Thermal desorption spectroscopy is used to measure the amount of residual chlorine on Al-Si-Cu. The Al-Si-Cu surface is etched by Cl-2 electron cyclotron resonance (ECR) plasma at substrate temperatures of 30 and -60 degrees C. The surfaces are then cleaned by one of the following methods : de-ionized water rinsing, H-2 ECR plasma irradiation, or annealing in H-2/N-2 mixed gas. AlCl molecules, which may be responsible for aftercorrosion, are detected. Chlorine compounds, however, cannot be detected by conventional x-ray photoelectron spectroscopy or Auger electron spectroscopy. The low substrate temperature etching reduces the amount of residual chlorine. Moreover, the cleaning ability of the H-2 ECR plasma treatment is no less than that of the de-ionized water rinse, and is superior to that of annealing in H-2/N-2 mixed gas.