JPS6240814B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a selected area diffraction imaging device for an electron beam device.

FIG. 1 is a block diagram of a conventional selected area diffraction imager. The electron beam generated by the electron gun 1 is deflected by a horizontal deflection lens 2 and a vertical deflection lens 3, and is deflected in the opposite direction by an electron lens 4 to irradiate a point on a sample 5. In this case, if the positions of the deflection lenses 2 and 3 are set as the object point positions of the electron lens 4, and one point on the sample 5 is set as the image point, the electrons can be directed in any direction by the deflection lenses 2 and 3. Even if the beam is deflected, the same point on the sample 5 can be irradiated with the electron beam, allowing so-called angular scanning.

Signals such as secondary electrons generated from the sample 5 in this way are detected by the detector 6 and the cathode ray tube 7
grid as a brightness modulation signal. On the other hand, the outputs of the horizontal deflection signal generation circuit 11 and the vertical deflection signal generation circuit 12 are amplified by amplifiers 8 and 9 to generate deflection currents applied to the horizontal deflection lens 2 and the vertical deflection lens 3. In addition, the horizontal deflection signal generation circuit 11
and the output of the vertical deflection signal generation circuit 12 is supplied to a correction signal generation circuit 14 to output a correction signal,
This is adjusted by a variable resistor 16 and added to the output of the DC signal generation circuit 15 by an adder 13. The output of this adder 13 is amplified by an amplifier 10 and supplied to the electron lens 4.

In the conventional selected area diffraction imager configured as described above, the output signal value of the correction signal generation circuit 14 was manually adjusted using the variable resistor 16.
In other words, it took a long time to manually correct the aberrations related to the deflection angle of the electron beam in order to satisfy the above-mentioned angle scanning conditions and to correct the aberrations related to the magnitude of the deflection angle of the electron beam. Ta. Furthermore, sample 5
It is necessary to change the current in order to change the focal length of the electron lens 4 each time the electron lens 4 is replaced, which has the drawback of requiring a large amount of time in the adjustment stage and reducing experimental efficiency.

The purpose of the present invention is to provide a high-performance selected area diffraction imager that is easy to adjust. The structure is such that a selected area diffraction image is obtained by setting the value of the correction current and superimposing this correction current on the current flowing through the electron lens.

FIG. 2 is a diagram illustrating the imaging relationship of the electron lens. In order to angularly scan the electron beam, the following conditions must be met as described above.

a ^-1 +b ^-1 = f ^-1 ...(1) However, a is the distance between the electron lens 4 and the horizontal deflection lens 2 and vertical deflection lens 3 b is the distance between the electron lens 4 and the surface of the sample 5 f is Let it be the focal length of the electronic lens 4. Also, the spherical aberration ds due to the electron lens 4
is proportional to the cube of the deflection angle α.

ds= ^Kfα3 ...(2) However, K is a coefficient. That is, if the paraxial electron beam shown by the broken line is made to intersect with the optical axis on the surface of the sample 5, the electron beam passing through the periphery of the electron lens 4 will intersect with the optical axis at a point P in front of the surface of the sample 5. fellowship,
On the surface of sample 5, it reaches a position ds away from the optical axis. This ds is called spherical aberration, and if the height of the electron lens 4 from the optical axis through which the electron beam passes is γ, then the spherical aberration ds is proportional to γ ³ , approximately proportional to α ³ . There is.

In order to remove the spherical aberration ds when the distance between the object point O and the surface of the sample 5 is constant, the focal length f of the electron lens 4 may be adjusted so that ds=O. Calculating Δf for this, Δf=f ² /b ² Δb...(3) Δb≒ds/α=Csα ² =Cs(γ/b) ² , so equation (3) becomes Δf=(f ² Cs/b ⁴ ) γ ² ...(4) That is, f may be adjusted so as to satisfy equation (4).
Now, if the lens current is I, Δf is approximately I
Since it is proportional to , equation (4) can be expressed as follows.

I∝γ ² ...(5) Also, the horizontal deflection current is I _h and the vertical deflection current is I _v
Then, I∝I _h ² + I _v ² ...(6) That is, if the lens current of the electron lens 4 is controlled in proportion to the square of the deflection current, the spherical aberration can be corrected and the One point can be angularly scanned.

FIG. 3 is a block diagram of a selected area diffraction imager according to an embodiment of the present invention, in which the same parts as in FIG. 1 are given the same reference numerals. In this case, variable resistor 16
A switch 18 is installed in place of the switch 18 to output the signal from the DC signal generation circuit 15 to the correction signal generation circuit 17.

FIG. 4 is a diagram illustrating the contents of the correction signal generation circuit 17 of FIG. 3. The output x of the horizontal deflection signal generation circuit 11 in FIG. 3 is squared by a squarer 19 and sent to an adder 21. On the other hand, the output y of the vertical deflection signal generation circuit 11 is squared by a squarer 20 and is then squared by an adder 2.
Sent to 1. The adder 21 calculates the value of x ² +y ² and sends it to the multiplier 22 , which multiplies the DC signal from the DC signal generation circuit 15 by a proportionality constant output k proportional to it and outputs it to the adder 13 . The above is equivalent to executing equation (6), so if the output of the adder 13 is amplified and used as a current to flow through the electron lens 4, spherical aberration is automatically removed and the deflection angle α is One point on the sample 5 can be accurately angularly scanned regardless of the angle.

Further, a correction current proportional to the direct current flowing through the electronic lens 4 is superimposed on the adder 13 via a switch 18. In this way, it becomes unnecessary to adjust the correction current applied to the electron lens 4 every time the surface position of the sample 5 changes (which seems to always occur when the sample 5 is replaced), and the angle scanning operation becomes unnecessary. becomes extremely simple. Therefore, the variable resistor 1 is connected between the correction signal generation circuit 17 and the adder 13.
6 is no longer necessary, and an on-off switch 18 is sufficient. This switch 18 makes it possible to instantly switch and observe the selected area diffraction image with and without the above correction on the cathode ray tube 7. This has the advantage of making it easier to deal with

In the selected area diffraction imager of this embodiment, the proportional constant output of the DC signal generation circuit 15 is corrected by the correction signal generation circuit 1.
By configuring the multiplier in 7 and directly outputting it to the adder 13, there is no need to adjust the correction signal even when the DC signal fed to the electron lens 4 is changed, greatly improving observation efficiency. You can get the effect that you can.

In the above embodiment, the correction current is passed through the electron lens 4, but the same effect can be obtained by providing an auxiliary lens equivalent to the electron lens 4 and passing the correction current through it. Therefore, the objective lens in the claims should be understood to include this auxiliary lens. Further, as shown in FIG. 4, a multiplier 22 is used to generate a correction signal proportional to the DC signal, but variable resistors that change the magnitude of the DC signal and the correction signal are connected in series, and an interlocking variable resistor is used. The same effect can be obtained even if it is used as This has the advantage that the electrical circuit is further simplified.

The selected area diffraction image device of the present invention has the advantage that a high performance selected area diffraction image can be obtained with simple operation.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional selected area diffraction imager, Fig. 2 is an explanatory diagram of the imaging relationship of an electron lens, and Fig. 3 is a block diagram of a selected area diffraction imager which is an embodiment of the present invention. FIG. 4 is a diagram illustrating the contents of the correction signal generation circuit of FIG. 3. 1...Electron gun, 2...Horizontal deflection lens, 3...
Vertical deflection lens, 4...electron lens, 5...sample, 6...detector, 7...cathode ray tube, 8,9,1
0...Amplifier, 11...Horizontal deflection signal generation circuit,
12... Vertical deflection signal generation circuit, 13, 21...
Adder, 14, 17... Correction signal generation circuit, 15
...DC signal generation circuit, 18...Switch, 1
9, 20... squarer, 22... multiplier.

Claims (1)

[Claims] 1. An electron beam generated from an electron gun is angularly scanned by a deflection lens and an electron lens around the same point on the sample, and a spherical aberration correction signal is sent to the electron lens in synchronization with the current flowing through the deflection lens.
In the selected area diffraction imager that detects the signal from the sample and displays it on a cathode ray tube, the proportionality constant of the spherical aberration correction signal is automatically changed in proportion to the current flowing through the objective lens. A selected area diffraction imaging device characterized in that it is configured as follows.