Effect of Film Composition on Low Temperature Processing of SBT Deposited by MOCVD
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149 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
EXPERIMENT SBT thin films of various compositions were grown on sputtered Pt(100nm)/Ti(1Onm)/SiO 2/Si substrates by a metalorganic chemical vapor deposition (MOCVD) process described previously [5]. The substrates were annealed in oxygen ambient prior to SBT deposition. Starting from a film composition of Sr/Bi/Ta = 0.85/2.20/2.00 the Sr and Bi contents were varied systematically from x=0.55 to 1.00 and y=1.95 to 2.50, respectively. Variables x and y equal the Sr and Bi content in the film when Ta content is fixed at 2 (SrBiyTa2O5 +x+3y/2). The film thickness was fixed at 90nm throughout this work. The films were deposited at a low temperature of about 350-450'C and annealed in oxygen ambient for an hour at 650 to 850'C. Crystallization of the as-deposited SBT films was performed using an ordinary furnace with a temperature ramp rate of 10°C/min. For comparison, SBT thin films were annealed in a rapid thermal annealing system (RTP) with a ramp rate of 25°C/s and an annealing time of an hour also. For electrical measurements, Pt top electrodes with a dot size of 2x 1041Pm2 and 200nm thickness were sputtered through a shadow mask. The fabricated Pt/SBT/Pt/Ti/SiO 2/Si capacitors were post-annealed at 700'C in oxygen ambient for 30 minutes to decrease leakage current and increase breakdown voltage [4,6]. Remanent polarization was measured using a Radiant-Technologies RT6000SI ferroelectric tester. The crystal structure and preferred orientation of the films was characterized by X-ray diffraction (XRD) employing Cu Kcc radiation with an Ni filter. For high-temperature X-ray diffraction (HTXRD) measurements we used in-situ heating while monitoring the crystallization. The heating ramp rate was 1°C/s. Film composition and thickness were determined by X-ray fluorescence (XRF) before crystallization. RESULTS Fig. 1 a) and b) show the 2 P, values (positive remanent polarization + negative remanent polarization) of the ferroelectric SBT films annealed at 700'C as a function of Sr and Bi content, respectively. Remanent polarization was read from hysteresis loops at a switching bias of 2V. The 2Pr values as a function of Sr content indicate a broad maximum of about 18XpC/cm2 at x=0.75 to 0.85 and rapidly decreasing remanent polarization for higher and lower Sr contents. SBT film compositions having maximum 2 Pr values were selected for RTP heat-treatment also. 25 -
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Fig. 1 2Pr values of SBT thin films as a function of (a) Sr content and (b) Bi content measured at a switching bias of 2V. SBT films were annealed at 700'C. 150
Compared to standard furnace annealing, RTP-annealed films have higher 2Pr values of about 2 0pC/cm 2. Varying the Bi content reve
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