Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-05-15
2004-09-21
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201900
Reexamination Certificate
active
06794625
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of auto-focusing techniques and relates to a method and apparatus enabling focus error correction to maintain a desired position of an article.
BACKGROUND OF THE INVENTION
There are a number of optical systems utilizing focus error correction techniques. Such optical systems, which typically comprise an illumination system equipped with suitable light directing optics and a detection system equipped with light collecting optics, are used for inspection/measurements of articles, reading/recording information in an information carrier, etc. The focus error correction technique ensures that the article under inspection (or an addressed layer of the information carrier) is located in a focal plane defined by the light directing optics.
According to one conventional auto-focusing technique disclosed, for example, in U.S. Pat. Nos. 4,123,652 and 5,563,702, a light beam scattered from an article is directed through anamorphic elements, and the shape of the light beam cross-section after passing these elements is monitored. The basic principle underlying the implementation is that a cylindrical lens produces astigmatism. A focused beam is first displaced from an in-focus point in a first direction, and then in a second direction perpendicular to the first direction. The beam cross section therefore changes from being oblong in the first direction, to being circular, to being oblong in the second direction. Thus, an auto-focus beam is typically split into a pair of sub-beams, which propagate along mutually perpendicular optical paths onto a pair of detectors through a pair of astigmatic elements, respectively. The detectors are quadrature photodiodes. When the article is in focus, both detectors are illuminated by a circular beam. As the article goes out of focus, the horizontal diameter of the beam decreases on one detector and increases on the other.
This changes the electrical output from the quadrature detectors and the focus correction signal is thus calculated accordingly.
SUMMARY OF THE INVENTION
There is a need in the art to facilitate focus error correction by providing a novel method and apparatus utilizing a dynamic auto-focusing technique. The present invention enables the detection of a position of the plane defined by an article under processing relative to the focal plane of an optical system, thereby enabling maintenance of accurate focusing of an incident beam from the optical system to the article.
The term “processing” used herein signifies any procedure applied to the article during which the article should be maintained in a preset plane. This procedure may be inspection, information reading/recording, etc.
One aspect of the present invention is based on the detection of at least two interference patterns formed by light components returned from an elongated illuminated region on the article and passing through at least two symmetrical peripheral regions of the optical axis of a focusing/collecting lens arrangement. The term “periphery regions” used herein signifies regions of light propagation along an optical axis of the lens making substantially large angles to and lying substantially far from the optical axis (as compared to paraxial regions of the optical axis).
Each of the interference patterns is created by interference between collected light formed of light components propagating within one of the periphery regions and collected light formed of light components propagating within the paraxial regions of the optical axis. The relation between the interference patterns is indicative of the phase difference between the collected light formed of light components propagating within the paraxial and periphery regions, and consequently, of the out-of-focus position of the illuminated region.
One concept underlying the present invention is based on the following known phenomena. Light waves returned from in-focus and out-of-focus planes and passed through a focusing lens have substantially flat and spherical wave fronts, respectively. Consequently, information indicative of distortions produced at any out-of-focus location is actually that contained in light propagating within a periphery region of the optical axis, rather than that associated with a paraxial area. Hence, it is desired to analyze light components propagating within the periphery regions, to detect the focus error. On the other hand, light components propagating within the paraxial and periphery regions of the optical axis have different optical paths, respectively, and therefore have a certain phase difference indicative of the out-of-focus position of the article.
In the inventive apparatus, the same focusing optics (typically composed of one or more objective lens) directs incident light onto an elongated region of the article, and collects light returned from the illuminated elongated region. This focusing optics presents an aperture stop whose physical dimensions define the collected light portion. Collected light, which contains information of the illuminated region and of the aperture stop, enters a focus detection unit, and is directed towards the sensing surface of a detector through an optical system.
When the collected light passes through the optical system, an image of the aperture stop is first created (by a first lens arrangement) in a plane conjugate to the aperture stop plane and extending in an X-Y plane perpendicular to the optical axis of light propagation through the optical system. Then, at least three spatially separated light components are picked up (selected) from the light indicative of this image by appropriately locating at least three slits in the image plane, and are imaged onto the sensing surface of the detector by suitable optics (second and third lens arrangements). The slits are accommodated such that at least one of the selected light components contains light propagated within the paraxial region of the optical axis of the focusing/collecting optics, and the at least two other light components contain light propagated within two symmetrical periphery regions of the optical axis. The slits are made in a blocking plate (located in the X-Y plane), extend along the X-axis (parallel to the elongated illuminated region), and are aligned in a spaced-apart parallel relationship along the Y-axis, such that the optical axis of the optical system intersects with an axis of the at least one central slit.
The central slit may be relatively long, extending across the entire blocking plate, or may be in the form of a two-part slit, the two parts being spaced apart from each other along the X-axis and being spaced from at least two side slits along the Y-axis. The side slitsmay be relatively short (as compared to the long central slit). The side slit(s) located at opposite sides of the central slit are centrally symmetrical relative to the intersection point between the slits' plane and the optical axis of the optical system, and are spaced from each other along both the X- and Y-axes.
According to one embodiment of the invention, there are provided two side slits located at opposite sides of the central slit. In this case, two interference patterns are created.
According to another embodiment of the invention, two pairs of side slits are located at respective opposite sides of the central slit. In this manner, a dynamic range of the detected signals can be extended. The slits of each pair may be spaced from each other along the Y-axis only, in which case two interference patterns are created, or may be spaced from each other along both the X- and Y-axis, in which case two pairs of interference patterns are created.
The slits are sufficiently thin to cause the diffraction of light emerging from the slits. Two interference patterns are created on the sensing surface by the interference between, respectively, the diffracted light ensuing from the upper slit(s) and the respective half (or part) of the central slit, and the diffracted light ensuing from the lower slit(s) and the other half (or part) of the central slit.
W
Applied Materials
Luu Thanh X.
Sughrue Mion LLP
LandOfFree
Dynamic automatic focusing method and apparatus using... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dynamic automatic focusing method and apparatus using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dynamic automatic focusing method and apparatus using... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3200365