We investigate the effect of passivation contouring, surface roughness, and sidewall etch tapering on the FEM modelling accuracy of mm-wave couplers in CMOS BEOL. It is found that accurate passivation contouring leads to a marginal improvement of 0.15 dB in peak coupling prediction accuracy, but introducing a sidewall taper of 104ᵒ can improve the prediction of peak coupling by up to 0.37 dB. Including surface roughness of 5 % and 10 % of metal thickness did not have a notable improvement on prediction of peak coupling.