Effect of ultra-low temperature treatments on the stress in aluminum metallization of silicon wafers

The stresses in Al-0.75w%Si-0.5w%Cu unpatterned metallization on silicon wafers have been measured using substrate curvature and x-ray diffraction techniques after quenching in liquid nitrogen. Stresses were measured with and without phospho-silicate glass overlayers and SiO₂ underlayers, and thermal cycling followed by relaxation at room temperature. It was found that cooling the substrates to 77 K and warming to room temperature caused the metallization stress to go from tensile to compressive. Subsequent heating of the substrates to above approximately 70 °C followed by cooling to room temperature caused the stress to become tensile. Both compressive and tensile stresses were found to relax at room temperature with a time constant of 2.3±0.2 hours. The magnitude of stress relaxation was a function of temperature, being about 20 MPa after heating to 240 °C. The metallization exhibited both compressive and tensile flow stresses of approximately 100 MPa near room temperature.