J. Phys. Chem. B 2002, 106, 4963-4966
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Deposition of Barium Titanate Films on Silicon by Barium Fluotitanate Powder M. K. Lee,* K. W. Tung, C. C. Cheng, H. C. Liao, and C. M. Shih Department of Electrical Engineering, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan 80824, Republic of China ReceiVed: September 7, 2001; In Final Form: February 21, 2002
In this study, the barium fluotitanate powder prepared by the precipitate of hexafluorotitanic acid and barium nitrate solution was used as the precursor for the deposition of barium titanate film on silicon substrate. The boric acid was incorporated into the deposition solution to enhance the deposition rate. A mirrorlike film can be obtained. The chemical reaction of liquid-phase deposited barium titanate is proposed. The leakage current density is as low as 5 × 10-9 A/cm2. The dielectric constant and the refractive index of the deposited film are 60 and 1.96, respectively.
I. Introduction In recent years, there have been increasing demands for high dielectric materials to replace silicon dioxide (SiO2) and for lowtemperature fabrication processes to meet the requirements of dimension decrease in high-density dynamic random access memories (DRAMs).1 There are many high dielectric constant materials such as TiO2, Si3N4, Ta2O5, ZrO2, etc. Barium titanate (BaTiO3, BTO) is a very promising candidate with high dielectric constant, high refractive index, and high chemical stability for applications in various devices.2 BTO thin films investigated so far show either amorphous or polycrystalline structures, depending on the deposition temperature or postheating treatments.3 There are many techniques to deposit BTO thin films, such as metal organic chemical vapor deposition (MOCVD)4,5 and molecular beam epitaxy (MBE),6 but these methods are high-temperature processes that would degrade the characteristics of the preexisting devices. In addition, the structure of BTO thin films are polycrystalline that have much higher dielectric constants and higher leakage current compared to that of amorphous BTO thin films. Sol-gel technology (SG),7 and reactive sputtering RF (radio frequency) magnetron sputtering8 allow the synthesis of homogeneous materials at low temperature.9 The amorphous BTO thin films prepared at low temperature show low leakage current.10 However, there is radiation damage in sputtered films.8 It needs high-temperature annealing to remove the damage. For sol-gel technology, the large shrinkage stress inherent limits the thickness to typically ∼0.7 µm without mechanical failure, unless high annealing temperature is employed.7 However, poor crystallinity of SG-BaTiO3 films after high temperature treatment was obviously observed.11 The high-temperature posttreatment also leads the poly-crystallization of the film and induces high leakage current. Therefore, the preparation of BTO amorphous film with low leakage current and high dielectric constant is noted.12 Liquidphase deposition (LPD) is almost a SG room-temperature technique for SiO2 amorphous film deposition in an aqueous solution. It has the advantages of low temperature deposition, low cost, and simple processes. Besides, high device quality LPD-SiO2 has been studied.13 In this study, we try to prepare * Corresponding author. Tel: 886-7-5252000, ext. 4120. Fax: 886-75254199. E- mail:
[email protected] BTO amorphous films on Si by LPD technique. We use barium nitrate [Ba(NO3)2] and hexafluorotitanic acid (H2TiF6) to prepare BTO films on Si. II. Experimental Section In this study, the precipitate of barium fluotitanate (BaTiF6 was obtained from a mixture of hexafluorotitanic acid (H2TiF6, 4.80-4.31 M) and barium nitrate (Ba(NO3)2, 0.20∼0.18 M). The precipitate was dried to be the powder, and dissolved with deionized water to be an aqueous solution. Then, 2 mL of boric acid (H3BO3, 0.19-0.26 M) was added to be the growth solution for the deposition of BTO thin films on Si substrate. Boron-doped, (100)-oriented silicon with a resistivity of 1525 Ω cm was used as the substrate. The deposition temperature was kept at 75 °C in this work. The flowchart of the LPD-BTO process is shown in Figure 1. After growth, the LPD-BTO film was rinsed in deionized water and dried with purified nitrogen gas. The thickness and refractive index of the LPD-BTO were measured by ellipsometry (a Gaertner model L116C auto-gain ellipsometer at a wavelength of 632.8 nm). The thickness was doubly checked by scanning electron microscopy (SEM, CamBridge S360). Atomic composition was examination by Auger electron spectroscopy (AES, VG. Microlab 310D). The composition distribution of the film was analyzed by the depth profile of secondary ion mass spectroscopy (SIMS, CAMECA IMS-4f). MOS (metal-oxide-semiconductor) capacitor with LPD-BTO as a dielectric was used for the electrical characterization. A HP4145B semiconductor-parameter analyzer was used for current-voltage (I-V) characterization. A high frequency (1 MHz) HP4280A capacitance-voltage meter was used for C-V characterization.
(s))
III. Results and Discussion The chemical reactions of LPD-BTO are proposed as the following chemical equilibriums:
H2TiF6 (l) + Ba(NO3)2 (l) S BaTiF6 (s) + 2HNO3 (l) (1) BaTiF6 (s) + 3H2O (l) S BaTiO3 (s) + 6HF (l)
(2)
H3BO3 (l) + 4HF (l) S HBF4 (l) + 3H2O (l)
(3)
10.1021/jp0134401 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/19/2002
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Figure 3. SIMS depth profile of as-deposited LPD-BTO film on silicon using the aqueous solution of Ba(NO3) and H2TiF6 directly.
and oxygen (O) throughout the film. The high concentration of fluorine (F) incorporated in the film is similar to that in LPDSiO2.15 In addition, there is a high concentration of Si in the LPD-BTO film. The Si distribution has a wide transition region about 1.5 µm from the Si substrate to the film. The wide transition region and high concentration of Si are the results of the etching of Si substrate by HNO3 and HF from the right sides of eq 1 and 2. The mechanisms are as follows:
Si (s) + HNO3 (l) + 6HF (l) S H2SiF6 (l) + HNO2 (l) + H2O (l) + H2 (g) (4) Figure 1. Flowchart of LPD-BTO film deposition.
H2SiF6 (l) + 2H2O (l) S SiO2 (s) + 6HF (l)
(5)
H3BO3 (l) + 4HF (l) S HBF4 (l) + 3H2O (l)
(6)
On the contrary, the mechanisms of the LPD-BTO deposition from the powder are as follows:
Figure 2. BaTiF6 powder X-ray diffraction.
If the aqueous solution of Ba(NO3)2 and H2TiF6 was directly used to deposit BTO thin films on Si substrate, the product of HNO3 and HF from eq 1 and eq 2 will strongly etch Si substrate.14 The composition of the powder from the precipitate is BaTiF6 examined by X-ray as shown in Figure 2. The growth solution prepared by the BaTiF6 powder without HNO3 can reduce the etching of Si substrate. The boric acid (H3BO3) will consume HF as in eq 3, and drives the BTO deposition on substrate. The structure of LPD-BTO film deposited on silicon substrate is amorphous checked by X-ray diffraction. Figure 3 shows the SIMS depth profile of the LPD-BTO film deposited from the aqueous solution of Ba(NO3) and H2TiF6 directly. It shows uniform distributions of barium (Ba), titanium (Ti), fluorine (F),
BaTiF6 (s) + 3H2O (l) S BaTiO3 (s) + 6HF (l)
(2)
Si + 6HF S H2SiF6 + 2H2
(7)
H2SiF6 + 2H2O S SiO2 + 6HF
(8)
HF from the H2TiF6 aqueous solution from eq 4 reacts with Si substrate to form H2SiF6.13 SiO2 is deposited from H2SiF6 and mixed with TiO2 deposited from H2TiF6. Without the incorporation of HNO3, the reaction rate of eq 7 is much slower compared with that of eq 4. Figure 4 shows the SIMS depth profile of the LPD-BTO film deposited from the powder. The SIMS depth profile shows that the Si distribution has a narrow transition region about 1.2 µm from the Si substrate. The concentration of Si in the LPD-BTO film is low. Figure 5 shows the atomic ratio of LPD-BTO film examined by AES. The atomic ratio of Ba, Ti, and O is 3.5:38.0:57.6. The composition of LPD-BTO is nonstoichiometry. It indicates that the chemical mechanisms proposed above are incomplete. The surface catalyst may dominate the chemical mechanisms. On the other hand, it could be from the high F and Si concentrations in the film. However, the LPD-BTO films show high quality optical and electrical properties. Figure 6 shows that the thickness and refractive index of LPDBTO films are functions of the deposition time at the deposition temperature of 75 °C. The film thickness increases with the deposition time. The refractive index decreases from 2.2 to 1.55
Deposition of Barium Titanate Films on Silicon
Figure 4. SIMS depth profile of as-deposited LPD-BTO film on silicon using the growth solution prepared by powder BaTiF6.
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Figure 6. Thickness and refractive index of as-deposited LPD-BTO film as a function of deposition time.
Figure 7. Thickness and refractive index of as-deposited LPD-BTO films as a function of deposition temperature.
Figure 5. Atomic concentration within LPD-BTO films as a function of annealing temperature.
with the film thickness increase from 500 to 2300 Å. The decrease of refractive index is due to the F concentration increase with the thickness16 as stated in eq 2 and also examined by SIMS depth profile as shown in Figure 3. Figure 7 shows that the thickness and refractive index of LPDBTO films are functions of the deposition temperature. The deposition time was kept at 1 h. The deposition rate increases from 510 Å/h to 1680 Å/h and the refractive index decreases from 1.96 to 1.60 as the deposition temperature increases from 65 °C to 85 °C. Therefore, LPD-BTO deposition is an endothermic reaction. According to Le Chatelier’s principle, a higher temperature drives the deposition faster. The decrease of the refractive index with increasing the deposition temperature is from the less dense film deposition by the faster deposition rate. Figure 8a and 8b show the SEM top and cross-sectional views of LPD-BTO film with the thickness of about 1360 Å. A mirrorlike surface and a high quality interface can be obtained.
The leakage current density of the as-deposited LPD-BTO film with a thickness of 1160 Å is about 7 × 10-9 A/cm2 at the electrical field intensity of 0.5 MV/cm as shown in Figure 9. Usually, the leakage current of the BTO film is from the dangling bonds.17,18 The very low leakage current of LPD-BTO film may be due to the high F or Si doping in the film.19 In this figure, we also check the improvement of the leakage current by the thermal annealing treatment in oxygen ambient for 20 min. The leakage current decreases with increasing the annealing temperature. The leakage current of LPD-BTO film is considerably improved to 2 × 10-10 A/cm2 after the thermal annealing at 600 °C. The leakage current improvement by the thermal annealing is from the decrease of oxygen vacancies. The static dielectric constant of the as-deposited LPD-BTO film with a thickness of 1160 Å is about 62 determined by a high-frequency C-V measurement as shown in Figure 10. The effective charges are about 2.30 × 1012 cm-3. The high effective charges could be from the nonstoichiometry structure. The static dielectric constant is much lower than that of polycrystalline BTO, but higher than that of SiO2 (3.9). The lower dielectric constant of LPD-BTO thin film is partly attributive to the amorphous structure and partly from the F incorporation in the film. IV. Conclusions In conclusion, a low temperature (about 20 °C-80 °C) LPDBTO film deposition process was developed. The barium fluotitanate powder prepared by the precipitate of hexafluorotitanic acid and barium nitrate solution was successfully used as
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Figure 10. High frequency (1 MHz) C-V curve of as-deposited LPDBTO MOS structure.
Acknowledgment. The authors acknowledge the support of the National Science Council of the Republic of China under Contract No. 90-2215-E-110-035, and Prof. K. M. Chen for helpful discussions. References and Notes Figure 8. SEM (a) top view and (b) cross-sectional view of as-deposited LPD-BTO film.
Figure 9. J-E curves as a function of annealing temperature.
the precursor for the deposition of LPD-BTO film on silicon substrate. A nonstoichiometric LPD-BTO film on silicon is obtained by Auger examination. However, it shows good optical and electrical properties. The refractive index of LPD-BTO is 1.96. From I-V and C-V measurements, the leakage current density reached as low as 5 × 10-9 A/cm2 and the dielectric constant can be as high as 60. These films show high potential in optical and electronic applications. The improvement of stoichiometry of LPD-BTO film needs a further study.
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