JP2000340628A - Method and device for detecting isolated pattern based on surface potential - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide a high-resolution scanning electron microscope, which can project a large-current electron beam with a low accelerating voltage and can be used for a method by which an isolated area caused by imperfect contacts that occurs in a semiconductor element and cannot be detected through visual inspections which are carried out by obtaining surface images is discovered from a potential contacted image. SOLUTION: In a method for detecting isolated pattern, a large-current electron beam projecting barrel, which is used for injecting charges into a sample at a shallow angle from the surface of the sample and an observatory electron beam projecting barrel for obtaining second electron images, are provided separately. An isolated pattern, which is not discriminated from the appearance of the sample, is detected by executing work for detecting secondary electron information by projecting a large- current electron beam 12 upon the surface of the sample for a fixed period of time and then projecting an electron beam 2 for observation upon the portion irradiated with the beam 12 and obtaining the microscopic image B of a potential difference contrast by performing scanning, in such a way that the portions which are irradiated with the beam 12 and 2 and from which the information is detected are shifted successively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for detecting an isolated pattern on a sample, and more particularly to a technique for detecting an isolated pattern of a semiconductor device.

[0002]

2. Description of the Related Art There is known a method of finding an isolated area due to a contact failure of a semiconductor element or the like, which cannot be found from an appearance inspection for obtaining a surface image, by a potential contrast image. In principle, the inspection area is scanned and irradiated with a low-acceleration, high-current electron beam, and the inspection area is observed. Inject negative charges. The electrons injected into the surface flow to the ground via the wiring pattern of the semiconductor element, and the current value varies depending on the resistance value of the path.
Therefore, a potential difference is generated on the surface of the sample in accordance with the resistance to ground for each part. When a potential difference occurs on the sample surface, it appears as a difference in the generation efficiency of secondary electrons based on the electron beam irradiation.Therefore, an image in which the potential difference has become luminance information is obtained. It is possible to find an isolated area due to an unknown contact failure of a semiconductor element.

A device of this type, which has been conventionally known,
It is also required to use a high-current electron beam to generate a potential difference in a short time, use a low accelerating voltage to prevent the surrounding insulator from being charged up, and use a high-resolution microscope to identify defective parts. Since these requirements are usually contradictory requirements, a scanning electron microscope satisfying the specifications has been extremely expensive.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and it is an object of the present invention to provide an isolated area due to a contact failure of a semiconductor element which cannot be found by an appearance inspection for obtaining a surface image. Is to provide a high-resolution scanning electron microscope which can be irradiated with a large current electron beam at a low acceleration voltage at a relatively low cost.

[0005]

SUMMARY OF THE INVENTION The present invention provides a high-current electron beam irradiation column for injecting electric charge into a sample surface from a shallow angle and an observation electron beam irradiation column for obtaining a secondary electron image. Separately provided, irradiate a large current electron beam for a certain period of time, irradiate the part with an observation electron beam to detect secondary electron information, and then irradiate the adjacent part with the position of irradiation and detection A microscope image of a potential difference contrast is obtained by sequentially shifting scanning, and an isolated pattern that cannot be seen from the appearance is detected.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is to detect an isolated floating region of a semiconductor element or the like which cannot be grasped from the external appearance as shown in a cross section in FIG. 1A. Assuming that there is a portion in contact with the conductive portion as shown on the left side and a conductive portion as shown on the left side, when a charge is injected by irradiating the surface of the sample with a large current electron beam, the portion connected to the conductive portion on the left side and properly Although the electric charge injected into the part immediately flows out to the ground via the conductive part, the electric charge injected into the part not in contact with the conductive part as shown on the right cannot flow directly into the electric conductor, and It flows out little by little at the current value corresponding to the resistance value between the two. Now, regarding the generation efficiency of secondary electrons by the irradiated electron beam,
When the value of (secondary electron amount / primary electron amount) is shown on the vertical axis as the secondary electron generation efficiency and the acceleration voltage value (Acc) on the horizontal axis, it is generally shown in FIG. 1C. A peak-shaped quadratic curve characteristic having a peak in a low acceleration voltage region of 1 kV or less is shown. Of course, this characteristic varies depending on the sample, but is shown as a qualitative characteristic when the beam angle is changed while the beam current is kept constant. a is a deep angle incidence, and b is a shallow angle incidence. As shown in C, the point b at which the incident angle of the electron beam is shallower has a higher characteristic peak, and the point of generation efficiency 1 is closer to the high voltage side. It is understood that this is because when the angle of incidence on the sample surface is small, electrons do not penetrate deeply into the sample and emit many secondary electrons from the surface. Considering the charging efficiency of the sample surface, an electron beam irradiated at a low acceleration voltage of 1 kV or less and at a shallow angle of incidence has a high secondary electron generation efficiency, which is advantageous. The sample is irradiated with a large current at a shallow angle of incidence.

[0007] In the charge injection at this time, 1 kV
When an electron beam in the following low acceleration voltage region is used, as can be seen from the graph of C, in that region, the number of secondary electrons that jump out is larger than the number of primary electrons that are irradiated. Therefore, although the charge of the irradiated electron is negative, the sample surface is eventually charged with a positive charge. On the surface of the unconnected portion on the right side, the injected electric charge cannot flow out to the conductor directly, stays there, takes on a positive electric charge, and becomes a positive potential. Although the design is similar, the charge on the surface of the normal part on the left flows out to the ground and reaches zero potential, and the charge on the surface of the abnormal part on the right becomes positive with residual charges. When a potential difference occurs on the sample surface, a difference occurs in the generation efficiency of secondary electrons from that portion, and on the secondary electron image, as shown in FIG. 1B, the left normal portion appears bright and the right abnormal portion appears dark. Become. The other semiconductor surfaces are insulated and darker than the abnormal part.

An electron beam most suitable for charge injection is several nA to several μA accelerated at a low acceleration voltage of 1 kV or less.
The scanning electron beam most suitable for observation uses a current of several pA to several nA accelerated by a high to low acceleration voltage. The present invention relieves the difficult problem of having to prepare a source that satisfies both conditions by dividing the different electron beam sources into two systems and preparing them according to the respective conditions. The cost can be greatly reduced. The two electron beams scan the observation area in a main scan and a sub-scan, for example, in a raster shape, but are not scanned separately but are synchronized so as to irradiate the same position. This main scan is shown in FIG.
As shown in FIG. 2A, the scanning is constantly performed in one direction, but when viewed microscopically, scanning is performed stepwise as shown in FIG. 2B. That is, a fixed time t for one irradiation point
When a large current electron beam for charge injection is irradiated, Δ
An electron beam for observation at time t2 is irradiated with a blank at time t, and during that time, secondary electrons corresponding to the potential of that portion are detected. This t1 + Δt + t2 is the working time of this part, and the scanning position is shifted so that the irradiation positions of both electron beams are shifted to the next every one unit working time. The observation electron beam is deflected and blocked by the blanking means so that the beam does not pass through the opening during the time between t1 and Δt when the large current electron beam for charge injection is irradiated. Between the blank time Δt and the observation period t2, the large current electron beam for charge injection is deflected so as not to pass through the opening.

[0009]

Embodiment 1 FIG. 4 is a conceptual diagram showing an entire configuration of a first embodiment of the present invention. FIG. 3 is a diagram for explaining the blanking operation. The irradiation column of the observation electron beam 2 includes an electron gun 1, an electron optical system 3 including a blanking means 4 and a blanking aperture 5 in addition to an electron beam acceleration, focusing, and scanning means. The irradiation column of the beam 12 comprises an electron gun 11, an electron optical system 13 including a blanking means 14 and a blanking aperture 15 in addition to an electron beam acceleration, focusing and scanning means. The charge injection electron beam is applied so that the surface of the sample stage 6 on which the sample 7 is placed is irradiated with the observation electron beam 2 at a deep incident angle.
Numeral 12 is provided with both lens barrels so as to be irradiated at a shallow incident angle. Reference numeral 8 denotes a secondary electron detector, and reference numeral 9 denotes a secondary electron energy filter. Reference numeral 20 denotes a signal processing unit that performs signal processing on the information of the detected secondary electrons, reference numeral 22 denotes a display that displays an observation image or the like, and reference numeral 21 denotes a controller that controls the system.

A semiconductor element as shown in FIG. 1A is placed on the sample stage 6, and one point on the surface of the semiconductor element is irradiated with a low-acceleration, high-current electron beam 12 for charge injection from an irradiation lens barrel. This irradiation is to cause a charge on the semiconductor surface and to obtain a secondary electron microscope image in that state. Therefore, as shown in FIG. 1C, electrons in a low acceleration voltage region of 1 kV or less having a high secondary electron generation efficiency are obtained. A beam is used and directed at the semiconductor surface from a shallow angle of incidence. At the point of this irradiation, the number of secondary electrons repelled is larger than the number of applied primary electrons, so that electrically positive charges are injected. If this point is the surface of the normal part as shown on the left, the charge immediately flows out to the ground through the lower conductor, but if it is the surface of the abnormal part on the right, the charge Cannot immediately flow out to ground via the lower conductor, but gradually flows out according to the resistance value of the insulating portion interposed between the semiconductor region and the conductor. If the irradiation point is an insulating part, the charge has no outflow path and remains charged there. As described above, the surface of the sample irradiated with the electron beam is charged with a charge corresponding to the resistance value between the irradiated portion and the ground.

The electron beam 12 for charge injection irradiates the irradiation point for the first t1 time, the electron beam 2 for observation is irradiated for t2 time with a blank of the time Δt, and the secondary electrons are detected by the secondary electron detector 8. Although detected, the irradiation and interruption of these beams are executed under the control of the blanking means 4 and 14. Specifically, as shown in FIG. 3, the electron beam irradiation column for observation and the electron beam irradiation column for charge injection have blanking means 4, 14 and blanking apertures 5, 15, respectively. , 11 are deflected by the blanking means 4, 14 so that the electron beams are not blocked through the openings of the blanking apertures 5, 15. That is, during the first time t1 + Δt, the electron beam is deflected by applying a deflection signal to the blanking means 4 of the electron beam irradiation column for observation, and the beam is cut off because the beam cannot pass through the opening of the blanking aperture 5. In the state, the blanking means 14 of the electron beam irradiation column for charge injection has a Δ
By applying a time deflection signal of t + t2 to deflect the electron beam and to make the opening of the blanking aperture 15 blocked so that the beam cannot pass, one irradiation point as shown in FIG. In this case, irradiation of a large current electron beam for charge injection for a fixed time t1 and irradiation of an electron beam for observation for a time t2 with a blank at time t are realized. Secondary electrons are emitted from the sample surface due to the beam irradiation, but detection by the secondary electron detector 8 is required only for the time t2 to obtain an observation image. Energy filter 9 like grid in front of vessel 8
And the time t during which the beam for charge injection is irradiated
1 blocks secondary electrons and does not saturate the detector. These two types of electron beams are continuously emitted from electron guns of separate lens barrels, and irradiation / irradiation of the sample surface is controlled by beam passing / shielding control by blanking means.
Since the interruption is performed, the optimal irradiation beam for each irradiation beam is compared with the conventional one in which the electron beam is controlled by controlling the electron beam under the condition that both charge injection and observation are compatible. Since the electron beam can be controlled under the conditions, the operability is excellent.

Next, in the beam scanning, the electron beams from both lens barrels need to be irradiated to the same point on the sample surface.
And it is scanned as shown in FIG. These two electron beams scan the observation area in a main scanning direction and a sub-scanning manner in the form of a raster.
As shown in FIG. 2A, scanning is performed in one direction at a constant speed, but when viewed microscopically, scanning is performed stepwise as shown in FIG. 2B as described above. That is, each irradiation point is assigned one unit work time of t1 + Δt + t2, and scanning is performed such that the irradiation point shifts to an adjacent irradiation point every time this time elapses. When the operation is sequentially performed and the operation of one line in the main scanning direction is completed, the scanning of the entire observation region is performed by deflecting and scanning the starting point in the main scanning direction shifted by one in the sub-scanning direction. This beam scanning and irradiation of a large current electron beam for charge injection for a certain time t1 in each unit working time, and a blank t2 for a time Δt
The irradiation of the electron beam for time observation and the energy filter need to be controlled in synchronization with each other. In this embodiment, the deflection scanning means and the blanking means 4, 14
The timing of the energy filter 9 is controlled so as to operate properly.

[0013]

The present invention relates to a method for discovering an isolated area due to a contact failure of a semiconductor element, which cannot be found from a visual inspection for obtaining a surface image, by a potential contrast image, and has a high resolution for observation. And a low-acceleration, high-current electron beam for charge injection are required.However, by obtaining these two types of electron beams from different lens barrels, both contradictory conditions are usually satisfied. The inspection can be carried out by a relatively inexpensive device without requiring an expensive electron microscope to satisfy.
In addition, since the electron beam with a low acceleration and a large current for charge injection is applied from a shallow angle of incidence, a clear secondary electron image with good charging efficiency on the sample surface can be obtained. Furthermore, since the two types of electron beams are continuously emitted from the electron guns of separate lens barrels, irradiation / interruption to the sample surface is performed by beam passing / shielding control by blanking means. Compared to the conventional one that controls electron beam under the condition that both charge injection and observation are compatible, beam irradiation can be performed under the optimum condition of each irradiation beam. It is also excellent from the point of view.

[Brief description of the drawings]

FIG. 1 is a diagram showing an operation principle of the present invention, in which A is a diagram schematically showing a charged state when a sample is irradiated with an electron beam;
B is a diagram showing the observed image, and C is a graph showing the secondary electron generation efficiency.

FIGS. 2A and 2B are diagrams showing beam scanning according to the present invention, in which A shows a main scanning waveform, and B shows a part thereof in a microscopic manner.

FIG. 3 is a diagram illustrating a beam blanking operation of the present invention.

FIG. 4 is a diagram showing a main configuration of an embodiment of the present invention.

[Explanation of symbols]

 1,11 electron gun 2,12 electron beam 3,13 electron optical system 4,14 blanking means 5,15 blanking aperture 6 sample stage 7 sample 8 secondary electron detector 9 energy filter

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Claims (4)

[Claims] 1. A high-resolution observation on a sample surface in which a large current electron beam is irradiated on the sample surface at a shallow incident angle to generate a potential difference distribution based on a difference in electric conductivity between each portion of the sample surface and ground. A secondary electron microscope image obtained by irradiating an electron beam for use from different directions, and detecting the isolated pattern having low electric conductivity based on the image. 2. A process for irradiating a certain portion of the sample surface with a high-current electron beam for a certain period of time and then irradiating the portion with an observation electron beam to detect secondary electron information, 2. The method for detecting an isolated pattern according to claim 1, wherein a secondary electron microscope image of the observation area is obtained by scanning in which the positions of irradiation and detection work are sequentially shifted. 3. An electron microscope comprising an electron beam irradiation column for observation for irradiating an electron beam toward a sample on a sample stage, a secondary electron detector, and a display for displaying an image. An isolated pattern detection electron device having a separately provided high current electron beam irradiation column for irradiating an electron beam, and means for controlling scanning of both electron beams and ON / OFF of irradiation of both electron beams in association with each other microscope. 4. The isolated pattern according to claim 3, wherein an energy filter is provided in front of the secondary electron detector, and a voltage is applied to the energy filter in association with both beam scanning and ON / OFF of irradiation. Electron microscope for detection.