Quasi-Optical Testbed for Wideband THz On-Wafer Measurements
Authors: Cui Yiran , Trichopoulos Georgios C. 
Published: June 2019

At mmW and THz band, on-wafer testing is very critical for on-wafer electronics devices and circuits as well as spectroscopy. Nevertheless, current measurement capabilities are limited by contact probe technology and vector network analyzers (VNAs). Recently, we proposed a non-contact method to tackle the issue of using expensive and brittle contact probes. Using quasi-optics and on-chip antennas, the signals from the THz VNA are coupled on the device under test (DUT) with low insertion loss and unmatched repeatability. However, the bandwidth and cutoff frequency limitations of VNAs limit the scope of THz measurements and increase the complexity. State-of-the-art VNAs use external frequency multipliers, namely VNA extenders, to up-convert the VNA signal to the THz band. The problem of using such extenders is three-fold: 1) The maximum cutoff barely breaks it into the THz band, 2) they are bandlimited, and 3) they are extremely expensive due to costs associated with waveguide micromachining and sophisticated semiconductor processes for the electronics.
Here, we propose the design of a novel quasi-optical on-wafer testbed that is compatible with photonics-based sources and detectors (e.g., photomixers) and use THz optical components instead of traditional waveguide structures to route the THz signals. With photomixers we can implement cost effective THz sources or receivers that can be efficiently integrated with quasi-optics. They feature a relatively simple topology and operate in a very wide bandwidth, typically from less than 100 GHz to more than 3 THz. Since the optical components are frequency independent, by adopting photomixers as THz emitter and detectors, the proposed quasi-optical system has a potential to achieve ultra-wideband on-wafer measurement capabilities. First, we talk about the design of the quasi-optical coupler consists of two beam splitters, which is used to discriminate between the reference and the measurement signals. Through a rigorous theoretical analysis and experiments we verified a minimum 60 dB isolation and less than 3 dB of insertion loss in the 330–500 GHz band. Then, we discuss how to use this quasi-optical coupler to implement one-port free-space measurements. We also discuss the calibration process and present three calibration standards that eliminate the error terms of the one-port free-space measurement topology. The experimental results we collected from our free-space measurements are shown afterwards. The results for two different DUTs show a good agreement between the measurement and the theoretical reflection coefficients. Next, we present the on-wafer testbed, which is constructed by a quasi-optical coupler and a non-contact probe. We also briefly introduce the on-wafer calibration approach and on-wafer standards. We notice that, in order to couple the THz beam to the on-wafer DUT, all the on-wafer standards and DUTs need to be integrated with probing antennas. Finally, we show the experimental results we obtained from one-port on-wafer measurements. For two different on-wafer DUTs, the measurement results agree with the simulations very well. Besides, with multiple measurements, we also verified that the proposed quasi-optical testbed has a good repeatability.