Optical: systems and elements – Compound lens system – Microscope
Reexamination Certificate
1999-10-28
2001-07-10
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
Microscope
C359S385000, C355S067000
Reexamination Certificate
active
06259557
ABSTRACT:
This application claims the benefit of Japanese Application No. 11-115647 which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dark field illumination apparatus and a dark field illumination method more particularly to an epi-illumination type of dark field illumination for an object to be observed in a microscope.
2. Related Background of the Invention
In semiconductor device manufacturing steps, to inspect dusts, flaws and the like on a wafer that is a photosensitive substrate, a microscope observation using a dark field illumination has been often used. In the microscopic observation using the dark field illumination, areflection light from an object to be observed and a transmission light therefrom are not directly observed, but a scattered light from the object is observed. In the case where the object to be observed is a wafer and the like, the object can be observed with a better contrast in a dark field illumination than in a bright field illumination.
In a conventional dark field illumination apparatus, incidentally, since an optical arrangement is constructed such that an object undergoes approximately a critical illumination, illumination light is diffused so that a light beam for illumination does not form an image on a surface of the object. To be more specific, in the case where the light beam for illumination is guided into the object through a ring-shaped concave reflection mirror, the illumination light is diffused by a diffuser arranged closer to a light source than the concave reflection mirror. In this case, the light diffused by the diffuser is not incident partially onto the concave reflection mirror. In other words, a part of the light diffused by the diffuser does not contribute to the illumination, thus causing a loss of a quantity of light.
On the other hand, in the case where the light beam for illumination is guided into the object through a ring-shaped condenser lens, a diffusing film is formed on an incidence plane of the condenser lens, and the illumination light is diffused by a function of the diffusing film. In this case, a fixed diaphragm for preventing a flare is disposed in an optical path between the condenser lens and the object so that the light beam diffused by the diffusion film does not illuminate the outside of a visual field when a low magnification microscope object lens is particularly used. As a result, the part of the light passing through the condenser lens is obstructed by the fixed diaphragm and lost without contributing to the illumination. At the same time, a working distance is shortened by the disposition of the fixed diaphragm, and an operability is deteriorated.
As described above, in the conventional dark field illumination apparatus, there has been a disadvantage that a sufficient brightness cannot be obtained in an illuminated field of the object due to a loss of a quantity of light created by the diffuser or the fixed diaphragm.
Moreover, since the conventional dark field illumination apparatus does not adopt an optical arrangement for Koehler illumination, there has been a disadvantage that evenness in brightness exists in a wide visual field to be observed particularly at the time of using a low magnification microscope object lens.
SUMMARY OF THE INVENTION
The present invention was made from the viewpoint of the foregoing problems. The object of the present invention is to provide a dark field illumination apparatus and a dark field illumination method which are capable of having a sufficient brightness and performing a dark field illumination in which unevenness in brightness is sufficiently suppressed.
To solve the foregoing problems, the present invention provides a dark field illumination apparatus comprising: a light source for supplying an illumination light; a shaping system for shaping a light beam from the light source to an approximately parallel beam having a ring-shaped section; a wavefront splitting type optical integrator having a plurality of elements, which forms a plurality of light source images based on the approximately parallel beam having the ring- shaped section which is incident onto incidence planes of the plurality of elements, the light source images being arranged circularly on a predetermined plane; and a light collection optical system for collecting light beams from the plurality of light source images formed on the predetermined plane to superimpose the collected light beams on an object plane.
Here, as the optical integrator, a fly-eye optical device and the like can be used.
In the foregoing apparatus, the illumination light from the light source is shaped to the approximately parallel light beam having the ring-shaped section, and then the approximately parallel beam is incident onto the optical integrator composed of the plurality of elements. The approximately parallel beam which is incident onto a ring-shaped incidence plane of the optical integrator forms the plurality of light source images arranged circularly on the predetermined plane. The light beams from the plurality of light source images formed so as to be arranged circularly are collected by the light collection optical system, and then superimposed on the object. In this case, the illumination light performs a Koehler illumination or an illumination akin to the Koehler illumination for the object plane so as to be superposed, so that an illuminated field in which unevenness in brightness is sufficiently suppressed is formed. Moreover, unlike a case where a light beam is diffused using a diffuser or the like in prior arts, the light beam incident onto a fly-eye element does not lose substantially its quantity of light and guided into the object plane through the light collection optical system. As a result, the illuminated field having a sufficient brightness is formed.
If such a dark field illumination apparatus is built into a microscope equipped with an object lens for collecting a light beam having a numerical aperture smaller than that of the light beams which is incident onto the object plane by the light collection optical system, a visual field can be made to be bright and even in brightness, and a performance of the microscope can be improved.
According to another aspect of the present invention, provided is a dark field illumination apparatus comprising:
a light source for supplying illumination light;
a shaping system for shaping light beam from the light source to an approximately parallel beam having a ring-shaped section;
a fly-eye optical device having a plurality of lens elements, which forms a plurality of light source images circularly arranged in the vicinity of exit planes of the plurality of lens elements, based on the approximately parallel beam having the ring-shaped section, the approximately parallel beam being incident onto incidence planes of the plurality of lens elements; and
a light collection optical system for collecting light beams from the plurality of light source images formed in the vicinity of an exit plane of the fly-eye optical device, and superimposing the light beams on an object plane.
In the foregoing apparatus, the illumination light from the light source is shaped to the approximately parallel beam having a ring-shaped section, and then incident onto the fly-eye optical device composed of the lens elements. The approximately parallel beam incident onto a ring-shaped incidence plane of the fly-eye optical device forms the plurality of light images circularly arranged in the vicinity of the exit plane, that is, in the vicinity of a focal plane on the rear side. The light beams from the plurality of light source images formed so as to be circularly arranged are collected by the light collection optical system, and then superimposed on the object plane upon another.
As described above, in the present invention, an approximately parallel beam or a light beam having a suitable divergence angle or a suitable convergent angle, which passes through the light collection optical system, performs a Koehl
Miyashita Tomohiro
Suenaga Yutaka
Nguyen Thong
Nikon Corporation
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