JPH0528465B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention focuses an electron beam emitted from an electron gun using a focusing lens, deflects it using an X-axis deflector and an X-axis deflector, and scans it over a sample. , relates to a scanning electron microscope that performs this scanning by digital scanning.

(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 Figure 3,
1 is an electron gun, 2 is a focusing lens, 3 is an X-axis deflector,
4 is a Y-axis deflector, and 5 and 6 are X-axis and Y-axis scanning signal generators. The electron beam emitted from the electron gun 1 is focused by a focusing lens 2 and deflected by deflectors 3 and 4 in response to scanning signals from scanning signal generators 5 and 6. The electron beam is further focused by an objective lens 7 to form an image on a sample 8. This electron beam is
The sample 8 is scanned by the axis deflector 3 and the Y-axis deflector 4. The electron beam that irradiated sample 8
emit secondary electrons, reflected electrons, etc. from
If it is a secondary electron, it is detected by the secondary electron detector 9 and displayed on the display device 10. The scanning signal generator 5
The output signals of 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 signal-to-noise ratio of an image has been achieved by integrating the images in an external storage device (frame memory) each 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 √, as is known as the improvement in the SN ratio by synchronous addition.

(Problem 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 y directions. come. Therefore, there is a limit to how short it can be. If the scanning time is T and the number of scanning is N, then
The time required to improve the S/N ratio by √ times is TN
becomes. In other words, in order to improve the SN ratio, a scanning time proportional to the square of the desired improvement rate is required, resulting in a long time required for image observation.

The present invention has been made in view of the above points, and its purpose is to provide a scanning electron microscope that can improve the signal-to-noise ratio in a short scanning time, and that can provide an easy-to-read screen without the vertical stripes characteristic of digital scanning. An object of the present invention is to provide a scanning electron microscope.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, focuses the electron beam emitted from the electron gun with a focusing lens, deflects it with an In a scanning electron microscope that scans the image and performs this scanning by digital scanning, there is a timing generation circuit that generates timing pulses and trigger pulses, and a timing pulse output from the timing generation circuit that receives the timing pulse and generates a digital scan signal in the X direction. an X-direction scan generator that generates a signal; a Y-direction scan generator that receives a timing pulse output from the timing generation circuit and generates a digital signal as a Y-direction scan signal; and the X-direction scan generator outputs a digital signal. a first D/A converter that converts an X-direction scan signal into an analog signal and outputs the same; and a second D/A converter that converts the Y-direction scan signal output from the Y-direction scan generator into an analog signal and outputs the analog signal. a converter, and the first and second D/
The X-direction and Y-direction scan signals output by the A converter are amplified, and the X-direction and Y-direction scan signals output by the A converter are
the timing generation circuit for each scanning point determined by the first and second amplifiers applied to the axis deflector and the Y-axis deflector, and the X-direction and Y-direction scan signals output from the X-direction and Y-direction scan generators; An A/D converter receives a plurality of consecutive trigger pulses from the scanning electron microscope, and each time the trigger pulse is received, the A/D converter converts the image signal outputted from the scanning electron microscope body into a digital signal and outputs the digital signal, and the timing generation circuit generates a digital signal. From a plurality of trigger pulses, a plurality of continuous integrated timing pulses for capturing the image signal digitally converted by the A/D converter are created, and the integrated timing pulses are used to generate consecutive integrated timing pulses at the same scanning point captured. an image signal integration circuit that adds image signals to obtain an image signal with an improved signal-to-noise ratio;
a frame memory for storing an image signal with an improved SN ratio;
As the plurality of consecutive integrated timing pulses for capturing the image signal digitally converted by the converter, the integrated timing pulse is generated at a timing after the output signal of the A/D converter is no longer affected by the glitches of the image signal. It is characterized by using only

(Function) In response to the timing pulse output from the timing generation circuit, the X-direction and Y-direction scan generators generate digital signals as X-direction and Y-direction scan signals, respectively. These scan signals are converted into analog signals, amplified, and then applied to the X-axis deflector and Y-axis deflector of the main body of the scanning electron microscope. For each scanning point determined by the X-direction and Y-direction scan signals, the A/D converter receives a plurality of consecutive trigger pulses from the timing generation circuit, and each time the A/D converter receives a trigger pulse, an output is output from the main body of the scanning electron microscope. Converts the amplified signal into a digital signal and outputs it. The image signal integration circuit is
A plurality of continuous integrated timing pulses are created from the plurality of trigger pulses generated by the timing generation circuit to capture the image signal to be digitally converted by this A/D converter, and the image signal is captured using the integrated timing pulses. , these consecutive image signals at the same scanning point are added together to obtain SN in one scan.
A new image signal with improved ratio has been obtained. This new image signal is stored in the frame memory. Here, as the plurality of continuous integrated timing pulses, only the integrated timing pulses that are generated at the timing after the influence of image signal glitches are eliminated on the output signal of the A/D converter are used, and the vertical stripes peculiar to digital scan are used. This makes it possible to obtain a screen that is easy to view.

(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, 11 is a timing generation circuit that generates timing pulses, and an X-direction scan generator 1.
2 and the Y direction scan generator 13. The output signal of the X-direction scan generator 12 is a digital ramp signal,
The signal is converted into an analog signal in the D/A converter 14 and becomes a step-like 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 as shown in FIG. In the figure, A is the timing pulse that is the output of the timing generation circuit 11, B is the waveform of the X-direction scan signal that is the output of the D/A converter 14 generated based on it, and C is the output of the D/A converter 15. This is a waveform of a certain Y direction scan signal. This scan signal is amplified by amplifiers 16 and 17, and enters the X-axis deflector 3 and Y-axis deflector 4 of the scanning electron microscope main body 18, and deflects the electron beam in the X-axis and Y-axis directions onto the sample 8. Let it scan. 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 is
In synchronization with the timing pulse for obtaining the direction and Y direction scan signals, a plurality of continuous trigger pulses are created for each scanning point determined by the X direction and Y direction scan signals, and the A/D converter 19 generates a plurality of consecutive trigger pulses.
I am sending it to The A/D converter 19 takes in the image signal using this trigger pulse and converts it into digital data. The image signal digitally converted by the A/D converter 19 is output to an image signal integration circuit 20. The image signal integration circuit 20 also receives a trigger pulse from the timing generation circuit 11 at the same time. Here, the trigger pulse is
The image signal integration circuit 20 is delayed by the time interval corresponding to the settling time of the A/D converter 19 with respect to the pulse, for example, at an interval of 80 ns. Generating integration timing pulse. The output signal of the A/D converter 19 is then taken into the image signal integration circuit 20 by this integration timing pulse. This timing relationship is shown in FIG. 2, and as is clear from FIG. A plurality of cumulative timing pulses are generated continuously. X direction scan signal
During the DT time, the electron beam from the scanning electron microscope main body 18 is stopped and irradiates only one point on the sample 8, and during this period, the image signal integration circuit 20 generates many integration timing pulses as shown in FIG. The image signal is captured by The image signal integration circuit 20 integrates the image signal the number of times the image signal is captured, divides the result by the number of integration times, and stores the obtained image signal in the frame memory 21. This state is shown in FIG. 2, E and F. In E, a state is shown in which image signals are captured the number of times corresponding to the cumulative timing pulse and the results are stored in one memory section of the frame memory 21. As a result of the above, it is possible to integrate signals many times during 1DT of the X-direction scan, and synchronous addition to improve the SN ratio can be performed in a single scan, which is necessary for improving the SN ratio. The scanning time is T, which is one scanning time, and the required time can be shortened while improving the SN ratio. The improvement rate of this SN ratio is determined by the number of integrated timing pulses in the IDT of the X-direction scan signal. The scanning speed of the X direction scan is determined by the settling time of the D/A converter 14, and the image capture is determined by the A/D
Since it is determined by the settling time of the converter 19, the trigger pulse from the timing generation circuit 11 is sent to the A/D converter 1 at the minimum time interval that takes into account the settling time of the A/D converter 19.
The signal-to-noise ratio can be maximized by feeding the signal so as to open the gate No. 9, and by scanning at a speed that takes into account the settling time of the D/A converter 14 for scanning in the X direction, the scanning speed can be improved. time can be shortened. However, since the shortening of the scanning time and the improvement rate of the SN ratio have a contradictory relationship, it is necessary to set them appropriately. Incidentally, a drawback of digital scan is that vertical stripes appear in the digital scan 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) in A to 0111111 (63) in B using a 7-bit digital signal.
When a binary number changes, if all the lower bits momentarily change to 1 before the top bit becomes 0, it temporarily changes to 1111111 (127) as shown in C.
, and the voltage will suddenly rise. In this state, as shown in FIG. 6, a peak called a glitch appears at the rise or fall time. In the case of 7 bits shown in the example, this is 1/2 of that, 63 to 64.
The voltage rise is the largest in terms of 31-32, 15-16
The same thing occurs in the case of This appears as vertical stripes in FIG. For this reason, an integrated timing pulse is created in the image signal integration circuit 20 using the trigger pulse from the timing generation circuit 11, but some pulses at the beginning of the integrated timing pulse are discarded and the image signal is not captured. By doing so, it is possible to obtain an easy-to-read screen without the vertical stripes that occur when using a digital scan.

Considering the relationship between the number of image integration timing pulses and the image signal, integrating the number of signals obtained and dividing by the number of times requires calculation by a computer and is difficult, so the number of times is selected as a number that is a power of two. If you store odd numbers beyond that number in a binary register, discarding them, then when you want to divide by that number, you only need to shift the register by a power of 2, so you don't need a special calculator. For example, if you integrate 16 times (2 ⁴ ) and have 19 times, you can discard 3 times to get 16 times, and shift it 4 times to the right to divide 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 scan waveform. Also, by discarding some of the integrated timing pulses, it is possible to eliminate glitches and eliminate vertical stripes from the image plane.

(Effects of the Invention) As explained in detail above, according to the present invention,
The image signal integration circuit creates a plurality of continuous integrated timing pulses for capturing the image signal digitally converted by the A/D converter from the plurality of trigger pulses generated by the timing generation circuit, and uses the integrated timing pulses. Since the image signals are acquired at the same scanning point and the consecutive image signals at the same scanning point are added, it is possible to obtain a new image signal with an improved signal-to-noise ratio in a short scanning time (single scanning).
As multiple consecutive integrated timing pulses,
Since only the integrated timing pulses generated at the timing after the influence of image signal glitches are eliminated on the output signal of the A/D converter are used, an easy-to-read screen without vertical stripes peculiar to digital scanning can be obtained.

[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 the embodiment of the present invention, FIG. 3 is a diagram of the main body of a scanning electron microscope in which the present invention is used, and FIG. 4 is a digital FIG. 5 is an explanatory diagram of the principle of generation of glitches, 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 device, 11...Timing generation circuit, 12...X direction scan generator, 13...Y direction scan generator,
14, 15...D/A converter, 16, 17...
...Amplifier, 18...Scanning electron microscope, 19...
A/D converter, 20... Image signal integration circuit, 2
0...Frame memory.

Claims (1)

[Claims] 1. In a scanning electron microscope, an electron beam emitted from an electron gun is focused by a focusing lens, deflected by an X-axis deflector and a Y-axis deflector, and scanned over a sample, and this scanning is performed using a digital scan.
a timing generation circuit that generates a timing pulse and a trigger pulse; an X-direction scan generator that receives the timing pulse output from the timing generation circuit and generates a digital signal as an X-direction scan signal; and an X-direction scan generator that generates a digital signal as an X-direction scan signal; a Y-direction scan generator that receives a timing pulse and generates a digital signal as a Y-direction scan signal; and a first D/D converter that converts the X-direction scan signal output from the X-direction scan generator into an analog signal and outputs the analog signal. A converter, a second D/A converter that converts the Y-direction scan signal outputted by the Y-direction scan generator into an analog signal and outputs it, and an direction,
First and second amplifiers amplify the Y-direction scan signal and apply it to the X-axis deflector and Y-axis deflector of the scanning electron microscope body, respectively, and the X-direction and Y-direction scan signals output from the X-direction and Y-direction scan generators. For each scanning point determined by the directional scan signal, a plurality of consecutive trigger pulses are received from the timing generation circuit, and each time the trigger pulse is received, the image signal output from the scanning electron microscope main body is converted into a digital signal. From the output A/D converter and a plurality of trigger pulses generated by the timing generation circuit, a plurality of continuous integrated timing pulses for capturing the image signal digitally converted by the A/D converter are created, and the integrated timing pulses are An image signal integration circuit that obtains an image signal with an improved SN ratio by adding consecutive image signals at the same scanning point captured using a timing pulse, and an image signal integration circuit that improves the SN ratio obtained by the image signal addition circuit. and a frame memory for storing image signals digitally converted by the A/D converter, and the plurality of continuous integration timing pulses for capturing the image signal digitally converted by the A/D converter in the image signal integration circuit include the A/D converter. A scanning electron microscope characterized in that only integrated timing pulses generated at a timing after the influence of image signal glitches are eliminated from the output signal of the scanning electron microscope are used.