# Necking

| Here is an X-ray topograph of the first part of crystal growth
|  | An X-ray topograph is similar to a transmission electron microscope image - it shows the interior of the sample and dislocations are visible as dark lines.
| You see a dislocation-free seed crystal, followed by region full of dislocations. This is unavoidable because dipping a solid seed in a melt that has by definition a higher temperature, always causes a "thermal shock" with stress and strain and therefore plastic deformation.
|  | The the diameter of the now growing crystal is made as small as possible (it still must be able to carry the weight of the finished crystal - up to 250 kg or so). This is the "necking" or Dash process.
|  | The dislocations disappear after a few cm, the question is why? The picture almost shows it. For the usual <100> oriented crystal, the glide planes of the dislocations (the {111} planes) are all inclined to the growth direction, and the dislocations, still feeling some stress, will simply move out of the crystal.
|  | This is where the art part comes in - or better came in. Keep enough stress to move the dislocations, but not that much that new ones will be generated.
Courtesy of Wacker / Siltronic; Burghausen, Germany

With frame

4.1.3 Silicon Crystal Growth and Wafer Production

Czochralski Crystal Growth Process

The Science and Art of Si Crystal Growth

Silicon Crystal

© H. Föll (Semiconductor Technology - Script)