U.S. patent application number 08/881747 was filed with the patent office on 2001-12-13 for on-site generation of ultra-high-purity buffered-hf and ammonium fluoride.
Invention is credited to CLARK, R. SCOT, HOFFMAN, JOE G..
Application Number | 20010051128 08/881747 |
Document ID | / |
Family ID | 56289776 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010051128 |
Kind Code |
A1 |
HOFFMAN, JOE G. ; et
al. |
December 13, 2001 |
ON-SITE GENERATION OF ULTRA-HIGH-PURITY BUFFERED-HF AND AMMONIUM
FLUORIDE
Abstract
Provided is a novel method and system for preparing
ultra-high-purity buffered-hydrofluoric acid or ammonium fluoride
controlled concentration. The method comprises bubbling purified
ammonia vapor into ultra-pure hydrofluoric acid. The inventive
method and system can be used as an on-site subsystem in a
semiconductor device fabrication facility for supplying the
buffered-hydrofluoric acid and ammonium fluoride to points of use
in the semiconductor device fabrication facility.
Inventors: |
HOFFMAN, JOE G.; (CARDIFF,
CA) ; CLARK, R. SCOT; (FALLBROOK, CA) |
Correspondence
Address: |
BURNS DOANE SWECKER & MATHIS L L P
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Family ID: |
56289776 |
Appl. No.: |
08/881747 |
Filed: |
June 24, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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08881747 |
Jun 24, 1997 |
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08674130 |
Jul 1, 1996 |
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5722442 |
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08674130 |
Jul 1, 1996 |
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PCT/US96/10388 |
Jun 5, 1996 |
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60018104 |
Jul 7, 1995 |
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Current U.S.
Class: |
423/470 ;
423/471; 423/484; 423/488 |
Current CPC
Class: |
C01B 7/196 20130101;
G05D 11/135 20130101; C01B 7/195 20130101; C01B 7/198 20130101;
C01C 1/024 20130101; C01C 1/162 20130101; H01L 21/67023 20130101;
C01B 7/0731 20130101; C01B 7/0712 20130101 |
Class at
Publication: |
423/470 ;
423/471; 423/484; 423/488 |
International
Class: |
C01B 007/19 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 1996 |
US |
PCTUS9610388 |
Claims
What is claimed is:
1. A method of preparing ultra-high-purity buffered-hydrofluoric
acid or ammonium fluoride of controlled concentration comprising
bubbling purified ammonia vapor into ultra-pure hydrofluoric
acid.
2. The method according to claim 1, wherein ultra-high-purity
bufferered-hydrofluoric acid is prepared.
3. The method according to claim 2, wherein said ultra-high-purity
buffered-hydrofluoric acid has a concentration of 10:1, 50:1 or
200:1 as measured by volume parts of 40% ammonium fluoride to 49%
HF.
4. The method according to claim 1, wherein ultra-high-purity
ammonium fluoride is prepared.
5. The method according to claim 4, wherein the ammonium fluoride
is a 40% by weight ammonium fluoride solution.
6. The method according to claim 1, wherein the ammonia vapor
bubbling is performed in a generator which is connected to a point
of use.
7. The method according to claim 6, wherein the point of use is
located in a semiconductor device fabrication facility.
8. The method according to claim 1, wherein the ultrapure
hydrofluoric acid is prepared by a process comprising the steps of:
removing a flow of hydrogen fluoride vapor from a source of
hydrogen fluoride; contacting said hydrogen fluoride vapor with a
recirculating volume of high-purity water containing a high
concentration of hydrogen fluoride in a hydrogen fluoride ionic
purifier unit, wherein said hydrogen fluoride ionic purifier unit
passes purified hydrogen fluoride gas; and combining the hydrogen
fluoride gas with acidic deionized water to produce the ultra-pure
hydrofluoric acid.
9. The method according to claim 8, wherein the source of hydrogen
fluoride is an anhydrous hydrogen fluoride source.
10. The method according to claim 8, wherein the source of hydrogen
fluoride is essentially arsenic-free.
11. The method according to claim 8, wherein the source of hydrogen
fluoride is ultra-pure arsenic-free aqueous hydrogen fluoride.
12. The method according to claim 8, wherein the ammonia vapor is
prepared by a process comprising the steps. of: removing a flow of
ammonia vapor from a source of liquid ammonia; contacting said flow
of ammonia vapor with a recirculating volume of high-purity water
containing a high concentration of ammonium hydroxide in an ammonia
ionic purifier unit, wherein said ammonia ionic purifier unit
passes said purified ammonia vapor.
13. The method according to claim 12, wherein said recirculating
volume of high-purity water in said hydrogen fluoride ionic
purifier and said recirculating volume of high-purity water in said
ammonia ionic purifier are free of additives.
14. The method according to claim 1, wherein the ammonia vapor is
prepared by a process comprising the steps of: removing a flow of
ammonia vapor from a source of liquid ammonia; contacting said flow
of ammonia vapor with a recirculating volume of high-purity water
containing a high concentration of ammonium hydroxide in an ammonia
ionic purifier unit, wherein said ammonia ionic purifier unit
passes said purified ammonia vapor.
15. The method according to claim 1, wherein the step of bubbling
the purified ammonia vapor into the ultrapure hydrofluoric acid is
performed in a generator, and wherein the ultra-pure hydrofluoric
acid is formed by introducing a 49% by weight hydrogen fluoride
solution into the generator, and diluting said hydrogen fluoride
solution with high-purity water.
16. The method according to claim 1, wherein additional
hydrofluoric acid is added to the solution after the ammonia
bubbling step, thereby forming said ultra-high-purity
buffered-hydrofluoric acid.
17. The method according to claim 16, wherein the ammonia bubbling
step forms a 40% ammonium fluoride solution product.
18. The method according to claim 1, wherein the ultra-pure
hydrofluoric acid is formed by introducing anhydrous hydrogen
fluoride into high purity water in a generator, and the ammonia
vapor is bubbled into the ultrapure hydrofluoric acid in the
generator.
19. The method according to claim 1, wherein the concentration of
the buffered-hydrofluoric acid or ammonium fluoride is controlled
by a step for detecting an endpoint of chemical mixing.
20. The method according to claim 19, wherein the step for
detecting an endpoint of chemical mixing is performed by acoustic
velocity measurement.
21. A system for preparing ultra-high-purity buffered-hydrofluoric
acid or ammonium fluoride of controlled concentration, comprising a
source of purified ammonia vapor, a source of ultrapure
hydrofluoric acid and a generator which combines said ammonia vapor
with said ultra-pure hydrofluoric acid to produce said
ultra-high-purity buffered-hydrofluoric acid or ammonium
fluoride.
22. The system according to claim 21, wherein the generator is
connected to a point of use through piping.
23. The system according to claim 22, wherein the point of use is
located in a semiconductor device fabrication facility.
24. The system according to claim 23, wherein the source of
ultrapure hydrofluoric acid comprises a reservoir connected to
receive a hydrogen fluoride source and to provide a flow of
hydrogen fluoride vapor therefrom, said flow of hydrogen fluoride
vapor being connected to pass through a hydrogen fluoride ionic
purifier unit which provides a recirculating volume of high-purity
water containing a high concentration of hydrogen fluoride in
contact with said flow of hydrogen fluoride vapor, wherein said
purifier passes purified hydrogen fluoride gas, and a hydrogen
fluoride generator unit, connected to receive said flow of hydrogen
fluoride gas from said purifier and to combine said hydrogen
fluoride gas with high-purity acidic deionized water to produce
said ultra-pure hydrofluoric acid.
25. The system according to claim 24, wherein the source of
hydrogen fluoride is an anhydrous hydrogen fluoride source.
26. The system according to claim 24, wherein the source of
hydrogen fluoride is essentially arsenic-free.
27. The system according to claim 24, wherein the source of
hydrogen fluoride is ultra-pure arsenic-free aqueous hydrogen
fluoride.
28. The system according to claim 24, wherein the source of
purified ammonia vapor comprises a reservoir connected to receive a
liquid source of ammonia and to provide a flow of ammonia vapor
therefrom, said flow of ammonia vapor being connected to pass
through an ammonia ionic purifier unit which provides a
recirculating volume of high-purity water, containing a high
concentration of ammonium hydroxide, in contact with said flow of
ammonia vapor, wherein said ammonia purifier passes said purified
ammonia vapor.
29. The system according to claim 24, wherein said recirculating
volume of high-purity water in said hydrogen fluoride ionic
purifier and said recirculating volume of high-purity water in said
ammonia ionic purifier are free of additives.
30. The system according to claim 21, wherein the source of
purified ammonia vapor comprises a reservoir connected to receive a
liquid source of ammonia and to provide a flow of ammonia vapor
therefrom, said flow of ammonia vapor being connected to pass
through an ammonia ionic purifier unit which provides a
recirculating volume of high-purity water, containing a high
concentration of ammonium hydroxide, in contact with said flow of
ammonia vapor, wherein said ammonia purifier passes said purified
ammonia vapor.
31. The system according to claim 21, further comprising means for
detecting an endpoint of chemical mixing.
32. The system according to claim 31, wherein the means for
detecting an endpoint of chemical mixing comprises an acoustic
velocity measurement sensor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The application is a continuation-in-part of copending
application Ser. No. 08/674,130, filed on Jul. 1, 1996, which
document is herein incorporated by reference, which application in
turn claims the benefit of priority of U.S. Provisional Application
Ser. No. 60/018,104, filed on Jul. 7, 1995. The present application
also claims benefit of PCT/US96/10388, filed on Jun. 5, 1996, which
documents are herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and system for
producing ultra-high-purity buffered-hydrofluoric acid (buffered-HF
or BHF) or ultra-high-purity ammonium fluoride (NH.sub.4F). The
invention has particular applicability in semiconductor fabrication
for providing ultra-high-purity materials to a semiconductor
manufacturing operation.
[0004] 2. Description of the Related Art
[0005] a. Contamination Control
[0006] Contamination is generally an overwhelmingly important
concern in integrated circuit (IC) manufacturing. A large fraction
of the steps used in modern integrated circuit manufacturing are
cleanup steps of one kind or another. Such cleanup steps are used,
for example, to remove organic contaminants, metallic contaminants,
photoresist (or inorganic residues thereof), byproducts of etching,
native oxides, etc.
[0007] The cost of a new IC wafer fabrication facility is typically
more than one billion dollars ($1,000,000,000). A large fraction of
the cost for such facilities is directed to measures for
particulate control, cleanup, and contamination control.
[0008] One important and basic source of contamination in
semiconductor fabrication is impurities in the process chemicals.
Since the cleanup steps are performed so frequently in and are so
critical to IC fabrication, contamination due to cleanup chemistry
is very undesirable.
[0009] b. Wet Versus Dry Processing
[0010] One of the long-running technological shifts in
semiconductor processing has been the changes (and attempted
changes) between dry and wet processing. In dry processing, only
gaseous or plasma-phase reactants come in contact with the wafer or
wafers being treated. In wet processing, a variety of liquid
reagents are used for a multitude of purposes, such as the etching
of silicon dioxide, silicon nitride and silicon, and the removal of
native oxide layers, organic materials, trace organic or inorganic
contaminants and metals.
[0011] While plasma etching has many attractive capabilities, it is
not adequate for use in cleanup processes. There is simply no
available chemistry with plasma etching to remove some of the most
undesirable impurities, such as gold.
[0012] Thus, wet cleanup processes are essential to modern
semiconductor processing, and are likely to remain so for the
foreseeable future.
[0013] Plasma etching is performed using a photoresist mask in
place, and is not immediately followed by high-temperature
processes. After plasma etching, the resist is stripped from the
wafer surface using, for example, an O.sub.2 plasma treatment.
Cleanup of the resist stripped wafer(s) is then necessary.
[0014] The materials which the cleanup process should remove
include, for example, photoresist residues (organic polymers),
sodium, alkaline earth metals (e.g., calcium, magnesium) and heavy
metals (e.g., gold). Many of these contaminants do not form
volatile halides. As a result, plasma etching will not remove such
contaminants from the wafer surface. Hence, cleanup processes using
wet chemistries are required.
[0015] Because any dangerous contaminants stemming from the plasma
etching process are removed prior to high-temperature processing
steps by wet chemical treatment, the purities of plasma etching
process chemicals (i.e., liquified or compressed gases) are not as
critical as those of the liquid chemicals used in cleanup
processes. This difference is due to the impingement rate of the
liquid chemical at the semiconductor surface typically being one
million times greater than that of the plasma species in plasma
etching. Moreover, since the liquid cleanup steps are directly
followed by high-temperature processes, contaminants on the wafer
surface tend to be driven (i.e., diffused) into the wafer.
[0016] Wet processing has a major drawback insofar as ionic
contamination is concerned. Integrated circuit devices generally
use only a few dopant species (e.g., boron, arsenic, phosphorus,
and antimony) to form the requisite p-type and n-type doped regions
of the device. However, many other species act as electrically
active dopants, and are highly undesirable contaminants. These
contaminants can have deleterious effects on the IC devices, such
as increased junction leakage at concentrations well below
10.sup.13 cm.sup.-3.
[0017] Moreover, some less desirable contaminants segregate into
the silicon substrate. This occurs when silicon is in contact with
an aqueous solution, and the equilibrium concentration of the
contaminants is higher in the silicon than in the solution.
Moreover, some less desirable contaminants have very high diffusion
coefficients. Consequently, introduction of such contaminants into
any part of the silicon wafer may result in diffusion of the
contaminants throughout the wafer, including junction locations
where leakage may result.
[0018] Thus, liquid solutions for treating semiconductor wafers
should have extremely low levels of metal ions. Preferably, the
concentration of all metals combined should be less than 300 ppt
(parts per trillion), and less than 10 ppt for any single metal.
Even lower concentrations are desirable. Contamination by anions
and cations should also be controlled. Some anions may have adverse
effects, such as complexed metal ions which reduce to mobile metal
atoms or ions in the silicon lattice.
[0019] Front end facilities typically include on-site purification
systems for preparation of high-purity water (i.e., "deionized" or
"DI" water). However, it is more difficult to obtain liquid process
chemicals in the purities required.
[0020] c. Purity in Semiconductor Manufacturing
[0021] Undetected contamination of chemicals increases the
probability for costly damage to a large quantity of wafers. The
extreme purity levels required by semiconductor manufacturing are
rare and unique among industrial processes. With such extreme
purity requirements, handling of chemicals is undesirable (though
of course it cannot be entirely avoided). Exposure of ultrapure
chemicals to air (particularly in an environment where workers are
also present) should be minimized. Such exposure risks the
introduction of particulates into the chemicals, which can result
in the contamination of those chemicals. Furthermore, shipment of
ultrapure chemicals in closed containers is not ideal, since such
containers increase the risk of contaminants being generated at the
manufacturer's or at the user's site.
[0022] Since many corrosive and/or toxic chemicals are used in
semiconductor processing, the reagent supply locations are commonly
separated from the locations where front-end workers are present.
Most gases and liquids can be transported to wafer fabrication
stations from anywhere in the same building (or in the same
site).
[0023] d. Uses of Buffered-HF and Ammonium Fluoride in
Semiconductor Processing
[0024] One of the important chemicals in the electronics industry
is hydrofluoric acid (aqueous HF). Hydrofluoric acid solutions are
used as cleaning and etching agents for silicon wafers, circuit
boards and high speed, high density chips for computers and optics.
In semiconductor manufacturing, those materials are very important
for deglazing (i.e., removal of thin native oxides) and for oxide
removal generally.
[0025] The reaction of HF with silicon produces fluosilicilic acid,
a strong acid which shifts the pH of the etching solution and hence
the etch rate. As a result, hydrofluoric acid is often used in
buffered form (Buffered-HF or BHF), to reduce shifts in pH as the
acid solution becomes loaded with etching by-products. In
buffered-hydrofluoric acid, the buffering in the acid solution is
usually provided by an ammonium component, such as ammonium
fluoride (NH.sub.4F).
[0026] Ammonium fluoride and buffered-HF differ in their respective
NH.sub.3 to HF molar ratios. Ammonium fluoride solutions have a
NH.sub.3 to HF molar ratio of 1.00, whereas buffered-HF solutions
have a molar excess of HF.
[0027] Buffered-HF solutions are identified by the ratio in volume
parts of 40%, ammonium fluoride to 49% HF. Thus, a 50:1 BHF
solution consists of 50 parts by volume 40% ammonium fluoride to 1
part by volume 49% HF. Typical BHF solutions used in the
semiconductor processing industry are 10:1, 50:1 and 200:1,
although other ratios are also used.
[0028] The requirement for buffering with ultra-high-purity
chemicals presents further problems, since the buffering agent too
is a source of contaminants, and must be sufficiently pure so as
not to degrade the cleaning or etching system.
[0029] e. Objects and Advantages of the Invention
[0030] To meet the requirements of the semiconductor processing
industry and to overcome the disadvantages of the related art, it
is an object of the present invention to provide novel methods and
systems for the preparation of ultra-high-purity
buffered-hydrofluoric acid and ultra-high-purity ammonium fluoride
in which the hydrofluoric acid and ammonium fluoride can be formed
at or introduced directly to a point of use. The system is very
compact, and can be located in the same building as the point of
use (or in an adjacent building), so that chemical handling can be
avoided. As a result, low impurity levels on a semiconductor wafer
surface can be achieved, resulting in better device characteristics
and increased product yield.
SUMMARY OF THE INVENTION
[0031] The foregoing objectives are met by the methods and systems
of the present invention. According to a first aspect of the
present invention, a novel method for preparing ultra-high-purity
buffered-hydrofluoric acid or ultra-high-purity ammonium fluoride
of controlled concentration is provided. The method comprises
bubbling purified ammonia vapor into ultra-pure hydrofluoric
acid.
[0032] According to a second aspect of the invention, a system for
preparing the ultra-high-purity buffered-hydrofluoric acid or
ammonium fluoride of controlled concentration is provided. The
system comprises a source of purified ammonia vapor, a source of
ultrapure hydrofluoric acid and a generator which combines the
ammonia vapor with the ultra-pure hydrofluoric acid to produce the
ultra-high-purity buffered-hydrofluoric acid or ammonium
fluoride.
[0033] The inventive system and method can be applied to an on-site
subsystem, in a semiconductor device fabrication facility for
supplying the buffered-HF or ammonium fluoride to points of use
therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The objects and advantages of the invention will become
apparent from the following detailed description of the preferred
embodiments thereof in connection with the accompanying drawings,
in which like reference numerals designate like elements, and in
which:
[0035] FIG. 1 is a process flow diagram of a unit for the
production of ultrapure ammonia;
[0036] FIG. 2 illustrates an on-site hydrofluoric acid
generator;
[0037] FIG. 3 is a process flow diagram of a unit for producing
buffered-hydrofluoric acid in accordance with the invention;
and
[0038] FIG. 4 is a block diagram of a semiconductor fabrication
line to which the hydrofluoric acid generator of FIG. 2 can be
connected.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0039] The inventors have found methods and systems for the
preparation of ultra-high-purity buffered-hydrofluoric acid
(buffered-HF) or ultra-high-purity ammonium fluoride (NH.sub.4F)
which have particular applicability in the semiconductor
manufacturing industry. In particular, the ultrapure chemicals can
be generated on-site, for example, at a semiconductor manufacturing
facility, so that they can be piped directly to or generated
directly at points of use. The disclosed systems are very compact
units which can be located in the same building as a front end (or
in an adjacent building), so that handling of the chemicals is
avoided.
[0040] The purities of the buffered-HF and ammonium fluoride
starting materials, ammonia and HF, are important to the final
product purity. Purification methods and systems for those
materials are described below.
[0041] On-Site Purification of NH.sub.3
[0042] In accordance with this invention, provided are methods and
systems for preparing ultra-high-purity ammonia which can be used
as a starting material in the manufacture of buffered-HF. The
system is an on-site system which can be located at a semiconductor
wafer production site.
[0043] A process flow diagram depicting one example of an ammonia
purification unit 100 in accordance with this invention is shown in
FIG. 1. Liquid ammonia 102 is stored in a reservoir 104 which acts
as an evaporation source for ammonia vapor 106. Ammonia vapor 106
is drawn from the vapor space 108 in the reservoir. Drawing vapor
in this manner serves as a single-stage distillation, leaving
certain solid and high-boiling impurities behind in the liquid
phase. The supply reservoir can be any conventional supply tank or
other reservoir suitable for containing ammonia, and the ammonia
can be in anhydrous form or an aqueous solution.
[0044] The reservoir can be maintained at atmospheric pressure or
at a pressure above atmospheric if desired to enhance the flow of
the ammonia through the system. The reservoir is preferably heat
controlled, so that the temperature is within the range of from
about 10.degree. to about 50.degree. C., preferably from about
15.degree. to about 35.degree. C., and most preferably from about
20.degree. to about 25.degree. C.
[0045] Impurities that will be removed as a result of drawing the
ammonia from the vapor phase include, for example, the following:
Metals of Groups I and II of the Periodic Table, as well as
aminated forms of these metals which form as a result of the
contact with ammonia; oxides and carbonates of these metals, as
well as hydrides such as beryllium hydride and magnesium hydride;
Group III elements and their oxides, as well as ammonium adducts of
hydrides and halides of these elements; transition metal hydrides;
and heavy hydrocarbons and halocarbons, such as pump oil.
[0046] The ammonia drawn from reservoir 104 is passed through a
shut-off valve 110 and through filtration unit 112 which can remove
any solid matter entrained with the vapor. Microfiltration and
ultrafiltration units and membranes are commercially available and
can be used for this purpose. The grade and type of filter can be
selected according to need. The presently preferred embodiment uses
a gross filter, followed by a 0.1 micron filter, in front of an
ionic purifier 118, and no filtration after the ionic purifier.
[0047] The filtered ammonia vapor 114, the flow of which is
controlled by pressure regulator 116, is directed to an ionic
purifier 118, which preferably takes the form of a scrubber unit.
In the exemplary ionic purifier, scrubbing column 118 contains a
packed section 120 and a mist removal pad 122.
[0048] Saturated aqueous ammonia 124 flows downward as the ammonia
vapor flows upward, the liquid being circulated by a circulation
pump 126, and the liquid level being controlled by a level sensor
128. Waste 130 is drawn off periodically from the retained liquid
in the bottom of the scrubber. Deionized water 132 is supplied to
scrubber 118, with an elevated pressure being maintained by a pump
134.
[0049] The vapor is scrubbed with high-pH purified (preferably
deionized) water. The high-pH water is preferably an aqueous
ammonia solution, with the concentration raised to saturation by
recycling through the scrubber. The scrubber can be conveniently
operated as a conventional scrubbing column in countercurrent
fashion.
[0050] Although the operating temperature is not critical, the
column is preferably run at a temperature ranging from about
10.degree. to about 50.degree. C., preferably from about 15.degree.
to about 35.degree. C. Likewise, the operating pressure is not
critical, although preferred operation is at a pressure of from
about atmospheric pressure to about 30 psi above atmospheric. The
column typically contains a conventional column packing to provide
for a high degree of contact between liquid and gas, and preferably
a mist removal section as well.
[0051] In one presently preferred example, the column has a packed
height of approximately 3 feet (0.9 meter) and an internal diameter
of approximately 7 inches (18 cm), to achieve a packing volume of
0.84 cubic feet (24 liters). The column of the preferred example is
operated at a pressure drop of about 0.3 inches of water (0.075
kPa) and less than 10% flood, with a recirculation flow of about
2.5 gallons per minute (0.16 liter per second) nominal or 5 gallons
per minute (0.32 liter per second) at 20% flood, with the gas inlet
below the packing, and the liquid inlet above the packing but below
the mist removal section.
[0052] Preferred packing materials for a column of this description
are those which have a nominal dimension of less than one-eighth of
the column diameter. The mist removal section of the column will
have a similar or a more. dense packing, and is otherwise
conventional in construction. It should be understood that all
descriptions and dimensions with respect to the preferred
embodiment are exemplary only. Each of the system parameters may be
varied.
[0053] In typical operation, startup is achieved by first
saturating deionized water with ammonia to form a solution for use
as the starting scrubbing medium. During operation of the scrubber,
a small amount of liquid in the column sump is drained periodically
to remove accumulated impurities.
[0054] Examples of impurities that will be removed by the scrubber
include reactive volatiles such as silane (SiH.sub.4) and arsine
(AsH.sub.3) halides and hydrides of phosphorus, arsenic and
antimony; transition metal halides in general; and Group III and
Group VI metal halides and hydrides.
[0055] The units described up to this point may be operated in
either batchwise, continuous or semi-continuous manner. Continuous
or semi-continuous operation is preferred. The volumetric
processing rate of the ammonia purification system is not critical
and may vary widely. In most operations, however, the flow rate of
ammonia through the system is preferably within the range of from
about 200 cm.sup.3/h to thousands of liters per hour.
[0056] The scrubbed ammonia 136 can be directed to one of three
alternate routes, including: (1) a distillation column 138 where
the ammonia is further purified, the resulting distilled ammonia
140 then being directed to the point(s) of use; (2) a dissolving
unit 142 where the ammonia is combined with deionized water 144 to
form an aqueous solution 146, which is directed to a point of use.
For plant operations with multiple points of use, the aqueous
solution can be collected in a holding tank from which the ammonia
is drawn into individual lines for a multitude of point-of-use
destinations at the same plant; and (3) a transfer line 148 which
carries the ammonia in gaseous form to a point of use.
[0057] The second and third of these alternatives, which do not
utilize the distillation column 138, are suitable for producing
ammonia with less than 100 parts per trillion of any metallic
impurity. For certain uses, however, the inclusion of the
distillation column 138 is preferred. Examples are furnace or
chemical vapor deposition (CVD) uses of the ammonia. If the ammonia
is used for CVD, for example, the distillation column would remove
non-condensables, such as oxygen and nitrogen, which might
interfere with the CVD process. In addition, since the ammonia
leaving the scrubber 118 is saturated with water, a dehydration
unit can optionally be incorporated into the system between the
scrubber 118 and the distillation column 138, depending on the
characteristics and efficiency of the distillation column.
[0058] With any of these alternatives, the resulting stream, be it
gaseous ammonia or an aqueous solution, can be divided into two or
more branch streams, each directed to a different use station. The
purification unit can thereby supply purified ammonia to a number
of use stations simultaneously.
[0059] In the presently preferred embodiment, the liquid volume of
the ammonia purifier is 10 l, and the maximum gas flow rate is
about 10 standard l/min. The scrubbing liquid is purged,
continuously or incrementally, such that it turns over at least
once in 24 hours.
[0060] In a batch operation, a typical operating pressure can be
300 psia (2,068 kPa), with a batch size of 100 pounds (45.4 kg).
The column in this example has a diameter of 8 inches (20 cm), aL
height of 72 inches (183 cm), operating at 30% of flood, with a
vapor velocity of 0.00221 feet per second (0.00067 meter per
second), a height equivalent to a theoretical plate of 1.5 inches
(3.8 cm) and 48 equivalent plates.
[0061] The boiler size in this example is about 18 inches (45.7 cm)
in diameter and 27 inches (68.6 cm) in length, with a reflux ratio
of 0.5. Recirculating chilled water enters at 60.degree. F.
(15.6.degree. C.) and leaves at 90.degree. F. (32.2.degree. C.).
Again, the above is merely exemplary, and distillation columns
varying widely in construction and operational parameters can be
used.
[0062] Depending on its use, the purified ammonia, either with or
without the distillation step, can be used as a purified gas or as
an aqueous solution. In the latter case, the purified ammonia is
dissolved in purified (preferably deionized) water. The proportions
and the means of mixing are conventional.
[0063] On-site HF Purification and Vaporization
[0064] Anhydrous HF is typically manufactured by the addition of
sulfuric acid to fluorspar, CaF.sub.2. Unfortunately, many
fluorspars contain arsenic, which leads to contamination of the
resulting HF. Other impurities, in conventional systems, are
contributed by the HF generation and handling system. These
impurities result from degradation of these systems, since they
were designed for applications much less demanding than the
semiconductor industry. These contaminants must be removed in order
to achieve good semiconductor performance.
[0065] FIG. 2 illustrates an on-site purification process flow and
system 200 for preparing ultra-high-purity HF which can be used as
a starting material in the manufacture of buffered-HF. The HF
process flow includes a batch process arsenic removal and
evaporation stage 202, a fractionating column 206 to remove most
other impurities, an ionic purifier column 208 to suppress
contaminants not removed by the fractionating column, and a
generator or supplier 210.
[0066] Arsenic is converted to the +5 state and held in the
evaporator 202 during distillation by the addition of an oxidant
(KMnO.sub.4 or (NH.sub.4).sub.2S.sub.2O.sub.8) and a cation source
such as KHF.sub.2 to form the salt K.sub.2AsF.sub.7. This should be
a batch process, as the reaction is slow and sufficient time for
completion must be allowed before the distillation takes place.
This process typically requires contact times of approximately one
hour at nominal temperatures. To achieve complete reaction in a
continuous process would require high temperatures and pressures
(which are undesirable from a safety standpoint) of very large
vessels and piping. HF is introduced into a batch process
evaporator vessel 202 and is treated with the oxidant while
stirring for a suitable reaction time. The arsenic in the HF is
oxidized into the +5 oxidation state and fractionation is performed
to remove the As.sup.+5 and metallic impurities. See, U.S. Pat. No.
4,929,435, which is herein incorporated by reference.
[0067] A variety of oxidizing reagents have been used for this
purpose, as shown in the literature. See e.g., U.S. Pat. Nos.
3,685,370, 5,047,226, 4,954,330, 4,955,430, 4,083,441; Canadian
Patent Document Nos. CA 81-177347, CA 74-101216, CA 78-23343, CA
81-177348t, CA98-P200672f; European Patent Document Nos. EP
351,107, EP 276,542; Japanese Patent Document No. JP 61-151002; and
U.S.S.R. Patent Document No. 379,533, all of which are herein
incorporated by reference.
[0068] Fluorine (F.sub.2) has been shown to work by the published
work of others, and is regarded as a preferred embodiment. Fluorine
requires expensive plumbing and safeguards, but has been shown to
be workable. An alternative preferred embodiment uses ammonium
persulfate ((NH.sub.4).sub.2S.sub.2O.sub.8), which is conveniently
available in ultra-high purity. In general, oxidizers which do not
introduce metal atoms are preferred. Thus other candidates include
H.sub.2O.sub.2 and O.sub.3.
[0069] A less preferred candidate is Caro's acid (persulfuric acid,
H.sub.2SO.sub.5, which produces H.sub.2O.sub.2 in solution).
Another option is ClO.sub.2, but this has the severe disadvantage
of being explosive. Other options include HNO.sub.3 and Cl.sub.2,
but both of these introduce anions which must be separated out. The
reduction of non-metallic anions is not as critical as the
reduction of metal cations, but it is still desirable to achieve
anion levels of 1 ppb or less. The initial introduction of anions
thus adds to the load on the ionic purification stage.
[0070] KMnO.sub.4 is a conventional oxidant, and is predicted to be
useable for ultrapurification if followed by the disclosed ionic
purifier and HF stripping process. However, this reagent imposes a
substantial burden of cations on the purifier, so a metal-free
oxidizer is preferred.
[0071] In an alternative embodiment, high-purity hydrofluoric acid,
for example 49% HF, which is essentially arsenic-free can be used
as a starting material. Such low-arsenic material can be used in
combination with an on-site ionic purification process without the
need for an arsenic oxidation step, to produce ultrapure HF
on-site. In this case, the arsenic removal step can be omitted.
[0072] The HF is then distilled in fractionating column 206 to
remove the bulk of the metallic impurities therefrom. Fractionating
column 206 acts as a series of many simple distillations. This is
achieved by packing the column with a high surface area material
with a counter-current liquid flow, thus ensuring complete
equilibrium between the descending liquid and the rising vapor.
Column 206 includes a reboiler 211 and a partial condenser 212
provides reflux. Elements showing significant reduction at this
step include the following:
1 Group 1 (I) Na Group 2 (II) Ca, Sr, Ba Groups 3-12 (IIIA-IIA) Cr,
W, Mo, Mn, Fe, Cu, Zn Group 13 (III) Ga Group 14 (IV) Sn, Pb Group
15 (VII) Sb.
[0073] The purified gaseous HF is then conducted to HF ionic
purifier 208. The HF prior to treatment in the ionic purifier is
pure by normal standards, except for the possible carryover of the
arsenic treatment chemicals or the quench required to remove these
chemicals.
[0074] The HF ionic purifier is utilized as an additional purity
guarantee prior to introduction of the HF gas into the supplier
system 210. Certain elements may be present in the treatment
solution or introduced into the ionic purifier to absorb sulfate
carried over in the HF stream. Ionic purifier testing has
demonstrated significant reductions in the HF gas stream
contamination for these elements:
2 Group 2 (II) Sr, Ba Groups 6-12 (VIA-IIA) Cr, W, Cu Group 13
(III) B Group 14 (IV) Pb, Sn Group 15 (V) Sb.
[0075] Many of the above elements are useful in suppressing the
arsenic contamination. Any carryover in the distillation column
arising from their excess in the arsenic treatment can be rectified
at this step.
[0076] The HF, once introduced into generator 210, can be mixed
with deionized water to provide an HF solution of desired
concentration. During mixing, the solution in generator is
continuously removed and transported by pump 214 through heat
exchanger 216 to remove the heat of reaction therefrom.
[0077] After the solution passes through heat exchanger 216, the
concentration thereof is monitored by sensor 218, which allows for
accurate chemical blending. Suitable generators, sensors, heat
exchangers and other components are described below with reference
to the buffered-HF generator system.
[0078] On-site Preparation of Ultrapure Buffered-HF and Ammonium
Fluoride
[0079] The methods for generating buffered-HF and those for
generating ammonium fluoride (NH.sub.4F) in accordance with the
invention differ only in their respective NH.sub.3 to HF molar
ratios. As a result, the same systems can be used in preparing both
types of solutions, the only difference being in concentration set
points to achieve the desired molar ratios. Thus, to obtain
ammonium fluoride solutions, the set point would be set such that
the NH.sub.3 to HF molar ratio is 1.00, while a molar excess of HF
would be used to prepare buffered-HF solutions.
[0080] On-site generation of buffered-HF and ammonium fluoride will
be described with reference to FIG. 3, which illustrates an
exemplary unit 300 and process flow for generating buffered-HF in
accordance with the invention.
[0081] According to one aspect of the invention, the buffered-HF or
ammonium fluoride can be prepared by bubbling ammonia vapor 302
into a hydrofluoric acid solution 304. The piping for transporting
the chemicals or gases, as well as other wetted surfaces of the
system should be constructed of materials which are compatible with
the chemicals or gases being contacted to avoid or minimize
contamination. Suitable materials include, for example,
polyfluorinated polymers such as Teflon.RTM. (tetrafluoroethylene),
polyfluoroethane (PFA) and polyfluoroethylene (PFE).
[0082] The buffered-HF/ammonium fluoride generation unit 300
includes a mixing tank 306 in which the starting materials are
mixed. In an exemplary embodiment, mixing tank 306 is a 20 gallon
Teflon.RTM. tank. In addition to Teflon.RTM., suitable materials of
construction for the mixing tank include but are not limited to
polyvinyldifluoroethylene (PVDF) and polyethylene.
[0083] While the mixing tank preferably has a volume of from about
1 to 20 gallons, the present invention can easily be applied to
substantially smaller (e.g., on the order of a few cubic
centimeters) or larger (e.g., on the order of several thousand
gallons) volumes.
[0084] The buffered-HF/ammonium fluoride generation unit 300
includes a high-purity deionized (DI) water supply line 308 for
feeding high-purity water into mixing tank 306. HF is fed through
supply line 310 into mixing tank 306. Transport of the HF into the
mixing tank is accomplished with the assistance of pump 312.
Suitable types of pumps are known in the art and include, for
example, double diaphragm pumps, centrifugal pumps and metering
pumps, the fluid contacting portions of which should be constructed
of a non-contaminating material, such as Teflon.RTM.. Suitable
pumps are commercially available from White Knight Corporation.
[0085] Ultra-high-purity ammonia gas is fed into mixing tank 306
via supply line 314. The ammonia can be fed directly from the ionic
purifier (including any subsequent processing) as described above
in reference to the ammonia purification unit, or an other
ultra-high-purity ammonia source. Each of the DI water, HF and
NH.sub.3 supply lines 308, 310 and 314 include a valve 316, 318 and
320, respectively, for regulating the amount of those materials
introduced into the mixing tank.
[0086] To roughly monitor the amount of chemicals introduced into
mixing tank 306, a first level sensor 322 is provided. Suitable
level sensors are known in the art and include, for example,
infrared (IR) or capacitance level sensors. Alternatively, any
suitable volumetric or gravimetric scale can be used.
[0087] During chemical mixing (including ammonia bubbling), the
solution in mixing tank 306 is continuously removed and transported
by pump 324 through heat exchanger 326 to remove the heat of
reaction therefrom. Suitable heat exchangers include, for example,
shell and tube, plate and frame, and jacket and tube-type heat
exchangers. The heat exchanger is preferably formed of a material
which allows for sufficient heat transfer and which does not add
contamination to the product chemical. Suitable materials of
construction for the heat exchanger include, for example,
Teflon.RTM., PVDF, PFA and polyethylene.
[0088] After the solution passes through heat exchanger 326, the
concentration thereof is monitored by sensor 328. Sensor 328 allows
for proper chemical blending during each chemical or gas addition
step to be achieved. That is, sensor 328 can detect the proper
endpoint for mixing the various components during formation of the
buffered-HF/ammonium fluoride solutions. For example, sensor 328
can detect the endpoint for HF dilution with deionized water as
well as during the step of bubbling ammonia vapor into the aqueous
HF solution.
[0089] An acoustic velocity sensor 328 can be used for this
purpose. Such equipment is commercially available from Mesa Labs.
The application of acoustic sensors to chemical blending is
described in detail in PCT Application No. PCT/US96/10389, Attorney
Docket No. 016499-263, filed on Jun. 5, 1996, the contents of which
are herein incorporated by reference. In place of acoustic velocity
measurement equipment, product concentration can be measured using,
for example, conductivity, density, index of refraction, or
infrared (IR) spectroscopy measurement equipment.
[0090] To further purify the chemical withdrawn from mixing tank
306, the chemical can optionally be passed through a filter 330.
The filter is preferably constructed of Teflon.RTM.. However, the
filter can be formed from other materials which do not contaminate
the formed chemical. Filter 330 preferably has a pore size of, for
example, from 0.05 to 0.1 .mu.m.
[0091] Depending upon the concentration measurement by sensor 328
and the particular mixing step being monitored, the chemical can be
withdrawn from the generation unit via line 332 as a final product
by opening valve 334 and by closing valve 336. If the chemical is
not of the proper final concentration, it can be reintroduced into
mixing tank 306 via recycle line 338 by opening valve 336 and by
closing valve 324.
[0092] The concentration measurement system can be connected to a
valve control system which will automatically operate the valves to
control material flow throughout the system. Those skilled in the
art will readily be able to design and integrate appropriate
controls in the inventive system by use of well known devices,
circuits and/or processors and means for their control. Further
discussion of this matter is omitted as it is deemed within the
scope of persons of ordinary skill in the art.
[0093] Mixing tank 306 further includes a vent (exhaust) line 340
in an upper portion thereof for removing vapors from the tank. Vent
line 340 can be connected to a downstream exhaust treatment
apparatus, such as a gas scrubber. To prevent contamination of the
chemicals in mixing tank 306 resulting from the backflow of
contaminants through the vent line, a flow of an inert gas, such as
nitrogen or argon, across the entrance to the vent line (i.e., an
inert gas pad) can be used.
[0094] According to a method for preparing buffered-HF or ammonium
fluoride according to a first aspect of the invention,
ultra-high-purity anhydrous HF is introduced into mixing tank 306
and is diluted to the proper concentration with deionized water.
Next, anhydrous ammonia can be added to the acid solution to an
appropriate endpoint as determined by concentration analysis to
obtain buffered-HF or ammonium fluoride.
[0095] The following example is provided to illustrate how 1 kg of
40% by weight ammonium fluoride solution can be generated according
to one aspect of the invention. At first, the total respective
amounts of HF and NH.sub.3 to be dissolved in water are determined.
1 kg of 40% by weight ammonium fluoride (NH.sub.4F) solution would
contain 400 g of NH.sub.4F and 600 g of ultra pure water. Since the
HF:NH.sub.3 molar ratio is 1:1 for pure NH.sub.4F, the 400 g of
NH.sub.4F would include 216 g of anhydrous HF and 184 g of
anhydrous NH.sub.3 (NH.sub.4F=37 g/mole; HF=20 g/mole; NH.sub.3=17
g/mole).
[0096] At completion of the HF formation cycle, 216 g of anhydrous
HF would be dissolved in 600 g of water, resulting in a 26.5% by
weight HF solution. On-board instrumentation in the system controls
the addition of HF to the water to achieve the proper HF
concentration. As an alternative to starting with anhydrous HF, a
49% HF starting solution can be diluted to this concentration.
After the 26.5% HF solution is formed, 189 g of NH.sub.3 are added
to mixing tank 306 via line 314 to form the 40% NH.sub.4F
solution.
[0097] Other concentrations and molar ratios can be set by the
concentration instrumentation for different applications simply by
adjustment of the instrumentation. As a result, ammonium fluoride
solutions and buffered-HF solutions of various concentrations can
be generated.
[0098] According to a method for preparing buffered-HF or ammonium
fluoride according to another aspect of the invention, premixed 49%
HF or HF of any other concentration can be added to the mixing
tank. The HF is then diluted with deionized water, if necessary, to
the appropriate concentration endpoint as determined by the
concentration sensor. Next, anhydrous ammonia can be added to an
appropriate endpoint as determined by concentration analysis to
obtain buffered-HF or ammonium fluoride.
[0099] According to the above-described methods, HF and high-purity
water are mixed to the desired concentration, followed by the
addition of ammonia to the requisite concentration to form
buffered-HF or ammonium fluoride.
[0100] According to a further aspect of the invention, buffered-HF
can be prepared by first forming an ammonium fluoride solution, for
example, a 40% NH.sub.4F solution, according to the above
procedures. This can then be followed by the addition of HF until
hydrofluoric acid of the desired concentration is obtained. The
concentrations during this final HF addition step can be controlled
gravimetrically or by using any of the concentration control
techniques described above.
[0101] The buffered-HF or NH.sub.4F generation system can be
positioned in close proximity to the point of use of the ultrapure
chemical in the production line, leaving only a short distance of
travel between the purification unit and the production line.
Alternatively, for plants with multiple points of use, the
ultrapure chemical from the generation unit can pass through an
intermediate holding tank before reaching the point(s) of use.
Further, the mixing tank of the buffered-HF or NH.sub.4F generator
system itself can be the point of use, in which the substrates are
processed.
[0102] Each point of use can be fed by an individual outlet line
from the holding tank. In either case, the ultrapure chemical can
therefore be directly applied to the semiconductor substrate
without packaging or transport and without storage other than a
small in-line reservoir, and thus without contact with the
potential sources of contamination normally encountered when
chemicals are manufactured and prepared for use at locations
external to the manufacturing facility.
[0103] In this class of embodiments, the distance between the point
at which the ultrapure chemical leaves the purification system and
its point of use on the production line will generally be a few
meters or less. This distance will be greater when the purification
system is a central plant-wide system for piping to two or more use
stations, in which case the distance may be two thousand feet or
greater. Transfer can be achieved through an ultra-clean transfer
line of a material which does not introduce contamination. In most
applications, stainless steel or polymers such as high density
polyethylene or fluorinated polymers can be used successfully.
[0104] Due to the proximity of the purification unit to the
production line, the water used in the unit can be purified in
accordance with semiconductor manufacturing standards. These
standards are commonly used in the semiconductor industry and are
well known among those skilled in the art and experienced in the
industry practices and standards.
[0105] Methods of purifying water in accordance with these
standards include ion exchange and reverse osmosis. Ion exchange
methods typically include most or all of the following units:
chemical treatment such as chlorination to kill organisms; sand
filtration for particle removal; activated charcoal filtration to
remove chlorine and traces of organic matter; diatomaceous earth
filtration; anion exchange to remove strongly ionized acids; mixed
bed polishing, containing both cation and anion exchange resins to
remove further ions; sterilization, involving chlorination or
ultraviolet light; and filtration through a filter of 0.45 micron
or less. Reverse osmosis methods will involve, in place of one or
more of the units in the ion exchange process, the passage of the
water under pressure through a selectively permeable membrane which
does not pass many of the dissolved or suspended substances.
[0106] Typical standards for the purity of the water resulting from
these processes are a resistivity of at least about 15 megohm-cm at
25.degree. C. (typically 18 megohm-cm at 25.degree. C.), less than
about 25 ppb of electrolytes, a particulate content of less than
about 150/cm.sup.3 and a particle size of less than 0.2 micron, a
microorganism content of less than about 10/cm.sup.3, and total
organic carbon of less than 100 ppb.
[0107] Wafer Cleaning
[0108] FIG. 4 illustrates exemplary wafer cleanup stations in a
conventional line 400 for semiconductor fabrication. The first unit
in the cleaning line is a photoresist stripping station 402, in
which aqueous hydrogen peroxide 404 and sulfuric acid 406 are
combined and applied to the semiconductor surface to strip off the
resist. This is followed by a rinse station 408, where deionized
water is applied to rinse off the stripping solution.
[0109] Immediately downstream of rinse station 408 is a cleaning
station 410 into which an aqueous solution of ammonia and hydrogen
peroxide are applied. This solution is supplied in one of two ways.
In the first, aqueous ammonia 412 is combined with aqueous hydrogen
peroxide 414, and the resulting mixture 416 is directed to cleaning
station 410.
[0110] According to the second method, pure gaseous ammonia 418 is
bubbled into an aqueous hydrogen peroxide solution 420 to produce a
similar mixture 422, which is likewise directed to cleaning station
410. Once cleaned with the ammonia/hydrogen peroxide combination,
the semiconductor passes to second rinse station 424 where
deionized water is applied to remove the cleaning solution.
[0111] The next station is a further cleaning station 426 where
aqueous solutions of hydrochloric acid 428 and hydrogen peroxide
430 are combined and applied to the semiconductor surface for
further cleaning. This is followed by a final rinse station 432
where deionized water is applied to remove the HCl and
H.sub.2O.sub.2.
[0112] At deglaze station 434, dilute aqueous HF or dilute
buffered-HF is applied to the wafer, for example, to remove a
native or other oxide film. The dilute buffered-hydrofluoric acid
can be supplied using a system as described above. For example, the
buffered-HF can be supplied directly, through sealed piping, from
generator 436. HF reservoir 438 holds anhydrous HF, from which a
stream of gaseous HF is fed through ionic purifier 440 into the
generator. To provide a buffered solution, gaseous ammonia can be
bubbled into generator 436 and ultrapure deionized water can be
added to achieve the desired dilution. This is followed by a rinse
in ultrapure deionized water at station 442, and drying at station
444.
[0113] The wafer or wafer batch 446 being treated is held on a
wafer support 448 and is conveyed from one workstation to the next
by a robot 450 or some other conventional means of achieving
sequential treatment. The means of conveyance can be totally
automated, partially automated or not automated at all.
[0114] The system shown in FIG. 4 is just one example of a cleaning
line which can be used in the manufacture of semiconductor devices.
In general, cleaning lines for high-precision manufacture can vary
widely from that shown in FIG. 4, either by eliminating one or more
of the units shown or by adding or substituting one or more units
not shown. The concept of the on-site preparation of high-purity
buffered-HF and ammonium fluoride, however, in accordance with this
invention is applicable to all such systems.
[0115] Modifications and Variations
[0116] While the invention has been described in detail with
reference to specific embodiments thereof, it will be apparent to
those skilled in the art that various changes and modifications can
be made, and equivalents employed, without departing from the scope
of the appended claims. For example, the disclosed innovative
techniques can be applied to the manufacture of products other than
ICs, such as discrete semiconductor components (e.g.,
optoelectronic and power devices), and to other manufacturing
technologies in which IC manufacturing methods have been adopted
(e.g., the manufacture of thin-film magnetic heads and
active-matrix liquid-crystal displays).
[0117] Furthermore, filtration units or stages in addition to those
described above can be combined with the disclosed purification
apparatus.
[0118] It should also be noted that additives can be introduced
into the purification water if desired, although this is not done
in the presently preferred embodiment.
[0119] According to a further aspect of the invention, the
disclosed methods and systems can be adapted to operate as part of
a manufacturing unit to produce ultra-high-purity chemicals for
packaging and/or shipment. In this case, however, the advantages
associated with the generation and purification of the chemicals
on-site would not be realized. While such applications are subject
to the above-discussed problems associated with the handling of
ultra-high-purity chemicals, the disclosed innovations nevertheless
provide an initial purity which is higher than that available by
other techniques.
[0120] Furthermore, although the primary embodiment is directed to
providing ultrapure aqueous chemicals which are most critical for
semiconductor manufacturing, the disclosed system and method
embodiments can also be used to supply purified gas streams. In
many cases, use of a dryer downstream from the purifier can be used
for this purpose.
[0121] It should also be noted that piping for ultrapure chemical
routing in semiconductor front ends may include in-line or pressure
reservoirs. Thus references to "direct" piping does not preclude
the use of such reservoirs, but does preclude exposure to
uncontrolled atmospheres.
* * * * *