New Technologies for Millimeter Waves
By John McNicol, Director of Business Development, MMIC Solutions UK

Today’s microwave systems, both commercial and military, use circuit board technology to interconnect active MMICs (Microwave Monolithic Integrated Circuits) and passive devices up to around 50GHz. However, when the wavelength approaches just a few millimeters, similar to the dimensions of the MMIC chips and the internal structure of the components, many electromagnetic effects, usually negligible at lower frequencies, become dominant. These effects, such as non-bulk conduction in the interconnections and resonance in the cavity the MMIC chips are housed in, can often render a millimeter wave module completely non-functional.

Using technology from the early days of microwave, millimeter wave modules for applications in the V-band (~60GHz), E-band (71-76GHz and 81-86GHz) and the W-band (up to 110GHz) are usually constructed from metal, machined to small dimensions with tight tolerances. Supporting functions such as power supplies and amplifier control are often implemented on separate circuit boards mounted in other cavities in the metal enclosure, far from the active chips. Glass feed-throughs make the connections to the MMICs.

Compared to lower microwave frequencies, where low loss RF materials allow multi-layer circuit boards that integrate both active MMIC devices and supporting surface mount, this step up to higher frequencies makes the modules larger, heavier, and more expensive. These factors also make automated assembly of millimeter wave components much more difficult, a roadblock to the benefits of repeatable performance and lower costs.

In addition to technology, size, weight, and costs, the contrast between microwave and millimeter wave can also be seen in the suppliers of components. There are numerous high volume suppliers of microwave sub-systems up to 50GHz, including most of the world’s largest telecom equipment manufacturers and their sub-contractors. However, building true millimeter wave systems at 60GHz and above often requires years of hard-won experience. With few exceptions, millimeter wave suppliers are small specialist companies of industry veterans, hand-building products for military, aerospace, and research applications.

However, the exploding market for media rich mobile applications is driving the need for higher capacity throughout mobile networks. Extending Gigabit Local Area Networks, and increasing use of CCTV, require much higher capacity in wireless backhaul in urban areas. If new technologies can be deployed for higher levels of integration, and volume manufacture at much lower cost, the large bandwidth available in the millimeter wave bands will release this pent-up demand.

New Chip Technology
Until recently, MMIC chips capable of operating at V-band or E-band were based on Gallium Arsenide (GaAs), or even on Indium Phosphide (InP) for some W-band applications. GaAs is now available on larger wafers and at lower cost from several suppliers, and the cost of fabrication and prototyping new chip designs is often reasonable due to a much smaller number of photolithography mask steps than, for example, CMOS silicon. However, designing for high gain at frequencies of 60GHz and higher requires advanced GaAs processes with gate widths around 100µm or less, which are only just becoming available. Also, highly integrated GaAs chips, such as entire transceiver circuits, are difficult to design successfully and still expensive to manufacture.

Recently, MMIC chips operating in the V-band have become available based on Silicon Germanium BiCMOS processes. This technology offers high levels of integration including amplifiers, mixers, various modulators, and digital control logic on a single MMIC chip. The performance of SiGe BiCMOS has been, and remains, less than GaAs, particularly with respect to transmitter output power and receiver noise figures. However, these new devices are targeted towards short range applications such as wireless personal area networks, and distributing high definition video (1.4Gigabits/second) around a room or a home where reasonable performance (Rx NF ~6dB and Tx P1dB ~10dBm) is perfectly adequate. In this case, the shorter range inherent to the V-band, where oxygen absorption in the atmosphere is a local maximum, actually benefits by reducing interference between networks.

This consumer equipment is looking to make use of the high bandwidth available in the V-band under some of the most extreme price pressure, so highly integrated chips on low cost processes are a must. In fact, V-band chips for this application have even been announced using a bulk CMOS process.

A disadvantage of these SiGe and CMOS processes with many mask stages is a huge cost of development. The cost of masks and wafer runs to fabricate prototypes can be up to $1M. In order to provide a justifiable return on investment, these high costs demand that companies manufacture in very large volumes. Even so, since these types of devices are becoming available for this short range market, they may be suitable to make a substantial reduction in the costs of some of the other millimeter wave applications noted above.

New Circuit Board Technology
Materials with low loss at microwave frequencies have been available for some time to construct circuit boards integrating MMIC chips and supporting surface mount circuitry. Implementing high frequency interconnections when stepping up to millimeter wave frequencies requires high precision and high stability matching structures. These are often still implemented using thin films deposited and patterned lithographically on substrates such as ceramic, or quartz. The high cost of these approaches forces manufacturers to resort to using separate circuit boards for surface mount components. This means increased inventory and more complex assembly and testing procedures. New board materials are now becoming available with losses at millimeter wave frequencies four times lower than was previously possible. This allows the integration of MMIC chips and surface mount components on a single multi-layer circuit board.

The stability of the material is another important factor. For example, under high temperature processing such as reflow soldering of surface mount components, different layers of some fiber-based microwave boards move with respect to each other. In addition, some microwave board materials change their characteristics due to water absorbed from the ambient atmosphere. Both these effects are usually catastrophic to the accuracy needed for millimeter wave modules.

Liquid Crystal Polymer materials are capable of good edge definition for the precise matching structures needed in the V and higher bands. LCP enables multi-layer circuit boards without fibers that do not absorb water. These RF multi-layers can be fabricated on conventional circuit substrates such as FR4, which offers the possibility of low cost integration with other electronics in the system, or on metal carriers which can allow MMIC chips such as power amplifiers to be mounted directly onto the conducting metal carrier to enhance heat dissipation. The latter can be a significant benefit in applications such as Point-to-Point millimeter wave radios, where high output power is needed to maximize link budget and range, but where the radio equipment should be as small as possible for unobtrusive deployment and also needs to avoid using cooling systems such as fans and heats sinks in the radio enclosure.

New Module Technology
Using new, more highly integrated chips, or assembling onto new low-loss multi-layer circuit boards, does not address the key barrier for millimeter wave modules noted earlier: the electromagnetic issues resulting from radiating wavelengths the same size as the bond-wires, structures, and MMIC chips.

Enclosing these components to protect them from both damage and electrical interference immediately results in unwanted feedback paths, and cavity resonances. The cavities are usually milled into the metal enclosure or lid and can sometimes be made small enough to prevent a resonance mode being set up at the fundamental operating frequency of the circuit.

However, resonance modes at higher frequencies can still couple into the devices and structures, and so seriously impact the circuit performance. Of course, at higher frequencies the cavity cannot be made small enough to avoid the resonances and still allow assembly of the MMIC chips. In addition, the trend towards larger, more integrated chips to achieve significant cost savings, is limiting the usefulness of this simple approach.

The conventional approach to these issues at microwave frequencies is adding material that absorbs the energy, (radar absorbent material = RAM), into the MMIC cavity. Either blocks of RAM are mounted inside the cavity, or the cavity walls are coated with absorber. Forming RAM blocks to fit at the appropriate location, and mounting inside cavities with dimensions of only a few millimeters is not easy, and often requires manual tuning by trained engineers. However, some of these materials must be thick to dissipate enough of the radiated energy, and many that absorb microwave frequencies are far less effective on millimeter waves.

MMIC Solutions (MMICS) in the UK is using a simple low cost technique to form a cavity for chips in a multi-layer circuit board, with a lid to encapsulate and protect the MMIC devices. The lid is easily fabricated, can be automatically assembled, and has a coated surface precisely placed to absorb unwanted millimeter wave energy and eliminate the resonance issues even at operating frequencies higher than 100GHz.

This solution was used on our recently announced MSx600-series receiver modules for V-band Point-to-Point radios. MMICS believes this approach is ‘future-proof’, as it is suitable for even the largest, most highly integrated, millimeter wave chips. The technique allows the use of a single, multi-layer circuit board to integrate entire millimeter wave transceiver modules, including several MMIC chips and supporting surface mount circuitry. This provides substantial reductions in size and weight and much lower cost for communications and radar applications, compared to the conventional module approach.

New LCP materials with very low loss at millimeter wave frequencies were used on the V-band modules. MMICS has also pioneered their use at higher frequencies, such as our W-band direct detection receivers, which are used as radiometers in millimeter wave imaging for security applications. The very low buried stripline loss of less than 0.2dB/mm at up to 110GHz is believed to be the world’s best performance for this type of circuit board material.

Conclusion
Many technologies currently used to build microwave components are no longer effective at millimeter wave, where the wavelength is close to the size of the chips and internal structures. The demand for commercial use of these bands, for high capacity communications and other applications, is driving the development and adoption of new technologies. These include new processes for complex MMIC chips, new low-loss materials for highly integrated circuit boards, and new module technologies to eliminate the unwanted, and potentially fatal, EM effects at millimeter wave frequencies.

MMIC Solutions is using novel resonance suppression technology, proven in modules operating in the V-band at 60GHz and even in the W-band up to 110GHz, and is developing additional highly integrated modules to enable and support the commercial use of the millimeter wave bands.

MMIC Solutions UK
www.mmicsolutions.com
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