JPH01183044A - Probe current stabilizing device - Google Patents

Abstract

PURPOSE:To stabilize the probe current regardless of the scale of the probe current by correcting the focus lens current with the exciting correction data responding to the variation of the output of a detector to the initial value. CONSTITUTION:By giving a probe current necessary to a controller 24, the controller controller 24 delivers a signal D0 to a D/A converter 23 responding to the probe current. The converter 23 delivers a signal to a focus lens power source 6 making reference to the voltage responding to the probe current, and the power source 6 drives a focus lens 5 by the lens current IL responding to the signal. On the other hand, a detector 12 detects the current with no relation to the lens current IL responding to the acceleration voltage, the bias of the electron gun, and the like. The detected signal is delivered to an A/D converter 17 through an amplifier 13. When the discharge current of the electron gun is varied, an adder 15 detects the condition to be VD'-VDnot equal to 0 to add a signal to correct to the focus lens power source 6, and the probe current is maintained constant regardless of the scale of the probe current value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a probe current stabilizing device in an analysis apparatus such as an electron microscope that analyzes a sample by irradiating the sample with an electron probe.

[Conventional technology]

6 and 7 are diagrams showing conventional examples of probe current stabilizing devices, where 41 is a cathode, 42 is a grid, and 43 is a cathode.
is an anode, 44 is an alignment coil, 45 is a focusing lens, 46 is a focusing lens power supply, 47 is a focusing lens aperture, 48
is a detection aperture, 49 and 50 are electron beams, 51 is an amplifier,
52 indicates an objective aperture.

In electron microscopes and the like that irradiate a sample with an electron probe and accurately analyze various information generated from the sample, it is essential to obtain a stable probe current. Conventional embodiments for stabilizing the probe current are shown in FIGS. 6 and 7.
As shown in the figure, the change in the emission current of the electron gun, which is composed of a cathode 41, a grid 42, an anode 43, etc., is corrected by detecting the change in the emission current between the focusing lens 45 and the objective aperture 52. A diaphragm 48 is provided, which is fed back negatively and added to the focusing lens power source 46 to stabilize the probe current.

[Problem to be solved by the invention]

However, in the conventional probe current stabilizing device as described above, when the focusing lens 45 is strongly excited to reduce the probe current to, for example, 1X10-''A or less, the detection aperture 4
There is a problem in that the influence of noise picked up by the amplifier 8 and the amplifier 51 cannot be ignored, making stabilization difficult. Also, the seventh
As shown in the figure, when the focusing lens 45 is weakly excited and the probe current is increased to, for example, lXl0-6A or more, the electron beam 49 passing through the detection aperture 48 does not increase in proportion to the electron beam 50. , finally electron beam 49
There was a problem in that the negative feedback was not performed and stabilization was difficult.

The present invention solves the above problems, and provides a probe current stabilizing device that does not reduce the detected current even when the probe current is large and is not affected by noise even when the probe current is small. The purpose is to

[Means to solve the problem]

To this end, the probe current stabilizing device of the present invention is an analyzer that analyzes a sample by irradiating the sample with an electron probe, in which a detector for detecting the emitted current is disposed between the electron gun and the focusing lens, and The device includes means for storing an initial value of the detector and excitation correction data of the focusing lens at a set probe current, and is configured to correct the focusing lens current using the excitation correction data in accordance with a change in the output of the detector with respect to the initial value. It is characterized by the fact that

[Effect]

In the probe current stabilizing device of the present invention, by storing the initial value of the detector and the excitation correction data of the focusing lens in the storage means while the set probe current is flowing, after that, when the output of the detector changes, The focusing lens current is corrected by excitation correction data in response to changes in the detector signal. The detector is placed between the electron gun and the focusing lens, and its signal has a value that is independent of the focusing lens current, so the probe current can be stabilized regardless of the magnitude of the probe current.

(Example〕 Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of the probe current stabilizing device according to the present invention, FIG. 2 is a diagram showing the probe current range and the corresponding code range, and FIG. 3 is a diagram showing the operation. This is a time chart for

In FIG. 1, ■ is a cathode, 2 is a grid, 3 is an anode, 5 is a focusing lens, 6 is a focusing lens power supply, 7 is a focusing lens aperture, 12 is a detector, 13 is an amplifier, 14.1
6.22 is a switch, 15 is an adder, 17 is an AD converter, I8 is a launch circuit, 19.21 and 23 are DA converters, 20 is a storage element, 24 is a control section, and 25 is an objective aperture.

The detector 12 detects changes in the emission current I, and is disposed between the anode 3 and the focusing lens 5, and its position may be above or below the alignment coil 4. Further, the shape of the detector 12 may be a bag-like shape having an electron beam passing hole as shown in the figure in order to efficiently detect the electron beam, or it may be a flat plate having such a passing hole. The amplifier 13 is
It is of a current-voltage conversion type and amplifies the emission current I detected by the detector 12. Switch 14.16.22
is for controlling the operation (on) or cancellation (off) of this device, and is controlled by the city control section 24. The AD converter 17 digitizes the analog signal, the DA converter 19 digitizes the digital signal with reference to a constant voltage, and the DA converter 21 .
Reference numeral 23 converts the digital signal into an analog signal by referring to a value corresponding to the acceleration voltage. The launch circuit 18 is a circuit that launches the digital signal of the AD converter 17,
The storage element 20 is a RAM that outputs stored contents to the DA converter 21 according to the digital signal of the AD converter 17.
It is. The control unit 24 determines the accelerating voltage and the excitation of the focusing lens 5, and also sends the content to be stored to the storage element 20.

When this device is in operation, the signals from the DA converters 21 and 23 are added by the focusing lens power source 6 to stabilize the probe current. Although not shown, there is an operation section attached to the control section 24, and based on commands from the operation section, the control section 24 controls on/off of the switches 14, 16, 22, and the storage section 20. Data is output to the DA converter 23.

Next, the operation will be explained.

Initially, interlock switches 14, 16, and 22 are set off and in the positions shown, respectively. Decide the required probe current 1p, and send the probe current I to the control unit 24 from the operation unit.
, the control unit 24 generates a digital signal corresponding to the probe current .
send to The DA converter 23 refers to the voltage corresponding to the acceleration voltage V ac and sends an analog signal AO to the focusing lens power source 6, and the focusing lens power source 6 drives the focusing lens 5 with a lens current IL according to this signal.

On the other hand, the detector 12 has an accelerating voltage ■□. A current (2) unrelated to the lens current IL is detected depending on the bias of the electron gun and the electron gun. This detection signal is converted into voltage ■ by amplifier 13,
and is sent to the AD converter 17.

The above is the operation with the switches 14, 16, and 22 set to OFF. Here, as shown in FIG. 3, when an operation start command is given from the operation section at time t1, the AD converter 1 digitizes the output voltage .
The output signal of 7 is sent to the latch circuit 18, and the latch circuit 1
8, the digital value ■ is latched at time t2. At the same time, the analog voltage -vll is applied to the adder 15. The control unit 24 also receives digital signals. , if necessary, outputs the contents to be stored in the memory element 20 by time t4, with reference to the acceleration voltages (2), (c), the bias of the electron gun, etc.

The content that the control unit 24 outputs to the storage element 20 is determined as follows. The code range for exciting the focusing lens 5 (which code range corresponds to the probe current range) is stored in the memory element 20 in the form of a function as shown in FIG.
is memorized and transferred to the DA converter 23. - When outputting -Dol, the probe current corresponds to IP -I.
The control unit 24 outputs to the storage element 20 a set of code data (DI) that can be changed by ±ΔIP+ with P+ as the center. Figure 2 (al shows the relationship between the probe current and the focusing lens excitation code D output to the DA converter 23; bl shows the set of code data [D1] stored in the storage element 20 and the probe current These function forms are calculated each time the control unit 24 outputs them to the DA converter 23 (using acceleration voltage, electron gun bias, and objective aperture diameter, etc. as parameters if necessary). and outputs the set [D,] to the storage element 20.

Next, at time t3, the switch 14 is turned on to reduce the output voltage V of the amplifier 13. ' and the output voltage -■ of the DA converter 19 are added by an adder 15.

Also, at time t3, the switch 16 is turned on and the AD
The converter 17 digitizes the signal added by the adder 15, adds the median value mid of the address of the storage element 20, and sends this signal to the storage element 20 at time t4. One data code D+ of the set [DI] stored in the storage element 20 corresponding to this digital signal is output to the DA converter 21. The DA converter 21 converts the data code D1 into an analog value with reference to the acceleration voltage ■llc. Then, by turning on the switch 22 at time t5, the probe current becomes stable.

Time required for the above procedure (from the command to start stabilization operation until stabilization is possible) 1. -1. can be done in a short time (several seconds). When the emission current of the electron gun changes, the adder 15 detects ■, '-■, ≠ 0 and adds a signal to be corrected to the focusing lens power supply 6, regardless of the magnitude of the probe current value. The probe current can be kept constant.

Note that the present invention is not limited to the above embodiments, and various modifications are possible.

FIGS. 4 and 5 are diagrams showing other embodiments of the probe current stabilizing device according to the present invention. For example, in the above embodiment, the detector 12 for detecting a part of the emitted current is shown in FIG.
As shown in the detector 12,' of A1, the focusing lens may be integrated with a focusing lens aperture, as long as the excitation change of the focusing lens does not affect the detection of the emission current. By doing so, it is possible to simplify the configuration and improve the detection rate.

Further, as shown in FIG. 4 (bl), a detection slit 26 with a pair of electrodes is placed below the electron gun alignment coil 4,
A detection slit 26 with another pair of electrodes perpendicular to the direction of the slit is placed below it, and automatic axis alignment is performed by detecting the amount of electron beam irradiated onto each of these electrodes (for example, 54-105685), the detector 12'' of the present invention may be placed between the two pairs of detection slits 26 and the focusing lens. In this device, the inclination of the electron beam emitted from the electron gun is Since it is always kept constant, the probe current can be stabilized with higher accuracy even when the emission current changes.

Furthermore, as shown in Fig. 5, a part of the electron gun automatic alignment device (e.g., Japanese Patent Application No. 105685/1982) is used to detect a part of the electron gun emission current and tilt the electron gun beam. In addition to correction, stabilization of the probe current may also be performed at the same time. First, the automatic alignment circuit will be briefly explained below.

In FIG. 5, a pair of detection electrodes 26 are separated by a certain distance in a horizontal plane perpendicular to the direction in which the electron beam is incident.
A, 26b, and another pair of detection electrodes 26C126d, which are arranged in a horizontal plane that is perpendicular to the slit direction and parallel to the detection electrode, are arranged below the alignment coil 4. These detection electrodes 26a, 26b, 26C126d
The respective detection currents ■X0, ■x2, Iyl,
■y2 is converted into voltages X0,
Xz, 'I'+ Yz, XI +X2, Y1+Y
2 is output. Then, in the dividers 37 and 38, (XI X2 ) / (XI +X2 ) (Y
l − Y2 ) / (Yl + Y2 ) is output. These two signals are connected to the alignment coil power supply 28 for driving the alignment coil 4, and are not affected by changes in the electron gun emission current (Xl - X2 > / (X
I 4-x, )(Yl Yz)/(Yl +Y
Negative feedback is performed to keep the value of 2) constant, and the inclination of the incident electron beam is automatically kept constant.

Next, the combination with the present invention will be explained.

The signals of the force u calculators 35 and 36 are added by an adder 39, and
Make 10X2 +Yl +Y2. By connecting this signal to the switch 14 as explained in FIG.
The probe current can be stabilized without the detector 12 explained in the figure. In addition, as a special case, the signal
+X2 +YI xl, X2, Y3 instead of +Y2
, Y2 and any combination thereof may be used. In the present invention, since the slope of the electron beam is kept constant, the probe current can be stabilized with higher accuracy even when the emission current changes.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, a detector for detecting the electron gun emission current is disposed between the anode and the focusing lens, and the excitation of the focusing lens is controlled according to the change in the emission current. Therefore, even when the probe current is extremely large, the detection current of the detector does not decrease, and the probe current can be stabilized. Moreover, even when the probe current is small, the detection current of the detector is large and the probe current can be stabilized without being affected by noise.

In addition, in response to the excitation of the focusing lens, a set of data for correcting the probe current is sent to the storage element, and the correction data is output from the storage element in accordance with the change in the emission current, so that the focusing lens can provide detailed information. A more stable probe current can be obtained, the number of addresses in the memory element can be reduced as much as possible, and the emission current value itself does not need to be used as a data parameter.

Furthermore, automatic axis alignment can further stabilize the probe current.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one embodiment of the probe current stabilizing device according to the present invention, FIG. 2 is a diagram showing the probe current range and the corresponding code range, and FIG. 3 is a time chart for explaining the operation. , FIG. 4 and FIG. 5 are diagrams showing other embodiments of the probe current stabilizing device according to the present invention, and FIG. 6 and FIG.
The figure shows a conventional example of a probe current stabilizing device. ■...Cathode, 2...Grid, 3...Anode, 5...Focusing lens, 6...Focusing lens power supply, 7
...Focusing lens aperture, 12...Detector, 13...
Amplifier, 14.16.22... Switch, 15...
Adder, 17... AC converter, 18... Launch circuit, 19.21 and 23... DA converter, 20.
. . . Memory element, 24 . . . Control unit, 25 . . . Objective aperture. Applicant JEOL Co., Ltd. Agent Patent Attorney Ryukichi Abe (3 others) Figure 5

Claims (1)

[Claims] (1) In an analyzer that analyzes a sample by irradiating the sample with an electron probe, a detector that detects the emission current is placed between the electron gun and the focusing lens, and the detector is activated at a set probe current. A probe current stabilizing device comprising means for storing an initial value and excitation correction data of a focusing lens, and configured to correct a focusing lens current using the excitation correction data in accordance with a change in the output of a detector with respect to the initial value. conversion device.