JPH0136668B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron microscope apparatus suitable for use in observing semiconductor devices in an operating state.

Conventionally, when checking the operation of integrated semiconductor devices such as ICs and LSIs, the semiconductor device is put into an operating state by applying power and clock pulses, and a probe is placed on the surface of the semiconductor device to check the point. The electric potential of the sensor is measured. However, as LSIs have become more highly integrated and operate at higher speeds in recent years, it has become difficult to set up the probe using the above method, and it has become difficult to accurately check the operation of high-speed devices using the capacitance of the probe. Problems such as the inability to do so have arisen, and for this reason, the above methods are already reaching their limits. Therefore, in recent years, a method using an electron microscope has been proposed, and attempts are being made to develop it into practical use in various fields. FIG. 1 shows a schematic configuration of a conventional strobe scanning electron microscope apparatus for observing semiconductor devices in an operating state. Inside the electron microscope 1, there are an electron gun 2, an X-direction deflection electrode 3, a Y-direction deflection electrode 4, a focusing lens 5, a scanning coil 6, an objective lens 7, and a secondary electron deflection electrode 8 (hereinafter referred to as the secondary electrode 8). etc., and below the secondary electrode 8 is a sample 9 such as an LSI.
is placed. Incidentally, the mirror body 1a is kept in a high degree of vacuum. Electron beam fired from electron gun 2
B ₁ is irradiated onto the surface of the sample 9 with the beam diameter narrowed down by the electron lens system. As a result, secondary electrons B ₂ are generated in an amount corresponding to the potential of the surface,
This secondary electron B ₂ is deflected by the secondary electrode 8 and applied to the detector 10 . Sample 9 is clock oscillator 1
A clock pulse is applied from 1 to put it into operation. This clock pulse is also applied to a pulse generator 13 through a variable delay circuit 12, thereby obtaining deflection voltages _V.sub.X and _V.sub.Y in the X and Y directions as shown in FIG. This deflection voltage
V _X and V _Y are synchronized with the clock pulse in a phase relationship corresponding to the amount of delay of the delay circuit 12. The electron beam _B1 is deflected in the ±X and ±Y directions by the deflection voltages _VX and _VY , as shown in FIG. in this case,
An image aperture 14 is substantially formed in the electron lens system, as shown by the dotted line in FIGS. 3 and 1, and at the moment when the beam B ₁ crosses this aperture 14 in the + beam 10
is dropped downward from the aperture 14 and reaches the sample 9. That is, the beam _B1 intermittently irradiates the sample 9 with a strobe while being synchronized with the clock pulse. Now, assuming that the scanning coil 6 is not operating, the beam 10 intermittently irradiates one point on the sample 9. The potential V _S at the irradiation point changes periodically in accordance with the clock pulse as shown in FIG. 4A, for example, due to the operation of the sample 9. In this state, when the beam is irradiated at the timing shown in FIG. 4B, the instantaneous potential V _S1 is sampled, and the amount of secondary electrons B ₂ generated according to this potential V _S1 is detected by the detector 10. be able to. Furthermore, if the delay amount of the delay circuit 12 is changed, the sampling timing is shifted as shown by the dotted line in FIG. 4B, so that the potential V _S2 can be detected.

Next, the scanning coil 6 and the horizontal and vertical coils 17 of the cathode ray tube 16 are driven by the deflection circuit 15. As a result, the predetermined area of the sample 9 becomes the beam B ₁
It is scanned while being irradiated intermittently. In this case, the scanning position on the sample 9 and the scanning position on the screen of the cathode ray tube 16 are always made to correspond. The potential of the element pattern and signal transmission pattern formed on the surface of the sample 9 scanned by the beam _B1 changes at a constant period according to the clock pulse.
The beam _B1 scans this surface intermittently at a constant period in synchronization with a clock pulse. Therefore, the beam
The potentials at each scanning position sampled by scanning _B1 are always approximately equal.
The secondary electron _B2 obtained by the above sampling is detected by the detector 28, and this detection signal is amplified by the video amplifier 18, noise is removed by the filter 19, and further amplified by the feedback amplifier 20. After that, it is applied to the cathode ray tube 16 from point a. Note that a part of the output of the amplifier 20 is fed back to the secondary electrode 8 from point a, which will be described later.
By applying the output from point a to the cathode ray tube 16, the potential pattern of the scanning surface of the sample 9 is gradually displayed on its screen. Therefore, by bringing a photographic film in close contact with this screen and exposing it to light, an image of the above-mentioned potential pattern corresponding to a certain phase of the clock pulse can be transferred onto the film. In this case, parts with high potential appear black, and parts with low potential appear white. Further, by changing the amount of delay of the clock pulse, and by changing the sampling phase as described with reference to FIG. 4B, it is possible to obtain potential patterns for different phases of the clock pulse. By observing this potential pattern, a defective element such as an element causing a delay inside the sample 9 can be easily found.

Next, FIG. 5 shows the structure of the secondary electrode 8, which is composed of an outer electrode 22 having a beam passage hole 21, an inner electrode 23, a grid 24, a retarding grid 25, etc. ing. Further, an anode electrode 26 and a Wehnelt electrode 27 are provided below the secondary electrode 8.
A secondary electron path 28 is formed by the outer electrode 22 and the inner electrode 23 in a curved manner as shown in the figure.
The anode electrode 26 and the Wehnelt electrode 27 are 2
It operates as a secondary electron extraction electrode, and is provided to eliminate the influence of the potential near the measurement point or measurement surface of the sample 9, and to accelerate the secondary electrons _B2 . A portion of the output of the amplifier 20 at point a in FIG. 1 is fed back and applied to the retarding grid 25. The potential of the grid 25 is controlled by this feedback voltage so that the amount of secondary electrons _B2 detected by the detector 10 is constant. Thereby, the potential change at the measurement point of the sample 9 can be quantified at point a, and the amount of secondary electron generation can be detected linearly.

Therefore, in the above-mentioned electron microscope device,
Since the beam B ₁ is irradiated intermittently in accordance with the operation of the high-speed element, the irradiation time is required to be extremely short, on the order of nanoseconds. In order to generate a sufficient amount of secondary electrons B ₂ to be detected in such a short period of time, a beam current that is about three orders of magnitude larger than that of a normal continuous irradiation type electron microscope is required. On the other hand, in order to increase resolution in an electron microscope, it is necessary to make the beam diameter as narrow as possible. However, the beam diameter r _s and beam current I ₁ are generally expressed as r _s = k ₁ [k ₂ I ₁ + k ₃ (1/V _a ) ² ] ³
₈ (However, k ₁ , k ₂ , k: constant, V _a = cathode voltage)
Because of this relationship, it has been difficult to increase the beam current sufficiently while reducing the beam diameter _rs .

In order to solve the above problem, the present applicant previously proposed the invention in Japanese Patent Application No. 55-43991. below,
The invention according to the above application will be explained.

In the above-described electron microscope apparatus shown in FIG. 1, a beam reduction ratio of M=α _c /α _s is generally used, and this M is usually M<<1. In addition, in the above equation regarding the beam diameter r _s , if the beam current I ₁ is large, the beam diameter _rs also becomes large. By making it smaller, the beam diameter can be made smaller while increasing the beam current. In order to make M smaller, α _s can be made larger in Fig. 1, but in the configuration shown in Fig. 1, since the secondary electrode 8 is located directly above the sample 9, α _s should be made larger. I can't.

The invention according to the above-mentioned application has been made by paying attention to the above-mentioned points, and an embodiment of the invention according to the above-mentioned application will be described below with reference to FIG. 6. In FIG. 6, the same parts as in FIG. 1 are given the same reference numerals.

In this embodiment, the objective lens 7 is placed directly above the anode 26, and the secondary electrode 8 is placed above the objective lens 7. According to the above configuration, since the secondary electrode 8 is arranged above the objective lens 7, the working distance, which is the distance between the objective lens 7 and the sample 9, can be reduced, and therefore _αs can be increased. You can take it. Therefore, the beam diameter can be made smaller while increasing the beam current than in the case of FIG. Furthermore, since the objective lens 7 is disposed directly above the sample 9, the secondary electrons _B2 are focused by the objective lens 7, resulting in very high efficiency.

In the above-mentioned electron microscope shown in FIG.
The primary electron beam B ₁ is affected by the secondary electron attraction voltage applied to the secondary electron drawing voltage. As a result, the beam _B1 may be bent or its cross-sectional shape may be distorted into an ellipse, which may impair the quality of the observed image.

The present invention solves the above problems, and embodiments of the present invention will be described below with reference to the drawings.

FIG. 7 shows an embodiment of the present invention, and parts corresponding to those in FIG. 6 are given the same reference numerals.

In this embodiment, a pair of secondary electron detectors 1
0a and 10b are provided at symmetrical positions with respect to the central axis of the mirror body 1a as shown in the figure. Furthermore, instead of the secondary electrode 8 shown in FIG. 6, a cylindrical secondary electron deceleration electrode (hereinafter referred to as secondary electrode) 29 as shown in FIG. It is arranged in The pair of detectors 10a and 10b are arranged above the secondary electrode 29 and the objective lens 7, and the secondary electrons B ₂ deflected and decelerated by the secondary electrode 29 are
is detected and added to the common video amplifier 18.

In FIG. 8, the secondary electrode 29 has cylindrical bodies 30 and 31 made of a semi-insulating material disposed coaxially, and a grid 24, a retarding grid 25 and an anode electrode 26 corresponding to those in FIG. It has a structure arranged like this. Although the anode electrode 26 is theoretically a part of the secondary electron extraction electrode and also serves as a part of the secondary electron deceleration electrode 29, it is generally treated as a part of the secondary electron extraction electrode. , it is common to handle the secondary electrons by excluding them from the secondary electron deceleration electrode 29.

According to the configurations shown in FIGS. 7 and 8, the working distance can be reduced and α _s can be increased, and therefore the beam diameter can be reduced while increasing the beam current. .
In addition, since the detectors 10a and 10b are arranged symmetrically, the 2
The beam _B1 is not bent or distorted by the secondary electron drawing voltage, and the image quality can be improved compared to that of FIG. Along with this, the secondary electron collection rate increases, and the S/N ratio can be improved.

Next, examples of other structures of the electron microscope will be described.

Such strobe scanning electron microscopes are generally used in two modes. The mode shown in FIG. 7 described above is a mode in which the potential of each part of the sample 9 is measured, but there is also a mode called an image mode in which changes in the potential of the sample 9 are observed as an image. 7th
In the figure, the secondary electrode 29 is removed when the electron microscope is used in image mode. Therefore, the secondary electrode 29 is detachably provided on the mirror body 1a.
When the secondary electrode 29 is attached and detached, air flows into the mirror body 1a, so after the attachment and detachment are completed, it is necessary to operate a vacuum pump to remove the air from the mirror body 1a to create a vacuum. In this case, a large pump must be used to remove all the air inside the mirror body 1a, and the work must be carried out over a long period of time.

Further, the sample 9 is placed in the mirror body 1a while being attached to a printed circuit board in which circuit components such as a capacitor and a resistor are incorporated. Therefore, when the mirror body 1a is placed in a highly vacuum state, gas is generated from organic materials such as resin used to mold the circuit components, reducing the degree of vacuum.

FIG. 9 shows an embodiment for solving the above problem, and the same parts as in FIG. 7 are given the same reference numerals.

In FIG. 9, the lower part of the mirror body 1a is partitioned by a partition plate 31 having an opening 30 and a shutter plate 32 which covers the opening 30 so as to be openable and closable, thereby forming a chamber 33. In this chamber 33, an objective lens 7 is provided, and a secondary electrode 29 is also provided in a detachable manner. An opening 34 is provided at the bottom of the chamber 33, and a housing 35 for storing the sample 9 is detachably provided at the lower surface of this bottom. The upper surface of this housing 35 has an opening 3 that coincides with the opening 34 described above.
6 are provided, and a large number of terminal pins 37 are led out from the bottom surface. The sample 9 is provided with a terminal pin 38 as shown in FIG. It will be installed. On the other hand, a printed circuit board 40 incorporating circuit components 39 is mounted from below the housing 35, and the terminal pins 37 are inserted into sockets of the printed circuit board 40.

In the above configuration, in order to attach and detach the secondary electrode 29, if the attachment and detachment operation is performed with the shutter plate 32 closed, air will flow only into the small space formed by the chamber 33 and the casing 35, and the attachment and detachment will be completed. After that, you can quickly remove the air using a small pump. Further, when the air in the chamber 33 is removed, the housing 35 is pressed against the bottom of the chamber 33, so that the two can be brought into complete contact with each other. Furthermore, printed circuit board 4
0 is placed outside, so no gas is generated. Furthermore, the printed circuit board 40 is connected to the sample 9 by inserting the terminal pin 37, so that
There is no need to route wiring between the two, and stray capacitance etc. do not occur due to routing the wiring.

As described above, the present invention provides two
Secondary electron extraction electrodes 26 and 27 are arranged, and an objective lens 7 and a cylindrical secondary electron deceleration electrode 29 arranged inside this objective lens 7 are arranged above the secondary electron extraction electrodes. A pair of secondary electron detectors 10 above the lens and the secondary electron deceleration electrode
This relates to an electron microscope device characterized in that a and 10b are arranged symmetrically with respect to the central axis of a mirror body 1a.

Therefore, according to the present invention, it is possible to increase the beam reduction ratio by increasing the work distance, so it is possible to reduce the beam diameter while increasing the beam current, and it is also possible to eliminate beam bending and distortion. can. Therefore, it is possible to obtain such an electron microscope device with excellent resolution, high sensitivity and precision, and excellent image quality. The present invention can provide excellent effects particularly when checking the operation of a semiconductor device.

[Brief explanation of drawings]

Figure 1 is a configuration and circuit diagram of a conventional electron microscope device, Figure 2 is a diagram of deflection voltage waveforms in the X and Y directions, Figure 3 is a plan view of the deflection unit in the X and Y directions, and Figures 4A and B.
5 is a waveform diagram showing the relationship between the potential on the surface of a semiconductor device and beam irradiation, FIG. 5 is a side view showing the configuration of the secondary electron deflection electrode and its vicinity, and FIG. 6 is a diagram of the prior invention to which the present invention can be applied. FIG. 7 is a configuration and circuit diagram of an electron microscope device showing an embodiment of the present invention; FIG. 8 is a configuration and circuit diagram of an electron microscope device showing an embodiment of the present invention;
The figures are a side view of the secondary electron deceleration electrode shown in FIG. 7, FIG. 9 is a configuration diagram showing another embodiment of the present invention, and FIG. 10 is a perspective view of a sample. In addition, in the symbols used in the drawings, 1...
...Electron microscope, 2...Electron gun, 7...Objective lens, 10a, 10b...Secondary electron detector, 29...
... Secondary electron deceleration electrode, 9 ... Sample, 26 ... Anode, 27 ... Wehnelt electrode.

Claims (1)

[Claims] 1. Sample 9, secondary electron extraction electrodes 26, 27, objective lens 7, and objective lens 7 in mirror body 1a.
A cylindrical secondary electron deceleration electrode 29 arranged inside the
are arranged substantially coaxially with respect to the central axis of the mirror body 1a, and at this time, the secondary electron extraction electrodes 26, 27 are arranged above the sample 9, and the secondary electron extraction electrodes 26, 27, The objective lens 7 and the secondary electron deceleration electrode 29 are arranged above the objective lens 7 and the secondary electron deceleration electrode 29, and a pair of secondary electron detectors 10a and 10b are placed above the objective lens 7 and the secondary electron deceleration electrode 29 at the center of the mirror body 1a. An electron microscope device characterized by being arranged symmetrically with respect to an axis.