In the semiconductor industry chain, especially in the third-generation semiconductor (wide bandgap semiconductor) industry chain, the distinction between substrate and epitaxial layer is crucial.
What is the significance of the epitaxial layer? What is the difference between it and the substrate?
First of all, the substrate is a wafer made of semiconductor single-crystal material. It can be used as a direct input in the wafer manufacturing process to produce semiconductor devices, or it can be processed by the epitaxial process to produce epitaxial wafers. The substrate is the foundation of the wafer, located at the bottom layer, and supports the entire wafer. In the chip manufacturing process, the wafer is cut into multiple independent dies, and after packaging, it becomes the chip we are familiar with. The substrate is the base at the bottom of the chip, and the complex structure of the chip is built on this base.
Secondly, epitaxy refers to the growth of a new single-crystal layer on a finely processed single-crystal substrate. This new single crystal can be the same as the substrate material or a different material, which is called homoepitaxial or heteroepitaxial respectively. Since the new single-crystal layer grows according to the crystal phase of the substrate, it is called an epitaxial layer. Its thickness is usually several microns. Taking silicon as an example, the significance of silicon epitaxial growth is to grow a single crystal layer with a good crystal structure with the same crystal orientation, different resistivity, and thickness on a silicon single-crystal substrate with a specific crystal orientation.
The substrate after epitaxial growth is called an epitaxial wafer, and its structure can be expressed as an epitaxial layer plus a substrate. The manufacturing process of the device is carried out on the epitaxial layer.
Epitaxy is divided into homoepitaxial and heteroepitaxial. Homoepitaxial refers to growing an epitaxial layer of the same material as the substrate on a substrate. The significance of homoepitaxial is to improve the stability and reliability of the product. Although the homoepitaxial layer is made of the same material as the substrate, the material purity and uniformity of the wafer surface can be improved through epitaxial treatment. Compared with the polished wafer with mechanical polishing, the substrate surface treated with epitaxial treatment has higher flatness, higher cleanliness, fewer micro defects, and fewer surface impurities, so the resistivity is more uniform, and it is easier to control defects such as surface particles, stacking faults, and dislocations.
Epitaxy not only improves the performance of the product but also ensures the stability and reliability of the product. In the CMOS silicon process, epitaxial growth on the wafer substrate is a crucial process step.
1. Improve crystal quality: The defects and impurities of the initial substrate can be improved by the growth of the epitaxial layer. The wafer substrate may produce certain defects and impurities during the manufacturing process. The growth of the epitaxial layer can generate a high-quality, low-defect, and impurity-concentration single-crystal silicon layer on the substrate, which is crucial for subsequent device manufacturing.
2. Uniform crystal structure: Epitaxial growth can ensure the uniformity of the crystal structure and reduce the influence of grain boundaries and defects in the substrate material, thereby improving the crystal quality of the entire wafer.
3. Improve electrical performance and optimize device characteristics: By growing an epitaxial layer on the substrate, the doping concentration and type of silicon can be precisely controlled to optimize the electrical performance of the device. For example, the doping of the epitaxial layer can accurately adjust the threshold voltage and other electrical parameters of the MOSFET.
4. Reduce leakage current: High-quality epitaxial layers have lower defect density, which helps to reduce leakage current in the device, thereby improving the performance and reliability of the device.
5. Support advanced process nodes and reduce feature size: In smaller process nodes (such as 7nm, and 5nm), the device feature size continues to shrink, requiring more refined and high-quality materials. Epitaxial growth technology can meet these requirements and support high-performance and high-density integrated circuit manufacturing.
6. Improve the breakdown voltage: The epitaxial layer can be designed to have a higher breakdown voltage, which is critical for manufacturing high-power and high-voltage devices. For example, in power devices, the epitaxial layer can increase the breakdown voltage of the device and increase the safe operating range.
7. Process compatibility and multi-layer structure: Epitaxial growth technology allows the growth of multi-layer structures on the substrate, and different layers can have different doping concentrations and types. This is very helpful for manufacturing complex CMOS devices and achieving three-dimensional integration.
8. Compatibility: The epitaxial growth process is highly compatible with existing CMOS manufacturing processes and can be easily integrated into existing manufacturing processes without significantly modifying the process lines.
