JPS62254350A - Improving method for sn ratio of digital scanning image in scanning type electron microscope - Google Patents

Abstract

PURPOSE:To improve a SN ratio by one-time scanning, by digitally converting image signals and synchronizing with digital scanning signals from a timing generator circuit, to give them to an integrating means, and then taking signals many times during one scanning step. CONSTITUTION:Pulses in a timing generator circuit 11 are given to X and Y directional scanning generators 12 and 13, and made to become analog in respective D/A converters 14 and 15, and then supplied to deflectors 3 and 4 in a scanning-type electron microscope 18, to scan a sample 8. The image signals are made to become digital signals in an A/D converter 19, and inputted to an image-signal integrating circuit 20, together with pulses in the timing generator circuit 11. And, synchronizing with the digital scanning signals, a plural number of integrating timing pulses are generated during one scanning step, to be added to the image signals, and then given to a memory 21. Therefore, a SN ratio can be improved by one-time scanning, and besides troubles on vertical fringes of images can be prevented.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an electron microscope, particularly a scanning electron microscope, and more particularly to an S
The present invention relates to a method for improving the /N ratio.

(Prior technology) A scanning electron microscope is a device that, like a transmission electron microscope, irradiates a sample with an electron beam and uses the short wavelength of the electron beam to observe extremely small substances, especially in the atomic and molecular regions. be. In Fig. 3, 1 is an electron gun, 2 is a focusing lens, 3 is an X-wheel deflector, 4 is a Y-axis deflector, 5.6 is an X-axis,
This is a Y-axis scanning signal generator. The electron beam emitted from the electron gun 1 is focused by a focusing lens 2, and then sent to a scanning signal generator 5.6.
The deflector 3.4 deflects the electron beam according to the scanning signal from the electron beam. The electron beam is further focused by an objective lens 7 to form an image on a sample 8. This electron beam is scanned over the sample 8 by an X-axis deflector 3 and a Y-axis deflector 4. The electron beam irradiating the sample 8 causes the sample 8 to emit secondary electrons 1 reflected electrons, etc. For example, if they are secondary electrons, they are detected by a secondary electron detector 9 and displayed on a display device @10. The output signals of the scanning signal generators 5 and 6 are sawtooth waves and are usually analog signals, and no digital signals are used. The advantages of digital scanning are as follows.

(b) Scan rate can be changed freely.

(b) Partial scanning can be easily performed.

(c) It is easy to stop the beam at one point, and the stopping position can be determined accurately.

Due to the above advantages, digital scanning has been used in some cases. By the way, in the case of digital scanning, the improvement of the SN ratio of an image has been carried out by a method of integrating images in an external storage device (frame memory) every time each frame (image of the screen) is scanned. If the number of scans is N, the improvement in the SN ratio is multiplied by ff, as is known as the improvement of the SN ratio by synchronous addition.

(Problems to be Solved by the Invention) In the case of digital scanning, the scanning time is determined by the settling time of the D/A converter for the scan signal in the X direction and the resolution (number of pixels) in the x and X directions.

Therefore, there is a limit to how short it can be. If the scanning time is T1 and the number of scanning is N, then the time required to improve the SN ratio by 1 is TN. In other words, in order to improve the S/N ratio, a scanning time proportional to the square of the desired improvement rate is required, and the image W
This results in a long AM time.

The present invention has been made in view of the above points, and its purpose is to provide a method that can improve the S/N ratio with one scan.

(Means for Solving the Problems) The present invention solves the above-mentioned problems in a method for improving the S/N ratio of a scanning electron microscope that obtains image signals by performing digital scanning. An X-direction and Y-direction scan generator that generates a scanning signal using pulses, a scanning signal generating means consisting of each D/A converter, an amplification means that supplies a scanning signal to a deflector, and an image signal from a scanning electron microscope. Then, an A/D converter for digital conversion and a timing generation circuit generate a plurality of integration timing pulses in synchronization with the X-direction scan and between 1X scan steps to perform synchronous addition of the image signals. The present invention is characterized by comprising an integrating means for improving the SN ratio and a recording means for storing it.

(Function) The present invention generates one digital scan step by using an MA calculation timing pulse that is generated in synchronization with a digital scan signal generated by a pulse oscillated by a timing generation circuit, and generates many pulses for one digital scan step. On the other hand, signals are taken in multiple times and synchronously added to achieve a good signal-to-noise ratio improvement with only one scan, and also remove the vertical stripe problem peculiar to digital images.

(Example) Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, reference numeral 11 denotes a timing generation circuit that oscillates timing pulses and supplies signals to the X-direction scan generator 12 and the Y-direction scan generator 13. The output signal of the X-direction scan generator 12 is a digital ramp signal, which is converted into an analog signal by the D/A converter 14 and becomes a stepped sawtooth wave (ramp waveform).

The Y-direction scan generator 13 is similar, and only has a different frequency. The timing of the above signals is the second
As shown in the figure. In the same figure, (A) shows the timing pulse that is the output of the timing generation circuit °11, (B) shows the waveform of the X-direction scan signal that is the output of the D/A converter 14 generated based on the timing pulse, and (C) shows the same waveform. This is the waveform of the Y-direction scan signal that is the output of the D/A converter 15. This scan signal is amplified by amplifiers 16 and 17, and is then amplified by the X-axis deflector 3 and Y-axis deflector 4 of the scanning electron microscope 18.
The sample 8 is scanned by deflecting the electron beam in the X-axis and Y-axis directions. An image signal caused by, for example, secondary electrons emitted from the sample 8 is detected by a secondary electron detector 9. This image signal is sent to the A/D converter 19. The timing generation circuit 11 generates trigger pulses synchronized with the timing pulses for the X-direction and Y-direction scan signals described above and sends them to the A/D converter 19, and the image signal is generated by the trigger pulses. It has been taken in.

The image signal converted into a digital signal by the A/D converter 19 enters an image signal integration circuit 20. Image signal integration circuit 20
, the trigger pulse from the timing generation circuit 11 is simultaneously received, and based on this trigger pulse, the A/D
Integral timing pulses are generated at time intervals of, for example, 80 ns, depending on the settling time of the converter 19, and the image signal output from the A/D converter 19 is taken into the image H4 calculation circuit 20 by the integrated timing pulses. It is rare. This timing relationship is shown in FIG. 2, and it can be seen in (d) of the figure that many image signal timing pulses are generated in one step (Dwell Time) of the X-direction scan signal. During the X direction scan signal, the electron beam of the scanning electron microscope 11118 is stopped and only one point on the sample 8 is irradiated.
During that time, the image signal integration circuit 20 captures the image signal with many pulses as shown in FIG. 2(d), so the image H
The quadrupling circuit 20 integrates the images by the number of times, divides the result by the number of times of integration, and stores the resulting value in the frame memory 21. This state is shown in FIGS. 2(e) and 2(f).

In (E), image signals are captured the number of times corresponding to the integrated timing pulse, and the results are stored in the memory 1 of the frame memory 21.
This shows the state stored in the partition. As a result of the above, it is possible to input the signal many times during 1DT of the X-direction scan, and synchronous addition to improve the S/N ratio can be performed in one scan. The required scanning time is equal to one scanning time, and the required time can be shortened while improving the S/N ratio. The improvement rate of this SN ratio is
It is determined by the number of integrated timing pulses in the IDT of the directional scan signal. The scanning speed of the X direction scan is D
The setting time of the A/D converter 14 determines the settling time of the A/D converter 14, and image capture is determined by the settling time of the A/D converter 19. The S/N ratio can be maximized by sending the signal to open the gate of the A/D converter 19 at minimum time intervals, and the settling time of the X-direction scan D/A converter 14 is also taken into account. By scanning at a high speed, the scanning time can be shortened. however,
Since the reduction in scanning time and the rate of improvement in the SN ratio are in a contradictory relationship, it is necessary to set them appropriately. Incidentally, a drawback of digital scanning is that vertical stripes appear in the digital scanning image as shown in FIG. This is based on the following reasons. An example of the process of converting a digital signal into an analog signal in the D/A converter 14 is shown in FIG. For example, as shown in the figure, 1000000 (64
) to 0111111 <63> in (b), and if all the lower bits change to 1 before the top bit momentarily becomes 0, it will temporarily change as shown in (c). As a result, the voltage becomes 1111111 (127>), and the voltage suddenly increases. In this state, as shown in Figure 6 (1), a peak called a glitch appears at the rise or fall time. This is shown in the example. In the case of 7 bits, the voltage increase is greatest at 1/2 of that point, 63 to 64, but at 3
1 to 32, and the same occurs in the case of 15 to 16. This appears as vertical stripes in Figure 4. For this reason, the image signal integration circuit 20 oscillates an integration timing pulse in response to a trigger from the timing generation circuit 11, but some pulses at the beginning of the oscillation pulse are discarded so as not to capture the image signal. If you do this, you won't get the vertical stripes that are typical of this digital scan.
can be done.

Statue 8! Calculating the number of signals obtained from the relationship between the number of I calculation timing pulses and the image signal by Vi and dividing by the number of times requires calculation by a computer and is difficult, so the number of times is equal to the number of brooms in 2. If you select a number that exceeds that number and store it in a binary register, discarding any odd numbers, when dividing by that number, you only need to shift the register by 2 zeros, so you don't need a special counter. For example, 16 times (24
), and if the 19 times product t>m, discard 3 times and get 1
If you take it as 6 times and shift it 4 times to the right, you have divided it by 16. This situation is shown in FIG.

As described above, according to the present invention, the SN ratio can be improved without increasing the scanning time by creating a large number of integrated timing pulses during 1DT of the X-direction scanning waveform.

Also, by discarding some of the integrated timing pulses, glitches can be eliminated and vertical stripes can be eliminated from the image.

(Effects of the Invention) As described in detail above, the SN ratio can be improved with one scan without repeating scanning to improve the SN ratio.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a signal timing diagram in an embodiment of the present invention, FIG. 3 is a diagram of a scanning electron microscope in which the present invention is used, and FIG. 4 is a digital scan. 5 is an explanatory diagram of the principle of glitch generation, FIG. 6 is an explanatory diagram of glitches, and FIG. 7 is an explanatory diagram of signal integration and average calculation. 1... Electron gun 2... Focusing lens 3...
・X-axis deflector 4...Y-axis deflector 5.6...
Scanning signal generator 7... Objective lens 8... Sample 9... Secondary electron detector 10... Display 11... Timing generation circuit 12... X direction scan generator 13... Y Directional scan generator 14.15...D/A converter 16.17...Amplifier 18...Scanning electron microscope 19...A/D converter 20...Image signal stack circuit 20...Frame memory Special applicant Nobuaki Kumagai, president of Osaka University, and one other person Patent attorney Fujiji Ijima No. 3
Electron gun 2; Focusing lens 3iXIk polarizer 4j Y-axis deflector 5.6; ) (c) ] 0 ] Figure 6 ■ d m ] ■■ " 1 Figure 7 1 N 3 4

Claims (1)

[Claims] In a method for improving the signal-to-noise ratio of a scanning electron microscope that obtains image signals by performing digital scanning, an X
direction and Y direction scan generators and respective D
A scanning signal generating means consisting of a /A converter, an amplifying means for supplying a scanning signal to a deflector, an A/D converter that takes in an image signal from a scanning electron microscope and converts it into a digital signal, and a trigger pulse from a timing generating circuit. A scanning electron microscope comprising an integrating means that synchronizes with the X-direction scan and generates a plurality of integrating timing pulses between 1X scan steps to improve the signal-to-noise ratio by synchronously adding the image signals, and a recording means for storing the integrating means. A method for improving the SN ratio of a digital scan image.