SQUID photoscanning: an imaging technique for NDE of semiconductor wafers and devices based on photomagnetic detection

The SQUID Photoscanning technique enables the noninvasive evaluation of semiconductor wafers and photovoltaic devices. The basic idea of the method is to detect photogenerated currents via their magnetic field by means of sensitive SQUID magnetometers. A magnetic imaging with high spatial resolution is performed by scanning the sample with a focused laser beam and synchronously measuring the magnetic field of the net photocurrents. Objects of analysis are semiconductor wafers with doping level fluctuations or electrically active defects, such as grain boundaries. Furthermore, the SQUID Photoscanning allows for the localization of artefacts in photovoltaic devices. The system uses sensitive, low-noise dc-SQUID magnetometers operated in a flux-locked loop (FLL) at 4.2 K or 77 K, respectively. The FLL electronics is adapted to the operation of the SQUID Photoscanning system in the presence of large, low-frequency interferences by implementing a frequency dependent feedback range. A digital signal processor (DSP) based control and data acquisition unit controls the amplitude modulation of the laser illumination, the xy-motion of the sample and the phase sensitive detection of the SQUID signal. The SQUID Photoscanning signal strengths obtained from the samples under investigation cover a range of about 100 fT for slight doping inhomogeneities in high purity silicon wafers up to several nT for photocurrent distributions in solar cells. The results of numerical simulations of SQUID Photoscanning signals are qualitatively and quantitatively in fair agreement with the experimental findings.