1. Field of Invention
This invention generally relates to an alignment system and method using dual opposing radiation beams to align one or more objects. The invention is particularly useful in aligning a seed crystal lifting cable with the center of a semiconductor melt crucible as used in the manufacture of monocrystalline semiconductor ingots.
2. Description of Related Art
Monocrystalline semiconductor ingots that are grown using the Czochralski method are typically formed by dipping a seed crystal into a rotating crucible of molten semiconductor material. As the seed crystal is slowly withdrawn from the melt by a cable, material in the melt crystallizes and forms a larger monocrystalline ingot. The diameter and length of the monocrystalline ingot that is formed are controlled based on various factors, including the pull rate of the crystal from the melt, and the melt surface level.
If the seed crystal lifting wire is not properly aligned with the center of the rotating melt crucible, the growing ingot will wobble in its rotation and not form properly. Accordingly, the seed crystal lifting cable must be properly aligned with the melt crucible.
One existing method for aligning a seed crystal cable with a crucible shaft is shown in FIG. 1. The seed crystal cable 21 is unwound from a seed crystal cable support 20 so that a plumb bob 22 attached to the end of the cable 21 nearly touches the crucible shaft 25. A technician views the position of the plumb bob 22 relative to the crucible shaft 25 and adjusts the position of the seed crystal cable support 20 accordingly.
The plumb bob method for aligning a seed crystal cable with a crucible shaft presents several problems. For example, several hours are required for the plumb bob and cable to stop movement after the cable and plumb bob are initially extended from the cable support. Further, air movement or vibration of the cable support tend to move the plumb bob and cable. In addition, there is no guarantee that the plumb bob point is coaxial with the cable. Thus, the plumb bob method does not always result in accurate alignment of the seed crystal cable and the crucible shaft.