III) The experimental techniques :

We can classified the techniques in three main types:

For each technique, specific equipment are involved.

Molecular Beam epitaxy

This technique consists in sending atoms or molecules at the surface of the substrate in an ultra high vacuum in order to prevent any contamination and the collisions along the path. Thee principle of the source is evaporation under vacuum with Knudsen cells by well controlled heating. The evaporating sources may be of several types; for each evaporated material, the heating must be specific to well control the material flux towards the substrates. From this control, a n atomic or molecular beam is produce in direction of the susbstrates. With a low flux, the growth can be very well monitored and the crystal can be grown atomic layer by atomic layer. By changing the nature and/or the doping of the material some very complex structures may be fabricated such as super lattice, laser diodes or high electron mobility field effect transisotrs (HEMT). Thus one can obtain a very good growth definition, abrupt junctions but this technique is very long, only one substrate is processed. The growth rate is in average 1nm per minute. As a consequence this technique is very expensive and is reserved for the fabrication of devices with a very high added-value.

This ultra-high vacuum system, 10^-10 Torr, allows all types of in-situ control techniques and all in-situ characterizations for which a high vacuum is required: electron diffraction, Auger spectroscopy, Electron Spectroscopy for Chemical Analysis-ESCA (XPS ou UPS), X-ray diffraction, etc... All along the process, the crystallinity of the grown layers can be checked.

Figure 16 : Molecular Beam Epitaxy reactor or MBE (After D.V Morgan et K. Board [3]).

Liquid phase epitaxy

This technique consists in growing the crystal from the contact between the substrate and a liquid source of the materail to grow. The priciple is really similar to the Czochralski ingot pulling method. The thermal exchanges have to be very well controled in order to avoid a melting of the substrate crystal. This technique has the great advantage to be very fast, the growth rate being as high as several microns per minutes. However, the accuracy of the concentration profiles of species is much lower than the MBE one.

Figure 17 : Liquid phase epitaxy apparatus for the growth of III-V coumpound heterojunction bipolar transistor.

Vapor phase epitaxy (Chemical Vapor Deposition)

This technique consists in growing the crystal from gas sources including material and its associated doping to deposit. In the chamber, the gas are dissociated to produce for example silicon atoms which are moving towards the substrate surface and thus bonded to the pre-existing lattice. In order to insure the epitaxial growth mechanism, the susbtrates have to be heated, usually at high temperature in the range of 1100°C. We will see, in the following, that the chemical reactions can be numerous and ,depending of the physical parameters some conditions, may lead to an etching of the surface. A strict control of the gas atomic ratio is then required. We give now some information on the reactions occuring during an epitaxial growth of silicon usually involved in the integrated circuit fabrication.

Figure 18 : Vapor phase epitaxy reactor. The injected gases include usually trichlorosilane, HCl, and hydrogen.

Vapor phase epitaxy of silicon

Several processes for silicon epitaxy are available in function of the silicon gas sources that can be SiCl₄,SiHCl₃, SiH₂Cl₂, or SiH₄.

1) from tetrachlorosilane, SiCl₄, the reaction is:

SiCl₄ _gaz + 2H₂ _gaz Si _sol + 4HCl _gaz

This reactio is usually performed at a temperature about 1250°C that leads a high doping redistribution during this step.

2) from trichlorosilane, SiHCl₃, the reaction is :

SiHCl₃ _gaz + H₂ _gaz Si _sol + 3HCl _gaz

It occurs usually at a temeprature close to 1100°C; this is the most industrially used technique presently.

3) from the pyrolyse of dichlorosilane, SiH₂Cl₂ , the reaction is :

SiH₂ Cl₂ _gaz Si _sol + 2HCl _gaz

This reaction leads to a good quality of the crystal with a high enough deposition rate.

The three presented techniques have however the disadvantage to involve chloride acid that can react with silicon surface and depending on the physical paramaters can etch the crystal instead to grow the crystal. In order to well controle this problem, the partial pression of this HCl gas is one of the main parameters of the process.

4) from silane, SiH₄ :

The pyrolyse reaction of silane is a non reversible reaction:

SiH₄ _gaz -> Si _sol + 2H₂ _gaz

This reaction occurs at 1000°C without chloride compounds. This techniqueCallows to fabricate abrupt junctions because the temperature is rather low in comparison with the other technique'ones, but the silane gas is expensive and dangerous (it burns instantaneously in the free atmosphere) and the deposition rate is low.