LOSS MECHANISMS IN FINE-GRAINED FERROELECTRIC CERAMIC THIN-FILMS FOR ULSI MEMORIES (DRAMS)

Ferroelectric thin films can now be prepared by a variety of technique s to have dielectric constants of 800, loss tangents of 0.01, and leak age currents as small as 1 nA cm(-2), which means that a capacitance o f 120 fF mu m(-2) can be achieved at 60 nm thickness (30 fF cell(-1) i n a 0.25 mu m(2) 256 Mbit DRAM). It is significant that these properti es are maintained up to 2 or 3 GHz, permitting operation of memories a t very high clock rates (greater than 100 MHz). High-frequency roll-of f of dielectric constants and loss tangents for barium strontium titan ate fine-grained ceramics are compared with those for lead zirconate-t itanate and discussed in terms of grain size and processing. The low l oss tangents for very fine-grained material (40-100 nm) are surprising in view of earlier work on strontium titanate. Our data support the N eumann-Hofmann theory [Ferreolectrics, 134 (1999) 201] of surface-laye r dominated loss.