Bhutani, Akanksha

Pittermann, Martin

Zwick, Thomas

English

KITopen

A 53 GHz - 125 GHz Chip to Double-RidgeRectangular Waveguide TransitionM. Pittermann, T. Zwick and A. BhutaniA novel chip to double-ridge rectangular waveguide transition,with an ultra-broadband operation bandwidth exceeding three standardrectangular waveguide (WR) bands of WR15, WR12 and WR10 isdemonstrated for the first time in this letter, to the best of authors’knowledge. This bandwidth is achieved by coupling the microstrip modedirectly to the fundamental TE mode of the waveguide. A unique featureof this transition is that the transition design can be implemented onsubstrates with a high dielectric constant, which makes it suitable forintegration with monolithic microwave integrated circuits (MMICs). Aback-to-back transition is realized using a split-block assembly and254 µm thickness alumina substrate (εr = 9.8 at 80GHz). The latteracts as an MMIC substrate substitute, including a tapered microstripand a 50Ω coplanar waveguide pad for probe-based measurement. Twonovel mechanical assembly concepts, involving die-attach-foil and indiumsolder, are used to build two split-block prototypes. The simulated andmeasured S-parameters of the transition are demonstrated from 35GHzto 125GHz.Introduction: There is an ongoing push for ultrawideband circuits, drivenby applications in wireless and optical communication. Monolithicmicrowave integrated circuits (MMICs) with bandwidths exceeding300GHz have recently been demonstrated [1, 2, 3]. At such bandwidths,packaging and interconnects become a key challenge. Due to their lowattenuation, metal waveguide split block designs are the gold standard forconnecting MMICs, passive elements and antennas. Therefore, a chip-to-waveguide transition with good impedance matching and low insertion lossover a large operational bandwidth is an essential building block for a high-performance waveguide-based split-block MMIC package.There are several common designs for microstrip-to-waveguidetransitions that use radiative elements, for example E-field probes [4, 5]or aperture coupled patches [6]. However, designs with radiative elementshave an inherently limited bandwidth, operating only in a single waveguideband. This is sufficient for rectangular waveguides, but higher bandwidthapplications require different waveguide geometry and transitions.Rectangular waveguides are limited in their usable bandwidth by thecutoff frequencies fc of their fundamental mode TE10 and their next-highermode TE20. Below fc,TE10 an electromagnetic wave cannot propagate.Above fc,TE20 multiple modes with different dispersion characteristicsare allowed to propagate, which leads to resonances. Due to theirgeometry, rectangular waveguides are limited to an operational bandwidthof fc,TE20/fc,TE10 = 2. In practice, only around 70% of the bandwidth istypically used since dispersion or evanescent wave effects can lead tounwanted behavior near the band edges.Ridged waveguides are used in applications where higher bandwidthsare required. One or two ridges create additional capacitive loading,which decreases the cutoff frequency of the fundamental mode. The cutofffrequency of their next-higher mode is also slightly increased. By choosingthe ridge dimensions accordingly, much greater single-mode bandwidthscan be achieved. In addition to the increased bandwidth, the electric fieldis concentrated between the ridges, which proves useful for the designpresented in this letter.Design: The waveguide transition is implemented on a 254 µm thickAlumina substrate (εr = 9.8, tan δ= 3× 10−2 at 80GHz [7]). Due tothe high dielectric constant of the alumina substrate, it serves as a goodchoice for replicating an MMIC substrate (for example, GaAs has εr =13, tan δ= 1× 10−3 at 100GHz [8]). Figure 1 shows the simulationmodel of the chip-to-double-ridge rectangular waveguide transition. Notethat the top-left corner of the double-ridge rectangular waveguide is kepthidden for clarity in representing the transition geometry for the reader.The bottom half of the split-block waveguide ends in a platform thatholds the microstrip substrate in a way that positions the microstrip signalline at the mid H-plane of the double-ridge rectangular waveguide. Thisplatform is in direct electrical contact with the metallized bottom of themicrostrip substrate. The top half of the waveguide ends at the positionwhere the microstrip substrate begins, i.e., y= 0mm. Only the waveguideridge extends beyond this point and makes contact with the microstripsignal line.xzy2 mm0.25 mm0.07 mm0.7 mm2.54 mm1.27 mmFig. 1: 3D model used for the back-to-back simulation with half of thetop split block half cut away. The waveguide taper has a length of 5mmbut is cut short in this illustration. The gap between the ridges measures0.07mm at the transition and tapers to 0.35mm (cut off). The red dotat the near end of the alumina substrate marks the coordinate origin. Theprobe model, outlined in red, consists of a short stub of 50Ω coax withthree needles.This approach works well because the field configuration of themicrostrip quasi-TEM mode is very similar to the dominant TE mode ofthe ridged waveguide. Figure 2 shows the evolution of the electric fieldstarting at the microstrip, at two points along the microstrip-ridge overlapand, finally, in the ridged waveguide. At a distance of 0.5mm from theedge of the alumina substrate, the microstrip mode is virtually undisturbed.The overlap region begins at y= 0.25mm. Since the field on top of themicrostrip is very weak, the emergence of the waveguide ridge does notdisturb the fields significantly. At position y= 0.05mm, the fields from thenarrow ridge gap are observed to extend outwards, curving into the aluminasubstrate. The conversion to the final TE mode of the ridged waveguide isvery gradual, which makes the bandwidth of this transition very wide. Thebest matching is achieved when the width of the microstrip matches thewidth of the waveguide ridge.The prototype is designed for a maximum frequency of 110GHz, areturn loss of greater than 10 dB and an insertion loss of less than 1 dB.WR10 (75GHz to 110GHz) was chosen for the outer dimensions ofthe rectangular waveguide. The addition of the ridges lowers the cutofffrequency of the fundamental mode to 35GHz according to numericalsimulation. Importantly, approaching the transition, the gap narrows toonly 0.07mm, as seen in fig. 1.A microstrip with a signal line width of 0.7mm on alumina substratehas a characteristic impedance of only 26Ω. With the gap of 0.07mm atthe transition, the parallel plate waveguide formed by the ridges (ignoringthe rectangular waveguide) has a line impedance of 30Ω. The impedancevalues are estimated on the basis of electromagnetic simulations carriedout in CST Studio Suite 2023. This is slightly higher than the impedance ofthe microstrip, but results in the best matching. The ideal size of the gap isfound on the basis of full-wave electromagnetic simulations.Figure 3 shows the scattering parameters for the single-sided chipto double-ridge rectangular waveguide transition without ground padsand microstrip taper, simulated using CST Studio Suite 2023. Thereflection loss is better than −10 dB from 40GHz to 125GHz andbetter than −15 dB from 43GHz to 103GHz. The insertion loss isbetter than −0.6 dB in the entire frequency range except for a narrowresonance at 124GHz. The simulation also shows good performance wellabove 125GHz. However, at 120GHz substrate modes and higher orderwaveguide modes emerge, necessitating thinner substrate and adjustmentsin the waveguide dimensions for practical use.The transition was also simulated with a 10 µm air gap between themicrostrip and the top ridge, resulting in only a < 0.1 dB increase ininsertion loss. Besides relaxing the manufacturing tolerances, this enablesthe integration of a DC-block by filling the air gap with a dielectric.Since the input and output pads of an MMIC are usually ina ground-signal-ground configuration with an impedance of 50Ω, acosinusoidal microstrip taper of length l= 2mm is added to serve asELECTRONICS LETTERS 17th January 2024 Vol. 00 No. 00Fig. 2: Absolute electric field strength |Ez | in kVm−1 at multiple planestransversal to the microstrip and waveguide. The colors correspond to themagnitude of the electric field in z-direction, the geometry of the aluminasubstrate and split block are overlaid. y= 0mm corresponds to the edgeof the alumina substrate.40 60 80 100 120−0.8−0.6−0.4−0.20−20−100Frequency in GHzS12 S11 S22S12 in dB S11, S22 in dBFig. 3: Simulated S-parameters of a single-sided transition, without themicrostrip taper (port 1 = waveguide, port 2 = microstrip). At the bottomend, the bandwidth is limited by the cutoff frequency of the waveguide.an impedance transformer. The width profile of the microstrip taperis w(x) = (a+ b)/2+(b− a)/2 · cos(πx/l), a= 0.19mm, b= 0.7mm.The tapered impedance transformer is connected to a GSG pad, wherethe ground pads are connected to the microstrip ground plane with vias.The GSG pad provides the benefit that a back-to-back configuration of thischip-to-waveguide transition can be characterized using two GSG probes.The chip-to-double-ridge rectangular waveguide transition shown infig. 1 is manufactured in a back-to-back configuration. The S-parametersof this transition are measured using two GSG probes on either end.Furthermore, the same back-to-back configuration chip-to-waveguidetransition, along with two idealized GSG probe tips, is simulated using CSTStudio Suite 2023. The measured and simulated S-parameters are shown infig. 5.Fabrication: The double-ridge rectangular waveguide is manufacturedusing brass in two split-block halves. Due to the geometry of the ridgeand to facilitate clamping of the microstrip, the waveguide is split in the H-plane. Since split-block waveguides are very sensitive to contact resistancein the H-plane, two prototypes are built. One prototype uses an electricallyconductive die-attach-foil (DAF) with a thickness of 10 µm [9], while theother prototype uses pure indium solder foil with a thickness of 50 µm[10] to minimize the contact resistance between the split-block halves.Simulations predict identical performance for both DAF and indium solder.Additionally, the upper part of the waveguide has relief channels to eitherside of the waveguide to decrease the contact area with the lower part.This results in increased contact pressure between the waveguide halves.Fig. 4: Assembled back-to-back transition. The 10mm long double-ridgewaveguide section consists of two back-to-back cosinusoidal tapers from aridge gap of 0.07mm to 0.35mm. From the outside, the two samples withdie-attach-foil and indium solder look identical.Two dowel pins are used for alignment and four M2 screws fasten the twohalves together.The split block halves are machined on a 5-axis micromachining center.After machining, critical dimensions are verified using a white lightinterferometer. Special attention is paid to the distance between the matingsurface of the split block halves and the surfaces that clamp the microstrip.The finished parts are within 3 µm of the nominal dimensions.Due to the requirement for vias of the microstrip launcher ground pads,the alumina pieces are manufactured in multiple steps: Alumina sheet witha thickness of (254 µm), covered on both sides with gold, is used as thebase material. Through-holes are created by a laser prototyping system.Then a thin layer of nickel (100 nm) is deposited via sputtering, followedby a galvanic layer of gold (2 µm). Finally, a laser prototyping system isused to structure the top side and to dice the samples.For final assembly, DAF or indium solder is applied to the bottom splitblock half in the areas that contact the top half. Then, DAF is applied tothe bottom side of the alumina substrate and the substrate is placed onthe bottom split block half. A small piece of DAF is placed on the endof the microstrip, where it contacts the top ridge. The alignment pins areused to precisely place the top split block half on the rest of the assembly.Finally, the assembly is heated to 120 ◦C for two hours to cure the DAFfor the first prototype, whereas the second prototype with indium solder isheated to 200 ◦C for reflowing. During curing, the screws are re-tightenedperiodically to ensure good contact pressure. The first prototype using themechanical assembly concept based on DAF is shown in fig. 4.Measurement and Results: The measurements are performed using aKeysight N5247B network analyzer and N5295A broadband frequencyextender modules. Two microwave probes are used to contact the device.The setup is calibrated at the probe tips with a short-open-load-through(SOLT) calibration on an impedance standard substrate.The measured scattering parameters of the first and second prototypebased on DAF and indium solder, respectively along with the simulationresults are shown in fig. 5. Both samples exhibit very similar scatteringparameters and the insertion loss lies within 1 dB to 2 dB in the frequencyrange of interest. Periodic resonances in the reflection loss, spaced by about13GHz, indicate reflections at the ends of the 10mm long waveguidesection. Except for a slightly higher loss, the simulations match themeasured scattering parameters very well. The DAF sample has noreflections exceeding 10 dB between 53GHz and 125GHz, a bandwidthof 1 : 2.3.Table 1 compares the presented chip-to-double-ridge rectangularwaveguide transition with microstrip/CPW to rectangular waveguidetransitions operating at similar or higher frequencies that have beenreported in literature to date. The previously demonstrated transitiondesigns have a much smaller bandwidth, all limited to a singlestandard waveguide band, whereas the transition presented in this letterdemonstrates the largest operating bandwidth of 53GHz to 125GHz,which translates to a relative bandwidth of 1 : 2.3. Regarding insertion loss,this transition also performs similar or better than other designs for 75 to110. No complex assembly steps such as wire bonding [4] or fabricationof very thin features [12] is necessary.240 60 80 100 120−3−2−10S12indB40 60 80 100 120−20−100Frequency in GHzS11indBB2B SimulationMeas. DAFMeas. IndiumFig. 5: Measured S-parameters of the two back-to-back samples. Thedevice is probed with the calibration plane at the probe tips. Since themicrostrip taper, ground pads and probe tips are included in the back-to-back simulation as well, the performance is slightly worse than what couldbe expected from fig. 3.Conclusion: In this work, a novel direct chip-to-double-ridge rectangularwaveguide transition is presented for the first time. The transitionestablishes a direct connection between a microstrip signal line and theridges of a double-ridge rectangular waveguide in its H-plane. This featuregives the transition a distinct advantage over previously demonstrated chip-to-waveguide transitions based on radiating elements. Implemented on ahigh dielectric constant alumina substrate, the design is well-suited forintegration into an MMIC substrate.Two prototypes, employing different mechanical assembly conceptsusing a die-attach-foil and indium solder, have been successfullyrealized in a back-to-back configuration. The measured S-parameters ofthese prototypes exhibit excellent agreement with their electromagneticsimulation results. Operating over a broad bandwidth from 53GHz to125GHz, covering three standard waveguide bands (WR15, WR12, andWR10), the transition demonstrates a return loss better than 10 dB and asingle-sided insertion loss of less than 1 dB across the entire operationalbandwidth.Table 1: Comparison of microstrip/CPW to waveguide transitionspresented in other works. The bandwidth is defined by S11 ≤−10 dBin this context. All designs are housed in a CNC machined split blockassembly. All characteristics are for a back-to-back transition.Ref. Method f (GHz) BW IL (dB)This work double ridge 53–125 1 : 2.3 1–2[4] wire bond 140–220 1 : 1.6 3–5[5] quartz probe 140–220 1 : 1.6 2.9[11] inline dipole 340–3801 1 : 1.1 6[12] thin ridge 75–110 1 : 1.5 3.5[13] stepped ridge 75–110 1 : 1.5 1–3[14] single ridge 60–90 1 : 1.5 1.5–2.71 defining the bandwidth based on S11 is difficult, so the bandwidthstated by the authors is used instead.Acknowledgment: The authors acknowledge the financial support bythe Federal Ministry of Education and Research of Germany in theprojects "ForMikro 2.0 - ECLIPSE" (grant number: 16ME0913) and"Open6GHub" (grant number: 16KISK010). Furthermore, the authorsthank A. Lipp for his help with indium soldering and assembling theprototypes.M. Pittermann, T. Zwick, A. Bhutani (Institute of Radio FrequencyEngineering and Electronics (IHE), Karlsruhe Institute of Technology(KIT), 76131 Germany)E-mail: martin.pittermann@kit.eduReferences1 Thome, F., Leuther, A.: First demonstration of distributed amplifiermmics with more than 300-ghz bandwidth.IEEE Journal of Solid-State Circuits 56(9), 2647–2655 (2021).doi:10.1109/JSSC.2021.30529522 Baeyens, Y., Mansha, M.W., Rücker, H.: A single-stage low-noise sigehbt distributed amplifier with 13 dbm output power and 20 db gain ind-band and over 170 ghz bandwidth.In: 2022 17th European Microwave Integrated Circuits Conference(EuMIC), , pp. 52–55. (2022)3 Tang, D., et al.: An ultra-wideband amplifier with a novel non- distributedbutterfly topology achieving 2-250 ghz bandwidth and 4.67 thz gbw in130nm sige bicmos.In: 2023 IEEE Custom Integrated Circuits Conference (CICC), , pp. 1–2.(2023)4 Stärke, P., Carta, C., Ellinger, F.: Direct chip-to-waveguide transitionrealized with wire bonding for 140–220 ghz g-band.IEEE Transactions on Terahertz Science and Technology 10(3), 302–308(2020). doi:10.1109/TTHZ.2020.29716905 Tessmann, A., et al.: 220 ghz low-noise amplifier modules for radiometricimaging applications.In: 2006 European Microwave Integrated Circuits Conference, , pp. 137–140. (2006)6 Lee, H.Y., et al.: Wideband aperture coupled stacked patch typemicrostrip to waveguide transition for v-band.In: 2006 Asia-Pacific Microwave Conference, , pp. 360–362. (2006)7 Kato, Y., Horibe, M.: Permittivity measurements and associateduncertainties up to 110 ghz in circular-disk resonator method.In: 2016 46th European Microwave Conference (EuMC), , pp. 1139–1142. (2016)8 Xu, H., et al.: Electromagnetic characterization techniques for dielectricmaterials at millimeter-wave range.In: 2023 International Conference on Microwave and Millimeter WaveTechnology (ICMMT), , pp. 1–3. (2023)9 The Furukawa Electric Co. , Ltd. : Electrically/thermally conductivedicing die attach film.https://www.furukawa.co.jp/uvtape/en/technology/cdaf.html (accessed February 12, 2024)10 Indium Corporation: Product data sheet solder ribbon and foil.https://www.indium.com/technical-documents/product-data-sheets/download/564/ (2023, (accessedFebruary 12, 2024))11 Leong, K.M.K.H., et al.: A 340-380 ghz integrated cb-cpw-to-waveguidetransition for sub millimeter-wave mmic packaging.IEEE Microwave and Wireless Components Letters 19(6), 413–415(2009). doi:10.1109/LMWC.2009.202004312 Shireen, R., et al.: A w-band transition from coplanar waveguide torectangular waveguide.In: 2008 IEEE Antennas and Propagation Society InternationalSymposium, , pp. 1–4. (2008)13 Wang, C., et al.: A wideband contactless cpw to w -band waveguidetransition.IEEE Microwave and Wireless Components Letters 29(11), 706–709(2019). doi:10.1109/LMWC.2019.294524214 Hannachi, C., Djerafi, T., Tatu, S.O.: Broadband e -band wr12 tomicrostrip line transition using a ridge structure on high-permittivity thin-film material.IEEE Microwave and Wireless Components Letters 28(7), 552–554(2018). doi:10.1109/LMWC.2018.28354753

A 53–125 GHz chip to double‐ridge rectangular waveguide transition

https://publikationen.bibliothek.kit.edu/1000169369/152876402

A 53–125 GHz chip to double‐ridge rectangular waveguide transition

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