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RF Electronics Chapter 11: Circuit Manufacture Page 380 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The low firing temperature for the ceramic green sheets allow low resistivity conductors like silver, gold, copper and alloys with palladium and platinum to be used [43]. The pastes are printed on the green-sheet using conventional thick film techniques. Special thick-film pastes must be used since the ceramic shrinks 10 to 15 percent in x/y-axis and about 10 to 45 percent in z-axis. If needed, shallow grooves, via holes or cavities can be milled in greentape and they can be cut using laser ablation cutters[44]. Shallow grooves allow thick tracks to be printed without any risk of air pockets resulting in surrounding areas. Any via holes are then filled with a gold based filler [45] that will then connect the required tracks on different layers. Figure 11.14. Typical LTCC structure [39,40]. It is also possible to integrate passive elements like resistors, capacitors, inductors, and transformers onto the substrate. Resistors are produced by printing tracks using resistive pastes of carefully controlled widths and lengths on the tape, just like conductor lines. Capacitors and inductors can be made by forming the conductive lines into plates or coils respectively. LTCC technology permits the construction of Stripline circuits, as opposed to Microstrip circuits. This reduces radiation and permits the construction of couplers with broadside rather than edge coupling, resulting in a high amount of coupling. Combining that with the ability to include active devices on the same small circuit opens up great possibilities. Many LTCC mixers [46], filters [47] and directional couplers [48] are produced by Mini- Circuits. Many other manufacturers also use LTCC technology. The filters and couplers use the same principles as outlined in chapters 7 and 4 respectively of this book. LTCC allows 3 dimensional layouts, so that broadside coupled lines can easily be made. The high dielectric constant of the ceramic material used also results in small circuits. LTCC technology is also good for low cost reliable hardware required for the automotive industry [49]. MMIC Fabrication At mm-wave frequencies, the track lengths needed for couplers and hybrids become less than 1 mm. As a result different techniques need to be used to produce the circuits. The same techniques used to produce semiconductors, can also be used to produce Microstrip lines. The RF or Microwave design techniques for couplers, hybrids and filters, described in this book can thus all be implemented at mm-wave frequencies as a MMIC on silicon or gallium arsenide. AWR DE can produce GDSII files that can be sent to a foundry to produce the MMIC. Figure 4.65 shows an MMIC realisation of a Marchand Balun using RF Electronics: Design and Simulation 380 www.cadence.com/go/awr

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