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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