Multicrystalline Silicon Ingot Pulling with Multiple Seeds
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Solution Overview
Problem
Conventional multicrystalline silicon ingot production by directional solidification results in reduced median grain size and high impurity content near the crucible, leading to inefficiencies in solar cell production and material recycling.
Innovation Solution
A method involving the use of multiple seed crystals and controlled growth conditions in a Czochralski-type crystal puller to produce multicrystalline silicon ingots with a constant diameter and reduced impurity segregation, utilizing a crucible with precise temperature control and axial heat flow to minimize dislocation multiplication and maximize usable ingot material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If directional solidification is used to produce multicrystalline silicon, then production throughput is increased, but median grain size is reduced near the crucible
Solution Approach 1:
The patent applies preliminary action by pre-heating the crucible to a temperature above the melting point of silicon before loading the charge, and by pre-forming a meniscus in the melt. This preliminary preparation creates optimal conditions for grain growth before the actual solidification begins, allowing larger grain sizes to form even at high production rates.
Solution Approach 2:
The patent changes key process parameters including crucible temperature (maintained above melting point), cooling rate (controlled to be less than 10°C per hour), and oxygen content (controlled at 10-1000 ppm). These parameter changes create a controlled solidification environment that promotes larger grain formation while maintaining high throughput production.
2Productivity
If directional solidification is used to produce multicrystalline silicon, then production throughput is increased, but impurity content increases in the top region
Solution Approach 1:
The patent controls the oxygen content parameter at 10-1000 ppm and maintains a slow cooling rate (less than 10°C per hour) during solidification. These parameter changes reduce impurity segregation and allow the top region of the ingot to meet device fabrication specifications, thereby increasing the proportion of usable material.
Solution Approach 2:
The patent implements feedback control by monitoring and adjusting the cooling rate and atmospheric conditions during solidification. This feedback mechanism ensures that impurity segregation is minimized throughout the process, maintaining quality in the top region of the ingot even at high production rates.
3Ease of manufacture
If conventional directional solidification is used, then multicrystalline silicon can be produced, but the peripheral portion near the crucible cannot be used for device fabrication
Solution Approach 1:
The patent applies local quality by creating different conditions in different regions of the crucible. The crucible is pre-heated to different temperatures in different zones, and the cooling rate is controlled to create a temperature gradient that promotes uniform grain growth throughout the ingot, including the peripheral regions near the crucible wall.
Solution Approach 2:
The patent uses preliminary action by pre-heating the crucible and establishing a meniscus before solidification begins. This preparation ensures that the peripheral regions near the crucible wall are already at optimal temperature and conditions for grain growth, preventing the formation of small grains and high impurity content in these regions.
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 enhances the median grain size and reduces impurity content, increasing the throughput of usable multicrystalline silicon for solar cell production while minimizing waste, thereby improving the efficiency and yield of the ingot production process.
Implementation Method 1
the silicon charge is heated to a temperature above about the melting temperature of the charge to form a silicon melt
Implementation Method 2
directional solidification (DS) process in which silicon is melted in a crucible and directionally solidified
Implementation Method 3
utilizing a crucible with precise temperature control and axial heat flow
Data Source
AI summary
Methods for producing muticrystalline silicon ingots by use of a Czochralski-type crystal puller and pulling assemblies that include a plurality of seed crystals for pulling multicrystalline silicon ingots.