Non-destructive AC permeability measurement of full wafer soft magnetic films

A novel high-sensitivity magnetic sensor for direct measurement of the AC initial permeability of full-wafer soft magnetic films (up to or larger than 5-in. diameter) is described. The sensor operates on the principle that the film permeability μ_f is proportional to the inductance value of a coil surrounding a ferrite core in which the thin film is part of the magnetic circuit. The magnetic flux produced by the coil in a main ferrite core of large cross section to avoid its saturation is linked inside the magnetic film. It is shown that a correct value of μ_f is given by such a sensor only when the magnetic leakage flux out of the film and ferrite core can be avoided, which implies a closed geometry for the ferrite core. In addition, the closed magnetic flux avoids demagnetizing effects in the film. The high sensitivity of the sensor offers the possibility of determining the film permeability spectra up to a few tens of megahertz. Measured frequency dependences of μ_f in soft amorphous thin films are presented