This invention relates generally to improvements in controlling the growth process of a monocrystalline silicon ingot and, more particularly, to a method and apparatus for accurately measuring the diameter of a monocrystalline silicon ingot during its growth process.
The Czochralski (CZ) process is used to obtain monocrystals, the most important application of which is to grow a monocrystalline silicon ingot. The silicon ingot is later sliced into silicon wafers for fabrication of semiconductor circuits thereon. Briefly described, the CZ process includes melting a charge of polycrystalline silicon in a quartz crucible and lifting a monocrystalline seed from the surface of the melt silicon. As the seed is lifted from the melted silicon, monocrystalline silicon grows from the seed and forms a cylindrical ingot.
In these days, the required standard for precisely and accurately controlling the intrinsic properties of silicon ingots during their growth has become much higher and stricter than it used to be. It is well known that the growth rate of a growing silicon ingot is one of the most important parameters which affect the intrinsic properties of the silicon ingot. In the typical CZ process, the growth rate control is achieved in a diameter control system for controlling the diameter of a growing silicon ingot. The diameter of a growing silicon ingot can be maintained constant only if the pull-speed accurately follows the growth rate of the silicon ingot.
More specifically, the typical diameter control system used in the CZ process employs a feedback controller to control the diameter of a growing silicon ingot. The controller receives a signal representing the diameter of the silicon ingot actually measured and transforms a deviation of the measured diameter from the target diameter into a pull-speed error. The pull-speed error is used to adjust the pull-speed of the seed. Further, the pull-speed error is integrated over time to derive a temperature error. Based on the derived temperature error, the temperature of the silicon melt is adjusted, resulting in adjusting the growth rate of the growing ingot. Thus, in the diameter control system used in the CZ process, an implementation of the diameter control is cascaded into an implementation of the growth rate control.
In the diameter control system used in the CZ process, therefore, the accuracy of the growth rate control depends on the accuracy of the diameter control. Any errors in inputs to the diameter control system will negatively affect not only the accuracy of the diameter control but also the accuracy of the growth rate control. Among the inputs to the diameter control system, the measured diameter is the input on which the diameter control system is designed to most sensitively react. An error in the measured diameter thus results in a serious error in the diameter control. The problem is that an error in the measured diameter propagates through the diameter control into the growth rate control and eventually negatively affects the intrinsic properties of the resulting silicon ingot. Thus, the diameter of a growing silicon ingot needs to be measured accurately to accurately and precisely control the intrinsic properties of the growing silicon ingot.
The diameter control system for the CZ process usually uses a camera to measure the diameter of a silicon ingot growing inside a furnace. The camera is set outside the furnace and observes the growth of the silicon ingot through a window of the furnace. The camera captures an image of the meniscus of the silicon ingot growing from the silicon melt. The meniscus of the growing silicon ingot is perceived as a bright ring in the crucible. The ring image captured by the camera is processed to obtain the diameter of the growing silicon ingot.
There are several known methods of measuring the diameter of a silicon ingot from its captured ring image. However, they all have problems in common. For instance, they look for pixels representing the meniscus, using only a limited number of partial scan images of the meniscus and thereby missing a large number of meniscus pixels which are not included in the limited number of partial scan images. Further, the conventional methods rely on assumptions of where to expect the meniscus pixels, which require prior knowledge of whereabouts of the meniscus pixels in the scan images or search algorithms to search the scan images for the meniscus pixels. Therefore, if the system is not set up correctly, the conventional methods relying on the assumptions can operate on serious errors during operation.
Other conventional methods rely on assumptions about brightness levels in the scan images. Like the conventional methods described above, these other conventional methods can operate on serious errors during operation if the system is not properly set up or if brightness levels change during operation, such changes being in fact likely to occur during operation. In addition, conventional image processing algorithms, due perhaps to the search algorisms used, are often not capable of operating at the full video frame rate. Consequently, one or more frames are dropped or skipped, resulting in missing valuable temporal information which could, if used, improve the signal to noise ratio. Because of these problems, the conventional methods all fall short of satisfactorily accurately measuring the diameter of a silicon ingot.
Also, there are factors presented in the CZ process which impede the accurate measurement of the diameter of a growing ingot. For instance, reflections from the silicon melt and the crown portion of the silicon ingot add noises to the captured ring image. The window when fogged obscures the ring image. Scratches on the window are another cause to add noises to the captured ring image. Also, heat shields being suspended inside the furnace usually partially block the ring image of the growing silicon ingot from the camera. There has been no apparatus or method so far to overcome the above factors and satisfactorily accurately measure the diameter of a growing silicon ingot.