JPS6016701B2 - Focusing devices in scanning electron microscopes and similar devices - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for focusing a scanning electron microscope and similar devices, and more particularly to a device suitable for automatically focusing.

Generally, in a scanning electron microscope, it is necessary to irradiate a sample with an extremely thin charged particle beam such as an electron beam or an ion beam; In this case, focusing operation is mainly performed.

Various methods have been proposed as conventional focusing methods of this type, ranging from manual methods to automatic methods, but manual methods lack objectivity as they depend largely on the operator's experience. In addition, with automatic means, the frequency component of the detection signal detected by the detection mechanism changes depending on the type of sample and the range of scanning of the sample, so it is not possible to perform appropriate processing. When obtaining focusing information from the high frequency components of the detection signal, it is difficult to obtain accurate information due to the influence of noise in the high frequency region. There is a problem that focusing cannot be performed.

The present invention aims to solve these problems.
In scanning electron microscopes and similar devices that enable accurate focusing by splitting the frequency components of the detection signal and performing signal processing in the optimal frequency range. The purpose is to share a focusing device.

For this reason, the focusing device of the present invention provides a detection system for detecting an image signal obtained from a sample when a charged particle beam is irradiated onto the sample through a focusing lens and a charged particle beam deflector. In a scanning electron microscope or similar device equipped with a mechanism, the detection signal from the detection mechanism is temporally differentiated by a differentiation mechanism, and the focus is adjusted so that the definite integral value of the absolute value of the differential amount shows the maximum value. A focusing control device is provided for controlling a lens for use with a frequency detector, and the focusing control device selects and changes a differential frequency region of the differential mechanism according to a frequency component of a detection signal from the detection mechanism. The differential mechanism is configured as a plurality of differentiators with different differential frequency regions, and the frequency detector operates one of the plurality of differentiators in an optimal differential frequency region. It is characterized in that it is equipped with an optimal differentiator operation control section.

The focusing device of the present invention also includes a detection mechanism for detecting an image signal obtained from the sample when the charged particle beam is irradiated onto the sample through the focusing lens and the charged particle beam deflector. In a scanning electron microscope and similar devices equipped with the above, the detection signal from the side of the detection element is temporally differentiated by a differentiating mechanism, and the focus adjustment is performed so that the definite integral value of the absolute value of the differential amount shows the maximum value. A focusing control device for controlling the lens is provided, and the focusing control device operates a frequency detector to select and change a differential frequency region of the differential mechanism according to a frequency component of a detection signal from the detection mechanism. The frequency detector is configured to select and change the differential frequency range of the differentiator to an optimal range, and is configured to include a time constant control section that can control the time constant of the differentiator. It is characterized by

Hereinafter, a focusing device for a scanning electron microscope and similar devices thereof as an embodiment of the present invention will be explained with reference to the drawings. FIG. 1 shows the entire diagram. Figure 2 is a schematic diagram for explaining the focusing operation by the focusing lens, Figure 3 is a graph showing the characteristics of the control current supplied to the focusing lens, and Figure 4a. is a block diagram showing the differential mechanism and frequency detector, the fourth
Figure b is a block diagram showing the frequency component detector of the frequency detector, Figure 5 is a graph showing the gain characteristics of the differential mechanism, Figure 6 is an explanatory diagram showing the method for determining the definite integral value,
FIGS. 7a to 7e are explanatory diagrams showing various signal waveforms at each focal point position. As shown in Fig. 1, an electron gun 1 is disposed above the mirror body of a scanning electron microscope, and below this electron gun 1 there is a focusing lens 2 and an electron beam deflector as a charged particle beam deflector. A line deflector 3 and a focusing lens 4 are arranged.

Further, a sample insertion section is formed below this focusing lens 4, and a sample 5 is placed in this sample insertion section.

The focusing lens 2, the electron beam deflector 3, and the focusing lens 4 are all constructed as Kasumi Shigeru lenses that perform their respective functions by controlling the current supply to the coils. The alignment lens 4 can change the focal point of the electron beam B as a charged particle beam, as shown in FIG. 2, by supplying a stepped current as shown in FIG. 3 to the lens 4. .

That is, when the supplied current (lens strength) is la shown in FIG. 3, the focal position a is far above the sample 5, resulting in a more over-focus state, and the supplied current is lb as shown in FIG. , lc, the focal position b,
c is located above sample 5 and is in an over-focus state, and the supplied currents are ld and le as shown in Fig. 3.
. . . If you gradually reduce it like ln, the current ld
At the exposure current le...ln, the focus position d becomes a just-focus state where it is located on the surface of the sample 5, and at the exposure current le...ln, the focus position e...n becomes an under-focus state where it is located below the sample 5. Become.

Further, the focusing lens 2, the electron beam deflector 3, and the focusing lens 4 may be configured as electrostatic lenses that control the V supply voltage to the opposing electrode plate and perform the action through each of them. In particular, the focusing gate lens 4 is supplied with a stepwise varying voltage.

Therefore, as shown in FIG. 1, when the electron beam B is emitted from the electron gun 1, it is concentrated and concentrated by the concentration lens 2, and then appropriately deflected in the X-Y direction by the electron beam deflector 3. After that, the light is focused by a focusing lens 4, and then the sample 5 is irradiated.

When the electron beam B is irradiated onto the sample 5 through the focusing lens 4 and the electron beam deflector 3 in this way, a video signal obtained from the sample 5, that is, electrons reflected from above the sample 5 and electrons transmitted through the sample 5, is generated. , the electrons flowing through sample 5 and leaving to the ground, and 2
Next electrons are generated, but in order to detect these electrons,
A detection mechanism 6 is arranged near the sample 5.

By the way, it is known that when the type of sample 5 or the range in which the sample 5 is scanned changes, the frequency component of the detection signal detected by the detection mechanism 6 also changes. Differential frequency region of the mechanism (refers to the frequency region in which proper differential calculations can be performed).

) must be selected and changed as appropriate depending on the type of sample 5, etc. For this reason, the detection mechanism 6 includes a differentiation mechanism 7 that temporally differentiates the detection signal from the detection mechanism 6 according to its frequency component, and a differentiation mechanism 7 that is connected to the detection mechanism 6 via a switch SW to generate the detection signal. A frequency detector 8 that analyzes the frequency components of the differential mechanism 7 and selectively changes the differential frequency region of the differential mechanism 7 according to the analysis result is connected in parallel with each other.

That is, this differential mechanism 7, as shown in FIG. 4a,
Four differentiators 7a to 7b with different differential frequency regions and a switch 7 connected to the input end of each differentiator 7a to 7d are connected to
7d', and the gain characteristics C, to G4 of each differentiator have such characteristics that the cutoff frequency is sequentially shifted to the higher frequency side, as shown in FIG. As a result, the differentiator 7a having the gain characteristic C is suitable for differentiating a detection signal whose main component is a low frequency. It can be seen that it is suitable for differentiating detection signals whose main components are low-medium frequencies, middle-high frequencies, and high frequencies. As shown in FIG. 5, each gain characteristic C, ~G4 has a characteristic in which the gain decreases as the frequency increases in the high frequency region, so that the detection mechanism 6 is connected to any of the differentiators 7a ~ 7d. Even if a detection signal from the sensor is input, high frequency noise included in the detection signal is attenuated and removed.

Further, as shown in FIG. 4a, the frequency detector 8 includes a frequency component detection section 8a that analyzes and detects the frequency component of the detection signal input from the detection mechanism 6 via the switch SW, and a frequency component detection section 8a that analyzes and detects the frequency component of the detection signal input from the detection mechanism 6 via the switch SW. Upon receiving the signal from the component detection section 8a, the switches 7a' to 7d' connected to the differentiators 7a to 7d located in the optimal differential frequency region are closed to operate the differentiators 7a to 7d located in the optimal differential frequency region. and an optimum differentiator operation control section 8b. That is, as shown in FIG. 4b, the frequency component detection unit 8a receives the detection signal from the detection mechanism 6 and has a cutoff frequency of about several tens of kratios for removing high frequency noise components contained therein. low-pass filter 8a, and low-pass filter 8
A high-pass filter 8a2 is connected to the high-pass filter 8a2, which has a cutoff frequency of about IH2 to cut the output DC component from this circuit 8a, and this high-pass filter is connected to the reference level (0 volt line) among the 8 outputs. It is provided with a Schmitt trigger circuit 8a3 which outputs a pulse train corresponding to the output frequency of the pass filter 8a2 and outputs a pulse train corresponding to the output frequency of the pass filter 8a2.
A counter 8a4 that counts the number of pulses from the Schmitt trigger circuit 8a3 and a digital-to-analog converter 8 that converts the digital signal from the counter 8a4 into an analog signal (hereinafter referred to as a D/A converter).

) and an analog peak memory 8 which can store and output the largest signal from the D/A converter 8. By the way, as mentioned above, since the frequency component of the detection signal differs depending on the type of sample 5, etc., it is necessary to determine the optimum differentiator for the sample 5 in advance before performing the focusing operation.

That is, in order to determine the optimal differentiator in this way, it is necessary to adjust the focusing lens 4 before performing the focusing operation.
It is necessary to supply a stepped current that can cover from a more focused state to an under focused state (a smaller step width is sufficient for this current than that shown in Fig. 3), and for this reason. A sampling circuit SP is provided. This sampling circuit SP is
It is configured to be able to supply a current having the above-mentioned rough step width to the focusing lens 4, and also to be able to reset the counter 8a4.

Note that this sampling circuit SP operates in conjunction with the opening and closing of the switch SW. Therefore, before performing a focusing operation, close the switch SW and operate the sampling circuit SP to supply a current with a rough step width to the end point alignment lens 4, and a detection signal will be detected for each current determined by each step width. However, these detection signals are sequentially input to the frequency component detection section 8a, and after noise etc. are removed by the filters 8a, 8a2, the signals corresponding to the frequencies of each detection signal are input to the Schmitt trigger circuit 8a3. After each pulse train is converted into a pulse train having a certain frequency, each pulse train corresponding to a new detection signal is counted by the counter 8a4 while the count is reset every time a pulse train corresponding to a new detection signal is received. 8 is outputted as a DC signal proportional to the frequency of the detection signal having the maximum frequency among these detection signals. Also,
The optimal differentiator operation control section 8b includes an analog peak memory 8
It consists of four groups of comparators connected together, and a different reference signal is input to each comparator, which causes one of the comparators to operate in response to the signal from the analog peak memory 8a6. Accordingly, one of the switches 7a' to 7d' closes. Note that these comparators can continue to operate even after the switch SW is turned off, making it possible to keep the switch with the optimal differentiator closed. There is.

Therefore, when the frequency component detecting section 8a detects that the frequency components of the detection signal are mainly low frequency components as a result of operating the sampling circuit SP for a certain sample 5, the optimal differentiator operation control is performed. The section 8b receives the information from the frequency component detection section 8a and switches the switch 7a'.
, and the differentiator 7a having a low frequency differential frequency region can be operated.

By the way, an absolute value circuit 9 is connected to the output side of the differentiation mechanism 7, and a definite integral value of the output signal of this register 9 over a certain period of time can be calculated at the output side of the absolute value circuit 9. An area calculation circuit 10 is connected.

This absolute value circuit 9 is composed of a so-called full-wave rectifier circuit, and the area calculation circuit 10 is configured to perform a predetermined sampling time Δt for the analog absolute value signal from the absolute value circuit 9.
The circuit includes an analog-to-digital conversion section that performs analog-to-digital conversion at each time, and a quadrature section that calculates the sum of the outputs from the analog-to-digital conversion section over a fixed time T.

Therefore, if the quadrature method is diagrammed, it will be as shown in FIG. 6, and this area calculation circuit 10 employs the piecewise quadrature method.

Note that the quadrature accuracy is determined by the sampling time Δt and the like.

Here, regarding the cases where the focusing state by the focusing lens 4 is under focus, just focus, over focus, and more over focus, the beam flea shape of the electron beam B, the detection mechanism 6
The waveform diagrams of the detection signal from the differential mechanism 7, the differential output from the differentiating mechanism 7, the absolute value output from the absolute value circuit 9, and the area signal from the area calculation circuit 10 are shown in FIG. T a to e.

As can be seen from Figure T e, the area signal is at its maximum value in the just-focus state.

By the way, as mentioned above, when the current supplied to the focusing lens 4 is controlled in stages from la to ln (see Figure 3), the focusing state gradually changes from more over-focused to under-focused. The state shifts to a focused state, but if you memorize the maximum value of the area signal at this time, you should know that the supply current value ld at that time is in a just focused state.

Therefore, in this device, "on the output side of the area calculation circuit 10,
A maximum value detection circuit 11 is connected to detect and store the maximum value of the signal from the circuit 10, and the maximum value detection circuit 11 receives the signal from the circuit 11 and calculates the maximum current ld. A lens intensity control circuit 12 is connected to supply the lens to the focusing lens 4.

Therefore, these differentiating mechanism 7, frequency detector 8, absolute value circuit 9, area calculation circuit 10 and maximum value detection circuit 1
The detection signal from the detection mechanism 6 is temporally differentiated by a factor 1, and the intensity of the focusing lens 4 is controlled via the lens intensity control circuit 12 so that the definite integral value of the absolute value of this differential amount shows the maximum value. A focusing control device F is configured.

In FIG. 1, illustrations of a sawtooth wave power source for scanning the electron beam B, a cathode ray tube as a display section, etc. are omitted. With the above-described configuration, in order to automatically perform focusing, the optimum differentiator for the sample 5 is first determined.

That is, by closing the switch SW to operate the frequency detector 8 and at the same time operating the sampling circuit SP, a stepped current having a rough step width is competitively supplied to the focusing lens 4. As a result, the focal position of the electron beam B gradually moves from an over-focused state to an under-focused state, and the frequency components of the detection signals corresponding to each step detected at this time are detected by the frequency detector 8. Closing a switch equipped with a differentiator that has a differential frequency range suitable for each detection signal with the highest frequency among these detection signals (a detection signal obtained in a state of almost just focus) as a reference. will be carried out.

After determining the optimal differentiator for sample 5 in this way, after turning off the switch SW, the current with a fine step width as shown in Fig. 3 is changed stepwise at regular intervals. The electron beam B is then supplied to the focusing lens 4, so that the focal position of the electron beam B becomes the second focal position.
Move from top to bottom as shown in the figure.

At this time, the electron beam B is repeatedly scanned over a predetermined area on the sample 5 by the electron beam deflector 3. In this way, when the electron beam B is irradiated onto the sample 5, the video signal is
It is detected by a detection mechanism 6 shown in the figure.

Next, the detection signal from the detection mechanism 6 enters the optimal differentiator selected by the operation described above and is differentiated.

In this way, the negative part of the detection signal differentiated by the differentiator having the optimum differential frequency region is inverted in the absolute value circuit 9, and then this absolute value signal is processed in the area calculation circuit 10 to maintain its constant value. The definite integral value in time is determined.

As described above, the area signal reaches its maximum value in the just-focus state, and this maximum value is stored in the maximum value detection circuit 11.

Then, the lens strength of the total point matching lens 4 is set from la to ln.
After the change has been completed, a current of ld is again supplied to the focusing lens 4 from the current control circuit 12 based on the signal from the maximum value detection circuit 11.

This means that focusing has been performed automatically. Note that it is of course possible to configure the optimal differentiator operation control section 8b with a group of digital type comparators instead of configuring it with a group of analog type comparators as in the above-mentioned embodiment, and in this case, frequency component detection is possible. The D/A converter 8 of the section 8a is also unnecessary, and instead of the analog peak memory 8, a digital peak memory may be directly connected to the counter 8a4.

Moreover, the number of differentiators 7a to 7d that constitute the differentiating mechanism 7 is as follows:
The number is not limited to four, and may be two, three, or five or more.

It goes without saying that when the number of differentiators is changed in this way, the number of switches, comparators, etc. is also changed.
Furthermore, instead of configuring the differentiating mechanism 7 with a plurality of differentiators 7a to 7d with different differential frequency regions as in the above embodiment, one variable time constant differentiator 7e (see FIG. 8) is used.
It can also be composed of

In this case, the frequency detector 8' is as shown in FIG.
A frequency component detection unit 8'a separates and detects the frequency components of the detection signal from the detection mechanism 6, and a differentiator 7e receives the signal from the frequency component detection unit 8'a to optimize the differential frequency region. and a time constant control section 8'c that can control the time constant of the differentiator 7e so that the time constant of the differentiator 7e is within the range.

Here, the frequency component detection section 8'a may be the same circuit as the frequency component detection section 8a used in the above embodiment, and the time constant control section 8'c may be a differentiator as shown in FIG. Resistance control means using FET 13 forming a part of 7e is adopted.

In other words, the output from the analog peak memory 8 shown in FIG. , since the resistance R between the drain terminal D and the source terminal S of the FET 13 changes from several ohms to several hundred ohms, the time constant of the differentiator 7e becomes finer, thereby changing the differential frequency range of the differentiator 7e. It can be done. Specifically, the larger the absolute value of the output value from the analog peak memory 8, the more the detection signal whose main component is a low frequency is passed through for differentiation. , it is suitable for differentiating a detection signal with a higher frequency principal component.

Also, if you want to soft control the time constant mentioned above,
Methods such as spectral analysis outside the scope and determination of a time constant based on the results of this analysis are being explored. In this case, it is easier to control the time constant of the differentiator 7e if it is of a digital filter type. Furthermore, even if the scanning area is changed on the same sample, the frequency components of the detection signal from the detection mechanism 6 will change, but in this case as well, before performing the focusing operation in the frequency detection units 8 and 8', Since the frequency component is detected and a differentiator having an optimal differential frequency region is operated, appropriate signal processing can be performed. Note that instead of changing the lens strength from an over-focus state to an under-focus state as in the previous embodiment, the lens strength is changed from an under-focus state to an over-focus state. You can change it.

As described above in detail, according to the focusing device of the scanning electron microscope and similar devices of the present invention, when a charged particle beam is irradiated onto a sample through a focusing lens and a charged particle beam deflector, , in a scanning electron microscope and similar devices equipped with a detection mechanism for detecting an image signal obtained from the sample, the detection signal from the detection mechanism is temporally differentiated by a differentiation mechanism, and the absolute value of the differential amount is calculated. a focusing control device that controls the focusing lens so that a definite integral value of indicates a maximum value; The differential mechanism is configured to include a frequency detector to select and change a differential frequency region, and the differential mechanism is configured as a plurality of differentiators having different differential frequency regions, and the frequency detector is configured to select and change a differential frequency region of the differential mechanism. It is equipped with an optimal differentiator operation control unit to operate the differentiator in the optimal differential frequency range, so that the charged particle beam is generated when the sample is irradiated and detected by the detection mechanism. The detection signal as a video signal is
It is differentiated and processed by a differentiator with an optimal differential frequency range.

Further, according to the scanning electron microscope and its similar device of the present invention, the detection signal from the detection mechanism is temporally differentiated by the differentiation mechanism, and the above-mentioned A focusing control device for controlling a focusing lens is provided, and the focusing control device selects and changes a differential frequency region of the differential mechanism in accordance with a frequency component of a detection signal from the detection mechanism. The frequency detector is configured to include a time constant control section capable of controlling a time constant of the differential mechanism in order to select and change a differential frequency region of the differential mechanism to an optimal range. Therefore, the detection signal is processed by the differentiator whose time constant is adjusted by the time constant control section and the differential frequency domain is adjusted to an optimal state.

In this way, signal processing can be performed in the optimal frequency domain, which ensures accurate focusing, and the detection signal can be processed while maintaining sufficient gain, thereby reducing the signal-to-noise ratio. It can also be significantly improved.

[Brief explanation of the drawing]

The figures show a focusing device in a scanning electron microscope and similar devices as an embodiment of the present invention. Fig. 1 is an overall block diagram thereof, and Fig. 2 shows a focusing operation using the focusing lens. Eishiki diagram for explanation,
Figure 3 is a graph showing the characteristics of the control current supplied to the focusing lens, Figure 4a is a block diagram showing the differential mechanism and frequency detector, and Figure 4b is the frequency detector. A block diagram showing the frequency component detection section, FIG. 5 is a graph showing the gain characteristics of the differential mechanism, FIG. 6 is an explanatory diagram showing the method for determining the definite integral value, and FIGS. FIG. 8 is an explanatory diagram showing various signal waveforms at the focal position, and FIGS. 8 and 9 show modified examples of the differential mechanism and frequency detector. FIG. 8 is a block diagram thereof, and FIG. 9 is a main part thereof. FIG. DESCRIPTION OF SYMBOLS 1... Electron gun, 2... Shuto lens, 3... Electron beam deflector as a charged particle beam deflector, 4... Lens for total point alignment, 5... Sample, 6... Detection mechanism, 7...Foundation of differentiator, 7
a to 7d... Differentiator, 7e... Variable time constant differentiator, 8, 8'... Frequency detection section, 8a, 8'a... Frequency component detection section, 8a. ...Low pass filter, 8a
2... High pass filter, 8a3... Schmitt trigger circuit, 8a4... Counter, 8a5... D/A converter, 84... Analog peak memory, 8b... Optimal differentiator operation control section, 8' c... time constant control section, 9...
Absolute value circuit, 10... Area calculation circuit, 11... Maximum value detection circuit, 12... Lens strength control circuit, 13... FET, B
. . . Electron beam as a charged particle beam, F . Figure 1 Figure 2 Figure 3 Figure 4 [o] Figure 4 tb1 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9

Claims (1)

[Claims] 1. A scanning device comprising a detection mechanism for detecting an image signal obtained from a sample when a charged particle beam is irradiated onto the sample through a focusing lens and a charged particle beam deflector. In a type electron microscope and similar devices, the detection signal from the detection mechanism is temporally differentiated by a differentiation mechanism, and the focusing lens is controlled so that a definite integral value of the absolute value of the differential amount shows a maximum value. A focusing control device is provided, and the focusing control device is configured to include a frequency detector to select and change a differential frequency region of the differential mechanism according to a frequency component of a detection signal from the detection mechanism. The differentiating mechanism is configured as a plurality of differentiators having different differential frequency regions, and the frequency detector is configured to operate an optimal differentiator in order to operate one of the plurality of differentiating devices in an optimal differential frequency region. 1. A focusing device for a scanning electron microscope and similar devices, characterized in that it includes a control section. 2 Scanning electron microscopes and similar devices equipped with a detection mechanism for detecting video signals obtained from the sample when the sample is irradiated with a charged particle beam through a focusing lens and a charged particle beam deflector. The apparatus includes a focusing control device that temporally differentiates the detection signal from the detection mechanism using a differentiation mechanism and controls the focusing lens so that a definite integral value of the absolute value of the differential amount shows a maximum value. The focusing control device is configured to include a frequency detector to select and change the differential frequency region of the differential mechanism according to the frequency component of the detection signal from the detection mechanism, in a scanning electron microscope and similar devices, characterized in that the device is configured to include a time constant control section capable of controlling the time constant of the differential mechanism in order to change the selection to the differential frequency region of the differential mechanism. Focusing device. 3. A focusing device in a scanning electron microscope and its similar devices according to claim 2, wherein the differential mechanism has a characteristic in which the gain decreases as the frequency increases in a high frequency region of the gain characteristic. .