JPH03183145A - Electron beam apparatus - Google Patents

Abstract

PURPOSE:To eliminate an influence caused by a low-frequency noise, to reduce a measuring error and to shorten a measuring time by finding a difference between the following: a signal component which has passed a high-pass filter out of pulsed secondary electronic signals; and a signal component which has sampled-and-held an output signal of the high-pass filter. CONSTITUTION:An EB pulse-generation control signal is input, as a strobe signal, to a sample-and-hold circuit 22 form a control circuit 11; an input secondary electronic signal whose noise has been eliminated by using a high-pass filter 21 is held at the timing of the strobe signal, i.e., at the timing to generate an EB pulse; a voltage is held for a definite time. Since a differential amplification output of a smaple-and-hold signal and a high-pass filter output signal always uses a level at the rise time of a pulse signal as a reference, a pulse height coincides with a peak value of a pulse. When an output signal of a differential amplifier 23 is sampled at the timing at which the secondary electronic signal becomes maximum and is A-D-converted, an influence by a low-frequency noise is eliminated and a measuring error can be reduced.

Description

[Detailed Description of the Invention] [Summary] Regarding an electron beam device, it is possible to reduce measurement errors and shorten measurement time by removing the effects of low frequency noise, and to obtain a good frequency when measuring periodically changing voltage waveforms. An irradiation means for irradiating an electron beam onto a measurement point of a sample, with the aim of providing an electron beam device that can obtain characteristics and can be fully put to practical use even when an MCP is used as a detector. a secondary electron detection means for detecting secondary electrons emitted from the sample and outputting a secondary electron signal; and a measuring means for processing the secondary electron signal to measure a physical quantity related to the sample. An electron beam apparatus comprising: a pulsing means for pulsing the electron beam; and a signal processing means disposed between the secondary electron detection means and the measurement means, the signal processing means comprising: A high-pass filter that passes only the alternating current component of the secondary electron signal output from the secondary electron detection means, a sample-and-hold circuit that samples and holds the output signal of the high-pass filter, and a sample-and-hold circuit that samples and holds the output signal of the high-pass filter and the sample-and-hold circuit. and a differential amplifier that outputs a difference from the output signal, and the measuring means is configured to measure a physical quantity related to the sample based on the output of the differential amplifier.

[Industrial application field]

The present invention relates to an electron beam device, and specifically, irradiates a sample with a narrowly focused electron beam, detects a secondary electron signal,
The present invention relates to an electron beam device that is applied to a scanning electron microscope (SEM), an electron beam tester, etc. that displays an enlarged image (secondary electron image) of a sample, measures line width, or measures voltage.

Electron beam devices can be used in various applications, but typically electron beam devices are used to diagnose the internal operation of VLSIs.
Electron Beaai: EB) Often used as a prober, the BB blower is a system that diagnoses the internal operating status of an LSI by irradiating an electron beam onto the surface of an operating LSI chip and detecting the emitted secondary electrons. be.

[Conventional technology]

As a conventional electron beam device, there is one shown in FIG. 6, for example. In the figure, l is an electron beam column;
2 is a sample chamber; 3 is a sample (test IC) placed inside the sample chamber 2; The electron beam emitted from the electron gun of the electron beam column 1 is pulsed by the EB pulse gate 4 and irradiated onto the sample 3 as an EB pulse, and secondary electrons 5 are emitted from the surface of the sample 3. The scintillator 6 accelerates and collides the secondary electrons 5, converts it into an optical signal, and outputs it to a photomultiplier (photomultiplier tube) 7, which converts this optical signal into an electric signal (current signal). do. The output (current signal) of the photomultiplier 7 is converted into a voltage signal by a current-voltage converter 8, then amplified by an amplifier 9, A/D converted by an A/D converter 10, and sent to a control circuit 11. Output. A/
The D converter 10 converts the analog signal into a digital signal at a timing defined by the sampling strobe signal from the sampling delay circuit 12, and the result is taken into the control computer 13 via the control circuit 11. The control circuit 11 controls each circuit for measuring the operating voltage of the IC based on the detection result of the secondary electrons 5 emitted from the sample 3, and sends a control signal (EB) for converting the electron beam into an EB pulse. pulse control signal) to the blanking driver 14
At the same time, this control signal is also output to the sampling delay circuit 12. Blanking driver 14
drives the EB pulse gate 4 based on a control signal from the control circuit 11, and the sampling delay circuit 12 generates the sampling strobe signal based on the control signal. In order to generate the signal, a certain time delay is given to the IIJII signal (EB pulse generation signal), and the A/D converter 10
Output to. The control computer 13 performs processing such as displaying a secondary electron image or measuring voltage based on the output result of the A/D converter 10.

[Problem to be solved by the invention]

However, in such a conventional electron beam device, a DC amplifier is used in the entire process of amplifying the secondary electron signal, which causes the following problems.

(1) Low-frequency noise, especially power supply frequency noise, is easily mixed in, causing stripes in the secondary electron image! ! This makes it difficult to observe details. Further, in line width measurement and voltage measurement, this results in an increase in measurement errors and an increase in measurement time.

(n) In order to cope with a high repetition frequency when measuring a periodically changing voltage waveform, a secondary electronic signal processing circuit is required to have higher frequency characteristics than a normal SEM. In this case, with a DC amplifier, it is difficult to obtain flat characteristics in a frequency domain ranging from DC to 100 MIIz.

(I[I) When an electron-electron multiplier device such as a microchannel plate (MCP), which has a wide detection surface, is -like, and has the potential for high detection efficiency, is used as a detector, the DC
Since the signal is output superimposed on the high voltage of

However, when using an isolation amplifier, the frequency band is limited to IMH2 at most, and additional switching noise is generated due to isolation, so it is hardly used in practice.

Therefore, the present invention aims to reduce measurement errors and shorten measurement time by removing the influence of low frequency noise, and also makes it possible to obtain good frequency characteristics when measuring periodically changing voltage waveforms, and further improves MCP. It is an object of the present invention to provide an electron beam device that can be put to practical use even when used as a detector.

[Means to solve the problem]

In order to achieve the above object, the electron beam apparatus according to the present invention includes an irradiation means for irradiating an electron beam onto a measurement location of a sample, and a secondary electron beam device for detecting secondary electrons emitted from the sample and outputting a secondary electron signal. In an electron beam apparatus comprising an electron detection means and a measurement means for signal processing the secondary electron signal to measure a physical quantity related to the sample, a pulsing means for pulsing the electron beam; A signal processing means is provided between the secondary electron detection means and the measurement means, and the signal processing means is a high-pass filter that passes only an alternating current component of the secondary electron signal output from the secondary electron detection means. The measuring means includes a filter, a sample-and-hold circuit that samples and holds an output signal of the high-pass filter, and a differential amplifier that outputs a difference between an output signal of the high-pass filter and an output signal of the sample-and-hold circuit. It is configured to measure physical quantities related to the sample based on the output of the dynamic amplifier.

[Effect]

In the present invention, the electron beam is pulsed and irradiated onto the sample, and the difference between the signal component of the secondary electron signal that has passed through the high-pass filter and the signal component obtained by sampling and holding the output signal of the high-pass filter is calculated. and a physical quantity related to the sample is measured based on the difference.

Therefore, it is possible to restore DC precepts while removing the influence of low-frequency noise, reducing measurement errors and shortening measurement time, and obtaining good frequency characteristics even when measuring periodically changing voltage waveforms. is possible, and further MC
Even when P is used as a detector, it can be sufficiently put to practical use.

〔Example〕

Hereinafter, the present invention will be explained based on the drawings.

Figures 1 to 4 are diagrams showing one embodiment of the electron beam device according to the present invention. In explaining this embodiment, the same components as the conventional example are denoted by the same numbers and duplicate explanations are omitted. do. 1st
In the figure, a high-pass filter 21. A sample and hold circuit 22 and a differential amplifier 23 are provided, and these constitute signal processing means 24.

A concrete circuit of the signal processing means 24 is shown in FIG. 2, and the current-voltage converter 8 is composed of operational amplifiers OPI and O20 and resistors Rz and R3. A capacitor C1 is used as the high-pass filter 21, and the output of the current-voltage converter 8, which is the first stage amplifier following the preamplifier (photomultiplier 7), is AC-coupled and sent to the differential amplifier 23. Note that RI is a resistor. Therefore, the output of the current-voltage converter 8 is filtered by the capacitor C1 to reduce low frequency components (for example, low frequency noise).
is canted and input to the manual input terminal of the second-stage differential amplifier 23 and the sample-and-hold circuit 22. The EB pulse control signal is input as a strobe signal to the sample and hold circuit 22, and the sample and hold circuit 22 samples and holds a secondary electronic signal input manually at the timing of this strobe signal, and sends the hold signal to one input of the differential amplifier 23. Output to the terminal. The differential amplifier 23 outputs the difference between each input signal to the A/D converter 10. The A/D converter 10 converts this difference signal into A/D, and the control computer 13 converts the difference signal into A/D.
Based on the output results of the converter 10, processing for displaying a secondary electron image and measuring voltage is performed. The rest is the same as the conventional example.

In this embodiment, the EB pulse gate 4 and the blanking driver 14 constitute a pulsing means 31 that pulses the electron beam, and the electron beam column 1 including the electron gun constitutes an irradiation means 32. Furthermore, the scintillator 6, photomultiplier 7, and current-voltage converter 8 constitute a secondary electron detection means 33, and the sampling delay circuit 12,
A/D converter 10, control circuit 11 and control computer 13
constitutes the measuring means 34.

Next, the effect will be explained.

The measurement sequence is as follows. For example, when measuring the signal voltage of the sample 3, first, the test vector generated by the control computer 13 is downloaded to the IC driver (the path shown), and at the same time the irradiation means 32 and the pulsing means 31
．． The secondary electron detection means 33 and the measurement means 34 are initialized. Next, a test pattern is repeatedly generated according to the test vector, and a trigger signal indicating the start timing of the test pattern is sent to sample 3 (i.e., DUT: d
evice undertest (test device)
) is supplied to the sample 3.

Thereafter, the secondary electron detection means 33 detects secondary electrons, and the signal processing means 24 processes the secondary electrons, and then the measurement means 34 measures the signal voltage of the sample 3.

Here, in this embodiment, in order to solve the above-mentioned problems, two steps are taken.
The influence of noise is removed by using an AC-coupled amplifier including a high-pass filter 21 as the amplifier of the secondary electron signal processing circuit. A problem called differential effect occurs. Therefore, in order to eliminate such differential effects, a sample and hold circuit 22 is provided as a means for restoring a DC offset signal from a signal containing only AC components. In addition, in order to make it possible to amplify the secondary electron signal with an AC coupled amplifier, the electron beam is pulsed and pulsed EB is always used instead of the continuous EB used in normal SEM. The second electronic signal is made into a pulse signal that can be amplified by an AC coupled amplifier.

By the way, when using an AC amplifier, there is the following problem.

In other words, the meaningful signal component in a pulsed secondary electron signal is the pulse height, but if this continues, the pulse 2
The next pulse rise reference level divided by the offset of the electronic signal becomes floating, and the current reference level is influenced by the previous pulse height and the duty of the pulse signal.

For example, when a secondary electron signal (DC amplified signal) as shown in Fig. 3 is observed when a secondary electron signal due to continuous EB is DC amplified, the DC power is canted in the AC amplifier, so the same The waveform observed as an AC amplified signal in the figure is distorted.

Therefore, by using pulsed EB, which is a pulsed electron beam,
When the secondary electron signal is made into a pulsed signal, the frequency spectrum of the pulse part increases as shown in Figure 4 as an AC amplified signal, and its height is the same as that of the C amplified signal as shown in Figure 4. Become. However, the profile of the signal that connects the sampling data of the peak value of this pulse still ends up being a differentiated version of the original signal.

If this situation is applied to a secondary electron image display, the contrast image of the LSI wiring pattern will be differentiated at the edge portions with respect to the horizontal direction. Moreover, the boundaries of wiring patterns extending in the horizontal direction become almost impossible to discern.

Therefore, in this embodiment, immediately before the rise of each pulse of the secondary electron signal (sample hold timing shown in FIG. 4)
A sample and hold circuit 22 is provided to hold the level of the sample and hold, and the secondary electronic signal pulse rises based on this held level, and the differential amplifier 23 performs differential amplification of the original secondary electronic signal and the sample and hold output. This will be done according to the following. In this way, the peak value of the pulse signal is entirely superimposed on the DC amplified signal, making it possible to restore DCtc.

Note that the EB pulse generation frequency is several IQkflz or higher even when considering various applications, so even if the cutoff frequency of the AC coupled amplifier is set to several kHz = 10 kHz, the signal component will not be lost. On the other hand, harmonic noise caused by power frequency noise can be almost completely removed by the high-pass filter 21.

To describe the specific operation of the signal processing means 24 from the above-mentioned viewpoint, the EB pulse generation control signal is inputted from the control circuit 11 as a strobe signal to the sample hold circuit 22, and the timing of this strobe signal, that is, the timing of the EB pulse generation is inputted to the sample hold circuit 22. At the appropriate timing, the high-pass filter 21 holds the input secondary electronic signal from which noise has been removed.
The voltage is maintained for a certain period of time (for example, one sweetfish). Changes in the signals of each part in this case are shown in FIG. In the signal that has passed through the high-pass filter 21 (signal at node Nl), the reference level of the pulse signal is different for each pulse,
Even if the pulse height is the same, the pulse peak value is not constant. On the other hand, since the differential amplification output of the sample-and-hold signal and the high-pass filter output signal always uses the level at the rising edge of the pulse signal as a reference, the pulse height and the pulse peak value match. Therefore, if the output signal of the differential amplifier 23 is sampled at the timing when the secondary electronic signal is at its maximum and A/D converted, the problem described in the previous section can be solved.

That is, it is possible to reduce the measurement error by removing the influence of low frequency noise, and since there is no influence of noise, it is easy to observe details, and as a result, the measurement time can be shortened. In addition, good frequency characteristics can be obtained when measuring a periodically changing voltage waveform, and measurement performance is improved.

Next, FIG. 5 is a diagram showing another embodiment of the present invention, in which an MCP 41 is used as the secondary electron detection means, and its output current signal is manually input to an AC coupling amplifier. That is, the secondary electrons 5 are detected by the MCP 41,
+100V is applied to the lower end surface of the MCP 41 in the figure, and +1 kV is applied to the upper end surface. The secondary electron signal is superimposed on a DC high voltage (+1 kV) and output, and is sent to an amplifier 43 via an isolation capacitor 42 for AC coupling.
and is amplified, and then connected to node N in FIG. Note that the amplifier 43 includes an operational amplifier OP and resistors R5, R, and R4 is a resistor. In this embodiment, the high voltage applied to the MCP 42 is canted by the isolation capacitor 42, and this capacitor 42 also functions as a high-pass filter, so that noise components are also removed and sent to the node N. Therefore, there is no need to use a so-called isolation amplifier, and it is possible to solve the problems mentioned in the conventional problems, that is, the effects of switching noise due to frequency band limitations and isolation thin. It can be used for practical purposes.

Further, there is an advantage that it becomes easier to speed up the processing of secondary electron signals.

The application of the present invention is not limited to SEMs, -electron beam testers, etc., but can be applied to anything that processes secondary electron signals to obtain physical quantities related to a sample.

〔Effect of the invention〕

According to the present invention, it is possible to remove the influence of low frequency noise, reduce measurement errors and shorten measurement time, and obtain good frequency characteristics when measuring periodically changing voltage waveforms. Furthermore, even when MCP is used as a detector, it can be sufficiently put to practical use.

[Brief explanation of drawings]

1 to 4 are diagrams showing an embodiment of the electron beam device according to the present invention, FIG. 1 is a block diagram thereof, FIG. 2 is a circuit diagram of its signal processing means, and FIG. 3 is its amplification. FIG. 4 is a diagram showing the waveform of the signal, FIG. 4 is a diagram showing the waveform for explaining sample and hold, FIG. 5 is a configuration diagram of the main part of another embodiment of the electron beam device according to the present invention, FIG. is a block diagram of a conventional electron beam device. l...Electron beam column, 2...Sample chamber, 3...Sample, 4...EB pulse gate, 5...2 Next electron, 6...Scintillator, 7...Photomultiplier, 8...
Current-voltage converter, 10... A/D converter, 11... Control circuit, 12... Sampling delay circuit, 13...
... Control computer, 14 ... Blanking driver, 21 ...
...High pass filter, 22...Sample hold circuit, 23...
... Differential amplifier, 24 ... Signal processing means, 31 ... Pulsing means, 32 ... Irradiation means, 33 ... Secondary electron detection means , 34... Measuring means, 41... MCP (Secondary electron detection means), 42
......Isolation capacitor (high pass filter), 43...Amplifier. Figure showing the waveform of the amplified signal of the secondary electronic signal AC amplified signal of one embodiment. Figure showing waveforms to explain sample hold

Claims (1)

[Claims] Irradiation means (32) for irradiating an electron beam onto a measurement location of the sample (3); and detecting secondary electrons emitted from the sample (3) and outputting a secondary electron signal. An electron beam apparatus comprising a secondary electron detection means (33, 41) and a measurement means (34) for signal processing the secondary electron signal to measure a physical quantity related to the sample (3), pulsing means (31) for pulsing the electron beam; the secondary electron detection means (33, 41); and the measuring means (34).
), and the signal processing means (24) is arranged between the secondary electron detection means (3).
A high-pass filter (21, 42) that passes only the alternating current component of the secondary electronic signal output from the secondary electronic signal (3, 41), and a sample-hold circuit (22) that samples and holds the output signal of the high-pass filter (21, 42). , a differential amplifier (23) that outputs the difference between the output signal of the high-pass filter (21, 42) and the output signal of the sample-and-hold circuit (22), and the measuring means (34) An electron beam device characterized in that it is configured to measure a physical quantity related to the sample (3) based on the output of (23).