Silicon nitride (Si_3N_4) has excellent mechanical, and optical properties etc. So silicon nitride thin films have been extensively used as support layer, insulating layer and surface passivation layer in infrared micro-bolometer detector. However, silicon nitride thin film prepared by plasma enhanced chemical vapor deposition (PECVD) has the complex chemical structure, the relationship between chemical structure and physical properties is unkown. So this thesis studied the microstructure of the films by FT...