JP2017049438A - Observation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an observation device such as an ellipsometer preventing enlargement of aberration of an image to be imaged by a life-size reflection type imaging optical system.SOLUTION: An ellipsometer 100 includes an imaging optical system 30 for imaging a light flux of reflection light from an object surface on a light-receiving surface 40. The imaging optical system 30 includes a concave primary mirror 32, a convex secondary mirror 34, and a drawing plane mirror 36. The imaging optical system 30 consists of a life-size reflex type imaging optical system capable of reflecting the light flux of the reflection light from the object surface in order of the concave primary mirror 32, the convex secondary mirror 34, and the concave primary mirror 32 and then imaging it via the drawing plane mirror 36 on the light receiving surface 40. The convex secondary mirror 34 is a backside mirror.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an observation device such as an ellipsometer.

  An ellipsometer is an apparatus that can measure the optical constants (refractive index, extinction coefficient, etc.) of a sample by observing changes in the polarization state when light is reflected or transmitted on the surface of the sample. For example, when a thin film exists on the surface of the sample, the thickness and optical constant of the thin film can be measured.

  Patent Document 1 discloses an example of a conventional ellipsometer. The ellipsometer disclosed in Patent Document 1 is an ellipsometer capable of magnifying and observing an image of an object to be observed using ellipsometry. This ellipsometer has a polarizer and a phase compensator on the optical path of a light source, and illuminates the object obliquely with parallel light that passes through the polarizer and the phase compensator, and reflected light from the object. An imaging system that enters and enlarges an object image to form an image, an analyzer provided in the optical path of the reflected light in the imaging system, and an image formed by the imaging system An image pickup device.

  The present inventor has also filed a patent application regarding an invention relating to an ellipsometer in which an Offner optical system, which is one of the equal-magnification reflective imaging optical systems, is applied to an imaging optical system for imaging reflected light from an object. (See Patent Document 2).

JP 2011-102731 A JP 2013-174844 A

  The present inventor has found that the following problem occurs when the equal magnification reflection type imaging optical system is applied to an observation device such as an ellipsometer.

  Optical elements such as a polarizer and a phase compensator are arranged on the optical path of the illumination optical system in the ellipsometer. An optical element such as an analyzer is also disposed on the optical path of an imaging optical system that forms an image of reflected light from the object. Furthermore, a glass window (an entrance window and an exit window) for preventing dust and the like from entering the interior of the lens barrel is provided at the entrance and exit of the lens barrel for storing a lens or mirror disposed on the optical path. ) Is installed. These optical elements have at least a pair of parallel planes. When an optical element having such a parallel plane is arranged on the convergent optical path, there is a problem that aberration of an image formed by the imaging optical system becomes large.

  Therefore, an object of the present invention is to solve the problem that an aberration of an image formed by an equal magnification reflection type imaging optical system becomes large in an observation device such as an ellipsometer.

Means for solving the above problems are the following inventions.
(1) An observation apparatus including an imaging optical system for forming an image of a reflected light beam from the surface of an object on a light receiving surface,
The imaging optical system includes a concave primary mirror, a convex secondary mirror, and an extraction flat mirror, and the reflected light beam from the surface of the object is converted into the order of the concave primary mirror, the convex secondary mirror, and the concave primary mirror. And is composed of a 1 × -magnification type imaging optical system that can form an image on the light receiving surface via the extraction flat mirror,
The observation apparatus, wherein the convex secondary mirror is a back mirror.

(2) The observation device according to (1), wherein the curvature of the front surface and the curvature of the back surface of the convex secondary mirror are different.

(3) The observation device according to (1) or (2), wherein the curvature of the surface of the convex secondary mirror is smaller than the curvature of the back surface of the convex secondary mirror.

(4) The observation apparatus according to any one of (1) to (3), wherein at least one optical element having a parallel plane is disposed on an optical path from the surface of the object to the light receiving surface. .

(5) The observation apparatus according to any one of (1) to (4), wherein the observation apparatus is an ellipsometer.

  According to the present invention, in an observation device such as an ellipsometer, it is possible to solve the problem that the aberration of an image formed by the equal magnification reflection type imaging optical system becomes large.

It is a figure which shows the whole structure of an ellipsometer. It is a cross-sectional enlarged view of a secondary mirror. 2 is a graph of Example 1. FIG. 10 is a graph of Example 2. 10 is a graph of Example 3. 10 is a graph of Example 4. 10 is a graph of Example 5. 10 is a graph of Example 6. 10 is a graph of Example 7. 10 is a graph of Example 8. 10 is a graph of Example 9. FIG. It is a graph of Example 10. FIG. 6 is a graph of Comparative Example 1. 10 is a graph of Comparative Example 2. 10 is a graph of Comparative Example 3. 10 is a graph of Comparative Example 4. 10 is a graph of Comparative Example 5. 10 is a graph of Comparative Example 6. 10 is a graph of Comparative Example 7. 10 is a graph of Comparative Example 8.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Below, the example which applied this invention to the ellipsometer is demonstrated.

  The ellipsometer 100 according to the present embodiment observes changes in the polarization state when light is reflected on the surface of the sample, and measures the film thickness, optical constant (refractive index, extinction coefficient), etc., which are the characteristics of the sample. It is a device that can do.

FIG. 1 shows the overall configuration of the ellipsometer 100.
As shown in FIG. 1, the ellipsometer 100 includes an illumination optical system 10 and an imaging optical system 30. The surface of the sample to be observed is represented as the object plane S.

  The illumination optical system 10 includes a light source 12, a collimator lens 14, a condenser lens 16, a crystal polarizer 18, and a liquid crystal polarizer 20. The illumination light L1 emitted from the light source 12 is converted by the collimator lens 14 into illumination light L2 that is parallel light. The illumination light L 2 is condensed on the object plane S by the condenser lens 16 after the amount of light is adjusted by the iris diaphragm 15. The illumination light L2 that has passed through the condenser lens 16 becomes linearly polarized light by the crystal polarizer 18. By electrically controlling the liquid crystal polarizer 20, linearly polarized light can be made circularly or elliptically polarized, or the direction of polarization can be rotated. Thereby, the illumination light L2 can be converted into illumination light L3 having various polarization states. The illumination light L3 is applied to the object plane S from an oblique direction. For example, the illumination light L3 is emitted from a direction of about 70 degrees with respect to the normal N extending from the object plane S. An ellipsometer provided with such an illumination optical system is known and disclosed in, for example, Japanese Patent Application Laid-Open No. 2011-102731.

Next, the imaging optical system 30 will be described.
In the ellipsometer 100 of this embodiment, the imaging optical system 30 is configured by an Offner optical system that is one of the equal-magnification reflective imaging optical systems.

  As shown in FIG. 1, the imaging optical system 30 includes a primary mirror 32 configured by a concave mirror, a secondary mirror 34 configured by a convex mirror, and a lead flat mirror 36. An analyzer 38 is disposed on the optical path between the object plane S and the primary mirror 32. The reflected light beam from the object surface S enters the analyzer 38. The reflected light beam that has passed through the analyzer 38 is reflected in the order of the primary mirror 32, the secondary mirror 34, the primary mirror 32, and the extraction plane mirror 36, and then forms an image on the light receiving surface 40 of the image sensor. Yes.

The object surface S and the light receiving surface 40 are in a conjugate relationship of equal magnification in the Offner optical system.
The object surface S is the surface of an object to be observed, for example, the surface of a substrate on which a thin film is formed.
The light receiving surface 40 is a surface on which a light flux of light reflected by the object surface S forms an image, and is, for example, a light receiving surface of an imaging element such as a two-dimensional CCD.
The secondary mirror 34 is a pupil of the optical system.
For example, a CCD element having a size of 10 μm square receives light from a 10 μm square region of the object plane having a 1 × conjugate relationship. Thereby, the film thickness and optical constant of the sample placed on the object plane can be measured in units of 10 μm square.
When the aberration of the imaging optical system is large, the light beam from the 10 μm square region of the object surface reaches not only the 10 μm square CCD element having a conjugate relationship but also the adjacent CCD element. As a result, the measurement accuracy of the film thickness per unit area and the optical constant deteriorates.

  As shown in FIG. 1, the light flux from the object surface S toward the concave primary mirror 32 is telecentric. The light reflected by the primary mirror 32 is reflected by the convex secondary mirror 34 that also serves as a stop. The light reflected by the secondary mirror 34 is reflected again by the concave primary mirror 32 and becomes telecentric. The light reflected by the main mirror 32 and made telecentric is reflected by the extraction plane mirror 36 and forms an image on the light receiving surface 40 at the same magnification.

  The crystal polarizer 18 and the analyzer 38 are constituted by a Glan-Thompson prism (calcite prism). Note that the crystal polarizer 18 and the analyzer 38 may be configured by a polarizer other than the Glan-Thompson prism.

  Glass windows (an entrance window 42 and an exit window 44) for preventing dust and the like from entering the interior of the barrel are provided at the entrance and exit of the barrel for storing the primary mirror 32 and the secondary mirror 34. is set up. The entrance window 42 and the exit window 44 are made of, for example, a glass plate having a predetermined thickness.

  A plurality of optical elements having at least a pair of parallel planes are arranged on the optical path of the ellipsometer 100 shown in FIG. For example, the crystal polarizer 18 disposed on the optical path of the illumination optical system 10 has at least a pair of parallel planes (light incident surface and light exit surface). The analyzer 38 disposed on the optical path of the imaging optical system 30 also has at least a pair of parallel planes (light incident surface and light exit surface). Further, the entrance window 42 and the exit window 44 arranged at the entrance and exit of the lens barrel are constituted by a glass plate having at least a pair of parallel planes. When an optical element having such a parallel plane is arranged on the convergent optical path, there is a problem that aberration of an image formed by the imaging optical system becomes large.

FIG. 2 is an enlarged cross-sectional view of the secondary mirror 34.
As shown in FIG. 2, the front surface 34a and the back surface 34b of the secondary mirror 34 are spherical and convex. For example, the secondary mirror 34 may be made of synthetic quartz glass having a predetermined thickness. The secondary mirror 34 may be made of a material other than quartz glass. For example, the secondary mirror 34 may be made of a glass material having a high transmittance in a desired wavelength region.

  In the ellipsometer 100 of the present embodiment, the secondary mirror 34 is constituted by a back mirror that reflects light at the back surface 34b.

  An antireflection film for preventing reflection of light may be formed on the surface 34 a of the secondary mirror 34. The antireflection film may be a known antireflection film, and may be, for example, a single-layer antireflection film with little wavelength dependency, or a multilayer antireflection film having high transmittance in a desired wavelength region. .

  The back surface 34b of the secondary mirror 34 is subjected to a process for reflecting light. For example, a metal reflective film typified by aluminum with little wavelength dependency may be deposited on the back surface 34b of the secondary mirror 34. Alternatively, a multilayer dielectric reflection film having a high reflectance in a desired wavelength region may be deposited on the back surface 34b of the sub mirror 34.

  The present inventor conducted research on a technique for solving an aberration problem caused by an optical element having a parallel plane arranged on an optical path in an ellipsometer to which an equal magnification reflection type imaging optical system is applied. It was. As a result, it has been surprisingly found that such a problem can be solved by using a back mirror as the convex secondary mirror 34.

  The inventor has also found that the aberration of the image formed by the imaging optical system 30 can be reduced by adjusting the thickness of the sub mirror 34. Here, “thickness” means “thickness T” at the center of the secondary mirror 34.

  Furthermore, the present inventor makes the aberration R of the image formed by the imaging optical system 30 smaller by making the curvature R1 of the front surface 34a of the secondary mirror 34 different from the curvature R2 of the back surface 34b of the secondary mirror 34. I found what I can do.

  According to the present invention, in an observation device such as an ellipsometer, it is possible to solve the problem that the aberration of an image formed by the equal magnification reflection type imaging optical system becomes large.

  As described above, the imaging optical system 30 for imaging the light from the object plane S on the light receiving surface 40 is constituted by an Offner optical system which is one of the equal magnification reflection type imaging optical systems. Such a reflective optical system has an advantage that the wavelength of light used for observation of the object plane S is not limited, unlike a refractive lens. For this reason, since the wavelength of light used for observation is not limited, the present invention may be applicable to various fields such as semiconductors and biotechnology.

The present invention can also be applied to observation devices other than ellipsometers.
The present invention can be applied to, for example, a microscope for observing an object plane.
The present invention can be applied, for example, to an apparatus for detecting an object plane defect.
The present invention can be applied to an apparatus for measuring the reflectance of an object surface, for example.
In addition to these devices, the present invention has the possibility of being applicable to optical observation devices in general.

[Example]
The theoretical value and design value of the resolution (MTF) when the object plane S is observed with the ellipsometer 100 shown in FIG. 1 were obtained by simulation. The incident angle of the illumination light was set to 60 degrees with respect to the normal of the object surface.

The simulation conditions of Examples 1 to 10 are as shown in Table 1 below.
In Table 1, “window thickness” means the thickness of the entrance window and the exit window.

[Comparative example]
Similar to the example, the theoretical value and design value of the resolution (MTF) when the object plane S was observed with the ellipsometer 100 shown in FIG. 1 were obtained by simulation. However, in the comparative example, the thickness of the secondary mirror was set to 0 mm, and the condition that the secondary mirror was not a back mirror but a front mirror was reflected.

The results of Examples 1 to 10 are shown in FIGS.
The results of Comparative Examples 1 to 8 are shown in FIGS.
3 to 20, the ST graph indicates the theoretical value of the sagittal plane. The SD graph shows the design value of the sagittal plane. The graph of TT shows the theoretical value of the tangential surface. The TD graph shows the design value of the tangential surface.

  3 to 20, when the difference between the theoretical value of the resolution (MTF) and the design value is large, it means that the aberration is large. When the difference between the theoretical value of the resolution (MTF) and the design value is small, it means that the aberration is small.

  As can be seen from comparison between Examples 1 to 4 and Comparative Examples 1 to 4, the use of a back mirror as a secondary mirror in the equal magnification reflection type imaging optical system reduced the aberration.

  As can be seen from comparison between Examples 6 to 8 and Comparative Examples 5 to 7, the use of a back mirror as a secondary mirror in the equal magnification reflection type imaging optical system reduced the aberration.

  As can be seen from a comparison between Example 4 and Example 5, the aberration was further reduced by making the curvature R1 of the surface of the secondary mirror different from the curvature R2 of the back surface of the secondary mirror. In particular, it has been found that the aberration becomes smaller by making the curvature R1 of the surface of the secondary mirror smaller than the curvature R2 of the back surface of the secondary mirror.

  As can be seen from Examples 1 to 10, the aberration can be reduced by adjusting the thickness of the sub-mirror according to the thickness of the entrance window and the exit window. For example, the aberration can be further reduced by adjusting the thickness of the secondary mirror so that it is substantially proportional to the thickness of the entrance window and the exit window.

DESCRIPTION OF SYMBOLS 10 Illumination optical system 12 Light source 14 Collimator lens 16 Condensing lens 18 Crystal polarizer 20 Liquid crystal polarizer 30 Imaging optical system 32 Primary mirror 34 Secondary mirror 36 Extraction plane mirror 38 Analyzer 40 Light-receiving surface 42 Entrance window 44 Output window 100 Ellipsometer

Claims (5)

An observation apparatus provided with an imaging optical system for imaging a light flux of reflected light from the surface of an object on a light receiving surface,
The imaging optical system includes a concave primary mirror, a convex secondary mirror, and an extraction flat mirror, and the reflected light beam from the surface of the object is converted into the order of the concave primary mirror, the convex secondary mirror, and the concave primary mirror. And is composed of a 1 × -magnification type imaging optical system that can form an image on the light receiving surface via the extraction flat mirror,
The observation apparatus, wherein the convex secondary mirror is a back mirror.   The observation apparatus according to claim 1, wherein the curvature of the surface of the convex secondary mirror is different from the curvature of the back surface.   The observation apparatus according to claim 1, wherein a curvature of a surface of the convex secondary mirror is smaller than a curvature of a back surface of the convex secondary mirror.   The observation apparatus according to any one of claims 1 to 3, wherein at least one optical element having a parallel plane is disposed on an optical path from the surface of the object to the light receiving surface.   The observation apparatus according to claim 1, wherein the observation apparatus is an ellipsometer.

Images