새로운 유기금속 전구체를 이용한 BST 박막의 화학증착

이정현, 박만영, 심재용, 이시우

포항공과대학교 화학공학과

Chemical Vapor Deposition of Barium Strontium Titanate Thin Films using New Organometalic Precursors

Jung-Hyun Lee, Man-Young Park, Jae-Young Shim, and Shi-Woo Rhee

Laboratory for Advanced Materials Processing(LAMP)

Department of Chemical Engineering, POSTECH

Introduction

In recent years, many kinds of dielectric thin films with a high dielectric constant have been studied for dielectrics of dynamic random access memory (DRAM) capacitors.(1-3) Among these materials, barium strontium titanate [(Ba1-xSrx)TiO3, BST] is the most promising material because of its higher dielectric constant, its paraelectric phase with an appropriate ratio of Ba / Sr composition, and its stable crystal structure.

BST thin films may be deposited by several methods. Metalorganic chemical vapor deposition (MOCVD) is unique in that it offers good conformality and scalability and can be carried out at higher effective oxygen partial pressure than physical vapor deposition (PVD) methods.(4-6)  Recently, there has been keen interest in the development of MOCVD precursors for BST thin films.

The success of MOCVD process depends critically on the volatility and stability of the precursor material.  Until recently, 2,2,6,6-tetreamethylheptane-

3,5-dione (thd) is a common ligand for Ba and Sr MOCVD precursors but this ligand has many disadvantages such as the decreased volatility and low thermal stability of the source near the sublimation temperature.

In this paper, in order to improve the thermal stability and the volatility of precursor, (Ba,Sr)TiO3 thin film was deposited on Pt substrate using new MOCVD precursors, Ba(methd)2 (methd = methoxyethoxytetramethylheptane-

dionate), Sr(methd)2, and Ti(mpd)(thd)2 (mpd = methylpentanediol, thd = tetramethylheptanedionate).

Experimental prodecures

BST thin films were deposited by MOCVD on Pt/TaOx/SiO2/Si wafers at 520℃ using a cold wall CVD reactor.  Ar carrier gas was used at a flow rate of 30 sccm and the precursors were injected via a liquid flow meter into a vaporizer. The mixture of the precursors and carrier gas entered the reactor through a vaporizer. Fig. 1 shows the molecular structures of Ba(methd)2, Sr(methd)2, and Ti(mpd)(thd)2. These precursors were supplied by Asahi Denka Co. With one of the butyl groups of ligand replaced with methoxy and ethoxy groups, spatial shield of Ba and Sr metals increases the stability of the precursor. The deposition rate was calculated from the increase in weight of the substrate and confirmed by cross-sectional scanning electron microscopy. Composition of the film was determined by Rutherford backscattering spectroscopy (RBS, NEC 3SDH) at 2,240MeV and the surface morphology and step coverage were observed using scanning electron microscopy (SEM, JEOL JSM-840A). Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the metal/insulator/metal (MIM) devices were measured using a HP4192A impedance analyzer and a HP4140B semiconductor parameter analyzer, respectively.  Aluminum electrodes were deposited for the upper electrode in a vacuum system.

Results & discussion

Fig. 2 shows the mass signals of the gas species obtained from Sr(methd)2 under isothermal conditions of 200℃ as a function of the retention time.  After 20 seconds, major peaks are observed at m/e 57 and 127 which can be assigned to +C(CH3)3 and +OC2COC(CH3)3, and the intensity of these peaks represents the free ligands and their cracked species.  After 180 seconds, m/z 345 and 948 peaks which can be assigned to +Sr(methd)2, and +Sr2(methd)3 were detected.  These peaks represent the metal organics and their oligomers.

Also similar results were obtained from Ba(methd)2, although the intensity of peaks were slightly different.  There are some dimeric forms detected with small peak intensity but no trimeric peak are observed.

The RBS spectrum of the BST film deposited at 520℃ is shown in Fig. 3. The spectrum was obtained using a collimated 2.240MeV beam of 4He2+ particles.  If the film thickness of BST layer is smaller than that of Pt layer, the peaks which can be assigned to Ba, Sr, and Ti metals were not detected.  We used the BST layer thickness of 6800Å(Pt 1300Å) and the compositional ratios of BST film were 0.5, 0.5, 1.2, and 4.4 for Ba, Sr, Ti, and O elements, respectively.  TaOx was not detected because RBS depth resolution range was from 50 to 300Å.

Fig.4 shows the planar and cross-sectional SEM images of BST films grown on Pt substrate at 520℃.  Control of grain structure is essential to improve the electrical properties such as leakage current, breakdown voltage, and reliability of these films for application to DRAMs.  The BST film deposited at 520℃ had a hillock-free, dense, and uniform microstructure with grain size below 100nm.  As shown in Fig. 4(b), the step coverage of a BST film was very excellent and the coverage dmin/dmas(dmin and dmax are film thickness on the side wall near the bottom and the top surface, respectively) was about 0.5.

A plot of small signal capacitance vs. bias voltage (C-V), shown in Fig. 5(a), represents double peaks corresponding to the hysteresis loop.  With increasing deposition temperature for Pt bottom electrode, the dielectric constant of BST films was increased.  The dielectric constant of a BST film deposited on Pt substrate at 520℃ was 295.

Fig. 5(b) represents current-voltage (I-V) characteristics of a BST film deposited at 520℃. The shape of current-voltage changed with the deposition temperature and the electrode materials.  With the 120nm BST film deposited at 520℃, the leakage current densities at 2.0V was about 2.0 ⅹ10-7 Acm-2.

Conclusion

The step coverage and electrical properties of BST thin film deposited on Pt/TaOx/SiO2/Si substrate by metalorganic chemical vapor deposition using Ba(methd)2, Sr(methd)2, and Ti(mpd)(thd)2 have been investigated. The BST film deposited at 520℃ had a hillock-free, dense, and uniform microstructure with grain size around 100nm and the step coverage of the BST film was very excellent.  The 120nm BST thin film deposited on Pt substrate at 500℃ has a dielectric constant of 295 and a leakage current density at 2.0V of about 2.0 ⅹ10-7 Acm-2.

References

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Fig. 1 The molecular structures of Ba(methd)2,

Sr(methd)2, and Ti(mpd)(thd)2.

Fig. 2 The mass signals of the gas species

obtained obtained from Sr(methd)2 under

isothermal conditions of 200℃ as a

function of retention time

(a) after 20 seconds (b) after 180 seconds

Fig. 3 The RBS spectrum of a BST thin

film deposited at 520℃. The line is

a computer-simulated spectrum.

Fig. 4 The planar and cross-sectional SEM images

of BST films grown on Pt substrate at 520℃.

Fig. 5 Capacitance-voltage (C-V) and current-voltage

(I-V) curves of the BST film deposited at 520℃

(Al/120nm-BST/Pt/TaOx/SiO2/Si)