U.S. patent number 3,728,866 [Application Number 05/160,664] was granted by the patent office on 1973-04-24 for exhaustless clean room work stations.
This patent grant is currently assigned to Interlab, Inc.. Invention is credited to Howard M. Layton.
United States Patent |
3,728,866 |
Layton |
April 24, 1973 |
EXHAUSTLESS CLEAN ROOM WORK STATIONS
Abstract
An exhaustless clean room work station in which work to be
treated is subjected to a vapor-generating wet process, which vapor
contaminates the atmosphere. To maintain clean room standards, the
vapor produced at the station is drawn through a chimney containing
chilling coils or other means functioning to condense the vapor to
form a liquid which is returned to the process, the resultant dry
air being forced through a sub-micronic filter to remove
particulate matter therefrom before the air is returned to the
station.
Inventors: |
Layton; Howard M. (Pound Ridge,
NY) |
Assignee: |
Interlab, Inc. (Pleasantville,
NY)
|
Family
ID: |
22577864 |
Appl.
No.: |
05/160,664 |
Filed: |
July 8, 1971 |
Current U.S.
Class: |
62/126;
55/DIG.18; 55/385.2; 55/473; 62/85; 62/129; 454/187; 55/490.2;
34/73; 55/DIG.29; 55/467; 62/78; 62/93; 454/56 |
Current CPC
Class: |
F24F
3/1405 (20130101); F24F 3/163 (20210101); F24F
3/167 (20210101); Y10S 55/29 (20130101); Y10S
55/18 (20130101) |
Current International
Class: |
F24F
3/14 (20060101); F24F 3/12 (20060101); F24F
3/16 (20060101); F25b 049/00 () |
Field of
Search: |
;34/72,73,79,82,27,32,130,131,132,76,77,78 ;62/78,85,93,126,129
;98/115LH,36 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wye; William J.
Claims
I claim:
1. In a clean room an exhaustless work station comprising:
A. a wet-processing machine for treating parts with a liquid, said
machine emitting residual vapors into the air in the vicinity of
the machine,
B. means including a chimney coupled to said machine for drawing
the air containing said residual vapors therefrom,
C. means in said chimney to condense said vapors and thereby
produce a dry air stream in the output of said chimney, and
D. means to force said dry air stream through a filter to remove
particulate matter therefrom to produce clean, dry air and to
direct same to said clean room
2. An arrangement as set forth in claim 1, further including means
to return the liquid derived from said cooling device to said
machine for re-use therein.
3. An arrangement as set forth in claim 1, wherein said condensing
means are constituted by refrigerated coils.
4. An arrangement as set forth in claim 3, wherein said machine is
a degreasing machine including a tank for containing a volatile
cleaning liquid, said tank having heating means to create a vapor
zone and being provided with a cooling jacket at a point adjacent
said zone to cool and condense said vapors, whereby the vapors
emitted from the machine are residual vapors not condensed by said
jacket.
5. An arrangement as set forth in claim 4, further including an air
plenum mounted above said tank to collect said residual vapors,
said chimney being coupled thereto.
6. An arrangement as set forth in claim 4, further including a
freeboard secured to said tank above said zone and provided with a
secondary cooling jacket to cool and condense vapors escaping
beyond the first cooling jacket.
7. An arrangement as set forth in claim 6, further including a
first refrigeration unit operatively coupled to said cooling
jackets, and a second refrigeration unit operatively coupled to
said refrigeration coils.
8. An arrangement as set forth in claim 5, further including
apertures in said plenum to create an air curtain for effectively
isolating said machine from the atmosphere in the clean room.
9. An arrangement as set forth in claim 5, further including a
sensor disposed above said plenum to detect the level of vapor
present and to increase the cooling effect of said cooling jacket
when the level exceeds a predetermined safety point.
10. An arrangement as set forth in claim 1, further including
additional wet-processing machines, each having said chimney and
said condensing means, said means to force said dry air stream
through a filter being common to all machines.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to clean rooms, and more
particularly to exhaustless work stations for critical applications
in "Class 100" clean room environments.
Clean rooms serve as work areas wherein control is exercised over
contaminating material subject to airborne transport. The degree of
control depends on the particular operations performed in the clean
room area. Clean room air is degraded by the presence of people in
the area as well as by activities and material that generate
particulate matter and fumes.
With the advent of micro-electronics, cleaning and wet processing
standards have been rendered far more critical, so that ordinary
air-conditioned environments suitable for less sophisticated
applications have been deemed inadequate for most applications in
the micro-electronics field.
The production of micro-electronic devices entails the preparation
of perfectly clean metallic surfaces so that further metallic
layers may be deposited or diffused into the substrate metal
itself. Since a complex device may have to be discarded because of
the failure of any one element within the device, in order to
obtain a high production yield, it is vital that preparatory
cleaning procedures remove all dirt, grease and other foreign
matter that might otherwise give rise to a defective element.
Critical cleaning steps are, therefore, entailed, as for example,
cleaning prior to etching of semiconductor substrates. If the
cleaning process is carried out imperfectly, the etching will be
impaired and the yield will be poor. To ensure effective cleaning,
use is made of elaborate washing, rinsing and drying machines
including degreasing and other vapor-producing machines.
In order to provide a controlled environment for laboratory and
manufacturing processes in the micro-electronics industry and for
sophisticated aerospace technology applications, clean rooms have
been developed which make use of heavy-duty, expensive
air-conditioning systems that function not only to govern humidity
and temperature conditions, but also incorporate means for
eliminating dust, lint, fibers and other forms of particulate
contamination so as to afford uniform contamination-free processing
conditions for the highly critical work involved.
Government committees in collaboration with industrial groups have
established certain standards for clean environments. Of these, the
most critical now in general use as a clean-room specification, is
Federal Standard No. 209A, Class 100, more simply known as a "Class
100" environment. The standard imposed by Class 100 is such that to
comply therewith, the environment must not exhibit more than 100
particles larger than 0.5.mu. in size, per cubic foot of air.
In one typical clean room manufactured in compliance with this
standard, the air stream flowing through the room first passes
through a bank of particulate filters capable of removing all
particles down to 0.3.mu. in size. In some of these arrangements,
the filter banks are disposed across one end wall of the room. In
another design which is becoming increasingly popular, the filter
banks are overhead, whereas the air flows vertically downward
through an open-mesh floor, to be collected in a lower plenum from
which it is returned to the starting point and re-used.
Individual work stations have also been developed to provide Class
100 environments for the processes housed therein. The use of banks
of such work stations offers a feasible alternative to the use of a
work room in which the entire atmosphere is subjected to continuous
sub-micron filtration. Such self-contained Class 100 work stations
are designed for critical applications in the electronics,
instrumentation, chemical, pharmaceutical and other sophisticated
industries.
The individual Class 100 work station, as above described, is also
popularly referred to as a laminar flow hood or as a clean work
station. Depending on the intended use, the flow of air into the
work area of the station, is arranged to be vertically downward,
horizontally forward from the rear of the work area. In another
arrangement, a combination of both vertical and horizontal air flow
patterns is used. To provide Class 100 conditions in the work area
of the station, the air stream which is forced into the work area
from above or from the rear, is caused to pass through a bank of
sub-micron filters so that particulate matter is removed, and after
leaving the work area, the air stream is then dispersed into the
external room atmosphere.
In the case of the horizontal laminar flow hood, the air flow path
is directed to the front of the work area and as this is open to
the room environment, diffusion of the clean air stream into the
environmental air is automatic. For the vertical laminar flow
arrangement, the surface of the work bench is usually perforated so
that the downflow clean air stream can pass directly through the
work area to be dispersed through the understructure of the console
and thence to the environment again.
The above-described Class 100 laminar flow consoles are only
suitable for applications which do not introduce contamination into
the air stream. These would include assembly, adjustment,
inspection and test procedures, but would exclude most of the wet
chemistry processes encountered in micro-circuit work.
Laminar flow consoles which provide a Class 100 environment for
micro-circuit processes, such as etching, developing, stripping and
critical cleaning, also incorporate means for collecting all of the
air that has passed through the work area, and for transferring
this air to a fume exhaust system so that it will not be re-mixed
with the room environment. The same is true of wet chemistry
processes installed in a Class 100 clean room, for again, the air
which encounters the work area and becomes contaminated with vapors
emanating from aqueous and solvent processes, is continuously drawn
through openings in the work surface itself for subsequent transfer
to a fume exhaust network.
It is important to note that the clean ambient air which flows
through a Class 100 room is seldom discarded after leaving the room
area. Bearing in mind that such air has been pre-conditioned and
sub-micron filtered at relatively high cost, it is usually an
economic necessity to recirculate the entire air volume and re-use
it continuously. Since the room itself is maintained at a moderate
positive air pressure, there is some loss through access doors as
well as through the fume exhaust networks which are associated with
the wet chemistry processes. Arrangements are made to supply makeup
air through the air-conditioning system of the room, to offset
these losses.
From the foregoing, it will be evident that chemical vapor
contamination levels which would be acceptable in ordinary
environments, cannot be tolerated at all in clean room
applications. Chemical vapors lost to the room environment are not
dispersed. They are, for the most part, trapped and recirculated
with the main body of room air and the mean vapor concentration
therefore builds up in the course of a working day to levels which
would not be encountered in ordinary environments. Further, a
stratification effect is sometimes experienced and this can result
in some areas of a clean room harboring heavier concentrations of
contaminating vapors than others.
To make matters worse, it has been reported that the health hazard
is not the only criterion to be taken into account in the design of
chemical vapor control systems for Class 100 applications. Many
micro-circuit processes are themselves much more sensitive to
vaporous contamination than are humans. For example, a very low
concentration of vapors of certain halogenated hydro-carbons can,
when impinging on hot surfaces such as baking ovens and hotplates,
produce by-products harmful to metallic deposition work. Similarly,
relatively low chemical vapor concentrations can, by
cross-contamination, interfere with etching processes and with the
efficacy of critical cleaning and drying procedures.
Hot solvent processing and vapor degreasing are widely used in
cleaning procedures in micro-electronic applications, but for
critical Class 100 processes, it becomes necessary to install these
machines in vertical laminar flow consoles equipped with suitable
fume exhaust facilities.
The conventional free-standing vapor degreaser has no place in such
applications because a laminar flow of clean air moving
continuously through the work area (at 50 to 100 feet per minute)
would entrain more vaporous contamination than could safely be
released into the environment. Whether the console is fitted with
an individual clean hood for Class 100 filtration, or whether it is
simply provided with an air collection plenum for use in a vertical
flow Class 100 clean room, there is always a forced air exhaust
chimney connected to the console to transfer the contaminated air
to a factory exhaust system.
Thus with vapor-generating work stations in which the contaminated
air is exhausted, there is a loss of conditioned air, the volume of
which must be replaced and reconditioned. Where a clean room makes
use of several stations of this type, the demand imposed on the
air-conditioning system is very heavy, thereby considerably raising
the cost of maintaining a clean room.
SUMMARY OF THE INVENTION
In view of the foregoing, it is the main object of this invention
to provide a clean room having work stations therein which emit
contaminating vapor and moisture into the work area, the
contaminated air from the stations being re-purified, recirculated
and reused is an exhaustless system.
More specifically, it is an object of this invention to provide an
exhaustless work station in which work to be treated is subjected
to a wet chemical process giving rise to residual vapors in the
work area, which vapors are drawn through into a cooling chimney
wherein the vapors are condensed and the resultant liquid removed,
the dry air being filtered to extract particulate matter before
being returned to the process work area for re-use.
Because of its "post-treatment" method for removing residual
contaminants from the air stream, the system in accordance with the
invention, is easier to control in that residual vapor removal is
effected after variable factors such as work-basket movement and
drag-out turbulence have taken place. Moreover, the forced air
movement imparted to air withdrawn from the work station can, with
appropriate design, result in an optimum dispersion of the exit
stream into the ambient atmosphere, and hence afford a double
safeguard against the build-up of hazardous vapor areas.
In addition, a system in accordance with the invention has the
following practical advantages:
a. Since all residual vapors from the process work area must pass
through the cooling and vapor removal chimney, reliable monitoring
of the final contamination level of the effluent air stream and its
continuous influence on the environment, is greatly
facilitated.
b. Connection to a fixed plant exhaust-forced air system is
avoided. This is of particular significance where an exhaust
network does not already exist.
c. Independence from a fixed plant exhaust connection results in
mobility of equipment -- comparable with the mobility of
free-standing vapor degreasers of the type used where exhaust and
toxicity regulations are not stringent.
d. Whereas conventional exhaustless vapor degreasing machines are
likely to produce ambient vapor concentrations in excess of 100
parts per million in the general environment around the work area,
concentrations produced by a system designed in accordance with the
present invention are easily limited to less than 20 parts per
million (using trichlorethylene for reference purposes).
OUTLINE OF THE DRAWING
For a better understanding of the invention as well as other
objects and further features thereof, reference is made to the
following detailed description to be read in conjunction with the
accompanying drawing, wherein:
FIG. 1 is a perspective view of a preferred embodiment of an
exhaustless work station in accordance with the invention;
FIG. 2 is a sectional view of the work station; and
FIG. 3 is a perspective view of a clean room including exhaustless
work stations in accordance with the invention.
DESCRIPTION OF THE INVENTION
Referring now to FIGS. 1 and 2, there is shown a laminar flow,
exhaustless work station in accordance with the invention, which
station includes a tank 10 for vapor degreasing of work pieces
placed therein.
Degreasing machines utilizing a volatile solvent are customarily
employed for flushing grease, wax, dirt or other foreign matter
from metal parts, in order to produce a chemically-clean surface
for the purpose of preparing the parts for subsequent fabrication,
plating and other operations. Among the volatile solvents commonly
used in degreasing machines are trichlorethylene, perchlorethylene
and other chlorinated solvents. Trichlorethylene has a boiling
point of about 188.degree. - 190.degree.F, the vapor of which is
several times heavier than air. Also in use for degreasing are
fluorocarbon solvents.
In order to prevent the escape of such vapor from the open tank in
which the parts to be cleaned are admitted, the tank is usually
provided with cooling coils in the upper region thereof. The
cooling coils act to chill and condense the vapor, the condensate
being returned to the liquid in the lower region of the tank where
it is reheated. Thus, the distillation cycle is closed and solvent
loss is minimized.
Various methods are currently in use with open-tank degreasers. In
the vapor method used in micro-electronics, work is suspended in
the vapor region above the boiling solvent, the solvent condensing
on the cooler parts with impurities dripping away with the
condensate. Particulate matter may be removed by a solvent spray in
the vapor region.
In the two-stage degreasing method, vapors condensed by the cooling
jacket or coils are collected in a trough in the form of fresh
distillate and then transferred in a continuous stream to the "dip"
tank which in turn, continuously overflows into the boil tank to
complete the cycle. The work is first submerged in the overflowing
distillate tank and then elevated in the vapor zone for rinsing
off. The present invention, for reasons of simplicity, will
describe a vapor method degreaser, but it is to be understood that
the invention is applicable to all vapor-generating machines as
well as to wet processing machines in general.
In the degreaser machine shown in FIG. 1, the solvent contained in
the sump of tank 10, is heated by suitable heater elements 11, to
create a vapor in a work zone 12 directly above the solvent pool.
The parts to be cleaned are supported within the zone in a work
basket or by other means appropriate to the work being treated.
The bulk of the solvent vapors is confined within the upper region
of the tank by means of a set of cooling coils or a cooling jacket
13, located at the upper boundary of the hot vapor zone 12. Vapors
which rise into this chilled region are immediately condensed back
into heavier driplets, thereby being prevented from rising to the
top of the vapor-degreasing vessel and escaping into the ambient
air. The condensed vapors fall directly back into the boil sump for
recycling.
In addition, a separating zone or freeboard 14 is established
between the chilled vapor line and the open top of degreasing tank
10. This helps to minimize disturbance of the solvent vapor zone by
atmospheric air movement and by the routine transposition of work
baskets. The freeboard behaves as a buffer zone between the hot
vapor region and the outside atmosphere and prevents excessive
drag-out of vapor when work baskets are raised out of the
vessel.
Condensation of vapors rising into the buffer zone is effected by a
secondary coil or cooling jacket 15 surrounding the freeboard, the
condensed vapors being collected in a suitable trough and returned
to the solvent pool. Cooling jackets 13 and 15 are supplied by a
standard refrigeration unit 16. In practice, the secondary cooling
jacket 15 is operated at a somewhat lower temperature than the
primary jacket 13. This has the effect of further chilling the air
in the freeboard zone, thereby trapping more of the residual low
concentration vapors before they can reach the lip of the tank.
Notwithstanding the fact that the vapors are condensed by both
primary and secondary jackets and that great care is exercised in
modern vapor degreaser designs to prevent the escape of vapors into
the atmosphere, the final ambient contamination level is still
materially influenced by operating procedures and by the particular
ambient air movement conditions of the work station location.
Vapors emanating from vapor degreasers or other vapor-producing
machines, are considered somewhat toxic and hazardous to health
unless their concentration is controlled and limited in accordance
with certain established standards. Limit values for a few of the
most common solvents are listed in the table below. The figures
appearing in the table represent the maximum permissible vapor
concentration to which human operators may be exposed on a
continuous basis without ill effect. Laws governing solvent vapor
control in industrial processes, vary from State to State, but the
figures given are commonly used. (Ref. American Conference of
Governmental Industrial Hygienists). ##SPC1##
Because of the possible health hazards involved, and the fact that
the vapors entering the clean room atmosphere are contaminating, it
has heretofore been the practice to couple vapor degreasers or
other vapor-producing machines to a forced air, outside exhaust
system. Since the air discharged from the room is conditioned air,
this increases the demand imposed on the conditioning system and
adds significally to operating costs.
To avoid these drawbacks, and to provide an exhaustless system for
the degreaser, a console is provided including a cabinet 17 for
containing the degreaser machine and the associated refrigeration
units. Tank 10 is vertically suspended from a plenum 18 mounted on
top of cabinet 17, the plenum being coupled to a rear chimney 19
leading to an overhead hood 20 having air blower 21 mounted
therein.
Thus in operation, the vapors in the air above the open mouth of
the tank 10 are not permitted to flow into the clean room, but are
drawn by blower 21 up chimney 19, as shown by the flow-direction
arrows. Placed within chimney 19 are evaporators or cooling coils
22 which are coupled to a second refrigeration unit 23 installed in
cabinet 17 below the processing tank. In practice, the heat
generated by refrigerator units 16 and 23 may be applied to the
bottom of tank 10 to supplement the heat furnished by heater
elements 11, rather than being wasted.
Thus residual vapors and moisture in the air drawn from the top of
the tank are condensed by chimney coil 22, and the resultant liquid
is conducted to a water separator 22A which is adapted to segregate
the cleaning solvent from the water, the cleaning solvent being
returned to tank 10 for recycling and the water being fed to a
drain. Thus the upwardly flowing air stream at the output of
evaporator coil 22 is dry, being free of solvent vapors and
moisture.
The forced dry air at the output of blower 21 is directed toward a
sub-micron or a so-called HEPA filter 24 having minute pores
therein, which act to trap substantially all particulate matter in
the air blown therethrough, and to render the pure, clean air
emitted therefrom, laminar in nature. The console arrangement is
made such as to facilitate replacement of the filter when it is
clogged with dirt.
Also provided in the hood is an adjustable air-intake grill or
louvre 25 which permits intermixing of the air drawn up chimney 19
with conditioned air from the clean room. Since very little makeup
air is required, grill 25 is provided with a suitable damper. On
the upper surface of the plenum 18, adjacent the front thereof, are
adjustable openings 26. Clean air flows downwardly from filter 24
through openings 25 into plenum 18 at relatively high velocity to
define an air curtain that acts as a barrier to prevent
contaminated air from entering the room atmosphere so that all
fumes from the processing tank are compelled to enter the chimney
for treatment. To minimize turbulence, the air collection plenum
may incorporate suitable dampers for this purpose. The work area is
made visible by means of a clear acrylic window 27.
In order to be sure that no toxic fumes succeed in escaping from
the tank, a monitoring sensor 28 is provided which is set to
provide a signal when the contamination level (parts per million)
exceeds a predetermined safety point. This signal may be used to
trigger an alarm 29 and to automatically cut off the degreaser
heater. Or the safety signal may be used to control the
refrigerator units 16 by means of a suitable control circuit 50 to
increase their cooling effect and thereby reduce the amount of
residual fumes present in the area above the degreaser tank.
Though the exhaustless console, in accordance with the invention,
has been described in connection with a vapor degreaser, it is
fully applicable to other wet chemistry processes, especially those
operated at elevated temperatures and acting to release significant
quantities of contaminating vapor into the incident air stream.
In many critical micro-electronic applications, it is even
necessary to discard air which has become heavily laden with plain
water vapor because failure to do so would place an excessively
heavy burden on the air-conditioning system in use. Or the ambient
humidity levels might reach an unacceptable level for the
processing sequences involved. In this case, the invention provides
a point-of-use de-humidifying service, permitting recirculation and
re-use of the same air. Many other wet chemistry processes
including acid etches, photo-resist stripping processes, etc. can
be dealt with in the same way so as to eliminate the need for a
permanent plant exhaust connection.
In clean rooms employing two or more wet processing stations, it
may be more efficient to make use of a combined exhaustless
purification system common to the several stations, rather than
making each station a self-sufficient exhaustless system of the
type illustrated in FIGS. 1 and 2.
FIG. 3 illustrates both a combined, exhaustless purification system
for several work stations in a clean room as well as a conventional
exhaust-type system typical of the prior art. In FIG. 3, work
station A represents a conventional wet chemistry exhaust-type
console having an air plenum 30 thereabove, which is coupled by a
duct 31 to an exterior exhaust chimney 32 which discharges fumes
into the atmosphere. The air volume exhausted by this arrangement
must be replaced in the clean room by a freshly air-conditioned,
filtered make-up air supply.
The wet-processing work stations B and C incorporate point-of-use
vapor removal consoles in accordance with the present invention,
thereby obviating the need for factory exhaust connections and also
avoiding the imposition of an extra load on the main
air-conditioning system.
The clean room is housed in a chamber having a raised open-grid
floor 32, supported on posts 33 above the base of the structure to
define a lower main air plenum 34. Work stations B and C are
provided at their tops with individual plenums 35 and 36,
respectively.
An air blower 37 is coupled by chimneys 38 and 39 to individual
plenums 35 and 36 to provide negative pressure for work stations B
and C. This blower acts to draw the contaminated air from these
work stations through vapor-removing refrigerated coils 40 and 41,
respectively.
The air in the clean room is properly conditioned by an
air-conditioner unit 42 which draws air from the lower main air
plenum 34 through grills 43, the air being forced upwardly through
passage 44 and being passed into an upper main plenum 45 together
with makeup air drawn from the exterior through inlet 46.
In the upper air plenum 45, a bank of blowers 47 forces the
conditioned air through sub-micron filters 48 back into the clean
room for recirculation. It will be seen that the output duct 49 of
the suction blower 37 coupled to work stations B and C leads to
passage 44, so that the air processed in chimneys 38 and 39 of
these stations remixes with the main recirculating air stream at
this point. Sensors may be placed at the remixing point to produce
a signal when the contamination level is unacceptable. This signal
is arranged to actuate a duct switching assembly (not shown) to
cause the air exhausted from consoles B and C to be directed to an
outside exhaust system.
Thus, in lieu of separate blowers and sub-micron filters for each
wet processing work station, an exhaustless system is provided in
the clean room in which the blowers and particulate filters are
common to two (or more) work stations exuding vapors. The essential
operating principles of this arrangement are no different from
those involved in the self-contained, single-station system
illustrated in FIGS. 1 and 2.
While there have been shown and described preferred embodiments of
an exhaustless clean-room work stations, in accordance with the
invention, it will be appreciated that many changes and
modifications may be made therein without, however, departing from
the essential spirit of the invention.
* * * * *