Wafer inspection using optimized geometry

#The present invention relates generally to optical inspection systems, and more specifically to optical inspection systems for obtaining data having very high signal-to-noise ratios.

#The present invention relates generally to methods of inspecting patterned and unpatterned semiconductor wafers for defect acquisition and defect classification. One known method of inspecting wafers involves illuminating a relatively small spot on a wafer using a light source such as a laser beam. This small spot of light is scanned over the wafer using either raster or spiral scanning until the surface of the wafer to be inspected has been covered. The light scatters from the wafer structures into a hemisphere in various directions with intensities depending upon the structure on the wafer's surface and any defects on the surface. The objective is to locate the defects on the wafer in the presence of scatter from both the defect and the intentional structures on the wafer's surface associated with the device (e.g., computer processor or memory chip) being fabricated on the wafer. The scatter from the structures can be considered noise to the signal that is the scatter from the defect. The scattered light is commonly detected by a fixed number of detectors in fixed positions about the hemisphere.

#In order to detect a defect, the signal-to-noise ratio of the collected scattered light must be sufficiently high. To obtain the most accurate defect analysis, it is desirable to locate the direction of the scattered light where the signal-to-noise ratio is the highest. The optimal region for the collection of scattered light can be anywhere on the hemisphere. By finding the optimum location in the hemisphere, the signal-to-noise ratios can easily be over 50 times greater than with fixed collection locations within the hemisphere. Therefore, to obtain the optimal signal to noise ratio, it is desirable to sample the entire hemisphere. Practically, it is not cost effective or easy to construct a complete hemisphere of detectors. Therefore, in current applications, a limited number of detectors are placed in what are hoped to be the optimal positions to obtain high signal-to-noise signals.

#These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.

#The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

#FIG. 1 illustrates a side plan, cross-sectional view of an optical inspection system according to one embodiment of the present invention.

#The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known operations have not been described in detail so not to unnecessarily obscure the present invention.

#The present invention pertains to an optical inspection system that is capable of obtaining both very high signal-to-noise ratio data and capable of high speed scanning rates. The ability to obtain high signal-to-noise data is contributed to by various aspects of the present invention. One aspect of the optical inspection system that contributes to a high signal-to-noise ratio is the set of lenses used to form a Fourier representation of the inspected specimen at a Fourier plane. Another aspect of the optical inspection system is a substantially hemispherical shaped mirror system that provides a large collection numerical aperture. Several embodiments of the present invention also disclose techniques for enhancing the signal-to-noise ratio of image data received from the optical inspection system.

#FIG. 1 illustrates a side plan, cross-sectional view of an optical inspection system 100 according to one embodiment of the present invention. The optical inspection system 100 is shown in FIG. 1 while being used in one of the typical uses of inspecting a semiconductor wafer 102 for defects, such as contaminating particulates or an incorrectly formed circuit structure. In order for inspection system 100 to inspect the entire surface of wafer 102, the light source is scanned relative to the inspection system 100 using scanning device 115, which scans the light source in an inspecting swath equal to the field of view of the inspection system optics. The wafer is moved underneath the inspection system 100 such that the wafer 102 will be inspected in a raster scanned manner of swaths. Alternatively, the wafer can be rotated while the lens system 100 is shifted such that the surface of the wafer is completely scanned in a spiral pattern of swaths. In this case, the radial and angular coordinates of the image can be transformed to Cartesian coordinates for computational purposes and to eliminate spatially periodic structures on the wafer. Alternatively, the inspection system 100 can be moved relative to the wafer 102 or both the inspection system 100 and the wafer 102 can be moved relative to each other. Inspection system 100 can also be used to inspect specimens other than semiconductor wafers.