액적화학증착법을 이용한 (Bi,Ce)4Ti3O12(BCT) 박막의 제조 및 특성 분석

Theories and Application of Chem. Eng., 2002, Vol. 8 No.1

화학공학의 이론과 응용 제8권 제1호 2002년

전민구, 김기웅, 우성일

한국과학기술원 생명화학공학과/ 초미세화학공정시스템연구센터

Characterization of (Bi,Ce)4Ti3O12(BCT) thin film prepared by

Liquid Source Misted Chemical Deposition(LSMCD)

Min Ku Jeon, Ki Woong Kim, Seong Ihl Woo

Department of Biomolecular and Chemical Engineering /

Center for Ultramicrochemical Process Systems, KAIST

Introduction

Ferroelectric thin films have been intensively studied for ferroelectric random access memory (FERAM), which realizes both nonvolatile operation and high-speed access. Recently, ferroelectric bismuth titianate Bi4Ti3O12 (BTO) thin films have attracted much attention for application to FERAM devices. Therefore, much effort has been made to form ferroelectric BIT thin films using several methods [1-5]. Ferroelctric BTO crystals have a typical perovskite-layered structure and show strong anisotropic properties: [6] coercive field values of 3.5 and 50 kV/cm, and spontaneous polarization of 4.0 and 50 mC/cm2 along the c- and a-axes, respectively. BTO has potential problems such as low spontaneous polarization along c-axis and fatigue failure in order to apply to memory [7]. Therefore, it is important to fabricate highly a-axis oriented and fatigue-free BTO thin films. Especially, fatigue failure of BTO should be removed in order to sustain a memory operation for a long time. Actually, it was reported that (Bi,La)4Ti3O12 (BLT) thin film, La substituted BTO thin film, prepared by pulsed laser ablation show fatigue free behavior [8]. From this report, we wanted to know if Ce which has smaller ionic radii than La is substituted instead of La electrical properties of BTO thin film will change. Though BLT thin film has been prepared by pulsed laser deposition, this research developed one of wet deposition techniques, Liquid Source Misted Chemical Deposition (LSMCD) to prepare (Bi,Ce)4Ti3O12 (BCT) thin films. LSMCD uses the ultrafine mists (diameter: 0.1-3 □) generated by ultrasonic nebulizer as the depostion source. Therefore, LSMCD is easy to control the film composition and to synthesize films that have various properties in various deposition conditions. In this research, the effect of crystallization temperature and thickness on the structure and electric properties of (Bi,Ce)4Ti3O12 films was investigated.

Experimental part

(1) Thin film Deposition (LSMCD)

Pt(1500□)/SiO2(3000□)/Si was used as a substrate. The precursor solution was 0.12M solution of Bi(II)-ethylhexanoate, Ce nitrate hexahydrate and Ti isopropoxide in 2-methoxyethanol and Bi was 25% excess because of volatility of Bi. The mists generated by using ultrasonic nebulizer (frequency: 1.63MHz) were deposited on the substrate at 700torr in Ar atmosphere for 5min. After deposition, the solution films were baked at 200oC for 2min and were dried at 400oC in air for 10min sequentially. And this procedure was repeated several times to increase film thickness. Heat treatment for crystallization of films was carried out at 650-750oC in Furnace for 1hr in the oxygen atmosphere.

(2) Characterization

The X-ray diffraction (XRD) was performed using a Rigaku D-MAX-RC diffractometer and Cu K□ radiation (□= 1.5405□) operated at 60 mA and 30 kV. The diffractograms were recorded in the range of 2 between 10 and 60 with 0.01 resolutions. Scanning electron microscope (SEM) images were obtained using a Philips 533M. In order to measure electrical properties, Pt upper electrodes of 400mm diameter was prepared by using shadow mask in the rf-magnetron sputter. Hysteresis properties were measured between 5 and 15 V and fatigue properties were obtained at 5 V by using RT66A tester and leakage current was measured by precision materials from Radiant Technology.

Results and Discussion

X-ray patterns of Bi3.25Ce0.75Ti3O12 thin films, thickness was 130nm, as function of annealing temperature between 600 and 750oC are shown in Fig.1., which shows that BCT films have been crystallized at 600oC. From this figure, it is observed that the intensity of (020) peak increased significantly with increasing annealing temperature though that of (117) peak didn't seem to increase with annealing temperature. Fig.2. shows the scanning electron microscope (SEM) images of the surface morphology and the cross section of the film. The grain size was increased and the morphology was smoother with raising annealing.

The composition of films was measured by wavelength dispersive spectroscopy (WDS) and the ratio of Bi/Ce was 3.63. So, the final composition was Bi3.22Ce0.78Ti3O12.

Fig.3.(a) shows hysteresis loops for 130nm BCT films annealed at 650, 700 and 750oC for 1hr in O2 atmosphere. Remanent polarizations (2Pr) were 7.7, 13.0 and 15.3 □C/cm2, respectively. Coercive fields (Er) were about 220 (kV/cm) for 700 and 750oC annealed films. 2Pr increase as annealing temperature increased. And it is significant that when the annealing temperature increased from 650oC to 700oC the hysteresis loop showed typical shape of ferroelectric materials and 2Pr increased almost 2 times. From this result, the annealing temperature above 700oC seems to be necessary for good electrical properties of BCT thin films prepared by LSMCD. However, the leakage current density of 130nm films was too large to measure. Therefore, electrical properties were measured again with 180nm films prepared by the same method with 130nm films. Fig.3.(b) shows hysteresis loops for 130nm and 180 nm BCT films annealed at 700oC for 1hr in O2 atmosphere. At the same electrical field, 2Pr was almost same. The leakage current density of 180nm film was 6x10-7 (A/cm2) at 1V.

Fatigue characteristic of 180nm film which annealed at 700oC was shown in Fig.4. 40kHz bipolar square wave of 5V was used to fatigue the film and the excellent fatigue endurance was observed until 1010 cycles. And the reduction of remanent polarization was less than 5% of initial value. This means that BCT film prepared by LSMCD have a good fatigue endurance.

Reference

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Fig.1. X-ray diffraction patterns of BCT films annealed at 600, 650, 700 and 750oC and non annealed film.

(a)	(b)	(c)

(d)	(e)	(f)

Fig.2. Scanning electron microscope images of the surface of (Bi,Ce)4Ti3O12 films synthesized as a function of  annealing temperature (a) non annealed, (b) 600oC, (c) 650oC, (d) 700oC, (e) 750oC and (f) cross section

( Annealed in furnace for 1hr in oxygen atmosphere )

(a)                                       (b)

Fig.3. Hysteresis loops (a) as a function of annealing temperature of 130nm films. and (b) as a function of film thickness of 130 and 180nm films.

Fig.4. Fatigue characteristics of BCT film of 180nm thickness.

( Annealed in furnace at 700oC in oxygen atmosphere )