Silicon Sheet Thickness Control via Side Cooling
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Solution Overview
Problem
Current methods for producing silicon sheets from a melt, such as sawing or pulling, result in significant material waste, high costs, and limitations in achieving thin, high-quality solar cells due to kerf losses and temperature gradients, which hinder the reduction of the cost/power ratio in the solar cell industry.
Innovation Solution
A method and apparatus involving a cooling plate and radiative heater to form and thin silicon sheets from a melt, minimizing material waste and improving crystal quality by controlling cooling parameters and solute concentrations, allowing for continuous sheet formation with reduced thermal gradients and stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pull rate is increased to improve productivity, then the sheet can be produced faster, but the temperature gradient increases causing poor quality multi-grain silicon
Solution Approach 1:
The patent transitions from vertical ribbon pulling to horizontal sheet formation. The melt is cooled from the sides rather than from the bottom, changing the thermal gradient direction. This allows the sheet to form horizontally with reduced temperature gradients, improving crystal quality while enabling higher production rates through continuous sheet formation.
Solution Approach 2:
The patent changes the cooling configuration from bottom-up vertical cooling to side-cooling in a horizontal configuration. The cooling plates are positioned on opposite sides of the melt, creating a more uniform temperature distribution. This parameter change reduces thermal stress and prevents multi-grain formation, allowing faster pull rates up to 18 mm/minute or higher.
2Manufacturing precision
If the sheet thickness is reduced to improve solar cell efficiency, then the cost/power ratio decreases, but the temperature gradient increases causing dislocations and warping
Solution Approach 1:
The patent uses separate cooling zones with different cooling rates. The regions closer to the cooling plates experience higher cooling rates for rapid solidification, while regions farther away experience lower cooling rates. This gradient in local cooling quality allows thin sheets to form without excessive temperature gradients, preventing dislocations and warping while achieving the desired thin thickness for high efficiency solar cells.
3Reliability
If sawing is used to cut wafers from monocrystalline silicon boules, then single crystal growth is maintained, but kerf loss increases material waste
Solution Approach 1:
The patent extracts the solidification process from the traditional vertical boule growth method. Instead of growing a large boule and cutting it into wafers, the melt is directly solidified into thin sheets horizontally. This extraction of the solidification step eliminates the need for subsequent sawing operations, removing kerf loss entirely while maintaining single crystal quality through controlled cooling.
4Productivity
If polycrystalline silicon ingots are used to reduce manufacturing complexity, then production speed increases, but wafer quality decreases due to defects and grain boundaries
Solution Approach 1:
The patent changes from vertical ingot growth to horizontal sheet formation. This dimensional change allows the entire solidification process to occur in a single pass, producing thin sheets directly at high speeds. The horizontal configuration with side cooling eliminates the formation of grain boundaries and defects associated with vertical polycrystalline growth, achieving both high productivity and high wafer quality simultaneously.
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
Enables the production of thinner, higher-quality silicon sheets with reduced material costs and improved efficiency, potentially lowering the cost/power ratio of solar cells and enabling wider availability of renewable energy technology.
Implementation Method 1
The removed latent heat during cooling and solidifying of the silicon must be removed along the vertical ribbon
Implementation Method 2
A method and apparatus involving a cooling plate and radiative heater to form and thin silicon sheets from a melt
Data Source
AI summary
A method and apparatus for forming a sheet are disclosed. A melt is cooled and a sheet is formed on the melt. This sheet has a first thickness. The sheet is then thinned from the first thickness to a second thickness using, for example, a heater or the melt. The cooling may be configured to allow solutes to be trapped in a region of the sheet and this particular sheet may be thinned and the solutes removed. The melt may be, for example, silicon, silicon and germanium, gallium, or gallium nitride.