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Characterization of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 thin film
Pergamon
Materials Research Bulletin 35 (2000) 1755–1761
Characterization of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 thin film Jae-Dong Byun*, Jung-Il Yoon, Sahn Nahm, Jin-Cheol Kim Department of Material Science and Engineering, Korea University, Seoul 136-701, South Korea (Refereed) Received 12 October 1999; accepted 20 January 2000
Abstract The crystal structure and dielectric properties of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 (BSTZ) films were investigated. The films were deposited on Pt/Ti/SiO2/Si(100) substrates by metal– organic decomposition (MOD) method and rf magnetron sputtering. The crystal structure of the BSTZ film was cubic perovskite, and the preferred orientation of the film varied with annealing temperature. The MOD film annealed at 750°C exhibited excellent dielectric properties of dielectric constant, k ⬇ 1,000 and dissipation factor, tan ␦ ⱕ 0.04. The films also showed a very stable leakage current behavior vs. applied field. The leakage current density, J, increased smoothly with field, up to E ⫽ 0.3 MV/cm, and was 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; D. Dielectric properties; D. Electrical properties; D. Ferroelectricity
1. Introduction High permittivity dielectric films with low leakage current are of great importance for a variety of integrated devices, such as storage capacitors in dynamic random access memory (DRAM). There are numerous reports on the electrical properties of perovskite-type thin films, such as (Ba,Sr)TiO3 (BST). The dielectric constant of BST films is about 300 –500 depending on the preparation conditions [1–3]. The characteristics of Ba(Zr,Ti)O3(BZT) films also have been reported [4,5]. They are highly insulative and have a dielectric constant comparable to or lower than that of BST films. These values are not sufficient for planar
* Corresponding author. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 3 9 0 - 1
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Fig. 1. Temperature dependence of dielectric constant of ceramics of various composition: (a) (Ba0.75Sr0.25)(Ti0.75Zr0.25)O3, (b) (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3, (c) (Ba0.5Sr0.5)(Ti0.5Zr0.5)O3, and (d) (Ba0.5Sr0.5)Ti0.5O3.
DRAM capacitors. Films with higher permittivity and proper insulative characteristics are needed. Preliminary investigations on the dielectric properties of (Ba1⫺xSrx)(Ti1⫺xZrx)O3 ceramics were carried out to select a suitable composition for thin film study and the results are given in Fig.1. From the experimental results, it was found that (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 (BSTZ) ceramics have very attractive dielectric properties. The Curie temperature of BSTZ was below ⫺30°C, and the dielectric constant at room temperature was about 2700. The temperature dependence of the dielectric constant was small in the DRAM operating temperature range. It was also found that the average grain size of BSTZ ceramics decreased from 6 to 3 m as Zr content increased from 25 to 30 m/o when they were sintered at 1450°C for 4 h. Similar results have been reported previously by other investigators [6]. Therefore, the composition of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 was selected for the thin film study, and the dielectric properties of BSTZ films deposited by MOD and sputtering were investigated.
2. Experimental The BSTZ precursor solution was prepared by dissolving Ba-2-ethylhexanoate, Sr-2ethylhexanoate, Ti-isopropoxide, and Zr-propoxide in xylene. The BSTZ metal– organic solution was spun onto Pt/Ti/SiO2/Si(100) substrates, and the films were post-annealed at various temperatures for 30 min in air. In addition, BSTZ films were deposited on the same
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
1757
Fig. 2. X-ray diffraction patterns of BSTZ thin films deposited by MOD and annealed at (a) 400, (b) 500, (c) 600, and (d) 750°C.
substrates by rf magnetron sputtering. The sputtering was carried out in 55%Ar– 45%O2 atmosphere with total pressure of 5 mTorr. The composition of the ceramic target was (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3. The crystal structure was analyzed by X-ray diffraction (XRD) and the microstructure by scanning electron microscopy (SEM). The surface roughness of the film was evaluated by atomic force microscopy (AFM). For the electrical measurements, a top Pt electrode was deposited through the metal shadow mask with a diameter of 0.2 mm. The capacitance and leakage current were measured using an HP 4284A LCR meter and an HP 4156A semiconductor parameter analyzer, respectively.
3. Results and discussion In Fig. 2, the XRD patterns of BSTZ films deposited by MOD are shown as a function of annealing temperature. After annealing at 400 and 500°C, the films showed amorphous XRD patterns. When annealing at higher temperatures, for example, at 600 or 750°C, the crystallinity of the film developed, and the crystal structure was cubic perovskite. It was observed that, after annealing at 750°C, the (110) peak from the BSTZ film became sharper and its intensity increased. This result seems to imply that the crystallinity of the film improved by annealing at higher temperatures. The SEM cross section and surface images of BSTZ films annealed at different temperatures are shown in Fig. 3. The thickness of the film was about 160 nm. The surface of the film annealed at 400°C was very smooth, without any distinct surface
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Fig. 3. SEM micrograph of (a) cross section and surfaces of BSTZ films deposited by MOD and annealed at (b) 400, (c) 600, and (d) 750°C.
topography, and by annealing at high temperature, a structure with very fine grains was developed. The XRD and SEM results indicate that annealing at higher temperatures induced crystallization of the films. The AFM images of the same films are shown in Fig. 4. The surface roughnesses of BSTZ films annealed at 400, 600, and 750°C were 17.36, 41.7, and 37.46 Å, respectively. Therefore, improvement in crystallinity of the films was accompanied by increase in surface roughness. In Fig. 4(c), we can see a few spikes along the c axis that might have been caused by shrinkage of the film during crystallization. These spikes may be responsible for the higher leakage current of the film annealed at 750°C, which is discussed later. Fig. 5 shows the dc leakage characteristics of BSTZ films deposited by MOD method and
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
1759
Fig. 4. AFM images of BSTZ films deposited by MOD and annealed at (a) 400, (b) 600, and (c) 750°C.
annealed at various temperatures. The excellent stability of the leakage current behavior vs. applied field was observed. At low field, the leakage current increased slowly with field, following J ⬀ En relation with n ⫽ 0.6⬃0.8. No sudden increase of current or breakdown phenomenon was observed in an extensive range of applied field. The leakage current densities seemed to be minimal when films were annealed at 400°C at both bias polarities. With increasing annealing temperature from 400 to 750°C, the leakage current density increased from 8 ⫻ 10⫺8 A/cm2 to 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. The increase of leakage current with annealing temperature is attributed to the increase of the surface roughness of the film [7]. The difference in leakage characteristics between films annealed at 600 and 750°C, despite similar surface roughness, could be attributed to the spikes observed in Fig. 4(c). The dielectric constants of the films deposited by MOD are plotted as a function of annealing temperature in Fig. 6. The dielectric constant of BSTZ films increased with increasing annealing temperature, and the maximum dielectric constant of 977 was achieved with the film annealed at 750°C. This is probably due to the improvement of crystallinity of BSTZ films. Up to this date, there have been no reports on the electrical properties of BSTZ films; therefore, we could not make direct comparison with other published data. However, the BSTZ films exhibited superior dielectric properties compared to similar materials, such as BST and BZT. A few remarks should be made about the characteristics of the BSTZ film deposited by
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J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
Fig. 5. I–V characteristics of BSTZ thin films deposited by MOD and annealed at (a) 400, (b) 600, and (c) 750°C.
sputtering and post-annealed at various temperatures. The thickness of the film was 300 nm. The leakage characteristic was very similar to that of MOD deposited films, as shown in Fig. 7, but the dielectric constant was only about 600. We believe, however, that the dielectric properties could be improved by adjusting the preparation conditions.
Fig. 6. Dielectric constants and loss of BSTZ thin films deposited by MOD and annealed at various temperatures.
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Fig. 7. I–V characteristics of BSTZ thin films deposited at 300°C by sputtering: (a) as-deposited, and annealed at (b) 500 and (c) 700°C.
4. Conclusions From the results of this work, it is clear that the BSTZ film prepared by MOD and annealed at 750°C exhibited excellent dielectric properties of dielectric constant, k ⬇ 1,000, and dissipation factor, tan␦ ⱕ 0.04. The film showed a very stable leakage current behavior vs. applied field. The leakage current density, J, increased smoothly with field, up to E ⫽ 0.3 MV/cm, and was 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. Therefore, considering their high dielectric constant and stable leakage characteristics, we believe that BSTZ films show promising potential for application in DRAMs.
References [1] T. Kuroiwa, Y. Tsunemine, T. Horikawa, T. Makita, J. Tanimura, N. Mikami, K. Sato, Jpn J Appl Phys 33 (1994) 5187–5191. [2] S.B. Krupanidhi, C.-J. Peng, Thin Solid Films 305 (1997) 144 –156. [3] Y. Shimada, A. Inoue, T. Nasu, K. Arita, Y. Nagano et al., Jpn J Appl Phys 35 (1996) 140 –143. [4] T.B. Wu, C.M. Wu, M.L. Chen, Appl Phys Lett 69 (1996) 2659 –2661. [5] N. Kamehara, M. Tsukada, J.S. Cross, K. Kurihara, J Ceram Soc Jpn 105 (1997) 746 –749. [6] I.-C. Ho, S.-L. Fu, J Mater Sci (1990) 4699. [7] P. Bhattacharya, T. Komeda, K. Park, Y. Nishioka, Jpn J Appl Phys 32 (1993) 4103– 4106.
Materials Research Bulletin 35 (2000) 1755–1761
Characterization of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 thin film Jae-Dong Byun*, Jung-Il Yoon, Sahn Nahm, Jin-Cheol Kim Department of Material Science and Engineering, Korea University, Seoul 136-701, South Korea (Refereed) Received 12 October 1999; accepted 20 January 2000
Abstract The crystal structure and dielectric properties of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 (BSTZ) films were investigated. The films were deposited on Pt/Ti/SiO2/Si(100) substrates by metal– organic decomposition (MOD) method and rf magnetron sputtering. The crystal structure of the BSTZ film was cubic perovskite, and the preferred orientation of the film varied with annealing temperature. The MOD film annealed at 750°C exhibited excellent dielectric properties of dielectric constant, k ⬇ 1,000 and dissipation factor, tan ␦ ⱕ 0.04. The films also showed a very stable leakage current behavior vs. applied field. The leakage current density, J, increased smoothly with field, up to E ⫽ 0.3 MV/cm, and was 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; D. Dielectric properties; D. Electrical properties; D. Ferroelectricity
1. Introduction High permittivity dielectric films with low leakage current are of great importance for a variety of integrated devices, such as storage capacitors in dynamic random access memory (DRAM). There are numerous reports on the electrical properties of perovskite-type thin films, such as (Ba,Sr)TiO3 (BST). The dielectric constant of BST films is about 300 –500 depending on the preparation conditions [1–3]. The characteristics of Ba(Zr,Ti)O3(BZT) films also have been reported [4,5]. They are highly insulative and have a dielectric constant comparable to or lower than that of BST films. These values are not sufficient for planar
* Corresponding author. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 3 9 0 - 1
1756
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
Fig. 1. Temperature dependence of dielectric constant of ceramics of various composition: (a) (Ba0.75Sr0.25)(Ti0.75Zr0.25)O3, (b) (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3, (c) (Ba0.5Sr0.5)(Ti0.5Zr0.5)O3, and (d) (Ba0.5Sr0.5)Ti0.5O3.
DRAM capacitors. Films with higher permittivity and proper insulative characteristics are needed. Preliminary investigations on the dielectric properties of (Ba1⫺xSrx)(Ti1⫺xZrx)O3 ceramics were carried out to select a suitable composition for thin film study and the results are given in Fig.1. From the experimental results, it was found that (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 (BSTZ) ceramics have very attractive dielectric properties. The Curie temperature of BSTZ was below ⫺30°C, and the dielectric constant at room temperature was about 2700. The temperature dependence of the dielectric constant was small in the DRAM operating temperature range. It was also found that the average grain size of BSTZ ceramics decreased from 6 to 3 m as Zr content increased from 25 to 30 m/o when they were sintered at 1450°C for 4 h. Similar results have been reported previously by other investigators [6]. Therefore, the composition of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 was selected for the thin film study, and the dielectric properties of BSTZ films deposited by MOD and sputtering were investigated.
2. Experimental The BSTZ precursor solution was prepared by dissolving Ba-2-ethylhexanoate, Sr-2ethylhexanoate, Ti-isopropoxide, and Zr-propoxide in xylene. The BSTZ metal– organic solution was spun onto Pt/Ti/SiO2/Si(100) substrates, and the films were post-annealed at various temperatures for 30 min in air. In addition, BSTZ films were deposited on the same
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
1757
Fig. 2. X-ray diffraction patterns of BSTZ thin films deposited by MOD and annealed at (a) 400, (b) 500, (c) 600, and (d) 750°C.
substrates by rf magnetron sputtering. The sputtering was carried out in 55%Ar– 45%O2 atmosphere with total pressure of 5 mTorr. The composition of the ceramic target was (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3. The crystal structure was analyzed by X-ray diffraction (XRD) and the microstructure by scanning electron microscopy (SEM). The surface roughness of the film was evaluated by atomic force microscopy (AFM). For the electrical measurements, a top Pt electrode was deposited through the metal shadow mask with a diameter of 0.2 mm. The capacitance and leakage current were measured using an HP 4284A LCR meter and an HP 4156A semiconductor parameter analyzer, respectively.
3. Results and discussion In Fig. 2, the XRD patterns of BSTZ films deposited by MOD are shown as a function of annealing temperature. After annealing at 400 and 500°C, the films showed amorphous XRD patterns. When annealing at higher temperatures, for example, at 600 or 750°C, the crystallinity of the film developed, and the crystal structure was cubic perovskite. It was observed that, after annealing at 750°C, the (110) peak from the BSTZ film became sharper and its intensity increased. This result seems to imply that the crystallinity of the film improved by annealing at higher temperatures. The SEM cross section and surface images of BSTZ films annealed at different temperatures are shown in Fig. 3. The thickness of the film was about 160 nm. The surface of the film annealed at 400°C was very smooth, without any distinct surface
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Fig. 3. SEM micrograph of (a) cross section and surfaces of BSTZ films deposited by MOD and annealed at (b) 400, (c) 600, and (d) 750°C.
topography, and by annealing at high temperature, a structure with very fine grains was developed. The XRD and SEM results indicate that annealing at higher temperatures induced crystallization of the films. The AFM images of the same films are shown in Fig. 4. The surface roughnesses of BSTZ films annealed at 400, 600, and 750°C were 17.36, 41.7, and 37.46 Å, respectively. Therefore, improvement in crystallinity of the films was accompanied by increase in surface roughness. In Fig. 4(c), we can see a few spikes along the c axis that might have been caused by shrinkage of the film during crystallization. These spikes may be responsible for the higher leakage current of the film annealed at 750°C, which is discussed later. Fig. 5 shows the dc leakage characteristics of BSTZ films deposited by MOD method and
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
1759
Fig. 4. AFM images of BSTZ films deposited by MOD and annealed at (a) 400, (b) 600, and (c) 750°C.
annealed at various temperatures. The excellent stability of the leakage current behavior vs. applied field was observed. At low field, the leakage current increased slowly with field, following J ⬀ En relation with n ⫽ 0.6⬃0.8. No sudden increase of current or breakdown phenomenon was observed in an extensive range of applied field. The leakage current densities seemed to be minimal when films were annealed at 400°C at both bias polarities. With increasing annealing temperature from 400 to 750°C, the leakage current density increased from 8 ⫻ 10⫺8 A/cm2 to 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. The increase of leakage current with annealing temperature is attributed to the increase of the surface roughness of the film [7]. The difference in leakage characteristics between films annealed at 600 and 750°C, despite similar surface roughness, could be attributed to the spikes observed in Fig. 4(c). The dielectric constants of the films deposited by MOD are plotted as a function of annealing temperature in Fig. 6. The dielectric constant of BSTZ films increased with increasing annealing temperature, and the maximum dielectric constant of 977 was achieved with the film annealed at 750°C. This is probably due to the improvement of crystallinity of BSTZ films. Up to this date, there have been no reports on the electrical properties of BSTZ films; therefore, we could not make direct comparison with other published data. However, the BSTZ films exhibited superior dielectric properties compared to similar materials, such as BST and BZT. A few remarks should be made about the characteristics of the BSTZ film deposited by
1760
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
Fig. 5. I–V characteristics of BSTZ thin films deposited by MOD and annealed at (a) 400, (b) 600, and (c) 750°C.
sputtering and post-annealed at various temperatures. The thickness of the film was 300 nm. The leakage characteristic was very similar to that of MOD deposited films, as shown in Fig. 7, but the dielectric constant was only about 600. We believe, however, that the dielectric properties could be improved by adjusting the preparation conditions.
Fig. 6. Dielectric constants and loss of BSTZ thin films deposited by MOD and annealed at various temperatures.
J.-D. Byun et al. / Materials Research Bulletin 35 (2000) 1755–1761
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Fig. 7. I–V characteristics of BSTZ thin films deposited at 300°C by sputtering: (a) as-deposited, and annealed at (b) 500 and (c) 700°C.
4. Conclusions From the results of this work, it is clear that the BSTZ film prepared by MOD and annealed at 750°C exhibited excellent dielectric properties of dielectric constant, k ⬇ 1,000, and dissipation factor, tan␦ ⱕ 0.04. The film showed a very stable leakage current behavior vs. applied field. The leakage current density, J, increased smoothly with field, up to E ⫽ 0.3 MV/cm, and was 3.47 ⫻ 10⫺7 A/cm2 at 1.25 V. Therefore, considering their high dielectric constant and stable leakage characteristics, we believe that BSTZ films show promising potential for application in DRAMs.
References [1] T. Kuroiwa, Y. Tsunemine, T. Horikawa, T. Makita, J. Tanimura, N. Mikami, K. Sato, Jpn J Appl Phys 33 (1994) 5187–5191. [2] S.B. Krupanidhi, C.-J. Peng, Thin Solid Films 305 (1997) 144 –156. [3] Y. Shimada, A. Inoue, T. Nasu, K. Arita, Y. Nagano et al., Jpn J Appl Phys 35 (1996) 140 –143. [4] T.B. Wu, C.M. Wu, M.L. Chen, Appl Phys Lett 69 (1996) 2659 –2661. [5] N. Kamehara, M. Tsukada, J.S. Cross, K. Kurihara, J Ceram Soc Jpn 105 (1997) 746 –749. [6] I.-C. Ho, S.-L. Fu, J Mater Sci (1990) 4699. [7] P. Bhattacharya, T. Komeda, K. Park, Y. Nishioka, Jpn J Appl Phys 32 (1993) 4103– 4106.