Electromagnetic field simulation of MMICs including RF probe tips

RF probe measurements are widely used for characterizing circuits in the millimeter-wave frequency range. Especially above 200 GHz the large dimensions of the RF probe, in comparison to the wavelength, lead to parasitic effects which will effect the device under test. Since the measurement is influenced by the electro-magnetic coupling between probe tip and the test structures on the substrate, the standard on-wafer calibration becomes unprecise. This paper discusses a method of simulating this influence by means of 3D electromagnetic simulations. Confirmation of the approach was done by comparing measured and simulated results of impedance substrate standards and galium arsenid (GaAs) monolithic millimeter-wave integrated circuits (MMICs). A very good agreement to measured results in the investigated WR3 frequency range (220-325 GHz) is presented, showing effects which were unobserved in earlier simulations.