Field of the invention

The invention relates to a vacuum measuring gauge for high vacuum applications.

Prior art

Vacuum measuring gauges of the Bayard-Alpert type are well known. Usually a hot cathode is used as an electron source. This has many disadvantages. A hot cathode which usually takes the shape of a filament needs to be heated by an electric current which requires a feed circuit capable of constantly supplying a relatively high feed voltage. The cathode emits electromagnetic radiation which may influence measurement results in not precisely predictable ways and may lead to undesirable interference with other measurements. A hot cathode also tends to release adsorbed gas molecules and to chemically react with gases when heated which again may influence measurement results . It is usually sensitive to vibrations which are often unavoidable in industrial processes. This may again lead to a deterioration of measuring accuracy and over time even to the destruction of the cathode. The kinetic energies of the emitted electrons are distributed over a relatively wide interval which reduces efficiency and may also lead to fluctuations of the sensitivity of the gauge.

US 5 278 510 A shows a Bayard-Alpert vacuum measuring gauge of the generic type with a field emission cathode where, in an essentially cylindrical housing, an ion collector in the

shape of an axial wire is arranged, surrounded by a cylindrical grid-like anode. Between the anode and the housing the electron-emitting field emission cathode facing the anode and the ion collector is arranged. Cathodes of this type offer considerable advantages. They are mechanically robust solid state elements which do not produce heat and function within a large temperature range. They do not as a rule cause side effects which might impact on measurement results in ways which are difficult to assess and compensate for.

In the vacuum measuring gauge as described in the above- mentioned patent publication the field emission cathode is a Spindt type cathode with isolated micropoints shaped as a square plate with a planar emitter area. Due to this planar configuration the emitter area can only be relatively small . Also, it covers only a small solid angle as seen from inside the anode, in particular from the position of the ion collector.

For high accuracy, in particular at very low pressure, a high ionization rate is required. This can only be achieved by a large electron density that must be created by extracting electrons at a high rate from the small emitter area which implies a high electron flow density there which in turn requires a high quality emitter surface as well as elevated electrical field strengths. The latter requirement may cause considerable strain on the material resulting in wear, e.g., due to sputtering, thereby reducing the service life of the gauge. Also, the need to provide high voltage differences places more stringent design requirements on the control circuit.

Summary of the invention

It is the object of the invention to provide a vacuum measuring gauge of the generic type which ensures a high yield of ions in the rarefied gas by achieving high electron density with moderate requirements on the electron-emitting cathode. This object is achieved by the features in the characterizing clause of claim 1.

In a vacuum measuring gauge according to the invention a larger emitter area can be accommodated in a housing of a given size. An electrode configuration with a larger emitter area. As the emitter surface can be made large the required electron flow density at the same may be comparatively small such that moderate field strengths will usually be sufficient. This alleviates requirements on the cathode and allows for a wider choice of material and structure for the emitter surface.

In many cases relatively cheap, easy to manufacture solutions will become practical. The emitter area may be formed by an emitter layer on the surface of the housing wall or even by the wall surface itself in which case the vacuum measuring gauge is especially simple and mechanically stable.

Where the emitter area is concave and in part surrounds a reaction region containing the ion collector, the electrons emitted by the cathode will remain in the reaction region until they eventually reach the anode. They will repeatedly collide with molecules of the rarefied gas in the reaction region and thereby produce ions which can be used in the measuring process. Only a small fraction of the electrons will escape and reach the housing along a relatively short

trajectory, even where no special measures, like keeping the housing at a voltage which is lower than the emitter voltage, are taken to prevent it. Even with a modest electron flow from the cathode, electron density in the reaction region will be high and a high yield of ions is assured.

The high achievable ion yield, together with the absence of side effects like desorption of gas molecules allow for extension of the measurement range to very low densities. At the same time, the required voltages are relatively small which allows the use of a rather simple and cheap control circuit with low power consumption.

Brief description of the drawings

Below, the invention is explained in more detail with reference to figures which show only embodiments.

Fig. 1 shows an axial section through a first embodiment ^■ of a vacuum measuring gauge according to the invention,

Fig. 2 shows a cross section through the vacuum measuring gauge along II-II in Fig. 1,

Fig. 3 shows an axial section through a second embodiment of a vacuum measuring gauge essentially like the configuration shown in Figures 1, 2, with a simplified control circuit,

Fig. 4 shows an axial section through a third embodiment of a pressure measuring gauge according to the invention, and

Fig. 5 shows a cross section through the vacuum measuring gauge along V-V in Fig. 4.

Description of the preferred embodiments

The embodiment of the vacuum measuring gauge according to the invention of Fig. 1, 2 comprises a housing with a base plate 1 and a side wall 2 mounted on the base plate 1 which laterally encloses a cylindrical reaction region 3 which is open at the top so as to be connected with a vacuum chamber where the gas pressure is to be measured. The base plate 1 and the side wall 2 together form the housing. Preferably they consist each of an electrically conductive material, in particular stainless steel or aluminium. In the axis of the reaction region 3 an ion collector 4 in the shape of a straight thin axial filament or wire is arranged which is surrounded at a distance by an essentially cylindrical grid- like anode 5, e.g. a helix-shaped wire. The ion collector 4 and the anode 5 each consist of a metal or alloy, e.g., the ion collector 4 of tungsten or stainless steel and the anode 5 of tungsten, platinum iridium alloy or stainless steel. The diameter of the reaction region is preferably between lcm and 8cm and in particular between lcm and 6cm, its height between 2cm and 8cm and in particular between 2cm and 5cm.

The greater part of the side wall 2 serves at the same time as a support wall for an electron-emitting field emission cathode 6 which is attached to its interior surface and covers it virtually completely. The field emission cathode 6 provides an emitter area, a large concave surface of cylindrical shape which concentrically surrounds the ion collector 4 and the anode 5. Seen from the centre of the reaction region 3, that is, from a point on the axis, i.e.,

on the ion collector 4, at medium height, the emitter area covers about two thirds of the full solid angle of 4π. The emitter area comprises an emitter surface formed by an emitter layer 7 of electrically conducting material which is attached to the side wall 2 and electrically connected to it and covers essentially the whole of the emitter area. Insulating spacers 8 are distributed over the emitter area, forming narrow strips which interrupt the emitter surface . The spacers 8 carry an electron-permeable gate 9 of electrically conducting material which is arranged at an essentially fixed distance in front of the emitter surface and electrically insulated from the emitter layer 7. The fixed distance may be anywhere between lμm and 5mm, but preferably it is rather small, i.e., between lμm and 200μm, in particular between 5μm and 200μm, e.g., approximately lOμm. Preferably, the distance between the emitter surface and the ion collector 4 is at least 20 times the distance of the gate 9 from the emitter surface .

The ion collector 4, the anode 5, and the gate 9 are connected via insulating feedthroughs which may consist of glass or glass-ceramics in the base 1 to a control circuit 10 which is also electrically connected to the grounded housing and via the same to the emitter layer 7. The control circuit 10 is schematically shown to comprise a current meter 11 between the ion collector 4 and ground for measuring an ion collector current lie, a voltage source 12 which keeps the anode 5 at an anode voltage V _A above ground voltage and is connected to ground via a current meter 13 measuring an anode current I _A and representing it by an output voltage V ₁ , and a controllable voltage source 14 which provides a gate voltage V _G - somewhere between ground voltage and anode voltage V _A - at the gate 9 and is connected to ground via a current meter 15 measuring a gate

current I ₅ - The controllable voltage source 14 is controlled by a regulator 16 which compares the output voltage Vi of current meter 13 with a reference voltage V _R . It causes the gate voltage V _G to be increased whenever the output voltage V ₁ of the current meter 13 which represents the anode current I _A falls below the reference voltage V _R and vice versa. So the anode current I _A which reflects the electron emission from the field emission cathode 6 is used as a control current for controlling the gate voltage V ₃ and thereby the said electron emission. It is, within narrow limits, kept constant at a set value by appropriate control of the gate voltage V _G . Alternatively, the gate voltage V _G can be pulsed.

The emitter layer 7 of the field emission cathode 6 can be realised in various ways. It can be an arrangement of carbon nanotubes, diamond-like carbon, a metal or a mixture of metals containing, e.g., molybdenum, tungsten or nickel or a semiconductor material, e.g., silicon which may be coated, e.g., with carbide or molybdenum. In any case the emitter surface should be rough with sharp edges or points where high local field strengths are achieved. Instead of cylindrical the shape of the emitter area can be prismatic or, at least in part, spherical. The emitter layer can be a thin layer applied using CVD or other known methods.

If the side wall 2 consists of metal, preferably stainless steel, it may, due to the large size of the emitter area, be sufficient to provide an adequate emitter surface simply by roughening the interior surface of the wall, either by mechanical means or, preferably, by etching, e.g., by plasma etching or, in particular, by chemical etching. A simple etching process will, even without masks or other structuring tools being used, produce a rough surface with

sharp edges and tips where the electric field will attain local maxima which are elevated enough to cause a sufficient level of electron emission simply by bringing out the grain of the wall material.

It is, of course, possible to provide walls with surface portions covered by emitter areas which have other shapes, in particular, with generally curved, that is, non-planar emitter areas of, e.g., concave spherical or even corrugated or convex shape. Apart from the small scale surface roughness which is required by the desired electrical properties, the surface may, on a larger scale, be essentially smooth or it may be piecewise smooth or even piecewise planar with edges where the pieces meet. The overall configuration is preferred to be non-planar in order that the area of the emitter surface be enhanced as compared with that of an overall planar configuration. The emitter area may cover, e.g., between 0.5cm ² and 80cm ² and in particular between lcm ² and 50cm ² .

The gate 9 can be a wire mesh made of metal wires or a grid, also of metal, e.g., nickel, molybdenum or stainless steel as the metal ought to be sputter-resistant. Another possibility consists in providing a relatively dense array of equally distributed spacers which rise above the emitter surface by a distance corresponding to the desired distance between the same and the gate and where the surfaces facing away from the support wall are covered with patches of thin metal film which together form the gate. In this case the emitter area is divided into an intermittent emitter surface and a complementary gate surface, the electric field strength being largest at the boundaries. The spacers can, in particular in the latter case, also be applied by CVD or similar methods and the patches electrically connected to

each other through the spacers by known semiconductor manufacturing methods and further to controllable voltage source 14.

Instead of isolated spacers other spacer means can be used like a porous spacer layer with a thickness of a few micrometers, e.g., a ceramic foil with microholes . The gate can then take the shape of a thin metal film covering the surface of the spacer layer.

In any case, due to the small distance between the emitter surface and the gate, even with a relatively small gate voltage V _G which may be between 200V and 1'OOOV, preferably between 200V and 600V, e.g., 300V a strong electric field is created at the emitter surface which, dependent on the type of gate used, can be relatively homogeneous or rather inhomogeneous . In any case, the high field strength at the emitter surface causes the extraction of electrons from the same which are further accelerated by the electric field between the field emission cathode 6 and the anode 5. The electrons collide with molecules of the rarefied gas in the reaction region 3, causing their ionisation.

The positive ions are attracted to the ion collector 4 where they take up electrons, thereby causing ion collector current I _IC which is essentially proportional to the rate at which ions are produced. The electrons emitted by the cathode 6 as well as those produced in the ionisation process are ultimately attracted to the anode 5, causing anode current I _A . A fraction of the electrons extracted from the emitter surface is caught in the gate 9, causing gate current I _G . Another fraction of electrons may escape to the base plate 1 or the side wall 2. This effect does not influence the result of the measurement but may result in

reduced efficiency of the ionisation process. However, due to the large size of the emitter area and with a solid angle covered by it of 2π or more, i.e., 50% or more of the total of 4π, as seen from the central part of the reaction region 3, losses of this type are kept very small.

With the anode current I _A kept constant, the ion collector current Ii _C is a measure of the density of gas molecules in the reaction region 3. It is, however, also possible to keep the cathode current or the gate current constant by appropriately regulating the gate voltage. The pressure measuring apparatus as described is suitable for measuring pressures in the range from 10 ^~1 mbar to 10 ^"13 mbar and in particular from 10 ^"2 mbar to 10 ^"1:L mbar.

The embodiment of the vacuum measuring gauge according to the invention shown in Fig. 3 is very similar to the one described above, only the control circuit 10 being somewhat simpler in that the anode voltage is not controlled separately and the anode current is not monitored. Corresponding parts are denoted by the same reference numerals and details may be found in the description of the pressure measuring apparatus according to Figs. 1, 2.

The controllable voltage source 14 of control circuit 10 which produces the gate voltage V _G is again controlled by a regulator 16 which here compares the output voltage of a current meter 17 measuring a cathode current I _c from the housing which serves as a control current with fixed reference voltage V _R . The anode 5 is connected to controllable voltage source 14 in parallel with gate 9 and is therefore kept at the same voltage. The ion collector current I _IC as measured by current meter 11 is again a measure of the density of gas molecules in the reaction

region 3. Anode 5 serves to shape the electrical field in the reaction region 3 but it is not absolutely required. An electrode configuration without an anode will usually be adequate as well .

Figs. 4, 5 show a particularly simple, robust and low-cost third embodiment of the vacuum measuring gauge according to the invention with just such a configuration. The control circuit 10 is similar to the one of the second embodiment but even simpler in that, apart from ion collector current lie which is again a measure of the pressure in the reaction region 3, only the gate current I _G is measured and used for controlling the gate voltage V _G . As a consequence, only two feedthroughs are present. The emitter surface is formed by a large portion of the inside surface of the side wall 2 which consists of stainless steel. The said surface portion has been roughened by chemical etching and is covered by the grid 8. Therefore no separate emitter layer is present.

List of reference symbols

1 base plate 2 side wall

3 reaction region

4 ion collector

5 anode

6 cathode 7 emitter layer

8 spacer

9 gate

10 control circuit

11 current meter 12 voltage source

13 current meter

14 controllable voltage source

15 current meter

16 regulator

17 current meter I _A anode current

Ic cathode current

I _IC ion collector current

I _G gate current

V _A anode voltage V _G gate voltage

V ₁ output voltage

V _R reference voltage