PURPOSE: To improve the durability of a semiconductor device having a multilayer interconnection structure under high temperature and high humidity by a method wherein a layer insulating film is subjected to a dehydration treatment.

CONSTITUTION: The multilayer interconnection structure of a semiconductor device has at least two wiring metal layers 6a and 8a and at least one layer insulating film 7a which isolates the wiring metal layers 6a and 8a from each other. In this structure, the layer insulating film layer 7a is subjected to a dehydration treatment. For instance, a PSG film is formed by a normal pressure CVD method as the interlayer film 7a and, after the film is patterned, resist is removed and extra-precise washing is performed with pure water. Then an Si substrate is placed on a hot plate to subject the PSG film as the interlayer film 7a to the dehydration treatment in an N2 atmosphere at 400°C for 30 minutes. Immediately after the dehydrating treatment is finished, pure A is deposited by an EB method to form the upper metal layer 8a which is then patterned. Finally, a PSG film is applied as a passivation film 9a and then patterned.