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Processing and dielectric propertiesof sol-gel derived bst thin filmsDanielle M. Tahan a , Ahmad Safari a & Lisa C. Klein aa Department of Ceramic Science and Engineering , RutgersUniversity , P.O. Box 909, Piscataway, NJ, 08855-0909Published online: 19 Aug 2006.

To cite this article: Danielle M. Tahan , Ahmad Safari & Lisa C. Klein (1997) Processingand dielectric properties of sol-gel derived bst thin films, Integrated Ferroelectrics: AnInternational Journal, 15:1-4, 99-106, DOI: 10.1080/10584589708015700

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lnreyrured Ferraeleczrics, 1997, Vol. 15, pp, 99-1 06 Reprints available directly from the publisher Photocopying permitted by license only

0 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands

under license by Gordon and Breach Science Publishers

Printed in India PROCESSING AND DIELECTRIC PROPERTIES OF SOL-GEL DERIVED BST THIN FILMS

DANIELLE M. TAHAN, AHMAD SAFARI, AND LISA C. KL,EIN Department of Ceramic Science and Engineering, Rutgers University, P.O. Box

909, Piscataway, NJ 08855-0909

(Received March I%, 19%; in final form June 15, I996)

Abstract The processing and dielectric properties of Ba$rl.,Ti03 (x=O.O-1 .O)

thin films prepared by a sol-gel method were investigated. The films were prepared using solutions consisting of acetate powders and titanium IV isopropoxide in a mixture of acetic acid and ethylene glycol. Processing parameters were optimized to develop stable solutions which yielded relatively low crystallization temperatures. Electron microscopy techniques were used to examine the grain size and microstructure of the thin films. The grain size was on the order of 50 nm and had a marked effect on the electrical properties of the films. Properties such as dielectric constant, dissipation factor and leakage current were also measured as a function of film thickness. A dielectric constant ranging from 200 to 625 was obtained for BST films with x = 0.6 over a thickness range of 100 to 900 nm. Leakage current densities of the films remained below 0.1 W c m 2 for extended time periods when measured at an applied field of 75 kV/cm.

INTRODUCTION

Today, with the ever increasing scale of integration, a need has arisen for new materials to replace conventional Si02/Si3N4 dielectrics as the storage medium in memory cells. Research has turned to the investigation of ferroelectric thin films for this purpose. Ferroelectric materials have a high dielectric constant which allows a given amount of charge to be stored in a smaller volume, and therefore promotes cell miniaturization.' Also, the necessity of complex stack and trench structures would be eliminated with the use of high dielectric constant materials because ultra-large scale integration could be achieved from planar structures.2

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Barium strontium titanate (BST) is an excellent material for dynamic random access memory capacitor cell applications due to its high dielectric constant, low dielectric loss, low leakage current and high dielectric breakdown strength.3 Also, the Curie temperature can be shifted out of the operating temperature range of DRAM operation by tailoring the Ba to Sr ratio, and therefore the electrical properties should remain relatively constant over this temperature range.

The main goal of this research is to study the processing of BST thin films of various compositions with properties sufficient for application in DRAM cells. Sol-gel was chosen as the preparation technique because it offers a homogeneous distribution of elements on a molecular level, ease of composition control, high purity, the possibility of low temperature processing, and the ability to coat large and complex area substrates.4 The electrical properties of the films with respect to processing conditions and microstructure were evaluated.

>

EXPERIMENTAL

BST solutions of 0.75 molar concentration were prepared by dissolving appropriate ratios of barium and strontium acetates in acetic acid under a nitrogen atmosphere. A stoichiometric amount of titanium IV isopropoxide was then added, followed by the addition of ethylene glycol in various proportions. Finally, the solution was heated to promote the condensation reaction between the acetic acid and the ethylene glycol, as follows:

HO-CH,-CH,-OH + CH3-COOH + HO-CH2-CH2-O-CO-CH3 + H20

Ba,Srl-,Ti03 thin films, where x was varied from 0 to 1, were prepared by spin coating the solutions onto Si (100) and PVTi/SiO,/Si (100) substrates* . The films were spun at 7500 RPM for 90 s and were immediately placed on a hot plate for drying and pyrolysis of organics. The films were then heat treated in air, at a temperature in the range of 500 to 700 "C. This process was repeated until the desired film thickness was achieved.

X-ray diffraction (XRD) profiles were obtained using a Ni-filtered CuKa radiation source to determine the crystallinity of the BST films. In preparation for electrical property measurement, a portion of the bottom electrode was exposed using a

* Silicon Quest International Incorporated, Santa Clara, CA

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DIELECTRIC PROPERTIES OF BST THIN FILMS [375]/101

chemical etch. Film thicknesses were measured using a Tencor Alpha-Step 200 profilometer. Pt top electrodes with a diameter of 0.6 mm were dc sputtered onto the surface of the films which were coated on the Pt/Ti/SiO2/Si substrates. The dielectric properties were measured using a Hewlett Packard 4194A Impedance/Gain-Phase Analyzer. Capacitance was measured at 1 kHz while sweeping through a bias voltage to obtain the capacitancdvoltage characteristics of the films. Leakage current was measured as a function of applied dc voltage and time using a Keithley 617 electrometer.

RESULTS AND DISCUSSION

Comparison of BST solutions prepared with varying ratios of acetic acid to ethylene glycol resulted in the observation that the acetic acid aided in the dissolution of the acetates, while the ethylene glycol stabilized the solution to precipitation. The optimum ratio was 3:1 acetic acid to ethylene glycol, in terms of solution stability, film uniformity and film crystallization temperature. In addition to increasing the stability of the BST solutions, ethylene glycol aided in decreasing the crystallization temperature of the films. The XRD patterns in figures la and b illustrate the effect of ethylene glycol on the crystallization of BST films. Films made from solutions with the ethylene glycol addition began to crystallize at 500 OC, which is 100 "C lower than that for the films without ethylene glycol.

Figures 2a, b, and c are field emission scanning electron micrographs of BST(xd.8) films which were heat treated to 500, 600, or 700 "C for 1 hour,

3

20 Z4 28 32 36 40 44 4

TWO-'MBTA (Dqrra)

FIGURE 1 from a) acetic acid and b) acetic acidethylene glycol solutions.

XRD patterns of films with various annealing temperatures made

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respectively. The film heat treated to 500 "C showed no evid,ence of grain structure. Upon heating to 600 "C, an extremely fine grain structure is evident, with grains on the order of 25 nm. An additional increase in the annealing temperature to 700 "C doubled the size of the grains to approximately 50 nm.

FIGURE 2 FESEM micrographs of BST(x=0.8) films bleat treated to a) 500, b) 600 and c) 700 "C for 1 hour.

TEM analysis of BST(x=0.7) and strontium titanate film surfaces was performed. Figures 3a and b are pictures taken at the same magnification for the two compositions, respectively. The BST film exhibited a grain s,ize which was in good agreement with the FESEM micrograph in Figure 2c. On the other hand, the grain size of the strontium titanate film was approximately 1.5 time larger, as seen in Figure 3b.

FIGURE 3 surfaces.

TEM pictures of a) BST(x=0.7) and b) strontium titanate film

The variation in the dielectric constant and the dissipation factor of BST(x=0.8) thin films with annealing temperature is illustrated in Figure 4. The films were heated to 500, 600 or 700 "C for 1 hour after spinning, and the dielectric measurements were taken at 1 kHz. The dielectric constant increased significantly with increasing annealing temperature, while the dissipation factor increased only slightly from 0.025 to 0.04 over that annealing temperature range. These results are consistent with the FESEM analysis, with an increase in grain size contributing to the increase in the dielectric constant of the films.

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DIELECTRIC PROPERTIES OF BST THIN FILMS I377111 0 3

Figure 5 plots the dielectric constant and the dissipation factor of BST(x=0.6) films with various thicknesses, annealed at 700 "C. It can be seen that the dielectric constant increases from 200 to 625 over thicknesses of 100 to 900 nm. The dissipation factor also increased with film thickness, with values between .04 to . I over the thickness range tested.

Dielectric constant measurement was performed on the BST films as a function of temperature. Measurements taken at 1, 10, and 100 kHz were not significantly different, and therefore only measurements at 1 kHz are plotted in Figure 6. For all compositions, the curves were relatively flat, with an increase in peak curvature with increasing strontium content. The addition of strontium to the barium titanate films increased the dielectric constant, with a maximum peak dielectric constant of 470 for the BST(x=0.6) composition at a corresponding peak temperature of -65 "C. A

maximum room temperature dielectric constant of 440 was obtained for the BST composition of x a . 7 5 . This result is similar to that found for single crystal or bulk BST, for which it is reported that the maximum room temperature dielectric constarlt is achieved for a BST composition of x=0.7.5

The dielectric constant versus temperature curves in Figure 6 are very flat as compared to those of the corresponding bulk and single crystal compositions. For example, the curve for the barium titanate film (BST(x=l .O)) does not exhibit a peak at 0 OC which corresponds to the temperature of the orthorhobic to tetragonal phase transition for single crystal barium titanate. Even upon performing the measurement to

'F 500 600 Annealing Temperature ( ) 0 500 lo00

Film Thickness (nm) Figure 4 Figure 5

FIGURE 4 temperature for BST(x=0.8) films with a thickness of approximately 400 nm.

FIGURE 5 BST(x=0.6) films.

Dielectric constant and dissipation factor versus annealing

Dielectric constant and dissipation factor versus thickness for

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higher temperatures about the Curie temperature, a dielectric peak was not observed. These results have been attributed to the nanoscale grain size of the BST films.6

A peak dielectric constant can be seen in the curves corresponlng to the films with a higher strontium content. The temperature of the peak dielectric constant decreases with increasing addition of strontium as does the Curie temperatures of single

250 d d -75 4 0 -5 30 65 100

Temperature ("C)

FIGURE 6 Dielectric constant measured at 1 kHz versus temperature for films 400 nm thick.

crystals of the same compositions, although the temperatures of maximum dielectric constant of the films are lower than the Curie temperatures of single crystals of the respective compositions. This is again expected to be the result of the very fhe grain size of the films.5

High leakage currents are detrimental for DRAM capacitor operation in that high leakage currents cause cells to require more frequent refreshing, use more power, and limit the maximum field that may be applied to the device. It is therefore important to characterize the leakage current of thin film capacitors with respect to applied voltage, and also time held under an applied electric field. Figure 7 plots the leakage current density versus applied voltage for the BST(x=0.6) films 'of various thicknesses. The leakage current decreased with increasing film thickness at higher voltages, with values of .09 and .02 pA/cm* for film thicknesses of 220 and 880 nm, respectively, under an applied voltage of 3 V. These values are below the values reported necessary for a practical 256 Mb DRAM.3

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DIELECTRIC PROPERTIES OF BST THIN FILMS [3791105

Leakage current deasity variations with time under an applied voltage of 3 V for BST(x=0.6) films of various thicknesses are illustrated in Figure 8. Again, the current density decreases with increasing film thickness. Also, the current density for the 220 nm film increases with time, while for the thicker films the current density first decreases with time and then levels or only slightly increases.

Another important characteristic necessary for DRAM operation is a high charge storage density of the capacitor film. A value on the order of 40 fC/pm2 has been reported to be required for a practical 256 Mb DRAM with a film thickness of 200

F 10

p 0.1

$ 1 3. W

.l j 0.01

! O-Oo1 .a o.oO01 U

0 1 2 3 4 5 6 Voltage (V) F i 7

0.M 1.00 2.00 3.00 Log Time (log s)

Figure8

FIGURE 7 Current density versus applied voltage for BST(x=0.6) films.

FIGURE 8 Current density versus time at a voltage. of 3 V for BST(x=0.6) films.

nm.3 The film capacitance was measured at 1 kHz as a function of applied voltage which was swept from - 10 V to I0 V, and then reversed. This data was used. to calculate the charge storage density of the film using the equation Q, = E&E, where Q, is the charge storage density, E, is the permittivity of free space, E is the relative permittivity, and E is the applied electric field. Figure 9 plots the charge storage density versus the applied electric field for a BST(x4.8) thin film with a thickness of approximately 400 nm. The Qc varied from 17 to 52 fUpm2 for an applied field of 50

to 250 kV/cm.

CONCLUSIONS

BST films of various compositions were prepared by a sol-gel spin coating technique. Ethylene glycol was a necessary component of the solutions to increase solution

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0 50 100 150 200 250 300 Electric Field (kVlcm)

FIGURE 9 Charge storage density versus electric field for a BST(x=0.8) film with a thickness of approximately 400 nm.

stability to precipitation and to decrease the crystallization temperature of the films. The grain size of the films ranged from 20 to 50 nm depending on the processing conditions and had a marked effect on the dielectric properties of the thin films. The dielectric constant and the dissipation factor were found to increase with film thickness, with a maximum room temperature dielectric constant of 440 and a dissipation factor of .04 for the BST(xd.75) composition. The leakage current density was determined to be below the maximum allowable limit for a practical 256 Mb DRAM capacitor when measured at an applied voltage of 3 V, and the charge storage density was sufficient under an applied field of 125 kV/cm.

t

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3 [ 6 ] 690 (1991). 3 D. Roy and S. B. Krupanidhi, ADD^. Phvs. Lett., a[ 101 1056 (1993). 4 G. Yi and M. Sayer, Ceramic Bulletin, m[7] 1173 (1991). 5 T. Horikawa, N. Mikami, T. Makita, J. Tanimura, M. Kataoka, K. Sato, and M.

Nunoshita, Jpn. J. ADD1. Phys .,24126(1993). 6 H. Frey and D. A. Payne, &pi. Phv s. Lett., Q[20] 2753 (1993).

Abt, EEE Transach 'ons On Ultrasonics. Ferroelectrics. And Frequency Con trol,

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