Heteroepitaxy
Heteroepitaxy in semiconductor manufacturing involves the growth of a material on a substrate where the material structure on the wafer surface differs from the growth crystal. In most cases, the “different substrate” onto which silicon is deposited is oxide, an amorphous (non-monocrystalline) material. Because of this amorphous nature, there is no preferred direction for the deposited silicon to align with. Consequently, during the deposition process, numerous small crystallization cores form independently in various lattice alignments. As the deposition progresses, these cores grow into what is known as polycrystalline silicon or polysilicon for short. Polysilicon consists of minute crystallites, often referred to as “grains,” making the polysilicon deposition a classic example of heteroepitaxy.
The size of these grains is influenced by the process parameters and can be as small as a few hundred nanometers. At grain boundaries, different lattice alignments meet, and these interfaces can result in current leakage. This property makes polysilicon unsuitable for producing p-n junctions, which are essential for many semiconductor devices. However, polysilicon serves a critical role in basic current conduction.
Despite its limitations, polysilicon finds extensive use in semiconductor fabrication. Some common applications include:
Control Electrodes: Polysilicon structures are used as control electrodes, often referred to as “gates,” in field-effect transistors (FETs).
Ohmic Resistors: Polysilicon can function as ohmic resistors in various circuit components.
Electrodes for Capacitors: It is also employed as electrodes for capacitors, where its properties are well-suited for use as a dielectric material.
Heteroepitaxy and Polysilicon
Structuring polysilicon for these applications involves photolithography followed by etching. Reactive ion etching (RIE) is a commonly used technique in modern semiconductor processes, as it enables the creation of precise and detailed structures. These polysilicon structures are typically established during the semiconductor fabrication process before the metallization layers. They are often insulated by oxide layers between the monocrystalline silicon and the metallization layers to prevent undesired electrical interactions.
In summary, heteroepitaxy involves growing silicon on different substrates, particularly oxide, resulting in polysilicon, which consists of small crystalline grains. Despite its limitations in terms of forming p-n junctions, polysilicon is invaluable in semiconductor manufacturing for various applications, including control electrodes, resistors, and capacitor electrodes. Polysilicon is structured through photolithography and etching techniques to create specific semiconductor device components.
What is the Difference between Homoepitaxy and Heteroepitaxy?
In homoepitaxy, the material used for the growth layers is the same as the material of the substrate. This means that both the substrate and the deposited layers have identical crystal structures and chemical compositions. Engineers often use homoepitaxy when they aim to extend or enhance the existing properties of the substrate material.
On the other hand, heteroepitaxy involves growing layers of a material different from the substrate material. This results in a heterostructure, where the crystal structures and chemical compositions of the substrate and the epitaxial layers do not match. Heteroepitaxy is employed when specific properties or characteristics of the epitaxial material are required for the device being fabricated. It enables the integration of different semiconductor materials into a single device or structure.
In summary, homoepitaxy entails growing layers of the same material as the substrate, while heteroepitaxy entails growing layers of a different material on the substrate. Both techniques have their own applications and advantages in semiconductor device manufacturing.
Why is polysilicon considered a polycrystalline material?
Answer: Polysilicon is polycrystalline because it is composed of many small crystallites or grains with different lattice alignments. The grain size in polysilicon can vary and be as small as a few hundred nanometers. Due to grain boundaries, polysilicon is not suitable for forming p-n junctions but is used primarily for basic current conduction.
What is the significance of the gate oxide (GOX)?
Answer: Gate oxide (GOX) is a thin oxide layer used as a dielectric for capacitors and field-effect transistors. It is referred to as the “gate oxide” because it serves as the dielectric layer in field-effect transistors, particularly in the formation of gate structures.