Thermal Analysis of Devices Using Hot Spot Measurements

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Thermal Analysis of Devices Using Hot Spot
Measurements

• Dimitri Kakovitch

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Outline

• Terms
• Liquid crystals Nematic liquid crystal 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 the
component of the electric field in the direction
of its transmission axis and blocks the
orthogonal component.
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WHY?

• As temperature rises on a device, its performance
  is degraded.
• Typical, heat sources arise from electronic
  components with larger power consumption than
  their surroundings.
• Exact temperature and location measurements of
  heat sources is a valuable tool in improving
  design 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 certain conditions.
• 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. The molecular
  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 29C, 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
  circuitry can lead to degradation of the image
  quality as the sensor heats itself up.
• The A/D converter is a major power consumer and
  it manifests itself as a heat source. An
  increase in the driving power of the A/D
  converter generates high temperature and the heat
  spreads out on the surface.

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Hot Spot
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