Coplanar Capacitors with High Dielectric Constant BaSrTiO 3 for Microwave Application
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ABSTRACT A coplanar structure is shown to be free of the asymmetrical I-V characteristics normally observed in MIM capacitors with BST inter-electrode dielectric. The low barrier height at the bottom interface is found to be responsible for the asymmetry, which causes increased leakage current and reduced breakdown voltage in one bias polarity. These problems can be eliminated using a coplanar structure, which has been shown elsewhere to have a very low parasitic inductance and a very high resonant frequency. However, these improvements are realized at the cost of a four times reduction of capacitance per unit area.
INTRODUCTION BaSrTiO 3 (BST) is a leading candidate for use as the inter-electrode dielectric for storage capacitors in advanced DRAMs and as on-chip capacitors in microwave monolithic circuits. For the successful development of this material, many improvements in film properties are still needed. Most important among them are the reduction of the leakage current and microwave loss in conjunction with an increase of the dielectric constant, linearity, and improved reliability. Many authors have reported on these issues [1]. In this work, we have investigated the asymmetrical I-V characteristics commonly observed with BST capacitors and we propose a new coplanar configuration to eliminate the asymmetry. The high frequency properties of this configuration are also discussed. To illustrate the asymmetry, the I-V characteristics of a typical BST capacitor are shown in
Fig. 1 where the absolute values of the current are plotted from negative to positive bias. As can be seen, the current at positive bias is higher than that at negative bias. When plotted on a linear scale, the characteristics appear to be diode-like with a lower breakdown voltage for the positive bias case. Such an asymmetric characteristic may complicate circuit design and has to be improved to win the approval of MMIC circuit designers. EXPERIMENTS Pt/BST/Pt MIM capacitors were fabricated with the BST films prepared by an automated sol-gel method [2]. Both Si and GaAs substrates were used for the film deposition. The GaAs wafer was protected by Si0 2 and SiNk before the Ti/Pt film for the bottom electrode was deposited. The BST deposition temperature was 400-500'C. After film deposition, a mask was used to define the BST film and to expose the bottom Pt metal for probing. The top Pt electrode was deposited by e-beamn evaporation and was defined by a second mask using a standard lift-off technique. The I-V characteristics were acquired using a voltage step method. The step duration was less than one second, which is not long enough to avoid the influence of the relaxation current at low voltage. In the following analysis, these low voltage data will be ignored.
469 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
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