Thermal conductivity is an important thermal property of silicon that changes with temperature. It determines the temperature distribution across a silicon wafer during semiconductor processing. Impurities in the silicon, both inherent and dopants introduced during manufacturing, decrease thermal conductivity as levels increase. When designing for temperature uniformity, adjustments must be made to account for the lower conductivity of doped silicon above 100 degrees Kelvin. Determining exact temperature uniformity requires complex 3D thermal modeling rather than just relying on conductivity properties. Liquid silicon has much higher thermal conductivity than solid silicon.

1. Silicon Thermal Conductivity
Material Properties

2. Facts
Thermal conductivity is one of many thermal properties of the
material silicon. Other properties include specific heat (0.70
Joules per gram degree Kelvin) which helps determine power
required to raise the temperature of the wafer, boiling point
(2,628 degrees Kelvin), melting point (1,683 K), critical
temperature (5,159 K), thermal diffusivity (.9 cm2 per second),
linear thermal expansion or thermal expansion coefficient (2.6 •
10-6 C-1), Debye temperature (640 K) and others.
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3. Values
Thermal conductivity changes as temperature changes. "You
need to keep the required operating temperature in mind when
using thermal conductivity to determine temperature distribution
across the wafer," warns Michael Klebig, a Silicon Valley sales
engineer specializing in the application of heat in semiconductor
wafer processing. "The wrong thermal conductivity means you
won't achieve the temperature uniformity you need and the
performance of the devices will be compromised. If it's bad
enough, you won't be able to continue to the next step of the
semiconductor processing sequence. The wafer will be useless."
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4. Impurities
There are two types of impurities in the silicon chip
manufacturing process. First, are impurities inherent in the
original silicon ingot. The second type of impurity is known as a
dopant, an impurity deliberately introduced in the manufacturing
process to change the electrical properties of silicon in specific,
exposed areas on a wafer. As the level of impurities increases,
thermal conductivity decreases. "When designing for temperature
uniformity, you must make adjustments in response to the lower
conductivity value of doped silicon," says Klebig. "Doped silicon
above 100 degrees Kelvin has negligible impact on thermal
conductivity."
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5. Expert Insight
Determining temperature uniformity by use of thermal
conductivity and other thermal properties is an extremely
complex task. "As a result," says Klebig, "you should utilize best
known methods when designing for temperature uniformity, such
as 3D thermal finite element analysis (FEA) computer modeling.
Large engineering organizations typically have their own FEA
modeling groups. If you don't have access to such a resource
within your company, contact an FEA engineering firm."
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6. Warning
Be aware that thermal conductivity material properties are
different for silicon in liquid form. Liquid silicon has three times
the thermal conductivity of solid silicon.
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