Handbook of air conditioning and refrigeration / Shan K

Manufacturing an integrated circuit involves photolithography, etching, and diffusion processes.
These processes may take place over many hours, even days. A stable temperature is extremely important.
Because clean-room production personnel wear smocks that fully cover them, clean-room
temperatures are controlled from 68 to 72°F (20 to 22.2°C) with tolerances of �0.1°F (0.056°C),
�0.2°F(0.11°C), �0.5°F (0.28°C), and 1.0°F (0.56°C). A closer tolerance is often maintained
within the manufacturing process itself; e.g., wafer reticle writing by electron beam technology
needs �0.1°F (0.056°C). whereas �1.0°F (0.56°C) is often used for the open-bay area.
In clean rooms, if the relative humidity is too low, static electricity is created and causes defective
products. If the relative humidity is too high, some chemicals may expand and may cause
equipment failure. For etching and diffusing areas, the humidity should be maintained at 40 to 45
percent with a tolerance of �5 percent, whereas in the photolithography area, a relative humidity of
35 to 40 percent with a tolerance of �2 percent is usually maintained.
The manufacture of metal oxide semiconductors requires large quantities of conditioned outdoor
air as makeup air, to replace processing exhaust air and to maintain clean-room pressurization. A
clean room is always maintained at a positive pressure to prevent the infiltration of contaminated air
from surrounding spaces. For some clean rooms, the process exhaust airflow may be as high as 10
cfm/ft 2 (182 m 3 /h� m 2 ) of floor area. The average process exhaust airflow for semiconductor fabrication
may be between 2 and 3 cfm/ft 2 (36 to 54 m 3 /h� m 2 ).
Energy Use of Components
System Description
AC SYSTEMS: CENTRAL SYSTEMS AND CLEAN-ROOM SYSTEMS 30.17
In Naughton (1990a, 1990b), the breakdown of energy use in a typical clean-room central system
with vertical unidirectional flow is approximately as follows:
Manufacturing equipment (75 W/ft 2
or 807 W/m 2 )
Air conditioning (HVAC&R)
Electric lights
Building envelope and others
50 percent
40 percent
6 percent
4 percent
The chiller and fans each consume 45 percent of the total HVAC&R energy use. Both plant and
building pumps use the remaining 10 percent.
Because of extremely high space sensible cooling load and very small space latent load, the sensible
heat ratio of the space conditioning line SHR s can often be taken as 0.99.
The operating cost of HVAC&R is only 5 to 20 percent of the total cost needed to produce an integrated
circuit (semiconductor wafer). The air cleanliness, temperature, relative humidity, airflow,
and pressurization required for successful fabrication are still extremely important goals of an
HVAC&R system design. Because of high utility rates, however, a reduction in the operating cost of
the clean-room system also becomes a very influential factor in clean-room design and operation.
Semiconductor clean rooms are of two types: open-bay design or clean tunnel. The clean tunnel
consists of narrow modular clean rooms which may be isolated from one another. Open-bay design
includes large open-construction clean rooms. A typical clean-room air conditioning system, or
simply clean-room system, for a class 10 clean room of the open-bay configuration is shown in Fig.
30.1. This system consists of the following components:
Recirculating Air Unit (RAU). The function of a recirculating air unit is to recirculate the space
air; to filter it, to cool it, to pressurize the mixture of recirculating and conditioned makeup air, and
to force the mixture to flow through the ULPA filters and the clean-room working area. An RAU