Epitaxy Techniques for Semiconductor Fabrication: ALD, VPE, and MBE Explained by Dr. G.S. Virdi

his lecture by Dr. G.S. Virdi (Ex-Chief Scientist, CSIR-CEERI Pilani) provides a clear and comprehensive introduction to epitaxy, a crucial thin-film growth process in semiconductor technology.

Topics covered:
• Definition and fundamentals of epitaxy
• Crystal alignment and role of substrate
• Types of epitaxy: Homoepitaxy & Heteroepitaxy
• Vapour Phase Epitaxy (VPE) & Atomic Layer Deposition (ALD)
• Molecular Beam Epitaxy (MBE) – system, process & monitoring
• Advantages, limitations and device applications

With 45+ years of semiconductor R&D experience, Dr. Virdi also connects fundamental concepts with real fabrication practices used in microelectronics, VLSI, MEMS and nanotechnology.

This lecture is ideal for students, researchers and professionals working in semiconductor materials and device fabrication.

1. EPITAXY
Dr. G.S.Virdi
EX-Chief Scientist
CEERI Pilani

2. 1)Epitaxyreferstothemethodofdepositingamono-
crystallinefilmonamono-crystallinesubstrate.The
depositedfilmisdenotedasepitaxialfilmorepitaxial
layer.
2)ThetermepitaxycomesfromtheGreekroots,Epi
means“above”andtaxismeans“depositioninordered
manner”.
3)Epitaxialfilmsmaybegrownfromgaseousorliquid
precursors.
4)The substrate acts as a seed crystal, the deposited film
takes on a lattice structure and orientation identical to
those of the substrate.
EPITAXY

3. Types of Epitaxial films
Epitaxial films can be classified into two broad categories:
(1)Homoepitaxy
-The film and the substrate are the same material.
-Epitaxiallygrown layers are purer than the substrate and can
be doped independently of it.
(2)Heteroepitaxy-
Film and substrate are of different materials

4. • Atomic layer deposition (ALD) is a thin film deposition
technique that is based on the sequential use of a gas phase
chemical process.
• ALD is considered a subclass of chemical vapour
deposition. The majority of ALD reactions use two
chemicals, typically called precursors.
• These precursors react with the surface of a material one at
a time in a sequential, self-limiting, manner.
• Through the repeated exposure to separate precursors, a
thin film is slowly deposited.
VAPOR -PHASE EPITAXY

5. Advantages
• VPE provides a very controlled method to produce a film to an
atomically specified thickness. Also, the growth of different
multilayer structures is straightforward.
• Due to the sensitivity and precision of the equipment, it is very
beneficial to those in the field of microelectronics and
nanotechnology in producing small, but efficient semiconductors
Disadvantages
• High purity of the substrates is very important, and as such, high
costs will ensue
• Once the layer has been made and the process is complete, there
may be a requirement of needing to remove excess precursors from
the final product.
EPITAXY

6. Molecular Beam Epitaxy
•To make an interesting new crystal using MBE, you start off with a
base material called a substrate, which could be a familiar
semiconductor material such as silicon, germanium, or gallium
arsenide
•First, you heat the substrate, typically to some hundreds of degrees
(for example, 500–600°C or about 900–1100°F in the case of gallium
arsenide)
•Then you fire relatively precise beams of atoms or molecules (heated
up so they're in gas form) at the substrate from "guns" called effusion
cells
•The molecules land on the surface of the substrate, condense, and
build up very slowly and systematically in ultra-thin layers, so the
complex, single crystal you're after grows one atomic layer at a time
•That's why MBE is an example of what's called thin-film deposition

7. Basic elements of MBE
system:
• Heated substrate
• Effusion cells and shutter
• Reflection High Energy
Electron
• Diffraction (RHEED system-
RHEED gun & screen)
• Ultra High Vacuum (UHV)
• Liquid Nitrogen cryopanelling

8. •The solid source material sublimates
•They provide angular distribution of atoms or molecules in beam
•The substrate is heated to the necessary temperature
•The gaseous elements then condense on the wafer where they
may react with each other to form a layer
•Atoms on clean surface are free to move until finding correct
position in the crystal lattice to bond
PROCESS

9. Advantages and Disadvantages of MBE
Advantages:
It's particularly good for making high-quality (low-defect, highly uniform)
semiconductor crystals from compounds or from a number of different
elements, instead of from a single element
It also allows extremely thin films to be fabricated in a very precise,
carefully controlled way
Disadvantages:
It's a slow and laborious method (crystal growth rate is typically a few
microns per hour), which means it's more suited for scientific research
laboratories than high-volume production, and the equipment involved is
complex and very expensive (partly because of the difficulty of achieving
such clean, high vacuum conditions)