JPS5946745A - Automatic focal point aligning unit for charged particle beam device - Google Patents

Abstract

PURPOSE:To perform correct focal point alignment even when astigmatism is not accurately corrected by presetting the strength of a focusing lens so that the conversion signal of beam diameter on the surface of a sample can be set to a specified value. CONSTITUTION:The surface of a sample 5 is irradiated with a particle beam 2 generated from a charged particle beam source 1 so that a small beam diameter can be formed on the surface of the sample 5 by a focusing lens 4 and then is scanned. A scanning picture is displayed on a picture display means 7 that synchronizes with this scanning using a picture signal detected from the sample 5. Such a device is provided with a means that obtains a conversion signal corresponding to the beam diameter on the surface of the sample 5 of the charged beam 2 based on the picture signal. In addition, the device is provided with a means that fixes the lens strength L1 and L2 of the focusing lens 4 by which a conversion signal lower by a specified amount than the maximum value of the said conversion signal that can be obtained when the lens strength of the focusing lens 4 is varied sequentially and a means that sets the lens strength of the focusing lens 4 to (L1+L2)/2.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in automatic focusing devices used for charged particle beam protection in scanning electron microscopes and the like.

In a scanning electron microscope, it is shown in Figure 1. The electron beam 2 generated from the electron gun 1 is focused onto the surface of the test stone 5 by a concentrating lens 3 to form a minute cross-sectional diameter, and at the same time the electron beam 2 is Deflect line 2 21 il 6x, 6
The sample surface 1- is scanned with fﾾ＾＾ﾆJ to J, which is supplied to Y, and from the sample O'≧1 at Brown 9';7, detected by the detector 8, J: is detected. By supplying the video signal amplified by the amplifier 9 as a brightness modulation signal, a scanned image of the specimen is displayed.

The image magnification of this test N'+1 sonic image was the deflection coil 6X.

Deflection of 6" Y-no-noun tube: 1"10X; Water 1 (scanning signal 1-1) between IOY and the output terminal of the scanning power supply 11 which supplies the vertical scanning signal V and the deflection "Ill 6X, 6" By changing the width of the inserted IIJ variable width scale 12, the area ratio of the electron beam scanning area on the sample surface and the cathode ray tube screen similar to the area can be changed. By changing the image magnification, the image magnification is adjusted.

Focusing of the scanning image displayed on the cathode ray tube screen is done by adjusting the output of the excitation power supply 13 of the focusing lens, especially the final focusing lens (objective lens) 4, to minimize the cross-sectional diameter of the electron beam (pulled onto the sample surface). 7) -U
However, since this adjustment is based on the naked eye observation of the sample scanned image, automatic focusing devices have recently been put into practical use to automate this adjustment. In Fig. 1, the rib block indicated by 1/I has an automatic focusing circuit C' for performing such automatic focusing, including U, and uses the vertical scanning signal from the scanning power supply 11 as a trigger signal. The appropriate output current of the excitation power source 13 is determined based on the results of monitoring the output of the amplifier 9 while sequentially changing the output of the excitation power source 13.

Figures 2 and 3 are for explaining the principle of the automatic focusing U circuit 1/I. Figure 2 (a) shows an IM i with a certain width on the sample surface viewed from the optical axis direction. In the direction of blue 15 and arrow 16 that crosses the structure (the cross-sectional shape of the electric r
l! i! Shows how 17 is horizontally scanned.il
Figure 2(b)(:'
d Uro 0. Fig. 3 (Jl) shows the same scanning as in Fig. 2 (a) performed using an electron beam 4i, -, 1 (j1 with a diameter of The signal waveform obtained by J is shown in Figure 33 (1)).From these figures, it can be seen that the electron beam cross-section of the sample 1'il is small, and the signal waveform is It can be seen that the height of the peak waveform is high and the width of the peak waveform is narrow and sharp.
The iJ system utilizes the phenomenon shown in A1, and the signal detected by the trial run of the electric F wire is converted into a differentiator circuit A) and a filter circuit. Take out only the high frequency components using (), and consider the high frequency component Beak's ratio or its top (9) as a signal corresponding to the cross-sectional diameter of the electron beam, and calculate the peak value or top of the peak n l + rJ to be at most 4T. In most cases, the objective lens is excitation (salmon is set).

Figure 4+31 represents the cross-sectional diameter of the electron beam in response to changes in the object lens current, and shows how the converted signal changes over the peak 1n or integrated value. When the relationship 111 is weakly expressed, the conversion 111 indicates a maximum of 1 focal length, and the objective lens current is J3, and the focal point is out of focus.

By the way, in the electron field of a scanning electron microscope, there is generally astigmatism.
Kome's xy 3', j non-Jj1 correction station 100 is combined J: Redeira. Adjust this astigmatism correction device correctly: 1
If the influence of 1-Pl, L, I') 2 is not removed, the relationship between the objective lens current and the converted signal will be as shown in Fig. 55. The maximum lil′t of the η conversion signal that is d3 in any of the current values
/+': The cow will be destroyed. However, this I
+) 1. Even if you set it to 11) or 2, the correct focus cannot be obtained, and the conventional automatic focusing system (U4D'5) (unprecedented astigmatism correction is performed) +IL
/ <tit! There were often serious injuries.

The present invention has a positive astigmatism correction (A11 (correct focusing even when not present, P) L!, and a stigma meter (J4>) in addition to the objective lens.
Focusing in the strict sense of the word I! The particle beam generated from the target and the previous electric particle beam was irradiated with a Toei lens so that a beam diameter of 1 cm was formed on each surface. , a certain area (11 planes) is scanned by the deflection means, and in synchronization with the scanning, the lens strength of the focusing lens is changed, and the image signal from the sample is used to detect the particle beam. On the sample surface (
) Detects a conversion signal that corresponds to the lens diameter, and detects a conversion signal that corresponds to the maximum value of the conversion signal, and detects the two lens intensities (1, -1, 1- The feature is to set it to () of (゛).

Hereinafter, 161 principles and embodiments/techniques of the present invention will be explained based on the drawings.

The cause of astigmatism J<E is shown in Fig. 6. The focus of the electrician's lens 1j: (the direction v'-' where the distance li1.It is orthogonal)
Ii/, 1a) There is so much. In the same figure (at 13, x, y
When the main surface of the lens passes through the intersection of the axes, j3, a focal line C is formed on the focal plane in the ! j1 random circle 1) is formed. This point D corresponds to the focal position after astigmatism has been compensated for. The interval of C is the so-called astigmatic interval.
It is.

Now, as shown in Fig. 7(a), the lens astigmatism expressed by the angle α with respect to the x-axis is Δ1. Consider using a supplementary IF for a y-type 'Jl fish correction device). In the figure, SX is a quadrupole lens located on x and yIIll indicated by ○.
y is rotated 45 degrees with the X and y axes! (Table 1 shows the astigmatism correction patterns 1 to 1 caused by J to the other /l 0 lances set at the indicated position.) This table makes it easy to consider the relationship between these patterns. In order to achieve this, all angles that each vector makes with the X axis are doubled, and Δ[-1S×, Sy is obtained, as shown in FIG. 7 (1)).

In Fig. 7(1)), ΔF is Δ)2× and Δ“y××
edge in the y-axis direction], and Δ' is expressed by the following equation 6.

AF = J Moromi T Yumata, minimum 1 (the diameter δ of one random circle is proportional to the astigmatism difference △1-, 5; k・ΔF −−〜−−・−1−・−−−−(1)
bawl. Here, K is a coefficient related to the hearing angle α of the electron beam with respect to the sample.

△[' in Fig. 7(C) indicates 2 when the astigmatism correction device is avoided.
′! , the i11 point vector is expressed by the following equation:

ΔL'・F shoulder;
From equation (1), the radius of the circle of least confusion in this case is as follows.

c'=yJ7 Ekaino-J mother---(3):J
From ν j s−1, astigmatism Z compensation 11 in the state of minimum 111 random circles
-Scan the two silk/I station lenses of the four parts at 1°f, respectively, and δ
′ is the minimum hit in order.
``) 0, that is, δ', and the astigmatism is incomplete.

By the way, the 0th rero J o [objective current 1+1i +-1
)l, 1. P2 corresponds to the focal line C, l-, which corresponds to the focal line C, l-, so that a minimum of 111 random circles 1) are formed. In order to achieve this, the value of the objective current 1 is set to L1 and 2 λ, but the method for determining this optimum value is shown in FIG. 8 and based on the apparatus according to the embodiment of the present invention. explain.

In FIG. 8, the same reference numeral 71 as in FIG. 1 represents the same component. In the mounting shown in Fig. 8, an xy-type astigmatism correction device consisting of two pairs of quadrupole lenses 15x and 15y and a 1G astigmatism correction power supply that supplies an excitation current to them is incorporated into the J3. , the current value to each 44fi lens is adjusted by the output of the current control circuit 17. Further, the excitation power source 13 for the objective lens is also adjusted by the output of the current control circuit 18. These current control circuits 17 and 18 are controlled by the output of the Sudezogiri irregular path 19. Secondary electrons, etc. generated from the sample 5 are detected by the detector 8 and sent to the camera 8 via the amplifier 9.
The signal level of the video signal is within a predetermined range, and the signal level of the video signal is within a predetermined range. A part of the output of the automatic brightness controller 1 to the last adjustment circuit 2o is used as a brightness modulation signal of the brightness tube 7.
(Supplied. The step variable circuit 19 receives input 2S and 1
Immediately after 194 imaging 5-), a conversion signal representing the surface diameter of the electron beam ttJi corresponding to the intensity signal of the high frequency component is generated, and the conversion signal is at maximum 11r1 and 4[J: sea urchin output. The control signal is increased or decreased in steps. Central control circuit 2
1 is a vertical loop (Pi It"i) as a timing signal) and then two current adjustment circuits 17, 1ε 3. Step angle changing circuit 1
9 and switches Si', S2.33.

Now, suppose that the relationship between the objective lens current and the conversion 6 is expressed by a waveform as shown in FIG. 1 due to the influence of astigmatism without using the astigmatism correction device. In this case, the device shown in FIG.
5 When the automatic focusing device is activated, the central control circuit 21 first sets the switches S1 and S2 to the A state, and the output of the current adjustment circuit 17 causes the 41mm lens 15x to be adjusted.
, 15y is set to zero. Next switch 5
3 is switched to the terminal a side, and the output of the step variable circuit 19 is supplied to the current adjustment circuit 18, and the step 1 current is set to a low current level h as shown in FIG. 10. It will increase sequentially. The time width t(, 1) of this Sdenno change corresponds to the period of vertical scanning (13), and the current change width Δ11. Set.

In the case of Fig. 10, the objective 1 nons current 11'i increases from the initial low value with a large change width Δ■1, and changes (when the β signal exceeds the predetermined L level, the current value changes to level 1). is set, and while increasing the current with a new small change width △I2, find the maximum value of the variable signal that is always 19, and set it by 2.
The value S of 0% reduction is memorized 51 times, and the current is increased rapidly until a conversion signal equal to the value S is 19 times (9th step).
The figure shows one shift of P to S after the conversion signal reaches its actual maximum value P. 〉In this way, when the lens current L2 corresponding to the level S is reached, the step variable circuit'
19t Lens current 1. , 2 (or the corresponding J signal), at the same time, return the lens current to l s again and convert S (increase the current by a minute width until i is obtained). Lens current (1 (or 1 for each)
find the value of the corresponding signal). Next, the switch L219 calculates each value and supplies the control signal to the current ÷ 11J adjustment circuit 18 so that the output current of 2 is obtained. R1, follow °η the device j' in Figure 8? 13-4 is a state in which astigmatism is evident, and the electric field f is in the state of the circle of least confusion b.
- It is possible to perform automatic focusing with high reliability when focusing the beam.

As described above, when the focusing l in the apparatus shown in Fig. ε3 is completed in step 1, the central control circuit 21 switches the
3 to the 1) side terminal, and switch S1 to A state.
/Start the operation to find the optimum excitation current intensity for the l-pole lens 15x1! Ru. /I pole lens 15 x G; L 15
When the IjlIIIi & current to y is continuously varied, the transformation change has a waveform similar to that shown in FIG. +/J Cao et al. sequentially increase in a small width in the shape of a block, and the conversion signal detected at the 8 block is the maximum Detect 1, memorize this value, and fix the excitation current to 1~1 nons 15x at this point.
Firstly, the control signal from the medium heat control circuit 19 is sent to switch $2.
to A shape (71-U/l Iij lens 16yl
The excitation current of N is increased sequentially in a straight-striped manner in the same manner as in the case of the 15x quadrupole lens, and the conversion fJ shows the maximum value.
Detects the excitation current and holds its value.

In this way, the xy-type point aberration corrector is adjusted while keeping the electron beam within the circle of least confusion, so astigmatism caused by the objective lens and other optical systems can be corrected accurately. It will be done. Also, in this state 'c4: What is the relationship between the L objective lens current value and the conversion signal? The waveform that has two peak values of J in Figure 5 changes to the waveform that has a peak value of 41 A as shown in Figure 4. (Therefore, the medium heat control circuit 21 is set to switch S.
3 to the FU and "a 111+1 end" to perform the second focusing adjustment to one step variable circuit.

This two-step focusing adjustment is equivalent to the first method described above, but it is good to have a focusing U of -b. It is a type that allows you to set the single-object lens current tl, so C is also acceptable.

As described above, according to the apparatus of the embodiment shown in FIG. Optimal system for lees microscopy 1
'Necessary for use in 1': i: ;IL! J Seiichi 1@
fl is significantly reduced compared to the conventional case.

Note that the present invention is limited to the embodiment 'A flJ shown in FIG.
Adjust the astigmatism correction again at tν when the focusing adjustment of i+ 11 is completed. It is also possible to omit or omit the circuit 20 without installing it.Alternatively, it is possible to install a number of adjustment circuits for adjusting the aberration compensation jl-equipment. Even if the device is equipped with only an adjustment mechanism for the focus, it is still necessary to use a conventional focusing device to achieve accurate focusing.Also,
What is the conversion signal of the real edict? The maximum value of the l:li frequency component included in the signal was used, and the LL Kozue Azusa 110 was used, but focusing on the fact that the peak field of the video signal becomes high at a jl-sh focal point, the single pole of the video in@ It is also possible to use the integrated value 1111 for a certain period of time to calculate the change to 1 to 1 or negative bias.Furthermore, the step-up variable turn V819 changes the excitation current to the objective lens or the L quadrupole lens in steps. However, it is also easy to adopt a system that increases and decreases continuously.

J, uni, the present invention, J, le, an electron microscope,
Ian Microanoriser 1, focusing (! operation) of charged particle beam equipment that uses a finely focused charged particle beam, such as an electron beam exposure equipment, can be automated with high reliability. If possible, it would have a great effect on the operation of charged particle beam devices.

[Brief explanation of drawings]

Figure 1 shows the conventional automatic focusing i! I got one piece of equipment [
Figures 2 to 1 are schematic diagrams showing the scanning electron microscope. Figures 2 to 1 are schematic diagrams for explaining the principle of automatic focusing U. Figures 5 to 7 are diagrams for explaining the effects of astigmatism and its correction method. FIG. 8 is a schematic diagram illustrating the operation of an embodiment of the present invention, and FIGS. 9 and 10 are schematic diagrams illustrating the operation of the apparatus of FIG. 8. 1: Electron gun, 3, 4: Focusing lens, 8: Detection j! :S,
11: Scanning power supply, 12: Variable amplifier, 13: Excitation {public power supply, 14: Automatic focusing circuit, 1! iX,'15y:l
141 lens, 16: Astigmatism ■circuit, 17.1E3=current 1. ■Adjustment circuit, 19: Variation circuit for SDP, 20: Automatic pyroxene-1-1-las {・Adjustment circuit, 21: Central control circuit. Special γ1 Applicant Stock Meeting 121 Electronic Dekunics Representative Tai [Tl Shinf) Moxibustion I Word Name

Claims (1)

[Claims] A particle beam generated from a 1.4Xl electric particle beam source is scanned over the sample surface with a silent lens so that a minute beam diameter is formed on one surface of the sample using a focusing lens, and an image is generated in synchronization with the scanning. At d3, test 1 of the particle beam is displayed on a device that displays a scanned image using the detected video signal of the sample on the display means.
The maximum ratio of the converted signal obtained by means of calculating a converted signal corresponding to the beam diameter in the +1 plane based on the video signal and by sequentially changing the lens strength of the focusing lens. , the lens strength 11J-(+-1,12) of the focusing lens to which a predetermined lower conversion signal is 1q
and the lens strength of the focusing lens L'
A charged particle beam device is installed at 1 ± L2, and is equipped with automatic focusing, 1 device. 2. Conversion corresponding to the beam diameter of means for obtaining a signal based on the video signal;
Incorporating a circuit for automatically setting the level and contrast of the video signal in the predetermined range Δ, DA'17+a'1 The electric particle beam device according to claim '1 is automatically set to J31J. Focus! Device. 3. A particle beam generated from a charged particle beam source is passed through a focusing lens and an xy-type astigmatism correction device onto the sample surface. At the same time, the sample surface is scanned by the particle beam, and detected from the sample by the image display means synchronized with the scanning. A device (1') for displaying a scanned image by supplying an image with two images, converts a conversion signal corresponding to the beam diameter of the particle beam onto the sample surface into the image. The means for obtaining 'C' based on the signal sequentially changes the lens 'intensity of the focusing lens such that a conversion signal that is 1t) less than the maximum value of the conversion signal obtained when (L.1.12
) and the lens strength of the focusing lens as L
1 + L2, J 1st step, x y type astigmatism correction is recommended (Pull two correction signal bullets 11)
The automatic focusing device is equipped with astigmatism correction control means for sequentially changing the conversion signal to set the maximum value of the conversion signal. .