Elastic and plastic strains in Al/TiW/Si contacts during thermal cycles

Abstract

Al/TiW/Si contacts are widely used in microelectronic devices. In spite of the thorough knowledge on the microstructure, composition and electrical properties of these contacts there is almost no knowledge on their elastic and plastic strains formed during and after thermal treatments. Such data is very valuable for optimization of the fabrication process and the operation conditions. This paper presents results of a systematic study of strains and stresses formed in the Al/TiW films on Si(100) substrate. The strain and stresses were determined by measuring the change in the specimen radius of curvature during heating and cooling cycles between 20 and 280°C under high vacuum of ∼10 −6 mbar. The measuring system is based on scanning the surface of the specimen with a laser beam and collecting the reflected beam on a position sensitive detector. It was found that during heating up to 90°C thermal elastic strains of ∼1×10 −3 are formed in the Al/TiW films. The initial compressive stress in the films increases linearly with increasing temperature and reaches the yield point under compressive stress of ∼200 MPa at 90°C. The biaxial elastic modulus of the Al/TiW bilayer structure was determined by comparing the calculated with the experimental slope of the stress vs. temperature curve in the elastic region. Above 90°C the compressive stress slightly and linearly decreases with increasing temperature due to plastic deformation in the films. The stress versus temperature curve in this region is analyzed based on a model of dislocation motion above obstacles in metals. During the third thermal cycle TiSi and TiSi 2 are formed during heating at temperatures between 230 and 280°C leading to a decrease in the compressive stress by ∼130 MPa. During an interval of 10 h between the thermal cycles, stress relaxation of ∼80 MPa occurred at room temperature.