GaN semiconductors are a key technology for the energy-efficient electric cars and 5G networks of the future. The Lund-based startup company Hexagem is further developing the next-generation semiconductor technology, which will contribute to increased electrification and a sustainable future, at the RISE testbed ProNano.
Semiconductor substrates, also known as wafers, control and convert electrical current. They are used in building components. The round wafers are trimmed down to stamp-sized pieces that are encapsulated in microchips. They can be described as the brain and memory that make electronic products work. The chips are in everything from personal computers to cars, and they contain millions of transistors. The more transistors a device has, the faster it is.
Most semiconductors are made of silicon, but to meet the needs of electrification and increased demand for fast and energy-efficient technology, new variants are being developed. To drive energy conversion in a conventional car, about 1500 silicon chips are required and over 2000 in an electric car. In the future, even more chips will be needed as more and more functions are introduced in cars, for instance, self-driving cars. Next-generation semiconductors contain more transistors and are made of other materials, such as gallium nitride (GaN).
Hexagem is the only Swedish company to develop semiconductors with gallium nitride on silicon wafers. It started back in 2015 when Lars Samuelson and Jonas Ohlsson took the step to commercialize the technology they had developed over several years by forming the company. They had produced nanowires from the semiconductor substance gallium nitride, which proved to have good characteristics for creating effective wafers.
The company’s CEO Mikael Björk says that they see tremendous advantages for businesses using the technology, mainly due to quality and compatibility.
“About ninety percent of all chips on the market are silicon-based,” says Hexagem’s CEO Mikael Björk. “They are also cheap to produce because they basically consist of sand. With our technique, companies get a technology that is compatible with the industry's manufacturing processes, and at the same time, they can continue to keep costs down. But above all, our product offers high material quality, which in turn leads to more savings with lower energy losses.”
Fewer defects result in climate-smart and efficient GaN semiconductors
When manufacturing semiconductors, dislocations are created, i.e. defects in the material. The more defects there are in a semiconductor, the more energy is wasted during electrical conversion and the chip thus becomes less energy efficient.
“Our patented technology is as efficient as the market's best GaN-on-silicon semiconductors,” says Mikael Björk. “At present, they have 100 million defects per square centimeter. But we believe that we can significantly improve the material quality and soon approach 10 million defects per square centimeter and thus surpass the competition.”
The energy consumption is increasing in society, both nationally and globally. In 2018, Sweden consumed 142 terawatt-hours (TWh) of electricity. The Swedish Energy Agency's forecast indicates that electricity use will increase to 234 TWh by 2050, i.e. close to a 100 percent increase.
“Society is in great need of energy-smart innovations, such as Hexagem's technology, to meet the needs for electrification and the growing electricity demand,” says Michael Salter, Senior Project Manager at RISE Test and Demo facility ProNano. “The next-generation, energy-efficient semiconductors will help us create new solutions for a sustainable future. In the long run, the new technology means less carbon dioxide emissions and waste heat, and that we can use more of the energy we produce. It’s an important piece of the puzzle in reaching the climate goals.”
Vertical semiconductors pave the way for faster 5G networks
A major challenge in the industry is developing larger wafers. With conventional semiconductor technology, the size has increased in diameter. But new technology, innovators, such as Hexagem, want to increase the thickness of the wafers, so-called vertical semiconductors. Two to four micrometer-thick layers are common today.
“We are now focusing on developing vertical semiconductors by manufacturing significantly thicker layers of gallium nitride,” says Mikael Björk. “Our goal is to make ten micrometer-thick tiles in 2022. Hexagem's vision is to offer a material so that our customers can build new types of vertical components.”
With the new technology, the current flows vertically instead of horizontally on the plane of the wafer. This entails that the components become smaller and more numerous per surface unit, leading to both material savings and costs savings. But the biggest advantage is that they become more powerful and can handle even higher currents and voltages.
“In the near future, we want to develop the technology so that we can attract the interest of industrial enterprises,” says Mikael Björk. “We can then also scale up the sizes of our tiles, from wafers of 50 millimeters in diameter to 150 millimeters, which is the size most companies want. With our wafers, we are primarily targeting companies that manufacture power electronics, such as Bosch and Infineon. They can then build components to control high currents and voltages through power conversion. This is important for the continued development of electric cars and the 5G network, for example, which requires fast and efficient components.”
Scaling up semiconductor technology at ProNano
RISE Test and Demo facility ProNano is a digital innovation hub that strives to strengthen digitalization among Swedish companies with the help of nanotechnology. Startups can establish their operations in the testbed to get underway with their ideas faster without having to invest in expensive equipment.
At RISE, Hexagem has received support from experts in epitaxial crystal growth, i.e. the production of gallium nitride on silicon substrates. With ProNano's equipment for metal-organic chemical vapor deposition (MOCVD), they have created nanowires that form thin layers of gallium nitride on the silicon wafer. The wafers are processed, both before and after crystal growth with lithography – before growth to create patterns where the gallium nitride can “grow”, and after to insulate individual components in the wafer. The material is thereafter analyzed with an electron microscope to examine the appearance of the material after growth. After the analysis, the experts manufacture components of the material to test the electrical quality of the material.
“Development of our product has gone faster thanks to the expertise and equipment at RISE,” says Mikael Björk. “In the lab, we basically work with the same types of machines as used by the large semiconductor manufacturers. This is something that would have been very expensive for a startup company like Hexagem to finance on its own. We attain entirely different stability with smoother production and reproducibility than we would have had in a university lab, for example, that would have more different materials in the machines. We have also gained access to a fantastic network at RISE and this has resulted in valuable contacts with large international companies and EU projects.”
Hexagem is a spin-off from Lund University and presently has five employees. The research within the company is conducted with support from Vinnova and the EU project UltimateGaN.
Next-generation semiconductors with broad bandgap
A semiconductor, as the name suggests, is neither a conductor nor an insulator but something in between. How much current they conduct is controllable and they can thus adapt to each application. The material is used to make transistors and other electronic components. The most common end-products are the microprocessors that are on the chips, for example in computers.
Silicon is the most widely used semiconductor material. A round, flat disk is created in silicon, called a silicon substrate. The substrate can be used to build components in pure silicon, but it can also be coated with other semiconductor substances to give the substrate new properties. Although silicon semiconductors are good enough for today's technology, other semiconductor materials can be used in more demanding applications. For example, to convert high voltages in the electricity grid, electric cars, or trains. In these applications, semiconductor materials with broader bandgaps, such as gallium nitride, are more advantageous. They can handle much higher voltages, frequencies, and temperatures than silicon. Semiconductor materials such as gallium nitride (GaN) and silicon carbide (SiC) have about three times the bandgap compared to silicon. This provides higher energy efficiency and lower energy losses in, for example, the form of waste heat during operation of electrical devices.
The lab process for making semiconducturs
To make transistors from semiconductor materials with a broad bandgap, RISE uses a process called MOCVD (Metal-organic Chemical Vapor Deposition) to create special top layers on top of the base substrate. In a MOCVD machine, specific gases (or vapors) are flowed over the surface of the substrate at controlled temperatures and pressures. In this way, top layers are “grown” in a crystal structure, one atomic layer at a time in several layers. The method is called crystal growth, or epitaxy. The layers can be grown over the entire substrate surface or on specific parts of the substrate to cultivate 3D nanostructures, such as nanowires.
The epitaxial layers can be cultivated by the MOCVD process on substrates in several different materials such as silicon, silicon carbide, gallium nitride, diamond, or sapphire. The materials have different degrees of difficulty for achieving few crystalline defects.
Thereafter, transistors and other electronic components of the wafer are created by additional processing steps common to semiconductor fabrication, such as lithography, etching, or deposition of metals or insulators. Lastly, the finished wafer is cut into stamp-sized pieces that are encapsulated in an electronic package, tested, and sent on along the supply chains to, for example, Samsung, Volvo, Apple, and Ericsson, to finally end up in a microprocessor in a mobile phone or car.
Step-by-step: Manufacturing GaN-on-silicon wafers at ProNano
Step 1: Substrate cleaning
In the chemistry lab, a lab engineer inspects and cleans the base plates in a fume cupboard. Here the lab engineer prepares them for either pattern growth or MOCVD.
Step 2: Pattern growth in cleanroom
If the substrate is to be cultured with pattern growth, the lab engineer creates a growth template; this is done in a cleanroom. A growth mask is created on top of the base plate by deposition, spinning, and etching.
Step 3: Additional cleaning of substrates during pattern growth
Step one is repeated, the personnel inspects and performs a second cleaning of substrates with pattern growth.
Step 4: Epitaxy with MOCVD
The substrate is placed in the MOCVD chamber to grow the crystal layers in different combinations of GaN, AlGaN, or InGaN based on a specific “recipe”. The recipe is crucial for material growth and controls the composition of gas as well as temperature and pressure profiles.
Step 5: Inspection with Scanning Electron Microscope (SEM)
An electron microscope is used to analyze the material after growth to check the results of the MOCVD process. With an electron beam, the crystalline and topographical properties of the material can be analyzed on a nanoscale.
Step 6: Characterisation tests
Other characterization tests can be performed to test, for example, the electrical conductivity of the material, element composition, and surface smoothness. Metal contacts can, for example, be deposited on top of the cultured material to test conductivity.
