A method includes measuring a first calibration kit in a wafer to obtain a first performance data. The wafer includes a substrate, and a plurality of dielectric layers over the substrate. The first calibration kit includes a first passive device over the plurality of dielectric layers, wherein substantially no metal feature is disposed in the plurality of dielectric layers and overlapped by the first passive device. The method further includes measuring a second calibration kit in the wafer to obtain a second performance data. The second calibration kit includes a second passive device identical to the first device and over the plurality of dielectric layers, and dummy patterns in the plurality of dielectric layers and overlapped by the second passive device. The first performance data and the second performance data are de-embedded to determine an effect of metal patterns in the plurality of dielectric layers to overlying passive devices.