JPS594826B2 - field emission electron gun - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a field emission type electron gun, and more particularly to an electron gun equipped with a safety circuit when a field emission electron flow is generated.

Field-emission electron guns have more than 1,000 times the brightness than conventional thermionic-emission electron guns, and their light source is extremely small, so they are used in electron microscopes and other devices, helping to dramatically improve resolution. , is an electron gun that is attracting a lot of attention.

However, in order to obtain a stable field emission current, it is necessary to maintain the electron gun chamber in an ultra-high vacuum, and in particular, the parts near the anode and cathode, which are subjected to the impact of the emission current, must be made of materials with low emission gas or treated. No.

This is because the emitted gas adheres to the cathode surface and disturbs the emitted current, or the emitted gas is bombarded by the field emission electron flow and becomes ionized, which impacts the cathode surface and disturbs the emitted electron flow. This is because it induces vacuum discharge and results in damage to the cathode.

In the prior art, there is no means to protect the cathode from the generation of the emitted gas and ions, so the cathode has the drawback of being easily damaged or worn out.

SUMMARY OF THE INVENTION An object of the present invention is to provide a field emission electron gun which eliminates the above-mentioned drawbacks of the prior art, has means for protecting against emitted gas, etc., and enables safe field emission.

In order to achieve the above object, the present invention measures the change in the degree of vacuum (, Δp) during emission of field emission electron current (hereinafter referred to as ic), and determines when Δp/ic exceeds a certain predetermined value. In such a case, a protection circuit is provided to cut off the voltage applied to the cathode and anode to prevent the cathode from being damaged by ion bombardment, vacuum discharge, etc.

FIG. 1 shows an embodiment of the present invention.

Inside the electron gun chamber 5, a first anode 3 and a second anode are arranged opposite to the cathode 1.
The structure in which the anode 4 is arranged is a field emission type electron gun.

The cathode 1 is supported by a filament 2 and connected to a power source through an insulator 7 attached to a part of the electron gun chamber wall 6.

The power source includes a filament heating power source 10, an electron extraction voltage power source (vt) 11, an electron acceleration voltage power source ffo) 12, and a radiation electron current measuring circuit 13.

The electron gun chamber 5 is evacuated by an ultra-high vacuum pump (generally a sputter ion pump), and the degree of vacuum is measured by a vacuum gauge 9 attached to a part of the electron gun chamber wall 6.

14 is a vacuum gauge power source, and 20 is a vacuum pump 80 control power source.

In the above structure, the field emission electron extraction voltage V
When 1 is applied, a field emission electron stream 23 is emitted from the cathode surface, and of this electron stream, electrons that have passed through the center hole of the first anode 3 are accelerated by the second anode 4 to an acceleration potential Vo.

Other electrons impact the surface of the first anode 30 and knock out the gas 24 attached to the surface.

A portion of the ejected gas is ionized by radiation electrons, and a portion of these ions 25 bombards the surface of the cathode.

Some other gas molecules are pumped out24
a, a part 24b that flies into the vacuum gauge 9, and a part that ejects gas molecules attached to the wall of the electron gun chamber.

In addition, some of the electrons that hit the surface of the first anode 30 become backscattered electrons 23a and hit the surfaces of other parts in the electron gun chamber and the electron gun chamber wall 6, removing gas attached thereto. It works to remove tiles.

As explained above, by emitting radiation electrons, the degree of vacuum p in the electron gun chamber becomes worse by Δp.

This degree of vacuum deterioration Δp is a value determined by the amount of gas held in all parts of the electron gun chamber and the magnitude of the radiation electron current IC.

That is, even if ic is constant, if the amount of gas held in the electronic needle internal component is large, Δp will be large, and if the amount of gas held is small, Δp will be a small value.

The value of Δp experimentally determined when IC is 10 μA is generally about 1 to 2 x 10-9 torr, but this value varies depending on the pump pumping speed and the volume of the electron gun chamber, and as an absolute value. cannot be decided.

Therefore, considering the various conditions mentioned above, Δp / i c
By determining −α and comparing it with the value β determined by the pump pumping speed and the structure of the electron gun, it is possible to determine whether the electron gun operates safely when the radiation current ic is applied. It turns out it can be done.

That is, if α/β≦1, it is a safe operating condition, and if it is larger than ①, there is a high risk of cathode damage, instability of radiation current, and vacuum discharge.

In order to determine the above conditions, as shown in FIG. β
A circuit 17 for comparing
), or a circuit 19 for ON-OFF control of one or both of them.

Furthermore, when an ion pump is used as the vacuum pump 8, since the ion current ip is proportional to the degree of vacuum in the electron gun chamber,
The change Δp in the degree of vacuum can be measured by the change Δ1pC in the ion current, and the Δi-u constant circuit 21 can be used in place of the vacuum gauge 9.

Using both Δp/ic and Δi p/ic, determine whether or not they exceed the predetermined reference values, and then determine whether the power supply is 0N by the logical sum or logical product of the determination results.
-OFF control can also be performed.

In the above structure, in the example, Δi is used and β is 10
-2.

As explained above, according to the present invention, the amount of gas emitted from the internal parts of the electronic needle in the state where the radiation electron current IC is flowing is determined from the change in degree of vacuum Δp and the pumping speed torr·l/sec, thereby ensuring safety. By operating the circuit, it is possible to protect the cathode from damage and provide a stable field emission type electron gun.

Furthermore, according to the present invention, even if the vacuum level of the electron gun deteriorates for some reason and only the vacuum level recovers thereafter, if the amount of gas retained in the electronic needle internal parts is small, the vacuum level at the time of emission of the radiation electron stream will be reduced. Since the temperature drop is large and the safety circuit is activated, it can work safely even in the vacuum environment of the electron gun chamber.

In other words, it is significant that it has been able to add safety that could not be achieved with the conventional high-voltage power supply safety circuit based only on the degree of vacuum.

[Brief explanation of the drawing]

FIG. 1 is a schematic block diagram showing one embodiment of the present invention. 1... cathode, 3... first anode, 4...
...Second anode, 5...Electron gun chamber, 8...
...Vacuum pump, 9...Vacuum gauge, 11...
...electronic extraction power supply, 12...electronic acceleration power supply,
13...Radiated electron current measurement circuit, 16...
- Comparison circuit, 18...Reference value setting circuit.

Claims (1)

[Scope of Claims] 1. An electron extraction voltage and an electron acceleration voltage are applied to a field emission cathode, first and second anodes disposed opposite to the field emission cathode, and both anodes, respectively, in an evacuated electron gun chamber. A field emission type electron gun having a power supply circuit, a circuit for measuring a radiation electron flow, a circuit for measuring a change in the degree of vacuum in the electron gun chamber when the electron flow is emitted, and a circuit for measuring the change in the degree of vacuum and the radiation electron flow. A field emission type electron gun characterized in that it is equipped with a circuit that cuts off both an electron extraction voltage and an electron acceleration voltage (one of which is cut off) when the ratio between The field emission type electron gun according to claim 1, characterized in that a vacuum gauge is used. 3. A change in the operating current of a pump used to evacuate the electron gun chamber is used to measure the change in the degree of vacuum. A field emission type electron gun according to claim 1, characterized in that: 4. Both a vacuum gauge and an operating current change of an electron gun chamber exhaust pump are used to measure changes in the degree of vacuum, and the degree of vacuum obtained by both is used. Claim 1, characterized in that at least one of the electron extraction voltage and the electron acceleration voltage is cut off when at least one of the ratios of the change and the emitted electron current exceeds a predetermined value. Field emission type electron gun.