JPS6329378B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for correcting astigmatism in scanning electron microscopes and similar devices.

In general, in scanning electron microscopes and similar devices, the spot of the electron beam irradiated onto the sample must be circular, but the condition of the lens (objective lens) used to focus it on the sample and the shape of the electron beam The spot of the electron beam often becomes oval rather than circular due to conditions along the path, especially due to dirt on the aperture inserted into the objective lens.

Such a state is said to have astigmatism in the electron beam.

Conventionally, as a means of correcting such astigmatism, as shown in Figure 1, first, the charged particle beam is synchronized with sawtooth waves for horizontal and vertical scanning in the horizontal and vertical directions. Sawtooth wave SX, SY
On the other hand, SX and X direction astigmatic aberration correction electric signals E ₁
are superimposed and supplied to the X-direction astigmatism correction coil, and the other sawtooth wave SY and the Y-direction astigmatism correction electric signal _E2 are superimposed to correct the Y-direction astigmatism. By supplying to the coil,
Astigmatism corrected image b on cathode ray tube screen a
appears locally.

Next, move the marker c to the position of this image b, align them, and then adjust the current value X, so that the absolute value is the same as the current value required to move the marker c to this matching point. The values of the electric signals E ₁ and E ₂ for Y-direction astigmatism correction are adjusted.

Thereafter, the supply of the sawtooth waves SX and SY to both astigmatism correction coils is stopped.

However, with such conventional astigmatic aberration correction means, since only one point of the optimal image appears on screen a, it is not possible to accurately determine where the optimal position is, which leads to the problem of poor correction accuracy. There is a point.

In addition, when using a sample whose entire surface is not uniform,
It is difficult to judge whether or not astigmatism has been corrected, and especially when the image appears only on a portion of screen a, it may not be possible to correct astigmatism at all.

The present invention aims to solve these problems, and includes a method for correcting astigmatism in scanning electron microscopes and similar devices, which makes it possible to easily and reliably correct astigmatism. The purpose is to provide equipment.

Therefore, the method of the present invention is characterized in that astigmatism is corrected by the following procedure in a scanning electron microscope equipped with a cathode ray tube for projecting a sample image and similar devices thereof.

(a) Each of the sawtooth waves synchronized with the horizontal and vertical scanning sawtooth waves for scanning the charged particle beam in the horizontal and vertical directions, is periodically Amplitude modulation is performed by multiplying modulated waves with varying amplitude values.

(b) A first direction astigmatic aberration correction electric signal is superimposed on the first amplitude modulated wave subjected to the amplitude modulation, and the electric signal is supplied to the first direction astigmatism correction member, and the above amplitude modulated wave is A second direction astigmatism correction electric signal is superimposed on the applied second amplitude modulated wave, and the electrical signal is supplied to the second direction astigmatism correction member.

(c) Next, by controlling the values of the electric signals for astigmatism correction in the first and second directions, the astigmatism correction image area appearing on the screen of the cathode ray tube is centered on the center of the screen. Adjust so that it changes symmetrically horizontally and vertically.

(d) Stopping the supply of the first and second amplitude modulated waves to the first and second direction astigmatism correction members.

Further, the apparatus of the present invention includes a cathode ray tube for projecting a sample image, and a horizontal beam for supplying sawtooth waves for horizontal scanning to a deflecting member for horizontal scanning of the cathode ray tube and a deflecting member for horizontal scanning in the mirror body. a horizontal scanning system including a directional sawtooth wave generator; and a vertical scanning system for supplying a vertical scanning sawtooth wave to the vertical scanning deflection member of the cathode ray tube and the vertical scanning deflection member in the mirror body. In a scanning electron microscope or similar device having a vertical scanning system including a sawtooth wave generator, a sawtooth wave synchronized with the horizontal scanning sawtooth wave and a vertical scanning sawtooth wave synchronized with the horizontal scanning sawtooth wave generator. An oscillator is provided that oscillates a modulated wave whose amplitude value changes periodically at a frequency lower than either of the synchronized sawtooth waves, and the modulated wave from the oscillator and the sawtooth shape of one of the two sawtooth waves are provided. a first multiplier that multiplies the wave by the first multiplier, and adds the output from the first multiplier and the electric signal from the first direction astigmatism correction power source, and applies this added output to the first multiplier in the mirror body. a second multiplier that multiplies the modulated wave from the oscillator and the other sawtooth wave of the sawtooth waves; , the output from the second multiplier and the electric signal from the second direction astigmatism correction power source are added, and the added output is supplied to the second direction astigmatism correction member in the lens body. a second adder that can be used, and a first switch is interposed between the first multiplier and the first adder; It is characterized in that a second switch is interposed between the second adder and the second adder.

The astigmatism correction device for a scanning electron microscope as an embodiment of the present invention will be explained below with reference to the drawings.
The figure is an electric circuit diagram showing the main parts, FIGS. 4a to 4f are waveform diagrams of each part, and FIGS. 5 and 6 are schematic diagrams for explaining the operation.

As shown in FIG. 2, sawtooth wave generators 1 and 2 for horizontal and vertical scanning are connected to deflection coils as deflection members for horizontal and vertical scanning of a cathode ray tube 5 for projecting a sample image via deflection circuits 3 and 4. 6 and 7, and via magnification switching circuits 8 and 9, an electron beam 10 as a charged particle beam is arranged inside the mirror body.
It is connected to deflection coils 11 and 12 as deflection members for horizontal and vertical scanning.

Therefore, the sawtooth wave from the generator 1 is sent to the deflection coil 11 by changing the amplitude appropriately by the magnification switching circuit 8.
, where the electron beam 10 is scanned in the horizontal direction, while this deflection coil 1
The sawtooth wave, which is synchronized with the sawtooth wave supplied to the cathode ray tube 1, is supplied to the coil 6 of the cathode ray tube 5, where it can be synchronized with the electron beam 10 to horizontally scan the electron beam inside the cathode ray tube 5. It is done. As a result, the generator 1, the deflection coils 6, 11,
The deflection circuit 3 and the magnification switching circuit 8 constitute a horizontal scanning system HS.

The sawtooth wave from generator 2 and the sawtooth wave synchronized therewith are also generated by the horizontal scanning system described above.
In almost the same way as in the case of HS, the deflection coil 12,
7, where the electron beam 10 inside the mirror body
Then, the electron beam inside the cathode ray tube 5 is scanned in the vertical direction. As a result, the generator 2, the deflection coils 7 and 12, the deflection circuit 4, and the magnification switching circuit 9 constitute a vertical scanning system VS.

By the way, an astigmatism correction circuit 13 is provided, and its three input ends 13a, 13b, 13
c, horizontal direction sawtooth wave generator 1,
An oscillator 14 and a vertical sawtooth wave generator 2 are connected, the two output ends 13d, 13
First and second direction astigmatism correction coils 15 and 16 are connected to e, respectively, as first and second direction astigmatism correction members.

Note that the first direction astigmatism correction coil 15 is
It is composed of four coil parts arranged with a phase difference of 90 degrees from each other, and by changing the magnitude and direction of the composite magnetic field by this correction coil 15, the cross section of the electron beam 10 is adjusted. The shape is expanded and contracted in the X direction (horizontal direction) and the Y direction (vertical direction) to bring the electron beam 10 closer to a perfect circular cross-sectional shape.

Further, the second direction astigmatism correction coil 16 is
It is composed of four coil parts that are arranged 45 degrees apart from each coil part of the first direction astigmatism correction coil 15 and with a phase difference of 90 degrees to each other. Therefore, by changing the magnitude and direction of the combined magnetic field, the cross-sectional shape of the electron beam 10 is expanded or contracted in directions rotated by 45 degrees from the X and Y directions, thereby causing the electron beam 10 to approach a perfectly circular cross-sectional shape. .

Further, this circuit 13 is provided with first and second potentiometers 17 and 18 that constitute power supplies for correcting astigmatism in the first and second directions.

The oscillator 14 also generates a sawtooth wave SX (see FIG. 4a) that is synchronized with the sawtooth wave for horizontal scanning and a sawtooth wave SY (see FIG. 4b) that is synchronized with the sawtooth wave for vertical scanning. Frequencies lower than either (e.g.
Modulated wave SM whose amplitude value changes periodically at a frequency of 0.5Hz
[See FIG. 4c].

Therefore, the sawtooth waves SX, SY and the modulated wave SM are input to the astigmatism correction circuit 13.

Furthermore, as shown in FIG. 3, this circuit 13 includes a first multiplier 19 that multiplies the modulated wave SM input through the terminal 13b and the sawtooth wave SX input through the terminal 13a; The signal output (first amplitude modulated wave) SM ₁ [see FIG. 4 d] multiplied by this multiplier 19 and the electric signal (DC) E ₁ from the first potentiometer 17 are added. The modulated wave SM is inputted through the terminal 13b and the modulated wave SM is inputted through the terminal 13c. sawtooth wave
A second multiplier 22 that multiplies SY and a signal output (second
(amplitude modulated wave) SM ₂ [see Figure 4e] and the electrical signal (DC) from the second potentiometer 18 E ₂
a second adder 23 that adds
3, and an amplifier 24 for power amplifying the output of No. 3.

Furthermore, a first multiplier 19 and a first adder 20
A first switch 25 is interposed between the second multiplier 22 and the second adder 23, and a second switch 26 is interposed between the second multiplier 22 and the second adder 23.

These switches 25 and 26 are configured to operate in conjunction with each other.

The outputs of the amplifiers 21 and 24 are supplied to the first and second direction astigmatism correction coils 15 and 16 via terminals 13d and 13e, respectively.

Further, as shown in FIG. 2, when the sample 27 is irradiated with the electron beam 10, secondary electrons, reflected electrons, etc. are emitted, and this signal is detected by the detector 28 and then amplified by the amplifier 29. Later, CRT 5
The sample image is projected onto the screen 5a of the cathode ray tube 5 (see Figures 5 and 6) by brightness modulation, and the signal is detected by the detector 28, amplifier 29, etc. System DS is configured.

With the above-described configuration, when astigmatism is corrected, the focal length of the objective lens is first adjusted, and then the first and second switches 25 and 26 are turned on. As a result, the first amplitude modulated wave is amplitude-modulated by multiplying the sawtooth wave SX by the modulated wave SM.
SM ₁ , the signal from the first potentiometer 17
A signal obtained by superimposing and amplifying E ₁ is supplied to the first direction astigmatism correction coil 15, and a second amplitude modulated wave which is amplitude-modulated by multiplying the sawtooth wave SY by the modulated wave SM is generated. A signal obtained by superimposing and amplifying the signal E ₂ from the second potentiometer 18 on SM ₂ is supplied to the second direction astigmatism correction coil 16 .

At this time, if the astigmatism is not eliminated, that is, if the areas α and β where the astigmatism is eliminated are shifted from the O levels L ₁ and L ₂ as shown in FIG. As is clear, the astigmatism-corrected image areas (clear areas of the image) P ₀ , ..., P _i , ..., P _j , ... appearing on the cathode ray tube screen 5a are asymmetrically located from the center of the screen 5a.
and finally the screen 5a.
After it protrudes from the center, it periodically returns to the center.

In this way, when astigmatism is not eliminated,
The reason why the astigmatism correction image areas P ₀ , ..., P _i , ..., P _j , ... move as described above is as follows.

Since the sawtooth waves SX and SY are amplitude-modulated by the modulation wave SM, the slopes of the sawtooth waves SX and SY are shown in FIG. 5 with the symbols A ₀ , ..., A _i , ..., respectively.
A _j , ..., A _o ; B ₀ , ..., B _i , ..., B _j , _... , Bo changes continuously and periodically, and as a result, in the case of Fig. 5, the above amplitude Modulated wave SM ₁ , SM ₂
The area where P intersects with the above sections α and β moves to the right and downward, resulting in the astigmatism corrected image area P ₀ ,...,
P _i ,..., P _j ,... move diagonally downward to the left from the center of the screen 5a, and finally amplitude modulated waves SM ₁ , SM ₂
Since there is no longer a region where . Thereafter, this astigmatic aberration corrected image instantaneously returns to the center of the screen 5a, and similar changes occur periodically.

With this kind of astigmatism not removed,
By adjusting the knobs of the first and second potentiometers 17 and 18 to change the values of the output signals E ₁ and E ₂ , a bias component is added to each amplitude modulated wave SM ₁ and SM ₂ . As shown in Fig. 6, when the center levels of the above sections α and β are matched to the above O levels L ₁ and L ₂ , in this matching state, by turning off the first and second switches 25 and 26, Therefore, even if the amplitude modulated waves SM ₁ and SM ₂ are removed, the matching state described above does not change. In other words, by establishing such a matching state, astigmatism can be corrected over the entire screen.

Incidentally, the changes in the astigmatism-corrected image areas P ₀ , ..., P _i , ..., P _j , ..., P _o in such a matching state are as follows. In other words, the astigmatism corrected image area
P ₀ appears at the center of the screen 5a, and then this image area gradually becomes image areas P _i , ..., P _j , ..., P _k , ... symmetrically in the horizontal and vertical directions, as shown in FIG. After expansion and expansion, the image area P ₀ momentarily returns to the center, which is periodically performed.

The reason for this is that, as is clear from Fig. 6, the slopes of the sawtooth waves SX and SY are of the same sign as shown in Fig. 6.
A ₀ ,…, A _i ,…, A _j ,…, A _k ,…, A _o ;B ₀ ,
..., B _i , ..., B _j , ..., B _k , ..., Bo changes continuously and periodically, and as a _result , the area where the amplitude modulated waves SM ₁ and SM ₂ intersect with the sections α and β This is because it spreads symmetrically in both the left and right directions.

Therefore, in order to correct astigmatism from a state where it has not been corrected, adjust the knobs of the first and second potentiometers 17 and 18 to If the astigmatic aberration correction image area that appears gradually expands and expands symmetrically from the center of the screen 5a in the left, right, up and down directions, and then returns instantaneously to the center again, periodically. good.

After adjusting in this way, the first and second switches 25 and 26 are turned off, and the first and second directional astigmatism correction coils 15 of the first and second amplitude modulated waves SM ₁ and SM ₂ are turned off. , 16 can be stopped.

Note that the moving speed of the astigmatism-corrected image area on the screen 5a at this time becomes slower as the state transitions from FIG. 5 to FIG.
The degree to which astigmatism has been corrected can also be estimated from this change in moving speed.

As a result, a clear sample image is projected on the screen 5a.

As described above, according to the astigmatism correcting means of the present invention, astigmatism can be corrected easily, reliably, and quickly with a simple circuit configuration.

Note that if the electron beam 10 does not pass through the center of the magnetic field of each astigmatism correction coil 15, 16, the electron beam 10 will be bent and the sample image will be synchronized with the modulated wave SM from the oscillator 14. Braun tube 5a
The image continues to move even after adjusting the expansion and contraction at the top to be symmetrical from the center, but if the electron beam 10 passes through the center of the magnetic field, the movement of the image will stop. The sample image becomes stationary. By utilizing this property, the axes of the correction coils 15 and 16 can be easily aligned.

Additionally, it can be used with high precision at both low and high magnifications.

Note that the potentiometers 17 and 18 may be operated both at the same time or separately.

By the way, it is also possible to use a modulated wave SM' as shown in FIG. It is adjusted so that it periodically contracts and contracts in a symmetrical manner toward the screen's periphery, and then instantaneously expands and expands to the periphery of the screen and returns.

Furthermore, as the modulated wave, one whose amplitude value changes periodically, such as a sine wave or a triangular wave, can also be used.

Further, the astigmatism correcting means of the present invention can be applied to an apparatus similar to a scanning electron microscope in substantially the same manner as in the above-mentioned embodiments.

As described in detail above, the astigmatic aberration correcting means in the scanning electron microscope and similar devices of the present invention provides the following effects and advantages.

(1) The astigmatic aberration correction operation, which has been difficult to understand, can be performed easily, reliably, and quickly with a simple configuration.

(2) Astigmatism can be corrected with high precision both at high and low magnifications.

(3) Can be used even if the sample is not uniform.

(4) It can also be used for alignment of the first and second direction astigmatism correction members.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram for explaining astigmatism correction means in a conventional scanning electron microscope.
Figures 2 to 6 show an astigmatic aberration correction device for a scanning electron microscope as an embodiment of the present invention. Figure 2 is a diagram of its overall configuration, and Figure 3 is an electric circuit showing its main parts. 4A to 4F are waveform diagrams of each part, and FIGS. 5 and 6 are schematic diagrams for explaining the operation thereof. 1, 2... sawtooth wave generator, 3, 4... deflection circuit, 5... cathode ray tube for projecting sample image, 6, 7...
Deflection coil as a deflection member for horizontal and vertical scanning of a cathode ray tube, 8, 9...Magnification switching circuit, 10
...Electron beam as a charged particle beam, 11, 12
... Deflection coil as a deflection member for horizontal and vertical scanning within the mirror body, 13... Astigmatism correction circuit,
13a, 13b, 13c...input end, 13d, 1
3e... Output end, 14... Oscillator, 15, 16...
...Correction coils as first and second direction astigmatism correction members, 17, 18...First and second potentiometers constituting power supplies for first and second direction astigmatism correction, 19 ...first multiplier, 20...first
adder, 21... amplifier, 22... second multiplier, 23... second adder, 24... amplifier, 2
5, 26...first and second switch, 27...sample, 28...detector, 29...amplifier, E ₁ ...
X direction astigmatism correction electric signal, E ₂ ...Y direction astigmatism correction electric signal, HS ... horizontal scanning system, VS ... vertical scanning system, DS ... signal detection system, SX, SY...Sawtooth wave, SM, SM'...Modulated wave, _SM1 , _SM2 ...First and second amplitude modulated waves.

Claims (1)

[Scope of Claims] 1. A scanning electron microscope and similar devices equipped with a cathode ray tube for projecting a sample image, characterized in that astigmatism is corrected by the following procedure. A method for correcting astigmatism in the device. (a) For each of the sawtooth waves SX and SY synchronized with the sawtooth waves for horizontal and vertical scanning for scanning the charged particle beam 10 in the horizontal and vertical directions, whichever of these two sawtooth waves SX and SY is used. Amplitude modulation is performed by multiplying modulation waves SM and SM' whose amplitude values periodically change at low frequencies. (b) First amplitude modulated wave subjected to the above amplitude modulation
Electrical signal E ₁ for correction of astigmatism in the first direction to SM ₁
The first direction astigmatism correction member 15
At the same time, a second direction astigmatism correction electric signal E 2 is superimposed on the second amplitude modulated wave SM ₂ subjected to the amplitude modulation, and the electrical signal E ₂ is supplied to the second direction astigmatism correction member 16. do. (C) Next, by controlling the values of the electric signals E ₁ and E ₂ for astigmatism correction in the first and second directions,
Adjustments are made so that the astigmatism-corrected image area appearing on the screen of the cathode ray tube 5 changes symmetrically in the left and right and up and down with the center of the screen as the center. (d) The first and second amplitude modulated waves SM ₁ , SM ₂
The above-mentioned first and second direction astigmatism correction member 1
5 and 16 will be stopped. 2. The astigmatism correction image area can periodically expand and expand symmetrically from the center of the screen in the left, right, up and down directions, and then return instantaneously to the center again. . The astigmatism aberration correction method in a scanning electron microscope and similar devices according to claim 1, wherein the sawtooth waves SX and SY are amplitude-modulated by a modulated wave SM. 3 The astigmatism correction image area expands and expands to the periphery of the screen, gradually shrinks and contracts horizontally and vertically symmetrically toward the center of the screen, and then instantaneously expands and expands again to the periphery. A scanning electron microscope and its analogues according to claim 1, wherein the sawtooth waves SX and SY are amplitude-modulated by a modulated wave SM' so that the sawtooth waves SX and SY can be changed in amplitude and returned to each other periodically. A method for correcting astigmatism in the device. 4 Equipped with a cathode ray tube 5 for projecting a sample image, and a deflection member 6 for horizontal scanning of the cathode ray tube 5.
and a horizontal scanning system HS including a horizontal sawtooth wave generator 1 for supplying a horizontal scanning sawtooth wave to the horizontal scanning deflection member 11 in the mirror body;
Vertical scanning including a vertical sawtooth wave generator 2 for supplying vertical scanning sawtooth waves to the vertical scanning deflection member 7 of the cathode ray tube 5 and the vertical scanning deflection member 12 in the mirror body. System VS
In a scanning electron microscope and similar equipment equipped with the above, the sawtooth wave SX synchronized with the horizontal scanning sawtooth wave and the sawtooth wave SY synchronized with the vertical scanning sawtooth wave are lower than either of the sawtooth wave SX synchronized with the vertical scanning sawtooth wave. Modulated wave SM whose amplitude value changes periodically with frequency,
An oscillator 14 for oscillating SM' is provided, and the modulated waves SM, SM' from the oscillator 14 and both the sawtooth waves
A first multiplier 19 that multiplies one sawtooth wave SX of SX and SY, and an output SM ₁ from the first multiplier 19 and a first directional astigmatism correction power source 17
a first adder 20 capable of adding the electric signal _E1 from the oscillator 14 and supplying the added output to the first direction astigmatism correction member 15 in the mirror body, and a first adder 20 capable of adding the electric signal E 1 from the waves SM, SM′ and the other sawtooth wave SY of the above sawtooth waves SX, SY
_A second multiplier ₂₂ for multiplying between the first multiplier 19 and the first adder 20; A scanning electron microscope characterized in that a first switch 25 is interposed, and a second switch 26 is interposed between the second multiplier 22 and the second adder 23. and astigmatism correction devices in similar devices. 5. Disadvantages of the scanning electron microscope and its similar devices according to claim 4, in which the first switch 25 and the second switch 26 are configured to operate in conjunction with each other. Sexual aberration correction device. 6 The first direction astigmatism correction member 15 is composed of four coil parts arranged with a phase difference of 90°, and the second direction astigmatism correction member 16 is comprised of four coil parts arranged with a phase difference of 90 degrees and shifted by 45 degrees from each coil part of the first direction astigmatism correction member 15, An astigmatic aberration correction device for a scanning electron microscope and similar devices thereof according to claim 4.