JPH0375974B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention protects the cathode that contributes to ion beam generation in a dual-plasmatron, mono-plasmatron type ion source, large accelerator ion source, etc. Concerning improvements to extend life.

<Prior Art> In recent years, various ion source devices such as those described above have come to be frequently used in various industrial fields.

Such ion source devices can be classified into two types depending on the structure of the cathode that contributes to ion generation.

One is that the main electron emitting section and the heater that heats the main electron emitting section are composed of completely separate members, like the relationship between the cathode and the heater in the electron tube of Sumiyoshi. The member constituting the emission surface covers the part of the heater that heats the main electron emission part. For convenience, a cathode that follows this structure will be referred to as a coated cathode.

The other one is, for example, a linear, rod-shaped, net-shaped member, etc.
By mechanically bending a single member of a suitable material,
As a result of using the bent tip as the main electron emitting surface, the main electron emitting surface becomes a part of the surface of the heater, or the tip of the heater is used with, for example, an oxide or metal attached, and this oxide surface Or, is it possible to define the metal surface part melted by heating as the effective main electron emitting part? Furthermore, while the cut surface of a short rod-shaped member made of a suitable material is defined as the main electron emitting part, it is also possible to define other parts as the main electron emitting part. For example, a linear heater may be wound around it.

Therefore, to summarize, this type of cathode is
It can be conceptualized as "an ion source cathode in which the main electron emitting part is integrated as part of the heater part that heats it, or has an exposed part in the heater part that heats it in the vicinity of the main electron emitting part." , it can be called an exposed type cathode if it corresponds to the above-mentioned covered type.

<Problems to be Solved by the Invention> The above-mentioned coated cathode has excellent ion bombardment resistance in the heater portion, and is often constructed to be quite strong in terms of physical structure.

Therefore, it naturally has a long lifespan, but it is inferior in terms of structure and ease of use, and in practice,
In cases where a relatively small current is required, exposed cathodes are often preferred because they are easier to handle.

However, in this exposed type cathode, even though consumption in the main electron emitting part is unavoidable due to its principle, the main electron emitting part is mechanically supported and heated by a heating power supply. The disadvantage was that the part that merely converts the current supplied from the filament into heat, that is, the part that only controls the original heater/filament function, breaks early, which limits its lifespan. . In other words, although the main electron emitting section was still usable, there were many cases where it became unusable due to line breakage.

Nevertheless, no effective countermeasure against this problem has been taken in the past.

This is because the cause of breakage in filament parts other than the main electric emission part as mentioned above has not been accurately elucidated. If we don't know the exact cause, there is no way we can take countermeasures that can be theoretically proven to be effective.

The present invention has been made in view of these circumstances, and uses measures whose validity can be fully proven theoretically to minimize useless filament breakage in areas other than the main electron emitting part of the exposed cathode in an ion source device. The purpose of the present invention is to provide a means to prevent this and thereby extend the life of the cathode.

<Means for Solving the Problems> In order to achieve the above object, the present invention began by obtaining the following knowledge.

In general, a considerable portion of the cathode is always exposed to plasma ions during its use, so wear and tear in the main electron emitting portion is inevitable in principle. However, as mentioned earlier, in the case of an exposed cathode, it is rare at present that the cathode becomes unusable due to the main electron emitting part being completely consumed, and its lifespan is limited to the main electron emitting part. This was limited by early breakage of heater parts (or filament parts) other than the above.

To simplify the explanation, an exposed cathode formed by simply bending a linear member will be taken as an example, and a typical broken state of the cathode is shown in FIG. 2. It is likely to occur several millimeters below, especially at a position on the low potential side with respect to the heating power supply 14.

In addition, as shown in the enlarged view of the copper plate, the parts on both sides of the fractured part 12 become tapered toward the fracture point, and the inner metal structure of the heater member is exposed by sputtering on the surface, and the inner metal structure of the heater member is exposed by sputtering, and At least one of the broken ends often has a spherical shape 13. This provides evidence that the break in question was a melt cut.

On the other hand, the main electron emitting part 11 is of course also worn out, but its shape is planar and some parts may show melting marks, but the degree of wear is still not so significant. Therefore, the first thing that can be said is that if the heater section had not been ruptured (fused), this cathode could have still been used for a considerable amount of time.

On the other hand, in view of the fact that the above-mentioned break was due to fusing 13, this is because the part 12 near the main electron emitting part that led to the break was selectively spattered by ion bombardment under usage conditions, and as a result, it became slightly thinner. As a result, the potential gradient in the surrounding space becomes higher than other areas, and a positive feedback phenomenon occurs in which ion bombardment and sputtering damage to this area further increase. The mechanism was that the electrical resistance value of the rapidly tapering portion 12 also increased rapidly, which caused a local abnormal temperature rise, which caused the melt to break at the thinnest portion 12 of the portion. I can see it.

Regarding this, the present inventor further conducted a more detailed study as described below.

The end of the cathode 10 on the heating power supply side is connected to the second
Although not shown in the figure, the ion source is cooled naturally or forcibly outside the ion source while being mechanically supported by a suitable support. Therefore, as you sandwich the main electron emitting part at the tip and go to the power supply connection end,
The temperature is going down. On the contrary, because it is
The main electron emitting section 11 is limited to a portion that maintains the temperature necessary for electron emission.

On the other hand, regarding the internal state of this type of ion source device, the measurement results regarding the curved cathode are included in the reference literature written by J. Kastemaker and others: Nuc. Instr. and Meth., 38 (1965) 1. Third
It is represented as in Figure A.

According to this, the potential distribution on the central axis is B
The fire ball (cathode side plasma) is generated by narrowing the front surface of the electron emitting surface of the cathode and the opening of the intermediate electrode.
Electric double layers A and B are respectively formed around the .

Therefore, a space charge limited electron current flows through these toward the high potential portion, and a space charge limited ionic current flows toward the low potential portion. In this case, the potential difference between each double layer varies depending on the ion species, plasma density, etc., but is generally on the order of several tens of volts.

Under such conditions, the electrons accelerated in each double layer undergo more active ionization in the next space, gradually increasing the number of ionized particles, and high-density plasma is formed in the anode hole.

In addition to the above, in the case of a dual plasmatron, a double layer C is also formed between the intermediate electrode and the anode due to convergence by the magnetic field.

Here, the amount of ions directly hitting the main electron emitting surface via the double layer A is given by the space charge limited ion current determined according to the potential of the layer region A.
This current is supplied by the ion current drifting through the positive column.

On the other hand, the space charge limited ion current supplied from the fire ball to the positive column by the double layer B, which has a potential difference of several tens of volts, far exceeds the above-mentioned drift current.

Therefore, the excess ion current that does not flow into the electron emitting surface through the double layer A has no choice but to flow into a portion other than the electron emitting surface through other paths.

Therefore, in the end, the shortest cathode part, that is, as shown in Figure 4, in the case of the exposed cathode structure in question here, is the shortest cathode part, which is adjacent to the main electron emitting part and on the lower potential side. For the part
A large amount of ions having energy equal to the potential difference between the double layers A and B will easily flow in. As already mentioned,
If double layer C exists, an ion flow with even higher energy will be added to it without passing through double layers A and B. In the exposed cathode structure,
Since only the cathode tip becomes the main electron emitting surface, the surface area of double layer A is much smaller than that of double layer B, and this tendency is further promoted.

Due to this mechanism, selective sputtering by a large amount of ions occurs in the vicinity of the main electron emitting region, and this tends to occur on the low potential side, and the degree of sputtering is much larger than that for the main electron emitting region. Become.

In plasmatron type ion sources, etc., the purpose is to make maximum use of the electrons emitted from the cathode to generate high-density ions.
It is a natural consequence that the number of back-diffused ions also increases accordingly.

As a result of fully understanding such phenomena caused by back-diffused ions, the present inventor has proposed a method for protecting an exposed ion source cathode having the following configuration.

A method for protecting an ion source cathode in which the main electron emitting part is integrated as a part of a heater part that heats the main electron emitting part, or has an exposed part in the heater part which heats it in the vicinity of the main electron emitting part, the method comprising: Covering the heater part that receives ion bombardment other than the main electron emitting part with a protective electrode while exposing the main electron emitting part; applying a relatively lower potential to the protective electrode than the potential of the main electron emitting part; A method for protecting an ion source cathode, characterized by;

<Functions and Effects> First, in the present invention, since a protective electrode is added, it is possible to mechanically or physically protect the portion covered by the protective electrode except for the main electron emitting portion.

In other words, the existence of this protective electrode makes it possible to physically absorb the back-diffusing ion flow for the time being.

However, simply mechanically covering the main electron emitting part cannot effectively prevent the ion flow flowing in through the gap exposing the main electron emitting part.

Therefore, in the present invention, a potential lower than that of the main electron emitting portion is further applied to the protective electrode. In this way, a potential barrier can be formed against the back-diffusion ion flow, and the back-diffusion ion flow flows more easily into the protective electrode than into the gap as described above, improving the shielding ability.

However, if this potential difference is too large, the main electron-emitting surface will be partially or completely covered with a lower potential, and electron emission will be inhibited or stopped. It is necessary to consider in the design how to set the level.

Note that, unlike the protection method of the present invention, based on the results of the above analysis, it is also possible to consider a method of coating parts other than the main electron emitting part with a metal, etc., which has high resistance to sputtering, by vapor deposition, etc. Therefore, it is thought that due to the difference in thermal expansion coefficient between the cathode material and the coating material, the coating material will easily peel off due to long-term operation or repeated operation and suspension. It is expected that the peeled pieces may clog each electrode hole or cause a short circuit between the electrodes. Therefore, such a method cannot be adopted at all.

On the other hand, the protection provided by the method of the present invention does not cause such problems, and only the originally desirable result of prolonging the life of the cathode can be obtained.

In practice, for example, when a 0.8 mmφ tungsten wire is bent as shown in the second figure and used as a cathode, the cross-sectional area of the region that can be defined as the main electron emitting region 11 is usually four to five times that of the broken part. Therefore, it can be seen that by applying the method of the present invention, the life span can be extended by at least several times.

Furthermore, if the life of the cathode itself is extended in this way, the ripple effect is that the number of resets of the ion source can be significantly reduced, which in turn contributes to improving the effective operating efficiency of ion equipment and lowering the cost of various products. It can also contribute to the development of

<Example> FIG. 1 shows an ion source constructed in accordance with the present invention. Of course, what is protected by the present invention is the exposed ion source cathode defined above, so the cathode 10 illustrated here is also such, and in particular, in its simplest form:
By bending a single linear member,
The bent portion is shown as the main electron emitting portion 11.

However, conversely, the present invention is also applicable to other forms as long as they can be defined as exposed cathodes using the above expressions.

The cathode 10 is mechanically supported by good supports 15, 15 by a known existing method, and the power supply voltage from the heating power source 14 is applied to the end portion supported by the supports.
At least the main electron emitting section 11 is sufficiently heated.

In this case, most of the surrounding parts except for the main electron emission part 11 are covered with a protective electrode 20, and the negative terminal side of the heating power supply 14 is connected to the protective electrode 20 as a common ground. A bias power supply 22 provides a further negative potential with respect to ground.

In practice, this structure has a small opening 21 at the top end.
In addition, a cylindrical protective electrode 20 having a closed ceiling surface is placed over the cathode 10, and the main electron emitting section 11 is assembled so as to protrude somewhat from the opening 21.

In this way, as explained in the operation section, the back-diffusion ion current generated in the ionization device is physically counteracted by the protective electrode 20 while being used.
Due to the bias relationship, it can also be electrically opposed.

As a result, as already mentioned, the cathode
It becomes something that can extend the life of 0.

However, it is not limited to the illustrated embodiment, and various modifications in accordance with the gist of the present invention are fully possible. For example, if the ion device to be incorporated is specified, or as a result of the analysis, the main electron emitting part of the cathode 10 If the part other than 11 that receives ion bombardment can be identified or almost identified, the 1st
It is not necessary to cover the entire cathode as shown in the figure, and the protective electrode 20 may be formed only on such a tip portion.

As for the bias power supply 22, in principle, it is sufficient if it can apply a bias of potential lower than the potential of the main electron emitting part 11 to the protective electrode 20. Therefore, strictly speaking, the heating power supply negative terminal potential of the cathode 10 (for example, the ground It may be better to set the potential higher than the current potential.

Therefore, while the illustrated method is of a fixed bias type, for example, a low resistance of an appropriate value is inserted between the protective electrode and the negative terminal of the cathode instead of the bias power supply 22 of FIG. By doing so, it is also possible to achieve self-biasing by the converted voltage of the ion flow within a range where the potential of the protective electrode does not become higher than that of the main electron emitting region.

However, regarding this bias application, the method illustrated is the simplest and most stable method.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an example of an apparatus configuration suitable for carrying out the ion source cathode protection method of the present invention, and FIG.
The figure is an explanatory diagram of a fracture that conventionally occurs in an exposed cathode, and FIGS. 3 and 4 are state-of-the-art and electrical diagrams of the inside of the ion source device for analyzing the cause of the fracture. In the figure, 10 is an exposed ion source cathode, 11 is its main electron emission part, 12 is a broken part, 13 is a spherical fused state, 14 is a heating power supply, 20 is a protective electrode, 21 is an opening, and 22 is a bias power supply , is.

Claims (1)

[Scope of Claims] 1. An ion source cathode in which the main electron emitting part is integrated as a part of the heater part that heats the main electron emitting part, or has an exposed part in the heater part which heats it in the vicinity of the main electron emitting part. A protection method, comprising: covering a heater part exposed to at least the main electron emitting part and receiving ion bombardment other than the main electron emitting part with a protective electrode; A method for protecting an ion source cathode, characterized by: applying a low potential to the ion source cathode.