U.S. patent application number 10/401631 was filed with the patent office on 2003-10-02 for method and apparatus for purifying a gas containing contaminants.
This patent application is currently assigned to EBARA CORPORATION. Invention is credited to Fujii, Toshiaki.
Application Number | 20030183503 10/401631 |
Document ID | / |
Family ID | 27457661 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030183503 |
Kind Code |
A1 |
Fujii, Toshiaki |
October 2, 2003 |
Method and apparatus for purifying a gas containing
contaminants
Abstract
A method and an apparatus for purifying a gas containing
contaminants are disclosed. The gas is irradiated with an
ultraviolet ray and/or a radiation ray so as to produce
microparticles of the contaminants. The resultant microparticles of
the contaminants are contacted with a photocatalyst. Then, the
photocatalyst is irradiated with light so as to decompose the
contaminants held in contact with the photocatalyst. Organic
compounds, organosilicon compounds, basic gas and the like can be
decomposed by the action of the photocatalyst. Even when these
species are present at a low concentration, they can be
concentrated locally by transforming into microparticles, and hence
can be removed.
Inventors: |
Fujii, Toshiaki; (Kanagawa,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
EBARA CORPORATION
Tokyo
JP
144-8510
|
Family ID: |
27457661 |
Appl. No.: |
10/401631 |
Filed: |
March 31, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10401631 |
Mar 31, 2003 |
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09853762 |
May 14, 2001 |
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09853762 |
May 14, 2001 |
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09147697 |
Feb 18, 1999 |
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09147697 |
Feb 18, 1999 |
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PCT/JP97/02863 |
Aug 19, 1997 |
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Current U.S.
Class: |
204/157.3 ;
422/186.3; 422/22; 422/24 |
Current CPC
Class: |
B01D 53/007 20130101;
B01D 2255/802 20130101; B01D 53/8668 20130101; B01D 53/66 20130101;
B01D 2259/804 20130101; B01D 2257/106 20130101; B01D 2258/0216
20130101 |
Class at
Publication: |
204/157.3 ;
422/22; 422/24; 422/186.3 |
International
Class: |
B01J 019/08; B01J
019/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 1996 |
JP |
235832/1996 |
Jan 22, 1997 |
JP |
21947/1997 |
Jan 31, 1997 |
JP |
31441/1997 |
Claims
1. A method for purifying a gas containing a contaminant,
comprising the steps of: irradiating the gas with an ultraviolet
ray and/or a radiation ray for producing microparticles from the
contaminant; contacting the contaminant in the form of
microparticles with a photocatalyst; and irradiating the
photocatalyst with a light for decomposing the contaminant being
contact with the photocatalyst.
2. The method of claim 1, wherein said contaminant includes at
least one species from the group consisting of an organic compound
(except for alkane), an organosilicon compound and a basic gas.
3. The method of claim 1, wherein said step for producing
microparticles comprises irradiating an ultraviolet ray having a
wavelength of not more than 260 nm and/or a radiation ray.
4. The method of claim 1, wherein said gas contains water in a
concentration of not less than 1 ppb or gaseous oxygen in a
concentration of not less than 1 ppb.
5. The method of claim 1, wherein said gas contains water in a
concentration of not less than 100 ppb or gaseous oxygen in a
concentration of not less than 100 ppb.
6. The method of claim 1, wherein said gaseous oxygen is present in
said gas in a concentration of not less than 100 ppb, said gas is
transformed into ozone in said step for producing microparticles
and the method further comprises a second decomposition step for
decomposing the resultant ozone.
7. The method of claim 1, wherein the method further comprises a
removal step for removing said contaminant.
8. The method of claim 7, wherein said contaminant includes an
acidic compound or a basic compound.
9. The method of claim 7, wherein said contaminant includes at
least one species among the group consisting of nitrogen oxide
(NOx), nitrogen oxide ion, nitrogen sulfide (SOx), nitrogen sulfide
ion, hydrogen chloride, hydrogen fluoride, ammonia and amines.
10. The method of claim 7, wherein said step for producing
microparticles is preceded by said removal step.
11. The method of claim 7, wherein said removal step is preceded by
said step for producing microparticles and followed by a first
decomposition step.
12. The method of claim 7, wherein said first decomposition step is
followed by said removal step.
13. The method of claim 7, wherein said removal step comprises the
use of at least one of a filter, an adsorbent, a photocatalyst and
an ion exchange material, said photocatalyst being used so as to
supply said contaminant with an electrical charge and trap the
resultant charged contaminant.
14. The method of claim 1, wherein said photocatalyst is composed
of a matrix and a catalytically active component carried on said
matrix, said catalytically active component being in the form of a
particle.
15. The method of claim 14, wherein said matrix has a shape
selected from the group consisting of a honeycomb structure
provided with at least one partition defining at least 2
through-holes, a bar body and a wall member, and said catalytically
active component consists of a semiconductor.
16. An apparatus for purifying a gas containing a contaminant,
comprising: a microparticle-producing section having a source for
emitting an ultraviolet ray and/or a radiation ray; and a
decomposition section composed of a photocatalyst and a light
source for irradiating said photocatalyst with light, said
decomposition section being connected to said
microparticle-producing section.
17. The apparatus of claim 16, further comprising a gas inlet and a
gas outlet, wherein said gas inlet, said microparticle-producing
section, said decomposition section and said gas outlet are
disposed successively downstream.
18. The apparatus of claim 16, wherein said microparticle-producing
section is followed downstream by an ozone-decomposing
material.
19. The apparatus of claim 16, further comprising another removal
section for removing acidic compounds or basic compounds.
20. The apparatus of claim 19, wherein said another removal section
includes one or more means selected from a filter, an adsorbent, an
ion exchange material as well as a photoelectron-emitting means and
a means for trapping a charged contaminant.
Description
BACKGROUND OF THE INVENTION
[0001] The disclosures of the Japanese Patent Applications Nos.
Hei-8-235832 filed on the Aug. 20, 1996 and Hei-9-31441 filed on
the Jan. 31, 1997 are incorporated herein by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and an apparatus
for purifying a gas containing contaminants. More specifically, the
present invention relates to a method and an apparatus for
purifying a gas by producing microparticles of the contaminants
present in a gas and decomposing the resultant microparticles of
contaminants with a photocatalyst for facilitating the removal
thereof.
[0004] 2. Related Art
[0005] It was considered satisfactory in semiconductor industries
in the past to remove only solid particles such as dust from a gas
such as air in a clean room. Methods for removing solid particles
can be classified broadly into 2 categories: (1) mechanical
filtration methods (e.g. HEPA (High Efficiency Particulate Air)
filter); and (2) methods for trapping microparticles
electrostatically (e.g. MESA filter). Methods included in the
category (2) comprise charging microparticles electrically with a
high electrical voltage and filtering the charged microparticles
with an electrically conductive filter. Gaseous contaminants,
however, cannot be removed by any method of either category.
[0006] Development of semiconductors of higher quality and finer
precision has made it necessary to remove not only dust-like solid
particles but also gaseous contaminants. Gaseous contaminants
include: organic compounds including phthalic esters; organosilicon
compounds including siloxane; acidic gases including sulfur oxides
(SOx), nitrogen oxides (NOx), hydrogen chloride (HCl) and hydrogen
fluoride (HF); as well as basic gases including NH.sub.3 and
amines. Amines may be included among organic compounds also. Anions
such as NO.sub.3.sup.-, NO.sub.2.sup.-, SO.sub.4.sup.2-, etc. have
characteristics and exert adverse effects similar to acidic gases,
and therefor, are considered as a member of acidic gases out of
convenience. Likewise, cations such as NH.sub.4.sup.+, etc. have
characteristics and exert adverse effects similar to basic gases,
and therefor, are considered as a member of basic gases for
convenience.
[0007] Organic compounds or organosilicon compounds, when deposited
onto the surface of a wafer (substrate), may have a negative effect
on the affinity (drapability) of a substrate for a resist.
Decreased affinity may exert a harmful influence on both the film
thickness of a resist and the adhesion of a substrate to a resist
("Air Cleaning", Vol. 33, No. 1, pp. 16-21, 1995). For example, SOx
may bring about defective insulation in an oxide layer. NH.sub.3
may produce ammonium salts that are responsible for the blooming
(poor resolution) of a wafer (Realize Inc., "Saishin Gijyutsu Koza,
Shiryo-shu", Oct. 29, 1996, pp. 15-25, 1996). For the
aforementioned reasons, such gaseous contaminants may diminish the
productivity (yield) of semiconductor products.
[0008] It was also considered satisfactory in the past to remove
gaseous contaminants to a level of ppm. It has become required now
to remove gaseous contaminants to a level of ppb. Among organic
compounds, alkanes such as methane and the like are not so reactive
as to exert an unfavorable influence on a semiconductor, and hence
are not required to be removed to a level of ppb.
[0009] Removal of contaminants including organic compounds,
especially gaseous organic compounds is described below in more
detail.
[0010] Known methods for removing organic compounds include
decomposition by combustion, catalytic decomposition, removal by
adsorption, decomposition with O.sub.3 and the like. These known
methods, however, are not effective in removing organic compounds
present in low concentrations in air for feeding a clean room.
[0011] In a clean room, contamination with organic compounds of an
extremely slight concentration cannot be ignored. External organic
compounds may be introduced into a clean room. For example, outdoor
air is contaminated with organic compounds originating from exhaust
gas of cars or those resulting from degassing of polymer products.
On the other hand, internal organic compounds may be generated in a
clean room. For example, polymer materials (e.g. polymeric
plasticizers, releasers, antioxidants and the like) which are used
for constructing a clean room are producers of organic gases ("Air
Cleaning", Vol. 33, No. 1, pp. 16-21, 1995). Synthetic polymers are
used in packing materials, sealants, adhesives and wall-forming
materials in a clean room. In addition, plastic containers are
disposed in a clean room. These synthetic polymers may evolve a
trace amount of organic gases. More particularly, sealants and the
production units thereof may give off gaseous siloxane, and plastic
containers may give off gaseous phthalic esters. It has recently
been found that gas evolves also from polymer materials employed in
a production unit. A process unit is partially or entirely
surrounded by plastic plates which also produce organic gas. A
variety of solvents (e.g. alcohols, ketones, etc., which are
necessary for operations in a clean room are also a contamination
source.
[0012] As stated above, a clean room is contaminated variously and
heavily with not only organic compounds attributable to external
air but also with organic compounds and organosilicon compounds
that are generated internally.
[0013] In view of energy saving considerations, recycling of air in
a clean room has become more frequent recently. In consequence,
organic gases are progressively concentrated in a clean room,
leading to heavier contamination of the base materials of a wafer
and a substrate. These organic compounds are likely to deposit onto
the bodies (e.g., starting materials and semi-fabricated products
of a semiconductor wafer, a glass substrate, etc.) placed in a
clean room, adversely affecting them.
[0014] A contact angle indicates a degree of contamination on a
wafer substrate with organic compounds and organosilicon compounds.
The contact angle refers to the angle formed by the water and the
surface of a substrate when the surface is wet with water. The
surface of a substrate, when covered with a hydrophobic (oily)
substance, becomes more water-repellent and less wettable, hence
the contact angle of water on the surface of a substrate becomes
larger. In other words, when the contact angle is larger, the
degree of contamination is higher. On the contrary, when the
contact angle is smaller, the degree of contamination is lower.
[0015] When a substrate is contaminated with organic compounds and
organosilicon compounds, its affinity (drapability) for a resist
decreases, imparting an unfavorable influence on the resist and the
film thickness or on the adhesion of the substrate to the resist,
that may result in lower quality and a lower yield.
[0016] Techniques in the high-technology field have made remarkable
progress in realizing semiconductor devices of a maximal precision
and a minimal size. In consequence, it has become necessary for a
clean room to be free from organic compounds normally present in
the air of the level that had conventionally been able to be
ignored (an extremely low concentration of the ppb level)
[Preparatory Manuscripts for the 39th Meeting of the Applied
Physical Society, p.86 (1992, Spring); "Air Cleaning", Vol. 33, No.
1, pp. 16-21, (1995)], as well as gaseous contaminants including
SO.sub.2, HF, NH.sub.3 ["Ultra Clean Technology", Vol. 6, pp. 29-35
(1994)]. Because, it has been revealed that the presence of these
gaseous contaminants diminished remarkably the productivity
(yield). The present invention is aiming to efficiently remove
these gaseous contaminants.
[0017] The present inventors have proposed a method for removing
hydrocarbons present in a gas comprising the steps of: irradiating
the gas with an ultraviolet ray and/or a radiation ray so as to
produce microparticles from the hydrocarbon; and trapping the
resultant hydrocarbon microparticles with a filter or charging the
hydrocarbon microparticles electrically with a photoelectron and
trapping the resultant charged microparticles (Laid Open Japanese
Patent Application No. Hei-5-96125). A similar method can be
applied also to noxious matter present in a gas (Laid Open Japanese
Patent Application No. Hei-4-243517).
[0018] Using the methods mentioned above, however, trapped
microparticles become accumulated on the filter or in the part for
trapping the charged microparticles, thus requiring frequent
changing of the filter or the trapping part. Further, when the
accumulated microparticles fall from the filter or from the
trapping part, the fallen microparticles, even if they are in
extremely small amounts, inadvertently contaminate a gas to be
purified. Therefore, it is considered preferable to decompose
contaminants than to remove them.
[0019] A conventional removing method is now described with
reference to FIGS. 16 and 17. As shown in FIG. 16, the air which is
fed to a clean room 1 in a recycled manner is composed of the
external air that is fed via a pipe 2 and is cleared of coarse
particles through a prefilter 3 and the internal air that is drawn
out of the clean room 1 through an air outlet 4. Both airs are
combined in a fan 5, conditioned in temperature and moisture with
an air conditioner 6 and cleared of microparticles with a HEPA
filter 7. The air in the clean room is kept at a purity (class) of
the order of 10,000. In this specification, the term "class" refers
to the number of particles having a particle diameter of not less
than 0.1 .mu.m that are present per cubic feet.
[0020] A clean bench 51 is disposed in the clean room 1 to trap and
remove a trace amount of hydrocarbons and microparticles
(particulate matter).
[0021] Organic compounds present in the clean room 1 may consist
presumably of those that originating in external air introduced
through the pipe 2 (those that are presumably discharged from cars
and synthetic resins) and those that are produced during operations
in the clean room.
[0022] The clean bench 51 comprises mainly a
microparticle-producing 48, a microparticle-charging section 49 and
a section for trapping charged microparticles 50. A highly pure air
(of class 10) that is freed of both organic compounds and
coexistent microparticles is fed above a working table 53, where
operations are being carried out.
[0023] In other words, air having a purity (class) in the order of
10,000 and containing a trace amount of organic compounds
originating in the clean room 1 is directed with a fan (not shown)
toward the clean bench 51. At the clean bench 51, the
microparticle-producing section 48 is provided for irradiating the
air with an ultraviolet radiation of a short wavelength so as to
produce microparticles of organic compounds contained in the air.
Then, in the microparticle-charging section 49, the microparticles
are electrically charged efficiently with photoelectrons emitted by
a photoelectron-emitting material as described hereinbelow. The
resultant charged microparticles are trapped and removed in the
section for trapping charged microparticles 50 that follows. In
this manner, air above the working table 53 can be maintained to be
highly pure and free of organic compounds.
[0024] A movable shutter is provided on the clean bench 51 for
facilitating introduction and/or withdrawal of instruments and
products into and/or out of the working table 53.
[0025] FIG. 17 shows schematically a microparticle-producing
section 48, a microparticle-charging section 49 and a section for
trapping charged microparticles 50. These sections are described
just below with reference to FIG. 17.
[0026] In other words, air 54 aspirated through a fan (not shown)
and containing a trace amount of organic compounds is filtered
through a prefilter (not shown), and then irradiated with an
ultraviolet radiation of a short wavelength in the
microparticle-producing section 48 that is mainly consisting of an
UV lamp 55. Organic compounds present in the air 54 are transformed
into microparticles 56 by UV irradiation. These microparticles 56,
together with naturally-occurring microparticles 57 already present
in the introduced air 54, are electrically charged in the
microparticle-charging section 49 so as to become charged
microparticles 58.
[0027] The microparticle-charging section 49 is mainly composed of
an UV lamp 59, a photoelectron-emitting material 60 (herein
consisting of a glass material having a surface coated with an Au
thin layer of a thickness of 5 to 50 nm, for example) and an
electrode material 61 for generating an electrical field. The
photoelectron-emitting material 60 is irradiated with the UV lamp
59 in the presence of an electrical field so as to emit
photoelectrons 62, which in turn supply the microparticles 56, 57
with an electrical charge so as to produce the charged
microparticles 58, which can then be trapped in the section for
trapping charged microparticles 50 that follows. The section 50
consists of a material for trapping the charged microparticles.
Reference numeral 63 indicates an UV-transmissive material.
Reference numeral 64 indicates a highly pure air that is dust-free
and free from organic compounds.
[0028] The arrangement as stated above is suffered from the
problems as set forth below:
[0029] (1) Microparticles that were produced from organic compounds
upon irradiation with an ultraviolet ray and/or a radiation ray
often failed to result in complete trapping with a filter or
complete charging and trapping with photoelectrons, depending on
the irradiation conditions and the kinds of organic compounds. This
is probably because some kinds of organic compounds tend to produce
microparticles of an extremely small size. Or else, the chemical
composition of organic compounds may be responsible for it. In case
when the trapping efficiency was low, a trapping section having a
larger volume was required, hence making the whole apparatus
larger.
[0030] (2) Produced particulate matter can be trapped at the
trapping section 50. Consequently, this particulate matter tends to
accumulate in the trapping section during a long-term continuous
operation. This requires a design of a trapping section 50 having a
higher trapping volume, thus making the size of an apparatus
larger.
[0031] On the other hand, the present inventors have proposed the
use of a photocatalyst in a system for removing organic compounds
(Japanese Patent Applications Nos. Hei-8-31230 and Hei-8-31231). In
this system, however, organic compounds in a low concentration are
decomposed with a photocatalyst so slowly that the decomposition
thereof is very time-consuming. In other words, diethylhexyl
phthalate (DOP) and siloxane present in the natural air and the air
in a clean room are only in a concentration as low as about 1 ppb
each.
[0032] Further, photocatalysts cannot effectively remove acidic
gases such as SO.sub.2, NO, HCl and HF. In particular,
sulfur-containing compounds such as sulfur oxides, hydrogen
sulfide, thiophene and thiols, when present at a high
concentration, may sometimes act as a catalytic poison to the
photocatalysts. Even if these compounds can avoid acting as a
catalytic poison, they might adversely influence on the
photocatalysts after a long-term operation.
SUMMARY OF THE INVENTION
[0033] The present invention is directed to solve the problems as
set forth above.
[0034] According to one aspect of the present invention, there is
provided a method for purifying a gas containing a contaminant
comprising a microparticle-producing step for irradiating the gas
with an ultraviolet ray and/or a radiation ray so as to produce
microparticles of the contaminant, a contact step for contacting
the microparticles of the contaminant with a photocatalyst and a
first decomposition step for irradiating the photocatalyst with a
light so as to decompose the contaminant being in contact with the
photocatalyst. Organic compounds (except for alkanes),
organosilicon compounds and basic gases can be oxidatively
decomposed with a photocatalyst. Even when contaminants are present
in small amounts, they can be concentrated locally by transforming
into microparticles, and hence can be oxidatively decomposed with a
photocatalyst efficiently.
[0035] In the microparticle-producing step, an ultraviolet ray
and/or a radiation ray having a wavelength of not more than 260 nm
is preferably used. Contaminants can aggregate through a radical
reaction to produce microparticles.
[0036] Preferably, the gas contains water or gaseous oxygen in a
concentration of not less than 1 ppb. More preferably, the gas
contains water or gaseous oxygen in a concentration of not less
than 100 ppb. It is believed that water or gaseous oxygen acts on
the surface of a photocatalyst by supplying it with OH radical to
induce activation of the photocatalyst. The OH radical probably
acts as an oxidant in the presence of the photocatalyst.
[0037] It is preferred that: a gas contains gaseous oxygen of at
least 1 ppm; the gaseous oxygen present in the gas is transformed
into ozone at the microparticle-producing step; and the method
further comprises a second decomposition step for decomposing the
resultant ozone.
[0038] More preferably, the method comprises a removal step for
removing contaminants. Preferably, the contaminants contain acidic
or basic compounds, and more preferably, the contaminants contain
at least one species selected from the group consisting of nitrogen
oxides (NOx), nitrogen oxide ions, sulfur oxides (SOx), sulfur
oxide ions, hydrogen sulfide, hydrogen fluoride, ammonia and
amines.
[0039] The removal step may precede the microparticle-producing
step. Alternatively, the removal step may follow the
microparticle-producing step and precede the first decomposition
step. The latter order is suitable when there is contained any
contaminant serving as poison to a photocatalyst. More precisely,
when sulfur-containing compounds such as sulfur oxides, hydrogen
sulfide, thiophene and thiols are present, it is preferred that
these compounds are removed prior to the treatment with the
photocatalyst. Alternatively, the removal step may follow
immediately after the first decomposition step.
[0040] Preferably, the removal step is carried out by means of one
or more of a filter, an adsorbent, an ion exchanger and a
photoelectron. Photoelectrons can supply contaminants with an
electrical charge so as to facilitate the trapping of the
contaminants.
[0041] Preferably, the photocatalyst is composed of a matrix and a
catalytically active component which is carried on the matrix and
which is preferably in the form a particle. More preferably, the
matrix is in the form of a honeycomb structure provided with
partitions defining at least 2 through-holes, a bar body or a wall
member, and the catalytically active component is
semiconductor.
[0042] According to the second aspect of the present invention,
there is provided an apparatus for purifying a gas containing a
contaminant comprising: a microparticle-producing section having a
source of an ultraviolet ray and/or a radiation ray; and a
decomposition section having a photocatalyst and a light source for
irradiating the photocatalyst, the decomposition section being
connected to the microparticle-producing section.
[0043] Preferably, the apparatus is provided with a gas inlet and a
gas outlet and is disposed in a manner that the gas inlet, the
microparticle-producing section, the decomposition section and the
gas outlet are arranged successively downstream.
[0044] More preferably, an ozone-decomposing material is provided
downstream to the microparticle-producing section.
[0045] In addition, it is preferred to provide a removal section
for removing acidic and/or basic compounds. Preferably, the removal
section comprises one or more means selected from a filter, an
adsorbent and an ion exchanger as well as a photoelectron-emitting
means and a means for trapping charged contaminants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a general view of a clean room having a
purification apparatus of the present invention disposed
therein.
[0047] FIG. 2 is a general view of a wafer stocker having a
purification apparatus of the present invention disposed
therein.
[0048] FIG. 3 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0049] FIG. 4 is a graph of the contact angle (degrees) versus
storage period (days) showing the results of Example 4.
[0050] FIG. 5 is a general view of a clean room having a
purification apparatus of the present invention disposed
therein.
[0051] FIG. 6 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0052] FIG. 7 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0053] FIG. 8 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0054] FIG. 9 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0055] FIG. 10 is a general view of a wafer stocker having a
purification apparatus of the present invention disposed
therein.
[0056] FIG. 11 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0057] FIG. 12 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0058] FIG. 13 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0059] FIG. 14 is a cross-sectional view showing another embodiment
of the purification apparatus of the present invention.
[0060] FIG. 15 is a graph of the contact angle (degrees) versus
storage period (days) showing the results of Example 14.
[0061] FIG. 16 is a general view of a conventional clean room.
[0062] FIG. 17 is an enlarged partial view of an air-purifying
section of the apparatus shown in FIG. 16.
[0063] FIG. 18 is a general view of a wafer stocker applying the
purification method of the present invention.
[0064] FIG. 19 is a general view of a wafer stocker applying
another purification method of the present invention.
[0065] FIG. 20 is a general view of a wafer stocker applying
another purification method of the present invention.
[0066] FIG. 21 is a general view of a wafer stocker applying
another purification method of the present invention.
[0067] FIG. 22 is a schematic view showing the purification of air
for feeding an air knife by applying another purification method of
the present invention.
[0068] FIG. 23 is a graph showing the change of the contact angle
(degrees) as the function of time (days).
[0069] FIG. 24 is a graph showing the number of days taken for
5-degree increase in the contact angle as the function of SOx
concentration.
[0070] FIG. 25 is a total ion chromatogram of hydrocarbons present
in the air obtained by gas chromatography/mass spectrometry (GC/VS)
method.
DETAILED DESCRIPTION OF THE INVENTION
[0071] According to the present invention, a gas containing
contaminants is purified. Contaminants which can be removed by the
present invention are mainly gaseous contaminants including:
organic compounds (except for alkanes such as methane);
organosilicon compounds such as siloxane; acidic gases such as
sulfur oxides (SOx), nitrogen oxides (NOx), hydrogen chloride (HCl)
and hydrogen fluoride (HF); as well as basic gases such as NH.sub.3
and amines.
[0072] Organic compounds include: aliphatic hydrocarbons,
especially lower aliphatic hydrocarbons having 1 to 40 carbon
atoms, such as alkene, alkyne and the like; aromatic hydrocarbons,
especially lower aromatic hydrocarbons having 1 to 40 carbon atoms,
such as benzene, naphthalene and the like; alcohols, especially
lower alcohols having 1 to 40 carbon atoms; phenols; carboxylic
acids such as higher fatty acids; carboxylic acid derivatives such
as esters, amides, acid anhydrides and the like; ethers; amines; as
well as sulfur-containing compounds such as sulfoxides, mercaptons,
thiols and the like. Examples of aromatic hydrocarbons which can be
removed by the present invention include benzene, toluene,
ethylbenzene and the like.
[0073] Carboxylic acid derivatives include phthalic esters such as
butyl phthalate originating in synthetic polymers the phthalic
esters and hence can be removed by the present invention.
[0074] Nitrogen-containing heterocycles such as pyrrole and
pyridine, oxygen-containing heterocycles such as furan and
tetrahydrofuran, as well as sulfur-containing heterocycles such as
thiophene can be removed as well.
[0075] Halogenated hydrocarbons include: halogen-containing
aliphatic compounds including trihalomethane such as chloroform as
well as trichlorofluoromethane, dichloromethane and dichloroethane;
halogen-containing aromatic compounds such as chlorophenol.
[0076] On the other hand, contaminants do not include alkanes such
as methane and the like, since they are less reactive and are
difficult to deposit onto a substrate made of a semiconductor and
the like. Hence, alkanes are not required to be removed by the
present invention. Alkanes having at most 4 carbon atoms are more
difficult to deposit onto a semiconductor substrate and those
having at most 3 carbon atoms are still more difficult to deposit
onto a semiconductor substrate.
[0077] The present invention will now be described in more detail
section by section.
[0078] According to the present invention, a gas containing
contaminants such as gaseous contaminants is irradiated with an
ultraviolet ray and/or a radiation ray. An apparatus of the present
invention comprises a microparticle-producing section having a
source of an ultraviolet ray and/or a radiation ray.
[0079] The microparticle-producing section has an irradiation
source for transforming gaseous contaminants such as organic
compounds present in a gas into microparticles (condensable matter
or particulate matter). Any irradiation source can be used,
provided that it allows organic compounds and coexistent gaseous
contaminants such as SO.sub.2 and NH.sub.3 to transform into
microparticles or particulate matter. Any of electromagnetic waves,
lasers, radiations can be used as desired in addition to
ultraviolet rays. Also, suitable irradiation sources can be
selected on the basis of the results of preliminary experiments,
depending on fields of application, compositions and concentrations
of the gaseous contaminants to be removed, sizes and shapes of
apparatuses as well as economical efficiency. Usually, irradiation
with an ultraviolet or a radiation ray is preferred.
[0080] Upon irradiation, the contaminants present in the gas are
transformed into particulate matter (condensable matter) optionally
accompanied by active matter (condensable matter), depending on the
constituents thereof and other matter coexistent therewith. For
example, when a gaseous mixture containing toluene, isopentane and
propylene as organic compounds is irradiated with an ultraviolet
ray, carboxylic acids and carbonyl compounds (condensable matter or
active matter) are produced.
[0081] The presence of higher aliphatic acids having a high
molecular weight, phenol derivatives, phthalic esters (e.g. DBP,
DOP) and siloxane in a clean room has recently become a serious
problem, since they are included among the organic compounds that
deposit onto a substrate including wafer and induce an increase in
the contact angle ("Air Cleaning", Vol. 33, No.1, pp. 16-21, 1995).
DBP is an abbreviation of dibutyl phthalate. DOP is an abbreviation
of dioctyl phthalate, which is called more accurately
di-(2-ethylhexyl)phthalate. Phthalic esters such as DBP, DOP and
the like are useful as a plasticizer of resins, in particular of
vinyl resins.
[0082] These organic compounds and organosilicon compounds
including siloxane are transformed into particulate matter upon
irradiation with an ultraviolet ray and/or a radiation ray. The
resultant particulate matter has a particle diameter of e.g. scores
of nanometers to several hundred nanometers. This may be probably
because the contaminants exposed to ultraviolet or other radiation
undergo a radical reaction with gaseous oxygen and water present in
a gas at a trace amount of the order of 1 ppb or more, whereby the
contaminants are brought into a state of association.
[0083] According to the present invention, contaminants are
transformed into microparticles as stated above. In other words,
contaminants are microscopically concentrated. Then, the resultant
microparticles of contaminants are decomposed with a
photocatalyst.
[0084] By way of another example, the reaction schemes of SO.sub.2
are set forth below:
H.sub.2O.sup.++e.sup.-.fwdarw..OH+H.
H.sub.2O.sup.++H.sub.2O.fwdarw.H.sub.3O.sup.++.OH
SO.sub.2+.OH.fwdarw.SO.sub.3.sup.-
SO.sub.3+H.sub.2O .fwdarw.H.sub.2SO.sub.4
H.sub.2SO.sub.4.fwdarw.microparticles (particulate matter)
[0085] H.sub.2O means the water contained in a gas at a trace
amount. During the final reaction, microparticles are produced from
sulfuric acid. It may be also probable in this reaction that when a
highly viscous liquid such as sulfuric acid is produced in a gas
even in only a small amount, other contaminants can associate
therewith to produce microparticles. The presence of a basic gas,
e.g. ammonia may induce a reaction of acidic gases.
[0086] Another example of the reaction schemes is shown below:
O.sub.2.sup.+(UV).fwdarw.O+O
O.sub.2+O.fwdarw.O.sub.3
O.sub.3.fwdarw.O+O.sub.2
O+H.sub.2O.fwdarw.2OH
OH+SO.sub.2+O.sub.2.fwdarw.SO.sub.3+HO.sub.2
SO.sub.3+H.sub.2O .fwdarw.H.sub.2SO.sub.4
H.sub.2SO.sub.4+H.sub.2O.fwdarw.microparticles (particulate
matter)
[0087] These reactions vary depending on the kinds of coexistent
gases, irradiation conditions and others. As shown in the above
schemes, water reacts with coexistent gaseous contaminants
(SO.sub.2 in this case) to afford a reaction product, which is
SO.sub.2.sup.- in the above. By transforming the reaction product
into microparticles followed by trapping and removing the resultant
microparticles, a gas is cleared of coexistent gaseous contaminants
such as SO.sub.2 and becomes highly pure. This action is not
limited to SO.sub.2, but is common to other various gaseous
contaminants such as NH.sub.3 and the like.
[0088] Transformation into microparticles (production of
microparticles) can be induced effectively with an irradiation
source having a wavelength of not more than 260 nm, preferably of
not more than 254 nm. Usually, irradiation sources of ultraviolet
and/or other radiation are preferred in view of effects and
operability.
[0089] Any UV source can be used, provided that the irradiation
therewith can produce microparticles of organic compounds and those
of coexistent gaseous contaminants including SO.sub.2 and NH.sub.3
(transformation into particulate matter or condensable matter or
transformation into microparticles and active matter). Suitable UV
sources can be selected on the basis of the results of preliminary
experiments, depending on the kinds of organic compounds and other
coexistent matter. UV sources that produce oxygen-activated species
(active radicals) such as active oxygen and OH radical may be
preferred depending on the field of application.
[0090] In general, suitable UV irradiation sources include a
mercury lamp, a hydrogen discharge tube (heavy hydrogen lamp) and
the like. It is preferred to use an UV irradiation source having
multiple wavelengths which can induce different actions depending
on the kinds of organic compounds and coexistent gaseous
contaminants including SO.sub.2, NH.sub.3, etc. or coexistent
matter. For example, a mercury lamp can use: (1) an ozone-producing
wavelength; in combination with (2) a wavelength for inducing
decomposition of the resultant ozone so as to promote the
production of oxygen-activated species. By way of an example, these
wavelengths are respectively of 184 nm and 254 nm. Production of
microparticles of organic compounds and coexistent gaseous
contaminants including SO.sub.2 and NH.sub.3 is mainly induced at
184 nm and decomposition of the produced ozone is mainly induced at
254 nm. Ozone is preferred because it has an ability to promote
transformation of gaseous contaminants into microparticles as shown
in the reaction schemes set forth above.
[0091] Radiation rays which can be suitably used are .alpha.-ray,
.beta.-ray, .gamma.-ray and the like. Irradiation means which can
be used are: a radiation source utilizing radioactive isotopes such
as cobalt 60, cesium 137 and strontium 90, radioactive wastes of a
nuclear reactor or radioactive materials obtained therefrom through
suitable processing; a radiation source utilizing directly a
nuclear reactor; a radiation source utilizing a particle
accelerator such as electron beam accelerator. Electron beam
irradiation with an accelerator can become highly dense and
effective by being applied at a low output power. An accelerating
voltage is at most 500 kV, preferably in the range of 50 kV to 300
kV.
[0092] According to the present invention, contaminants in the form
of microparticles are made contact with a photocatalyst. Contact
includes deposition and adsorption. The present invention is
intended for the removal of contaminants having a tendency to
increase the contact angle on the surface of a substrate such as a
wafer. These contaminants are likely to deposit not only onto the
wafer surface but also onto the photocatalyst. In addition,
contaminants in the form of microparticles may deposit onto the
photocatalyst through Brownian motion.
[0093] According to the method of the present invention, a
photocatalyst is irradiated with light. In other words, the
apparatus of the present invention comprises a decomposition
section including a photocatalyst and a light source for
irradiating the photocatalyst.
[0094] Photocatalysts are now described. Photocatalysts are
described in U.S. patent application Ser. No. 08/733,146, the
disclosure of which.is herein incorporated by reference.
[0095] Any photocatalyst can be used, provided that the excitation
thereof by irradiation with light can promote an oxidative
reaction. Photocatalysts can oxidatively decompose organic
compounds, organosilicon compounds, basic gases such as ammonia.
For example, organic compounds are decomposed into low molecular
non-toxic substances, such as carbon dioxide and water.
Organosilicon compounds are decomposed into carbon dioxide and
water. It is not completely elucidated whether atomic silicium is
oxidatively decomposed to produce SiO.sub.2. Ammonia is thought to
be oxidatively decomposed into gaseous nitrogen.
[0096] Photocatalysts are not required to oxidatively decompose
organic compounds down to carbon dioxide. When a photocatalyst is
used to prevent the increase in the contact angle on the surface of
a semiconductor wafer, it is required only to transform organic
compounds into the compounds playing no part in the increase in the
contact angle, in other words, stable compounds having no adverse
effects even if they are deposited on the surface of the
semiconductor wafer.
[0097] On the other hand, photocatalysts are thought to oxidize
acidic gases such as sulfur oxides, nitrogen oxides and the like,
too. By way of an example, sulfur oxides (SOx) are oxidized into
SO.sub.2, which may possibly react with water in the air to produce
sulfuric acid. Acidic gases transformed into the form of
microparticles are preferably removed with any of a filter, an
adsorbent or an photoelectron.
[0098] Photocatalysts contain catalytically active components,
which are preferably semiconductors. Such semiconductors include:
elementary semiconductors such as Si, Ge and Se; compound
semiconductors such as AlP, AlAs, GaP, AlSb, GaAs, InP, GaSb, InAs,
InSb, CdS, CdSe, ZnS, MoS.sub.2, WTe.sub.2, Cr.sub.2Te.sub.3, MoTe,
Cu.sub.2S and WS.sub.2; oxide semiconductors such as TiO.sub.2,
Bi.sub.2O.sub.3, CuO, Cu.sub.2O, ZnO, MoO.sub.3, InO.sub.3,
Ag.sub.2O, PbO, SrTiO.sub.3, BaTiO.sub.3, Co.sub.3O.sub.4,
Fe.sub.2O.sub.3, NiO, WO.sub.3 and SnO.sub.2. Preferable oxides are
titanium oxide, titanium strontium trioxide, cadmium sulfide, zinc
oxide, tungsten oxide and tin oxide, and, more preferable oxides
are titanium oxide, titanium strontium trioxide and zinc oxide.
Titanium oxides respectively of rutile structure and anatase
structure are useful.
[0099] Cocatalysts such as Pt, Ag, Pd, RuO.sub.2, Co.sub.3O.sub.4
and the like can be added to catalytically active components so as
to improve the catalytic action of photocatalysts. Cocatalysts can
be used alone or in combination of two or more. Cocatalysts can be
added by using any of suitable well known methods such as
impregnation, photoreduction, sputtering evaporation, kneading and
the like.
[0100] Catalytically active components are preferably formed in the
shape of a particle so as to increase the surface area thereof.
When a cocatalyst is used, individual particles are composed of a
photocatalyst and a cocatalyst.
[0101] A photocatalyst is preferably composed of a matrix and a
catalytically active component carried on the matrix. The
catalytically active component can be fixed to the matrix by being
coated onto the surface of the matrix or by being wrapped in the
matrix or inserted into the matrix. Matrices can be made of
ceramics, fluororesins, glass, glassy materials or various metals.
A matrix may be formed in the shape of a honeycomb, a wire cloth, a
fiber, a rod and a filter. The term "honeycomb" as used herein
means a structure provided with through-holes of any shape in
cross-section. The cross-sectional shape of a through-hole may be
selected from a circle, an ellipse and a polygon, for example.
[0102] By way of an example, a matrix may be a honeycomb structure
provided with partitions defining two or more through-holes. A
photocatalyst in the form of particles can be carried on the
partitions of the honeycomb structure. The honeycomb structure may
be made of ceramics. Alternatively, a metallic matrix having a net
structure may have a surface coated with TiO.sub.2. Or, a fibrous
matrix made of glass may have a surface coated with TiO.sub.2.
Alternatively, a photocatalyst can be carried on a surface of a
light source so as to integrate the photocatalyst with the light
source as stated hereinafter (Japanese Patent Application No.
Hei-8-31231, the disclosure of which is incorporated herein by
reference).
[0103] A photocatalyst can be carried on a matrix by any of well
known processes including a sol-gel process, sintering process,
evaporation process, sputtering process, coating process, baking
finish process and the like. Materials and shapes of these matrices
as well as the manner of carrying catalytically active components
can be selected as appropriate, depending on the size and shape of
an apparatus, types and shapes of a light source, kinds of
catalytically active components, desired effects, economical
efficiency and the like. A method for supporting catalytically
active components on a linear article such as a fiber according to
sol-gel process is described in the Laid Open Japanese Patent
Application No. Hei-7-256089, the disclosure of which is herein
incorporated by reference.
[0104] A photocatalyst may be disposed in a space where a gas to be
treated flows through. Catalytically active components may be
coated on the surface of the walls, floors and ceilings that define
a space prevailed by the flow of a gas to be treated.
[0105] Any light source may be used for light irradiation, provided
that it can emit light having a wavelength absorbable with a
photocatalyst. Light belonging to the visible and/or ultraviolet
regions is effective, and hence an UV lamp or the sunlight can be
appropriately applied. Examples which can be mentioned are a
bactericidal lamp, a black light, a fluorescent chemical lamp, an
UV-B lamp and a xenon lamp. Radiations as mentioned hereinbefore
can be appropriately used. Materials of photocatalysts, materials
and shapes of matrices, the presence or not of additives, types of
irradiation sources and their installation mode in a gas to be
treated can be appropriately selected on the basis of the results
of preliminary experiments, depending on fields of application as
well as sizes, shapes and required specifications of an
apparatus.
[0106] The mechanisms by which organic compounds can be transformed
into particles and the resultant particles of particulate matter
can be decomposed in the presence of a photocatalyst are generally
believed to be as described below, though many details are
remaining unclear due to the fact that organic compounds are
thought to be present in the air in the form of a mixture of more
than hundreds or thousands of constituents.
[0107] Organic compounds in the air can be activated themselves
upon exposure to an ultraviolet ray and/or a radiation ray. When
water is present in the air even at a slight concentration, the
water may induce the production of OH radical and/or provoke an
ionic nucleation reaction. Reaction products resulted from these
many complicated reactions are associated to become microparticles.
Even when the organic compounds are present in an extremely low
concentration, reactions can occur effectively. For example,
phthalic esters such as siloxane, DOP and DBP can be easily
transformed into microparticles.
[0108] The surface of a photocatalyst is activated upon irradiation
of light and/or radiation. Contaminants that can easily deposit on
a substrate such as a wafer can easily deposit also on a
photocatalyst also. Organic compounds such as phthalic esters are
hydrophilic, and therefor can easily deposit onto an active
surface. Moreover, those organic compounds are concentrated in the
form of microparticles, and therefore more easily deposit onto the
surface of a photocatalyst. Subsequently, the contaminants are
decomposed on the surface of the photocatalyst into a stable form
of a low molecular weight.
[0109] Purified gas in a clean room contains organic compounds that
have been decomposed and hence cannot deposit on a wafer and a
glass substrate. Even though a component in the purified gas
deposits on a wafer, the component is hydrophilic. Therefore in
either case, the contact angle does not increase. In other words,
exposing a wafer and a glass substrate to the purified gas
according to the present invention, does not increase the contact
angle.
[0110] Since organic compounds comprise many constituents as
mentioned above, it is practically not possible to analyze and
estimate completely the composition thereof. Contamination of a
substrate with organic compounds depends on the activity of each
surface (e.g. film-forming species). In other words, contamination
is different depending on the surface state of the substrate. A
sensitive substrate is largely influenced by contaminants.
According to the present invention, by taking non-methane organic
compounds as an indicator, the non-methane organic compounds may be
removed to a level preferably of not more than 0.2 ppm, more
preferably of not more than 0.1 ppm.
[0111] Non-methane organic compounds are taken as an indicator
because they can be easily measured by gas chromatography (GC). In
contrast the constituents that can easily deposit on a substrate
and consequently become troublesome in a clean room, such as
siloxane and DOP (practically problematic matter) are present in an
extremely low concentration of at most 1 ppb, and therefore the
measurement and analysis thereof are complicated and laborious as
well as are difficult to be monitored.
[0112] According to the present invention, when oxygen is present
in a gas, ozone is generated upon irradiation of an ultraviolet ray
and/or a radiation ray. The resultant ozone is preferably removed
by any of well known ozone-decomposing materials, depending on
fields of application. In the fabrication of a silicon substrate
for example, the surface of the silicon substrate may be oxidized
into silicon dioxide in the presence of ozone.
[0113] Ozone-decomposing materials which can be used are composite
oxide catalysts including manganese dioxide catalysts,
MnO.sub.2/TiO.sub.2--C, MnO.sub.2/ZrO--C, as previously proposed by
the present inventors (Laid Open Japanese Patent Application No.
Hei-6-190236). Well-known active charcoals can be suitably used as
well. Ozone-decomposing materials are described in the Laid Open
Japanese Patent Application No. Hei-6-190236, the disclosure of
which is incorporated herein by reference.
[0114] Ozone can be decomposed also in the presence of a
photocatalyst used in the present invention, but it is preferred to
use any of ozone-decomposing materials set forth above, in the case
when the resultant ozone reaches a high concentration or when a
tolerable ozone leakage is at only a low level.
[0115] Preferably, the method of the present invention comprises
further a step of removing microparticles of the contaminant from a
gas. Preferably, the apparatus of the present invention has a
section for removing microparticles of the contaminant. More
precisely, it is preferable to trap and remove microparticles of
the contaminant with a filter or an adsorbent or by charging with
photoelectrons. This removal step can eliminate mainly gaseous
contaminants other than organic compounds, such as sulfur oxides
(SOx), nitrogen oxides (NOx) and ammonia (NH.sub.3) in the form of
microparticles.
[0116] It is preferred according to the present invention that
acidic and basic gases as well as microparticles are removed.
Removal can be carried out by using one or more of a filter, an
adsorbent and an ion exchanger and/or by electrically charging the
contaminant with photoelectrons followed by trapping the charged
contaminant. Organic compounds and organosilicon compounds that had
been transformed into microparticles can also be removed by these
removing means.
[0117] Filters which can be used include a HEPA filter, an ULPA
(Ultra Low Penetration Air) filter, an electrostatic filter, an
electret material, an ion exchange filter and the like. The ion
exchange filter is preferred depending on fields of application,
since it can trap a toxic gas, an odorous gas and the like which
partly flows out without transformed into microparticles if
present.
[0118] Adsorbents which can be used include active charcoal, silica
gel, synthetic zeolite, molecular sieve and alumina. Alternatively,
the adsorbents composed of a glass and a fluororesin that were
previously proposed by the present inventors for trapping
non-methane organic compounds can be suitably used (Laid Open
Japanese Patent Application No. Hei-6-324). The disclosure about
adsorbents described in the Laid Open Japanese Patent Application
No. Hei-6-324 is herein incorporated by reference. According to the
present invention, gaseous contaminants difficult to remove as such
can be effectively removed by these adsorbents, since the
contaminants are transformed into particles by means of an
ultraviolet ray and/or a radiation ray.
[0119] In UV/photoelectron systems, microparticles are electrically
charged with photoelectrons emitted from a photoelectron-emitting
material and the resultant charged microparticles are trapped and
removed. The UV/photoelectron systems previously proposed by the
present inventors can be applied as desired. Methods for removing
contaminants in the form of charged microparticles are described in
the Japanese Patent Publications Nos. Hei-3-5859, Hei-6-34941,
Hei-6-74909, Hei-6-74910, Hei-8-211, Hei-7-121369 and Hei-8-22398,
all of which are herein incorporated by reference.
[0120] Any photoelectron-emitting material can be used, provided
that it can emit photoelectrons upon UV irradiation. Those
materials having a lower value of photoelectrically working
function are preferred. In view of effects and economical
efficiency, any of Ba, Sr, Ca, Y, Gd, La, Ce, Nd, Th, Pr, Be, Zr,
Fe, Ni, Zn, Cu, Ag, Pt, Cd, Pb, Al, C, Mg, Au, In, Bi, Nb, Si, Ti,
Ta, U, B, Bu, Sn and P as well as compounds, alloys and mixtures
thereof can be preferably used alone or in combination of two or
more of them. Physical composite materials such as amalgam can be
used as well.
[0121] Suitable compounds include oxides, borides and carbides.
Examples of oxides are BaO/SrO, CaO, Y.sub.2O.sub.5,
Gd.sub.2O.sub.3, Nd.sub.2O.sub.3, ThO.sub.2, ZrO.sub.2,
Fe.sub.2O.sub.3, ZnO, CuO, Ag.sub.2O, La.sub.2O.sub.3, PtO, PbO,
Al.sub.2O.sub.3, MgO, In.sub.2O.sub.3, BiO, NbO, BeO and the like.
Examples of borides are YB.sub.6, GdB.sub.6, LaB.sub.6, NdB.sub.6,
CeB.sub.6, BuB.sub.6, PrB.sub.6, PrB.sub.6, ZrB.sub.2 and the like.
Examples of carbides are UC, ZrC, TaC, TiC, NbC, WC and the
like.
[0122] Alloys which can be used are brass, bronze, phosphor bronze,
an alloy of Ag with Mg (2 to 20 wt % of Mg), an alloy of Cu with Be
(1 to 10 wt % of Be) and an alloy of Ba with Al. Ag--Mg alloy,
Cu--Be alloy and Ba--Al alloy as mentioned above are preferred.
Oxides can be obtained also by heating only the surface,of a metal
in the air or by oxidizing it with a chemical agent.
[0123] Alternatively, an oxide layer having a good long-term
stability may be formed on the surface of a metal alloy by heating
the metal alloy prior to use. By way of an example, a surface oxide
layer can be formed by heating a Mg-Ag alloy in steam at a
temperature of 300.degree. C. to 400.degree. C. The obtained oxide
thin layer exhibits a good long-term stability.
[0124] Photoelectron-emitting materials having a multi-layer
construction as previously proposed by the present inventors can
also be used. The disclosure about photoelectron-emitting materials
described in the Laid Open Japanese Patent Application No.
Hei-1-155857 is herein incorporated by reference. Alternatively, a
photoelectron-emitting substance can be fabricated by forming a
thin layer carried on a suitable matrix. For example, a thin layer
of Au carried on a glass matrix may be used.
[0125] These materials can be used in any shape of a plate, a
pleat, a curved surface, a net and the like. Shapes having a larger
UV-exposed area and a larger air-contacting area are preferred.
[0126] Photoelectrons can be effectively emitted from a
photoelectron-emitting material by suitably applying a reflective
surface or a curved reflective surface, as previously proposed by
the present inventors. The disclosure about photoelectron-emitting
materials described in the Japanese Patent Publication No.
Hei-6-34941 is herein incorporated by reference. Alternatively, an
integral photoelectron-emitting device may be formed by coating an
UV lamp with a photoelectron-emitting material as stated below. The
disclosure about photoelectron-emitting devices described in the
Laid Open Japanese Patent Application No. Hei-4-24354 is herein
incorporated by reference. Shapes of photoelectron-emitting
materials and those of reflective surfaces may be varied depending
on shapes and constructions of a device or desired efficiency and
can be determined as appropriate.
[0127] Any ultraviolet radiation may be used, provided that it can
irradiate a photoelectron-emitting material so as to emit
photoelectrons. A mercury lamp, a hydrogen discharge tube, a xenon
discharge tube, a Lyman discharge tube, etc. are generally
suitable. Those having a bactericidal (sterilizing) action at the
same time are preferred depending on fields of application. Types
of ultraviolet rays can be selected as appropriate depending on
fields of application, operation details, purposes of use, economic
efficiency and the like. In the biological field for example, the
combined use of far infrared rays is preferred in view of
bactericidal action and efficiency. A bactericidal lamp (main
wavelength of 254 nm) is preferable, because the electrically
charging action of the present invention is added with a
bactericidal action. Any UV source can be used, provided that it
can emit an ultraviolet radiation. The UV source can be selected as
appropriate depending on fields of application, shapes and sizes of
a device, effects and economical efficiency.
[0128] By irradiating the photoelectron-emitting material with an
ultraviolet radiation in an electrical field, photoelectrons can
efficiently supply microparticles with an electrical charge.
Disclosures about electrical charging in an electrical field
described in the Laid Open Japanese Patent Applications Nos.
Sho-61-178050 and Sho-62-244459 as well as Laid Open Japanese
Patent Application No. Hei-1-120563 are herein incorporated by
reference. In the present invention, an electrical field may range
from 0.1 V/cm to 5 kV/cm and the suitable intensity thereof can be
determined as appropriate based on the results of preliminary
experiments and examinations.
[0129] Since photoelectrons can electrically charge even extremely
minute microparticles (e.g. those having a particle size of <0.1
.mu.m) with a high efficiency, microparticles can be trapped and
removed efficiently. Prior to electrical charging, microparticles
may be made to grow to a larger particle diameter. A method for
growing and charging microparticles electrically has been
previously proposed by the present inventors (the Japanese Patent
Application No. Hei-1-120564) and can be suitably used for
electrically charging extremely minute microparticles as desired,
depending on fields of application.
[0130] Any trapping material can be used, provided that it can trap
the electrically charged microparticles. Dust-collecting plates
(dust-collecting electrodes) and electrostatic filter systems are
of general use, but an effective trapping material can also be
constructed in such a way that the trapping section itself forms an
electrode made of a woolen material such as steel wool or tungsten
wool (woolen electrode material). Electret materials can be
suitably used, too. An ion exchange filter (fiber) that has been
previously proposed by the present inventors may be effective
depending on fields of application. Ion exchange filters can trap
coexistent gaseous contaminants and odorous gases that are
difficult to be trapped by the present invention and hence the use
of ion exchange filters is preferred depending on fields of
application. These trapping materials may be used alone or in
combination of two or more of them as appropriate, depending on
fields of application, sizes and shapes of a device, economic
efficiency and the like.
[0131] An ion exchange material refers to a support having the
surface attached by an ion exchanger or an ion exchange group. Ion
exchangers include cationic ion exchangers and anionic ion
exchangers, a combination of both being preferred. Ion exchange
groups include cationic ion exchange groups and anionic ion
exchange groups, a combination of both being preferred.
[0132] Ion exchange materials are preferably ion exchange fibers
having a support composed of a fiber. Fibers which can be used are
natural fibers, synthetic fibers and the mixtures thereof.
[0133] Ion exchangers are now be described mainly with reference to
ion exchange fibers. Ion exchangers may be supported on a fibrous
support directly or on a support composed of a woven, knitted or
filled fiber. Any form of fibers can be used, provided that ion
exchangers supported on a fiber can be finally obtained.
[0134] Preferable methods for fabricating an ion exchange fiber
suitable for use in the present invention are those that imply a
graft polymerization, in particular radiation graft polymerization.
This is because these methods can make use of starting materials
having various properties and sizes.
[0135] Natural fibers which can be used include wool, silk etc. and
synthetic fibers which can be used include hydrocarbon
polymer-based fibers, fluorine-containing polymer-based fibers or
polyvinyl alcohol, polyamide, polyester, polyacrilonitrile,
cellulose, cellulose acetate, etc.
[0136] Hydrocarbon polymers which can be used include: aliphatic
polymers such as polyethylene, polypropylene, polybutylene and
polybutene; aromatic polymers such as polystyrene and
poly-.alpha.-methylstyrene; cycloaliphatic polymers such as
polyvinyl cyclohexane; or copolymers thereof. Fluorine-containing
polymers which can be used include polyethylene tetrafluoride,
polyvinylidene fluoride, ethylene-ethylene tetrafluoride copolymer,
ethylene tetrafluoride-propylene hexafluoride copolymer, vinylidene
fluoride-propylene hexafluoride copolymer and the like.
[0137] Any material can be used as a support, provided that it: has
a large contact area with a gas stream; is shaped to have a
diminished resistance; is readily to be grafted; has a good
mechanical strength; is less likely to produce and fall waste
fiber; and is less influenced by heat. A suitable support material
can be selected depending on purposes of use, economical efficiency
and effects, but generally a support is made of polyethylene, and
most preferably made of polyethylene or a composite of polyethylene
and polypropylene.
[0138] Various cationic ion exchangers and anionic ion exchangers
can be used as an ion exchanger without particularly being limited.
Examples which can be mentioned are cationic ion exchangers that
contain a cationic ion exchange group such as a carboxyl,
sulfonate, phosphate or phenolic group as well as anionic ion
exchangers that contain a cationic ion exchange group such as any
of primary to tertiary amino groups or a quaternary ammonium group
or ion exchangers having both of the aforesaid cationic and anionic
ion exchange groups.
[0139] More precisely, fibrous ion exchangers having a cationic or
anionic ion exchange group can be obtained by graft polymerizing on
the aforesaid fiber a styrene compound such as acrylic acid,
methacryric acid, vinylbenzene-sulfonic acid, styrene,
halomethylstyrene, acyloxystyrene, hydroxystyrene or aminostyrene,
or, vinyl pyridine, 2-methyl-5-vinylpyridine,
2-methyl-5-vinylimidazole or acryronitrile followed by reacting
with sulfuric acid, chlorsulfonic acid or sulfonic acid as
required. Optionally, these monomers may be grafted on a fiber in
the presence of a monomer having two or more double bonds such as
divinyl benzene, trivinyl benzene, butadiene, ethylene glycol,
divinyl ether, ethylene glycol dimethacrylate and the like.
[0140] Ion exchange fibers can be fabricated in the manner as
described above. Ion exchange fibers have a diameter of 1 to 1,000
.mu.m, preferably 5 to 200 .mu.m and the diameter can be
appropriately selected depending on types and uses of a fiber.
[0141] The way of using cationic ion exchange groups and anionic
ion exchange groups in these ion exchange fibers can be determined
depending on the kinds and the concentrations of the components to
be removed in a gas to be purified. By analyzing the gas previously
and estimating the components to be removed, adequate types and
amounts of ion exchange fibers can be selected. More precisely,
when an alkaline gas is to be removed, fibers having cationic ion
exchange groups (cation exchangers) are suitable, whereas, when an
acidic gas is to be removed, fibers having anionic ion exchange
groups (anion exchangers) are suitable. And, when both an alkaline
and acidic gases are to be removed, both of anionic ion exchange
groups and cationic ion exchange groups may be used.
[0142] An effective way of flowing a gas through an ion exchange
fiber is to generate a gas stream perpendicular to a filter made of
the ion exchange fiber.
[0143] The flow rate of a gas passing through an ion exchange fiber
can be suitably determined by conducting preliminary experiments.
Since having a high removal rate, this fiber can be used generally
at a SV of the order of 1,000 to 100,000 (h.sup.-1). Ion exchange
fibers that are produced by a radiation graft polymerization, as
previously proposed by the present inventors, can be suitably used
with a particularly high efficiency (Japanese Patent Publications
Nos. Hei-5-9123, Hei-5-67325, Hei-5-43422 and Hei-6-24626).
[0144] Ion exchange fibers are effective in trapping of ionic
matter (constituents) and hence can efficiently trap and remove the
ionic matter intended by the present invention.
[0145] In particular, ion exchange filters (fibers) that are
fabricated by radiation graft polymerization are practicably
effective, since the radiation can reach deep in a support
uniformly and the ion exchanger (anion and/or cationic exchanger)
can firmly attach over a large area thereof (at a high density) so
that the exchange volume is made larger and hence ionic matter in a
low concentration can be removed at a high rate and with high
efficiency.
[0146] Fabrication process using a radiation graft polymerization
is advantageous in that: the polymerization can be conducted on a
support having the shape near to that of finished product; it can
be conducted at room temperature; it can be conducted in vapor
phase; it can realize a larger grafting ratio; and it can give an
adsorption filter having a low impurity content.
[0147] As the result, following characteristics can be
obtained:
[0148] (1) Ion exchange fibers made by radiation graft
polymerization can exhibit a higher adsorption rate and a larger
adsorption amount, since an ion exchanger (adsorptive part) is
added thereto more uniformly and more abundantly (a higher addition
density).
[0149] (2) Pressure loss can be diminished.
[0150] A means for trapping and removing microparticles may be used
alone or in combination of two or more, which can be suitably
selected on the basis of preliminary experiments, depending on the
properties of the produced microparticles.
[0151] Schemes for producing microparticles (irradiation source,
way of producing microparticles, conditions and the like) and for
trapping and removing microparticles (trapping means, conditions
and the like) can be suitably selected on the basis of preliminary
experiments, depending on various factors including application
field, gas type, apparatus design, production scale, performance
requirement, economical efficiency and the like.
[0152] The method of the present invention for removing
microparticles of contaminants can be carried out according to any
of the 6 schemes shown below by way of examples.
[0153] (1) A.fwdarw.B.fwdarw.C
[0154] (2) A.fwdarw.B.fwdarw.A.fwdarw.C
[0155] (3) A.fwdarw.C.fwdarw.A.fwdarw.B
[0156] (4) A.fwdarw.C.fwdarw.B
[0157] (5) A.fwdarw.B.fwdarw.+C
[0158] (6) A+B.fwdarw.C
[0159] In the schemes above:
[0160] the member A represents a step for irradiating a gas with an
ultraviolet ray and/or a radiation ray; or a
microparticle-producing section having a source of an ultraviolet
ray and/or a radiation ray;
[0161] the member B represents a step for contacting the
microparticles of contaminants with a photocatalyst and for
irradiating the photocatalyst being kept in contact with the
microparticles; or a decomposition section having a photocatalyst
and a light source for irradiating the photocatalyst;
[0162] the member C represents a step for clearing the gas of the
microparticles of contaminants; or a removal section for removing
the microparticles of contaminants;
[0163] the member C isn't essential to the present invention; the
symbol .fwdarw. indicates a temporal sequence of the steps; or a
spatial sequence of the sections downstream; and
[0164] the symbol + indicates concurrence of 2 steps; or
integration of 2 sections.
[0165] Suitable schemes can be selected on the basis of preliminary
experiments and examinations, depending on fields of application,
types of apparatus, states of the gas to be treated, required
removal performances, economical efficiency and the like.
[0166] Generally speaking,
[0167] (a) when an apparatus is of a large size, the schemes are
preferred in the order of (2), (3)>(1), (4)>(5),
[0168] (b) when organic compounds are present in a concentration
higher than that of other gaseous contaminants, (1) or (2) is
preferred,
[0169] (c) when a gas contains a high concentration of constituents
having a possibility of becoming a catalytic poison to
photocatalysts during a long-term operation, such as
sulfur-containing compounds, (3) or (4) is preferred,
[0170] (d) when a high removal performance is required, (2) or (3)
is preferred: and
[0171] (e) when an apparatus is of a small size, (5) or (6) is
preferred.
[0172] The member C may precede the members A and B. This order is
preferred because acidic gases having a possibility of adversely
influencing on photocatalysts can be preliminarily eliminated. Such
acidic gases include SO.sub.2, NO, HCl and HF. Moreover, when
sulfur-containing compounds, such as sulfur oxides, hydrogen
sulfide, thiophene and thiols are present in a high concentration,
the compounds might act on photocatalysts as catalytic poison.
EXAMPLES
[0173] The present invention will now be described in more detail
by way of examples, which should not be construed as limiting the
scope of the present invention.
Example 1
[0174] FIG. 1 represents an embodiment of the apparatus of the
present invention applied to purify air for feeding an air knife in
a semiconductor production factory. A purification apparatus 12 of
the present invention is placed in a clean room 1 of class
1,000.
[0175] First, a device for feeding air 14 into the clean room 1 is
described. An air inlet pipe 2 is connected to a pre-filter 3 for
filtering off coarse solid particles present in the outside air.
The pre-filter 3 and an air outlet 4 of the clean room 1 are
connected to a fan 5 for feeding air into the clean room 1. The fan
5 is connected to an air-conditioner 6 for controlling the
temperature and the humidity of the air. The pipe leaving the
air-conditioner is divided into several branches, each branch being
connected to a HEPA filter 7 for removing solid microparticles. The
HEPA filter 7 is provided at the interface with the internal space
of the clean room 1, and preferably at the ceiling of the clean
room. The air outlet 4 is provided at the bottom of the clean room
1.
[0176] The outside air 2 to be fed into the clean room 1 is treated
first with the coarse filter 3 and the air-conditioner 6. The air
is then freed from dust with the HEPA filter 7 before entering into
the clean room 1, to become air 14 of class 1,000 containing an
extremely low concentration of organic compounds 30. In other
words, the organic compounds of an extremely low concentration
originating in organic materials (polymer materials) including cars
and plastics can be removed with neither coarse filter 3, nor
air-conditioner 6, nor HEPA filter 7, and hence are brought to
enter into the clean room 1. On the other hand, the structural
components of the clean room 1 are evolving organic compounds in
the clean room 1. In consequence, the air in the clean room 1 has a
concentration of organic compounds higher than that of the outside
air. The air 14 in the clean room 1 has a concentration of organic
compounds of 0.8 to 1.2 ppm in terms of non-methane hydrocarbons
taken as an example.
[0177] Preferably, the purification apparatus 12 of the present
invention is horizontally disposed. As the result, the air 14 can
flow horizontally through the apparatus 12. Preferably, an air
knife device 29 is provided with an air inlet in the direction of
exhaust air discharged from an air outlet of the purification
apparatus 12. And preferably, the air outlet 4 of the clean room 1
is provided in the direction of exhaust air discharged from the air
outlet of the air knife 29.
[0178] As shown in FIG. 1, the air 14 is fed through the HEPA
filter 7 into the clean room 1 downwardly from the top thereof.
Then, while passing through the purification apparatus 12
horizontally, the air 14 is freed from dust and is purified to
become clean air 28 in which organic compounds had been decomposed.
The resultant clean air 28 is fed into the air knife 29 for
cleaning a wafer. Exhaust air of the air knife 29 is drawn out of
the air outlet 4 provided at the lower part of the clean room.
[0179] As shown in FIG. 1, the purification apparatus 12 is
provided successively with a coarse filter 25a for dust-removing, a
microparticle-producing section 8, a decomposition section 26, a
coarse filter 25b and an ozone-decomposing section 27.
[0180] Preferably, the purification apparatus 12 has the coarse
filter 25a. Dust that is eventually present in the clean room 1
tends to contaminate and deteriorate a photocatalyst 32. The coarse
filter 25a are useful for removing such dust.
[0181] The microparticle-producing section 8 consists of a housing
and an UV lamp 15 received therein. The UV lamp is a low-pressure
mercury lamp 15, for example.
[0182] Organic compounds 30 in the air 14 have a possibility of
increasing the contact angle by depositing onto a substrate such as
a wafer. But, when irradiating the air 14 with the UV radiation
emitted by the UV lamp 15, these organic compounds 30 can be
transformed into particulate matter 16. Upon exposure to the UV
radiation, gaseous oxygen present in the air also is transformed
into gaseous ozone.
[0183] The decomposition section 26 consists of an UV lamp 31 and a
pair of photocatalysts 32 placed opposite each other on the either
side of the UV lamp in the air-flowing direction. The photocatalyst
32 shown in FIG. 1 is composed of a honeycomb structure provided
with the partitions defining two or more through-holes and a
catalytically active component in the form of particles. The
catalytically active component is made of titanium dioxide in the
form of particles that are coated on the surface of the partitions
of the honeycomb structure. Preferably, the honeycomb structure has
the length in the direction parallel to the partitions shorter than
that in the radial direction. This is to avoid the interception of
UV radiation by the partitions.
[0184] The air which contains organic compounds passes through the
through-holes of the honeycomb structure and are introduced into
the decomposition section 26. While the air 14 is passing through
the holes of the honeycomb structure of the photocatalyst 32,
particulate matter 16 contained in the air 14 is brought into
contact with the catalytically active components in the form of
particles present on the surface of the through-holes of the
honeycomb structure. Since the photocatalyst 32 is activated by
irradiation with the UV lamp 31, particulate matter is decomposed
under catalytic action. Ultimately, organic compounds are reduced
to a level of not more than 0.2 ppm, preferably 0.1 ppm in terms of
non-methane organic compounds taken as an indicator. Organic
compounds having a high molecular weight or active organic
compounds that are likely to increase the contact angle on a wafer
are decomposed into organic compounds of a low molecular weight
that aren't likely to increase the contact angle, or carbon dioxide
and water.
[0185] The dust-removing filter 25b consists of an ULPA filter, for
example. The air of class 1,000 in the clean room 1 can be cleared
of microparticles with the ULPA filter to become class 10 or less.
The dust-removing filter 25b can also efficiently trap
microparticles escaped at or near the microparticle-producing
section 8 in case of emergency. Preferably, the ozone-decomposing
material 27 has a honeycomb shape provided with the partitions
defining at least 2 through-holes. Ozone that was generated upon
irradiation with the UV lamp 15 can be decomposed to a level of not
more than 0.01 ppm by means of the photocatalyst 32 and the
ozone-decomposing material 27. Since generation of ozone cannot be
ignored in semiconductor production factories, ozone is decomposed
to a level equal to or less than that in natural air through the
two-step decomposition.
Example 2
[0186] Example 2 is described with reference to FIG. 2.
[0187] As shown in FIG. 2, a wafer stocker 36 for storing wafers is
disposed in the clean room 1 of class 1,000 and a purification
apparatus 12 is disposed in the stocker 36. The purification
apparatus 12 comprises a microparticle-producing section 8 for
transforming organic compounds into microparticles by UV
irradiation, a decomposition section 26 for decomposing the
resultant microparticles of organic compounds by the action of a
photocatalyst and an ozone-decomposing section 27. The air 14 in
the stocker 36 containing organic compounds 30 is treated with the
purification apparatus 12 of the present invention to become clean
air 28. Organic compounds in this air 28 are decomposed to a level
of not more than 0.01 ppm, at which level the organic compounds can
avoid increasing the contact angle on the surface of a wafer.
[0188] This example will be described in more detail below.
[0189] The air 14 in the stocker 36 is first irradiated with an UV
lamp 15 (low-pressure mercury lamp) so that organic compounds 30
contained therein are transformed into particulate matter 16. This
particulate matter 16 are made to deposit onto the surfaces of the
photocatalyst 32 (carried on and fixed to the surrounding wall
surfaces and the glass rods) which had been irradiated with an UV
lamp 31 (bactericidal lamp). The deposited particulate matter is
decomposed and removed through photocatalysis to afford a clean air
28.
[0190] As shown in FIG. 2, the photocatalyst 32 is composed of at
least 2 glass rods and titanium oxide in the form of particles
applied onto the surface of each glass rod. This photocatalyst is
hanging. The photocatalyst is further applied on the wall surfaces.
More particularly, titanium oxide in the form of particles
dispersed in a suitable solvent are applied on the wall
surfaces.
[0191] By opening the wafer stocker 36, the air of class 1,000 in
the clean room enters into the wafer stocker 36. This air contains
organic compounds in a concentration of 0.8 to 1.5 ppm in terms of
non-methane organic compounds. The air in the form of air streams
33a, 33b and 33c containing these organic compounds are made to
contact with the photocatalyst 32, and consequently microparticles
of organic compounds having a high molecular weight or those of
active organic compounds can be effectively decomposed into carbon
dioxide and water. Organic compounds are decomposed to a level
below 0.1 ppm in terms of non-methane organic compounds taken as an
indicator.
[0192] Irradiation with the UV lamps 15, 32 makes the temperature
elevate and gives rise to air convection. As a result of this air
convection, air flows upwardly from the bottom towards the top of
the apparatus 12, and outside of the apparatus, air streams 33a,
33b and 33c are generated.
[0193] Since the air stream inside the apparatus 12 gives rise to
Brownian movement on a molecular level, contaminants are brought
into collision with the glass rods or the wall surfaces and hence
microparticles of organic compounds are made into contact with the
photocatalyst. Since contaminants are easy to deposit onto a wafer,
contaminants can deposit also onto the photocatalyst.
[0194] In the manner as described above, the part for storing the
wafers 34 set in a wafer case 35 is effectively cleaned.
[0195] Ozone generated upon irradiation with the UV lamp 15 for
producing particles can be decomposed and removed to a level of not
more than 0.01 ppm by the action of the photocatalyst 32 and an
ozone-decomposing material 27 of a honeycomb shape.
[0196] In FIG. 2, reference numerals identical to those in FIG. 1
denote the same elements as those in FIG. 1.
[0197] Though the cases when a gas is composed of air are described
in the examples 1 and 2, the present invention can of course be
applied as well to the cases in which gas is another gas such as
nitrogen or argon that contains gaseous contaminants such as
organic compounds as impurities. The present invention can be
applied not only under atmospheric pressure, but also under an
increased or decreased pressure.
Example 3
[0198] FIG. 3 shows a purification apparatus 12 of a type different
from that shown in FIG. 2. As shown in FIG. 3, the order of the
ozone-decomposing material 27 and the decomposition section 26
provided with the photocatalyst are reversed so that the
ozone-decomposing material 27 is situated next to the
particle-producing section 8. The apparatus thus arranged has the
effects comparable to those of the apparatus shown in FIG. 2.
Example 4
[0199] A sample of gas as specified below was charged into a
stocker arranged as shown in FIG. 2, a wafer was placed therein,
and then the contact angle on the wafer, the concentrations of
non-methane organic compounds and ozone present in the stocker were
measured.
[0200] Experimental conditions:
[0201] (1) sample gas: air of class 10 in a semiconductor
production factory containing non-methane organic compound in a
concentration of 0.8 to 1.2 ppm;
[0202] (2) stocker volume: 30 liters;
[0203] (3) UV lamp for producing particles: low pressure mercury
lamp (184 nm);
[0204] (4) photocatalyst: titanium dioxide carried on a glass fiber
matrix by sol-gel process;
[0205] (5) light source: bactericidal lamp (254 nm) (for
irradiating a photocatalyst);
[0206] (6) ozone-decomposing material: composite oxide. catalyst,
MnO.sub.2/ZrO--C.
[0207] More precisely, the ozone-decomposing material is composed
of a honeycomb structure provided with the partitions defining at
least 2 through-holes and a manganese oxide coated on the surfaces
of the partitions of the honeycomb structure. The honeycomb
structure is substantially composed of a zirconium oxide and carbon
atoms. At least a portion of carbon atoms may be present in the
form of zirconium carbide. Manganese oxide has the shape of a
particle, for example. These particles are not always required to
cover all the surfaces of the partitions of the honeycomb
structure.
[0208] (7) wafer: a highly pure silicon wafer of 5-inch diameter
was cut into pieces of 1 cm.times.8 cm and placed in the
stocker;
[0209] (8) pretreatment of the wafer: washing with a detergent and
alcohol on a clean bench in the clean room, followed by UV/O.sub.3
cleaning;
[0210] (9) measurement of the contact angle: the contact angle was
measured with a CA-D type contact angle feeler manufactured by
Kyowa Kaimen Kagaku, Inc.;
[0211] (10) concentration of non-methane organic compounds: was
measured by gas chromatography (GC) method;
[0212] (11) ozone concentration: was measured with a
chemiluminescent ozone densitometer;
[0213] (12) opening of the stocker: the stocker was disposed in a
clean zone (class 10) of a semiconductor production factory and
open-close cycles were repeated 6 times per day.
[0214] Results:
[0215] (1) FIG. 4 shows the contact angle on a wafer as the
function of the number of days during which the wafer was stored in
a stocker.
[0216] In FIG. 4, the symbol .smallcircle. represents the values
obtained according to the present invention and the symbol
.circle-solid. represents the values obtained from a (comparative)
test in which the wafer was exposed to the air of class 10 in a
clean room. The symbol .dwnarw. means that the obtained value is
below the limit of detection.
[0217] (2) Concentrations of non-methane organic compounds and
ozone present in the stocker are shown in the Table 1.
1TABLE 1 storing period organic compound ozone (days) concentration
(ppm) concentration 1 <0.1 ppm <0.01 ppm 2 <0.1 ppm
<0.01 ppm 10 <0.1 ppm <0.01 ppm
[0218] Switching the UV lamp for producing particles generates a
ozone concentration of 15 to 20 ppm.
[0219] (3) In a stocker without the unit of the present invention,
a wafer was stored for 2 and 7 days. Then, the wafer was taken out
of the stocker, heated to desorb the organic compounds deposited
thereon. Analysis of the wafer by gas chromatography/mass
spectrometry (GC/MS) showed the presence of phthalic esters such as
DOP. The unit of the present invention was disposed in the stocker
and a wafer was treated and analyzed in the same way. Phthalic
esters such as DOP was undetectable.
Example 5
[0220] FIG. 5 shows an embodiment of the apparatus of the present
invention applied in a semiconductor production factory to purify
air for feeding an air knife. The purification apparatus 12 shown
in FIG. 5 is different from that shown in FIG. 1 in having further
a section C for removing particulate matter.
[0221] As shown in FIG. 5, reference numeral 1 represents a clean
room of class 100. The air 14 in the clean room 1 is treated with
the purification apparatus 12 of the present invention comprises a
section 8(A) for producing microparticles of organic compounds and
coexistent gaseous contaminants such as SO.sub.2 by UV irradiation,
a section 26(B) for decomposing the microparticles of organic
compounds with a photocatalyst, an ozone-decomposing section 27 and
the sections 9, 10(C) for trapping the microparticles of
particulate matter with photoelectrons. The air 14 having passed
through the purification apparatus 12 of the present invention
results in the clean air 28 which is free from dust as well as
organic compounds and coexistent gaseous contaminants. The clean
air 28 is fed to an air knife device 29 for cleaning a wafer
(substrate).
[0222] This embodiment is, as described above, arranged according
to the following scheme. section for producing microparticles by UV
irradiation (A).fwdarw.decomposition section for decomposing
microparticles of organic compounds with a photocatalyst
(B).fwdarw.removal (dust-removal) section for trapping and removing
particulate matter in the form of microparticles (C).
[0223] This embodiment is described hereinafter in more detail.
[0224] The outside air 2 is treated first with a coarse filter 3
and an air-conditioner 6 before entering into the clean room 1.
Then, the air is cleared of dust with a HEPA filter 7 at the inlet
of the clean room to become air 14 having a microparticle
concentration of class 100. In the air 14, there exist organic
compounds 30a together with gaseous contaminants 30b which are
composed of acidic gases including SOx, NOx, HCl, HF, etc. and
basic gases including NH.sub.3, amines, etc. Acidic and basic gases
contained in the outside air 2 are carried into the clean room 1 in
company with the outside air 2. Only a little amount of acidic and
basic gases generate in the clean room 1. The air 14 in the clean
room has an organic compound concentration of 0.8 to 1.2 ppm in
terms of non-methane organic compounds.
[0225] In the air 14 in the clean room 1, organic compounds 30a as
well as acidic and basic gases 30b are transformed into particulate
matter 16 upon irradiation of the UV lamp (low-pressure mercury
lamp) 15 (A).
[0226] Among the particulate matter 16, particles that are derived
from organic compounds 30a and basic gas tend to deposit onto the
surface of the photocatalyst (TiO.sub.2), and hence can deposit on
the surface of the photocatalyst 32 activated by irradiation with
the UV lamp 31 so as to be decomposed and removed through
photocatalysis (B).
[0227] On the other hand, among the particulate matter 16,
particles derived from acidic gas are difficult to decompose with
the photocatalyst 32, and consequently pass through the
photocatalytically decomposing section B into the section (C) for
trapping and removing with the aid of photoelectrons. The section
(C) is composed of an UV lamp 19, a photoelectron emitting material
20, an electrode (part 9 for supplying particulate matter with an
electrical charge) and a material 10 for trapping the electrically
charged particulate matters 10.
[0228] In the trapping and removing section (C), influent
particulate matter 16 are electrically charged with photoelectrons
(not shown) that are emitted from the photoelectron emitting
material 20, and the electrically charged particulate matter is
trapped and removed with the material 10. Photoelectrons can be
efficiently emitted by covering the UV lamp 19 with the
photoelectron emitting material 20 and forming an electrical field
of 50 V/cm between the material 20 and the electrode 21.
[0229] In this embodiment, air contaminated with organic compounds
is introduced into the decomposing section 26 (B) which is composed
of a photocatalyst 32 (formed by coating titanium oxide on the
surface of the partitions of a honeycomb-shaped ceramic matrix) and
an UV lamp (bactericidal lamp) 32. In this section, organic
compounds are decomposed to a level of not more than 0.2 ppm,
preferably 0.1 ppm in terms of non-methane organic compounds taken
as an indicator.
[0230] More precisely, organic compounds of a high molecular weight
or active organic compounds that are responsible for an increase in
the contact angle are decomposed into organic compounds of a low
molecular weight that avoid any increase in the contact angle, or
into carbon dioxide and water.
[0231] The resultant air has a concentration of acidic and basic
gases by an amount tenfold lower than that of the air 14 in the
clean room. In terms of SO.sub.2 taken as an indicator, an average
SO.sub.2 concentration which had been 0.001 ppm (10 ppb) in the
clean room 1 can be decreased to 1 ppb or less.
[0232] In a manner as described above, microparticles present in
the air 14 of class 100 in the clean room are, together with
particulate matter 16 originating from gaseous contaminants, are
electrically charged and trapped at the removal section (C) for
removing contaminants with photoelectrons. By virtue of the
apparatus of the present invention 12, the air 28 becomes an
extremely clean air which is superior to air of class 1 and are
free from organic compounds and coexistent gaseous
contaminants.
[0233] Ozone that is generated by irradiation of the UV lamp 15 for
producing particles can be decomposed to a level of not more than
0.01 ppm by means of the photocatalyst 32 and a honeycomb-shaped
ozone-decomposing material 27.
[0234] In other words, since effluent ozone should not be ignored
in semiconductor production factories, the ozone is decomposed to a
level equal to or less than that in natural air through the
two-step decomposition.
[0235] In FIG. 5, reference numeral 4 denotes an air outlet of the
clean room 1 and reference numeral 5 denotes a fan.
Example 6
[0236] FIG. 6 represents an another embodiment in which the
apparatus of the present invention is applied to purify air for
feeding an air knife in a clean room 1 of class 100 of the same
type as that in Example 5.
[0237] As shown in FIG. 6, the apparatus of this example is
different from that of Example 5 (FIG. 5) in that decomposition
section (B) for decomposing the microparticles of organic compounds
with a photocatalyst is followed by another microparticle-producing
section (A) using UV irradiation.
[0238] In other words, the apparatus of this example is arranged in
accordance with the scheme of A.fwdarw.B.fwdarw.A.fwdarw.C.
[0239] Owing to this arrangement, the apparatus of this example can
subject acidic gas to microparticle-producing step twice, thus
facilitating the removal thereof in the trapping section. The air
14 in the clean room is freed of gaseous contaminants more
efficiently than in Example 1. Gaseous contaminants are decreased
to one fiftieth or less of the concentration at the inlet.
[0240] In FIG. 6, reference numerals identical to those in FIG. 5
denote the same elements as those in FIG. 5.
Example 7
[0241] FIG. 7 represents another embodiment in which the apparatus
of the present invention is applied to purify air for feeding an
air knife in a clean room 1 of class 100 of the same type as that
in Example 5.
[0242] As shown in FIG. 7, the apparatus is arranged in accordance
with the scheme of A.fwdarw.C.fwdarw.A.fwdarw.B. This type of
apparatus is effective for treating air having a high concentration
of acidic or basic gases, hence suitable for use in acid washing or
alkali treating operation in a clean room.
[0243] As shown in FIG. 7, gaseous contaminants are transformed
into microparticles (A), and then the resultant microparticles of
particulate matter is trapped and removed (C), thus enabling to
decrease the concentrations of acidic and basic gases.
[0244] Then, gaseous contaminants such as organic compounds are
transformed into microparticles again (A), and then the resultant
microparticles originating in organic compounds and basic gas are
decomposed with a photocatalyst (B). Reference numeral 25b denotes
a HEPA filter which can act in case of emergency for collecting an
eventually present microparticles (particulate matter) in the upper
stream.
[0245] Since acidic or basic gases at high concentration can be
trapped and removed prior to reaching the photocatalytically
decomposing section (B), possible sources (e.g. acidic gas) of a
catalytic poison to the photocatalyst 32 can be removed, thus
allowing for a long-term stable operation.
[0246] By virtue of the apparatus of the present invention, organic
compounds are decomposed to a level of not more than 0.1 ppm in
terms of non-methane organic compounds taken as an indicator.
Operations as stated above makes gaseous contaminants increase to a
level of 100 to 500 ppb in terms of SO.sub.2 concentration, which
can be removed to a level of not more than 1 ppb owing to the
apparatus of the present invention.
[0247] In FIG. 7, reference numerals identical to those in FIG. 5
denote the same elements as those in FIG. 5.
Example 8
[0248] FIG. 8 represents another embodiment in which the apparatus
of the present invention is applied to purify air for feeding an
air knife in a clean room 1 of class 100 of the same type as that
in Example 5.
[0249] As shown in FIG. 8, the apparatus is arranged in accordance
with the scheme of A.fwdarw.C.fwdarw.B. This type of apparatus is
effective for treating air having a high concentration of acidic or
basic gases.
[0250] As shown in FIG. 8, gaseous contaminants are transformed
into microparticles (A), and then the resultant microparticles of
particulate matter is trapped and removed (C), thereby enabling to
decrease the concentrations of acidic and basic gases.
[0251] Then, microparticulate matter originating in organic
compounds and basic gas are decomposed with a photocatalyst (B).
Reference numeral 25b denotes a HEPA filter which can act in case
of emergency for collecting an eventually present microparticles
(particulate matter) in the upper stream.
[0252] Since acidic or basic gases at high concentration can be
trapped and removed prior to reaching the photocatalytically
decomposing section (B), possible sources (e.g. acidic gas) of a
catalytic poison to the photocatalyst 32 can be removed, thus
allowing for a long-term stable operation.
[0253] By virtue of the apparatus of the present invention, organic
compounds are decomposed to a level of not more than 0.1 ppm in
terms of non-methane organic compounds taken as an indicator.
Operations as stated above makes gaseous contaminants increase to a
level of 100 to 500 ppb in terms of SO.sub.2 concentration, which
can be decreased to reach a level of not more than 1 ppb owing to
the apparatus of the present invention.
[0254] In FIG. 8, reference numerals identical to those in FIG. 5
denote the same element as those in FIG. 5.
Example 9
[0255] FIG. 9 represents another embodiment in which the apparatus
of the present invention is applied to purify air for feeding an
air knife in a clean room 1 of class 100 of the same type as that
in Example 5.
[0256] As shown in FIG. 9, the apparatus 12 of the present
invention of Example 5 is modified in such a way that an integral
section (B+C) is formed by integrating the decomposition section
(B) for photocatalytically decomposing the microparticles of
organic compounds and basic gas with the removal section (C) for
trapping and removing acidic and basic gases with
photoelectrons.
[0257] In other words, the apparatus is arranged in accordance with
the scheme of A.fwdarw.B+C.
[0258] The air 14 in the clean room 1 is irradiated with an UV lamp
(low-pressure mercury lamp) 15 so as to transform organic compounds
30a as well as acidic and basic gases 30b into particulate matter
16 (A).
[0259] An integral section composed of the photocatalytical
decomposition section (B) and the trapping section (C) for trapping
microparticles of particulate matter comprises an UV lamp 31 (19),
a photoelectron-emitting material 20 coated on the surface of the
UV lamp 31 (19), an electrode 21, a photocatalyst 32 coated on the
surface of the electrode 21 and a trapping material 10 which is
placed downstream to the UV lamp 31 for trapping charged
particulate matter. The UV lamp 31 (19) is a bactericidal lamp and
has a dual function of irradiating the photoelectron-emitting
material 20 (for emission of photoelectrons) and the photocatalyst
32 (for induction of photocatalysis). The photoelectron-emitting
material 20 is coated over the UV lamp 31 (19). By forming an
electrical field of 50 V/cm between the photoelectron-emitting
material 20 and the electrode 21, photoelectrons can be efficiently
emitted towards the electrode 21. Among the particulate matter 16,
particles originating in the acidic and basic gases 30b can be
electrically charged with the photoelectrons emitted from the
photoelectron-emitting material 20. The resultant charged
particulate matter is trapped and removed with the trapping
material 10 for trapping the charged particulate matter.
[0260] Among the particulate matter 16, microparticles originating
in organic compounds 30a and basic gas can deposit on the surface
of the photocatalyst 32 which had been activated by irradiation of
the UV lamp 31, and subsequently they can be decomposed and removed
through photocatalysis (B).
[0261] In a method for removing microparticles in an electrical
field that comprises: irradiating a photoelectron-emitting material
with ultraviolet radiation so as to induce the emission of
photoelectrons; supplying microparticles with an electrical charge
by the photoelectrons; and trapping the charged microparticles
while forcing them to move by the action of the electrical field,
the effects of the photocatalyst can be further improved by
incorporating the photocatalyst into an electrode that forms the
electrical field (Japanese Patent Application No.
Hei-8-231290).
Example 10
[0262] Purification of the air in a wafer stocker (wafer receiving
stocker) 36 in a clean room of class 100 in a semiconductor
production factory is described with reference to a basic
arrangement shown in FIG. 10.
[0263] Organic compounds 30a and coexistent acidic and basic gases
30b including SO.sub.2 and NH.sub.3 contained in the air present in
the stocker 36 are treated by a purification unit 12 of the present
invention disposed in the stocker 36. The purification unit 12
comprises a microparticle-producing section 8 (A) for producing
microparticles of organic compounds and coexistent gaseous
contaminants by UV irradiation, a decomposition section 26 (B) for
decomposing the microparticles of organic compounds with a
photocatalyst, an ozone-decomposing section 27 and a trapping
section (C) composed of sections 9, 10 for trapping the
microparticles of particulate matter with photoelectrons.
[0264] The air 14 in the stocker 36 containing organic compounds
30a as well as coexistent gases 30b including acidic gas such as
SO.sub.2 and basic gas such as NH.sub.3 are treated in the unit 12
of the present invention to become clean air 28. This air 28 is
extremely clean and is superior to class 1 and has a concentration
of organic compounds below 0.01 ppm and concentrations of acidic
and basic gas respectively of below 1 ppm.
[0265] By placing the wafers 34 held on a wafer case 35 into the
wafer stocker 36 of this embodiment and exposing the wafers to the
extremely clean air as mentioned above, the wafers can be
maintained without the increase in the contact angle (no increase
in the contact angle occurs on the wafer 34 received in the part D
of the wafer stocker 36) and the change of the electrical
properties.
[0266] This embodiment is arranged in accordance with the scheme
below: microparticle-producing section by UV irradiation
(A).fwdarw.decompositio- n section for decomposing the
microparticles of organic compounds with a photocatalyst
(B).fwdarw.removal (dust-removal) section for trapping the
microparticles of particulate matter (C).
[0267] This embodiment is described in more detail.
[0268] By opening the stocker 36 disposed in the clean room 1, the
air 14 of class 100 in the clean room 1 flows into the stocker 36.
This air 14 is contaminated with organic compounds in a
concentration of 0.8 to 1.5 ppm in terms of non-methane
hydrocarbons as well as with acidic gas such as SO.sub.2 and basic
gas such as NH.sub.3. SO.sub.2 is present in a concentration of 10
to 15 ppb and NH.sub.3 is present in a concentration of 30 to 50
ppb.
[0269] When organic compounds 30a in air in the wafer stocker 36
are deposited onto a substrate such as a wafer, the organic
compounds 30a increase the contact angle. When acidic and basic
gases 30b in air in the wafer stocker 36 are deposited onto a
substrate, the acidic and basic gases 30b may adversely influence
the electrical properties of the wafer. These organic compounds 30a
and acidic and basic gases 30b can be transformed into particulate
matter 16 upon irradiation with an UV lamp (low-pressure mercury
lamp) (A).
[0270] Among particulate matter 16, particles originating in the
organic compounds 30a and the basic gas which can be readily
adsorbed onto an adsorptive surface such as that of photocatalytic
material (TiO.sub.2) may deposit (be adsorbed) on the surface of
the photocatalyst 32 which has been activated by irradiation with
an UV lamp 31, and then be decomposed and removed through
photocatalysis (B).
[0271] On the other hand, among particulate matter 16, particles
originating in acidic gas which cannot be readily decomposed with
the photocatalyst 32 pass through the photocatalytically
decomposing section B into the trapping and removal section (C)
that follows. The section (C) is composed of an UV lamp 19, a
photoelectron-emitting material 20, an electrode 21 (a charging
part for supplying particulate matter with an electrical charge)
and a material for trapping the charged particulate matter 10 with
photoelectrons.
[0272] In the section (C) for trapping and removing with
photoelectrons, inflow particulate matter 16 can be electrically
charged with photoelectrons (not shown) emitted from the
photoelectron-emitting material 20 and the resultant charged
particulate matter can be trapped and removed with a trapping
material 20. The photoelectron-emitting material 20 is coated on
the UV lamp 19. By forming an electrical field of 50 V/cm between
the photoelectron-emitting material 20 and the electrode 21,
photoelectrons can be efficiently emitted.
[0273] As stated above, particles 16 originating in organic
compounds 30a and basic gas can deposit on the surface of the
photocatalyst 32 (carried and fixed on the surfaces of surrounding
walls and glass rods) that had irradiated with the UV lamp 31
(bactericidal lamp) and can be decomposed and removed efficiently
through photocatalysis. In consequence, organic compounds can be
decomposed to a concentration of not more than 0.2 ppm, more
preferably 0.1 ppm in terms of non-methane organic compounds taken
as an indicator.
[0274] In other words, organic compounds having a high molecular
weight and active organic compounds that are responsible for the
increase in the contact angle are decomposed to organic compounds
having a low molecular weight that can avoid increasing the contact
angle or carbon dioxide and water, depending upon the types of the
compounds.
[0275] Further, acidic and basic gases present in the air
prevailing in the wafer stocker 36 are decreased to the one tenth
of the initial concentration. In terms of SO.sub.2 and NH.sub.3,
the concentration of each of SO.sub.2 and NH.sub.3 in this stocker
is decreased to a value of not more than 1 ppb.
[0276] Microparticles present in the air 14 of class 100 in the
wafer stocker 36 are, together with particulate matter 16
originating in gaseous contaminants as stated above, can be trapped
and removed in the section (C) for trapping and removing with
photoelectrons in the same manner as described above.
[0277] The air 20 which was obtained by using the purification
apparatus 12 of the present invention is freed from organic
compounds and coexistent gaseous contaminants, resulting in an
extremely clean air superior to class 1.
[0278] The air 14 in the wafer stocker 34, while flowing in the
form of the air streams 28, 33a, 33b and 14, can be effectively
treated during successive passages through the
microparticle-producing section (A), the decomposition section for
decomposing the microparticles of organic compounds with a
photocatalyst (B) and the section for trapping and removing the
particulate matter in the form of microparticles (C). These air
streams 28, 33a, 33b and 14 are generated as the result of the
difference between the temperatures above and below of the
purification unit 12, this difference being caused by irradiation
with the UV lamp 15, 31 and 19. In the manner as described above,
the part D storing the wafer 34 set in a wafer case 35 can be
effectively cleaned.
[0279] By exposing the surfaces of the wafer 34 to an extremely
clean air as stated above, the wafer surfaces can escape from
contamination. In consequence, the contact angle on a wafer can
avoid increasing. This avoidance of increase in the contact angle
has the effects of reinforcing the adhesion of a thin film formed
on a wafer substrate (Air Purification, Vol. 33, No. 1, 16-21,
1995).
[0280] Ozone generated by irradiation with the UV lamp 15 for
producing microparticles can be decomposed and removed to a level
of not more than 0.01 ppm by the action of the photocatalyst 32 as
stated above and a honeycomb-shaped ozone-decomposing material
27.
[0281] Though this example describes about the case in which gas is
air, the present invention can of course be applied as well to the
cases in which gas is another gas such as nitrogen or argon which
is contaminated with organic compounds and gaseous contaminants
including acidic and basic gases. The present invention can be
applied not only under atmospheric pressure, but also under an
increased or decreased pressure.
[0282] In FIG. 10, reference numerals identical to those in FIG. 5
denote the same elements as those in FIG. 5.
Example 11
[0283] FIG. 11 shows a purification apparatus 12 of a type
different from that of Example 10 shown in FIG. 10.
[0284] As shown in FIG. 11, an ozone-decomposing material 27 and a
photocatalytically decomposing section 26 are disposed in the
reversed order so that the ozone-decomposing material 27 comes next
to the particle-producing section 8 (A). The arrangement shown in
FIG. 10 can exhibit the effects comparable to those of the
apparatus shown in FIG. 10. In FIG. 11, reference numerals
identical to those in FIG. 10 denote the same elements as those in
FIG. 10.
Example 12
[0285] FIG. 12 represents a purification unit 12 of a type
different from that of Example 10 shown in FIG. 10.
[0286] The unit 12 shown in FIG. 12 is modified in such a way that
the decomposition section (B) for decomposing the microparticles of
organic compounds with a photocatalyst is integrated with the
section (C) for trapping and removing the acidic and basic gases in
the form of microparticles with photoelectrons.
[0287] In other words, the unit is arranged in accordance with the
scheme of A.fwdarw.B+C.
[0288] Organic compounds 30a as well as acidic and basic gases 30b
are transformed into particulate matter 16 (A) by irradiation with
an UV lamp (low-pressure mercury lamp) 15.
[0289] An integral section composed of photocatalytically
decomposing section (B) and the section (C) for trapping the
microparticles of particulate matter comprise an UV lamp 31 (19), a
photoelectron-emitting material 20, an electrode 21 carrying a
photocatalyst 32 and a material 10 for trapping the charged
particulate matter.
[0290] Among the particulate matter 16, particles originating in
organic compounds 30a and basic gas can deposit (be adsorbed) on
the surface of the photocatalyst which had been activated by
irradiation with the UV lamp 31, and hence can be decomposed and
removed through photocatalysis (B).
[0291] In a method for removing microparticles by irradiating a
photoelectron-emitting material with an ultraviolet radiation in an
electrical field, incorporation of a photocatalyst into an
electrode forming the electrical field may improve the action of
the photocatalyst (Japanese Patent Application No.
Hei-8-231290).
[0292] On the other hand, among the particulate matter 16,
particles originating in acidic and basic gases 30b can be
electrically charged with photoelectrons (not shown) emitted from
the photoelectron-emitting material 20. The resultant charged
particulate matter can be removed by the material 10 for trapping
the charged particulate matter. The photoelectron-emitting material
20 is coated onto the UV lamp 19. By forming an electrical field of
50 V/cm between the photoelectron-emitting material 20 and the
electrode 35, photoelectrons can be efficiently emitted. Charged
particulate matter can be removed by the trapping material 10.
[0293] In this manner, the air 14 in the wafer stocker 36 is
treated with the unit 12 of the present invention, resulting in the
clean air 28. This air 20 is an extremely clean air superior to
class 1 in which organic compounds have been decomposed and removed
to be a level of not more than 0.01 ppm and the acidic and basic
gases 30 such as SO.sub.2 and NH.sub.3 that had been present in the
air 14 in the clean room have been removed to be a level of not
more than 1 ppb.
[0294] In FIG. 12, reference numerals identical to those in FIG. 10
denote the same element as those in FIG. 10.
Example 13
[0295] FIGS. 13 and 14 represents a purification unit 12 of a type
different from that of Example 10 shown in FIG. 10.
[0296] The unit 12 shown in the FIGS. 13 and 14 is modified in such
a way that the microparticle-producing section 8 (A) is integrated
with the decomposition section 26 (B) for decomposing the
microparticles of organic compounds by means of a
photocatalyst.
[0297] In other words, the apparatus of this example is arranged in
accordance with the scheme of A+B.fwdarw.C.
[0298] The photoelectron-emitting material 20 is formed on the
surface of wall-forming materials by Au plating.
[0299] In FIG. 13, the photocatalyst 32 is coated on the surface of
a wall. The photocatalyst 32 is activated to perform photocatalytic
action by irradiation with the UV lamp 15 for producing
particles.
[0300] In FIG. 14, the photocatalyst 32 is provided on or near the
surface of an ozone-decomposing material 27. The photocatalyst 32
is activated to perform photocatalytic action by irradiation with
the UV lamp 15 for producing particles.
[0301] In FIGS. 13 and 14, reference numerals identical to those in
FIG. 10 denote the same element as those in FIG. 10.
Example 14
[0302] A sample of gas as specified below was charged into a
stocker arranged as shown in FIG. 10, a wafer was placed therein,
and then the contact angle on the wafer, the concentrations of
non-methane organic compounds, SO.sub.2, HCl, NH.sub.3 and ozone
present in the stocker were measured.
[0303] Experimental conditions
[0304] (1) sample gas: air of class 10 in a semiconductor
production factory;
[0305] concentration of non-methane organic compound: 0.8 to 1.2
ppm;
[0306] SO.sub.2 concentration: 10 to 30 ppb;
[0307] HCl concentration: 3 to 5 ppb;
[0308] NH.sub.3 concentration: 10 to 20 ppb
[0309] (2) stocker volume: 80 liters;
[0310] (3) UV lamp for producing particles: low-pressure mercury
lamp (184 nm);
[0311] (4) photocatalyst: titanium dioxide carried on a glass fiber
plate by sol-gel process;
[0312] (5) light source: bactericidal lamp (254 nm) (for
irradiating a photocatalyst and for emitting photoelectrons);
[0313] (6) photoelectron-emitting material: an Au layer having a
thickness of 8 nm plated on the above-mentioned bactericidal lamp
p1 (7) electrode materials and electrical field:
[0314] for charging: Cu--Zn, 50 V/cm;
[0315] for trapping particulate matter: Cu--Zn, 500 V/cm
[0316] (8) ozone-decomposing material: honeycomb-shaped composite
oxide catalyst, MnO.sub.2/ZrO--C;
[0317] (9) wafer: a highly pure 5 inch silicon wafer was cut into
pieces of 1 cm.times.8 cm and placed in the stocker;
[0318] (10) pretreatment of the wafer: washing with a detergent and
alcohol on a clean bench in the clean room, followed by UV/O.sub.3
cleaning;
[0319] (11) measurement of the contact angle: the contact angle was
measured with a CA-D type contact angle feeler manufactured by
Kyowa Kaimen Kayak, Inc.;
[0320] (12) concentration of non-methane organic compound: was
measured by gas chromatography (GC) method;
[0321] (13) SO.sub.2 concentration: was measured by solution
conductivity method;
[0322] (14) HCl concentration: was measured by absorption liquid
method;
[0323] (15) NH.sub.3 concentration: was measured by
chemiluminescent (LCD) method and liquid absorption method;
[0324] (16) ozone concentration: was measured with a
chemiluminescent ozone densitometer;
[0325] (12) opening of the stocker: the stocker was disposed in a
clean zone (class 10) of a semiconductor production factory and the
open-close cycles were repeated 6 times per day.
[0326] Results:
[0327] (1) FIG. 15 shows the contact angle on a wafer as the
function of the number of days during which the wafer was stored in
the stocker.
[0328] In FIG. 15, the symbol .smallcircle. represents the values
obtained according to the present invention and the symbol
.circle-solid.represent- s the values obtained from a (comparative)
test in which the wafer was exposed to the air of class 10 in a
clean room. The symbol .dwnarw. means that the obtained value is
below the limit of detection.
[0329] (2) Concentrations of non-methane organic compound,
SO.sub.2, HCl, NH.sub.3 and ozone present in the stocker are shown
in the Table 2.
2TABLE 2 storage organic period compound SO.sub.2 conc. HCl conc.
NH.sub.3 conc. ozone conc. (days) conc. (ppm) (ppb) (ppb) (ppb)
(ppb) 1 <0.1 <1 <0.5 <1 <0.1 2 <0.1 <1 <0.5
<1 <0.1 10 <0.1 <1 <0.5 <1 <0.1
[0330] Besides, the concentrations of organic compounds and
SO.sub.2 (after 1 day of storage) were determined in a similar
manner, except that the removal section (C) for trapping with
photoelectrons was omitted. The obtained values were
respectively<0.1 ppm and 10 to 25 ppb.
[0331] The concentration of the microparticles in the stocker was
undetectable (after 30 minutes, 1 day and 10 days of storage) with
a particle counter. Therefore, the air in the stocker had a purity
superior to class 1.
[0332] Switching the UV lamp for producing particles generates the
ozone concentration of 15 to 20 ppm.
[0333] (3) In a stocker without the unit of the present invention,
a wafer was stored for 2 or 7 days, Then, the wafer was taken out
of the stocker, heated to desorb the organic compounds deposited
thereon. Analysis of the wafer by gas chromatography/mass
spectrometry (GC/MS) showed the presence of phthalic esters such as
DOP. The unit of the present invention was disposed in the stocker
and a wafer was treated and analyzed in the same way. Phthalic
esters such as DOP was undetectable.
[0334] In the embodiment described below, after a gas is cleared of
acidic and/or basic gases, organic compounds and residual basic gas
can be decomposed with a photocatalyst. Acidic gases which can be
mentioned are nitrogen oxides (NOx), nitrogen oxide ion, sulfur
oxides (SOx), sulfur oxide ion, hydrogen chloride and hydrogen
fluoride. Basic gases which can be mentioned are ammonia and
amines.
Example 15
[0335] FIG. 18 represents a wafer stocker 71. This wafer stocker is
disposed in a clean room of class 10,000 in a semiconductor
production factory.
[0336] 1.0 to 1.5 ppm of non-methane hydrocarbons, 30 to 40 ppb of
SOx and 60 to 80 ppb of NH.sub.3 are present in the clean room. A
wafer is placed in the stocker 71 so as to be protected against
these gaseous contaminants.
[0337] In other words, a wafer carrier 3 holding the wafer 72 can
be introduced into or withdrawn from the stocker 71 by opening the
door of the stocker 71. At every opening of the door, contaminants
present in the clean room penetrate into the stocker 71. These
organic compounds 74 may cause the contact angle to increase. SOx
may bring about defective insulation of an oxide film. NH.sub.3 may
bring about defective resolution of a wafer.
[0338] The apparatus of the present invention is provided with an
ion exchange fiber 76 and a decomposition section situated above
the ion exchange fiber. The ion exchange fiber is preferably in the
form of a knitted filter.
[0339] The decomposition section consists of an UV lamp 77, a
catalytically active material coated in the form of a film on the
surface of the UV lamp 77 and a light-screening material 80.
Catalytically active material is preferably TiO.sub.2. The
light-screening material 80 is provided for protecting the wafer 72
from irradiation with slight UV leakage of the UV lamp 77.
[0340] The UV lamp 77 irradiates the thin film of TiO.sub.2 coated
on the surface thereof, allowing the TiO.sub.2 to perform the
photocatalytic action for decomposing organic compounds
effectively.
[0341] UV irradiation causes, a slight temperature difference
between the above and the below of the photocatalyst 78. This
temperature difference gives rise to air convection in the stocker
71, thus generating the air streams 79a, 79b and 79c circulating in
the stocker. As a result, air passes through the ion exchange fiber
76 and comes into contact with the photocatalyst 78 that
follows.
[0342] First, acidic and basic gases are removed with the ion
exchange fiber 76. Acidic gases can be removed with an anionic ion
exchange fiber, whereas basic gases can be removed with a cationic
ion exchange fiber. Organic compounds 74 present in the air, having
passed through the ion exchange fiber, are decomposed with the
photocatalyst.
[0343] The air in the clean room contains organic compounds in a
concentration of 1.0 to 1.5 ppm, SOx in a concentration of 30 to 40
ppb and NH.sub.3 in a concentration of 60 to 80 ppb. These
contaminants penetrate into the stocker 71 on opening of the
stocker 71. By virtue of the apparatus of the present invention,
organic compounds can be decomposed to a level of not more than 0.1
ppm in terms of non-methane hydrocarbons taken as an indicator. At
the same time, SOx and NH.sub.3 can be trapped with the ion
exchange fiber 76, thereby removed to a level of not more than 1
ppb. As the result, the wafer 72 stored in the stocker 71 can be
maintained with no increase in the contact angle, no defective
insulation and no defective resolution.
[0344] The present invention can be applied to not only air in a
clean room as usual but also various gases, e.g. N.sub.2 and Ar, as
well.
[0345] In FIG. 18, it is preferred to dispose between the ion
exchange fiber 76 and the UV lamp 77 an additional section having
an UV lamp for producing microparticles of organic compounds and
organosilicon compounds. Alternatively, the UV lamp 77 and the
photocatalyst 78 shown in FIG. 18 can be replaced by the
microparticle-producing section 8 and the decomposition section 26
shown in FIG. 2.
Example 16
[0346] FIG. 19 represents another embodiment of a wafer stocker 71.
This wafer stocker is disposed in a clean room of class 10,000 of a
semiconductor production factory in a manner similar to Example
15.
[0347] In example 16, by opening the stocker 71, organic compounds
74, SOx and NH.sub.3 75 as well as particulate matter 81
(microparticles) including ions such as NO.sub.3.sup.-,
NO.sub.2.sup.- and SO.sub.4.sup.- penetrate into the stocker
71.
[0348] Thus, unlike the apparatus of example 15 shown in FIG. 18,
the section C is disposed for charging and trapping the
microparticles 81 with photoelectrons. More precisely, the section
C consists of an UV lamp 82, a photoelectron-emitting material 83
coated on the surface of the UV lamp 82, an electrode 84
surrounding the UV lamp 82 for emitting photoelectrons and a
trapping material 85 disposed downstream to the UV lamp for
trapping charged microparticles.
[0349] The photoelectron-emitting material 83 of this example is
coated on the surface of the UV lamp 82 so as to form an integrated
device (Laid Open Japanese Patent Application No. Hei-4-243540).
The electrode 84 serves to generate an electrical field
(photoelectron-emitting material (-) and electrode (+)) so that the
photoelectron-emitting material 83 may emit photoelectrons
efficiently.
[0350] In the stocker 71, microparticles 11 are carried by air
streams 79a to 79c that are circulating in the apparatus of the
present invention. First, SOx and NH.sub.3 are trapped and removed
with an ion exchange fiber 76. Second, particulate matter including
NO.sub.3.sup.-, NO.sub.2.sup.- and SO.sub.4.sup.2- are trapped and
removed in the section C after being electrically charged with
photoelectrons. In the section C of this embodiment, microparticles
are electrically charged with the photoelectrons that had been
emitted from the photoelectron-emitting material 83 upon
irradiation with the UV lamp 82, to become charged microparticles,
which can be in turn trapped and removed with the trapping material
85 placed downstream. Organic compounds 74 present in the air that
had passed through the trapping material 85 can be decomposed with
the photocatalyst 78.
[0351] As stated above, purified air that has been cleared of
acidic and basic gases, ion-containing particulate matter as well
as gaseous organic compounds is present in the space B shown in
FIG. 19. In other words, SOx and NH.sub.3 are decreased to a level
of not more than 1 ppb and organic compounds are decreased to a
level of not more than 0.1 ppm, thus creating a purified space
superior to class 1.
[0352] In FIG. 19, reference numerals identical to those in FIG. 18
denote the same element as those in FIG. 18.
[0353] In FIG. 19, it is preferred to dispose an additional section
between the trapping material 85 and the UV lamp 77 for producing
microparticles of organic compounds and organosilicon compounds.
Alternatively, the UV lamp 77 and the photocatalyst 78 shown in
FIG. 19 may be replaced by the microparticle-producing section 8
and the decomposition section 26 shown in FIG. 2.
Example 17
[0354] FIG. 20 represents an embodiment other than that shown in
FIG. 19.
[0355] In the embodiment shown in FIG. 20, the ion exchange fiber
76 and the trapping section 85 shown in FIG. 19 are formed as an
integral section for trapping and removing acidic and basic gases.
In FIG. 20, reference numerals identical to those in FIG. 19 denote
the same element as those in FIG. 19.
[0356] As shown in FIG. 20, the UV lamp 82 is placed upstream to
the ion exchange fiber 76, which in turn is placed upstream to the
trapping section 85. The ion exchange fiber 76 and the trapping
section 85 are integrated. A photoelectron-emitting material 83 is
coated on the wall opposite to the UV lamp 82, not on the surface
of the UV lamp 82. A photocatalyst 78 is coated also on the wall
opposite to the UV lamp 77.
[0357] In this example, particulate matter 81 including
NO.sub.3.sup.-, NO.sub.2.sup.- and SO.sub.4.sup.2- are first
trapped and removed in the trapping section (C) by being charged
electrically with photoelectrons. Second, acidic and basic gases
including SOx and NH.sub.3 are trapped and removed with the ion
exchange fiber 76. And finally, organic compounds 74 are decomposed
with the photocatalyst 78 and removed.
[0358] As stated above, purified air that is free from acidic and
basic gases, ion-containing particulate matter as well as gaseous
contaminants (organic compounds) is present in the space B shown in
FIG. 20 (SOx and NH.sub.3: not more than 1 ppb; organic compounds:
not more than 0.1 ppm), thus creating an extremely purified space
superior to class 1. A wafer placed in this space B can be
protected from an increase in the contact angle, defective
insulation of oxide film, defective resolution, as well as circuit
breakage and shortage.
[0359] In FIG. 20, it is preferred to dispose between the trapping
material 85 and the UV lamp 77 an additional section having an UV
lamp for producing microparticles of organic compounds and
organosilicon compounds. Alternatively, the UV lamp 77 and the
photocatalyst 78 in FIG. 20 can be replaced by the
microparticle-producing section 8 and the decomposition section 26
shown in FIG. 2.
Example 18
[0360] FIG. 21 represents an embodiment of a type other than that
shown in FIG. 19.
[0361] In the embodiment shown in FIG. 21, ion exchange fiber is
omitted, and hence acidic and basic gases are removed in a trapping
section C only by being charged electrically by photoelectrons.
[0362] This embodiment is suitable for use in purifying a gas of
the type that contains acidic and basic gases such as SOx and NHx
at a relatively low concentration and ion-containing microparticles
such as NO.sub.3.sup.-, NO.sub.2.sup.- and SO.sub.4.sup.3- at a
relatively high concentration.
[0363] An UV lamp 82 irradiates a photoelectron-emitting material
83, allowing the latter to emit photoelectrons. The photoelectrons
can supply the particles with an electrical charge and the
resultant charged particles can be removed in a trapping section
85. Then, a photocatalyst 78 irradiated with an UV lamp 77 can
oxidatively decompose the organic compounds and basic gases.
[0364] In FIG. 21, it is preferred to dispose between the trapping
material 85 and the UV lamp 77 an additional section having an UV
lamp for producing microparticles of organic compounds and
organosilicon compounds. Alternatively, the UV lamp 77 and the
photocatalyst 78 shown in FIG. 21 may be replaced by the
microparticle-producing section 8 and the decomposition section 26
shown in FIG. 2.
[0365] In FIG. 21, reference numerals identical to those in FIGS.
18 to 20 denote the same element as those in FIGS. 18 to 20.
Example 19
[0366] FIG. 22 represents an embodiment of the purification
apparatus 70 of the present invention applied to purify air for
feeding an air knife device 89. The purification apparatus 70 and
the air knife device 89 are disposed in a clean room of class
10,000 of a semiconductor production factory.
[0367] The purification apparatus of the present invention
comprises successively downstream an ion exchange fiber in the form
of a filter 76, a dust-removing filter 87a, a decomposition section
and a dust-removing filter 87b. The decomposition section is
composed of an axially extending UV lamp 77 and a photocatalyst 78.
The photocatalyst 78 is coated on the inner surface of a housing. A
glass rod is placed in the axial direction of the decomposition
section in parallel to the UV lamp and has the surface which is
also coated with a catalytically active material.
[0368] Non-methane hydrocarbons are present in the concentration of
1.1 to 1.3 ppm in the clean room of class 10,000. SOx, NOx and
NH.sub.3 are present in the concentration of 40 ppb, 30 ppb and 150
ppb (average concentration), respectively.
[0369] From the air 16 in the clean room, acidic and basic gases
are removed first by trapping action of the ion exchange fiber 76
until the concentrations of SOx and NH.sub.3 taken as contamination
indicators are decreased each to a level of not more than 1 ppb.
Then, microparticles present in the air in the clean room are
removed with the dust-removing filter 87a.
[0370] Then, with the aid of the photocatalyst (TiO.sub.2) 78 which
had been activated by UV radiation emitted from the UV lamp 77,
organic compounds are decomposed until the concentration of
non-methane hydrocarbons taken as contamination indicator are
decreased to a level of not more than 0.1 ppm.
[0371] Filters suitable for use as the dust-removing filter 87b
(dust-removing section) are those that in case of emergency can
efficiently trap the microparticles which eventually flow out at or
near the ion exchange fiber and the organic compound-decomposing
section. An ULPA filter is used in this embodiment.
[0372] An extremely pure air 88 that is superior to class 1 (NOx,
SOx, NH.sub.3: below 1 ppb; organic compounds: below 0.1 ppm) and
is free from acidic and basic gases as well as particulate matter
and gaseous contaminants (organic compounds) can be obtained in a
manner as described above. The obtained extremely pure air 88 is
fed to the air knife device 89.
[0373] In FIG. 22, it is preferred to dispose an additional section
between the filter 87a and the decomposition section composed of
the UV lamp 77 and the photocatalyst 78, the additional section
being a microparticle-producing section having an UV lamp for
producing microparticles of organic compounds and organosilicon
compounds. Alternatively, the UV lamp 77 and the photocatalyst 78
shown in FIG. 22 can be replaced by the microparticle-producing
section 8 and the decomposition section 26 shown in FIG. 2.
Example 20
[0374] A sample of gas as specified below was charged into a
stocker as shown in FIG. 18 and the open-close cycles were repeated
10 times per day. The contact angle on the wafer stored in the
stocker was measured under a long-term continuous operation. The
concentrations of non-methane hydrocarbons and SOx in the air
present in the stocker were measured and the hydrocarbons deposited
on the wafer in the stocker were identified.
[0375] Sample gas: air of class 10 in a clean room
[0376] concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm
[0377] Sox concentration: 40 to 60 ppb;
[0378] Stocker volume: 80 liters;
[0379] Light source: low-pressure mercury lamp (with peaks at 184
nm and 254 nm);
[0380] Photocatalyst: TiO.sub.2;
[0381] The way of carrying photocatalyst on the light source:
TiO.sub.2 was coated onto the surface of the mercury lamp to a
thickness of 50 nm by sol-gel method;
[0382] Means for trapping and removing acidic and basic gases: an
ion exchange fiber (anion-type) or a fibrous active charcoal;
[0383] Fabrication of the ion exchange fiber: anion exchange fiber:
graft polymerization of fibrous polypropylene was carried out by
exposing to an electron beam of 20 Mrad under nitrogen and then
immersing in a solution consisting of hydroxystyrene monomer and
isoprene. Quaternary amination of the reaction product afforded an
anion exchange filter; Measurement of the contact angle: a method
for measuring the contact angle of a water drop (manufactured by
Kyowa Kaimen Kagaku Inc., CA-DT type);
[0384] Concentration of non-methane hydrocarbons in the stocker:
was measured by gas chromatography (GC) method; Concentration of
SOx in the stocker: was measured by solution conductivity
method;
[0385] Identification of the hydrocarbons adsorbed on the wafer:
GC/MS method;
[0386] Wafer stored in the stocker: a highly pure 5-inch silicon
wafer was cut into pieces of 1 cm.times.8 cm and pretreated as
stated below prior to introduction into the stocker;
[0387] Pretreatment of the wafer: washing with a detergent and
alcohol on a clean bench in the clean room, followed by UV/O.sub.3
cleaning. The wafer was exposed to UV radiation under the
conditions allowing O.sub.3 evolution.
[0388] Results:
[0389] (1) The Contact Angle on the Wafer
[0390] FIG. 23 shows the contact angle (in degrees) as the function
of the number of days when an ion exchange fiber was employed. In
FIG. 23, the symbol -.largecircle.- represents the combination of
an ion exchange fiber and a photocatalyst (present invention), the
symbol -.circle-solid.- represents a control without ion exchange
fiber and the symbol -.tangle-soliddn.- represents a control
without photocatalyst.
[0391] The control without photocatalyst showed the contact angle
of 20 degrees at 20 hours after and 40 degrees at 60 hours after,
thus no preventive effect was observed on the increase in the
contact angle. The measured values as long as the 350th day are
shown in FIG. 23. When an ion exchange fiber was disposed at the
inlet in the manner as shown in FIG. 22, the apparatus could
maintain a high performance without deterioration with time.
[0392] The control applying a fibrous active charcoal in place of
an ion exchange fiber for trapping and removing acidic and basic
gases showed the results similar to those shown in FIG. 23 and
exhibited an increase in the contact angle of below 2 degrees after
300-day operation.
[0393] (2) Organic Compounds in the Stocker
[0394] The concentrations of non-methane hydrocarbons and SOx as
well as the identification of the hydrocarbons adsorbed on the
wafer (presence or absence of the identified components) are shown
in the table 3. The results obtained by using a control having an
ion exchange fiber without photocatalyst or a control having a
photocatalyst without ion exchange fiber are also shown in the
table 3.
3TABLE 3 storage hydrocarbon period conc. in the deposition on SOx
conc. in (days) conditions air (ppm) the wafer the air (ppb) 1
invention <0.1 absent <1 no photocatalyst 1.2.about.1.5
present <1 no ion exchange <0.1 absent 5.about.7 fiber 10
invention <0.1 absent <1 no photocatalyst 1.2.about.1.5
present <1 no ion exchange <0.1 absent 5.about.7 fiber 100
invention <0.1 absent <1 no photocatalyst 1.2.about.1.5
present <1 no ion exchange <0.1 absent 10.about.15 fiber 300
invention <0.1 absent <1 no photocatalyst 1.2.about.1.5
present <1 no ion exchange 0.5.about.0.8 absent 15.about.20
fiber
[0395] In table 3, the hydrocarbon deposited on the wafer was
phthalic ester such as DOP.
Example 21
[0396] The same apparatus as that used in the example 21 was used,
except that a photocatalyst was employed and an ion exchange fiber
was omitted. The number of days required for a 5 degree increase in
the contact angle was observed as the function of the SOx
concentration by using a sample gas adjusted to a SOx concentration
of 1 to 50 ppb. Other experimental conditions were same as those
used in Example 20.
[0397] Sample gas: air of class 10 in a clean room
[0398] Concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm
[0399] SOx concentration: 1 to 50 ppb
[0400] The concentration of SOx was appropriately adjusted by
regulating the passing speed of the gas through the ion exchange
fiber for removing a part of SOx as desired.
[0401] Results
[0402] FIG. 24 represents the number of days required for a 5
degree increase in the contact angle versus the SOx concentration
in the sample gas. It was obvious that the less was the
concentration of sulfur oxides in a sample gas, the more was the
number of days required for increase in the contact angle. In other
words, the air was kept clean for longer.
[0403] FIG. 25 is a total ion chromatogram obtained by gas
chromatography/mass spectrography of organic compounds in the air.
The axis of abscissa x represents the mass of an ion and the axis
of ordinate y represents a relative strength. 30 liters of sample
air was flown into an adsorbent (TENAX-GR) at a rate of 0.5
liter/min. The adsorbent was heated in a device for concentration
and introduction (manufactured by CHROMPACK Inc., CP4010 model) so
as to desorb the adsorbed gas, which was cooled with liquid
nitrogen, concentrated and measured in a gas chromatography/mass
spectrometry device (manufactured by Shimazu Seisakusho Inc.,
QP-1100EX model).
[0404] (a) is a graph showing the results of the air of class
10,000 in a clean room. Non-methane hydrocarbons are present in a
concentration of 1.1 to 1.3 ppm and SOx, NOx and NH.sub.3 are
present in a concentration of 40 ppb, 30 ppb and 150 ppb (average
concentration) respectively. Each peak represents the presence of
an organic compound.
[0405] (b) is a graph showing the results of the air of class
10,000 in a clean room after treated by the apparatus of the
present invention. It is obvious that the amount of organic
compounds was drastically decreased.
[0406] The present invention, by producing microparticles of
contaminants and concentrating them locally, can decompose the
contaminants with a photocatalyst effectively, even when the
contaminants are present at low concentration. Contaminants such as
organic compounds in the air, even when they are present at low
concentration, can be efficiently decomposed because they are made
into microparticles and brought into contact with a photocatalyst
in the concentrated form. As a result, the decomposition rate of
organic compounds was improved. The photocatalyst is effective in
decomposing oxidizable compounds including gaseous organic
compounds such as phthalic ester, gaseous organosilicon compounds
such as siloxane and basic gas such as ammonia. The present
invention is particularly advantageous in view of the fact that
gaseous organic compounds and gaseous organosilicon compounds are
difficult to remove by photoelectronical charging or with a filter
or an ion exchange fiber.
[0407] When a removal section utilizing a filter, an adsorbent, a
photoelectron and an ion exchange resin is provided, other gaseous
contaminants which are present together with organic compounds,
including.acidic gases such as SO.sub.2, NOx, HCl, HF and the like
as well as basic gases such NH.sub.3, amines and the like can also
be removed. In other words, a wide variety of contaminants ranging
from gas to particles can be removed.
[0408] In addition, when acidic and basic gases are removed prior
to the treatment with a photocatalyst, adverse effects on the
photocatalyst by the acidic and basic gases can be avoided. In
consequence, the photocatalyst can act reliably over a long period,
thus allowing long-term operation.
[0409] By way of an illustrative application, the present invention
was described mainly with reference to a clean room of
semiconductor production factories.
[0410] The present invention are also suitable to other
applications as set forth below:
[0411] (1) Air purification for public welfare, for example,
purification of air in offices, buildings, houses, hospitals,
hotels and the like;
[0412] (2) Purification of exhaust gases originating in various
industries including sewer drains and waste disposal sites;
purification of industrial atmosphere; as well as purification of
exhaust gases originating in underground parking areas and tunnel
ventilators; and
[0413] (3) Purification of air and gases including nitrogen, oxygen
and the like for use in gas production units for feeding clean
rooms, clean booths, clean tunnels, clean benches, safety cabinets,
bioclean boxes, sterile rooms, path boxes, precious article
stockers, transportation spaces, interfaces, air curtains, air
knives, drying sections, production lines and the like in pioneer
industries such as semiconductor industries, electronic industries,
pharmaceutical industries, food industries, agricultural and
forestry industries, medical industries, precision machinery
industries and the like.
[0414] Public welfare was mentioned in consideration of the fact
that gaseous contaminants present in the air may adversely affect
human health producing so-called sick building syndrome.
* * * * *