Thermal Failure Analysis by IR Lock-in Thermography

Thermal infrared (IR) microscopy has experienced a decisive technical improvement by the application of Lock-in Thermography (LIT), which is commercially available for failure analysis by different vendors. Due to its averaging nature, this technique allows the detection of local heat sources at the surface of a few µW corresponding to a local temperature modulation of a few µK. Thus it outperforms other thermal imaging methods, like liquid crystal imaging, fluorescent microthermal imaging, Raman IR-thermography, and steadystate IR thermography, by 2 to 3 orders of magnitude. Thereby, LIT allows to extend the application field of thermal imaging for failure analysis drastically. The emissivity contrast, which obscures the thermal contrast in conventional IR thermography, can be avoided in LIT by displaying the phase image or the 0°/-90° image in a 2-phase measurement. Due to its dynamic nature, lateral heat diffusion (blurring) is considerably reduced in LIT compared to steady-state techniques, depending on the chosen lock-in frequency. The usefulness of Lock-in Thermography is given mainly by the fact that it allows a though relatively coarse (3-5 µm) but very sensitive lo calization of any leakage current or other local heat source in an IC with a very high success rate without any preparation expense. LIT is also applicable for backside inspection and for detecting sub-surface heat sources. Its spatial resolution can be improved down to 1 µm by applying a solid immersion lens. By using LIT some faults can be localized which are not visible in OBIRCH or light emission microscopy. In this contribution the technique of microscopic IR Lock-in Thermography is described, the basic principles of the interpretation of the results are reviewed, and some typical results illustrating the application of this technique are introduced.