JP2001053122A - Method and apparatus for detecting isolation pattern based on surface potential - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect defective contact in a semiconductor element by irradiating an adjacent part with a focused ion beam, scanning the positions to be detected while shifting sequentially and obtaining a microscopic image of potential difference contrast on the surface of a sample, thereby detecting an isolation pattern based on an internal defect which cannot be detected by visual inspection. SOLUTION: The surface of a sample stage 6 for mounting a sample 7 is irradiated at a deep incident angle, with a focused ion beam for making observation from an irradiation lens-barrel comprising an ion gun 1, a blanking means 4 and an ion optical system 3 and is irradiated at a shallow incident angle, with an beam 12 for charge injection from an irradiation lens-barrel comprising an electron gun 11, a blanking means 14 and an electrooptic system 13. Detection positions of the sample 7 are then scanned while being shifted sequentially, and a microscopic image of potential difference contrast on the surface of the sample 7 is obtained thus detecting a floating region, i.e., an isolation pattern, based on an internal defect which cannot be detected by visual inspection.

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 by using a scanning ion microscope, 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 bad internal contact of a semiconductor element or the like by a potential contrast image on the element surface, which cannot be found from an appearance inspection for obtaining a surface image. In principle, the inspection area is scanned and irradiated with a low-acceleration, high-current electron beam to inject the irradiation negative charges onto the surface of the inspection object, and the potential distribution in the inspection area at that time is observed. That is, the electrons injected into this surface flow to the ground via the wiring pattern of the semiconductor element, but the current value differs 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 distribution 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. This makes it possible to find an isolated area due to a contact failure of a semiconductor element which cannot be understood.

A device of this type, which has been conventionally known,
A high-current electron beam is used to generate a potential difference in a short time.However, it is necessary to use a low accelerating voltage to avoid charging up the surrounding insulators, and to use a high-resolution microscope to identify defective parts. Are required at the same time, but these are usually conflicting requirements,
Scanning electron microscopes that satisfy the specifications have been extremely expensive. In addition, a method of irradiating a focused ion beam to the sample surface instead of the electron beam to obtain a potential difference contrast image has been considered. This is a method in which a constant or signal voltage is applied between the terminals of a semiconductor chip as a sample to generate a potential difference distribution on the semiconductor surface, and then a focused ion beam is applied to the semiconductor chip to irradiate secondary electrons or secondary ions. An image is obtained based on the difference in the generation rate of the secondary charge particles. This method requires troublesome operations such as power supply to terminals, comparison with an image of a normal chip, or observation of a temporal change corresponding to an applied signal.

[0004]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems of the prior art, and it is not possible to find out from a visual inspection for obtaining a surface image and to isolate the semiconductor device due to internal contact failure or the like. In the method of finding a region with a potential contrast image, it is possible to irradiate a large current electron beam at a low accelerating voltage and provide a high-resolution scanning microscope at relatively low cost. It is intended to provide means that can be implemented.

[0005]

According to the present invention, there is provided a dedicated high-current electron beam irradiation column for injecting charges into a sample surface from an oblique angle, and a focused ion irradiation column for obtaining a scanning microscope image. After irradiating a high-current electron beam for a certain period of time, irradiate the focused ion beam to that part and perform the work of detecting secondary charged particle information, and then sequentially shift the position of irradiation and detection to the next part A microscopic image of the potential difference contrast on the surface of the sample is obtained through the scanning, and an isolated pattern based on an internal defect that cannot be seen from the appearance is detected. Injecting into the sample surface at a shallow angle of 45 degrees or less, particularly as an oblique angle, is convenient because the generation efficiency of secondary electrons increases.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is to detect an isolated floating region based on an internal defect of a semiconductor element or the like which cannot be grasped from the external appearance as shown in the cross section in FIG. 1A. There is a part that is properly connected to the part, such as the one shown on the left, for example, a foreign substance intervening and incompletely connected to the conductive part, and a part shown on the right, for example, a shallowly etched part that is not in contact with the conductive part. As
When a charge is injected by irradiating a large current (several nA to several μA) electron beam on the sample surface, the charge injected into the central conductive part and the part which is properly connected immediately flows to the ground via the conductive part. However, if the conduction relationship on the left is incomplete, the electric charge flows out to ground via the resistance between them, and the electric charge injected into the part not in contact with the conductive part as shown on the right is applied to the conductor. It cannot flow directly and cannot easily flow out due to the insulation between them. Regarding the generation efficiency of secondary electrons by the irradiated electron beam, 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) is shown on the horizontal axis. ), A peak-shaped quadratic curve characteristic that generally has a peak in a low acceleration voltage region of 1 kV or less as shown in FIG. 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. The smaller the angle b at which the electron beam is incident is, the higher the peak of the characteristic is, and the cross point of the 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 appropriate charging efficiency for obtaining a potential difference contrast image on the sample surface, an electron beam irradiated with a low acceleration voltage of 1 kV or less at a small incident angle and having a high secondary electron generation efficiency is advantageous. The sample is irradiated with a low acceleration voltage, a large current and a shallow angle of incidence on the sample.

[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 FIG. 2, in that region, the number of emitted secondary electrons becomes larger than the number of irradiated primary electrons. 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 structure is similar in design, the charge on the surface of the normal part in the center flows out to ground and reaches zero potential, and the charge on the defect part on the left gradually flows out through a resistor and abnormalities on the right side. The charge remains on the surface of the portion and becomes positive potential. When a potential difference occurs on the sample surface, a difference occurs in the generation efficiency of secondary electrons from that part. On the secondary electron image, as shown in FIG. 1B, the center normal part is bright and the left defect part is gray and right abnormal. The portion appears dark, and the other semiconductor surface is insulated and darker than the abnormal portion. When a secondary ion is detected as a secondary charged particle, the relationship between the charges is reversed, and the relationship between the brightness on the microscope image is reversed as shown in FIG. 1C.

The electron beam for charge injection has a large current of several nA to several μA accelerated by a low acceleration voltage of 1 kV or less, and the scanning focused ion beam for observation has a high resolution of 0.1 pA to several μA. A current of nA is used. The present invention divides the different charged particle beam sources into two systems and prepares them according to the respective conditions.
This relieves the difficult task of preparing an electron beam source that satisfies both conditions, and significantly reduces costs. The two charged particle beams scan the observation region in a main scanning direction and a sub scanning direction, for example, in a raster shape, but are not scanned separately but are synchronized so as to irradiate the same position. This main scanning is constantly swept in one direction as shown in FIG. 3A, but is scanned stepwise as shown in FIG. 3B when viewed microscopically. That is, when one irradiation point is irradiated with a large current electron beam for charge injection for a fixed time t1, an electron beam for observation for a time t2 with a blank for a time Δt is irradiated, and during that time, the potential is changed according to the potential of that portion. Secondary electrons 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 electron beam for observation 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. 5 is a diagram for explaining the blanking operation. The irradiation column of the focused ion beam 2 for observation includes an ion gun 1, a blanking means 4 and a blanking aperture 5 in addition to acceleration, focusing, and scanning means of the focused ion beam.
The irradiation column of the electron beam 12 for charge injection includes a blanking means 14 and a blanking aperture 15 in addition to the electron gun 11, acceleration, focusing, and scanning means of the electron beam. It comprises an electron optical system 13. Both surfaces of the sample stage 6 on which the sample 7 is placed are irradiated with the focused ion beam 2 for observation at a deep incident angle and the electron beam 12 for charge injection is irradiated at a shallow incident angle. A tube is provided. Reference numeral 8 denotes a secondary electron detector as a secondary charged particle 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 device as shown in FIG. 1A is mounted on a sample stage 6, and a point on the surface of the semiconductor device is irradiated with a low-acceleration, large-current electron injection electron beam 12 from an irradiation lens barrel. This irradiation is for charging the semiconductor surface to obtain a secondary electron microscope image in that state, and as shown in FIG. 2, electrons in a low acceleration voltage region of 1 kV or less where secondary electron generation efficiency is high. 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, and the focused ion beam 2 for observation is irradiated for t2 time after leaving a blank for Δt time. The irradiation and interruption of these beams are executed under the control of the blanking means 4 and 14. Specifically, as shown in FIG. 5, the focused ion 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. By deflecting the electron beams continuously emitted from the electron beams 11 and 11 by the blanking means 4 and 14, the electron beams are blocked without passing through the openings of the blanking apertures 5 and 15. That is, during the first time t1 + Δt, a deflection signal is applied to the blanking means 4 of the observation focused ion beam irradiation column to deflect the electron beam, and the beam is blocked because the beam cannot pass through the opening of the blanking aperture 5. The electron beam is deflected by applying a time deflection signal of Δt + t2 to the blanking means 14 of the electron beam irradiation column for charge injection to deflect the electron beam.
By setting the opening 15 to a state in which the beam cannot pass and is blocked, the irradiation of one irradiation point as shown in FIG.
Irradiation of a focused ion beam for t2 time observation with a blank for t time is 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. An energy filter 9 such as a grid is installed in front of the detector 8 to block the secondary electrons during the time t1 during which the charge injection beam is irradiated so as not to saturate the detector. These two types of charged particle beams are continuously emitted from the electron gun ion gun in separate barrels, and irradiation / interruption to the sample surface is performed by beam passing / shielding control by blanking means. Therefore, the beam can be controlled under the optimum condition of each irradiation beam as compared with the conventional one in which the electron beam is controlled by controlling the electron beam under the condition that both the charge injection and the observation are compatible with one lens barrel. Therefore, it is excellent also in terms of operability.

Next, in the beam scanning, the charged particle beams from both lens barrels need to be irradiated to the same point on the sample surface, and are scanned as shown in FIG. The two charged particle beams scan the observation region in a main scanning direction and a sub scanning direction in a raster fashion. The main scanning is performed at a constant speed in one direction as shown in FIG. As described above, scanning is performed in a stepwise manner as shown in FIG. 3B when viewed microscopically. 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 fixed time t1 in each unit working time, irradiation of a focused ion beam for observation for a time t2 with a blank of Δt time, and the energy filters are synchronized with each other. Although it needs to be controlled, in this embodiment, the controller 21
The deflection scanning means, the blanking means 4, 14 and the energy filter 9 of the two lens barrels are controlled in timing on the basis of the command from the camera so as to operate properly. In addition, irradiation of both charged particle beams is turned on / off in time series.
However, there is no problem even if the focused ion beam is continuously irradiated during the electron beam irradiation period t1. Since it is not possible to obtain a secondary electron image by the focused ion beam while the electron beam is being irradiated, it is necessary to interrupt the irradiation of the electron beam at the time t2 when the focused ion beam is irradiated. This is because there is no adverse effect on beam irradiation.

[0013]

Embodiment 2 Next, an embodiment in which a secondary ion detector is used as a secondary charged particle detector will be described. The difference from the previous example of the apparatus is that the secondary ion detector 8 'is used as the secondary charged particle detector and the time t1 during which the beam for charge injection is irradiated is such that the secondary electrons are blocked. Energy filter that does not saturate the laser is unnecessary. In addition, a blanking function for switching between both charged particle beams at high speed is not necessarily required. Since it is unlikely that secondary ions are generated based on electron beam irradiation, if the secondary charged particles to be detected are secondary ions, there are no secondary ions that cause noise, and the generation of secondary electrons Is not a detection noise at all. In this embodiment, the secondary ion detection timing may be performed during a time t2 after the time t1 when the charge injection beam is irradiated. Irradiation of the focused ion beam may be OFF or continuous during time t1. Then, as opposed to the secondary electron image, the obtained microscopic image in which the secondary ions are obtained is darkest in the central normal part, gray in the left defect part and bright in the right abnormal part as shown in FIG. 1C. Since the other semiconductor surface is in an insulating state, it becomes brighter than the abnormal portion.

[0014]

The present invention relates to a method for finding an isolated area due to a contact failure or the like based on an internal defect of a semiconductor element, which cannot be found by a visual inspection for obtaining a surface image, using a potential contrast image. Requires a high-resolution focused ion beam for scanning and a low-acceleration, high-current electron beam for charge injection. However, the two types of charged particle beams are obtained from different lens barrels, which are usually contradictory. An expensive electron microscope that satisfies both conditions is not required, and the inspection can be carried out by a relatively inexpensive device without troublesome work. 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. Further, the two types of charged particle beams are continuously emitted from the ion gun electron gun in separate lens barrels. Irradiation / interruption or continuous irradiation of the sample surface by beam passing / shielding control by blanking means. Optimizing each irradiation beam compared to the conventional one that controls the electron beam under the condition that both electron beam injection and observation are compatible with a single lens barrel Because the beam can be controlled under the conditions, it is excellent in terms of operability. Further, since the present invention is provided with the focused ion beam source, not only the intended isolated region can be found in the potential contrast image, but also the etching process using the ion beam can be performed. is there.

[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 a secondary electron observation image, and C is a diagram showing a secondary ion observation image.

FIG. 2 is a graph showing a secondary electron generation efficiency corresponding to a beam incident angle.

3A and 3B are diagrams showing beam scanning according to the present invention, in which A shows a main scanning waveform and B shows a part of the waveform in a microscopic manner.

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

FIG. 5 is a diagram illustrating a beam blanking operation of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 ion gun 2 ion beam 3 ion optical system 4 blanking means 5 blanking aperture 6 sample stage 7 sample 8 secondary charged particle detector 9 energy filter 11 electron gun 12 electron beam 13 electron optical system 14 blanking means 15 blanking Aperture

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Claims (5)

[Claims] 1. A sample surface which is irradiated with a large current electron beam at an oblique angle to a sample surface to generate a potential difference distribution based on a difference in electric conductivity between each portion of the sample surface and ground.
A method of irradiating a high-resolution focused ion beam for observation from different directions to obtain a secondary charged particle microscope image, and detecting the isolated pattern having low electric conductivity based on the image. 2. A process of 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 a focused ion beam for observation to detect secondary charged particle information,
2. The method for detecting an isolated pattern according to claim 1, wherein a secondary charged particle microscope image of the observation area is obtained by scanning the position of the irradiation operation and the detection operation sequentially on the adjacent part. 3. A scanning ion microscope comprising a focused ion beam irradiation column for observation for irradiating a focused ion beam toward a sample on a sample stage, a secondary charged particle detector, and a display for image display. On the other hand, a large current electron beam irradiation column that irradiates the electron beam at an oblique angle is separately provided, and scanning and irradiation of the focused ion beam are turned ON / OFF.
OFF: ON / OFF of scanning and irradiation of high current electron beam
A scanning ion microscope for detecting an isolated pattern, comprising: 4. An energy filter is provided in front of a secondary electron detector used as a secondary charged particle detector, and the energy filter has ON / OF of scanning and irradiation of both beams.
The scanning ion microscope for detecting an isolated pattern according to claim 3, wherein a voltage is applied in association with F. 5. The oblique angle is 4 degrees with respect to the sample surface.
5. The scanning ion microscope for detecting an isolated pattern according to claim 3, wherein the angle is a shallow angle of 5 degrees or less.