Patent Description:
Raman spectroscopy is advantageous in that measurement can be executed on samples of any form, for example, a gaseous form, a liquid form, a crystal form, and an amorphous form, irrespective of whether temperature is low or high, without using a special measurement atmosphere such as a vacuum atmosphere. In addition, the Raman spectroscopy is advantageous in that pretreatment of the sample can be omitted, and the sample can be measured as it is. Therefore, various measurements have been carried out utilizing these advantages. The application of the Raman spectroscopy enables identification of substances, measurement of concentration, and measurement of crystallinity, stress etc..

An optical microscope for carrying out the Raman spectroscopy is disclosed (Patent Literature <NUM> and <NUM>). The optical microscope disclosed in Patent Literature <NUM> focuses a laser light to a sample and illuminates the sample with the laser light. Then a spectroscope disperses a Raman scattered light from the sample to thereby observe Raman spectrum. Further, this optical microscope deflects a laser light, scans a beam spot on the sample, then performs measurement, to thereby measure spectrum distribution in a specific region of the sample. Further, in order to shorten the measurement period, the beam spot is extended in one direction, the resultant beam spot is scanned, the sample is illuminated in a line form, and the Raman scattered light is detected by a CCD camera. Since the sample is illuminated in a line form, a wide area can be illuminated at one time, whereby the measurement period can be reduced. Further, by extending the beam spot, it is possible to prevent the sample from being damaged.

There is a problem in the optical microscope disclosed in Patent Literature <NUM>, however, that since the light beam is refracted by the lens, chromatic aberration occurs, whereby it becomes difficult to perform measurement in a wide wavelength band. Specifically, when a common optical system is used from a deep ultraviolet region of <NUM> to a near-infrared region of <NUM> in the configuration shown in <FIG> of Patent Literature <NUM>, chromatic aberration occurs in an optical system including lenses. Chromatic aberration occurs, for example, in lenses <NUM> and <NUM> (hereinafter, these lenses are referred to as a relay optical system A) for guiding a light beam deflected by a Y-directional scanning unit <NUM> to an X-directional scanning mirror <NUM>, lenses <NUM> and <NUM> (hereinafter, these lenses are referred to as a relay optical system B) for guiding the light beam reflected in the X-directional scanning mirror <NUM> to an objective lens <NUM>, and a lens <NUM> (hereinafter it will be referred to as a focus optical system) configured to focus the outgoing light from the sample <NUM> on the entrance slit <NUM>.

In Patent Literature <NUM>, it is difficult to correct chromatic aberration that occurs in the relay optical system A, the relay optical system B, and the focus optical system to near the diffraction limit. This is because, while chromatic aberration that occurs in lenses is typically corrected by combining a plurality of types of lens materials, correction of chromatic aberration including a deep ultraviolet region from <NUM> cannot be done since many optical glasses are opaque in a deep ultraviolet region, and available lens materials are limited to a small number of types such as synthetic quartz glass or calcium fluoride.

The present disclosure has been made in view of the aforementioned problem, and aims to provide an optical microscope and a spectroscopic measurement method capable of reducing aberrations.

An optical microscope according to one aspect of the present invention is disclosed in claim <NUM>.

According to this configuration, it is possible to reduce aberration.

In the aforementioned optical microscope, a focal length of the first off-axis parabolic mirror may be equal to a focal length of the second off-axis parabolic mirror.

In the aforementioned optical microscope, when a distance from the focal point of the light beam between the first off-axis parabolic mirror and the second off-axis parabolic mirror to the first off-axis parabolic mirror is denoted by L2 and a distance from the focal point to the second off-axis parabolic mirror is denoted by L3, the ratio of L2 to L3 may be equal to the ratio of the focal length of the first off-axis parabolic mirror to the focal length of the second off-axis parabolic mirror.

In the aforementioned optical microscope, when a distance from the first scanner to the first off-axis parabolic mirror is denoted by L1 and a distance from the second off-axis parabolic mirror to an entrance pupil of the objective lens is denoted by L4, L1=L2 and L3=L4 may be satisfied. According to this configuration, it is possible to cause the light beam to pass substantially the center of the entrance pupil of the objective lens, whereby it is possible to prevent the laser intensity and the spatial resolution from being changed in the entire measurement area.

In the aforementioned optical microscope, the first relay optical system may further include: a first correcting lens having a positive power provided between the first off-axis parabolic mirror and a focal point of the light beam; and a second correcting lens having a positive power provided between the focal point and the second off-axis parabolic mirror. According to this configuration, it is possible to reduce aberrations.

In the aforementioned optical microscope, when a distance from the first correcting lens to the focal point is denoted by L5 and a distance from the focal point to the second correcting lens is denoted by L6, L5=L6 is preferably satisfied.

The aforementioned optical microscope may further include a focus optical system configured to focus the outgoing light descanned by the first scanner on the slit of the spectroscope, in which the focus optical system may include: a first concave mirror configured to reflect the outgoing light; and a first convex mirror configured to reflect the outgoing light reflected in the first concave mirror. It is therefore possible to correct the astigmatism.

In the aforementioned optical microscope, the first concave mirror and the first convex mirror may be spherical mirrors whose curvature radii are substantially equal to each other. According to this configuration, it is possible to reduce the Petzval sum and to reduce the curvature of field.

In the aforementioned optical microscope, an image plane of the sample imaged by the focus optical system may be inclined with respect to the incident plane of the slit. According to this configuration, it is possible to prevent a ghost image from being generated.

In the aforementioned optical microscope, a line that passes the center of curvature of the first concave mirror and the center of curvature of the first convex mirror may be inclined from a reference axis of the outgoing light that enters the first concave mirror.

The aforementioned optical microscope may further include: a second scanner that is provided in an optical path from the light source to the first scanner, deflects the light beam, and scans the spot position of the light beam on the sample; and a beam splitter that is provided in an optical path between the first scanner and the second scanner and separates the outgoing light emitted from the sample toward the spectroscope from the light beam emitted from the second scanner toward the first scanner, in which the first scanner may scan the spot position in a first direction that corresponds to a direction orthogonal to a longitudinal direction of the slit of the spectroscope, and the second scanner may scan the spot position in a second direction that corresponds to the longitudinal direction of the slit.

The aforementioned optical microscope may further include: a second relay optical system arranged in an optical path between the second scanner and the first scanner, in which the second relay optical system may include: a second concave mirror configured to reflect a light beam from the second scanner; a second convex mirror configured to reflect the light beam reflected in the second concave mirror; a third convex mirror configured to reflect the light beam reflected in the second convex mirror; and a third concave mirror configured to reflect the light beam reflected in the third convex mirror, and the second concave mirror and the second convex mirror may be arranged to be symmetrical with the third concave mirror and the third convex mirror with respect to an intermediate image plane which is between the second convex mirror and the third convex mirror. According to this configuration, it is possible to correct coma aberration, distortion aberration, and astigmatism.

In the aforementioned optical microscope, the second concave mirror, the second convex mirror, the third concave mirror, and the third convex mirror may be spherical mirrors whose curvature radii are substantially equal to one another. It is therefore possible to correct the curvature of field.

In the aforementioned optical microscope, a line that passes the center of curvature of the second concave mirror and the center of curvature of the second convex mirror may be inclined from a reference axis of the light beam that enters the second concave mirror, and a line that passes the center of curvature of the third concave mirror and the center of curvature of the third convex mirror may be inclined from the reference axis of the light beam reflected in the third concave mirror.

The aforementioned optical microscope may further include: a second relay optical system arranged in an optical path between the second scanner and the first scanner, in which the light source may be capable of using the laser light beams having laser wavelengths different from each other by switching the laser light beams, the optical path of the laser light may be provided with a beam expander configured to adjust the degree of focus or the degree of divergence, the second relay optical system may include: a first relay lens configured to refract the light beam from the second scanner; a second relay lens configured to refract the light beam from the first relay lens to obtain a collimated light beam, the second relay lens causing the collimated light beam to be made incident on the first scanner; and a diaphragm arranged between the first relay lens and the second relay lens. According to this configuration, it is possible to correct chromatic aberration.

The aforementioned optical microscope may further include a third scanner that is provided just before the first scanner and scans the spot position of the light beam on the sample in the second direction, in which the third scanner may scan the light beam in the second direction depending on the angle of the first scanner so as to cancel a change in the spot position that occurs by distortion aberration of the first relay optical system. According to this configuration, it is possible to correct distortion aberration.

The aforementioned optical microscope may further include a processor configured to correct distortion aberration that occurs in the first relay optical system, in which spots of the light beam on the sample may be extended in a line form along the second direction, a plurality of pixels that detect the outgoing light from the linear area may be arranged in the two-dimensional array photodetector, and one-dimensional measurement data detected by the plurality of pixels may be interpolated, whereby distortion aberration may be corrected. According to this configuration, it is possible to easily correct distortion aberration.

The aforementioned optical microscope may further include an optical member configured to shrink a cross-sectional shape of the light beam in the second scanner in the second direction. According to this configuration, it is possible to prevent the sample from being damaged.

In the aforementioned optical microscope, the second direction that corresponds to a longitudinal direction of the slit may be along a direction that corresponds to a geometric symmetry axis of the first off-axis parabolic mirror.

A spectroscopic measurement apparatus according to this embodiment may be a spectroscopic measurement method for performing spectroscopic measurement by performing the following processing of: causing a light beam to be deflected by a first scanner; causing the light beam from the first scanner to be made incident on an objective lens via a first relay optical system; focusing the light beam by the objective lens and illuminating a sample with the light beam; collecting an outgoing light emitted from the sample by the objective lens; dispersing the outgoing light from the objective lens by a spectroscope; and detecting the outgoing light dispersed by the spectroscope, in which the first relay optical system includes a first off-axis parabolic mirror that is arranged in an optical path from the first scanner to the objective lens and reflects the light beam deflected by the first scanner and a second off-axis parabolic mirror that reflects the light beam reflected in the first off-axis parabolic mirror. According to this configuration, it is possible to reduce aberration. Advantageous Effects of Invention.

According to the present disclosure, it is possible to provide an optical microscope and a spectroscopic measurement method capable of reducing aberrations.

An embodiment to which the present disclosure can be applied will be now described. The following description explains the embodiment of the present disclosure and the disclosure is not limited to the following embodiment. For ease of explanation, the following description is given in an abbreviated and simplified manner as appropriate. Those skilled in the art will recognize that components in the following embodiment can be readily changed, added, and replaced within the scope of the disclosure. Here, the same components are denoted by identical reference numerals, and description thereof is omitted as necessary.

Referring to <FIG>, an optical microscope according to an embodiment of the present disclosure will be described. <FIG> is a diagram schematically showing the overall structure of an optical system of an optical microscope <NUM> according to this embodiment. The optical microscope <NUM> includes, as the structure for observing a sample <NUM>, a light source <NUM>, a beam expander <NUM>, a Y-directional scanning unit <NUM>, a first relay optical system <NUM>, a beam splitter <NUM>, an X-directional scanning mirror <NUM>, a second relay optical system <NUM>, an objective lens <NUM>, a stage <NUM>, a focus optical system <NUM>, a spectroscope <NUM>, a detector <NUM>, a stage driver <NUM>, and a processor <NUM>. Further, the spectroscope <NUM> is provided with an entrance slit <NUM> on an incident side thereof. Even when the scanning angles by the X-directional scanning mirror <NUM> and the Y-directional scanning unit <NUM> are changed, a reference axis (optical axis) is the Z axis, the light traveling direction is a positive direction, and the X axis and the Y axis are the left-handed system. Even when the scanning angles by the X-directional scanning mirror <NUM> and the Y-directional scanning unit <NUM> are changed, the Z axis is made constant.

The optical microscope <NUM>, which is a Raman microscope, allows a light beam from the light source <NUM> to enter the sample <NUM> and detects a Raman scattered light from the sample <NUM> by the detector <NUM>. Further, the spectroscope <NUM> disperses the Raman scattered light, and thus spectroscopic measurement can be performed on Raman spectrum. In addition, the optical microscope <NUM> enables scanning in XY direction (horizontal direction) and the Z direction (vertical direction). Hence, a three-dimensional Raman spectrum image can be measured.

First, the overall structure of the optical microscope <NUM> is described with reference to <FIG>. The light source <NUM> is a laser light source that emits a monochromatic laser light. As the light source <NUM>, for example, Millennia available from Spectra Physics, inc. can be used. The light source <NUM> is an Nd/YVO4 laser with a laser wavelength of <NUM>, a laser linewidth of <NUM>, and the maximum power of <NUM> W. The light source <NUM> emits a laser light of the above laser wavelength. Further, a plurality of laser light sources may be used as the light source <NUM> while switching the laser wavelength depending on the band where spectroscopic measurement is required. That is, it is possible to use the light source <NUM> that is available by switching laser light beams having laser wavelengths different from each other.

The light beam from the light source <NUM> is expanded by the beam expander <NUM> and then enters the Y-directional scanning unit <NUM>. The Y-directional scanning unit <NUM> is, for example, an acousto-optic device or a galvano mirror. The Y-directional scanning unit changes an output angle of the incident light beam to deflect the light beam. As a result, an incident position of the light beam is moved along the Y direction on the sample <NUM>. That is, the Y-directional scanning unit <NUM> scans the light beam in the Y direction. Incidentally, a deflection angle of the light beam of the Y-directional scanning unit <NUM> is controlled in accordance with electric signals from the processor <NUM>. The light beam deflected by the Y-directional scanning unit <NUM> enters the relay optical system <NUM>. The details of the relay optical system <NUM> will be explained later.

The light beam from the relay optical system <NUM> enters the beam splitter <NUM>. The beam splitter <NUM> is, for example, a dichroic mirror, which reflects light of a laser wavelength toward the sample <NUM>. As the dichroic mirror, "edge filter" available from Semrock Inc. can be used. The light reflected by the beam splitter <NUM> enters the X-directional scanning mirror <NUM>. The X-directional scanning mirror <NUM> is, for example, a galvano mirror, and an angle of a reflection surface is changed to thereby deflect the light beam. That is, an angle of the reflection surface of the X-directional scanning mirror <NUM> to the optical axis is changed, so the output angle of the light beam can be changed. As a result, the incident position of the light beam is moved along the X direction on the sample <NUM>. Hence, the light beam can be moved in the X direction. Incidentally, the deflection angle of the light beam in the X-directional scanning mirror <NUM> is controlled in accordance with electric signals from the processor <NUM>. Here, the X direction and the Y direction are orthogonal to each other. The X-directional scanning mirror <NUM> and the Y-directional scanning unit <NUM> scan the sample in the XY direction, and thus a two-dimensional area can be scanned on the sample <NUM>.

The light beam scanned by the X-directional scanning mirror <NUM> enters the relay optical system <NUM>. The details of the relay optical system <NUM> will be explained later. The light beam from the relay optical system <NUM> enters the objective lens <NUM>. The objective lens <NUM> focuses the light beam, and causes this light beam to be made incident on the sample <NUM>. That is, the objective lens <NUM> focuses the light beam onto the sample <NUM> to illuminate the sample <NUM>. As a result, a spot-like area of the sample <NUM> is illuminated. As the objective lens <NUM>, a Schwarzschild-type reflective objective lens can be used, as will be explained later. Alternatively, a plurality of objective lenses attached to a microscope nose piece turret or the like may be used while switching them depending on the laser wavelength.

A part of the light incident on the sample <NUM> is Raman-scattered. Out of the light incident on the sample <NUM>, light emitted toward the objective lens <NUM> due to Raman scattering is referred to as "outgoing light". That is, out of the Raman scattered light, light incident on the objective lens <NUM> is the outgoing light. The wavelength of the Raman-scattered outgoing light is different from that of the incident light. That is, the outgoing light is scattered with a frequency that deviates from an incident light frequency by Raman shift. A spectrum of the outgoing light is Raman spectrum. Therefore, by measuring the spectrum of the outgoing light, the chemical structure and the physical state of substances included in the sample <NUM> can be identified. That is, the Raman spectrum includes information about the vibrational information of a substance that composes the sample <NUM>. Hence, if the outgoing light is dispersed by the spectroscope <NUM> and then detected, substances in the sample <NUM> can be identified.

Then, a focal position of the incident light is scanned in the XYZ direction to measure the spectrum of the outgoing light from all or a part of the sample <NUM>, and it is possible to execute three-dimensional measurement of the Raman spectrum. By observing a specific wavelength out of the measured Raman spectrum, three-dimensional spatial distribution of a specific substance can be measured. To be specific, if the sample <NUM> is a living cell, spatial distribution of nucleic acids or lipids can be measured.

Incidentally, the sample <NUM> is placed on the stage <NUM>. The stage <NUM> is, for example, an XYZ stage. The stage <NUM> is driven by the stage driver <NUM>. The stage driver <NUM> moves the stage <NUM> in the XY direction to thereby illuminate a desired portion of the sample <NUM>. Further, the stage driver <NUM> moves the stage in the Z direction to thereby change a distance between the objective lens <NUM> and the sample <NUM>. Accordingly, the focal position of the objective lens <NUM> can be changed along the optical axis direction. The optical microscope <NUM> of the present disclosure constitutes a laser confocal microscope as described below. Thus, Z directional scanning is realized by changing the focal position. That is, if the stage is moved in the Z direction, a tomographic image of the sample <NUM> can be taken. Further, the Raman scattered light from a predetermined height of the sample <NUM> can be detected to enable measurement of a three-dimensional Raman spectrum image. The processor <NUM> outputs a control signal to the stage driver <NUM> to control the driving of the stage <NUM>.

The outgoing light that is Raman-scattered on the sample <NUM> placed on the stage <NUM> and then enters the objective lens <NUM> propagates through the same optical path as that for the incident light. That is, the outgoing light is refracted or reflected by the objective lens <NUM> to enter the X-directional scanning mirror <NUM> via the relay optical system <NUM>. The X-directional scanning mirror <NUM> reflects the incident outgoing light toward the beam splitter <NUM>. At this time, the outgoing light is descanned by the X-directional scanning mirror <NUM>. That is, the outgoing light is reflected by the X-directional scanning mirror <NUM> and thus propagates in a direction opposite to a traveling direction of the incident light that is incident on the X-directional scanning mirror <NUM> from the light source <NUM>. Further, the Rayleigh scattered light from the sample <NUM> propagates through the same optical path as that for the Raman scattered light.

The outgoing light reflected by the X-directional scanning mirror <NUM> enters the beam splitter <NUM>. The beam splitter <NUM> is, for example, a dichroic mirror. The beam splitter <NUM> splits the outgoing light from the sample <NUM> and the incident light emitted from the light source <NUM> to the sample <NUM> based on a wavelength. That is, the beam splitter <NUM> is set with its reflection surface inclined with respect to the optical axis of the incident light. The outgoing light from the sample <NUM> is transmitted through the beam splitter <NUM>, so the optical axis of the outgoing light from the sample <NUM> is different from the optical axis of the incident light emitted from the light source <NUM> to the sample <NUM>. Hence, the outgoing light from the sample <NUM> can be separated from the incident light emitted from the light source <NUM> to the sample <NUM>.

Further, the beam splitter <NUM> as a dichroic mirror has characteristics of reflecting light of a laser wavelength and transmitting the Raman scattered light. Accordingly, Rayleigh scattered light from the sample <NUM> is reflected by the beam splitter <NUM>, and Raman scattered light is transmitted through the beam splitter <NUM>. That is, since a dichroic mirror is used as the beam splitter <NUM>, the Rayleigh scattered light can be removed by utilizing a wavelength difference between the Rayleigh scattered light and the Raman scattered light. Further, almost all of the laser light from the light source <NUM> is reflected by the beam splitter <NUM> toward the sample <NUM>. Therefore, a loss of the laser light can be minimized, and only the Raman scattered light can be efficiently detected. Incidentally, reflection characteristics of the dichroic mirror may be determined in accordance with a spectrum range as a measurement target. Here, the beam splitter <NUM> is provided between the sample <NUM> and the Y-directional scanning unit <NUM>. Thus, the beam splitter <NUM> separates the outgoing light before descanning with the Y-directional scanning unit <NUM> from the light beam from the light source <NUM>.

The outgoing light transmitted through the beam splitter <NUM> enters the entrance slit <NUM> provided on the incident side of the spectroscope <NUM> via the focus optical system <NUM>. At this time, the focus optical system <NUM> focuses the outgoing light on the entrance slit <NUM>. That is, the focus optical system <NUM> forms an enlarged image of an illuminated area of the sample <NUM> on the entrance slit <NUM>. The details of the focus optical system <NUM> will be explained later. The entrance slit <NUM> has a linear opening. This opening extends along a direction corresponding to the Y direction. That is, the opening of the entrance slit <NUM> extends along a direction corresponding to a scanning direction (Y direction) of the Y-directional scanning unit <NUM> on the sample <NUM>. Therefore, the scanning direction of the Y-directional scanning unit <NUM> corresponds to the longitudinal direction of the opening of the entrance slit <NUM>.

The focus optical system <NUM> focuses the outgoing light on the entrance slit <NUM>. Here, the incident light is focused into a spot-like image on the sample <NUM>, so the outgoing light is condensed into a spot shape on the entrance slit <NUM>. The direction in which the opening of the entrance slit <NUM> extends is matched with the scanning direction of the Y-directional scanning unit <NUM>. The outgoing light enters the beam splitter <NUM> without being descanned by the Y-directional scanning unit <NUM>. Hence, when scanning is performed by the Y-directional scanning unit <NUM>, a spot position of the light beam is moved on the entrance slit <NUM> toward the linear opening of the entrance slit <NUM>. The entrance slit <NUM> is provided such that the light moved on the sample <NUM> is focused into the opening of the entrance slit <NUM>. In other words, the entrance slit <NUM> and the illuminated area of the sample <NUM> are arranged in such a way that they are in a conjugated relation. Therefore, Raman microscope is formed as a line confocal (slit confocal) optical system.

Then, the scattered outgoing light that exits from the sample <NUM> is condensed into a spot shape on the entrance slit <NUM>. The entrance slit <NUM> has an opening extending along the Y direction, and the opening allows only the incident outgoing light to pass therethrough toward the detector <NUM>. An illumination optical system from the light source <NUM> to the sample <NUM> and an observation optical system from the sample <NUM> to the detector <NUM> are configured as the above imaging optical system to thereby complete a confocal Raman microscope. This enables measurement with high resolution in the Z direction. Moving the stage <NUM> in the Z direction, the Raman scattered light from a desired height of the sample <NUM> can be separated from the Raman scattered light from the other heights thereof and then detected.

The outgoing light passed through the entrance slit <NUM> enters a main body of the spectroscope <NUM>. The spectroscope <NUM> includes a dispersive device such as grating or prism. Thus, the spectroscope <NUM> spatially disperses the incident light from the entrance slit <NUM> in accordance with its wavelength. Regarding the spectroscope <NUM> that uses a reflective grating, there is additionally provided an optical system inclusive of a concave mirror guiding light from the entrance slit <NUM> to the dispersive device and a concave mirror guiding the light diffracted by the dispersive device to the detector <NUM>. Needless to say, the structure of the spectroscope <NUM> is not limited to the above one. The outgoing light is dispersed by the spectroscope <NUM> toward a direction perpendicular to the direction of the entrance slit <NUM>. That is, the spectroscope <NUM> executes wavelength dispersion of the outgoing light toward a direction perpendicular to the linear opening of the entrance slit <NUM>. The outgoing light dispersed by the spectroscope <NUM> enters the detector <NUM>. The detector <NUM> is an area sensor where light receiving devices are arranged in matrix. More specifically, the detector <NUM> is a two-dimensional array photodetector having arrayed pixels such as a two-dimensional CCD camera. The spectroscope <NUM> can disperse, for example, the wideband Raman scattered light from a deep ultraviolet region of <NUM> to a near-infrared region of <NUM>. The spectroscope <NUM> may be a Fourier spectroscope. In this case, the outgoing light is not spatially dispersed in accordance with its wavelength and the detector <NUM> detects the outgoing light.

The detector <NUM> may be, for example, a cooled CCD for detection of light in a range from <NUM> to <NUM>. Specifically, <NUM>×<NUM>-pixel electric cooled CCD (cooling temperature -<NUM>) available from Princeton Instruments, inc. can be used as the detector <NUM>. Further, an image intensifier may be attached to the detector <NUM>. An InGaAs camera can be used for detection of light in a range from <NUM> to <NUM>. By switching the detector <NUM> depending on the bandwidth where spectroscopic measurement is to be performed, the detector <NUM> can be used for wideband spectrum measurement. Pixels of the detector <NUM> are arranged along a direction that corresponds to the entrance slit <NUM>. Therefore, one arrangement direction of the pixels of the detector <NUM> coincides with the direction of the entrance slit <NUM>, and the other arrangement direction coincides with the dispersion direction of the spectroscope <NUM>. The direction of the detector <NUM> that corresponds to the direction of the entrance slit <NUM> is the Y direction (second direction), and the direction perpendicular to the entrance slit <NUM>, that is, the direction in which the outgoing light is dispersed by the spectroscope <NUM>, is the X direction (first direction).

As described above, the spectroscope disperses the wideband Raman scattered light from the deep ultraviolet region of <NUM> to the near-infrared region of <NUM> in the X direction. The detector <NUM> has sensitivity for the light from the deep ultraviolet region of <NUM> to the near-infrared region of <NUM>. The detector <NUM> outputs a detection signal that corresponds to the intensity of the outgoing light received by each pixel to the processor <NUM>. The processor <NUM> is, for example, an information processing unit such as a personal computer (PC). The processor stores the detection signal from the detector <NUM> in a memory or the like. Then, the processor <NUM> executes predetermined processing on the detection result, and displays the resultant on a monitor. Further, the processor <NUM> controls the scanning with the Y-directional scanning unit <NUM> and the X-directional scanning mirror <NUM> and the driving of the stage <NUM>. Here, the X direction of the detector <NUM> corresponds to the wavelength (frequency) of the outgoing light. That is, a pixel at one end of a pixel line in the X direction detects outgoing light of a long wavelength (low frequency), and a pixel at the other end detects outgoing light of a short wavelength (high frequency). In this way, the distribution of the light intensity in the X direction of the detector <NUM> is a Raman spectrum distribution.

Here, during a period in which the detector <NUM> takes an image of one frame, the Y-directional scanning unit <NUM> scans the light beam in the Y direction one or more times. That is, a scanning period of the Y-directional scanning unit <NUM> is made shorter than an exposure period, and the beam is scanned in the Y direction one or more times within the exposure period of one frame of the detector <NUM>. According to this configuration, it is possible to measure Raman spectrum in the linear area in accordance with the scanning range in one frame of the detector <NUM>. That is, the whole scanning range of the Y-directional scanning unit <NUM> is scanned within the exposure period. It is therefore possible to reduce the measurement period. Even if the Raman spectrum of, for example, a three-dimensional large area is measured, it is possible to prevent the measurement period from being long and to improve its practicability. Further, the Y-directional scanning unit <NUM> scans the light beam at a fast speed, whereby it is possible to prevent the sample from being damaged.

In this embodiment, the optical microscope <NUM> performs spectroscopic measurement on the wideband Raman scattered light from, for example, a deep ultraviolet region of <NUM> to a near-infrared region of <NUM>. Therefore, the optical system for reducing aberrations is used. For example, the relay optical system <NUM>, the relay optical system <NUM>, and the focus optical system <NUM> are formed of reflecting mirrors, as will be described later. According to this configuration, it is possible to reduce chromatic aberration, whereby the optical microscope <NUM> can be applied to spectroscopic measurement in a wide band. In the following, each of the relay optical system <NUM>, the relay optical system <NUM>, and the focus optical system <NUM> will be explained.

First, with reference to <FIG>, a configuration of the relay optical system <NUM> will be explained. <FIG> is a diagram showing a configuration of the relay optical system <NUM>. While a off-axis symmetrical optical system will be explained in the following description, the line that the light that enters the pupil of the objective lens <NUM> at an incident angle of <NUM> degrees or the principal ray of the Raman scattered light (outgoing light) emitted at <NUM> degrees from the pupil of the objective lens <NUM> passes is referred to as a reference axis OX1. Even when the scanning angles by the X-directional scanning mirror <NUM> and the Y-directional scanning unit <NUM> are changed, the reference axis is set to be the Z axis, the light traveling direction is set to be the positive direction, and the X axis and the Y axis are the left-handed system. Unless otherwise specified, the distance between the optical elements is along the reference axis.

The relay optical system <NUM> is a reflection optical system including a first off-axis parabolic mirror <NUM> and a second off-axis parabolic mirror <NUM>. The first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM>, which are concave mirrors having paraboloids, are provided in such a way that the geometric focal point P1 of the paraboloid of the first off-axis parabolic mirror <NUM> and that of the second off-axis parabolic mirror <NUM> coincide with each other. The first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> are provided in such a way that the geometric symmetry axes SX are parallel to each other and the paraboloids face in directions opposite to each other. For example, in <FIG>, the paraboloid of the first off-axis parabolic mirror <NUM> faces on the left side and the paraboloid of the second off-axis parabolic mirror <NUM> faces on the right side. In <FIG>, the geometric symmetry axis SX of the first off-axis parabolic mirror <NUM> coincides with the geometric symmetry axis SX of the second off-axis parabolic mirror <NUM>. Further, the reference axis of the light beam that enters the first off-axis parabolic mirror <NUM> and the reference axis of the light beam reflected in the second off-axis parabolic mirror <NUM> are parallel to the geometric symmetry axis SX. The Y axis is within a plane that includes the geometric symmetry axis SX of the paraboloid. Specifically, the Y axis shown in <FIG> is parallel to the symmetry axis SX between the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM>.

The light beams, which are collimated light beams reflected in the X-directional scanning mirror <NUM>, enter the first off-axis parabolic mirror <NUM>. When the light beams enter the first off-axis parabolic mirror <NUM> along the reference axis OX1 (parallel incident), the light reflected in the first off-axis parabolic mirror <NUM> is focused on the geometric focal point P1 (intermediate focal point) of the paraboloid. Then the light reflected in the first off-axis parabolic mirror <NUM> enters the second off-axis parabolic mirror <NUM>. The light reflected in the second off-axis parabolic mirror <NUM> becomes collimated light beam that is parallel to the reference axis, and enters the objective lens <NUM>. The objective lens <NUM> is arranged in such a way that the optical axis of the objective lens <NUM> becomes parallel to the geometric axis of the second off-axis parabolic mirror <NUM>.

In the relay optical system <NUM>, the focal point distance of the first off-axis parabolic mirror <NUM> and that of the second off-axis parabolic mirror <NUM> are preferably made the same. It is therefore possible to minimize the curvature of field under a condition that the scanning range in the sample <NUM>, the beam diameter of the light beam that enters the objective lens <NUM>, and the distance from the X-directional scanning mirror <NUM> to the pupil of the objective lens <NUM> are made constant. The distance from the first off-axis parabolic mirror <NUM> to the focal point P1 along the reference axis is denoted by L1, the distance from the focal point P1 to the second off-axis parabolic mirror <NUM> is denoted by L2, the distance from the X-directional scanning mirror <NUM> to the first off-axis parabolic mirror <NUM> is denoted by L3, and the distance from the second off-axis parabolic mirror <NUM> to the entrance pupil of the objective lens <NUM> is denoted by L4. More preferably, L1=L2=L3=L4 is satisfied.

When, for example, the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> have paraboloids having a focal length of <NUM> and light that enters in parallel to the reference axis is reflected at an angle of <NUM> degrees, L1=L2=L3=L4=<NUM> is satisfied. When L1=L2=L3=L4 is satisfied, even when the angle of the X-directional scanning mirror <NUM> is changed, the light beam passes substantially the center of the entrance pupil of the objective lens <NUM>. As a result, even when the sample <NUM> is scanned by the X-directional scanning mirror <NUM>, the changes in the laser intensity and spatial resolution can be prevented in the entire measurement area. As long as a slight increase in aberration is allowed, L1 and L4 may not be equal to each other and L2=L3 may be satisfied. Even under these conditions, when the angle of the X-directional scanning mirror <NUM> is changed, the light beam can be made to pass substantially the center of the entrance pupil of the objective lens. The focal length of the first off-axis parabolic mirror <NUM> and that of the second off-axis parabolic mirror <NUM> may be different from each other. In this case, the ratio of L2 to L3 is made equal to the ratio of the focal length of the first off-axis parabolic mirror <NUM> to the focal length of the second off-axis parabolic mirror <NUM>, and L1=L2 and L3=L4 are satisfied.

When the off-axis parabolic mirror alone is used, a large aberration is generated except for the case of the parallel incident. However, by providing the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> in such a way that they face in directions opposite to each other with respect to the intermediate focal point (focal point P1), it is possible to provide the relay optical system <NUM> in which aberrations are canceled and spherical aberration, coma aberration, and astigmatism are corrected. When, for example, the angle of the X-directional scanning mirror <NUM> is changed, the light beams input to the first off-axis parabolic mirror <NUM> are not parallel incident. In this case, while the spot shape spreads due to an influence of aberration in the intermediate focal point, the light beams after being reflected in the second off-axis parabolic mirror <NUM> become a substantially complete collimated light beams since aberrations are canceled.

While the angle between the light that is made incident on the first off-axis parabolic mirror <NUM> and the principal ray of the reflected light (hereinafter this angle will be referred to as a reflection angle of the first off-axis parabolic mirror <NUM>) is <NUM> degrees in <FIG>, the optical system having the aforementioned properties can be obtained also when the reflection angle of the first off-axis parabolic mirror <NUM> is other than <NUM> degrees. For example, the reflection angle of the first off-axis parabolic mirror <NUM> may another angle such as <NUM> degrees or <NUM> degrees. However, in this case, the optical component and the optical path become close to the objective lens, whereby it becomes difficult to measure a large sample. When a large sample is measured, the reflection angle is preferably set to about <NUM> degrees and the optical component and the optical path are preferably separated from the objective lens. Since the distortion aberration increases when the reflection angle is made large, when a similar relay optical system is used in a telescope or the like, the reflection angle is designed to be as small as possible. In the relay optical system <NUM>, distortion aberration can be corrected by adjustment of the angle of the X-directional scanning mirror <NUM> and data processing (correction of distortion aberration will be explained later). Therefore, the reflection angle can be set to about <NUM> degrees.

While the light beams from the X-directional scanning mirror <NUM> toward the objective lens <NUM> have been described above, in the relay optical system <NUM>, the outgoing light from the objective lens <NUM> toward the X-directional scanning mirror <NUM> has a similar property as well. In <FIG>, a configuration in which lenses are not used can be employed, whereby chromatic aberration can be eliminated.

With reference to <FIG>, a modified example of the relay optical system <NUM> will be explained. In <FIG>, a first correcting lens <NUM> and a second correcting lens <NUM> are added to the components shown in <FIG>. That is, the relay lens optical system <NUM> shown in <FIG> is a catadioptric system including the first off-axis parabolic mirror <NUM>, the second off-axis parabolic mirror <NUM>, the first correcting lens <NUM>, and the second correcting lens <NUM>. The descriptions of the configurations similar to those shown in <FIG> will be omitted as appropriate. In <FIG> as well, the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> are arranged in such a way that the geometric symmetry axes of the paraboloids become parallel to each other and the paraboloids face in directions opposite to each other. The Y axis is in a plane including the symmetry axis.

The first correcting lens <NUM> and the second correcting lens <NUM> are lenses having a positive power. The first correcting lens <NUM> is arranged in the optical path from the first off-axis parabolic mirror <NUM> to the focal point P1. The second correcting lens <NUM> is arranged in the optical path from the focal point P1 to the second off-axis parabolic mirror <NUM>. That is, the light beam reflected in the first off-axis parabolic mirror <NUM> enters the first correcting lens <NUM>. The light beam refracted by the first correcting lens <NUM> enters the second correcting lens <NUM>. The light beam refracted by the second correcting lens <NUM> enters the second off-axis parabolic mirror <NUM>. The light beam reflected in the second off-axis parabolic mirror <NUM> enters the objective lens <NUM>. The first off-axis parabolic mirror <NUM> and the first correcting lens <NUM> focus the light beam on the focal point P1.

By adding the first correcting lens <NUM> and the second correcting lens <NUM>, each having an optimized positive power, it is possible to correct the curvature of field that the optical system shown in <FIG> has. The first correcting lens <NUM> and the second correcting lens <NUM> are preferably arranged in the vicinity of the focal point P1 as much as possible. According to this configuration, the chromatic aberration occurred by the first correcting lens <NUM> and the second correcting lens <NUM> can be made negligibly small. Further, the material of the first correcting lens <NUM> and the second correcting lens <NUM> may be preferably synthetic quartz glass or calcium fluoride. According to this configuration, the relay optical system <NUM> can be used in a wide wavelength band including a deep ultraviolet region.

The first correcting lens <NUM> and the second correcting lens <NUM> are added between the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM>. Therefore, the geometric focal point of the paraboloid of each of the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> does not coincide with the focal point P1 of the light beam, unlike <FIG>.

The distance from the first correcting lens <NUM> to the focal point P1 is denoted by L5 and the distance from the focal point P1 to the second correcting lens <NUM> is denoted by L6. Under a condition that the reflected light of the second off-axis parabolic mirror <NUM> becomes collimated light beam that is parallel to the geometric axis of the second off-axis parabolic mirror <NUM> when collimated light beams enter the first off-axis parabolic mirror <NUM> in parallel to each other, L5=L6 and L2=L3 are preferably satisfied.

By arranging the first off-axis parabolic mirror <NUM> and the first correcting lens <NUM> to be symmetrical with the first correcting lens <NUM> and the second correcting lens <NUM> with respect to the focal point P1, coma aberration and astigmatism can be canceled. In the relay optical system <NUM> in <FIG> as well, when L1=L4, L1 is preferably such a length that the light beam passes substantially the center of the entrance pupil of the objective lens <NUM> even when the angle of the X-directional scanning mirror <NUM> is changed. This L1 can be obtained by repeating tracing of rays that are inclined from the position of the X-directional scanning mirror <NUM> and acquisition of the intersection with the reference axis by changing L1. According to this configuration, it is possible to prevent the laser intensity and spatial resolution from being changed in the entire measurement area even when the X-directional scanning mirror <NUM> scans the sample <NUM>.

It is assumed, for example, that light beams that enter the reference axis OX1 in parallel to each other are reflected in the direction of <NUM> degrees by the first off-axis parabolic mirror <NUM> using the first off-axis parabolic mirror <NUM> and the second off-axis parabolic mirror <NUM> having paraboloids with a focal length of <NUM>. Further, a plano-convex lens made of synthetic quartz glass having a focal length of <NUM> and a central thickness of <NUM> is used for the first correcting lens <NUM> and the second correcting lens <NUM>. In this case, when L1=L4=<NUM>, L2=L3=<NUM>, and L5=L6=<NUM> are satisfied, the aforementioned conditions can be satisfied. Note that each of L5 and L6 is a distance from the focal point P1 to the plane side of the correcting lens.

Further, as long as a slight increase in aberration is allowed, L1 and L4 may not be equal to each other. In this case as well, when the angle of the X-directional scanning mirror <NUM> is changed, the light beam can be made to pass substantially the center of the entrance pupil of the objective lens <NUM>.

The curvature of field can be corrected by providing a biconvex lens in the position of the focal point P1. When the biconvex lens is provided in the position of the focal point, the laser light is focused in the biconvex lens. Therefore, light emission such as a Raman scattered light is generated from a lens material, which may interfere with measurement of the Raman scattered light from the sample. Therefore, it is preferable to provide the first correcting lens <NUM> and the second correcting lens <NUM> in such a way that they are separated from each other with the focal point P1 provided therebetween, as shown in <FIG>. When the first correcting lens <NUM> and the second correcting lens <NUM> are used, the light emission from the lens material does not interfere with measurement of the Raman scattered light from the sample since there is no lens material provided in the focal position.

The configuration shown in <FIG> or <FIG> may be used for the relay optical system <NUM>. That is, the relay optical system shown in <FIG> or <FIG> may be arranged in the optical path from the Y-directional scanning unit <NUM> to the beam splitter <NUM>. In this case, the light beam scanned by the Y-directional scanning unit <NUM> enters the first off-axis parabolic mirror <NUM>.

Next, with reference to <FIG>, a configuration of the focus optical system <NUM> will be explained. <FIG> is a diagram showing a configuration of the focus optical system <NUM>. Specifically, <FIG> shows an optical system from the X-directional scanning mirror <NUM> to the entrance slit <NUM>. In <FIG>, the beam splitter <NUM> is omitted. In <FIG>, the Y axis is perpendicular to the paper surface. The focus optical system <NUM> is a reflection optical system including a first concave mirror <NUM>, a first convex mirror <NUM>, and a plane mirror <NUM>.

The outgoing light generated in the sample <NUM> is descanned by the X-directional scanning mirror <NUM>. The outgoing light reflected in the X-directional scanning mirror <NUM> is reflected in the first concave mirror <NUM>. The outgoing light reflected in the first concave mirror <NUM> is reflected in the first convex mirror <NUM>. Then the outgoing light reflected in the first convex mirror <NUM> is reflected in the plane mirror <NUM>. The outgoing light reflected in the plane mirror <NUM> enters the entrance slit <NUM>. The first concave mirror <NUM> and the first convex mirror <NUM> focus the outgoing light in the entrance slit <NUM>. Each of the first concave mirror <NUM> and the first convex mirror <NUM> may be a spherical mirror. In this case, the curvature radius of the first concave mirror <NUM> and the curvature radius of the first convex mirror <NUM> may be substantially equal to each other. That the curvature radii are substantially equal means that the difference between the two curvature radii is, for example, within <NUM>%. When this difference is within <NUM>%, a sufficiently high aberration reduction effect can be obtained. In <FIG>, the reflection angle of the first concave mirror <NUM> is denoted by θ1 and the reflection angle of the first convex mirror <NUM> is denoted by θ2.

When light does not incident normal to the first concave mirror <NUM> and the first convex mirror <NUM>, which are spherical mirrors, astigmatism occurs. However, in the focus optical system <NUM>, astigmatism can be corrected by optimizing the reflection angle θ1 in the first concave mirror <NUM> and the reflection angle θ2 in the first convex mirror <NUM>. When astigmatism is corrected in this way, the line (symmetry axis) that passes the center of curvature of the first concave mirror <NUM> and the center of curvature of the first convex mirror <NUM> is not parallel to the reference axis of the light beam that enters the first concave mirror <NUM>. That is, the line (symmetry axis) that passes the center of curvature of the first concave mirror <NUM> and the center of curvature of the first convex mirror <NUM> is inclined from the reference axis of the light beam that enters the first concave mirror <NUM>. While an optical system in which aberration increases is obtained in an area outside a narrow area along the Y axis, a good image can be obtained along the opening part of the entrance slit <NUM>. Further, in the focus optical system <NUM>, the first concave mirror <NUM> and the first convex mirror <NUM> are used. By using the first concave mirror <NUM> and the first convex mirror <NUM> having curvatures close to each other, the Petzval sum and the curvature of field can be reduced. The Y axis is perpendicular to the plane including the reference axis and the symmetry axis.

In the focus optical system (lens <NUM>) disclosed in Patent Literature <NUM>, an image plane with respect to the surface perpendicular to the reference axis of the sample (hereinafter it will be simply referred to as an image plane) coincides with the incident plane of the entrance slit of the spectroscope. Therefore, the optical system can be easily adjusted. On the other hand, in the focus optical system <NUM> shown in <FIG>, at the position of the entrance slit <NUM>, the image plane of the sample and the entrance slit <NUM> are not parallel to each other. That is, the image plane of the sample imaged by the focus optical system <NUM> is inclined with respect to the incident plane of the entrance slit. The image plane of the sample is rotated about the Y axis with respect to the incident plane of the entrance slit. In other words, at the position of the entrance slit <NUM>, the plane perpendicular to the reference axis OX1 and the image plane of the sample are not parallel to each other.

This is because the spectroscope <NUM> (see <FIG> since it is not shown in <FIG>) is provided in such a way that the entrance slit <NUM> becomes perpendicular to the reference axis OX1. A spectroscope where the incident angle to the entrance slit <NUM> becomes equal to the incident angle to the detector <NUM> is used as the spectroscope <NUM>. According to this configuration, although the procedure of adjusting the optical system becomes complicated, by setting the incident angle to the detector <NUM> to be <NUM> degrees, it is possible to prevent a ghost image from being generated due to reflection in the window of the detector <NUM> and the light receiving surface of the detector <NUM>.

In the position of the entrance slit <NUM>, the image plane of the sample and the entrance slit <NUM> do not become parallel to each other. Therefore, only the linear part of the sample <NUM> focuses in the slit. However, no problem occurs since only the area in which the entrance slit <NUM> is in focus is measured at one time. The linear area on the sample <NUM> illuminated with the laser light focuses in the entrance slit <NUM> regardless of the angle of the X-directional scanning mirror <NUM>. When the angle at which the outgoing light is made incident on the entrance slit <NUM> is not equal to the angle at which the light is made incident on the detector <NUM>, the outgoing light is made incident on the entrance slit <NUM> in such a way that the incident angle to the detector <NUM> becomes <NUM> degrees.

The distance from the X-directional scanning mirror <NUM> to the first concave mirror <NUM> is preferably a distance at which the principal rays of the outgoing light from the respective points on the sample <NUM> illuminated in a line form become perpendicular to the entrance slit <NUM>. In other words, the distance from the X-directional scanning mirror <NUM> to the first concave mirror <NUM> is preferably a distance at which the imaging from the sample to the entrance slit <NUM> becomes telecentric. The distance at which the imaging becomes telecentric can be obtained by repeating ray tracing while changing the distance. A spectroscope whose inner optical system is a telecentric optical system is used as the spectroscope <NUM>. According to this configuration, the outgoing light beams from the respective points on the line can be made normal incident on the detector <NUM>. Therefore, it is possible to make the detection efficiency of the light from the respective points uniform and to prevent a ghost image from being generated.

A design example will be explained below. The curvature radius of the first concave mirror <NUM> and that of the first convex mirror <NUM> are made the same, that is, <NUM>. The distance from the first concave mirror <NUM> to the first convex mirror <NUM> along the reference axis OX is set to <NUM>, and the distance from the first convex mirror <NUM> to the slit is set to <NUM>. Further, by setting θ1 to be equal to <NUM> degrees and setting θ2 to be equal to <NUM> degrees, various aberrations are properly corrected. At this time, by setting the distance from the X-directional scanning mirror <NUM> to the first concave mirror <NUM> to be equal to <NUM>, telecentricity can be achieved.

In the focus optical system <NUM>, the optical elements are provided in the order of the first concave mirror <NUM> and the first convex mirror <NUM> from the side of the sample <NUM> along the reference axis OX1. Since the optical elements are provided in this order, even when the distance is set to the one at which the imaging from the sample <NUM> to the entrance slit <NUM> becomes telecentric, the distance from the X-directional scanning mirror <NUM> to the focus optical system <NUM> can be increased. It is therefore possible to provide a space where the beam splitter <NUM>, filters and the like are provided between the X-directional scanning mirror <NUM> and the focus optical system <NUM>. If the optical elements are provided in the order of the first convex mirror <NUM> and the first concave mirror <NUM> to correct various aberrations, the distance between the X-directional scanning mirror <NUM> and the optical element is reduced at the distance at which telecentricity is achieved. Therefore, it becomes difficult to provide the space where the beam splitter <NUM>, the filters and the like are provided. The optical elements are preferably provided in the order of the first concave mirror <NUM> and the first convex mirror <NUM> from the sample <NUM>, as described in this embodiment.

Next, with reference to <FIG>, a configuration of the relay optical system <NUM> will be explained. <FIG> is a diagram showing a configuration of the relay optical system <NUM>. Specifically, <FIG> shows an optical system from the Y-directional scanning unit <NUM> to the X-directional scanning mirror <NUM>. In <FIG>, the beam splitter <NUM> is omitted. In <FIG>, the Y direction is perpendicular to the paper surface. The relay optical system <NUM> is a reflection optical system including the second concave mirror <NUM>, the second convex mirror <NUM>, the third convex mirror <NUM>, and the third concave mirror <NUM>.

The light beam of the collimated light scanned by the Y-directional scanning unit <NUM> is reflected in the second concave mirror <NUM>. The light beam reflected in the second concave mirror <NUM> is reflected in the second convex mirror <NUM>. The second concave mirror <NUM> and the second convex mirror <NUM> focus the light beam on an intermediate image plane P2 in a spot shape. The light beam reflected in the second convex mirror <NUM> enters the third convex mirror <NUM>. The third convex mirror <NUM> reflects the light beam toward the third concave mirror <NUM>. The third concave mirror <NUM> reflects the light beam toward the X-directional scanning mirror <NUM>. The light beam reflected in the third convex mirror <NUM> and the third concave mirror <NUM> becomes a collimated light beam. Therefore, the X-directional scanning mirror <NUM> scans the light beam of the collimated light.

A diaphragm <NUM> may be provided in the intermediate image plane P2. The diaphragm <NUM> has, for example, a circular opening, and shields external light beams. That is, the passage of the light beams deviated from the opening is restricted. The area in which the scanning speed of the Y-directional scanning unit <NUM> is not constant is preferably shielded by the diaphragm <NUM>. According to this configuration, it is possible to make the illumination intensity in the linear area uniform.

The second concave mirror <NUM>, the second convex mirror <NUM>, the third convex mirror <NUM>, and the third concave mirror <NUM> are spherical mirrors. The second concave mirror <NUM> and the second convex mirror <NUM> are arranged to be symmetrical to the third convex mirror <NUM> and the third concave mirror <NUM> with respect to the intermediate image plane P2. For example, the curvature radius of the second concave mirror <NUM> and that of the third concave mirror <NUM> are made substantially equal to each other. The curvature radius of the third convex mirror <NUM> and that of the second convex mirror <NUM> are made substantially equal to each other. That the curvature radii are made substantially equal to each other indicates that the difference between the two curvature radii is within <NUM>%. When the difference is within <NUM>%, a sufficiently high aberration reduction effect can be obtained. Further, the distance from the second concave mirror <NUM> to the second convex mirror <NUM> is made equal to the distance from the third convex mirror <NUM> to the third concave mirror <NUM>. The distance from the second convex mirror <NUM> to the intermediate image plane P2 is made equal to the distance from the intermediate image plane P2 to the third convex mirror <NUM>.

It is assumed that the reflection angle of the second concave mirror <NUM> is equal to the reflection angle of the third concave mirror <NUM> (hereinafter, this angle is referred to as a reflection angle θ3) and the reflection angle of the second convex mirror <NUM> is equal to the reflection angle of the third convex mirror <NUM> (hereinafter, this angle is referred to as a reflection angle θ4). By optimizing the distance from the Y-directional scanning unit <NUM> to the second concave mirror <NUM> and the distance from the third concave mirror <NUM> to the X-directional scanning mirror <NUM>, even when the Y-directional scanning unit <NUM> has changed the angle of the light beam, the position in the X-directional scanning mirror <NUM> where the principal ray passes can be made substantially unchanged.

Patent Literature <NUM> discloses a relay optical system used between two scanning mirrors that are orthogonal to each other. The relay optical system disclosed in Patent Literature <NUM> is a reflection optical system that uses a concave mirror. However, the optical system disclosed in Patent Literature <NUM> cannot be used with a large beam diameter since it cannot sufficiently correct aberrations. In the relay optical system <NUM> shown in <FIG>, astigmatism can be corrected by optimizing the reflection angle θ3 and the reflection angle θ4.

In this case, the line (symmetry axis) that passes the center of curvature of the second concave mirror <NUM> and the center of curvature of the second convex mirror <NUM> are not parallel to the reference axis of the light beam that enters the second concave mirror <NUM>. That is, the line that passes the center of curvature of the second concave mirror <NUM> and the center of curvature of the second convex mirror <NUM> is inclined from the reference axis of the light beam that enters the second concave mirror <NUM>. Further, the line (symmetry axis) that passes the center of curvature of the third convex mirror <NUM> and the center of curvature of the third concave mirror <NUM> is not parallel to the reference axis of the light beam that is reflected in the third concave mirror <NUM> and travels toward the X-directional scanning mirror <NUM>. That is, the line that passes the center of curvature of the third concave mirror <NUM> and the center of curvature of the third convex mirror <NUM> is inclined from the reference axis of the light beam that is reflected in the third concave mirror <NUM> and travels toward the X-directional scanning mirror <NUM>. According to this configuration, astigmatism can be corrected. Note that the Y axis is perpendicular to the plane including the reference axis and the symmetry axis.

Further, by using the second concave mirror <NUM>, the second convex mirror <NUM>, the third convex mirror <NUM>, and the third concave mirror <NUM> having curvatures close to one another, the curvature of field can be corrected, and by arranging them to be symmetrical, coma aberration and distortion aberration are corrected. Accordingly, the relay optical system <NUM> according to the present disclosure can be used with a beam diameter larger than that of the optical system disclosed in Patent Literature <NUM>.

A design example of the relay optical system <NUM> will be explained. The curvature radius of the second concave mirror <NUM>, that of the second convex mirror <NUM>, that of the third convex mirror <NUM>, and that of the third concave mirror <NUM> are made the same, that is, <NUM>. The distance from the second concave mirror <NUM> to the second convex mirror <NUM> along the reference axis OX is set to <NUM>, the distance from the second convex mirror <NUM> to the intermediate image plane P2 is set to <NUM>, the distance from the intermediate image plane P2 to the third convex mirror <NUM> is set to <NUM>, and the distance from the third convex mirror <NUM> to the third concave mirror <NUM> is set to <NUM>. Further, θ3 is set to be <NUM> degrees and θ4 is set to be <NUM> degrees. According to this configuration, it is possible to properly correct various aberrations. At this time, each of the distance from the Y-directional scanning unit <NUM> to the second concave mirror <NUM> and the distance from the third concave mirror <NUM> to the X-directional scanning mirror <NUM> is set to <NUM>, whereby the position in the X-directional scanning mirror <NUM> where the principal ray passes can be made substantially unchanged even when the angle of the beam is changed by the Y-directional scanning unit.

A plurality of laser light sources having wavelengths different from each other may be used, and a beam expander capable of adjusting the degree of focus or the degree of divergence of the light beam for each laser wavelength may be provided in each of the laser light sources. <FIG> shows a modified example of the relay optical system <NUM> configured to change the degree of focus and the degree of divergence of the light beam in accordance with the wavelength.

Three light sources <NUM>, <NUM>, and <NUM> having laser wavelengths different from one another are provided as the light source <NUM>. The light source <NUM> is a laser light source that generates a laser light having a wavelength λ1. The light source <NUM> is a laser light source that generates a laser light having a wavelength λ2. The light source <NUM> is a laser light source that generates a laser light having a wavelength λ3.

The light beam having the wavelength λ1 generated in the light source <NUM> enters a dichroic mirror <NUM> via the beam expander <NUM> formed of lenses <NUM> and <NUM>. By adjusting the positions of the lenses <NUM> and <NUM>, the degree of focus or the degree of divergence of the light beam having the wavelength λ1 can be adjusted. The light beams having the wavelength λ2 generated in the light source <NUM> enters a dichroic mirror <NUM> via the beam expander <NUM> formed of lenses <NUM> and <NUM>. By adjusting the positions of the lenses <NUM> and <NUM>, the degree of focus or the degree of divergence of the light beam having the wavelength λ2 can be adjusted. The light beam having the wavelength λ3 generated in the light source <NUM> enters a dichroic mirror <NUM> via the beam expander <NUM> formed of lenses <NUM> and <NUM>. By adjusting the positions of the lenses <NUM> and <NUM>, the degree of focus or the degree of divergence of the light beam having the wavelength λ2 can be adjusted.

The dichroic mirror <NUM> causes the light beam having the wavelength λ1 to transmit therethrough and reflects the light beams having the wavelengths λ2 and λ3. The dichroic mirror <NUM> causes the light beam having the wavelength λ3 to transmit therethrough and reflects the light beams having the wavelength λ2. The dichroic mirror <NUM> reflects the light beams having the wavelength λ3. Accordingly, the optical paths of the light beams having the wavelengths λ1, λ2, and λ3 are combined. It is also possible to combine the optical paths of the light beams using a beam splitter instead of using the dichroic mirrors <NUM>, <NUM>, and <NUM>. By adjusting the positions of the lenses <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, it is possible to change the degree of focus and the degree of divergence of the respective light beams independently. Then the light beam from the dichroic mirror <NUM> enters the Y-directional scanning unit <NUM>.

In <FIG>, the relay optical system <NUM> can be formed of two lenses <NUM> and <NUM>, as disclosed in Patent Literature <NUM>. The light beam deflected by the Y-directional scanning unit <NUM> is refracted by the lens <NUM> and then enters the diaphragm <NUM>. The lens <NUM> focuses the light beam onto the surface of the diaphragm <NUM>. The diaphragm <NUM>, which includes, for example, a circular opening, shields external light beams. That is, the passage of the light beams deviated from the opening are restricted. It is preferable that the area of the Y-directional scanning unit <NUM> where the scanning speed is not constant be shielded by the diaphragm <NUM>. According to this configuration, it is possible to make the illuminance intensity of the linear area uniform.

The light beam that has been transmitted through the diaphragm <NUM> is refracted by the lens <NUM> to become collimated light beam. The beam splitter <NUM> shown in <FIG> is provided in the subsequent stage of the lens <NUM>, although the beam splitter <NUM> is not shown in <FIG>. The lenses <NUM> and <NUM> are made of synthetic quartz glass and calcium fluoride and these lenses are designed to be optimized for light with the degree of focus and degree of divergence that vary depending on the wavelength, whereby it is possible to obtain an optical system in which chromatic aberration is sufficiently corrected in a wide wavelength band from a deep ultraviolet region to a near infrared region.

In the relay optical system <NUM> shown in <FIG> or <FIG>, distortion aberration occurs. <FIG> is a diagram for describing the distortion aberration that occurs in the optical system shown in <FIG>. The square lattice shown in <FIG> is obtained by connecting the focal positions when light having an angle in the X direction and the Y direction with respect to the reference axis is directly made incident on the objective lens having no distortion aberration by a line. The points plotted in <FIG> are the positions of the focal point when light having an angle with respect to the reference axis is made incident on the objective lens <NUM> having no distortion aberration from the position of the X-directional scanning mirror <NUM> shown in <FIG> via the relay optical system <NUM>.

The plots shown in <FIG> can be obtained by performing ray tracing on design data. As shown in <FIG>, when the angle of the X direction is changed while fixing the angle of the Y direction, the spot position is changed in the Y direction depending on the angle of the X direction. On the other hand, when the angle of the Y direction is changed while fixing the angle of the X direction, there is little change in the X direction of the spot position. In other words, the spot position is changed only in the Y direction in response to the change in the angle of the Y direction. Therefore, the linear area where the sample <NUM> is illuminated is preferably an area extended along the Y axis. In other words, the linear area scanned by the Y-directional scanning unit <NUM> is preferably along the direction that corresponds to the geometric symmetry axis of the first off-axis parabolic mirror <NUM>. That is, in the configuration shown in <FIG>, the linear area is parallel to the Y axis, and the geometric symmetry axis of the first off-axis parabolic mirror <NUM> is parallel to the Y axis.

According to this configuration, distortion aberration can be corrected by performing calculation of data for each measurement of the linear area. That is, if the optical system is rotated about the Z axis by <NUM> degrees between the X-directional scanning mirror <NUM> and the first off-axis parabolic mirror <NUM> in the relay optical system <NUM> shown in <FIG>, the linear area where the sample <NUM> is illuminated is extended along the horizontal axis shown in <FIG>. According to this technique, when, for example, data in an area along one line on the sample is acquired, measurement of the linear area curved due to distortion aberration is repeated a plurality of times by changing the measurement area, and the obtained data needs to be interpolated, thereby obtaining the measurement results. At this time, it is required to interpolate two-dimensional data for each measurement wavelength. When the linear area is made to have the direction along the vertical axis shown in <FIG>, it is not required to perform interpolation from the measurement data of the linear area obtained from the measurement performed a plurality of times. The distortion aberration can be corrected by performing interpolation processing on the measurement data of each linear area. Further, the calculation can be made simple since the results can be obtained from interpolation processing of one-dimensional measurement data.

When the angle of the Y direction is fixed and the angle of the X direction is changed in <FIG>, the spot position is changed in the Y direction depending on the angle of the X direction. This change can be corrected by calculation using interpolation processing, as described above. For example, the processor <NUM> executes this interpolation processing, whereby the distortion aberration can be corrected. Specifically, the outgoing light from the linear area illuminated with the light beam is dispersed by the spectroscope <NUM>. The detector <NUM> includes a plurality of pixels arranged in the direction orthogonal to the direction in which the dispersed outgoing light is dispersed. The plurality of pixels arranged in one line along the direction orthogonal to the dispersion direction detect the outgoing light in one specific wavelength. Then one-dimensional measurement data detected by the plurality of pixels is interpolated in the direction orthogonal to the dispersion direction. The processor <NUM> performs interpolation processing on the measurement data for each linear area. The processor <NUM> acquires a Raman scattered light image in which the distortion aberration has been corrected.

Alternatively, the distortion aberration may be corrected by adding a Y-directional scanning mirror <NUM> just before the X-directional scanning mirror <NUM>, as shown in <FIG>. The Y-directional scanning mirror <NUM>, which is a third scanner, scans the spot position of the light beam in the Y direction on the sample. In the configuration shown in <FIG>, the scanning angle of the Y-directional scanning mirror <NUM> is changed in accordance with the scanning angle of the X-directional scanning mirror <NUM>, whereby the distortion aberration can be corrected. The overall structure of the optical microscope when the Y-directional scanning mirror <NUM> is arranged just before the X-directional scanning mirror <NUM> as shown in <FIG> is similar to that shown in <FIG> of Patent Literature <NUM>.

<FIG> shows a relation between the incident angle in the X-axis direction and the spot position in the Y-axis direction by the distortion aberration. It is sufficient that the angle of the Y-directional scanning mirror <NUM> be changed so as to cancel the change in the spot position shown in <FIG>.

<FIG> is a graph showing a relation between the scanning angles when the scanning angle of the Y-directional scanning mirror <NUM> is changed so as to cancel the change in the spot position by the distortion aberration shown in <FIG>. In <FIG>, the horizontal axis indicates the angle of the X-directional scanning mirror <NUM> and the vertical axis indicates the angle of the Y-directional scanning mirror <NUM>. It is sufficient that the scanning angle of the X-directional scanning mirror <NUM> and that of the Y-directional scanning mirror <NUM> be changed in association with each other so as to cancel the change in the spot position by the distortion aberration.

In order to prevent damages in a sample, it is preferable to extend the beam spot in the sample <NUM> in one direction. The beam spot in the sample <NUM> can be extended, for example, by using an optical system shown in <FIG>. In <FIG>, a cylindrical lens <NUM> is added just before the Y-directional scanning unit <NUM>. The cylindrical lens <NUM> focuses the light beam in the position of the Y-directional scanning unit <NUM>.

The light beam that has been focused is shrunk in the Y direction in the position of the Y-directional scanning unit <NUM> to have a cross-sectional shape extended along the X direction. At this time, the beam spots extended in parallel to each other along the Y direction are formed in the sample <NUM>. That is, the beam spots that are extended in the direction the same as the scanning direction by the Y-directional scanning unit <NUM> can be obtained.

When the laser light beams having different wavelengths are used, it is sufficient that the cylindrical lens be moved in the Z direction depending on the wavelength in such a way that the position of the Y-directional scanning unit <NUM> is focused. Alternatively, by focusing the position of the Y-directional scanning unit <NUM> using the reflecting mirror having a cylindrical surface, the above adjustment can be omitted. Alternatively, a cylindrical lens having a concave surface may be arranged after the Y-directional scanning unit <NUM>, as disclosed in Patent Literature <NUM>.

When the available laser intensity is not too large, even when the amount of extension of the beam spot in the sample <NUM> is small, it is possible to prevent the sample from being damaged. When the amount of extension of the beam spot is small, the configuration shown in <FIG> may be used. In <FIG>, anamorphic prisms <NUM> and <NUM> are added just before the Y-directional scanning unit <NUM>. By using the two anamorphic prisms <NUM> and <NUM>, the beam cross-sectional shape can be shrunk in the Y direction, and this beam is made incident on the Y-directional scanning unit <NUM> having an elliptical beam cross-sectional shape.

Alternatively, in order to shrink the light beam in one direction, as shown in <FIG>, two cylindrical lenses <NUM> and <NUM> may be used. The cylindrical lenses <NUM> and <NUM> are arranged just before the Y-directional scanning unit <NUM>.

The cylindrical lenses <NUM> and <NUM> and the like shrink the beam cross-sectional shape at the position of the Y-directional scanning unit <NUM> in the Y direction. Then the beam spot at the position of the sample <NUM> is extended in the Y direction in accordance with the amount of shrinkage of the beam cross-sectional shape at the position of the Y-directional scanning unit <NUM>. For example, the cylindrical lenses <NUM> and <NUM> and the like may shrink the beam to <NUM>/<NUM> or <NUM>/<NUM>. When the beam is shrunk to <NUM>/<NUM>, the beam spot is extended more than that when the beam is shrunk to <NUM>/<NUM>. In this way, the amount of extension of the beam spot in the sample <NUM> can be adjusted.

In the optical system shown in <FIG>, the beam spot in the sample <NUM> can be extended more than that in the optical system shown in <FIG> and <FIG>. On the other hand, when the beam spot is extended too much, the intensity of the laser irradiated onto the measurement area may become too small. By extending the beam spot relatively small so as to prevent the damage in the sample <NUM> by the optical system shown in <FIG> and <FIG>, the area to be measured can be illuminated with the laser light more efficiently. At this time, the scanning range by the Y-directional scanning unit <NUM> is preferably adjusted in accordance with the area to be measured.

While a case in which the beam cross-sectional shape at the position of the Y-directional scanning unit <NUM> is shrunk in the Y direction has been described in the above description, the cross-sectional shape of the beam may be changed by extending the beam cross-sectional shape in one direction using the optical systems shown in <FIG> and <FIG> in the opposite manner. At this time, the expansion rate of the beam in the beam expander <NUM> is adjusted.

As shown in <FIG>, by shrinking the cross-sectional shape of the light beam in the Y-directional scanning unit <NUM> in the Y direction, it is possible to prevent the sample <NUM> from being damaged. In <FIG>, the cylindrical lens or the anamorphic prism is not the only one to be used as the optical member configured to shrink the cross-sectional shape of the light beam in the Y-directional scanning unit <NUM> in the Y direction, and various other optical members may be used.

The objective lens <NUM> can be switchably used for each wavelength band to be measured. Otherwise, when a wide wavelength band is measured at one time, a Schwarzschild-type reflective objective lens may be, for example, used.

While the configurations of the relay optical system <NUM>, the relay optical system <NUM>, and the focus optical system <NUM> have been described in the above description, not all the configurations may be used. That is, only a part of the configurations shown in <FIG> may be used for the optical microscope <NUM> shown in <FIG>.

If, for example, the relay optical system <NUM> has the configuration shown in <FIG> or <FIG>, the configurations of the relay optical system <NUM> and the focus optical system <NUM> are not limited to those shown in <FIG>, <FIG>, or <FIG>. Further, the relay optical system <NUM> may have a configuration shown in <FIG> or <FIG>. In this case, the configuration of the relay optical system <NUM> is not limited to the one shown in <FIG> or <FIG>. As a matter of course, the relay optical system <NUM> preferably has the configuration as shown in <FIG> or <FIG> and the relay optical system <NUM> preferably has the configuration as shown in <FIG> or <FIG>. Further, the focus optical system <NUM> preferably has the configuration shown in <FIG>. The optical system other than those shown in <FIG> may be, for example, the optical system that uses the lens, as disclosed in Patent Literature <NUM> and <NUM>.

As described above, the optical microscope according to this embodiment includes the light source; the first scanner configured to deflect the light beam from the light source and scan a spot position of the light beam on a sample; the objective lens configured to focus the light beam deflected by the first scanner and cause the light beam to be made incident on the sample; the spectroscope configured to spatially disperse the outgoing light emitted from an area on the sample onto which the light beam has been illuminated in accordance with the wavelength; the two-dimensional array photodetector including light-receiving pixels arranged in an array, the two-dimensional array photodetector detecting the outgoing light dispersed by the spectroscope; and the first relay optical system including the first off-axis parabolic mirror that is arranged in the optical path from the first scanner to the objective lens and reflects the light beam deflected by the first scanner and the second off-axis parabolic mirror that reflects the light beam reflected in the first off-axis parabolic mirror. The first relay optical system may be at least one of the relay optical system <NUM> and the relay optical system <NUM> shown in <FIG>.

According to the aforementioned optical microscope, Raman spectrum may be measured. While the optical microscope <NUM> configured to perform spectroscopic measurement on the Raman scattered light has been described in the above description, the present disclosure is not limited thereto. It is sufficient that a spectroscopic measurement apparatus configured to detect an outgoing light emitted from a sample in a wavelength different from a laser wavelength of an incident light be used. A spectroscopic measurement apparatus configured to detect fluorescence excited by an excitation light or a spectroscopic measurement apparatus configured to detect infrared absorption may be, for example, employed. According to these spectroscopic measurement apparatuses as well, aberrations can be prevented.

Claim 1:
An optical microscope comprising:
a light source (<NUM>) configured to generate a light beam;
a first scanner (<NUM>) configured to deflect the light beam and scan a spot position of the light beam on a sample;
an objective lens (<NUM>) configured to focus the light beam deflected by the first scanner and cause the light beam to be made incident on the sample;
a spectroscope (<NUM>) including a slit on an incident side which an outgoing light emitted from an area on the sample onto which the light beam has been illuminated enters;
a two-dimensional array photodetector (<NUM>) including light-receiving pixels arranged in an array, the two-dimensional array photodetector detecting an outgoing light from the spectroscope; and
a first relay optical system (<NUM>) including a first off-axis parabolic mirror (<NUM>) and a second off-axis parabolic mirror (<NUM>), the first off-axis parabolic mirror (<NUM>) being arranged in an optical path from the first scanner to the objective lens and reflecting the light beam deflected by the first scanner, and the second off-axis parabolic (<NUM>) mirror reflecting the light beam reflected in the first off-axis parabolic mirror (<NUM>),
wherein the first off-axis parabolic mirror (<NUM>) and the second off-axis parabolic mirror (<NUM>) are concave mirrors having paraboloids,
geometric symmetry axes (SX) of the first and second off-axis parabolic mirrors are parallel to each other and the paraboloids face in directions opposite to each other,
a second direction corresponds to a longitudinal direction (Y) of the slit in a plane perpendicular to a reference axis (OX1) of the light beam between the first and the second off-axis parabolic mirrors is within a plane including the geometric symmetry axes (SX), the reference axis (OX1) of the light beam between the first and the second off-axis parabolic mirrors is a line that the light that enters a pupil of the objective lens (<NUM>) at an incident angle of <NUM> degrees or a principal ray of the outgoing light emitted at <NUM> degrees from the pupil of the objective lens passes.