Traceability for On-Wafer S-Parameter Measurements of Membrane Technology Devices Established up to 110 GHz

Within the scope of the European project PlanarCal, PTB has developed a comprehensive uncertainty budget for on-wafer S-parameter measurements of membrane technology devices up to 110 GHz. The budget includes instrumentation errors, connector repeatability and calibration standard uncertainties.

High-frequency on-wafer science, engineering and metrology are underpinning technologies for almost all applications that employ micro- and nano-electronics. Integrated circuits operated in the microwave and mm-wave frequency range are in widespread use, in applications ranging from mobile communications to sensors. The ubiquitous presence of wireless data transmission that we are used to would not be possible without them. However, the ever-increasing demand for higher data rates, the growing number of services to be covered, and the development of high-resolution radar imaging have been continuously pushing up the frequency of operation. 60 GHz short-range high-data rate communications and automotive radar at 77 GHz are examples for important applications beyond 50 GHz which are now being deployed. Beyond this, various applications for imaging, material testing, and ultra-broadband wireless links are envisaged above 100 GHz.

Although on-wafer HF measurements already have an economic impact on chip fabrication costs, industrial assurance and traceability have not yet been established. Boundary conditions of the measurement system setup and parasitic modes are often not sufficiently considered, leading to inconsistent results. Within the PlanarCal project, which started in October 2015, the most advanced vector network analysers (VNAs) currently available are used together with state-of-the-art numerical simulation techniques to fully capture all relevant effects. The overall aim of this project, which is funded through the European EMPIR initiative, is to enable the reliable measurement and electrical characterization of integrated planar circuits and components from radio-frequency (RF) to sub-mm wavelengths (i.e. 325 GHz).

Multiline TRL as one of the most accurate algorithms for on-wafer S-parameter calibrations requires planar waveguides of different lengths as calibration standards. While low fabrication costs of planar waveguides are desirable, the transmission of signals should not be significantly degraded over a wide frequency range. Unfortunately, coplanar waveguides (CPWs) and microstrip lines fabricated in conventional technology exhibit dispersion and radiation losses as well as the excitation of parasitic modes at higher frequencies.

To circumvent these problems, CPWs built in membrane technology can be employed, as the influence of the thin supporting dielectric material is significantly reduced in comparison to the influence of several-hundred-µm thick substrates which are conventionally used. The air-line-like coplanar waveguides (CPWs) built in membrane technology have an effective dielectric constant close to one and show almost ideal quasi-TEM behavior up to at least 110 GHz. With the aid of a mathematical model developed at PTB, the wideband propagation properties of these waveguides can be calculated very accurately.

Such CPWs in membrane technology were successfully fabricated by Rohde & Schwarz, Munich. The Ferdinand-Braun-Institute, Berlin, carried out numerical simulations to optimize the layout of the test structures. Figure 1 shows an interconnect structure serving as "Thru" calibration standard consisting of silicon-to-membrane transitions at both sides of a 500 µm-long CPW section supported by a thin membrane. The silicon-to-membrane transitions contain contact pads, which allow for ground-signal-ground microwave probing, and a short interconnect segment on silicon.

PTB developed a comprehensive uncertainty budget for on-wafer S-parameter measurements of membrane technology devices, which includes instrumentation errors of the vector network analyzer, connector repeatability and calibration standard uncertainties. A lot of effort has been put into the characterization of the uncertainty components. For example, the dimensional characterization of the membrane CPW cross section was carried out with a high-precision optical coordinate measuring machine and an atomic-force microscope.

Figure 2 shows measured and model-based values of the transmission of a 500 µm-long matched line. Solid lines indicate nominal values, shaded areas indicate the expanded uncertainty intervals at a coverage probability of 95% (k=2). The S-parameters are normalized to 50 Ω. The expanded uncertainty intervals fully comprise the model values in the frequency range from 1 to 110 GHz.
A conference paper summarizing the results of this research was recently presented at the 90^th Automatic RF Techniques Group (ARFTG) Conference in Boulder, Colorado, and received the Best Oral Presentation Award, as voted on by the conference attendees.

Figure 1: Thru calibration standard fabricated in membrane technology.

Figure 2: Comparison between measured (black) and modeled (blue) magnitude of transmission S-parameter of a 500 µm-long matched line.