Influence of parasitic probe effects in on-wafer measurements of coplanar waveguides with backside metallisation

PTB has performed investigations of coplanar waveguides with backside metallisation and developed empirical formulas to describe parasitic effects including probe properties. The empirical formulas were validated by full-wave 3D electromagnetic simulations (EM) and checked with measurements up to 110 GHz.

On-wafer measurements are of fundamental importance for characterizing active and passive devices at millimetre-wave frequencies. It is well known that they exhibit various parasitic effects stemming from measuring probes, multi-mode propagation, crosstalk between adjacent structures, and radiation. While many studies have been performed for conventional coplanar waveguides (CPW), the parasitic effects of coplanar waveguides with backside metallisation have not been fully investigated.

Conductor-backed coplanar waveguides are of greater interest for industrial applications because they are used in many applications for monolithic microwave integrated circuits (MMICs), hybrid integrated circuits, and PCBs. They also offer advantages in terms of mechanical stability and heat dissipation.

However, due to the backside the so-called parallel plate line (PPL) mode propagates, which adversely affects the transmission behaviour of the CPW line (Figure 1). The interaction of the PPL mode with the desired CPW mode leads to destructive interference as well as resonance effects that affect the accuracy of the measurements (Figure 2).

To describe these resonance effects, a systematic study was carried out. The geometrical dimensions of the measuring probes as well as the material properties of the substrates were varied. From the results of this study, an analytical formula was developed to predict the PPL resonance effects, taking into account the probe properties.

Figure 1: Top view: simulated vertical electric field component of an offset short on metal chuck to demonstrate the propagation of the PPL mode.

Figure 2: Measurement and EM simulation of the CPW with l = 11400 μm on different chuck materials: Magnitude of transmission coefficient |S₂₁| (top) and difference Δ|S₂₁| between transmission on metal and ceramic chuck (bottom)

Figure 3: Analytical description of resonant frequency as a function of CPW line length, compared to measured data.

Figure 3 shows the results of the analytical formula compared to data extracted from measurements. For the first time, PTB has developed an empirical formula to describe the parasitic effects in on-wafer measurements of coplanar waveguides with backside metallization. The formula helps to better understand the underlying physical phenomena and can also be used for the design of integrated circuits.

Contact Person:

Gia Ngoc Phung

Department 2.2 „High Frequency and Electromagnetic Fields“

gia.phung@ptb.de