1. Field of the Invention
The present invention relates generally to optical analysis systems, and more particularly, to an automated optical inspection system capable of making a three-dimensional analysis of an object, and even more specifically, an automated optical analysis system for inspecting the integrity and quality of electrical lead connections (wire bonds) between the pads of an integrated circuit and the conductive fingers of a lead frame prior to encapsulation.
2. Brief Description of the Prior Art
Because of their extremely small physical size, integrated circuit (IC) devices are normally packaged in an assembly which includes a protective housing for the IC and a plurality of conductive strips which facilitate connection of the IC to other electrical and electronic devices.
More specifically, during the packaging operation, the IC dies are attached to a lead frame which consists of an elongated strip of metal which has been etched to form a plurality of arrays of elongated conductors. Each array generally has one conductor with a central area (the lead frame island) upon which one die rests, and which acts as a grounding plane or heat sink for the die, and a number of connection fingers. After the die is placed on the island of the lead frame, the electrodes or pads of the die are connected to corresponding fingers of the lead frame plate by extremely thin bonding wires. Automatic wire bonders are capable of creating an interconnection of fine wire between an electrode of the die and a finger of the lead frame plate in 250 microseconds or less. See Dreibelbis et al, U.S. Pat. No. 4,586,642, issued May 6, 1986.
Although the production of integrated circuits is highly automated, trained human inspectors are still typically utilized to analyze the integrity and quality of the wire bonds before encapsulation. The process of inspecting the quality and integrity of the bonds and bonding wires placed by the wire bonders on the lead frame assemblies is commonly referred to as "bond inspection."
Because humans have relatively short attention spans and are prone to health related problems when employed for inspecting minute objects, the human inspectors have proven to be too unreliable for such exacting work. The high volume of integrated circuit manufacture, the high cost of human labor, and the unreliability of human inspectors has also typically resulted in unsatisfactory quality yields for semiconductor products. At the present time, bond inspection is merely a statistical quality control device rather than an accurate method for detecting bonding failures prior to encapsulation of the assembly.
The inability to detect bonding failures prior to encapsulation can result in significant increases in the cost of producing integrated circuits. Automated bond inspection systems, which would eliminate the human error inherent in the present bond inspection process, have been created which are capable of making two dimensional optical measurements of the electrical connections of the semiconductor assembly. However, because the measurements are only in two dimensions, the prior art systems have been unable to determine the placement and size of bonds, as well as the path or height of the bonding wires.
Methods of maximizing the gradient of an imaged object to bring the object into focus and thereby determine the distance from the imaging system to the object are well known. In an article by Gerd Hausler and Eva Korner, "Imaging with Expanded Depth of Focus," Zeiss Information, Oberkochen, 29, 9-13 (1986-/87), No. 98E, it is suggested that by increasing the depth of focus of the imaging system by taking a number of different images of the same object at different heights from the object and then recombining those images, a single image of greatly improved clarity can be produced. However, even this improved quality two-dimensional image does not reduce the noise associated with the maximized gradient to a point that such a method can be practically utilized in a number of applications where a high optical contrast is required between the imaged object and its background.
A three dimensional bond inspection system has been developed by View Engineering, Inc., of Chatsworth, Calif. This system uses a number of different video cameras and standard illuminations to create three dimensional images of the assembly and its connections by correlating images taken from different optical points of view. A significant problem associated with using a standard illumination technique in bond inspection and other applications is that the background, in this case the lead frame leads, fingers and pads, and the object being analyzed, the wires, ball bonds or crescent bonds, are quite often made from similar materials and reflect relatively equal quantities of light. Thus, although a three dimensional inspection system may be able to make three dimensional comparisons of the assembly, the poor contrast between the desired object and the undesired background makes such systems of limited usefulness. In addition, multi-camera systems which image and illuminate from a variety of different optical points of view can be as slow, unreliable and costly as human inspectors, thereby providing semiconductor manufactures with little incentive to automate the bond inspection process.
In Sanderson, Weiss, and Nayar's article, "Structured Highlight Inspection of Specular Surfaces," IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 10, No. 1, January 1988, a method is suggested for illuminating and imaging specular surfaces of a three-dimensional object which will yield information regarding the surface height of the object and its orientation. Two approaches are suggested by this article, a single optical perspective structured highlight approach and a stereo structured highlight approach. Both approaches use one or more fixed imaging devices and a multiple of different light sources, each illuminating the object at different angles of incidence, to recreate a three-dimensional object in two-dimensions. However, because both systems require multiple images of the same portion of the object from different illumination perspectives, a significant amount of time and computation is required to create the desired imaging information.