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Liquid crystal hot spot detection with infinitesimal temperature control
| Details |
Inventors: Tan, Peng;
Assignee:
Primary Examiner: Griffin; Donald A.
Assistant Examiner:
Attorney, Agent or Firm:
This improvement is the process of using a few well collimated and even radiative heating lights to heat up the liquid crystal film, which is spread over the surface of the die of an integrated circuit. The radiative heating of the liquid crystal film from the top helps to form an even temperature profile on the liquid temperature film. The rapid time response of the heating filament temperature and the radiative heating process induces a rapid response in the liquid crystal film temperature. By repeatedly turning on and turning off the heating lights at an appropriate power setting, the temperature of the liquid crystal film will repeatedly rise above and drop below the liquid crystal phase transition temperature. During this temperature rise and drop process, the liquid crystal film temperature is brought to infinesimally close below to the liquid crystal phase transisiton temperature, for a limited length of time. We call this temperature control process the infinitesimal temperature control method. During this limited length of time, a small ohmic heating dissipated from the die into the liquid crystal film would induce a localized phase transition in the liquid crystal film. Under a cross polarized light, the nematic liquid phase transition process exhibit a change in the liquid crystal's transparency and colors. The transition process is most easily visible when the die is periodically dissipating heat into the liquid crystal film at a 1.2 Hz and at 50% duty cycle. At this periodic heat dissipating mode, the periodic phase transistion induces a blinking appearance at the region where the phase transition is taking place. I call this blinking the `hot spot induced blinking`. This periodic ohmic heating is accompanied with periodic voltage change in the die. The voltage changes will induce a blinking appearance similar to the hot spot induced blinking. With the use of the infinitesimal temperature control method to vary the ambient temperature of the liquid crystal film the blinking spot size of the hot spot induced blinking increases as the ambient temperature increases. But the blinking spot size of the voltage induced blinking does not respond to temperature changes, as long as the liquid crystal's temperature is not beyond the phase transition temperature. Thus, the varied heating light provide a means to differentiate the two types of blinkings. We have illustrated heating lights as the means of heating up the liquid crystal film. However, if the heating lights were replaced by other heating means, such as a conductive hot plate, or a convective oven, the infinitesimal temperature control method would still work, as long as these alternate heating means were operated at a repeatedly turning on and turning off mode. Therefore, this hot spot detection process will also work well with other heating means. |
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DETAILED DESCRIPTION OF THE INVENTION.
It should be emphasized that it is not the detailed design of the heating means that contributes to the primary success of bringing the liquid crystal film temperature infinitesimally close below to the liquid crystal phase transition temperature, rather it is the invented method of operating the heating means at a repeatedly turning on and turning off mode.
The sensitivity of this invention was helped by using a pulsing input to the hot spot.
The pulsing input induces a voltage induced blinking which can be mistaken for the hot spot induced blinking.
The difficulty arising from the inability to differentiate between a voltage induced blinking and a hot spot induced blinking was solved by a differentiation method invented by this invention, that is, by observing the temperature responses of these two kinds of blinkings: The hot spot induced blinking increases in blinking spot size as the liquid crystal temperature rises, the voltage induced blinking remains in blinking spot size as the liquid crystal temperature rises.
Reference 1: John Hiatt, "A Method of Detecting Hot Spots on Semiconductors using Liquid Crystals.
" 19th Annual Proceedings of the IEEE Reliability Physics Symposium, 1981, Pg.
130-133.
Reference 2: E.
M.
Fleuren, "A very sensitive, simple analysis technique using nematic liquid crystals," 21st Annual Proceedings of the IEEE Reliability Physics Symposium, 1983, Pg.
148-149.
Reference 3: J.
L.
Fergason, "Liquid crystals in nondestructive testing," Applied Optics, Vol.
7, No.
9, Sept.
1968 Pg.
1729-1737.
Reference 4: G.
D.
Dixon, "Cholesteric liquid crystal in nondestructive testing," Material Evaluation, June 1977, Pg.
51-55.
SUMMARY OF THE INVENTION This invention invented a few processes that significantly improve the effectiveness of the liquid crystal hot spot detection method in terms of the ability to detect the lowest power hot spot on a die or wafer.
These invented processes, new use of certain liquid crystals, and hardware include, but are not limited to, the following items: Item (1): The new use of a few nematic liquid crystals for the liquid crystal hot spot detection method; Item (2): A new process to judge and obtain an optimal thickness of nematic liquid crystal film that is most suitable for the liquid crystal hot spot detection method; Item (3): A new process to differentiate the voltage induced blinking from the hot spot induced blinking when current input to the die or wafer is operated at a pulsing mode
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