JPS58137948A - Method of adjusting focus of scanning electron microscope or the like - Google Patents

Abstract

PURPOSE:To enable the focus adjustment of a scanning electron microscope or the like to be performed over a relatively wide range of the frequency by subjecting a picture signal obtained every time when the excitation is changed to Fourier conversion carried out in an electronic computer so as to obtain a power spectrum, and comparing areas surrounded by such power spectra and a given reference line with each other. CONSTITUTION:Every time when the excitation of an objective lens 5 is switched by supplying a control signal from an electronic computer 15, a horizontal scanning signal is generated from a scanning-signal generating circuit 12, a certain specific line on a sample is scanned by electron rays 2, and a detection signal obtained from a detector 7 as the result of the above scanning is converted with an AD-converter 19 into a digital signal, which is supplied to the electronic computer 15. In the electronic computer 15, the supplied signal is subjected to Fourier conversion so as to obtain a power spectrum, an area surrounded by the line of the spectrum and, for example, the reference line of 20dB is computed every time when the excitation of the objective lens 5 is switched, and excitation intensity which maximizes the above area is found out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for automatically focusing a scanning electron microscope or the like.

Various attempts have been made to automatically focus scanning electron microscopes. A typical example is to sample the image signal every time the excitation of the objective lens is changed and integrate the peak value of the signal.
The peak value integration method takes the point where this integrated value is maximum as the focal point.Each time the excitation of the objective lens changes, the image signal is supplied to a high-pass band-pass filter, and the point where the filter output is maximum is determined as the focal point. There is a method using a band-pass filter to do this, but in the former method, there is a problem in accuracy because peak determination and peak value integration are performed in an analog manner using a capacitor.

In addition, in the latter method, if the sample does not have a spatial frequency distribution in the passband of the filter, the signal cannot be extracted from the filter, so automatic focusing is not possible, and it is difficult to focus on a specific sample. It can only be applied to

The present invention has been made with the aim of solving these conventional drawbacks, and the basic idea of the present invention will be explained below by taking as an example the case of obtaining a transmission scanning image.

The image signal ψ of the transmission scanning image is obtained as the convolution integral of the distribution function i of the incident electron beam with the electron transmittance of the sample.

ψ= IPs% i ... ■ Generally, the density distribution of an incident electron beam can be approximated by a Gaussian distribution, so its frequency 1 wave number distribution in the spatial frequency domain I (k) I (k ) = i・exp (-k” / 2 trk''
)...■. However, fk is the dispersion in the spatial frequency domain, and the dispersion in the spatial domain (x”L (y”〆= 1
/ ((2g)7a”). When I(k) is expressed in a Bode diagram, it is as shown in Figure 1, which shows the density of the electron beam irradiated onto the sample in a transmission scanning electron microscope. The fact that there is some spread in the distribution corresponds to the fact that this conversion system acts as a kind of low-pass filter when converting the sample into an image via this electron beam. For I in the out-of-focus state as shown by A′ in b),
As the electron beam comes into focus, the density distribution of the electron beam approaches one concentrated at a single point, so I becomes one in which the passage area expands to the higher frequency side, as shown by A in Figure 2 <a>. .

Therefore, the distribution of the transmittance distribution ψS of the sample in the spatial frequency domain is expressed by a function as shown in Fig. 2 (!l) and (b). S, the expression φ in the spatial frequency domain of ψ is as shown by C in Figure 2 (a) when it is in focus, and as shown in Figure 2 (b) when it is out of focus. ), the result will be as shown by C′, and if you compare the area of the area surrounded by φ and the baseline (hatched), you will find that the area when it is in focus is not in focus. This is clearly larger than the case. Since the present invention is based on such a principle, the excitation intensity of the objective lens is changed in successive step operations, and each time this change is performed, the same part of the sample is scanned with an electron beam, and the image obtained along with the scanning is Gatoru's captive medicine 1 or 4a
It is characterized in that the areas are compared with each other and the excitation of the objective lens is fixed at the point where the pattern area becomes maximum.An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 3 shows an example of an apparatus for carrying out the present invention. In the figure, 1 is an electron gun, and the electron beam 2 from the electron gun 1 is 1,2-stage converging lens 3°4. After being converged by, the objective lens 5 irradiates the sample 6 as a narrow electron beam bundle. The electron beam transmitted through the sample 6 enters the transmission electron detector 1 and is detected. In addition, 11.51, 10
．． Reference numerals 11 denote a focusing lens, an intermediate lens, a projection lens, and a fluorescent plate, each of which is used to obtain a normal transmission electron microscope image.

12 is a scanning signal generating circuit, and horizontal and vertical scanning signals from this circuit 12 are supplied to horizontal and vertical deflection coils 13X and 13Y of the electron microscope, and 1 cathode ray tube 1.
4 horizontal and vertical deflection coils DX, DYi (supplied). Generation of scanning signals by the scanning signal generating circuit 12 is controlled by control signals from the electronic computer 15. 16 is an objective lens power supply; The excitation current from the power supply is supplied to the objective lens 5 via the amplifier 11. The current supplied to the objective lens 5 from the objective lens power supply 16 is also controlled by a control signal from the electronic computer 15. The output signal of the electron detector 1 is sent to the amplifier 1
8 to the grid G of the polar ray tube 14 and also to the computer 15 via the person converter 19. In such a configuration, the computer 1
Each time a control signal is supplied from 5 to switch the excitation of the objective lens, a horizontal scanning signal is generated from the scanning signal generating circuit 12, and the same line on the sample is scanned by the electron beam 2. The obtained detection signal is sent to an AD converter 1s
The digital signal is converted into a digital signal and supplied to the electronic computer 15. The electronic computer 15 performs a Fourier transform on this supplied signal to obtain a power spectrum, calculates the area F surrounded by the 29 dB reference line and this spectrum line each time the excitation of the objective lens 5 is switched, and , find the excitation intensity that maximizes the product P.

In order to efficiently search for the excitation intensity at which F is maximum, it is carried out as shown in the flowchart of FIG. 5, for example. That is, for example, suppose that the value of P changes as shown in FIG. 4 with the value of excitation intensity. First, a fourth lens is placed near the approximate focal point position.
As shown in the figure, there are three exciting current values 0L(1) that differ by d.

OL(ri) is set, and η is calculated for each excitation state.
are respectively *F(0), P(1), P(2), these are compared in the electronic computer 15, and P(0) or P(2
) is maximum, there is no peak of F in the range from 0L(Q) to 0L(2), so F(0)
is maximum, 0L(0), 0L(1), 0L(2
) respectively by d, and if F(2) is maximum, then 0
L(0), 0L(t).

0L(2) are each moved by -d. Also, when P(1) is maximum, the peak is OL (0)-IIs et al. 0L()c
Since it exists between o, F(0) and F(2) are compared and F(
0)>Fl) then OL (0) Th et 0L (1)
Since there is a peak between them, 0L(Q) is left as is, and the points where 0L(1) and 0L(z) are moved by -d/2 are set as new 0L(1) and 0L(2), and then d cut in half. Also, if F (2) > F (0) (7) then Ha0L (1)
Since there is a peak between 0L(2) and OL(2)
is shifted from 0L(2) by -d/2, and d is halved. In this way, newly set 0L(0), 0L(1), and 0L(2) for each case, and if the set excitation current is the first time, scan the electron beam at this excitation intensity to find F. ,' (o).

Repeat the comparison of F(1) and F(2), and see how the values of 0L(0), 0L(1), and 0L(2) shift below the horizontal axis in Figure 4. This is explained in detail in up to three steps. If d becomes smaller than the specified step d, this routine is exited, then F(1) is the peak value, and this excitation current 0L(1) is the excitation current at the focused point. Therefore, a control signal is supplied from the electronic computer 15 at cc to fix the excitation of the objective lens power source 1g.

As described above, according to the present invention, focusing can be performed automatically, but in the present invention, the image signal obtained every time the excitation is changed is Fourier-transformed by an electronic computer to obtain a power spectrum. Since the area from the reference line consisting of this power spectrum is compared with each other,
Since the comparison can be made over a wider frequency range than before, accurate automatic focusing can be performed on any sample without being affected by the special distribution of the sample's spatial frequency. Furthermore, since all calculations are performed digitally, highly accurate calculations can be performed, and therefore accurate focusing can be achieved.

Although the above-mentioned embodiments have been explained with respect to the case of obtaining a transmission scanning image, the principle of the present invention can be analogized to the case of obtaining a secondary electron image or a backscattered electron image, so the same applies to both cases. Applicable.

Further, in the above-described embodiment, the area of the power spectrum was calculated, but the same method can be implemented by calculating the area of the amplitude of the spectrum obtained by Fourier transforming the signal.

[Brief explanation of drawings]

! Figure 11 is a diagram for explaining the present invention in detail, Figure 2 is a diagram for explaining Φ in focused and out-of-focus cases, and Figure 13 is a diagram for implementing the present invention. FIG. 4 is a diagram showing the relationship between the area value of the power spectrum and the excitation intensity, and FIG. 5 is a flowchart for searching for the excitation intensity that maximizes the area of the power spectrum. 1: Electron gun, 2: Electron beam, 5: Objective lens, 6: Sample,
1: Transmission electron detector, 12: Scanning signal generation circuit, 13X
, 13Y: Deflection coil, 15: Electronic computer, 16: Objective lens power supply. Patent applicant JEOL Ltd. Representative Tadao Kase

Claims (1)

[Claims] The excitation intensity of the objective lens is changed in successive step operations,
Each time this change is made, the same part of the sample is scanned with an electron beam,
The image signals obtained with the wrinkle scanning are supplied to an electronic computer and subjected to Fourier transformation to obtain the amplitude or power spectrum of each signal, and the areas of each amplitude or power spectrum are compared with each other to calculate the wrinkle area. A focusing method for a scanning electron microscope, etc., characterized in that the excitation of an objective lens is fixed at a point where it becomes maximum.