Mechanical Behavior of Pt and Au thin films

Strengthening mechanisms and residual stress control in Pt and Au thin films

Richard P. Vinci, Seungmin Hyun, Richard Chromik, Ming-Tzer Lin, Terry Delph, Walter Brown

Department of Materials Science and Engineering, Lehigh University

Collaborators: Stanford University, Applied Materials, Inc., and the US Army Research Laboratory

Introduction

Platinum and gold electrodes are often used in circumstances that require high oxidation resistance. Platinum is particularly suited for high temperature applications.

One such application is platinum integrated with high permittivity ceramic materials for capacitive storage devices such as the DRAMs (Dynamic Random Access Memory chips), and for piezoelectric actuators (with PZT - Lead Zirconium Titanate). The processing conditions for the complex oxides typically involve elevated temperatures and oxidizing conditions. Electrical leakage should be low for low-term stability (reduced "memory" loss). Unfortunately, platinum electrodes are susceptible to adhesion problems, hillock growth, and high residual stresses, all of which are detrimental to electrode performance and lifetime. These problems are ultimately related to the mechanical behavior of the platinum films. Understanding and controlling the mechanical properties of the metal films will allow improvements in electrode behavior.

Gold is highly susceptible to creep and stress relaxation, even at moderately high temperatures. Methods for fabricating creep-resistant gold electrodes are needed. This requires an understanding of the deformation mechanisms acting in thin gold films.

Sample Images

Schematic of an on-chip capacitor.	Thermal stress behavior of Pt on Si.

Goals

Develop new alloy systems to improve the performance and reliability of metal electrodes integrated with high-K capacitor dielectric materials.
Gain understanding of plasticity mechanisms and strengthening effects in this films at a variety of temperatures.

Findings

A model for stress relaxation in Pt thin films has been developed. It explains the role of grain structure and film thickness in determining relaxation rate.
A method for creating low-residual stress Pt metallization has been developed. It is currently not suited for mass production, but further development is under way.
Solid solution strengthening effects were evaluated in Pt-Ru alloys. It was found that the effect could be described by a bulk strengthening theory, but that the grain size refinement associated with the addition of the Ru solute dominated the total strengthening.
Reduction in stress relaxation rate and extent in Au was demonstrated at 200 and 400 C by alloying with V.

Related Publications

S. Hyun, R. P. Vinci, K.P. Fahey, B.M. Clemens, Effect of grain structure on the onset of diffusion-controlled relaxation in Pt thin films, Appl. Phys. Lett., 83 (14), 2003, pp. 2769-71.
S. Hyun, O. Kraft, and R.P. Vinci, Solid solution alloy effects on microstructure and indentation hardness in Pt-Ru thin films, Mater. Res. Soc. Proc., 673, 2001, pp. P3.2.1-P3.2.6.
S. Hyun, O. Kraft, and R.P. Vinci, Mechanical behavior of Pt and Pt-Ru solid solution alloy thin films, Acta Mat., 52(14), 2004, pp 4199-4211.
R.R. Chromik, T. Bannuru, and R.P. Vinci, Internal oxidation and mechanical properties of Pt-IrO2 thin films, Proc. Mater. Res. Soc. Symp., 2004, U8.13.1-6.

This is work has been supported by:

the National Science Foundation, grant number DMR-0072134, Subcontract No. PY-0826
the U.S. Army Research Office and U.S. Army Research Laboratory, Cooperative Agreement Number DAAD19-02-2-0030.

The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office, Army Research Laboratory, or the U.S. Government.

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Last updated: August 2, 2005