A study of barium strontium titanate thin films for use in bypass capacitors
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A study of barium strontium titanate thin films for use in bypass capacitors B. A. Baumert, L-H. Chang, A. T. Matsuda, and C. J. Tracy Materials Technology Center, Materials Research and Strategic Technologies, Motorola Semiconductor Products Sector, 2200 West Broadway Road, Mail Drop M360, Mesa, Arizona 85202
N. G. Cave, R. B. Gregory, and P. L. Fejes Materials Characterization Laboratory, Materials Research and Strategic Technologies, Motorola Semiconductor Products Sector, 2200 West Broadway Road, Mail Drop M360, Mesa, Arizona 85202 (Received 3 September 1996; accepted 16 July 1997)
Physical and electrical characterization techniques have been applied to the problem of developing a lower temperature process for spin-on Ba0.7 Sr0.3 TiO3 thin films and capacitors compatible with on-chip aluminum metallization. The films were prepared by spin-coating from carboxylate precursors and were processed at temperatures between 650 ±C and 450 ±C. Capacitors annealed at higher temperatures have a dielectric constant (k) of 382, a CyA of 20 fFymm 2 , and a leakage current density of 2 3 1027 Aycm2 at 3.3 V. Those processed at 450 ±C show occasionally promising but inconsistent results, correlated using TEM images with locally variable crystallization into the perovskite phase. The kinetics of the spin-on solution chemical decomposition and crystallization has been investigated through the use of x-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy.
I. INTRODUCTION
Barium Strontium Titanate (BST) is of interest for use in bypass capacitors1,2 and in DRAM’s3–11 because of its high dielectric constant. This value can be as high as 250012 in bulk ceramic BST. At room temperature, the composition Ba0.7 Sr0.3 TiO3 has the highest dielectric constant.13 In addition, this composition is desirable for study because it is cubic (with a space group of Pm3m) and paraelectric at room temperature. The more Ba-rich phases are ferroelectric and tetragonal at room temperature (see Fig. 1), whereas the more Sr-rich phases have lower dielectric constants. BST has the perovskite structure ABX3 , which is shown in Fig. 2. It derives its high dielectric constant from an ionic displacement and therefore differs from lower dielectric constant materials such as SiO2 , which experience an electronic displacement only with changing applied voltage. It can be seen from Fig. 2 that the Ti ions are each surrounded by six oxygen ions. At zero applied volts, the Ti ions are centered in the oxygen octahedra and the dipole moments cancel. With the application of voltage, however, the Ti ions will be displaced and a dipole moment m will be induced where m qd (q is the charge and d is the separation between charge centers). The polarization, P, is directly proportional to the dipole moment: P Nm where N is the number of molecules per unit volume and P e0 xE where e0 is the permittivity of free space, x, the dielectric susceptibility is x er 2 1 where er is the relative dielec
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