Epitaxial Growth Process - Silicon Wafers

Epitaxy can be defined as a deposition process whereby a thin layer (0.5 - 20 microns) of monocrystalline doped semiconductor is grown on a super clean wafer, usually by chemical vapour deposition.

The purpose of epitaxy is to grow a silicon layer of uniform thickness and accurately controlled electrical properties that will provide a perfect wafer substrate for the subsequent device processing.

All the individual microelectronic circuits and components are fabricated in the epitaxial layer while the p - type substrate supports the epitaxial layer and electrically isolates the circuits from each other.
Silicon epitaxy is done to improve the performance of bipolar devices.

Disadvantages of Epitaxy:

1.    Higher cost of wafer fabrication
2.    Additional process complexities
3.    Problems associated with defects in the epitaxial layer.

There are three types of epitaxial processes that I know of which are:

1.    Vapour Phase Epitaxy
2.    Molecular Beam Epitaxy
3.    Liquid Phase Epitaxy

Vapour Phase Epitaxy

This is an epitaxial process whereby reduction of silicon tetrachloride is carried out in a horizontal tube reactor. Clean wafer substrates is placed on the graphite susceptor in the reactor and silicon tetrachloride vapour with hydrogen is passed over the wafers at very high temperature (1200°C).

Equation : SiCl₄ + 2H₂ => Si + 4HCl

The growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometer per minute produce polycrystalline silicon and etching may occur.

Liquid Phase Epitaxy

This epitaxial process deposits a monocrystalline film from the liquid phase, typically at a rate of 0.1 to 1 micrometer per minute. The process occurs at temperatures well below the melting point of the deposited semiconductor. The semiconductor is then dissolved in the melt of another material. At conditions that are close to the equilibrium, semiconductor crystal is deposited on the substrate.
The equilibrium condition depends on the temperature and the concentration of the dissolved semiconductor in the melt.
The growth of the layer from the liquid phase can be controlled by a fixed cooling of the melt.
This process is mainly used for the growth of compound semiconductors, also very thin, uniform and high quality layers can be produced.


The epitaxial layer usually has a different doping concentration from the substrate. The core of an epitaxial reactor is the reaction chamber, typically made of quartz. Inside the chamber is the susceptor, a holder for the silicon substrate typically made of graphite coated with silicon carbide.

The susceptor provides two things which are:

1.    Mechanical support for the wafers
2.    An environment with uniform thermal distribution

Four Point Probe

This is a method commonly used to measure the sheet resistance of epitaxial layers.

Limitations of Four Point Probe Method

1.    The probes must be able to make ohmic contact with the material and GaAs cannot be probed that way.

2.   Very low resistive material requires the maximum current from the current source to achieve a reading on display.

3.   Only very thin films can be measured --- The current through the probe is best restricted to 10mA because of heating effects and excessive current density at the probe tips.

4.   Low level measurements are negatively affected by various sources.

5.   High sheet resistivity material can be measured using very low currents and avoiding a greater voltage indication than 200mV.