by Tudor Williams, Director of Technology, Filtronic
The microwave industry is rapidly advancing toward harnessing the potential of ultra-high frequencies, reaching 300 GHz and beyond. This journey, while filled with promise, is laden with challenges, and I’ll detail some of the most important here.
The Internet has become an integral part of our daily lives, facilitating activities such as information exchange, social networking, banking, and online shopping. As we step further into the era of the Internet of Things (IoT) — where a web of smart devices seamlessly connects — the possibilities are endless. These IoT applications are poised to reshape everything from the way that our households run to modern businesses and manufacturers.
To meet these needs and keep up with the increasing demand for data, 5G and 6G wireless networks must push the boundaries further. The most likely early implementations will be in backhaul, the data pipe between the base-station and the core network.
Manufacturing tolerances stand out as a significant hurdle when dealing with high-frequency designs. As frequency increases the demands for tighter tolerances on component placement, interconnects and metalwork increase as well. Traditional manufacturing methods, such as CNC machining used to create the waveguide for signal conditioning, might lack the required precision. This forces the adoption of new production methods such as additive manufacturing to achieve the required tolerances.
Moreover, the journey from lower frequencies to 300 GHz involves the development of smaller semiconductor features, especially for high-power devices. For compound semiconductor devices, this progress is slower because of increased complexity. The development of these processes is driven by the emergence of new applications and business cases, making it a lengthy and costly journey.
One of the paramount challenges in high-frequency design is the management of parasitics. A good example would be wire bonding at lower frequencies. The impact of parasitic elements can be mitigated with relative ease, but as we move to millimeter-wave frequencies, it becomes necessary to design a matching network to cancel out the parasitic effects. And when we move to 300 GHz, this becomes impractical as manufacturing tolerances mean that a different matching network would be required for each new bond — clearly impossible in a volume production environment.
To overcome these challenges, a shift towards alternative packaging and interconnect solutions for semiconductor devices becomes imperative, and work has already begun on such designs, including flip-chip, hot vias, and chip scale packaging.
At these high frequencies, semiconductors become a critical focal point in the overall design. Each semiconductor material has unique properties that can support the critical design requirements. At lower frequencies, a mix of performance and cost often leads to integrating several individual semiconductors. In contrast, for higher frequencies, the aim will be to integrate as much functionality as possible into a single chip to minimize interconnects and the related problems outlined above.
So, much of the integration will likely be in the form of a silicon chip with high-performance elements, such as the power amplifier and LNA, remaining in a compound semiconductor material to maintain the required performance. Today, the processes do not exist with the performance to meet the power required to form wireless data links, but these will evolve in the coming years.
With each stride toward higher frequencies, the efficiency of electronic devices experiences a noticeable decline. This means that more DC power is required to achieve the same RF output power. This clearly calls for larger power supplies, but more critically, it leads to more power dissipation in the form of heat.
Compounding this challenge is the continual miniaturization of electronic circuits. As components and devices become smaller and more densely packaged, the heat generated is concentrated within a reduced physical area. This concentration of heat amplifies the importance of effective thermal management. Failing to dissipate this heat adequately can lead to various issues, including reduced device performance, lifetime, and even device failure.
To address these thermal management challenges, the industry is actively exploring innovative solutions. This starts at the interface between the semiconductor device and the heatsink, leading to a requirement for innovative materials in epoxies, sinters, and solders to remove heat from the device while maintaining an expansion coefficient with the other materials around it to remove stresses and strains, which can again lead to device aging or failure.
Once the interface is controlled, heat must be quickly spread. Here, heat-spreaders are deployed with materials such as copper, molybdenum, and even diamond for their enhanced thermal properties. Once the heat is spread, it must be effectively removed, leading to requirements for forced air convection or liquid cooling.
Manufacturing at high frequencies introduces a whole new realm of intricacies. Precision becomes the bedrock of this endeavor, with a demand for tight tolerances in critical areas such as component placement and epoxy dispensing. The high-frequency landscape leaves little room for error, and even minuscule variations in component placement can result in severe consequences. Therefore, the challenge lies in developing machinery capable of producing high volumes with impeccable precision.
The pursuit of precision in manufacturing for high frequencies often necessitates the creation of custom machinery. Off-the-shelf solutions may prove inadequate when dealing with the exacting requirements of these designs. As a result, manufacturers are driven to invest in tailor-made, precision-focused equipment to meet high-frequency production’s stringent demands. This tailored approach, while addressing the need for precision, adds an additional layer of complexity to the manufacturing process.
The challenge in high-frequency manufacturing extends beyond the design and engineering phases. It’s not just about creating high-frequency designs but making them viable for mass production. The intersection of precision, custom machinery, and evolving manufacturing techniques creates a nexus of complexity and opportunity. As the industry strives to lead the way in RF and mmWave communications, it must navigate these manufacturing constraints with ingenuity and unwavering commitment to achieving the desired precision in high-frequency electronic systems.
In the high-frequency landscape, it’s not merely a journey of technological progress; it’s a relentless pursuit of precision, precision in design, manufacturing, and achieving the desired performance at these as-yet-unreachable frequencies. Although designing solutions to 300 GHz is possible, achieving production at scale for such high frequencies is still an elusive goal — it’s not today’s technology but a vision for the future.