JPS5825050A - Focus adjustment of electron-ray device - Google Patents

Abstract

PURPOSE:To carry out the focus adjustment automatically by changing the excitation of an objective lens in step-like form, scanning electron rays over a sample every time when the excitation of the lens is changed, and computing the entropies of thus obtained secondary electrons so as to fix the excitation at the maximum point. CONSTITUTION:The excitation of an objective lens 5 is changed in step-like form by controlling an excitation power source 15 by means of an electronic computer 13. At the same time, horizontal scanning signals are produced from a scanning- signal generator 7 synchronously with the step-like changes of the excitation, and scanning are performed over a sample 6 every time when the focus position of the lens 5 changes when the excitation increases a step by a step. Next, thus obtained signals of a secondary-electron detector 9 are stored in the computer 13 through an AD converter 12, and the entropies of the signals are computed and compared with each other so as to obtain a point at which the maximum entropy is given. As a result, the excitation intensity of the lens 5 can be automatically adjusted to the regular focus by controlling the excitation intensity to be fixed at the maximum point.

Description

Detailed Description of the Invention The present invention relates to a method for automatically focusing an objective lens in an electron beam device such as a scanning electron microscope. Controlling the excitation current to minimize the diameter of the electron beam irradiated onto the sample surface is extremely important in order to obtain a high-quality sample. Various methods have been proposed in the past to automatically perform this focusing, but a typical method is to remove the image signal and fix the excitation of the objective lens at the point where the differential value is maximum. Signal differential method, peak value detection method that fixes excitation of the objective lens at the point where the peak value of the AC component of the image signal is maximum, and peak value detection method that integrates the peak values of the image signal and sets the focal point at the point where the integrated value is maximum. There are value integration methods, etc., but the first method is extremely susceptible to noise due to differentiation, and J
The llEZth and 3rd methods are strong against noise, but
When the magnification is high, the difference in the peak value of the detection signal due to the difference in the diameter of the electron beam probe becomes extremely small. In the third method, it is impossible to focus accurately at high magnification lζ.

The present invention solves the drawbacks of the conventional focusing method for electron beam equipment, and provides an electronic S* position that is resistant to noise and capable of automatic focusing with high precision even at high magnifications. This method provides a focusing method in which the excitation intensity of the objective lens is changed in steps, the electron beam is scanned across the sample each time the excitation intensity is changed, and the secondary electrons or reflections obtained as a result of this scanning are The entropy of the electron detection signal is calculated every scan at different excitation tC and compared with each other, and the excitation intensity of the objective lens is fixed at a point where the entropy becomes maximum. O The principle of the present invention will be described below. The inventor of the present application is an optical system.
In the device, we found that the entropy of the image signal obtained from the secondary electron detector, etc. is maximized when the electron beam is scanned in the state where the focus is closest. Now consider a set of independent events (, i), When the probability of occurrence of each event xt is P(xi), this event system (
b entropy (average information amount) of xtt [((X) is generally defined as follows.

H(X)=-! 'P(xi)logP(xi)***a
(11 However, !P(xi) = 1, (L≦P
(xi)≦1 (0) If this definition is expanded from a disjoint system to a continuous system, I(X) can be expressed as follows.

H(X)=-ノ: P(x)logP(x)dx
asss t2) On the other hand, the human power given in the steady process X* * Xs *...

It is widely known that the entropy H (ωh..., ωn) in the same wavenumber region of the output signal obtained when Xo is passed through a filter with the transmission same wavenumber characteristic K(ω) is expressed by the following equation. It is being

H(ul, @war@, ω.) = H(Xh..., xn
) 10 fW logIK (df in ω...・(3) However, and 2E, W is the bandwidth to be integrated 0 The image information possessed by a sample in a scanning electron microscope is spatially stationary, In addition, since the electron beam probe of a scanning electron microscope can be regarded as a kind of spatial filter with the same spatial wavenumber K (u, v), the entropy in the frequency plane of the output image that has passed through this filter is expressed as follows: It is expanded as follows.

H(ule v+ e 11@119 un, Vl
)=H(ξjtW1m””*However, in equation (4), u
l, ma1. ...*ufiema1 is the coordinate on the same wave number plane, ξt e Ll t-..., ξ. , va are the coordinates on the sample plane, and U and V are the bandwidths to be integrated.

The first term on the right side is the entropy of the image information that the sample initially has, so if we set this as Ho and write the left side of the father's equation as H7, equation (4) can be changed by 1 as follows.

"-H1= U'VfU j'g IK("s
' ) I "d u, dv...φ. This is the number of information that is lost when the image information held by the electron beam probe is converted to the scanning microscope. If we express this number of lost information as H1oss, then O≦K(u,
v)≦1, so H1oss= ,, fOfy
JoglK(u,v)l”du,dv≧0−−−+61
holds true, kllo@B is always greater than or equal to O, and it is clear that it meets the actual situation.

By scanning a sample with an electron beam probe in this way, the IIT intensity information of the sample is determined only by the spatial iso-wavenumber characteristics of the scanning probe.

On the other hand, the spatial isowave number response of the probe in the secondary electron field IEICE Technical Report GNSO-49°f'3
3-40 (1980-6) etc., the electron beam probe's electric ILI! When the F degree distribution is a Gaussian distribution and there is no astigmatism, the spatial frequency characteristic of the probe is given by the following equation.

K(ρ) = exp (-56 doρ”)
m5ss 17) However, do is the half width of the probe, and ρ=fJ-1.

If the diameter of the probe normalized by the spatial constant wave number f of the image information possessed by the sample is D, then do = "-, so Hj+oss =--# /, V log 1exp
(-ass ()” tl l l” dρ・・・・
(8) Use this standardized probe diameter as a measuring meter,
If we perform the integral calculation of equation (8) using an electronic computer and plot the results as a graph, we get something like the one shown in Figure 1.
Here, the calculation is performed assuming f=128.

As is clear from FIG. 1, in the case of a scanning electron microscope, HIO@11 increases monotonically with respect to D. From this, it can be seen that in the case of a scanning electron microscope, the diameter of the probe becomes thicker (1), so the amount of information lost in the output image increases.In other words, in the case of a scanning electron microscope, the smaller the diameter of the probe, the more information is lost in the output image. Equation (4) shows that the entropy of the output image or output signal increases.
). Therefore, every time the excitation intensity of the objective lens is changed, the entropy of the secondary electron or backscattered electron detection signal detected by scanning the sample surface with the four electron beam probes is calculated, and this calculated entropy is maximized. Focusing can be achieved by finding such an excitation intensity.

FIG. 2 shows an example of an apparatus for carrying out the present invention based on the principle lζ mentioned above. In the figure, 1 is an electron gun, and an electron beam 2 from the electron gun 1 passes through a converging lens 6. After passing, horizontal and vertical deflector 4K.

4Ylζ and further focused onto the surface of the sample 6 by the IC objective lens 51ζ. Horizontal and vertical scanning signals are supplied from a scanning signal generator 7 to the horizontal and vertical deflectors 4X, 4Y and the cathode ray tube 8. o9 is a secondary electron detector for detecting secondary electrons generated from the sample 6. and
The output signal from the secondary electron detector 9 is supplied to the grid G of the cathode ray tube 8 via an amplifier 10 and an attenuator 11 . The signal is supplied to the electronic computer 131ζ via the AD converter 12. 14 is a display means for displaying the output signal of the electronic computer 16. Reference numeral 15 denotes an excitation power source for the objective lens 5, and the excitation power source 151ζ is supplied with a signal from a computer 16 for controlling the excitation intensity.

Control signals for controlling the generation of horizontal and vertical scanning signals from the scanning signal generator 7 are also supplied from the computer 16.

In the apparatus having such a configuration, the computer 15 sends a control signal to the excitation power source 15 to increase the excitation current of the objective lens 5 from the initial value in steps as shown in FIG. 3 (tal). On the other hand, the electronic computer 16 sends a control signal to the scanning signal generator 71, and the slope scanning signal generator 7 performs horizontal scanning as shown in FIG. Generate a signal.

At this time, the output value of the vertical scanning signal is fixed. As a result, the excitation current supplied to the objective lens 5j is 1
Each time the focal position of the objective lens 5 changes by increasing steps, the sample 6)1. The same horizontal line is horizontally scanned once. The detection signal obtained from the secondary electron detector 9 by such a plurality of horizontal scans 1 differs slightly because the excitation intensity of the objective lens 5 differs for each scan. Come on, number 4
The detection signal as shown in FIG. 1cl, and the detection signal as shown in FIG. 4 1cl in the n + 1st scan,
+21g In the first scan, as shown in Fig. 4, 1cl.
k detection signals are obtained. These detection signals for each scan are amplified in an amplifier 10, attenuated in an attenuator 11, and then converted in a D-converter 12 into a digital signal consisting of, for example, 512 point separation values, and then sent to an electronic computer 16. Each scanning signal is stored in the internal storage device. The detection signal stored in the storage device for each scan is Fourier-transformed using a tc#
Calculate the electric sludge vector 8(fi) at (i==1, 2°..., 512). By the way, the following two equations hold true for this electric waste vector 8(fi)i.

11 Therefore, in Equation 11) above, we consider the assembly (,i) to be especially the set (fil), and let P(xi) meet as 8(fi).
If it corresponds to l, the calculated electric waste vector 8(fi)
The following equation is calculated in the electronic computer 16 using
Calculate.

As a result of such entropy calculation, for example, the above-mentioned (
1, n+1 e n+21g The entropy of the signal obtained by the 1st scan Cζ is each! 4, A7, 40
The correction gradually increases and decreases to 3 in the n + 3rd scan.
7, the electronic computer 16 compares the entropy calculated for each signal obtained by each of these scans, and selects the scan that gives the maximum entropy as n.
+ Find out that it is the signal obtained by the second scan. During image observation, control is performed so that a current equal to the excitation current supplied to the objective lens 5I is supplied from the excitation power supply 15 when performing the n+2th scan. In this way, the excitation of the objective lens 5 is Intensity is automatically adjusted to normal focus.

In the focusing method for the IC of the present invention described above,
Since the detection signal is not differentiated as in the conventional method, it is resistant to noise, and unlike the peak value integration method, it is possible to detect a part of the elm of the signal (peak value) that changes as the focus is focused. Rather than detecting the degree of focus only from When performing high-magnification observation using a scanning deaf electron microscope, focusing can be performed with high precision.

The embodiment described above is only one embodiment of the present invention, and the present invention may be implemented in many other ways.

For example, in the embodiment described above, the electric scum vector 8 (fi) was used to calculate the entropy of the signal obtained for each scan, but the signal values were spread over a number of equally spaced areas vj. When divided, the signal sent from the SC detector is included in each region vi. First, calculate the frequency 1 [Q (mi), and use this Q (mi) to calculate the entropy given by equation may be calculated.

In the embodiment described above, one line of the sample was repeatedly scanned to calculate the entropy of the drawing signal, but it is also possible to repeatedly scan multiple lines or the -ij plane and use the detection signal obtained at that time Entropy may also be calculated.

[Brief explanation of drawings]

#I1 diagram is a diagram showing the relationship between the electron beam probe diameter and lost entropy, Figure 2 is a diagram showing an example of an apparatus for carrying out the present invention, and Figure 3 is a diagram showing the relationship between the excitation current and the entropy lost. FIG. 4 is a diagram for showing the relationship with the scanning signal, and is a diagram for explaining the change in the detection signal due to the change in the excitation current. 1: Electron gun, 2: Electron beam! I: Converging lens, 4X. 4Y: X direction and Y direction deflector, 5: Objective lens,
6: Sample, 7: Scanning signal generator, 8: Cathode ray tube, 9-electron detector, 10: Amplifier, 11: Attenuator, 12: ^
D converter, 16: Electronic computer, 14: Display means, 15:
Excitation power supply. Patent applicant JEOL Ltd. Representative Tadao Kase

Claims (1)

[Claims] The excitation intensity of the objective lens is changed stepwise, and each time the excitation intensity is changed, the electron beam is scanned with the sample J-s, and the entropy of the secondary electron or backscattered electron detection signal returned with the scanning is varied. Focusing method for an electron beam device 0, characterized in that the excitation intensity of the objective lens is fixed at the point where the entropy reaches the maximum, by calculating and comparing each scan for each excitation.