Electrical properties of PZT thin films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition

MAC-AND

PHYSICS

Materials Chemistry and Physics 45 (1996) 155-158

Materials Science Communication

Electrical properties of PZT thin films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition S.T. Kim a, J.W. Kim a, S.W. Jung ‘, J.S. Shin a, S.T. Ahn b, W.J. Lee a aDepartment ofMater&

Science and Engineering, hSemiconductor

Korea Advanced Institute of Science and Technology,

R&D Center, Sumsung Elecrronics

Taejon, South Korea

Co., Suwon, South Korea

Received 19 December 1994: accepted 8 September 1995

Abstract Ferroelectric Pb(Zr,Ti)03 thin films were successfully fabricated on Pt-coated Si substrates by the electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) method using metal-organic (MO) sources. Perovskite structures with well-developed crystalline grains are obtained at a substrate temperature of 500 “C. These PZT films, with thicknesses of about 1000 A, show high charge storage densities (P,,,,, - P, = lo-15 PC cm - 2 for 1.5 V operation) and low leakage current densities ( = 10m6 A cm-* at 1.5 V). The effects of the Zr/Ti concentration ratio in the film and the rapid thermal annealing on the electrical properties of the films were also studied. Keywords:

PZT thin films; Electrical properties; Ferroelectric thin films

1. Introduction

522 -

One of the major issues in 256 Mbit and higher density dynamic random-access memory (DRAM) is to obtain an adequate amount of capacitance in a small cell area [ 1,2]. To overcome this problem, three-dimensional capacitor structures, such as microvillus patterning (MVP) and cylinder structures, and the use of high dielectric materials, including Ta,O,, BST and PZT, have also been investigated [371. In this study, we have fabricated PZT thin films for a charge storage capacitor of a DRAM device by the electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) method. Because ECR PECVD is performed at a lower pressure than other CVD processes, homogeneous reactions are greatly inhibited. In addition, the high density of the ECR plasma makes it possible to deposit high quality films with good stoichiometric controjlability even at a low temperature. We have obtained 1000 A level PZT films having good crystallinity and stable electrical properties utilizing the ECR PECVD method at a deposition temperature of 500 “C. The effects of the Zr/Ti concentration ratio in the film and the post-annealing on the electrical properties of the films were investigated. 2. Experimental A schematic diagram of the ECR PECVD system is shown in Fig. 1. The 2.45 GHz microwave radiation generated from 02%0584/96/$15.00

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1.Schematic diagram of the ECR PECVD system.

a magnetron is introduced through a waveguide into the quartz bell jar discharge chamber. ECR plasma is generated when the orbital frequency of the electrons in the magnetic field is the same as the microwave frequency. PZT films were deposited at 500 “C on Pt(70 nm) /Ti ( 100 nm) /SiO,( 600 nm) ISi substrates. The precursors used were lead @diketonate (Pb(DPM),, Pb( C, rHr902) 2), titanium iso-propoxide (TiIP, Ti(i-OC,H,),), zirconium t-butoxide (ZrTB, Zr(tOC,H,),) and 0,. The flow rates of O2 and Ar (carrier gas of metal-organic (MO) sources) were controlled by mass flow controllers (MFCs) . The flow rates of MO sources were calculated from the equilibrium vapor pressures (P,) of the

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Chemistry and Physics 45 (1996) 155-158

sources and the conductances of the fine metering valves. Pb( DPM) 2, ZrTB and TiIP were introduced independently into the reaction chamber by passing Ar gas through each MO source bubbler, where a constant bubbling temperature was maintained: Pb( DPM)* at 155 “C (Peg = 0.86 torr), ZrTB at 70 “C (P, = 2 torr) and TiIP at 85 “C (P_, = 1.73 torr) (measured experimentally in this laboratory, and Ref. [ 81) . The deposition parameters used in this study are summarized in Table 1. The structural phases of the PZT thin films were characterized using an X-ray (Cu Ka) diffractometer (XRD) . The chemical compositions of the films were analyzed by wavelength dispersive X-ray spectroscopy (WDS) . The characTable 1 Conditions

for the deposition of the PZT films by ECR PECVD

Substrate Source

PtlTi/SiOz/Si

02 Pb(DPM), ZrTB TiIP Substrate temp. Process pressure Microwave power

29 seem 0.43 seem 0.10 seem 0.12-0.22 seem 500 “C 3 mtorr 200 w

Table 2 Concentration ratios and thicknesses “C by ECR PECVD Sample no.

Concentration

3. Results and discussion

of the PZT thin films prepared at 500

ratio in the film

Film thickness (A)

I 2 3

Pb/(Zr+Ti)

Zr/Ti

0.98 0.98 0.98

0.35 0.57 0.63

teristic X-rays detected from WDS were Pb Mo (2.346 keV) , Zr La (2.042 keV) and Ti Mo (4.511 keV). Rutherford back-scattering spectroscopy (RBS) was used to calibrate the chemical compositions analyzed with WDS. The thicknesses of the films were measured by a high resolution scanning electron microscope (SEM) . A MIM structure was constructed to measure the electrical characteristics of the films. The Pt film (2000 A thick) deposited by the r.f. magnetron sputtering method was used as the upper electrode. The leakage current (Z-V) was measured with an HP 4145B semiconductor parameter analyzer by applying a positive bias to the upper electrode. The measurement conditions were as follows: a holding time of 10 s, a delay time of 5 s, and a poling voltage that increases with a step voltage of 0.1 V. The capacitance of the films was measured with an HP 4192A LCR meter by varying the a.c. frequency. The P-E hysteresis was obtained by using an RT66A ferroelectric tester in a virtual ground mode.

1100 1000 900

Table 2 shows the chemical compositions and thicknesses of the PZT films used in this study. The Pb/ (Zr + Ti) ratios for all films are close to unity. The Zr/Ti ratios of the three films were 0.35, 0.57 and 0.63, r:spectively, and the thicknesses were 1100, 1000 and 900 A, respectively. The XRD patterns obtained from the films of Table 2 are shown in Fig. 2. Perovskite single phase was obtained for all of them. As the Zr/Ti ratio decreases, peak separations at about22” ((OOl), (100)) and44.5”( (002), (200)) become prominent because the characteristics of a tetragonal structure become stronger. Fig. 3 shows the dependence of the leakage current of the films on the Zr/Ti ratio. At an applied field of 0.2 MV cm-’ 104

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Chemistry and Physics 45 (1996) 155-158

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caused by the charges trapped in defects. Therefore we think that the defects in the as-deposited PZT film are diminished by the 650 “C RTA treatment. The increase in the leakage current for the specimen RTA-treated for 10 min is thought to result from the microcracks caused by the thermal stress between the fully crystallized PZT film and the substrate. Fig. 7 shows the C-Vcharacteristics for the films with and without RTA treatment. The C-V curve of the as-deposited PZT film is shifted toward negative field. However, the curve of the PZT film RTA-treated for 10 min has a symmetrical shape with respect to 0 V and the capacitance is increased by

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Frequency ( kHz ) Fig. 4. Dielectric constant of PZT films as a function of signal frequency. The ax. amplitude is 1.OV; the d.c. bias is 0 V. ( VW = 2 V), the leakage current density is about 10e6 A cm-*. As the Zr/Ti ratio increases from 0.35 to 0.63, the leakage current characteristics improve and the ‘take-off point’ of the leakage current curve appears at higher electric field. The dielectric constants of the films were measured with various signal frequencies. The dielectric constants of the films whose Zr/Ti ratios were 0.35-0.63 are above 500 at all frequencies from 10 Hz to 1 MHz, as shown in Fig. 4. It has been reported that as the Zr/Ti ratio increases to the morphotropic phase boundary (Zr/Ti = 1.13)) the dielectric constant increases [ 91. On the other hand, it is also known that as the thickness of the film decreases below a certain value, the dielectric constant decreases rapidly [ lo]. Among the three films in Fig. 4, sample no. 2, which has a Zr/Ti ratio of 0.57 and a thickness of 1000 A, shows the highest dielectric constant. This is considered to result from the combined effects of both the film composition and the film thickness. Fig. 5 shows the hysteresis loop for a 1.5 V operating voltage. The charge storage densities (P,,, - P,) of the films are lo-15 pC cm-*, which are large enough to meet the requirements of 256 M DRAMS (4.2-8.3 p,C cm-*) [ 111. The effects of post-annealing treatment on the deposited PZT films were also investigated. Annealing was conducted in ambient atmosphere at 650 “C for 1 and 10 min using a rapid thermal annealing (RTA) method. The RTA-treated film was sample no. 3, which has a ZrlTi ratio of 0.63 and a thickness of 900 A. The leakage current characteristics of the RTA-treated films were measured and are shown in Fig. 6. After 1 min of RTA treatment, the humps that appeared in the Z-V curve of the as-deposited PZT films are diminished and the ‘take-off point’ is shifted to higher electric field. After 10 min of RTA treatment, the humps have disappeared completely but the leakage current at above 0.2 MV cm-’ increases. The humps of the Z-V curves are considered to be

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S.T. Kim et al. /Materials Chemistry and Physics 45 (1996) 155-158

158 7

4. Conclusions

.,.,.,.,,(.,.,.,.,.

.

without RTA i PZT films ( = 1000 A thick) that had perovskite single phase were prepared at 500 “C by the ECR PECVD method. The leakage current densities are about lop6 A cm-’ at 1.5 V. As the Zr/Ti ratio increases, the leakage current characteristics improve and the ‘take-off point’ in the Z-V curve appears at higher electric field. The dielectric constants of the films are 500-1200 for all frequencies from 10 Hz to 1 MHz. The charge storage densities (P,, - P,) for a 1.5 V operating voltage are 10-l 5 pC cm-‘. Appropriate RTA treatment may enhance the leakage current and capacitance characteristics by reducing trapped and fixed charges in the PZT film.

Acknowledgements

Applied Voltage (V)

The tronics Co.,

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Fig. 7. Changes in the C-V characteristics of the PZT film (sample no. 3) after RTA treatment at 650 “C.

References [ 11 T. Ema, S. Kswanago, T. Nishi, S. Yoshida, H. Nishibe, T. Yabu, Y. Kodama, T. Nakano and M. Taguchi, Tech. Digest, International

20%. In general, the shift ( 6V) of the C-V curve is caused by the fixed charges in the film and the difference in work functions between the upper and lower electrodes:

Electron Devices Meeting (IEDM), Dec. 11-14, 1988, San Francisco, CA, USA, p. 592. [2] N. Shinmura, S. Kakimoto, K. Iguchi, K. Uda and J. Takagi, 22nd Solid State Devices and Materials (SSDM). August 22-24, 1990, Sendai, Japan, p. 833. [3] J.H. Ahn,Y.W. Park, J.H. Shin,S.T.Kim, S.P. Shin, S.W.Nam, W.M.

[4] [5]

where V& and V;, are the voltages at the maximum capacitance ( C,,) in a positive bias and in a negative bias, respectively, &,,i and & are the work functions of the upper and lower electrodes, respectively, and Q, is a fixed charge density. The difference in work functions between the upper and the lower electrodes can be ignored, because the same material (Pt) was used for the electrodes. From Eq. (2), the negative shift of the C-V curve is caused by negative fixed charges. Therefore we think that the RTA treatment reduces the negative fixed charges, such as Pb vacancies, at the Pt/ PZT interface.

[ 61

[7]

[8]

Park, H.B. Shin, C.S. Choi, K.T. Kim, D. Chin, O.H. Kwon and C.G. Hwang, Tech. Digest, Symp. on VLSI Technology, May 26-28, 1992, Seattle, Washington, USA, p. 12. D. Temmler, Tech. Digest, Symp. on VLSI Technology, May 28-30, 1991, Oiso, Japan, p. 13. K. Sagara, T. Kure, S. Shukuri, J. Yugami, N. Hasegawa, H. Shingiki, H. Goto, H. Yamashita and E. Takeda, Tech. Digest, Symp. on VLSI Technology, May 26-28, 1992. Seattle, Washington, USA, p. 10. S. Hayashi, M. Huffman, M. Azuma, Y. Shimada, T. Otsuki, G. Kano, L.D. McMillan and C.A. Paz de Araujo, Tech. Digest, Symp. on VLSI Technology, June 7-9, 1994, Honolulu, Hawaii, USA, p. 153. J.W. Kim, S.T. Kim, SW. Chung, J.S. Shin, K.S. No, D.M. Wee and W.J. Lee, edited by K. Barmak, M.A. Parker, J.A. Flora, R. Sinclair and D.A. Smith, Mater. Res. Sot. Symp. Proc., 343 (1994) 493. T. Nakai, T.T. Tabuchi, Y. Sawado, I. Kobayashi and Y. Sugimori,

Jap. J. Appl. Phys., 31 (1992) 2992. [9] B. Jaffe, R.S. Roth and S. Marzullo, J. Appl. Phys., 25 (1954) 809. [lo] T. Hase, T. Sakuma, Y. Miyasaki, K. Hirata and N. Hosokawa, Jpn. J. Appl. Phys., 32 (1993) 4061. [ 1l] S.C. Lee, Ph.D. Thesis, The University of Arizona, Tucson, AZ, 1993.