Investigation of Variation in on-Si on-Wafer TRL Calibration in sub-THz

This work presents possible variations in TRL calibrated on-wafer S-parameters measurements with change in the on-wafer reflect design (pad open to pad short or vice-versa) in the TRL calibration method. The variations in each error-term values due to the change in the on-wafer reflect in the TRL calibration are examined which shows the source match and reflect tracking as possible error-terms bringing the observed variations in the on-wafer TRL calibrated S-parameter. In order to identify sources introducing these variations in the error-terms values and the S-parameters, TRL calibrated on-wafer S-parameters (S11 and S22) of symmetric structures (both reflects, i.e., pad open and pad short) are compared. Further, to develop a deep understanding about variations in error-terms values and in the S-parameters, a comprehensive 3D electromagnetic (EM) simulation study is performed. First, EM simulation analyses considering only one structure in simulation setup are performed to examine role of probe-to-probe and probe-to-substrate couplings. Later, EM simulation study is carried out to analyze the impact of the on-wafer neighbors, and the spatial position of the on-wafer neighbors. The EM simulation study shows a strong impact of the coupling of the DUT with the on-wafer neighbors, the spatial placement of the on-wafer neighbors, and the design of the reflect whereby the influence of the probe-to-probe coupling is partially masked by the aforementioned effects.