JP2009302014A - Electron microscope, observing test-piece, test-piece base of electron microscope, and semiconductor wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electron microscope by which its noise-attenuation improving effect is obtained without depending on the states of the external environment. <P>SOLUTION: In the electron microscope 20 which generates observing signals based on such secondary electrons 14 generated from an observing test-piece 4 so as to scan the observing test-piece 4 by scanning an electron beam 16 along a plurality of lines , a standard signal is generated just before the scan of the lines or between the mutual scans of the lines based on the secondary electrons 14 generated by scanning a test-piece 5 for generating the standard signal by the electron beam 16. Further, the standard signal is converted into its spectrum of frequency space so as to generate a difference-signal spectrum between the standard-signal spectrum and an ideal-signal spectrum obtained when the standard signal has no noise. Moreover, a filter for attenuating the difference-signal is set in a filter processing portion 15 so as to perform a filter processing to the observing signal by using the filter. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electron microscope that scans an electron beam along a plurality of lines, an observation sample observed by the electron microscope, a sample stage of the electron microscope, and a semiconductor wafer.

  In the electron microscope, an image of an observed sample such as a semiconductor wafer is displayed. The displayed image includes noise. This noise is noise from the outside mainly due to electromagnetic waves, depending on thermal noise and the installation environment of the electron microscope. Therefore, in the electron microscope, frame integration processing and pixel integration processing are performed on the image signal of the image, or the scanning time is extended to increase the irradiation amount per unit area of the electron beam and emitted from the observation sample. The noise of the image (signal) is attenuated by increasing the secondary electrons and increasing the signal intensity of the image signal.

In addition, the pilot specimen (standard signal generation sample) is scanned a plurality of times to acquire an image signal, and the estimated relative characteristic function of the electron optical column of the electron microscope is estimated from the image signal. There has been proposed a method of correcting an image using the method (for example, see Patent Document 1).
JP-A-6-150866 (paragraphs 0009 and 0010)

  However, the noise generation situation may change depending on the surrounding environment, for example, the temperature, the operating condition of the equipment around the installation environment, etc. In such a case, the noise attenuation effect cannot be sufficiently obtained with the conventional technology. It was.

  Therefore, an object of the present invention is to provide an electron microscope, an observation sample, a sample stage for an electron microscope, and a semiconductor wafer in which the effect of improving noise attenuation can be obtained without depending on the state of the external environment.

  The present invention that has solved the above problem is an electron microscope that scans an electron beam along a plurality of lines and generates an observation signal based on secondary electrons generated from an observation sample on the scan, immediately before the scanning of the line. Alternatively, during scanning of the lines, the electron beam is scanned with a sample for generating a standard signal, a standard signal is generated based on the generated secondary electrons, and the standard signal is converted into a spectrum in a frequency space. And generating a signal spectrum of a difference between the spectrum of the standard signal and the spectrum of the ideal signal when there is no noise of the standard signal, setting a filter for attenuating the signal of the difference, and using the filter A filter process is performed on the observation signal.

  Further, the present invention is for generating a standard signal capable of obtaining a spectrum in the frequency space of an ideal signal when there is no noise of a standard signal generated based on secondary electrons generated by scanning an electron beam of an electron microscope. It is an observation sample (semiconductor wafer) provided with a sample (pattern) and a sample stage of an electron microscope.

  ADVANTAGE OF THE INVENTION According to this invention, the improvement effect of noise attenuation can be provided without depending on the condition of an external environment, and the electron microscope, observation sample, the sample stand of an electron microscope, and a semiconductor wafer can be provided.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted. In the embodiments described below, a scanning electron microscope will be described as an example of the electron microscope. However, the present invention is not limited to an electron microscope, but an inspection apparatus, a measurement apparatus, an exposure apparatus using a scanning electron beam, an ion implantation apparatus using a scanning ion beam, charged particles such as electrons, ions, and charged clusters. The present invention can be applied to a charged particle beam apparatus such as a processing apparatus using a scanning charged particle beam.

  FIG. 1 shows a configuration diagram of an electron microscope 20 according to an embodiment of the present invention. In the electron microscope 20, an electron beam 16 is emitted from the electron gun 1, and on the observation sample 4 and the standard signal generating sample 5 arranged on the sample stage 17 by the electron optical system of the converging lens 2 and the objective lens 3. It is converged finely and irradiated. The electron beam 16 is deflected by supplying a scanning signal from the electron beam scanning control means 10 to the deflection coil 12, and can scan the observation sample 4 and the standard signal generating sample 5. Note that, in the scan on the observation sample 4 and the scan on the standard signal generating sample 5, the center point position of the range scanned with the electron beam 16 is changed. The so-called image shift is performed when the scanning on the observation sample 4 and the scanning on the standard signal generating sample 5 are shifted to each other.

  When the electron beam 16 scans the observation sample 4 and the standard signal generation sample 5, secondary electrons 14 are generated, and the secondary electrons 14 are detected by the secondary electron detector 6. The detected secondary electrons (detection signal) 14 are converted into digital signals by the AD converter (unit) 7. A digital signal converted from the secondary electrons 14 generated when the electron beam 16 scans the observation sample 4 is an observation signal, and is generated when the electron beam 16 scans the standard signal generating sample 5. A digital signal converted from the secondary electrons 14 is a standard signal. The observation signal and the standard signal are received by the filter processing unit 15. In the observation signal and the standard signal, there is noise that has come to the secondary electrons 14 and the electron beam 16 that are themselves and their generation sources. Depending on the noise source, the frequency (distribution) and (spectrum) intensity of noise varies with time. However, if the time interval is shortened, the change can be regarded as small and constant. The frequency (distribution) and (spectrum) intensity of noise on the observation signal can be regarded as equal to noise on the standard signal generated before (immediately before) the observation signal. Therefore, the filter processing unit 15 specifies noise from the standard signal, sets a filter for attenuating the specified noise, and filters the observation signal with this filter, so that after the generation of the standard signal (immediately after ) Noise is attenuated from the observation signal generated. The standard signal may be generated after (immediately after) the observation signal. In this case, the observation signal may be stored until a filter is set based on the standard signal. The standard signal is preferably generated immediately before and immediately after the observation signal. However, the time interval between the time when the standard signal is generated and the time when the observation signal is generated depends on the lifetime of the noise to be attenuated. It can be set accordingly.

  FIG. 2 shows a block diagram of the filter processing unit 15. The filter processing unit 15 includes a signal switching unit 15a, an inverse filter processing unit 15b, and an inverse filter setting processing unit 15c. The electron beam scanning control means 10 outputs a timing signal to the signal switching unit 15a and the image processing device 9 in synchronization with the deflection signal output to the deflection coil 12 (see FIG. 1). Based on the timing signal, the signal switching unit 15a performs signal switching such that the observation signal is transmitted to the inverse filter processing unit 15b and the standard signal is transmitted to the inverse filter setting processing unit 15c.

  The inverse filter setting processing unit 15c receives a standard signal and converts the standard signal into a spectrum in a frequency space. By the way, as shown in FIG. 3A, the surface of the standard signal generating sample 5 is provided with a concavo-convex pattern in which the concavo-convex changes in a scanning direction of the electron beam 16 like a substantially sinusoidal curve, for example. . With such a concavo-convex pattern, the spectrum of the ideal signal 21 in the frequency space when there is no noise of the standard signal as shown in FIG. 3B can be calculated by simulation. The inverse filter setting processing unit 15c stores the spectrum of the ideal signal 21. Then, the inverse filter setting processing unit 15c generates a signal spectrum that is a difference between the spectrum of the standard signal and the spectrum of the ideal signal. Since the difference signal is noise that is currently generated, the inverse filter setting processing unit 15c uses a filter for attenuating the difference signal (noise) based on the spectrum of the difference signal (noise) in FIG. The inverse filter processing unit 15b shown in FIG. The inverse filter processing unit 15b performs filter processing on the observation signal using this filter. By this filter processing, noise included in the observation signal can be attenuated. The observation signal with the attenuated noise is transmitted to the image processing device 9.

  An image processing apparatus 9 shown in FIG. 1 receives an observation signal based on the timing signal from the electron beam scanning control means 10 and performs various image processing, or stores the observation signal in the image memory 8. Or The observation signal stored in the image memory 8 is read out at an arbitrary timing by the control of the image processing device 9 and further by the control of the computer 11 and integrated in units of one pixel data for further noise reduction of the observation signal. Processing, integration processing in units of frames, Fourier transform / inverse transform processing may be performed, and the observation signal that has undergone these processes may be stored in the image memory 8 again. The observation signal stored in the image memory 8 is appropriately transmitted to the display device 13 such as a liquid crystal monitor via the computer 11 and displayed on the display device 13.

  Next, a method for reducing the noise of the observation signal in the electron microscope 20 will be described. FIG. 4 shows a flowchart of a noise reduction method for reducing the noise of the observation signal. The operation subject of this flowchart is the electron beam scanning control means 10.

  First, prior to step S1, the computer 11 displays a selection request for selecting the image quality improvement mode or the normal image quality mode on the display device 13 using the GUI. The user sees this display and makes a selection using a GUI using an input device such as a keyboard and a mouse, and the computer 11 receives an input of the selection result. Based on this input, the computer 11 determines whether the mode set in the electron microscope 20 is the image quality improvement mode. If the image quality improvement mode is set, the filter processing unit 15 functions. If the image quality improvement mode is not set, the filter processing unit 15 does not function, and the observation signal input to the filter processing unit 15 attenuates noise. It will be output as it is.

  Next, the computer 11 displays a selection request as to whether or not to start observation on the display device 13. The user sees this display and makes a selection using the GUI, and the computer 11 receives the input of the selection result. Based on this input, the computer 11 determines whether or not to start observation. When it is determined that the observation is started, the computer 11 transmits an observation start signal to the electron beam scanning control means 10, and in step S1, the electron beam scanning control means 10 receives the observation start signal. When it is not determined that the observation is started, the computer 11 does not transmit an observation start signal to the electron beam scanning control means 10 and waits until it is determined that the observation is started.

  In step S <b> 2, the electron beam scanning control unit 10 determines whether an observation end signal is received from the computer 11. When the observation end signal is received (step S2, Yes), the noise reduction method is stopped, and when the observation end signal is not received (step S2, No), the process proceeds to step S3.

  In step S3, the electron beam scanning control unit 10 sets 1 to the number of lines N.

  In step S4, the electron beam scanning control means 10 supplies a scanning signal corresponding to the image shift to the deflection coil 12, and executes the image shift. In the image shift, the center point position of the scanning range of the electron beam 16 is set to the center point position of the scanning range when scanning the standard signal generating sample 5. Note that the electron beam 16 is blanked and not emitted so that the electron beam 16 is not irradiated onto the observation sample 4 or the like during the image shift.

  In step S5, the electron beam scanning control means 10 before scanning the standard signal generating sample 5, for example, at a timing parallel to the execution of the image shift in step S4, that is, at the start, middle or end timing of the image shift. The timing signal is generated and transmitted to the signal switching unit 15a. The signal switching unit 15a switches the connection to the inverse filter setting processing unit 15c at the timing when the timing signal is received. That is, the electron beam scanning control means 10 switches to the inverse filter setting processing unit 15c at a timing parallel to step S4 (step S5). As a result, the processing time of the noise reduction method can be shortened.

  In step S6, the electron beam scanning control means 10 supplies a scanning signal corresponding to the scanning of the standard signal generating sample 5 to the deflection coil 12, and the standard signal generating sample 5 on the standard signal generating sample 5 as shown in FIG. The sample scan 26 for signal generation is executed. The standard signal generating sample scan 26 generates secondary electrons 14 from the standard signal generating sample 5, and the secondary electrons 14 are detected by the secondary electron detector 6. The detected secondary electrons 14 are converted into a digital standard signal by the AD converter (unit) 7 to generate a standard signal. FIG. 6 shows a data pattern of a signal input to the filter processing unit 15. A standard signal is input after a signal generated at the time of image shift.

  In step S7, the electron beam scanning control means 10 supplies the deflection coil 12 with a scanning signal corresponding to the number N of return lines, and executes the return line 24 shown in FIG. Note that an image shift is performed on the return line 24. In the image shift, the center point position of the range in which the electron beam 16 is scanned is set to the center point position of the scanning range when the observation sample 4 is scanned. In the data pattern of the signal input to the filter processing unit 15 in FIG. 6, a signal generated at the time of return is input after the standard signal. Then, at the timing of this blanking, switching by a signal switching unit 15a described later and setting of an inverse filter described later are performed. Note that the electron beam 16 is blanked so as not to be emitted so as not to irradiate the observation sample 4 and the like with the electron beam 16 during the return.

  In step S8, the inverse filter setting processing unit 15c converts the received standard signal into a spectrum in the frequency space when the return line 24 is executed. For example, the standard signal 28, 29a as shown in FIG. Generate a spectrum of ~ 29d. In this data conversion, real-time characteristics are required, and it is desirable to use FFT (Fast Fourier Transform) in order to perform data conversion quickly. Step S8 ends when the input of the standard signal to the inverse filter setting processing unit 15c ends.

  In step S9, the inverse filter setting processing unit 15c performs the spectrum of the standard signals 28, 29a to 29d and the ideal signal 21 shown in FIG. 7B (FIG. 3B) when the return line 24 is executed. The spectrum is compared, and the spectrum of the difference signal is generated from the difference in signal strength between the standard signals 28 and 29a to 29d and the ideal signal 21. The spectrum of the difference signal is composed of standard signals 29a to 29d which are noises as shown in FIG. 7C, and the ideal signal 21 is omitted.

ここで、各周波数の振幅成分（信号強度）X（ejω）は下記の式２で表される。各周波数の振幅成分X（ejω）について、ノイズが無い状態で取得される理想信号２１と、実際に走査した場合における標準信号とで大小が比較される。そして、両者の振幅成分が一致しない周波数上の信号をノイズと見なすことで、ノイズ（差分信号）のスペクトルを生成することができる。

This comparison is performed by the following equation 1 where the non-periodic discrete time signal is x [n].

Here, the amplitude component (signal intensity) X (ejω) of each frequency is expressed by the following Equation 2. For the amplitude component X (ejω) of each frequency, the magnitude is compared between the ideal signal 21 acquired in the absence of noise and the standard signal when actually scanned. A spectrum of noise (difference signal) can be generated by regarding a signal on a frequency at which the amplitude components of the two do not match as noise.

  In step S10, the electron beam scanning control unit 10 uses the inverse filter setting processing unit 15c to filter the single or multiple filters so that the difference signal (noise) is attenuated when the return line 24 is executed. To decide.

  In step S11, the inverse filter setting processing unit 15c sets a filter constant in the inverse filter processing unit 15b, for example, a low-pass filter (LPF), a high-pass filter (HPF), a band-pass filter (BPF), a band elimination It is possible to set a nation filter (BEF), or a filter combining these. Specifically, as shown in FIG. 7C, so-called inverse filters 30a to 30d are set in the inverse filter processing unit 15b so as to be masked in correspondence with the standard signals 29a to 29d as noise.

  In step S12, the electron beam scanning control means 10 determines the timing before scanning the observation sample 4, for example, at a timing parallel to the execution of the return line in step S7, that is, at the start, middle or end timing of the return line. A signal is generated and transmitted to the signal switching unit 15a. The signal switching unit 15a switches the connection to the inverse filter processing unit 15b at the timing when the timing signal is received. Step S12 is performed in parallel with step S7 and in parallel with steps S8 to S11, and this can shorten the processing time of the noise reduction method.

  In step S13, the electron beam scanning control means 10 supplies a scanning signal corresponding to scanning on the observation sample 4 having the number N of lines to the deflection coil 12, and the observation sample 4 is observed on the observation sample 4 as shown in FIG. Scan 22 is performed. The observation sample scanning 22 generates secondary electrons 14 from the observation sample 4, and the secondary electrons 14 are detected by the secondary electron detector 6. The detected secondary electrons 14 are converted into an observation signal of a digital signal by the AD converter (unit) 7 to generate an observation signal. The observation signal includes, for example, observation signals 31a to 31c that are not caused by noise and observation signals 32a to 32c that are caused by noise as shown in FIG. The observation signal 31c shows an example in which an observation signal caused by noise overlaps with an observation signal not caused by noise. In the data pattern of the signal input to the filter processing unit 15 in FIG. 6, the observation signal is input after the signal generated at the time of return. Then, an inverse filter process is performed on the observation signal in the next step S14.

In step S14, the inverse filter processing unit 15b performs inverse filter processing on the observation signal. In the inverse filter processing, convolution integration is used as in a general digital filter. A specific calculation formula is expressed as follows with the output signal of the inverse filter (observed signal after noise attenuation) being g (t).

g (t) = ∫h (τ) t (t + τ) dτ

Here, h (t) (= ∫H (ω) exp (jωt) dω) is an impulse response, and H (ω) is a transfer function of the inverse filter.

  The observation signals 32a to 32c and a part of the observation signal 31c due to the noise shown in FIG. 7D are standard signals 29a to 29d due to the noise shown in FIG. The intensity is consistent. Therefore, as shown in FIG. 7 (e), the inverse filters 30a to 30d set in FIG. 7 (c) are used to standardize the observation signals 32a to 32c and a part of the observation signal 31c caused by noise in terms of signal intensity. It can be attenuated by the signal intensity of the signals 29a to 29d. Then, observation signals 31a to 31c that are not caused by noise can be obtained as the observation signals. The inverse filter processing unit 15 b transmits the observation signals 31 a to 31 c subjected to the inverse filter processing to the image processing device 9. Since the observation signals 31a to 31c do not include noise, the image quality of the image produced by the image processing device 9 based on the observation signals 31a to 31c is improved. According to the present embodiment, even if the noise changes in response to changes in the surrounding environment (for example, the temperature, the operating status of the equipment around the installation environment, etc.) Is extracted and compensated with an inverse filter, noise in the observation signal can be reduced and the image quality of the image can be improved in real time.

  In step S15, the electron beam scanning control means 10 determines whether or not the number of lines N has reached a predetermined maximum number of lines. With this determination, it can be determined whether the line has reached the last line of the frame. If the number of lines N has reached the predetermined maximum number of lines (step S15, Yes), the line has reached the last line of the frame, so the process returns to step S2, and the number of lines N has reached the predetermined maximum number of lines. If not (No in step S15), the line has not reached the final line of the frame, and the process proceeds to step S16.

  In step S16, the electron beam scanning control means 10 adds 1 to the number of lines N, and returns to step S4 and step S5. When returning to step S4 and step S5, as shown in FIG. 5, the image shift 27 is executed in step S4, the standard signal generating sample scan 26 is executed again in step S6, and the return line 25 is displayed in step S7. The observation sample scanning 23 is executed in step S13. Thus, the so-called lines of the observation sample scans 22 and 23 reach the final line. This is the end of one frame. One image obtained by observing the observation sample 4 can be formed with an observation signal for one frame. When the final line is reached, the observation signal is acquired from the line having the number of lines of 1 in order to acquire the observation signal for one frame again.

  In this embodiment, every time one line is scanned, the standard signal generating sample 5 is scanned to generate a standard signal and an inverse filter. If the generation of the inverse filter ends in the period of the return lines 24 and 25, the time required for producing an image formed in one frame does not become longer than in the conventional case. Conversely, if the generation of the inverse filter does not end in the period of the return lines 24 and 25, if one inverse filter is generated every time a plurality of lines are scanned, further every frame, every plurality of frames, Since the period of the observation sample scanning 23 as well as the period of the retrace lines 24 and 25 can be applied to the generation of the inverse filter, the time required for producing an image formed in one frame is longer than in the past. There is no. In this case, it is considered that the noise generated before a plurality of lines and before a plurality of frames is also generated in the line to be scanned next. That is, the noise life varies depending on the noise source, and if an inverse filter can be generated at a time interval shorter than the noise life, the noise in the observation signal can be attenuated. Conversely, the number of lines or frames may be set by the user or the designer based on the life of the noise, how many inverse filters are generated for each line, and how many inverse filters are generated for each frame. .

  In the present embodiment, the standard signal generating sample 5 is simultaneously placed as a separate sample on the sample stage on which the observation sample 4 is placed. However, the present invention is not limited to this. A generation sample 5 may be provided. More specifically, it may be a semiconductor wafer provided with a pattern that functions as the standard signal generating sample 5.

It is a block diagram of the electron microscope which concerns on embodiment of this invention. It is a block diagram of a filter process part. (A) is a perspective view of a standard signal generating sample, and (b) does not include noise with respect to the standard signal generated based on secondary electrons generated by scanning the standard signal generating sample with an electron beam. It is the spectrum of the ideal signal frequency space. It is a flowchart of the noise reduction method which reduces the noise of an observation signal. FIG. 4 is a perspective view of an observation sample and a standard signal generation sample arranged on a sample stage, and shows a scanning path of an electron beam by a deflection coil. It is an example of the data pattern of the signal input into a filter process part. (A) is a spectrum in the frequency space of a standard signal generated based on secondary electrons generated by scanning a sample for generating a standard signal with an electron beam, and (b) does not include noise with respect to the standard signal. (C) is a spectrum of a frequency space of a filter provided for each difference signal (noise) obtained by subtracting the ideal signal from the standard signal, and (d). Is a spectrum in the frequency space of the observation signal generated based on secondary electrons generated by scanning the observation sample with an electron beam, and (e) is a frequency space spectrum of the observation signal in which noise is attenuated by a filter. It is a spectrum.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electron gun 2 Converging lens 3 Objective lens 4 Observation sample 5 Standard signal generation sample 6 Secondary electron detector 7 AD converter 8 Image memory 9 Image processing device 10 Electron beam scanning control means 11 Computer 12 Deflection coil
DESCRIPTION OF SYMBOLS 13 Display apparatus 14 Secondary electron 15 Filter process part 15a Signal switching part 15b Reverse filter process part 15c Reverse filter setting process part 16 Electron beam 17 Sample stage 20 Electron microscope 21 Ideal signal 22, 23 Observation sample scanning (line)
24, 25 Return line 26 Sample scan for standard signal generation 27 Image shift 28 Standard signal 29a, 29b, 29c, 29d Standard signal (noise)
30a, 30b, 30c, 30d Inverse filter (mask)
31a, 31b, 31c Observation signal 32a, 32b, 32c Observation signal (noise)

Claims (6)

In an electron microscope that scans an electron beam along a plurality of lines and generates an observation signal based on secondary electrons generated from an observation sample on the scan,
Before scanning the lines or during the scanning between the lines, the electron beam is scanned with a sample for generating a standard signal, and a standard signal is generated based on the generated secondary electrons,
Converting the standard signal into a spectrum in frequency space;
Generating a spectrum of the difference signal between the spectrum of the standard signal and the spectrum of the ideal signal when there is no noise of the standard signal;
Set a filter to attenuate the difference signal,
An electron microscope characterized in that the observation signal is filtered using the filter.   The electron microscope according to claim 1, wherein the standard signal generating sample has an uneven pattern capable of calculating a spectrum of the ideal signal by simulation.   The image shift for changing a center point position of a range scanned with the electron beam is performed between the scanning of the line and the scanning of the standard signal generating sample. Electron microscope.   Provided with a standard signal generating sample capable of obtaining the spectrum of the ideal signal frequency space when there is no noise of the standard signal generated based on secondary electrons generated by scanning an electron beam of an electron microscope Characteristic observation sample.   Provided with a standard signal generating sample capable of obtaining the spectrum of the ideal signal frequency space when there is no noise of the standard signal generated based on secondary electrons generated by scanning an electron beam of an electron microscope An electron microscope sample stage.   A semiconductor comprising a pattern capable of obtaining a frequency space spectrum of an ideal signal in the absence of noise of a standard signal generated based on secondary electrons generated by scanning an electron beam of an electron microscope Wafer.

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