Origin of the Name “Epitaxial Wafer”
Wafer preparation consists of two main steps: substrate preparation and epitaxial process. The substrate is made of semiconductor single crystal material and is typically processed to produce semiconductor devices. It can also undergo epitaxial processing to form an epitaxial wafer. Epitaxy refers to the process of growing a new single crystal layer on a carefully processed single crystal substrate. The new single crystal can be of the same material as the substrate (homogeneous epitaxy) or a different material (heterogeneous epitaxy). Since the new crystal layer grows in alignment with the substrate’s crystal orientation, it is called an epitaxial layer. The wafer with the epitaxial layer is referred to as an epitaxial wafer (epitaxial wafer = epitaxial layer + substrate). Devices fabricated on the epitaxial layer are called “forward epitaxy,” while devices fabricated on the substrate are referred to as “reverse epitaxy,” where the epitaxial layer serves only as a support.
Homogeneous and Heterogeneous Epitaxy
▪ Homogeneous Epitaxy: The epitaxial layer and substrate are made of the same material: e.g., Si/Si, GaAs/GaAs, GaP/GaP.
▪ Heterogeneous Epitaxy: The epitaxial layer and substrate are made of different materials: e.g., Si/Al₂O₃, GaS/Si, GaAlAs/GaAs, GaN/SiC, etc.
Polished Wafers
What Problems Does Epitaxy Solve?
Bulk single crystal materials alone are insufficient to meet the increasingly complex demands of semiconductor device fabrication. Therefore, in late 1959, the thin single crystal material growth technique known as epitaxy was developed. But how did epitaxial technology specifically help the advancement of materials? For silicon, the development of silicon epitaxy occurred at a critical time when the fabrication of high-frequency, high-power silicon transistors faced significant difficulties. From the perspective of transistor principles, achieving high frequency and power requires that the collector region’s breakdown voltage be high, and the series resistance be low, meaning the saturation voltage should be small. The former requires high resistivity in the collector material, while the latter requires low resistivity, which creates a contradiction. Reducing the thickness of the collector region to reduce series resistance would make the silicon wafer too thin and fragile for processing, and lowering the resistivity would conflict with the first requirement. The development of epitaxial technology successfully resolved this issue. The solution was to grow a high resistivity epitaxial layer on a low-resistivity substrate. The device is fabricated on the epitaxial layer, ensuring the high breakdown voltage of the transistor, while the low-resistivity substrate reduces the base resistance and lowers the saturation voltage, solving the contradiction between the two requirements.
Additionally, epitaxial technologies for III-V and II-VI compound semiconductors such as GaAs, GaN, and others, including vapor phase and liquid phase epitaxy, have seen significant advancements. These technologies have become essential for the fabrication of many microwave, optoelectronic, and power devices. In particular, techniques like molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) have been successfully applied to thin layers, superlattices, quantum wells, strained superlattices, and atomic-scale thin epitaxial layers, laying a solid foundation for the development of new semiconductor fields such as “band engineering.”
In practical applications, most wide-bandgap semiconductor devices are fabricated on epitaxial layers, with materials like silicon carbide (SiC) being used solely as substrates. Therefore, controlling the epitaxial layer is a critical factor in the wide-bandgap semiconductor industry.
Epitaxy Technology: Seven Key Features
1. Epitaxy can grow a high (or low) resistivity layer on a low (or high) resistivity substrate.
2. Epitaxy allows the growth of N (or P) type epitaxial layers on P (or N) type substrates, directly forming a PN junction without the compensation issues that arise when using diffusion to create a PN junction on a single crystal substrate.
3. When combined with mask technology, selective epitaxial growth can be performed in specific areas, enabling the fabrication of integrated circuits and devices with special structures.
4. Epitaxial growth allows for the control of doping types and concentrations, with the ability to achieve abrupt or gradual changes in concentration.
5. Epitaxy can grow heterogeneous, multi-layered, multi-component compounds with variable compositions, including ultra-thin layers.
6. Epitaxial growth can occur at temperatures below the melting point of the material, with a controllable growth rate, allowing for atomic-level precision in layer thickness.
7. Epitaxy enables the growth of single crystal layers of materials that cannot be pulled into crystals, such as GaN and ternary/quaternary compound semiconductors.
Various Epitaxial Layers and Epitaxial Processes
In summary, epitaxial layers offer a more easily controlled and perfect crystal structure than bulk substrates, which is beneficial for the development of advanced materials.
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