Optical: systems and elements – Compound lens system – With curved reflective imaging element
Reexamination Certificate
2002-05-20
2003-05-06
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
With curved reflective imaging element
C359S365000, C359S434000, C359S728000
Reexamination Certificate
active
06560011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for multiple mode imaging, and more particularly to catadioptric optical systems used for dark field imaging applications.
2. Description of the Related Art
High precision optical instruments and imaging systems used in many different applications must operate effectively and efficiently. To accommodate optical functionality under varied conditions, precision lenses are often employed in different complex combinations.
Many different imaging modes exist for optical inspection. These imaging modes include bright field, confocal, and a variety of dark field imaging modes. Typically each different mode requires a different machine. Full inspection of an object, such as a semiconductor wafer, requires several separate very expensive machines. Combining many different imaging modes into one machine can dramatically reduce inspection costs as well as provide performance advantages.
The bright field imaging mode 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. This technique can be more easily used with image comparison and processing algorithms for computerized object detection and classification.
The confocal imaging mode has been successfully used for optical sectioning to resolve the height differences of object features. Most imaging modes have difficulty detecting changes in the height of features. The confocal mode forms separate images of object features at each height of interest. Comparison of the images then shows the relative heights of different features.
The dark field imaging mode has been successfully used to detect features on objects. 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.
Dark field illumination provides a large signal for small features that scatter light. This large signal allows larger pixels to be used for a given feature size, permitting faster object inspections. Fourier filtering can also be used to minimize the repeating pattern signal and enhance the defect signal to noise ratio.
Each dark field mode consists 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 modes including laser directional dark field, double dark field, and central dark ground.
One prior method for achieving laser directional dark field imaging is disclosed in U.S. Pat. No. 5,177,559, issued Jan. 5, 1993 to Batchelder and Taubenblatt and assigned to International Business Machines, which is hereby incorporated by reference. This method uses a collimated beam of monochromatic light to illuminate a semiconductor wafer from outside the objective between an angle of 8 degrees from the horizontal and the numerical aperture, or NA, defined by the imaging objective. Before forming a dark field image, the collected light passes through a Fourier filter to attenuate the spatial frequency components corresponding to repeating array patterns.
This laser directional dark-field method illuminates the wafer outside the NA of the imaging objective. For this reason, the illumination angles are limited to between 8 degrees from the horizontal and the NA defined by the imaging objective. Collection angles are also limited to the range of angles within the NA of the objective. A long working distance objective is necessary to allow access by the laser to the area of interest on the semiconductor wafer. Objectives used in dark field applications of this type are generally limited to NAs less than 0.7, which corresponds to collection angles of only up to 44 degrees from normal.
Another prior method for achieving laser directional dark field imaging is disclosed in U.S. Pat. No. 5,428,442, issued Jun. 27, 1995 to Lin and Scheff and assigned Optical Specialties, which is hereby incorporated by reference. This method uses a collimated beam of monochromatic light illuminating the wafer from inside the optical system, within the NA defined by the objective. If the system will encounter a specific range of defect sizes, the illumination angle on the wafer is chosen so the optical system collects spatial frequencies of interest.
This is a laser directional dark field method wherein the laser illuminates the wafer from inside the NA as defined by the objective. The system uses the same objective pupil plane for injecting the illumination and processing the light collected by the objective. This objective pupil feature seriously limits the types of illumination and Fourier filtering that are possible. Systems using objectives of this type are generally limited to NAs of less than approximately 0.9. This means illumination angles are limited to less than approximately 64 degrees. Illumination at angles above 64 degrees is often necessary to obtain optimum defect sensitivity. This high angle illumination is not possible without a higher NA objective. The available objectives of this type with a high NA have very small fields, relative to that of a lower NA objective. This seriously limits the number of resolvable points in the image and the achievable inspection speed. Another problem with this technique is small amounts of scattered and reflected light from lens elements in this system have the ability to produce noise at levels that compromise the sensitivity. Introducing laser illumination from inside this type of objective can cause a significant amount of scattered and reflected light from the multiple lens surfaces. The system must deal with scattered light from the lenses, illumination beam and the specular reflection from the wafer, which is a tremendous potential problem.
A third known dark field imaging method designed to detect particles on a periodic patterned object is disclosed in U.S. Pat. No. 4,898,471, issued Feb. 6, 1990 to Stonestrom et al. and assigned to Tencor Instruments, which is hereby incorporated by reference. This method uses a single light beam scanned at a shallow angle over the object. The position of the collection system as well as the polarization of the light beam may be arranged to maximize the particle signal compared to the patterned signal.
This single beam/shallow angle system uses an off axis collector to image the area of interest onto a detector. The position of the collection system as well as the polarization of the illumination is arranged to maximize the signal scattered by particles. Only a single angular position is used for illumination and another angular position for collection. Such a dark field system uses a single spot to scan across the wafer in conjunction with a single detector. If the system uses a small spot for high sensitivity detection, the inspection speed tends to decrease dramatically. If a larger spot size is used to increase inspection speed, the overall system sensitivity degrades.
In the practical industrial application for object inspection the scattered and diffracted light is collected from either side of the plane of incidence. Since this dark field mode collects light outside of the plane of incidence, this mode is categorized as double dark field. The double dark field technique often obtains maximum sensitivity when the collection angle is greater than 70 degrees from normal, which is well outside the range of a 0.9 NA objective. This makes the combination of the double dark field mode and other imaging modes such as bright field and laser directional dark field difficult.
Three physical
Armstrong J. Joseph
Chuang Yung-Ho
Shafer David
Tsai Bin-Ming B.
KLA-Tencor Corporation
Nguyen Thong
Smyrski & Livesay, LLP
LandOfFree
High NA system for multiple mode imaging does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High NA system for multiple mode imaging, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High NA system for multiple mode imaging will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3033677