Papers by Author: Chung Seog Oh

Abstract: The accurate characterization of linear coefficient of thermal expansion (CTE) of thin films is vital for predicting the thermal stress, which often results in warpage and failure of a MEMS structure. In this paper, special emphasis is placed on the development of novel test method to extend an ISDG (Interferometric Strain/Displacement Gage) technique to the direct and accurate CTE measurement of MEMS materials, AlN and Au. The freestanding AlN and Au films are 1 μm thick and 5 mm wide. Strain is directly measured by a brand-new digital type ISDG with two Cr lines deposited on the specimen while heating a specimen in a furnace. The whole test system is verified first by measuring the CTE for the NIST’s SRM (Standard Reference Material) 736 (Cu) block. The measured CTE is 17.3 με/oC up to 167 oC, which agrees well with the NIST’s certified value. The CTE of Au is 25.4 ± 1.15 με/oC and that of AlN film is 3.77 ± 0.12 με/oC. The in-plane displacement resolution is about 5 nm at the best circumstances.

Abstract: This investigation aims at doing some durability and damage tolerant (DaDT) tests with 2124-T851 aluminum specimens having corner cracks under a random history, correlating with the simulation results from AFGROW and then drawing some conclusions from those comparisons. Two hydraulic actuators and a homemade Wood’s alloy grip are employed to do the test. The surface crack lengths are measured by a traveling microscope and used as a reference for the later fractography. The crack penetration and the total lives are about 17 and 27 blocks, respectively. The crack length and depth are evaluated by the fractography after completing each test and used to grasp the shape change. The Forman equation is used to simulate the fatigue crack growth behavior according to a bearing stress ratio extensively. The crack penetration life is decreased as the bearing stress ratio increases. The crack aspect ratio is very dependent on the bearing stress ratio. The LBH loading accelerates the fatigue crack growth in the crack depth direction but decelerates that in the length direction until crack penetration.