JPS62223960A - Potential contrast image device of stroboscopic method - Google Patents

Abstract

PURPOSE:To observe a stroboscopic image continuously without losing the contrast, by preparing two sets of voltage patterns to drive the sample, converting the two patterns alternatively synchronously to the scanning of electron beams, and displaying selectively only the image responding to one side of the two input voltage patterns. CONSTITUTION:A sample (IC) 10 is driven by a driving power source 11. A trigger output signal generated from the driving power source 11 in every cycle of the driving pulse pattern receives a delay at a phase adjustor 5 controlled by a control computer 18, and inputs to a pulse oscillator 12. The electron beams are scanned two-dimenally by a scanning coil 8 driven by a scanning power source 14. The secondary electrons produced from the sample are stored in an image memory 15 through an energy analyzer 13, a detector 9, and an amplifier 17, and the contents are displayed on a CRT 16. The control computer 18 receives a vertical synchronous signal from the scanning power source 14, makes the output driving pulse pattern of the power source 11 into a potential contrast pattern of the same phase, and prohibits to write the image memory 15 alternatively in every one frame.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a strobe-type potential contrast image that performs all two-dimensional distribution images of potential on a sample using a charged beam of a scanning electron microscope or the like. Regarding equipment.

[Conventional technology]

FIG. 2 is a basic configuration diagram of a strobe scanning path electron microscope that pulses a beam, measures a voltage waveform, and observes a two-dimensional potential distribution image at a specific phase point of the voltage waveform. An electron beam 2 emitted from a sub-gun 1 is focused onto a sample 10 using an electron lens 6, and a scanning coil 8
The sample 10 is two-dimensionally scanned with the electron beam 2 using the . f
The principle of the 5-scanning electron microscope is that a detector 9 detects secondary electrons emitted from a sample 10 by irradiation with the fi-son beam 2, and displays an image of the sample on a display device 7. However, when using this method to observe a sample that changes rapidly,
The scanning speed of the electron beam by the scanning coil 8 cannot follow the changing speed of the sample, and all changes are displayed overlappingly. Therefore, a pulse gate (combination of deflection plate 3 and aperture 4) synchronized with the drive power source 11 that changes the sample is added. With this configuration, it is possible to control the electron beam scanning the sample so that it is irradiated only when the sample is in a certain phase of change, and it is possible to detect only the state of the sample at the moment of irradiation. This will be explained in FIG. Same figure (
When the input voltage waveform of period T shown in Figure 3 (B) is repeatedly applied to the input of the IC (integrated circuit) in A), the voltages of patterns A and H in the integrated circuit are shown in Figure 3 (B). It shall change as follows. If the aforementioned pulse gate is opened instantaneously with a delay of one time from the starting point of the input voltage and this is repeated, only information at the phase at point a in the figure can be detected. The energy distribution of secondary electrons emitted from a sample by electron beam irradiation changes depending on the potential of the sample.
By placing the energy analyzer 13 between
The amount of secondary electrons detected can be correlated with the sample potential (see Japanese Patent Publication No. 47-51024). One of the main uses of this strobe scanning electron microscope is as shown in Figure 3C.
In B), the time t1 is fixed, the electron beam is scanned two-dimensionally over the sample, and the sample image is observed wt.
Since the contrast of the image depends on the voltage of each part in the image, it becomes a potential distribution image at the phase of point a in FIG. 3(B). In this example, the potential of pattern A is -5V, and the potential of pattern B
Since the potential of is +5V, the resulting image has pattern A as bright and pattern B as dark, as shown in FIG. Also, if the delay time from the start point of the input voltage to the pulse gate is t2 in the same figure (B), the potential distribution image at this time corresponds to the phase of point b in the same figure, so the figure (c) On the contrary, pattern B is observed brightly.

This method is generally called the strobe image mode, and although the measurement accuracy of the sample potential is low, it is possible to determine the approximate potential of the circuit pattern over a wide area.

This is particularly effective for analyzing defects such as circuit breaks. Another method is to fix the electron beam, for example, at one point on pattern A in FIG. 3(A), and vary the time tl from O to T in FIG. Waveforms can be measured.

(Nikkei Electronics, March 15, 1982 issue.

(See pp. 172-201) This method is generally called waveform measurement mode.

Incidentally, the objects to be measured by this strobe scanning electron microscope are mainly ICs (semiconductor integrated circuits), and in many cases, ICs are coated with an insulating film called passivation to prevent dirt and moisture. In this case, the passivation film is interposed between the electron beam and the metal electrode within the IC, so it functions equivalently as a capacitor. Due to the intervention of this capacitor, in the waveform measurement mode described above, the measurement result is a derivative with a time constant determined by the equivalent capacitance and the current (leakage current) that is the difference between the incident electron beam and the emitted secondary electrons. It is known that the waveform [Scanning Electron Microscopy, 1983. V
o 1.2 Pages 561 to 568 (Scanning
Electron Microscopy, 198
3゜Vol, 2. pp. 561-568)], and it is known that in the strobe image mode, the potential contrast disappears over time. This development is explained as follows. 4 shows actual voltage waveforms A and B of the two patterns in FIG. 3(A) and voltage waveforms A' and B' detected through the passivation film. In this example, the electron beam is irradiated at the timing of point a in the figure, and the electron beam irradiation generates a leakage current and the amplitude decreases, causing the A at the time of electron beam irradiation.
The potential difference between ' and B' gradually decreases, and the contrast difference between patterns A and B disappears. One way to deal with this problem is to synchronize with the scanning of the electron beam to . Il! The phase of the child beam irradiation point is alternately switched between two values, first and second, so that an image can be displayed only when the electron beam is irradiated in the first phase, and as the second phase. reported a method of selecting a phase in which the potential of the part to be observed in the sample is inverted with respect to the first phase. [Materials from the 82nd meeting of the 132nd Committee of the Japan Society for the Promotion of Science, 57°10.2
5. Pages 14 to 19] This means that a sudden change in voltage is
This is because it is transmitted to the electron beam side without being attenuated via capacitance. Figure 4 A#.

The waveform B' shows this, and the electron beam has two points: a and b.
Since the points are irradiated with alternating phases, the potential contrast as a whole is preserved without attenuation.

[Problem that the invention seeks to solve]

However, according to this method, each time the phase point to be amned is changed, a phase point whose potential is inverted from this point must be set according to the convenience, and a large-scale IC requires more than ten points. This becomes even more difficult when driving with an input voltage pattern of

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a strobe-type potential contrast imaging device that allows continuous strobe image observation without loss of contrast.

[Means for solving problems]

As mentioned above, contrast disappearance can be prevented by alternately irradiating the electron beam when the potential in the sample is reversed.

Instead of changing the irradiation phase of the electron beam, by alternating the input voltage waveforms so that the sample potential is reversed in one phase, this can be realized more easily and with a wider range of applications.

[Effect]

Therefore, we prepared two types of input voltage patterns to drive the sample, switched these two patterns alternately in synchronization with the scanning of the electron beam, and selectively displayed only the image corresponding to one of the two input voltage patterns. Make it visible.

〔Example〕

FIG. 5 shows an embodiment of the present invention. Sample 10 is an IC,
It is driven by a drive power source 11. A trigger output signal generated from the drive power supply 11 for each cycle of the drive pulse pattern is delayed (phase set) by a phase adjuster 5 controlled by a control computer 18, and then sent to a pulse oscillator 1 that pulses the electron beam.
2 is input. The electron beam is also two-dimensionally scanned by a scanning coil 8 driven by a scanning power source 14.

Two-dimensional electrons generated from the sample are detected by the detector 9 through the energy analyzer 13, and stored in the image memory 15 through the amplifier 17, and the contents of the image memory are read out and sent to the D/A converter. It is displayed on the CRTL via. The control calculator 18 receives a vertical synchronization signal from the scanning power supply 14.

The output drive pulse pattern of the drive power supply 11 is rearranged into the second pattern, and then returned to the original pattern with the next vertical synchronization signal.If this is repeated, for each frame scan,
The sample 10 is driven with different drive pulse patterns, and the obtained image signals are potential contrast images of the different drive pulse patterns at the same phase (delay time). If this continues, two different images will be displayed alternately, so
At the same time as the drive pulse pattern is rearranged, the control calculator 18 alternately puts the image memory 15 in a write-inhibited state for each frame, and when the second pattern is used for the purpose of recovering potential contrast.

Although the electron beam is irradiated, the displayed image is controlled so that the image of the original pattern is maintained. The two drive pulse patterns are determined as follows. The first pattern is a pattern that originally drives the IC that is the sample,
The second pattern is as wide as possible in the phase range,
The sample potential is determined to be inverted from that when driven in the first pattern. This work is possible because the potential change corresponding to the input pulse pattern at each node inside the sample IC is predicted by the design simulation of that IC. As the driving power source 11, there is a commercially available device that generates a pattern of several hundred to several thousand steps on output lines of several channels to several tens of channels, and this can be used. This type of device allows you to freely create patterns using a computer program.
This is controlled by the control calculator 18 mentioned above. Also, instead of rearranging the pattern every frame scan.

If the first and second drive patterns are installed in succession in advance and the trigger output signal generation points are set alternately at the start point of the first pattern and the start point of the second pattern, It can be controlled more easily.

An example is shown in FIG. This is an example of driving a simple type flip-flop IC as shown in figure (C), and the top waveform in both figures (A) and (B) is the internal clock of the driving power supply.

Its output can be programmed every clock.

The clock input and data input waveforms are the output of the driving power supply,
In the flip-flop IC, two complementary outputs Q and Q output change according to the data input at the rising edge of the clock input. When driving alternately using the drive pulse pattern shown in FIG. 5A as the first pattern and the drive pulse pattern shown in FIG. In all phases, and the clock input lines except for internal clocks 4, 5, 12.13,
Since the potentials are reversed, the above-mentioned contrast loss prevention effect can be achieved.

As described above, according to one embodiment of the present invention, it is possible to solve the problem of contrast loss that has been a problem in observing potential contrast images of an IC covered with a passivation film.

When analyzing the operation or failure of an IC using this type of device, the need to remove the passivation film is not only time-consuming.

Since there is a risk of damaging the IC itself, this is a very big constraint.By solving this problem, the embodiments of the present invention are of great help in IC analysis.

In the embodiment of the present invention, switching is performed for each frame scan, but it may be performed for each horizontal scanning line. Furthermore, without using one image memory, the output of the amplifier 17 shown in FIG.
Even if 16 is blanked, the purpose can be similarly achieved.

〔Effect of the invention〕

According to the present invention, the above-described conventional problems can be solved and strobe image observation can be continued without loss of contrast.

[Brief explanation of drawings]

Fig. 1 is an example of a pulse pattern according to an embodiment of the present invention, Fig. 2 is a basic configuration diagram of a strobe scanning electron microscope, Fig. 3 is a diagram explaining a potential contrast image of the strobe method, and Fig. 4 is a diagram illustrating a potential contrast image of a strobe type scanning electron microscope. A diagram showing the principle of a conventional example, and FIG. 5 is a configuration diagram showing an embodiment of the present invention. 1... Electron gun, 2... Electron beam, 3... Deflection plate, 4... Aperture, 5... Phase 1+1111 device,
6...electronic lens. 7...Display device, 8...Scanning coil, 9.
...Detector, 10...Sample, 11...Drive power supply, 1
2... Pulse oscillator, 13... Energy analyzer, 14... Scanning power supply, 15... Image memory, 16
. . . CRT, 17 . . . Amplifier, 18 . . . Control calculator.

Claims (1)

[Claims] 1. The sample is driven with a pulse pattern voltage having a repeating period, and the sample is two-dimensionally scanned with a pulsed charged beam synchronized with the pulse pattern voltage with an arbitrary phase difference. In a strobe-type potential contrast imaging device that obtains a potential contrast image at a phase of , the pulse pattern voltage is alternately switched between two different patterns in synchronization with the scanning of the charged beam, and On the other hand, a strobe type potential contrast imaging device is characterized in that it is possible to select and display only a potential contrast image when a sample is being driven.