Float-Zone Silicon Crystal Growth Using Alternating Rotation and Magnetic Field
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Solution Overview
Problem
Existing methods for producing single crystals, such as the Czochralski and float-zone processes, face challenges in controlling crystallization, particularly for larger diameters, leading to imperfections and non-uniform resistivity in silicon rods, which are critical for high-quality semiconductor wafers.
Innovation Solution
Applying an axially oriented magnetic field in conjunction with an alternating rotational pattern during the float-zone process to stabilize the molten zone and ensure uniform resistivity across the cross-sectional plane of the crystal, using a solenoid coil to generate the magnetic field and adjusting rotational rates and durations to optimize crystal growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the diameter of the single crystal rod is increased, then the productivity and output are improved, but the manufacturing precision and uniformity of resistivity deteriorate due to increased imperfections and difficulties in controlling crystallization
Solution Approach 1:
The patent applies an axial magnetic field to the molten zone during float-zone crystallization, changing the physical parameters of the system. This magnetic field influence modifies the crystallization process, enabling better control over crystal growth and dopant distribution even in larger diameter rods, thus maintaining resistivity uniformity while allowing increased productivity
Solution Approach 2:
The patent employs periodic rotation of the crystal rod during growth, alternating between different rotational directions or speeds. This periodic mechanical action, combined with the axial magnetic field, helps distribute impurities and dopants more uniformly throughout the molten zone, preventing segregation and ensuring homogeneous resistivity across the cross-section of larger diameter crystals
2Device complexity
If conventional float-zone process is used without magnetic field, then the device complexity is low, but the manufacturing precision and control over crystallization deteriorate
Solution Approach 1:
The patent introduces a magnetic field as an intermediary element between the heating system and the crystal growth process. This magnetic field acts as a mediator that influences the behavior of charged particles (ions) in the molten silicon, improving crystallization control without requiring direct mechanical or chemical intervention in the melt, thus maintaining relative process simplicity while enhancing manufacturing precision
3Device complexity
If permanent magnets are used to generate axial magnetic field, then the device complexity and cost are reduced, but the reliability deteriorates due to demagnetization at high temperatures
Solution Approach 1:
The patent replaces the mechanical permanent magnet system with an electromagnetic field generation system (such as solenoid coils or electromagnets). This substitution eliminates the demagnetization problem inherent in permanent magnets at high temperatures, as electromagnets can maintain their magnetic field strength regardless of temperature, thus improving reliability while the system design keeps the overall device complexity manageable
Solution Approach 2:
The patent changes the operational parameters of the magnetic field generation from static (permanent magnets) to dynamically controllable (electromagnets with adjustable current). This allows the magnetic field strength to be optimized for each specific crystallization condition, ensuring stable and reliable field application throughout the process, and enabling compensation for any temperature-induced variations in the system
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 combined approach significantly improves the uniformity of single crystals, as evidenced by reduced relative resistivity values, ensuring high-quality wafers with minimal resistivity variations, particularly effective for silicon doped with boron or phosphorus.
Implementation Method 1
Applying an axially oriented magnetic field in conjunction with an alternating rotational pattern during the float-zone process to stabilize the molten zone
Implementation Method 2
the movement of the molten zone from one end of the polycrystalline rod to the other
Implementation Method 3
rely upon the formation of a single crystal during resolidification of molten material
Data Source
Figure 1
Figure 2~3
AI summary
A method is disclosed for producing a single crystal comprising passing a polycrystalline rod (1) through a heating region 12) to create a molten zone (3), applying a magnetic field to the molten zone, and inducing growth of a single crystal (5) upon solidification of the molten material on a single crystal seed (4). The growing single crystal is rotated in a pattern alternating between clockwise and counter-clockwise rotational directions. The method is useful for producing silicon single crystals having uniform electrical characteristics. Also disclosed is an apparatus for performing said method.