FRAM 소자 응용를 위한 SrBi2Ta2O9 박막의 고밀도 플라즈마 식각

박진수,1 임연호,1 최창선,2 한윤봉1,*

1. 전북대학교 화학공학부, 2. 전북대학교 반도체과학기술학과

High Density Plasma Etching of SrBi2Ta2O9 Thin Films

for FRAM Application

Jin Soo Park,1 Yeon Ho Im,1 Chang Sun Choi,2  and Yoon-Bong Hahn1,*

1. School of Chemical Engineering and Technology,

2. Dept. of Semiconductor Science and Technology, Chonbuk National University, Chonju 561-756, Korea

INTRODUCTION

Recently, non-volatile memory devices have received much attention because they retain information when power is interrupted. The ferroelectric random access memories (FRAMs) require low operating voltage, fast switching speed, wide operating temperature range, and high radiation hardness.1-3 For the FRAMs application, ferroelectric materials such as Pb(Zr,Ti)O3 and Bi-layered structure oxides have been studied intensively. Among the ferroelcetric Bi-layered materials, the layered perovskite SrBi2Ta2O9 (SBT) is known as a promising candidate for FRAM device due to its excellent properties such as a high fatigue resistance against polarization switching up to 1012 cycles.1,3-5 In the fabrication of the non-volatile memory devices, dry etching of the ferroelectric thin films is an important issue for pattern transfer. However, to our knowledge, there are few studies on etching behavior of the ferroelcetric Bi-layered materials reported.

In this work, we examined plasma chemistries such as Cl2/Ar, Cl2/NF3/Ar and Cl2/NF3/O2/Ar for achieving practical etch rates for the SBT films in a planar-type inductively coupled plasma etcher. The influences of plasma chemistries and plasma parameters were analyzed in terms of the etch rate, the surface morphology, the crystal structure, and the polarization-electric field (P-E) loops.

EXPERIMENTAL

The SBT films were prepared on Pt(1500Å)/Ti(500Å)/SiO2/Si substrates using a radio frequency (rf) sputtering system equipped with a planar magnetron sputtering source. Etching was performed in a Vacuum Science ICP system (VSICP-1250A), in which the ICP source operated at 13.56 MHz. The ion energy was controlled by the applied rf

* Author for all correspondence. Email: ybhahn@moak.chonbuk.ac.kr.

chuck power at 13.56 MHz. Etch rates were obtained from stylus profilometry measurements.The surface morphology and crystalline structure were analyzed using atomic force microscope (AFM) and X-ray diffraction (XRD) patterns, respectively. Electrical characterization of the films was carried out using a Pt/SBT/Pt capacitor structure.

RESULTS AND DISCUSSION

Figure 1 shows the effect of ICP source power on etch rates of SBT films with different chemistries of 10sccm Cl2/10sccm Ar, 10sccm Cl2/10sccm NF3/5sccm Ar, and 10sccm Cl2/5sccm NF3/15sccm O2/5sccm Ar. During these experiments, the reactor pressure and the rf chuck power were held constant at 5 mTorr and 150 W. The Cl2/Ar plasma showed a gradual increase in etch rate with increasing the ICP source, while Cl2/NF3/Ar and Cl2/NF3/O2/Ar plasmas showed a maximum etch rate of ~1600 Å/min at 700 W. The increase in the etch rate with the ICP power is attributed to an ion enhanced etch mechanism. The etch rates of SBT films in Cl2/NF3/Ar and Cl2/NF3/O2/Ar plasmas obtained in this work is about six times higher than those reported previously.6,7

Figure 2 shows the AFM images of the surfaces of SBT films etched at 5 mTorr, 700 W source power and 150 W rf chuck power with Cl2/Ar, Cl2/NF3/Ar, and Cl2/NF3/O2/Ar discharges. Compared to the surface morphology of the unetched control sample (rms roughness 10.3 nm), Cl2/NF3/Ar and Cl2/NF3/O2/Ar plasmas resulted in fairly smooth morphologies of 4.1 and 7.5 nm, respectively. By contrast, Cl2/Ar plasma showed a rougher morphology of etched SBT surface (rms = 9.26 nm).

Figure 3 shows the P-E hysteresis loops of control and etched SBT films. It is seen that the as-grown or control sample exhibits typical ferroelectric hysteresis loop (remnant polarization, Pr = 9.04 □C/cm2; coercive field, Ec = 33.5 V/cm). The etched films indicated some substantial damage to electrical property, which may be attributed to the structural change of the etched samples, that is, no (200) peaks in XRD pattern (Fig. 2). All chemistries also showed a positive shift of the P-E hysteresis loops, compared to that of the control sample. This voltage shift might be attributed to defects such as trap charges at the interface between the ferroelectrics and electrodes. The ICP etching of SBT films with Cl2/NF3/O2/Ar showed overall a best P-E hysteresis loop compared to other chemistries, resulted in Pr = 6.13 □C/cm2 and Ec = 41.9 V/cm.

CONCLUSIONS

In conclusion, the SBT films were etched for achieving practical etch rates in a planar type inductively coupled plasma etcher with different chemistries of Cl2/Ar, Cl2/NF3/Ar and Cl2/NF3/O2/Ar. Cl2/NF3/Ar and Cl2/NF3/O2/Ar plasmas showed a maximum etch rate of ~1600 Å/min at 5 mTorr, 700 W ICP power and 150 W rf chuck power. These chemistries also resulted in better surface morphology than that of the control and Cl2/Ar-etched samples. The SBT films etched with Cl2/Ar and Cl2/NF3/Ar exhibited strong (115) peaks in XRD patterns. However, the SBT films etched with Cl2/NF3/O2/Ar showed new peaks of (110), (111) and (008), and the best P-E curve compared to the samples etched with Cl2/Ar and Cl2/NF3/Ar discharges.

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Figure 1. Effect of ICP source power on the etch rates with

different plasma chemistries.

Figure 2. XRD patterns of unetched control and etched SBT films with      different plasma chemistries at 5 mTorr, 700 W ICP, and 150 W rf.

Figure 3. P-E hysteresis loops of SBT films etched in different plasma           chemistries: (a) 10 sccm Cl2/10 sccm Ar, (b) 10 sccm Cl2/10            sccm NF3/5 sccm Ar, and (c) 10 sccm Cl2/5 sccm NF3/15               sccm O2/5 sccm Ar at 5 mTorr, 700 W ICP, and 150 W rf.