0'
`
`Europaisches Patentamt
`
`European Patent Office
`Office européen des brevets
`
`® Publication number:
`
`0 242 028
`A2
`
`®
`
`EUROPEAN PATENT APPLICATION
`
`@ Application number: 873013395
`
`69 Int. CI.4: C01B 23/00
`
`@ Date of filing: 17.02.87
`
`@ Priority: 16.04.86 GB 8809325
`
`Date of publication of application:
`21.10.87 Bulletin 87/43
`
`Designated Contracting States:
`DE FR GB
`
`@ Applicant: Hewlett-Packard Company
`Mail Stop 20 8-0 3000 Hanover Street
`Palo Alto California 94304(US)
`
`® Inventor: Lovelock James Ephraim
`Combe Mill St Giles on the Heath
`
`Launceston Cornwall(GB)
`
`
`
`
`
`
`
`Representative: Squibbs, Robert Francis
`Hewlett-Packard Limited Nine Mile Ride
`Wokingham Berkshire HG11 3LL(GB)
`
`
`
`63 Method and apparatus for purifying a continuous flow of helium and/or neon gas.
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`@ Continuous production of ultra-pure helium and
`neon gases is achieved by utilising the fact that
`these gases have the highest ionisation and excita-
`tion potentials of
`all chemical
`substances. The
`helium/neon gas stream to be purified is excited by.
`for example, a corona discharge not only to directly
`ionise some of the impurities but also to produce
`rare-gas ions and metastable atoms. These ions and
`metastables subsequently collide with and ionise im-
`purities. A polarising field is used to draw the impu-
`rity ions out of the main gas flow and into a subsid-
`iary flow bled from the main flow.
`In a preferred
`embodiment, the polarising field is established be-
`tween a central
`rod-shaped anode(l3) and a sur-
`rounding tubular cathode(l0) through which the main
`gas flow passes. The cathode is made of a porous
`material so that positive impurity ions can be re-
`moved by the radial outwards ilow of gas thereth-
`rough.
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`Xerox Copy Centre
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`TSMC-1215
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`TSMC v. Zond, Inc.
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`Page 1 of 5
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`TSMC-1215
`TSMC v. Zond, Inc.
`Page 1 of 5
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`D 242 028
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`2
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`§
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`METHOD AND APPARATUS FOR PURIFYING A CONTINUOUS FLOW OF HEUUM AND/OR NEON GAS
`
`The present invention relates to a method and
`apparatus for purifying a continuous flow of helium
`and/or neon gas; in particular, but not exclusively,
`the invention is concerned with the continuous pro-
`duction of ultra pure helium or neon (or mixtures
`thereof) where 'ultra pure' means that contamina-
`tion with any impurity gas or gases is less than one
`part in ten million by volume. Helium and neon
`gases of this purity have many uses including as
`sample carrier gases in modem analytical tech-
`niques, filling gases for discharge tubes and lasers,
`and purge gases in the semiconductor industry.
`It is an object of the present invention to pro-
`vide for the production of ultra pure helium and
`neon gases on a continuous basis and in a manner
`relatively easy to implement.
`According to one aspect of the present inven-
`tion,
`there is provided a method of purifying a
`continuous flow of helium or neon gas or a mixture
`thereof, said method involving the operations of:
`-passing a stream of the gas to be purified
`along a predetermined flow path;
`-ionising impurities in the gas both dimctly and
`by exciting the gas to generate rare—gas ions and
`metastable atoms which thereafter ionise said im-
`purities; and
`-removing the positive impurity ions from the
`main gas flow by setting up a polarising field
`across said stream to cause migration of said im-
`purity ions out of the main gas flow.
`The effectiveness of this purification method
`rests on the fact that helium and neon have the
`
`highest ionization and metastable excitation poten-
`tials of all chemical substances. As a result.
`if
`impure helium or neon is subject to an ionising
`energy input,
`the more easily ionized impurity
`atoms and molecules are preferentially ionised.
`Furthermore, the helium and neon metastables and
`ions produced both readily exchange their stored
`energy and charge respectively to impurity mol-
`ecules they encounter by collision which greatly
`enhances the efficiency of the ionisation process
`and leaves the helium or neon as uncharged, uneXs
`cited atoms. The
`ionising
`effect of
`rare-gas
`'metastable' atoms is, of course, a known effect
`that has previously been exploited in sensitive de-
`tectors of the form described in UK Patent Speci-
`fication No. 882,977.
`
`According to the another aspect of the present
`invention, there is provided apparatus for purifying
`a continuous flow of helium or neon gas or a
`mixture thereof, said apparatus comprising:
`-an inlet for the gas to be purified;
`an outlet for purified gas;
`-means defining an elongate gas flow path
`
`between said inlet and outlet;
`
`-exhaust means for bleeding off a subsidiary
`flow of gas from the main gas flow passing along
`said gas flow path;
`’
`-ionisation means operative to cause ionisation
`of impurities in the gas passing along said flow
`path, both by direct ionisation of the impurities and
`by exciting the rare gas atoms such as to generate
`rare-gas ions and metastable atoms which sub-
`quently ionise the impurities in the gas; and
`-polan'sing-field means operative to generate a
`polarising field across the gas flow path for drawing
`positive impurity ions out of the main gas flow and
`into said subsidiary flow for removal through said
`exhaust means.
`
`Preferably, the polarising-field means includes
`a central elongate anode coaxially surrounded by a
`tubular cathode that also serves as said means
`defining the gas flow path between said inlet and
`outlet.
`
`Advantageously, the tubular cathode is porous
`and additionally serves as said exhaust means, the
`gas passing outwards through the cathode con-
`sfituting said subsidiary gas flow and including the
`impurities drawn to the cathode by the polarising
`field.
`
`Conveniently the ionisation means may take
`the form of a corona discharge device constituted
`by the anode and cathode of said polarising-field
`means and by a source of high potential, short
`duration pulses connected between the anode and
`cathode. Other forms of ionisation means can also
`be used such as radioactive sources, ultraviolet
`sources. or extemal X-ray or gamma-ray sources.
`A helium/neon gas purifier embodying the in-
`vention will now be described by way of non-
`limiting example with reference to the accompany-
`ing diagrammatic drawing, the sole Figure of which
`is a longitudinal cross-section through the purifier.
`Although the purifier to be described is suitable
`for purifying helium or neon gases individually or in
`mixture, for ease of explanation the following de-
`scription will be given in terms of the purification of
`a helium gas flow only.
`As shown in the drawing, the purifier comprises
`a tubular member l0 defining a main gas flow path
`between an inlet It for impure helium and an outlet
`l2 for purified helium. The tubular member ID is
`formed of a porous conducting material such as
`sintered metal or sintered metallized glass or
`quartz. The tubular member ID constitutes a cath-
`ode of the purifier, the complementary anode l3
`being formed by a metal rod positioned coaxially in
`the tubular cathode l0 by insulating end seals 14.
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`0 242 028
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`A polarising field is set up between the anode
`l3 and cathode ID by a DC polarising source 15.
`This source l5 applies a potential between the
`anode and cathode which is substantially less than
`would cause an are or discharge in helium at the
`pressure existing within the purifier during opera-
`tion (generally slightly above atmospheric pres-
`sure); the applied potential is, however, sufficient to
`cause a rapid migration of any positive ions
`present to the cathode it).
`in order to ionise impurities in the gas flowing
`through the cathode lo. a pulse source l6 is pro-
`vided for supplying high frequency (I00 kHz to
`50MHz), high potential, short duration (less than
`one microsecond) pulses between the anode and
`cathode whereby to set up an ionising corona di-
`scharge between the electrodes. Such an arrange-
`ment is disclosed in UK Patent Specification No.
`l,597,623; and will not.
`therefore, be further de-
`scribed herein except to note that since to initiate
`corona discharge at least one ion or electron must
`be present, it is prudent to provide some means for
`generating the required ion or electron (although
`background radiation could be relied upon). Con-
`sequently,
`the polarising source could be made
`adjustable to cause an initial discharge before be-
`ing set back to its normal operating potential.
`Surrounding the tubular cathode IO is a cylin-
`drical jacket I7 for collecting gas which,
`in opera-
`tion, passes radially outwards through the porous
`cathode IO. This jacket I? has an exhaust port l8
`adjacent the inlet end of the main gas flow path
`defined by the cathode K).
`A diffuser IQ is provided in the main gas flow
`path adjacent to inlet ii to establish streamline flow
`down the inside of the cathode l0.
`
`In operation, impure helium typically, one part
`per million impurity) is supplied to the inlet ii at a
`slight over-pressure relative to the surrounding at-
`mosphere to establish a streamlined main gas flow
`coaxially down the inside of the cathode ID in the
`direction of arrow A to the outlet
`l2. Due to the
`
`porous nature of the cathode to, a small subsidiary
`gas flow is set up that passes radially outwards
`through the cathode and then in the direction of
`arrow B to the exhaust port IS.
`The corona discharge established between the
`anode l3 and cathode l0 by the pulse source l6
`causes ionisation of any impurity atoms or mol-
`ecules within the main gas stream either directly or
`by first generating helium metastable atoms and
`helium ions which subsequently collide with and
`ionise any impurities. Ultra-violet raditation emitted
`from the transitions of the helium ions and metasta-
`
`impurities by
`bles will also produce ionisation of
`photo-ionisation. The predominant impurity-ionising
`
`process will generally be that due to the collision of
`helium metastables with impurity atoms and mol-
`ecules; however at high field intensities ionisation
`by electron impact may predominate.
`The polarising field set up between the anode
`l3 and cathode II by the polarising source is causes
`positive impurity ions to migrate to the cathode
`where they pass out of
`the main gas stream
`through the porous cathode II as part of the subsid-
`iary gas flow.
`The purified helium typically with an impurity
`level of less than one part in ten million exits the
`purifier through the outlet l2 while the impurities are
`exhausted through exhaust port I8.
`The actual dimensions of the purifier will de-
`pend upon the desired flow rate of helium and the
`degree of purity sought. However, typical dimen-
`sions would be a tube l0 mm in internal diameter
`
`with a length of 0.3 metres.
`An ion current of 4.3 microamperes would be
`needed to remove all of a one part per million
`impurity in helium flowing at one millilitere per
`second at N.T.P. This assumes that all of the ion
`current
`is carried by impurity ions and that the
`impurities are completely removed from the main
`helium flow after the impurity ions encounter the
`cathode. In practice, ionising efficienes of | to IO%
`are more usual.
`
`The purifier can be thought of as analogous to
`a distillation column with gaseous diffision pro-
`cesses tending to hinder the smooth operation of
`the separation. For a cathode I0 mm in internal
`diameter and an anode 2 mm diameter and with
`
`the flow rate and impurity level quoted in the
`preceding paragraph, each 5 to lo mm section of
`the purifier corresponds to a plate of a distillation
`column and 67% of impurity will be removed in
`each of the sections. Thus,
`five sections should
`theoretically remove about 99% of the impurity. A
`typical purifier is in fact 0.3m or more in length to
`extend the range of purification and the maximum
`flow rate.
`The structural and electrical characteristics of
`
`the purifier are preferably such as to avoid the
`development of ion densities in the purifier in the
`plasma region.
`In plasma,
`ions and electrons are
`closely associated in a gaseous cloud and the ions
`cannot be so readily moved to the electrodes by
`the applied polarising field. For a lo mm section of
`purifier, with a cathode having an internal diameter
`of IO mm,
`the polarising field current should not
`exceed one microampere if plasma conditions are
`to be avoided.
`
`it will be appreciated that various modifications
`can be made to the described form of purifier.
`Thus. instead of providing an ionising source in the
`form of a corona discharge device, other forms of
`ionising source could be employed such as, for
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`5
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`O 242 028
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`6
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`for drawing positive impurity ions out of the main
`gas flow and into said subsidiary flow for removal
`through said exhaust means (l0,l7,l8).
`3. Apparatus according to claim 2, wherein said
`polarising-field means includes a central elongate
`anode(13) ooaxially surrounded by a tubular cath-
`ode (lO) that also serves as said means defining the
`gas flow path between said inlet and outlet.
`4. Apparatus according to claim 3, wherein the
`tubular cathode ([0)
`is porous and additionally
`serves as said exhaust means. the gas passing
`outwards through the cathode constituting said
`subsidiary gas flow and including the impurities
`drawn to the cathode (ID) by the polarising field.
`5. Apparatus according to claim 4, wherein the
`cathode (I0)
`is surrounded by a jacket
`(17)
`for
`collecting the subsidiary gas flow passing through
`the cathode, the jacket (I?) being provided with an
`exhaust port (l8) positioned such that the subsid-
`iary gas flow passes along the outside of the oath-
`ode (ID)
`in the opposite direction to the main gas
`flow within the cathode (l0).
`6. Apparatus according to claim 3 or claim 4,
`wherein said ionisation means comprises a corona
`discharge device constituted by the anode (l3) and
`cathode (ll!) of said polarising field means and by a
`source (16) of high potential, short duration pulses
`connected between the anode and cathode.
`7. Apparatus according to any one of the
`claims 2 to 6 further comprising a diffuser (l9)
`positioned in the gas flow path adjacent the inlet to
`streamline the flow of gas along said flow path.
`
`example, a radioactive source located within the
`
`cathode lo, an ultraviolet source generated by a
`subsidiary discharge, an external X-ray or gamma-
`ray source.
`
`Furthermore, whilst the use of a porous cath—
`ode is a particularly convenient and efficient way of
`removing the impurity atoms and molecules,
`it
`would be possible to use a non-porous cathode
`and then to separate off from the main gas flow,
`the gas stream that pases directly over the cathode
`and within which the neutralised impurities will be
`concentrated.
`
`Although the heavier rare gases such as argon
`could be subject to the same purification process
`as described and illustrated above for helium and
`neon, the effectiveness of the process would be
`greatly reduced since many of the common impuri-
`ties ot the heavier rare gases would not be pref-
`erentially ionised.
`
`Claims
`
`I. A method of purifying a continuous flow of
`helium or neon gas or a mixture themat. charac-
`terised in that said method involves the operations
`of:
`
`-passing a stream of the gas to be purified
`along a predetermined flow path;
`-ionising impurities in the gas both directly and
`by exciting the gas to generate rare-gas ions and
`metastable atoms which thereafter ionise said im-
`purities; and
`-removing the positive impurity ions from the
`main gas flow by setting up a polarising field
`across said stream to cause migration of said im-
`purity ions out of the main gas flow.
`2. Apparatus for implementing the method of
`claim I, characterised in that said apparatus com-
`prises:
`-an inlet for the gas to be purified;
`-an outlet (l2) for purified gas;
`-means ([0) defining an elongate gas flow path
`between said inlet and outlet;
`
`exhaust means (ID) for bleeding off a subsidiary
`flow of gas from the main gas flow passing along
`said gas flow path;
`-ionisation means (l0,|3,l6) operative to cause
`ionisation of impurities in the gas passing along
`said flow path. both by direct
`ionisation of the
`impurities and by exciting the rare gas atoms such
`as to generate rare-gas ions and metastable atoms
`which subsequently ionise the impurities in the gas;
`and
`
`-polarising-field means (l0,|3,15) operative to
`generate a polarising field across the gas flow path
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