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Metrology and Machine Vision: When Enough Is Never Enough
by Winn Hardin, Contributing Editor - AIA Posted 02/24/2010
In many ways, machine vision and metrology are synonymous. While metrology is the science of measurement, machine vision is the reality of measurement in industrial applications. Perhaps meaningful comparisons lie in the differences between the two.
Coordinate measurement machines (CMM) are precision instruments for making measurements, but they solve a different problem set than most machine vision systems. Most CMM machines are not in-line systems, while the “OEM inspection and measurement systems that use our cameras are typically in line solutions that make precision measurements on high-volume production lines,” explains Marcel Dijkema, Marketing Manager at Adimec (Stoneham, Massachusetts), camera supplier to OEM metrology system designers.
Developing in-line metrology systems for complex, precision parts in the flat panel, semiconductor, electronics, and general manufacturing industries places extreme burdens on component and system designers. Customers routinely ask for more speed, higher resolutions, and lower prices with each new generation of in-line metrology equipment. The only solution for machine vision companies is to figure out what customers really need, often before they know themselves, and deliver it at a price point and with a feature set that meets today’s manufacturing metrology needs.
Metrology Starts at the Sensor
The requirements for machine vision metrology applications vary considerably, from measuring 20-micron wide scribe lines in solar panels, to millimeter-sized capacitors on electronics boards measuring several centimeters on a side. While the scale may be quite different, the problem is essentially the same: how do you acquire images that place a pair of pixels on the smallest possible feature, while covering a relatively large area compared to the minimum critical dimension.
“We started with CCD cameras, but about 5 years ago, customers demanded speed that CCDs couldn’t match without complex read out schemes,” explains Jochem Herrmann, Chief Technology Officer at Adimec. “So we started making CMOS products. At first, there was a gap between the image quality - particularly in uniformity – that CMOS sensors delivered versus customer’s expectations based on CCD image quality. We spent quite some time in optimizing the camera for CMOS to compensate for its weaknesses while utilizing its strengths. We realized that the speed requirements in semiconductor and AOI [electronics] inspection at high resolutions could not be meet with CCD’s in the future.”
Adimec manufactures area arrays for metrology applications, eschewing line scan cameras, although they admit that both sensor types have their place. “Area arrays have advantages because they don’t have jitter and alignment issues, while line scan can produce high-resolution images” explains Adimec’s Dijkema. “We’ve answered by developing area array camera solutions that combine high resolutions with high frame speed and imaging accuracy”
“There are applications where both line scan and area scan can solve the problem,” continues Dijkema. “For high precision applications in flat panels, solar, front end and back end semiconductors, and (3D)-AOI/SPI electronics inspection, we think an area array is the best answer for OEMs looking for easy to use, robust solutions with low cost of ownership.”
As customers demand higher resolution cameras that work faster for increased system throughput, camera makers struggle with the right pixel size, array size, filling the pixel well with electrons, and finally, how to get all that image data off the sensor as quickly as possible. While pixel sizes have shrunk and stabilized at 5 microns2 on average for metrology applications, quantum efficiency, dark noise ratios, and application-specific signal processing remain the major differentiators when it comes to choosing a camera for metrology applications.
As mentioned earlier, the common thread connecting all metrology machine vision applications is higher resolution and higher speed, which translates to higher throughput on the inspection system and increases bandwidth demands on the image transmission medium.
Solar and flat panel are two applications where the camera is often moving, and can be located 30 meters or more from the processing unit. To deliver the data from the sensor to the processing unit, Adimec worked together with partners in a consortium to initiate a new vision industry interface standard: CoaXPress. CoaXpress combines transmission of video, communication, triggering and power at a datarate of 6.25 Gbps across a standard single flexible coaxial cable. Cables can be combined to deliver up to 25 Gpbs or even higher transmission speeds for a single camera. While maximum throughput of 6.25 Gbps is similar the full 2-cable configuration mode of Camera Link, CoaXpress coaxial cable can run over 100 m without a repeater, similar to GigE if power were included in the current GigE Vision specification.
“The solar industry is a unique opportunity for machine vision because of all the manufacturing steps requiring inspection,” explains Matt Huff, Technical Sales Representative for North America Industrial Applications at VITRONIC Machine Vision Ltd. (Louisville, Kentucky). “You go from silicon ingot, to wafer, then printed cell, the turn cells into strings, and strings into modules. There are a lot of steps for handling fragile silicon wafers, and damage can occur at any step. That’s why VITRONIC offers nine different inspection systems as part of its VINSPEC solar line for both amorphous silicon and thin-film solar panels. Some are area arrays, others line scan, but people always need more resolution. In the past, 40 micron minimum resolutions for 80 micron scribe lines were okay, but now the fingers or conductors [also called the P1, P2, and P3 lines] can be 30 microns wide, which means you need resolutions of 10 to 15 microns.”
Achieving resolutions on this scale across hundreds of millimeters is difficult enough, Huff says, but this is simply the price of admission for consideration. What customers really want is repeatability and stability greater than 99.7%. Proving that to prospective customers is a challenge, but experience and proof of concepts can make a bit difference. And while solar inspection is similar to semiconductor inspection, the differences are important. “Unlike semiconductors, which are pass/fail, solar products are often classified into different classes depending on their quality,” adds Vitronic’s Huff.
Front end semiconductor inspection, or wafer inspection, remains more challenging in performance than back-end or semiconductor packaging inspection, despite the growth of high density bump packages over leaded frame packages. “The big explosion in terms of speed and resolution comes from the front end, wafer inspection where critical dimensions keep shrinking as the wafer size increases,” says Adimec’s Herrmann. “Back end inspection systems are seeing additional cost pressures as more suppliers of back-end semiconductor inspection equipment enter the market.”
Electronics inspection, often referred to as Automated Optical Inspection (AOI), is the most challenging, volume metrology application on the market today, according to Adimec’s Herrmann. “The components are getting smaller, boards are more complicated, and customers are demanding 3D inspection for many applications. It really puts the pressure on the camera, lighting, and image processing elements.”
Know Thy Customer (Better Than They Know Themselves)
For machine vision suppliers in precision metrology applications, finding the right mix of high performance and cost-effectiveness can be a difficult task. “If you ask a customer what they want out of the next generation metrology camera, they say, "more of everything but cheaper and smaller". This is of course not helping us to optimize our product. If it comes to choose the 10 most important qualities – resolution and speed are easy. But specifying the rest is very difficult for many. It’s our job to understand our customers imaging inspection and business requirements, and work together with them and the sensor makers to figure out the right mix of camera functionality, performance and cost. That’s what we do,” concludes Herrmann.
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