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Semiconductor Wafer Fabrication Equipment
Epitaxial Deposition Equipment
The equipment used in depositing epitaxial layers on the wafer is often referred to as ‘epitaxial reactor’. Epitaxial reactors are high-temperature chemical vapor deposition systems. There are two major types of epitaxial deposition equipment, namely, the ‘pancake reactor’ and the ‘barrel reactor’. These two types got their names from the shapes of their susceptor, which is the part that holds the wafers during the epitaxial deposition process.
A
basic epitaxial reactor should consist of at least the following:
a) a reactor tube or chamber to isolate the epitaxial deposition
environment; b) a system that distributes the various chemical species
for epitaxial deposition in a very controlled manner; c) a system for
heating the wafers; and d) a system for scrubbing the effluent gases.
Oxidation Systems
The formation of silicon dioxide (SiO2) on a silicon substrate is known as oxidation. Generally this is accomplished by thermal oxidation, wherein the wafer is exposed to an oxidizing environment at elevated temperature. Thus, an oxidation system has to have a heat source to elevate the temperature of the oxidizing environment and a system for delivering the oxidizing gases to the wafers.
A basic oxidation system would consist of the following: a) a cabinet that houses the various parts of the furnace; b) a heating system; c) a temperature measurement and control system; d) process tubes where the wafers undergo oxidation; e) a system for moving the oxidizing gases into and out of the process tubes and f) a loading station where the wafer boats are loaded into and unloaded from the process tubes.
Note that
thermal oxidation is basically a diffusion process, so it can also be
accomplished by diffusion
systems designed not only for depositing dopants into wafers, but
for oxidation purposes as well.
Diffusion Systems
Diffusion is the transfer of a species resulting from concentration gradients. Diffusion can pertain to either oxidation or dopant deposition, but it is generally used to refer to the latter.
In the early days of semiconductor manufacturing, diffusion was extensively used in the controlled deposition of impurity atoms or dopants into the silicon substrate, which is the foundation of p-n junction formation. Ion implantation has become the primary means for dopant deposition in recent years, but diffusion is still necessary in certain applications.
A
typical diffusion system, also known as a diffusion furnace, is very
similar to (and in some cases, the same as) an oxidation furnace.
It is an equipment designed to provide an
environment of high temperature and controlled gas flow to wafers. It
consists of a heating element, a diffusion tube, a diffusion boat, and a dopant delivery system. The wafer is exposed to the dopant gases at high
temperature inside the furnace, a mechanism that is very similar to
oxidation for growing SiO2 films.
Ion Implantation Equipment
Ion implantation is used in wafer fabrication to selectively deposit dopant ions into the surface of the wafers. This process involves the direct introduction of highly energetic, charged atomic species onto the target substrate. Its application to semiconductor manufacturing requires a great deal of control to ensure that the dopants are introduced in precise quantities at the correct location and depth without inducing any damage to the silicon lattice structure of the substrate. Needless to say, an ion implantation equipment is very complex, needing to accurately implant and monitor the species being introduced.
A typical ion implantation equipment consists of a feed source, an ion source, a device for extracting and analyzing ions, an acceleration tube, a scanning system, a system end station, a high vacuum system, and a computerized control system.
The
feed source contains the material where the species for implantation
will come from. The acceleration tube determines the energy content of
the ions while the scanning system ensures uniform distribution of the
ions over the target. The system end station measures the implant dose
and minimizes dose errors.
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