KR910007806B1 - Electro-static lens structure for focusing ion beam - Google Patents

Abstract

The equipotential einzel lens composed of three electrodes, is used for focusing the charged particles such as electron or ion. The lens comprises upper projection (8) on third electrode (3), for removing the line of sight (12) to lower insulating material (5); connecting position, between third electrode (3) and lower insulating material (5), being lower than the center position, which suppresses charge-up of lower insulating material (5) generated by second electron or ion, and prevents the breakdown of insulating material.

Description

Structure of electrostatic lens for focusing charged particles (electrons or ions)

1 is a conceptual diagram of an equipotential electrostatic lens.

2 is an embodiment of a conventional equipotential electrostatic lens.

3 is an embodiment of an equipotential electrostatic lens according to the present invention.

* Explanation of symbols for main parts of the drawings

1: first electrode of electrostatic lens 2: second electrode of electrostatic lens

3: third electrode of electrostatic lens 4: upper insulator of electrostatic lens

5: Lower insulator of electrostatic lens 6: Lower protruding portion of first electrode

7: lower protrusion of second electrode 8: upper protrusion of third electrode

9 connection portion of third electrode 10 positive electrode of electrostatic lens

11 insulator of electrostatic lens 12 line of sight

The present invention relates to the structure of an equipotential electrostatic lens (einzel lens) consisting of three electrodes for focusing charged particles.

An electrostatic lens for focusing charged particles is composed of two or three positively applied voltages, and an equipotential blackout is applied in which the voltages of the first electrode and the last electrode are equally applied (generally, ground potential). It is called an einzel lens.

1 is a conceptual diagram of an equipotential electrostatic lens, in which the vertical solid line between the electrodes represents the equipotential distribution, the dotted line represents the incident ion beam, and in the case of the electron beam, the polarity of the applied voltage is reversed.

That is, the ion beam and the electron beam can be focused by changing the polarity of the applied voltage in the electrostatic lens having the same structure.

In general, an equipotential electrostatic lens is implemented by applying the first electrode and the last electrode as the ground potential, applying an appropriate voltage to the center electrode, and installing an insulator between the electrodes for inter-electrode insulation and fixing of the electrode.

In this case, the degassed stainless steel is generally used as the electrode, and the insulator is made of a material (eg, ceramic) which emits little particles or gases in vacuum.

The reason for this is that systems using charged particle beams (ion beams or electron beams) (eg Secondary Electron Microscopy, Focused Ion System) are used in the system where the beam remains in the sample as it travels through the sample. This is because they operate in high vacuum to minimize scattering by collisions with gas particles.

As such, consideration should be given to the material constituting the electrostatic lens, but consideration of electrical aspects such as prevention of discharge by breakdown is very important.

The electrostatic lens has a function of focusing by an electrostatic force applied to charged particles by an electric field formed by an appropriately applied voltage, thereby preventing breakdown of the insulation by a focused electrode (voltage applied to an electrode). Is an important issue.

The two most important considerations must be considered to prevent breakdown.

One of them is the consideration of the discharge between the electrodes due to the dielectric breakdown of the vacuum.

Insulation breakdown of vacuum breaks down the insulation of vacuum due to excessive voltage applied to the electrode, and discharge occurs.

Such vacuum breakdown is generally known to occur at a potential difference in the range of 6-10 kV / mm.

In order to prevent such breakdown of the vacuum, the distance between electrodes should be maintained so that the maximum potential difference in the lens is smaller than the above value.

The threshold voltage (6kV / mm-10kV / mm) of the vacuum insulation breakdown is kept evenly spaced, and the ideal vacuum value is such that the breakdown occurs below the above threshold value in the actual electrostatic lens.

Therefore, in order to increase the threshold voltage of the vacuum dielectric breakdown in the actual implementation of the electrostatic lens, the electric field distribution formed between the electrodes is increased by increasing the surrounding environment of the electrostatic lens, that is, the degree of vacuum and maintaining the spacing between the electrodes very uniformly. The castle must be maintained.

In other words, it is necessary to prevent the occurrence of insulation breakdown at a potential difference lower than the expected value of the dielectric breakdown threshold voltage of the expected vacuum due to the increase of the electric field strength at a specific portion.

In addition to the vacuum breakdown described above, an important consideration in the implementation of the electrostatic lens is to prevent the breakdown caused by the breakdown of the insulator used for inter-electrode insulation and fixing.

Insulation breakdown of an insulation can be divided into two cases.

One is the bulk breakdown of the insulator.

This is a case where the dielectric breakdown due to the voltage applied to the electrodes across the insulator does not take into account the dielectric breakdown, that is, the surface effect or skin effect.

The breakdown threshold voltage at this time is considerably reduced when the inside of the insulator is uniformly constructed.

The threshold voltage of volume breakdown in the absence of defects or cracks inside the insulator varies depending on the material, but typically reaches several tens of kV.

The second insulation breakdown is insulation breakdown due to the surface effect of the insulator.

This is generally caused by electron hopping, in which electrons (secondary electrons) generated externally or at the surface of the electrode move the surface by an electric field formed by a voltage applied to the electrode. It is known.

That is, electrons moving along the surface of the insulator generate another secondary electron, and this phenomenon is continuously amplified, resulting in a path through which a large current flows along the surface of the insulator, leading to breakdown.

The threshold voltage of the dielectric breakdown due to the insulator surface effect is lower than that of the previous volume dielectric breakdown and is about 2 kV / mm in the ceramic case.

Considering the dielectric breakdown phenomenon of the insulator as described above, in order to implement a stable electrostatic lens, it is necessary to design the insulator by the threshold voltage of the surface dielectric breakdown, not the threshold voltage of the volume dielectric breakdown.

In order to prevent dielectric breakdown due to electron hopping on the surface of the insulator, the cause of the generation of secondary electrons generated in the electrode should be minimized.

That is, the charged particle beam must be prevented from colliding with the electrode, or the secondary electrons already generated must be prevented from moving to the surface of the insulator.

For this purpose, the line of sight to the insulator should be eliminated as much as possible in the electrode as shown in FIG.

In addition to the dielectric breakdown of the insulator by the above-described process, that is, the volume breakdown and the surface insulation breakdown of the insulator, the charged jib passing through the electrostatic lens for the purpose of focusing is the scattering of the charged particles due to the collision with the electrode, or 2 As the electrons or ions accumulate in the insulator, an insulation breakdown for charge-up occurs.

In the breakdown caused by the charge-up of the insulator, the charge that reaches the insulator does not escape to the outside and the charge continuously accumulates inside the insulator, causing the breakdown. Do not allow secondary electrons or ions to reach the insulator.

This is not only to prevent the breakdown caused by the charge accumulation phenomenon of the insulator, but also to prevent the breakdown caused by the surface electron transfer described above.

For this purpose, the line of sight must be eliminated as described above.

Up to now, the electrical aspects have been described to prevent the discharge phenomenon due to dielectric breakdown, which should be considered in actual electrostatic lens implementation.

Further consideration with this electrical aspect is that care must be taken in connection with other ionoptical modules.

That is, in order to improve the shape (circle) of the charged particle beam focused by the electrostatic lens, the shape of the beam incident on the electrostatic lens should be corrected.

There are many reasons for distortion of the cross section of the charged particle beam incident on the electrostatic lens, but the main reason is a module (eg, a module which passes through a beam from a beam source until it reaches a sample). This is because the distortion caused by uneven distribution of the electric field in the accelerator and the central axis of the charged particle beam do not coincide with the central axis of the modules through which the charged particle beam passes.

A shape corrector is provided directly above the electrostatic lens to correct the cross-sectional shape of the charged particle beam in which the dent is generated.

In addition to such a beam stator, a deflector for scanning the charged particle beams onto a sample is usually installed at the bottom of the electrostatic lens.

A beam cross-section corrector is typically composed of two pairs of parallel flat deflectors consisting of four or eight electrodes.

It is constructed similar to one in a configured electrostatic scarnner, a beam stigmator or a magnetic scanner by a magnetic coil.

Such deflectors should be located as close as possible to the exit of the electrostatic lens in order to increase the deflection range.

Therefore, to secure the space necessary for the installation of such modules (beam shape corrector, deflector, etc.) should be considered in the configuration of the actual electrostatic lens.

2 shows an example of a conventional electrostatic lens, wherein the distance between electrodes is maintained to withstand vacuum breakdown, but the line of sight 12 to the insulator 11 at the electrode 10 is opened. Therefore, the dielectric breakdown of the insulator may be caused by charge-up or surface electron hopping by secondary electrons or ions, which may cause the dielectric breakdown at a voltage lower than the ideal dielectric breakdown voltage of the insulator. The lens is very unstable.

SUMMARY OF THE INVENTION An object of the present invention is to maintain an even interval of each electrode in the implementation of an equipotential electrostatic lens (einzel lens) to suppress the occurrence of a discharge phenomenon by vacuum insulation breakdown, and also to prevent any line of In order to suppress the insulator from the sight and at the same time eliminate the inclined surface, the charge-up in the insulator by the secondary electrons or ions is suppressed.

In this way, it is possible to design and manufacture a more stable electrostatic lens by increasing the dielectric breakdown voltage of the insulator.

Hereinafter, preferred embodiments for achieving the object of the present invention will be described.

3 shows an embodiment of an equipotential electrostatic lens according to the present invention.

As shown in the drawing, a voltage is applied to the second electrode 2 with the first electrode 3 and the third electrode 3 as the ground potential.

In such a configuration, the distance G between the electrodes must be uniformly maintained so that vacuum breakdown does not occur due to the connection voltage applied to the second electrode 2.

At this time, the inter-electrode gap G should be maintained so that the narrowest part of the entire electrostatic lens can withstand the highest focusing voltage.

In addition, if the electrode has a pointed portion, the dielectric breakdown may occur even at intervals capable of withstanding the dielectric breakdown, because in the pointed portion, an electric field is concentrated.

In other words, the distribution of the electric field formed between the electrodes must be maintained uniformly to obtain the dielectric breakdown threshold voltage close to the ideal limit threshold voltage (6-10 kV / mm) at a given inter-electrode spacing.

Therefore, the distance G between the electrodes constituting the electrostatic lens must be maintained to withstand the focusing voltage at the minimum distance, and the distance must be uniform.

In addition, the sharp edges should be rounded to prevent concentration of the electric field in the part.

In addition, the length L of the insulators 4 and 5 should also be determined very carefully in order to prevent breakdown of the insulator used for inter-electrode insulation.

As described above, the threshold voltage of the bulk breakdown of the insulator is quite high, but careful attention is required because the breakdown of the insulator in the electrostatic lens is driven by the surface breakdown.

In general, the length L of the insulator is made several times the distance G between the electrodes.

However, insulation breakdown of an insulator is also caused by surface electron hopping or charge-up of the insulator, as described above, and this phenomenon usually causes breakdown below a known threshold voltage.

Therefore, as described above, the insulator is not visible to any line of sight in the charged particle beam, thereby preventing the charged particle beam from reaching the insulator due to secondary electrons or ions generated by collision with the electrode. Should be.

Therefore, in the first electrode 1 of FIG. 3, secondary electrons or ions generated at the upper surface of the second electrode 2 are formed by forming a lower protrusion 6 rounded at the end of the inclined surface of the electrode. Reaching the insulator 4 was prevented.

That is, the line of sight to the upper insulator 4 above the second electrode 2 was removed.

Also similar to that of the first electrode at the lower end of the second electrode 2 and the upper end of the third electrode 3 to remove the line of sight to the lower insulator 5. To form an upper projection 8 rounded.

However, the spacing in the protrusions 7 and 8 thus formed should also be kept equal to or greater than the minimum spacing G so as not to cause breakdown of the vacuum at the applied focus voltage.

Such a structure of the electrostatic lens can improve the dielectric breakdown voltage of the insulator, so that the electrostatic lens can be operated more stably.

In addition, since the line of sight to the lower insulator 5 is removed by the upper protrusion 8 formed at the upper end of the third electrode 3, unlike the conventional second embodiment, the third electrode 3 and the lower portion of the third electrode 3 are removed. The connection 9 of the insulator 5 can be lowered.

By this effect, the depth D of the first electrode 1 can be reduced with respect to the length L of the same insulator.

Therefore, since the depth D, which is a problem in manufacturing the first electrode 1, is reduced, the processing of the first electrode 1 is easy and the depth D is reduced, so that the installation, maintenance, and repair of the beam shape regulator is easy. It also has the advantage of losing.

As described above, the configuration of the equipotential electrostatic lens of the present invention has the effect of improving the dielectric breakdown voltage of the insulator by eliminating the line of sight to the insulator in the charged particle beam while maintaining a uniform interval between the electrodes. As a result, the stability of the electrostatic lens can be improved.

In addition, the depth D of the first electrode 1 is reduced to facilitate the installation, maintenance, and repair of the beam shaper, and also facilitate the processing of the first electrode 1.

Claims (5)

In the electrostatic lens composed of three electrodes, the line of sight of the lower insulator 5 is removed by the upper protrusion 8 formed on the upper end of the third electrode 3, and the third electrode 3 and the lower insulator ( 5) lowering the connection portion 9 of the lower portion than the center portion to suppress the charge accumulation of the lower insulator (5) by the secondary electrons or ions to prevent the breakdown of the electrostatic charge The structure of the stove. 2. The end of each electrode according to claim 1, wherein the protrusions 6 of the first electrode 1, the protrusions 7 of the second electrode 2, and the upper protrusions 8 of the third electrode 3 are formed at the end of each electrode. The structure of the electrostatic stove for focusing charged particles, characterized by having more than two protrusions to prevent the charge accumulation of the upper and lower insulators (4,5) by the secondary electrons or ions to prevent the breakdown. . The method according to claim 1, wherein the inclined planes are removed from the first electrode 1, the second electrode 2, and the third electrode 3, and the gap between the electrodes is uniformly treated in the vertical plane and the horizontal plane, so that the secondary electrons or ions The structure of the electrostatic range for focusing the charged particles, characterized in that the charge accumulation of the upper and lower insulators (4,5) to suppress the breakdown. In a microwave oven composed of three or more electrodes by using two electrodes of the first electrode 1, the second electrode 2, and the third electrode 3 or by adding a new electrode, the lower protrusion part of the first electrode (6) The second electrode lower projections 7 and the upper projections 8 of the third electrode have at least one protrusion at the end of the electrode, and the inclined surface is removed from the electrode surface and treated in the vertical and horizontal planes. The structure of the electrostatic range for focusing charged particles, characterized in that to prevent the breakdown by preventing the charge accumulation of the upper and lower insulators (4,5) by the secondary electrons or ions by uniformly spaced. 5. The method of claim 4, wherein the line of sight of the lower insulator 5 is removed by the upper protrusion 8 formed on the upper end of the third electrode 3, and the third electrode 3 is connected to the lower insulator 5. A structure of an electrostatic range for focusing charged particles, characterized in that the portion (9) is lowered from the center portion to prevent charge breakdown by inhibiting charge accumulation of the lower insulator (5) by secondary electrons or ions.

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