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EPITAXY

15.6 WAFER PROCESSING

15.1.4 Chemical Vapor Deposition
Chemical vapor deposition occurs at or near atmospheric pressure depending on the application. The
sources are gaseous and generally carried by high-purity hydrogen. The CVD growth process is
much more complex and usually carried out at higher temperatures than MBE. It involves gas-phase
and surface mechanisms.
Only recently, a full understanding of these two processes through simulation and experimental
works has emerged. For example, in Si epitaxy using trichlorosilane (SiHCl or TCS), up to eleven
3
9
species of Si H Cl and nine reactions have been identified in the gas-phase reactions. A first-order
y
z
x
approximation of the TCS and H reaction for silicon epitaxy is
2
SiHCl + H → Si + 3HCl
3 2
10
MOCVD is very similar to CVD in most aspects. The difference is in the precursors; metal-
organic sources such as trimethyl gallium (TMG), indium (TMI), and aluminum (TMA) are used
with hydride group five elements such as AsH , PH , and NH . The growth conditions are slightly
3 3 3
different from those of silicon. The growth rate can be described by the same equations (see next sec-
tion) with multiple chemical species. However, for III-V compound semiconductor epitaxy an addi-
tional parameter, the V/III ratio (mole fraction ratio of group V elements over group III elements)
must be taken into account. The V/III ratio needs to be high enough to compensate for the low
decomposition rates of hydrides and to drive carbon and oxygen from group V lattice sites. For GaAs
grown at a typical temperature of 700°C, a V/III of about 100 is sufficient, for GaN using ammonia
for group V, a V/III of about 1000 at 1050°C is needed for high-quality materials.
For GaAs, the simplified reaction using TMGa and AsH is given by
3
Ga(CH ) + AsH → GaAs + 3CH
3 3 3 4
CVD epitaxial systems consist of a gas manifold, a reaction chamber, and an exhaust system. The
gas manifold is made of semiconductor-grade stainless steel tubing with pneumatic valves, mass
flow controllers, and pressure regulators and transducers. The growth chamber is made of either
quartz or stainless steel and consists of a susceptor and elements of heating (resistive, induction heat-
ing, and lamp heating) and finally the exhaust system.
Two reactor designs are available in the market—vertical reactors in which the gas flow is per-
pendicular to the substrate surface and horizontal reactors in which the gas flow is parallel to the sub-
strate. In addition, for batch or multiwafer processes, two types of susceptor designs exist—the
barrel type and the pancake type. These reactors are suitable for small diameter substrates (less than
200 mm) standard complementary metal oxide semiconductors (CMOS) and for discrete device
applications where the requirements in thickness and resistivity uniformity are not tight. Advanced
CMOS epitaxy systems for 200 and 300 mm diameter wafers are of the horizontal type equipped
with cassette load locks, a transfer chamber, and a cool-down chamber.

15.2 SILICON EPITAXY FOR ADVANCED CMOS TECHNOLOGY

Silicon epitaxy of 200 and 300 mm is still the technique of choice for virtually defect-free epilayers with
thickness and resistivity that can be tuned to customer specifications. Recently, Czochalski (CZ)-
prepared Si substrates combined with processes such as Ar/H anneal produced top silicon with charac-
2
teristics similar to epilayers. These techniques are very cost effective and will compete with epitaxial
layers in terms of device performances. However, epilayer characteristics such as the absence of oxygen
and tight control of resistivity cannot be obtained so easily with CZ bulk crystal growth and annealing.
15.2.1 Silicon Epitaxy Process: An Overview

In general, the CVD process includes: 11
1. Transport of bulk gases to the growth region
2. Transport of reactants to the substrate surface through a “stagnant layer” or boundary layer
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