Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1994-11-23
1997-03-04
Phan, James
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359206, 359662, 359792, G02B 2608
Patent
active
056085648
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a scanning objective for the linewise or pointwise three-dimensional scanning of object surfaces with high resolution and high testing speed.
The increasing packing density on an electronic assembly requires a correspondingly adapted testing. As a rule, this testing takes place by means of a high-resolution optical inspection system. In principle, the triangulation principle can be used in conjunction with a system for the rapid scanning of a surface. However, as a rule in the case of reflecting surfaces of structured objects, the confocal principle is more suitable. In this case, a point light source which is usually defined by an aperture diaphragm, is imaged onto the object surface and the back-scattered light is imaged onto what is almost a point detector. In this connection, reference is made by way of example to the European patent application bearing the official file reference 91 120 863.5. The depth of field of a confocal optical configuration is a measure of the height resolution of the system. In this case, the depth of field is inversely proportional to the square of the numerical aperture. A scanning objective having a high numerical aperture and a very rapid beam deflection unit, for example a polygonal mirror rotating at a high speed of rotation, satisfies the requirements with respect to high resolution for an automatic testing system. At the present time, however, a scanning objective having a sufficiently high numerical aperture is not available. A sufficiently high numerical aperture should exhibit at least a value of 0.15.
The quantity which is essential in the design of a diffraction-limited scanning objective, and which reproduces the theoretically attainable imaging performance, is the Lagrange invariant L. This is formed by the product of half the beam diameter D at the location of a beam deflection unit and the scanning angle or, respectively, deflection angle .theta.. This is synonymous with the product of numerical aperture NA and half the scan length S of the scanning objective. proportional to the number of scanned points per scan line. A high scan rate is thus achieved by a high speed of deflection in the course of scanning and the greatest possible Lagrange invariant L of the scanning system. In accordance with the abovementioned dependencies, both beam deflection unit and scanning objective must be adapted to one another.
With respect to the beam deflection unit, it results that for rotating polygonal mirrors as compared with other beam deflectors, the feasible angular velocity, with at the same time a large beam diameter, is very high and thus the pixel data rate, is very high. Other beam deflectors are understood to include, for example, acousto-optic deflectors, resonance scanners or galvanometer mirrors. A limitation of the data rate in an upward direction is provided by the increasing moment of rotational inertia with increasing beam cross-section or increasing mirror facet diameter of the mirror. Depending upon the material used and the mechanical construction of the polygonal mirror, the result is accordingly an optimal dimensioning of the polygonal mirror.
The fundamental design of a scanning objective must take into account the following dependencies: field), the more difficult it is to minimize geometric imaging defects and to design a scanning objective in production-oriented fashion. which is synonymous with an enlargement of the numerical aperture, the scan length is reduced with an unchanged Lagrange invariant. aperture is increased with constant focal length, then the beam diameter increases with a reduction of the scan angle. An increase in the beam diameter at a small scan angle does, however, increase the number and the size of the facets of the polygonal mirror, whereby again the attainable speed of rotation is reduced. aperture is increased while maintaining the scan angle, then the objective focal length is reduced. This means that in the case of conventional scanning objectives, the front and rear focal planes pass very close to
REFERENCES:
patent: 4734578 (1988-03-01), Horikawa
patent: 4893008 (1990-01-01), Horikawa
patent: 4953927 (1990-09-01), Vedder
Applied Optics and Optical Engineering, vol. X, Copyright 1987, W. B. Wetherell, "Use of Afocal Lenses in Scanning Systems", pp. 174 and 175.
Phan James
Siemens Aktiengesellschaft
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