JPH05297146A - Method and device for measuring corpuscular beam - Google Patents

Abstract

PURPOSE:To reduce the ratio of counting omission in detected pulse signals of secondary electrons of an electron microscope used for evaluation of semiconductor integrated circuit device. CONSTITUTION:The detected pulse signal detected by a secondary electron detector 3 is first transmitted to a discriminator 5. The discriminator 5 determines a threshold voltage and conducts the digital processing of the detected pulse signal. When the threshold voltage to the detected pulse signal is determined, the integrated type pulse wave height distribution showing the change of the digital pulse signal number obtained when the threshold voltage to the detection pulse signal is gradually changed is determined, and its peak value is taken as the threshold voltage. Thereafter, the number in the sampling period of the digital pulse signal outputted from the discriminator 5 is counted by a counter, and the data of the number is transmitted to an image memory 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a particle beam measuring method and apparatus technology, and for example, a scanning electron microscope (Scanning Electron Microscope) used in a manufacturing process of a semiconductor integrated circuit device.
croscope: hereinafter also referred to as SEM) and a technique effective when applied to a method and apparatus for measuring secondary electrons.

[0002]

2. Description of the Related Art SEM detects secondary electrons emitted from a sample when a finely focused electron beam is scanned on a sample surface at a constant speed, and then performs predetermined signal processing on the detected signal. The apparatus is capable of observing the sample surface by displaying the secondary electron image thus obtained on a CRT or the like.

By the way, conventionally, in the SEM, the secondary electrons have been measured as follows. First, the above-mentioned secondary electrons are detected by the secondary electron detector attached to the SEM, and this is converted into an electric signal. At this time, for example, in FIG.
It is assumed that the detection pulse signal as shown in FIG. In addition,
In FIG. 8, a pulse signal having a relatively small peak value is
Noise such as thermal noise of the secondary electron detector.

Then, the output signal of the secondary electron detector is transmitted to the discriminator. In the discriminator,
The detection pulse signal is converted into a digital pulse signal with the threshold level automatically set so that the signal and the noise are separated.

Conventionally, the threshold level at this time is obtained by, for example, obtaining an integrated pulse wave height distribution representing a change in the number of digital pulse signals obtained when the threshold level is variously changed, and then further calculating the integrated pulse wave height distribution. The voltage value V ₁ giving the minimum value of the curve obtained by differentiating as shown in FIG. 9 is automatically set.

It is known that the voltage value V ₁ giving the minimum value is at a level slightly higher than the peak value of the noise described above. Therefore, if the voltage value V _{1 is set} to the threshold level, the signal and the noise are reduced. This is because it is possible to effectively separate and.

After that, the output signal of the discriminator is transmitted to the counter. The counter counts the number of digital pulse signals within a predetermined period and stores the result in an image memory or the like. Then, by transmitting the information stored in the image memory to the CRT or the like, the secondary electron image, that is, the image of the sample surface becomes visible.

The SEM is described, for example, in "Ultrafine Machining Introduction" P164 to P168, issued by Ohmsha Co., Ltd., June 20, 1989, and the principle of SEM and the structure of SEM are explained. ..

[0009]

However, the present inventor has found that the above-mentioned conventional technique has the following problems.

Conventionally, in the case of automatically setting the threshold level at the time of digitizing the detection pulse signal of the secondary electron, consideration was given to separating the signal and noise, but the incident secondary electron As a result of the temporal density becoming higher than the temporal resolution of the measurement system, the density of the detection pulse signals becomes high, and sufficient consideration has not been given to the case where adjacent detection pulse signals may overlap.

Therefore, when the density of the detection pulse signals is high and the adjacent detection pulse signals are superposed, if the threshold level is automatically set by the conventional method, the number of the detection pulse signals is counted down, There is a problem that the resolution of the secondary electron image is reduced. This will be described with reference to FIGS. 10 and 11.

For example, assume that a detection pulse signal as shown in FIG. 10 is detected by the secondary electron detector. FIG. 10 shows a state in which a plurality of (for example, seven) detection pulse signals are superimposed in the period T ₁ . In addition,
T _X1 indicates a sampling period corresponding to one pixel.

In the conventional method, even in such a case, when the detection pulse signal is digitized, a voltage value slightly higher than the peak value of the thermal noise is applied to the threshold voltage V.
It will be automatically set as _TH0 .

However, as described above, the threshold voltage V
If set to _TH0 performs digital processing, in a period T _1, originally, for example, for seven digital pulse signal must be generated, as shown in FIG. 11, for example, one of the digital pulse signal It only happens to occur.

As a result, the counter originally has to count, for example, nine digital pulse signals in the sampling period T _X1 , but only three, for example, so that the resolution of the secondary electron image is increased. The problem of diminishing occurs.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a technique capable of reducing the rate of counting down detection pulse signals obtained by detecting particle beams. It is in.

Another object of the present invention is to provide a technique capable of quantitatively measuring the count-down ratio of detection pulse signals obtained by detecting a particle beam.

The above and other objects and novel features of the present invention will be apparent from the description of the specification and the accompanying drawings.

[0019]

Among the inventions disclosed in the present application, a representative one will be briefly described below.
It is as follows.

That is, the invention according to claim 1 is a pulse which represents a change in the number of digital pulse signals obtained when the threshold level for the detection signal is changed when digitally processing the detection signal of a predetermined particle beam. Obtaining the wave height distribution, automatically setting the threshold level that gives the peak value of the pulse wave height distribution as the true threshold level, converting the detection signal into a digital pulse signal, and the predetermined period obtained by the digital processing. And a step of counting the number of digital pulse signals in the particle beam measuring method.

According to a second aspect of the present invention, there is provided a particle beam measuring method for calculating a counting rate of detection pulse signals of the particle beam based on the state of the pulse height distribution and displaying the calculation result. Is.

[0022]

According to the invention described in claim 1, when the threshold level is set when the detection pulse signal is converted into the digital pulse signal, the level at which the number of the digital pulse signals becomes maximum within a predetermined period is set. By automatically setting the true threshold level, for example, in the SEM secondary electron measurement, the temporal density of incident secondary electrons becomes higher than the temporal resolution of the measurement system, resulting in an increase in the density of the detected pulse signal. Even if adjacent detection pulse signals are superposed, it is possible to reduce the number of detection pulse signals that are missed.

According to the second aspect of the present invention, the operator can quantitatively measure the rate of counting down the detection pulse signal of the particle beam.

[0024]

1 is an explanatory view of a particle beam measuring apparatus according to an embodiment of the present invention, FIG. 2 is a waveform diagram of a detection signal detected by a particle beam detector, and FIG. 3 is an integral type pulse height distribution. Graphs, FIGS. 4 and 5 are graphs showing other examples of the integrated pulse wave height distribution, and FIGS. 6 and 7 are signal waveform diagrams for explaining the digital processing of the detection signal waveform of FIG.

The particle beam measuring apparatus of this embodiment is, for example, SE
It is a secondary electron measuring device of M. FIG. 1 shows the configuration of the secondary electron measuring apparatus 1 and the SEM 2.

Secondary electron detector 3 of secondary electron measuring apparatus 1
Is a detector that detects secondary electrons ejected from the irradiation portion when the main surface of the semiconductor wafer W is irradiated with a narrowed electron beam E from the electron gun 2a of the SEM 2 at a constant speed and converts it into an electric signal. Is.

The secondary electron detector 3 has, for example, a detector having a structure in which a scintillator and a photomultiplier layer are combined or a high-speed response such as a microchannel plate in the multiplication part of the photomultiplier. A detector provided with a device is used.

A detection pulse signal, which will be described later, detected by the secondary electron detector 3 is transmitted to the amplifier 4. The amplifier 4 is a component for amplifying the detection pulse signal, and, for example, a high speed high band amplifier is used.

The detection pulse signal amplified by the amplifier 4 is
It is adapted to be transmitted to a discriminator (discrimination means) 5. The discriminator 5 sets, for example, a predetermined threshold voltage to the detection pulse signal, “1” is a detection pulse signal having a voltage equal to or higher than the voltage value, and “1” is a detection pulse signal having a peak value smaller than the voltage value. 0 "
By this, the detection pulse signal is converted into a digital pulse signal, and the signal and noise are discriminated.

The output signal of the discriminator 4 may be transmitted to the monitor counter (counting means) 6 or may be transmitted to the counter (counting means) 7. The transmission control is performed by a computer (control means, calculation means) 8.

The monitor counter 6 and the counter 7 are
In each of the input digital pulse signal waveforms,
Although it is a component that counts the number of digital pulse signals in a sampling period corresponding to one pixel, the monitor counter 6 is provided to create data of an integrated pulse height distribution described later, and the counter 7 is observed by an operator. It is different in that it is provided for creating the data of the secondary electron image.

The monitor counter 6 is a digital pulse output from the discriminator 5 in accordance with each threshold voltage when the threshold voltage value for the detection pulse signal input to the discriminator 5 is gradually changed. The number of digital pulse signals in the sampling period is counted for each signal waveform, and the data of that number is transmitted to the computer 8.

The computer 8 obtains an integral pulse wave height distribution representing the change in the number of digital pulse signals with respect to the change in the threshold voltage, based on the data of the number of digital pulse signals transmitted from the monitor counter 6, and calculates the peak value thereof. It is designed to automatically determine the true threshold voltage.

That is, in the present embodiment, when setting the threshold voltage when converting the detection pulse signal into the digital pulse signal, the voltage value with the maximum number of digital pulse signals in the sampling period is set to the true threshold voltage. Since, for example, the temporal density of incident secondary electrons becomes higher than the temporal resolution of the measurement system and the adjacent detection pulse signals may overlap with each other, the detection pulse signals will not be counted. It is possible to reduce the ratio.

Further, the computer 8 determines the secondary electron detector 3 from the state of the above-mentioned integral type pulse height distribution, for example, the shape and the number of digital pulse signals counted by the counter 7.
The count-down ratio of the detected pulse signals detected in step 1 is calculated, and the calculation result is displayed on the display unit 9.

The display unit 9 is composed of, for example, a liquid crystal display plate or the like, and is adapted to display the count-down ratio of the detection pulse signal by a numerical value. As a result, in this embodiment, the counting rate can be quantitatively measured.
It is possible to evaluate the reliability of secondary electron measurement.

On the other hand, the counter 7 outputs the digital pulse output from the discriminator 5 when the true threshold voltage automatically determined by the computer 8 is set for the detection pulse signal input to the discriminator 5. The number of signals is counted and that number of data is transmitted to the image memory 10.

The image memory 10 is a storage unit for storing data of the number of digital pulse signals transmitted from the counter 7. The data in the image memory 10 is D / A converted and then transmitted and displayed on the monitor screen 11. As a result, the operator can view the secondary electron image, that is, a predetermined region of the main surface of the semiconductor wafer W, through the monitor screen 11.

Next, the secondary electron measuring method of this embodiment is shown in FIG.
~ It demonstrates by FIG.

First, the semiconductor wafer W is irradiated with the electron beam E that is narrowed down from the electron gun 2a of the SEM 2. The secondary electron detector 3 detects the secondary electrons ejected from the irradiation unit and converts the secondary electrons into an electric signal.

At this time, for example, as shown in FIG. 2, it is assumed that a signal waveform in which adjacent detection pulse signals are superimposed is obtained in a predetermined period. The detection pulse signal having a relatively low peak value in FIG. 2 is noise such as thermal noise of the secondary electron detector 3. Further, T _X is a sampling period corresponding to one pixel.

Then, the secondary electron detector 3 transmits the obtained detection pulse signal to the amplifier 4. The amplifier 4 amplifies the detected pulse signal so as to enable subsequent signal processing, and then transmits the detected pulse signal to the discriminator 5.

After that, in this embodiment, the following signal processing is performed. First, the computer 8 gradually changes the threshold voltage for the detection pulse signal shown in FIG.
V _{TH1 to} V _{TH4 in} FIG. 2 are threshold voltages.

Then, the discriminator 5 generates a digital pulse signal waveform corresponding to the threshold voltage and transmits it to the monitor counter 6. The monitor counter 6 counts the number of digital pulse signals in the sampling period T _X for each transmitted digital pulse signal waveform, and transmits the number of data to the computer 8.

Based on the data of the number, the computer 8 obtains an integral type pulse height distribution representing the change in the number of digital pulse signals with respect to the change in the threshold voltage, as shown in FIG. 3, for example. Then, the threshold voltage V _TH3 that gives the peak value of the distribution is automatically determined as the true threshold voltage.

That is, in the case of the present embodiment, paying attention to the number of digital pulse signals output from the discriminator 5, the detection pulse signal is digitally processed by the voltage value that maximizes the number of digital pulse signals within the sampling period. It is determined as the threshold voltage when

However, the integral type pulse height distribution is not limited to the distribution in the state shown in FIG. 3, but varies depending on the detected pulse signal waveform, and for example, as shown in FIG. 4, the distribution in which the count number decreases. In some cases, the distribution may have a peak value in the latter half, as shown in FIG. 5, for example. The threshold voltage V _{TH in} FIGS. 4 and 5 indicates the threshold voltage that gives the peak value of the integrated pulse height distribution.

Next, the computer 8 transmits the data of the determined threshold voltage V _{TH3 to} the discriminator 5. As shown in FIG. 6, the discriminator 5 receives the threshold voltage V _{TH3 with} respect to the detection pulse signal.
Is automatically set. At this time, in the case of the present embodiment, the voltage value that crosses the most detection pulse signals is set as the threshold voltage.

Then, the detection pulse signal is digitally processed with the threshold voltage V _TH3 as a boundary, and a digital pulse signal waveform as shown in FIG. 7 is output. In the case of the present embodiment, as shown in FIGS. 6 and 7, the signal and the noise can be excellently separated by the digital processing, and sampling is performed in comparison with the number of detection pulse signals in the sampling period T _X. The number of digital pulse signals in the period does not decrease extremely.

Subsequently, the discriminator 5 transmits the digital pulse signal to the counter 7. The counter 7 counts the number of digital pulse signals in the sampling period T _X and transmits the data to the image memory 10. The image memory 10 stores the data as image data.

Further, the computer 8 has a counter 7 with the integral pulse wave height distribution state shown in FIG.
The ratio of the number of detected pulse signals that has been counted down is calculated from the number of digital pulse signals counted in step 1, and the calculation result is displayed on the display unit 9 as a numerical value. As a result, in this embodiment, the counting rate can be quantitatively measured,
It is possible to evaluate the reliability of secondary electron measurement.

Thereafter, the image data stored in the image memory 10 is D / A converted, and then the monitor screen 11
Will be transmitted and displayed.

As described above, according to this embodiment, the following effects can be obtained.

(1). When setting the threshold voltage when converting the detection pulse signal to the digital pulse signal, the voltage that maximizes the number of digital pulse signals in the sampling period is automatically set as the true threshold voltage. Even if adjacent detection pulse signals are superposed, it is possible to reduce the number of detection pulse signals that are missed. Therefore, the SN ratio of the detection signal of the secondary electrons can be improved, and the resolution of the secondary electron image can be improved.

(2). By calculating the count-down ratio of the detected pulse signal from the shape of the integral pulse wave height distribution and displaying the calculation result as a numerical value on the display unit 9, the operator can
Since it is possible to quantitatively measure the rate of counting down the detection pulse signal of secondary electrons, it is possible to recognize the measurement accuracy of secondary electrons and to evaluate the reliability in secondary electron measurement. ..

(3) Since the evaluation of the semiconductor integrated circuit device can be improved by the above (1) and (2), the reliability of the semiconductor integrated circuit device can be improved.

Although the invention made by the present inventor has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Needless to say.

For example, in the above embodiment, the case where the present invention is applied to the evaluation of a semiconductor wafer has been described.
The present invention is not limited to this, but various applications are possible, and for example, it is also applicable to evaluation of micromachines and exposure masks.

In the above description, the SEM, which is the field of application of the invention mainly made by the present inventor, was the background.
However, the present invention is not limited to this, and various applications are possible, such as ST
The present invention can also be applied to other particle beam measuring devices such as a secondary electron measuring device of EM or spin SEM, an optical measuring device used for a photocounting image device, or an X-ray measuring device used for an X-ray detection device.

[0060]

The effects obtained by the typical ones of the inventions disclosed in the present application will be briefly described as follows.
It is as follows.

(1) According to the first aspect of the invention, when the threshold level is set when the detection pulse signal is converted into the digital pulse signal, the number of the digital pulse signals becomes maximum within a predetermined period. By automatically setting the level as the true threshold level, for example, in the SEM secondary electron measurement, the temporal density of the incident secondary electrons becomes higher than the temporal resolution of the measurement system, and as a result, the density of the detection pulse signal is increased. Even when the detection pulse signals become high and adjacent detection pulse signals are superposed, it is possible to reduce the rate of counting down the detection pulse signals. Therefore, for example, the SN ratio of the detection signal of the secondary electrons can be improved, and the resolution of the secondary electron image can be improved.

(2) According to the invention described in claim 2, since the operator can quantitatively measure the rate of counting down of the detection pulse signal of the particle beam, the operator can recognize the measurement accuracy of the particle beam. Therefore, it is possible to evaluate the reliability of particle beam measurement.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a particle beam measuring apparatus according to an embodiment of the present invention.

FIG. 2 is a waveform diagram of a detection signal detected by a particle beam detector.

FIG. 3 is a graph showing an integrated pulse wave height distribution.

FIG. 4 is a graph showing another example of the integral pulse height distribution.

FIG. 5 is a graph showing another example of integral pulse height distribution.

FIG. 6 is a signal waveform diagram for explaining digital processing of the detection signal waveform of FIG.

FIG. 7 is a signal waveform diagram for explaining digital processing of the detection signal waveform of FIG.

FIG. 8 is a signal waveform diagram showing an example of a conventional secondary electron detection pulse signal.

FIG. 9 is a graph diagram for explaining a conventional threshold voltage setting method for a detection pulse signal.

FIG. 10 is a waveform diagram of a detection pulse signal for explaining the problem of the conventional method of setting the threshold voltage for the detection pulse signal.

11 is a waveform diagram of a digital pulse signal obtained by digitizing the detection pulse signal of FIG.

[Explanation of symbols]

1 Secondary Electron Measuring Device (Particle Beam Measuring Device) 2 Scanning Electron Microscope 2a Electron Gun 3 Secondary Electron Detector (Detecting Means) 4 Amplifier 5 Discriminator (Discriminating Means) 6 Monitor Counter (Counting Means) 7 Counter (Counting Means) 8 Computer (Control Means, Calculation Means) 9 Display Unit (Display Means) 10 Image Memory 11 Monitor Screen V _TH Threshold Voltage V _TH1 Threshold Voltage V _TH2 Threshold Voltage V _TH3 Threshold Voltage V _TH4 Threshold Voltage V _TH0 Threshold Voltage T _X sampling period T _X1 sampling period T ₁ period V ₁ voltage value E electron beam W semiconductor wafer (substrate)

Claims (5)

[Claims] 1. When digitally processing a detection signal of a predetermined particle beam, a pulse wave height distribution representing a change in the number of digital pulse signals obtained when a threshold level for the detection signal is changed is obtained, and the pulse wave height is obtained. The threshold level giving the peak value of the distribution is automatically set as a true threshold level, the step of converting the detection signal into a digital pulse signal, and the number of digital pulse signals within a predetermined period obtained by the digital processing are And a step of counting the particle beam. 2. Based on the state of the pulse height distribution,
The ratio of counting down of the detection pulse signals of the particle beam is calculated, and the calculation result is displayed.
The described particle beam measuring method. 3. The particle beam measuring method according to claim 1, wherein the predetermined particle beam is a secondary electron emitted from an irradiation part when a predetermined substrate is irradiated with an electron beam. .. 4. A detection means for detecting a predetermined particle beam, and a detection signal detected by the detection means is converted into a digital pulse signal using a predetermined threshold level that is set to discriminate between a signal and noise. And a pulse wave height distribution representing the change in the number of digital pulse signals obtained when the predetermined threshold level is changed, and the peak value of the pulse wave height distribution is automatically applied to the discriminating means as the true threshold level. Particle beam measuring apparatus, which comprises: a control unit that sets the target pulse number and a counting unit that counts the number of digital pulse signals output from the discriminating unit within a predetermined period. 5. Based on the state of the pulse height distribution,
The particle beam measuring apparatus according to claim 4, further comprising: a calculating unit that calculates a counting rate of the detection pulse signals of the particle beam, and a display unit that displays a calculation result of the calculating unit.