Optical: systems and elements – Having significant infrared or ultraviolet property – Having ultraviolet absorbing or shielding property
Reexamination Certificate
2000-04-26
2002-01-15
Henry, Jon (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Having ultraviolet absorbing or shielding property
C359S350000, C359S363000
Reexamination Certificate
active
06339498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an illumination optical system for an ultraviolet microscope apparatus which uses wavelengths in an ultraviolet region and to an optical filter for obtaining only light with desired wavelengths in the ultraviolet region, and in particular, to an illumination optical system and an optical filter for an ultraviolet microscope apparatus which uses wavelengths in a deep ultraviolet region, less than 300 nm. This invention also relates to an illumination optical system for an ultraviolet microscope apparatus which is used in combination with a conventional microscope optical apparatus for making observations with wavelengths in a visible region.
2. Description of Related Art
In keeping with the development of high-density ICs, microminiaturization of wiring patterns is rapidly advanced, and thus there are strong and growing demands that optical microscopes used for their observations and examinations should have high resolution. As is well known, there are two techniques of obtaining high resolution in an optical microscope. One is to increase the numerical aperture of an objective lens, and the other is to shorten wavelengths used. However, since under the present situation the numerical aperture already reaches a limited value as high as 0.95, it is very difficult to desire a higher numerical aperture. Hence, the technique of reducing wavelengths must be used, but with wavelengths in the visible region, it is difficult to accommodate microminiaturization of recent ICs, and it become necessary to use ultraviolet rays which have wavelengths shorter than those in the visible region.
Various ultraviolet microscopes have been suggested, and examples of ultraviolet microscopes using light sources, such as mercury-vapor lamps, are set forth in Japanese Patent Preliminary Publication Nos. Sho 64-62609 and Hei 5-127096. A microscope disclosed in Sho 64-62609 is designed so that all lenses placed in optical paths of ultraviolet rays are constructed of quartz. On the other hand, a microscope disclosed in Hei 5-127096 is such that an illumination lens and an objective lens system are corrected for chromatic aberration in wavelength regions extending from a visible region to a near-ultraviolet region, and is equipped with means for observing a visible image and an ultraviolet image.
A light source used for illumination has a specialty with respect to each of the characteristics of its spectral intensity distribution and emission point. For an illumination optical system taking account of the characteristic of a special emission point of the light source itself, it is disclosed in Japanese Patent Preliminary Publication No. Hei 6-289301 that a nearly afocal, variable-magnification lens system is interposed to be removable between a lamp house containing a light source and collector lenses and a projection optical system. This variable-magnification lens system is provided for the purpose that when a halogen lamp is replaced by an arc lamp, the magnification of projection of the lamp on the pupil of an objective lens is changed. The lens system includes two lens units with two lens components, comprised of a positive lens and a negative lens which are arranged in this order from the collector lens side.
Here, a brief description of the arc lamp is given. An arc lamp, like a high-pressure mercury-vapor lamp, is adapted to cause an electric discharge between an anode and a pointed cathode so that extremely bright light is produced. In the arc lamp, a space between the anode and the cathode is narrow, and the discharge is not maintained at a uniform intensity between them. Truly high luminance is kept only in a very small region close to the pointed cathode. Thus, in Hei 6-289301, when the light source is switched to the arc lamp, the variable-magnification lens system is inserted to increase the magnification of projection of an arc on the pupil of the objective lens, thereby casting bright illumination upon the pupil of the objective lens.
In order to make observations with ultraviolet rays, it becomes necessary to use an optical filter for selecting only light in a particular wavelength region from among various wavelengths emitted from a light source. In a conventional, optical microscope used in a visible region, an optical filter (namely a band-pass filter, which is hereinafter abbreviated to BPF) placed in an illumination optical system or an observation optical system to obtain only light in a desired wavelength region, as shown in
FIG. 1
, is such that its spectral transmittance characteristic curve is symmetrical about a wavelength &lgr; c in the vicinity of the desired wavelength region. Hence, this spectral transmittance distribution is not set to take account of a special, spectral intensity distribution characteristic of the light source itself.
Another technique of obtaining light with desired wavelengths in an ultraviolet wavelength region is that, in a microscope illumination system using deep ultraviolet light from a light source such as an arc lamp, light other than desired deep ultraviolet light is transmitted through a cold mirror and only deep ultraviolet light reflected by the cold mirror is used.
For still another technique, an example of a combination of a transmission-type element and a reflection-type element is disclosed in Japanese Patent Preliminary Publication No. Hei 8-313728.
In the case where the technique disclosed in Sho 64-62609 is employed, however, lens material used is limited to quartz, and therefore correction for chromatic aberration is impossible. Thus, there is the problem that a substantially usable wavelength is restricted to a certain particular wavelength (or a very narrow wavelength region). Moreover, because chromatic aberration cannot be corrected, the resolution and contrast of an image may be degraded due to the chromatic aberration unless light with wavelengths other than this wavelength is cut off by a filter. In the illumination optical system, aberration also causes uneven illumination. Because the usable wavelength is limited to the above wavelength, only a dark image can be observed. Furthermore, this prior art publication fails to give a description of a combination with a conventional microscope for visible-region observation, and thus the possibility of this combination is unclear. Since a specific means or condition for selecting wavelengths in the ultraviolet region, shorter than 300 nm, is not described, the optimum resolution, contrast, and brightness cannot be obtained directly even if the technique is used as it is.
In contrast to this, with the technique disclosed in Hei 5-127096, chromatic aberration is corrected in the range from the visible region to the near-ultraviolet region, and hence it is possible to combine a visible-image observation with an ultraviolet-image observation. Since, however, chromatic aberration is not corrected in the region of wavelengths shorter than 300 nm, the resolution and contrast of an image in this wavelength region are considerably deteriorated and uneven illumination is produced. In addition, this publication provides a means for separating a visible image from an ultraviolet image, but fails to suggest a specific means or condition for selecting wavelengths in the ultraviolet region, less than 300 nm. Therefore, the optimum resolution, contrast, and brightness cannot be obtained directly even if the technique is used as it is.
The technique disclosed in Hei 8-313728 is provided for use in semiconductor exposure so that the transmittance of a wavelength used is increased and those of the other wavelengths are decreased. However, since the technique is not directed to photography and observation for which a TV camera is used, this publication does not in any way suggest to what extent the transmittances of the other wavelengths must be held or a consideration for the transmittances of wavelengths in the infrared region. Moreover, an illumination optical system and an imaging optical system are not specifically des
Kashima Shingo
Nishida Hiroyuki
Henry Jon
Olympus Optical Co,. Ltd.
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