플라즈마-MOCVD를 이용한 PbTiO3 박막의 전기적, 구조적 특성에 대한 급속열처리의 영향

김 종 화, 한 윤 봉

전북대학교 화학공업공학부

Effect of Rapid Thermal Annealing on the Structural and Electrical Properties of PbTiO3 Thin Films Prepared by Plasma Enhanced MOCVD

Jong-Wha Kim and Yoon-Bong Hahn

School of Chemical Engineering and Technology, Chonbuk National University

664-14 Duckjin-Dong 1 Ga, Chonju 561-756, Korea

INTRODUCTION

Lead based ferroelectric materials such as PbTiO3, PZT and PLZT have attracted great interest in recent years because of their unique set of electrical and optical properties. These properties are being utilized in applications to ULSI memory devices, integrated optical devices, piezoelectric devices and infrared devices.1,2

In this work, to study the structural and electrical properties of PbTiO3 thin films, films were deposited on p-type Si(100) substrate by plasma enhanced MOCVD using Ti(O-i-C3H7)4, Pb(tmhd)2 and oxygen. The as-deposited PbTiO3 films were treated by rapid thermal annealing (RTA) method in oxygen ambient. The effects of annealing conditions on the properties of PbTiO3 films were investigated in terms of crystallinity, microstructure, leakage current and dielectric constant.

EXPERIMENTAL

In experiments, the home-made PEMOCVD and RTA equipments were used.3 PbTiO3 films were deposited on the Si substrate using Ti(O-i-C3H7)4 (99.999%), Pb(tmhd)2 (99.99%) and oxygen (99.999%). The bubbler temperatures were controlled to 60℃ and 175℃ for Ti(O-i-C3H7)4 and Pb(tmhd)2, respectively. Metalorganic vapors were introduced into the reaction chamber by passing Ar gas through the bubblers, and the stainless steel feed lines were heated to 220℃ to prevent the condensation of the vaporized reactants. The substrates were placed on the susceptor which contains a resistive heater with a PID controller. Ti(O-i-C3H7)4, Pb(tmhd)2 and O2 were simultaneously introduced into the chamber to start the deposition of PbTiO3. As-deposited PbTiO3 films were annealed in the RTA system in oxygen ambient.

The physical and chemical properties of the film were measured by using □-step, high resolution SEM, EDS, X-ray diffraction system. Electrical properties were measured using Al/PbTiO3/Si (MOS) structure. The current density vs electric field (I-V) characteristics were measured using a HP4145 picoam-meter. The the dielectric constant was calculated using the film thickness and the capacitance measured with an LCR meter at 1 MHz.

RESULTS AND DISCUSSION

Dielectric PbTiO3 thin films were successfully prepared on p-Si(100) substrate by PEMOCVD using high purity Ti(O-i-C3H7)4, Pb(tmhd)2 and oxygen. The deposition rate increased with increasing substrate temperature and RF power. The as-deposited films showed PbO structure at 350∼400℃, but (100) and (101) PbTiO3 orientations started to appear at 450℃.

In order to study the effect of annealing conditions, the RTA treatment was carried out with varying annealing time, temperature and oxygen flow rate. First, the effect of post-annealing time on the crystallinity of the PbTiO3 films was analyzed for the films deposited for 40 minutes at 450℃ and 1.08 W/cm2. Figure 1 shows the XRD patterns of the PbTiO3 films annealed at 700℃ with oxygen flow rate of 2.0 mol/min under variation of annealing time from 10 to 120 s. The PbO peak was still observed for the film annealed for 10 s, but it was disappeared and instead the PbTiO3 peaks were observed after 60 s or longer annealing time. It is also seen that the peak intensity of (001) orientation increased with increasing the annealing time from 60 to 120 s. Figure 2 shows the scanning electron microscopy (SEM) photographs of the films annealed with varying the annealing time. The PbTiO3 film annealed for 60 s showed densely packed spherical grains of 0.06 □m diameter. However, the microstructure of the film annealed for 120 s consisted of relatively large grains and nonuniform surface, and had white grains of 0.083□m diameter that represented the Pb diffused out to the surface due to volatilization. It was also found that the films annealed for longer than 60 s showed poor leakage current as shown in Fig. 2. This results indicate that the optimum annealing time is 60 s for the PbTiO3 thin films.

Annealing the oxide films in oxygen ambient is generally accepted, but the effect of oxygen concentration is little known especially for the case of rapid thermal annealing of PbTiO3 films. Figure 3 shows the effect of oxygen concentration on the crystallinity of the PbTiO3 films annealed at 700℃ for 60 s. The oxygen molar flow rates of 0.05, 0.5 and 2.0 mol/min are equivalent to the RTA chamber pressures of 0.1, 1 and 4 Torr, respectively. It is seen that the crystallinity of perovskite PbTiO3 increases as oxygen flow rate increases. This result reveals that the concentration of oxygen plays an important role in crystallization of the PbTiO3 thin films. It is believed that oxygen diffuses from the ambient to the surface, makes up for the oxygen deficiency in the film, and consequently enhances the crystallinity of the PbTiO3 film.

Figure 4 shows the effect of annealing temperature on the crystallinity of the PbTiO3 thin films annealed for 60 s with oxygen flow rate of 2.0 mol/min. The XRD patterns showed that the intensity ratio of (001) to (100) reflections increases with annealing temperature and the PbO peak disappeared at 600℃ or higher temperature. It is also interesting to note that the (200) peak was observed for the films annealed at 700℃ or higher temperature. The leakage currents of annealed PbTiO3 films were shown in terms of I-V characteristics in Fig. 5. The leakage current density increased with increasing the annealing temperature. This poor leakage with annealing temperature is believed mainly due to the presence of defects caused by the vaporization of lead at higher temperature. It may be also due to the interface layer such as lead silicate formed during the high temperature annealing.

The dielectric constants were also measured for the films annealed at various temperatures. As shown in Fig. 6, a linear relationship was found between the dielectric constant and the RTA temperature. In general, an interface layer of lead silicate growing at high temperatures reduces the value of dielectric constant of the films. However, as shown in Fig. 4, no peaks of lead silicate appeared at high temperatures, implying that the amount of lead silicate formed is negligibly small; instead stronger peaks of (001) and (101) orientations were obtained with higher annealing temperature. These results indicate that the increased intensity of (001) and (101) peaks gains the predominating influence on the dielectric constant, and results in a linear relationship between the dielectric constant and the RTA temperature. However, it is worthy to note that greater dielectric constant with higher annealing temperature does not necessarily mean lower leakage current density because of the presence of defects caused by volatilization of lead and the interface layer formed during the high temperature annealing.

REFERENCES

1. M. Shimizu, M. Sugiyama, H. Fujisawa, and T. Shiosaki,Integrated Ferroelectrics, 6, 155(1995).

2. B. S. Kwak, E. P. Boyd, and A. Erbil, Appl. Phys. Lett., 53, 1702(1988).

3. I.-S. Lee, J.-W. Kim, C.-J. Youn, S.-K. Park, Y.B. Hahn, Korean J. Chem. Eng.,13, 473(1996).

Fig 1. XRD patterns of the PbTiO3 films annealed at 700℃

with varying annealing time.

10s                        60s	120s

Fig 2. SEM photographs of the PbTiO3 films annealed at 700℃ with varying

annealing time.

Fig 3. Effect of Oxygen concentration on   Fig 4. Effect of annealing temperature on

crystallinity of the PbTiO3 films            crystallinity of the PbTiO3 films.

annealed at 700℃.                         (Oxygen flow rate of 2 mol/min)

Fig 5. Effect of annealing temperature on        Fig 6. Dielectric constants of the PbTiO3 films

I-V characteristic of the PbTiO3 films.           annealed at various temperatures.