Fatigue Characteristics And Life Prediction For MEMS Electroplated Copper Films

MEMS is not a simple geometric reduction of the traditional machinery. If thecomponentsaccess tomicro-nano-size,themechanics andphysicalpropertiesofthesematerials will significantly change. The elastic modulus, tensile strength, fracturetoughnessandfatigue strength ofmaterialsmeasuredunderthemacro-blockconditionmay not be applicable to MEMS design. Whether the existing theory and methods ofbulkmechanicalfatigue canbeusedinMEMSstillneedstobestudiedfurther.In this paper, notched and smooth specimens were produced for free-standingelectroplated copper films with LIGA-like process at micron dimension. With theobservationofthree-dimensionalvideo,thespecimensmetthetestrequirements.The tensile fatigue testswere completed by using MMT-11Nmechanical fatiguetesting machine. S-N curve, cyclic???curve and ??N curve were obtained.Basedonthemodifiedlocalstress-strainapproach,fatiguelivesofnotchedspecimenswere predicted. The results were found in good agreement with the experimentalfatiguelives,which provedthatsomeofthetraditionalfatigueresearchmethodscouldbe also used in MEMS. Fatigue fractograghy observation showed that fracture modewas intergranular fracture form. Cyclic hardening behavior was found during thefatigue process. A comparison of coarse-grained, fine-grained electroplated copperfilms andbulkcooperisalsodescribedinthepaper.Finally, according to the characteristics of micro-mechanical fatigue test, amicro-mechanical fatigue testing device based on magnetostrictive principle wasinvented. Three kinds of fatigue specimens, tension, bending and bending-torsion,were designed. Especially during the past micro-mechanical fatigue testing programs,bending-torsion fatigue tests were difficult to achieve, therefore, the methods in thispaper couldlead anewsubjectandthenare innovative.