JP3965325B2 - Microstructure observation method and defect inspection apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a method for observing a fine structure, and a high-resolution microscope optical system that realizes the same, and particularly for high-definition observation and inspection of fine pattern defects and foreign matters in a semiconductor manufacturing process and a flat panel display manufacturing process. The present invention relates to a resolution optical system and a defect inspection apparatus using the same.
[0002]
[Prior art]
In a semiconductor manufacturing process, a flat panel display manufacturing process, and the like, observation and inspection of fine pattern defects and foreign matters using an optical microscope are performed. In recent years, it has become necessary to improve the performance of microscope optical systems as the degree of integration of semiconductor devices increases.
[0003]
Methods for increasing the resolution of the optical microscope include shortening the wavelength of light used for imaging, increasing the numerical aperture (NA) of the objective lens, and increasing the high frequency of the transfer function (MTF) of the imaging system. There are methods using a resolution technique. Of these, shortening the wavelength and increasing the NA are direct methods, but there are various restrictions in practical use, and there are cases where it cannot be realized. Therefore, a method that can observe a fine structure with high contrast without changing the wavelength and NA, that is, a super-resolution technique that raises the high region of the MTF of the imaging system has been attracting attention.
[0004]
As an example of the super-resolution technique, Japanese Patent Laid-Open No. 2000-155099 discloses a method for improving MTF by controlling the polarization state. Illuminate the sample with linearly polarized light, guide the reflected light from the sample to the imaging system through the analyzer, and illuminate the sample with elliptically polarized light, and only the linearly polarized component reflected by the polarizing beam splitter from the reflected light from the sample A method is shown for guiding to the imaging system. In the former, by adjusting the direction of linearly polarized light that illuminates the sample with respect to the direction of the linear pattern on the sample and the direction of the analyzer, the light quantity ratio between the higher-order diffracted light and the 0th-order light by the pattern on the sample is adjusted. ing. By reducing the amount of zero-order light, the high-frequency MTF is improved, and the difference in the amount of light between the part with and without the pattern can be reduced, making it easy to see the fine pattern and using the observation image. The performance of defect inspection can be improved. Since it is necessary to use a non-polarizing beam splitter to separate the optical path of the imaging system of the illumination system, there are drawbacks in that the light utilization efficiency is low and the image becomes dark, but the change in polarization during reflection on the sample is significant. Therefore, a large MTF improvement effect can be obtained. In the latter method, the same MTF improvement effect can be obtained by optimizing the orientation and ellipticity of elliptically polarized light that illuminates the sample with respect to the direction of the linear pattern on the sample. It is possible to obtain higher light use efficiency than a system using linearly polarized illumination, and a bright image can be obtained. In this system, linearly polarized illumination can be realized. However, in this case, since the reflected light from the sample does not return to the imaging system, the image cannot be observed with linearly polarized illumination that maximizes the MTF improvement effect.
[0005]
An example of using the latter elliptically polarized illumination in the basic configuration of the system realizing this method is shown in FIGS. Light from the light source 8 reaches the aperture stop 11 through the concave mirror and the lens 9, and further enters the polarization beam splitter 15 through the lens, the wavelength selection filter 12, and the field stop 13. The linearly polarized light transmitted through the polarizing beam splitter 15 is converted into elliptically polarized light through the λ / 2 plate 16 and the λ / 4 plate 17 with λ as a wavelength, and is irradiated onto the sample 1 by the objective lens 20. By rotating the λ / 4 plate 17, the direction of the major axis of the elliptically polarized light can be controlled, and by rotating the λ / 2 plate 16, the ellipticity of the elliptically polarized light can be controlled. The light reflected by the sample 1 passes through the objective lens 20, the λ / 4 plate 17, and the λ / 2 plate 16 and again enters the polarization beam splitter 15, and only the s-polarized component is reflected, and the imaging lens 30 and the zoom lens are reflected. 50 to the imaging system. In this system, when the angle of the two wave plates is determined so as to illuminate the sample 1 with circularly polarized light, only the component whose polarization state has not changed when reflected on the sample surface is reflected by the polarization beam splitter 15. Reflected and guided to the imaging system.
[0006]
On the other hand, when the angle of the two wave plates is determined so that the sample 1 is elliptically polarized, a part of the component whose polarization state has changed when reflected by the sample 1 is also reflected by the polarization beam splitter 15. Guided to the imaging system. In general, the light diffracted in a linear pattern may change the polarization state, but the zero-order light does not change. Therefore, the diffracted light component is enhanced by elliptical illumination and guided to the imaging system. As a result, it is possible to obtain an image with improved high-frequency MTF.
[0007]
[Problems to be solved by the invention]
The method using the linearly polarized illumination or the elliptically polarized illumination described in the conventional example is an effective method that can improve the high-frequency MTF. However, in order to obtain a large MTF improvement effect, it is necessary to change the direction of polarization of illumination light depending on the direction of the pattern on the sample, and there is a problem that the condition setting is complicated. In addition, when patterns with different orientations coexist on the sample, there is a problem that it is difficult to obtain the same MTF improvement effect for all patterns. This is because the improvement effect of MTF is not isotropic and depends on the relationship between the polarization direction of the illumination light and the direction of the pattern on the sample. SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus capable of obtaining an MTF improvement effect in a high range without depending on the direction of a pattern on a sample.
[0008]
In addition, the method using elliptically polarized illumination is an excellent method with high light utilization efficiency and an MTF improvement effect, but has a problem that coexistence with the method using linearly polarized illumination is difficult. The method using linearly polarized illumination has a problem in regular use because the light use efficiency is low and the image becomes dark. However, since a larger MTF improvement effect can be obtained than in the case of elliptically polarized illumination, it is desirable that it can be used when necessary. However, since the characteristics required for the beam splitter used for separating the optical path of the imaging system of the illumination system are different, it is very difficult to realize both with one optical system. Accordingly, another object of the present invention is to realize an optical system that can switch the polarization state of illumination light to linearly polarized light as required.
[0009]
[Means for Solving the Problems]
In an embodiment of the present invention, an illuminating unit that irradiates the sample with illumination light, an imaging unit that forms an image of the illuminated sample, an imaging unit that captures the image of the sample, and the acquired image In a defect inspection apparatus comprising a defect detection means for detecting a defect of the sample in comparison with an image stored in advance, and a display means for displaying the defect detected by the defect detection means, the imaging means includes λ A wavelength plate switching means for individually moving the objective lens, the λ / 4 plate, the λ / 2 plate, the λ / 4 plate and the λ / 2 plate into and out of the optical path, the partial polarization beam splitter and the analyzer. An image lens is provided.
[0010]
In order to obtain the MTF improvement effect regardless of the orientation of the pattern on the sample, a method for obtaining the MTF improvement effect under circularly polarized illumination is provided. Specifically, a partially polarized beam splitter is used to separate the optical path between the illumination system and the imaging system, and a λ / 4 plate is added to illuminate the sample with circularly polarized light. Add an analyzer. The component generated by changing the polarization when reflected by the sample is incident on the partially polarized beam splitter as p-polarized light, and thus a part of the component is reflected and guided to the imaging system. By adjusting the orientation of the analyzer of the imaging system, this component can be guided to the imaging system by a necessary amount, and an MTF improvement effect can be obtained. Furthermore, in the present invention, a phase compensator is added between the polarizing beam splitter and the analyzer as necessary. By adjusting the phase difference between the p-polarized component and the s-polarized component reflected by the partially polarized beam splitter by the analyzer using the phase compensator, it is possible to further increase the MTF improvement effect.
[0011]
The present invention also provides a method for realizing all of the conventional elliptically polarized illumination system, the above-mentioned circularly polarized illumination system, and the linearly polarized illumination system that can obtain the greatest MTF improvement effect by simple switching with a single optical system. . Specifically, a partial polarization beam splitter is used to separate the optical path of the illumination system and the imaging system, and a λ / 2 plate and a λ / 4 plate, which can be put in and out individually, are added to the specimen so that the sample has an arbitrary polarization state. Illuminate with light and add an analyzer to the imaging system immediately after the partially polarized beam splitter. When realizing a system compatible with the conventional elliptically polarized illumination system, the analyzer orientation is set so that only the s-polarized component of the light reflected by the partially polarized beam splitter is guided to the imaging system. In other cases, the degree of the MTF improvement effect can be adjusted by adjusting the orientation of the analyzer. Further, when linearly polarized illumination is used, since the component generated by the change in polarization when reflected by the sample is incident on the partially polarized beam splitter as s-polarized light, almost all is reflected and the imaging system is reflected. Led to. Since this component is led to the imaging system with about twice the efficiency compared to the case of using a non-polarizing beam splitter, it is possible to obtain a greater MTF improvement effect than the conventional linear polarization illumination method. Become.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of an optical defect inspection apparatus using the microstructure observation method of the present invention is shown in FIG. This is an example using circularly polarized illumination. The sample 1 is vacuum-adsorbed on the chuck 2, and the chuck 2 is mounted on the θ stage 3, the Z stage 4, the Y stage 5, and the X stage 6. The optical system 111 disposed above the sample 1 captures an optical image of the sample 1 in order to inspect the appearance of the pattern formed on the sample 1, and mainly includes the illumination optical system and the sample 1. An imaging optical system that forms and captures an image and a focus detection optical system 45 are included. The light source 8 disposed in the illumination optical system is an incoherent light source, for example, a xenon lamp.
[0013]
The light emitted from the light source 8 passes through the opening of the aperture stop 11 through the lens 9, and further reaches the field stop 13 through the lens and the wavelength selection filter 12. The wavelength selection filter 12 considers the spectral reflectance of the sample 1 and limits the illumination wavelength region in order to detect an image of the sample 1 with high resolution. For example, an interference filter is disposed. The light transmitted through the field stop 13 enters the optical path separation unit 210.
[0014]
The optical path separation unit 210 includes a partial polarization beam splitter 200 (for example, Tp: Rp = 6: 4, Ts / Rs <0.05), a λ / 4 plate 17, a phase compensation plate 220, and an analyzer 22. The optical path of illumination light from the light source 8 toward the sample 1 and the optical path from the sample 1 toward the image sensor are separated. Its function is shown in FIG. Illumination light (random polarization) incident on the optical path separation unit 210 passes through the partial polarization beam splitter 200 by the transmittance Tp (˜0.6) and becomes p-polarized linearly polarized light. Further, the sample 1 is irradiated with the λ / 4 plate 17 through the objective lens 20 as circularly polarized light. The light illuminated on the sample 1 is reflected, scattered, and diffracted on the sample 1, and the light within the NA of the objective lens 20 enters the objective lens 20 again, passes through the λ / 4 plate 17, and passes through the partial polarization beam splitter 200. And is incident on the analyzer 22. Of the light reflected by the sample, the component whose rotation direction is reversed at the time of reflection (regular reflection component, that is, the 0th-order light component) becomes linearly polarized light by the λ / 4 plate 17 and is converted into s-polarized light by the partial polarization beam splitter 200. Incident and reflected with almost no loss.
[0015]
On the other hand, a component of the light reflected by the sample whose rotation direction did not change at the time of reflection (a component generated by a change in the polarization state of the reflected light, that is, a part of higher-order diffracted light) is linearly polarized by the λ / 4 plate 17. Then, it enters the partially polarized beam splitter 200 as p-polarized light, and is reflected at the reflectance Rp. (This component does not reach the imaging system because it is Rp˜0 in a normal polarization beam splitter.) After these components are adjusted in phase difference by the phase compensator 220, they enter the analyzer 22, They are combined (added) at a ratio determined by the azimuth and guided to the imaging optical system. The phase compensation amount of the phase compensation plate 220 and the orientation of the analyzer 22 are determined so that the high-order diffracted light is guided to the imaging system more efficiently than the 0th-order light. Since the high-order diffracted light component whose rotational direction of the circularly polarized light does not change at the time of reflection can also be taken into the imaging optical system, it is possible to improve the high-frequency MTF. In addition, the phase compensation plate optimizes the phase difference between the component whose circular polarization is rotated during reflection and the component that did not change at the time of reflection, and adjusts the azimuth of the analyzer. Since the ratio of these components can be adjusted, the improvement degree of the high frequency MTF can also be adjusted.
[0016]
The light transmitted through the analyzer 22 forms an image of the sample 1 on the light receiving surface of the image sensor 70 through an imaging optical system including the imaging lens 30 and the zoom lens 50. As the image sensor 70, a linear sensor, a TDI image sensor, an area sensor (TV camera), or the like is used. Further, a part of the reflected light from the sample is guided to the focus detection optical system 45 by a light splitting means 25 such as a dichroic mirror and used for signal detection for automatic focusing.
[0017]
The focus detection light forms an optical image having the height information of the sample 1 on the sensor 41 by the imaging lens 40, and the sensor output signal is input to the focus detection signal processing circuit 90. The focus detection signal processing circuit 90 detects the amount of deviation between the height of the sample 1 and the focal position of the objective lens 20 and sends data on the amount of focus deviation to the CPU 75. In accordance with the amount of focus shift, the CPU 75 instructs the stage control unit 80 to drive the Z stage 4 and sends a predetermined pulse from the stage control unit 80 to the Z stage 4 to perform automatic focusing.
[0018]
Further, the image data of the optical image of the sample 1 detected by the image sensor 70 of the detection optical system is input to the image processing circuit 71 and processed, and the defect determination circuit 72 determines the defective portion. The result is displayed on a display means such as a display and transmitted to an external storage / control device such as a workstation or a data server via a communication means.
[0019]
As a specific processing method of a series of image processing from the image sensor 70 for determining the defective portion based on the detected image data to the defect determination circuit 72, for example, Japanese Patent Laid-Open No. 2-170279 or Japanese Patent Laid-Open No. 3-33605. As described in the Gazette, etc., a method of comparing adjacent image data of adjacent chips, a method of comparing corresponding image data of adjacent chips, and comparing image data of adjacent patterns And a method of comparing design data and image data.
[0020]
The movement of the sample 1 in the XY directions is performed by two-dimensionally controlling the movement of the X stage 6 and the Y stage 5 by the stage controller 80. The θ stage 3 is used when θ alignment of the movement direction of the XY stages 6 and 5 and the pattern formed on the sample 1 is performed.
[0021]
Next, FIG. 2 shows an embodiment of the optical path separation unit 210 of the illumination system and the imaging system when realizing linearly polarized illumination in an optical system capable of circularly polarized illumination and elliptically polarized illumination.
[0022]
In this embodiment, the optical path separation unit 210 includes a partial polarization beam splitter 200 (for example, Tp: Rp = 6: 4, Ts / Rs <0.05), and a λ / 2 plate 16 and a λ / 4 that can be taken in and out, respectively. The plate 17, the phase compensation plate 220, and the analyzer 22 are included.
[0023]
When image observation is performed using linearly polarized illumination, the λ / 2 plate 16 is inserted into the optical path and the λ / 4 plate 17 is removed as shown in the figure. Illumination light (random polarization) incident on the partial polarization beam splitter 200 is transmitted by Tp (˜0.6) to become p-polarized light, and the polarization direction can be changed in accordance with the pattern on the sample by the λ / 2 plate 16. After that, it enters the sample through the objective lens 20. The component of the light reflected by the sample whose polarization direction has not changed is returned to the original polarization direction by the λ / 2 plate 16 and is incident on the partial polarization beam splitter 200 as p-polarized light, and the reflectance Rp (˜0). Reflected in 4).
[0024]
On the other hand, the component generated by the rotation of the polarization direction of the reflected light of the light reflected from the sample becomes linearly polarized light that is orthogonal to the original polarized light by the λ / 2 plate 16 and is converted into s-polarized light by the partial polarization beam splitter 200. Incident and reflected with almost no loss. These components are adjusted in phase difference with each other by the phase compensation plate 220, then enter the analyzer 22, and are combined (added) at a ratio determined by the azimuth and guided to the imaging optical system. Since the component generated by the change in the polarization direction during reflection can be efficiently taken into the imaging optical system, not only the conventional method using elliptically polarized illumination but also the image observation using linearly polarized illumination is possible. Compared with a method using a polarizing beam splitter, a larger MTF improvement effect can be obtained.
[0025]
In addition, the phase compensation plate optimizes the phase difference between the component whose polarization direction did not change during reflection and the component caused by the change in polarization direction during reflection, and adjusted the orientation of the analyzer, leading to the imaging optical system. Since the ratio of these components in the light to be burned can be adjusted, it is possible to further emphasize and adjust the MTF improvement effect.
[0026]
In order to switch to the method using the circularly polarized illumination shown in FIG. 1, the λ / 4 plate 17 is inserted into the optical path and the λ / 2 plate 16 is removed. Furthermore, in order to be compatible with the conventional method using elliptically polarized illumination, both the λ / 2 plate 16 and the λ / 4 plate 17 are inserted in the optical path, and the s-polarized light out of the light reflected by the partial polarization beam splitter. The orientation of the analyzer may be set so that only the components are guided to the imaging system. In this way, the observation conditions can be changed as necessary by simple switching.
[0027]
In these embodiments, an example in which the phase compensation plate 220 is inserted in the optical path separation unit 210 is shown, but this is not essential. Even without the phase compensator 220, this system is established, and the effect of improving the MTF in the high band can be obtained under circularly polarized illumination. In addition, as an example of the characteristics of the partial polarization beam splitter 200, the number Tp: Rp = 6: 4, Ts / Rs <0.05 is shown, but these are not essential conditions. Any p-polarized component may be reflected.
[0028]
As described above, even in the case of circularly polarized illumination, an effect of improving the MTF at a high frequency can be obtained, so that high-resolution image observation independent of the pattern direction on the sample and highly sensitive defect detection can be performed. Further, if necessary, it is possible to selectively use linearly polarized illumination, circularly polarized illumination, and elliptically polarized illumination by simple switching, so that it is possible to deal with a wider variety of samples.
[0029]
【The invention's effect】
According to the present invention, the effect of improving the MTF at a high frequency can be obtained without depending on the direction of the pattern on the sample. In addition, an optical system that can switch the polarization state of illumination light to linearly polarized light as required can be realized.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing a configuration of a defect inspection apparatus.
FIG. 2 is a longitudinal sectional view showing a configuration of an optical path separation unit when circularly polarized illumination is used.
FIG. 3 is a longitudinal sectional view showing a configuration of an optical path separation unit when switching between linearly polarized illumination, circularly polarized illumination, and elliptically polarized illumination.
FIG. 4 is a longitudinal sectional view showing a configuration of an optical system of a conventional defect inspection apparatus.
FIG. 5 is a longitudinal sectional view showing a configuration of an optical path separation unit in a conventional defect inspection apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Sample, 8 ... Light source, 16 ... (lambda) / 2 board, 17 ... (lambda) / 4 board, 20 ... Objective lens, 22 ... Analyzer, 30 ... Imaging lens, 50 ... Zoom lens, 70 ... Image sensor, 71 ... Image Processing circuit 72... Defect determination circuit 200. Partial polarization beam splitter 210 210 Optical path separation unit 220 Phase compensation plate

Claims (7)

In a method of observing a fine pattern formed on a sample surface with light, illumination light having a substantially circular polarization state is irradiated on the sample surface through an objective lens, and the light reflected by the sample is λ / 4 plate with λ as a wavelength. A fine structure observing method, characterized in that the image is guided to an imaging optical system through a partial polarization beam splitter and an analyzer, and an image formed by the imaging optical system is observed or imaged. In a method of observing a fine pattern formed on a sample surface with light, illumination light having a substantially linear polarization state is irradiated onto the sample surface through an objective lens, and the light reflected by the sample is λ / 2 plate with λ as a wavelength. A method for observing a fine structure, characterized in that the image is guided to an imaging optical system through a partial polarization beam splitter and an analyzer, and an image formed by the imaging optical system is observed or captured. A phase compensation element is provided between the partial polarization beam splitter and the analyzer, and the phase difference between the s-polarized component and the p-polarized component reflected by the partial polarization beam splitter is adjusted and guided to the imaging optical system. The microstructure observation method according to claim 1 or 2. An illumination unit that irradiates the sample with illumination light, an imaging unit that forms an image of the illuminated sample, an imaging unit that captures the image of the sample, and an image in which the acquired image is stored in advance In a defect inspection apparatus comprising a defect detection means for detecting a defect of the sample by comparison and a display means for displaying a defect detected by the defect detection means, the illumination means converts the illumination light in a substantially circular polarization state into an objective lens A defect inspecting device comprising: a circularly polarized light generating means incident on the light source, and the imaging means having an objective lens, a λ / 4 plate, a partial polarization beam splitter, an analyzer, and an imaging lens with λ as a wavelength. apparatus. An illumination unit that irradiates the sample with illumination light, an imaging unit that forms an image of the illuminated sample, an imaging unit that captures the image of the sample, and an image in which the acquired image is stored in advance In a defect inspection apparatus comprising a defect detection means for detecting a defect of the sample by comparison and a display means for displaying a defect detected by the defect detection means, the illumination means converts illumination light in a substantially linear polarization state into an objective lens Defect inspection, characterized in that it comprises linearly polarized light generating means to be incident on the image forming means, and the imaging means comprises an objective lens, a λ / 2 plate, a partial polarization beam splitter, an analyzer, and an imaging lens with λ as a wavelength. apparatus. An illumination unit that irradiates the sample with illumination light, an imaging unit that forms an image of the illuminated sample, an imaging unit that captures the image of the sample, and an image in which the acquired image is stored in advance In a defect inspection apparatus comprising a defect detection means for detecting a defect of the sample by comparison and a display means for displaying a defect detected by the defect detection means, the imaging means has an objective lens and λ / 4 plates, λ / 2 plates, λ / 4 plates and λ / 2 plates are individually provided in and out of the optical path, wave plate switching means, a partial polarization beam splitter, an analyzer, and an imaging lens. Defect inspection equipment. 7. The defect inspection apparatus according to claim 4, wherein the imaging unit includes a phase compensation unit.

Images