Patent Application
20080037933
A full and enabling disclosure, including the best mode of practicing the appended claims, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended figures, in which:
Use of like reference numerals in different features is intended to illustrate like or analogous components.
Reference will now be made in detail to various and alternative exemplary embodiments and to the accompanying drawings, with like numerals representing substantially identical structural elements. Each example is provided by way of explanation, and not as a limitation. In fact, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the scope or spirit of the disclosure and claims. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the instant disclosure includes modifications and variations as come within the scope of the appended claims and their equivalents.
Inspection systems may use a fiber optic bundle or bundles to reduce speckle and other irregularities. See, for example, the bundle arrangement set forth in U.S. Pat. No. 6,892,013, filed Jan. 15, 2003 and assigned to Negevtech, Ltd., and U.S. patent application Ser. No. 10/345,097, filed Jan. 15, 2003, both of which are hereby incorporated by reference for all purposes herein. Although such prior solutions provide advantages such as coherence breaking, various disadvantages and points for further improvement exist. For instance, the spatial distribution of illumination on the wafer or other object that is illuminated may not be uniform, and the resolution of the image may be degraded.
Therefore, since more energy may be lost in longer fibers, there may be an appreciable variance in the intensity of light at the end of a fiber bundle.
The above-mentioned Kohler and/or critical illumination may be used in systems that utilize a fiber bundle. However, there are certain disadvantages that become apparent. For instance, in Kohler illumination, although irregularities in the angular distribution are smoothed out, the angular distribution at the source is mapped to the spatial distribution of illumination at the object plane. Therefore, the non-uniform angular distribution from the fiber bundle can result in non-uniform illumination of the wafer or other object being inspected. Furthermore, in Kohler illumination, spatial non-uniformities are mapped to the angular distribution at the object plane, and the resolution is degraded.
Critical illumination may avoid problems introduced by Kohler illumination, but may substitute others. If critical illumination is used instead of Kohler illumination, the non-uniform spatial distribution of light intensity will be imaged to the object plane. Furthermore, the non-uniform angular distribution of the light will be mapped to the object plane.
When light is used to illuminate an object, an image of that object will be dependent upon light that is reflected or scattered back to an imaging detector or detectors. The resolution of such imaging is dependent on the angular distribution of the light that is reflected and/or scattered from the object. The angular distribution of such reflected/scattered light is dependent on the angular distribution of the light illuminating the object. For example, a wide angular distribution gives finer resolution than a narrower distribution. In most cases, a uniform angular distribution is preferred, either as is, or as a controlled starting point for a more elaborate angular distribution achievable, for example, by adding additional optical components.
As discussed above, use of a bundle along results in both angular and spatial intensity non-uniformities due to the nature of the bundle—i.e., that it is constructed of a number of optical fibers, each of which having non-uniform angular distribution and spatial intensity characteristics. Accordingly, use of Kohler illumination may not be sufficient to address such non-uniformities, and may instead simply switch one problem for another by rearranging the underlying illumination problems. Therefore, further improvements to the uniformity of the underlying angular and spatial distribution of the light are desirable.
The skilled artisan will note that, although a tapered end facet 14 and first light guide 16 are illustrated, the source may be arranged to provide light directly to the end facet of fiber optic bundle 20 in any suitable fashion, such as by a lens, fiber bundle(s), light guides, direct coupling, an air gap, or any other suitable arrangement.
As shown in
For example, if the coherence length of source 12 is approximately 8 mm, the difference in length between any two fibers (or fiber groups) will be approximately 8 mm or less, equal to 8 mm, or greater than 8 mm. Alternatively, the fibers (or groups) may vary in length in a non-uniform fashion, or may vary in length such that the difference between individual fibers (or groups) are all greater than, less than, or equal to the coherence length of the source.
Fibers 18 of fiber optic bundle 20 are optically linked to light guide 22, as illustrated at the dotted box B. Light guide 22 (and light guide 16 for that matter) may comprise any single optical element, such as a single multi-mode fiber, a transparent rod, a hollow fiber, or a wave guide. The core diameter of the light guide 22 may be selected so that it is substantially equal to the diameter of the bundle 20. The light guide may be constructed of any suitable material or combinations of materials. For example, the light guide may comprise silica. As a further example, the light guide may be constructed of the same material as that used for the fiber optic bundle or of a different material.
Addition of an output light guide between the fiber optic bundle and the illumination source serves to substantially reduce or eliminate non-uniformities in both the spatial distribution of light and the angular distribution of light illuminating the object under inspection. The fiber optic bundle concept is retained for its advantageous coherence-breaking effects; alternatively, two or more serial bundles may be used, provided the ultimate output of such bundle(s) passes into a light guide.
Dotted boxes A and B illustrate transitions between input light guide 16 and fiber optic bundle 20, and between fiber optic bundle 20 and output light guide 22, respectively. The fiber optic bundle and light guides may be connected to one another in a variety of ways. For instance, in both connection areas A and B, the fibers 18 are fused to light guides 16 and 22, for example by being hot fused into a single optical unit.
This and other means of connection are illustrated schematically in
Returning to
For instance, the illumination system 10-1 could be implemented using an input light guide 16 hot fused to an input taper 14. For instance, the input taper 14 may have an input diameter between 4-6 mm and an output diameter of about 1.35 mm and a length of 100-200 mm. The input taper 14 may be hot-fused to an input light guide 16 having a core diameter matched to the input taper 14 and a length of about 1 m. The input light guide 16 may be hot-fused to fiber optic bundle 20 with a matching numerical aperture (NA). Bundle 20 may comprise 256 fibers, with the shortest fiber being 2800 mm in length and each fiber stepping up in length by 80 mm. Bundle 20 may be hot-fused to an output light guide 22 having a matching NA and core diameter of 1.35 mm, with a length of 14 m. The output end of light guide 22 may be positioned as a source in an optical inspection system directly, or may be positioned so that light first passes through a diffuser and/or other elements such as lens 301 for Kohler illumination, for example. Alternatively, suitable lenses, such as lenses corresponding to L2 and L3 as shown in
Tapers may be advantageous as inputs and/or outputs on light guide by allowing injection of high-energy beams into or out of the light guide while avoiding high energy density per-area at the light guide facet where the light guide material encounters the ambient environment (for example, at the interface between silica and air). In the ideal case, the taper is configured so that as the diameter changes, the output beam's numerical aperture changes relative to the input beam so that brightness remains substantially constant inside the taper. Furthermore, the taper may be advantageous, for example, when critical illumination is used, since the relative size of areas of surface non-uniformity will be smaller as compared to the larger facet area.
An illumination system such as 10-2 may be implemented, for example, using an input taper 14 having an initial NA of 0.22 and a final NA of 0.12 matched to the input light guide 16. The input light guide may have, for example, a core diameter of about 0.95 mm and a length of 1.0 meters, and be fused to fiber bundle 20. Fiber bundle 20 may comprise 128 fibers varying in length from about 2800 mm in steps of 160 mm. Bundle 20 may be hot fused to output light guide 22, which may have a length of 25 meters and be fused to output taper 26. Output taper 26 may have an initial diameter and NA matching light guide 22, and taper to an output diameter of 1.35 mm and NA of 0.22 over a length of 100 mm.
Illumination systems such as 10-3 may be implemented, for example, using an input taper 14, input light guide 16, and fiber bundle 20 similar to those discussed above in conjunction with
In various alternative embodiments, as noted above, fiber bundle 20 may comprise multiple groups of fibers with identical-length fibers within groups, but different lengths between groups. For instance, 256 fibers may be divided into 65 length groups with length variance steps of 625 mm. The number of fibers within each group may be equal, or may vary, for instance with between 3-5 fibers in each group. In a variant of the embodiment shown in
Of course, the skilled artisan will recognize that particular values discussed herein, such as the numerical apertures, fiber lengths, group lengths, and core diameters, light guide lengths and core diameters, materials, and other figures are presented for purposes of example only. Such values should be selected based on the characteristics of the light source(s) with which the illumination system will operate, keeping in mind the optical characteristics and arrangement of the inspection system in which the illumination system will operate, as well as the characteristics of the objects to be illuminated by the system.
At least partially coherent light energy is provided by source 12 into a first end of a fiber optic bundle 20. As discussed herein, the light energy may be provided by way of an input light guide 16 that includes an input taper 14, although other or additional components may be included between the source and the first end of the fiber optic bundle 20. The light is then directed into the first end of a light guide 22, which may be connected to the fiber optic bundle 20 in any suitable manner, for example by hot fusing.
Light output from the light guide 22 may be directed towards an object, such as wafer 100 as illustrated in
In bright field illumination in general, the illumination is incident on the sample through the same objective lens as is used for viewing the sample. As discussed above,
Of course, the particular arrangements of the lenses may be varied by one of skill in the art depending on the optical arrangement of the system to achieve Kohler, semi-critical, critical, and/or other illumination as desired.
The illumination returned from the wafer is collected by the same objective lens 201, and is deflected from the illumination path by means of a beam splitter 202, towards a second beam splitter 500, from where it is reflected through the imaging lens 203, which images the light from the wafer onto the detector 206. The second beam splitter 500 is used to separate the light going to the imaging functionality from the light used in other aspects of the inspection tool, such as the auto-focus detector 502 and related components.
When conventional dark field illumination is required for imaging, a dark field side illumination source 231 is used to project the required illumination beam 221 onto the wafer 100. When orthogonal dark field, or obscured reflectance dark field illumination is required for the imaging in hand, an alternative dark field illumination source 230 is used to project the required illumination beam 232 via the obscured reflectance mirror 240 onto the wafer 100 orthogonally from above. Alternatively, rather than three separate sources 12, 230, and 231, a single source or multiple sources in combination may be used. The source(s) may be repositioned, and/or have its output light redirected in order to achieve the different illumination effects.
Although the exemplary systems of
For example, another type of illumination may include semi-critical illumination, which is similar to critical illumination, with the difference being that the end facet is defocused. Use of semi-critical illumination may advantageously reduce the effects of surface non-uniformities, such as scratches and digs in the glass or other material(s) making up portions of the illumination system. Of course, still further types of illumination are also suitable.
It will be noted by one skilled in the art that the inspection system discussed in the present disclosure is for purposes of example only, and the illumination systems 10 discussed herein and variants thereof are applicable for use in a wide variety of inspection and other systems. The light guide and fiber bundle combination may be used alone or as part of a larger illumination system in other types of inspection tools and in other applications which benefit from uniform illumination, for example.
It is appreciated by persons skilled in the art that what has been particularly shown and described above is not meant to be limiting, but instead serves to show and teach various exemplary implementations of the present subject matter. As set forth in the attached claims, the scope of the present invention includes both combinations and sub-combinations of various features discussed herein, along with such variations and modifications as would occur to a person of skill in the art.
