This invention relates to a cleanbox suitable for use in storing
or transporting objects to be processed such as semiconductor wafers, photomasks
and hard discs in an ultra-clean environment.
When transporting/storing substrates to be processed such as semiconductor
wafers and photomasks used in production plants for semiconductor devices, minute
amount of dust particles adhering to the objects such as semiconductor wafers
and gaseous impurities existing in the atmosphere surrounding the objects lead
to lower product yield. Such contamination problems become more critical as the
density of circuit integration increases. For magnetic discs also, the advent of
magnetic reluctance head has resulted in significantly accelerating high density
recording so that product cleanliness is being demanded in terms of not only dust
particles but gaseous impurities. From the viewpoint of reducing reaction of substrates,
convenient means for maintaining low humidity are also important during storage
The demand for such a clean environment in which to transport/store
substrates has led to the development of cleanboxes equipped with high-efficiency
particle air (HEPA) filter and ultra-low penetration air (ULPA) filter working
in conjunction with a circulation fan. Also, other types of storage devices include
replacing the air surrounding the wafers in the cleanbox with high purity nitrogen
so as to maintain non-reacting environment and prevent the growth of native oxide
film on the wafer surface.
However, although those cleanboxes based on HEPA and ULPA filters
are able to remove particulate contaminants, they are not capable of removing minute
amount of organic or inorganic gases. Furthermore, the efficiency of sweeping
the environment is reduced and the effects of clean air in preventing contamination
cannot be expect, when the circulation system allows a large volume of circulated
air to flow into those regions that do not contribute to cleanliness of the environment,
or if stagnation occurs within the cleanbox. As for the cleanboxes using a nitrogen-based
atmosphere, such boxes are not able to remove impurities emanating from the wafers
having some type of coating such as photoresist film, for example, and it also
raises a concern related to the safety of nitrogen atmosphere to workers.
Disclosure of Invention
In view of the problems existing in the current technologies, the
object of this invention is to provide a cleanbox to prevent not only contamination
that can be introduced from external environment but also contamination that can
be produced internally from the objects stored in the cleanbox and component parts
of the cleanbox.
The invention disclosed in claim 1 relates to a cleanbox comprising:
a casing container comprised of a container main body and a lid member for hermetically
sealing a top opening section of the container main body; a dividing wall for
forming circulation paths having an upstreaming path and a downstreaming paths
within the casing container; a substrate holding section disposed in the upstreaming
path for holding broad surfaces of substrates approximately parallel to the upstreaming
path; an air filter and a gas removal filter disposed upstream in relation to
the substrate holding section in the upstreaming path; and a motor-driven fan housed
in the casing container for producing air streams circulating in the circulating
Accordingly, circulation paths are created within the container,
and the flowing streams are purified first by using air filter and a gas removal
filter to remove contaminants physically and chemically and then the air streams
are directed to the substrate holding section, so that, even if there are particulate
and gaseous sources of contaminants adhering to the inner walls of the container
or to the wafers themselves, contamination of the wafers stored in the substrate
holding section can be prevented. Because the air filter and the gas removal filter
in the upstreaming path are placed upstream in relation to the substrate holding
section, objects to be cleaned can be readily loaded or unloaded from the top section
of the container without the fear of causing contamination to the objects. Also,
because the container is quite susceptible to contamination when the lid is opened
or closed, the container lid is placed downstream in relation to the substrate
holding section, so that any contaminants that may have been brought into the
container can be removed long before they can reach the objects.
The filtering membrane of the gas removal filter may be comprised
singly of ion exchange fibers or activated charcoal fibers, or by combining these
fibers or by activated charcoal particulate embedded in a urethane foam carrier,
or by using integrated filtering unit made by weaving these fibers together. The
filtering system efficiently removes and adsorbs ions such as ammonia present
in the air and ionic substances contained in a mist such as hydrofluoric acid and
hydrochloric acid by using ion exchange fibers and activated charcoal fibers produced
by activation reaction of carbons produced from cellulose, acrylic and lignin
group fibers. Ion exchange fibers may be produced by radiation-induced graft polymerization.
Claim 2 discloses a cleanbox recited in claim 1, in which the upstreaming
path is formed approximately centrally in the casing container, and merges with
the downstreaming paths formed between the dividing walls and the container main
body, by being redirected at a top section and/or a bottom section having an arc
surface or a spherical surface. By shaping the top section and/or bottom section
into a dome shape having an arc radius or spherical radius, the overall flow is
made smooth with no stagnating points, and therefore, the energy required to form
circulating paths is reduced so that one battery is sufficient to operate the
cleanbox for a prolonged period of time. It is preferable that the radius of curvature
be not less than 1.2R when the object wafer radius is R.
Claim 3 discloses a cleanbox recited in claim 2, in which the lid
section at the inner surface and the container main body at the bottom surface
are provided with guiding protrusions for separating the streaming path. Normally,
the profile of such a protrusion is triangular.
Claim 4 discloses a cleanbox recited in claim 1, in which the circulation
paths are provided with a reverse flow preventing filter in a downstream location
to the substrate holding section so as to prevent back streaming of contaminants
from the motor-driven fan. Accordingly, even if the fan is stopped, there is no
danger of contaminating the wafers due to contaminants in the fan unit.
It is preferable that the motor for the motor-driven fan is a dc
brushless motor, because there are no moving parts to generate contaminants. Furthermore,
the motor may be canned in a stainless steel casing of 0.2 mm thickness, so that
the motor is dustless and gas tight, thereby preventing contamination by gaseous
substances emitted from the coils and other parts of the fan.
Claim 5 discloses a cleanbox recited in claim 1, in which the substrate
holding section is a carrier receiving section for freely detachably seating a
carrier having openings at the top and the bottom sections, at least, for holding
a plurality of substrates. This arrangement enables the wafers to be transported
in carrier-based quantities.
Claim 6 discloses a cleanbox recited in claim 1, in which the substrate
holding section is provided with guide grooves for directly supporting the plurality
of substrates. Thus, the objects are stored without using the carrier so that
the cleanbox can be made more compact and light weight.
Claim 7 discloses a cleanbox recited in claim 1, in which the substrate
holding section houses a plurality of substrates, and the substrate holding section
is provided with a diffuser plate for dispersing circulating air in the spaces
between the substrates. Accordingly, cleaning of the wafers can be carried out
efficiently by producing uniform flow of air in the spaces between the wafers.
Claim 8 discloses a cleanbox recited in claim 1, in which the motor-driven
fan is operated by a control section. Accordingly, sequences for different mode
of operation may be programmed into a microchip, so that intermittent operation
may be carried out, or the fan speed may be varied to prolong the service life
of the battery to improve the efficiency and economy of operating the cleanbox.
Claim 9 discloses a cleanbox recited in claim 1, in which the container
main body is structurally integrated with constituting component parts in the container
main body without using fasteners. Accordingly, contamination generated from the
fasteners and other means of joining are prevented while preserving the ease of
handling the components of the cleanbox for maintenance and replacement.
Claim 10 discloses a cleanbox recited in claim 1, in which the container
main body is maintained at a low humidity by using a dehumidifying agent made in
a sheet form or stored in a bag. Accordingly, low humidity levels in the cleanbox
can be achieved quickly by circulating the air inside the cleanbox by means of
Claim 11 discloses a cleanbox recited in claim 1, in which the gas
removal filter has a filtering membrane made of a non-woven ion exchange fabric
or a woven ion exchange fabric produced by radiation-induced graft polymerization.
Claim 12 discloses a cleanbox recited in claim 1, in which the gas
removal filter has a filtering membrane comprising activated charcoal produced
by imbedding activated charcoal particulate in activated charcoal fibers or urethane
Claim 13 discloses a cleanbox recited in claim 1, in which the gas
removal filter has a filtering membrane comprising a combination of a gas removal
filter having a filtering membrane made of a non-woven ion exchange fabric or
a woven ion exchange fabric produced by radiation-induced graft polymerization
and a filtering membrane comprising an activated charcoal produced by imbedding
activated charcoal particulate in activated charcoal fibers or urethane foam.
Brief Description of Drawings
Best Mode for Carrying Out the Invention
- Figure 1 is a cross sectional front view of a cleanbox in a first embodiment
of this invention;
- Figure 2 is a cross sectional view through a plane A-A in Figure 1;
- Figure 3 is a perspective view of the components of the cleanbox shown in Figure
- Figure 4 is a cross sectional front view of a cleanbox in a second embodiment
of this invention;
- Figure 5 is a cross sectional view through a plane B-B in Figure 4;
- Figure 6 is a cross sectional front view of a cleanbox in a third embodiment
of this invention;
- Figure 7 is a cross sectional view through a plane C-C in Figure 6;
- Figure 8 is a front view of flow patterns obtained by computer analysis in
the third embodiment;
- Figure 9 is a side view of the flow patterns obtained by computer analysis
in the third embodiment;
- Figure 10 is a graph showing changes in the concentration of gaseous ammonia
in the third embodiment;
- Figure 11 is a cross sectional front view of a cleanbox in a fourth embodiment
of this invention;
- Figure 12 is a cross sectional view through a plane D-D in Figure 11; and
- Figure 13 is a graph showing changes in the humidity in the fourth embodiment.
Preferred embodiments of this invention will be presented in the
following with reference to the drawings.
Figures 1-3 show a first embodiment of the cleanbox used for holding,
storing and transporting a plurality of semiconductor wafers (substrates for further
processing) Wf in a carrier 10. The cleanbox is comprised by a container main
body 12 of a square cylindrical shape; a lid member 14 for covering the freely
openable lid section at the top end of the container main section; and a bottom
section 16 covering the open section at the bottom end; and hermetically sealing
these components together to form a hermetically sealed casing container 18. There
is no need to make the casing container 18 (including the main section 12; lid
section 14 and the bottom section 16) from expensive materials that are known to
produce low gas emission, e.g., engineering plastics or fluoride resins, which
include polycarbonate, polytetrafluoroethylene, polybutylene terephthalate and
polyether etherketone, for example. The structures used in this invention can be
made from relatively low cost materials that are easily fabricated or formed such
as polypropylene, acrylonitrile butadiene styrene resins and their alloys. Antistatic
agents may be included lin the raw materials.
The interior of the casing container 18 is divided into a central
chamber 22a and a pair of side chambers 22b on both lateral ends of the central
chamber 22a by means of a pair of left and right dividing walls 20 leaving spaces
at the top and bottom ends for the lid section 14 and the bottom section 16. A
substrate holding section (carrier support section) 24 having an upwardly expanding
taper section is integrally provided above the dividing walls 20 in such a way
to couple with the tapered bottom section of the carrier section 10.
An air filter 26 for removing primarily particulate matters and a
gas removal filter 28 for removing gaseous impurities are freely detachably disposed
below the substrate holding section 24 of the central chamber 22a so as to permit
free flow of air in the vertical direction. In each of the lateral chambers 22b,
a dc motor-driven brushless fan 30 is provided so as to direct the air downwards,
and a reverse flow prevention gas removal filter 32 is disposed immediately above
the fan 30.
The ceiling section 14a of the lid section 14 is shaped to form a
smoothly curving inside surface, and an upper laminar plate 34 of a triangular
profile shape is provided in the center region. Similarly, the inner bottom section
16a of the bottom section 16 is shaped to form a smoothly curving surface, and
a bottom laminar plate 36 of a triangular profile shape is provided in its center
region. A carrier entry laminar plate 38 is also provided below the substrate
holding section 24.
Using a commonly available wafer carrier 10 of a 25-wafer storing
capacity, the spacing between the 1st wafer and the container main body and the
spacing between the 25th wafer and the container main body are made wider than
the spacings between the wafers Wf so as not to supply the same volume of air
for all the wafers. By providing a carrier entry laminar plate 38, air flow rates
through the spaces between the container main body and the 1st and 25th wafers
are adjusted to be equalized in such a way that the air is supplied for all these
A power feeding slot 40a is provided on the lateral surface of the
container main body for freely detachably holding an electrical power unit 40 having
a terminal for connecting the power terminal to the motor-driven fan 30. The power
unit 40 has an internal control device to operate the fan 30 according to control
programs to start/stop the operation and to control the rotational speed of the
As shown in Figure 3, the component elements inside the cleanbox
are assembled by coupling the components successively, without using fasteners
such as rivets, except for the dividing plates which are integrally attached to
the container main body. The filters 26, 28 disposed in the central chamber 22a
are successively stacked on the upper frame 16b of the bottom section 16, so as
to be retained inside the central chamber 22a. In the lateral chambers 22b, a
motor support 44 detachably attaches the motor-driven fan 30 above the upper frame
16b, and the gas removal filter 32 is placed on top of the motor support 44, thereby
eliminating fasteners that are potential sources of dust particles as well as
facilitating disassembly of the container 18 for washing and cleaning.
In this embodiment, gas removal filter 28 is made of a woven mixture
of ion exchange fibers and activated charcoal fibers, but such a part may also
be made by wrapping a non-woven ion exchange fabric around a urethane form serving
as the carrier for embedding particles of activated charcoal. Activated charcoal
fibers may be produced by subjecting carbon fibers made from rayon, kainol, polyacrylonitrile,
kerosene, and tar pitch and reacting the fibrous carbon with steam and carbon
dioxide gas and the like at a temperature in excess of 800 °C to carry out the
so-called gasification activation process. Some activated charcoal fibers contain
binders, that do not contribute to adsorption, for the purpose of strengthening
and dust prevention, but it is preferable that the fibers do not contain such binders
from the viewpoint of their performance.
Activated charcoal contains numerous fine pores in fundamental carbon
crystals because unincorporated carbons are removed in the process of activation.
The fine pores together with the large surface area are the contributing factors
for the physical adsorption action exhibited by the activated charcoal. Some activated
charcoal filters are available commercially, and these use the property of strong
adsorption action by imbedding activated charcoal particulate. Other commercially
available filters for filtering air are based on activated charcoal fibers whose
pores are finer than the those in the particulate types, and are readily formable
and produce less dust and offer a higher specific surface area. Other commercially
available types include open pore structure fibers made of urethane fibers, which
are imbedding activated charcoal particulate of approximately 0.5 mm diameter.
On the other hand, ion exchange fibers may be produced by introducing
ion exchange radicals into polymeric fibers by radiation-induced graft polymerization
reaction. Such fibers are high molecular weight polymers including polyethylene,
polypropylene, and natural polymeric fibers such as cotton, wool fibers or fabrics,
which are subjected to radiation from electron beam and gamma radiation and the
like to generate activation sites. These activated sites are radical sites, which
are highly reactive, however, the properties of the base fiber can be altered by
chemically combining a monomer to the radical sites to produce a filter material
having different properties than the base fibers.
This technology is called graft polymerization because the monomers
are grafted to the base material. By radiation-induced graft polymerization process,
it is possible to produce non-woven ion exchange material having much improved
ion exchange rates than ion exchange beads which are generally called ion exchange
resins. Polyethylene non-woven fabric may be treated by this technique by attaching
ion exchange species such as sulfone, carboxyl, and amino group monomers represented
by sodium styrene sulfonic acid, acrylic acid or aryl amines.
Similarly, monomers that can accept ion exchange radicals such as
styrene, chlormethyl styrene, glycidylmethacrylate, acrylonitrile, or acrolein
as the base may be radiation grafted and ion exchange radicals introduced afterwards
to produce a filter that retains the original shape.
In this embodiment, gas removal filter 28 was produced by weaving
ion exchange fibers and activated charcoal fibers concurrently, but ion exchange
fibers and activated charcoal fibers may be used singly or in combination to produce
a gas removal filter.
Next, air filter 26 will be explained. The HEPA filter has a particle
capture efficiency in excess of 99.97 % for particle of 0.3 µm size at the standard
flow rate. In the 1980s, however, with the advent of high density circuit integration
in LSI circuits, it has become necessary to develop filters capable of handling
class 10 (10 particles/ft3) environment, and filters of higher performance
than the HEPA filters. In response to such a demand, ULPA filters are now available
At the beginning, glass fibers were used for ULPA filters, but a
problem was discovered that glass fibers produce BF3 by reacting with
hydrogen fluoride (HF) gas used in semiconductor processing. Recently, ULPA filters
using polytetrafluoroethylene (PTFE) as the filtering fibers that do not contain
boron and metallic impurities and do not react with acid, alkaline and organic
solvents have become commercially available. In this invention, glass fibers or
PTFE fibers are used as appropriate.
Next, dehumidifying agents will be explained. There are several commercial
brands available, some of the representative substances are listed below. Physical
adsorbers available include: silica gel, molecular sieve, synthetic zeolite, and
chemical adsorbers include calcium chloride, magnesium chloride. Any of these dehumidifying
agents may be used in the cleanbox, but it is preferable to use physical adsorbers
that give off lesser amount of contaminants in the form of dust and organic or
The operation of the cleanbox of such a construction will be explained.
The interior space of the box is maintained cleanly, and filters 26, 28, 32 and
the electrical power unit are installed in their respective places. The lid section
14 is removed in a highly clean environment such as a cleanroom so that the carrier
10 with wafers Wf can be placed on the substrate holding section 24, and the lid
section 14 is reinstalled tightly. The carrier 10 can be placed easily inside because
the substrate holding section (carrier support section) 24 is situated above the
filters 26, 28.
The external switch is turned on, then the motor-driven fan 30 starts
operating according to some pre-installed program. Accordingly, circulation paths
are formed inside the container, descending along the lateral chambers 22b, and
ascending the central chamber 22a, after which the central stream is split in the
upper region into two lateral streams so as to descend along each of the lateral
chambers 22b and return separately to the fan 30. More specifically, the air stream
directed downward by the dc brushless motor-driven fan 30 flows along the floor
16b of the bottom section 16, and is redirected by the lower laminar plate 36
so that each stream flows upward and the air streams merge together to form an
ascending air stream through the central region.
The air is cleaned by flowing through both gas removal filter 28
and the ULPA filter 26, and is guided between the wafers Wf by the carrier entry
laminar plate 38. By providing the carrier entry laminar plate 38, excessive flow
of air between the wafers is prevented. The air streams that passed through the
spaces between the wafers Wf are directed along the upper laminar plate 34 and
the inner surface 14a of the lid section 14 to reverse the direction, and flow
along the lateral chambers 22b to pass through the filter 32 to be cleaned and
returned to the fan 30.
In the process of air circulation inside the casing container, solid
particles that may be adhering to various sections and gaseous substances emitted
from various materials are swept by the circulated air, and are cleaned in the
two filters 26, 28 disposed in the upstream location, and the air streams are then
allowed to flow between the wafers Wf. Therefore, the casing container not only
prevents contamination from external sources, but it aids in cleaning any substances
that may be present in the interior space, thus preventing so-called self-contamination.
Also, because the wafers are in the upstreaming location and upstream side of
the lid section 14 and the lid section 14 is susceptible to contamination from
external air, contamination of the wafers caused by external sources can be prevented.
Operational mode for the fan 30 may be chosen from a variety of program
modes depending on the usage of the cleanbox. Generally, at the beginning of the
operation, the motor may be operated continuously or at a high flow rate so as
to provide active cleaning of the air brought into the casing container. After
cleaning the air in this mode, the flow rate may be lowered or the fan may operated
intermittently to prevent contamination of the wafers from the contaminants that
may be generated from the wafers Wf themselves or components present in the casing
container. This mode of operation prolongs battery life.
Here, even if contaminants are generated from the motor when the
fan 30 is stopped, they are prevented from reaching the wafers Wf because of the
presence of reverse flow prevention filter 32.
Figures 4 and 5 show a second embodiment of this invention. The difference
between this embodiment and the first embodiment is that the shape of the ceiling
section 14a of the lid section 14 is made approximately arc shaped, whose radius
of curvature is greater than the radius of curvature of the wafers. For example,
if the wafer radius R, the lid curvature is higher than 1.2R so that the air stream
flowing along the ceiling section 14a will be redirected smoothly. Also, in this
embodiment, the carrier 10 serving as the object holder is made integral with the
object holding plate 50 so that a carrier is not needed. Opposing guide grooves
50a are provided in the object holding plate 50 to support the wafers Wf at their
Figures 6 and 7 show a third embodiment of this invention. The difference
between this embodiment and the first embodiment is that the motor-driven fan 30
is disposed on the bottom section 16, below the central chamber 22a, so as to
direct the air upwards. Specifically, the bottom section is provided with a support
for the fan 30, and a diffuser plate 52 is disposed between the fan 30 and the
gas removal filter 28 so as to disperse the air uniformly through the entire width
of the central chamber 22a.
This design permits to use only one motor-driven fan to lighten the
weight of the cleanbox. Specifically, the dimensions of the casing container 18
are 250 mm width, 200 mm depth and 380 mm height to give a total weight of 6 Kg,
including twenty-five 6-inch wafers so that the casing container can be carried
by a person. The cleanbox is designed so that the air circulating at 0.1 m3/min
inside the casing container 18 will produce an air speed of 0.1 m/s when flowing
through the center region between the wafers Wf.
Figures 8 and 9 show the results of computer analysis of the flow
patterns in the casing container of the third embodiment as seen in a front view
and a side view, respectively. The flow patterns relate to the distribution of
air speed at the exit of the fan 30 and the flow lines inside the container main
body. These flow diagrams demonstrate that the air filtered through the gas removal
filter 28 made of weaving the ion exchange fibers and activated charcoal fibers
together and the ULPA filter 26 flows between the wafers Wf uniformly without generating
any stagnation inside the casing container 18.
Next, Figure 10 shows changes that occur during the cleaning process
for a given initial concentration of ammonia. The method used was the impinger
method. It can be seen in the diagram that even when the ambient concentration
is low, the concentration is lowered to 1 ppb within 10 minutes of operation.
Figures 11 and 12 show a fourth embodiment of this invention. The
differences between this embodiment and the third embodiment are that the power
unit 40 is disposed on the bottom section 16, the gas removal filter 32 for prevention
of reverse flow is disposed at the entrance to the fan 30, a pocket 53 for storing
the dehumidifying agent is provided, and the casing container 18 is designed to
be made by injection molding to permit mass production of the cleanbox.
Such a design provides a low center of gravity in a compact and light
weight structure. The dimensions of the casing container 18 are 280 mm width, 220
mm depth and 360 mm height to give a total weight of 6 Kg, including twenty-five
8-inch wafers so that the casing container is lighter weight than the weight of
casing container in the third embodiment, even though the diameter of wafer has
Next, the results of measuring changes in the humidity by providing
100 g of dehumidifying agent inside the fourth embodiment of the cleanbox will
be presented in Figure 13. The results relate to a comparison of two cases, when
the fan 30 was operated continuously and when the fan 30 was stopped. The results
in Figure 13 demonstrate that it is possible to lower the relative humidity to
about 25 % by operating the fan 30 for several minutes.
As explained above, this invention provides a cleanbox that prevents
not only external contaminants from entering the environment but also prevents
contaminants emitted from the substrates themselves, because the air directed to
the wafers is first cleaned by passing the air streams through the air filter
and gas removal filter, to perform physical and gas removal filtering, before the
air streams are allowed to come into contact with the wafers. The cleanbox thus
contributes significantly to improving the production yield and performance qualities
of those objects that must avoid contaminants caused by particulate and gaseous
substance at all cost, such as semiconductor wafers and photomasks. Because the
air filter and the gas removal filters are disposed upstream in relation to the
wafer holding section, loading/unloading the wafers can be carried out from the
top section of the casing container, and because the container lid is disposed
downstream in relation to the wafer holding section, it is possible to avoid contamination
caused by the wafer holder which is susceptible to contamination while the lid
is open. Further, by including the humidifying agent inside the box and circulating
air in the box, the humidity inside the box can be lowered in a short time to a
relative humidify value that is less than 25 % to contribute to reducing the chemical
reaction of the wafers with an ambient atmosphere.
This invention is useful for cleanbox used to store or transport
objects such as semiconductor wafers and photomasks, for example, until they are
needed for processing in cleanrooms.