JPS6155735B2 - - Google Patents

Description

[Detailed description of the invention] FIELD OF THE INVENTION The present invention relates to scanning electron microscopy.

In a scanning electron microscope, an electron beam is used as a probe with a minute diameter to irradiate and scan a sample, and information about the sample is obtained using signals from the sample generated by the irradiation. In such an apparatus, when a sample is irradiated with an electron beam at a high acceleration voltage and an electron beam at a low acceleration voltage,
The information obtained from the sample is different. In other words, when a sample is irradiated with an electron beam with a low acceleration voltage, information near the sample surface can be obtained, and when a sample is irradiated with an electron beam with a high acceleration voltage, information from deeper parts of the sample can be obtained. can get. There was a difference 2
It would be of great significance if it were possible to compare and compare the images of a sample obtained when the sample is irradiated with an electron beam with an acceleration voltage higher than that of the electron beam.

In view of these points, the present invention provides a scanning electron microscope that can clearly and simultaneously observe images at different depths of the same area of a sample, and will be described in detail below with reference to the drawings.

FIG. 1 is a schematic structural diagram showing an embodiment of the present invention, and 1 is a mirror body of a scanning electron microscope. Reference numeral 2 denotes an electron gun placed above the mirror body 1, and the electron beam E generated from the electron gun is narrowly focused by focusing lenses 3 and 4 and irradiated onto the sample 5. At least one stage of X, Y deflection coils 6X and 6Y is placed between the focusing lenses 3 and 4, and each deflection coil is provided with a sawtooth signal from a scanning signal generating circuit 8 via a magnification adjuster 7. A wave scanning signal is provided. As a result, the electron beam is two-dimensionally scanned over the sample 5. Secondary electrons and reflected electrons generated from the sample by the scanning of the electron beam are detected by a detector 9, and are supplied to the brightness modulation grids of cathode ray tubes 12 and 13 via an image intensifier 10 and a switching circuit 11. The deflection coils 14X, 14Y and 15X, 15Y of the cathode ray tubes 12 and 13 are supplied with a scanning signal synchronized with the signal sent from the scanning signal generating circuit 8 to the deflection coils 6X, 6Y. When the switching circuit 11 is supplying a video signal to one cathode ray tube 12, for example, the switching circuit 11 switches the switching circuit 11 to the other cathode ray tube 13,
is configured to provide a blanking signal. Reference numeral 16 denotes a focusing auxiliary coil 22 placed close to or inside the focusing lens 4.
This is a lens control circuit for controlling the excitation current supplied to the lens. Reference numeral 17 denotes an astigmatism correction coil for correcting astigmatism of the electron beam, and a correction current is supplied from the astigmatism correction circuit 18. Reference numeral 19 denotes an acceleration voltage control circuit for controlling the acceleration voltage applied to the electron gun 2, and the control circuit is provided with two setting means 20a and 20b for specifying the acceleration voltage.
With these two setting means, two different acceleration voltages Va and Vb can be set as the acceleration voltage applied to the electron gun 2: a high acceleration voltage such as 10KV and a low acceleration voltage such as 1KV. Further, the setting means 20a and 20b include the magnification adjuster 7, the lens control circuit 16, the astigmatism correction circuit 18, and the image intensifier 10.
The lens control circuit 16 generates a current Ia such that the electron beam E accelerated at the acceleration voltage Va set by the setting means 20a is focused on the sample 5 and an acceleration voltage set by the setting means 20b. A current Ib is set so that the electron beam accelerated by Vb is focused on the sample 5. Further, the magnification adjuster 7 divides the amplitude of the scanning signal into two stages so that a constant magnification is always maintained at any acceleration voltage set by the setting means 20a and 20b, that is, a constant area on the sample 5 is scanned. Set. Further, the astigmatism correction circuit 18 sets two correction currents for correcting astigmatism at each acceleration voltage set by the setting means 20a and 20b. Further, in the video amplifier 10, the amplification degree and the DC level are set in two stages so that the video signals supplied to the cathode ray tubes 12 and 13 are always constant at any set acceleration voltage. A timing signal from a timing circuit 21 is sent to the acceleration voltage control circuit 19, magnification adjuster 7, lens control circuit 16, astigmatism correction circuit 18, and video intensifier 10. The two set signals are alternately sent out from the circuits 19, 16, and 18. The timing circuit 21 generates a timing signal synchronized with the X-axis (horizontal) scanning signal from the scanning signal generating circuit 8. The timing signal from the timing circuit is also sent to the switching circuit 11, whereby video signals are alternately supplied to the cathode ray tubes 12 and 13.

The operation of such a device will be explained. First, two desired acceleration voltages Va and Vb are designated by the setting means 20a and 20b. Next, when the scanning signal generating circuit 8 is turned on, a horizontal scanning signal and a vertical scanning signal (not shown) as shown in FIG.
Y and each deflection coil 14 of the cathode ray tubes 12 and 13
It is supplied to X, 15X, 14Y, and 15Y. At the same time, a portion of the horizontal scanning signal is supplied to the timing circuit 21, so that the timing circuit 21 generates a timing signal as shown in FIG. 2b.
The generated timing signal is applied to the acceleration voltage control circuit 1.
9, a magnification adjuster 7, a lens control circuit 16, an image intensifier 10, and a switching circuit 11, respectively. However, in the timing signal shown in Figure 2b, the period
When T ₁ , the acceleration voltage control circuit 19 outputs the second
Since the higher accelerating voltage is applied to the electron gun 2 as shown by V _a in Figure c, the accelerating voltage V _a from the electron gun is
The accelerated electron beam is extracted. Further, from the lens control circuit 16, a current I a suitable for focusing the electron beam accelerated by an accelerating voltage _{V a on} the sample 5 as shown by I _a in FIG.
sent to. Similarly, the astigmatism correction circuit 18 sends a current to the astigmatism correction coil 17 to correct the astigmatism of the electron beam at the acceleration voltage V _a . Therefore, the focused electron beam with the accelerating voltage _V a horizontally scans a certain area on the sample 5 and is astigmatically corrected. By scanning the electron beam _, the detector 9 detects a video signal as indicated by S _a in FIG. After being amplified as described above, the light is introduced into the cathode ray tube 12 via the switching circuit 11. Next, during period T2 of the timing signal shown in FIG. _2b , the acceleration voltage control circuit 19 applies a lower acceleration voltage to the electron gun 2 as shown by _Vb in FIG. An electron beam accelerated by an accelerating voltage V _b is extracted from. Further, from the lens control circuit 16, a current I _b (I _b <I _a ) (FIG. 2 d) suitable for focusing the electron beam accelerated by the accelerating voltage V _a on the sample 5 is sent to the auxiliary lens 22. Furthermore, the astigmatism correction control circuit 18 also sends a current to the astigmatism correction coil 17 to assist in the astigmatism of the electron beam accelerated by the acceleration voltage V _b . Further, the magnification adjuster 7 supplies the deflection coils 6X, 6Y with a scanning signal having a smaller amplitude than the scanning signal used when scanning the electron beam accelerated by the accelerating voltage V _a . Therefore, the focused electron beam at the accelerating voltage V _b horizontally scans over a certain area of the sample 5 with the same scanning width as at the accelerating voltage V _a .
By scanning the electron beam, a video signal as shown by S _b in FIG. 2e is detected from the detector 9 and sent to the video intensifier 10. At this time, the video intensifier ₁₀ is switched by the timing signal of the middle period _T2 in FIG. As a result, the video signal output from the video amplifier 10 has approximately the same level and amplitude during the _T1 period and the _T2 period, as shown in FIG. 2(f). The video signal S _b ' from the video amplifier 10 is then introduced into the cathode ray tube 13 via the switching circuit 11. After one frame of scanning is completed by repeating the above-mentioned operations, a sample image based on the accelerating voltage V _a is displayed on the cathode ray tube 12, and a sample image based on the accelerating voltage V a is displayed on the cathode ray tube 13.
Sample images based on _b are displayed. As a result, sample images based on two different accelerating voltages can be observed simultaneously.

Although two cathode ray tubes are used in this embodiment, the configuration may be such that images of two different accelerating voltages are independently displayed on the left and right halves of one cathode ray tube.

Alternatively, the accelerating voltage may be changed every time one frame or several frames are scanned.
Furthermore, the switching of the acceleration voltage is not limited to two stages, and may be set to a desired number. Furthermore,
Although the auxiliary lens for focusing is provided, the excitation current supplied to the focusing lens 4 or 4 and 3 may be changed without using the auxiliary lens.

As explained in detail above, according to the present invention, the lens means is changed between two different types in synchronization with the switching of the acceleration voltage so that the focused point by the lens means is maintained regardless of the switching of the acceleration voltage. means for switching to the above lens strength, and a deflection sent to the deflection system in synchronization with switching of the accelerating voltage so that the same area of the sample is scanned by the electron beam regardless of switching of the accelerating voltage. By including means for switching the signals, a scanning electron microscope is provided that allows images of different depths of the same area of the sample to be observed simultaneously and clearly.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing one embodiment of the present invention, and FIGS. 2 a to 2 f are diagrams for explaining the operation of the present invention. In Fig. 1, 1 is a mirror body, 2 is an electron gun, 3 and 4 are focusing lenses, 5 is a sample, 6X and 6Y are deflection coils, 7 is a magnification adjuster, 8 is a scanning signal generator, 9 is a detector, 10 is a video intensifier, 11 is a switching circuit, 1
2 and 13 are cathode ray tubes, 16 is a lens control circuit,
17 is an astigmatism correction coil, 18 is an astigmatism correction control circuit, 19 is an acceleration voltage control circuit, 20a and 20b
2 is a setting means, 21 is a timing circuit, and 22 is an auxiliary coil.

Claims (1)

[Claims] 1 Equipped with an electron beam generating means for generating an electron beam, a lens means for narrowly focusing the electron beam from the electron beam generating means and irradiating it onto the sample, and a deflection system for scanning the electron beam on the sample. in the apparatus, means for alternately switching and applying two or more different accelerating voltages to the electron beam generating means in synchronization with scanning of the electron beam; means for switching the lens means to two or more different lens strengths in synchronization with the switching of the accelerating voltage so that a focused point is maintained; means for switching a deflection signal sent to the deflection system in synchronization with switching of the accelerating voltage so that the same area is scanned; displaying each sample image based on the electron beam accelerated by the respective accelerating voltage; A scanning electron microscope consisting of display means for obtaining.