Guiding, Localizing and IMaging confined GHz acoustic waves in GaN Elastic waveguides and Resonators for monolithically integrated RF front-ends

As smartphones become the dominant mechanism for information transfer and processing in modern society, our expectations on what we hope to achieve with them also increases proportionally. In particular, the smart phone has become our portal to the internet, replaced our television, radio and music devices, and also serves as our credit card and personal guide (GPS). We also expect our mobile phones to work seamlessly as we travel across international borders. All of this is enabled by the separation of the various functions into different wireless (RF) frequency bands, and the development of sophisticated analog and digital circuitry, that enables the phone to simultaneously carry out these communications. As we move towards 5G and other technologies that increase the data throughput available, these channels must increase. While on the digital signal processing side, the steady advance of Moore's law and microelectronic integration has enabled silicon technology to keep up with the demand, this is not the case for the RF front-end circuitry, which is primarily analog.

The RF front-end circuit, receives the signal from the antenna and separates it into different channels (based on RF filters), amplifies it with a low noise amplifier (LNA) and then hands it over to the DSP for baseband signal processing. Currently, RF filters and LNAs are primarily discrete devices that are co-packaged together. While this hybrid approach has certain advantages (mainly the choice of piezoelectric materials for the filter), as demand for filters continuously rises, it is known that a co-packaging approach will not scale. The main reason is that the available smartphone footprint (in terms of chip area) for the RF front-end has remained roughly the same across generations, while the filtering demand has continuously increased. As the microelectronics industry has repeatedly taught us, monolithic integration is the only long-term solution to address these problems.

In this project, we will demonstrate that gallium nitride (GaN) is the ideal platform for achieving monolithic integration by exploiting a key advantage that GaN provides over traditional solutions: acoustic waveguiding. GaN allows us to guide high-frequency sound on the surface of chip with low acoustic attenuation. By routing sound in nanoscale waveguides and localising it in micron-scale resonators, one can re-design RF system components from the ground up realizing a massive reduction in component footprint, which is key to enabling monolithic integration. By applying ideas from integrated photonics to high-frequency acoustics, we hope to realize for RF systems the same benefits (in terms of size, weight and performance) that silicon photonics has achieved for optical telecommunication systems. We will show that high quality RF passive devices (in particular, piezoelectric resonators and filters) can be built on the same GaN substrate as the active transistor devices. We will implement a process flow and design the associated process development kit to implement these ideas in commercial GaN RF foundries (for ex: the Newport wafer fab) in collaboration with our project partners.

This project fundamentally relies on recognising and exploiting the universality of wave phenomena. By applying methods developed for (silicon) integrated photonics to acoustic waves which share the same wavelength, we can vastly reduce the on-chip footprint of high-frequency acoustic wave resonators and filters and achieve monolithically integrated RF front-ends. As such, we believe the ideas developed as part of this work will serve to underpin future radio frequency (RF) technology and will have a wide impact on societal communications and daily life.

Impact on economy: We foresee our work will impact the UK economy in two complementary ways. In the near term, we will add the novel passive acoustic wave devices developed in this work to the gallium nitride (GaN) RF foundry process development kit (PDK). GaN RF foundry processes are slowly maturing worldwide. With the UK's extensive investments in GaN, including a GaN RF foundry process being developed at the Newport wafer fab in Cardiff, this project provides the necessary toolkit for the UK GaN foundries to differentiate themselves from the competition. This work also builds on the growing industry trend towards 'more than Moore' type systems that are complementary to traditional silicon microelectronics. More specifically, the work will exploit the multifunctional capabilities of the versatile gallium nitride (GaN) platform by showing that its electronic and acoustic properties can be simultaneously harnessed to produce devices with unmatched performance. In the longer term, we will work with our project partners (TI, Qorvo and IQE) to bring such highly integrated GaN chipsets with their inherent size, weight and performance (SWaP) benefits to their future product roadmap.

Impact on knowledge: One of the key pillars of this project is the development of novel visualization tools (in particular the gated Raman microscope) that allow us to map the acoustic energy density at the nanoscale with high spatial and temporal resolution. The development of novel metrological tools has always been a key precursor to scientific progress and we believe the acoustic microscopes developed during this project will open up new avenues of scientific understanding and enquiry beyond the specific project for which they are being developed. More broadly, this project aims to illustrate the power of engineering integrated platforms in materials whose multifunctional properties (in this case electrical and mechanical) can be simultaneously exploited. We believe these paradigms of thinking will broadly influence the academic community, both in the UK and worldwide.

Impact on people: This project combines ideas from integrated photonics, ultrasonics, piezoelectric devices, nanofabrication and RF and microwave device engineering. This interdisciplinary view will greatly benefit the three research associates, as they use this project as a stepping-stone for their future academic careers. We also believe this project will encourage UK academics to take a fresh look at RF systems from the device perspective. While the UK was the world leader in RF device engineering in the 1980s, over time, most of the RF innovation has been occurring at the systems and architecture level with fundamental device innovation being relatively neglected. Going back to the fundamentals of engineering RF devices and rethinking RF systems from the ground up, as this project aims to do, will be a shot in the arm for UK engineering.

Impact on wider society: This project, at its core, is built on the idea of universality of wave phenomena and the similarity of effects that can be observed and exploited across seemingly different experimental engineering platforms. Recognising this similarity will serve the engineers of the future (the high school students and undergraduates of today) in good stead.