Crucible Retaining Sleeve with Anisotropic Thermal Conductivity
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Solution Overview
Problem
The directed solidification of semiconductor ingots, such as silicon and germanium, faces challenges due to impurity diffusion, thermomechanical stresses, and crystalline defects, which degrade the optical and electrical properties of the ingots, making it difficult to produce ingots of greater height and width with uniform quality.
Innovation Solution
A crucible with a mold and retaining sleeve having anisotropic thermal conductivity, where the radial thermal conductivity is significantly higher than the axial conductivity, is used to regulate heat fluxes and reduce thermomechanical stresses, maintaining the shape of the mold and controlling the solidification front.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the height and width of the ingot are increased to improve productivity, then the output per batch is improved, but the thermomechanical stresses increase leading to more crystalline defects
Solution Approach 1:
The retaining sleeve is provided with anisotropic thermal conductivity where the radial thermal conductivity is higher than the axial thermal conductivity. This creates different thermal characteristics in different directions: radially, heat is conducted more efficiently to maintain uniform temperature distribution, while axially, heat propagation is limited to reduce thermomechanical stresses. This local differentiation of thermal properties allows producing taller and wider ingots with reduced crystalline defects.
2Reliability
If uniform heat distribution is maintained to reduce thermomechanical stresses, then crystalline defect density is reduced, but the ability to control solidification front becomes compromised
Solution Approach 1:
The anisotropic thermal conductivity of the retaining sleeve creates different thermal behaviors in radial and axial directions. Radially, high thermal conductivity ensures uniform heat distribution to reduce thermomechanical stresses. Axially, lower thermal conductivity allows temperature gradients to be maintained for controlling the solidification front position and shape. This directional differentiation resolves the contradiction between reducing defects and maintaining solidification control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for the production of ingots with reduced crystalline defects and improved optical and electrical properties, enabling the manufacture of ingots with greater height and width while maintaining a uniform density of defects, compared to traditional methods.
Implementation Method 1
the sleeve material having anisotropic thermal conductivity and radial components and axial components such that the radial component of the thermal conductivity is higher than the axial component of the thermal conductivity
Implementation Method 2
the sleeve material, being more insulating axially than radially, limits the axial propagation of heat from the heat exchanger block within the sleeve
Implementation Method 3
heat flows applied radially to the ingot and the molten silicon bath
Implementation Method 4
These stresses result from temperature gradients that develop first in the nuclei during the melting of the initial charge to form the bath, and then between the central and peripheral regions of the solidifying ingot
Implementation Method 5
local temperature variations within the molten silicon bath induce natural convection currents that promote the segregation of impurities
Implementation Method 6
These impurities diffuse into the molten silicon bath and degrade the optical and electrical properties of the ingot
Implementation Method 7
a mold comprising a mold bottom wall and a mold side wall extending from the mold bottom wall along an axial direction, the mold bottom wall and the mold side wall together defining a cavity to contain the molten ingot material
Data Source
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AI summary
Crucible for directional solidification of an ingot made of an ingot material chosen from silicon, germanium and mixtures thereof, comprising: - a mold comprising a mold bottom wall and a mold side wall extending from the mold bottom wall along an axial direction, the mold bottom wall and the mold side wall together defining a cavity for containing the molten ingot material, and - a support sleeve extending in the axial direction and comprising a sleeve material, the thermal conductivity of which is anisotropic and has radial (I) and axial (II) components such that: (III, IV) being the thermal conductivity of the mold material, V being the thermal conductivity of graphite, the support sleeve encircling the mold side wall around the axial direction. <sp /> <sb /> <sp /> <sb /> <sp />