New Developments in IR Lock-in Thermography

Lock-in thermography based on an infrared (IR) camera has proven to be a useful tool for failure analysis of integrated circuits (ICs) [1, 2]. This technique is based on the application of a supply voltage to the IC which is periodically pulsed in the 3 Hz to 1 kHz range. Any faults dissipating electrical energy lead to periodically oscillating local heat sources, which cause a periodic temperature (T-) modulation at the surface. These heat sources are imaged by the IR camera with its output digitally on line lock-in processed to yield the amplitude and phase of the local T-modulation. The sensitivity of this a.c. technique can be considerably better than that of any other direct thermal imaging technique due to its averaging nature and suppression of the steady-state IR image. Nevertheless, the T-modulation amplitude images are modulated by the IR emissivity contrast, which may complicate the interpretation of the images. However, this can be avoided by displaying the phase image [1, 2]. Thus, lock-in thermography has the potential to considerably expand the application field of thermal failure analysis. In this contribution four new technical developments of lock-in thermography will be introduced. These developments are (1) blackening the IC surface with colloidal bismuth, (2) the synchronous undersampling technique allowing the use of higher lock-in frequencies, (3) displaying the 0°/-90° signal as a novel high resolution emissivity corrected image type, and (4) removing the thermal blurring effect by mathematically deconvoluting the 0°/-90° signal. The effect of these techniques is demonstrated by using a regularly working operational amplifier (μA 741) and a damaged capacitor as test devices. It will be shown that blackening the IC surface improves the detection sensitivity in metallized regions by up to a factor of 10, whereas the other methods allow improvement of the effective spatial resolution. We will also discuss which of the spatial resolution improvement techniques is most appropriate in different situations. Methods and Analysis