Process and composition for purifying semiconductor process gases to remove Lewis acid and oxidant impurities therefrom

Scavenger compositions useful for purifying process gas streams, such as process gas streams, such as hydrogen, nitrogen, noble gases, diborane, and hydride gases from Groups IVA-VIA of the Periodic Table, such as arsine, phosphine, silane, germane, hydrogen selenide, and hydrogen telluride, and mixtures thereof, to remove water, oxygen, and other oxidant and Lewis acid impurities therefrom, such scavenger comprising a porous, high surface area inert support having thereon an active scavenging species, formed by the deposition on the support of a Group IA metal and pyrolysis thereof at a selected elevated temperature on said support. In another aspect, the present invention relates to a method of making a scavenger useful for purifying process gas streams, to remove water, oxygen, and other oxidant and Lewis acid impurities therefrom, and a process for purifying process gas streams to remove water, oxygen, and other oxidant and Lewis acid impurities therefrom, such process comprising contacting the impurity-containing process gas stream with a scavenger of the general type described above. In a further aspect, the invention relates to a method for using scavengers of the general type described above as back-diffusion scrubbers to protect the manufacturing process or gas supply system from inadvertent introduction of impurities, such method comprising contacting the impurity-containing process gas stream with a scavenger of the general type described above and providing in the scavenger bed one or more endpoint detectors so that back-diffusion events are observed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a composition and method for removing 
water, oxygen, and other oxidant and Lewis acid impurities from a flowing 
gas stream. The purification method does not contaminate the gas stream 
with added hydrocarbon impurities. 
2. Description of the Related Art 
The provision of high purity gas streams is critically important in a wide 
variety of industrial and research applications. The rapid expansion of 
vapor-phase processing techniques, e.g., chemical vapor deposition, in the 
semiconductor industry has been associated with the deployment and use of 
manufacturing equipment that is totally reliant on the delivery of 
ultra-high purity process gases at the point of use in the semiconductor 
manufacturing facility. Currently, over 5 billion dollars worth of such 
equipment is in use. 
Considering the impurities which are present in gas streams involved in 
semiconductor manufacturing, it is to be noted that the growth of high 
quality thin film electronic and opto-electronic cells by chemical vapor 
deposition or other vapor-based techniques is inhibited by a variety of 
low-level process impurities. These impurities can cause defects that 
reduce yields by increasing the number of rejects, which can be very 
expensive. These impurities may be particulate or chemical contaminants. 
Particulates are typically filtered out of the gas stream using extremely 
efficient commercially available particle filters, with particle 
filtration generally being employed at the point of use. 
Chemical impurities may originate in the production of the source gas 
itself, as well as in its subsequent packaging, shipment, storage, and 
handling. Although source gas manufacturers typically provide analyses of 
source gas materials delivered to the semiconductor manufacturing 
facility, the purity of such gases may change because of leakage into or 
outgassing of the containers, e.g., gas cylinders, in which the gases are 
packaged. Impurity contamination may also result from improper gas 
cylinder changes, leaks into downstream processing equipment, or 
outgassing of such downstream equipment. 
Chemical impurities that are of special concern in semiconductor 
manufacturing processes include water, oxygen, and other oxidant and Lewis 
acid species such as aluminum, boron or zinc-containing species. In 
general, the key chemical impurities must be held at levels of a few parts 
per billion or lower. 
In support of the requirement for high purity process gases, a number of 
types of gas purifiers have been introduced that remove chemical 
contaminants from the semiconductor process gases at the point of use. 
These gas purifiers employ a variety of sorption processes to remove 
impurities, including physisorption processes, e.g. gas adsorption by 
zeolites or activated carbon, or various chemisorption processes, where 
the impurities adsorb to and chemically react with a component or 
components of the purifier. 
Particularly useful in-line purifiers are based on passive sorption 
processes, wherein the impurity species are adsorbed and chemically 
reacted with scavengers bound to or incorporated in porous inert support 
materials. Such purifiers are described in U.S. Pat. Nos. 4,603,148, 
4,604,270, 4,659,552, 4,800,189. Because of their usefulness in purifying 
semiconductor process gas streams, where the requirements for purity are 
stringent, such purifiers have been the subject of much research and 
development activity, as well as significant commercial success. U.S. Pat. 
Nos. 4,761,395 (composition for purification of arsine, phosphine, ammonia 
and inert gases); 4,853,148 (hydrogen halide purification); 4,797,227 
(hydrogen selenide purification); 4,781,900 (method of purifying arsine, 
phosphine, ammonia and inert gases); 4,950,419 (inert gas purification); 
4,685,822 (hydrogen selenide purification); 4,925,646 (hydrogen halide 
purification method); 4,983,363 (apparatus for purifying arsine, 
phosphine, ammonia and inert gases); and 5,015,411 (inert gas purification 
method) describe this type of purifier and their disclosures are hereby 
incorporated herein. This class of purifiers is quite versatile, since the 
immobilized scavenger species may be varied and tailored to react with a 
large number of different impurities. Because the support material is 
usually porous, contact of the scavenger with the gas stream is extensive. 
Such gas purifiers are used to remove Lewis acid and oxidant impurity 
species, particularly water and oxygen, which have deleterious effects on 
the semiconductor manufacturing process. By varying the chemical identity 
of the scavenger, they may also be used to remove undesirable dopant 
species from the gas stream. Such gas purifiers can be very simple in 
design and operation, since purification occurs passively, simply by 
contact of the process gas stream with the scavenger 
As described in applicant's copending U.S. patent application Ser. No. 
07/898,840, the disclosure of which is hereby incorporated herein, these 
purifiers can also be used advantageously in the back-diffusion scrubber 
mode. The purifier is outfitted with one or more endpoint detectors, and 
is positioned to purify the process gas stream before its entrance into 
the process tool, and also serves as an impurity scrubber that protects 
the gas supply against contamination caused by diffusion of one or more 
foreign components back into the supply lines. Back-diffusion can occur 
when mechanical components such as check valves and shut-off valves fail. 
Additionally, in low flow conditions, impurities can successfully diffuse 
against the convective forward flow. An example of a situation where 
back-diffusion is of concern is the case where an inert gas such as 
nitrogen is used to pressurize vessels containing liquids used in 
semiconductor manufacturing processes. Such liquids include sulfuric acid, 
isopropanol, acetone and the like, which can cause corrosion and 
contamination of the nitrogen supply system by back-diffusion under low 
flow conditions. 
When the purifier is used for a back-diffusion scrubber, endpoint detection 
is critical. Back-diffusion is not planned for, and therefore it is 
impossible to predictively calculate the purifier's lifetime on the basis 
of flowrates, expected impurity concentrations, and so forth. Endpoint 
detection allows the immediate detection of a serious back-diffusion 
event, and the appropriate precautions to protect the gas supply may be 
mobilized. Use of two endpoint detectors disposed at separate points in 
the gas purifier's scavenger bed allows back-diffusion to be distinguished 
from normal exhaustion of the purifier. If the downstream endpoint 
detector signals purifier depletion before the upstream one does, 
back-diffusion can be diagnosed in a straightforward and simple way. 
While the gas purifiers of the types described above are very effective at 
removing impurities from the process gas streams to very low levels, the 
scavengers may contribute low levels of hydrocarbon impurity to the gas 
streams being purified. U.S. Pat. Nos. 4,604,270 and 4,603,148 to G. M. 
Tom disclose scavengers in which alkyl metal compounds are immobilized by 
coupling them to an organic polymeric support, followed by pyrolysis to 
yield a dispersed phase of the metal hydride in the organic polymer 
matrix. For example, dried, porous styrene-divinylbenzene copolymer 
(PSDVB) beads are mixed with butyllithium and heated for a prolonged 
period in an oven to immobilize butyllithium species on the resin and 
largely convert the butyllithium to lithium hydride which is immobilized 
in the porous polymer beads. Such scavengers can contribute the butane 
elimination reaction by-product to gas streams being purified. Other 
purifiers that have alkylmetal-based scavengers or scavengers prepared 
from alkylmetal starting materials may manifest this same behavior. 
Hydrocarbon impurities, even at very low levels, are highly undesirable in 
semiconductor process gas streams. In chemical vapor deposition processes, 
the high temperatures or plasmas in the reactor can cause decomposition of 
the hydrocarbon impurity and incorporation of carbon in the growing film. 
Carbon, a Group IVA element, is a dopant in compound semiconductors of the 
III-VI type. 
Process gas purifiers based on other sorption principles such as metal 
eutectic alloy getters are sometimes employed, and these purifiers avoid 
the hydrocarbon impurity problem. For example, European Patent Application 
EP 470,936 describes removal of impurities from hydride gases by passing 
the hydride gas over a hydrogenated getter metal in a chamber. In 
particular, disiloxane may be removed from silane using hydrogenated 
Zr-V-Fe getter alloy. Gases which may be purified in this fashion include 
SiH.sub.4, GeH.sub.4, NH.sub.3, AsH.sub.3, SbH.sub.3 and PH.sub.3, all of 
which are used in semiconductor manufacturing. European Patent Application 
EP 365,490 describes a method for removing impurity gases from inert gases 
such as argon or nitrogen using a first sorbent of either a non-evaporable 
getter alloy of Zr-V-Fe or Zr-Fe and a second sorbent of a non-evaporable 
getter alloy of 5-50% Al, balance Zr. Both sorbents are pellets formed 
from alloy powder of average particle size below 125 microns, with the 
first sorbent being located at the gas inlet and the second at the gas 
outlet. 
These metal eutectic alloy getters, while avoiding the problem of 
hydrocarbon contribution to the process gas stream, are not the simple, 
elegant systems that the passive sorption-based purifiers described 
earlier are. The getters must be operated at high temperatures, in the 
range of 300.degree. C. to 500.degree. C. At lower temperatures, e.g. 
about 25.degree. C., the scavenging capacities of the getters are low. 
Because the gases being purified can be highly flammable, e.g., silane or 
hydrogen, and because of the added complexity and expense involved in 
their use, the metal eutectic alloy getters are not the most desirable 
solution to the problem. Impurity reduction using the getters has been 
shown to be less efficient than competing sorption-based purifier 
technology. 
The presence of even small concentrations of impurity species in the 
process gas streams employed in semiconductor manufacturing is potentially 
deleterious. Even small levels of impurities on the order of parts per 
million (ppm) can cause inconsistent electrical properties in 
semiconductor devices manufactured by deposition techniques using 
impurity-containing gas streams. 
It therefore is an object of the present invention to provide a simple, 
rapid, and versatile purification system able to provide a high level of 
purification efficiency, with regard to removal of water, oxygen, and 
other oxidant and Lewis acid impurities, such as is required to protect 
semiconductor manufacturing processes. 
It is a further object of the present invention to provide an improved 
scavenger characterized by high scavenging capacity with regard to removal 
of water, oxygen, and other oxidant and Lewis acid impurities, and that 
avoids previous problems of hydrocarbon contamination. 
It is still another object of the invention to provide a method of making 
the aforementioned scavengers, and a process and apparatus for using the 
same to purify process gas streams, to remove water, oxygen, and other 
oxidant and Lewis acid impurities therefrom. Such scavengers and purifier 
systems can also be used in a back-diffusion scrubber mode to protect the 
integrity of the manufacturing process or gas supply system. 
SUMMARY OF THE INVENTION 
The present invention relates in one aspect to a scavenger composition 
useful for purifying process gas streams, such as hydrogen, nitrogen, 
noble gases (helium, neon, argon, krypton, and xenon), diborane, and 
hydride gases from Groups IVA-VIA of the Periodic Table, such as arsine, 
phosphine, silane, germane, hydrogen selenide, and hydrogen telluride, and 
mixtures thereof, to remove oxidants and Lewis acid impurities, 
particularly water and oxygen, therefrom, such scavenger comprising: 
(a) an inert support having a surface area in the range of from about 50 to 
about 1000 square meters per gram (as measured by the conventional BET 
surface area determination), and thermally stable up to at least about 
250.degree. C.; and 
(b) an active scavenging species, present on the support at a concentration 
of from about 0.01 to about 1.0 moles per liter of support, and formed by 
the deposition on the support of a Group IA metal selected from sodium, 
potassium, rubidium, and cesium and their mixtures and alloys and 
pyrolysis thereof at a selected elevated temperature on said support. 
In another aspect, the present invention relates to a method of making a 
scavenger useful for purifying process gas streams, to remove oxidants and 
Lewis acid impurities, particularly water and oxygen, therefrom, such 
method comprising: 
(a) providing a support having a surface area in the range of from about 50 
to about 1000 square meters per gram, and thermally stable up to at least 
about 250.degree. C.; 
(b) depositing on such support, at a concentration from about 0.01 to about 
1.0 moles per liter of support, a Group IA metal selected from the group 
consisting of sodium, potassium, rubidium and cesium; and 
(c) pyrolyzing the metal on the support, at a temperature of from about 
125.degree. C. to about 225.degree. C., under a blanketing atmosphere of 
an inert gas such as argon, nitrogen, or helium, with intermittent or 
continuous mixing, to distribute the alkali metal throughout the porous 
support. 
A further aspect of the invention relates to a process for purifying gas 
streams to remove oxidants and Lewis acid impurities, particularly water 
and oxygen, therefrom, such process comprising contacting the 
impurity-containing process gas stream with a scavenger of the general 
type described above. 
In a further aspect, the invention relates to a method for using scavengers 
of the general type described above as back-diffusion scrubbers to protect 
the manufacturing process or gas supply system from inadvertent 
introduction of impurities, such method comprising contacting the 
impurity-containing process gas stream with a scavenger of the general 
type described above and providing in the scavenger bed one or more 
endpoint detectors so that back-diffusion events are observed.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention overcomes the deficiencies of the prior art 
sorption-based process gas purification systems, as described in the 
"Background of the Invention" section hereof by the provision of a 
non-hydrocarbon-releasing gas purification system which is specifically 
adaptable for use in the manufacture of semiconductor devices including 
vapor deposition based processes. 
In semiconductor manufacturing operations, water vapor and oxygen are 
regarded as the critical impurities, whose presence often indicates 
atmospheric contamination of the process system. Accordingly, the 
invention will be illustratively described hereinafter primarily with 
reference to removal of water or oxygen as the impurity species of 
interest. It will be recognized, however, that such focus is for 
descriptive purposes only and that the invention is broadly practicable in 
scavenging other impurity species that are oxidants or Lewis acids, since 
these materials all react rapidly and avidly with the scavengers of the 
present invention. 
First, the gas purification system must itself be non-contaminating in 
character, with respect to the gas stream being processed. Since the 
flowing gas stream after its purification is flowed to the deposition 
reactor or other locus of use, any contaminants deriving from the 
scavenger will subsequently be distributed throughout the process system. 
Any contributed impurities may have a deleterious effect on the products 
being manufactured. Accordingly, any impurities introduced from the 
purification system itself should be suitably low, e.g., in the parts per 
billion range or lower. 
The gas purification system must be mechanically tight and leak-free in 
character. This requirement dictates the use of correspondingly suitable 
materials of construction in the purification system, with the parts and 
components of the detector system having a high finish on those parts and 
components which are in contact with the gas stream, and with all seals of 
the purifier being of a face seal, leak-tight character. 
Preferred materials of construction for the purifier housing, connections 
and valving are stainless steel, glass, or chemically resistant epoxies. 
If any particulates are generated in the use and operation of the 
purifier, particle filters may be required components of the system. This 
requirement is readily met in actual practice, since most commercially 
available gas purifiers incorporate a particle filter as an integral part 
of the design. Any endpoint detector sensing unit incorporated in the 
purifier design should be positioned upstream of the particle filter. 
The scavenger must react rapidly and essentially irreversibly with the 
impurities of interest. The scavenger should incorporate a highly reactive 
species immobilized on a porous support medium, so that the contact time 
of the gas with the reactive species will be sufficient for adequate 
purification to occur. 
Further, the scavenger system should be chemically stable when stored for 
substantial periods of time, e.g., at least six months, and preferably on 
the order of one year or more, without the scavenger becoming degraded and 
losing its high reactivity with impurities. 
Additionally, the cost of the process gas purification system should be 
suitably low to ensure ready commercial deployment, with economic, readily 
available gas purifier devices being utilizable in present and foreseeable 
semiconductor processing systems. 
The foregoing criteria are accommodated in the broad practice of the 
present invention by the provision of a gas purification system in which 
the flowing process gas stream is passed over a scavenger material that is 
highly reactive with the impurity or impurities of interest, and that 
reacts with the impurities to form involatile products that are held 
within the porous support medium and not released into the purified 
process gas stream. 
The present invention relates in one aspect to a scavenger composition 
useful for purifying process gas streams comprising hydrogen, nitrogen, 
noble gases (helium, neon, argon, krypton, and xenon), and hydride gases 
(tabulated below), or mixtures thereof, to remove oxidants and Lewis acid 
impurities, particularly water and oxygen, therefrom, such scavenger 
comprising: 
(a) an inert support having a surface area in the range of from about 50 to 
about 1000 square meters per gram, and thermally stable up to at least 
about 250.degree. C.; and 
(b) an active scavenging species, present on the support at concentration 
ranging from about 0.01 to about 1.0 moles per liter of support, 
preferably 0.01-0.25 moles per liter of support, and formed by the 
deposition on the support of a Group IA metal and pyrolysis thereof at a 
selected elevated temperature on said support. 
Hydride gases that can be purified using the scavenger of the present 
invention include the following, tabulated according to Periodic Group of 
the nonhydrogen element: 
______________________________________ 
Group IIIA 
Group IVA Group VA Group VIA 
______________________________________ 
B.sub.2 H.sub.6 
CH.sub.4 NH.sub.3 
diborane methane ammonia 
SiH.sub.4 PH.sub.3 H.sub.2 S 
silane phosphine hydrogen sulfide 
GeH.sub.4 AsH.sub.3 H.sub.2 Se 
germane arsine hydrogen selenide 
SnH.sub.4 SbH.sub.3 H.sub.2 Te 
stannane stibine hydrogen telluride 
______________________________________ 
Properties of metals that are useful as the active scavenging species are 
shown below (from R. C. West, Ed., CRC Handbook of Chemistry and Physics, 
65th Ed., CRC Press, Inc., Boca Raton, Fla., pp. D155-D159, 1984): 
______________________________________ 
Melting Vapor Forms Reactivity 
Point Pressure Passive 
with Inert 
Metal (.degree.C.) 
(atm) E (V) Oxide Gases 
______________________________________ 
Na 97.8 3 .times. 10.sup.-14 
-2.71 No None 
K 63.7 2 .times. 10.sup.-11 
-2.93 No None 
Rb 38.9 3 .times. 10.sup.-10 
-2.98 No None 
Cs 28.7 3 .times. 10.sup.-9 
-2.92 No None 
______________________________________ 
The metal for the active scavenging species can be chosen from the Group IA 
metals, or mixtures or alloys of the same. Na, K, Rb, and Cs or mixtures 
or alloys of the same are preferred, with K most preferred. These metals 
are soft, have low vapor pressure, are extremely sensitive to O.sub.2 and 
H.sub.2 O, and are readily and inexpensively available. Note, however, 
that lithium will react with nitrogen and may be unsatisfactory for 
service in this gas stream, and in addition, lithium has a much higher 
melting point (180.5.degree. C.) and therefore could not be 
vapor-deposited on the resin beads as conveniently as Na, K, Rb, and Cs. A 
further consideration is the effect that any trace of the scavenging metal 
would have on the semiconductor process. 
The support medium must be inert in the intended application, where "inert" 
means that the support is non-reactive with the Lewis acid and oxidant 
impurities which are reactively removed by the active scavenging species 
present on the support, and that the support is also non-reactive with the 
gases being purified by the scavenger. The support must be compatible with 
the gas mixtures being purified, and the reaction products of the impurity 
removal, and any intermediates involved with conditioning or otherwise 
preparing the scavenger, and must be stable under the conditions of use. 
The preferred characteristics of supports which are useful for scavengers 
of the invention include (a) high surface area, for example, a surface 
area in the range of from about 50 to about 1000 square meters per gram of 
support (as measured by the conventional BET surface area determination), 
(b) high porosity from pores of a diameter in the range of from about 3 to 
about 200 .ANG.ngstroms, and (c) good thermal stability, e.g., thermally 
stable at temperatures up to about 250.degree. C. 
Illustrative support materials which may be potentially useful in the broad 
practice of the invention include macroreticulate polymers, such as those 
formed from monomers such as styrene, vinyltoluene, vinylisopropylbenzene, 
ethylvinylbenzene, vinylnaphthalene, alpha-methylstyrene, 
beta-methylstyrene, and mixtures thereof. Such polymers may suitably be 
polymerized in the presence of a cross-linking agent such as 
divinylbenzene or divinylnaphthalene. 
A particularly preferred macroreticulate polymer is 
poly(styrenedivinylbenzene), commercially available as Amberlite XAD4 (50 
.ANG.ngstrom pore size) and Amberlite XAD2 (100 .ANG.ngstrom pore size), 
from Rohm and Haas Corp., Philadelphia, Pa. 
In general, the scavengers of the present invention are prepared by a 
process comprising: 
(a) providing a support having a surface area in the range of from about 50 
to about 1000 square meters per gram, and thermally stable up to at least 
about 250.degree. C.; 
(b) depositing on such support, at a concentration from about 0.01 to about 
1.0 moles per liter of support, preferably 0.01-0.25 moles per liter of 
support, a Group IA metal selected from the group consisting of sodium, 
potassium, rubidium and cesium and mixtures and alloys of the same; and 
(c) pyrolyzing the metal on the support, at a temperature of from about 
125.degree. C. to about 225.degree. C., under a blanketing atmosphere of 
an inert gas such as argon, nitrogen, or helium, with intermittent or 
continuous mixing, to distribute the alkali metal throughout the porous 
support. 
The support is selected as described above and is provided in a reactor 
vessel in a dry state, which can be obtained by heating the support in a 
dry atmosphere. The addition of the metal for preparing the active 
scavenging species is carried out in an inert atmosphere such as in an 
inert atmosphere glove box or the like. The metal used for preparing the 
active scavenging species, selected as described above, is added to the 
support in a liquid or solid state. The reactor vessel is then protected 
from contact with oxygen or moisture in the atmosphere by sealing or some 
other method, and heated to a temperature sufficient to melt the metal and 
disperse it throughout the pores of the support. The reactor vessel may be 
periodically or continuously agitated during this heating step to 
facilitate dispersion of the metal. The heating step may be carried out 
initially at a lower temperature, i.e. on the order of about 100.degree. 
C., to melt the metal, and then after the molten metal has been dispersed 
throughout the porous support, the temperature may be increased to a 
higher level, i.e., on the order of 200.degree.- 230.degree. C., and held 
there for several hours, to immobilize the metal on the porous support. 
When the heating step is complete, a color change is generally observed, 
indicating a strong interaction of the metal with the porous support 
medium. 
It should be noted that, by contrast with the methods for preparing prior 
art scavengers for passive sorption based gas purification, such as those 
described in the Related Art section, e.g. U.S. Pat. Nos. 4,603,148, 
4,761,395, and 4,950,419, the method of the present invention does not 
require solvents such as benzene or toluene. This advantage, possible 
because the scavenger is loaded onto the inert support medium as a molten 
metal rather than as a solution of an organometallic precursor compound, 
provides cost and environmental benefits. Solvents such as benzene and 
toluene require careful control to prevent exposure to personnel, and 
solvent wastes must be disposed of through regulated, sometimes costly, 
means. 
The scavengers of the invention may be readily formed into a bed through 
which the impurity-containing gas stream is flowed for purification 
thereof, thereby providing a highly efficient removal system for water, 
oxygen, and other oxidant and Lewis acid impurities from process gas 
streams, such as hydrogen, nitrogen, noble gases, diborane, and hydride 
gases from Groups IVA-VIA of the Periodic Table, such as arsine, 
phosphine, silane, germane, hydrogen selenide, and hydrogen telluride, or 
mixtures thereof. The capacities of the scavengers for water removal can 
range from about 0.2 to about 20 liters gaseous water per liter of a bed 
of said scavenger, with the corresponding oxygen removal capacities being 
approximately equal to the values for water removal capacities. 
The scavenging capacity depends upon the loading of the Group IA metal on 
the porous support. In use with hydride gases, scavengers of the invention 
may, upon initial contact with the gas being purified, undergo exothermic 
"preconditioning" reactions to form active scavenging species. Thus it 
should be appreciated that in service with inert gases, high capacity and 
hence high loading may be desirable, whereas with hydride gases, because 
the initial preconditioning reaction of the hydride gas with the scavenger 
is exothermic, a lower loading may be preferred. 
The process gas stream purification can take place at the point at which 
the gas is loaded into cylinders for delivery or at a central point in the 
process plant where the gas is used ("bulk purification"). Alternatively, 
the scavenger may be incorporated into a smaller-scale unit and 
purification may take place immediately before the gas stream enters the 
process tool, i.e., the chemical vapor deposition reactor, reagent 
pressurization unit, or other gas-requiring process unit. Because of the 
simplicity of the passive sorption-based purification provided by the 
scavengers of the present invention, such "in-line purification" is easily 
facilitated, in contrast to prior art, non-hydrocarbon contaminating 
methods such as the metal eutectic getters described above in the "Related 
Art" section, which must be heated to a high temperature to be effective 
scavengers. 
In a further aspect, the invention relates to a method for using scavengers 
of the general type described above as back-diffusion scrubbers to protect 
the manufacturing process or gas supply system from inadvertent 
introduction of impurities, such method comprising contacting the 
impurity-containing process gas stream with a scavenger of the general 
type described above and providing in the scavenger bed one or more 
endpoint detectors so that back-diffusion events are observed. The 
endpoint detectors signal the occurrence of a major back-diffusion event 
through coupling to alarms or lights that notify personnel that the 
purifier has been exhausted and needs to be replaced. The purifier 
essentially serves as an impurity scrubber that protects the gas supply 
against contamination caused by diffusion of one or more foreign 
components back into the supply lines. Back-diffusion can occur when 
mechanical components such as check valves and shut-off valves fail. 
Additionally, in low flow conditions, impurities can successfully diffuse 
against the convective forward flow. An example of a situation where 
back-diffusion is of concern is the case where an inert gas such as 
nitrogen is used to pressurize vessels containing liquids used in 
semiconductor manufacturing processes. Such liquids include sulfuric acid, 
isopropanol, acetone and the like, which can cause corrosion and 
contamination of the nitrogen supply system by back-diffusion under low 
flow conditions. 
When the purifier is used for a back-diffusion scrubber, endpoint detection 
is critical. Back-diffusion is not planned for, and therefore it is 
impossible to predictively calculate the purifier's lifetime on the basis 
of flowrates, expected impurity concentrations, and so forth. Endpoint 
detection allows the immediate detection of a serious back-diffusion 
event, and the appropriate precautions to protect the gas supply may be 
mobilized. Use of two endpoint detectors disposed at separate points in 
the gas purifier's scavenger bed allows back-diffusion to be distinguished 
from normal exhaustion of the purifier. If the downstream endpoint 
detector signals purifier depletion before the upstream one does, 
back-diffusion can be diagnosed in a straightforward and simple way. 
FIG. 1 shows a schematic representation of an apparatus for carrying out 
the gas purification method of the invention. The vessel 10 comprises an 
upper cylindrically shaped block 12 joined to the cup-like receptacle 14 
by means of circumferentially extending weld 16. In the lower portion of 
receptacle 14 is disposed a bed 18 of the scavenger according to the 
present invention. 
The vessel features means for introducing the impurity-containing gas 
mixture, comprising one or more gases from the group hydrogen, nitrogen, 
noble gases, diborane, and hydride gases from Groups IVA-VIA of the 
Periodic Table, such as arsine, phosphine, silane, germane, hydrogen 
selenide, and hydrogen telluride, into the interior space of the 
receptacle 14 for contact with the scavenger in bed 18. Such introduction 
means comprise the conduit 20, provided at its exterior end with an 
appropriate fitting 22 for joining with the supply line 32 to inert gas 
mixture source 30. The conduit 20 passes through the block 12 as shown, in 
a generally horizontal direction toward the center of the block and then 
downwardly extending from the block into the bed 18. At its lower portion 
in contact with the bed, this conduit has a plurality of gas distribution 
openings 34, through which the gas mixture flows outwardly and upwardly 
through the scavenger in the bed. 
Above the bed in the receptacle 14, the impurity-depleted gas flows into 
the outlet conduit 24, provided with a suitable fitting 26 for connection 
to the product gas discharge line 28, from which the purified gas may be 
supplied to a downstream end-use processing facility. 
FIG. 2 shows two perspective views of a purification system 200, comprising 
an endpoint detector connected through a sensor port 203 constructed in 
the body of an inline gas purifier 201 connected to a downstream particle 
filter 208. The gas flow stream to be purified enters the purifier through 
the inlet 202. The sensor port 203 is constructed in the purifier body, 
presenting the sensor element to the gas flow stream. The detector 
feedthroughs pass through the sensor port 203 and are connected via a 
fitting 204 to the detector control module 205, which provides LED display 
206. The gas flow stream continues through the connection fitting 207, 
passing through particle filter 208 and finally exiting through outlet 
209. The purifier configured as shown in FIG. 2 is useful as a back 
diffusion scrubber, able to protect both purge lines from potential 
contaminant backstreaming and to purify the gas stream used in the 
downstream process. The provision of the in-situ sensor allows back 
diffusion events to be detected, and thus the need for purifier changeout 
or the system problems can be noted and addressed promptly. 
In semiconductor manufacturing, many of the gases used are toxic or 
flammable, and all must be of exceptionally high purity. Therefore, in the 
practice of the present invention, all connections and fittings must be of 
high integrity and non-contaminating, such as VCR or Swagelok fittings. 
Other gas flow streams in which the gas purifier of the present invention 
can provide analogous service include nitrogen flow streams used to 
pressurize liquid reagents and solvents for delivery into semiconductor 
processes and the various types of chemical vapor deposition processes 
that not only require extremely high gas purity but also use gases such as 
arsine, silane or phosphine that are toxic and/or flammable. 
The features and advantages of the present invention are further shown with 
respect to the following non-limiting examples. 
EXAMPLE 1 
Preparation of Cesium-PSDVB Scavenger 
Inside an inert atmosphere glove box, about 100 ml of Amberlite XAD4 
(previously dried by calcining at 200.degree. C.) was placed into a 
long-necked, 250 ml glass round bottle-type reaction vessel having a 3 mm 
inner diameter constriction near the top of its neck. The dried Amberlite 
XAD4 beads were cream-white in color. About 2 g cesium metal was melted 
into the vessel. The vessel was sealed with a serum vial top. The reaction 
mixture was a two-phase mixture of the polymer beads and the metallic 
cesium. The sealed vessel was brought out of the glove box. About 100 
ml/minute of dry nitrogen was flowed into the vessel through a needle 
inserted through the serum vial top above the constriction. The nitrogen 
was not passed through the bed of polymer beads. The bottom of the vessel 
was heated to about 200.degree. C. in an oil bath for a total of about 19 
hours. The vessel was agitated periodically, by gently swirling 
approximately once every hour. After about five minutes of heating, the 
appearance of the reaction mixture changed from the initial cream-white 
polymer bead/grey metal two-phase appearance to the appearance of polymer 
beads being coated with a grey metallic film. After about two hours of 
heating, the mixture began to change to a purple-red color, becoming a 
uniform mass of deeply purple-colored beads. The reaction vessel was 
cooled to room temperature and the vessel was sealed by melting the glass 
at the constriction. 
EXAMPLE 2 
Preparation of Potassium-PSDVB Scavenger 1 
250 ml of Amberlite XAD4 (previously dried by calcining at 200.degree. C.) 
was added to a clean 1-liter single-ended Hoke.TM. sample cylinder inside 
an inert atmosphere glove box. 2.5 g liquid (100.degree. C.) potassium was 
added to the cylinder, which was then sealed with a Nupro.RTM. BW-series 
bellows valve. Between the sample cylinder and the Nupro valve was placed 
a 0-60 psig pressure gauge. The sample cylinder was then removed from the 
glove box, placed in an oven, and heated to 100.degree. C. for 50 minutes. 
The cylinder was shaken several times to disperse the molten potassium. 
After 50 minutes, the oven temperature was slowly raised to 220.degree. 
C., by raising the temperature in 20.degree. C. increments and then 
holding temperature constant for 30 minutes at each new temperature. The 
cylinder was shaken for about one minute with every temperature increase, 
again in order to disperse the potassium. The oven was then held at 
220.degree. C. for 15 hours. The cylinder was then allowed to cool to room 
temperature and then was transferred into an inert atmosphere glove box. 
The resulting K/Amberlite material was then poured from the cylinder 
through a funnel into a 500 ml Wheaton media bottle. The K/Amberlite 
material was uniformly dark brown, free-flowing beads with a bulk density 
of 0.3 g/ml. 
EXAMPLE 3 
Preparation of Potassium-PSDVB Scavenger 2 
Liquid potassium (100.degree. C., 0.25 g) was transferred from a 50 g 
potassium ampoule (Strem Chemical) to a 75 ml single-ended stainless steel 
sample cylinder (Hoke) which contained 50 ml of Amberlite XAD4 previously 
dried by calcining at 200.degree. C. for 15 hours. The sample cylinder was 
held at 210.degree. C. for 15 hours. Periodically the sample cylinder was 
shaken to disperse the molten potassium. The final product was 
free-flowing, reddish-brown Amberlite-like beads. 
EXAMPLE 4 
Preparation of Sodium/Potassium Alloy-PSDVB Scavenger 
Amberlite XAD4 (25 ml, previously dried by calcining at 200.degree. C. for 
24 hours) was added to a clean 50 ml alumina crucible inside an inert 
atmosphere glove box. Sodium-potassium alloy (0.25 g, Na/K mass ratio 1:6) 
was added to the crucible, which was then heated to 200.degree. C. using a 
muffle furnace with the glove box. Periodically the mixture was stirred to 
distribute the Na/K. After an hour of heating at 200.degree. C., the 
Amberlite color changed from its initial cream color to a dark brownish 
red. The crucible and its contents were then allowed to cool to room 
temperature. Still in the inert atmosphere glove box, the flee-flowing, 
brownish-red reaction product was poured into a 125 ml glass Wheaton media 
bottle. Upon close visual inspection, some small residual Na/K droplets 
were present in the product, probably as a consequence of insufficient 
heating time. When removed from the glove box and exposed to room air, the 
Na/K/Amberlite scavenger reacted to yield off-white spherical beads. 
EXAMPLE 5 
Reaction of Potassium-PSDVB Scavenger 1 with 1000 ppm Water 
The K/Amberlite scavenger material prepared in Example 2 was allowed to 
react with a 1000 ppm gaseous H.sub.2 O (balance helium) stream, in order 
to measure the amount of free potassium or other H.sub.2 -producing 
species. Any free K would react to produce gaseous H.sub.2 by the 
reaction: 
EQU K+H.sub.2 O.fwdarw.KOH+1/2H.sub.2 
The H.sub.2 should then elute from the scavenger bed with minimal 
chromatographic retention because of its low boiling point. Thus, 
measurement and integration of the gas phase H.sub.2 concentration in the 
effluent gas stream should yield a reasonably quantitative estimate of the 
free K content of the scavenger beads. 
100 standard cubic centimeters per minute (sccm) of helium was bubbled 
through a 100 ml serum vial containing 20 ml of deionized water. The 
temperature and pressure of the serum vial bubbler were measured, and, 
assuming that the helium gas stream became saturated with water vapor, the 
gas phase water concentration was calculated to be 1000 parts per million 
(ppm). The resulting H.sub.2 O-laden helium was then passed over a bed of 
10 g K/Amberlite Scavenger 1 at a rate of 100 sccm. The H.sub.2 in the 
effluent gas stream was quantified using a Gow-Mac 590 gas chromatograph 
with discharge ionization detector, under the following chromatographic 
conditions: 
______________________________________ 
Column: 6 foot .times. 1/8 inch 5A Molecular Sieve 
in series with a 20 foot .times. 1/8 inch 
Porapak Q.S. precolumn 
Carrier flow rate: 
30 ml/minute helium 
Analysis temperature: 
80.degree. C. isothermal 
Detector temperature: 
100.degree. C. 
Detector flow: 
10 sccm 
Detector range: 
10.sup.-11, attenuation 1 
______________________________________ 
This chromatographic method could detect CO, O.sub.2, and CH.sub.4 in 
addition to H.sub.2. No CO, O.sub.2, or CH.sub.4 were detected in the 
course of the experiment, with a detection limit of 50 parts per billion 
(ppb). 
The H.sub.2 concentration in the effluent gas stream approached a constant 
level of about 10 ppm and stayed at that level for about 2 hours, 
suggesting that the hydrogen-producing sites were uniformly distributed 
throughout the bed of scavenger beads, in agreement with the visual 
observation that the bed was very uniform in color. Free, metallic 
potassium reacts with water to give hydrogen and potassium hydroxide, by 
the reaction shown above. Thus, if the potassium present in the bed were 
in the free, metallic form, the concentration of hydrogen in this plateau 
phase would have been expected to be about 500 ppm, half of the original 
1000 ppm. Based on the amount of H.sub.2 released by the bed at a plateau 
level of about 10 ppm, the free potassium in the scavenger bed was 
calculated to be about 2% of the potassium that was originally added to 
the Amberlite in the preparation of the scavenger bed. It must be 
concluded that the remainder of the potassium present in the scavenger bed 
was in a chemical form other than free, metallic potassium, in agreement 
with the observation that a color change occurred upon heating the 
potassium/Amberlite mixture. The bulk of the oxygen scavenging capacity of 
the bed was not accounted for by free potassium, and thus most of the 
scavenger sites comprise the chemically reacted potassium. 
EXAMPLE 6 
Analysis of C.sub.2 -C.sub.5 Hydrocarbon Emissions from Potassium-PSDVB 
Scavenger 1 
The K/Amberlite scavenger prepared in Example 2 was tested to determine if 
it released hydrocarbons during purging with helium. Analysis was carried 
out by gas chromatography using a discharge ionization detector. The limit 
of detection for C.sub.1 to C.sub.4 hydrocarbons was about 10 parts per 
billion (ppb). The instrument was calibrated for the range of zero to 1000 
ppb C.sub.2 -C.sub.4 n-alkanes using a calibration standard gas prepared 
by serial dilution of a 1 part per million certified standard gas mixture 
using mass flow controllers to effect dilution. The accuracy of this 
dilution method is .+-.5%. The gas chromatographic conditions were: 
______________________________________ 
Column: 6 foot .times. 1/8 inch Hayesep Q 
Carrier gas flow: 
30 ml per minute helium 
Analysis temperature: 
Isothermal at 125.degree. C. 
Detector temperature: 
100.degree. C. 
Detector range: 10.sup.-11, attenuation 1 
Run time: 15 minutes 
Order of elution: 
C.sub.1 0.93 minutes, C.sub.2 1.54 minutes, 
C.sub.3 3.6 minutes, C.sub.4 9.6 minutes 
______________________________________ 
The C.sub.1 -C.sub.4 hydrocarbon emissions were studied at three stages: 
1. Freshly prepared K/Amberlite (10 ml) which had never been purged, as 
obtained directly from the reaction vessel, was placed in a tube and 
helium was passed through the material at a flow rate of 250 sccm for two 
minutes to pick up any residual hydrocarbons. 
2. K/Amberlite during continuous purging with helium at 250 sccm. 
3. Depressurization of a vessel containing the K/Amberlite sample after a 
24-hour hold at elevated pressure, 100 psig. 
The results of the analysis showed: 
1. Freshly prepared K/Amberlite that had never been purged, as obtained 
directly from the reaction vessel, off-gassed methane, ethane, propane, 
butane and butene at low levels. C.sub.1 -C.sub.4 hydrocarbons were 
detected at levels of 500 to 1000 parts per billion for a period of about 
thirty minutes. 
2. After the K/Amberlite sample (10 ml) was purged with helium for one hour 
at a flow rate of 250 sccm, the C.sub.1 -C.sub.4 hydrocarbons that had 
initially been detected declined to levels below the detection limit of 
the discharge ionization detector (i.e., less than 10 ppb each). 
3. In order to determine whether any of the C.sub.1 -C.sub.4 hydrocarbons 
that were detected in the initial purge were regenerated or desorbed upon 
standing, the same 10 ml sample of K/Amberlite analyzed in steps 1 and 2 
above was pressurized to 100 psig with helium and then valved off and held 
at room temperature for 24 hours. After the 24 hour hold, the gas in the 
sample tube was analyzed by the discharge ionization detector. No C.sub.1 
-C.sub.4 hydrocarbons were detected. 
EXAMPLE 7 
Measurement of Oxygen Scavenging Capacity of Potassium-PSDVB Scavenger 2 
The oxygen scavenging capacity of the K/Amberlite scavenger prepared in 
Example 3 was measured. A stream of 1% O.sub.2 in N.sub.2 at a flow rate 
of 100 sccm was passed through a 30 ml sample of the scavenger. Oxygen in 
the effluent stream was measured with a Model 100 Delta F Trace Oxygen 
Analyzer (Delta F Corp., Woburn, Mass.). The 1% O.sub.2 flow was continued 
through the sample with no O.sub.2 detectable in the effluent for about 50 
minutes, whereupon O.sub.2 breakthrough occurred. On this basis, the 
O.sub.2 capacity of the scavenger was calculated to be 1.7 liters O.sub.2 
per liter of the K/Amberlite scavenger resin. The spent resin from this 
O.sub.2 capacity test did not appear to undergo reaction upon exposure to 
room air. By contrast, unused K/Amberlite scavenger resin underwent a 
dark-brown to beige color change and a temperature rise of about 
10.degree. C. when exposed to room air. 
EXAMPLE 8 
Analysis of C.sub.1 -C.sub.4 Hydrocarbon Emissions from Sodium/Potassium 
Alloy-PSDVB Scavenger 
The scavenger prepared in Example 4 was analyzed for C.sub.1 -C.sub.4 
hydrocarbon emissions by the method described in Example 6. No off-gassing 
of C.sub.1 -C.sub.4 hydrocarbons was observed when the scavenger was 
purged with ultra-high purity helium (detection limit 10 ppb for each 
C.sub.1 -C.sub.4 hydrocarbon component). 
EXAMPLE 9 
Measurement of Oxygen Scavenging Capacity of Sodium/Potassium Alloy-PSDVB 
Scavenger 
The oxygen scavenging capacity of the scavenger prepared in Example 4 was 
measured by the method described in Example 7. The O.sub.2 scavenging 
capacity of the Na/K/Amberlite was 1.7 liters O.sub.2 per liter scavenger 
bed. 
EXAMPLE 10 
Comparison of C.sub.1 -C.sub.4 Hydrocarbon Emissions from Three Resin-Based 
Purifiers 
FIG. 3 shows the infrared absorbances of gases purged from the headspaces 
of a resin-based purifier according to U.S. Pat. No. 4,950,419 (Trace A), 
a commercially available resin-based purifier, the "Nanochem.RTM. Gas 
Purifier" (Semigas Systems, San Jose, Calif.) (Trace B), and a purifier 
according to the present invention (Trace D). Trace C shows the infrared 
absorbances of a gas standard mixture containing 1 ppm each of C.sub.1 
-C.sub.4 hydrocarbons (Matheson Gas Products, San Jose, Calif.). The 
hydrocarbon emissions from the three purifiers were compared by 
experiments performed as follows: 
Trace A 
A 150 ml sample of purifier resin prepared according to U.S. Pat. No. 
4,950,419 was sealed off in a sample cylinder for about 8 weeks. A 500 
sccm flow of highly pure helium was started through the purifier to a 10 
meter path length infrared (IR) absorbance cell and thence to a gas 
chromatograph (GC) fitted with a discharge ionization detector. The GC 
column was a 50 meter alumina porous layer open tube capillary column of 
0.53 mm inner diameter. The IR was the primary analytical tool and the GC 
was confirming. The frequency of the carbon-hydrogen stretching band (2970 
cm.sup.-1) was used because it is common to all hydrocarbons except 
methane, which was quantified separately. The detection limit for methane 
was about 30 ppb and for total hydrocarbons it was about 60 ppb. The GC 
detection limit was about 100 ppb for each hydrocarbon. After being valved 
off for 8 weeks, the purifier head space contained about 3.1 ppm total 
hydrocarbons and 8.9 ppm methane. 
Trace B 
A 300 ml purifier (Nanochem.RTM. Purifier, Ser. No. 6413) was valved off 
for 4 days. As was described for Trace A, a 500 sccm flow of highly pure 
helium was started through the purifier to a 10 meter path length infrared 
(IR) absorbance cell and thence to a gas chromatograph (GC) fitted with a 
discharge ionization detector. The GC column was a 50 meter alumina porous 
layer open tube capillary column of 0.53 mm inner diameter. After being 
valved off for 6 weeks, the purifier head space contained no detectable 
C.sub.1 -C.sub.4 hydrocarbons. 
Trace C 
The IR analysis as described for Traces A and B was standardized with a 
mixture of 1 ppm each of C.sub.1 through C.sub.4 hydrocarbons (Matheson 
Gas Products, San Jose, Calif.). 
Trace D 
A 300 ml sample of purifier resin (1 mol K/liter Amberlite) was prepared as 
described in Example 3, heated to 210.degree. C. for 30 hours, and then 
heated at 210.degree. C. and purged with a 300 sccm flow of highly pure 
argon for an additional 100 hours, and then sealed off in a sample 
cylinder for about 6 weeks. As was described for Trace A, a 500 sccm flow 
of highly pure helium was started through the purifier to a 10 meter path 
length infrared (IR) absorbance cell and thence to a gas chromatograph 
(GC) fitted with a discharge ionization detector. The GC column was a 50 
meter alumina porous layer open tube capillary column of 0.53 mm inner 
diameter. After being valved off for 6 weeks, the purifier head space 
contained no detectable C.sub.1 -C.sub.4 hydrocarbons. Helium flowed 
through the purifier for 20 minutes at room temperature at 500 sccm with 
no change in hydrocarbon emissions. 
Although the invention has been described with respect to particular 
features, aspects, and embodiments thereof, it will be apparent that 
numerous variations, modifications, and other embodiments are possible 
within the broad scope of the present invention, and accordingly, all 
variations, modifications, and embodiments are to be regarded as being 
within the spirit and scope of the invention.