JPH0619960B2 - Electron beam apparatus having thermal field emission electron source - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electron beam apparatus having a thermal field emission electron source which is optimal for use in an electron beam length measuring machine, a scanning electron microscope and the like.

[Prior Art] In an electron beam length measuring machine, a scanning electron microscope, etc., it is necessary to change the SN ratio of an image to be obtained, and to reduce damage and contamination depending on the sample. It is desirable to be able to control the amount of electron beam current applied. FIG. 5 shows an example of an optical system of a scanning electron microscope, in which 1 is an electron gun, 2 is a converging lens, and 3 is.
Is an objective lens, 4 is a deflection coil, 5 is a diaphragm, and 6 is a sample. An electron beam from the electron gun 1 is converged by a converging lens 2 as shown by a broken line, an image of a virtual object point (crossover) of the electron gun 1 is formed at a point P, and the electron beam is an objective. It is converged finely on the sample 6 by the lens 3. The position of the electron beam irradiated on the sample is determined by the deflection coil 4
The surface of the sample 6 is scanned by the electron beam, which is changed according to the scanning signal applied to the sample.

Here, when changing the current amount of the electron beam with which the sample 6 is irradiated, for example, the excitation of the converging lens 2 is strengthened, and the position of the crossover image is changed from the point P to the point P as shown by the dotted line in the figure. Move to ′. As a result, the amount of electron beam limited by the diaphragm 5 changes, and as a result, the amount of current of the electron beam with which the sample 6 is irradiated is changed.

[Problems to be Solved by the Invention] As described above, when the current amount of the electron beam is changed, the excitation of the converging lens 2 is changed and the crossover position is changed, so that the objective lens is changed each time. The excitation strength of 3 must be changed to focus. Further, if a deflection coil, which is an electron beam scanning system, is positioned in front of the objective lens 3, the image rotation, that is, the rotation of the electron beam in the scanning direction on the sample surface, is caused by the change in the excitation intensity of the objective lens. It is not desirable. Particularly, in the electron beam length measuring machine, when the deflection direction of the electron beam changes with respect to the pattern to be measured, sufficient measurement accuracy cannot be obtained. In order to solve this problem, there is a method of using a three-stage lens system as shown in FIG. In FIG. 6, the same components as those in FIG. 5 are designated by the same reference numerals, but the difference between the configuration of this figure and the configuration of FIG. This is the point where the converging lenses 7 and 8 are used. That is, with the crossover position in front of the objective lens 3 fixed at point P, the amount of current of the electron beam with which the sample 6 is irradiated is controlled, so that the lens strengths of the two-stage convergent lenses 7 and 8 are interlocked. Can be changed. In the figure, when the electron beam path I ₁ has a relatively large current amount, the path I ₂ has a small current amount. Here, when a thermionic emission type electron gun using _a tungsten filament or L _a B ₆ is used as the electron gun 1, the angular current density of the electron gun is large, and the electron gun crossover (virtual object point) is used. ) Is very large, the image reduction rate by the converging lenses 7 and 8 must be increased,
It is necessary to use a magnetic field type lens as the converging lens.

Here, when the field emission type electron gun is used, since the angular current density of the electron gun is small and the diameter of the virtual object point is extremely small, the converging lens is used at a magnification of about 1 to several times. Therefore, in the optical system that forms the image (crossover) of the virtual object point in the middle of the first and second converging lenses 7 and 8 in FIG. The convergence is worse than in the case of stages. Further, in the configuration of FIG. 6, since the acceleration part of the electron gun and the three parts of the first and second converging lenses are separated, the entire system becomes long,
The electron beam converging property is deteriorated. Further, if the two-stage converging lens is a magnetic field type, the lens system becomes large due to cooling of the lens and the electron optical optical path becomes long, and the electron beam is easily affected by an external magnetic field. If the accelerating voltage is low, the resolution and length measurement accuracy of the obtained image are adversely affected.

The present invention has been made in view of the above points, and controls the current amount of the electron beam with which the sample is irradiated without controlling the objective lens and without deteriorating the convergence characteristic of the electron beam. It is an object of the present invention to provide an electron beam apparatus having a thermal field emission electron source having a compact structure.

[Means for Solving Problems] An electron beam apparatus having a thermal field emission type electron source according to the present invention includes a field emission emitter, an extraction electrode for extracting an electron beam from the emitter, and the extraction electrode. And an acceleration electrode for accelerating electrons, and first and second electrostatic converging lens electrodes connected to their own lens power sources between the extraction electrode and the acceleration electrode. An electron beam limiting diaphragm is provided in the subsequent stage of the electric focusing lens electrode, and the electron beam current amount is changed by changing the voltage intensity applied to each of the first and second focusing lens electrodes by each of the original lens power supplies. , The applied voltage intensity of the first and second converging lens electrodes is
The position of the crossover image formed by each converging lens formed between the extraction electrode and the acceleration electrode can be changed in conjunction with the acceleration voltage and the electron beam current amount so that the position is always constant. It is characterized by being configured in.

[Embodiment] An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 shows a scanning electron microscope equipped with a thermal field emission electron gun according to the present invention, 11 is a field emission emitter, 12 is a filament holding the emitter 11,
13 is a heating power supply for supplying a heating current to the filament 12, 14 is a thermoelectron suppression electrode, 15 is a suppression voltage power supply for applying a suppression voltage between the emitter 11 and the thermoelectron suppression electrode 14, 16 is an extraction electrode, 17 is the emitter 11
And an extraction electrode 16, an extraction voltage power supply for applying an extraction voltage, 18 is an acceleration electrode having a ground potential, 19 is an acceleration voltage power supply for applying an acceleration voltage between the emitter 11 and the acceleration electrode 18, 20 , 21 are the first and second electrostatic converging lens electrodes, and 22 is the first electrostatic converging lens electrode 2.
0 is a power source, 23 is a power source for the second electrostatic converging lens electrode 21, 24 is a diaphragm for limiting the amount of current, 25 is a diaphragm arranged at the crossover point P ₀ , and 26 is an electron beam deflector. For deflection, 27 is an objective lens, 28 is a sample, 29 is a detector for detecting backscattered electrons generated by irradiation of the sample 28 with an electron beam, 30 is a digital scanning signal generating circuit, and 31 is A control device such as a computer for controlling the scanning signal generating circuit 30 and measuring the length of the measured pattern of the sample based on the signal from the detector 29 and the scanning signal, 32 is a detector A cathode ray tube to which the detection signal from 29 and the scanning signal from the scanning signal generating circuit 30 are supplied, 33 is a control device such as a microprocessor, 34 is a memory, 35 is an acceleration voltage instructing means, and 36 is an electron beam current setting means. is there. The field emission emitter 1
FIG. 2 shows specific configurations of the extraction electrode 16, the acceleration electrode 18, the first electrostatic converging lens electrode 20, and the second electrostatic converging lens electrode 21.

In the configuration as described above, the filament 12 is heated by the supply of the heating current from the heating power source 13, so that the emitter 11 supported by the filament 12 is heated.
Will be indirectly heated. The degree of heating is such that when the emitter is, for example, Zr / O / W, the emitter is heated to about 1800 ° C. With the emitter maintained at this temperature, the extraction voltage for maintaining optimal emission radiation is uniquely determined with respect to the distance between the emitter tip and the extraction electrode. A voltage is applied from the power source 17 between the extraction electrode 16 and the emitter 11.
Electrons are extracted from the emitter 11 by such an extraction voltage. Here, thermoelectrons are also generated from the emitter 11 with heating, but the thermoelectrons are generated.
It is suppressed by the suppression voltage applied to No. 4, and eventually only electrons based on field emission are mainly obtained through the extraction electrode 16. The extracted electrons are accelerated by the accelerating electrode 18, and each converging lens formed between the extracting electrode 16 and the accelerating electrode 18, that is, the extracting electrode 16, the converging lens electrode 20, and the converging lens electrodes 20 and 21. , And the converging lens formed between each of the converging lens electrode 21 and the accelerating electrode 18, and the objective lens 27 finely converges on the surface of the sample 28. The electron beam irradiation position of the sample 28 is two-dimensionally or linearly scanned by the scanning signal from the scanning signal generating circuit 30.
The backscattered electrons and secondary electrons generated by the scanning are detected by the detector 29, and the detection signal is sent to the cathode ray tube 32 to which the scanning signal is supplied, and the backscattered electron image and the secondary electron image of the sample. Or line profile is displayed.
Further, the detection signal is supplied to the control signal 31, and the controller measures the width of the pattern on the sample and the like. The length measurement result is displayed on the screen of the cathode ray tube 32 or is recorded by a recording device such as a recorder (not shown).

FIG. 3 is an optical conceptual diagram of the scanning electron microscope shown in FIG. 1, in which the electron beam emitted from the emitter 11 is a converging lens formed between the extraction electrode 16 and the acceleration electrode 18, that is, the extraction electrode. The converging lens formed between each of the electrode 16 and the converging lens electrode 20, the converging lens electrodes 20 and 21, and the converging lens electrode 21 and the accelerating electrode 18 (actually, three converging lenses are formed as described above. However, for convenience of description, a crossover image is formed at a point P ₀ by either of the two stages of the converging lenses A and B. However, the voltage intensity applied to the first converging lens electrode 20 is The strength is such that a crossover image is not formed between the converging lenses. Here, in FIG. 1, the accelerating voltage of the electron beam is set by the accelerating voltage setting means 35, and the control device 33 controls the accelerating electrode 19 according to the signal from the setting means 35 to set a desired accelerating voltage. It is applied to the emitter 11 between the acceleration electrodes 18. Further, the current amount of the electron beam with which the sample 28 is irradiated is input from the current setting means 36, and the control device 33 causes the memory 34 to output the first and second convergent lens electrodes 20, by a signal from the current setting means 36. The signal related to the strength of the voltage applied to 21 is read out. The memory 34
In the table, a combination of voltage intensities applied to the first and second converging lens electrodes is stored according to each acceleration voltage. This combination has a value such that the crossover image is always tied to P ₀ even when the voltage intensity applied to each of the converging lens electrodes is changed in order to change the electron beam current amount, and at a specific acceleration voltage. , the applied voltage intensity V ₁ of the the first converging lens electrode 20 applied voltage intensity V ₂ to the second focusing lens electrode 21 has, for example, a relationship as shown in Figure 4. In this figure, the first converging lens electrode 20
The applied voltage intensity V ₁ to V corresponds to the desired electron beam current amount. Based on the signal related to the applied voltage intensity read from the memory 34, the control device 33 causes the desired voltage to be applied to the first converging lens electrode 20 and the second converging lens electrode 21. Power source 22 for the converging lens electrode
And control 23. The data related to the applied voltage intensity to the converging lens electrode stored in the memory 34 is
Although the combinations are discontinuous in relation to the capacity of the memory, when an electron beam current amount in the middle is required, the control device 33 calculates a signal of a desired combination by the interpolation method, and the calculation is performed. The power supply of each converging lens electrode may be controlled based on the result.

Referring again to FIG. 3, the electron beam passing through each of the converging lenses A and B has its electron beam current amount limited by the diaphragm 24, but the voltage intensity applied to the converging lens electrode 20 and the second converging lens electrode is changed. As a result, the electron beam current amount is changed in a state where the electron beam crossover is imaged at the point P ₀ . For example, in the case of a large current, the electron beam in the path indicated by I ₃ is irradiated on the sample, and the amount of the electron beam blocked by the diaphragm 24 increases as I ₄ and I _5, and as a result, the sample 28 is irradiated. The amount of current of the generated electron beam is small. In this way, no matter what acceleration voltage or electron beam current amount is selected, the crossover image can always be formed at the point P ₀ .

In the embodiments described in detail above, two stages of electrostatic focusing lens electrodes are arranged between the extraction electrode and the accelerating electrode, and the voltage intensity applied to the two stages of the focusing lens electrode is the acceleration voltage and the desired electron. The position of the crossover image formed by each converging lens formed between the extraction electrode and the accelerating electrode is always constant because it is changed according to the amount of current in the line, so that the amount of accelerating voltage or electron beam current changes Also, it is not necessary to adjust the objective lens. Therefore, the image is not rotated by the objective lens, and the scanning direction of the electron beam can be kept constant. As a result, with the electron beam length measuring machine, even if the accelerating voltage or the electron beam current amount is changed according to the sample, the scanning direction of the electron beam with respect to the measured pattern does not change, and highly accurate measurement can be performed. it can. Further, since the electrostatic type converging lens electrode is arranged between the field emission emitter 11 and the acceleration electrode 18, the device can be made compact and the structure is not easily affected by the external magnetic field, and the optical path length can be reduced. Can be shortened, so that the electron beam converging characteristic can be improved. Further, since no crossover is formed between the converging lenses formed between the extraction electrode and the accelerating electrode, even if a plurality of converging lenses are formed, the converging lens can be operated substantially as one stage. Therefore, the aberration of the lens is not added, and the convergence characteristic of the electron beam due to the aberration is not deteriorated.

The distance between the position of the virtual object point of the electron beam generation source and the position of the crossover image formed by each converging lens is 160 m in the case of using the conventional magnetic field type converging lens.
However, according to the configuration of FIG.
I was able to shorten it to mm. Also, the length of the converging lens electrode itself in the optical axis direction is 40 m from the conventional length of 110 mm.
m, and a compact structure can be achieved.

[Effect] As described in detail above, the present invention includes the first and second electrostatic converging lens electrodes connected to their own lens power sources between the extraction electrode and the accelerating electrode. An electron beam limiting diaphragm is provided in the subsequent stage of the converging lens electrode, and the voltage intensity applied to each of the first and second converging lens electrodes is changed by each of the unique lens power supplies to change the electron beam current amount. The applied voltage strengths of the first and second converging lens electrodes are
The position of the crossover image formed by each converging lens formed between the extraction electrode and the acceleration electrode can be changed in conjunction with the acceleration voltage and the electron beam current amount so that the position is always constant. It is not necessary to adjust the objective lens even if the accelerating voltage or the electron beam current amount is changed, so that the image is not rotated by the objective lens, and the crossing between the converging lenses is eliminated. Since an over image is not formed, even a multi-stage converging lens can be operated as a single-stage converging lens, so that the aberration of the lens is not added. , The convergence characteristic of the electron beam due to aberration does not deteriorate.

[Brief description of drawings]

FIG. 1 is a diagram showing a scanning electron microscope according to the present invention, and FIG.
FIG. 4 is a sectional view showing a specific structure of each electrode and a converging lens electrode, FIG. 3 is an electron optical conceptual diagram of the scanning electron microscope of FIG. 1, and FIG. 4 is a diagram showing a relationship between voltages applied to the converging lens electrode. 5 and 6 are views showing an example of an optical system of a conventional scanning electron microscope. 11 ... Emitter, 12 ... Filament 13 ... Heating power supply, 14 ... Thermoelectron suppression electrode 15 ... Suppression voltage power supply, 16 ... Extraction electrode 17 ... Extraction voltage power supply, 18 ... Acceleration electrode 19 ... Acceleration Voltage power source 20, 21 ... Electrostatic converging lens electrode 22 ... Converging lens electrode 20 power source 23 ... Converging lens electrode 21 power source 24, 25 ... Aperture, 26 ... Deflection coil 27 ... Objective lens, 28 ...... Sample 29 ...... Detector 30 ...... Digital scanning signal generation circuit 31 ...... Control device, 32 ...... Cathode ray tube 33 ...... Control device, 34 ...... Memory 35 ...... Acceleration voltage indicating means 36 ...... Electron beam current Quantity setting means.

Claims (1)

[Claims] 1. A field emission emitter, an extraction electrode for extracting an electron beam from the emitter, an acceleration electrode for accelerating the extracted electrons, and an independent electrode between the extraction electrode and the acceleration electrode. First and second connected to the lens power supply
Electrostatic converging lens electrode, and an electron beam limiting diaphragm is provided in the latter stage of the second electrostatic converging lens electrode, and the first and second converging lens electrodes are provided by the respective individual lens power sources. The applied voltage intensity of each of the first and second converging lens electrodes is changed by changing the applied voltage intensity of each electron beam current amount, and the convergent lens formed between the extraction electrode and the accelerating electrode. Electron beam apparatus having a thermal field emission electron source configured so that the position of the crossover image formed by .