Micromachined Circuits For Terahertz Communications

EPSRC have a delivery plan to align their portfolio to areas of UK strengths and national importance and have designated a number of 'Grow' areas. This application addresses two of these areas: 'RF and microwave communications' and 'RF and microwave devices', specifically matching the terahertz technology aspect of the latter.

Why has EPSRC highlighted these areas? The answer is that society is evolving with a continuously increasing demand for the exchange of digital information. There is an expectation that everyone will be permanently connected to the Internet, no matter where they are. People are expecting that more information of a higher quality is delivered immediately: therefore newer services are requiring higher and higher data volumes and transfer rates. On demand video is an excellent example, with in-home delivery with standard definition now common place and demonstrations of new 4k on demand video now taking place. The data rates expected for these services are vast and the infrastructure needs adapt to cope.

One way to achieve this is to move to higher frequencies for wireless links. We propose to demonstrate new building block components for such a communications system, designing and building these on an entirely new basis. A frequency of 300 GHz is chosen as it is at the cusp of technology; systems are now being deployed at frequencies below about 100 GHz where as systems approaching 1000 GHz are some years away because of the lack of active circuits. The components will also be applicable in radar and sensing scenarios. Once the individual components have been demonstrated, a full communications system will be designed, built and tested. There are very few demonstrations of communication systems at 300 GHz and the unique design methodology will provide a world-class demonstration.

Three groups are collaborating in this project: the Fraunhofer Institute in Freiburg, Germany (IAF), and it the UK the Rutherford Appleton Laboratory (RAL) and Birmingham University. All partners have substantial design and measurement capabilities at these very high frequencies. IAF are world leaders in the production of submillimetre wave integrated circuits and will be supplying transistors for the amplifiers. RAL will deliver world class Schottky barrier and the University of Birmingham has advanced micromachining capabilities.

At Birmingham a new interconnect principle has been developed to link the Schottky diodes and transistors. Instead of using wires and their analogues, hollow waveguide tube based resonant cavities will be used. Currently 300 GHz components are mounting in conventionally milled gold pated blocks. The required waveguide dimensions are about 0.8 mm by 0.4 mm. Although conventional milling machines can machine this, once internal structures for resonators are required, milling becomes difficult or impossible. A technology that can be used for the waveguide cavities, and for smaller resonators at higher frequencies, is micromachining. Birmingham University have demonstrated micromachined waveguides, filters, diplexers and antennas at and above 300 GHz. This technology is now ready for the next step, which is the inclusion of active and non-linear devices. The micromachining work at Birmingham has been done by a number of techniques, the primarily technique is by etching an ultraviolet sensitive photoresist called SU8. This allows a pattern to be defined photolithographically by a mask and then etching sections produces the waveguide. The final structure is made by bonding a number of SU8 etched layers together and then metal coating them. The performance of the SU8 waveguides has been shown to be as good as metal. Other techniques for micromachining circuits will be investigated in order to find the optimum solution.

The project will deliver components for a 300 GHz communications system with mixers, amplifiers and antennas designed around a new resonant cavity embedding principle and integrated using micromachining technology. These components will be integrated into a demonstrator communications system, but are also required in many other systems; some of which are described below. The micromachining technology is applicable well above the 300 GHz demonstrator frequency, and is scalable to produce number of components in parallel. We expect this technology to be disruptive and have impact in a number of applications areas including radar, imaging, spectroscopy, and industrial process control.

The novel approach offers a higher performance, lower cost, method of achieving functionality at submillimetre wave frequencies: additionally it includes miniaturisation because of the integration. The maximum bandwidth is limited by the waveguide bandwidth. This is not narrow, and permits a wide spectrum of applications. The project outcomes will require investment for industrialisation: demonstration will be a solid start for future commercial approach.

Beneficiaries from the communications system would include military users of a low mass, directional, short range, high bandwidth system that penetrates smoke and fog. Selection of the frequency permits operation in atmospheric windows for extended range, or close to absorption lines where covertness is critical. Similar advantages might attract mobile phone companies, where applications would be short inter-tower links, possibly when assisting the emergency services in temporary situations such as natural or man-made disasters, or for revenue generation at music festivals, or the local downloads to mobile devices. There is considerable interest in short-range wide bandwidth Wi-Fi type connection to and between computers, tablets and multimedia components. Applications also include replacement of wide bandwidth physical and optical connectors where reliability is paramount.

Considering the technology more generally, the public sector, including law enforcement and health care, would benefit from the gas phase spectroscopic applications, e.g. breath analysis for disease diagnosis and ozone monitoring for predicting surges in hospital admissions. One can also envisage reflectometry being applied for skin disorders, including carcinoma, and transmission instrumentation for the study of small volume samples in non-aqueous solvents, with the option of time resolution. Multi-pixel security imaging will also benefit from the ease of production of detector and illumination units, and applications are well known, local and standoff imaging having been widely demonstrated. UK society will benefit from improved forecasts and climate models. This will arise from more affordable and therefore more extensive sensor networks, comprising ground based mm-wave cloud radars and passive multi-channel mm-wave radiometers. Better weather forecasts improve the quality of life, and reduce the cost to society of extreme weather events, as warnings can be issued and the NHS and emergency services prepared.

UK industry will benefit from the availability of an alternative process monitoring technology, one that might replace some applications of X-rays, or ultrasound, which requires physical contact. Wide instantaneous spectral bandwidth, and the ability to detect amplitude and phase, open applications such as layer thickness measurement, e.g. when curing paint or producing paper. The latter includes liquid levels in individual consumer packages, as well as high-resolution radar-based industrial tank and silo measurements.

The authors are already engaged in some of these areas of impact, and are well positioned to adopt the developments. RAL can apply the technology for Earth observations and radio astronomy. At Birmingham, Jaguar Land Rover is supporting work into a submillimetre wave imaging car radar.