Mechanical Stress as a Function of Temperature in Thin Aluminum Films and its Alloys
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Mechanical Stress as a Function of Temperature in Thin Aluminum Films and Its Alloys Donald S. Gardner and Paul A. Flinn**
*C.I.S., Stanford University, Stanford, Calif. 94305
"**Intel
Corporation and
Dept. of M.S. & E., Stanford University, Stanford, Ca 94305 Abstract Aluminum alloys have virtually replaced aluminum for interconnections in VLSI because of their improved reliability. Mechanical stress is a problem of growing importance in these interconnections. Stress as a function of temperature was measured for thin aluminum films and several aluminum alloys and layered films consisting of silicon, copper, titanium, tungsten, tantalum, vanadium, and TiSi 2. Solid-state reactions of the aluminum with the additives and with the ambient during thermal cycling will occur and depending on what compounds have formed and at what temperature, this will determine the morphology and reliability of the metallization. The measurement technique, based on determination of wafer curvature with a laser scanning device, directly measures the total film stress and reflectivity in situ during thermal cycling. Changes in stress were detected when film composition and structure varied and were correlated using x-ray diffraction with the formation of aluminides. Other phenomena that contribute to stress changes including elastic behavior, recrystallization, grain growth, plastic behavior, yield strength, and film hardening from precipitates.
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Introduction
Several failure mechanisms associated with mechanical stress are becoming increasingly important in thin films used for interconnections in integrated circuits. Open circuits from metal cracking and voiding, short circuits due to hillocks, and both open and short failures due to electromigration are phenomena related to mechanical stress. Temperature is important because the conductors and dielectrics have relatively large thermal mismatches with silicon resulting in stress. Solid-state reactions and changes in structure normally involving volume changes may also occur during thermal cycling causing variations in stress. More details can be found in our other publications [1,2,3,4,5]. Two types of stresses (or strain) occur in thin films-intrinsic and thermally induced. Intrinsic (or growth) stress is observed in as-deposited films and can be a result of defects or structural mismatch between the film and substrate [6]. Thermally induced stress is introduced by temperature changes after deposition. The origin of these stresses has been reviewed by Campbell [7,8], Kinosita [9], and Hoffman [10]. Studies of stress have helped to achieve a better understanding of hillocks and voids in thin films. Hillock suppression [5,11,12] is one primary advantage of layered films and knowledge of the internal stresses has assisted in explaining the improvements [1,2]. The detection of these changes when film composition and structure change has proven to be very useful because the precise temperature at which precipitate/compound formation, or grain growth occurs can be determined.
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