Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/Fabrication and Evaluation of SrTa2O6 Thin Films
Using Chemical Solution Deposition Method
Li Lu1, Masahiro Echizen1, Takashi Nishida1,
Kiyoshi Uchiyama1 and Yukiharu Uraoka1,2
1

Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan
2CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan

Keywords: SrTa2O6, Chemical Solution Deposition, dielectric constant, loss tangent, leakage
current

Abstract. SrTa2O6 (STA) thin films were fabricated using two Chemical Solution Deposition
methods, the Metal-Organic Decomposition (MOD) method and the Sol-Gel method. Electricalproperties of STA thin films were investigated with a metal-insulator-metal structure. Both of
the STA thin films deposited by the two methods were crystallized at 800 oC. The dielectric
constants (ε) of crystalline thin films were much higher than those of amorphous thin films. Theε of about 150 for crystalline STA thin films fabricated by the Sol-Gel method is lower than thatby the MOD method, but its leakage current density was improved markedly from 10-3 to 10-8
A/cm2 at 250 kV/cm. The larger leakage current is regarded as the reason of higher ε.

Introduction
Along with the miniaturization of electronic devices high-k materials are needed to replace
conventional SiO2 in a broad range of application areas, such as information storage capacitors
for future dynamic random access memories (DRAMs), embedded capacitors, tunable devices
and gate oxides for field effect transistors [1, 2]. The research and application of some high-kmaterials including Al2O3, HfO2, ZrO2 and Ta2O5 have already performed [2-7]. Recently,SrTa2O6 (STA) has attracted much attention because of its high dielectric constant and low
leakage current density [2, 8-13]. Several deposition methods have been used to investigate its
fundamental electrical properties with a metal-insulator-metal (MIM) structure, such as Metal
Organic Chemical Vapor Deposition (MOCVD) [2], Plasma-Enhanced Atomic Layer
Deposition [9] and Sputtering [8]. However, reports on using the Chemical Solution Deposition(CSD) method are rare in the literature.
The CSD method possesses many advantages in preparing oxide thin films, such as good
homogeneity, lower processing temperature, precise control of composition, simplicity andlower equipment cost [14]. However, the fabrication conditions of the CSD method greatly
affect the micro-structures [15] and the electrical properties. Therefore, in this study, weinvestigated the capacitance vs. frequency and leakage current properties of STA thin films by
two CSD methods, the Metal-Organic Decomposition (MOD) method and the Sol-Gel method,under various fabrication conditions.

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ExperimentalRepetition

In the MOD method, the composition and concentration of the STA precursor was 0.1 mol/L
SrO-TaO2.5 solution (Kojundo Chemical Lab.). To
increase wetness, the solution was diluted with
Pt/TiOx/SiOx/Si(100)butyl acetate in a 1:1 ratio. In the Sol-Gel method,
the composition and concentration of the STA
Spin-coatingprecursor were set at Sr/Ta =1/2 and 7 wt%,respectively. The prepared precursors were(3000 rpm/ 30 s)
spin-coated on Pt/TiOx/SiOx/Si (Pt/Si) substrates
at 3000 rpm for 30 sec. Then the MOD-prepared
Dryingfilms were dried at 150 oC for 1 min ando(T1 C/ t1 min; HP*)
pyrolyzed at 250 oC for 4 min; theSol-Gel-prepared films were dried at 100 oC for 5Pyrolysis
min and pyrolyzed at 450 oC for 3 min on hoto
(T2 C/ t2 min; HP*)
plates in air. This coating and heating treatment
was repeated four times, until the thicknesses
reached about 150 nm for the MOD method andAnnealing
200 nm for the Sol-Gel method. After theseo(T C/ t hr.; TF**)
processes, samples were annealed from 500 to800 oC for 30 min in air using a tube furnace.
Figure 1 shows the fabrication process of STA
STA thin films
thin films.
*HP: Hot Plate,To measure the electrical properties, Pt top**TF: Tube Furnace
electrodes with a diameter of about 0.09 mm weredeposited through shadow masks using the rf
sputtering method.
Fig.1: Fabrication process of STA thin
The crystal structure and composition of films by the MOD and the Sol-Gel
obtained films were evaluated by X-ray method. T1, T2 and T are the drying,Diffractometry (XRD: X’pert MRD, PANalytical) pyrolysis and annealing temperatures,and X-ray fluorescence (XRF: MagiX Pro, respectively. t1, t2 and t are the drying,
PANalytical). Surface morphologies were pyrolysis
andannealing
times,
observed with Atomic Force Microscopy (AFM: respectively.
SPA400, Seiko Instruments). For accuracy several
points were measured on each sample. Deposited
film thicknesses were determined by Transmission
Electron Microscope (TEM: JEM-3100FEF, JEOL). Leakage currents were measured with
Programmable Electrometer (Keithley617, Keithley). A hold time of 30 sec was set for eachmeasuring point to eliminate relaxation process effects. Capacitance vs. frequency andcapacitance vs. voltage properties were measured by Impedance Analyzer (HP4149A,Hewlett-Packard). All of the electrical properties were measured at room temperature.
Results and Discussions
Figure 2(a) and 2(b) show the XRD patterns using the thin film diffraction technique of STA
thin films annealed at various temperatures by the MOD method and the Sol-Gel method,respectively. Definite regularity diffraction peaks were observed at 800 oC annealing, and broad
peaks in a halo pattern were observed from 500 to 700 oC annealing using the two methods,indicating that STA thin films crystallized at 800 oC and were in amorphous states below this100228043-2

551

Pt 200061
541

Pt 111140131
231

230

Intensity (a.u.)

Intensity (a.u.)

130

temperature. This crystallization temperature(a)agrees well with what has been reported inRefs. [2] and [9], but is lower than thatreported in Ref. [10]. These two crystalline
800 oC
thin films are polycrystalline and have thesame phases, belonging to the orthorhombic
700 oCtungsten bronze structure [2].
600 oC
Dielectric constants (ε) and loss tangents(tan δ) at 100 kHz of MOD derived STA thin
500 oCfilms are compared in figure 3(a). ε
15 20 25 30 35 40 45 50 55 60
increased slightly with increasing annealing2θ (deg.)
temperatures for thin films in amorphousstates. However, it increased rapidly to(b)about 260 after crystallization, which isabout 3~4 times higher than those of800 oC
amorphous films. This value is much higher
than those reported in Refs. [2] and [9].
700 oC
Sol-Gel derived STA thin films showed thesame characteristic, as shown in figure 3(b).
600 oCThe ε value in the crystalline state of about
500 oC
150 is comparable to that of other reports.
Tan δ decreased as annealing temperature
15 20 25 30 35 40 45 50 55 60
increased for both methods. In particular,2θ (deg.)
low tan δ values of 0.02 were found for two
crystalline films which agree with otherFig.2: XRD patterns by the thin film diffraction
reports [2, 9] and satisfy the low tan δ technique of STA thin films annealed atparameter for capacitor applications. various temperatures by (a) the MOD method
Therefore, dielectric properties of crystalline
and (b) the Sol-Gel method.
STA thin films, which possess higher ε andlower tan δ, are more suitable for capacitor applications than amorphous STA thin films.

0.3

150

0.2

1000.1500500600700Dielectric constant ε0.42000.5300tan δDielectric constant ε250(a)0.0800Temperature (oC)

250(b)

0.4

2000.31500.2100tan δ

0.5

3000.1

5000.0500

600

700800Temperature (oC)Fig.3: Dielectric constants (ε) and loss tangents (tan δ) at 100 kHz of STA thin films by(a) the MOD method and (b) the Sol-Gel method as a function of temperature.

Leakage currents of MOD derived STA thin films are shown in figure 4(a) indicating that

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leakage currents of amorphous films are lower than that of the crystalline film. The lowest
value of about 10-7 A/cm2 at 250 kV/cm was obtained by the 700 oC annealed film, which wasdegraded up to 10-3 A/cm2 at 800 oC annealing. The leakage current of the crystalline STA thinfilm is much larger than that reported by Regnery et al. [2] and is too large to fabricate actualdevices. Leakage currents of Sol-Gel derived STA thin films are shown in figure 4(b). As in theamorphous state, the leakage current of the film annealed at 700 oC is lower than those annealed
at 500 and 600 oC. However, a considerable difference between thin films by the two methods
was observed at crystallization. The leakage current of the Sol-Gel derived thin film wasimproved after crystallization, which is even lower than that of the 700 oC-annealed thin film.
The value of about 10-8 A/cm2 at 250 kV/cm is comparable to that by the MOCVD method [2].

600 oC

500 oC-610
1.00E-06

700 oC-8
10
1.00E-08

(a)

10-10

1.00E-10050-2
1.00E-0210(b)2

800 oC-410
1.00E-04

Current Density (A/cm )2

Current Density (A/cm )-2
1.00E-02
10100

150200250-4
1.00E-04

10500 oC

-61.00E-06
10600 oC
700 oC-81.00E-08
10800 oC-10101.00E-10300

0Electric Field (kV/cm)

50100150200

250300Electric Field (kV/cm)

Fig.4: Leakage currents of STA thin films fabricated by (a) the MOD method and (b) theSol-Gel method as a function of temperature.

For the requirements of application such as high ε, low tan δ and low leakage current, thecrystalline STA thin films by the Sol-Gel method (hereinafter: C-SG) is a promising candidate.
Although the crystalline STA thin film by the MOD method (hereinafter: C-MOD) has a higherε, its leakage current is very large. To gain a deeper understanding, the cause of leakage
currents of C-MOD and C-SG was investigated.
RMS=0.96 nm

RMS=1.24 nm

(a)

(b)

(c)

(d)

Fig.5: Surface morphologies of crystalline STA thin films by (a) the MOD method, (b) theSol-Gel method at an area of 5×5 μm2 and (c) the MOD method, (b) the Sol-Gel method at
an area of 1×1 μm2.

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The difference in crystalline structure can be excluded because both of them arepolycrystalline and no secondary phase was observed, as shown in figure 2. The composition
ratios of Sr to Ta were also evaluated by XRF and were found to be about 1 to 2. Hence, theinfluence of nonstoichiometry could be excluded, too.
Surface morphologies with the scan area of 5×5 μm2 for C-MOD and C-SG were shown infigure 5(a) and 5(b), respectively. Almost the same surface morphologies were observed. Thenthe scan area of 1×1 μm2 were also carried out for detailed observation, as shown in figure 5(c):C-MOD and 5(d): C-SG. The surface root mean square (RMS) roughness of C-MOD and C-SG
were 1.24 nm and 0.96 nm, respectively. The lower RMS indicates the surface of C-SG issmoother than that of C-MOD. Thus, we attribute the larger leakage current of C-MOD to theworse surface morphology and the enormous high ε is presumed to be the result of the leaky
property and not a real value.

Summary
SrTa2O6 thin films were fabricated on Pt/TiOx/SiOx/Si substrates using the Metal-Organic
Decomposition method and the Sol-Gel method. They were crystallized at the annealing
temperature of 800 oC in air by both methods. The crystalline SrTa2O6 thin film by the Sol-Gel
method showed a high ε of about 150 and a low leakage current density of about 10-8 A/cm2 at
250 kV/cm, both of which are comparable to those reported on the MOCVD derived thin film.

Acknowledgement
We would like to thank for the support of The Murata Science Foundation.
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