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EPITAXY

EPITAXY 15.15

15.3.2 Control Procedure and Methodologies for Thickness and Resistivity
Thickness and resistivity are also controlled; these two parameters on the other hand exhibit tight run-
to-run control (<1 percent) with uniformity in the range of 1 to 2 percent for thickness and run-to-run
reproducibility in the 2 to 3 percent range for resistivity. Because of the high stability of the epitaxy
process, the capabilities often exceed the 1.33 C value allowing for a looser control plan. Current epi-
pk
taxial reactors are stable for a large number of epitaxial wafers without any intervention for reactor tun-
ing. In addition, integrated supervision systems in modern epitaxy reactors allow for monitoring and
flagging any anomaly that occurs during the process, giving the operator the ability to abort the
process and identify the source, thus minimizing the losses.

15.3.3 Environmental Needs for Epitaxial Growth
As discussed previously, it is critical to maintain the incoming substrates under tight surface defects
control. This is accomplished by maintaining an environment free of particulate and contamination
around the epitaxy operation. State-of-the-art epitaxy facilities usually include a class-1 cleanrooms
environment, and a mini-environment around certain inspection tools with additional features
such as:

1. Through the wall mounting: All reactors are installed using the “through the wall” approach
where the reactor service area (gray area) is separated from the loading area (class-1).
2. Front opening unified pod: In order to minimize the environment effect, wafers handling in the
pre-epi and post-epi process sequence is usually carried out using automatic loading and unload-
ing, completely eliminating the operator factor in manual load/unload processes that have shown
to contribute to wafer contamination. This procedure though costly is very effective in producing
a low particle count that will meet next generation products. Figure 15.16 shows a photograph of
a FOUP cassette and opener.

15.4 SAFETY AND ENVIRONMENTAL HEALTH

The main concern in the operation of CVD epitaxy systems is safety. A substantial part of the cost
associated with setting up a CVD operation is in the toxic/flammable gas monitoring and alarms sys-
tems along with emergency trained personnel, usually as part of the facility department. In addition,
since only a fraction of the inlet gases is consumed in deposition, the un-reacted gases along with
the gas-phase reaction by-products are run through the exhaust system. Abatement systems are
installed down stream of the CVD system to reduce the emission to the atmosphere. Epi-foundries
must comply with OSHA (Occupational Safety and Health Act) codes, Toxic Gas Ordinance,
Uniform Fire Code and Clean Air Act and city/state regulations.

15.5 FUTURE EPITAXY TRENDS

It is obvious that epitaxy will evolve at the same pace as the electronic and photonic industry. Next gen-
eration devices will require tighter epilayers specifications and complex structures with abrupt transition
layers and doping profiles. Heterogeneous epitaxy with low thermal budgets on patterned substrates will
see more acceptance. Epitaxy systems also need to evolve in the same direction with new reactor designs
to meet the flatness, nanotopography, and LPDs required for small geometry devices. In addition, in situ
real-time process control of thickness, doping, and composition will be sought for potential elimination
of post-epi inspection, thus increasing the throughput and reducing the overall cost.

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