Magnetic field and current density imaging using off-line lock-in analysis

In the current paper the application of a custom developed 2-dimenional scanning magnetic field microscope based on tunnel-magnetoresistive sensors and subsequent qualitative and quantitative analysis is described. To improve sensitivity and to enable the detection and evaluation of phase deviations, an off-line lock-in approach was employed by driving the samples under test with an injected current at a fixed signal frequency. Amplitude and phase evaluation was based on simultaneous acquisition of the reference and the measurement signal obtained from the magnetic field sensor. This off-line lock-in approach enables not just the detection but also the estimation of changes in signal phase caused by capacitive, inductive or ohmic coupling of the induced currents. Furthermore assessed magnetic fields were converted into the current density by solving the inverse magnetic problem and post processing of the acquired signals. For verification of the developed set-up the current density distribution was computed from experimentally acquired magnetic fields for a two-wire test structure. In addition quantitative values of the current density were derived for a calibration pattern containing defined structures. Finally, to evaluate the practical relevance a power MOSFET with unknown defect was analysed and an area of unexpectedly increased magnetic field intensity was observed.