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.

When performing Liquid Crystal Hot Spot Detection, the following must be observed:

The correct amount of liquid crystal must be dropped on the die surface. Too thick a film of liquid crystal would appear uniformly black, making it impossible to detect nematic phase changes from temperature increases. Too thin a film makes the surface appear as streaks of dark and light grays, also making hot spots less visible. The perfect amount would make the general area of the die surface look rainbow-colored, which would offer the best contrast to a hot spot.

The bias must be chosen such that the defect site carries enough current to heat the liquid crystal at the hot spot, but not enough current to heat the entire liquid crystal film. If the hot spot generates so much heat that the entire die surface is darkened even at minimum power setting, the excitation must be 'pulsed' or oscillated. This will allow the analyst to locate the hot spot, which is the point of origin and return of the oscillating color contrast on the die surface.

The polarizing filters of the microscope must be adjusted to provide the best rainbow color for the liquid crystal film.

Care must be taken when interpreting the presence or absence of hot spots on the die. Although an abnormal hot spot very likely means that something wrong, the hot spot itself is not always the actual failure site. Some hot spots come from good components that are just forced to conduct high currents by an anomaly somewhere else in the circuit. It is important to complement microthermography results with those of other FA techniques in order to arrive at the right conclusion.

Figure 1. Photo of a hot spot during

liquid crystal analysis; note the rainbow

color of the liquid crystal

Microthermography is used for detecting the following: Dielectric Shorts or Breakdowns, Metallization Shorts, Junction Leakages, Mobile Ionic Contamination, etc.

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

Package Failures; Die Failures

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