Television – Special applications – Hazardous or inaccessible
Reexamination Certificate
1997-06-10
2001-05-01
Kelley, Chris (Department: 2613)
Television
Special applications
Hazardous or inaccessible
C348S086000, C117S014000
Reexamination Certificate
active
06226032
ABSTRACT:
TECHNICAL FIELD
Diameter control systems for pulling monocrystalline rods by the Czochralski (CZ) method.
BACKGROUND
Several control systems have been developed to control the diameter of a monocrystalline rod being pulled by the CZ method. These involve optical scanning systems and CCD camera systems that derive crystal diameter information from a reflective meniscus around the crystal's emergence from the melt surface. Although these systems work fairly well in controlling the diameter of the body of the crystalline rod, improved precision is always welcome, especially since the completed rod must be ground to a uniform diameter before it is sliced into wafers.
The existing systems have significantly more difficulty in accurately controlling the diameter of the neck of the crystal, which is formed as the pulling process is initiated. The crystal growth from a seed crystal is necked down to a diameter of 3 to 4 millimeters before the diameter is enlarged to a shoulder and thereafter kept constant as the cylindrical body of the rod is pulled. Forming larger diameter cylindrical bodies of 300 millimeters as desired by the wafer processing industry requires that the neck of the crystal be made somewhat larger, at a diameter of about 5 millimeters. One of the functions of the crystal neck forming process is to reduce crystal defects in the early stages of crystal growth. Larger neck diameters are less effective at reducing defects, but defect reduction is improved by maintaining well-controlled diameters in the crystal neck region. Thus, improving the accuracy of diameter control of the neck region of the crystal becomes especially important for reducing defects in necks that are necessarily somewhat larger in diameter, because of the increased weight the necks must support as the diameter of the body of the crystal being pulled increases.
The improvements we have devised in a crystal diameter control system especially improve the accuracy of the diameter maintained in the neck region of the crystal. Our system also improves on the accuracy and versatility of the diameter control of the cylindrical body of the crystalline rod.
SUMMARY OF THE INVENTION
Our improved control system uses a CCD camera aimed into the crystal pulling crucible so as to view light reflected from a meniscus formed around the crystal's emergence from the melt level. Light from the meniscus is imaged as a luminous arc on a pixel array of the CCD camera, and the output signal from the camera is preferably adjusted using the reflectance of the melt level surface as a standard and selecting luminance of the meniscus arc as a predetermined value above the luminance of the melt surface. The signal from the image of the arc is then processed to determine the location and dimensional configuration of the luminous arc, preferably several times per second.
The diameter of the crystal is calculated by a computer from the optical information derived from the arc imaged on the CCD camera. This involves selecting one or more chords across the luminous arc and determining the chord lengths from signals produced by the output of the CCD camera. From the chord lengths, the diameter of the crystalline rod is calculated, with the computer also receiving an input from a melt level measurement system. This preferably involves a laser beam reflected off the melt level at an angle that results in light movement relative to a sensor as the melt level changes. Computer control of the diameter measurement calculation allows software control so that the diameter measurement process can be varied, calibrated, and improved based on experience.
Calibration of the diameter calculation can be done with a movable viewing telescope that sights opposite edges of the crystal diameter. The light reflecting from the meniscus has a width and a diameter larger than the crystal so that the crystal diameter calculation corrects for this.
For controlling crystal diameter at the neck region of the crystal, the process uses as an additional input the total number of pixels in the CCD camera that are illuminated by the image of the meniscus arc around the crystal neck. We have discovered that changes in crystal diameter as the neck region is being pulled produce stronger signal variations in the total pixel illumination count than in the crystal diameter calculation. Our control system exploits this by responding in a short time duration manner to changes in the pixel illumination count and in responding over a longer time duration to changes in the crystal diameter calculation. This is preferably done by forming a ratio of the total pixel illumination count divided by the diameter measurement. This ratio is filtered over a brief duration and applied to a controller adjusting the pull rate for the crystal.
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“Pulling of Silicon Mono Crystals” by M. Dreckmann, IBM Technical Disclosure Bulletin, vol. 11, No. 10, Mar. 1969, New York , pp. 1374-1375.
Latka Henry C.
Lees David K.
General Signal Corporation
Kelley Chris
Pepper Hamilton LLP
Vo Tung
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