JP3206014B2 - Electron detector and scanning electron microscope using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope and similar devices, and more particularly to a scanning electron microscope using a pulse counting type detector as a sample image signal detector for secondary electrons or the like.

[0002]

2. Description of the Related Art In a scanning electron microscope, a finely focused electron beam is two-dimensionally scanned on a sample, a signal such as secondary electrons generated from the sample is detected and amplified, and this signal is converted into a luminance modulation signal as a cathode ray tube. The enlarged image of the sample is displayed on the screen. As means for detecting a signal, a scintillator (electron-light conversion element) that emits light when electrons are incident
And a photomultiplier tube (scintillator-
Photomultiplier type detector) is generally used, and the output current of a photomultiplier tube is amplified by an amplifier and directly used as a luminance modulation signal of a cathode ray tube, or once by an A / D converter. A signal is passed through a digital circuit such as an image memory and then converted into an analog signal again by a D / A converter to form a luminance modulation signal of a cathode ray tube, thereby forming an image.

In this method, a signal is treated as a continuous amount (analog amount). However, as another method, a signal to be detected is originally a set of electrons as particles, and the magnitude of the signal is the number of electrons. Based on this, there is a method (pulse counting method) in which individual electrons are separately separated and detected, and the number of detected electrons is counted to generate a signal.

In the pulse counting method, since the incident electrons are separately detected, it is necessary that the detector, the amplifier, and the counting circuit operate at a speed higher than the interval between the incident electrons. However, the secondary electron signal amount of the scanning electron microscope is at least several pA, and 1 pA is 6250 per second.
Since the number of electrons corresponds to 000 electrons, the above detection system needs an operation speed of more than 100 MHz (taking into account that the generation of electrons is a stochastic process) in order to count this without counting down. Since it was not easy to satisfy this, the pulse counting method was not used conventionally. However, in recent years, a high-speed detection system that satisfies the above conditions has been made possible, and the pulse counting method has been used in a scanning electron microscope.

The pulse counting method is described, for example, in the document "Secondar
y Electron Counting Images inSEM, Proceedings of th
e XIIth International Congress for ElectronMicrosc
opy, 1990, pp. 402-403, "(1) In the case of the same amount of secondary electrons, the S / N ratio is improved by several times in the case of the same amount of secondary electrons, Data with less noise can be obtained.

(2) In order to obtain data having the same S / N ratio, the amount of secondary electrons may be small. That is, the amount of primary electrons irradiating the sample can be reduced, and the spatial resolution can be improved, charging, sample contamination can be reduced, and sample damage can be reduced.

This is an excellent method.

A pulse counting type electronic detector is disclosed in the document "SE.
Electronic Counting Image Real-time Processing System in M, 132nd Committee of Japan Society for the Promotion of Science
As described in the 113th Meeting of the Society, 1990, pp. 131-136, it is composed of a scintillator and a photomultiplier tube, and this output pulse uses a discriminator for removing low-level electrical noise. It is counted by the counter via. The counter outputs a count value between clocks generated by the electron beam scanning circuit for each pixel time for each clock, and this data is sequentially written to the image storage circuit, and is converted into an analog signal by the D / A converter. It is converted into a signal and displayed as an image. At this time, in order to make the image contrast appropriate, for example, the data between the minimum value and the maximum value of the data of each pixel in the image storage circuit is calculated and corrected so as to fall within an appropriate data range, and then output. It is common to do.

[0009]

In the conventional method of treating a signal as an analog quantity, the magnitude of the voltage of the detector output becomes the image contrast as it is. Therefore, as a means for adjusting the image contrast, The voltage of the photomultiplier is adjusted or the gain of the amplifier is adjusted thereafter.

However, in the pulse counting type, the magnitude of the number of pulses of the detector output per pixel becomes the contrast of the image. In the above method, only the pulse peak value changes, and the number of pulses is It does not change. Therefore, another means is needed to adjust the number of pulses at the detector output.

Another problem peculiar to the pulse counting type is that when the scanning speed is changed, the time per pixel changes, so that the output value of the above counter, that is, the value obtained by counting the detector output during one pixel time, is obtained. Is to change.

[0012]

In order to solve this problem, means for freely increasing or decreasing the numerical value of the data of each pixel somewhere after the output of the detector is added to increase or decrease the contrast and scan. What is necessary is just to control the increase / decrease value in order to correct a change in data due to a change in time.

[0013]

The numerical multiplication means is inserted after the detector. The multiplied value of this multiplying means can be changed at appropriate intervals by means of (1) contrast adjusting means.

(2) When the scanning speed is changed, the scanning speed is changed in inverse proportion to one pixel time.

Furthermore, a numerical value adding means is inserted into the output of the detector or the output of the multiplying means in order to adjust the offset value of the image data, that is, the brightness of the image. This added value can be changed at appropriate intervals by means for adjusting the brightness of the image.

With these two means, the contrast of the displayed image can be adjusted to an appropriate value.

[0017]

FIG. 2 shows an example of a conventional pulse counting type detector. Secondary electrons emitted from the sample 2 by the irradiation of the primary electron beam 1 enter the scintillator 3 and emit light.
The emitted light is amplified by the photomultiplier tube 4 and output as a pulse train. This pulse train is amplified by the amplifier 5 and input to the counter 7 via the discriminator 6. The discriminator 6 eliminates the influence of electrical noise by passing only pulses having a certain magnitude or more and blocking pulses having a small peak value. The scanning circuit 11 drives the deflection coil 12 to scan the electron beam 1, generates a clock pulse at a time interval corresponding to one pixel of an image, and inputs the clock pulse to the counter 7. The counter 7 outputs the count value at the moment when the clock pulse is input and is reset, and starts counting the data of the next pixel. The image memory 8 stores output data of the counter 7 according to address data generated by the scanning circuit 11. When the image data of one screen has been stored or the data of the set multiple scans have been added, the data in the image memory 8 is adapted to the D / A converter 9 and the image display monitor 10. The image data is output at the speed, and the image is displayed on the image display monitor 10.
The computer 13 performs overall control and, as described above, calculates data for making a display image have an appropriate contrast.

FIG. 1 shows one embodiment of the present invention, wherein a digital multiplier 14 and a digital multiplier 14 are provided before the image memory 8 of FIG.
The digital adder 15 is inserted. Multiplier 14
Multiplies the output (S) of the counter 7 by a multiplier (M) given from the computer 13. The adder 15
The output (S ') of the multiplier 14 is also connected to the computer 13
And outputs the result to the image memory 8. The computer 13 calculates a multiplier (M) and a numerical value (B) from the input from the operation switch 16 for adjusting the contrast and brightness of the image, the scanning speed of the scanning circuit 11, and the number of image additions. Are set in the multiplier 14 and the adder 15, respectively.

[0019]

[Table 1]

Table 1 shows an example of setting a standard value of the multiplier (M). In this example, the minimum scanning speed is 100 seconds, and a numerical value obtained by multiplying this by the reciprocal of the scanning speed ratio is used. The value of 10 at 100 seconds is
As described above, this value is used for contrast adjustment.
This is because the adjustment can be performed in 10% steps (between 1 and 100). When repeatedly adding images, a value obtained by dividing this value by the number of additions N is set. In addition,
The reason for limiting the value of N at 1/30 seconds to 30 or more is to prevent the value of M from becoming too large and the number of bits of the circuits after the multiplier from becoming too large. At the scanning speed, for example, as described above, the incident electron amount 1p
In A, the number of electrons arriving during 130 ns averages 0.1.
This is because there are only eight, and an image is not obtained unless the addition is substantially repeated.

FIG. 3 shows an input / output value conversion circuit 17 which replaces the functions of the multiplier 14 and the adder 15 with a simpler configuration. In this method, the memory element 18
Write data proportional to the address value. The memory element 18 is set to the reading mode during operation, the output data of the counter 7 is connected to the address input, and the data written at the address corresponding to the value is output to the image memory 8. . The address input of the memory element 18 can be connected to the computer 13 by the switching switch 19, and when changing the multiplication value M and the addition value B, it is connected to the computer 13 to rewrite the data.

[0022]

[Table 2]

Table 2 shows an example of data to be set. This example is a case where the counting capacity of the counter 7 is 8 bits. Although high-speed digital multipliers and adders are quite large circuits, this method can achieve the same function with relatively simple circuits.

[0024]

According to the present invention, it is possible to freely adjust the contrast and brightness of an image in a pulse counting type detector, similarly to a conventional detector which treats a signal as an analog quantity. When an image is once stored in an image memory and then displayed, it is possible to perform an operation on the data in the memory, as described above. However, when adjusting the focus, it is necessary to be able to observe the image in real time. According to the method, it can be realized without any problems.

Also, in this type of apparatus, unlike a television or the like, the signal amount is small, so that a very low scanning speed is required to obtain a high quality image. In general, it is necessary to make the scanning speed drastically changeable because of the need to make adjustments while the scanning speed is high. However, this can also be corrected in real time by implementing the present invention, which has a very great effect on improving operability and, consequently, data quality.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention applied to a scanning electron microscope.

FIG. 2 is a block diagram showing a configuration of a conventional pulse counting method.

FIG. 3 is a block diagram showing another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Primary electron beam, 2 ... Sample, 3 ... Scintillator, 4 ... Photomultiplier tube, 5 ... Amplifier, 6 ... Discriminator, 7
... Counter, 8 ... Image memory, 9 ... D / A converter, 10
... image display monitor, 11 ... scanning circuit, 12 ... deflection coil, 13 ... computer, 14 ... digital multiplier, 15
... Digital adder, 16 ... Operation switch, 17 ... Input / output conversion circuit, 18 ... Memory element, 19 ... Replacement switch.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koshio Kageyama 882 Ma, Katsuta-shi, Ibaraki Hitachi, Ltd. Naka Plant (56) References JP-A-3-46743 (JP, A) (58) Investigated Field (Int.Cl. ⁷ , DB name) H01J 37/28 G01T 1/164

Claims (5)

(57) [Claims] 1. A pulse counting type electron detector, the output of the detector, via the image recording circuit, or directly,
You supply collector as a luminance signal of the display device is converted into a continuous amount
In the child detector, a time per pixel of the electron beam scanning is added to an output of the detector.
Equipped with a numerical multiplying means for multiplying a numerical value that is inversely proportional to
Electron detector characterized by comprising Te. 2. A method according to claim 1 , wherein the multiplication value of said numerical multiplication means is calculated for each pixel of electron beam scanning.
Value that is inversely proportional to the product of the time
An electronic detector, characterized in that: 3. The method according to claim 1 , wherein the multiplication value of said numerical multiplication means is further manually or automatically calculated.
Multiplied by any number generated by contrast setting means
An electronic detector, characterized in that: 4. The method according to claim 1 , further comprising adding or subtracting an arbitrary value to an output of said detector.
An electronic detector characterized by adding means for calculating . 5. The scanning electronic device according to claim 1 , further comprising the electronic detector.
Child microscope .