Page 31 - Semiconductor Manufacturing Handbook

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SILICON SUBSTRATES FOR SEMICONDUCTOR MANUFACTURING

3.10 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

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1. SIMOX Process. SIMOX, which stands for silicon separation by oxygen implantation, involves
oxygen implantation into a silicon wafer using high-energy implanters to form a buried oxide
film beneath the surface of the wafer (Fig. 3.8). This technique uses high-energy implanters
to implant the oxygen followed by a thermal annealing process to promote oxygen-silicon reac-
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tions to form a buried oxide. Implant doses of 10 to 10 cm −2 are typically used to implant
sufficient oxygen beneath the surface of the wafer. Following implantation the wafer is annealed
at very high temperatures, of the order of 1350°C, to promote the formation of a buried silicon-
dioxide film. There have been many developments in this process to improve the quality of the
top silicon layer, which can be compromised as a consequence of the high-dose implantation
process. Additional developments include implanting at elevated temperatures, to reduce defect
densities, post-implantation in-diffusion of oxygen for improved oxide integrity, and interface
properties. 7,8
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2. Wafer bonding and layer transfer. The second approach for SOI wafer manufacture involves bond-
ing thin single crystal films of silicon to oxidized substrates as shown in Fig. 3.9. This approach is
more flexible as compared to SIMOX as it allows a wider range of thicknesses for the top silicon
and dielectric (oxide) layers and also makes possible the use of other dielectrics such as diamond
and the potential for using lower-cost substrates besides single crystal silicon. The approach,
called the layer transfer process, depends on two key technologies—the ability to bond silicon
wafers to each other and the process whereby very thin layers of silicon are separated from a bulk
of silicon wafers using a combination of hydrogen implantation to create a two-dimensional layer
of microvoids beneath the silicon surface and the subsequent separation of the thin film of silicon
(after bonding to a handle wafer) from the bulk of the wafer by thermal shocking or mechanical
means.
The fundamental problem with SOI wafers is the high manufacturing cost even at high volumes,
resulting in very high prices (5X to 10X epitaxial wafers). If wafer prices can be brought down to
parity with conventional epi wafers, SOI applications and their uses will proliferate. Manufacturing
cost reduction requires new and innovative approaches.
In addition, the bonding and layer transfer process lends itself to a large range of new materials
combinations. One can combine dissimilar materials on a common silicon substrate to enable com-
bining optical, optoelectronic, digital, analog, and memory functions on a common silicon substrate.
Dissimilar materials such as silicon, gallium arsenide, silicon carbide, and indium phosphide can be
combined on a common silicon substrate.

High-quality single
crystal wafer

O ions
Implantation of oxygen
and annealing to form
a buried oxide layer
Silicon

Silicon dioxide
insulator
Silicon
FIGURE 3.8 SOI wafer manufacturing based on the SIMOX
process.

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