This might be due to not suffiicently resolve the geometry, or mesh triangle quality is too high. This can happen when the device has thin and long layers or other small feature. You may try to increase the iteration limit, reduce the triangle quality. Please refer to this article for more details:
https://optics.ansys.com/hc/en-us/articles/8355464963987-Troubleshooting-convergence-errors-in-CHARGE
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