Silicon-On-Insulator (SOI) is silicon on an insulating substrate, which is a material and technology that plays an important role in the semiconductor field.
1. Structure and Principle
SOI presents a three-layer structure similar to a sandwich. The top layer is the top silicon, also called the device layer, which is used to make various semiconductor devices, such as transistors; the middle layer is the buried oxide layer, usually an insulating SiO₂ layer, which plays a key role in electrical isolation and can effectively reduce interference between devices; the bottom layer is the silicon substrate, which provides mechanical support for the entire structure. The principle is to introduce an insulator material between silicon transistors to greatly reduce parasitic capacitance, thereby significantly improving device performance.
2. Preparation technology
（1）Oxygen injection isolation technology (SIMOX): By injecting oxygen ions into the surface of the silicon wafer, followed by annealing at high temperature, the oxygen ions are diffused to form an oxide layer, and finally the SOI structure is constructed. However, due to the high cost of this process, it has gradually been replaced by other technologies.
（2）Wafer bonding and reverse etching technology (BESOI): First, the polished silicon wafers with high-quality thermal oxide layers grown on the two surfaces are strictly cleaned, and then bonded by van der Waals force in an ultra-clean environment, and annealed at high temperature to enhance the bonding strength. After that, one of the wafers is used as a substrate, and the other wafer is ground and polished to the required thickness, thereby forming a silicon single crystal film on an insulator.
（3）Smart Cut technology: This is the mainstream technology for preparing SOI silicon wafers. First, prepare two silicon wafers, oxidize one of them and inject hydrogen ions to form a hydrogen ion layer; clean the two silicon wafers and bond them; after appropriate heat treatment, the hydrogen-implanted wafer is completely split from the hydrogen ion layer to form an SOI structure; finally, the SOI surface is chemically mechanically polished to remove residual damage and provide a smooth surface for subsequent device preparation.
3. Performance advantages
（1）Excellent electrical performance: effectively reduce parasitic capacitance. Compared with traditional bulk silicon materials, the operating speed of SOI devices can be increased by 20% - 35%, while reducing leakage and reducing power consumption by 35% - 70%. It also completely eliminates the parasitic latch effect in bulk silicon CMOS circuits, suppresses the pulse current interference of the substrate, reduces the occurrence of soft errors, and ensures the stability and reliability of the circuit.
（2）Obvious process and integration advantages: good compatibility with existing silicon processes, can reduce about 13% - 20% of the process, help reduce production costs and improve production efficiency. And can achieve higher integration density, in line with the development trend of miniaturization of electronic equipment.
（3）Strong environmental adaptability: can work in a high temperature environment, the maximum operating temperature can reach 300 ° C, can effectively reduce overheating problems, and can also operate stably in some environments with harsh temperature requirements.
4. Application areas
（1）Integrated circuit manufacturing: widely used in the manufacture of high-performance processors, memory and radio frequency devices. It can provide better insulation performance, reduce crosstalk between electronic devices, improve the performance and reliability of integrated circuits, while reducing power consumption and increasing working speed.
（2）Microelectromechanical systems (MEMS): used to manufacture accelerometers, pressure sensors, micromechanical switches and other devices. Its insulating layer can provide good mechanical isolation and reduce the impact of mechanical vibration on device performance.
（3）Optoelectronics: suitable for manufacturing optoelectronic devices such as optical modulators, optical switches, and optical detectors. SOI silicon wafers have low light absorption loss and high optical conductivity, which helps to improve the performance of optoelectronic devices.
（4）Biosensors: can be used to manufacture biochips, DNA sequencers, and biosensor arrays. Its insulating layer can provide better biocompatibility, reduce the impact of background noise, and improve the sensitivity and accuracy of sensors. It plays an important role in medical diagnosis and environmental monitoring.
5. Development direction
（1）RF-SOI: It has a unique silicon/insulating layer/silicon three-layer structure, and achieves full dielectric isolation between devices and substrates through an insulating buried layer. It can achieve higher linearity and lower insertion loss with a higher cost-effectiveness, bringing users faster data speeds, longer battery life and more stable and smooth communication quality. It is widely used in the RF field, such as switches and antenna tuners for smartphones.
（2）Power - SOI: The main structure includes a single crystal top silicon wafer, an intermediate oxide buried layer and a bottom silicon substrate. Due to its wafer-thickened buried oxide structure, it can effectively overcome the problem of high voltage penetration components and achieve stability in the use of power components. It is mainly used for high-voltage component integration in BCD power integrated circuit manufacturing technology.
（3）FD - SOI (Fully Depleted Silicon on Insulator): As a planar process technology, from a structural point of view, the electrostatic characteristics of its transistors are better than traditional bulk silicon technology. The buried oxide layer can reduce the parasitic capacitance between the source and the drain, effectively inhibit the flow of electrons from the source to the drain, and greatly reduce the leakage current that causes performance degradation. In addition, it has unique advantages such as back bias capability, excellent transistor matching characteristics, the ability to use low power supply voltage close to the threshold, ultra-low sensitivity to radiation, and very high transistor intrinsic operating speed. It is suitable for application fields such as smartphones, Internet of Things, 5G, and automobiles that require high reliability, high integration, low power consumption, and low cost.
