Epitaxy means "on top" or "assigned to", and represents a process in which a layer is created on top of another layer and inherits its crystal structure. If the deposited layer is of the same material as the substrate one speaks of homoepitaxy, if it's another material it's so-called heteroepitaxy. The most significant process in the homoepitaxy is the deposition of silicon on silicon, in heteroepitaxy usually a silicon layer is deposited on an insulator such as oxide (Silicon On Insulator: SOI).
Depending on the process, the wafers can be delivered from the wafer manufacturer with an epitaxial layer (e.g. for CMOS technology), or the chip manufacturer has to make it himself (for example in the bipolar technology).
As a gas for generating the epitactical layer, pure hydrogen is used in conjunction with silane (SiH4), dichlorosilane (SiH2Cl2) or silicon tetrachloride (SiCl4). At about 1000 °C, the gases cleave off the silicon, which is deposited on the wafer surface. The silicon inherits the structure of the substrate and is growing, for energy reasons, layer by layer successively on. To not grow up a polycrystalline silicon, one must always prevail a shortage of silicon atoms, e.g. it is always slightly less silicon available as material could actually grow up. When silicon tetrachloride is used, the reaction proceeds in two steps:
In order to inherit the substrate's orientation the surface must be absolutely clear. So one can utilize the equilibrium reaction. Both reactions can occur in the other direction, depending on the ratio of the gases. If there is only few hydrogen in the atmosphere, as in the trichlorosilane process for the purification of raw silicon, material is removed from the silicon wafer surface due to the high chlorine concentration. Only with increasing concentration of hydrogen growth is achieved.
With SiCl4 the deposition rate is approximately 1 to 2 microns per minute. Since the monocrystalline silicon grows only on the bare surface, certain areas can be masked with oxide where the silicon grows as polycrystalline silicon. This polysilicon, however, is etched very easily compared to single-crystalline silicon through the backward-running reaction. Diborane (B2H6) or phosphine (PH3) are added to the process gases, to create doped layers, since the doping gases decompose at high temperatures and the dopants are incorporated in the crystal lattice.
The process to create home-epitactical layers is realized under vacuum atmosphere. Therefor the process chamber is heated to 1200 °C to remove the native oxide, which is always present on the silicon surface. As mentioned above, due to a low hydrogen concentration there occurs a back etch on the silicon surface. This can be used to clean the surface before the actual process starts. If the gas concentration is varied post this cleaning the deposition begins.
Illustration of a barrel reactor for epitactical processes
Due to the high process temperatures there's a diffusion of dopants in the substrate or impurities, which have been used in earlier processes, can move to the substrate. If SiH2Cl2 or SiH4 are used there's no need for such high temperatures, so these gases are used primarily. To achieve the etch back process to clean the surface, HCl has to be added separately. The disadvantage of this silanes is that they form germs in the atmosphere right before deposition, and thus the quality of the layer is not as good as with SiCl4.