Guide: Semiconductor wafer with gallium nitride (GaN) – the manufacturing processes
Semiconductor wafers with gallium nitride (GaN) materials create more energy-efficient electronic components than their silicon counterparts and it is a key technology for the energy-smart electric cars and 5g networks of the future. In this story, you will get a step-by-step guide to the manufacturing process of GaN semiconductor wafers.
Do you want to further develop your innovation with next-generation nano and semiconductor wafer technology? RISE testbed ProNano is a Digital Innovation Hub where you can pilot test and scale up your prototype, without having to invest in expensive equipment or infrastructure, and here, you get access to cleanrooms for industrial semiconductor manufacturing.
From semiconductor wafers in silicon to next-generation GaN wafers
Most electronic products, such as smartphones, computers, and cars, contain electrical components made with semiconductor wafers. 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 individual application.
Silicon is the most widely used semiconductor material and is found in virtually all electronics. Although silicon is good at solving most tasks, it sometimes needs the help of other semiconductor materials such as gallium nitride (GaN), for example, at high temperatures or at high frequencies. Although new materials, in some cases are replacing silicon, it is still by far the dominant semiconductor base material at the lowest cost. For this reason, a silicon wafer is often used as the base carrier material for next-generation semiconductor materials.
Silicon is manufactured in long logs or cylinder-shaped “ingots” which are then sliced into very thin, often less than a millimeter, circular semiconductor disks, also called wafers. On such a semiconductor wafer, electronic circuits can be defined or used as a base substrate coated with other semiconductor materials, such as GaN.
The method of coating is called epitaxy. During the process, gases and metals react with the substrate material under well-controlled conditions and high temperatures. In this way, thin layers or nanowires of GaN can be “grown” on a wafer. The next-generation semiconductor wafers have elements of GaN material.
The finished wafer is then trimmed down to stamp-size pieces that are encapsulated. In everyday language, the encapsulated semiconductor is called a microchip or just a chip. These can be described as the brain and memory that make electronic products work. The chip contains millions of transistors. The more transistors a device has, the faster it performs its tasks.
GaN is a semiconductor material with a wide bandgap and this is the secret behind its success. The wide bandgap allows GaN to be used at significantly higher temperatures and higher electrical voltages while retaining functionality, compared to silicon. An electronic device with GaN semiconductors can, for example, be designed smaller and lighter, at the same time as it is more energy-efficient, which means longer battery life.
Step-by-step manufacturing of semiconductor wafers in GaN material
In the experimental development of GaN semiconductors, sapphire can be used instead of silicon. The manufacturing steps described below are largely the same for both substrates. However, the cultivation, or growth, of GaN is more difficult on silicon because the disks are prone to cracking due to mechanical stress unless specific layers are grown to counteract the pressure. The different layers can, for example, be combined by GaN, AlGaN or InGaN. The “recipe” is, among other things, what makes GaN semiconductors unique. The recipe is crucial for material growth and controls the composition of gas as well as temperature and pressure profiles. RISE crystal growth experts start the development process with sapphire and when the recipe is refined for the specific innovation, it is used on silicon substrates.
Step 1. Cleaning of substrate before MOCVD growth
In the chemistry lab, a Laboratory Engineer inspects and cleans the base plates in a fume cupboard. Here he prepares them for either pattern growth or MOCVD, which stands for Metal-Organic Chemical Vapor Deposition. The large capacity of ProNano's MOCVD reactor means that seven 50-millimeter disks can be grown epitaxially in one round, or alternatively one 150-millimeter disk in one round. When the smaller disks are grown, the full capacity is not always utilized and only one or two disks are used by the seven positions. This can be, for example, for experimental purposes in process development or calibration of the growth reactor's process parameters, such as temperature, pressure, or gas flows. For uniform growing conditions, the empty positions need to be filled with disks of the same size and substrate material, in the exemplified case below: sapphire.
From a sustainability perspective and to manage resources, the sapphire disks are reused. This is done by etching, or cleaning, removing all gallium nitride (GaN) from the top surface of the sapphire disk with heated potassium hydroxide (KOH). The sapphire disks are placed in a holder and dipped in the etchant.
Since the etchant needs to be heated to achieve quick and efficient cleaning of the sapphire disks, the disks are immersed in a heated water bath where temperature and volume are controlled.
Once the cleaning process is complete, the sapphire disks are rinsed in filtered deionized water and dried with pressurized nitrogen. However, this leaves a thin layer of water on the surface which is removed by heating the disks in an oven above 100°C.
Before nanostructures can be grown, the clean disks are subjected to a number of processes in the cleanroom, such as deposition, spinning, and etching. This creates a pattern of cavities, on a nanometer scale, over the entire surface of the disk. The cavities are there to determine where the nanostructures will be created or not created. The quality, such as size, distribution, and purity, can be effectively measured with scanning electron microscopy. The dimensions of the pattern affect the input parameters of the epitaxial growth and are therefore a necessary step in creating high-quality semiconductor material.
Step 3. Epitaxial growth of semiconductor wafers with MOCVD
ProNano's epitaxial growth reactor is of the MOCVD type. The principle of crystal growth is that gases of different types flow into a chamber with stringent control over pressure, temperature, and gas conditions, which causes the gases to decompose on the surface of the substrates that act as the starting material.
The pattern and crystal plane of the substrate directs growth to assume a certain direction so that high-quality films or nanostructures can be grown on top of the substrates. All handling of the substrates is carried out in a controlled environment of nitrogen gas, an inert gas that does not affect the surface of the substrates before and after crystal growth. The substrates are moved with gloves towards the nitrogen-filled chamber. The disks are placed close together to attain uniform conditions in terms of temperature, pressure, and gas conditions during epitaxial crystal growth.
The substrates, in this case, sapphire disks with a diameter of 50 millimeters, are handled by gloves towards a nitrogen-filled chamber. The 50-millimeter disks are placed close together to attain uniform conditions in terms of temperature, pressure, and gas conditions during epitaxial crystal growth.
Step 4. Further characterization of semiconductor wafers with scanning electron microscope (SEM)
The scanning electron microscope (SEM) used in step two is used in this step to study the shape of nanostructures after the epitaxial growth of GaN on the substrate. With the SEM equipment, it is possible to accurately and quickly study nanostructures and verify the quality of the epitaxially grown semiconductor material.
In epitaxial crystal growth, defects, also called dislocations, are created in the material. The more dislocations there are in a semiconductor wafer, the more energy is wasted during electrical conversion, and it then becomes less energy efficient. RISE experts work together with customers to realize unconventional growth methods, such as coalescing of nanowires, which have shown promising results in achieving lower defect densities in GaN material.
The SEM equipment can be used to examine and determine the defect density in the semiconductor material. The microscope method is based on electrons being pushed towards the sample to be studied, for example, a sapphire disk. The thin epitaxial semiconductor material is placed with metal clips against a grounded holder so that the electron beam in the microscope does not create blurred microscope images. The interaction between the electrons causes electrons or roentgen rays to be scattered from the sample. These signals can be captured with detectors, and images with a resolution of less than one nanometer can be seen.
After the GaN crystals have been analyzed with SEM, RISE experts manufacture electronic components, such as Schottky diodes or p-n diodes, from the semiconductor material. Characterization tests are performed to test the electrical conductivity of the material, the composition of the elements, and surface smoothness. The components are produced in cleanrooms using lithographic methods, etching, and metal contacts that are deposited on top of the cultured material. The experts then measure the electrical capacity of the component, such as current and capacitance, and evaluate its performance based on parameters such as breakdown voltage and leakage current.
Electrical components are manufactured from the epitaxial semiconductor material. The components are the square pattern on the semiconductor and they have been manufactured using lithographic methods, etching as well as metal contacts that are deposited on top of the cultured material.
BONUS: The startup company Hexagem is developing semiconductors with GaN at ProNano
Read more in the next story about the Lund company Hexagem, which is further developing the next generation of semiconductor technology with GaN material at the RISE testbed. Hexagem's CEO, Mikael Björk explains:
“At ProNano, 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 levels with smoother production and reproducibility than we would have had in a university lab, for example, that would have a variety of different materials in the machines.”