Liquid Crystal Hot Spot Detection | Equipment Considerations

equipmentVery little equipment is required for excellent liquid crystal hot spot detection capability.  The equipment shown above includes optional as well as essential elements.

(1) Metallurgical Microscope
A metallurgical microscope equipped with a polarizing set is essential.  The microscope on an analytical probe station is typical and ideal.  A polarizing set consists of two polarizers.  One polarizes the illumination and the other is placed in the reflected light path.  Common microscopes on probe stations are the Mitutoyo, the Bausch and Lomb MicroZoom, or the A Zoom.  Polarizer sets are available for all of these.

(2) Curve Tracer
Any DC power supply can be used to power the device under test (DUT), but a curve tracer is best.  The current/voltage status of the device can be monitored constantly with the curve tracer.
(3) Additional Power Supply
Additional power supplies may be necessary to set up bias  conditions that activate the abnormal power.

(4) External Heating Device
If the power associated with a defect is less than about 20 mW, the DUT must be heated a few degrees.  A point defect dissipating as little as 10uW can often be detected if its ambient temperature is increased to just below the liquid crystal transition temperature.  A precisely controlled hot chuck is perfect.  Temptronic Corporation and Trio-Tech International each offer well controlled hot chucks for wafers.   A copper disc with a center post can extend the advantage of a hot chuck to sockets machined to accept the post.  Nippon Scientific Co offers temperature controlled sockets.  The cost of these sophisticated systems should not be allowed to preclude the advantage of external heating.  Less expensive, but very acceptable “home made” heating schemes can be devised.  A variable voltage power supply and a few resistors or nichrome wire is one possibility.  A Peltier device offers a better solution because it can cool and heat.  Detection requires only a few degrees rise. Precise temperature control is necessary, but precise temperature measurement is not.   The liquid crystal is its own best temperature indicator.

(5) Variable Power Switch
A switch that varies the power delivered to the device under test is essential.   A hot spot generated from 1 mW is small enough to be camouflaged within a large mosaic of liquid crystal. The purpose of the conditioning is to create an oscillation on the DC voltage.  The resulting power variation makes the hot spot come and go in an eye catching manner.  Turning the power on and off can have the same effect for voltages below three volts. On/off switching above three volts induces visible effects due to electric field changes.  Voltage induced effects reduce visibility of the hot spot.  The switch position between the collector supply of the curve tracer and the DUT maintains a common ground with other test equipment.

(6) Video Camera & Monitor
No other equipment is required to perform liquid crystal analysis. However, a means to document and share results should be included.  A video camera and monitor allow more than one person to view results.

(7) Video Capture Capability
The best way to communicate dynamic results is by videotape. However, some editing capability is needed to make the video tape suitable for an effective report.   Another solution is a frame capture board that can provide digital input to a video printer and/or a computer network. The photograph below was captured from video tape by a “Snappy” accessory to a personal computer.

hot spot_5alt

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