorphous structure 共Fig. 10兲. The reason for the presence of metallic Bi, i.e., Bi0 and not Bi3+, in the films deposited at a higher Tg is attributed to the redox potential difference of Bi3+ /Bi0 and Ti4+ /Ti0. The redox potential of Bi3+ /Bi is +0.20 V and that of Ti4+ /Ti is −0.91 V at standard state.23 This means that Bi3+ has a tendency to reduce to Bi0, whereas the higher oxidation state of Ti4+ is more stable than the metallic Ti0 at standard conditions. Ti is a transition metal which belongs to group 4A in the periodic table. Therefore, Ti ions may exist in the BTO film with


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Figure 5. AES depth profile results of BTO films grown on Ru substrates at a Tg of 共a兲 225 共C concentration ⬃1.0%兲, 共b兲 275 共C concentration ⬃0.7 atom%兲, and 共c兲 two-step growth at 225°C.

Figure 6. XRD patterns of BTO films 共a兲 grown at a Tg of 225, 250, 275, and 300°C, respectively, on Ru substrates and 共b兲 grown at a constant Tg of 250°C on Ru substrates with different Bi ratios of 59, 46, and 43%, respectively.

Figure 7. XPS results of the 共a兲 Bi 4f peak and 共b兲 Ti 2p peak variation with Tg in BTO films grown on Ru and 共b兲 shows deconvoluted Ti 2p peak in BTO films 共upper: 300°C, lower: 225°C兲. In 共a兲 the metallic Bi peak area increases with Tg共 225°C :5.1%, 250°C:5.5%, 275°C:6.3%, 300°C:6.9%兲.

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Figure 8. 共a兲 Variation in k 共closed symbol兲 and dissipation factor 共open symbol兲 of the films as a function of Tg. The film thicknesses were 14–17 nm, and 共b兲 leakage current vs electric field 共J-E兲 plots of the 14–17-nm-thick films grown at a Tg of 225, 250, 275, and 300°C, respectively. The films were postannealed at 400°C.

many oxidation states, such as Ti2+ and Ti3+ in addition to Ti4+. However, all of the Ti ions with lower oxidation states have a negative redox potential for Ti4+.23 Considering the fact that the tendency of most metals to oxidize increases with increasing temperature, it can be expected that all the redox potentials of Bi3+ /Bi, Ti4+ /Ti3+, Ti4+ /Ti2+, and Ti3+ /Ti2+ decrease with increasing temperature. The rate of decreasing redox potential is determined by the number of electrons which are involved in the redox reaction, the activities of the chemical species, and some other factors. In this case, the rate of decrease in the redox potential of Bi3+ /Bi is smaller than that of Ti4+ /Ti3+, Ti4+ /Ti2+, and Ti3+ /Ti2+, so that more active oxidation of Ti2+ and Ti3+, if any, to Ti4+ occurs for high Tg that induced the reduction of Bi3+ to Bi0. This is further discussed in Fig. 7. The overall Bi concentration of the film decreases with increasing Tg. Therefore, the formation of metallic Bi in the films grown at high Tg is not due to the excessive presence of Bi in the films. Figure 6b shows the XRD patterns of the BTO films grown on Ru at 250°C with various Bi concentrations. Here, the cation composition was changed by changing the ratio of the constituent metal pulses. Although all of these samples were deposited at the same Tg, the metallic Bi peak also appeared in the film with a high Bi concentration. Thus, it can be understood that the oxidation of incorporated Bi at a low Tg and lower Bi concentration 共Bi/共Bi + Ti兲 ⬍ ⬃50%兲 is due to the catalytic activity of a Ti–O layer and is not due to its own oxidation potential as described above. When the Bi chemisorption increases on the surface and some of the Bi atoms do not meet the underlying Ti–O layer, some of the Bi atoms remain nonoxidized and form the metallic Bi as shown in Fig. 6. This behavior becomes more evident with in-

creasing Tg because the relative oxidation potential of Ti atoms is even higher than that of Bi atoms at a higher Tg. This can be confirmed by XPS analysis as shown below. Figure 7 shows the high-resolution XPS spectra of the 共a兲 Bi 4f and 共b兲 Ti 2p core levels for as-deposited BTO films at various temperatures. The measurements were referenced to the C 1s signal which was observed at 284.5 eV. The 5/2 and 7/2 spin-orbit doublet components of the Bi 4f core-level photoemission locate at 164.5 and 159.2 eV, respectively, and their position hardly changes with Tg 共Fig. 7a兲. The relative intensity ratio is 3:4, and the energy spacing between the peaks is 5.3 eV. Bi 4f7/2 peak position of crystalline Bi2Ti2O7 was reported to be 159.4 eV,24 which is slightly different from this result, maybe due to the structural difference 共crystalline vs amorphous兲. 159.2 eV is also different from the reported Bi 4f7/2 peak positions of Bi2O3 共159.0 eV24兲 and Bi4Ti3O12 共158.1 eV25兲. It can be further noted from Fig. 7a that tiny peaks formed at binding energies of 162 and 156.5 eV, which correspond to the 5/2 and 7/2 spin-orbit doublet components of the metallic Bi 4f core level as Tg increases. Although intensities of these peaks are quite weak, the presence of metallic Bi at high Tg was confirmed by the XRD results shown in Fig. 6a. Therefore, it is believed that these XPS signals should not be regarded as noise. The Ti 2p1/2 core-level photoemission peak partially overlaps the Bi 4d3/2 core-level peak 共Fig 7b兲. The Ti 2p3/2 locates at 458.2 eV, which also hardly varies with Tg. This value is a little different from that of TiO2, which ranges from 458.5 to 459.2 eV.25 Therefore, the binding energies of the Bi 4f and Ti 2p core electrons are different from those of TiO2 and Bi2O3, suggesting that the grown BTO films are not simply the physical mixture of simple binary oxides, Bi2O3

Figure 9. 共a兲 Variation in k 共closed symbol兲 and dissipation factor 共open symbol兲 of the films as a function of the Bi concentration grown at a constant Tg of 250°C. The film thicknesses were 14–17 nm. 共b兲 Leakage current vs electric field 共J-E兲 plots of the 14–17-nm-thick films which have a Bi concentration of 43, 46, and 59%, respectively. The films were postannealed at 400°C.


Figure 10. Cross-sectional HRTEM image of BTO films grown at 250°C on a planar Si substrate.

and TiO2, but a new chemical compound was formed. In order to further understand the change in the oxidation states of the constituent metallic elements, the Ti 2p XPS peak was deconvoluted under the assumption that the peaks are comprised of Ti2+, Ti3+, and Ti4+, with Bi 4d peaks. The Ti 2p XPS signal was mainly composed of Ti4+ with some trace amount of Ti2+ and Ti3+ peaks. Because of the weak intensities of the Ti2+ and Ti3+ peaks, the quantification of the relative concentration of the various Ti peaks was not possible. However, it is certain that a certain difference, to be regarded as a noise, exists between the spectra of the samples grown at the Tg of 225 and 300°C. The Ti2+ and Ti3+ peak intensities decrease with increasing Tg by the sufficient oxidation of the Ti ions. As discussed

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above, this reduces the Bi3+ ions to the Bi0 metallic component. The formation of metallic Bi has a fatal influence on the leakage current of the films. The electrical properties of 14–17-nm-thick BTO films deposited on Ru electrode at various Tg were measured using Pt top electrodes. Figure 8a shows the variations in the dielectric constant and dissipation factor as a function of the applied voltage of BTO films grown at 225, 250, and 275°C, respectively. The dielectric constant of the film grown at 300°C could not be measured because of a too-high leakage current as shown in Fig. 8b. The dielectric constant of the films grown at 225 and 250°C, of which the Bi concentration is 47 and 46%, is approximately 42 and does not show any applied voltage dependence, suggesting that the BTO films have paraelectric properties. The film grown at 275°C shows a little higher dielectric constant of ⬃45, although its Bi concentration is ⬃37%. Considering the results shown in Fig. 9, this slight increase in the dielectric constant must originate from the larger leakage current of this sample as shown in Fig. 8b; capacitance–voltage measurement artifact. Figure 8b shows the leakage current density vs applied electric field 共J–E兲 curves of the films grown at different temperatures. The BTO films deposited at 225 and 250°C showed low enough J values 共⬍10−7 A/cm2 at ±1 MV/cm兲 for a DRAM capacitor dielectric. However, the dissipation factors of these BTO films are ⬃2% 共Fig. 8a兲, which is a little larger for DRAM capacitor dielectric. The films grown at higher Tg 共275 and 300°C兲 show too-high J due to the presence of metallic Bi, as shown by XRD and XPS. In order to understand the variation in the dielectric properties with changing cation composition, the Bi concentration of the BTO film was varied at a given Tg of 250°C by changing the constituent metal cycle ratio. Figure 9a shows that the dielectric constant and dissipation factor of the BTO film with a higher Bi concentration 共46%兲 has a slightly higher dielectric constant 共k = 42兲 than the film with a lower Bi content 共k = 39 Bi concentration 43%兲. The dielectric constant of the film with a much higher Bi content 共59%兲 cannot be measured by a too-large J 共Fig. 9b兲. This is due to the formation of metallic Bi in this film. This corresponds to the report that pyrochlore-type Bi2Ti2O7 has a tendency to Bi deficiency,16 so that excess Bi exists in metallic form. The BTO films with a Bi

Figure 11. 共a兲 Cross-sectional TEM image of BTO films grown at 250°C on a trench–Ru/SiO2 substrate and 共b兲 Bi/共Bi + Ti兲 ratio at the various points shown in 共a兲.

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concentration of 43 and 46% have a low enough J for DRAM capacitor dielectrics as shown in Fig. 9b. In addition, the dissipation factor of the film with a lower Bi content 共Bi concentration 43%兲 is ⬃1%, which is low enough for DRAM application. The higher dielectric constant of the BTO film with a higher Bi content is a reasonable result considering the higher polarizability of Bi ions compared to Ti ions. Figure 10 shows an HRTEM image of a BTO film grown on a Si substrate at a Tg of 250°C of which the Bi content is 46%. It can be observed that the film has an amorphous microstructure with an interfacial oxide layer. The interface layer might be composed of native oxide, which was not removed before film deposition, and a small fraction of silicate that was formed during the film growth. Figure 11a shows a cross-sectional TEM picture of the BTO film deposited at 250°C on a contact hole with a hole diameter of 0.15 ␮m and a depth of 1.1 ␮m 共aspect ratio ⬃7兲 over which the Ru metal electrode was deposited by CVD before BTO ALD. The inset figures show enlarged images of the BTO films grown on the Ru electrode on the top, middle, and bottom parts of the hole, respectively. It can be observed that the BTO film maintains its amorphous structure irrespective of the locations along the hole surface. The estimated thickness conformity was ⬃80%. The BTO film in the middle region shows a dark contrasted portion which may be related to the presence of metallic Bi, as can be assumed by the local cation concentration measurement by TEM–EDS, which is shown in Fig. 11b. The ICP–MS analysis of this film showed that the average Bi concentration is 46.1%. The local Bi content variation along the hole depth direction is shown in Fig. 11b. The estimated error of the present TEM–EDS is ⬃3 atom % due to the statistical error and local geometry of the sample. It can be observed that the Bi content in the middle region reaches a maximum value 共⬃53%兲 which is a high enough value to segregate metallic Bi in the film. The BTO film top and bottom part of the hole has a lower Bi content. The cation concentration uniformity over the hole surface is ⬃78%. This nonperfect cation composition step coverage might originate from the different sticking probability of Bi and Ti precursor molecules, or from insufficient precursor supply for the larger surface area of the wafer having numerous holes, although the precursor supply was sufficient for the planar surface.21 Further research and improvements are necessary regarding the cation conformity, as has been done for BST MOCVD12 and SrTiO3 ALD.26 Conclusion An alternative high-dielectric material, thin amorphous Bi2Ti2O7,and its ALD process were studied for DRAM capacitor dielectric application using Bi共mmp兲3 and TTIP as Bi and Ti precursor, respectively, and H2O as oxidant. The catalytic activity of the growing Ti–O surface facilitates the Bi-oxide incorporation into the film with the help of the already-present Bi–O bond in the Bi共mmp兲3 precursor. The BTO films have to be grown at a wafer temperature between 225 and 250°C with a well-controlled Bi concentration 共43–46 atom %兲 in order to ensure that no segregation of metallic Bi in the film takes place, which is fatal for the dielectric property of the film, and to ensure a high dielectric constant with a low enough leakage current. The acquired dielectric constant was ⬃40 and J ⬍ 10−7 A/cm2 at 1 MV/cm at a film thickness of

⬃15 nm. The ALD–BTO film grown at 250°C showed a reasonable thickness 共⬃80%兲 and composition 共78%兲 step coverage over the contact hole structure with a Ru bottom electrode. In this regard, further improvement is necessary for the adoption of this material into DRAM capacitors with a design rule ⬍70 nm. Acknowledgment The work was supported by the System IC 2010 program and the Korean Ministry of Science and Technology through the National Research Laboratories program of the Korean government. Seoul National University assisted in meeting the publication costs of this article.

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