JP2013174844A - Equal-magnification reflection-type imaging optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an equal-magnification reflection-type imaging optical system that enables reflection light from a measured thin film surface of a predetermined wavefront aberration and a broad frequency band to be imaged on a light receiving element (for example, two-dimensional CCD) over a wider area.SOLUTION: An imaging optical system 40 is composed of the equal-magnification reflection-type imaging optical system that causes a flux of light from an object surface to be reflected in order of a concave surface main mirror 44, a convex surface sub-mirror 46 and the concave surface main mirror 44 via an introduction plane surface mirror 42 and thereafter enables the flux of light to be imaged on a light receiving surface of a light receiving element 30 via a drawing plane surface mirror 48. The object surface and the light receiving surface are inclined with respect to an optical axis of light incident on the respective surfaces. A perpendicular line of the object surface is inclined at 45 to 70 degrees with respect to an optical axis of incident light from an illumination optical system. A perpendicular line of the light receiving surface is inclined at 45 to 70 degrees with respect to an optical axis of the reflection light from the object surface.

Description

  The present invention relates to an equal magnification reflection type imaging optical system.

  In the manufacturing process of a semiconductor or a liquid crystal panel, patterning is performed on a thin film formed on the surface of a substrate by photolithography. An ellipso method is used to measure the optical constants such as the thickness, refractive index, and extinction coefficient of the thin film formed on the substrate.

  In a general ellipsometer, light is irradiated with an incident angle of 45 to 70 degrees with respect to the thin film surface to be measured, and reflected light from the surface of the thin film and the substrate surface is received by the light receiving element with a reflection angle of 45 to 70 degrees. For example, when linearly polarized light such as a laser is incident on the measurement thin film surface, the reflected light on the surface of the thin film and the reflected light on the substrate surface interfere in various ways and are reflected on the optical path as reflected light having different phases. It reaches the light receiving element through the element. By measuring changes in the polarization states (= Ψ and Δ) of incident light and reflected light, optical constants such as the thickness of the thin film, the refractive index of the thin film, and the extinction coefficient can be obtained by calculation.

  An ellipsometer that measures a two-dimensional plane of a finely patterned semiconductor or liquid crystal panel at multiple points does not appear to have been proposed.

  Even at one point measurement, if the ellipsometer or the measurement thin film surface is moved in the XY two dimensions, multipoint measurement on a two-dimensional plane becomes possible. However, it takes an enormous amount of time, is inefficient, and cannot be measured dynamically.

  The ellipsometer requires an incident-reflection angle imaging optical system of 45 to 70 degrees, but the reflected light from the thin film surface to be measured having a large area and a predetermined wavefront aberration and a wide wavelength band is applied to a light receiving element (for example, a two-dimensional CCD). There was a problem that it was difficult to form an image.

  An ellipsometer that measures a collective area using a two-dimensional light receiving element is called an imaging ellipsometer. However, since the performance of the incident / reflecting angle imaging optical system is limited to a small area, it is necessary to move the imaging ellipsometer or the measuring thin film surface in XY two dimensions in order to measure a large area.

JP 2009-92389 A

  The present invention has been made in view of the above circumstances, and forms an image of reflected light from a thin film surface to be measured having a predetermined wavefront aberration and a wide wavelength band in a large area on a light receiving element (for example, a two-dimensional CCD). An object of the present invention is to provide an equal-magnification reflective imaging optical system that can be used.

Means for solving the above problems are the following inventions.
The reflected light beam from the object surface is reflected in the order of the concave primary mirror, the convex secondary mirror, and the concave primary mirror via the introduction plane mirror, and then reflected on the light receiving surface of the light receiving element via the extraction plane mirror. An equal-magnification reflective imaging optical system that can form an image,
An equal magnification reflection type imaging optical system, wherein the object surface and the light receiving surface are inclined with respect to an optical axis of light incident on each surface.

  In the equal-magnification reflective imaging optical system of the present invention, the normal of the object plane is inclined by 45 to 70 degrees with respect to the optical axis of incident light from the illumination optical system, and the normal of the light receiving surface is It is preferable to be inclined by 45 to 70 degrees with respect to the optical axis of the reflected light from the object surface.

  In the equal magnification reflection type imaging optical system of the present invention, the illumination optical system preferably includes a rod, a condenser lens, an aperture stop, and a collimator lens.

  According to the present invention, an equal magnification reflection type imaging optical system capable of forming an image of reflected light from a thin film surface to be measured having a predetermined wavefront aberration and a wide wavelength band in a large area on a light receiving element (for example, a two-dimensional CCD). Can be provided.

1 is an overall view of a spectroscopic ellipsometer including an equal magnification reflection type imaging optical system. It is a YZ plan view of an imaging optical system. FIG. 3 is an XZ plan view of the imaging optical system. It is a calculation result of the wavefront aberration in the imaging surface of a substrate surface. The resolution (MTF) is calculated with NA = 0.04, wavelength: 0.550 nm (weight = 1.0), 0.250 nm (weight = 1.0), 0.800 nm (weight = 1.0). . 1 is an overall view of a spectroscopic ellipsometer in which an illumination optical system is configured by an Offner optical system.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is an overall view of a spectroscopic ellipsometer 10 provided with an equal magnification reflection type imaging optical system according to an embodiment of the present invention.
As shown in FIG. 1, the spectroscopic ellipsometer 10 is an illumination for making polarized light (hereinafter referred to as “polarized light”) incident on a substrate S on which a thin film (either a single layer film or a multilayer film) is formed. Optical system 20, light receiving element 30 for receiving reflected light of polarized light from substrate S (or thin film), and light reflected from substrate S (or thin film) to form an image on the light receiving surface of light receiving element 30. The imaging optical system 40 is provided. The surface of the substrate S corresponds to the “object plane” of the present invention.

  The spectroscopic ellipsometer 10 according to the present embodiment is characterized in that the imaging optical system 40 is configured by an Offner optical system which is one of the equal-magnification reflective imaging optical systems.

FIG. 2 is a YZ plan view of the imaging optical system 40. FIG. 3 is an XZ plan view of the imaging optical system 40.
As shown in FIGS. 2 and 3, the imaging optical system 40 constituted by an Offner optical system includes an introduction plane mirror 42, a primary mirror 44 constituted by a concave mirror, a secondary mirror 46 constituted by a convex mirror, and a lead plane. A mirror 48 is provided. Reflected light from the surface of the substrate S (or thin film) is reflected in the order of the introduction plane mirror 42, the main mirror 44, the sub mirror 46, the main mirror 44, and the extraction plane mirror 48, and then received by the light receiving element 30. An image is formed on the surface.

The surface of the substrate S and the light receiving surface of the light receiving element 30 are in a conjugate relationship of the same magnification in the Offner optical system.
The secondary mirror 46 is a pupil of the optical system.
The light receiving element 30 is configured by an image pickup element such as a two-dimensional CCD.

  As shown in FIG. 2, the perpendicular line N <b> 1 on the surface of the substrate S is inclined by 60 degrees with respect to the optical axis L <b> 1 of incident light from the illumination optical system 20. The perpendicular line N2 of the light receiving surface of the light receiving element 30 is also inclined by 60 degrees with respect to the optical axis L2 of the reflected light from the substrate S.

  As shown in FIG. 3, the region R1 where the surface of the substrate S is irradiated with polarized light is substantially rectangular and has a size of 24.6 mm × 18.5 mm. The short side of the region R1 is inclined by 90-60 = 30 degrees with respect to the optical axis L1 of the incident light from the illumination optical system 20. Further, the long side of the region R1 is line symmetric with respect to the optical axis L1 of the incident light from the illumination optical system 20.

  As shown in FIG. 3, the region R2 where the light reflected from the substrate S is irradiated onto the light receiving surface of the light receiving element 30 is substantially rectangular and has a size of 24.6 mm × 18.5 mm. The short side of the region R2 is inclined by 90-60 = 30 degrees with respect to the optical axis L2 of the reflected light from the substrate S. Further, the long side of the region R2 is line symmetric with respect to the optical axis L2 of the reflected light from the substrate S.

  As shown in FIG. 1, the illumination optical system 20 includes a rod lens 22, a condenser lens 24, an aperture stop 26, and a collimator lens 28.

  Light emitted from a light source (not shown) enters the incident end 22 a of the rod lens 22. The light incident on the incident end 22 a of the rod lens 22 is emitted from the exit end 22 b of the rod lens 22 after the illuminance is made uniform by repeating multiple reflections inside the rod lens 22.

  The light emitted from the exit end 22 b of the rod lens 22 passes through the aperture stop 26 and the collimator lens 28 and then enters the surface of the substrate S at an incident angle of 60 degrees.

  In the present embodiment, the imaging optical system 40 is configured by a telecentric optical system. The illumination optical system 20 is also configured by a telecentric optical system in accordance with the imaging optical system 40. The illumination optical system 20 is designed so that a telecentric beam with NA (numerical aperture) = 0.04 can be incident on the substrate S.

  The surface (object surface) of the substrate S and the emission end 22b of the rod lens 22 have an optically conjugate relationship.

  As shown in FIG. 1, since the surface of the substrate S is inclined with respect to the optical axis of the incident light, the exit end 22b of the conjugate rod lens 22 is also inclined with respect to the optical axis of the incident light.

  The size of the imaging surface on the surface of the substrate S (that is, the imaging surface of the illumination optical system 20) is 5.5 times the size of the exit end 22b of the rod lens 22. That is, the reduction magnification from the imaging surface to the exit end 22b of the rod lens 22 is 1 / 5.5. Therefore, when the incident angle of the incident light on the surface of the substrate S is 60 degrees, the inclination of the exit end 22b of the rod lens 22 is 16 degrees with respect to the optical axis of the emitted light.

  The size and exit angle (= NA) of the exit end 22b of the rod lens 22 are determined by the magnification with respect to the image plane. The length of the emission end 22b in the coordinate X direction (longitudinal direction) is 24.6 mm / 5.5 = 4.5 mm. The length of the output end 22b in the coordinate Y direction (short direction) is 18.5 mm × cos 60 degrees / 5.5 = 1.7 mm. The NA required for the emission end 22b is 0.04 × 5.5 = 0.22 in both the longitudinal direction and the lateral direction.

Here, a case is considered where the light beam incident on the incident end 22a of the rod lens 22 is a light beam having a narrow wavelength width emitted from a slit of a spectrometer (not shown).
It is assumed that the optical characteristics of the spectroscope are f = 320 mm, F / 4.1 (NA = 0.06), and grating: 1200 lines / mm.

A light flux having a wavelength width of 5 nm is emitted from a 1 mm wide slit of the spectroscope.
The light beam emitted from the slit of the spectroscope is received by a bundle fiber (a bundle of optical fibers).
The bundle fiber has an incident end shape of 1 mm × 20 mm and an output end shape of 2.79 mm × 7.17 mm.
The shape of the incident end of the rod lens 22 is 1.75 mm × 4.5 mm.
The cross-sectional shape of the exit end of the bundle fiber is similar to the cross-sectional shape of the incident end 22 a of the rod lens 22.

NA = 0.06 at the exit end of the bundle fiber.
Using a lens system or the like, the light beam emitted from the exit end of the bundle fiber is optically reduced to ¼ and converted to NA = 0.24.
The light beam emitted from the exit end of the bundle fiber is incident on the entrance end 22 a of the rod lens 22. The size of the incident light irradiation surface at the incident end 22a is 0.7 mm × 1.79 mm.
The light beam incident on the rod lens 22 is repeatedly reflected inside the rod lens 22. Thereby, the illuminance of the light emitted from the emission end 22b of the rod lens 22 is made uniform.
NA = 0.24 at the exit end 22 b of the rod lens 22.

(Image conjugate)
The exit end 22 b of the rod lens 22 is image conjugate with the surface (object surface) of the substrate S and the light receiving surface of the light receiving element 30. The illuminance between the surface of the substrate S in a conjugate relationship and the light receiving surface of the light receiving element 30 is kept uniform.

(Pupil conjugate)
The aperture stop 26 and the secondary mirror 46 are pupil conjugates.
The illumination light flux is limited by the diaphragm placed on the aperture diaphragm 26, and the NA (= 0.04) of the light irradiated on the surface of the substrate S is determined.
The light beam reflected by the surface of the substrate S is again limited by the sub mirror 46 of the imaging optical system 40 and is irradiated on the light receiving surface of the light receiving element 30 at an angle of NA (= 0.04).

(Meaning of image conjugation)
If the illuminance unevenness on the surface (object surface) of the substrate S is small, thin film information necessary for measurement by the ellipso method can be obtained uniformly at each point on the measured side.
Similarly, if the illuminance unevenness on the light receiving surface of the light receiving element 30 is small, the received light amount width becomes uniform and the calculation of the thin film information becomes easy.

(Meaning of pupil conjugate)
The light beam incident on the incident end 22a of the rod lens 22 includes a light beam exceeding NA necessary for image formation. The extra light beam is repeatedly reflected by the imaging optical system 40 (Offner optical system) and reaches the light receiving surface of the light receiving element 30 as stray light, which causes the S / N of the light receiving element to deteriorate. The pupil stop has a role of cutting such extra light flux.

FIG. 4 shows the calculation result of the wavefront aberration in the imaging plane of the surface of the substrate S. At each point in the plane, the wavefront aberration is within 0.01 Waves, and measurement by the ellipso method is possible.

In FIG. 5, the resolution (MTF) is calculated with NA = 0.04, wavelength: 0.550 nm (weight = 1.0), 0.250 nm (weight = 1.0), 0.800 nm (weight = 1.0). It is the result.
The higher curve is the MTF in the coordinate X direction (longitudinal direction), which matches the theoretical value.
The lower curve is the MTF in the coordinate Y direction (short direction), and the MTF value decreases as NA decreases, but it matches the theoretical value.

In the conventional ellipso measurement at one point, the size of the measurement point is rarely discussed.
The ellipsometer of the present invention aims to finely measure a thin film at each point in the plane of the thin film. Therefore, the resolution at each point to be measured must be good.

In the present invention, the resolution of the light receiving element 30 is preferably about 10 μm × 10 μm.
It is desirable that the resolution of each point on the surface to be measured (object surface) having the same magnification conjugate relationship is also on the order of 10 μm.
At NA = 0.04, the MTF value of 10 μm decomposition in the coordinate X direction (longitudinal direction) is 58%, which is sufficient, and the MTF value of 10 μm decomposition in the coordinate Y direction (short direction) is 25%, which is a little short. .

What should be noted here is the angle of incidence on the substrate S (object plane).
In general, a two-dimensional light receiving element placed on an inclined imaging surface must have a large light receiving sensitivity characteristic.

A commonly used CCD has a condensing lens called a micro lens incorporated in front of each element in order to increase the light receiving efficiency.
However, it is difficult to collect a light beam having a large inclination with a micro lens, and many light beams do not reach the light receiving surface of the CCD.
Therefore, the two-dimensional CCD used as the light receiving element 30 is preferably a type in which a micro lens is not incorporated.

To increase the resolution, the NA can be increased by a factor of approximately 0.07.
In this case, the 10 μm resolution MTF in the short direction is 50%, and the wavefront aberration in the surface to be measured is within 0.2 Waves, so that the ellipso measurement is possible.
However, if NA is increased too much, the angular width of the light beam impinging on the thin film becomes large, and the S / N of the thin film information used for ellipso measurement may be deteriorated.

Reflective optical systems have the feature of being independent of wavelength.
Since the imaging optical system 40 of the present invention is also an equal magnification reflection type imaging optical system, it has a feature that does not depend on the wavelength.

FIG. 4 shows wavefront aberrations at wavelengths of 0.550 μm, 0.250 μm, and 0.800 μm. As shown in FIG. 4, the wavefront aberration increases in proportion to the reciprocal of the wavelength.

In the ellipso measurement, it is desirable to use a wavelength width of at least one cycle because light and dark occur for each multiple of wavelengths.
When discussing the wavelength width, it is necessary to consider that all of the constraints on the light source wavelength, the transmittance and extinction coefficient of the thin film to be inspected, and the spectral sensitivity of the light receiving element satisfy the specifications.
Assuming one cycle, the wavelength width may be limited to 250-500 nm or 400-800 nm.

The illumination optical system 20 shown in FIG. 1 is generally called a Koehler illumination system, and gives the merit of the conjugate relationship to the imaging system.
However, the illumination optical system 20 does not need to be configured only by a refractive lens, and may be configured by, for example, an Offner optical system that is one of the equal-magnification reflective optical systems, as shown in FIG. .

  In the embodiment described above, an example in which a spectroscope is used as a light source has been shown, but other light sources may be used. For example, instead of a spectroscope, a light source in which a plurality of interference filters are combined with a white light source may be used.

  In the above embodiment, the normal line N1 on the surface of the substrate S is tilted by 60 degrees with respect to the optical axis L1 of the incident light. It may be an angle.

In the above embodiment, an example in which the perpendicular line N2 of the light receiving surface of the light receiving element 30 is tilted by 60 degrees with respect to the optical axis L2 of the reflected light from the substrate S has been shown. Any tilt angle may be used.

  In the above-described embodiment, an example in which the equal-magnification reflective imaging optical system of the present invention is applied to the spectroscopic ellipsometer 10 has been described. It is also possible to apply the same magnification reflection type imaging optical system.

The present invention may be configured as follows.
A spectroscopic ellipsometer,
An illumination optical system for making polarized light incident on a substrate on which a thin film is formed;
A light receiving element for receiving reflected light from the substrate,
The reflected light beam from the substrate is reflected in the order of the concave primary mirror, the convex secondary mirror, and the concave primary mirror through the introduction plane mirror, and then the light receiving surface of the light receiving element through the extraction plane mirror A spectroscopic ellipsometer comprising an equal-magnification reflection-type imaging optical system capable of forming an image on a mirror.

DESCRIPTION OF SYMBOLS 10 Spectroscopic ellipsometer 20 Illumination optical system 22 Rod lens 24 Condenser lens 26 Aperture stop 28 Collimator lens 30 Light receiving element 40 Imaging optical system (equal magnification reflection type imaging optical system)
42 Introduction plane mirror 44 Primary mirror 46 Secondary mirror 48 Drawer plane mirror

Claims (3)

The reflected light beam from the object surface is reflected in the order of the concave primary mirror, the convex secondary mirror, and the concave primary mirror via the introduction plane mirror, and then reflected on the light receiving surface of the light receiving element via the extraction plane mirror. An equal-magnification reflective imaging optical system that can form an image,
An equal magnification reflection type imaging optical system, wherein the object surface and the light receiving surface are inclined with respect to an optical axis of light incident on each surface. The normal of the object plane is inclined by 45 to 70 degrees with respect to the optical axis of incident light from the illumination optical system,
2. The equal-magnification reflective imaging optical system according to claim 1, wherein a perpendicular of the light receiving surface is inclined by 45 to 70 degrees with respect to an optical axis of reflected light from the object surface.   3. The equal-magnification reflective imaging optical system according to claim 2, wherein the illumination optical system includes a rod, a condenser lens, an aperture stop, and a collimator lens.