MECHANICAL-STRESS AS A FUNCTION OF TEMPERATURE FOR ALUMINUM-ALLOY FILMS

Aluminum alloys have virtually replaced aluminum thin films for interconnections in very large-scale integration 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 on an oxidized silicon substrate and several aluminum alloys and layered films consisting of silicon, copper, titanium, tungsten, tantalum, vanadium, and TiSi2. 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 as a function of temperature 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 include elastic behavior, recrystallization, grain growth, plastic behavior, yield strength, and film hardening from precipitates.