JPS6114631B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus monitoring method that facilitates focus alignment in a scanning electron microscope.

It is well known that in order to perform high-resolution observation in a scanning electron microscope, it is necessary to reduce the diameter of the probe, and that focusing the objective lens is the most important operation for this purpose. Conventionally, various studies have been conducted regarding this focusing, and automatic focusing devices have already appeared. However, since such devices are expensive, they can only be used in high-end machines, and are only used in popular and simple models.
A method has been used in which a video signal is displayed as a waveform on a cathode ray tube by amplitude modulation (deflection modulation), and the objective lens current is adjusted so that the peak value of the waveform is maximized. However, with this method, it is not possible to observe images on the same cathode ray tube, and it is not easy to confirm slight changes in the peak value when adjusting the objective lens current. It was a cumbersome operation.

In order to overcome these drawbacks, a saw-tooth signal, for example, which changes continuously as the cathode ray tube screen is scanned in one direction, is supplied to the excitation coil of the objective lens. A device has also been considered in which a scanned image based on the current value is observed and the excitation current of the objective lens is adjusted so that the sharp image portion is located at the center in the scanning direction. However, in such devices, the focused area is only an extremely narrow linear area, so it is difficult to pinpoint that area, and it is impossible to accurately focus. .

Therefore, the present invention proposes a focus monitoring method that allows even an amateur to easily adjust the focus while observing an image.

FIG. 1 is a block diagram showing an embodiment of the focus system, and numeral 1 indicates a column of a scanning electron microscope. An electron gun 2 is provided in the upper part of the column,
An electron beam is emitted toward the sample 3 placed at the bottom of the column. This electron beam is focused onto the sample 3 by a focusing lens 4 and an objective lens 5 to have a minimum spot diameter, and is deflected by X, Y deflection coils 6X and 6Y provided between the focusing lens and the objective lens. . Horizontal and vertical sawtooth wave signals are sent to this deflection coil from a horizontal and vertical scanning signal generation circuit 7 via a magnification adjustment circuit 8, so that the electron beam covers a certain area on the sample two-dimensionally. Scan to. Secondary electrons and reflected electrons scattered from the sample by the scanning are detected by a detector 9 and applied to a brightness modulation grid of a cathode ray tube 12 via an amplifier 10 and an adder circuit 11. Since horizontal and vertical scanning signals are also supplied from the scanning signal generation circuit 7 to the deflection coils 13X and 13Y of the cathode ray tube, the scanning on the sample 3 is synchronized, and the detector 9 is displayed on the screen. A sample image whose brightness is modulated by the signal from is displayed. Reference numeral 5a denotes an auxiliary coil for the objective lens, to which a focusing current is supplied.
The main excitation coil of the objective lens 5 may be used without providing this auxiliary coil. Clipping circuits 14a and 14b clip the vertical scanning signals from the scanning signal generation circuit 7 to different values, respectively, and output the signals to the switches S ₁ and S _{2 .}
The signal is supplied to the adder circuit 15 via. 16a, 16
b is a comparison circuit to which the vertical scanning signal is supplied;
It is compared with the reference signals of the reference power supplies 17a and 17b, and when the two match, an output signal is generated. This signal is passed through the adder circuit 15 via switches _S3 and _S4 , respectively.
is supplied to The switches S ₁ , S ₂ , S ₃ and S ₄ are interlocked, and each has three connection terminals a, b,
c, S ₁ and S ₂ have terminals a and c not connected, and b
The terminals are connected to clip circuits 14a and 14b. In addition, in switches S ₃ and S ₄ , terminal a
and b are not connected, and terminal c is the comparison circuit 16a, 16b.
It is connected to the. 18 is a variable DC power supply;
It can be set to any value between +E and -E, and its output is supplied to the adder circuit 15. The output signal of this adder circuit is the auxiliary coil 5a of the objective lens.
and is used to substantially vary the focal length of the objective lens. The comparison circuit 16a, 1
A part of the output of 6b is sent to the differentiating circuit 19, and the pulse signal generated at the rising edge of the output is sent to the switch _S5 ,
It is applied to the brightness modulation grid of the cathode ray tube 12 through the summing circuit 11.

In such a configuration, when the switches S ₁ to _{S 4} are connected to a, only the variable DC power supply 18 is connected to the adder circuit 15, so that a uniform scanning image can be obtained over the entire area on the cathode ray tube 12. Therefore, after adjusting the variable DC power supply so that the output becomes zero,
Switch the switches S ₁ to S ₄ to the b terminal. As a result, the clip circuits 14a and 14b are connected to the adder circuit 15.
Since the auxiliary coil 5a is connected to the auxiliary coil 5a, a fluctuating current is supplied to the auxiliary coil 5a. in this regard,
This will be explained with reference to FIG. FIG. 2a shows a vertical scanning signal, and this signal is transmitted to the clip circuit 14.
When the signals are supplied to the terminals a and 14b, the clipping circuit 14a clips the signal of _-V1 or higher and outputs a signal as shown in FIG. 2b. This signal is a signal whose intensity changes over time only during one-third of the period on the negative side of one vertical scanning signal, and has a constant intensity during the other periods. The other clip circuit 14a outputs a signal whose intensity changes temporally only during the positive 1/3 period of the vertical scanning signal below + _V1 as shown in FIG. 2c.
The output signals from both clip circuits 14a and 14b are sent to an adder circuit 15, where they are added to the output from the variable DC power supply 18 (zero in the current state), resulting in the signal shown in FIG. 2d, which is supplied to the auxiliary coil 5a. . This causes the auxiliary coil to be energized to a continuously varying negative intensity during the first third of each vertical scan, zero during the middle period, and continuously energized during the last third of each vertical scan. Excited to varying positive intensity. As a result, on the screen of the cathode ray tube 12, as shown in FIG. , the upper area A displays an image when the magnet is excited to a continuously changing negative intensity, and the lower area C displays an image when the magnet is excited to a continuously changing positive intensity. . Therefore, the images displayed on the cathode ray tube are all images with the same focus state in the central region B, but in the regions A and C, they are images with consecutively different focuses in the vertical direction.
In order to simplify the display, in the figure, the focus state is expressed by the sparseness and density of the horizontal lines, and the denser the horizontal lines, the better the focus is.

By the way, since the vertical scanning signal from the scanning signal generation circuit 7 is also sent to the comparison circuits 16a and 16b, the voltages of the reference power supplies 17a and 17b are set to -V ₁ and +V ₁ , which are the same as the clip voltage. Then the second
As shown in Figures e and f, a pulse signal that rises when the scanning signal matches the reference voltage is obtained. This pulse signal is differentiated in a differentiating circuit 19, and as shown in FIG. As a result, bright lines (or dark lines) are displayed at the boundaries between areas A and B and B and C, as shown by Y ₁ and Y ₂ , and each area is clearly divided.

In FIG. 3a, the image in the central area B is uniformly blurred as a whole, and the blur in area C is even more intense, and the blur becomes larger as it goes downward. On the other hand, in area A, the horizontal lines are denser than in area C, and the area near line P is extremely dense, indicating that the focus is aligned along this line. Note that, in reality, such a line is not displayed, but an image that is best focused along the line, and an image that becomes increasingly blurred as it moves away from the line in the vertical direction.

Since the line P corresponds to P' of the signal shown in FIG. As shown in Figure b, the image displayed in central area B is in the optimal state, and both areas A and C are in B
The blur increases as the distance from the area increases, and an image with approximately symmetrical blur between A and C is obtained. Then, when switches S ₁ to _{S 4} are switched to terminal a and switch S ₅ is turned OFF, cathode ray tube 1
2, a focused image is displayed on the entire screen.

On the other hand, when switches S ₁ to _{S 4} are switched to terminal C,
Comparison circuit 1 replaces clip circuits 14a and 14b
6a and 16b are connected to the adder circuit 15. In this adder circuit, the output signals from the comparator circuits 16a and 16b shown in FIG. 2e and f and the variable DC power supply 1
The output signal from 8 (initially zero) is added, and the signal shown in FIG. 2h is sent to the auxiliary coil 5a. When excited with such a signal, the cathode ray tube 1
Images with uniform focus are obtained in each of the areas A, B, and C on the two screens. Then, the images of each region are observed, and the variable DC power supply 18 is adjusted so that the image of the central region B is in the highest focus state. Thereafter, by switching the switches S ₁ to _{S 4} to the a terminals, an image in full focus can be obtained on the screen of the cathode ray tube.

FIG. 4 shows a waveform supplied to the auxiliary coil 5a for explaining another embodiment, and FIG.
In the illustrated embodiment, the clip circuits 14a, 14
b, comparison circuits 16a, 16b and variable DC power supply 1
When all the output signals of 8 are added in the adding circuit 15, the signals on both sides change stepwise and continuously over time with respect to the signal at the center. In addition, in the case of figure b, the signals on both sides (signals corresponding to image areas A and C) are changed intermittently. This can be achieved by

With the configuration detailed above, the divided area A,
The focus state can be monitored from the image states of B and C, and at that time, focus can be easily adjusted by adjusting the DC current supplied to the objective lens auxiliary coil so that the image in the central area B shows the optimum contrast. Even an amateur can perform accurate correction operations. In particular, since the central region B is an image showing a wide and uniform focus state, the focus state can be monitored regardless of the size of image grain.

In the above embodiment, the output signal of the adder circuit 15 is introduced into the auxiliary coil 5a, but the objective lens 5
It may be introduced into the main coil of. Further, although the case where the image is divided into three areas has been described, the case where the image is divided into an odd number of areas may also be used. In this case, at least the central region needs to be an image generated by excitation with a constant signal value. Furthermore, vertical scanning signals are supplied to the clipping circuits 14a, 14b and comparison circuits 16a, 16b, and the screen is divided vertically in synchronization with the vertical signals, but each horizontal scanning signal is supplied to the above circuits. In this way, it is possible to obtain images partitioned in the horizontal direction by horizontal synchronization. Furthermore, it is convenient if the range of current change in both side regions is not fixed, but is varied manually or automatically in conjunction with variable magnification or high voltage switching.

Furthermore, the present invention can also be applied to monitoring two-dimensional focus deviation, that is, astigmatism. FIG. 5 is a diagram for explaining this, and unlike the above, it shows a case where the screen is divided into three parts in the horizontal direction in synchronization with horizontal scanning. First, suppose that an image as shown in FIG. 5a is obtained on the cathode ray tube 12 when a three-step stepped wave synchronized with horizontal scanning as shown in FIG. 2h is supplied to the auxiliary coil 5a. In this state, focus shift and astigmatism appear simultaneously, and area D is an image when the sample is irradiated with an electron beam spot near the circle of least confusion.
There is no image flow, and overall blurring occurs depending on the size of the spot. Areas E and F are irradiated with a spot above or below the circle of least confusion, and because the spot is elliptical due to astigmatism, the image flows according to the direction of the ellipse. Moreover, the size of the flow at E and F,
and the degree of blur of the image is different. When such an image is obtained, adjust the variable DC power supply 18 to move the image with no flow to the central area E as shown in FIG. I'll do what I do. As a result, the focus (one-dimensional focus) adjustment is completed, so the intensity of the variable DC power source 18 is kept fixed as it is, and the astigmatism correction device (not shown) is controlled. When an image as shown in FIG. 5c is obtained by this control, that is, when there is no flow in areas D and F and the central area becomes an extremely sharp image, the astigmatism has been corrected. In addition, in the above, the second
Although the screen is divided using the signals shown in FIG. 2H, it goes without saying that the signals shown in FIGS. 2D and 4 may also be used.

As described above, in the present invention, one period of horizontal or vertical scanning is temporally divided into three parts, and the central area is a constant value signal whose level can be arbitrarily varied, and both peripheral areas are the central area. a signal that is low on one side and high on the other side and that varies substantially continuously in relation to said scanning;
Since this signal is supplied to the excitation coil or auxiliary coil of the objective lens, when the objective lens current is adjusted so that the linear portion that appears to be sharp at the periphery of the screen moves to the center of the screen, the screen Since the image corresponding to this linear portion is developed in a relatively wide area in the center, the sharpness of the image in this central area is maximized, and the adjustment is made so that the blur increases symmetrically around this central area. By doing this, the objective lens can be focused easily and accurately.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIGS. 2 and 3 are diagrams explaining its operation, FIGS. 4a and 4b are signal waveform examples showing another embodiment, and FIG. It is a figure which shows another application example. DESCRIPTION OF SYMBOLS 1... Column, 2... Electron gun, 3... Sample, 4... Focusing lens, 5... Objective lens, 5a... Auxiliary coil, 6
X and 6Y...deflection coil, 7...scanning signal generation circuit, 8...magnification adjustment circuit, 9...detector, 11...addition circuit, 12...cathode ray tube, 14a, 14b...clip circuit, 15...addition circuit, 16a, 16b ...Comparison circuit, 17a, 17b...Reference power supply, 18...Variable DC power supply, 19...Differentiating circuit, _S1 to _S5 ...Switch.

Claims (1)

[Claims] 1 One period of horizontal or vertical scanning is temporally divided into three parts, and the center area has a constant value signal whose level can be arbitrarily varied, and both peripheral areas have one side lower than the center area signal. , on the other side to produce a signal that changes substantially continuously in relation to the scanning, and this signal is supplied to an excitation coil or an auxiliary coil of the objective lens, so that each of the divided regions in the signal is A focus monitoring method in a scanning electron microscope, characterized in that a sample image corresponding to a signal is divided and displayed on a cathode ray tube screen.