The origin of the name epitaxial wafer
First, let’s popularize a small concept: wafer preparation includes two major links: substrate preparation and epitaxial process. The substrate is a wafer made of semiconductor single crystal material. The substrate can directly enter the wafer manufacturing process to produce semiconductor devices, or it can be processed by epitaxial processes to produce epitaxial wafers. Epitaxy refers to the process of growing a new layer of single crystal on a single crystal substrate that has been carefully processed by cutting, grinding, polishing, etc. The new single crystal can be the same material as the substrate, or it can be a different material (homogeneous) epitaxy or heteroepitaxy). Because the new single crystal layer extends and grows according to the crystal phase of the substrate, it is called an epitaxial layer (the thickness is usually a few microns, taking silicon as an example: the meaning of silicon epitaxial growth is on a silicon single crystal substrate with a certain crystal orientation. A layer of crystal with good lattice structure integrity and different resistivity and thickness with the same crystal orientation as the substrate is grown), and the substrate with the epitaxial layer is called an epitaxial wafer (epitaxial wafer = epitaxial layer + substrate). When the device is made on the epitaxial layer, it is called positive epitaxy. If the device is made on the substrate, it is called reverse epitaxy. At this time, the epitaxial layer only plays a supporting role.
Polished wafer
Epitaxial growth methods
Molecular beam epitaxy (MBE): It is a semiconductor epitaxial growth technology performed under ultra-high vacuum conditions. In this technique, source material is evaporated in the form of a beam of atoms or molecules and then deposited on a crystalline substrate. MBE is a very precise and controllable semiconductor thin film growth technology that can precisely control the thickness of deposited material at the atomic level.
Metal organic CVD (MOCVD): In the MOCVD process, organic metal and hydride gas N gas containing the required elements are supplied to the substrate at an appropriate temperature, undergo a chemical reaction to generate the required semiconductor material, and are deposited on the substrate on, while the remaining compounds and reaction products are discharged.
Vapor phase epitaxy (VPE): Vapor phase epitaxy is an important technology commonly used in the production of semiconductor devices. The basic principle is to transport the vapor of elemental substances or compounds in a carrier gas, and deposit crystals on the substrate through chemical reactions.
What problems does the epitaxy process solve?
Only bulk single crystal materials cannot meet the growing needs of manufacturing various semiconductor devices. Therefore, epitaxial growth, a thin-layer single crystal material growth technology, was developed at the end of 1959. So what specific contribution does epitaxy technology have to the advancement of materials?
For silicon, when silicon epitaxial growth technology began, it was really a difficult time for the production of silicon high-frequency and high-power transistors. From the perspective of transistor principles, to obtain high frequency and high power, the breakdown voltage of the collector area must be high and the series resistance must be small, that is, the saturation voltage drop must be small. The former requires that the resistivity of the material in the collecting area should be high, while the latter requires that the resistivity of the material in the collecting area should be low. The two provinces are contradictory to each other. If the thickness of the material in the collector area is reduced to reduce the series resistance, the silicon wafer will be too thin and fragile to be processed. If the resistivity of the material is reduced, it will contradict the first requirement. However, the development of epitaxial technology has been successful. solved this difficulty.
Solution: Grow a high-resistivity epitaxial layer on an extremely low-resistance substrate, and make the device on the epitaxial layer. This high-resistivity epitaxial layer ensures that the tube has a high breakdown voltage, while the low-resistance substrate It also reduces the resistance of the substrate, thereby reducing the saturation voltage drop, thereby resolving the contradiction between the two.
In addition, epitaxy technologies such as vapor phase epitaxy and liquid phase epitaxy of GaAs and other III-V, II-VI and other molecular compound semiconductor materials have also been greatly developed and have become the basis for most microwave devices, optoelectronic devices, power It is an indispensable process technology for the production of devices, especially the successful application of molecular beam and metal organic vapor phase epitaxy technology in thin layers, superlattices, quantum wells, strained superlattices, and atomic-level thin-layer epitaxy, which is a new step in semiconductor research. The development of “energy belt engineering” in the field has laid a solid foundation.
In practical applications, wide bandgap semiconductor devices are almost always made on the epitaxial layer, and the silicon carbide wafer itself only serves as the substrate. Therefore, the control of the epitaxial layer is an important part of the wide bandgap semiconductor industry.
7 major skills in epitaxy technology
1. High (low) resistance epitaxial layers can be epitaxially grown on low (high) resistance substrates.
2. The N (P) type epitaxial layer can be epitaxially grown on the P (N) type substrate to form a PN junction directly. There is no compensation problem when using the diffusion method to make a PN junction on a single crystal substrate.
3. Combined with mask technology, selective epitaxial growth is performed in designated areas, creating conditions for the production of integrated circuits and devices with special structures.
4. The type and concentration of doping can be changed according to needs during the epitaxial growth process. The change in concentration can be a sudden change or a slow change.
5. It can grow heterogeneous, multi-layered, multi-component compounds and ultra-thin layers with variable components.
6. Epitaxial growth can be performed at a temperature lower than the melting point of the material, the growth rate is controllable, and epitaxial growth of atomic-level thickness can be achieved.
7. It can grow single crystal materials that cannot be pulled, such as GaN, single crystal layers of tertiary and quaternary compounds, etc.
Post time: May-13-2024