Electrical properties of O-2 and N-2 annealed (Ba,Sr)TiO3 thin films

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Electrical properties of O

2 and

N

₂

annealed (Ba, Sr)TiO

₃

thin

films

M. S. Tsai a , S. C. Sun ab & T. Y. Tseng a a

Department of Electronics Engineering , National Chiao Tung University , Hsinchu, Taiwan, R.O.C. b

Research and Development, Taiwan Semiconductor Manufacturing Company , Hsinchu, Taiwan, R.O.C. Published online: 19 Aug 2006.

To cite this article: M. S. Tsai , S. C. Sun & T. Y. Tseng (1998) Electrical properties of O2 and N2 annealed (Ba, Sr)TiO3 thin films, Integrated Ferroelectrics: An International Journal, 21:1-4, 173-183, DOI: 10.1080/10584589808202061

To link to this article: http://dx.doi.org/10.1080/10584589808202061

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Inrqrutrd Ferroi.lewrcs. 1998. Vol. 21. pp. 173-183 Reprints available directly from the publisher Photocopying permitted by license only

8 1998 OPA (Overseas Publishers Association) N.V. Published by llcense under

the Gordon and Breach Science

Publishers imprint. Printed in Malaysia.

Electrical Properties

of

0 2

and

N2

Annealed

(Ba,Sr)Ti03 Thin

Films

M.

S. Tsai,

S.

C. Sun*

and

T.

Y. Tseng

Department of Electronics Engineering, National

Chiao

Tung University Hsinchu, Taiwan, R.O.C.

*Research and Development, Taiwan Semiconductor Manufacturing Company Hsinchu, Taiwan,

R.O.C.

(Received 4 Murch 1998; In jinalform 3 May 1998 j

The dielectric constant and the leakage current of (Ba,Sr)TiO,

(BST)

thin films deposited on Pt boaom electrodes before and after annealing in 0, and N2 ambients were investigated. The crystallinity was improved after post- deposition annealing. The refractive index, dielectric constant, and leakage current of the films were strongly dependent on the annealing conditions. The 02-annealed BST films have higher dielectric constant and lower leakage current than those annealed in

N2

ambient.

Keywords:

BST;

annealing; oxygen vacancy; leakage current

INTRODUCTION

In recent years, thin films made &om high dielectric constant materials have attracted great attention for use in fabricating cell capacitors for high density dynamic random access memories (DRAMS), since employment of high- dielectric constant films can lower the height of the storage node and simplify the cell structure. The (Ba,Sr)TiO, (BST) films have been reported to be the most promising capacitors materials for giga-bit DRAMs because of its high dielectric constant, low leakage current density,

high

dielectric breakdown strength, paraelectric perovskite phase that does not exhibit fatigue and aging and the ease of compositional control [ 1-41,

Pt

is the most studied electrode material because it has a large work function which lowers the leakage current

I73

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I74 M. S . TSAl et al.

strength, paraelectric perovskite phase that does not exhibit fatigue and aging and the ease of compositional control [1-4].

Pt

is the most studied electrode material because it has a large work function which lowers the leakage current of BST capacitors. In this paper we have studied the electrical characteristics of annealed BST films on

Pt.

Electrical and reliability characteristics of BST films annealed at different conditions are compared.

EXPERIMENTAL

Metal-BST-metal capacitors were

used

as test devices. Pt bottom electrode with thickness of 100 nm was directly deposited on SiO, coated Si-substrates by RF magnetron sputtering at 350 "C

.

BST thin films (100 nm) were deposited by RF magnetron sputtering at 450 and 27

"C

in ambient of 50% 0,

and 50% Ar. After BST deposition, some samples were treated in 0, and N,

at 550,650 and 750°C for 1 to 3 min using rapid thermal annealing and 30 min using furnace annealing. Finally, Pt (50 nm) top electrodes with diameters of 165, 255 and 350 p were formed by electron beam evaporation through a

shadow mask. The refractive index and the film thickness of BST films before and after annealing were measured by Rudolph Auto

EL-III

ellipsometer at a wavelength of 6328

A.

RESULTS AND DISCUSSION

Fig. I depicts the change of refractive index with annealing time for BST thin films deposited at 27 "C , The refractive index of BST annealed at 550

"C

approaches a constant value

-

1.85. The refractive indexes of BST annealed at 650 and 750°C increase with increasing annealing time, but the index of BST annealed at 750 'C attains saturation when annealed over 1 min.

Fig, 2 shows the variation of thickness with annealing time for BST thin films deposited on Pt at 27°C. The film thickness of BST annealed at 550 and 650°C decreases with increasing annealing time. Also the film thickness of BST annealed at 750°C decreases with increasing annealing time, but it attains saturation when annealed over 1 min. This thickness reduction can be explained as the result of an increased density and elimination of porosity [5].

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ELECTRICAL PROPERTIES OF 0, AND N2 175

-0

T=65O0C

.

f j

I

g

2.0 a

0

n 1 L L

0

1

10

100

1 .u

Time

(

min)

FIGURE

1 The refraction index of BST films deposited at 27

"C

as a fimction of annealing time in N,.

FIGURE 2 The thickness of BST films deposited at 27 "C as function of annealing time in

N,.

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176 M. S. TSAI rt 01.

Fig. 3 indicates that the dielectric constant of 27 "C -deposited BST after

annealed at 550 'C in

N,

ambient approachs a constant value

-

23. The dielectric constant of BST annealed at 650 and 750 "C increases with increasing annealing time, which may be due to the increase of crystallinity with annealing. However, the dielectric constant of BST annealed at 750°C attains saturation after annealing over 1 min. The results shown in Fig. 3 are in good agreement with those indicated in Fig. 1. Fig. 4 shows that the dielectric constant of BST annealed in 0, have the similar result with Fig. 3.

The dielectric constant of BST annealed in 0, is larger than that of BST

annealed in N,. These results may be attributed to the lowering of the oxygen vacancy concentration in BST annealed in O,, which may lead to an increase in dielectric constant.

Fig. 5 illustrates the leakage current measured at 100 kV/cm (with 30 sec delay time) as a function of N2 annealing time. The leakage current of BST

annealed at 550

"C

decreases with increasing annealing time, because of increasing densification of BST films after post-annealing, as shown in Fig. 2. The leakage current of BST annealed at 650 "C decreases with increasing annealing time, but it slightly increases for BST films annealed for 30 min.

The possible reason is that the increase in densification of BST

films

after post-annealing leads to a decrease of leakage current, but the increase of crystallinity for annealing over 30 min will increase the leakage current. The leakage current of BST annealed at 750°C increases with increasing annealing time, because the crystallinity effect is more dominant, as compared to the effect of densification. Also, morphology changes such as hillocks can greatly influence leakage after annealing at high temperature or long time; or increase in oxygen-vacancy concentration as well. Fig. 6 shows that the lezkage current of BST annealed in 0, has the similar behavior as that in Fig. 5. The leakage current is slightly increased for BST annealed in 0, at 650°C over 3 min, whereas after annealed in N, at 650'C for over 30 min (Fig. 5) the leakage current was also slightly increased. The dielectric constants of BST annealed in 0, (Fig. 4) are larger than those of the BST annealed in N, (Fig. 3), and the leakage currents of BST annealed in 0, (Fig. 6) are smaller than that in N,

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ELECTRICAL PROPERTIES OF O? AND N,

0

1

10

1

0 Time

(

min

)

1 I 1

10

FIGURE 3 The dielectric constant of 27 "C -deposited films as a function of annealing time in N,. I 55OoC

I

0,

ambient

65OoC

I

75OoC

1 *

A -

A

/

0

,/

1

,

I

0' ,

FIGURE 4 The dielectric constant of 27

"C

-deposited films as a function of annealing time in 0,.

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I78 M. S . TSAI et al.

Time

(

min)

FIGURE 5 The leakage current of 27

"C

-deposited films as a function of annealing time in N,, at 100 kV/cm.

in-2,

Time

(

min

)

I0

FIGURE

6

The

leakage current of 27 "C -deposited films as a h c t i o n of annealing time in O,, at 100 kV/cm.

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ELECTRICAL PROPERTIES OF 0, AND N1 179 (Fig. 5). The reason may be due to the decrease of oxygen vacancies of BST after annealed in O,, and hence the dielectric constant can be enhanced and the leakage current can be suppressed. These results suggest that the BST films annealed in 0, at 650

"C

is a suitable process condition for obtaining high dielectric constant and low leakage current.

Fig. 7 shows that the dielectric constant of BST deposited at 27°C and 450

"C

and annealed at 650°C in N, and 0,. The dielectric constant of BST films increases with increasing annealing time. The dielectric constant of 450 "C -deposited BST annealed at 650'C in 0, is larger than that of the BST annealed in

N,.

Therefore, the polarization or crystallization of BST annealed in 0, ambient is expected to be more enhanced than in

N,

ambient. Fig. 8 shows the leakage current of

27°C-

and 450°C-deposited BST annealed at 650

"C

in

N,

and 0,. The leakage current of 0,-annealed BST does not change with annealing time, probably due to the balance of crystallization effect and the reduction of oxygen-vacancy concentration. The leakage current of N,- annealed BST increases slightly with increasing annealing time, because higher crystallization of Npnuealed BST has enhanced the electrons transport and there is no compensation of reduction of oxygen vacancy concentration. The dielectric constant of BST films deposited at 450

"C

after annealing in 0,

and N, ambient at 650

"C

for 30 min are 476 and 390, respectively. The leakage current density of BST films deposited at 450

"C

after annealing in 0,

and N, ambient at 650

'

C

for 30 min are 2 . 5 ~ 1 0 - ~ and 1.35x10-' Alcm2, respectively.

Fig. 9 shows the leakage current of as-deposited BST films (27°C and 450 "C ). The leakage current at positive bias of BST deposited at 27°C is larger than that of BST deposited at 450"C, because the BST film deposited at 27'C is less dense than at 450

"C

, Fig. 10 depicts the leakage current of BST deposited at 450

"C

and annealed at 650

"C

in N, and 0, for 3 min. The leakage currents in both conditions have almost the same value. Fig. 11 shows the leakage current of BST deposited at 450°C

under

650°C annealing in N,

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180 M. S. TSAI rt ul.

Time

(

min

)

FIGURE 7 The dielectric constant of BST films deposited at 27 "C and 450

"C

as a function of annealing time at 650 "C with 0, and

N,

ambient as a

parameter.

FIGURE 8 The leakage current of BST films deposited at 27 "C and 450 "C as a function of annealing time at 650 "C with 0, and N, ambient as a parameter.

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Electric Field

(

V/cm

)

FIGURE 9 The leakage current of BST films deposited at 27

"C

and 450 "C.

Electric Field

(

V/cm

)

FIGURE 10 The leakage current of 450 "C-deposited BST films after annealed for 3 min at 650 "C in 0,

and

N, ambient .

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182 M. S. TSAI et a1

Electric Field

(

V/cm

)

FIGURE

11 The leakage current of 450 "C-deposited BST films after annealed for 30 min at 650

"C

in 0, and N, ambient

.

and 0, for 30 min. The leakage current of 0,-annealed BST is smaller than that of N2-annealed BST. Again, 0, ambient is believed to reduce the oxygen vacancies, hence the leakage current of 0,-annealed BST is smaller

than

that of BST annealed in N,.

We have also examined the BST films deposited

on

F't bottom electrode at 27

"C

and annealed in N,O ambient. The capacitors showed a very large leakage current and therefore their dielectric constant could not be measured. CONCLUSIONS

The dielectric constant and the leakage currnet of 0,- and N,- annealed BST thin films were examined. The post-annealing process can improve the crystallinity of BST films. The dielectric constants of BST films deposited at 450

"C

after annealing in 0, and N, ambient at 650 "C for 30 min are 476 and

390, whereas the leakage currents of the films are 2.5x10-' and 1 . 3 5 ~ 1 0 - ~

A/cmz, respectively. The BST films annealed in 0,-ambient have a higher

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ELECTRICAL PROPERTIES OF O2 AND N2 183

dielectric constant and a lower leakage current as compared to the films annealed in N,-ambient.

This

may be attributed to the decrease of oxygen vacancies of 0,- annealed BST.

References

[l] T. Horikawa, N. Mikami, T. Makita, J. Tanimura, M. Kataoka, K. Sato, and M. Nunoshita, Jpn.

J

Appl. Phys., 33, p. 4126,1993.

[2] T. Eimori, Y. Obno, H. Kimura, J. M a t s h s a , S. Kishimura, A. Yoshida, H. Sumitani, T. Maruyama, K. Moriizui, T. Katayama,

M.

Asakura, T.

Horikawa, Y. Maruyama, T. Shiano, H. Itoh, K. Sato, K. Namba, T. Nishiimura, S. Satoh and H. Miyoshi, IEDM Tech. Digest, p. 63 1,1993. [3] E. Fujii, Y. Uemoto, S. Hayashi, T. Nasu, Y. shimada, A. Matsuda, M.