Optimizing use and performance of optical systems implemented with telecentric on-axis dark field illumination
View Patent ↗Systems and methods are provided for imaging a planar specular object such as a semiconductor wafer. In one embodiment, an imaging system for imaging a defect on a planar specular object includes a telecentric lens having a sufficiently aspherical surface such that the telecentric lens is substantially corrected for an optical aberration. The imaging system also includes a telecentric stop including an aperture therein to block light reflected from the planar specular object while allowing light reflected from the defect to pass through the aperture. The imaging system further includes a lens group having a system stop positioned between the telecentric stop and the lens group. The lens group is substantially corrected for the optical aberration independent of the telecentric lens.
1. A method for aligning a telecentric on-axis darkfield (TOAD) lighting device with a substantially specular surface, the TOAD lighting device having a plurality of concentric illumination arrays, the method comprising:
adjusting an angle of incidence between the TOAD lighting device and an object plane in a first direction until an area of intense brightness is substantially removed from a first side of an image of the specular surface;
recording the adjusted angle of incidence as a first measurement;
adjusting the angle of incidence between the TOAD lighting device and the object plane in an opposite direction as compared to the first direction until the area of intense brightness is substantially removed from a second side of the image of the specular surface;
recording the readjusted angle of incidence as a second measurement;
determining an aligned angle of incidence for the first direction as an approximate difference between the first measurement and the second measurement; and
aligning the TOAD lighting device according to the determined aligned angle of incidence for the first direction.
2. The method of claim 1 , wherein adjusting the angle of incidence between the TOAD lighting device and the object plane comprises:
selecting an innermost concentric illumination array; and
adjusting an angle of incidence between the innermost concentric illumination array and the TOAD lighting device.
3. The method of claim 1 , further comprising:
adjusting the angle of incidence between the TOAD lighting device and an object plane in a second direction until the area of intense brightness is substantially removed from a third side of the image of the specular surface;
recording the adjusted angle of incidence as a third measurement;
adjusting the angle of incidence between the TOAD lighting device and the object plane in an opposite direction as compared to the second direction until the area of intense brightness is substantially removed from a fourth side of the image of the specular surface;
recording the readjusted angle of incidence as a fourth measurement;
determining an aligned angle of incidence for the second direction as an approximate difference between the third measurement and the fourth measurement; and
aligning the TOAD lighting device according to the determined aligned angle of incidence for the second direction.