thermal analysis of devices using hot spot
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Thermal Analysis of Devices Using Hot Spot
Measurements
Dimitri Kakovitch
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Outline
• Terms
• Liquid crystals & Nematic liquidcrystal method
• WHY? & HOW?
• Examples
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Terms
• The crystal structure is said to be isotropic if the spacing of the atoms
is the same along each axis.
• If the spacing or arrangement of atoms along one axis is different
from that of another axis, then the crystal is said to be anisotropic. • If the atoms or molecules of a solid are not arranged in a regular
periodic structure, then the solid is said to be amorphous.
• Crystals that are anisotropic are also optically anisotropic. (An
optically isotropic medium is the same in all directions).
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Polarizer
A polarizer is a device that transmits thecomponent of the electric field in thedirection of its transmission axis andblocks the orthogonal component.
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WHY?
• As temperature rises on adevice, its performance isdegraded.
• Typical, heat sources arise fromelectronic components withlarger power consumption thantheir surroundings.
• Exact temperature and locationmeasurements of heat sources isa valuable tool in improvingdesign to prevent device failure.
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Techniques to measure the temperature on the
device surface.
• Infrared thermal imaging: a quick and
easy method for measuring temperature and
temperature gradients. – Limitation: the resolution of the image, so this
technique is not adequate for a micron-scale
device.• Nematic liquid crystal method
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Nematic liquid crystal method
• Advantages:
– non-destructive
– easy to apply
– high resolution that is limited only by the
resolution of the microscope used to
observe it.
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Liquid Crystal
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Typical Setup
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HOW?• The nematic liquid crystal is an optically anistropic medium
which can be used as a polarization rotator under certainconditions.
– Note: A polarization rotator rotates the plane of polarization
of linearly polarized light by a fixed angle, maintaining its
linearly polarized nature.
• A thin layer of liquid crystal is polished on the device.
• The nematic liquid crystal is placed between two linear
polarizers, which are on the microscope.
• The transmitted intensity of light can be modulated depending
on the rotation angle of the liquid crystal molecules. Themolecular orientation can be changed thermally or by an applied
electric field.
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HOW? (cont.)
• The liquid crystal becomes an isotropic medium at the
transition temperature. In other words, a thin layer of
liquid crystal below transition temperature actually twists
or rotates the light as it passes through.
• Above the transition temperature, the orientation of
molecules becomes random and the liquid crystal becomes
an optically isotropic medium.
• Linearly polarized light is required to examine the
transition temperature (or clearing point) boundary of the
liquid crystal through a cross polarizer.
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Nematic under optical microscope
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Propagation of light in a twisted
nematic liquid crystal.
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In other words:
• If a spot of liquid crystal is locally heated due to device
failure or power consumption above the clearing point
temperature, such spot will appear as dark gray or black.
• For example, if we use a liquid crystal with a clearing
point temperature of 29°C, then the device surface will
look dark gray if the surface temperature of the device is
higher than the clearing point temperature.
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Example
• GaN-based electronic devices (i.e. AlGaN/GaN HFETs)
– Suffers from serious heating issues due to their poor
thermal conductivity
– Thermal analysis for these devices has been
increasingly sought after since their heat dissipation can
sufficiently degrade the DC and the RF performances
of the device; in particular during the high power
operation. It locates the heat source associated with
anomalous leakage or power associated with a device
failure.
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Another Example• Increased system temperature
arising from dense circuitrycan lead to degradation of theimage quality as the sensor
heats itself up.• The A/D converter is a major
power consumer and itmanifests itself as a heatsource. An increase in the
driving power of the A/Dconverter generates hightemperature and the heatspreads out on the surface.
Y Media's new 3.17 C3D™
megapixel image sensor
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Hot Spot
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