Gas-phase epitaxy - obtaining epitaxial layers of semiconductors by deposition from the vapor - gas phase. Most often used in the technology of silicon , germanium and gallium arsenide semiconductor devices and integrated circuits [1] , [2] .
The process is carried out at atmospheric or reduced pressure in special reactors of vertical or horizontal type. The reaction proceeds on the surface of substrates ( semiconductor wafers ) heated to 400β1200 Β° C (depending on the deposition method, process speed, and pressure in the reactor ). The heating of the substrates is carried out by infrared radiation , by induction or resistive method. Lowering the process temperature below the limit for these specific deposition conditions leads to the formation of a polycrystalline layer. On the other hand, it makes it possible to reduce the width of the diffusion transition region between the epitaxial layer and the substrate, the presence of which worsens the characteristics of the resulting devices .
There are two main methods for producing epitaxial silicon layers by gas phase epitaxy:
- hydrogen reduction of silicon tetrachloride (SiCl 4 ), trichlorosilane (SiHCl 3 ) or dichlorosilane (SiH 2 Cl 2 );
- pyrolytic decomposition of monosilane
Content
- 1 Chloride method
- 2 Silane method
- 3 Doping
- 3.1 Gaseous impurities
- 3.2 Liquid impurities
- 3.3 Solids
- 3.4 Auto doping
- 4 Physical methods of HFE
- 5 See also
- 6 notes
Chloride Method
When using silicon tetrachloride as a source , the total reaction can be written as:
SiCl 4 + 2H 2 (dry) = Si + 4HCl
The reaction is reversible, and with increasing temperature and / or chloride concentration begins to go in the opposite direction. The trichlorosilane and dichlorosilane reduction reactions are intermediate in the hydrogen reduction reaction of silicon tetrachloride. Therefore, their use as sources of silicon can improve the technical and economic indicators of the process. At the same time, when choosing a source, take into account the specifics of the substances used. Trichlorosilane and silicon tetrachloride are liquid at room temperature and gaseous dichlorosilane . Silicon tetrachloride is less dangerous during storage and transportation, therefore trichlorosilane is usually used in the presence of its own production.
In general, the process of hydrogen reduction of silicon tetrachloride can be described by the following reaction system [3] , [4] :
- SiCl 4 + H 2 <--> SiHCl 3 + HCl;
- SiHCl 3 + H 2 <--> SiH 2 Cl 2 + HCl;
- SiH 2 Cl 2 <--> SiCl 2 + H 2 ;
- SiHCl 3 <--> SiCl 2 + HCl;
- SiCl 2 + H 2 <--> Si + 2HCl
The layer growth rate is 0.1-2.0 ΞΌm / min depending on the source of silicon, temperature and pressure. It is proportional to the concentration of the silicon-containing component in the vapor-gas phase.
Limitations of the method: it is impossible to grow an epitaxial film on sapphire substrates, since hydrogen chloride etches sapphire under these conditions.
Silane Method
SiH 4 = Si + 2H 2
Decomposition occurs at t = 1050 Β° C, which, compared with the chloride method, slows down diffusion and reduces the harmful effect of self-alloying. Due to this, this method manages to obtain sharper transitions between layers.
Doping
The doping of epitaxial layers is carried out simultaneously with their growth in a reactive manner (by adding dopants to the vapor-gas mixture).
Gaseous impurities
Gaseous impurities in most cases make it possible to build a simpler installation, but are unstable during storage and highly toxic ( phosphine , diborane , arsine )
Arsine AsH 3 is most commonly used in this capacity.
Liquid Impurities
Liquid alloying impurities are poured into a separate thermostatically controlled sparger of the bubble type (if the impurity does not evaporate well) or of the evaporative type (if it evaporates well), into which the carrier gas H 2 is supplied. However, in this case, it is more difficult to control the concentration of the impurity in the epitaxial layer.
Solid Impurities
Solid alloying impurities are sprayed with a spark and then transported to the reaction chamber with hydrogen, or evaporate in the low-temperature zone of the furnace (for this method, dual-zone furnaces are built).
Auto Doping
Along with targeted doping during epitaxy, self-doping also occurs - impurity transfer from a heavily doped layer to a lightly doped one. The main mechanism of self-doping is the diffusion of impurities. However, during the deposition of lightly doped layers, sublimation of an impurity from a heavily doped substrate and its transfer through the gas phase with subsequent incorporation into a growing lightly doped layer is also possible [5] , [4] .
Physical Methods of HFE
The methods of gas-phase epitaxy, in which the starting materials are vaporized in various ways and then condensed onto the substrate without participating in chemical reactions, include technologies of deposition from molecular beams in vacuum ( Molecular-beam epitaxy ), flash evaporation, βhot wallβ, as well as methods cathodic sputtering and deposition.
See also
- Solid Phase Epitaxy
- Liquid phase epitaxy
- Magratron
- Molecular beam epitaxy
- Precipitation of organometallic compounds from the gaseous phase
- Epitaxy
- CVD process
Notes
- β Gusev A.I. Nanomaterials, nanostructures, nanotechnologies. Ed. 2nd, corrected and supplemented. Moscow: Nauka-Fizmatlit, 2007
- β Nashelsky A. Ya., Technology of semiconductor materials, M., 1987
- β VLSI Technology / Ed. S. Zee, per. from English, book 1-2, M., 1986
- β 1 2 Bakhrushin V. Ye. Obtaining and physical properties of lightly doped layers of multilayer compositions. - Zaporozhye, 2001
- β Bakhrushin V.E. Production and properties of lightly doped layers of silicon structures. - Zaporozhye, 1997