Epitaxial Growth

Epitaxial Growth

The epitaxy means 'arranged upon'. In epitaxy a monocrystalline film is formed on the top of a monocrystalline surface. Thus epitaxy is crystalline growth process in which the foundation layer i.e. substrate works as seed crystal. The epitaxial layer formed on the substrate may be either n-doped, p-doped or intrinsic. The polarity and the concentration of the dopant to be used is not determined by the foundation layer. In general for p-type doping bi-borane (B₂H₆) and for n-type doping phosphine (PH₃) are used with the steam of silicon tetrachloride hydrogen gas.

The epitaxial growth of pure silicon can be represented with the help of following chemical reaction.

There are two types of epitaxy as given below.

1. Homoepitaxy : When the epitaxial layer and the substrate on which the epitaxial layer is to be formed, are of same materials, then the process is called homoepitaxy. The silicon process in which silicon is grown or formed over silicon substrate is an example of homoepitaxy.

2. Heteroepitaxy : When the epitaxial layer and the substrate on which the epitaxial layer is to be formed are not of identical material, then the process is called heteroepitaxy. But for crystalline growth with heteroepitaxy, the materials must have identical crystal structure.

While studying epitaxy one must note that this process is not restricted to a single layer formation on the substrate. But multiple epitaxial layers are also possible which are widely found in various applications such as microwave p-i-n diodes.

In general bipolar integrated circuits use epitaxial layer process in which high resistivity epitaxial layer is formed over a low resistivity substrate. To provide isolation between the epitaxial growth and the substrate, the doping used in both layers is of opposite type. Due to this a heavily doped buried layer is formed. The buried layer is also called diffusion layer as shown in the Fig. 1.6.1.

 The advantages of epitaxy are as follows -

1. Because of one or more buried layers, the designer can control the doping in the structure.

2. The properties of the epitaxial layer and the bulk material are different.

3. Using epitaxial structures the performances of RAMs (Random access memories) and CMOS ICs can be improved.

The two main epitaxial processes are

1. Chemical Vapour Deposition (CVD) and

2. Molecular Beam Epitaxy (MBE)

1. Vapour Phase Epitaxy

In chemical vapour deposition (CVD), the film is formed on the surface of the substrate by thermal decomposition and or the reaction of various gaseous compounds. As in CVD, the epitaxial layer is formed from the gaseous vapour phase, hence it is called vapour phase epitaxy.

The process of CVD growth takes place in the reaction chamber in which a long quartz tabe with RF induction coil wound on it. The silicon wafers are placed in the graphite rod which acts as base. It is also called graphite susceptor which supports the wafer. Then the susceptor is heated by RF induction to temperature between 1100°C to 1300^oC. The four important compounds for the reaction are silicon tetrachloride (SiCl4), trichlorosilane (SiHCl₃), dichlorosilane (SiH₂Cl₂) md silane (S1H₄). Out of these compounds, silicon tetrachloride (SiCl₄) is the compound used widely in the industries.

The temperature for SiCl₄ reaction is controlled between 1150 to 1250 °C and respectively lower temperatures for remaining vapours. The different gases required for epitaxial layer formation are allowed in the chamber through different channels. This doping is done by using hydrides of arsenic, boron, phosphorus. These are Arsine (A5H₃), diborane (B2H₆) and phosphine (PH₃). In general, the film grows at 1 pm/min typically. Now a days, instead of RF induction, heating with quartz halogen lamps is preferred as it provides more uniform heating. The doping capability of an epitaxial layer lies in the range 10¹² to 10²² atoms/cm³ .

2. Molecular Beam Epitaxy

The molecular beam epitaxy (MBE) is based on evaporation. In MBE, the film is evaporated and deposited one layer at a time. In the process, no chemical reactions are considered. Instead of that the evaporation of silicon and other dopant is carried out under ultra high vacuum (UHV) pressures of the orders of "10^-8 to "10^-10 Torr. The through put is slow. The growth rate is also very slow typically, 0.01 to 0.3 pm/min. The molecular beam epitaxy is carried out under temperature ranging between 600°C to 900°C which is comparatively low temperature. As this process is very expensive, it is extensively used in only special applications such as GaAs technology, silicon on insulator (SIC) and silicon on sapphire (SOS).

3. Advantages of MBE Over CVD Process

1) MBE is low temperature process which is advantageous for VLSI technology.

2) While preparing thin layers using MBE process, autodoping and autodiffusion both are minimized.

3) The MBE process can be used for generating complicated doping profiles as it regulates the amount of dopant.

4) As MBE process is based on the evaporation of silicon and the dopants, hence no chemical reactions are involved in it.

5) For MBE process safety precautions are not required extensively as compared to those required in CVD process.

4. Disadvantages of MBE Process

1) For overall perfect and pure film, it is necessary to maintain a very low pressure of the order of 10^-10 Torr which is slightly difficult.

2) This process is very expensive as compared to CVD process.

3) The growth rate in MBE process is 0.01 - 0.3 µm/min which is very small compared to the growth rate of 1 pm/min in CVD process. 

Review Questions

1. Explain the word epitaxy and describe the epitaxial growth process.

Dec.-06, Marks 8

2. Explain the process of epitaxial growth in IC fabrication process with neat diagrams.

Dec.-11, May.-07, 13, 15, Marks 8

3. With the neat sketches describe epitaxial growth process in IC fabrication

May-05, Dec.-03, 16, Marks 6,May-08, 11, Marks 8

4. What is vapour phase epitaxy? Explain the same with suitable diagram.

5. Write advantages and disadvantages of molecular beam epitaxy.

6. Describe about epitaxial growth process.

Dec.-16, Marks 6