Process and apparatus for controlling the melt level while pulling single crystals

An apparatus for controlling the melt level of molten material in a crucible covered by a crucible cover while pulling a single crystal according to the Czochralski process, comprising:

• a) said crucible cover having at least one characteristic shaped portion in a distance S to the melt surface, said shaped portion causing a reflection from the melt surface;
• b) an optical recording device which records an image of an evaluation area from the melt surface, said evaluation area, at least in part, containing said reflection;
• c) an electronic image-processing device which determines with the aid of said recorded image the distance S and generates an actual signal proportional to said distance S;
• d) a set point generator which generates a set point signal proportional to a predetermined value;
• and
• (e) a control device which compares the actual signal with the set point signal and, if a difference is observed, generates a control signal to change the melt level.