Space charge measurements by the alternating thermal wave method: thermal analysis and simulations for data processing improvement

Since more than 25 years, various nondestructive methods for space charge measurements have been developed in many laboratories. The presence of space charge in insulating structures is recognized to be a proof of weakness and an initiating factor to fatal breakdowns. The present work deals with the recent improvements of the Alternating Thermal Wave Method (ATWM), a technique we finalized for space charge measurement in dielectric films for capacitors. We recall the experimental protocol. The thermal wave is produced by a thermo-electrical module applied on one face of the film. Attenuated sinusoidal temperature waves penetrate by conduction into the sample. The wavelength and the attenuation factor depend on frequency and thermal properties of the material. Evidently, only the space charges within the penetration depth of the thermal wave are concerned with the induced pyroelectric current and may be evaluated. Our purpose is to scan the successive thickness layers by using increasing penetration depths. The cross checking of information at several wave frequencies, taking into account a wave efficiency function, and the use of an adapted mathematical model for the electric field representation increase the accuracy of the method. We give numerical simulations in order to show the new resolving power of our technique.