How to eliminate polysilicon stress
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I would like to ask the experts how to eliminate the internal stress in polycrystalline silicon rods. The polycrystalline silicon rods we produce develop cracks or even break during the cutting process after being taken out of the furnace. What are the reasons behind this phenomenon, and are there any ways to solve it?
Control the cooling temperature; the temperature change per unit of time should not be too large.
Perhaps making the grains larger could reduce the likelihood of cracking, but overly large grains can cause stress concentration, making cracking more likely, especially when the cutting tools are not optimal； Smaller grains allow for more effective release of internal stress. This question is really a bit contradictory; I’m not sure how to approach it.
Control the cooling rate during shutdown carefully. Note that the temperature range of 800–400 degrees is the period when stress generation is most active, and it is essential to keep this range under control.
The main issue of thermal stress concentration is that, when cooling the reduction furnace, the temperature gradient should not be too large, and the cooling time should not be too short.
Everyone knows that the rate of temperature drop should not be too fast, and the temperature gradient should not be too large. But how can we ensure that the temperature gradient inside and outside the silicon rod is not too large? Applying an electric current is equivalent to internal heating; what we measure is the temperature on the surface of the silicon rod. The temperature difference in that case cannot be determined