Micromachined Waveguide Interposer for the Characterization of Multi-port Sub-THz Devices
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This paper reports for the first time on a micromachined interposer platform for characterizing highly miniaturized multi-port sub-THz waveguide components. The reduced size of such devices does often not allow to connect them to conventional waveguide flanges. We demonstrate the micromachined interposer concept by characterizing a miniaturized, three-port, 220–330-GHz turnstile orthomode transducer. The interposer contains low-loss micromachined waveguides for routing the ports of the device under test to standard waveguide flanges and integrated micromachined matched loads for terminating the unused ports. In addition to the interposer, the measurement setup consists of a micromachined square-to-rectangular waveguide transition. These two devices enable the characterization of such a complex microwave component in four different configurations with a standard two-port measurement setup. In addition, the design of the interposer allows for independent characterization of its sub-components and, thus, for accurate de-embedding from the measured data, as demonstrated in this paper. The measurement setup can be custom-designed for each silicon micromachined device under test and co-fabricated in the same wafer due to the batch nature of this process. The solution presented here avoids the need of CNC-milled test-fixtures or waveguide pieces that deteriorate the performance of the device under test and reduce the measurement accuracy.
KeywordsWaveguide Silicon micromachining Terahertz Multi-port Measurement Interposer Test-fixture DRIE Orthomode transducer
Waveguide components working in the millimeter-wave (mmW) and sub-mmW frequency ranges of the electromagnetic spectrum are of particular interest for many applications due to their high performance and reduced size . Silicon micromachining of waveguide components and systems in this frequency range enables high-complexity, high-performance, and drastically miniaturized devices [2, 3, 4]. Although silicon-micromachined waveguide devices are usually substantially smaller than waveguide flanges, it is often necessary to connect them to such flanges in specific system architectures, or solely for their characterization.
A smart design can allow for the direct mounting of highly miniaturized micromachined components between two waveguide flanges, if axial and opposite-face port arrangement is featured, as demonstrated for micromachined filters [5, 6] or switches [7, 8]. However, such direct connection cannot be used for devices with an off-axis port arrangement or multi-port devices if the device is smaller than the area required for the individual flanges.
The conventional solution to characterize sub-THz devices, or to utilize them in systems with internal flange connections, is to manufacture them in a CNC-milled split-block configuration [9, 10], or to mount micromachined devices in CNC-milled test fixtures [3, 11, 12, 13]. Such CNC-milled parts can be costly and difficult to fabricate, and significantly add to the overall losses of the system due to inferior manufacturing tolerances and the long waveguide sections required .
Another common approach for characterizing multi-port devices with a two-port measurement system is to co-fabricate multiple devices in different port configurations. For each configuration, two of the ports are accessible to standard waveguide flanges and the remaining ports are, for instance, terminated with integrated absorbers [12, 14, 15]. This avoids the need for long waveguide sections to re-route each port but requires the fabrication of several prototypes that should be almost identical, as it is otherwise not possible to extract accurate S-parameters. Moreover, the loads in these measurement setups are integrated into the component and cannot be characterized individually, thus their non-ideal behavior cannot be properly de-embedded. This, together with the fact that the fabricated devices are not identical, results in significant uncertainty in the measured S-parameters.
Silicon micromachining is an enabling fabrication technology for sub-THz waveguide devices [1, 2, 3, 4, 8, 11, 12, 16, 17, 18]. These cited works have demonstrated complex and low-loss waveguide geometries, MEMS integration, or system-on-chip architectures. The authors have recently introduced a very low-loss micromachined waveguide technology (0.02–0.07 dB/mm in the 220–330-GHz band, ), which is a near-ideal candidate for routing waveguide signals in micromachined test fixtures.
In this paper, we report for the first time on a measurement setup for the characterization of miniaturized multi-port waveguide components at sub-THz frequencies. The silicon-micromachined interposer setup was used for the first time to characterize a recently published turnstile orthomode transducer (OMT) in the 220–330-GHz frequency range . The setup is composed of a silicon-micromachined interposer and a square to rectangular waveguide transition, which replaces the alternative non-optimal CNC-milled test fixtures. Besides a low-loss micromachined waveguide routing network, the interposer contains integrated micromachined loads to terminate the unused ports in specific measurement configurations. This paper describes the concept of the measurement setup, the interposer design with the integrated micromachined matched-loads, reports on the characterization of all individual sub-components, and discusses the effect of their performance on the characterization of the device under test (DUT).
2 Measurement Requirements and Interposer Concept
The characterization of this device requires routing the rectangular ports (P3 and P4 in Fig. 1) with an offset of 20 mm (size of a standard test-port flange), and to independently access ports P1 and P2 in the square waveguide. Furthermore, to characterize the device in a two-port measurement setup, the test fixture must offer the possibility of terminating the unused rectangular port in the different measurement configurations.
(a) connect the V-pol channel in the square port and measure the V-pol rectangular port, H-pol terminated by a matched load (S11, S31, S13, and S33);
(b) connect the H-pol channel in the square port and measure the H-pol rectangular port, V-pol terminated by a matched load (S22, S42, S24, and S44);
(c) connect the H-pol channel in the square port and measure the V-pol rectangular port, H-pol terminated by a matched load (S23 and S32);
(d) connect the V-pol channel in the square port and measure the H-pol rectangular port, V-pol terminated by a matched load (S14 and S41).
The waveguide section that routes the signal in the interposer can be independently characterized and thus de-embedded from the measurements. The de-embedding process allows for the extraction of all the relevant RF parameters of the single multi-port DUT from the measured raw data of the different dual-port measurements. The lower uncertainty of microchip-to-metal flange interfaces as compared with metal-to-metal-flange interfaces allow for significantly more accurate de-embedding of the DUT performance from the measurements. The tighter tolerances of the silicon-to-metal connection relies on a novel alignment hole approach, demonstrated by Campion et al. by developing high-precision calibration shims for THz frequencies [20, 21].
The interposer mounted on one of the VNA ports is shown in Fig. 5b, which interfaces to two of the DUT ports. The flange to chip alignment accuracy is better than 5 μ m, which is achieved by using a tight-fitting hole for one pin and an elliptical hole for the other pin, as reported by Campion et al. in .
2.2 Rectangular-to-Square Waveguide Transition
Once the transition is mounted on the test port, as shown in Fig. 6c, it provides a square-waveguide interface for the common port in the OMT and loads one of the two polarization channels, as shown in Fig. 6d, with mounted OMT on the transition. The loss and mismatching generated by the transition cannot be de-embedded from the measured data since a square waveguide calibration kit would be necessary. Therefore, such compact micromachined transitions with low insertion loss and low standing wave ratio are beneficial to obtain reliable measurement results.
3 Interposer Characterization
The S-parameters of the individual sub-components in the interposer must be measured in order to de-embed their effects from the measurements and obtain the S-parameters of the DUT. This is one of the main advantages of this technique as compared with the alternative approach of including the loads and long waveguide sections directly into the DUT, where their effect cannot be accurately deducted.
4 DUT Measurements
The measurement procedure of the DUT, a turnstile orthomode transducer, is described in Section 2. The four different configurations shown in Fig. 2 are needed for the characterization of most of the S-parameters of the device, i.e., the insertion loss, return loss, and cross-polarization for both channels. Isolation data (S12, S21, S34, and S43) cannot be extracted using this measurement setup because ports P1 and P2 or P3 and P4 cannot be accessed simultaneously.
The de-embedding of the S-parameters of the waveguide section in the interposer is done using the inverse T-matrix method . The extracted S-parameters from the DUT include the effect of the square-to-rectangular transition and assume a perfect load in the non-measured ports. This assumption is only valid if the non-measured ports belong to the isolated channel, and the de-embedding process could not be done if low cross-polarization levels were observed. The measured cross-polarization for this OMT is always better than 35 dB, as reported in ; therefore, this condition is met.
A silicon micromachined interposer for the characterization of highly miniaturized multi-port waveguide components has been reported for the first time in this paper. This measurement setup provides lower loss, at a much lower cost, and better flange-to-chip alignment than conventional CNC-milled test fixtures, which provides better de-embedding accuracy. The higher accuracy, lower cost, ability to integrate loads, and versatility of silicon micromachined test interposers make them an enabling tool for the development of sub-THz waveguide components.
The entire test setup was manufactured simultaneously on the same wafer as the device under test, taking advantage of the batch processing nature of silicon micromachining, making possible the fabrication of custom-made measurement setups for every component. This solution was successfully tested for the characterization of a silicon micromachined wideband OMT, thus allowing for the first time, to the best of our knowledge, the characterization of a compact multi-port sub-THz waveguide component without the need of any CNC-milled flanges or fittings.
Open access funding provided by Royal Institute of Technology. This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 616846), and the Swedish Foundation for Strategic Research Synergy Grant Electronics SE13-007.
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