JPH0339377B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention applies a first voltage and a second voltage to both ends of a sample, and calculates the difference between two types of potential distributions formed when the first and second voltages are applied. The present invention relates to a scanning electron microscope capable of displaying contrast images.

For example, let's compare the potential distribution image when a first voltage slightly lower than the rated voltage planned in the element design is applied to both ends of an integrated circuit element, and the potential distribution image when a slightly higher second voltage is applied. If you do,
Conventionally, a sample to which a first voltage is applied is scanned with an electron beam to obtain an image when the first voltage is applied and photographed, and then a scanned image is obtained when a second voltage is applied. They also took photos of both statues and compared the photos. Therefore, conventionally, it has not been possible to intuitively grasp in real time the portion where the potential distribution differs due to a change in the applied voltage. Further, problems such as the sample drifting occur while obtaining both images, so it is necessary to adjust the sample position each time. Furthermore, the contrast based on the potential distribution of the sample is generally relatively small and may be disturbed by the diffusion of the incident electron beam. Therefore, in order to increase this small contrast as much as possible, in conventional equipment, the acceleration voltage of the electron beam irradiated onto the sample is lowered during observation. However, when the accelerating voltage of such an electron beam is lowered, the resolution is poor and the amount of detected signal is also small, making it impossible to obtain a high-quality image.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, it is an object of the present invention to provide a scanning electron microscope capable of observing a clear, high-resolution potential distribution image of a sample in real time with a good S/N ratio.

In order to achieve this object, the present invention scans an electron beam two-dimensionally on a sample, and guides the signal obtained from the sample along with the scanning to a display device that is scanned in synchronization with the scanning. In the apparatus for obtaining an image, means for applying a constant frequency voltage higher than the horizontal scanning frequency of the electron beam to both ends of the sample, and synchronizing the signal obtained from the sample with the constant frequency voltage. The present invention is characterized in that it includes a lock-in amplifier that performs detection based on the supply of a reference signal, and displays a sample image on the display device based on the output signal of the lock-in amplifier.

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 is for showing one embodiment of the present invention. In the figure, 1 is an electron gun, and an electron beam 2 from this electron gun 1 is focused by a focusing lens 3, and further narrowed by an objective lens 4. It is focused and irradiated onto the integrated circuit element 5, which is the sample. Reference numeral 6 denotes a deflection coil, to which sawtooth X (horizontal direction) and Y (vertical direction) scanning signals can be supplied from a scanning signal generating circuit 8 via an amplifier 9. The scanning signal from the scanning signal generating circuit 8 is also supplied to the deflection coil D of the cathode ray tube 10, and the cathode ray tube 10 is scanned in synchronization with the electron beam 2.
One end A of the integrated circuit element 5 is grounded, and the other end B is connected to an amplifier 1 that amplifies the output signal of the oscillator 11.
2 output voltages are applied. This oscillator 11
The oscillation frequency f is selected to be sufficiently higher than the frequency of the H scanning signal generated by the scanning signal generating circuit 8. The DC level and AC voltage width of the output voltage of this amplifier 12 can be changed by adjusting a level adjuster 13 and a gain adjuster 14, respectively. 15 is a secondary electron detector for detecting secondary electrons generated from the integrated circuit element 5, and the signal from this detector 15 is sent to the preamplifier 1.
After being amplified by the oscillator 6, the signal from the oscillator 11 is supplied to a lock-in amplifier 17, which is supplied as a reference signal. Further, the output signal of the lock-in amplifier 17 is supplied to the cathode K of the cathode ray tube 10 via the video amplifier 7.

In such a configuration, let us take as an example a case where an image is to be obtained that emphasizes the difference between the potential distribution when, for example, 5V is applied to one end B of the integrated circuit element 5 and the potential distribution when 9V is applied. Let me explain.

First, the gain adjuster 14 and the level adjuster 13
The amplifier 12 applies a voltage that periodically changes between 5V and 9V as shown in FIG. 2a. Now, 5V is applied to the terminal B of the integrated circuit element 5.
When 9V is applied, a certain −X on this element 5
It is assumed that potential distributions as shown by solid lines and dotted lines in FIG. 3 occur along the scanning lines. When a scanning signal is supplied from the scanning signal generation circuit 8 to the deflection coil 6 and the integrated circuit element 5 is scanned by the electron beam 2, an amount of secondary electrons corresponding to the potential at that point is emitted from the irradiation point of the electron beam 2. However, since the frequency of the applied voltage is selected to be sufficiently high compared to the frequency of the H scanning signal, the electron beam 2 is irradiated onto the position corresponding to P ₁ in FIG. 3 on the X scanning line. 2b from the secondary electron detector 15.
When _a signal as shown in _FIG . The result will be as shown in d. Therefore, the signal whose phase is detected by the lock-in amplifier 17 becomes a signal as shown in FIG. When displayed, the cathode ray tube 10 has an image as shown in FIG. 5 in which the difference between the potential distribution when 5V is applied to the terminal B of the integrated circuit element and the potential distribution when 9V is applied is emphasized. can be obtained. In addition, in Fig. 5, the dotted line U
is the X scanning line.

According to the apparatus of the present invention, an image that emphasizes the part where the difference in potential distribution occurs can be obtained in real time. Since the secondary electron detection signal is detected in synchronization with the voltage, a contrast signal representing the difference in the potential distribution can be detected with a high S/N ratio. Therefore, according to the apparatus of the present invention, even when a sample is irradiated with a highly accelerated electron beam, an image with sufficient contrast can be obtained, the resolution of the obtained image can be increased, and the amount of detected signals can be increased. Therefore, high-quality images can be obtained.

In addition, in the above embodiment, if the voltage applied to terminal B is periodically changed between OV and a certain voltage V, it is possible to obtain a potential distribution image when voltage V is applied to the sample. can.

Further, in the above-described embodiments, the case where an integrated circuit element is observed is illustrated, but the present invention is similarly applicable to the case where other samples are observed.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining one embodiment of the present invention, FIG. 2 is a diagram for illustrating output signals from each circuit shown in FIG. 1, and FIG. 3 is a diagram for explaining the potential on the -X scanning line. A diagram for illustrating the distribution, FIG. 4 is a diagram for illustrating the output signal of the lock-in amplifier,
FIG. 5 is a diagram for illustrating a display image of a cathode ray tube. 1: Electron gun, 2: Electron beam, 3: Focusing lens,
4: Objective lens, 5: Integrated circuit element, 6: Deflection coil, 7, 9, 12, 16: Amplifier, 8: Scanning signal generation circuit, 10: Cathode ray tube, 11: Oscillator, 1
3: Level adjuster, 14: Gain adjuster, 15:
Secondary electron detector, 17: Lock-in amplifier.

Claims (1)

[Claims] 1 Scan the electron beam two-dimensionally over the sample,
In an apparatus that obtains a sample image by guiding a signal obtained from the sample during the scanning to a display device that is scanned in synchronization with the scanning, a constant frequency voltage higher than the horizontal scanning frequency of the electron beam is applied. means for applying voltage to both ends of the sample, and a lock-in amplifier that detects a signal obtained from the sample based on the supply of a reference signal synchronized with the constant frequency voltage, A scanning electron microscope characterized in that a sample image is displayed on the display device based on the display device.