Next generation Hall-Effect sensors

5 min read

Working with CERN’s Magnetic Measurements section Paragraf is applying its new graphene-based Hall-effect sensors

For over 15 years the unique properties of graphene has been widely discussed yet, due to the lack of a contamination-free, transfer-free, large-area graphene source, its commercialisation has been limited. It’s been a frustrating few years for those looking to develop and apply this material, but that could be about to change.

Paragraf is a technology company that looks to deliver commercial quality, graphene-based electronic devices, and as Dr Ellie Galanis, the company's Business Developer explains, commercially produced graphene is currently created by either exfoliating graphite or depositing the graphene onto a metal substrate – which is most commonly a copper foil.

“The process produces good quality graphene, but it tends to be multi-layered, non-homogeneous and non-customisable, making it unsuitable for electronic devices.

“The advantages of graphene are well documented,” continues Dr Galanis. “It’s a remarkable material with a combination of exceptional properties - whether that’s its conductivity, mechanical strength, light weight, flexibility or its chemical stability. For electronic devices it is capable of carrying a charge very quickly, making devices much faster while using less power.

“Its mechanical strength means that it has much higher voltage tolerances, so that even at high voltages it remains stable and, due to its chemical stability, it can be used in extreme conditions. These properties make it possible to not only improve existing technologies but to create new ones.”

To date many companies, including the likes of Intel, IBM, and Samsung, have collectively invested billions in attempting to bring electronic devices made from graphene to market but, as yet, with limited success.

Graphene, which is currently grown on copper foil, needs to be transferred to an electronics compatible substrate after being synthesised, that involves various wet and dry transfer processes. This can affect how the graphene functions in an electronic device and can result in the graphene being contaminated by the copper.

These metallic impurities can then change the electronic and electrochemical properties of graphene and detrimentally impact on the integration of graphene with silicon technologies.

“Paragraf has been able to address this problem with a new process that grows graphene directly onto a semiconductor substrate,” explains Dr Galanis. “This unique and patented process uses scalable processes to allow the manufacturing of large-area, high-quality graphene (currently up to 8” diameter).

“We use a modified deposition method that removes the need for the transfer processes commonly used in most synthesis methods. As a result, the graphene can be grown in a uniform, single layer directly onto a wide range of substrates, including silicon, silicon-carbide (SiC), sapphire, gallium-nitride (GaN) and other semiconductor-compatible substrates and it is free from metallic contamination.”

The company’s approach has been to focus on semiconductor devices, rather than simply the graphene, and this has enabled it to open the door to the commercialisation of graphene.

“We’ve invested in new R&D and production facilities and our funding is now in place to scale up and commercialise our first product,” says Dr Galanis.

Hall-Effect sensor

That first commercial product is a new Hall-effect sensor.

“The technology was attractive in the sense that there hasn’t been much innovation with these devices over the past 40 years, and with the growth in electrification and the demand for magnetic sensing accelerating it seemed like an attractive first product.”

Hall-effect sensors are used for measuring magnetic fields and electric currents, as well as timing the speed of wheels and shafts.

“It’s a relatively simple device which makes it easier to incorporate graphene into the design and provides us with with a technical platform from which we can develop more sophisticated technologies,” according to Dr Galanis.

The market for magnetic sensors spans scientific research, healthcare, aerospace, industry and automotive, but existing products tend to have a limited temperature range, sensitivity, accuracy and magnetic field range.

“As we see further innovation in these markets they will require magnetic sensors that can operate in much harsher conditions and at higher temperatures.”

The graphene-based GHS series Hall-effect sensors from Paragraf look to address this and have a range of features that include the lack of a planar Hall-effect, much higher sensitivity, and the ability to be used in wide temperature ranges and harsh environments.

The lack of a planar Hall-effect is a result of the thinness of the monolayer graphene. As a single layer of carbon atoms, there is a negligible planar Hall-effect, which means that false signals are not induced. This enables only the actual perpendicular magnetic field value to be obtained, allowing for much higher precision mapping of magnetic fields.

“These sensors also have high mobility for faster sensing and a large dynamic range. The sensor also has a highly linear voltage response (less than 0.5 % nonlinearities) and there is no need for integrated signal processing electronics as the capabilities mentioned are possible with the bare sensor alone.

“As the Paragraf Hall-effect sensor is a bare analogue sensor without amplification, there is also the potential for the performance to be improved once it has undergone further product development,” explains Dr Galanis.

Field testing

These Hall-effect sensors have undergone field testing with a number of partners to prove their suitability in commercial and working environments and commercial availability is expected later this year.

Partners include the National Physical Laboratory (NPL) as part of an Innovate UK funded project, investigating the suitability of Paragraf’s sensors in harsh environments while, earlier this month, the company announced that it was working with the Magnetic Measurement Laboratory of the European Organization for Nuclear Research (CERN).

Above: Paragraf is working with CERN's Magnetic Measurement Laboratory

“With CERN we’re looking at how measurements can be opened up through using our graphene sensors and their unique properties, especially the negligible planar Hall-effect,” says Dr Galanis.

CERN operates the world’s largest particle accelerators, and physicists at the facility use the Large Hadron Collider (LHC) to understand how the world was built at the fundamental level by colliding sub-atomic particles in particle accelerators.

“In order to measure their work they rely on large numbers of normal and superconducting magnets to steer and focus the particle beam to their collision points.

“The Magnetic Measurements section uses the latest-available techniques and instruments but are always looking to improve their measurement methods and accuracy, so we’re working with them to better understand the potential of graphene-based Hall-effect sensors to improve accuracy in magnetic measurement applications.,” explains Dr Galanis.

The company’s Hall-effect sensors will look to provide highly accurate measurements of local field distributions in accelerator magnets, while eliminating artefacts and reducing uncertainties stemming from the sensors.

“Existing Hall-effect sensors exhibit planar Hall effects leading to false signals that, together with the non-linear response to the field strength, increases the measurement uncertainty and can limit their application.

“By using our Hall-effect sensors without planar effect it will be possible to open the door to new mapping techniques by mounting a stack of sensors on a rotating shaft. The compelling advantage would be measurements of the harmonic content in accelerator magnets almost point-like along the magnet axis”, explains Dr Galanis.

Because Paragraf’s Hall effect sensor are also able to operate in a wide temperature range from +80°C down to cryogenic temperatures of 1.5 Kelvin, it means that the fields inside the superconducting magnets can now be measured with much greater high accuracy.

CERN’s Magnetic Measurement section is looking to perform more in-depth tests on the Hall-effect sensors, with the eventual aim of using them to build a novel mapping system for magnetic fields.

“The hype that that has surrounded graphene and the lack of commercialisation to date, has been a problem,” suggests Dr Galanis. “We’re working against some cynicism that this technology can actually be used. But I think our collaboration with CERN is the highest validation of this technology and we will begin to see the benefits of graphene, in many different electronic devices, being realised in the not too distant future.”