Abstract:
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 comprises an electron-emitting cathode ( 31 ) made by exploiting the nanotube technology, a grid ( 13; 33; 133; 133 ′) for accelerating the electrons emitted by the cathode, and a plate ( 15; 35 ) collecting the ions and/or the ionised positive molecules of the gas. Measuring the plate current by a galvanometer allows for determining the value of the residual pressure inside the container.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ionisation vacuum gauge. 
     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 ionised positive molecules of the gas, wherein the measurement of the plate current by a galvanometer allows for determining the value of the residual pressure inside the container. 
     BACKGROUND OF THE INVENTION 
     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. 
     In thermionic emission vacuum gauges, the electron source comprises 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 the 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. 
     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. 
     A plate, which is designed as a simple wire allows for pressure measurements as low as about 10 −9  Pa. Indeed, 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. 
     The example of vacuum gauge is disclosed for instance in U.S. Pat. No. 2,605,431 “Ionisation Vacuum Gauge”. 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, where directionality of the electron beam is an essential parameter for vacuum gauge sensitivity. 
     The vacuum gauge sensitivity is not constant, since the distribution of the electron emission changes direction as the temperature along the emitting cathode filament changes, this filament typically reaching temperatures up to about 2000° C. 
     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. 
     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. 
     The above and other objects are achieved by a vacuum gauge as claimed in the appended claims. 
     Advantageously, the gauge according to the invention exploits the nanotube technology for making the electron-emitting cathode. 
     According to such a solution, electron emission takes place by field effect, and not by thermionic effect: application to a nanotube film of a strong electric field, whose flow lines are concentrated at the ends of the nanotubes, results in electron emission. 
     A nanotube cathode is a so-called “cold” electron source, requiring very low power consumption and having high directionality. 
     According to a preferred embodiment of the invention, due to the use of such a cathode, it is possible to utilize not only cylindrical geometry of the conventional Bayard-Alpert vacuum gauge but to use different geometries, allowing miniaturising the ionisation vacuum gauge. 
     More particularly, according to some embodiments of the invention, the electrons continue moving in the space between the grid and the plate, without any appreciable electron amount passing again through the grid. 
     The preferred embodiments of vacuum gauge according to the invention, given by the way of non-limiting examples, is disclosed in greater detail hereinafter, with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematical perspective representation of a nanotube; 
         FIG. 2A  is a schematical representation of a assembling nanotubes for manufacturing a nanotube film for electron emission; 
         FIG. 2B  is another schematical representation of assembling nanotubes for manufacturing a nanotube film for electron emission; 
         FIGS. 3 to 8  are schematical perspective views of different embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , reference numeral  1  denotes a single-wall carbon nanotube. 
     Carbon nanotubes are one of the possible forms of crystalline carbon, together with graphite, diamond and fullerenes. 
     Generally, a single-wall carbon nanotube  1  can be considered as a carbon tube made of a graphite layer rolled up into a cylinder, closed at its ends by two hemispherical caps  1   b . The nanotube body is formed only by hexagonal carbon structures  3   a , where the end caps are generally formed by both hexagonal structures  3   a  and pentagonal structures  3   b  of carbon atoms. 
     The diameter of a nanotube is generally in a range 0.8 to 10 nm and usually is smaller than 2 nm. The length of a nanotube is generally of the order of 10 4  to 10 5  nm, so that nanotubes can be considered monodimensional structures. 
     Multiple nanotubes assembled into a thin film exhibit optimum field emission capability, i. e. capability of emitting electrons due to the action of a strong electric field, whose flow lines are concentrated at ends  1   b  of the nanotubes. 
     In order to exhibit good field emission capability, the nanotubes in the thin film must be arranged in ordered manner. 
       FIGS. 2A and 2B  show two typical modalities for assembling the nanotubes. 
     In  FIG. 2A , a plurality of nanotubes  5 ,  5 ′,  5 ″ are arranged inside one another, so that they are concentric and form a so-called multiple-wall nanotube. 
     In  FIG. 2B , on the contrary, a plurality of nanotubes  7 ,  7 ′,  7 ″ are arranged parallel and adjacent to one another, so that they form an ordered bundle. 
     By using either arrangement described above, a nanotube film with optimum electron emission properties can be obtained. 
     Turning now to  FIG. 3 , a vacuum gauge according to the invention is shown. The ionisation vacuum gauge is of the so-called Bayard-Alpert type, which uses a cathode  31  capable of emitting electrons and formed by a nanotube film  29  arranged on a substrate  27 . 
     The vacuum gauge is housed inside a vacuum chamber  10  and it comprises, a nanotube cathode  31  for electron emission, an anode  13  in a shape of a cylindrical grid, capable of accelerating the electrons emitted by the cathode, and a wire-shaped plate or collecting electrode  15 , located centrally of anode  13 , for collecting the gas ions and ionised positive molecules. 
     Cathode  31 , formed by the thin nanotube film  29  arranged on the substrate  27  according to the arrangement shown in either  FIG. 2A  or  FIG. 2B , is a low-temperature, highly directional, field-emission electron source. An extraction grid  30  is located opposite to film  29 , at short distance therefrom, and is connected to a power supply  17  keeping the grid at a potential V 30  higher than that of the substrate  27 , which is grounded. The potential difference between the substrate  27  and the extraction grid  30  generates an electric field, in which nanotube film  29  is immersed and which causes field-effect electron emission by the nanotubes. 
     The electrons emitted by the cathode  31  are accelerated by grid-shaped anode  13 , connected to a second power supply  19  and kept at a potential V 13 &gt;V 30 . The electrons accelerated in this manner pass through the grid  13  and move towards collecting electrode  15  that, however, being grounded, repels the electrons, causing them to pass again through grid  13 . This motion in and out of the grid  13  continues until the electrons are absorbed by the grid itself. During this motion, the electrons ionise the molecules or atoms of the residual gas contained in vacuum chamber  10 , so that the ionised molecules or atoms are attracted by the plate  15 . The ion current generated on said plate  15  can be measured by means of a galvanometer  21 . Suitable signal processing means  23  allows for obtaining the residual gas pressure inside chamber  10  from the value of the ion current, once the current intensity of the electron source of cathode  31  is known. 
     It is clear that using a nanotube cathode allows for solving many problems inherent in the use of ionisation vacuum gauges. The nanotubes are highly directional electron sources, whereas the conventional heated filament is a substantially isotropic source. Moreover, the power required to apply to the cathode a potential difference sufficient to cause field emission by the nanotubes is far lower than that required to bring the filament to incandescence. 
     Referring to  FIG. 4 , there is shown a second embodiment of the vacuum gauge according to the invention. A chamber  10  encloses the volume containing a residual gas, the pressure of which is to be measured. The vacuum gauge substantially comprises: a cathode  31  capable of emitting electrons, which cathode is formed by a nanotube film  29  arranged on a substrate  27  and is provided with an extraction grid  30 ; a grid-shaped anode  33 , capable of accelerating the electrons emitted by cathode  31 ; and a plate or collecting electrode  35 , which is to collect the ions produced by the electron collisions with the gas atoms or molecules. 
     In that embodiment, anode  33  is made as a substantially plane grid placed opposite to said cathode  31 , at a short distance therefrom. Thus, the electrons emitted by cathode  31  are focussed into a beam oriented according to a preferential initial direction (denoted by arrow F), substantially perpendicular to the plane of the grid  33 . 
     It is therefore advantageous to make plate  35  as a plane plate, in register with and substantially parallel to the grid  33 . 
     Note that, in the embodiment shown, cathode  31  and plate  35  are made as plane plates. Such members could however have a different shape as well, e.g. a concave or convex shape. Moreover, plate  35  could be also 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 the plate. 
     The electrons, once the cathode  31  due to field effect emits them, are accelerated through holes  34  of grid  33  in a direction perpendicular to the grid, towards plate  35 . To this end, like in the previously disclosed embodiment, the grid  33  is suitably biased at a potential V 33  higher than potential V 30  at which extraction grid  30  of cathode  31  is set and such that the electrons passing through grid  33  come out therefrom with a kinetic energy preferably in a range 100 to 150 eV, that is, in the most favourable energy range for ionisation of residual gas present in chamber  10 . 
     In order to keep extraction grid  30  of cathode  31  and anode  33  at different potentials, two d.c. power supplies  17 ,  19  connected in series are provided. 
     Extraction grid  30  is connected to power supply  17 , which keeps the grid at a potential V 30  higher than that of grounded substrate  27  of nanotube film  29 . 
     The electrons emitted by the cathode  31  are accelerated by grid-shaped anode  33 , connected to the second power supply  19  and kept at a potential V 33 &gt;V 30 . 
     During their motion between said grid  33  and said plate  35 , the electrons collide with the atoms or the molecules of the residual gas, ionising them. When arriving close to the plate  35 , the electrons are repelled by the plate, since the plate is grounded. The electrons are also repelled by the walls of chamber  10 , which also are grounded, and are directed again towards grid  33 , by which they are eventually absorbed after further collisions with the atoms or the molecules of the residual gas. 
     The ions of the residual gas are on the contrary collected by plate  35 , which is connected with a galvanometer  21  for measuring the absorbed ion current. Suitable means  23  for processing the analogue signal generated by galvanometer  21  allows for obtaining the residual gas pressure in chamber  10  from the value of the ion current, once the current intensity of the source consisting in cathode  31  is known. 
     To obtain a more accurate measurement, also grid  33  can be connected to a second galvanometer (not shown), for measuring the grid electron current due to the electrons absorbed by said grid. 
     Residual pressure P x  can thus be obtained according to relation:
 
 p   x   =c·i   p   /i   g ,
 
where:
 
     c is a constant typical of the apparatus and of the gas nature; 
     i p  is the plate current intensity; 
     i g  is the grid current intensity. 
     Note that using a plane geometry allows for placing the collecting plate at a greater distance from the grid (which, on the contrary, surrounds said 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. 
     Moreover, using the plane geometry for grid-shaped anode  33 , together with using a nanotube emitting cathode  31 , allows for further miniaturising the vacuum gauge according to the invention. 
     The cathode  31  and the grid  33  may be spaced apart by tens of micrometres (for instance, 20 to 50 μm), and the distance between the grid  33  and the plate  35  may be for instance in a range from 100 to 500 μm, depending on the sensitivity needed. Clearly indeed, the greater the spacing between the grid  33  and the plate  35 , the greater the probability of ionisation of the residual gas contained in chamber  10 . 
     In order to further reduce the size of the vacuum gauge according to the invention, in the embodiment shown in  FIG. 4 , two magnets  25  (for instance, electromagnets or permanent magnets), formed by grounded plane discs or plates, are located between the grid  33  and the plate  35 , in planes perpendicular to both the electrodes  33 ,  35  and hence parallel to the initial direction of the electron beam. 
     The magnetic field produced by the 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 the gas, and hence the sensitivity of the vacuum gauge according to the invention, are increased. In the alternative, the distance between grid  33  and plate  35  (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. 
     Turning now to  FIG. 5 , there is shown a third embodiment of the vacuum gauge according to the invention, which differs from the previous ones in a shape of the grid-shaped anode, here denoted by reference numeral  133 . 
     The anode  133  is made as a substantially parallelepiped cage, having a face  133   a  parallel to the cathode  31  and located at a short distance therefrom. Thus, the electrons emitted by cathode  31  are accelerated through the face  133   a  of anode  133  according to a preferential initial direction (denoted by arrow F), substantially perpendicular to the plane of said face  133   a.    
     Collecting plate  35  is placed opposite to face  133   a , in correspondence of open base  132  of grid  133 . 
     Note that using a parallelepiped grid  133  allows for increasing the vacuum gauge sensitivity. Actually, being both plate  35  and the walls of chamber  10  grounded, the ions could be attracted by the walls rather than by plate  35 , thereby creating an ion dispersion effect. Use of the parallelepiped grid  133 , which is closed except for the opening  132  in correspondence of the plate  35 , allows for avoiding ion dispersion and consequently increasing the vacuum gauge sensitivity. 
     Turning now to  FIG. 6 , there is shown a fourth embodiment of the vacuum gauge according to the invention, which differs from the previous ones in the arrangement of collecting plate  35 . 
     In the previously disclosed embodiments, the plate  35  is placed opposite to cathode and lies in a plane substantially parallel to the cathode itself and perpendicular to preferential direction F of the electron beam. 
     On the contrary, in the embodiment of the vacuum gauge according to the invention shown in  FIG. 6 , plate  35  lies in a plane substantially perpendicular to the plane of cathode  31 , 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 the plate  35  move towards the plate in a direction substantially perpendicular to that of the electron beam. 
     Thus, interactions between the electron source (cathode  31 ) and collecting plate  35  are limited. More particularly, the photoelectric effect on plate  35  due to X-rays emitted by grid  133 ′ is significantly limited, whereby the sensitivity of the vacuum gauge according to the invention is further increased. 
     In this embodiment, grid-shaped anode  133 ′ is suitably equipped with a side opening  132 ′ in correspondence with collecting plate  35 . 
     An extracting device  37  may be provided in correspondence with opening  132 ′ to make ion channelling towards plate  35  easier. The extracting device may comprise, for instance, in an electrostatic lens and it is connected to a power supply  16 , such that the extraction device can be brought to a potential intermediate between the potentials of plate  35  (that is grounded) and grid  133 ′. 
     This embodiment may be provided with a pair of magnets  25  in order to create a magnetic field causing the electrons to move along spiral paths. In the plate-shaped magnets  25  are advantageously located in planes perpendicular to both cathode  31  and plate  35 . 
     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  35  at a greater distance from grid-shaped anode  133 ′. To this aim, means such as magnets, capacitor plates, electrostatic lenses, radiofrequency devices, capable of deflecting a beam of charged particles, could be used. 
     Turning to  FIG. 7 , there is shown a fifth embodiment of the invention, in which a capacitor  45  is provided, of which plates  47  are suitably biased so as to channel between them the ions or the ionised molecules, so as to deflect their advancing direction by about 90°. 
     More particularly, one of the plates  47  may be grounded and the other may be brought to a suitable potential to obtain ion paths with the desired curvature radius. 
     The electrons accelerated by anode  133 ′ collide with the atoms or the molecules of the residual gas and ionise them. The ions or the ionised molecules are channelled into the space between plates  47  of the capacitor  45  and are deflected by 90° towards a plate  35  placed at the exit from the passageway defined between the plates  47 . 
     Similarly, in a sixth embodiment of the vacuum gauge according to the invention, shown in  FIG. 8 , a capacitor  49  may be provided, located between extracting device  37  and collecting plate  35  and having plates  51  that are shaped so as to deflect the direction of the ions or the ionised molecules by about 180°. 
     The ions or the ionised molecules produced by the collisions of the electrons accelerated by anode  133 ′ are channelled between plates  51  of the capacitor  49  and are deflected by 180° towards the plate  35  placed at the exit from the passageway defined between the plates  51 . 
     Advantageously, according to the latter two embodiments, collecting plate  35  is isolated from the electron beam and the electron source, so that the photoelectric effect due to X-rays produced on grid  133 ′ is significantly reduced. 
     In the latter two disclosed embodiments described above, a pair of shaped magnets might be used 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 133′   −V   m ),
 
where
 
     m and q are the mass and the charge, respectively, of the ions to be deflected; 
     r is the desired curvature radius; 
     B is the strength of the magnetic field generated by said magnets and; 
     V 133′ is the potential of grid  133 ′. 
     The skilled in the art will immediately appreciate that the use of the vacuum gauge of the present invention gives important advantages. First, the possibility of constructing a vacuum gauge of substantially reduced size makes the vacuum gauge suitable for any field of application. Still due to its reduced size, the vacuum gauge does not perturb the environment where pressure is to be measured, so that said measurement is more reliable and accurate. 
     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.