Integrating ferroelectrics

02/01/1998

Integrating ferroelectrics

One symposium at the recent MRS Fall Meeting in Boston, MA, was devoted to the properties and processing of ferroelectric thin films. The high dielectric constant of these materials makes them an attractive alternative for DRAM manufacturers attempting to achieve the required cell capacitance in a smaller space.

As Scott Summerfelt of Texas Instruments` Components and Materials Research Center explained, perovskite materials such as (Ba, Sr)TiO3 (BST) can achieve much greater capacitance densities than either traditional SiO2/Si3N4 dielectrics or medium dielectric constant materials like Ta2O5, but perovskites require oxygen-stable lower electrodes such as Pt, Ir, or conductive oxides. Since all of these materials are new to CMOS processing, the relationships among device structure, processing, and material properties are not yet fully understood.

According to Sufi Zafar and co-workers in Motorola`s Materials Research and Strategic Technologies group, the dielectric constant (k) and dispersion parameter (m) are the two key dielectric properties. The dielectric constant reflects the amount of charge the materials can store, while m reflects charge loss over time. The dispersion parameter depends on the film thickness, temperature, and the reciprocal of k, and is related to the capacitance by

C = C0f-m

where:

f = frequency.

Researchers often model ferroelectric capacitors as an interfacial capacitance, Ci, in series with the bulk capacitance Cb. When Pt electrodes were used, Zafar`s group found that Ci increases with temperature, but has no frequency dependence -the interface is nondispersive. The dispersion parameter increased with thickness, but Zafar emphasized that this effect is purely volumetric - the bulk properties dominate as thickness increases. As temperature increased, the bulk k decreased, so m increased. In contrast, the interface was dispersive when Ir electrodes were used.

Angus Kingon and S.K. Streiffer, both of the Department of Materials Science and Engineering at North Carolina State University, also studied the thickness dependence of dielectric behavior in an effort to distinguish between extrinsic effects, resulting from processing or sample preparation, and the intrinsic limits of the material. They studied BST metal-insulator-metal (MIM) capacitors, Kingon said, because a stable deposition process is available to make capacitors with consistent properties and a flat frequency response, and because the Curie temperature of BST is below room temperature.

The researchers found that k decreased with thickness, and also decreased as Ti content increased. According to Kingon, the interfacial capacitance often invoked to explain the thickness dependence could result from a number of causes, ranging from extrinsic sources like a discrete second phase or charge trapping at the interface to intrinsic sources like a "dead layer" or Schottky barrier effects. The extrinsic causes probably contribute, but do not fully account for the thickness dependence seen experimentally. Instead, Kingon said, most of the observed thickness dependence is probably due to a "dead layer" near the interface in which the polarization is less than in the bulk. Therefore, most of the decrease in k with thickness is intrinsic to the material, and improved bulk properties will be needed to reach acceptable capacitances.

Another member of the ferroelectric family, SrBi2Ta2O9 (SBT), has attracted great interest for nonvolatile memory applications. According to Christian D. Gutleben, reporting on research done at Sony, high resolution XPS results as a function of Pt coverage show no structural change in Sr or Ta bonding. However, Pt reacts disruptively with residual Bi to form a 10-? thick low-melting point Pt-Bi alloy layer between the Pt electrode and the SBT. As a result, the SBT layer adjacent to the electrode is Bi-deficient. These effects contribute to degraded adhesion and electronic properties at the interface. Gutleben said the XPS data was taken from unannealed samples - he expects much more dramatic reactions to occur during the required crystallization anneal.

Norifumi Fujimura, presenting work done at N.C. State, pointed out that the Pt-Bi eutectic occurs at 760?C, while the Ir-Pt eutectic occurs at 1400?C. Thus, less dramatic effects are seen with Ir electrodes.- K.D.