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49 January 1998 Oki Technical Review Vol. 63 Special Issue on Global Environment: UDC [628.83 : 621.63-831] : 621.315.592.2-034.782-416.006.3 Energy Saving System for Air Conditioning of
Clean Room for Semiconductor Factory
(Estimation of FMU System) Mikio MATSUKI*, Norio TANAKA** Abstract
The total power consumption of all semiconductor plants exceeds 100 million kWh per year, with about 43% of this
consumption to operate air conditioning facilities. This high power consumption is due to the requirement of the
cleanliness kept for clean rooms, with extremely high power consumed to the air transferring power. Oki developed the
fan module unit (FMU) type air conditioning system to decrease the air transferring power, in overall vertical laminar
flow type clean rooms. We installed this system in the latest semiconductor manufacturing plants. This air conditioning
system has an improved FMU type fan, and considerably decreased running costs to about 53% of the recycle fan type.
The air transferring power of this FMU air conditioning system is 6.5W/CMM (m3/min). We also designed an air
conditioning system without air conditioning ducts, which implemented saving energy, saving resources and mainte-
nance savings, contributing to environmental protection. 1. Introduction The clean room air conditioning system for a wafer process
semiconductor factory (hereafter WP factory) has changed
considerably due to the requirements for the high perfor-
mance (improved cleanliness, impurity removal, etc.) of
clean rooms as the integration of semiconductors increases.
On the other hand, companies have been responding to the
necessity of energy saving, because total costs had to be
decreased amidst severe price competition in the semicon-
ductor market, and energy consumption had to be de-
creased to solve global environmental problems. In Oki’s
WP factories, power consumption exceeds 100 million
kWh per year. Figure 1 shows a breakdown of the power
consumption of each facility at a WP factory. The air
transfer facility for air conditioning is 30% and the heat
source facility is 13%, indicating that air conditioning
related facilities make up about half the power consump-
tion of an entire factory. One factor that increases the power consumption of an air conditioning facility is the air conditioning facility
operation for air cleaning inside a clean room. Air cleaning
is required because air quality free of impurities is de-
manded, therefore, this is closely related to microfabrication
and quality of semiconductor devices. For air cleaning,
temperature / humidity control, room air-pressure control
and cleanliness control have respective requirements, for
which enormous electric energy is necessary. To insure
cleanliness for cleanliness control, the number of times of
air circulation in a clean room must be increased, which
consumes a lot of power. The air conditioning system of a WP factory is classified into an overall vertical laminar flow system, tunnel clean-
ing system, and open bay system. In the clean rooms of
current WP factories, the overall vertical laminar flow type
and open bay type air conditioning system are primarily
used. For these types of clean rooms, the layout changes of
a process equipment and the maintenance of fans are easy,
but the expenses for construction and operations are higher
than those of the tunnel clean system. For an air circulation system to maintain cleanliness of a clean room, an FMU (Fan Module Unit) system and FFU
(Fan Filter Unit) system is more frequently used than an
RCF (Recycle Fan) system, because of energy saving and
maintenance saving. To meet the demand for quality of semiconductor de- vices and environmental protection, Oki constructed an
FMU type air conditioning system which uses a overall
vertical laminar flow system that can keep an entire clean Figure 1: Ratio of electric power consumption in WP factory Air conditioning facility (heat source facility) 13% Illumination 2% Production facility 40% Air conditioning facility (air transfer facility) 30% Facility other than air conditioning 15% * Environmental Technology Team, Environmental Protection Division, Device
Production Center ** Leader, Environmental Technology Team, Environmental Protection Division, Device
Production Center 50 Energy Saving System for Air Conditioning of Clean Room for Semiconductor Factory (Estimation of FMU System) controlled by the dry coil to remove the heat load inside the
clean room, and air is circulated back to the fan (FMU). By
designing an electric energy conversion efficiency for FMU
and other components on the circulation path in the clean
room (by improving efficiency of fan and motor of FMU,
and by low pressure loss design of the return chamber), air
transferring power of the air conditioning system can be
decreased, and an energy saving type air conditioning
system can be implemented. 2.2 Comparison of air circulation systems Overall vertical laminar flow type air circulation sys-
tems used in the clean rooms of WP factories are classi-
fied into three types: FMU, RCF and FFU systems.
Figure 3 shows a schematic of the air circulation of each
system. The features of each system are described below.
1. FMU system An FMU system is an air circulation system that has
a fan on the top of the Ultra Low Penetration Air
filter (hereafter ULPA filter). One fan has the capa-
bility to supply air for two or more ULPA filters. The
air circulation transfer system is an air conditioning
ductless type, and is integrated into the building. The
air circulation transfer speed is slow to decrease pres-
sure loss. The supply chamber is separated into a fan
chamber and filter chamber, which makes the main-
tenance of fans and filters easier. This system has the
smallest air transferring power of all air conditioning
circulation systems. 2. RCF system A RCF system is generally called a “central system”.
The air circulation system is mainly an air condition-
ing duct type. For this system. design and construc-
tion are easy, but a large fan is required and the high
pressure fan must be necessary because the pressure
loss of the circulation system is high. The running
cost, therefore, becomes high. Figure 2: Air circulation flow in clean room by FMU system OAC D C D C Filter chamber Fan chamber Cleanroom Filter FMU Supply chamber Return chamber Dry coil room area at high cleanliness, and has no air conditioning
ducts, which decreases air transferring power 40% or more. This paper describes an FMU type air conditioning system, and compares its performance with other air circu-
lation systems. Also the installation of an FMU system at
the latest WP factory is evaluated, and future expansion
plans are described. 2. Selection of air conditioning system 2.1 Overview of air conditioning systems In order to maintain class 1 cleanliness in the air condition-
ing systems of clean rooms of WP factories, 200~500 times
air circulation is necessary in clean rooms. Figure 2 shows
an air circulation flow in a clean room when an FMU type
air conditioning system is used. First, air pressurized by a
fan (FMU in this case) inside the fan chamber is filtered by
the filters inside the filter chamber. The filtered air flows
from the filters on the ceiling of the clean room to the clean
room as vertical laminar flow, and enters the return cham-
ber. In the return chamber, the temperature of the air is Figure 3: Schematic diagram of air circulation systems FMU system RCF system FFU system Supply chamber Fan chamber FMU Filter chamber Corridor Clean room Return chamber Dry coil Silencer Duct Corridor Clean room Return chamber Dry coil Supply chamber FMU Corridor Cleanroom Return chamber Dry coil Supply chamber Fan Silencer 51 January 1998 Oki Technical Review Vol. 63 3. FFU system Just like the FMU system, the FFU system has a fan on
top of the filter. In this system, one fan is used for one
filter, so fan and filter are integrated into one unit. This
makes the number of fans 5~10 times more than the
FMU system, and insuring cleanliness is difficult when
system operation trouble occurs. The air circulation
transfer system is an air conditioning ductless type, and
the running cost is equivalent to the FMU system. 2.3 Evaluation of FMU system Table 1 shows a comparison of the performance of air
circulation systems in an overall vertical laminar flow
type clean room, and the features and qualitative results
of performance are described below.
1. Performance Concerning vibration transmitted to a building, the
FMU system, which has a smaller fan and less excita-
tion than the RCF system, does not need vibration
isolation measures. For the FMU and FFU systems, it
is easy to adjust cleanliness and to install the air
circulation system in stages when layout is changed.
Air volume can be easily changed for the FMU system
by controlling the number of FMUs because pressure
is controlled to be uniform in the filter chamber. For
upgrading, the FMU system is relatively easy because
the supply chamber is separated into a fan chamber
and filter chamber. For maintainability, the FMU
system, where one fan supplies air to two or more
filters, is also good because even if one fan stops it
will have minimal effect on the clean room. In the
RCF and FFU systems, however, insuring cleanliness
of a clean room is difficult when a fan stops. As a
result, the performance of the FMU system is better
than the other two systems in terms of vibration,
flexibility and maintainability. 2. Cost The initial cost is higher in the sequence of RCF
system < FMU system < FFU system in large scale
clean rooms. For the running costs, the FMU and
FFU system are cheaper than the RCF system because
their pressure loss of the air circulation system is 1 / 2 ~ 1 / 4 that of the RCF system. The FMU and FFU systems require more fans than the RCF system but
the fans have high reliability and do not require
maintenance. Therefore, FMU and FFU are less ex-
pensive than RCF in maintenance as well. As a result,
for the FMU system, power consumption per unit of
air volume can be decreased much more than the
other two systems. 3. Delivery FMU and FFU systems can be manufactured at as-
sembly shops by unit house and pre-fabrication. For
the RCF system, however, a large crane is necessary to
install large fans and ducts. The construction of the
RCF system is mostly shop work and there are also many restrictions in the construction periods, for
example ducts in the ceiling must be completed be-
fore constructing the clean room ceiling. For the
FMU system, the construction period is longer than
the FFU system because the supply chamber has a
double structure, but construction is easy compared
to the RCF system, and shortening the construction
period is possible. 4. Safety Comparing safety when fan operation trouble occurs,
the influence of trouble on cleanliness of clean rooms
is the least (safest) in the FMU system because pres-
sure is is controlled to be uniform in the filter cham-
ber. For the RCF system, the influence of fan opera-
tion trouble on clean rooms is large because the air
volume of one fan is high. For the FFU system, where
one fan is used for one filter, the air flow blown from
a filter stops when the fan stops. This causes turbu-
lent flow, a drop in cleanliness, and other negative
influences. For the safety in the fan operation trouble
period, the FMU system, which hardly affects clean-
liness keep in clean rooms, is safer than the other two
systems. From the above (1) ~ (4) performances, we judged that the FMU system is the most effective energy
saving type air circulation system to implement tar-
gets. 3. Evaluation of FMU system by actual
installation at factory We installed an FMU type air conditioning system at
Oki’s latest WP factory. The result is as follows. When
the FMU system was first installed in the research and
development building, the air transferring power reduc-
tion was only 3.2W/CMM(m 3 /min). The air circulation system at that time had low pressure loss, but efficient Item System FMU system Layout change Air volume change Local cleaning Upgrading Maintainability Initial costs Running costs Maintenance costs Energy saving support Constructability Construction period Problem handling Table 1: Comparison of performance of air circulation systems RCF system FFU system Performance Cost Term of
delivery Safety Vibration Flexibility Maintainability General evaluation 52 Energy Saving System for Air Conditioning of Clean Room for Semiconductor Factory (Estimation of FMU System) fans for that circulation system were not available, which
is why a dramatic energy saving effect was not imple-
mented. To further decrease the air transferring power,
we reviewed the design values, and experimented with
the most efficient FMU type air conditioning system.
Table 2 shows the experiment results for the air circula-
tion system. In the experiment, the air transferring
power of the FMU system was 7.2W/CMM, and we
confirmed that it is possible to develop FMU fans close
to experiment values. As a result, we installed an FMU system with improved fans as the air conditioning system at the latest WP factory.
Table 3 shows a comparison of initial costs and running
costs of air conditioning circulation systems. For the FMU
system with improved fans, the air transferring power is
6.5W/CMM, which is a 53% decrease compared with the Item System FMU system (improved type) Table 3: Comparison of initial and running costs of air circulation systems RCF system FFU system Initial costs ratio Running costs ratio Transferring power (W/CMM) 1.04 0.47 6.5 1.00 1.00 13.9 1.17 0.91 12.6 Item System FMU system Power consumption of fan (kW/unit) Table 2: Comparison of experiment result for air circulation systems RCF system FFU system Fan Performance Air volume (CMM) External static pressure Overall efficiency Transferring power (W/CMM) CMM: m 3 /min, W/CMM: power (W; watt) to transfer 1m 3 of air per minute Overall efficiency: electric energy conversion efficiency
External static pressure: blast pressure on filter surface 50 Fan efficiency Motor efficiency (0.58 0.67) 53 Fan efficiency Motor efficiency (0.76 0.70) 21 Fan efficiency Motor efficiency (0.34 0.61) 92 22 1620 40 15 10 0.661 7.2 20.0 12.3 0.117 7.8 air transferring power of the RCF system. In terms of air
transferring power in a 2000m 2 clean room, the FMU system with improved fans can reduce 230kW of power
consumption compared with the RCF system. 4. Conclusion We installed an FMU system with improved fans in the latest
WP factory as an air conditioning system for overall vertical
laminar flow type clean rooms, and implemented dramatic
energy savings, resource savings and maintenance savings.
The initial cost of this air condition system was about 4%
more than that of the RCF system. The air transferring power
of this system was about 53% less than that of the RCF
system, which led to considerable reduction of operations
costs. In the future, we will install FMU type air conditioning
systems as a standard, and will expand this system to all WP
factories. For this, further energy savings must be imple-
mented by studying the optimum air volume, optimum
sectional area, the shape of air ducts and other matters, and by
developing high efficient motors and fans. 5. References 1. Norio Tanaka. “Clean Room,” Electronics materials,
December, Supplemental Issue, (1995): 122~123.

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