Anelasticity in Thin Film Metals

Time-dependent behavior of thin metal films

Richard P. Vinci, Paul El-Deiry, Seungmin Hyun, and Walter Brown

Department of Materials Science and Engineering, Lehigh University

Introduction

Time-dependent deformation of materials may be recoverable (anelastic/viscoelastic) or permanent (viscoelastic/creep/stress relaxation). This behavior is generally undesirable, but can be controlled to some degree through control of composition and microstructure. These phenomena tend to be stronger in thin metal films than in bulk due to the small scales inherent in thin films.

Characterization of time-dependent thin film behavior is challenging, and customized equipment must often be designed specifically for this purpose.

Sample Images

	A schematic of a "bulge" specimen geometry used for stress relaxation measurements either under gas pressure or under electrostatic actuation (as shown).
	Stress relaxation behavior for 33 nm thick pure Al films as a function of temperature.

Goals

Design and fabrication of apparatus for characterization of time-dependent behavior.
Characterization of behavior and mechanisms active in sub-micron thick films.
Investigation of the role of anelastic behavior in the "modulus deficit" often observed in microtensile testing.
Development of mechanism-based models for time-dependent behavior, and schemes for increased thin film stability.

Findings

Apparatus has been constructed that allows for time-dependent testing of thin metal films as thin as 30 nm.
Anelastic response of thin Al films has been shown to obey the laws of Linear Viscoelasticity.
High strain-rate testing of freestanding film (via microtensile testing) has shown that significant relaxation can occur over 100 msec, and may account for some of the modulus deficit commonly observed in slower microtensile tests.

Related Publications

R.P. Vinci, G. Cornella, J.C. Bravman, Anelastic Effects in Freestanding Al Thin Films, Proc. 5th International Workshop on Stress Induced Phenomena in Metallization, Stuttgart, Germany, June, 1999, pp. 240-8. (Invited)
P. A. El-Deiry and R. P. Vinci, Anelastic Behavior Of Pure Aluminum and Copper Micro-Wires, Mater. Res. Soc. Proc., 695, 2002, p. 159.
S. Hyun, W.L. Brown, and R. P. Vinci, Thickness and temperature dependence of stress relaxation in nanoscale aluminum films, Appl. Phys. Lett., 83 (21), 2003, pp. 4411-13.
N. Barbosa, P. El-Deiry, and R.P. Vinci, Monotonic testing and tension-tension fatigue testing of freestanding Al microtensile Beams, Proc. Mater. Res. Soc. Symp., 2004, U.11.39.1-6.
S. Hyun, W.L. Brown, and R. P. Vinci, Stress relaxation in nanoscale aluminum films, Proc. SPIE, 5343, 2004, 154-162.
P. A. El-Deiry, N. Barbosa III, W. L. Brown, R. P. Vinci, “Effective Modulus and Stress Relaxation of Freestanding Aluminum Microtensile Beams”, submitted to Proc. Mater. Res. Soc., 2004.
S. Hyun, Tejpal K. Hooghan, W.L. Brown, and R.P. Vinci, Linear Viscoelasticity in Aluminum Thin Films, to appear in Appl. Phys. Lett., August 2005.

Portions of this work were performed collaboratively at Lehigh University and Stanford University. Support for Lehigh has been provided by:

NSF CAREER DMR-9876261
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