JPS5998447A - Electrostatic lens - Google Patents

Abstract

PURPOSE:To enable the captioned lens to be actuated always in the state of small aberration, by modulating the electric potential of the intermediate electrode with AC signal, detecting the focus position of charged particle beam which is fluctuated by modulation, and further detecting and displaying the phase difference between the detected signal and the modulated signal. CONSTITUTION:The focal distance of a lens L is periodically changed due to repetitive application of the voltage of AC power supply 5 and fluctuation of the focus position is detected by means of a detector 6. AC signal having the frequency which corresponds to the modulation by the power supply 5 of the electrostatic lens L is sent out from the detector 6. This output is sent to a phase difference discriminator 10 through an amplifier 9, and its phase difference from that of a reference signal sent from the AC power supply 5 through a phase adjustor 11 is discriminated. The output of this phase difference detector 11 is displayed on a display device 12. Any display device will do, so long as it can display whether the phases of two signals entering the phase difference discriminator are equiphase or antiphase. Consequently, the operating point of the lens can be securely set by observating the display condition of the display device 12.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrostatic lens used as a focusing means for a charged particle beam.

For example, in ion microscopes, ion microanalyzers, and the like, electrostatic immersion lenses are used to focus ions. FIG. 1 is a diagram showing the electrode portion of the electrostatic immersion lens, and 1.2. Reference numerals 3 and 3 indicate electrodes, and voltages ①0, ②2, and ①1 are applied to 1, 2 and 3, respectively. The electrodes 1, 2. The charged particle beam 4 parallel to the optical axis Z is curved as shown in the figure by the electric field caused by the beams 3 and 3, and is focused at a point F. The position of the focal point F (distance Zo from the end surface of the first electrode 1) generally changes as shown in FIG. 2 according to V2/V1=V. As can be seen from the figure, the same focal length Zo is made smaller at two different Vs. That is, Vl,
The same focal length Zo+ is produced at V2. Since the focal lengths are the same, it seems that there is no problem in using either one, but if the above ■ is large, that is, in Fig. 2, v
In the case of 2, spherical aberration and chromatic aberration that are more than one order of magnitude larger than vl occur, and it is preferable to provide the desired focal length with the smaller V.

However, the focal length for the above V is 1! Since the It Zo curve changes depending on the potential of the first electrode 1, the position of the object point, etc.,
The V cannot be fixed to a specific value and used.

For this reason, in conventional devices, the lens often operates in a region where ■ is large, which also causes a reduction in resolution.

The present invention eliminates the above-mentioned conventional drawbacks and provides a new electrostatic lens that can operate with small aberrations at all times.

For the above purpose, the present invention provides means for modulating the potential of the intermediate electrode with an arbitrary alternating current signal in an ordinary electrostatic immersion lens, and substantially detecting the focal position of the charged particle beam that changes due to the modulation. The present invention is characterized by an electrostatic lens comprising means, a circuit for detecting and outputting a phase difference between the output signal from the detection means and the modulation signal from the modulation means, and means for displaying the output signal of the phase difference detection circuit. .

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an example of the present invention, in which a predetermined DC voltage is applied to each electrode 1, 2, 3 of the electrostatic lens L, and the charged particle beam 4 is focused as shown in the figure. It is assumed that Reference numeral 5 denotes an AC power supply, which applies an AC voltage of a predetermined frequency and amplitude to the intermediate electrode 2 of the lens. In an actual device, an AC power source and a DC power source are prepared, and the AC power source is connected to the lens by a switch only when necessary, and is configured to be superimposed on the DC voltage. Due to the superimposed application of this AC voltage, the focal length of the lens L changes periodically,
The fluctuation of the focal position is detected by the detector 6. Although any detector can be used as long as it can detect changes in the focal position of the charged particle beam, a Faraday cup will be described here. Fig. 4 shows an example of this. is focused ahead of the slit 8 (hyperfocal).

As can be seen from the figure, in the case of (a), almost no charged particle beam is blocked by the slit 8, so the Faraday cup 7
The output of the Faraday cup 7 is the maximum, and since a portion of the charged particle beam is cut by the slit 8 in the figure (b), the output of the Faraday cup 7 is smaller than that of the figure (a). Therefore, detector 6
An alternating current signal having a period corresponding to the modulation of the electrostatic lens L by the power source 5 and a period corresponding to the modulation of the lens L by the power source 5 is outputted from. The output is sent to a phase difference discriminator 10 via an amplifier 9, and the AC power source 5
The phase difference between the reference signal and the reference signal sent through the phase adjuster 11 is discriminated. The output of the phase difference detector is displayed on the display device 1.
2. This display device only needs to be able to display whether the phases of the two signals entering the phase difference detector are in the same phase or in opposite phases, and therefore can display a simple meter that displays + and - or two different colors. A lamp etc. that can be used is sufficient.

The operation of the apparatus shown in FIG. 3 will be explained below with reference to FIG.

FIG. 5 shows the state of modulation of lens strength, that is, focal length, and the horizontal axis is the second. The voltage ratio V of the third electrode is shown, and the vertical axis shows the output signal S from the detector 6. In the figure,
Curve A corresponds to the curve in FIG. 2, and AC signal S+++ is the modulation voltage of lens L.

Due to this modulation, the signal Sl or S2 is obtained from the detector 6, and is sent to the phase difference discriminator 10. Since the phases are reversed, if a positive output is obtained from the discriminator 10 during sl, a negative output is obtained during S2. Therefore, by observing the display state of the display device 12, it is possible to reliably set the lens operating point when ■ is small.

Incidentally, the above is one embodiment of the present invention, and various modifications are possible in reality. For example, the above is a case where the charged particle beam is used for focusing after being emitted, but it can also be applied to a field emission type electron gun or a field emission type ion source, and the first electrode 1 is used as an electron or ion extraction electrode. You may do as you like. In addition, when detecting the focal position using the Faraday cup in Fig. 4, if the focal state is not only positive focus and hyperfocal state (Fig. 4(b)) but also insufficient focus, the frequency of the detection signal will be twice the modulation frequency. Therefore, the phase adjustment circuit 11
It is good to have a frequency multiplier inside.

With the configuration described above, it is possible to determine the polarity of the slope of the change in focus position due to the intermediate electrode potential, so the lens can always be focused in an area with little spherical aberration and chromatic aberration, which reduces the reduction in resolution. can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the formation of the electrode part of an electrostatic immersion lens, Fig. 2 is a diagram showing the focal characteristics of the lens, and Fig. 3 is a diagram showing the formation of the electrode part of the electrostatic immersion lens.
The figure is a block diagram showing one embodiment of the present invention, and the M4 figure is the third block diagram.
FIG. 5 is a diagram showing an example of the focal position detector used in the embodiment shown in the figure, and is a diagram for explaining the operation of the embodiment shown in FIG. 3. 1: First electrode 2: Second electrode 3: Third electrode 5: AC power supply 6: Focus position detector 7: Faraday cup 8 Nislit 10: Phase difference discriminator 12: Display device Patent applicant Representative of JEOL Ltd. Ito - Husband

Claims (1)

[Claims] In a lens having at least three electrodes and whose focal length is varied by varying the voltage applied to the intermediate electrode, means for modulating the potential of the intermediate electrode with an arbitrary alternating current signal; a circuit for detecting a phase difference between an output signal from the detection means and a modulation signal from the modulation means, and an output signal of the phase difference detection circuit; An electrostatic lens characterized by comprising means.