Fatigue and Reliability of Metals for MEMS Applications

Richard P. Vinci, Paul El-Deiry, Nicholas Barbosa, Guido Cornella*, and John C. Bravman*

Department of Materials Science and Engineering, Lehigh University

*Stanford University

Introduction

Knowledge of mechanical properties is essential for the sucessful modeling and design of MicroElectro Mechanical Systems (MEMS). Fatigue behavior, for instance, may ultimately limit product lifetime in certain applications. Fatigue is of particular interest in devices such as RF switches which operate under cyclic loading at high frequencies (kHz to GHz). Cyclic loading can cause behavior evolution over as few as 100 cycles due to strain hardening or softening, thereby causing device performance to change well before failure is reached. Failure under cyclic loading will depend on the magnitude of the applied strain, but will also be affected by the magnitude of the intrinsic tensile stress common to many beams and membranes used in MEMS applications.

Results for MEMS structures which have been cycled billions of times have not shown fatigue failure as it is observed in macroscopic samples under similar conditions. A thickness threshold for fatigue failure seems plausible.

Goals

• Design and microfabrication of fatigue test samples.
• Development of high frequency piezo-driven fatigue testing equipment.
• Investigation of S-N curves in strain controlled experiments for samples of different thicknesses, leading to a thickness threshold estimation for fatigue failure.
• Determination of changes in fatigue mechanisms with changing sample size as observed via Transmission Electron Microscopy.

Sample Images

	Shown on the left is an SEM micrograph of a portion of a micromachined fatigue test sample. Dimensions of the beam are 1 micron thick x 100 microns wide x 600 microns long. It has been notched by a focused ion beam to localize crack formation. The pictured beam material is pure aluminum.
	Pictured is a series of optical micrographs of a 2 micron thick aluminum beam illustrating crack growth as a slow test progresses. Note the clearly visible necked region in which the crack nucleates and grows.

Findings 

• Apparatus has been constructed that allows for testing of freestanding thin films at strain rates approaching 0.1/sec.
  
• Significant strain rate sensitivity has now been measured at room temperature in freestanding Al thin films. This phenomenon is apparently due to anelastic (time-dependent, recoverable) processes. These processes are capable of relaxing the film stress by over 50% in 30 minutes even at very low stresses.  By extension, it is reasonable to assume that similar effects may be present in other thin films. Cu films on Si substrates are known to relax rapidly at room temperature, so it would not be surprising if the low modulus observed in freestanding Cu films by other investigators could be attributed to similar anelastic processes. The implications of this finding are that micro-tensile tests at practical strain rates will always result in a relaxed modulus for such materials.
  
• Fatigue failure has been characterized in freestanding pure Al microbeams. An S/N curve for this behavior has been developed. Comparisons to bulk Al fatigue show that thin film fatigue can be related to bulk behavior with allowances for factors like grain size strengthening.
  

Related Publications

1. G. Cornella, R.P. Vinci, R. Suryanarayanan, R.H. Dauskardt, and J.C. Bravman, Observations of Low Cycle Fatigue of Al Thin Films for MEMS Applications, Mater. Res. Soc. Proc., 518, 1999, pp. 81-88.
2. 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)
3. 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.
4. 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.
5. 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.
6. N. Barbosa III, P. A. El-Deiry, W. L. Brown, R. P. Vinci, “A system for microtensile testing of freestanding thin films", in preparation.
7. N. Barbosa III, P. A. El-Deiry, W. L. Brown, R. P. Vinci, “Fatigue behavior of freestanding Al thin films", in preparation.
8. P. A. El-Deiry, N. Barbosa III, W. L. Brown, R. P. Vinci, “Anelastic behavior of freestanding Al thin films", in preparation.

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

• NSF CAREER DMR-9876261
• NSF ECS-0322702
  

Last updated: August 2, 2005