JPS5834551A - Signal processing circuit of field emission scan electronic microscope - Google Patents

Abstract

PURPOSE:To improve S/N ratio of the signal considerably by amplifying the input signal considerably at the prestage of an amplifier circuit (processing circuit) while to eliminate the unstable factor of the electron beam through the division. CONSTITUTION:In the drawing A'4 will receive the output signal from a current- voltage conversion amplifier A2 at one input terminal while receive the output signal from a current-voltage conversion amplifier A1 at the other input terminal through a level regulation amplifier AVL thus to amplify the differential signal, or it is a differential amplifier. The variable resistor RVL is for regulating the amplification of the amplifier AVL. When constituting the amplifier A'4 in such differential manner, the D.C. component contained in the output signal from the current-voltage conversion amplifier A2 can be eliminated properly, the dynamic range is widened thereby the differential amplifier A' can sufficiently amplify said A.C. component, resulting in the considerable improvement of S/N.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention manually calculates the detection current corresponding to the amount of secondary electron beam and the flow of the monitor 11E corresponding to the amount of electron beam near the probe, and calculates the Marugame pressure related to the amount of secondary electron beam. This invention relates to a signal processing circuit for a field emission scanning electron microscope that obtains and outputs a signal (video signal).

In general, the electron beam emitted from an electron gun usually accompanies some fluctuations. This is caused by fluctuations in electron emission from the tip of the electron gun, and is unavoidable to some extent.

This fluctuation in the electron beam is not a particular problem when the magnification of the electron microscope is low because the overall contrast is strong, but when the magnification is high, the field of view is generally limited to areas with weak contrast, so the fluctuation of the electron beam is The effect of fluctuations appears, causing the problem that the image is shaky and difficult to see. In order to avoid this kind of influence, the signal processing circuit of a conventional field emission scanning electron microscope has a configuration as shown in Fig. 7. The output monitor current, ts(t), is a detection current corresponding to the amount of secondary electron beam. and M3 are each equal to this current 1x(t
), 1s(t) This is a current-to-voltage conversion amplifier that converts tg voltage signals.

As the conversion amplifier Al t AI, a low noise amplifier, a photomultiplier, or the like is usually used. A.

and M4 are conversion amplifiers M1 and A, respectively.

A divider divides the output signal of the amplifier A4 by the output signal of the Al ri amplifier A1. A voltage divider 8 appropriately divides the output voltage of the divider A, and a variable resistor is used as this voltage divider, for example. Wound variable resistors are usually used to improve voltage division integrity and stability. A is a differential amplifier that receives the output signal of the voltage divider R and the DC signal V and outputs a difference signal between them, and the output voltage Vo of this amplifier A6 becomes the output of the signal processing circuit and is displayed on the CRT. It turns out.

The operation of the conventional circuit configured in this way will be outlined.

First, a detection current port (1) corresponding to the amount of secondary electron beam
We will explain the principle by which the influence of electron beam fluctuations can be removed by dividing the voltage signal related to (output of amplifier A4) by the signal related to the monitoring voltage fiix(t) (output of amplifier A, ).

The nine monitor '1tfi1. (t) includes fluctuations specific to the electron gun, and its DC component is Ix, li excitation rate k(t
) is expressed by the following formula.

1x(t) = lx (/+k(t))t:
On the other hand, the globe' direct current 1p(t) can be expressed similarly, and is expressed by the following equation, where the direct current component is Ip and the fluctuation rate is t-ki.

By irradiating a sample with a lunar electron beam, the yield of signals such as secondary electrons emerging from the sample is expressed as tp (t) = Ip
t) is expressed by the following formula.

It (t)=1(t) i p (t)=η(suI
p(/+kz(su)(3) Here, after converting [41!(t) and ig(su) into voltage signals, the voltage is amplified to the human power level of division #As and the division is performed. The output voltage 0 of calculator A is expressed by the following formula.
4 in the vicinity of the probe, the fluctuation rate k(t) can be considered to be almost the same as the fluctuation rate /(. Therefore, equation (4) can be expressed as the following equation. t) -= K'η(t) (X KI S constant, that is, the output voltage of the divider. It can be seen that the fluctuation is removed from the output voltage of the divider, and only the component η(1) to be observed remains. .

Now, in the conventional circuit shown in FIG. 1, the signal converted into a voltage by the current-voltage conversion amplifiers Al and Mo1 is directly amplified by the voltage amplifier A and Mo4. Therefore, in addition to the alternating current component that contributes most to my contrast, the direct current component included in the output signal such as 7 otomaru is also amplified.

□ Generally, the signal is #S in the front stage of the amplifier circuit.
Although the N ratio is improved, in the above conventional example, the amplifier Alt
Coupled with the limited dynamic range of A4, it is not possible to sufficiently amplify the alternating current component that contributes to the contrast of a CRT image in the front stage amplification section. For this reason,
In the circuit shown in FIG. 1, the signal cannot be amplified sufficiently in the front stage, resulting in a poor 87N ratio. Even if the DC component Wb is subtracted and amplified by the amplifier A- in the vk installation section, an improvement in the 87N ratio cannot be expected.

The present invention has been made in view of these points, and it improves the 8/N ratio by appropriately subtracting the DC component and amplifying it significantly in the front stage, and also increases the electronic power by performing division processing. This is the result of removing the beam instability factors.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

The gλ diagram is an electrical circuit diagram showing an embodiment of the signal processing circuit according to the present invention (the same parts as those in the circuit of FIG. 1 are given the same reference numerals and explanations are omitted).

In the figure, AS receives the output signal of the current-voltage conversion amplifier M1 at one input terminal, and receives the output signal of the current-voltage conversion amplifier A at the other input terminal via the level adjustment amplifier AvL. .

This is a differential amplifier that amplifies the difference between these two signals.The variable resistor 8 is used to adjust the amplification factor of the amplifier AVI. By differentially configuring amplifier A in this way, current-voltage conversion amplifier A! Since the DC component contained in the output signal of the amplifier can be excessively removed, a large dynamic range can be obtained, and the differential amplifier A- can sufficiently amplify the AC component. Therefore, the input signal can be greatly amplified in the front stage of the amplifier circuit The output signal of the differential amplifier A, from which the direct current component has been significantly removed, is supplied to one input terminal of the sub-jlL amplifier A. On the other hand, the other input terminal of the divider A is connected to the amplifier A.
3 output signals are provided. Output signal of divider A,
. is a signal from which fluctuations in the electron beam have been removed for the same reason as mentioned above. The output signal of this divider A. After being appropriately divided by a subsequent voltage divider R, it enters an amplifier A; The output signal VO of amplifier 52 is
← (not shown), and the CRT Kt! , a high-quality image with a good 8/N ratio is displayed, with noise due to tS instability peculiar to field emission guns eliminated.

The reason why the amplifier 2 is not configured differentially as in the amplifier A shown in FIG. 1 is because the DC component has already been removed in the previous stage, so there is no need to remove it further.

As described in detail above, according to the present invention, the human input signal can be greatly amplified in the front stage of the amplifier circuit (processing circuit), so the 87N ratio of the signal can be significantly improved. Furthermore, since division processing is performed, causes of instability of the electron beam can be removed.

[Brief explanation of the drawing]

FIG. 1 is an electrical circuit diagram showing a conventional example of the Fi signal processing circuit. FIG. 1 is an electrical circuit diagram showing an embodiment of the signal processing circuit according to the present invention. Ale Muu...Current-voltage cost conversion amplifier Muhata, Mu4...
・Amplifier A2...Differential amplifier M,...Divider M-...Differential amplifier M21...Amplifier
8...Voltage divider b...Level adjustment amplifier 凰mad...variable resistor! xtt zi horse 1 diagram

Claims (1)

[Claims] A detection current corresponding to the amount of secondary electron beam and a monitor current corresponding to the amount of electron beam near the probe are received, and these are converted into first and second voltage signals, respectively, and these W4
The first and second voltage signals are input to a differential amplifier, and the output signal of the differential amplifier is divided by a third voltage signal obtained by amplifying the second voltage signal. A signal processing circuit for a field emission scanning electron microscope.