How to simulate undercut heater?

Hi,

I am trying to simulate a doped silicon heater with substrate silicon removal. The cross-section is as follows. Silicon substrate under SiO2 box is removed.

The simulation file is attached here.
Undercut_heater.ldev (6.0 MB)

The air is filled in the removal space. In previous topics, I found that Lumerical could not simulate heat transport in fluid like air. Air can only be the thermal boundary condition. I tried to set the convection boundary. But it still failed to converge with error 9002. Could anyone help check the simulation file and solve the problem ?