Guilin Sun
Ansys Employee
The first step is to make sure it is necessay to simulate 30um long. If there is a material interface in 30um, then change all the boundary conditions to Metal, and see if it diverges.
if it still diverges, then reduce dt factor and use PML again till it converges.
if using Metal BCs does not diverge it is due to PML:
if the device has higher diffraction orders, large diffraction angles, please use "Steep angle PML".
if no, you may need to use more number of PML layers, and make sure the PML thickness is about half wavelength or more. Or you may need to move away the PML from material interfaces. Please use only mesh accuracy 2 for the test. if you have override mesh, make sure its aspect ratio is not more than 5; and it is not too fine and close to PML.

More details can be found here