Improved calibration structures for frequencies up to 110 GHz
Fig. 1: Measurement system with probes and short CPW line (indicating the propagation of a parasitic PPL mode)
Fig. 2: Measured and simulated CPW attenuation as a function of frequency
Network analyzer measurements are the most important means for the characterization of RF circuits in the upper GHz range. For on-wafer measurements, appropriate probes (Fig. 1) are used that connect to coplanar pads on the chip. The raw measurement data do not only reflect the behavior of the structure under investigation but also show influences of the probes, the measurement setup as well as electromagnetic effects of the environment. These effects must be deducted from the measured data of a device under test (DUT) in order to describe it correctly. This is obtained by a so-called calibration. Several dedicated CPW structures are measured and special algorithms are applied to the results to correct for the undesired effects.
However, a critical evaluation of the state-of-the-art calibration procedures shows that not all effects are taken into account properly. This introduces unknown errors and deviations in both, correction terms and results. In order to improve understanding and calibration accuracy, a small group of scientists is working on a description of these effects using simulations and measurements, focusing on the CPW case so far. Various electromagnetic effects like multi-mode propagation, surface and substrate modes, and radiation are considered for a better explanation of the phenomena observed. In the frequency range up to 110 GHz, the root causes for the parasitic effects have already been identified. Essentially, this is due to different parallel-plate modes (PPL modes) and surface modes radiating into the environment, which withdraw power from the system and increase losses (Fig. 2, beyond 90 GHz). In the lower frequency range, the description can be restricted to the CPW mode and the first PPL mode (rippled shape in Fig. 2). Here, a relatively simple model is valid and quantitatively analyzed at present.
Publication:
F.J. Schmückle, R. Doerner, G.N. Phung, W. Heinrich, D. Williams, U. Arz, "Radiation, Multimode Propagation, and Substrate Modes in W-Band CPW Calibrations", European Microwave Conf. (EuMC 2011), Manchester, UK, Oct. 10-13, pp. 297-300 (2011).
FBH research: 20.10.2011