JPS5951540A - Voltage contrast image display apparatus - Google Patents

Abstract

PURPOSE:To obtain an apparatus for displaying a voltage contrast image under the condition that a protection film is covered through the use of a low speed energy as the primary beam by respectively providing specific deflection oscillator, a circuit for applying voltage to sample and a signal selection circuit. CONSTITUTION:A voltage is applied to the specified Al wiring of semiconductor of a sample 8. When the sample is irradiated with slowly accelerated primary beams 1, the secondary electrons are generated from the surface of sample 8. The secondary electron signal generated also includes an general SEM data and a voltage contrast data applied to the Al wiring. This signal is detected by a detector 5 and amplified and is then inputted to a signal selection circuit 6. When a rectangular wave is applied to the Al wiring, a signal having equal positive and negative values is generated as an output. The signal selection circuit 6 selects either one signal and performs switching by the synchronous signal of X-scanning sent from the deflection oscillator 4. An output is inputted as an intensity modulated signal of CRT 9 and is displayed on the CRT screen as an SEM image.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to improvements in voltage contrast image display devices.

[Background of the invention]

SEM is used as a non-destructive testing device for semiconductor devices such as LSI.
The use of has been increasing recently. and low acceleration (~2
By using a primary electron beam (kV), it is possible to observe an LSI surface covered with an insulating film as an image without a charge-up phenomenon without any pretreatment. This 1 indicates that it is possible to inspect the shape of the LSI at the final stage after applying the protective film. .

Further, in the SEM, a voltage contrast image is possible in which a voltage is applied to the LSI from the outside and information on the voltage inside the LSI is displayed as a SEM image. For example, the development of strobe SEM and S
With the development of EM, information on voltage contrast has been obtained, and L
Pulse analysis inside SI and AI! Dynamic inspection is becoming possible by observing broken wires. However, these developments that are generally carried out are based on primary beams with a high energy of several tens of KeV or L
Most of the development is done without a protective film covering the SI.

[Purpose of the invention]

The present invention has been made focusing on these points, and has the following features:
Using low-velocity (~2kV) energy as the secondary beam,
It is an object of the present invention to provide a voltage contrast image display device that displays a voltage contrast image covered with a protective film.

[Summary of the invention]

It is a very important question how the voltage information on substances under the protective film, such as AJ wiring, changes when the protective film passes through it.
One is schematically shown in FIGS. 1(a) and (b). (
As shown in a), the primary beam ■ is stopped and irradiated to one point on the Aj5 wiring covered with the crushing film, and the AI! The voltage v applied to the wiring is changed using a rectangular wave. Then, the secondary electrons I3 generated from the protective film are captured and photomuled (P, M
) to obtain the output voltage vo. Vd and V at this time.

The relationship is shown in figure (b) based on the experimental results. From these experiments, AI! Did you apply it to the wiring? 1. It was found that the pressure is differentiated and output by passing through the protective membrane.

In other words, the protective film acts as a kind of capacitor. This shows that even if a DC voltage is applied to the AJ wiring under the protective film, it cannot be detected through the protective film, and that it is necessary to apply an AC voltage.

[Embodiments of the invention]

In view of the above experimental results, the method of the present invention for constantly and stably displaying voltage contrast information under a protective film will be described in detail with reference to the following examples.

FIG. 2 shows a block diagram of this device. The signal from the deflection oscillator 4 synchronizes the CRT 9 and the reconnaissance device 3
- Scan to Y. Further, only the X-scan synchronization signal is taken out from the deflection oscillator 4 and inputted to the sample application voltage circuit 7.
do. The sample application voltage circuit 7 generates a predetermined rectangular wave voltage in synchronization with the X-scan, and applies the voltage to a predetermined AJ wiring of the semiconductor of the sample 8. When the sample is irradiated with a low-power 41st-order beam (~2 KeV), secondary electrons 2 are generated from all 8 surfaces of the sample. The generated secondary electron signal is a general S
EM information and AI! It also includes information on the voltage contrast applied to the wiring. The secondary electronic signal containing these two pieces of information is detected by the detector 5 and amplified, and then the signal selection circuit 6
is input. As described above, the voltage contrast information is due to the function of the protective film, and the AI! It is known that the voltage applied to the wiring is differentiated and output.

Therefore, when a rectangular wave is applied to the AJ wiring, a signal having equal positive and negative values is generated as an output.

Therefore, it becomes necessary to use one of the signals as the brightness modulation signal. The signal selection circuit 6 is a circuit that selects either the positive or negative signal, and performs switching based on the X-scan synchronization signal from the deflection oscillator 4. Therefore, the signal selection circuit 6 is a circuit that outputs a signal only when the X-deflection is odd-numbered or even-numbered, and the opposite platform is forcibly set to O potential. Signal selection circuit 6
The output from CRT9 is input as a brightness modulation signal, and
It is displayed as a SEM image on the RT plane.

FIG. 3 shows examples of waveforms at each part. A shows the sawtooth wave of the X deflection, and B shows the square wave applied to the sample synchronized therewith. C indicates the generated secondary electron signal. AI with applied voltage! The voltage contrast component is displayed superimposed only on the wiring portion. D is a brightness modulation signal of the CRT, and the signal is set to 0 by the signal selection circuit 6 during even-numbered X deflection.

As explained above, by using this method according to the above embodiment, it is possible to display information on the voltage contrast under the protective film in tSEM images.

Next, FIG. 4 shows a block diagram of another embodiment of the present invention. The direction method for primary electrons and the method for detecting secondary electrons are the same as those described above. In particular, we will focus on the method of applying voltage to the sample.

A lock-in amplifier 11 is provided, and its reference (R
ference) signal is input to the sample applied voltage modulation circuit 12. The output of the sample applied voltage modulation circuit 12 is
Lock-in amplifier 11 reference (Re-fere)
A predetermined voltage modulated at a frequency (for example, 30KI4z) is generated and supplied to the A/wirings of the eight samples. AI! The secondary electron signal generated by the primary beam irradiated through the protective film on the wiring is not only a general SEM signal but also a voltage contrast component (Re
(components modulated at the reference frequency) are also included. This is because the protective film acts as a capacitor, and since the modulation frequency (reference frequency: 30 kHz) is high, voltage contrast information is transmitted directly onto the protective film.

A signal containing these two pieces of information is input to the lock-in amplifier 11 through the detector 5. lock-in amplifier 11
performs detection and integration at the modulation frequency (Reference frequency). Then, a signal containing only that frequency component becomes an output signal of the lock-in amplifier 11 and becomes a luminance signal of CBr4. That is, the output signal from the lock-in amplifier 11 does not include a general SEM signal, and only information about the voltage contrast applied to the A/' wiring is transmitted to the CRT screen.
In F, there is a statue.

When the above method is used in this way, AI! Since only the portion of the wiring or the like to which a voltage is applied becomes an image, it is convenient because it is easy to inspect the portion where the wire is broken, but it has the disadvantage that the shape of the unevenness cannot be observed. Therefore, an example in which the drawbacks of this method are improved is shown below. FIG. 5 shows its block diagram. This method is a combination of the methods shown in FIGS. 2 and 4. The sample applied voltage modulation circuit 12 is a lock-in amplifier 1
It is modulated at a high reference frequency from 1.

The output is then input to the switching circuit 13.
The switching circuit 13 also receives an X-scan synchronization signal from the deflection oscillator 4 to perform switching in synchronization at a low frequency, for example, 50 kHz. Therefore, for example, at the odd numbered X deflection, the switch is ON and a predetermined voltage modulated at 30kHz is output, and conversely, at the even numbered X scanning, the switch is OFF and the output is O. Become. The output voltage of sample 8 is
! When applied to the wiring, the secondary electron signal generated from the sample by the primary beam is divided into two signals: odd-numbered X deflection signals are mainly voltage contrast information, and even-numbered signals are general SEM information. Therefore, the signal from the detector 5 is input to the signal selection circuit 6 and selected in synchronization with the X-scan, and the odd-numbered input signal is input to the lock-in amplifier 11 and integrated.
Used as a luminance modulation signal for RTlo. Furthermore, the even-numbered input signal is used as a CBr4 brightness modulation signal. From the above, CBr4 is a general SEM (I, CRT
The IO can display alternating voltage contrast images. Of course, it can be easily assumed that the images can be displayed overlappingly on one CRT.

〔Effect of the invention〕

As explained above, by using the method according to the present invention, it has become possible to simultaneously observe the shape of unevenness and display the voltage contrast at the final stage when a semiconductor such as an LSI is covered with a protective film.

[Brief explanation of drawings]

11u(a) and (b) are diagrams explaining the basic principle of the present invention, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is a diagram showing each part of the above block diagram. The waveform diagram, FIG. 4, and FIG. 5 are block diagrams showing other embodiments of the present invention, respectively. In the figure, 1... electron beam, 2... secondary electron, 3... deflector, 4... deflection oscillator, 5
. . . Detector, 6 . . Signal selection circuit, 7 . . . Sample application voltage circuit, 8 . . . Sample, 9. ,,CRT0 spear 2
Press figure + 3 figure Δtry. Jusuke + 4 questions

Claims (1)

[Claims] 1. A detection means for detecting a secondary electron signal generated by scanning a low-acceleration primary electron beam on a sample; - generating a signal for causing the electron beam to scan in the X direction and the Y direction of the sample -F; a deflection optical oscillator; a sample application voltage circuit for applying a predetermined rectangular wave voltage to the sample in synchronization with the X-direction scanning by the deflection oscillator; A signal selection circuit that allows even or odd numbered signals to pass through in the X direction scan, and image display means that displays an image by using the output signal of the signal selection circuit as a brightness modulation signal in synchronization with the X direction scan. A voltage contrast image display device comprising: