Abstract:
The present invention relates to an ionisation vacuum gauge for measuring the residual pressure of a gaseous material remaining in a container ( 10 ), more particularly after operation of a vacuum pump. The gauge comprises an electron-emitting cathode ( 17 ), a grid ( 13 ) for accelerating the electrons emitted by the cathode and a plate ( 15 ) collecting the ions and/or the ionised positive molecules of the gas, wherein said plate is placed outside said grid. Measuring the plate current by a galvanometer allows determining the value of the residual pressure inside the container.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an ionisation vacuum gauge.  
         [0002]     More particularly, the present invention relates to an ionisation vacuum gauge for measuring the residual pressure of a gaseous material remaining in a container, for instance, after operation of a vacuum pump, the gauge being of the kind comprising an electron-emitting cathode, a grid for accelerating the electrons emitted by the cathode and a plate collecting the ions or the ionised positive molecules of the gas, wherein the measurement of the plate current by a galvanometer allows determining the value of the residual pressure inside the container.  
       BACKGROUND OF THE INVENTION  
       [0003]     Two kinds of vacuum gauges are known: thermionic emission vacuum gauges (also called hot cathode vacuum gauges), and field emission (or cold cathode) vacuum gauges.  
         [0004]     In thermionic emission vacuum gauges, the electron source consists in one or more filaments, for instance of tungsten, brought to incandescence. A typical example of hot cathode vacuum gauge is the Bayard-Alpert vacuum gauge. That kind of vacuum gauge comprises a wire-shaped ion collecting plate, a cylindrical grid surrounding said plate and an incandescent tungsten filament for electron emission, located outside the grid. The electrons emitted by the filament and accelerated by the grid ionise the residual gas, and the ions and/or the ionised positive molecules are collected by the plate, which is kept at lower potential than the electron source and the grid.  
         [0005]     In the disclosed design, the electrons pass several times through the grid and, during such in and out movement, they ionise the residual gas until they hit the grid and are absorbed by it.  
         [0006]     Due to that design including a plate reduced to a simple wire, pressures as low as about 10 −9  Pa could be measured. Because of the reduced plate wire surface, the background current is minimised due to photoelectric effect from the plate (electron emission) caused by X rays produced by electrons hitting the grid.  
         [0007]     Such a vacuum gauge is disclosed for instance in U.S. Pat. No. 2,605,431, “Ionisation Vacuum Gauge”.  
         [0008]     The major drawback of that kind of vacuum gauges is related to the nature of the electron-emitting cathode. Actually, a heated filament is an isotropic electron source, whereas directionality of the electron beam is an essential parameter for vacuum gauge sensitivity.  
         [0009]     In the disclosed design, vacuum gauge sensitivity is not constant, since the distribution of the electron emission direction changes as the temperature along the emitting cathode filament changes, said filament typically reaching temperatures up to about 2000° C.  
         [0010]     Moreover, the phenomenon of electron emission by thermionic effect entails high power consumption, long response times and a non-negligible pollution of the surrounding environment due to the release of impurities.  
         [0011]     In order to improve the performance of Bayard-Alpert vacuum gauges, use of more recent technologies has been proposed for making the electron-emitting cathode.  
         [0012]     U.S. Pat. No. 5,278,510 “Ionisation Vacuum Gauge Using a Cold Micropoint Cathode” discloses a vacuum gauge wherein, in order to obviate the drawbacks mentioned above, the heated filament is replaced by a microtip cold cathode.  
         [0013]     A microtip cathode comprises a set of very small tip-shaped emitters, located beneath an extraction grid provided with corresponding openings. The microtips are made of metal (molybdenum, niobium) or of silicon-polysilicon and are placed on supports of the same material, or of silicon or glass. By applying a potential difference between the microtips and the extraction grid, an electric field is produced that is strong enough to produce electron emission. This microtip cathode is an extremely directional electron source, with low consumption and very short response time.  
         [0014]      FIG. 1  schematically shows a microtip disclosed above. On a plane substrate  1 , for instance of niobium or molybdenum, a tip  3  of the same metal, about 1 μm high, is grown. An extraction grid  5  is located above substrate  1  and parallel thereto, and has openings  7  in correspondence with each tip  3 . Said openings  7  typically have diameters of about 1 μm. Said grid  5  is kept separated from said substrate  1  and from tips  3  by an insulating layer  9 , having a cavity  11  in correspondence with each tip  3  to allow electron emission from tip  3  through the corresponding opening  7  in extraction grid  5 .  
         [0015]     Usually, the microtips are produced in arrays and adjacent microtips are spaced apart by few micrometers, so that densities of the order of 10 6  to 10 8  microtips/cm 2  can be achieved.  
         [0016]     The teaching of U.S. Pat. No. 5,278,510 provides, however, only a partial response to the problems inherent in ionisation vacuum gauges.  
         [0017]     More particularly, the vacuum gauge described above, which reproduces the geometry of a conventional Bayard-Alpert vacuum gauge, is cumbersome and difficult to use in many applications. Moreover, said vacuum gauge is by itself a non-negligible distortion in pressure measurement, taking also into account the high vacuum degrees it is intended for.  
         [0018]     This is mainly due to the fact that, even if the microtip cathode is a highly directional electron source, electron movement into and out of the cylindrical grid does not allow reducing the vacuum gauge size.  
       SUMMARY OF THE INVENTION  
       [0019]     It is the main object of the present invention to overcome the above drawbacks, by providing a miniaturised vacuum gauge, which has a great sensitivity and which does not appreciably perturb the pressure measurements.  
         [0020]     It is another object of the present invention to provide an ionisation vacuum gauge with a cold electron source having high directionality, low consumption and short response time.  
         [0021]     The above and other objects are achieved by a vacuum gauge as claimed in the appended claims.  
         [0022]     Advantageously, the arrangement according to the invention, in which the collecting plate is located outside the grid-shaped anode accelerating the electrons, allows a considerable size reduction.  
         [0023]     Moreover, by using a substantially plane grid-shaped anode, an electron beam can be obtained that leaves said anode according to a predetermined direction, substantially perpendicular to the plane on which said anode lies.  
         [0024]     The advancing direction of the electrons is then modified by the collisions with the atoms or molecules of the residual gas and by interaction with the plate. The electrons however continue moving in the space between the anode and the plate, without any appreciable electron amount passing again through the grid-shaped anode.  
         [0025]     Advantageously, the vacuum gauge performance can be enhanced by acting on the characteristics of the electron motion, by introducing magnetic and/or electric fields.  
         [0026]     Introducing a magnetic field allows for imparting to said electrons a spiral motion, thereby allowing incrementing the collisions with the residual gas atoms or molecules for a given linear distance travelled.  
         [0027]     By introducing means capable of deflecting an ion beam, for instance a capacitor with shaped plates or a pair of permanent magnets, the ions or the ionised molecules can be taken from the region where they are produced and transferred to the collecting plate.  
         [0028]     The emitting cathode is preferably manufactured by using the microtip technology. Thus, due to the plane grid anode and to a cold emitting cathode, the overall size of the vacuum gauge according to the invention can be limited to a few hundreds of micrometres.  
         [0029]     Some preferred embodiments of the vacuum gauge according to the invention, given by way of non-limiting example, will be disclosed in greater detail hereinafter, with reference to the accompanying drawings, in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a schematic perspective representation of a microtip of an electron emitting cathode according to previously discussed prior art.  
         [0031]     FIGS.  2  to  6  are schematic representations of the vacuum gauge according to different embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     Referring to  FIG. 2 , there is shown a first embodiment of the vacuum gauge according to the invention. A chamber  10  encloses the volume containing a gaseous material, the residual pressure of which is to be measured. The vacuum gauge comprises a cathode  17  capable of emitting electrons, a grid-shaped anode  13 , capable of accelerating the electrons emitted by cathode  17 , and a plate or collecting electrode  15 , which is intended to collect the gas ions produced by electron collisions with the atoms or molecules of said gaseous material.  
         [0033]     Anode  13  is made as a substantially plane grid and is placed opposite cathode  17 , at short distance therefrom. Thus, the electrons emitted by cathode  17  are accelerated into a beam oriented according to a preferential initial direction (denoted by arrow F), substantially perpendicular to the plane of anode  13 . Plate  15  is made as a plane plate, in register with and substantially parallel to grid-shaped anode  13 .  
         [0034]     According to the first embodiment as shown in  FIG. 2 , cathode  17 , grid  13  and plate  15  are made as plane plates. Such members could however have different shapes as well, e.g. a concave or convex shape. Moreover, plate  15  could also be made as a small bar or a wire. The plane plate shape is however preferable since increasing the plate surface directed towards the electron source results in increasing the sensitivity of said plate. According to the embodiment shown, cathode  17  is made by using the microtip technology. As discussed above, a microtip cathode comprises a set of very small tips  3 , placed onto a conducting substrate  1  beneath an extraction grid  5  having corresponding openings  7 . By applying a suitable potential difference between grid  5  and microtips  3 , an electric field is produced that is strong enough to cause electron emission.  
         [0035]     The electrons, once emitted, are accelerated through holes  23  of grid-shaped anode  13  in a direction perpendicular to anode, towards plate  15 . To this end, grid-shaped anode  13  is biased at a potential V 13  higher than potential V 5  at which extraction grid  5  is set, and such that the electrons passing through the anode come out therefrom with a kinetic energy preferably in the range 100 to 150 eV, being a favourable energy range for ionisation of residual gas present in chamber  10 .  
         [0036]     In order to keep substrate  1 , extraction grid  5  and anode  13  at different potentials, the vacuum gauge according to the invention includes two D.C. power supplies  29 ,  31  connected in series. The first power supply  29  keeps extraction grid  5  at a potential V 5  higher than the potential of tips  3 , which are grounded, so as to create the electric field required for electron emission. The second power supply  31  keeps grid-shaped anode  13  at a potential V 13  higher than potential V 5  of extraction grid  5 , so as to accelerate the electrons, preferably to an energy in a range 100 to 150 eV.  
         [0037]     Along their paths between grid-shaped anode  13  and plate  15 , the electrons collide with the atoms or the molecules of the residual gas, ionising them. When arriving close to plate  15 , the electrons are repelled by the plate, since said plate is grounded. The electrons are also repelled by the walls of chamber  10 , which also are grounded, and are directed again towards grid-shaped anode  13 , by which the electrons are eventually absorbed after further collisions with atoms or molecules of the residual gas.  
         [0038]     The ions of the residual gas are on the contrary collected by plate  15 , which is connected with a galvanometer  33  allowing measuring the plate ion current. Suitable means  35  for processing the analogue signal generated by galvanometer allows for the residual gas pressure in chamber  10  from the value of the ion current, once the current intensity of the electron source consisting in cathode  17  is known.  
         [0039]     To obtain a more accurate measurement, also grid  13  can be connected to a second galvanometer, for measuring the grid electron current due to the absorbed electrons.  
         [0040]     Residual pressure p x  can thus be obtained according to relation: 
 
 p   x   =c·i   p   /i   g ,
 
 where 
 
         [0042]     c is a constant typical of the apparatus and of the gas nature;  
         [0043]     i p  is the plate current intensity;  
         [0044]     i g  is the grid current intensity.  
         [0045]     Note that using a plane geometry allows for placing the collecting plate at a greater distance from the grid (which, on the contrary, surrounds the plate in the Bayard-Alpert vacuum gauge), thus limiting the background current due to the photoelectric effect of the plate, caused by X rays produced on the grid. Consequently, in the vacuum gauge according to the invention, the plate does not need to be reduced to a wire (as in the Bayard-Alpert vacuum gauge), but its surface can be advantageously increased so as to enhance the measurement sensitivity.  
         [0046]     Moreover, using a plane geometry for grid-shaped anode  13 , together with using a microtip emitting cathode, allows for obtaining a vacuum gauge with greatly reduced size.  
         [0047]     Cathode  17  and grid-shaped anode  13  may be spaced apart by some tens of micrometres (for instance, 20 to 50 μm), and the spacing between grid-shaped anode  13  and plate  15  may for instance in a range from 100 to 500 μm, depending on the desired sensitivity. Clearly indeed, the greater the spacing between grid-shaped anode  13  and plate  15 , the greater the probability of ionisation of the residual gas contained in chamber  10 .  
         [0048]     In order to further reduce the vacuum gauge size, according to the embodiment shown in  FIG. 2 , two magnets  25  (for instance, electromagnets or permanent magnets), formed by grounded discs or plane plates, are located between grid-shaped anode  13  and plate  15 , in planes perpendicular to grid-shaped anode  13  and plate  15  and hence parallel to the initial direction of the electron beam.  
         [0049]     The magnetic field produced by magnets  25  affects the motion of the electrons, which follow spiral paths. Thus, the number of collisions of each electron with the atoms or the molecules of the residual gas per unit of linear distance travelled is increased. In other words, with a same geometry, the ionisation degree of said gas and hence the vacuum gauge sensitivity are increased. In the alternative, still due to the provision of the magnets described above, the spacing between grid-shaped anode  13  and plate  15  (and hence the overall dimensions of the vacuum gauge according to the invention) can be reduced while leaving the ionisation degree of the residual gas and the vacuum gauge sensitivity unchanged.  
         [0050]     Turning to  FIG. 3 , there is shown a second embodiment which differs from the previous one in the shape of the grid-shaped anode, here denoted by reference numeral  113 .  
         [0051]     Anode  113  is a substantially parallelepiped cage, having a face  113   a  parallel to cathode  17  and placed at a short distance therefrom. Thus, the electrons emitted by cathode  17  are accelerated through face  113   a  of anode  113  according to a preferential initial direction (denoted by arrow F), substantially perpendicular to the plane of face  113   a.    
         [0052]     Collecting plate  15  is placed opposite face  113   a , in correspondence of open base  114  of grid  113 .  
         [0053]     Note that using a parallelepiped grid  113  allows for increasing the vacuum gauge sensitivity. Actually, being both plate  15  and the walls of chamber  10  grounded, the ions could be attracted by the walls rather than by plate  15 , thereby creating an ion dispersion effect. Using a parallelepiped grid  113 , which is closed except for opening  114  in correspondence of plate  15 , allows for avoiding ion dispersion and consequently increasing the vacuum gauge sensitivity.  
         [0054]     Turning now to  FIG. 4 , there is shown a third embodiment of the vacuum gauge according to the invention, which differs from the previous ones in the arrangement of collecting plate  15 .  
         [0055]     In the previously disclosed embodiments, plate  15  is placed opposite the cathode and lies in a plane substantially parallel to the cathode itself and perpendicular to preferential direction F of the electron beam.  
         [0056]     In the embodiment shown in  FIG. 4 , plate  15  lies in a plane substantially perpendicular to the plane of cathode  17 , and hence it is located in a plane parallel to preferential initial direction F of the electron beam. Thus, the ions and the ionised molecules attracted towards plate  15  move towards the plate in a direction substantially perpendicular to that of the electron beam.  
         [0057]     Thus, interactions between the electron source (cathode  17 ) and collecting plate  15  are limited. More particularly, the photoelectric effect on plate  15  due to X rays emitted by grid  113 ′ is significantly limited, whereby the sensitivity of the vacuum gauge according to the invention is further increased.  
         [0058]     In this embodiment, grid-shaped anode  113 ′ is equipped with a side opening  114 ′ in correspondence with collecting plate  15 .  
         [0059]     An extracting device  19  may be provided in correspondence of opening  114 ′ to make ion channelling towards plate  15  easier. The extracting device may, for example comprise electrostatic lens and it is connected to a power supply  27 , such that the extraction device can be brought to a potential intermediate between the potentials of plate  15  (that is grounded) and grid  113 ′.  
         [0060]     In this embodiment, a pair of magnets  25  may be provided in order to create a magnetic field causing the electrons to move along spiral paths. In the present case, magnets  25  are advantageously located in planes perpendicular to both cathode  17  and plate  15 .  
         [0061]     In order to limit the background current due to photoelectric effect of the plate caused by X rays produced on the grid and, hence, to improve the sensitivity of the vacuum gauge according to the invention, it might be advantageous to place collecting plate  15  at a greater distance from grid-shaped anode  113 ′. To this aim, means such as magnets, capacitor plates, electrostatic lenses, radio frequency devices, capable of deflecting a beam of charged particles, could be used.  
         [0062]     Turning to  FIG. 5 , there is shown a fourth embodiment of the invention, in which a capacitor  45  is provided, of which plates  47  are suitably biased and shaped so as to channel between them the ions or the ionised molecules, thereby deflecting their advancing direction by about 90°.  
         [0063]     More particularly, one of plates  47  may be grounded and the other may be brought to a suitable potential to obtain ion paths with the desired curvature radius.  
         [0064]     The electrons accelerated by anode  113 ′ collide with the atoms or the molecules of the residual gas and ionise them. The ions or the ionised molecules are channelled by extracting device  19  into the space between plates  47  of capacitor  45  and are deflected by 90° towards plate  15  placed at the exit from the passageway defined between said plates  47 .  
         [0065]     Similarly, in a fifth embodiment of the vacuum gauge according to the invention, shown in  FIG. 6 , a capacitor  49  may be provided, located between extracting device  19  and collecting plate  15  and having plates  51  that are suitably biased and shaped so as to deflect the direction of the ions or the ionised molecules by about 180°.  
         [0066]     The ions or the ionised molecules produced by the collisions of the electrons accelerated by anode  113 ′ are channelled between plates  51  of capacitor  49  and are deflected by 180° towards plate  15  placed at the exit from the passageway defined between plates  51 .  
         [0067]     Advantageously, according to the latter two embodiments, collecting plate  15  is isolated from the electron beam and the electron source, so that the photoelectric effect due to X rays produced on grid  113 ′ is significantly reduced.  
         [0068]     The fourth and fifth embodiments described above may utilise a pair of shaped magnets in place of a capacitor for deflecting the ions. In such case, electrical potential V m  between said magnets will be chosen depending on the curvature radius desired for the ion paths, according to relation: 
 
 m/q=r   2   B   2 /2( V   113′   −V   m ),
 
 where 
 
         [0070]     m and q are the mass and the charge, respectively, of the ions to be deflected;  
         [0071]     r is the desired curvature radius;  
         [0072]     B is the strength of the magnetic field generated by said magnets and;  
         [0073]     V 113′  is the potential of grid  113 ′.  
         [0074]     The skilled in the art will immediately appreciate that the use of the vacuum gauge described above gives important advantages. First, the possibility of constructing a vacuum gauge of extremely reduced size makes the vacuum gauge according to the invention suitable for any field of application. Still due to its reduced size, the vacuum gauge according to the invention does not perturb the environment where pressure is to be measured, so that said measurement is more reliable and accurate. Moreover, while operating according to principles similar to a Bayard-Alpert vacuum gauge, the vacuum gauge according to the invention has a wider operating pressure range, whereby it can be used with advantage for measuring very high vacuum of the order of 10 −10  to 10 −11  Pa.  
         [0075]     It is clear as well that the above description has been given only by way of non limiting example and that changes and modifications are possible without departing from the scope of the present invention as claimed.  
         [0076]     In particular, even though using a cold cathode with microtips for electron emission is advantageous, any electron source (including the conventional incandescent wire) could be used, without thereby departing from the scope of the present invention.