JPS608580B2 - Automatic focus adjustment device for scanning electron beam equipment, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic focus adjustment device for an apparatus such as a scanning electron microscope that sweeps a sample surface with an electron beam or the like to obtain an image signal.

When adjusting the automatic focus, it is necessary to detect the critical condition.
Various methods have been proposed for this purpose.

The present invention does not propose such a method of detecting disease, nor is it limited to any particular method.
The purpose is to improve the accuracy of focus adjustment.
The present invention improves the accuracy of focus adjustment by detecting focus while changing the focus of the lens, and by repeating this operation and calculating the average.

The present invention will be explained below with reference to Examples. The embodiment described in (a) shows a scanning electron microscope, and the electron microscope part is simplified in the figure.

In this embodiment, a rectified and smoothed signal of an alternating current component in a video signal is used as a focus evaluation signal when performing focus detection by analyzing a video signal. 1 is an objective lens, 2 is a deflection coil, 3 is a sample, and B is an electron beam.

4 is a detector that detects secondary electrons etc. emitted from the sample;
Reference numeral 5 denotes an amplifier for amplifying the output of the detector, and when the sample 3 is swept with an electron beam, a video signal is output from the amplifier 5.

In this case, when the excitation current of the objective lens is changed from 0 to the maximum value while sweeping the electron beam, the in-focus state is passed midway through, and the image signal in this case becomes as shown in FIG. 2a. If only the alternating current component is taken out from this video signal and rectified and smoothed, the signal shown in FIG. 2b is obtained. Since the alternating current component of the video signal is at its maximum in the in-focus state, the center of the peak in FIG. 2b represents the in-focus state. Since the waveform in FIG. 2b is symmetrical, it is cut at a constant level by a waveform shaping circuit and converted into a signal as shown in FIG. 2c. The axis in Figure 2 is the sweep distance, and since the excitation current of lens 1 is monotonically changed as the sweep is performed, the axis also corresponds to the excitation current, and the excitation point at the rising point of the rectangular wave in Figure 2c is Assuming that the current is na and the excitation current at the falling point is blood, the excitation current of the lens 1 that provides a concomitant state is (cloth+nb)/2. Alternatively, if the current difference between na and nb is N, it can also be written as na+N-2.
The excitation current for the objective lens amount is determined by the above formula.
In order to improve the accuracy of this determination, the excitation current is determined by repeating the above operation over m circuits and averaging the above formula.The present invention automatically performs the operation to obtain this average. The output pulse of the oscillator is input to the digital-to-analog converter DA through the gate G, and a stepwise changing signal is obtained from tDA.This signal controls the excitation power supply S of the objective lens mother, increasing the excitation current from 0 to the maximum. Change the value stepwise.

During this time, the video signal output from the amplifier S is input to the waveform shaping circuit (level comparator) 6, and the waveform shown in FIG. 2c is obtained from the mackerel. G2 is a gate that is normally open and is closed at the rise of the output of 6, that is, at the position M in FIG. 2c.
It is inputted to the counter C through the counter C and counted. Kausota C...When the count reaches m in the m-adjustable count, it issues a carry signal and returns to the river. This carry signal is also counted by the counter C. Therefore, C2 is 1 of the number of pulses from the start of the electron beam sweep until the excitation current reaches na.
nom, that is, 1/m of the excitation current na is given. G3 is from na to n in Figure 2c when the output of the waveform shaping circuit 6 is high.
The pulses of the pulse oscillator PG are sent to the counter C3 and counted through the gate which is open until b. C3 is a 2h counter, and when the count exceeds 2, it issues a carry signal and returns to 0. This carrier IJ- signal is also input to counter C2. Gate G2 is open from excitation current 0 to na, and gate G3 is open from na to nb, so
The counter G2 continuously counts from the beginning of the sweep until the excitation current reaches nb', and the counted value becomes 2h/mN during - times of the sweep.

Now, the gate GI is open while the count output of the counter C4 is from 0 to m, and the gate G4 increments the count by the fall of the output of the waveform shaping circuit 6 (the blood in Figure 2 c). It is supposed to be done.

Further, the pulses passing through the gate GI are counted by a counter C5. C5 is designed to carry at count no, and this carry signal resets the power converter C, C3 and the digital analog converter port A. The deflection input of the deflection coil 2 is controlled using the carry signal as a synchronization signal. No is determined so that the digital-to-analog conversion output corresponding to count no becomes the maximum excitation current. With this kind of configuration, ``electron beam B is pumped m times,'' and each time during this period, M skin and N/2h for that time are determined by counter CI? C3. This is input to counter C2. Thus, when the output of the waveform shaping circuit 6 falls at the m-th bridge, a signal is output from the counter C4 and the gate G is closed.The striped signal is a sweep signal in a normal scanning electron microscope. On the other hand, after the gate G is closed, the count of the counter C2 is 2 (na no 10 N 2 h), but 2 holds the sum of m times. This held value is m It is the average of the lens excitation current (na 10N/2) at the time of focusing in each sweep, and indicates the value of the excitation current that gives the most accurate convergence condition. The excitation current of the objective lens is determined by applying it to the converter DA through the gate G which is opened by the output of the tortoise count m.The gate G2 is opened during the sweep process from the beginning of the sweep until the excitation current reaches na. ing.

For this reason, it is opened at the zero count output of the G-shape counter C5 and closed at the rising edge of the output of the waveform shaping circuit 6.The embodiment shown in FIG. So, the digital to analog converter is DA turtle?
There are two of them, 2, and they can be selected and switched using a switch Sw. DA amount, output of DA2 is added to adder A
d and sent to the exciting current power source S. Components corresponding to those in the embodiment of FIG. 1 are given the same reference numerals. First, when the switch Sw is set to the coarse adjustment side and the device is operated, the first
The focus is adjusted in exactly the same way as the circuit shown in the figure. Next, switch Sw is switched to the fine adjustment side. Every time one pulse is input to DA2, the output increases by 1/no of that of DA. Therefore, during the sweep, DA2 changes by one step of the output of DAI. The output of this DA2 is added to the output of the DA tortoise, which is already fixed at the approximate position, and is sent to the exciting current power source S. Thus, when the circuit is operated again, DA2
The same operation as in the circuit shown in Figure 1 is performed again using , and the focus adjustment is completed after m sweeps. When changing the lens excitation current, there is a delay in establishing the lens magnetic field, so the sweep speed cannot be increased very much. In the embodiment shown in Fig. 3, two circuits are used, one for coarse adjustment and one for coarse adjustment, but since the range of change in the excitation current in the second adjustment is narrow, it is possible to follow the current by increasing the sweep speed, and the first adjustment is for coarse adjustment. The sweep speed may be fast, and therefore the overall adjustment time can be shortened. The automatic focus adjustment device of the present invention has the above-described configuration, and the lens strength that provides the in-focus state is determined by the average of several focus detection operations, so the accuracy of focus determination is high. As can be seen, there is an advantage that multi-stage adjustments such as coarse adjustment and fine adjustment can be easily performed with one device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the circuit configuration of an embodiment of the present invention; FIG. 2 is a signal waveform diagram illustrating the operation of the above circuit;
FIG. 3 is a block diagram showing another embodiment of the present invention. 1...Objective lens, 2... Deflection coil, 3...
...Sample, B...Electron beam "PG...Pulse oscillator, GI~G4...Gate, DA, DA1, D
A2...Digital to analog converter, CI to C5...
...Counter, 4...Electron beam detector, 5...Amplifier, 6...Waveform shaping circuit (level comparison circuit).
Next' figure is 2 figure is 3 figure

Claims (1)

[Claims] 1 Sweep the sample surface with an electron beam, etc., count the output pulses of the pulse oscillator during the sweeping process, convert the count signal from DA to change the lens strength, and measure the output pulses of the pulse oscillator with an m-ary counter. A counter is provided to count the carry signals of the same counter, and the count output of the counter that counts the carry signals until and while the focus evaluation signal for the video signal exceeds the reference level is counted by the electron beam. A scanning type that performs focus adjustment by integrating a signal obtained by adding 1/2 of the integrated output of the former and 1/2 of the integrated output of the latter during m sweeps, etc., as a digital signal indicating lens strength. Automatic focus adjustment device for electron beam equipment, etc.