Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-10-18
2001-01-30
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C702S159000, C250S559230, C250S559310
Reexamination Certificate
active
06181425
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and systems for high speed measuring of targets and, in particular, to methods and systems for high speed measuring of microscopic targets which may be “non-cooperative.”
BACKGROUND ART
Recognition of Need
A class of three-dimensional imaging and measurement applications now requires unprecedented demonstration of capability to support new microelectronic and micromechanical fabrication technologies. For example, emerging semiconductor fabrication technologies are directed toward establishing a high density of interconnection between the chip and package. The “bumped wafer” and miniature ball grid array (“&mgr;-BGA”) markets are emerging, and large scale growth is predicted. For instance, NEMI (National Electronics Manufacturing Initiative) has clearly indicated that the miniature array technologies are to replace traditional wire bonding interconnects. Manufacturers are experimenting with new processes. Measurement tools to support their efforts will require versatility.
For example, “dummy wafers” are used for many experiments, which have a specular and featureless surface onto which interconnects are placed. The appearance is much different than patterned wafers seen in typical production environments. This imaging phenomena is of little concern to the process engineer. In fact, the most difficult imaging problems may coincide with the best choice of process. Industry process development engineers indicate that reflowed spherical solder bumps with a smooth surface finish, sometimes a nearly perfect mirror, may be the preferred technology for the chip interconnects. The surface reflectance will vary because of process engineers' choices of relative content of lead and tin. Such targets are often “uncooperative.”
The chips onto which the balls are placed are subsequently attached to printed circuit boards where both flattened and spherical mating interconnects can be expected, with either a dull or smooth surface finish. All combinations are expected. Other geometric shapes (wire with flat top, cones) can be expected in the future which will pose measurement challenges, particularly when the surface is specular with spherical or cylindrical geometry, including concavities.
Such “non-cooperative” targets, (i.e. those which present challenges for measurement systems as a result of light reflection, scattering, and geometry), are and will continue to be growing in occurrence for semiconductor, micromachining, and mass storage imaging applications. A specific growing need is recognized for an imaging system capable of improving dimensional measurement of &mgr;-BGAs and bumped wafers (i.e. “spherical mirrors” on variable wafer backgrounds) and other such targets, which are “non-cooperative” with respect to traditional imaging systems. As inspection and measurement requirements for industries requiring microscopic measurement capabilities, for instance semiconductor and mass storage, become more demanding, extraordinary versatility will be needed for handling wide variation in scale, target geometry, and reflectivity. Similarly, inspection and measurement of circuit boards and the dielectric and conductive materials requires a versatile imaging system, particularly for fine geometries and densely populated component boards.
As mentioned previously, imaging requirements for the semiconductor packaging industry include defect detection as part of Package Visual Inspection (PVI), measurement of &mgr;-BGA height, coplanarity, diameter, and wafer defects. High resolution and image clarity obtained from reduction of image artifacts are both required for adequate process characterization. Problems similar to those in the semiconductor area are also present when measuring other miniature parts like micromachined (micromechanical) assemblies, like miniature gears and machines, and components utilized in the mass storage industry, including substrates, disk heads, and flexures.
For example, as illustrated in
FIGS. 1 and 7
, inspection of a very fine solder bump or ball
20
with a “pin” or tip
22
necking down to about 1-3 &mgr;m in dimension mounted on a solder pad
24
, poses a measurement problem. Manufacturers often examine the tip
22
with an electron microscope for initial evaluation, but such a tool is much too slow for detailed process characterization or real time control.
Also, detection of small “hairline” burrs on IC leads is often successful using gray or 3D data using only triangulation, but false alarms are common because background noise and reflection from a container, such as a tray wall
26
, can appear similar to the defect such as a burr
27
, as illustrated in FIG.
2
. Conversely, IC leads
28
of an IC chip
30
may be indistinguishable from the background noise
32
. These false alarms are unacceptable and lower yields, thereby decreasing the value of inspection equipment.
&mgr;-BGA inspection can be roughly equivalent to measuring a tiny “spherical mirror” (solder ball) mounted on a plane “mirror” (wafer) background; yet, in other cases, where the wafer is patterned and the ball has a lower tin content, is a completely different imaging problem. Solutions to such measurement problems will require versatility for handling the geometric shape and reflectance variation.
Hence, with wafer scale and other sub-micron measurement tasks, the challenges with material properties will grow, not diminish. There is a need to measure substrates, conductors, and thickness of films, or the geometry of micromechanical assemblies such as miniature gears having deep, narrow dimensions and varying optical properties, including partially transparent layers.
Prior Art Technology
Early work on defect detection of features having specular components using camera-based inspection is described in U.S. Pat. No. 5,058,178 and the references cited therein. The method is primarily directed toward lighting and image processing methods for defect detection of bumped wafers. The lighting system included combinations of bright and dark field illumination. Measurement of the diameter can be done with a camera system and appropriate illumination, but accuracy is often limited by light scattering and limited depth of focus when high magnification is required. However, in addition to defect detection and bump presence, there is a need to measure the three dimensional geometry of the bumps for process characterization. The bumps must be coplanar to provide a proper connection, and the diameter within tolerance for a good connection with the bonding pads.
Triangulation is the most commonly used 3D imaging method and offers a good figure of merit for resolution and speed. U.S. Pat. Nos. 5,024,529 and 5,546,189 describe the use of triangulation-based systems for inspection of many industrial parts, including shiny surfaces like pins of a grid array. U.S. Pat. No. 5,617,209 shows an efficient scanning method for grid arrays which has additional benefits for improving accuracy. The method of using an angled beam of radiant energy can be used for triangulation, confocal or general line scan systems. Unfortunately, triangulation systems are not immune to fundamental limitations like occlusion and sensitivity to background reflection. Furthermore, at high magnification, the depth of focus can limit performance of systems, particularly edge location accuracy, when the object has substantial relief and a wide dynamic range (i.e. variation in surface reflectance). In some cases, camera-based systems have been combined with triangulation systems to enhance measurement capability as disclosed in the publication entitled “Automatic Inspection of Component Boards Using 3D and Grey Scale Vision” by D. Svetkoff et al., P
ROCEEDINGS
I
NTERNATIONAL
S
YMPOSIUM
ON M
ICROELECTRONICS
, 1986.
Confocal imaging, as originally disclosed by Minsky in U.S. Pat. No. 3,013,467, and publications: (1) “Dynamic Focusing in the Confocal Scanning Microscope” by T. Wilson et al.; (2) “Digital Image Processing of Confocal Images” by I. J. Cox and C. J. R. Sheppard; (3) “Three-Dime
Ehrmann Jonathan S.
Kilgus Donald B. T.
Lin Warren
Svetkoff Donald J.
Brooks & Kushman P.C.
General Scanning Inc.
Pham Hoa Q.
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