JP2009170427A - Electron beam device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a beam with high luminance exceeding a Langmuir limit and high emittance for achieving a multiple beam inspection device with high resolution and a high throughput. <P>SOLUTION: An electron gun includes a flat-face cathode, a drawer electrode or an anode, and a Wehnel electrode shaped like a portion of cone. An electron gun current Ie (mA) is set by a relation with a distance Dac (mm) between a cathode and anode in a range of: 0.388/Dac-0.046≤Ie≤92.8/Dac+9.28, Dac≥3 mm, or 0.388/Dac-0.046≤Ie≤22/Dac+32.7, Dac<3 mm. Alternatively, the electron gun current Ie (mA) is limited in a numerical value (a formula abbreviated) by a relation with a cathode radius. Of the relation between luminance B and cathode current density Jc, a simulation value 571 (dashed line) and an actual measurement value 572 match each other comparatively well, and the luminance exceeds a Langmuir limit 573. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electron gun capable of obtaining ultra-high brightness, and relates to an electron beam apparatus for evaluating a sample having a pattern formed on the surface using such an electron gun. An electron beam is irradiated to a sample such as a wafer after the process, etc., and secondary electrons that change according to the surface properties are captured to form image data, which is formed on the sample surface based on the image data. The present invention relates to an electron beam apparatus for evaluating pattern defects and the like with high throughput.

In the semiconductor manufacturing process, the design rule is about to reach 45 nm, and the production form is shifting from small-quantity mass production represented by DRAM to high-variety small-quantity production such as SOC (Silicon on chip). Along with this, the number of manufacturing processes increases, and it is essential to improve the yield for each process, and defect inspection due to the process becomes important.
Along with the high integration of semiconductor devices and the miniaturization of patterns, high resolution and high throughput inspection apparatuses are required. In order to examine defects in wafers having a 45 nm design rule, a resolution of 40 nm or less is required, and the amount of inspection increases due to an increase in manufacturing process due to high integration of devices, so high throughput is required. In addition, as the number of devices increases, the inspection apparatus is also required to have a function of detecting a contact failure (electrical defect) of a via that connects wirings between layers.
In such a situation, an apparatus for forming a plurality of beams in the vicinity of one optical axis and increasing the throughput has been studied. (Mamoru Nakasuji et al, Jpn. J, Appl., Phys., Vol 44, No. 7B 2005, P5570) Such a multi-beam apparatus requires an electron gun with high emittance as well as high luminance.
The electron source used for the incidence of the ERL radiation source requires an electron gun with an extremely high brightness and a large beam current. (Nishitani et al., 53rd Applied Physics Related Lecture Preliminary Lecture No2, 2006 Spring p798)
With regard to the brightness of the electron gun, it was traditionally believed that there was a maximum value called the Langmuir limit. It was a value indicated by the following formula.
If the cathode current density is Jc, the maximum current density Jmax obtained is
Jmax = Jc (1 + eφ / kT) sin ² α (1)
Maximum brightness Bmax is
Bmax = Jmax / πsin ² α = Jc (1 + eφ / kT) / π (2)
For example, when Jc = 10 A / cm ² and φ = 4500 V, e = 1.6 × 10 ⁻¹⁹ , k = 1.38 × 10 ⁻²³
Is substituted into equation (2), Bmax = 9.23 × 10 ⁴ A / cm ² sr.
However, these equations are obtained on the assumption that the electron gun matches the optical model, and if it is out of the optical model, the existence of such a limit is unknown. In other words, in order to obtain a brightness much higher than such a value, an electron beam that deviates significantly from the optical model may be obtained.

As the electron gun in the above-described conventional apparatus, a three-electrode electron gun having a convex spherical surface with a small curvature radius, a flat anode, and a flat Wehnelt electrode, in which a high electric field is applied to the cathode surface, has been mainstream. However, the brightness of such an electron gun is limited, and there is a problem that it is much smaller than the desired brightness.
Further, a simulation result exceeding the Langmuir limit has been proposed for a three-electrode electron gun having a spherical cathode with a curvature radius of 25 to 40 μR, a planar anode and a planar Wehnelt electrode. (JP 2003-2977272, JP 2004-22235, JP 2003-323860) However, in such an electron gun, the cathode dimensions are small. Had a problem that was not obtained.

  Further, the conventional type electron gun has a problem that the current density is high in the vicinity of the cathode, so that when the beam energy is small in the vicinity of the cathode, electrons interact with each other and the energy width is widened. An object of the present invention is to solve the above-described problems, and to provide an electron gun that can reliably obtain luminance exceeding the Langmuir limit, and to provide an electron gun that is suitable for multi-beam generation and has a small energy width. It is another object of the present invention to provide a multi-beam electron optical system that can fully utilize the performance of the electron gun obtained in the present invention.

In order to achieve the above-described object, in the electron beam apparatus according to the present invention, an electron gun having a circular flat cathode, an extraction electrode or anode having a convex shape, and a Wehnelt electrode having a partial cone shape is used. Gun current
0.388 / Dac -0.046 ≤ Ie ≤ 92.8 / Dac + 9.28, Dac ≥ 3 mm, or
0.388 / Dac-0.046 ≤ Ie ≤ 22 / Dac + 32.7, Dac <3 mm. Further, in the above means, the cathode radius is in the range of 20 μm to 500 μm.

In addition, when it has a cathode of a photocathode having a negative electron affinity, an extraction electrode or an anode having a part of a plane shape, and a Wehnelt electrode having a truncated cone shape, when the cathode current is mA and the cathode radius Rc is μm Cathode current,
The value was within the range of 0.5 + 0.0098Rc <cathode current <2.3 + 0.026Rc.
Further, in the above means, the cathode curvature radius is a concave surface, a convex surface or a flat surface of 1.5 mm or more.

  Furthermore, it has a circular cathode, an extraction electrode or anode having a shape of a part of a plane, and a frustoconical Wehnelt electrode, and the electric field strength formed between the anode and the cathode is in the range of 1.6 to 5.53 kV / mm. It was inside.

  Furthermore, it has a circular cathode, an extraction electrode or anode having a shape of a part of a plane, and a frustoconical Wehnelt electrode, the cathode diameter is in the range of 20 μm to 500 μm, and the distance between the cathode and anode is 0.8 to The range was 3 mm.

  It has a circular cathode, an extraction electrode or anode having a shape of a part of a plane, and a frustoconical Wehnelt electrode, and the angle between the frustoconical Wehnelt electrode and the beam periphery is 69.4 to 93.2 degrees did. However, the beam periphery was approximated by a cone connecting the cathode periphery and the optical axis at the anode position.

It has a photocathode cathode, Wehnelt electrode, extraction electrode or anode electrode, and the electron beam emitted from the cathode monotonously decreases the beam diameter between the cathode and the extraction electrode, and the minimum beam diameter behind the extraction electrode Was controlled to form.
Further, in the above means, the cathode diameter is in the range of 40 μm to 1000 μm.
Further, a lens is provided behind the electron gun of the first means, and the excitation or excitation voltage of the lens is adjusted to adjust the brightness of the electron gun.

Furthermore, it has a cathode of cathode, Wehnelt electrode, extraction electrode or anode electrode, and cathode current Ie
0.5 + 0.0098Rc>Ie> 2.4 + 0.026Rc
It was. In this means, the cathode radius was in the range of 20 to 500 μm.

Furthermore, it has a cathode of cathode, Wehnelt electrode, extraction electrode or anode electrode, and cathode current Ie
Ie> 1.4 + 0.019Rc
It was. Note that the electric field strength applied between the cathode and the anode in the first means was 1.6 kV / mm or more.

  Furthermore, the electron gun has a circular plane photoelectron emission cathode, Wehnelt electrode, a planar extraction electrode or anode, and a cone-shaped Wehnelt electrode so that the electron gun current flows at a predetermined value determined by the cathode radius. did. Further, a lens is provided behind the electron gun of the above means, and the excitation or excitation voltage of the lens is adjusted to adjust the electron gun brightness.

Further, the aperture is irradiated with the electron beam emitted from the electron gun of the first means, the sample surface is scanned with the electron beam shaped by the aperture, the secondary electrons emitted from the scanning point are detected, and the sample is detected. The information of the face was obtained.
Further, in the last means, a two-stage condenser lens is provided between the electron gun and the opening, and the current density for irradiating the opening is adjusted without forming a crossover between the first condenser lens and the opening.

When the Planck constant is h, the irradiation laser wavelength is ν, and the work function of the cathode is eφ, the cathode excitation laser wavelength is within ± 20% of the limit wavelength determined by hν = eφ.
Further, the cathode was cooled, and the wavelength of the cathode excitation laser was used within the range of 0 to −10% of the limit wavelength determined by hν = eφ.
Furthermore, it has a cathode of a photocathode, an extraction electrode or anode having a shape of a part of a plane, and a frustoconical Wehnelt electrode. When the cathode current is mA and the cathode radius Rc is μm, the cathode current is 0.0107 Rc. <Cathode current <0.06 Rc.

Also, cathode current: Ie
0.0062Rc -0.0487 ≤ Ie ≤ 0.101Rc + 4.73 Rc ≥ 0.3mm, or
The value was within the range of 0.0062Rc-0.0487 ≤ Ie ≤ 0.0266 Rc -0.225 Rc <0.3 mm. further,
Electron gun current: Ie
The value was within the range of 0.204 / Dac-0.0086 ≤ Ie ≤ 40 / Dac-2.24.

Table 1 shows an example of an electron gun model used to obtain luminance exceeding the Langmuir limit. Calculations were performed using MEBS simulation software “Source” with such a model. The conditions for performing “sourcea” are also listed at the end of Table 1. In Tables 1 and 2, the bold letters are the parts changed as parameters. The simulation procedure is
・ When the Wehnelt voltage is set to a certain value and “SourceV” is executed, the cathode current Ie is calculated.
• When “SourceA” is executed, the cathode current density Jc, the crossover diameter Dco, and the crossover position: Zco are calculated.
・ When “SourceB” is executed, the emission angle dependency of luminance is output. With on-axis brightness B. The emission angle at which the luminance is 90% or 110% of the on-axis luminance is read, and the emittance is calculated from the product of the emission angle and the crossover.

FIG. 1 is a conceptual cross-sectional model diagram of an electron gun according to the present invention. The shape is a rotationally symmetric shape around the optical axis 4 and is a planar extraction electrode electron gun. The cathode 1 is a flat disk-shaped electron beam emission surface, and is a low work function material such as LaB ₆ , CeB ₆ , W-ZrO, or a photocathode. The Wehnelt electrode 2 has a shape of a part of the inner surface of the truncated cone in common with the conventional electron gun, but its position and size have the following characteristics. Assuming a virtual cone that intersects the optical axis at the anode position from the outer circumference of the cathode, this cone has an angle difference of θw degrees, and the same Z coordinate and R coordinate as the cathode outer circumference are the cone 2 starting from the cathode outer circumference + 100 μm position. The indicated Wehnelt was provided. The cone 7 is approximately the outer circumference of the beam. Since the outer circumferential surface of the beam changes when the beam current is changed, the virtual cone is defined as described above. Here, the Wehnelt angle: θw was 90.5 degrees. The cathode side end 5 of the anode electrode was curved to avoid discharge. The hole through which the beam passes was a 0.2 mmR hole parallel to the optical axis. The cathode 1 is structured so that it can be aligned with the Wehnelt 2 in the atmosphere before being installed. The two electrodes and the anode can be adjusted while viewing the beam by a fine adjustment mechanism provided outside the electron gun (not shown). I made it.

FIG. 2 shows the simulation results when the cathode radius Rc is changed to 80, 100, 120, 140, 160, 180, and 200 μmR in the electron gun of the model of FIG. Anode voltage: 4.5 kV, cathode: 0 V, electron gun current was changed by changing the Wehnelt voltage. The brightness of the electron gun 631 was changed by changing the current. In FIG. 2, the horizontal axis represents the electron gun current Ie (mA), and the vertical axis represents the luminance (x10 ⁵ A / cm ² sr), emittance (μmmrad), and cathode current density Jc (A / cm ² ). In FIG. 1, the distance between the cathode and the anode electrode: Dac is 1.2 mm.
20 is a cathode radius of 80 μm, 21 is a cathode radius of 100 μm, 22 is a cathode radius of 120 μm, 23 is a cathode radius of 140 μm, 24 is a cathode radius of 160 μm, 25 is a cathode radius of 180 μm, and 26 is an electron gun with a cathode radius of 200 μm. The dotted line is emittance, the solid line near the straight line is the cathode current density, and the solid curve is the luminance. Although it changes depending on the cathode radius, the brightness of each electron gun other than 80 μm Rc increases at a specific Ie, and a value far exceeding the Langmuir limit value is obtained. Since the simulation used ignores the initial velocity distribution of electrons at the cathode, the absolute value of luminance is somewhat unreliable, but the initial energy width is used in the calculation, so the relative value is close to the true value. Conceivable.
High luminance is obtained at 6.4 mA at 100 μm Rc, 7.26 mA at 120 μm Rc, 8.4 mA at 140 μm Rc, 10.1 mA at 160 μm Rc, 12 mA at 180 μm Rc, 12.8 mA at 200 μm Rc. While these electron gun currents are several hundred μA or less in the conventional high-intensity electron gun, this result is characterized in that the electron gun current is 10 times or more. That is, it can be said that a value far exceeding the Langmuir limit value was obtained by increasing the electron gun current and changing the condition from the optical model.
In the 100, 120, and 140 μm Rc electron guns, the luminance increases as Ie increases, and rapidly decreases immediately after the super high luminance is achieved. On the other hand, at 180 μm Rc and 200 μm Rc, it can be seen that once the electron gun current Ie is increased, the luminance is greatly reduced, and when Ie is further increased, ultra-high luminance is obtained. Therefore, in the 180 μm Rc and 200 μm Rc electron guns, in order to search for the ultra-high brightness condition, it is only necessary to search for the condition for decreasing the brightness and to check the electron gun condition slightly larger than the electron gun current.
Further, when the ratio (Iemax / Rc) of the cathode current Iemax and the cathode radius Rc at which the maximum luminance can be obtained is calculated, 0.064, 0.0605, 0.06, 0.0631, 0.066, 0.064 (in order of 100, 120, 140, 160, 180, 200 μm Rc). mA / μm) was obtained. Accordingly, a luminance far exceeding the Langmuir limit value can be obtained with a cathode current obtained by multiplying the cathode radius (μm) by 0.0638.

FIG. 3 is a graph obtained by changing the luminance-emittance curve and luminance-cathode current density characteristics from FIG. The curve to the lower right is the luminance-emittance curve, where the right or upper curve in the figure is the high performance. The solid line data near the horizontal is the luminance-cathode current density characteristic, and the lower line in the figure shows the high performance. The curves 31, 32, 33, and 34 are slightly raised to the right because the lower electron gun current side is graphed from the maximum brightness value in FIG. The reason why each curve of 35 and 36 is slightly lower right is that the high electron gun current side is graphed from the maximum luminance value in FIG. 30, 31, 32, 33, 34, 35, and 36 are 80 μm Rc, 100 μm Rc, 120 μm Rc, 140 μm Rc, 160 μm Rc, 180 μm Rc, and 200 μm Rc in this order. 37 is the Jc dependence of the Langmuir limit, and according to conventional theory, there should be no B-Jc curve to the right of 37. 38 is a value proportional to (luminance) x (emittance) ^2. The line parallel to this dotted line is the luminance-emittance characteristic. At the upper right operating point, the maximum current obtained when the beam is not blocked by the aperture is I can estimate. Since the slopes of the curves 34, 35, and 36 are closer to the vertical than the slope of 38, the same electron gun can provide a beam current with a low luminance and a high emittance condition. Since the slopes of curves 31, 32, and 33 are closer to the horizontal than slope of 38, the same electron gun can produce a beam current with high brightness and low emittance conditions.
1) A planar cathode planar anode electrode gun can obtain characteristics far exceeding the Langmuir limit value at 100 μm Rc to 200 μm Rc.
2) When the cathode radius is large, high luminance exceeding the Langmuir limit value can be obtained at a small cathode current density.
3) At 80 μRc, high luminance exceeding the Langmuir limit value was not obtained under the above conditions.
4) To search for ultra-high brightness conditions, the cathode radius Rc (μm) should be used, and the electron gun current of Rc x 0.0638 (mA) should be examined.
Three
FIG. 4 shows an example in which high brightness is not obtained, and FIG. 5 is an example in which electron trajectories when high brightness is obtained are compared with an electron gun having a cathode radius of 80 μm. The cathode 1, Wehnelt 2, anode 3, and optical axis 4 are the same as in FIG. 42, 43, and 44 are equipotential lines of 0, 100, and 4400V, respectively. In FIG. 4, a crossover 41 is formed between the cathode and the anode. In FIG. 5, the trajectory 40 close to the optical axis is a trajectory close to the laminar flow model without crossover until the anode electrode is passed. From the comparison between FIG. 4 and FIG. 5, controlling the electron gun condition so that the electron orbit is close to the laminar flow model rather than the electron orbit close to the optical model that creates the crossover is one condition for obtaining ultra-high brightness. It can be said.
As described above, it was shown that ultra-high brightness can be obtained by applying a predetermined electron gun current depending on the cathode radius. However, since these currents are large, it was investigated whether high luminance exceeding the Langmuir limit value could be obtained with a smaller current. FIG. 6 shows a case where an electrostatic lens is provided behind the model of FIG. The simulation was performed by providing three electrodes 61, 62, 63, applying the same potential to the anode 61, 63, and applying various voltages to the central electrode 62. An example model of FIG. 6 is shown in Table 2. The .com data for running “Sourcea” is the same as in Table 1.

The results are shown in FIGS. FIG. 7 shows the dependence of the luminance (solid line), emittance (dotted line), and cathode current density (solid line) on the electron gun current Ie. The potential applied to the central electrode 62 is 12.5 kV for 71, 14 kV for 72, 15 kV for 73, 12 kV for 74, and 4.5 kV for 75, respectively. When the anode potential is 0V, 4.5 kV must be subtracted from these values. The cathode radius Rc is the result for the case of 140 μm. The 75 without lens excitation is 9.4 mA, and the brightness is high at 15.4 kV, 4.75 mA at 14 kV; 5.7 mA at 14 kV, 7.25 mA at 15 kV, and 9.23 mA at 12 kV. Brightness is obtained. That is, the maximum brightness is obtained with a smaller current by passing the lens.
In common with all lens conditions, the brightness simply increases as the electron gun current Ie is increased, and an ultrahigh brightness is obtained when the electron gun current Ie is further increased. It can be seen that the emittance is minimized under the condition where the luminance is maximized and then maximized in accordance with the sudden decrease in luminance. Therefore, in order to search for the ultra-high luminance condition, an aperture capable of measuring luminance is provided and luminance measurement is performed, or a condition for increasing emittance is searched for, and an electron gun condition slightly smaller than the electron gun current is examined.

  FIG. 8 shows FIG. 7 in terms of luminance B − emittance E characteristics (substantially lowering curve group) and B-Jc (cathode current density). Reference numerals 81, 82, 83, 84, 85, and 86 denote cases where voltages of 12.5 kV, 13 kV, 14 kV, 12 kV, and 4.5 kV are applied to the central electrode of the lens, respectively. These voltages are values when the cathode is set to 0 V. When the cathode is −4.5 kV, these voltages are values obtained by subtracting 4.5 kV. 37 is the Langmuir limit, and in the conventional theory, there should be no B-Jc characteristic data on the right side of this line. Reference numeral 38 denotes the straight line described with reference to FIG. 3. Here, since the slope of the B-E characteristic is larger than the slope of 38, the maximum beam current obtained is larger under the low luminance and high emittance conditions. Since the BE curve 86 without lens action is above 81, 82, 83, 84, and 85 with lens action, unfortunately the maximum beam current that can be obtained with the same brightness is reduced when the lens action is used.

  FIG. 9 is a diagram showing the trajectory of electrons emitted perpendicularly from the surface of the cathode in order to investigate why the luminance is improved by passing through the lens. As shown by 91, the orbits emitted from the cathode at equal intervals are seen to have a lower density at the center due to the space charge effect. However, after passing through the lens, it can be said that the density distribution on the evaluation surface 6 is greatly improved as shown at 92, and the decrease in luminance due to the space charge effect is compensated. Reference numeral 93 denotes an equipotential distribution of the positive voltage driving lens.

  FIG. 10 shows the trajectory when a negative voltage driving lens is used. As in the case of FIG. 9, the density distribution after passing through the lens is significantly improved as compared with 101 before entering the lens, which supports high luminance. Reference numeral 103 denotes an equipotential distribution of the negative voltage driving lens.

FIG. 11 shows the result of simulation by changing the angle of the conical Wehnelt. The result is that the cathode radius Rc is 80 μm. The angles of the Wehnelt electrode and the approximate surface of the beam periphery are shown as 110, 111, 112, 113, and 114 in the order of 93.2 degrees, 90.5 degrees, 83.6 degrees, 69.4 degrees, and 57.6 degrees, respectively. The dotted line is emittance, the solid line rising to the right is the cathode current density, and the brightness is sharply rising on the right shoulder. Ultra high brightness exceeding 1x10 ⁶ A / cm ² sr is obtained when the angle between the Wehnelt electrode and the approximate surface of the beam is 93.2 ° to 69.4 °, but the maximum brightness is obtained at 57.6 °. Was 4.6 × 10 ⁵ A / cm ² sr. For this reason, the angle between the Wehnelt electrode and the approximate surface of the beam periphery needs to be larger than 69.4 degrees.
Here, when the Wehnelt angle: 90.5 degrees: 111 and 69.4 degrees: 113 are compared, the ratio of the cathode current at which the luminance exceeding the Langmuir limit is obtained is 2.8 mA / 3.6 mA = 0.777 times.

  FIG. 12 is a graph showing the result of FIG. 11 in terms of luminance B-emittance E characteristics (curved downwards) and B-Jc characteristics. 120, 121, 122, 123, and 124 are cases where the angles of the Wehnelt electrode and the approximate surface of the beam periphery are 93.2 degrees, 90.5 degrees, 83.6 degrees, 69.4 degrees, and 57.6 degrees, respectively. 37 is the Langmuir limit at a cathode temperature of 1800K. According to the conventional theory, there is no B-Jc characteristic on the right side. At 121: 92 degrees, the B-E characteristic is on the right side, and the same emittance provides high brightness, which is a good performance. At 120: 90.5 degrees, the cathode current density is too high. 123: 69.5 degrees is characterized by low cathode current density. From the above, the optimum angle between the Wehnelt electrode and the approximate surface of the beam periphery is 69.4 to 90.5 degrees, and if it is larger than 67.5 degrees, luminance exceeding the Langmuir limit can be obtained. 124: At 57.6 degrees, the value is small but the luminance exceeds the Langmuir limit.

FIG. 13 shows the result of simulation by changing the electron gun current Ie for electron guns having cathode radii Rc of 40, 80, 120, 160, and 200 μm. The lens voltage is 12.5 kV, and as described in the explanation of FIG. 7, this is a condition in which the cathode current at which the luminance exceeding the Langmuir limit is obtained is the smallest. The Wehnelt angle is 90.5 degrees, and the distance between the cathode and the anode Dac is 1.2 mm. These results are shown in the order of 130, 131, 132, 133, and 134. The solid line near the straight line is the cathode current density (unit: A / cm ² ) The solid curve is the luminance (unit: 10 ⁵ A / cm ² sr) The dotted curve is the emittance (μmmrad), and the horizontal axis is the cathode current Ie, The unit is mA. The cathode current values at which the luminance is maximized are 1.82, 2.93, 4.24, 5.4, and 6.58 mA in order for 40 to 200 μm Rc. These current values are
Ie = 0.5 + 0.0304Rc
It is almost on the straight line. Therefore, the condition for obtaining luminance exceeding the Langmuir limit may be the cathode current (mA) = 0.5 + 0.0304 X cathode radius (μm). As a result of not using the lens, considering the cathode current = 0.0658 X Rc, if the lens condition is not optimal, it is between these two formulas,
0.5 + 0.0304Rc <Cathode current <0.0658Rc (3)
It is.

FIG. 14 is a graph showing the result of FIG. 13 in terms of luminance B-emittance E characteristics (curved downward curve) and B-Jc characteristics. 140, 141, 142, 143, and 144 correspond to electron guns having cathode radii Rc of 40, 80, 120, 160, and 200 μm. When the luminance-cathode current density characteristic is almost horizontal, it is on the right side of the straight line 37 corresponding to the Langmuir limit, and also corresponds to the condition where the luminance increases rapidly. It is sufficient that the cathode current is greater than 0.5 + 0.0304 X cathode radius (μm).
In the luminance-cathode current density characteristics, the larger the cathode radius, the higher the luminance exceeding the Langmuir limit with a small cathode current density. In addition, although it is not remarkable, there is a tendency to obtain a large emittance with the same luminance when the cathode radius is small with BE characteristics.

FIG. 15 shows a simulation result by changing the electron gun current Ie with the distance between the cathode and the anode: Dca as a parameter. The cathode radius Rc was 120 μm, the lens voltage was 14 kV, and the Wehnelt angle was 90.5 degrees. Dac was performed with dimensions of 3, 2.8, 1.8, 1.6, 1.2, and 1 mm. These results are shown in the order of 150, 151, 152, 153, 154 and 155. The dotted curve is emittance (μmmrad), the solid curve is luminance (x10 ⁵ A / cm ² sr), the curve close to the straight line is the cathode current density (A / cm ² ). The value is near the axis. Distance between cathode and anode: With an electron gun with a Dca of 3 mm, the brightness was only 5.6 × 10 ⁵ A / cm ² sr at maximum. The distance between the cathode and the anode: When the Dca was 2.8 mm or less, the luminance was much higher than 1 × 10 ⁷ A / cm ² sr. Since a voltage of 4.5 kV is applied between the cathode and the anode, if the electric field strength between the cathode and the anode is approximately 1.6 to 4.5 kV / mm, the luminance exceeds the Langmuir limit. It can be said that
Further, when Ie that obtains luminance exceeding the Langmuir limit is compared between 151 and 154, it is 2.5 mA at Dac: 1.2 mm and 4.5 mA at Dac: 2.8 mm. Therefore, when the result of FIG. 13 with Dac: 1.2 mm is converted into the case of Dac 2.8 mm, equation (3) becomes equation (4).
0.278 + 0.0169Rc <Cathode current <0.0658Rc (4)
Furthermore, using the optimal Wehnelt angle, considering the ratio of 0.777 times described in [0026], (4)
0.168 + 0.0131Rc <cathode current <0.0658Rc (5)
become.

FIG. 16 is a graph showing the result of FIG. 15 in terms of luminance B-emittance E characteristics (curved downward curve) and B-Jc characteristics. The characteristics with dimensions of Dca of 3, 2.8, 2.2, 1.8, 1.4, and 1 mm correspond to 160, 161, 162, 163, and 164, respectively. From the luminance B-emittance E characteristic, it is not good that Dac of 160 is 3 mm. Under this condition, the luminance is 5.6 × 10 ⁵ A / cm ² sr at the maximum, but the luminance exceeding the Langmuir limit is obtained because the B-Jc characteristic is on the right side of the straight line 37.
When the electric field strength between the cathode and the anode is large, there is a tendency that high emittance tends to be obtained at the same luminance, but there is a problem that the cathode current density at high luminance is large. Therefore, also from this figure, the optimum electric field strength between the cathode and the anode is approximately 1.6 to 4.5 kV / mm.

Next, the case where the cathode temperature is changed will be described with reference to FIGS. 1 model without lens, cathode radius 140μmRc, anode-cathode distance Dac: 1.2mm, Wehnelt angle: 90.5 degrees, cathode temperature calculated for 1800K, 1100K, 450K, 292K, and the results in order 170, 171, 172, and 173. At 1800K, CeB6 cathode, the work function is 2.35eV, 1100K is an oxide cathode such as Ba, and the work function is 1.15eV, 450K, the photocathode cathode is assumed, but the difference between work function and irradiation laser energy Was assumed to be 0.1 eV or less of laser and cathode material. 292K. Assuming a photocathode having a negative electron affinity, the work function was set to -0.01 eV.
The solid line is the luminance (x10 ⁵ A / cm ² sr), the dotted line is the emittance (μmmrad), the solid line close to the straight line is the cathode current density, and the cathode current density at the same temperature: Ie is almost the same value. At 170 of 1800K, as Ie increases, the luminance begins to decrease, reaches a minimum value at 9.2 mA, immediately thereafter reaches a maximum value, and then decreases rapidly. In contrast to this, the emittance drops rapidly from a constant value and settles to a smaller value at the beginning. At 1100 K of 171, the luminance starts to decrease as Ie increases, reaches a minimum value at 7.25 mA, immediately thereafter reaches a maximum value at 7.55 mA, and then decreases rapidly. In contrast to this, the emittance drops rapidly from a constant value and settles to a smaller value at the beginning. At 450K 172, unlike the above two, as Ie increases, the luminance increases monotonically, reaches a maximum at 4.2 mA, and then decreases rapidly. In contrast, the emittance rapidly decreases from a certain value, increases rapidly, and takes a maximum value. In 173 at 292K, the same Ie dependency as in 450K is shown, and the electron gun current Ie at which the luminance becomes maximum is 4.02 mA, and in the case of 1800 K, it is greatly reduced compared to 9.45 mA. Therefore, it can be said that an oxide cathode, a photoelectron cathode, or a cathode having a negative electron affinity can easily obtain luminance exceeding the Langmuir limit as compared with a LaB6 or CeB6 thermal cathode.
Comparing 292K and 1800K with Ie that gives a luminance exceeding the Langmuir limit,
It is 4.02 / 9.45 = 0.425 times. Therefore, in the case of a photocathode having a negative electron affinity, the condition for obtaining a luminance exceeding the Langmuir limit is
0.0714 + 0.00557Rc <Cathode current <0.0658Rc (6)
become. This will be described in more detail with reference to FIG.

FIG. 18 is a graph showing the results of FIG. 17 in terms of luminance B-emittance E characteristics (curved downwards) and B-Jc characteristics. For the B-Jc characteristics, only the data on the larger Ie side from the value with the maximum luminance was used. In FIG. 18, the results of the cathode temperatures of 1800K, 1100K, 450K, and 292K are shown as 180, 181, 182, and 183 in order. The Langmuir limit when the cathode temperature is 1100 K, 450 K, and 292 K was calculated from the equation (2) and shown in 39,185,186.
From B-E characteristics, the condition of about luminance 1x10 6 A ^/ cm2sr, optoelectronic cathode obtained large emittance compared to CeB6 or oxide cathode.
B-Jc characteristics, 180 is right from line 37, 181 is right from line 39, 182 is right from 185, 184 is right from 186, and brightness exceeding the Langmuir limit at low temperature is obtained. It can be said.

FIG. 7 shows that high luminance exceeding the Langmuir limit value can be obtained with a smaller current when the electrostatic lens is provided. We investigated whether high luminance exceeding the Langmuir limit value was obtained when the electrostatic lens was changed to an electromagnetic lens. A simulation was performed by applying various excitation AT to an electromagnetic lens having a Bohr diameter of 1 mmR and a gap of 1 mm. The results are shown in FIGS. FIG. 19 shows the electron gun current Ie dependence of luminance (solid line), emittance (dotted line), and cathode current density. Excitation AT is 191, 500 AT, 192 is 550 AT; 193 is 600 AT; 194 is 400 AT; 195 is 0 AT, respectively.
The 195 without lens excitation has a high brightness at 9.4 mA, whereas the 500AT is 4.6 mA; the 550AT is 5.85 mA, the 600AT is 7.4 mA, and the 400AT is 8.6 mA. ing. In other words, the maximum luminance is obtained with a smaller current by passing the magnetic lens.
In common with all lens conditions, the luminance increases monotonously when the electron gun current Ie is increased, and when the electron gun current Ie is further increased, ultra-high luminance is obtained, and then the luminance decreases rapidly and becomes the minimum luminance. It can be seen that the emittance increases monotonously as Ie increases, and increases rapidly after reaching the minimum emittance. Therefore, in order to search for the ultra-high luminance condition, an aperture capable of measuring luminance is provided and luminance measurement is performed, or a condition for increasing emittance is searched for, and an electron gun condition slightly smaller than the electron gun current is examined.

  FIG. 20 shows FIG. 19 in terms of luminance: B − emittance: E characteristics (substantially downward curve group) and B-Jc (cathode current density). Reference numerals 201, 202, 203, 204, 205, and 206 denote cases where excitation of 500, 550, 600, 400, and 0 AT, respectively, is given to the lens. 37 is the Langmuir limit, and in the conventional theory, there should be no B-Jc characteristic data on the right side of this line. 38 is the straight line described with reference to FIG. 3. Here, since the slope of the BE characteristic is larger than the slope of 38, the maximum beam current obtained is larger under the low luminance and high emittance conditions. Since the B-E curve 206 without lens action is above 201, 202, 203, 204, and 205 with lens action, unfortunately, if the lens action is used, the beam current obtained with the same brightness is reduced.

  FIG. 21 shows an electron trajectory in order to investigate why a high brightness condition may be obtained when an electromagnetic lens is used. In this case, the voltage applied to the central electrode 211 of the electrostatic lens is the same as the electrode voltage before and after this, and is 4.5 kV. The vertical line 210 is an isomagnetic potential line and indicates the lens position. As in the case of the electrostatic lens in FIGS. 4 and 5, the variation in the density of the latter orbit is greatly improved before entering the lens and on the evaluation surface 6. Further, it can be seen that the crossover 212 of the track near the optical axis is formed closer to the cathode than the crossover 213 of the track far from the optical axis. This means that negative spherical aberration has occurred. Negative spherical aberration occurs due to the lens action of the anode hole and the space charge effect, and the lens action of the anode hole does not depend on the beam current. It can be interpreted that high luminance is generated under the condition that the large negative spherical aberration caused by the space charge effect due to the large is canceled by the positive spherical aberration generated by the electron gun and the magnetic lens. I don't know if this interpretation is correct, but the common sense that the brightness cannot be changed by the lens has been denied at least in the simulation.

FIG. 22 shows an embodiment of an electron gun using a photocathode electron gun. The cathode 1, Wehnelt 2 and anode 3 are axial objects around the optical axis 4. Since the angle between the Wehnelt and the beam outer periphery is relatively large at 69.4 degrees or more, the lens 222 is passed through between the anode and the Wehnelt, and the laser beam from the laser light source 223 is irradiated onto the electron beam emission surface of the cathode 1. The electrode voltage is supplied from the hermetic seal 221. Reference numeral 224 denotes a part of an optical system connected to the electron gun.

FIG. 23 shows an electron optical system in which a high-intensity electron gun is used. A high current multi-beam can be formed on a single optical axis using the high luminance effectively. Of course, since a beam is also formed at a position away from the optical axis, it is necessary to be more careful than in the case of a single beam. The electron gun is a three-electrode electron gun having a cathode 1, Wehnelt 2 and an anode 3. The beam emitted from the electron gun is aligned with the condenser lens 235 by the alignment deflector 234. The beam converged by the condenser lens 235 is aligned with the second condenser lens 238 and the multi-aperture 239 by the two-stage alignment deflectors 236 and 237. The multi-beam formed by the multi-aperture is reduced by the rotation adjusting lens 243, and the two-stage alignment deflectors 245 and 247 are used to align the NA aperture 242 and the reduction lens 243. The offset of the optical axis is corrected by the electrostatic deflector 245 and the electromagnetic deflector 246, and is incident perpendicularly on the objective lens 248, and the sample surface 250 is irradiated with a multi-beam and scanned. The sample surface is scanned by the scanning signal superimposed on the electrostatic deflector 245 and the electrostatic deflector 247. Reference numeral 249 denotes an axially symmetric electrode for reducing the longitudinal chromatic aberration of the multi-beam, and a positive high voltage is applied thereto. Secondary electrons emitted from the sample surface are separated from the primary optical system by electromagnetic deflectors 246 and 251 and are vertically corrected by electrostatic deflector 255. The object is enlarged by the objective lens 248, the electrostatic lens 253, and the two-stage lenses 256 and 258, and the detector 259 detects secondary electrons from each multi-beam independently.
The objective lens 248 is an electromagnetic lens in which a lens gap 467 is formed on the side of the sample 250. The axial chromatic aberration is small. It has the composition which is. A cross-sectional view of the objective lens is shown in FIG. The lens excitation coil 461 is surrounded by an outer magnetic pole 460 made of a permalloy core 460 and an inner magnetic pole 462 made of a pamentule core, and includes a non-magnetic metal member 463 having an L-shaped cross section and two O-rings 465. Vacuum sealed. The O-ring contact surface 464 may have a tapered shape as shown in the drawing or a concave curved surface on the O-ring side in order to reduce the magnetic resistance between the core 460 and the core 462. Since the two cores 460 and 462 can be fixed in the optical axis direction by the fixing screw 468, it can be manufactured with high accuracy. To summarize, the magnetic gap formed by the inner magnetic pole and the outer magnetic pole is a magnetic lens in the sample direction. What is necessary is just to make it have a ferromagnetic ring having a triangular cross section or a concave shape on the maximum side of the triangle at the connection portion with the outer magnetic pole core outside the magnetic pole. As a result, the magnetoresistance at the connection between the two magnetic poles is reduced, the axial magnetic field distribution of this lens has a single peak, and the aberration of this lens can be reduced.
FIG. 24 is a detailed diagram showing only the primary system of the electron optical system of FIG.
In FIG. 24, 244 is a line showing the image of the multi-aperture 239. Reference numeral 241 denotes a line indicating crossover imaging. The crossover of the beam emitted from the electron gun is enlarged by the two-stage lenses 235 and 238 at the tip of the multi-aperture 239, and the magnification can be adjusted. As a result, the current density in the multi-aperture is increased, and an adjustment with a wide uniformity of irradiation intensity over the entire multi-aperture is made possible. In this case, since no crossover is formed between the two-stage condenser lenses, the increase in the energy width due to the space charge effect is small, and the effect of using the photocathode having a negative electron affinity is great.

FIG. 25 shows not only the radius of curvature of the cathode as flat: 252 but also a convex surface of 5 mmR: 253, concave surface: 251;
The simulation results are shown with the convex surface of 1.5 mm: 250 and the concave surface: 254. The cathode radius was fixed at 200 μm. The lens voltage is 12.5 kV, the Wehnelt angle θw is 90.5 degrees, and the Dac is 1.2 mm. The flat cathode 252 has a maximum luminance at a cathode current of 7.6 mA, while the 1.5 mm convex surface has a maximum luminance at 6.4 mA. Cathode current conditions that can achieve brightness exceeding the Langmuir limit are reduced to 6.4 / 7.6 = 0.842 times. Therefore, in equation (5) with Dac: 2.5 mm and optimum Wehnelt angle: 69.5 degrees, the left side is multiplied by 0.842.
0.141 + 0.0142Rc <cathode current <0.0658Rc (7)
become.
Cathode current condition Ie for obtaining luminance exceeding the Langmuir limit is as large as 9.38 mA with a concave surface of 1.5 mmR, but the cathode current density Jc in this case is characterized by decreasing from 26 A / cm 2 to 23 A / cm 2. Since Ie is multiplied by 9.38 / 7.6 = 1.234, the left side of Equation (5) is multiplied by 1.234,
0.207 + 0.0162Rc <cathode current <0.06Rc (8)
become.

  FIG. 26 shows FIG. 25 with luminance: B − emittance: E characteristics (substantially lowering curve group) and B-Jc (cathode current density) characteristics: substantially horizontal lines. 260, 261, 262, 263, 264, 265 & 266 respectively correspond to cathode curvature radii: 1.5mm convex, 3mm convex, 5mm convex, flat, 5mm concave, 3mm concave, 1.5mm concave. In the 260mm 1.5mm convex cathode, the BE curve is located on the left side, the maximum current that can be obtained with the same brightness is small and worst, and the B-Jc characteristic is also located on the upper side. The 1.5mm concave cathode is the best with BE characteristics located on the right side and B-Jc characteristics located on the bottom side.

FIG. 27 shows a simulation result of an electron gun having a cathode of a photocathode having a negative electron affinity, an extraction electrode or anode having a shape of a part of a plane, and a Wehnelt electrode having a truncated cone shape. Dac: 1.2 mm, θw: 90.2 degrees, lens; 6.5 kV, cathode temperature 293 K, cathode work function: −0.01 eV, cathode radius Rc was changed to 30, 40, 80, 120, 160, 200 μm It was. The results are shown in order at 270, 271, 272, 273, 274, 275, 276, 277. The dotted line is the emittance (μmmrad), the solid line is the luminance (1 × 10 ⁶ A / cm ² sr), and the slightly upward line is the cathode current density (A / cm ² ). When the cathode current Ie that gives the maximum luminance at each cathode radius was read from the figure, 1.85, 2.2, 2.95, 3.55, 4.4, 6.1, 7.05, and 8.9 mA were obtained in this order. These numbers will be used later (FIG. 45).
FIG. 28 shows FIG. 27 with luminance: B − emittance: E characteristics (substantially lowering curve group) and B-Jc (cathode current density) characteristics: substantially horizontal lines. The results of changing the cathode radius Rc to 30, 40, 80, 120, 160, and 200 μm are shown in order as 280, 281, 282, 283, 284, 285, 2876, and 287, respectively.

FIG. 29 shows the simulation results with cathode work function: −0.01 eV, operating temperature 293 K, Dac: 2.5 mm, θw: 90.5 degrees, and cathode radius Rc as a parameter. The results obtained when Rc was 20, 100, 200, 300, 400, and 500 μm were shown in order of 290, 291, 292, 293, 294, and 295, respectively. For each cathode radius, the cathode current Ie that gives the maximum luminance was 0.361, 1.8, 2.45, 3.39, 4.34, and 5.36 mA in this order. These numbers will be used later (FIG. 45).
FIG. 30 shows FIG. 29 with luminance: B − emittance: E characteristics (substantially lowering curve group) and B-Jc (cathode current density) characteristics: substantially horizontal lines. The results of changing the cathode radius Rc to 20, 100, 200, 300, 400, and 500 μm are shown in order of 300, 301, 302, 303, 304, and 305, respectively. On the right side of the Langmuir limit line: 186, there is a B-Jc curve with a very small cathode current density Jc. That is, when Rc is changed to 20, 100, 200, 300, 400, and 500 μm, the cathode current density Jc is 30, 13, 7.7, 6, 5.1, 4.6 A / cm2 in order, exceeding the Langmuir limit. Yes. Thus, it is very practical to obtain a luminance exceeding the Langmuir limit with a small current density.

FIG. 31 shows the simulation results with cathode work function: −0.01 eV, operating temperature 293 K, Dac: 0.8 mm, θw: 90.5 degrees, and cathode radius Rc as a parameter. The results of setting Rc to 20, 30, 40, 120, 200, 300, 400, and 500 μm are shown in 310, 311, 312, 313, 314, and 315 in this order. For each cathode radius, the cathode current Ie that gave the maximum brightness was 1.78, 2.98, 3.42, 5.38, 7.69, 10.2, 12.76, and 15.4 mA in this order. These numbers will be used later (FIG. 45).
FIG. 32 shows FIG. 31 with luminance: B − emittance: E characteristic (substantially downward curve group) and B-Jc (cathode current density) characteristic: substantially horizontal line. The results of setting the cathode radius Rc to 20, 30, 40, 120, 200, 300, 400, and 500 μm are shown in 320, 321, 322, 323, 324, and 325 in this order. Langmuir limit line: B-Jc curve exists on the right side of 186. That is, when Rc is changed to 20, 30, 40, 120, 200, 300, 400, and 500 μm, the cathode current density Jc is 160, 100, 49, 38, 30, 27, 26 A / cm in order. ² exceeds the Langmuir limit. A feature of this cathode-anode distance Dac: 0.8 mm is that the emittance E at a luminance exceeding the Langmuir limit is very large. For example, when FIG. 30 and FIG. 32 are compared at a luminance of 1 × 10 ⁷ A / cm ² sr, the former Dac: 2.5 mm is 1.4 to 9 μmmrad, whereas the latter Dac: 0.8 mm is 12 to 65 μmmrad. is there. It is very practical to obtain high emittance with high brightness.
Here, the photocathode is examined. Assuming that the work function of the cathode is eφ, the irradiation laser wavelength is λ, the photon energy is hν (eV), and the cathode temperature is T (K), the beam energy width δE ignoring the Bercher effect is
δE ² = (kT) ² + (hν-eφ) ² (7)
become. Therefore, in order to reduce the energy width ΔE, the energy of the laser wavelength may be set to a value slightly larger than the work function of the cathode material and the cathode temperature may be lowered.
The results when the cathode temperature is low are shown below. FIG. 33 shows the simulation results with cathode work function: −0.01 eV, operating temperature 77 K, Dac: 2.5 mm, θw: 90.5 degrees, and cathode radius Rc as a parameter. The results of setting Rc to 2, 100, 200, 300, 400, and 500 μm are shown in 330, 331, 332, 333, 334, and 335 in this order. For each cathode radius, the cathode current Ie that gives the maximum luminance was 0.321, 0.923, 1.485, 2.13, 2.83, and 3.69 mA in this order. These numbers will be used later (FIG. 45).
FIG. 34 shows FIG. 33 with luminance: B − emittance: E characteristic (substantially lowering curve group) and B-Jc (cathode current density) characteristic: substantially horizontal line. The results of setting the cathode radius Rc to 20, 100, 200, 300, 400, and 500 μm are shown in order of 340, 341, 342, 343, 344, and 345. Langmuir limit line: B-Jc curve exists on the right side of 186. That is, when Rc is changed to 20, 100, 200, 300, 400, and 500 μm, the cathode current density Jc is 28, 91, 59, 46, 30, 27 A / cm ² in order and exceeds the Langmuir limit. ing. A feature of the cathode-anode distance Dac: 2.5 mm is that the current density Jc and the cathode current Ie at a luminance exceeding the Langmuir limit are very small. Thus, it is very practical to obtain a high emittance with the current density Jc and the cathode current Ie having a small high luminance.
FIG. 35 shows the relationship between eφ and the photoelectric limit wavelength as the work function of the cathode material. 350, 351, 352, 353, 354, 355, and 356 are Cs on Pt, K, Ba, Al, Ag, Pt, and W on Pt in this order (Institute of Electrical Communication, Communication Engineering Handbook, Maruzen, 1957) Year, p470). The thick line is the critical wavelength determined by hν = eφ. For metallic materials, except W, the critical wavelength is shifted about 20% from the straight line toward the long wavelength side. Conversely, tungsten: W is shifted to the short wavelength side. It can be inferred that these are the result of changing the work function due to surface contamination, and the effect of allowing electron emission with smaller energy photons due to thermal motion depending on the cathode temperature. Thus, considering the change in work function, it is preferable to excite the cathode with a laser wavelength within the range indicated by 367 and 358. For example, in the CeB ₆ cathode 359, in the range of 490 to 550 nm, 527 nm and 532 nm; in LaB ₆ in the range of 410 to 550 nm and at wavelengths of 532 nm and 527 nm, both are semiconductor-pumped solid lasers (green). When the Planck constant is h, the wavelength is ν, and the work function of the cathode is eφ, the cathode excitation laser wavelength may be a wavelength within ± 20% of the limit wavelength determined by hν = eφ. When the cathode is cooled, the cathode excitation laser wavelength may be 0 to -10% of the limit wavelength determined by hν = eφ.

Next, in order to investigate why high brightness is obtained, normal brightness, medium brightness under the conditions of cathode work function −0.01eV, operating temperature 293K, Dac: 0.8 mm, θw: 90.5, cathode radius Rc: 0.5mm Compare electron trajectories at brightness and high brightness. When these trajectories are created, only the trajectory near the center of the cathode is output, so that the number written as 22 in italic letters in Table 2 is changed to 55. The lens is not excited.
In this connection, in order to reduce the space charge effect, it is preferable to provide a small opening on the rear surface of the anode to remove unnecessary peripheral beams as soon as possible.
FIG. 36 shows normal brightness and a disturbance in 360 orbits.
FIG. 37 shows medium brightness, with the trajectories being too dense at 370 and 371 and the trajectory being sparse at 372.
FIG. 38 shows a trajectory with high brightness and a trajectory close to a laminar flow without any disturbance. In orbits close to laminar flow, the orbits do not cross each other, so there is little impact between electrons and the effect of expanding the energy width due to the Bercher effect, and the beam has a minimum diameter of 1.5 mm from the cathode. Position: Behind 0.8mm, the beam is sufficiently accelerated, so the interaction between electrons is even smaller. Naturally, such an electron orbit close to the laminar flow is far from the optical model, so there is no contradiction even if a luminance exceeding the Langmuir limit is obtained. After all, if the Wehnelt voltage is controlled so that the current density is small at the cathode and the beam diameter monotonously decreases until the beam diameter passes the anode, a beam with high brightness and a small energy width can be obtained, the axial chromatic aberration is small, and the large current Can be squeezed small.

FIG. 39 shows the result of examining the minimum value at each cathode radius of the cathode current Ie having a luminance exceeding the Langmuir limit in the thermoelectron cathode. Simulation was performed by changing the electron gun current Ie for an electron gun having a cathode radius Rc of 80, 100, 200, 300, 400, and 500 μm, lens conditions of 12.5 kV, Dac: 2.5 mm, 2.35 eV, and Wehnelt angle: 90.5 degrees. . The results are shown in order at 390, 391, 392, 393, 394, 395 and 396. The solid line near the straight line is the cathode current density (unit: A / cm ² ) The solid curve is the luminance (unit: 10 ⁵ A / cm ² sr) The dotted curve is the emittance (μmmrad), and the horizontal axis is the cathode current Ie, The unit is mA. The cathode current values for increasing the brightness are 1.81, 2.31, 3.26, 4.69, and 6.25 mA in this order. The 500 μm radius cathode could not exceed the above limit under these conditions. These numbers will be used later (FIG. 45).
FIG. 40 is a graph showing the result of FIG. 39 in terms of luminance B-emittance E characteristics (curved downward curve) and B-Jc characteristics. 400, 401, 402, 403, 404, and 405 correspond to electron guns having cathode radii Rc of 80, 100, 200, 300, 400, and 500 μm. When the luminance-cathode current density characteristic is almost horizontal, it is on the right side of the straight line 37 corresponding to the Langmuir limit. “A luminance exceeding the Langmuir limit is obtained. The curve of 405 with a cathode radius of 500 μm is on the right side of 37. Did not come out and the above limit was not exceeded.

FIG. 41 shows the result of investigating the maximum value at each cathode radius of the luminance exceeding the Langmuir limit in the thermoelectron cathode. Simulation by changing the electron gun current Ie for an electron gun with cathode radius Rc of 80, 100, 200, 300, 400 and 500 μm, no lens excitation, Dac: 0.8 mm, work function: 2.35 eV, Wehnelt angle: 90.5 degrees did. The results are shown in 410, 411, 412, 413, 414, 415 and 416 in this order. The solid line near the straight line is the cathode current density (unit: A / cm ² ) The solid curve is the luminance (unit: 10 ⁵ A / cm ² sr) The dotted curve is the emittance (μmmrad), and the horizontal axis is the cathode current Ie, The unit is mA. The cathode current value that increases the brightness
1.81, 2.31, 3.26, 4.69, and 6.25 mA. The 500 μm radius cathode could not exceed the above limit under these conditions. These numbers will be used later (FIG. 45).
FIG. 42 is a graph showing the result of FIG. 41 in terms of luminance B-emittance E characteristics (curved downwards) and B-Jc characteristics. 420, 421, 422, 423, 424, and 425 correspond to electron guns having cathode radii Rc of 80, 100, 200, 300, 400, and 500 μm. When the luminance-cathode current density characteristic is almost horizontal, it is on the right side of the straight line 37 corresponding to the Langmuir limit. “A luminance exceeding the Langmuir limit is obtained. The curve of 405 with a cathode radius of 500 μm is on the right side of 37. Did not come out and the above limit was not exceeded.

FIG. 43 shows a simulation result in which the cathode work function is −0.01 eV, the operating temperature is 77 K, the Dac is 2.5 mm, θw is 90.5 degrees, and the cathode radius Rc is a parameter, as in FIG. However, here, in order to reduce the space charge effect, a small opening (43.1 μmR) was provided on the rear surface of the anode, and the beam current was reduced to 1/10 or less. The results of setting Rc to 2, 100, 200, 300, 400, and 500 μm are shown in the order of 430, 431, 432, 433, 434, and 435. The cathode current Ie at which the maximum luminance was obtained at each cathode radius was 0.323, 0.868, 1.47, 2.038, 2.791, and 3.561 mA in this order. These numbers are almost the same as the results of FIG. Further, in this figure, the value of the crossover Z position: Zco (substantially solid mountain line) is also displayed. At 20 μm Rc, Zco had a negative value, but when Rc was 100 to 400 μm, the value was around 10 mm. Not displayed at 500 μm Rc. Since the aperture position is the anode rear surface Z = 3.6 mm, even when the beam current is reduced to 1/10 or less before the crossover is formed, the luminance exceeding the “Langmuir limit” is obtained.
FIG. 44 shows FIG. 43 with luminance: B − emittance: E characteristics (substantially lowering curve group) and B-Jc (cathode current density) characteristics: substantially horizontal lines. The results of setting the cathode radius Rc to 20, 100, 200, 300, 400, and 500 μm are shown in order of 440, 441, 442, 443, 444, and 445. Langmuir limit line: B-Jc curve exists on the right side of 186. That is, when Rc is changed to 20, 100, 200, 300, 400, and 500 μm, the cathode current density Jc is 28, 91, 59, 46, 30, 27 A / cm ² in order and exceeds the Langmuir limit. ing. Comparing this case with FIG. 4 in terms of emittance, the emittance does not decrease but slightly increases despite the beam current decreasing to 1/10 or less. If the beam current is reduced to 1/10 or less, the space charge effect is surely reduced to 1/10 or less. Thus, an opening that removes most of the beam before the crossover created by the electron gun is provided. Is practical.
FIG. 45 summarizes the cathode radius dependence of the cathode current at which the luminance exceeding the Langmuir limit described so far can be obtained.
450: The results of FIG. 41 are summarized. ; Ie = 10.5 + 0.0296Rc 120 <Rc ≤ 500
451: The results of FIG. 41 are summarized. ; Ie = 0.116Rc Rc ≤ 120 μm
452: The results of FIG. 2 are summarized. ; Ie = 0.0645Rc
453: The results of FIG. 31 are summarized. ; Ie = 2.6 + 0.0254Rc
454: The results of Figure 13 are summarized. ; Ie = 0.9 + 0.027Rc
455: The results of Figure 27 are summarized. ; Ie = 1.5 + 0.0183Rc
456: The results of Figure 29 are summarized. ; Ie = 1 + 0.0086Rc
457: The results of Fig. 33 & 34 are summarized. ; Ie = 0.4 + 0.0064Rc
From this figure, when the cathode radius Rc is less than or equal to 120 μm,
In the range of 0.4 + 0.0064Rc ≤ Ie ≤ 0.116 Rc, you can create a condition that provides brightness that exceeds the Langmuir limit.
When the cathode radius Rc is larger than 120 μm,
In the range of 0.4 + 0.0064Rc ≤ Ie ≤ 10.5 + 0.0296 Rc, conditions can be created to obtain brightness exceeding the Langmuir limit.
Furthermore, when high brightness and high emittance are desired, the cathode current in the range between 451 or 450 and 453 may be set, that is, when the cathode radius Rc is smaller than or equal to 120 μm,
2.6 + 0.0254Rc ≤ Ie ≤ 0.116 Rc, and when cathode radius Rc is greater than 120μm,
2.6 + 0.0254Rc ≤ Ie ≤ 10.5 + 0.0296 Rc may be set.
In order to obtain luminance exceeding the Langmuir limit with a relatively small cathode current or a relatively small cathode current density, a cathode current in the range between 457 and 453 may be used. That is, the range may be 0.4 + 0.0064Rc ≦ Ie ≦ 2.6 + 0.0254Rc.
Furthermore, the cathode radius is 20 ≦ Rc (μm) ≦ 200, and the brightness exceeding the Langmuir limit can be obtained with a relatively small cathode current, and the brightness exceeding the Langmuir limit can be obtained with a relatively small cathode current density. Is 200 ≦ Rc (μm) ≦ 500.

    In the above embodiment, ultra-high brightness was obtained in a relatively large range of electron gun current. In order to obtain an easy-to-use electron gun, the conditions for obtaining ultra-high brightness exceeding the Langmuir limit with a smaller electron gun current were investigated. The electron gun model is shown in FIG. The electrostatic lens: 474 is provided behind the cathode: 471, Weinert: 472, convex spherical anode: 473, and the voltage applied to the electrostatic lens electrode 474 and the angle between the Wehnelt and the optical axis are optimized. 475 is an optical axis, and 476 is an evaluation surface.

FIG. 48 shows the results when the cathode radius Rc is fixed at 10 μm and the distance Dac between the cathode and the anode is changed to 10 mm, 6 mm, 4 mm and 3 mm, respectively at 481, 482, 483 and 484. 53 and 54 are graphs in which the abscissa (1 / Dac) is plotted on the horizontal axis when the electron gun current is 13.3, 24, 40.8, and 61 μA under these Dac conditions and the super high brightness is obtained.

FIG. 49 shows the results when the cathode radius Rc is fixed to 300 μm and the distance Dac between the cathode and the anode is changed to 10 mm, 6 mm and 4 mm, respectively at 491, 492 and 493. Under these Dac conditions, ultra high brightness is obtained at electron gun currents of 1.82, 4.55 and 7.76 mA, respectively. FIGS. 53 and 54 are graphs in which the electron gun current Ie (max B) at which ultra-high luminance is obtained is plotted on the vertical axis and the reciprocal of the distance between the cathode and the anode (1 / Dac) is plotted on the horizontal axis.

FIG. 50 shows the results obtained by fixing the cathode-anode distance Dac to 10 mm and changing the cathode radius Rc to 10 μm, 300 μm and 500 μm, respectively at 501, 502 and 503. Under these Rc conditions, ultra-high brightness is obtained at electron gun currents of 0.00133, 1.82 and 3.05 mA, respectively. FIG. 52 shows a graph in which the electron gun current Ie (max B) at which ultra-high luminance is obtained is plotted on the vertical axis and the cathode radius Rc is plotted on the horizontal axis.

FIG. 51 shows the results when the cathode-to-anode distance Dac is fixed to 4 mm and the cathode radius Rc is changed to 10 μm, 100 μm, 200 μm, 300 μm, 500 μm and 700 μm, respectively at 511, 512, 513, 514, 515 and 516. . Under these Rc conditions, ultra high brightness is obtained at electron gun currents of 0.0408, 2.4, 5.23, 9.75 and 11.8 mA, respectively. A graph in which the electron gun current Ie (max B) at which ultra-high brightness is obtained is plotted on the vertical axis and the cathode radius Rc is plotted on the horizontal axis is shown by straight lines 522 and 523 in FIG.

In FIG. 52, when the distance between the cathode and the anode Dac is 10 mm, the relationship between the cathode radius Rc and the electron gun current Ie giving the maximum luminance is almost on a straight line 521. When the distance between the cathode and the anode Dac is 4 mm, the relationship between the cathode radius Rc and the electron gun current Ie giving the maximum luminance is almost on the straight line 522 when Rc is 300 μm or less, and almost on the straight line 523 when Rc is 300 μm or more. Straight lines 521, 522, and 523 are given by the following equations, respectively.
521: Ie = 0.0062Rc-0.0487,
522: Ie = 0.0266Rc -0.225,
523: Ie = 0.0101Rc + 4.73, where Ie: mA, Rc: μm
Therefore, in the range of the cathode-anode distance Dac from 4 mm to 10 mm, the Ie condition for obtaining ultra-high brightness is
0.0062Rc -0.0487 ≤ Ie ≤ 0.101Rc + 4.73 Rc ≥ 0.3mm, or
A value in the range of 0.0062Rc−0.0487 ≦ Ie ≦ 0.0266Rc−0.225 Rc <0.3 mm.

53 and 54, when the electron gun current Ie for obtaining the maximum luminance is displayed on the vertical axis and the reciprocal (1 / Dac) of the cathode-anode interval Dac is displayed on the horizontal axis, the cathode radius is shown as a straight line 531 when the cathode radius is 300 μm. In the case of 10 μm, it is almost on the straight line 532. Straight lines 531 and 532 are given by the following equations, respectively.
531: Ie = 0.0204 / Dac-0.0086,
532: Ie = 40 / Dac-2.24, where Ie: mA, Dac: mm
Therefore, the condition of Ie for obtaining ultra-high luminance when the cathode radius Rc is in the range of 10 μm to 300 μm is:
Electron gun current: Ie
A value within the range of 0.204 / Dac-0.0086 ≤ Ie ≤ 40 / Dac-2.24 may be used.
The vertical axis in FIG. 54 is 100 μm or less, and ultra-high brightness is obtained with a very small electron gun current. Under such conditions, the increase in energy width due to the space charge effect is also small.

The above describes only the simulation. The results of comparing past experimental results with simulation results are shown below.
FIG. 55 is a diagram published in M. Nakasuji and H. Wada, J. Vac. Sci. & Technol., 17 (6) Nov./Dec. 1980 p1367. Simulation was performed on the actual measurement data 551 and 552. The results are shown in FIGS. Here, since the cathode current density is not actually measured, a simulation value is used. In FIG. 56, 561 is a Langmuir limit at a beam energy of 20 keV and a cathode temperature of 1800 K, 562 is a simulation value, and 563 is an actual measurement. The measured luminance value is slightly smaller than the simulation value, but the Langmuir limit is well exceeded. It can be predicted that the luminance is smaller than the simulation value due to insufficient assembly accuracy of the electron gun electrode.
In FIG. 57, 573 is a beam energy of 20 keV, a cathode temperature of 1800 K, a Langmuir limit, 571 is a simulation value, and 572 is an actual measurement. Since the simulation value 571 and the actual measurement value 572 are relatively well matched in the low luminance region, these experimental values are reliable, and there are some actual measurement values that sufficiently exceed the Langmuir limit. From these comparisons, it can be said that the simulation described above is feasible.

  As described above, as can be understood from the description of the best mode for carrying out the invention of the electron beam apparatus according to the present invention, the present invention makes it possible to use an electron gun capable of obtaining luminance exceeding the Langmuir limit, An electron gun that is expected to be small in width is obtained. Therefore, a plurality of narrowly focused multi-current multi-beams are formed around one optical axis, and secondary electron detection from a beam far from the optical axis is possible without any problem. Can be shortened, and the blur due to the space charge effect can be reduced. Furthermore, if a small opening is provided on the back surface of the anode and a beam far from the optical axis is removed at a position close to the electron gun, the increase in energy width due to the space charge effect is reduced. Furthermore, since ultra-high brightness can be obtained with a small electron gun current of 100 μm or less, the increase in energy width due to the space charge effect is also reduced.

It is a model figure of the electron gun which concerns on this invention. 2 is a simulation example using the cathode radius as a parameter in the electron gun shown in FIG. The figure which made the result of FIG. 2 the B-E display and the B-Jc display. It is an electron orbit diagram when not exceeding the Langmuir limit. It is an electron orbit diagram when the brightness | luminance exceeding Langmuir limit is acquired. FIG. 2 is a model diagram in which an electrostatic lens is added to the electron gun shown in FIG. 1. 7 is a simulation example using the lens voltage as a parameter in the electron gun shown in FIG. 6. The figure which made the result of FIG. 7 the BE display and the B-Jc display. The horizontal axis is luminance (x10 ⁵ A / cm ² sr), and the vertical axis is emittance (μmmrad) and cathode current density: Jc (A / cm ² ) In the electron trajectory diagram when the brightness exceeding the Langmuir limit is obtained, the lens gives a positive high voltage. In the electron trajectory when luminance exceeding the Langmuir limit is obtained, the lens is given a negative high voltage. 7 is a simulation example using the Wehnelt angle as a parameter in the electron gun shown in FIG. 6. The figure which made the result of FIG. 11 the B-E display and the B-Jc display. FIG. 7 is a simulation example using the cathode radius as a parameter in the electron gun shown in FIG. 6. FIG. The figure which made the result of FIG. 13 the BE display and the B-Jc display. The horizontal axis is luminance (x10 ⁵ A / cm ² sr), and the vertical axis is emittance (μmmrad) and cathode current density: Jc (A / cm ² ) FIG. 7 is a simulation example using the distance between the cathode and the anode as a parameter in the electron gun shown in FIG. 6. FIG. The figure which made the result of FIG. 15 the BE display and the B-Jc display. The horizontal axis is luminance (x10 ⁵ A / cm ² sr), and the vertical axis is emittance (μmmrad) and cathode current density: Jc (A / cm ² ) 2 is a simulation example using the cathode temperature as a parameter in the electron gun shown in FIG. 1. The figure which made the result of FIG. 17 the BE display and the B-Jc display. The horizontal axis is luminance (x10 ⁵ A / cm ² sr), and the vertical axis is emittance (μmmrad) and cathode current density: Jc (A / cm ² ) This is a simulation example in which an electromagnetic lens is added to the electron gun shown in FIG. 6 and the lens excitation AT is a parameter, and the electrostatic lens is not operating. The figure which made the result of FIG. 19 BE display and B-Jc display. The horizontal axis is luminance (x10 ⁵ A / cm ² sr), and the vertical axis is emittance (μmmrad) and cathode current density: Jc (A / cm ² ) When the electromagnetic lens is operated in the electron orbit diagram when the brightness exceeding the Langmuir limit is obtained. 1 is an embodiment of an electron gun using a photocathode electron gun. This is an electron optical system in which a high-intensity electron gun is used. This is the primary optical system of the electron optical system that uses a high-intensity electron gun. 7 is a simulation example using the cathode radius of curvature as a parameter in the electron gun shown in FIG. 6. The figure which made the result of FIG. 25 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). This is a simulation result with an electron gun using a photocathode and a cathode radius: Rc as a parameter. The figure which made the result of FIG. 25 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁶ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). This is a simulation result with an electron gun using a photocathode and a cathode radius: Rc as a parameter. Dac: 2.5mm. The figure which made the result of FIG. 29 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). This is a simulation result with an electron gun using a photocathode and a cathode radius: Rc as a parameter. Dac: 0.8mm. The figure which made the result of FIG. 31 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). The relationship between the cathode radius Ic and the cathode current Ie, which gives a luminance exceeding the Langmuir limit, with an electron gun using a photocathode cooled to 77K. The figure which made the result of FIG. 33 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). Relationship between work function and photoelectric limit wavelength Electron trajectory with a normal-brightness electron gun Electron trajectory high brightness with medium brightness electron gun Electron trajectory with high-intensity electron gun Cathode current dependence of brightness, emittance, and cathode current density at a thermionic cathode The figure which made the result of FIG. 39 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). 1800K, Dac :. Simulation results with an electron gun using a photocathode at 8 mm and θw: 90.5 degrees, with the cathode radius: Rc as a parameter. The figure which made the result of FIG. 41 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). This is an electron gun in which a cathode using a photocathode is cooled to 77K and a small opening is provided on the rear surface of the anode, and is a simulation result with a cathode radius: Rc as a parameter. Dac: 2.5 mm. The figure which made the result of FIG. 43 the BE display and the B-Jc display. The horizontal axis represents luminance (x10 ⁵ A / cm ² sr), and the vertical axis represents emittance (μmmrad) and cathode current density: Jc (A / cm ² ). Cathode radius: The relationship between Rc and the cathode current Ie at which luminance exceeding the Langmuir limit is obtained. An objective lens with a magnetic gap on the sample side. Electronic gun model. 47 is a simulation example of the electron gun shown in FIG. 47 with the cathode-anode distance Dac as a parameter. 47 is a simulation example of the electron gun shown in FIG. 47 with a cathode radius Rc of 0.3 mm and a cathode-anode distance Dac as a parameter. 47 is a simulation example with the cathode radius Rc as a parameter in the electron gun shown in FIG. 47 is a simulation example in which the distance between the cathode and the anode Dac is fixed to 4 mm and the cathode radius Rc is a parameter in the electron gun shown in FIG. Cathode radius: The relationship between Rc and the cathode current Ie at which luminance exceeding the Langmuir limit is obtained. Relationship between cathode-anode spacing Dac and cathode current Ie that provides brightness exceeding the Langmuir limit. The figure which expanded the vertical axis | shaft of FIG. Past measured data. Comparison between past actual measurement data and simulation. Comparison between past measurement data and simulation at a cathode curvature radius of 480 μm.

7: Virtual frustum, 5: Curved surface for avoiding discharge of anode hole, 6: Evaluation surface, 61, 62, 63: Electrostatic lens, 254: Two-stage static for correcting axis in synchronization with scanning of primary beam Electro-deflector, 259: Multi-beam detector, 242: Line indicating electron gun cross-over imaging, 244: Beam imaging line from multi-aperture,

Claims (6)

A device provided with a lens behind the electron gun, having a flat cathode, an extraction electrode or anode, an electron gun having a Wehnelt electrode in the shape of a part of a cone, and the unit of the electron gun current: Ie is mA, Distance between cathode and anode: Dac, when unit is mm,
0.388 / Dac -0.046 ≤ Ie ≤ 92.8 / Dac + 9.28, Dac ≥ 3 mm, or
0.388 / Dac-0.046 ≤ Ie ≤22 / Dac + 32.7, Dac <3mm.
Or when the unit of the cathode radius Rc is μm, the electron gun current Ie is 0.0062Rc -0.0487 ≤ Ie ≤ 0.101Rc + 4.73 Rc ≥ 0.3 mm, or
0.0062Rc-0.0487 ≤ Ie ≤ 0.0266Rc -0.225 Rc <0.3mm.
An electron beam apparatus characterized by having a value within the range. 2. The electron beam apparatus according to claim 1, wherein the brightness of the electron gun is adjusted by adjusting excitation or excitation voltage of the lens. 2. The electron beam apparatus according to claim 1, wherein the electric field strength applied between the cathode and the anode is 1.6 kV / mm or more. In claim 1, the electron gun current: Ie
An electron beam apparatus characterized by having a value within a range of 0.0107Rc <Ie <0.06Rc. 2. The lens according to claim 1, wherein the magnetic gap formed by the inner magnetic pole and the outer magnetic pole is a magnetic lens in the sample direction, the inner magnetic pole is made of a ferromagnetic material having a high saturation magnetic flux density, and the outer magnetic pole is made of a high permeability material. An electron beam apparatus characterized by having a ferromagnetic ring having a triangular cross section or a concave shape on the maximum side of the triangular shape at the connection portion with the outer magnetic pole core outside the inner magnetic pole. 2. The electron beam apparatus according to claim 1, wherein the inner magnetic pole is a permendur and the outer magnetic pole is a permalloy.