Eddy current thickness measurement apparatus
A sheet of a material is disposed in a melt of the material. The sheet is formed using a cooling plate in one instance. An exciting coil and sensing coil are positioned downstream of the cooling plate. The exciting coil and sensing coil use eddy currents to determine a thickness of the solid sheet on top of the melt.
1. A sheet-forming apparatus comprising:
a melt of a material;
a sheet of said material disposed in said melt, wherein said sheet has a higher resistivity than said melt;
a cooling plate disposed a distance above a surface of said melt that is configured to form said sheet;
an eddy current measurement system for providing real-time monitoring and feedback of a thickness of said sheet as it is formed in said melt, the eddy current measurement system comprising:
an exciting coil and a sensing coil disposed a second distance above said surface of said melt downstream of said cooling plate; and
a power source connected to said exciting coil; and
a controller connected to said eddy current measurement system, said controller programmed to execute instructions to increase a temperature of said cooling plate if said sheet is thicker than desired and to decrease a temperature of said cooling plate if said sheet is thinner than desired.
2. The sheet-forming apparatus of claim 1 , wherein said sheet floats on said melt.
3. The sheet-forming apparatus of claim 1 , wherein said material is silicon.
4. The sheet-forming apparatus of claim 1 , further comprising a quartz sleeve disposed around said exciting coil and said sensing coil.
5. The sheet-forming apparatus of claim 1 , wherein said exciting coil and said sensing coil are composed of copper or molybdenum.
6. The sheet-forming apparatus of claim 1 , further comprising a vessel configured to contain said melt and a spillway configured to separate said sheet from said melt.
7. The sheet-forming apparatus of claim 1 , further comprising a pump configured to make said sheet and a surface of said melt flow at an equal speed.
8. The sheet-forming apparatus of claim 1 , wherein said power source is configured to energize said exciting coil to produce a time-varying magnetic field.
9. The sheet-forming apparatus of claim 1 , wherein said controller is configured to measure an induced magnetic field in said sensing coil, wherein said controller interprets a signal from said sensing coil to estimate a thickness of a higher resistivity layer disposed on top of a lower resistivity layer.
10. The sheet-forming apparatus of claim 1 , wherein said exciting coil and said sensing coil are contained within a single coil.
11. The sheet-forming apparatus of claim 1 , wherein one of said exciting coil and said sensing coil has a diameter, and wherein said second distance is less than said diameter.
12. The sheet-forming apparatus of claim 1 , wherein said cooling plate is configured to use radiation cooling.
13. A sheet-forming apparatus comprising:
a melt of a material;
a sheet of said material that floats on said melt, wherein said sheet has a higher resistivity than said melt;
a cooling plate disposed a distance above a surface of said melt that is configured to form said sheet, wherein said cooling plate is configured to use radiation cooling;
a pump configured to make said sheet and said melt flow; and
an eddy current measurement system configured for real-time monitoring and feedback of a thickness of said sheet as it is formed on said melt, the eddy current measurement system comprising:
an exciting coil and a sensing coil disposed a second distance above said surface of said melt downstream of said cooling plate and; and
a power source connected to said exciting coil; and
a controller connected to said eddy current measurement system, said controller programmed to execute instructions to increase a speed of said melt if said sheet is thicker than desired and to decrease a speed of said melt if said sheet is thinner than desired.
14. The sheet-forming apparatus of claim 13 , wherein said material is silicon.
15. The sheet-forming apparatus of claim 13 , further comprising a quartz sleeve disposed around said exciting coil and said sensing coil.
16. The sheet-forming apparatus of claim 13 , wherein said controller is configured to measure an induced magnetic field in said sensing coil, wherein said controller interprets a signal from said sensing coil to estimate a thickness of a higher resistivity layer disposed on top of a lower resistivity layer.
17. The sheet-forming apparatus of claim 13 , wherein one of said exciting coil and said sensing coil has a diameter, and wherein said second distance is less than said diameter.
18. The sheet-forming apparatus of claim 13 , wherein said power source is configured to energize said exciting coil to produce a time-varying magnetic field.