Logo of the Physikalisch-Technische Bundesanstalt

Commercially available calibration substrates suitable as transfer standards for the traceability of on-wafer microwave measurements


PTB has provided evidence that on-wafer calibration structures manufactured on commercially available substrates can be used as transfer standards for reference calibrations. This enables traceable planar scattering measurements in industrial applications for the first time.



In modern high-frequency technology, measuring the scattering parameters by means of vector network analysis is one of the most important methods when it comes to characterizing components manufactured on wafers (planar), such as those required for applications in 5G communications technology (Figure). The measurement methods preferred by industry for this purpose involve automated wafer probers, which typically work at a constant distance of the probe tips and perform systematic error correction (calibration) with commercially available calibration substrates (Impedance Standard Substrate, ISS). However, the error correction performed with these substrates has, to date, been too inaccurate in numerous applications, especially at high frequencies.

Within the scope of the PlanarCal European Metrology Research Project, PTB has developed traceable calibration methods for coplanar waveguides (CPWs) which are manufactured on fused silica substrates. These reference calibration procedures are based on multiline TRL (Thru Reflect Line), which is widely recognized as one of the most accurate calibration methods for planar on-wafer scattering parameter measurements. The novel procedures are also particularly well-suited to characterize structures on commercially available ISS substrates. The results of the investigations have demonstrated that commercially available calibration structures indeed can be used as transfer standards, thus enabling high-precision industrial calibrations after being characterized.

These novel reference calibration procedures also have other advantages: the reference plane of the calibration may be shifted to adapt to the device under test; moreover, they minimize the wear of the reference calibration standards since reference calibration standards are only necessary to characterize the transfer standards.

In principle, the verified methods used for CPWs on fused silica can be transferred to other substrate materials and waveguides. Depending on the application at hand, this can, however, be highly complex. PTB therefore offers support within the scope of research cooperation projects. In addition, PTB characterizes transfer standards manufactured on ISS substrates as a calibration service in accordance with EN ISO 17025.

Measurement setup at PTB for the measurement of planar scattering parameters

Figure: Typical measurement setup at PTB for the measurement of planar scattering parameters: On-wafer microwave probes over test structures on a silicon wafer.




Department 2.2 “High Frequency and Electromagnetic Fields”