Local Microstructure and Stress in Al(Cu) Thin Film Structures Studied by X-Ray Microdiffraction
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Local Microstructure and Stress in Al(Cu) Thin Film Structures Studied by X-Ray Microdiffraction B.C. Valek1, N. Tamura2, R. Spolenak3, A.A. MacDowell2, R.S. Celestre2, H.A. Padmore2, J.C. Bravman1, W.L. Brown3, B. W. Batterman2,4 and J. R. Patel2,4 1
Dept. Materials Science & Engineering, Stanford University, Stanford CA 94305 USA ALS/ LBNL, 1 Cyclotron Road, Berkeley CA 94720 USA 3 Agere Systems, formerly of Bell Laboratories, Lucent Technologies, Murray Hill NJ 07974 USA 4 SSRL/SLAC, Stanford University, P.O.BOX 43459, Stanford CA 94309 USA 2
ABSTRACT The microstructure of materials (grain orientation, grain boundaries, grain size distribution, local strain/stress gradients, defects, …) is very important in defining the electromigration resistance of interconnect lines in modern integrated circuits. Recently, techniques have been developed for using submicrometer focused white and monochromatic x-ray beams at synchrotrons to obtain local orientation and strain information within individual grains of thin film materials. In this work, we use the x-ray microdiffraction beam line (7.3.3) at the Advanced Light Source to map the orientation and local stress variations in passivated Al(Cu) test structures (width: 0.7, 4.1 µm) as well as in Al(Cu) blanket films. The temperature effects on microstructure and stress were studied in those same structures by in-situ orientation and stress mapping during a temperature cycle between 25°C and 345°C. Results show large local variations in the different stress components which significantly depart from their average values obtained by more conventional techniques, yet the average stresses in both cases agree well. Possible reasons for these variations will be discussed. INTRODUCTION Extensive study has been conducted on the mechanical properties of thin films and structures. Thermal expansion mismatch between materials and/or transport of material during electromigration lead to large stresses in these structures, often much higher than those sustainable by bulk materials. Conventional techniques such as wafer curvature and x-ray diffraction only provide a macroscopic average of strain/stress or film texture. As integrated circuit device dimensions shrink to submicrometer sizes, one can expect that local microstructural and stress variations do play a prominent role in determining the materials failure modes. X-ray microdiffraction techniques developed at synchrotron sources [1,2,3] have been shown to be a promising new method in the study of mechanical behavior at the micrometer scale. These techniques allow for the determination of the orientation of single grains in a material as well as the complete strain/stress tensors with micrometer scale resolution. X-rays are particularly advantageous because they are penetrating and can probe buried structures. This means that special sample preparation is unnecessary, allowing the investigation of passivated interconnect structures without altering the stress state. P7.7.1
EXPERIMENTAL This study was conducted at the x-ray microd
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