The Epitaxy (Epi) Process in Semiconductor Fabrication
Key Takeaways
The process of epitaxy allows the growth of a higher purity layer on a substrate of the same material.
In the epitaxy process, the orientation of the growth is determined by the underlying base crystal.
The epi process influences the device quality, characteristics, electrical properties, etc.
Epitaxially growing gallium arsenide on a silicon substrate
Building integrated circuits or semiconductor devices on a perfect crystalline foundation layer is ideal. The epitaxy (epi) process in semiconductor fabrication aims to deposit a fine layer of single crystal, usually around 0.5 to 20 microns, on a single crystal substrate. The epi process is a significant step in semiconductor device manufacturing, especially in silicon wafer fabrication.
The Epitaxy (Epi) Process in Semiconductor Fabrication
An Overview of the Epitaxy Process in Semiconductor Fabrication | |
What it is | The epitaxy (epi) process in semiconductor fabrication allows the growth of a thin crystal layer on top of the crystal substrate in a given orientation. |
The goal | In semiconductor manufacturing, the goal of the epitaxy process is to make electron transmission more effective through the device. In the construction of semiconductor devices, the epitaxial layers are included to refine and make the structure homogenous. |
The process | The process of epitaxy allows the growth of a higher purity layer on a substrate of the same material. In some semiconductor materials, such as HBT or MOSFET, the epitaxy process is employed for growing material layers different from the substrate. It is the epitaxy process that makes it possible to grow a low-density doped layer over a highly doped material layer. |
Types of Epi Processes in Semiconductor Fabrication
In the epitaxy process, the orientation of the growth is determined by the underlying base crystal. There can be either one or many epitaxy layers depending on the repetition of the deposition. The epitaxy process can be employed to form a thin layer of material that can be either the same or different from the underlying substrate in terms of chemical composition and structure.
The Two Types of Epi Processes | ||
Characteristic | Homoepitaxy | Heteroepitaxy |
Grown layer | The epitaxially grown layer is of the same material as the substrate layer | The epitaxially grown layer is of a different material than the substrate layer |
Crystal structure and lattice | The crystal structure and lattice constant of the substrate and epitaxial layer are the same | The crystal structure and lattice constant of the substrate and epitaxial layer are different |
Example | Epitaxial growth of highly pure silicon over substrate silicon | Epitaxially growing gallium arsenide on a silicon substrate |
Application | Semiconductor device construction where layers of different doping levels are required or pure films on substrates that are less pure | Semiconductor device construction where layers of different materials are needed or to build a crystalline film of a material that is not available as a single crystal |
Factors Influencing the Epi Process in Semiconductor Fabrication
Factor | Description |
Temperature | Affects the epitaxy rate and epitaxial layer density. The temperature required for the epitaxy process is higher than room temperature, and the value depends on the type of epitaxy. |
Pressure | Affects the epitaxy rate and epitaxial layer density. |
Defects | Defects in the epitaxy lead to faulty wafers. The physical conditions required for the epi process should be maintained for non-defective epitaxial layer growth. |
Desired position | The epitaxial growth should be in the correct positions on the crystal. The regions that should be excluded from the epitaxial process should be properly filmed to prevent growth. |
Autodoping | As the epitaxy process is conducted at high temperatures, dopant atoms may be capable of bringing variations in the material. |
Epitaxy Density and Rate
The density of the epitaxial growth is the number of atoms of the material per unit volume in the epitaxially grown layer. Factors such as temperature, pressure, and type of semiconductor substrate affect the epitaxial growth. Generally, the density of the epitaxial layer varies with the aforementioned factors. The speed at which the epitaxial layer grows is called the epitaxy rate.
If the epitaxial growth is in the proper position and orientation, the growth rate will be high, and vice versa. Similar to epitaxial layer density, the epitaxy rate is also dependent on physical factors such as temperature, pressure, and substrate material type.
The epitaxy rate increases at high temperatures and low pressures. The epitaxy rate is also dependent on the substrate structure orientation, the concentration of the reactants, and the growth techniques employed.
Epi Process Methods
There are several methods to perform the epitaxy process: liquid phase epitaxy, hybrid vapor phase epitaxy, solid phase epitaxy, atom layer deposition, chemical vapor deposition, molecular beam epitaxy, etc.
Let’s compare the two epitaxy processes: CVD and MBE.
Chemical Vapor Deposition (CVD) | Molecular Beam Epitaxy (MBE) |
Chemical process | Physical process |
Involves a chemical reaction that takes place when gaseous precursors meet the heated substrate in the growth chamber or reactor | The material to be deposited is heated under vacuum conditions |
Precise control over the film growth process | Precise control over the thickness of the growth layer and composition |
Employed in applications requiring an epitaxial layer of high-quality | Employed in applications requiring an extremely fine epitaxial layer |
Most commonly used method | Expensive |
The epi process in semiconductor fabrication is crucial; it optimizes the performance of semiconductor devices and integrated circuits. It is one of the major processes in semiconductor device manufacturing that influences the device's quality, characteristics, and electrical properties.
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