Thermal Analysis of Devices Using Hot Spot

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Thermal Analysis of Devices Using Hot Spot

Measurements

Dimitri Kakovitch



Outline

• Terms

• Liquid crystals & Nematic liquidcrystal method

• WHY? & HOW?

• Examples



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).



Polarizer

A polarizer is a device that transmits thecomponent of the electric field in thedirection of its transmission axis andblocks the orthogonal component.



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.



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



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.



Liquid Crystal







Typical Setup



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.



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.







Nematic under optical microscope



Propagation of light in a twisted

nematic liquid crystal.



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.



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.



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



Hot Spot



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Thermal Analysis of Devices Using Hot Spot