JP6232195B2 - Sample inspection apparatus and sample inspection method - Google Patents

Description

  The present invention relates to an inspection apparatus and an inspection method for inspecting and evaluating a very fine state of a sample surface, such as inspection of a defect in a circuit pattern of a semiconductor substrate or a circuit pattern on a wafer, and the presence of foreign matter.

In the manufacturing process of a semiconductor integrated circuit, a semiconductor substrate is inspected by irradiating a target sample with an electron beam, and secondary electrons emitted from the sample (including mirror electrons and reflected electrons in addition to secondary electrons). An electron microscope is used in which an electron image of a sample surface is acquired by inspecting the sample by detecting secondary electrons).
In this case, when a sample including a dielectric or an insulator as a constituent material is irradiated with an electron beam, the potential of the sample surface fluctuates, making it difficult to obtain a stable electron image or a reproducible electron image. In particular, when the surface of the sample is negatively charged by electron beam irradiation, there is a problem that the detection accuracy of defects and foreign matters is reduced due to deterioration of the electron image.

  In order to solve such problems, the sample surface is irradiated with ultraviolet rays through a path different from that of the electron beam, thereby suppressing the sample surface from being negatively charged due to the photoelectric effect generated on the sample surface. An inspection apparatus is known in which a stable electronic image is obtained by controlling the above (see, for example, Patent Document 1).

JP 2009-4114 A

The conventional inspection apparatus acquires an electron image by secondary electrons emitted from a sample by irradiation of an electron beam and a photoelectron image by photoelectrons emitted by a photoelectric effect by irradiation of ultraviolet rays, and the electrons are overlapped with each other. It is configured to acquire the electron image by irradiating the electron beam after adjusting the irradiation area of the line and adjusting the ultraviolet irradiation condition to remove the negative charge of the sample and then acquiring the electron image. However, it is not configured to switch and acquire the electron image of the sample by irradiation with the electron beam and the photoelectron image by irradiation of ultraviolet rays.
If an electron image and a photoelectron image can be acquired selectively according to the inspection and evaluation conditions such as the state and constituent materials of the sample to be inspected or what kind of sample surface it is desired to obtain, It is extremely useful for improving the accuracy of inspections and evaluations, but the conventional inspection apparatus can selectively acquire an electron image and a photoelectron image of a sample, and inspect and evaluate the sample based on one of the images. I couldn't.

  Further, since the conventional inspection apparatus is equipped with an electron beam generation source and an ultraviolet generation source which are provided separately, the apparatus configuration has to be large.

  In view of such problems of the prior art, the present invention can selectively acquire a sharp electron image and a photoelectron image by controlling the potential of the sample surface, and can generate a common source of electron beam and ultraviolet light. It is an object of the present invention to simplify the configuration of the apparatus so that it can be supplied.

In order to solve the above problems, the present invention includes a primary electron optical system that irradiates a sample surface with an electron beam, and a secondary electron optical system that forms an image of secondary electrons emitted from the sample on an electron detection surface of a detector. In a sample inspection apparatus for inspecting a sample by acquiring an electron image of the sample surface from a signal detected by a detector,
A means for irradiating the sample surface with ultraviolet rays together with an electron beam or irradiating ultraviolet rays alone, and selectively irradiating the sample surface with an electron beam or ultraviolet rays, whereby a secondary electron image of the sample by electron beam irradiation or It has the structure which acquired the photoelectron image by irradiation of an ultraviolet-ray, It is characterized by the above-mentioned.

In the sample inspection apparatus having the above configuration, the laser beam emitted from one laser device is split by a beam splitter, and the split laser beam is incident on the electron beam supply means as the electron beam generation source, It is preferable that the laser beam is incident on a triangular mirror disposed in the secondary electron optical system and provided with a pinhole, and the sample surface is irradiated with the reflected light of the triangular mirror.
It is preferable that an output adjustment mechanism for laser light incident on the triangular mirror is provided on the path of the divided laser light between the beam splitter and the triangular mirror.

  The sample inspection method of the present invention is a sample inspection method using the sample inspection apparatus having the above-described configuration, and controls the potential of the sample surface by irradiating the sample surface with ultraviolet rays together with an electron beam. And

According to the present invention, the sample is irradiated with ultraviolet rays through a path different from that of the electron beam, and the negative charging of the sample surface due to the irradiation of the electron beam is suppressed by the photoelectric effect generated on the sample surface, and the potential of the sample surface is sharpened. It is possible to acquire a reproducible clear and stable electronic image by controlling stably so as to be suitable for image acquisition.
Further, the laser beam emitted from the laser device is divided, and the divided laser beam is used as an electron beam and ultraviolet light source, respectively, so that the device configuration can be made compact. In this case, as a means to irradiate the sample with ultraviolet rays, a triangular mirror with a pinhole is arranged in the secondary electron optical system, and the sample surface is irradiated with ultraviolet rays so as to go back to the secondary electrons to be imaged. By doing so, it is possible to easily adjust the illumination field of the ultraviolet rays to be irradiated by adjusting the incident angle of the ultraviolet rays to the triangular mirror. Furthermore, by providing a laser light output adjustment mechanism in front of the triangular mirror, it is possible to control the output of ultraviolet rays applied to the sample without changing the amount of laser light applied to the electron beam supply means. It is possible to precisely adjust the surface potential.

  According to the present invention, laser light is used as an electron beam generation source and at the same time as a light generation source for controlling the potential of the sample surface, and the sample is irradiated with laser light (ultraviolet rays) for this potential control. Since it is configured to detect photoelectrons generated by irradiation with a detector and acquire a photoelectron image of the sample surface from the detected signal, control of the potential of the sample surface, and the type, constituent material, and sample of the sample By selectively irradiating the sample with either or both of an electron beam and light according to the purpose of surface observation, etc., an electron image or photoelectron image of the sample surface that matches the purpose of observation is acquired and evaluated. Thus, the inspection accuracy of the sample can be increased.

It is a schematic block diagram of the electron optical system of the electron microscope which is one Embodiment of the sample inspection apparatus of this invention, and a laser introduction system. It is a schematic block diagram of the whole electron microscope of FIG. It is the figure which showed the change of the surface potential of a sample when an electron beam or light (ultraviolet rays) is irradiated to a sample using the electron microscope of FIG. It is the figure which showed the electronic image of the sample surface acquired with the electron microscope of FIG. It is a schematic block diagram of the electron optical system of the electron microscope which is other embodiment of this invention, and a laser introduction system.

Preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a schematic configuration of an electron optical system and a laser introduction system of a projection type electron microscope which is an embodiment of a sample inspection apparatus of the present invention. In the figure, reference numeral 1 denotes a lens barrel, and 2 denotes primary electron optics. A system 3, 3 is a secondary electron optical system, 4 is a detector, and 5 is a sample holding device, which are installed on a vacuum chamber 9 controlled in a vacuum atmosphere described later. Reference numeral 6 denotes a laser device installed outside the lens barrel 1.

The primary electron optical system 2 includes an electron gun 21 as electron beam supply means, a converging lens 22 that controls the shape of the electron beam, an aperture 23, an aligner 24 that controls the traveling direction of the electron beam, and the like.
The electron gun 21 can use, for example, an electron gun that emits electrons (generates photoelectrons) by irradiating the photocathode with DUV laser light, and emits an electron beam extracted by incidence of laser light from a laser device 6 to be described later. It is provided to do. The primary electron optical system 2 adjusts the shape of the electron beam emitted from the electron gun 21 with a converging lens 22 and an aperture 23, and further controls the traveling direction with an aligner 24 to an E × B filter 32 described later. It is configured such that it enters and the direction of travel is bent by the E × B filter 32 so that the sample held on the sample holding device 5 is irradiated.

The secondary electron optical system 3 includes a lens 31, an E × B filter 32, a lens 33, a lens 34, and an aligner group for controlling the trajectory of the electron beam, which is disposed above from the sample holding device 5 side (see FIG. The secondary electrons emitted from the sample surface when the electron beam is irradiated onto the sample by the primary electron optical system 2 are detected by the lens groups and the aligner group. 4 is formed on the four detection surfaces 41.
The secondary electron optical system 3 has an electron beam near the E × B filter 32 among the points for calibrating the crossover between the electron beam emitted from the electron gun 21 and the secondary electron emitted from the sample. A triangular mirror 35 having a pinhole large enough to pass is disposed. The secondary electrons emitted from the sample surface are guided to the detector 4 through the pinhole of the triangular mirror 35.

  The detector 4 includes an MCP (microchannel plate) that doubles the secondary electrons, a fluorescent plate that converts the doubled electrons into light, and a TDI (Time Delay Integration) -CCD that captures the converted optical signal as an image signal. It consists of a camera. Secondary electrons emitted from the sample are imaged on the detection surface 41 of the TDI-CCD camera by the secondary electron optical system 3. You may comprise using EB (Electron Beam) -CCD camera or an EB-TDI sensor.

  The sample holding device 5 is installed on a stage 10 to be described later, holds a sample such as a semiconductor wafer to be inspected on its upper surface, and moves the sample holding surface to X, Y as the stage 10 moves. It is configured so that it can be continuously displaced in the direction. Note that a photomask, EVU mask, semiconductor wafer, or the like is used as a sample to be inspected.

As the laser device 6, a device that emits ultraviolet laser light is used, and is installed outside the lens barrel 1. The laser light emitted from the laser device 6 is divided by the beam splitter 7, and one of the divided laser lights is guided by a plurality of mirrors 8 and enters the electron gun 21 to be used as an electron beam generation source. .
On the other hand, the other divided laser beam is introduced into the barrel 1 and is incident on the triangular mirror 35 disposed in the secondary electron optical system 3, and is reflected by the triangular mirror 35 to be reflected on the sample holder 5. The sample surface is guided to irradiate the sample surface.
As described above, the triangular mirror 35 is arranged in the secondary electron optical system 3 and the sample is irradiated with the ultraviolet ray reflected by the triangular mirror 35, so that the irradiation angle of the ultraviolet ray on the sample surface is substantially vertical. The quantum efficiency generated on the sample surface by ultraviolet irradiation can be maximized, and by adjusting the incident angle of the laser beam to the triangular mirror 35, the irradiation field of the ultraviolet light on the sample surface can be adjusted. It becomes easy.
Although not shown, a means for switching between a state in which laser light is incident on the triangular mirror 35 and a state in which the incidence of laser light is blocked is provided on the laser light path between the beam splitter 7 and the triangular mirror 35.

The electron optical system and the laser introduction system shown in FIG. 1 are installed on, for example, the vacuum chamber 9 shown in FIG.
Specifically, a stage 10 that is displaced in the X and Y directions is installed in the vacuum chamber 9, and the sample holding device 5 is installed on the stage 10. A rotary stage may be installed on the stage 10. The stage 10 is configured so that, for example, a mirror is installed on the stage, and the amount of displacement and position of the X and Y coordinates can be measured by a laser interferometer. The coordinate measurement position accuracy can be about 0.1 nm to 0.6 nm.
Further, a necessary potential, for example, a potential of about −1 kV to 10 kV is provided on the surface of the sample supported on the stage 10 on the side of the lens barrel 1 by a power source. Since the reference potential of each lens and aligner installed in the lens barrel 1 is GND, secondary electrons emitted from the sample move in the lens barrel 1 due to electron energy caused by the difference potential, and the lens aligner. An image is formed on the detector 4 by (not shown).

Further, a Faraday cup FC for measuring the electron beam amount of the primary electron optical system 2 is installed on the stage 10 and used for creating conditions for the primary electron optical system 2 and the secondary electron optical system 3. Standard pattern samples are installed.
Specifically, the conditions of the electron axis are created by adjusting the laser irradiation position and adjusting the electron beam irradiation position of the primary electron optical system 2 as follows.

First, laser light is introduced from the triangular mirror 35 of the secondary electron optical system 3 to irradiate the sample, and photoelectrons from the sample are imaged by the secondary electron optical system 3 and imaged by the detector 4.
Next, whether or not photoelectrons from the sample pass through the center of the lens 31 is confirmed by changing the laser irradiation position, and the laser irradiation position where the image center does not move by the wobbling of the lens 31, that is, the lens 31 The laser irradiation position (object center) through which photoelectrons pass through the center is obtained. Next, the electron trajectory is adjusted using an aligner arranged at the rear stage of the lens 31 so that the photoelectrons pass through the centers of the lens 33 and the lens 34.
Thereafter, with the E × B condition for performing electron irradiation by the primary electron optical system 2 applied to the E × B filter 32, the centers of the lenses 33 and 34 arranged at the subsequent stage of the E × B filter 32 are again formed. Aligner conditions are obtained and set so that photoelectrons can pass.
Then, if the condition for irradiating the electron beam by the primary electron optical system 2 to the object surface position (position where the photoelectron from the sample passes through the center of the lens 31) obtained in this way is obtained, the primary electron optical system is obtained. Secondary electrons (secondary electrons, reflected electrons, part of back-scattered electrons and combinations thereof) emitted from the sample by electron irradiation by 2 pass through the center of the lens 31 and pass through the E × B filter 32 in the subsequent direction. In the state of going straight (Vienna condition), the light passes through the centers of the lens 33 and the lens 34 and is imaged on the detector 4.

Such an electron axis condition creation method not only makes it possible to specify the center of the object surface efficiently, but also the position of the object surface center of the secondary electron optical system 3 and the result of photoelectrons from the sample by laser irradiation. Since the image condition and the Wix condition of E × B can be obtained, the adjustment of the electron beam of the primary electron optical system 2 can be performed very simply and efficiently.
Further, in the above electron axis condition creation process, the irradiation size of the laser beam on the sample is a two-dimensional surface shape, which is a circular shape or an elliptical shape. Irradiation conditions on the sample reflecting the shape can be set by providing an aperture with a through hole such as a square, hexagon, or octagon. The electron beam of the primary electron optical system 2 is irradiated on the sample in a circular or elliptical two-dimensional planar beam shape. A laser is irradiated to the secondary electron generation part (photocathode) of the sample by irradiation of this electron beam. By providing an aperture having a through hole of an arbitrary shape in the introduction part of this laser beam, The electron generator can be irradiated with a laser beam shape reflecting the shape, and this makes it possible to generate a laser beam-shaped electron. Furthermore, it becomes possible to irradiate the sample with the electron beam having the shape.

  The inspection of the sample using the electron microscope having the above-described configuration is performed by irradiating the sample with the electron beam emitted from the electron gun 21 and imaging the secondary electrons emitted from the sample surface on the detection surface 41 of the detector 4. If the sample surface potential fluctuates and a stable electron image cannot be obtained because the sample contains a dielectric or insulator as a constituent material, A sharper electronic image is acquired after compensating the potential of the sample surface by irradiating ultraviolet rays.

  That is, FIG. 3 shows changes in the potential (LE: landing energy) of the sample surface when the sample is irradiated with an electron beam and ultraviolet rays, and the brightness (standard value of luminance) of the electronic image detected by the detector 4. It shows a change. In the figure, (1) is a state in which only the electron beam is irradiated, (2) is a state in which only the laser beam (ultraviolet light) is irradiated, (3) is a state in which the electron beam and the laser beam are simultaneously irradiated, (4 ) Indicates a change in luminance obtained by adding (1) and (2).

Comparing the brightness of the electron image when the sample is irradiated with the electron beam and the ultraviolet light simultaneously with the sum of the brightness of the electron image when the electron beam and the ultraviolet light are individually irradiated as shown in FIG. The sum of the brightness when the beam and ultraviolet rays are individually irradiated is a simple sum of the brightness that is not affected by the potential fluctuation of the sample surface, whereas the negative charge due to the electron beam irradiation when simultaneous irradiation is performed. Action and positive charging effect due to UV irradiation occur on the sample surface, respectively, and the brightness of the electronic image becomes in accordance with the compensated surface potential. Compared to single irradiation and simultaneous irradiation, positive charging due to UV light is greatly affected. Thus, it is possible to distinguish between the LE region to be subjected to, and the LE region to which negative charging is greatly applied by the electron beam. From the measurement results shown in the figure, it can be understood that, with LE = B as a boundary, the low LE side (LE ≦ B) is positively charged, and the high LE side (LE ≧ B) is negatively charged.
This is because, at low LE (LE <A), the electron beam does not reach the sample, so that the amount of secondary electrons emitted from the sample surface decreases, while the amount returned as mirror electrons increases and the electron image becomes It appears brighter and less negatively charged, which is a change in surface potential, and irradiates with ultraviolet light to qualitatively match the effect of positive charging.
In addition, at high LE (LE> C), the electron gun 21 is dominantly irradiated with an electron beam, and the amount of secondary electrons emitted from the sample surface is increased, resulting in a negative charge. Even if it acts on the positive charge, it can be qualitatively matched with the strong influence of the negative charge.

In the electron microscope of the illustrated form, one laser beam emitted from the laser device 6 is divided by the beam splitter 7 and distributed to the electron gun 21 and the triangular mirror 35 to irradiate the electron beam and ultraviolet rays. The generation source can be shared by a single laser device 6, which is effective in reducing the size of the inspection device and suppressing production costs.
In this case, a means for changing the irradiation ratio of the laser light to the electron gun 21 and the triangular mirror 35 is provided, and the state of the surface potential of the sample is set to 0 eV by adjusting the irradiation ratio appropriately, or as necessary. Thus, it can be set to a positively charged state or a negatively charged state.

  For example, the state of potential distribution on the sample surface is examined in advance, and when the sample surface is negatively charged, the amount of UV irradiation is such that the negative charge on the sample surface is canceled by adjusting the distribution ratio of the laser beam. By increasing the ratio of the laser beam, a positive charging effect is exerted on the sample surface. Conversely, when the sample surface is positively charged, the laser beam is also affected so that a negative charging effect is similarly exerted. By adjusting the distribution ratio, the potential of the sample surface at the time of inspection can be set to 0 eV.

FIG. 4 shows the result of confirming the positive charging effect by ultraviolet irradiation in the inspection image. The upper part of the figure is an electron image when the electron beam alone is irradiated to the sample whose potential is LE = A, B, C. The lower row is an electron image when the sample of each potential is irradiated with ultraviolet rays and then irradiated with an electron beam.
From the figure, it can be understood that an image having the same quality as an electron image obtained with a high LE by an electron beam alone can be obtained even with a lower LE due to a positive charging action by ultraviolet irradiation.

Further, as shown in FIG. 5, the output adjustment mechanism 11 of the laser light incident on the triangular mirror 35 on the path of the divided laser light between the beam splitter 7 and the triangular mirror 35, for example, the output of a polarizing plate or the like. Adjustment means may be provided. In this way, it is possible to control the output of the ultraviolet rays applied to the sample without changing the amount of laser light applied to the electron gun 21, and the surface potential of the sample surface can be precisely adjusted.
For example, even if the potential distribution on the surface of the same sample surface is examined in advance, if the degree of negative charge is not uniform and there are sparsely charged portions scattered on the sample surface, the distribution By irradiating the ultraviolet rays while adjusting the output of the ultraviolet rays based on the position information by the output adjusting mechanism, the potential of the sample surface can be set constant (0 eV). Can be suppressed.

Further, in the electron microscope of the above form, in addition to the function of acquiring an electron image by secondary electrons emitted from the sample surface, according to the difference in the quantum efficiency of the material constituting the sample surface as the sample surface is irradiated with ultraviolet rays. The emitted photoelectrons may be imaged on the detection surface 41 of the detector 4 by the secondary electron optical system 3 to obtain a photoelectron image of the sample surface.
In this way, if the electron image of the sample obtained by electron beam irradiation and the photoelectron image of the sample obtained by ultraviolet irradiation can be selectively obtained, the sample type, constituent materials, and the purpose of observing the sample surface Depending on the sample, the sample surface is selectively irradiated with either or both of an electron beam and ultraviolet rays to obtain an electron image or photoelectron image of the sample surface that matches the observation purpose. Inspection accuracy can be increased.

When the sample surface is observed by irradiation with ultraviolet rays, the sample surface is generally positively charged. In this case, it is possible to slightly operate the electron gun 21 to slightly irradiate the electron beam, thereby alleviating or canceling the positive charge generated by the irradiation of ultraviolet rays. For example, when a sample is a semiconductor element, a photoelectron image is acquired by irradiating the sample with ultraviolet rays, and the contrast of the sample surface is observed, an increase in the potential of the sample surface (surface charge) is observed over a long period of time. Accumulation) may destroy the semiconductor element itself. In such a case, the irradiation of the electron beam by the electron gun 21 is effective for the purpose of relaxing or canceling the positive charging due to the irradiation of ultraviolet rays.
In addition, observation of the sample surface by electron beam irradiation irradiates the sample surface with a small amount of charged particles having a mass, but depending on the sample, physical damage due to collision of charged particles on the surface may be a problem. There is. Therefore, in such a sample, for example, a material that cannot be directly irradiated with an electron beam such as a resist surface for electron beam exposure, observation by ultraviolet irradiation is effective.

  Thus, the inspection accuracy of the sample can be improved by making it possible to select the observation system from the electron beam and the ultraviolet ray based on the condition of the sample to be observed. In this case, the observation by electron beam irradiation reduces or cancels the potential change (negative charge) on the sample surface by ultraviolet irradiation, and the observation by ultraviolet irradiation reduces the potential change (positive charge) on the sample surface. The cancellation can be performed by irradiation with an electron beam.

  In addition, the form of each component of the illustrated inspection apparatus is an example, and the present invention is not limited to this, and can be configured in other appropriate forms.

1 lens barrel, 2 primary electron optical system, 21 electron gun, 3 secondary electron optical system, 35 triangular mirror, 4 detector, 41 detection surface, 5 sample holding device, 6 laser device, 7 beam splitter, 8 mirror, 9 Vacuum chamber, 10 stages, 11 Output adjustment mechanism

Claims (3)

A primary electron optical system that irradiates the sample surface with an electron beam and a secondary electron optical system that forms an image of the secondary electrons emitted from the sample on the electron detection surface of the detector. The sample is detected from the signal detected by the detector. An apparatus for inspecting a sample by acquiring an electron image of a surface , comprising means for irradiating the sample surface with ultraviolet rays together with an electron beam or irradiating ultraviolet rays alone, and selectively irradiating the sample surface with an electron beam or ultraviolet rays In a sample inspection apparatus having a configuration that acquires a secondary electron image of a sample by irradiation of an electron beam or a photoelectron image by irradiation of ultraviolet rays by irradiating ,
The laser beam emitted from one laser device is split by a beam splitter, one split laser beam is incident on the electron beam supply means as the electron beam generation source, and the other laser beam is sent to the secondary beam. A sample inspection apparatus characterized by having a configuration that is arranged in an electron optical system and is incident on a triangular mirror provided with a pinhole so as to irradiate the sample surface with the reflected light of the triangular mirror . 2. The sample inspection apparatus according to claim 1, further comprising an output adjustment mechanism for laser light incident on the triangular mirror on a path of the divided laser light between the beam splitter and the triangular mirror. 3. A sample inspection method using the sample inspection apparatus according to claim 1 , wherein the sample surface potential is controlled by irradiating the sample surface with ultraviolet rays together with an electron beam. .

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