Fundamental Issues and Practical Problems Associated with the Use of Barium Strontium Titanate films for DRAM capacitors

The high charge storage densities attainable using the parraelectric phase of ferroelectric materials make them attractive as dielectrics for DRAM capacitors. For example charge storage densities as high as 140 fF/mm2 have been achieved in barium strontium titanate BSTO films making it a leading candidate for use is stacked DRAM capacitor structures. Evaluation of the properties of BSTO dielectrics shows that films in the thickness range 10-30nm will be required to meet the requirements of future generations of DRAM memories. The electrical properties BSTO films in this thickness range differ significantly from those of the more conventional oxynitride dielectrics currently in use. In particular the measured permitivity BSTO films is found vary significantly with stress, temperature, film thickness, electric field and frequency. In addition to understanding the basic properties BSTO films the impact of annealing and etching processes on the electrical properties is required to integrate capacitors into DRAM device structures. In the talk I will present the results of studies we have made of the leakage, dielectric relaxation and stress and temperature dependence of the dielectric properties of different thickness MOCVD grown BSTO films. The studies indicate that the dielectric properties of the thin films can be understood in terms of simple model in which the interior of the films have dielectric properties similar to that of bulk material where as a layer of lower pemitivity material forms at each electrode interface. The dielectric relaxation observed in films will also be discussed in relation to the properties of bulk BSTO dielectrics. The results of the studies will be used to discuss the effect integration processes can have on the properties of the films and hence the performance of DRAM capacitors.