U.S. patent number 3,742,343 [Application Number 05/085,157] was granted by the patent office on 1973-06-26 for ion gauges.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Lawrence Graham Pittaway.
United States Patent |
3,742,343 |
Pittaway |
June 26, 1973 |
ION GAUGES
Abstract
An ion guage comprising a conducting cylinder closed at one end;
a pair of cathodes within the cylinder; a cylindrical grid between
the cathodes and having a wire mesh peripheral walls relatively
transparent to electrons, which grid is bounded at one end by a
wire mesh wall and at the opposite end nearer the closed end of the
conducting cylinder by an apertured metal wall; means adjacent to
the aperture for extracting and converging ions, and an electrode
for collecting extracted ions.
Inventors: |
Pittaway; Lawrence Graham
(Crawley, Sussex, EN) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
10466254 |
Appl.
No.: |
05/085,157 |
Filed: |
October 29, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Oct 29, 1969 [GB] |
|
|
53007/69 |
|
Current U.S.
Class: |
324/462 |
Current CPC
Class: |
H01J
41/04 (20130101); H01J 41/06 (20130101) |
Current International
Class: |
H01J
41/00 (20060101); H01J 41/06 (20060101); H01J
41/04 (20060101); G01n 027/00 (); G01n
027/62 () |
Field of
Search: |
;324/33 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Krawczewicz; Stanley T.
Claims
What is claimed is:
1. An ion gauge for measuring low gas pressures present in a high
vacuum system, comprising a conducting cylinder closed at one end,
said cylinder being operative at ground potential to screen said
gauge from extraneous fields, at least one pair of cathodes
radially spaced within said cylinder, said cathodes being operative
at voltages that are positive relative to said grounded cylinder, a
cylindrical grid positioned between said cathodes and having wire
mesh peripheral walls that are relatively transparent to electrons
from said cathodes when operated at positive voltages relative to
said cathodes, said grid being bounded by a wire mesh wall at an
end thereof opposite the closed end of said cylinder and by a metal
wall having an aperture at the end thereof nearer the said closed
end of said cylinder, the metal wall of said grid providing a
shield so that only ions dependent upon the pressure of said gas
are in proximity with the aperture of the said nearer end of said
grid, means adjacent to the aperture of said grid for extracting
and converging ions in proximity with the aperture of metal wall of
said grid, and an electrode cooperating with said extracting means
for collecting said extracted ions to provide a current dependent
upon the pressure of gas within said grid.
2. An ion gauge as claimed in claim 1 wherein said pairs of
cathodes are electron emissive filaments.
3. An ion gauge as claimed in claim 1 wherein said wire mesh
peripheral walls and wire mesh end wall opposite the closed end of
said grounded cylinder comprise closely woven tungsten wire.
4. An ion gauge as claimed in claim 1 wherein said extracting and
converging means comprises a first and second electrostatic lens
system.
5. An ion gauge as claimed in claim 4 wherein said first
electrostatic lens system comprises a cylindrical conducting
extension of the aperture of said metal end wall of said
cylindrical grid, and an axially symmetrical extractor electrode
adjacent said extension, said extractor electrode operating at
negative voltages relative to said cathodes, and said second
electrostatic lens system comprises said extractor electrode and a
cylindrical reflector electrode adjacent to and axially aligned
with and extractor electrode, said reflector electrode having an
aperture for said collecting electrode and operating at a positive
voltage equal to or greater than the voltages of said grid.
6. An ion gauge as claimed in claim 1 wherein said extracting and
converging means comprises a rim surrounding the aperture of said
grid, a cylindrical extractor electrode adjacent to but insulated
from said rim, said extractor electrode having an inside diameter
greater than the diameter of said rim to provide screening from
extracted ions when operating at negative voltages relative to said
cathodes, and a cylindrical reflector electrode surrounding said
rim and said extractor electrode, said reflector electrode being
fastened to the metal end wall of said grid.
7. An ion gauge as claimed in claim 1 wherein said collecting
electrode is a tungsten wire.
Description
This invention relates to ion gauges.
An ion gauge is frequently employed for measuring very low
pressures in high vacuum apparatus. One form of ion gauge known as
the Bayard-Alpert gauge which was proposed by R. T. Bayard and D.
Alpert in the Review of Scientific Instruments 21, 1950 pages
571-2, has the advantages that it is relatively easy to outgas, it
exhibits a high sensitivity and a good linearity and does not
require a magnetic field. The gauge was developed in order to
reduce the residual output current caused by the soft X-rays and
desorbed ions generated by impact of electrons with the electron
accelerating grid. The soft X-rays and desorbed ions tended to be
intercepted by the collector electrode, the former causing a
photo-electron emission current which together with the desorbed
ion current, tend to swamp the ion output current at the lower
pressure.
In "Modulated Bayard-Alpert Gauge," P. A. Redhead, Review of
Scientific Instruments 31, 1960 pages 343-344, a form of
Bayard-Alpert gauge is described in which a modulator electrode is
added to provide the output ion current with a modulated component
which can be separated from the normal ion current which contains
the X-ray emission and desorbed ion currents. In this way the lower
limit to the measurable pressure can be further extended.
In "Gauge manometrique a collecteur exterieur pour pressions
tresbasses" by J. Groszkowski, Bulletin de l'Academie Polonais des
Sciences, Volume 14, 1966, pages 169-177 (1023-1031), there is
illustrated in FIG. 6(a) on page 175 (1029) a form of ion gauge in
which the collector electrode is withdrawn from the interior of the
ionisation space and hence out of the region permeated by the
majority of the soft X-rays and desorbed ions produced by electron
collision with the accelerator and thus allowing lower pressures to
be measured than with the modulated Bayard-Alpert gauge. The gauge
illustrated in FIG. 6(a) on page 175 of the above reference will be
referred to herein as the Groszkowski gauge. Reference will now be
made to FIG. 1 of the drawings which is a sectional diagram
illustrating the Groszkowski Gauge. Two electron emissive cathodes
1 and 2 are mounted on either side of a cylindrical electron
accelerating grid 3. The grid 3 is made up of a peripheral wall 4
comprising a helix of molybdenum wire, an end wall 5 comprising a
sprial of molybdenum wire and an end wall 6 comprising a metal disc
having an ion exit aperture 7 formed therein. The walls 4, 5 and 6
are conductively joined together and connected to a source of
biasing potential not shown. The cathodes 1, 2 and the grid 3 are
mounted in a glass envelope 8 which can be connected via a tubular
neck 9 to a vacuum system the gas pressure in which it is desired
to measure.
A thin wire ion collector electrode 10 is also present within the
envelope 8 and is mounted outside the grid 3 in the region facing
the ion exit aperture 7. A glass shield 11 attached to the inner
surface of the envelope 8 surrounds the collector electrode 10 and
is provided with an aperture 12 arranged in the ion path from the
ion exit aperture 7 to the ion collector electrode 10.
The ion collector electrode 10 is normally connected to earth via
the input circuit of ion current measuring means. The cathodes 1
and 2, which conveniently are electron emissive filaments and of
which one is connected to a source of heater current, are
maintained at a potential positive with respect to earth, typically
+ 210 volts. The electron acceleration grid 3 is maintained at a
positive potential with respect to the cathodes 1 and 2 so that
emitted electrons are accelerated towards the grid 3 and pass
through the spaces between the turns of the helix 4 with sufficient
kinetic energy to ionize gas present within the space 13 enclosed
by the grid 3. Typically the grid 3 is maintained at a potential of
+ 100 volts with reference to the potential of the cathodes 1, 2.
The negative potential on the collector 10 with respect to the grid
3 is said to cause ions formed within the space 13 to be drawn out
via the apertures 7 and 12 towards the collector 10 to form the ion
ouput current. Because the ion collector 10 is substantially
shielded by the disc 6 from the peripheral wall 4 with which a
proportion of the incident electrons collide generating soft
X-rays, the X-ray photo-emission current component of the collector
current is maintained at a low level.
It is common nowadays to provide an ion gauge without an envelope,
the gauge being mounted on a flange adapted for attachment to a
vacuum system for measuring the pressure therein. Such a gauge is
commonly called a nude gauge and will be referred to herein as
such. Such a vacuum system normally presents a grounded electrical
environment to the gauge and at least some walls of the system are
normally made of a metal such as stainless steel.
A nude form of the Groszkowski gauge was constructed and tested
employing the electrode dimensions given in the article referred to
above, and surrounded by a grounded cylindrical metal screen to
simulate a grounded environment. It was found however that as the
potential of the grid 3 was increased positively with respect to
the grounded cylindrical metal screen, the cathode being biased 5
volts positively with respect to ground and the collector being
maintained at 200 volts negatively with respect to the cathode, the
sensitivity of the gauge increased until the potential difference
between grid 3 and the grounded metal screen was approximately 60
volts, after which the sensitivity fell away rapidly. In addition
the sensitivity under the above conditions was found to be very
dependent on the ionizing electron current.
It is an object of the invention to provide a nude form of ion
gauge having an external collector electrode, in which the above
difficulties are substantially overcome. It is a further object of
the invention to provide an improved form of ion extraction and
collection means for withdrawing ions from the electron
acceleration grid of an ion gauge.
According to the present invention there is provided a nude ion
gauge for use in a grounded environment including a source of
electrons; an ionization space surrounded by a conducting surface
including a peripheral region that has a plurality of small
apertures and that is relatively transparent to electrons directed
thereat from the electron source mounted exterior to the ionization
space to ionize gas present in the space, said conducting surface
having an end portion substantially opaque to the passage of soft
X-rays and provided with an aperture through which ions formed in
the ionization space by the electrons, can be extracted by the
application of an extraction field; and means for collecting the
extracted ions to provide a current dependent on the pressure of
gas in the ionization space, the apertures in the electron
transparent region being sufficiently small and of such a shape
that the normal electric field present outside the ionization space
is substantially prevented from penetrating the electron
transparent region and from extracting a substantial proportion of
the ions present in the ionization region via the electron
transparent region. To ensure repeatable performance in a variety
of different vacuum systems the gauge can be surrounded by a
grounded conducting cylindrical screen.
According to another aspect of the present invention an ion gauge
comprises a conducting member enclosing a region within which ions
are formed from the collision of electrons with gas present in the
region, the enclosing member has an end wall formed of a material
substantially opaque to soft X-rays and contains an aperture
through which the ions can be extracted and fed to an ion collector
electrode situated outside the enclosure, the ion gauge includes in
addition to the collector electrode a conducting extractor
electrode and a reflector electrode, the extractor electrode being
provided with means for feeding an ion extracting potential thereto
and being so disposed adjacent the ion exit aperture in the end
wall, that, on the application of extracting potential to the
extractor electrode ions are extracted from the enclosure towards
the collector electrode substantially without striking the the
extractor electrode the reflector electrode is disposed adjacent
the collector electrode and has means for applying a potential to
reflector electrode to cause ions to be directed towards the
collector electrode. At the ion exit aperture in the end wall the
enclosing member can include a cylindrical extension located
external to the enclosure which cylindrical extension co-operates
with a cylindrical extractor electrode to form a converging
electrostatic lens. The extractor electrode and a cylindrical
reflector electrode can co-operate to form a second converging
electrostatic lens. A grounded conducting metal screen can surround
the gauge.
In order that the present invention may be clearly understood and
readily carried into effect, embodiments thereof will now be
described, by way of example, with reference to the figures of the
accompanying drawings of which:
FIG. 1 is a sectional diagram of a Groszkowski Gauge which is known
in the prior art,
FIG. 2 is a longitudinal section of one form of gauge employing the
invention,
FIG. 3 is a detail in longitudinal section illustrating an
alternative embodiment of the invention, and
FIG. 4 is a longitudinal sectional diagram illustrating a nude form
of the Groszkowski gauge embodying the invention.
Consideration will now be given to the use of a nude ion gauge in a
vacuum system. By virtue of the electrically conducting nature of
the walls of such a vacuum system, since they are normally
constructed of stainless steel, such a nude gauge is usually
surrounded by an earthed conductor formed by the said walls. The
potential difference between the electron acceleration grid and the
walls results in an electrostatic field therebetween said grid and
said walls, which is known to affect the passage of ionizing
electrons through the ionizing space formed within the electron
acceleration grid, thus causing variations in the sensitivity of
the gauge. For this reason it is desirable that the gauge should be
arranged in a cylindrical electrically conducting screen to ensure
repeatability in pressure measurement in various configurations of
the vacuum system. Furthermore when a similar ion gauge is
alternatively enclosed in a glass envelope and connected to a
vacuum system via a glass tube, it is desirable and common practice
for an electrically conducting screen to be formed by a layer of
tin oxide deposited on the inside wall of the glass envelope,
thereby defining the electrostatic field between the grid and the
envelope and also shielding the gauge from electrical disturbances
outside the glass envelope.
It is also desirable that the electron accelerator grid should be
maintained at a potential of at least 100 volts positively with
respect to the grounded environment, to enable the electron
accelerating potential between the grid and the electron emitting
cathode to provide optimum ionization efficiency of the gas
molecules in the said space by the ionizing electrons.
As has been described above, in attempting to employ the
Groszkowski gauge in nude form in a grounded environment, it was
found that the sensitivity fell rapidly on increasing the electron
acceleration grid voltage beyond about 60 volts above ground and
also that the sensitivity was very dependent on the ionising
electron current.
These disadvantages were overcome in accordance with one aspect of
the invention by replacing the helical wound grid wall 4 and the
spiral wound grid wall 5 of prior art ion gauges with a closely
woven wire mesh, conveniently of Tungsten, and denoted by 20 in
FIG. 4 to which reference will now be made and which illustrates a
nude form of exterior collector ion gauge employing the invention.
The tungsten mesh employed in one embodiment had a mesh of 50 wires
per inch and an electron transparency of 92 percent. The cathodes
1, 2 and the grid 3 were arranged in a cylindrical conducting
screen 14.
In the nude gauge shown in FIG. 4, the potential of the grid 3 was
raised until it was approximately 300 volts positive with respect
to the screen 14 which was connected to ground, and the sensitivity
was found to increase until it was approximately twice the maximum
value reached when using a helical wound grid.
It is thought that when the Groszkowski gauge is employed in nude
form, the electrostatic field produced between the helical grid and
the grounded environment penetrates the helical grid and as the
grid potential is raised, ions formed within the ionization space
13 tend to be drawn out of the ionization space via the walls 4 and
5 (FIG. 1) by this field, instead of via the aperture 7 by the
collector field, thus depleting the ion current flowing to the
collector 10. By employing the invention, a form of electron
transparent grid is provided having a plurality of relatively small
apertures and this is effective in reducing penetration of the
field outside the ionization space 13, which field would
extractions in preference to the collector 10.
When the Groszkowski gauge is employed in a glass envelope 8 not
having a conducting internal coating, the inside walls tend to be
charged by the electrons emitted by the cathode 1 or 2 (FIG. 1)
until they reach cathode potential. The field distribution within
the gauge environment is thus made relatively predictable but not
necessarily compatible with the requirements for optimum
sensitivity. Furthermore the walls of the glass shield 12 (FIG. 1)
also become charged to approximately the potential of the cathodes
1, 2 and the shield 12 therefore acts as an extractor electrode
which draws ions through the aperture 7 from the ionisation region
13 towards the collector electrode 10, a considerable proportion of
the said ions being intercepted by the said shield 12 before
reaching the said collector 10. It is a further object of the
invention to provide an external collector gauge having improved
means for extracting ions from the ionisation space, which do not
depend on the accumulation of electric charges on glass
surfaces.
One embodiment of the invention is illustrated in longitudinal
section in FIG. 2, to which reference will now be made.
Cathodes 21, 22 in the form of electron emissive filaments are
connected to support and connection wires 41 mounted in a glass
seal 45, and which project therefrom so that heating and biasing
potentials can be fed thereto. The glass seal 45 is attached to and
forms a seal with a metal ring 46 which latter is sealed to a
conducting cylinder 44. A metal flange 43 welded to the cylinder 44
enables the ion gauge to be attached to a vacuum system to form a
seal therewith. The glass employed for the seal 45 is chosen to
have a coefficient of thermal expansion which is compatible with
that of the ring 46.
The ionization space 13 is bounded by a conductive electron
acceleration grid assembly 23 comprising a peripheral wall 24 and
one end wall 25, both formed of a closely woven wire mesh,
conveniently of Tungsten, and a further end wall 26, substantially
opaque to soft X-rays, and formed of thin sheet metal, conveniently
stainless steel. An aperture 27 is formed at the center of the wall
26 which is provided with a cylindrical conducting extension 47.
The electron acceleration grid assembly 23 is conductively attached
to a support and connection wire 48 which passes through the seal
45 and is attached thereto.
An axially symmetrical ion extractor electrode 49 formed of
conducting material, conveniently stainless steel, is mounted so
that the forward end projects into the rearward end of the
extension 47. The diameter of the extractor electrode 49 is
increased in a rearward direction beyond the end of the extension
47, and the electrode 49 is supported by a conductive connecting
wire 50.
The extractor electrode 49 forms, together with the rearward
cylindrical extension 47 of the end plate 26, a converging
electrostatic lens system having a focal point located at the point
F.sub.1.
A reflector electrode 51, cylindrical in form and having an end
wall 52, is mounted on a conducting support 53 which extends
through the seal 45. The reflector 51, conveniently of stainless
steel, is arranged behind the extractor 49 and is coaxial with the
extractor 49 and the cylindrical extension 47. The reflector 51 and
the extractor 49 form together a further converging electrostatic
lens having a focal point at the point F.sub.2.
A collector electrode 30 in the form of a wire, conveniently of
Tungsten, is mounted on a conductive support member 54 which
extends through the seal 45 to form a terminal connection. The
collector 30 projects through an aperture 55 in the wall 52 of the
reflector 51. The support member 54 and that part of the collector
30 situated behind the reflector 51 are shielded by a glass
cylinder 56 extending from the seal 45, from the incidence of
charged particles present outside the extractor and reflector lens
system.
In operation the cathodes 1, 2 are biased negatively with respect
to the grid 23 so that electrons are accelerated towards the mesh
wall 24 and penetrate the apertures therein with sufficient energy
to ionise gas present in the space 13. The screen 44 is connected
to ground and the cathodes 1, 2 are biased positively with respect
thereto. The extractor 49 and the collector 30 are biased
negatively with respect to the cathodes 1 or 2. The reflector 51 is
biased positively with respect to the collector 30, to a potential
equal to or greater than that of the grid 23. Electrons present
within the grid 23 form a space charge which, together with the
screening effect of the closely woven mesh walls 24 and 25, produce
an electric field within the ionizing region 13 which prevents
positive ions formed therein from escaping via apertures in the
walls 24 and 25.
The potential on the extractor 49 sets up an ion extraction field
which draws positive ions out of the ionisation region 13 towards
the collector 30. The converging effect of the electron lens formed
between the extractor 49 and the extension 47 causes the ions to
follow paths which, in general, avoid collision with the extractor
49. The further converging lens formed between the extractor 49 and
the reflector 51 further assists in directing ions away from the
extractor 49 and towards the collector electrode 30.
The collector electrode 30 is so situated that soft X-rays
generated by the collision of electrons with the peripheral grid
portion 24 of the acceleration grid 23 are substantially shielded
therefrom by the end wall 26.
By employing the form of construction shown in FIG. 2, the
extraction field can be predictably maintained with changes in the
environment in which the nude form of gauge is employed, and a good
ion collection efficiency can be provided.
An alternative embodiment of the invention is shown in FIG. 3, to
which reference will now be made. FIG. 3 is a sectional detail
showing an alternative form of ion extraction and collection means
which can be employed in an ion gauge otherwise as shown in FIG. 2.
The end wall 26 of the electron accelerator grid 23 is provided
with an aperture 27 and a rim 60 is formed surrounding the aperture
27. A cylindrical reflector electrode 61 having an end wall 62,
formed conveniently from stainless steel, is conductively attached,
for example by a welding process, to the rearward face of the wall
26.
A cylindrical extractor electrode 69, formed conveniently of
stainless steel, is mounted adjacent the aperture 27 inside the
reflector 61 but is insulated therefrom and provided with
connection to a source of bias potential. The diameter of the
extractor electrode 69 is greater than that of the rim 60 of the
aperture 27 so that it is at least partly screened by the rim 60
from ions passing through the aperture 27.
A collector electrode 30, conveniently a tungsten wire
approximately 200 .mu.m in diameter and mounted as before on a
conductive support 54, extends through an aperture 70 in the wall
62 along the axis of the reflector electrode 61. The arrangement is
such that the collector is substantially shielded by the wall 26
from soft X-ray radiation emitted by electron collision with the
peripheral wall 24 of the grid 23. A potential negative with
reference to that of the grid 23 is applied to the ion extractor
electrode 69 and similarly to the collector electrode 30. The
extractor electrode 69 thus sets up an ion extraction field which
draws positive ions out of the ionisation space 13 in the direction
of the collector electrode 30 to which the positive ions are
attracted by the field resulting from the potential supplied
thereto. The reflector 61, by being connected to the grid 23, tends
to repel the positive ions towards the collector 30, and also
screens the collector from any charged particles present in the
vacuum space outside the region 13. Exterior to the reflector 61,
the collector electrode 30 and support 54 are shielded as before by
a surrounding glass tube 56.
The extremities of the extractor electrode 69 are shown to have
rims 71 and 72 which are desirable for purposes of rigidity. The
extractor 69 can be modified by shortening it while maintaining a
similar proximity to the wall 26.
Thus by employing the invention, a nude form of external collector
ion gauge can be manufactured, the operation of which is
substantially independent of the electric field conditions produced
by the surrounding vacuum system. The use of extractor and
reflector electrodes enables the various electrodes of such a gauge
to be biased so that an optimum sensitivity can be obtained in a
repeatable manner.
* * * * *