JPH0644473B2 - Sample potential contrast image observation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention observes a two-dimensional distribution image of a potential on a sample by using a charged beam such as a scanning electron microscope. The present invention relates to a stroboscopic potential contrast imaging device.

[Conventional technology]

FIG. 2 is a basic configuration diagram of a stroboscopic scanning electron microscope for pulsing an electron beam to measure a voltage waveform and observe a two-dimensional potential distribution image at a specific phase point of the voltage waveform. An electron beam 2 emitted from an electron gun 1 is focused on a sample 10 using an electron lens 6, and a scanning coil 8
Is used to two-dimensionally scan the sample 10 with the electron beam 2. The principle of the scanning electron microscope is to detect the secondary electrons emitted from the sample 10 by the irradiation of the electron beam 2 by the detector 9 and display the sample image on the display device 7. However, when a sample that changes at high speed is observed by this method, 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 in duplicate. Therefore, a pulse gate (combination of the deflection plate 3 and the aperture 4) synchronized with the driving power supply 11 which gives the sample change is added. With such a configuration, the electron beam scanning the sample can be controlled so as to be irradiated only when the sample has a constant phase with a change, and only the sample state at the moment of the irradiation can be detected. This will be explained with reference to FIG. I in FIG.
When the input voltage waveform of the period T shown in FIG. 3 (B) is repeatedly applied to the input of C (integrated circuit), the voltages of patterns A and B in the integrated circuit change as shown in FIG. 3 (B). It shall be. If the above-mentioned pulse gate is instantaneously opened with a delay of time t ₁ from the starting point of the input voltage and this is repeated, only the information at the phase at point a in the figure can be detected. The energy distribution of the secondary electrons emitted from the sample due to the irradiation of the electron beam changes depending on the potential of the sample. Therefore, between the detector 9 and the sample 10 in FIG.
By providing the energy analyzer 13, the amount of secondary electrons detected can be associated with the sample potential (see Japanese Patent Publication No. 47-51024). One of the main uses of this strobe scanning electron microscope is shown in Fig. 3 (B).
The time t ₁ is fixed and the sample image is observed by two-dimensionally scanning the sample with an electron beam, and the image contrast depends on the voltage of each part in the image. ) Is a potential distribution image at the phase of point a. In this example, the potential of the pattern A is −5 V and the potential of the pattern B is +5 V, so that in the obtained image, the pattern A is bright and the pattern B is dark as shown in FIG. Also, the delay time from the starting point of the input voltage to the pulse gate is shown in FIG.
At t ₂ , the potential distribution image at this time corresponds to the phase of point b in the figure, so that the pattern B is brightly observed, contrary to FIG. This method is generally called the strobe image mode, and although the measurement accuracy of the sample potential is low, it is possible to know the approximate potential of the circuit pattern over a wide area.
It is especially effective for analyzing defects such as circuit breaks. In another method, the electron beam is fixed at one point on the pattern A in FIG. 3 (A) and the time t ₁ in FIG. 3 (B) is changed from 0 to T. The potential waveform can be measured. (Nikkei Electronics, March 1982
This issue is generally called the waveform measurement mode.

By the way, the measurement object of this strobe scanning electron microscope is mainly an IC (semiconductor integrated circuit), but in many cases, the IC is covered with an insulating film called passivation in order to prevent dirt and moisture. In this case, the passivation film intervenes between the electron beam and the metal electrode in the IC, and thus equivalently functions as a capacitor. Due to the interposition of this capacitor, in the above-mentioned waveform measurement mode, the measurement result has a differential with a time constant determined by the equivalent capacitance and the current (leakage current) of the difference between the incident electron beam and the emitted secondary electron. It is known to have a waveform [Skyyanning Electron Microscopy, 1983, Vol.
2 561 to 568 (Scanning Electron Micros
copy, 1983, Vol. 2, pp 561 to 568]], and in the strobe image mode, it is known that the potential contrast disappears with time. This development is explained as follows. FIG. 4 shows actual voltage waveforms A and B of the two patterns shown in FIG. 3A 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 a leakage current is generated by the irradiation of the electron beam to reduce the amplitude, and the potential difference between A ′ and B ′ at the time of electron beam irradiation. Gradually decreases to pattern A
The contrast difference between B and B will disappear. As one of the methods that can deal with this problem, the phase of the electron beam irradiation point with respect to the input voltage waveform is alternately switched between the first value and the second value in synchronization with the scanning of the electron beam. The image can be displayed only when the electron beam is irradiated in the phase, and the potential of the portion to be observed in the sample is inverted with respect to the first phase as the second phase. Methods for choosing the phase have been reported. [Japan Society for the Promotion of Science, 132nd Committee, 82nd Workshop, 57.10.2
5.14 to 19] This is because the sharp change in voltage is
This is because they are transmitted to the electron beam side without being attenuated via the electrostatic capacitance. The waveforms A ″ and B ″ in FIG. 4 show this. Since the electron beam is irradiated by alternately changing the phase at the points a and b, the potential contrast is preserved as a whole without being attenuated. .

[Problems to be solved by the invention]

However, according to such a method, every time the phase point to be observed is changed, it is necessary to set the phase point at which the potential is inverted with respect to the phase point.
Then, when it is driven by applying a dozen or more input voltage patterns, it becomes difficult.

An object of the present invention is to solve the above-mentioned problems and to provide a stroboscopic potential contrast imaging device capable of continuously performing stroboscopic image observation without disappearance of contrast.

[Means for solving problems]

As mentioned above, contrast annihilation can be prevented by alternately irradiating the electron beam while the potential is reversed in the sample. Further, by alternately combining the input voltage waveforms so that the potential inside the sample is inverted, it is possible to realize the invention more easily and with a wider application range.

[Action]

Therefore, two types of input voltage patterns for driving the sample are prepared, these two patterns are alternately switched in synchronization with the scanning of the electron beam, and only the image corresponding to one of the two input voltage patterns is selectively selected. To be able to display.

〔Example〕

FIG. 5 shows an embodiment of the present invention. Sample 10 is an IC,
It is driven by the drive power supply 11. The trigger output signal generated from the drive power source 11 for each cycle of the drive pulse pattern is delayed (phase setting) by the phase adjuster 5 controlled by the control computer 18, and the pulse oscillator 1 for pulsing the electron beam is generated.
Entered in 2. The electron beam is also two-dimensionally scanned by the scanning coil 8 driven by the scanning power supply 14. The two-dimensional electron generated from the sample is the energy analyzer 1
3 is detected by the detector 9 and stored in the image memory 15 via the amplifier 17 and the contents of the image memory are read out and displayed on the CRT 16 via the D / A converter. The control calculator 18 receives the vertical synchronizing signal from the scanning power source 14, changes the output drive pulse pattern of the driving power source 11 into the second pattern, and restores the original pattern with the next vertical synchronizing signal. By repeating this, the sample 10 is driven with different drive pulse patterns for each frame scanning, and the obtained image signal becomes a potential contrast image of different drive pulse patterns in the same phase (delay time). Under this condition, two different images will be displayed alternately. Therefore, the control calculator 18 changes the drive pulse pattern and simultaneously sets the image memory 15 to the write-inhibited state alternately for each frame to recover the potential contrast. In the second pattern, the electron beam is irradiated, but the display image is controlled so that the image in the original pattern is maintained. The two drive pulse patterns are determined as follows. The first pattern is
The second pattern is determined so that the potential inside the sample is inverted from that when the IC is driven by the first pattern, in the widest possible phase range. This work is possible because the potential change of each node inside the IC as the sample corresponding to the input pulse pattern is predicted by the design simulation of the IC. As the driving power supply 11, a device for generating a pattern of hundreds to thousands of steps with an output number of several channels to several tens of channels is commercially available, and this can be used. In this type of device, a pattern can be freely formed by a computer program, and this is controlled by the control calculator 18. Further, instead of changing the pattern for each frame scanning, the first and second drive patterns are continuously incorporated in advance, and the generation points of the trigger output signal are alternately changed to the start point of the first pattern and the second point. If it is set to the start point of the pattern, the control can be performed more easily.

FIG. 1 shows an example thereof. This is an example of driving a simple D-type flip-flop IC as shown in FIG. 7C, and the waveforms at the uppermost stage in FIGS. 7A and 7B are the internal clocks of the driving power supply. The output can then be programmed for each clock. The clock input and the data input waveform are the output of the driving power supply, and the flip-flop IC changes the two complementary outputs Q, according to the data input at the rising edge of the clock input. Same figure (A)
When the drive pulse pattern of No. 1 is used as the first pattern and the drive pulse pattern of FIG. 6 (B) as the second pattern, the data input line, the Q line, and the output line are The clock input lines are internal clocks 4, 5, 1
Since the potentials are inverted with respect to each other except 2 and 13, the above-mentioned effect of preventing the disappearance of the contrast can be enhanced.

As described above, according to the embodiments of the present invention, it is possible to solve the problem of the disappearance of contrast, which has been a problem in observing the potential contrast image of the IC covered with the passivation film. When performing the operation analysis or the failure analysis of the IC by using this kind of device, it is necessary to remove the passivation film, not only it is time-consuming but also the risk of damaging the IC itself. It is a very big constraint to be satisfied. By solving this problem, the embodiment of the present invention is very useful for IC analysis.

In the embodiment of the present invention, switching is performed for each frame scanning, but this may be performed for each horizontal scanning line. Further, without using the image memory, the output of the amplifier 17 shown in FIG. 5 is directly used as the brightness modulation signal of the CRT 16, and the frame driven by the second pattern is the CRT.
Even if 16 is blanked, the same purpose can be achieved.

〔The invention's effect〕

According to the present invention, the above-mentioned conventional problems can be solved, and the stroboscopic image observation can be continuously performed without disappearing the contrast.

[Brief description of drawings]

FIG. 1 is an example of a pulse pattern of an embodiment of the present invention, FIG. 2 is a basic configuration diagram of a strobe scanning electron microscope, FIG. 3 is a diagram for explaining a potential contrast image of a strobe system, and FIG. FIG. 5 is a diagram showing the principle of a conventional example, and FIG. 5 is a configuration diagram showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Electron beam, 3 ... Deflection plate, 4 ... Aperture, 5 ... Phase adjuster, 6 ... Electron lens, 7 ... Display device, 8 ... Scanning coil, 9 ... Detector, 10 ... Sample,
11 ... Driving power supply, 12 ... Pulse oscillator, 13 ... Energy analyzer, 14 ... Scanning power supply, 15 ... Image memory, 1
6 ... CRT, 17 ... amplifier, 18 ... control calculator.

Claims (1)

[Claims] 1. A sample is driven by a pulsed voltage pattern having a predetermined period, and the potential of the sample surface is two-dimensionally scanned by a charged beam having an arbitrary phase difference from the pulsed voltage pattern. In a sample potential contrast image observation apparatus for obtaining a contrast image, the pulsed voltage pattern is alternately switched between two different pulsed voltage patterns in synchronization with the scanning of the charged beam, The sample surface potential at the time of irradiating the charged beam with a first voltage pattern and the sample surface potential at the time of irradiating the charged beam with a second voltage pattern are mutually inverted, and the two pulse shapes Sample potential characterized by selecting and displaying only the potential contrast image of the sample surface when the sample is driven in one of the voltage patterns Contrast image observation device.