Gaining a better understand of Silicon Epitaxy

The word epitaxy is derived from Greek and loosely means thin skin. When a crystalline substrate or wafer receives a thin crystalline overlay, it is often referred to as epitaxy. The over-layment must be ordered with a preferred or specific orientation as it relates to the substrate. The thin layer of crystal material usually ranges from .5 to 20 microns. If the growth orientation is random, it is non-epitaxial. When silicon is used as the over-layment material, it is termed silicon epitaxy. There are different kinds of epitaxy, homoepitaxy, heteroepitaxy, and heterotopotaxy.

Homoepitaxy means that the material used for the substrate or wafer is the same material used to create the crystalline overlay. For instance, when you use a silicon substrate and deposit a film of silicon over it, it is homoepitaxy. This results in a film that is even more pure than the substrate. Different doping levels can be achieved with this process. Most wafers or substrates used are 50 mm, 100 mm or 150 mm in diameter. They are usually made of silicon that is very pure and is grown into a cylindrical ingot that is then sliced into wafers and polished. There can be one or two thin layers or there can be a build-up of multiple layers to make the entire piece thicker.

Heteroepitaxy combines two materials, one for the substrate wafer and another material for the epitaxial layer. This may be used to create a crystalline film over a substance that isn't in crystal form. In other words, the crystal material actually mimics the layer of the substrate but in crystal form. This method can also be used to create layers of different materials that can be integrated.

Why do we need Silicon Epitaxy?

When a lightly doped crystalline layer is grown over a substrate that is heavily doped, then a higher breakdown voltage is achieved, and at the same time the collector resistance is kept low. This basically provides for a higher operating speed and improved bipolar performance. We need silicon epitaxy to produce JBTs and CMOS integrated circuits, as well as for the production of compound semiconductors. It has made it affordable to create high quality crystal growth, particularly for materials that are important technologically. With certain CMOS circuits, epitaxy helps to minimize the occurrence of latch-up. In addition, better control of the concentrations of doping on the devices can be gained. Also, the performance of RAM devices can be improved.

How is Silicon Epitaxy used?

Epitaxy is often a part of nanotechnology. Epitaxy is used for integrated circuits, IGBT, ultra-fast diodes, DMOS, low-signal transistors and diodes, Multilayer Epi, Power transisters and diodes, and buried layer epi for customized applications. You will find that epitaxy has been used to grow layers of silicon on silicon substrates prior to their becoming semiconductor devices and being used in power devices for such things as automobile computers, pacemakers, and vending machine controllers.

What are the advantages of Silicon Epitaxy?

It has been shown that using epitaxial substrates for IC designs allows them to have less of a problem with latch-up than when bulk wafer material is used as a starting point. In order to get a low noise performance, there must be an excellent amount of isolation between analog circuits. With the low resistivity in the substrate material, it can work more efficiently to couple noise to circuits that are in close proximity. The technology is simpler, which makes it preferable to other technologies. They also result in the consumption of less power while at the same time providing high RF capabilities.

When silicon epitaxy is used for layer growth of high purity, thicker structures can be formed. These are often used in power devices which results in fewer defects. Power devices have special requirements, and must have the thicker doped structures. CMOS technology frequently uses thinner structures, with the substrate only being coated with a few thin layers. The ability to totally customize the application is greatly enhanced through the use of epitaxy, as the materials for the substrates can be specified and the number of layers required can also be laid out by the customer. This means that practically any IC application can be catered to.

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