Microthermography or Hot Spot Detection

Microthermography or Hot Spot Detection is a semiconductor failure analysis technique used to locate areas on the die surface that exhibit excessive heating. Excessive heating indicates a high current flow, which may be due to die defects or abnormalities like dielectric ruptures, metallization shorts, and leaky junctions.

An inexpensive yet effective Hot Spot Detection technique consists of dropping liquid crystal on the die surface of a biased device. The bias is chosen such that the defect site is allowed to conduct enough current to generate the amount of heat needed to change the visual characteristics of the liquid crystal. This technique is popularly known as the 'Liquid Crystal Technique.'

Liquid crystals undergo several phase changes as the temperature increases. At very low temperatures, liquid crystals are solid and are said to be in its crystalline phase. At a higher temperature liquid crystals become liquid while retaining the 'crystalline' nature of its optical properties, i.e., their elongated molecules tend to align parallel to each other.

There are two liquid crystal phases wherein the physical phase is liquid but the optical characteristics are crystalline, namely, the 'smectic' phase and the 'nematic' phase. At even higher temperatures, the liquid crystal goes into its 'isotropic phase,' wherein its molecules are randomly located and randomly oriented.

Only two phases are important in liquid crystals that are commercially available for Hot Spot Detection, i.e., the isotropic and the nematic phases. When in its isotropic phase, the liquid crystal appears as a clear liquid, offering no distinctive optical characteristics because of the homogeneous distribution of its molecules. When it is in its nematic phase, the liquid crystal has a milky appearance and exhibits optical properties similar to those of crystal structures.

When viewed using an optical microscope equipped with a polarizing filter in the illumination path and a cross polarized filter (analyzer) in the viewing path, thin films of liquid crystal in isotropic and nematic phases appear very different. Isotropic films appear black, since the cross polarized light is blocked by the analyzer.

On the other hand, nematic films appear to be rainbow-colored, since light reflected from nematic films 'twist' such that they are able to pass through the analyzer. Hots spots raise the temperature of the liquid crystal, making it appear black at that spot. The die surface of a well-prepared sample with a hot spot will therefore appear as rainbow-colored, except for the hot spot which will appear as a blackened area.

Curve Tracing; LEM; Microprobing; FA Lab Equipment; Basic FA Flows;

Package Failures; Die Failures

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