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Singular Matrix, Void Equations, Scatterer on Substrate

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Hi,

I am trying to simulate 'scattering on substrate' example in RF module.
My model is a little bit modified.
It is so simple 3D structure, just air and Si.
My study node has two steps.
The first step solves in full field mode to calculate background E field for the second step field by using port and periodic boundary conditions, and the second step uses the solution of the first step as the background E field just to simulate the same structure but in scattered field mode and using PML boundary conditions.
I am using swept mesh just exactly like the example.
I tried different sizes of mesh.
It never solves, and makes error:

Failed to find a solution.
Singular matrix.
There are 127476 void equations (empty rows in matrix) for the variable comp1.E210.
at coordinates: (3.3e-007,2.25e-007,-3.43333e-007), (3.3e-007,2.25e-007,-3.46667e-007), (3.3e-007,2e-007,-3.43333e-007), (3.3e-007,2e-007,-3.46667e-007), (3.36667e-007,2.25e-007,-3.4e-007), ...
and similarly for the degrees of freedom (empty columns in matrix).
Returned solution is not converged.

In 2D case it works well, but in 3D it always make abovementioned error.

Has somebody faced this kind of problem already?
I appreciate if you share your experience with me, guys.
I also attached the file.

Thanks,
Mohammad

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