Optical: systems and elements – Having significant infrared or ultraviolet property – Lens – lens system or component
Reexamination Certificate
1999-07-07
2003-01-28
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Lens, lens system or component
C359S364000, C359S365000, C359S366000, C359S385000
Reexamination Certificate
active
06512631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for bright-field and dark-field imaging applications.
2. Description of the Related Art
Many optical and electronic systems exist to inspect object surfaces for defects such as those on a partially fabricated integrated circuit or a photomask. Defects may take the form of particles randomly localized on the surface of the circuit or photomask, as well as scratches, process variations, and so forth. Various imaging techniques used to perform surface inspection for such defects provide different advantages depending on the types of defects present.
Two well known imaging techniques for detecting defects are bright field imaging and dark field imaging. Bright field imaging is commonly used in microscope systems. The advantage of bright field imaging is the image produced is readily distinguishable. The size of image features accurately represents the size of object features multiplied by the magnification of the optical system. Bright field imaging can be more easily used with image comparison and processing algorithms for computerized object comparison, defect detection, and classification.
Dark field imaging has been successfully used to detect irregularities on object surfaces. The advantage of dark field imaging is that flat specular areas scatter very little light toward the detector, resulting in a dark image. Any surface features or objects protruding above the object scatter light toward the detector. Thus, in inspecting objects like semiconductor wafers, dark field imaging produces an image of features, particles, or other irregularities on a dark background.
One advantage of dark field imaging is that it provides a large signal for small features that scatter light. This large signal allows dark field imaging to detect smaller object features and provide faster object inspections than bright field imaging. Another advantage is that Fourier filtering can be used to minimize repeating pattern signals and enhance the defect signal-to-noise ratio.
Many dark field imaging techniques have been developed to enhance the detection of different types of defects. These techniques consist of a specific illumination scheme and collection scheme such that the scattered and diffracted light collected from the object provides the best signal. Several optical systems have been developed that use different dark field imaging techniques including laser directional dark field, double dark field, central dark ground, and ring dark field.
When employing either bright field or dark field imaging it is often desirable to use short wavelength illumination in the 300-400 nm ultraviolet (UV) range, 200-300 nm deep ultraviolet (deep UV or DUV) range, or 100-200 nm vaccuum ultraviolet (vacuum UV or VUV) range. For bright field imaging short wavelength illumination provides improved resolution allowing the detection of smaller object features. For dark field imaging short wavelength illumination provides greatly increased scattering signals that allow the detection of smaller objects, an increase in inspection speed, or a decrease in the illumination power requirements. In addition, both bright field and dark field imaging can take advantage of changes in material absorption and reflectivities at short wavelengths. The changes in absorption and reflectivity of different materials at short wavelengths can help to identify these different materials. Also, many materials have greatly increased absorption at wavelengths in the DUV and VUV. Increased absorption can help improve optical inspection of upper surfaces, such as in semiconduictor wafer inspection, by minimizing reflections interference from underlying layers.
Optical systems supporting bright field and dark field imaging typically require correction over some finite spectral bandwidth or wavelength range. Correction is necessary because different wavelengths have different glass indexes, known as dispersion. Conventional designs usually use two or three glass types to compensate for dispersive effects. Compensating for these dispersive effects is called color correction. Color correction in the UV, DUV, and VUV wavelength ranges is increasingly difficult. At shorter wavelengths, the glass dispersion greatly increases and is difficult to correct. In addition, at shorter wavelengths fewer and fewer glass materials may be used for correction.
At wavelengths shorter than 365 nm there are very few glass materials having high transmission. These materials typically include silica, CaF
2
, MgF
2
, and LiF
2
. Of these materials, silica is most desirable to use in high end optical systems. Silica is a hard glass with low thermal expansion, no birefringence, high UV damage threshold, and is not sensitive to humidity. CaF
2
, MgF
2
, and LiF
2
are soft glasses which are difficult to polish, have high thermal expansion, some birefringence, and can be sensitive to humidity. Of these fluoride glasses, CaF
2
is the most desirable to use as an optical glass.
Minimizing the number of glass materials used in a UV, DUV or VUV optical system produces special challenges for correcting color aberrations. This is especially true in the VUV wavelength range where both silica and CaF
2
are extremely dispersive. Even a narrow spectral bandwidth at very short wavelengths can require the correction of numerous distinct color aberrations. Some important color aberrations that need to be corrected may include primary and secondary axial color, primary and secondary lateral color, chromatic variation of spherical aberration, and chromatic variation of coma.
At a wavelength of 157 nm, for example, CaF
2
is the only reasonable glass material that has high transmission and does not have severe problems with birefringence, water solubility, or mechanical softness. Standard color correction is not possible because no other glass material is available.
Another problem with currently available systems is that such systems provide a relatively short working distance between the optical system and the surface being inspected. Photomask inspection requires the working distance of the imaging system to be greater than approximately 6 millimeters due to the protective pellicle present on the photomask. A long working distance is also desirable in laser dark-field inspection environments. An imaging system having a long working distance makes it possible to directly illuminate the surface being inspected from outside the objective. Under typical circumstances, a working distance greater than 4 millimeters presents generally desirable attributes, while a working distance greater than 8 millimeters is preferred.
Further, a high numerical aperture (NA) provides advantages for high resolution imaging and collecting as large a solid angle as possible. It is highly desirable to achieve numerical apertures of 0.8, which corresponds to collecting angles above the surface from normal to 53 degrees.
A further problem with currently available systems is that while some relatively high NA systems exist, the central returning rays may be obscured due to apertures and other optical components. Such a central obscuration blocks low frequency information from the image and is undesirable.
Additionally, some presently available systems include internal pupil planes. A system having an internal pupil plane is undesirable because it does not readily support aperturing, particularly variations in aperture shapes, and Fourier filtering.
Finally, some currently available systems have limited field sizes. A large field size is often important for area imaging and to allow high speed inspection such as for semiconductor wafers and photomasks. Field size is typically limited by aberrations such as lateral color and chromatic variation of aberrations. Aberration correction is especially difficult if combined with chromatic correction for a large spectral bandwidth, high NA, long working distances, no central obscuration, and
Armstrong J. Joseph
Chuang Yung-Ho
Shafer David R.
KLA-Tencor Corporation
Sikder Mohammad
Smyrski & Livesay, LLP
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