Confocal scanning microscope using two Nipkow disks

A confocal scanning microscope using a Nipkow disk prevents deterioration of performance in an optical axis direction while maintaining a high measurement speed. The confocal scanning microscope includes a light source, an illuminating device to pass the light from the light source toward a certain direction, and two Nipkow disks each having slit-shaped apertures formed thereon such that the light incident from the illuminating device travels in a form of light which passed through a single aperture. In addition, the confocal scanning microscope includes a first optical system to form an image on a sample by the light passed through the Nipkow disks, and a second optical system to form a second image by the light reflected from the sample and passed through the Nipkow disks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No. 2004-75996, filed on Sep. 22, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present general inventive concept relates to a confocal scanning microscope to measure critical dimensions of a semiconductor device and an image output device to perform a real time check in a production process of the semiconductor device and image output devices.

2. Description of the Related Art

Confocal microscopes are apparatuses for irradiating a sample with light having a certain wavelength, controlling the light to be reflected from the sample through a confocal aperture, such as a pin hole, and detecting the light only emitted from a focal plane of an objective lens using a photo-detector (PD), the principle of which is disclosed in Korean Patent Laid-Open No. 2002-0084786.

As disclosed in the above mentioned reference, since light reflected from a portion outside the focal plane of the objective lens of the confocal microscope does not pass through the pin hole, and hence, is not detected in the photo-detector, the confocal microscope has not only a high resolution in an optical axis direction but also a resolution higher than that of existing optical microscopes in an direction perpendicular to the optical axis direction. In addition, with the confocal microscope disclosed in the above reference, it is possible to observe a desired plane on the sample and obtain a three-dimensional image of the sample.

Owing to the high resolution and the capability of obtaining the three-dimensional image, confocal microscopes has been widely used in the fields of cell biology and semiconductor chip testing.

As one of methods for obtaining a two-dimensional plane image using such a confocal microscope, Japanese Patent Laid-Open No. Hei 06-018786 discloses a confocal microscope for scanning every point of a measurement area with light using point scanning of a television scan line system. The confocal microscope employs a method of deflecting light in two perpendicular axis directions using two optical deflectors, and scanning each point of the measurement area with the deflected light. However, such a confocal microscope has a problem in that it takes a long time to obtain the two-dimensional image due to a limited mechanical speed of the optical deflectors and a calculation load of serial signal processing.

As a method for obtaining higher image acquisition speed, compared to the method using the optical deflectors, U.S. Pat. No. 5,067,805 discloses a confocal scanning microscope using a Nipkow disk, the principle of which will be described with reference toFIG. 1.

Referring toFIG. 1, the confocal scanning microscope using the Nipkow disk includes a light source1, a collimating lens2for transforming light emitted from the light source1into a parallel beam, a beam splitter3for changing a direction of the parallel beam incident from the collimating lens2, a Nipkow disk4having a plurality of apertures formed therein such that only a portion of a beam incident from the beam splitter3passes therethrough, a motor5for rotating the Nipkow disk, a tube lens6for transforming a beam, which has passed through the Nipkow disk4, into a parallel beam, an objective lens7for irradiating a sample8with the parallel beam incident from the tube lens6, a first lens9for transforming the beam reflected from the sample8and passed through the Nipkow disk4into a parallel beam, and a second lens10for concentrating the beam passed through the first lens9onto a two-dimensional photo-detector11for acquisition of an image.

The light emitted from the source of light1becomes the parallel beam after passing through the collimating lens2. This parallel beam illuminates a top surface of the Nipkow disk4after being reflected at the beam splitter3. As shown inFIG. 2A, a plurality of apertures4aeach having a pin hole shape are distributed (formed) in the Nipkow disk4. With this distribution of the apertures4a, only a portion of the parallel beam irradiating the Nipkow disk4passes through the apertures4a. A beam passed through the apertures4apropagates at various angles by diffraction, thereby causing an effect as if a point of the light source is placed on each aperture4a. The tube lens6and the objective lens7form an image on the sample8by irradiating the sample8with the beam passed through the apertures4a. Only a plurality of point regions of an overall observation region on the sample8, which correspond to the apertures4a, are illuminated. In order to illuminate the overall observation region on the sample8, positions of the apertures4amust be varied. To this end, the motor5connected to a center of the Nipkow disk4rotates the Nipkow disk4. When the positions of the apertures4aare varied with respect to the sample8according to rotation of the Nipkow disk4, the overall observation region on the sample8is illuminated.

The beam illuminating on the sample8is reflected from the sample8and passes through the objective lens7and the tube lens6for formation of an image on the Nipkow disk4. At this time, some beam, reflected from a focal plane (f) of the objective lens7, of the beam reflected from the sample8, passes through the apertures4aof the Nipkow disk4, however, some portion of the beam, reflected from points deviated from the focal plane (f) in the optical axis direction, of the beam reflected from the sample8, does not pass through the apertures4a. This accounts for the so-called confocal principle through which high resolution in the optical axis direction can be obtained.

The beam passed through the apertures4ais incident into the two-dimensional photo-detector11through the first lens9and the second lens10so that an image is formed on the photo-detector11. As the positions of the apertures4aare varied according to the rotation of the Nipkow disk4by the motor5, a position on the photo-detector11at which the image is formed is varied. Accordingly, an optical signal is transported on the overall region of the two-dimensional photo-detector11, so that a two-dimensional image can be at once obtained with respect to the sample8.

FIG. 2Billustrates a shape of the Nipkow disk4, where slit-shaped curve apertures4bare formed. In the case of the confocal scanning microscope using the Nipkow disk4having the slit-shaped curve apertures4bformed thereon, an illumination beam passing through the Nipkow disk4takes a line shape, and a region to be illuminated on the sample8also takes a line shape. When the Nipkow disk4is rotated, the illumination beam of the line shape for forming an image on the sample8is moved, and accordingly, the image of the line shape formed on the photo-detector11is also moved to obtain a two-dimensional image of the sample8.

However, although the confocal scanning microscope using the Nipkow disk4has an advantage of an image acquisition speed higher than that of the confocal scanning microscope using the optical deflector, it has a problem of deterioration of the resolution in the optical axis direction since it illuminates not a point but a plurality of point regions or a line region on the sample for parallel processing of signals.

As shown inFIG. 3, the beam reflected from the focal plane (f) of the objective lens7is exactly concentrated on and exits through the apertures4aof the Nipkow disk4through which the illumination beam has been emitted, after passing through the objective lens7and the tube lens6. However, the beam reflected from the points deviated from the focal plane (f) and moved deeper in the optical axis direction is not exactly concentrated on the apertures4a, forms an image before the apertures4a, and then, passes through adjacent apertures4aas well as the apertures4athrough which the illumination beam is emitted. The beam illuminated on the adjacent apertures41acts as a kind of noise, which deteriorates optical performance in the optical axis direction.

FIG. 4is a graph showing a resolution in an optical axis direction when a size of the aperture in confocal scanning microscopes using a single aperture and multiple apertures is changed. In order to obtain the amount of beam of a range within which an image can be measured, it is required to increase the size of the aperture over a prescribed size. However, it can be seen fromFIG. 4that a resolution value in the confocal scanning microscope using the multiple apertures is abruptly increased as the size of the aperture is increased, which results in deterioration of performance of the confocal scanning microscope.

SUMMARY OF THE INVENTION

The present general inventive concept provides a confocal scanning microscope using a Nipkow disk, which is capable of preventing deterioration of performance in an optical axis direction while maintaining a high measurement speed.

The foregoing and/or other aspects and advantages of the present general inventive concept may be achieved by providing a confocal scanning microscope using two Nipkow disks, the confocal scanning microscope comprising a light source, an illuminating device to pass the light from the light source toward a certain direction, two Nipkow disks each having slit-shaped apertures formed thereon such that the light incident from the illuminating device travels in a form of light which has passed through a single aperture, a first optical system to form an image on a sample by the light passed through the Nipkow disks, and a second optical system to form a second image by the light reflected from the sample and passed through the Nipkow disks.

The illuminating device may include a collimating lens to transform the light emitted from the light source into a parallel beam, and a beam splitter to change a direction of the parallel beam incident from the collimating lens2.

The two Nipkow disks may be disposed adjacent to and overlap each other in the certain direction.

The confocal scanning microscope may further comprise a relay optical system disposed between the two Nipkow disks to guide the light passing between the two Nipkow disks.

The two Nipkow disks may be rotated at different rotational speeds.

The two Nipkow disks may be arranged in such a manner that rotation axes thereof are laid on a straight line.

The first optical system may include a tube lens to transform the light passed through the Nipkow disks into a parallel beam, and an objective lens to collect the parallel passed through the tube lens.

The second optical lens may include a first lens to transform the reflected light passed through the Nipkow disks into a parallel beam, and a second lens to collect the parallel beam passed through the first lens.

The confocal scanning microscope may further comprise a photo-detector to convert an image formed in the second optical system into an electrical signal.

The foregoing and/or other aspects and advantages of the present general inventive concept may also be achieved by providing a confocal scanning microscope including two Nipkow disks each having slit-shaped apertures formed thereon such that the light incident from the illuminating device travels in a form of light which has passed through a single aperture.

The foregoing and/or other aspects and advantages of the present general inventive concept may also be achieved by providing a confocal scanning microscope comprising two Nipkow disks each having slit-shaped apertures formed thereon, wherein a common aperture is formed by a combination of apertures formed in the two Nipkow disks and the two Nipkow disks are rotatably provided such that position of the common aperture can be varied with respect to center axes of the Nipkow disks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a description will be given of a confocal scanning microscope using Nipkow disks according to embodiments of the present general inventive concept with reference to the accompanying drawings.

Referring toFIGS. 5 and 6, a confocal scanning microscope according to an embodiment of the present general inventive concept comprises a light source31, a collimating lens32to transform light emitted from the source of light31into a parallel beam, a beam splitter33to change a direction of the parallel beam incident from the collimating lens32, a first Nipkow disk34and a second Nipkow disk35having respective slit-shaped apertures34aand35aformed thereon, through which a portion of the light incident from the beam splitter33passes, a first motor36and a second motor37to rotate the first Nipkow disk34and the second Nipkow disk35, respectively, a tube lens38to transform the light passed through an intersecting point h of the two apertures34aand34ainto a parallel beam, a first lens41to transform the light passed through the first and second Nipkow disks34and35into a parallel beam, and a second lens42to concentrate the light passed through the first lens41into a two-dimensional photo-detector43for acquisition of an image.

The confocal scanning microscope according to this embodiment of the present general inventive concept uses the two Nipkow disks34and35, as described above, unlike a conventional confocal scanning microscope. Hereinafter, an operation of the confocal scanning microscopeFIG. 5will be described.

The light emitted from the light source of light31is transformed into the parallel beam by the collimating lens32. The parallel beam is reflected by a beam splitter33and illuminates the first and second Nipkow disks34and35. As shown inFIG. 6, the slit-shaped apertures34aand35aare formed on the first and second Nipkow disk34and35, respectively, and the light passes through only a small hole, i.e., the intersection hole h, formed at an intersecting point of the apertures34aand35aof the first and second Nipkow disks34and35. This hole h acts as a single aperture. The light passed through the first and second Nipkow disks34and35travels in diverse directions by diffraction phenomenon, thereby causing an effect as if a point source of the light is placed in the hole through which the light passes. The light passed through the first and second Nipkow disks34and35illuminates the sample40through the tube lens38and the objective lens39. The light reflected from the sample40is concentrated in the hole h, through which the light has passed, through the objective lens39and the tube lens38. At this time, only the light reflected from a focal plane (f) of the objective lens39passes through the hole h, however, the light reflected from a point deviated from the focal plane (f) in an optical axis direction does not pass through the hole h by being interrupted by the first and second Nipkow disks34and35. Accordingly, a confocal effect with high resolution in the optical axis direction can be obtained.

As shown inFIG. 6, since the hole h formed at the intersecting point of the apertures34aand35aof the first and second Nipkow disks34and35exists uniquely, not in multiple, resolution deterioration in the optical axis direction due to multiple apertures can be prevented.

The light passed through the first and second Nipkow disks34and35forms an image on the two-dimensional photo-detector43through the first lens41and the second lens42. The two-dimensional photo-detector43has an imaging device such as a charge coupled device (CCD) to convert the light into an electrical signal.

In order to obtain a two-dimensional image with respect to an overall observation region on the sample40, when the first motor36and the second motor37are rotated at different speeds, a position of the hole h formed at the intersecting point of the apertures34aand35aof the first and second Nipkow disks34and35is moved with respect to the sample40or centers of the first and second Nipkow disks34and35, so that the two-dimensional image of the sample40can be obtained.

FIG. 7is a schematic view illustrating a confocal scanning microscope according to another embodiment of the present general inventive concept, where the same reference numerals refer to the same elements as the confocal scanning microscope ofFIG. 5, explanation of which will be omitted for the purpose of brevity.

In the confocal scanning microscope ofFIG. 5, in order that the hole h formed at the intersecting point of the apertures34aand35aof the two Nipkow disks34and35acts as a single aperture, a gap between the two Nipkow disks34and35must be very small and a thickness of the Nipkow disks34and35must be very thin.

The confocal scanning microscope ofFIG. 7may solve such a shortcoming of the embodiment ofFIG. 5, by providing a first relay lens51and a second relay lens52between the first and second Nipkow disks34and35to transport the light passed through the first Nipkow disk34to the second Nipkow disk35, The first and second Nipkow disks34and35are spaced apart from each other by a certain distance. The first relay lens51transforms the light passed through the second Nipkow disk35into the parallel beam and the second relay lens52concentrates the parallel beam passed through the first relay lens51on the first Nipkow disk34. Thus, by using the first and second relay lenses51and52, the light can be passed only at the intersecting point of the apertures34aand35aof the first and second Nipkow disks34and35.

FIG. 8is a schematic view illustrating a confocal scanning microscope according to another embodiment of the present general inventive concept andFIG. 9is a plan view illustrating a Nipkow disk of the confocal scanning microscope ofFIG. 8, where the same reference numerals refer to the same elements as the confocal scanning microscopes according to the embodiments of theFIGS. 5 and 7, explanation of which will be omitted for the purpose of brevity.

The confocal scanning microscope according to this embodiment of the present general inventive concept modifies arrangement of the two Nipkow disks34and35and the corresponding driving motors36and37, compared to the confocal scanning microscope ofFIG. 7. Rotation axes of the two Nipkow disks34and35are disposed on a straight line. The confocal scanning microscope according to this has a merit in that a size thereof can be reduced compared to that of the confocal scanning microscope ofFIG. 7.

FIG. 10is a schematic view illustrating a confocal scanning microscope according to another embodiment of the present general inventive concept, where the same reference numerals refer to the same elements as the confocal scanning microscopes ofFIGS. 5,7, and8, explanation of which will be omitted for the purpose of brevity.

In the confocal scanning microscope according to this embodiment of the present general inventive concept, the first and second relay lenses51and52are provided between the two Nipkow disks34and35of the confocal scanning microscope ofFIG. 8, in a manner similar to the confocal scanning microscope ofFIG. 7.

As is apparent from the above description, with the confocal scanning microscope according to the present general inventive concept, illumination light can be passed through only an intersecting portion of the apertures of the two Nipkow disks and the interesting portion of the apertures are moved when the Nipkow disks are rotated, thereby obtaining an effect as if a single aperture is moved at a high speed. Accordingly, the performance deterioration in the optical axis direction occurring when disks having multiple apertures are used can be prevented.