Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
1998-01-22
2001-01-16
Mehta, Bhavesh (Department: 2721)
Image analysis
Applications
Manufacturing or product inspection
C382S149000, C356S239800, C356S237400
Reexamination Certificate
active
06175645
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of optical inspection techniques and relates to a method and a system for inspecting patterned objects such as, for example, photomasks, printed circuit boards (PCBs) or the like.
BACKGROUND OF THE INVENTION
There is a great variety of optical inspection systems having a common goal for locating defects existing on the patterned surface of an inspected object. The term “patterned surface” signifies such a surface which is formed with regions having different optical properties in respect of an incident radiation.
An inspection system of the kind specified typically comprises means for illuminating an object to be inspected, acquiring images formed by light reflected from the illuminated object, and image processing. However, if the inspected object is a photomask, flexible printed circuit board (PCB) or the like, whose patterned surface typically comprises transparent and opaque regions, the acquired images formed of light reflected from the illuminated surface are not indicative of such ‘defects’ as foreign particles, for example, of dirt or dust, which may occasionally be located in the transparent regions. Indeed, it is known that a surface of such particle is not mirror like, and, accordingly, light returned from the particle is irregularly reflected, scattered light. The problem is very essential when using the photomask as a phototool in PCB, graphic arts and printing industries.
There have recently been developed methods and systems wherein the inspection is performed by means of illuminating an object and acquiring and processing images formed of reflected and transmitted beams of light. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,572,598 and 5,563,702. The systems in both patents employ a so-called ‘scanning technique’, wherein an illuminating laser beam is generated and focused onto a pixel defining spot on the surface of an object to be inspected. The illuminated beam is deflected in an oscillatory fashion so as to sweep the spot across the inspected surface. The system is adapted for three different modes of operation. According to the first and second modes, so-called “Transmitted Light Inspection Mode” and “Reflected Light Inspection Mode”, the object is point-by-point inspected by means of detecting either transmitted or reflected light, respectively. These modes of operation are timely separated. The third mode of operation, which is aimed at defects classification, is based on detecting both reflected and transmitted beams of light. A single laser beam of incident radiation is directed onto the patterned surface of an object through light deflection means and is either reflected or transmitted, or partly reflected and partly transmitted by the object. This intensity of the incident beam is determined before its interaction with the object. Two separate detectors are accommodated at opposite sides of the object and detect transmitted and reflected beams resulting from this interaction. To this end, the system comprises separate directing optics for receiving the transmitted and reflected beams, respectively, and directing them onto the detectors.
This approach is based on that the interaction of an incident beam with an object to be inspected causes changes in beam's intensity, which changes depend on reflectivity and transmission of the respective region of the object. Hence, by appropriately detecting the intensities of the incident beams and reflected and transmitted beams, respectively, before and after the interaction, each inspectable point, or pixel, on the surface can be represented in a so-called ‘T-R space’, namely by a point with coordinates corresponding to the transmitted and reflected signal values produced at that point.
However, the system requires very complicated arrangements for illuminating and collecting optics. Indeed, the illuminating arrangement should be provided with the light deflection means and detector appropriately accommodated in the optical path of the incident beam so as to determine the beam intensity prior to the interaction with the object. This complicates and extends the optical path of the incident beam. Moreover, the use of a single beam of incident radiation results in an unavoidable requirement for locating the collecting optics, as well as the detectors for sensing the reflected and transmitted beams, at opposite sides of the object.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide novel method and apparatus for automatic optical inspection of an object by means of detecting reflected and transmitted light components of an incident radiation.
There is provided, according to one aspect of the invention, a method for an optical inspection of an object, having upper and lower face, so as to detect defects existing on the object, the method comprising the steps of;
(a) providing first and second beams of an incident radiation;
(b) directing the first beam of the incident radiation onto the object and sensing a light component reflected from one face of the object;
(c) directing the second beam of the incident radiation onto the object and sensing a light component transmitted through the upper and lower faces of the object;
(d) simultaneously acquiring first and second images of the object, wherein the first image is formed by the reflected light component and the second image is formed by the Transmitted light component; and
(e) analyzing said first and second images so as to provide data indicative of said defects.
The term “defects” used herewith signifies certain undesirable conditions of the object such as, for example, existence of foreign particles located on the object.
Thus, the idea of the present invention is based on the following main features. The first and second beams of incident radiation are produced and directed towards the object for focusing them onto the upper face. It is understood that, generally, each of the incident beams can be both reflected and transmitted by the different regions of the object. In other words, each of the incident beams, depending on the region of its interaction with the object, may be partly transmitted and partly reflected resulting in, respectively, transmitted and reflected light components. To this end, what is actually detected by two image sensors are, respectively, that light component of the first incident beam which is reflected from the object and that light component of the second incident beam which is transmitted through the object.
Generally, the first and second incident beams can be directed onto the object from the same side thereof, that is from either the upper or the lower face. In this case the sensors and associated directing optics are located. at opposite sides of the object. It should be noted that it is advantageous to illuminate the object from opposite faces. This enables the sensors to be placed at one side of the object and, accordingly, a common directing optics to be employed for receiving both the reflected and transmitted light components and directing them onto the respective sensors.
Thus, the reflected and transmitted light components are, preferably, directed onto the different sensors via a common optical system appropriately accommodated in the optical paths of the both light components. It is understood that, accordingly, means should be provided for successfully separating the different light components so as to be sensed by the different image sensors. To this end, two alternating embodiments of the invention are exemplified.
According to one embodiment, the first and second beams of the incident radiation are simultaneously directed onto different portions of the object. More specifically, they illuminate, respectively, first and second spaced-apart, parallel, identical strips of the upper face. The relationship between the two illuminated strips and the common optical system is such that the strips extend symmetrically relative to the optical axis of the common optical system. The optical system actually projects the st
Elyasaf Emanuel
Tirosh Ehud
Applied Materials Inc.
Mehta Bhavesh
Sughrue Mion Zinn Macpeak & Seas, PLLC
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