High-cycle fatigue and durability of polycrystalline silicon thin ®lms in ambient air C.L. Muhlstein a,* , S.B. Brown b , R.O. Ritchie a a Department of Materials Science and Engineering, University of California, LBNL, 1 Cyclotron Road, MS 62-203, Berkeley, CA 94720-1760, USA b Exponent Incorporated, Natick, MA 01760, USA Received 17 April 2001; received in revised form 28 July 2001; accepted 3 August 2001 Abstract To evaluate the long-term durability properties of materials for microelectromechanical systems MEMS), the stress-life S/N) cyclic fatigue behavior of a 2-mm thick polycrystalline silicon ®lm was evaluated in laboratory air using an electrostatically actuated notched cantilever beam resonator. A total of 28 specimens were tested for failure under high frequency 40 kHz) cyclic loads with lives ranging from about 10 s to 34 days 3  10 5 to1:2  10 11 cycles) over fully reversed, sinusoidal stress amplitudes varying from 2.0 to 4.0 GPa. The thin-®lm polycrystalline silicon cantilever beams exhibited a time-delayed failure that was accompanied by a continuous increase in thecomplianceofthespecimen.Thisapparentcyclicfatigueeffectresultedinanendurancestrength,atgreaterthan10 9 cycles,of 2GPa, i.e. roughly one-half of the single cycle) fracture strength. Based on experimental and numerical results, the fatigue process is attributed to a novel mechanism involving the environmentally-assisted cracking of the surface oxide ®lm termed reaction-layer fatigue). These results provide the most comprehensive, high-cycle, endurance data for designers of polysilicon micromechanical components available to date. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fatigue; Stress-life; Durability; Silicon; Polysilicon; MEMS 1. Introduction Over the past decade, microelectromechanical systems MEMS) and the enabling technologies of surface micro- machininghaveevolvedfromacademiclaboratoryexercises toestablishedcommercialfabricationstrategies.Duringthis period of rapid innovation, a vast array of MEMS applica- tions have emerged and commercial products have entered the marketplace. These applications range from memory, mass storage and display applications for personal comput- ing to critical sensor applications such as pressure transdu- cers in medical devices and inertial sensors for passive restraint systems e.g. airbags) and active automotive sus- pensions. Two-key points have become clear as MEMS are emerging from their infancy. First, silicon-based ®lms are still the dominant structural material for micromachines. Second, the reliability of these components is critical for both product performance and safeguarding human life. Consequently, studies that characterize the failure modes that will ultimately dictate the long-term durability of MEMS components are critical in this maturing ®eld. The silicon-based ®lms commonly used in micromecha- nical structures fall into the category of brittle structural materials and therefore inherit many of the traits, in parti- cular low fracture toughness and damage tolerance, that preclude their use in conventional macro-scale applications. Research into the toughening of brittle structural materials, especially related to structural ceramics and intermetallics, has been extensive over the past couple of decades. The result is that there is now an extensive body of literature on how processing may be used to enhance the fracture tough- ness of these materials [1]. An unfortunate consequence of these so-called extrinsic) toughening schemes, however, is that the material invariably becomes more) susceptible to premature failure by fatigue when subjected to cyclic stres- ses [2]. 1 Since it will be the damage tolerance, i.e. fracture andfatigueresistance,ofthethin-®lmsiliconthatwillmost affect the reliability and long-term durability of MEMS Sensors and Actuators A 94 2001) 177±188 * Corresponding author. Tel.: 1-510-486-4584; fax: 1-510-486-4881. E-mail address: cmuhlstn@uclink4.berkeley C.L. Muhlstein). 1 An example of this is the cyclic fatigue of polycrystalline ceramics, such as Al 2 O 3 , Si 3 N 4 and SiC, which fail intergranularly and are thus toughened by the bridging of interlocking grains in the crack wake; under cyclic loading, frictional wear in the sliding interfaces can lead to a progressive degradation in the potency of these grain bridges, thereby effectively ``detoughening'' the material and causing premature fatigue) failure [2]. 0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-424701)00709-9