Multi-Stage Cooling System for Monocrystalline Ingot Edge Band Defects
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Solution Overview
Problem
Existing crystal pulling systems for producing monocrystalline ingots, such as those used in semiconductor and solar-grade materials, face challenges in reducing edge band defects, which affect the quality and yield of silicon wafers, particularly due to the incorporation and agglomeration of vacancies and oxygen precipitates during the Czochralski process.
Innovation Solution
A crystal pulling system incorporating a multi-stage cooling system with an annular heat shield and a fluid-cooled housing that applies varying cooling rates to the ingot as it is pulled from the melt, including enhanced and reduced cooling zones to control the nucleation and transport of defects, thereby reducing the size and concentration of defects in the edge band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the v/G ratio is controlled near a critical value to suppress point defects, then vacancy and interstitial defects are incorporated in low concentrations, but an annular ring of agglomerated defects (voids and oxygen precipitates) forms in the edge band
Solution Approach 1:
The cooling process is segmented into multiple stages with different cooling rates. The multi-stage cooling system applies different cooling rates to different axial segments of the crystal during growth, allowing suppression of defect formation in the edge band while maintaining overall crystal quality.
Solution Approach 2:
The cooling rate is made dynamic rather than constant. The system adjusts cooling rates during the crystal growth process, transitioning between different cooling stages to control defect nucleation and transport in real-time, preventing edge band defect formation.
2Productivity
If conventional single-stage cooling is used, then the crystal growth process is simple, but the edge band contains large and concentrated agglomerated defects that reduce wafer yield
Solution Approach 1:
The cooling system is divided into multiple stages, each with optimized cooling rates for specific growth phases. This segmentation allows precise control over defect formation in the edge band, reducing defect size and concentration to improve wafer yield.
Solution Approach 2:
The cooling rate parameter is changed throughout the growth process rather than remaining constant. By varying the cooling rate according to the growth stage and crystal conditions, the system optimizes defect characteristics and improves overall productivity.
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
The system effectively minimizes the size and concentration of defects in the edge band of monocrystalline ingots by controlling cooling rates and the melt/crystal interface profile, leading to improved wafer quality and increased yield without the need for post-growth thermal treatments.
Implementation Method 1
a multi-stage heat exchanger positioned within the passage defined by the heat shield, and including a fluid-cooled housing that defines a central passage for receiving the ingot
Implementation Method 2
controlling the v/G ratio near a critical v/G value at which vacancy and interstitial defects are incorporated into the growing crystal ingot in very low and comparable concentrations
Data Source
AI summary
A crystal pulling system for growing a monocrystalline ingot from a melt of semiconductor or solar-grade material includes a crucible for containing the melt of material, a pulling mechanism configured to pull the ingot from the melt along a pull axis, and a multi-stage heat exchanger defining a central passage for receiving the ingot as the ingot is pulled by the pulling mechanism. The heat exchanger defines a plurality of cooling zones arranged vertically along the pull axis of the crystal pulling system. The plurality of cooling zones includes two enhanced-rate cooling zones and a reduced-rate cooling zone disposed vertically between the two enhanced-rate cooling zones.