Shrinking Vision System: Smaller Is Better For Electronics Inspection

As ICs shrink, halving their critical dimensions every 24 months or so, the machine vision market has had to develop new solutions to meet more demanding product inspection requirements. Many solutions have come from the upper echelons of high technology, combining the science of automation and machine vision, with X-ray, e-beam and other non-destructive technologies to microscopic and now, nanoscopic, features; but these systems are expensive and are not suited to the majority of inspection nodes in a semiconductor manufacturing process. Today, the majority of inspection is still down in the visible spectrum, prompting an evolution of standard machine vision systems to smaller footprints with greater capability.

Make it Fit: The Right Tool for the Job
‘‘In the semiconductor inspection field, small size is important; high resolution is important; high speed is important,’‘ explains Marty Furse, CEO of Prosilica (Burnaby, BC, Canada), a manufacturer of high-performance, compact cameras.

‘‘Small is important because the camera often has to fit into existing equipment,’‘ Furse continues. ‘‘So the smaller the camera is, the more likely it'll fit without a problem.’‘ This has led to the use of remote head cameras connected to the sensor control electronics via high speed cable, and of cameras that are just plain small. ‘‘Our cameras tend to be smaller than most cameras, so we haven't felt any pull to develop remote head cameras since our complete camera is small enough to do the job. But that is where a lot of remote head cameras are used.’‘

Speed and Optics
Small is also important because it implies lightweight. The electronics industry is a big user of pick-and-place equipment and, for many applications, the easiest way to guide the pick-and-place machine to a part is to mount the camera directly on the pick-and-place arm, particularly when the part is small enough to challenge the accuracy of the pick-and-place machine. In these cases, vision mounted on the arm can help improve the overall accuracy of the system, but if the camera becomes too large or heavy, it can slow down the pick-and-place machine, require a machine with greater payload and higher cost, or slow down overall throughput if no changes are made.

Prosilica's EC-Series cameras have a footprint smaller than half of a standard business card, are lighter weight than a typical chocolate bar, and offer a variety of high-bandwidth cabling options, including GigE and IEEE-1394 (FireWire). High bandwidth is important because of the continuing and growing need for high-resolution imaging in semiconductor and electronics inspection.

Unfortunately, it's not as easy as simply installing megapixel cameras, as Prosilica's Furse explains. ‘‘To get the detail and accuracy per placement that electronics manufacturers need for manufacturing and inspection, one must use higher resolution optics as the camera's resolution goes up. End users often don't appreciate that the lens has a finite resolution; so they don't achieve the full performance of high-resolution camera if they're using standard CCTV type lenses.’‘

Furse adds that Prosilica steers customers towards Tamron's (Commack, New York) line of high resolution lenses for electronics applications.

‘‘When you use a large format lens on a small format sensor, you operate in the middle of the lens, which minimizes spherical distortion. Higher resolution lenses tend to have an aspherical element that reduces some of the aberrations, but there will always be some depending on the angle of view, but you can reduce that effect in software by calibrating out the distortion,’‘ Furse says

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Historically, high-resolution lenses used a lot of glass to try and reduce the distortion. In photography 20 years ago, a lens with very little distortion was very heavy and that's because it might have 10 or more glass elements in there – all trying to correct for various aberrations. Today, a high quality lens is very light, and that's because they use aspherical elements that correct for aberrations within a single element; this also speaks to the lightweight need for cameras and inspection systems in semiconductor inspection.

Fiber Comes Back to Electronics
Light emitting diodes (LEDs) quickly capitalized on the smaller-is-better trend prevalent in machine visible systems for the electronics industry. Although not as efficient in electric-to-optical conversion as traditional lamps, LEDs are very small and have long lifetimes, which answers another need for the electronics industry, namely, the need for uptime and fear of downtime.

‘‘The introduction and application of LED lighting in the semiconductor market has had a profound effect on our business because they offer some great operational value, in terms of longevity, and a lot of the electronic applications don’t require a bright, intense light,’‘ says Steve Giamundo, Vice President of Sales and Marketing at Fiberoptics Technology Inc. (Pomfret, Connecticut), a supplier of fiber optic lighting components. Prior to the introduction of LEDs, typical lighting scenarios for electronics assembly and inspection used remotely located lamps that used fiber optics to deliver the light to the region of interest (ROI). The lamps offered high output, but were typically too large and produced too much heat to incorporate directly into the equipment.

After falling away from the main stage in the electronics industry, however, fiber optic delivery is positioned to come back, according to Giamundo.

First, LEDs are small, but not as small as fiber. LEDs provide definable points of light instead of homogenous illumination, as most inspection systems prefer. Also, bright LEDs have thermal dissipation issues, which can be acerbated in small spaces and make light uniformity a greater problem. All of these issues are aided or solved by adopting a hybrid solution that uses LEDs as light sources, and fiber optics for transmission and light delivery.

‘‘There are really two parts to semiconductor processing where lighting is really important: automated inspection and the ability of the machines to locate a part,’‘ Giamundo adds. ‘‘Fiber optics still has a presence in the off-line inspection area. People still do quality control inspection of wafers and chips by looking through a microscope. In this case, it's better to use fiber optics to deliver light from a lamp because the lighting system has a better color temperature and it’s bright, and that relieves stress and fatigue on human operators. Brighter light lets you use higher magnification without increasing the size of the aperture, keeping objective costs down and resolution optimum.’‘

As the electronics industry evolves from a microscopic to a nanoscopic world, end users will continue to look for technologies that can catch defects as early in the manufacturing process as possible. While development has picked up in technologies that use smaller wavelengths of light to improve resolution and reveal hidden defects, it seems that visible inspection will continue to play an important – if smaller only in size – role.