Optics: measuring and testing – Lamp beam direction or pattern
Reexamination Certificate
1999-07-01
2001-02-06
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Lamp beam direction or pattern
C250S201900
Reexamination Certificate
active
06184974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an apparatus and method for evaluating an object, particularly an object larger than an aperture of a sensor.
2. Description of Related Art
The use of wavefront sensors, including Shack-Hartmann wavefront sensors, is a known technique for measuring the wavefront of light. The features of a surface, such as a wafer, an optic, etc., may be measured by reflecting light from the surface and directing it to the wavefront sensor. Wavefront sensors determine wavefront error through slope measurement.
In a Shack-Hartmann test, a plurality of lenslets arranged in an array are used to sample the wavefront. Each lenslet creates a corresponding sub-aperture. The resulting array of spots, which may be interpreted as a physical realization of an optical ray trace, are focused onto a detector. The position of a given focal spot is dependent upon the average wavefront slope over the sub-aperture. The direction of propagation, or wavefront slope, of each of the samples is determined by estimating the focal spot position shift for each lenslet. The wavefront may then be reconstructed from the detected image in a number of known manners. The resolution and sensitivity of the sensor are determined by the lenslet array.
There are several applications of the Shack-Hartmann wavefront sensor. Several of these applications have been extensively developed, with specific devices developed for adaptive optics, measurement of pulsed lasers and laser beam quality, ocular adaptive optics and measurement, and a wide variety of metrology applications. For some applications, the Shack-Hartmann sensor is advantageously applied, since it is relatively insensitive to vibration, independent of source light wavelength, and can be arranged in a simple, compact and robust assembly. A summary of uses of Shack-Hartmann wavefront sensors is set forth in D. R. Neal et al. “Wavefront Sensors for Control and Process Monitoring in Optics Manufacture,”
Lasers as Tools for Manufacturing II
, SPIE Volume 2993 (1997).
However, there are a number of metrology applications where the size of the target is a limiting factor in the application of wavefront or other metrology technology. Examples include large mirrors or optics, commercial glass, flat-panel displays and silicon wafers. While some previous methods have been developed, e.g., U.S. Pat. No. 5,563,709 to Poultney, which is hereby incorporated by reference in its entirety for all purposes, these suffer from a loss of spatial resolution when applied to large elements; and from difficulties in size and calibration.
An example of such a metrology application is the measurement of a silicon wafer. In such a measurement, the key result is the determination of surface defects that affect the fabrication of small features on the silicon wafer. The minimum feature size for microelectronic circuits has steadily decreased since their inception. Where 0.35 &mgr;m features are currently the norm, the next generation of circuits will need 0.18 &mgr;m or even 0.13 &mgr;m. Fabrication of these small features requires the detection (and elimination) of ever smaller size defects. At the same time, the wafer size is getting larger. The current generation of 200 mm wafers is rapidly being supplanted by the 300 mm wafer, with 450 mm wafers planned for the near future. The need for ever better resolution, combined with larger wafers places extremely difficult demands upon the metrology tools.
The current generation of metrology methods is clearly not scalable to the needs of these new processes. Such scaling to larger sizes requires extremely large optics with their associated high cost, large footprint and difficulty of fabrication. Furthermore, the required resolution cannot reasonably be obtained with such methods. The Shack-Hartmann method requires at least four pixels per lenslet. Thus, the resolution over a given aperture is limited. Scaling to larger areas with methods such as disclosed in Poultney, requires the use of cameras with an extremely large number of pixels. While the interferometry methods may be applied to larger areas with less loss in resolution, modern practical methods required the acquisition of 4-6 frames of data. This leads to difficulties in automated inspection in a clean-room environment because of vibration and to throughput reduction when analyzing a large object.
Other applications may be even more stressing than the wafer analysis discussed above. While silicon wafers may be scaled to 300 mm or even 450 mm, flat panel displays are currently being fabricated at 1500×600 mm. Scaling of existing metrology tools for single aperture measurement is clearly impractical. Automotive or commercial glass is manufactured in even larger areas, with 4 m wide segments not uncommon. Clearly an alternative technique is needed.
As the feature size to be analyzed decreases, the size of tolerable distortions decreases, and high resolution measurements must be made to insure sufficient surface flatness. This high resolution requirement is incompatible with making measurements over a large area. Further, the calibration of a system for measuring flatness over a large area in a single measurement requires a reference of similar dimensions, which is difficult to produce.
While some solutions, such as those set forth in U.S. Pat. No. 4,689,491 to Lindow et al., U.S. Pat. No. 4,730,927 to Ototake et al. and U.S. Pat. No. 5,293,216 to Moslehi disclose point by point analysis of surfaces, the analyzing disclosed in these patents is very time consuming.
SUMMARY OF THE INVENTION
The present invention is therefore directed to a method and apparatus for evaluating the surface of an object which is larger than an aperture of the sensor which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
It is therefore an object of the present invention to combine the advantages of the Shack-Hartmann sensor (namely insensitivity to vibration, measurement of surface slope directly, and invariance with wavelength) when applied to measure a small area with a large area measurement. For a small area, a very good reference flat may be obtained, and hence an extremely accurate measurement may be made. Off the shelf cameras and lenslet array technology may be employed. A number of adjacent and overlapping regions are measured using this technique over the whole surface of interest. In order to measure a large area, in accordance with the present invention, these regions are then “stitched” together with an appropriate algorithm that may take advantage of the slope information to provide a characterization of the whole surface. As used herein, the tem “stitching” means assembling a wavefront from the derivatives of wavefronts in the overlapping regions. In this way high resolution, yet large area measurements may be made without the need for extremely large optics or detectors. The method is scalable to any size that may be measured with appropriate translation devices.
At least one of these and other objects may be realized by providing a method for reconstructing a wavefront from a target having a plurality of subregions including illuminating a subregion, delivering light from the subregion to a lenslet array, detecting positions of focal spots from the lenslet array, determining a wavefront from the subregion from detected focal spot positions, repeating steps the preceding steps until all subregions have been measured, and stitching together wavefronts thereby reconstructing the wave front from the target.
The target may be ideally a flat surface. The method may include calibrating using a reference surface. The repeating may include moving the object and a system providing said illuminating, delivering and detecting relative to one another. The moving may include moving by an integral number of lenslets. The moving may result in a 10-50% overlap of adjacent measurements. When each subregion extends along an entire first direction of the target, the
Armstrong Darrell J.
Mansell Justin D.
Neal Daniel R.
Rammage Ron R.
Turner William T.
Jones Volentine, L.L.C.
Pham Hoa Q.
Wavefront Sciences Inc.
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