Clean room atmosphere control

A method and apparatus for providing a clean respirable atmosphere in a clean room. A liquefied breathable gas is supplied from a source thereof and filtered. The filtered liquid is vaporised and the gas produced by the vaporisation is filtered. Thereafter the filtered gas is introduced into the clean room.

This invention relates to methods and apparatus for providing a clean, 
respirable atmosphere in a clean room. 
Clean rooms are used extensively in the production of semiconductor 
devices, where it is essential to have an ultra clean environment. It is 
usual to classify the clean room atmosphere requirement according to a US 
Federal Standard, which classifies the cleanliness of the atmosphere 
required according to the number of particles entrained therein: for 
example, "Class 100" signifies that the clean room atmosphere contains 
less than 100 particles per cubic food (per 0.028 cubic meters) of size 
greater than 0.5 microns. 
In conventional clean rooms, it is normal to have a Class 100 atmospheric 
cleanliness requirement; this is achieved by filtering and recirculating 
the air in the clean room using a large air filtration and conditioning 
unit. In a clean room where highly sensitive processes are carried out 
which necessitate a Class 1 level of atmospheric cleanliness, and/or in 
clean rooms in which it is necessary for a very large volume ol air to be 
processed, extremely sophisticated and expensive air handling systems are 
required to filter and condition the air. 
Conventional clean room air handling systems are also incapable of rapidly 
accommodating changes in the rate of particle production within the clean 
room, such as when the number of human operators within the clean room 
changes, or when particular manufacturing processes are carried out. 
Moreover, the recirculation of air back to the clean room after filtration 
is inefficient; not only must particulates produced by people and machines 
within the clean room have to be removed, which places a greater load on 
the filtration system, but also the air handling system has to be 
sufficiently sophisticated to prevent the possibly harmful recirculation 
of gaseous pollutants, such as phosphine and arsine which might leak from 
machines commonly used in semiconductor wafer manufacture. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to ameliorate and, to some extent, 
eliminate the above problems. 
Accordingly, the present invention provides a method of providing a clean 
respirable atmosphere in a clean room comprising supplying a liquefied 
breathable gas from a source thereof, filtering the liquid, vaporising the 
filtered liquid, filtering the gas and introducing the filtered gas into 
the clean room. 
Such an arrangement takes advantage of the fact that liquids are inherently 
easier to filter than gases. Consequently, it is possible to filter out at 
least the coarser particles from the liquid as it leaves the source, or 
storage vessel. The filtered liquid can then be piped close to the point 
of use before starting a heat exchange process, allowing the liquid to 
vaporised and expand in volume. The resulting gas can then be filtered to 
remove any remaining particles and thus leave an ultra-clean gas for 
introduction into the clean room. 
Preferably the filtered gas is introduced in to the clean room at a rate 
sufficient to maintain an overpressure therein. 
This overpressure need only be slightly greater than ambient atmospheric 
pressure within the clean room, in order to prevent the ingress of "dirty" 
air into the room. The air within the clean room may be vented to 
atmosphere, in which case the overpressure within the clean room creates a 
constant flow of clean air out of the clean room. This flow can be used 
advantageously to carry out from the clean room any particles generated by 
people and/or machines within the clean room and any harmful gaseous 
leakages from machines will be removed from the clean room. 
The vents from the clean room to the atmosphere outside are preferably 
located adjacent any actual or potential sources of particulate or gaseous 
air pollutants in the clean room, such as adjacent human work stations or 
particular machines. The filtered gas may also be introduced into the 
clean room adjacent any such sources of pollutants, so as to create a 
localised overpressure and a localised flow of breathable gas which is 
effective to carry the pollutants away from the locality and out of the 
clean room. 
Such "blanketing" of a source of pollutants is effective in preventing 
contamination of the clean room atmosphere and degradation of the 
manufacturing processes therein. The breathable gas is, of course, 
substantially free of particulate contaminants and its composition may be 
determined so as to optimise its benefits to the people and/or the 
manufacturing processes within the clean room. 
The present invention is also beneficial as a result of the low temperature 
at which a breathable gas must be maintained if it is to be in the liquid 
state: heat is required to vaporise the liquefied gas, and this can be 
used to cool the clean room. Moreover, the filtered gas can be introduced 
into the clean room at such a temperature as to provide a cooling effect, 
to counteract the heat generated by people, machines and/or processes 
within the clean room and thus aiding, or even eliminating the need for, a 
clean room air conditioning/cooling system.

DETAILED DESCRIPTION 
The sole FIGURE illustrates a clean room provided with an apparatus for 
providing a clean respirable atmosphere thereto. A source 3 of liquefied 
respirable gas, such as that provided by the Applicants under the Trade 
Mark SLA, supplies liquid gas through a pipeline 5 and a filter 7, adapted 
to remove impurities which are particulate (or at least particulate at the 
cryogenic temperature of the liquid gas), to a heat exchanger 9 located 
within the clean room 1. There, a heat exchange process takes place (which 
can be useful for cooling the clean room) allowing the filtered liquid gas 
to vaporise and expand in volume, typically by about 600 times. The 
vaporised gas then leaves the heat exchanger 9 and flows first through a 
further filter 11 for removing any remaining particulate impurities and 
thence to an outlet 13 within the clean room 1. 
Within the clean room 1 is a pressure sensor 15 adapted to measure the 
difference in atmospheric pressure inside and outside the clean room. The 
pressure sensor 15 sends an electronic signal to a controller 17 
indicative of the sensed pressure difference, the controller 17 being 
adapted to actuate a valve 19 in pipeline 5, to vary the amount of gas 
flowing therealong so as to maintain a substantially constant overpressure 
(typically about 1 psi, or 7 kPa) in the clean room. This ensures that no 
impurities enter the clean room, against the tendency of the atmosphere to 
flow therefrom. 
Many straightforward modifications to the apparatus shown in the drawing 
will immediately bound to the forefront of a skilled person's mind. For 
example, there could be any number of outlets for the vaporised gas and 
these could be located so as to "blanket" a particular item of equipment 
or workstation within the clean room with an ultra-clean respirable 
atmosphere, thus avoiding risk to operators. The clean room could be 
provided with outlets to atmosphere in locations such that there is a 
controlled and predetermined flow of air within the clean room and the 
controller 17 can be provided with further sensors (not shown) and 
programmed to vary the flow of gas to take account of variations in 
particle production--due to changing numbers of operators within the clean 
room 1, for example.