JPS6324617Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for focusing in scanning electron microscopes and similar devices.

Generally, in scanning electron microscopes, etc., it is necessary to irradiate a sample with an extremely thin charged particle beam such as an electron beam or ion beam; Focusing operations are mainly performed.

In this way, focusing is essential for scanning electron microscopes, etc., and how quickly and accurately this focusing operation is performed is a barometer for evaluating the performance of scanning electron microscopes, etc. It also becomes.

By the way, conventionally, when performing this type of focusing, the current value to the focusing lens (objective lens) is changed in steps, and the detection signal obtained at each step is subjected to signal processing such as differentiation. Finally, the current value corresponding to the maximum step among these is supplied again to the objective lens.

Conventionally, when supplying this stepwise changing current to the objective lens, a means was used to scan the electron beam over the entire scanning range on the sample and change the current value in a stepwise manner accordingly.
A method is used in which a cathode ray tube for projecting a sample image is placed in a blanking state, and the current value is changed in steps while repeatedly scanning the same line on the sample surface with an electron beam.

However, these conventional means have the following problems. First, with the former method, since the electron beam scans a different line on the sample surface at each step, there is a problem that correct focusing cannot be achieved if the density on the sample surface changes rapidly. In the latter method, the cathode ray tube is kept in a blanking state while searching for the focal position, so there is a problem that the sample image cannot be observed during that time.

This invention attempts to solve these problems, and while observing the sample image on a cathode ray tube,
It is an object of the present invention to provide a focusing device for a scanning electron microscope and similar devices that enables quick and accurate focusing.

Therefore, in scanning electron microscopes and similar devices, the present invention has developed an image signal obtained from the sample surface when a charged particle beam is irradiated onto the sample surface through a focusing lens and a charged particle beam deflector. A detection mechanism for detecting the image, a cathode ray tube that receives a signal from the detection mechanism and displays a sample image, and a focus accuracy detection circuit that detects the degree of focus accuracy based on the signal from the detection mechanism; a charged particle beam scanning control device that controls a charged particle beam scanning signal to cause a horizontal retrace line blanked on the cathode ray tube to be scanned on the same scanning line on the sample surface; The present invention is characterized in that a focusing control device is provided that controls the focusing lens based on a signal from the accuracy detection circuit.

Below, a focusing device in a scanning electron microscope and similar devices as an embodiment of the present invention will be explained with reference to the drawings. FIG. 1 is an overall configuration diagram thereof, and FIG. The circuit diagram and FIGS. 3a to 3e are waveform diagrams for explaining the operation, FIG. 4 is a schematic diagram for explaining the focusing operation by the focusing lens, and FIG. 5 is the focusing operation. 3 is a graph showing the characteristics of a control current supplied to a lens for use in the present invention.

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 a charged particle beam deflector in the X direction. (horizontal direction) and Y direction (vertical direction) electron beam deflectors 3X, 3Y and a focusing lens 4 are provided.

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

Note that the focusing lens 2, electron beam deflectors 3X, 3Y
The focusing lens 4 and the focusing lens 4 are each configured as an electromagnetic lens that performs an appropriate action by controlling the current supply to the coil.
By supplying a step current as shown in FIG. 5 to the electron beam, the focal position of the electron beam B as a charged particle beam can be changed as shown in FIG.

That is, when the supplied current (strength of the lens) is Ia shown in Fig. 5, the focal position a is in a more over-focus state slightly above the sample 5, and the supplied current is as shown in Fig. 5. When Ib and Ic, the focal positions b and c are located above the sample 5, resulting in an overfocus state, and the supplied current gradually decreases as shown in Id, Ie, ..., In shown in Figure 5. As a result, the current Id causes the focal point d to be located on the surface of the sample 5, resulting in a just-focus state, and the current Ie,...,In causes the focal point e,...,n to be located below the sample 5. This results in an under-focus state.

Also, a focusing lens 2, electron beam deflectors 3X, 3Y
The focusing lens 4 may also be configured as an electrostatic lens that controls the voltage supplied to the opposing electrode plates to perform an appropriate action for each. A voltage that changes is supplied.

Therefore, as shown in FIG. 1, when the electron beam B is emitted from the electron gun 1, it is focused by the focusing lens 2, and then sent to the electron beam deflectors 3X, 3Y.
After the beam is appropriately deflected in the X-Y directions, the beam 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,
Video signals obtained from sample 5, that is, electrons reflected from sample 5, electrons transmitted through sample 5, sample 5
Electrons and secondary electrons that flow through the interior and leave the ground are generated, but in order to detect these electrons, a detection mechanism 6 is installed.
is placed near sample 5.

The signal from this detection mechanism 6 is amplified by an amplifier 7 and then supplied to the cathode or grid of a cathode ray tube 8, so that a sample image is projected on the screen of this cathode ray tube 8 by brightness modulation. .

Incidentally, a sawtooth wave generator 9 for the horizontal direction and a sawtooth wave generator 10 for the vertical direction are provided. The sawtooth wave from the generator 9 is transmitted to the deflection coil 8 of the cathode ray tube 8 via the deflection circuit 11.
The electron beam is supplied to the electron beam X in a waveform as shown by the symbol SX in FIG. It's becoming like that.

Further, the sawtooth wave from the generator 10 is supplied to the deflection coil 8Y of the cathode ray tube 8 through the deflection circuit 12 in a waveform as shown by the symbol SY in FIG. The electron beam is modified and supplied to the electron beam deflector 3X via the electron beam scanning control device 13 and the magnification switching circuit 15, as shown by the symbol SY' in FIG. 3e.

By the way, the electron beam scanning controller 13 transforms the sawtooth wave from the vertical sawtooth wave generator 10 into a waveform as shown by the symbol SY' in FIG. 3e.

As shown in FIG. 2, this electron beam scanning control device 13 includes a constant value signal generator 16 that outputs a signal at a predetermined level (including the ground level), a predetermined level signal E ₁ from this generator 16, and A changeover switch 17 receives sawtooth wave signals from the generator 10 inputted through the input terminal 13a and alternately outputs these signals at predetermined timings.
It is composed of the following.

This changeover switch 17 has a second switch section 17b connected to the generator 16 and an input terminal 13.
The first switch section 17a is connected to the generator 10 through a terminal A, and this first switch section 17a receives a cathode ray tube blanking control signal SB as shown in FIG. 3c from the control end 13b. It closes when this signal SB is at a high level and opens otherwise.The switch section 17 receives a signal obtained by inverting the control signal SB by an inverter 17c, and closes when this inverted signal is at a high level and opens otherwise. be.

In this way, each switch section 1 of the changeover switch 17
7a and 17b are cathode ray tube blanking control signals
Since the signals are alternately switched at the timing shown in FIG. Signal E ₁
A signal SY' (see Fig. 3e) obtained by mixing the signals is output.

Therefore, as is clear from the signal waveform shown in FIG. 3e, the horizontal retrace of the electron beam B is blanked on the cathode ray tube 8, so that when blanking the scanning signal of the cathode ray tube 8 [see FIG. When the signal SB is at a low level at c), the horizontal retrace line is made to scan the same line on the surface of the sample 5.

Furthermore, the position of this same scan line is
It can be set as appropriate by changing the value of the constant value signal _E1 from .

In this way, the horizontal retrace line of the electron beam B is scanned on the same line as the surface of the sample 5 during blanking of the cathode ray tube, so that the sample image appears on the screen of the cathode ray tube 8 regardless of the scanning mode described above. is being projected.

By the way, as shown in FIG. 1, the focus accuracy of the signal from the detection mechanism 5 is detected by the focus accuracy detection circuit 18.
The signal from 8 is adapted to be supplied to a focusing control device 19.

Here, as the focus accuracy detection circuit 18, for example, a circuit that differentially processes the signal from the detection mechanism 5, takes the absolute value of this differential signal, and outputs an area signal based on this absolute value signal is used. ing.

Further, as the detection circuit 18, a circuit that differentiates the detection signal from the detection mechanism 5 and outputs the differential signal can be considered.

Furthermore, the focusing control circuit 19 supplies a current that changes in steps as shown in FIG. 5 to the focusing lens 4 to change the focus from an over-focus state to an under-focus state. Accordingly, the focus accuracy detection circuit 18
The current value at the step where this signal takes the maximum value is supplied again to the focusing lens 4, and the lens 4 is controlled to be in a just-focus state.

With the above configuration, when the electron beam B is scanned by the electron beam polarizers 3X and 3Y, the electron beam B is scanned by the cathode ray tube 8, as is clear from the waveforms SX' and SY' shown in FIGS. 3d and 3e. When blanking, sample 5
Different lines on the surface are sequentially scanned, but when blanking the cathode ray tube 8, the horizontal retrace line is
The same line on the surface of the sample 5 is repeatedly scanned.

In this way, when blanking the cathode ray tube, sample 5
Since the horizontal retrace line of the electron beam B is repeatedly scanned on the same line in the plane of In addition, when blanking the cathode ray tube, sample 5
Since the electron beam B sequentially scans the surface to be observed, the material image can also be projected onto the screen of the cathode ray tube 8.

Note that the predetermined level set to cause the horizontal retrace line of the electron beam B to scan on the same line of the surface of the sample 5 during cathode ray tube blanking may be the ground level or any other level.

Furthermore, it goes without saying that this device can also be applied to devices similar to electron microscopes.

As described in detail above, according to the focusing device in the scanning electron microscope and similar devices of the present invention, the horizontal return line of the charged particle beam scans the same line on the sample surface during blanking of the scanning signal of the cathode ray tube. This has the advantage that focusing can be performed quickly and accurately while observing the sample image on the cathode ray tube screen.

[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 configuration diagram thereof, and FIG. 2 is an electric circuit showing the electron beam scanning control device. Figure, 3rd
Figures a to e are all waveform diagrams for explaining their effects, Figure 4 is a schematic diagram for explaining the focusing operation by the focusing lens, and Figure 5 is a waveform diagram for explaining the focusing operation by the focusing lens. 3 is a graph showing characteristics of control current. DESCRIPTION OF SYMBOLS 1... Electron gun, 2... Focusing lens, 3X, 3Y... Electron beam polarizer, 4... Focusing lens, 5... Sample, 6... Detection mechanism, 7... Amplifier, 8... Braun tube, 8X, 8Y... Deflection coil, 9... Horizontal direction sawtooth wave generator, 10... Vertical direction sawtooth wave generator, 11, 12... Deflection circuit, 13... Electron beam scanning control device as charged particle beam scanning control device, 13a
...Input end, 13b...Control end, 13c...Output end, 1
4, 15... Magnification switching circuit, 16... Constant value signal generator, 17... Changeover switch, 17a, 17b... First and second switch sections, 17c... Inverter, 18
... Focus accuracy detection circuit, 19... Focusing control device, B... Electron beam as a charged particle beam.

Claims (1)

[Scope of utility model registration request] In a scanning electron microscope and similar devices, a detection mechanism for detecting an image signal obtained from a sample surface when the sample surface is irradiated with a charged particle beam through a focusing lens and a charged particle beam deflector. It is equipped with a cathode ray tube that receives signals from the same detection mechanism and projects an image of the sample.
A focus accuracy detection circuit detects the focus accuracy based on a signal from the detection mechanism, and a charged particle beam scan is provided to scan the blanked horizontal retrace line on the cathode ray tube on the same line as the sample surface. A charged particle beam scanning control device for controlling signals, and a focusing control device for controlling the focusing lens on the same scanning line on the sample surface based on the signal from the focus accuracy detection circuit. A focusing device for a scanning electron microscope and similar devices.