Fault Localization and Functional Testing of ICs by Lock-in Thermography

In this paper new thermographic techniques with significant improved temperature and/or spatial resolution are presented and compared with existing techniques. In infrared (IR) lock-in thermography heat sources in an electronic device are periodically activated electrically, and the surface is imaged by a free-running IR camera. By computer processing and averaging the images over a certain acquisition time, a surface temperature modulation below 100 μK can be resolved. Moreover, the effective spatial resolution is considerably improved compared to stead-state thermal imaging techniques, since the lateral heat diffusion is suppressed in this a.c. technique. However, a serious limitation is that the spatial resolution is limited to about 5 microns due to the IR wavelength range of 3 -5 μm used by the IR camera. Nevertheless, we demonstrate that lock-in thermography reliably allows the detection of defects in ICs if their power exceeds some 10 μW. The imaging can be performed also through the silicon substrate from the backside of the chip. Also the wellknown fluorescent microthermal imaging (FMI) technique can be be used in lock-in mode, leading to a temperature resolution in the mK range, but a spatial resolution below 1 micron.