JPH07159977A - Photomask inspecting device - Google Patents

Abstract

PURPOSE:To form images with stressed patterns by installing an equiv. optical system in a position equiv. to an imaging optical system even when viewed from the imaging plane of a photomask and superposing both linearly polarized light beams on each other just before the imaging plane. CONSTITUTION:The parallel beams emitted from a light source 101 are polarized by linearly polarized beams by a polarizer 102 and are divided to two optical paths by a beam splitter 103. The one linearly polarized light passes an optical path adjusting device composed of 108 from a mirror 105, illuminates a photomask 109 and is passed through an objective lens 110, by which 113 is obtd. The other linearly polarized light advancing rectilinearly in the beam splitter 103 is bent in the optical path by the mirror 104 and the polarization direction is rotated 90 deg. by a half wave plate 112. Thereafter, the polarized light passes the optical system 115 and is superposed on the linearly polarized light past the photomask 109 by a mirror 114 and a translucent mirror 113. Further, the image stressing the pattern in particular is obtd. when the direction of the major axis of the elliptically polarized light formed by superposition and the direction of an analyzer 117 are aligned.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photomask inspection apparatus for detecting a pattern shape in a photomask surface used in a lithography technique or a structural defect in a pattern thickness direction.

[0002]

2. Description of the Related Art In order to miniaturize a pattern of a semiconductor integrated circuit or the like, a reduction projection exposure apparatus used in a lithography technique has been improved in resolution. As one method for this purpose, a commonly used method is to form a chromium film on a glass substrate having a thickness that absorbs almost all of the exposure light, and then pattern this into a photomask to be used. Instead, a technique using a photomask that gives a phase difference to the light passing through the transmitting portion or a photomask that gives the pattern portion a certain degree of transparency and phase change is in the spotlight. The improved former technique using a photomask is called a phase shift technique and is disclosed in the monthly publication "Nikkei Microdevice", July 1990, pp. 103-114 and JP-A-58-173744. Also,
The technique using the improved latter photomask is described in "Electronic,
The 36th International Symposium on Electron and Photon Beams
n, Ion and Photon Beams) J4 paper and Japanese Patent Laid-Open No. 4-1
It is disclosed in Japanese Patent No. 36854 and Japanese Patent Application Laid-Open No. 4-162039.

The technique using these improved photomasks is a technique for improving the light intensity distribution on the image plane as a result of the interference of light from each part of the photomask. For example, a phase shift technique is used as a standard photomask. The description will be made in comparison with the case where it is used.

FIG. 4A is a diagram showing an amplitude distribution when a line / space pattern is imaged by using a normal photomask. The dotted line shows the amplitude distribution of transmitted light from each space, and the solid line shows the amplitude distribution as a result of interference. The light intensity distribution is the square of the amplitude distribution shown by the solid line. FIG. 4B is a diagram showing a case where a phase shift photomask is used. As in the case of FIG. 4A, the dotted line shows the amplitude distribution of transmitted light from each space, and the solid line shows the amplitude as a result of interference. The distribution is shown. The light intensity distribution is the square of the amplitude distribution shown by the solid line. As is clear from FIG. 4, in the normal photomask, the diffracted light from the adjacent transmissive portions are superposed in the same phase, so the intensity of the light shielding portion does not become zero. On the other hand, in the phase shift photomask, the diffracted light from the adjacent transmissive portions are superposed in opposite phases, so that the intensity of the light shielding portion becomes zero. As a result, the image contrast is improved in the phase shift photomask.

It should be noted here that the more the phase of light from the adjacent transmitting portions deviates from π, the more the amplitude of light transmitted through the phase shift member due to absorption of the phase shift member. The larger the decrease is, the less the contrast improving effect is. Furthermore, unless the pattern for shifting the phase is formed in a desired position and in a desired shape, the desired contrast improving effect cannot be obtained.

[0006]

According to the above description, in the improved photomask, it is extremely important to control the phase and transmittance of the transmitted light of each part. Further, it is important that each pattern of the photomask is formed in a desired shape at a desired position with a controlled phase and transmittance.

Here, the phase / amplitude transmittance / energy transmittance can be controlled by controlling the thickness of each material if the refractive index and extinction coefficient of each material used for manufacturing the photomask are accurately known. However, it is often difficult to accurately measure the refractive index and extinction coefficient of each material, and in manufacturing, impurities are mixed into each material and errors occur in the refractive index and extinction coefficient. Moreover, there is always a manufacturing tolerance associated with the thickness of each material. Therefore, the phase, amplitude transmittance, and energy transmittance of each part of the manufactured photomask are actually measured, and the work of feeding back the results to the manufacturing process is repeated to realize a desired photomask.
However, there is a problem that there is no effective means for measuring the phase / amplitude transmittance / energy transmittance. Further, in order to confirm that each pattern of the photomask is formed in a desired shape at a desired position with a controlled phase and transmittance, only the phase and transmittance pattern of interest is imaged,
Alternatively, it is necessary to emphasize the pattern of the phase and the transmittance of interest to form an image, but there is a problem that there is no such technique.

An object of the present invention is to obtain a simple and accurate measuring means of phase, amplitude transmittance and energy transmittance and image only the phase and transmittance pattern of interest, or emphasize the pattern. To obtain an inspection device for the photomask to be imaged.

[0009]

The first object of the present invention is to divide a linearly polarized light of parallel light into a light source and an inspection light and a reference light whose polarization directions are orthogonal to each other.
Semi-transparent mirror, and the inspection light optical system through which the inspection light passes,
In a photomask inspection apparatus having a reference light optical system through which the reference light passes, a second semitransparent mirror that combines the inspection light and the reference light, and a photodetector, the inspection light optical system includes the inspection light. Has an illumination optical system for illuminating the inspection target photomask by means of an optical system, and an image forming optical system for forming an image of the inspection light transmitted through the inspection target photomask, and the reference light optical system is one of the optical axis and the optical axis. And an optical system equivalent to the image forming optical system at a position equivalent to the image forming optical system when viewed from the image forming position of the image forming optical system on the optical axis. A photodetector arranged at an image forming position of the image forming optical system and an analyzer arranged between the second semitransparent mirror and the photodetector, and the inspection light optical system. The difference in optical distance from the reference light optical system is smaller than the coherence length of the linearly polarized light. It is achieved.

Further, the photodetector is a device in which elements for converting the intensity of incident light into the intensity of an electric signal are arranged two-dimensionally, and the linearly polarized light is ultraviolet light, and The second semitransparent mirror is a plane-parallel plate having a metal thin film formed on the surface thereof, and is achieved by performing antireflection treatment on at least the light emitting surface side.

Further, the difference in optical distance between the inspection light optical system and the reference light optical system can be achieved by adjusting with an optical distance adjusting device having a precision displacement mechanism.

[0012]

As described above, according to the present invention, the bisected light is passed through different optical paths to be linearly polarized light having polarization directions orthogonal to each other in a plane perpendicular to the traveling direction. An imaging optical system for imaging a photomask and a photomask is installed, and in other linearly polarized light, an equivalent optical system is installed at a position equivalent to the imaging optical system when viewed from the imaging plane of the photomask, Moreover, since both linearly polarized lights are superposed immediately before the image plane, the superposed lights are generally elliptically polarized lights. From the measurement of the polarization state of the elliptically polarized light at the point of interest of the photomask, the amplitude transmittance or energy transmittance of the point of interest of the photomask and the amount of phase change due to light passing through each part of the photomask can be obtained. In addition, an analyzer is installed between the image plane and a place where two linearly polarized light beams are superposed, and the linearly polarized light beam that has passed through the pattern of interest is superposed on another linearly polarized light beam. If the direction and the direction of the analyzer are matched, an image in which the pattern of interest is emphasized can be obtained. Alternatively, of the two types of patterns of interest, the optical path difference between the two linearly polarized lights is adjusted so that the linearly polarized light that has passed through one type of pattern is combined with the other linearly polarized light to become linearly polarized light. By matching the direction of the linearly polarized light generated as a result and the direction of the analyzer, it is possible to obtain an image of only the other type of pattern of interest.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a diagram showing a first embodiment of a photomask inspection apparatus according to the present invention, FIG. 2 is a diagram showing a method of calculating δ and a _x , and FIG.
FIG. 3 is a diagram showing a second embodiment of the photomask inspection device according to the present invention.

First Embodiment In FIG. 1 showing a first embodiment of the present invention, 101 is a light source, 102 is a polarizer, 103 is a beam splitter, and 10
4 to 108 are mirrors, 109 is a photomask to be inspected, 110 is an objective lens, and 111 is the photomask 10 described above.
It is the image of 9. 112 is a half-wave plate, 113 is a semi-transparent mirror,
Reference numeral 114 denotes a mirror, 115 denotes an optical system having a configuration equivalent to that of the objective lens 110 when viewed from the image 111, and 116.
Is an imaginary plane that is at a position equivalent to a photomask when viewed from the image 111. Further, 117 is an analyzer. Light source 10
The reference numeral 1 may be one that supplies light of a wavelength required for inspection as parallel light. For example, when inspecting at the wavelength of KrF excimer laser light, a KrF excimer laser may be used,
When inspecting at a wavelength of 365 nm, a laser that oscillates at 365 nm or a combination of a mercury lamp, a monochromator and a collimator may be used. The parallel light emitted from the light source 101 is linearly polarized by the polarizer 102 and split into two optical paths by the beam splitter 103. One of the linearly polarized lights passes through an optical path adjusting device composed of mirrors 105 to 108, illuminates a photomask 109, passes through an objective lens 110, and becomes an image 111. The other linearly polarized light that has passed straight through the beam splitter 103 has its optical path bent by the mirror 104 and has its polarization direction rotated 90 degrees by the half-wave plate 112.
After that, the linearly polarized light that has passed through the optical system 115 and the photomask 109 is superposed by the mirror 114 and the semitransparent mirror 113. For convenience of explanation, the linearly polarized light that illuminates the photomask will be referred to as inspection linearly polarized light 118, and the other linearly polarized light will be referred to as reference linearly polarized light 119.

The above inspection linearly polarized light 118 and the reference linearly polarized light 1
If x and y are 19 respectively, the above-mentioned inspection linearly polarized light 11
8 is the phase delay at the semitransparent mirror 113 is δ _x

[0016]

[Equation 1]

The reference linearly polarized light 119 is expressed as follows, where δ _y is the phase delay at the semitransparent mirror 113.

[0018]

[Equation 2]

Is expressed as The combined light

[0020]

[Equation 3]

[0021] Here, E _x and E _y are the time-varying functions of the linearly polarized light intensity in the x direction and the y direction at a certain location, respectively, a _x and a _y are the amplitudes of the above E _x and E _y , and ωt is the time t And δ is the difference between δ _x and δ _y , which are the initial phases of linearly polarized light in the x and y directions,

[0022]

[Equation 4]

It is defined as Now, considering the beam splitter 103 as a starting point, the phase delay of the inspection linearly polarized light 118 in the photomask 109 is δ _x1 , the phase delay due to passing through the inspection target portion of the photomask is δ _x2 , and after passing through the photomask, up to the image plane 111. The phase delay of the optical path of is set to δ _x3 . Further, if the phase delay of the reference linearly polarized light 119 in the virtual plane 116 is δ _y1 and the phase delay of the optical path from the virtual plane 116 to the image plane 111 is δ _y3 , then the above equation (4) becomes

[0024]

[Equation 5]

[0025] Here, δ _x3 is a function of the position on the photomask, but is constant with respect to the diffracted light emitted from one point on the photomask and corresponds to 1 on the virtual surface 116.
It coincides with δ _y3 of the reference linearly polarized light passing through the point. Therefore, the above equation (5) is

[0026]

[Equation 6]

It is expressed as follows. In this embodiment, since the two-dimensional CCD camera is installed on the image plane 111, the analyzer 11
The output intensity of the CCD element that receives the image of one point of the photomask of interest while rotating 7 is monitored, and the volume of Applied Optics, Vol. 1, No. 3 (1962) is monitored.
Year), page 201, and δ in equation (6) and a _{x in} equation (1) were determined. That is, as shown in FIG. 2, an analyzer installed perpendicularly to the optical axis is used to
/ N rotation × n times = 1 rotation (n is an integer), and the output of the light intensity detector at each inclination of the analyzer is read. The direction of the analyzer at the first rotation (i = 1, 2, 3, ..., N) of the analyzer is (360 ° / n) with respect to the reference direction assumed in the plane perpendicular to the optical axis. ) × i = α _i (n is a positive integer), the output is I _i ,

[0028]

[Equation 7]

When k ₀ , k ₁ and k ₂ are respectively calculated by
And a _x

[0030]

[Equation 8]

It is represented by Next, the photomask was removed and the same procedure was followed to obtain δ and a _x . As is clear from the equation (6), the difference between both δ is the phase delay due to passing through the inspection target portion of the photomask, the ratio of both a _x is the amplitude transmittance, and if it is further squared, it becomes the energy transmittance. Become.

Further, as is apparent from the equations (3) and (6), the polarization state of the elliptically polarized light is determined according to the optical properties of the pattern, so that only a specific pattern, for example, the phase shift is given. When the direction of the major axis of the elliptically polarized light generated in the image portion of the pattern that has the pattern of ∘ is matched with the direction of the analyzer 117, an image in which a specific pattern is emphasized is obtained.

Alternatively, for a specific pattern, for example, a pattern that serves only to provide a phase shift, the mirror 105
To drive the optical path adjusting device constituted by Nos. 108 to 108 by a precision displacement mechanism using a piezo element (not shown),
When δ _x1 in the equation (6) is adjusted and δ is set to nπ (n is an integer), the polarized light generated by superimposing the image portions of the specific pattern is linearly polarized light. Therefore, when the direction of the linearly polarized light generated by the superposition and the direction of the analyzer 117 are matched, a more emphasized image of the specific pattern is obtained.

When the photomask 109 to be inspected is made up of an object for delaying the phase with respect to the substrate portion, a so-called shifter, a light shield, and an exposed portion of the substrate, as shown in FIG. Adjust δ _x1 in equation (6),
If δ is nπ (n is an integer), the polarized light of the image on the substrate becomes linearly polarized light. Therefore, when the direction of the analyzer 117 is set to the direction orthogonal to the direction of the linearly polarized light, an image of only the object whose phase is delayed with respect to the substrate portion can be obtained.

Second Embodiment In FIG. 3 showing a second embodiment of the present invention, 201 is a light source, 202 is a polarizer, 203 is a beam splitter, and 20.
4 is a mirror, 205 is a photomask to be inspected, 20
6 is an objective lens, and 207 is an image of the photomask 205. Further, 208 is a half-wave plate, 209 is a semi-transparent mirror, 21
Reference numeral 0 is a mirror, 211 is an optical system having a configuration equivalent to that of the objective lens 206 when viewed from the image 207, and 212 is a virtual surface which is equivalent to the photomask 205 when viewed from the image 207. 213 is an analyzer. The light source 201 may be any light source that supplies light of a wavelength required for inspection as parallel light. For example, when inspecting at the wavelength of KrF excimer laser light, a KrF excimer laser may be used, and when inspecting at light of a wavelength of 365 nm, even a laser oscillating at 365 nm is a combination of a mercury lamp, a monochromator and a collimator. It may be one. The parallel light emitted from the light source 201 is linearly polarized by the polarizer 202 and split into two optical paths by the beam splitter 203. One linearly polarized light illuminates the photomask 205, and an image 207 is formed by the objective lens 206. The other linearly polarized light that has passed straight through the beam splitter 203 has its polarization direction rotated 90 degrees by the half-wave plate 208. After that, the linearly polarized light that has passed through the optical system 211 and the mirror 210 and the semitransparent mirror 209 that has passed through the photomask 205 is superimposed. In this embodiment, a two-dimensional CCD camera is installed at the position of the image 207 to serve as an image observing means. The operating principle of this embodiment is the same as that of the first embodiment, but it is generally impossible to make the polarized light obtained by superimposing the image portions for a specific pattern into linear polarized light. However, this embodiment is devised so that the optical path adjusting device in the first embodiment is omitted. Except for the phase change caused by passing through the photomask 205, vibrations orthogonal to each other passing through the two optical paths are provided. There is no optical path difference between the linearly polarized light in the direction. Therefore, the restriction on the coherence length of the light from the light source 201 is extremely loose.

In each of the above embodiments, 103 and 203 are used when the wavelength of the light used for inspection is outside the visible range in the ultraviolet.
Each of the beam splitter and the semi-transparent mirrors 113 and 209 is composed of a plane parallel plate on which a thin film of a metal such as chrome or aluminum is vapor-deposited, and at least the emission surface side is subjected to antireflection treatment. . The optical element through which the linearly polarized light passes was corrected for chromatic aberration according to the wavelength of the linearly polarized light.

[0037]

As described above, the photomask inspection apparatus according to the present invention splits the linearly polarized light of the parallel light into the light source and the inspection light and the reference light whose polarization directions are orthogonal to each other. A first semi-transparent mirror, an inspection light optical system through which the inspection light passes, a reference light optical system through which the reference light passes, and a second semi-transparent mirror that combines the inspection light and the reference light. In the photomask inspection device having a light detection means, the inspection light optical system includes an illumination optical system for illuminating the inspection target photomask with the inspection light, and an imaging optical system for forming an image of the inspection light transmitted through the inspection target photomask. The reference light optical system shares a part of the optical axis with the imaging optical system, and is located on the optical axis at a position equivalent to the imaging optical system when viewed from the imaging position of the imaging optical system. , An optical system equivalent to the above-mentioned imaging optical system, The means includes a photodetector arranged at an image forming position of the image forming optical system, and an analyzer arranged between the second semitransparent mirror and the photodetector. Since the difference in optical distance between the system and the reference light optical system is smaller than the coherence length of the linearly polarized light, the transmittance and the amount of phase change of each part of the photomask can be simply and accurately obtained. Further, since only a specific pattern can be imaged or an image in which a specific pattern is emphasized can be formed, it is possible to inspect the shape of the specific pattern. INDUSTRIAL APPLICABILITY The present invention can be applied not only to the above photomask but also to all objects that are transparent to the light of the wavelength to be handled. Furthermore, if the thickness of the object is known, the refractive index, the absorption coefficient, or the extinction coefficient can be obtained from the obtained amplitude transmittance and the amount of phase change.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a photomask inspection apparatus according to the present invention.

FIG. 2 is a diagram showing a method of calculating δ and a _x in the above embodiment.

FIG. 3 is a diagram showing a second embodiment of the photomask inspection device according to the present invention.

4A and 4B are diagrams showing an amplitude intensity distribution and a light intensity distribution when a photomask is used. FIG. 4A is a diagram showing a case where a conventional normal photomask is used, and FIG. 4B is a diagram showing a case where a phase shift photomask is used. .

[Explanation of symbols]

 101, 201 Light source 103, 203 First semi-transparent mirror 109, 205 Photomask for inspection 110, 206 Objective lens 111, 207 Imaging 113, 209 Second semi-transparent mirror 117, 213 Analyzer

Claims (4)

[Claims] 1. A light source which generates linearly polarized light of parallel light, a first semi-transparent mirror which splits the linearly polarized light from the light source into inspection light and reference light whose polarization directions are orthogonal to each other, and the inspection light In a photomask inspection device having an inspection light optical system that passes through, a reference light optical system through which the reference light passes, a second semi-transparent mirror that combines the inspection light and the reference light, and a light detection means, The inspection light optical system has an illumination optical system that illuminates the inspection target photomask with the inspection light, and an imaging optical system that forms an image of the inspection light that has passed through the inspection target photomask,
The reference light optical system shares a part of the optical axis with the image forming optical system, and forms an image on the optical axis at a position equivalent to the image forming optical system when viewed from the image forming position of the image forming optical system. An optical system equivalent to an optical system, wherein the photodetection means is provided between the photodetector arranged at the image forming position of the image forming optical system, the second semitransparent mirror and the photodetector. A photomask inspection apparatus, characterized in that the optical distance difference between the inspection light optical system and the reference light optical system is smaller than the coherence length of the linearly polarized light. 2. The photomask inspection device according to claim 1, wherein the photodetector is a device in which elements for converting the intensity of incident light into the intensity of an electric signal are arranged two-dimensionally. 3. The linearly polarized light is ultraviolet light, and the first and second semitransparent mirrors are parallel plane plates having a metal thin film formed on the surface thereof, and at least a light emitting surface side is subjected to antireflection treatment. The photomask inspection device according to claim 1, wherein the photomask inspection device is provided. 4. A photomask inspection apparatus according to claim 1, wherein a difference in optical distance between the inspection light optical system and the reference light optical system is adjusted by an optical distance adjusting device having a precision displacement mechanism.