Vision-based semiconductor inspection speeds die processing

Automated vision-based defect-inspection system performs 25,000 pass/fail quality wafer and die checks a day compared to manually screening and classifying a few thousand die per day.

By Lawrence J. Curran,Contributing Editor

Until recently, the visual inspection of wafers and die at the back end of semiconductor production was a manual process, performed by inspectors with microscopes who made thousands of pass/fail quality decisions a day. Both throughput and defect detection were limited by the skills and fatigue of the inspectors. Those problems confronted IC Services Corp. (Roseville, MN), when, almost five years ago, the company decided to expand its range of services to include wafer and die inspection. By adopting an automated vision-based system, the company solved the problems and achieved an inspection rate of 25,000 a day.

Click here to enlarge image

FIGURE 1. In operation, the NSX-80 automated defect-inspection system has hiked throughput at IC Services from a few thousand die per day when inspected manually to approximately 25,000 per day when inspected automatically.

The back end of wafer processing includes back-grinding the wafer, sawing it into individual die, and mounting the die on film frames, gel/waffle packs, multichip modules, or other packages. Accordingly, the wafers and die have to be inspected and cataloged for defects to maintain a high-quality semiconductor process.

Improve yields

Seeking to speed wafer throughput and boost yields, IC Services officials began evaluating the use of an automated inspection system. Jeff Miller, general manager, says the company looked first to a neighbor—August Technology Corp. (Edina, MN). As it turned out, August Technology was already at work on a prototype of an automated inspection system, which the company contends is the first optical wafer and die-defect-inspection system designed for back-end semiconductor manufacturing, known in the industry as second optical inspection.

Miller says IC Services placed the first $250,000 order for what has evolved into the NSX-80 automated defect-inspection system (see Fig. 1). IC Services currently operates two such systems. Armed with these inspection systems, Miller reports, the company hiked throughput from "a few thousand" die inspected per day manually to 25,000 per day automatically almost immediately after installing the system. That rate has remained fairly constant, even though the product mix has changed substantially, as IC Services has expanded beyond its early customer list to handle more product varieties and more-complex devices.

Click here to enlarge image

FIGURE 2. August Technologies NSX-80 automated defect-inspection system incorporates a black-and-white and a color camera to inspect semiconductor die that have been sawed but not separated from the wafer. Newer inspection systems use a dual-DSP architecture to boost image processing to 20 or 60 Mbytes/s. The color review monitor allows an operator to use microscope objectives to more closely inspect red-tagged die to classify suspected defects for later sorting into fail or pass bins.

"We inspect and classify 2000 die per hour, depending on the magnification required," Miller says. This capacity is traceable to the fact that the NSX-80 system takes a picture every half second—a function of the system's image-processing rate of 4 Mbyte/s. August Technology's newer systems are faster, claims Tom Verburgt, chief technical officer. He says the NSX-90 and NSX-100 can inspect tens of thousands of die per day using rates of 20 and 60 Mbytes/s, respectively.

IC Services' Miller stresses that for now, the NSX-80 system is equal to the present tasks at the company. "Scanning a wafer faster doesn't buy us enough because we still have to classify the defects," he says.

Essential components

The system's vision process begins with a commercial black-and-white area-scan CCD camera (see Fig. 2). The camera operates in continuous-light, electronic shutter or strobe modes as the wafer moves back and forth on the stage under custom-developed optics from Olympus America Inc. (Melville, NY). A second commercial three-chip color CCD camera provides in-line defect review as displayed on one of the system's two monitors. One is a 17-in. color monitor from Panasonic Medical & Industrial Video Co. (Secaucus, NJ), says Verburgt, that eliminates the need for a separate review station. The second monitor is a black-and-white, 17-in. flat-panel display from Viewsonic Corp. (Walnut, CA).

"We use a number of different frame grabbers," Verburgt says, some of which are DSP-based to help achieve the faster image-processing speeds of the NSX-90 and NSX-100 systems. The NSX-80's image processing is all done on the host PC, however; the camera data are delivered to a 400-MHz Pentium-based PC running the Windows NT operating system. The PC's motherboard uses MMX processing code, Verburgt says.

Click here to enlarge image

FIGURE 3. Automated inspection system displays and classifies semiconductor defects, such as this void in a metal run. Inspection data can be displayed, stored, printed, or exported onto other systems, a network, or the Internet.

The NSX systems take advantage of the higher image-processing speed of a DSP architecture. "We tried to do all the host-based image-processing," Verburgt says, "but soon realized that we needed dedicated embedded DSPs to reach the higher speeds of the newer systems."

The NSX-80's image-processing software is a combination of the Matrox Imaging Library (MIL; Dorval, Quebec, Canada) and algorithms developed by August Technology for both standard defect detection and "bump inspection." The MIL software executes functions such as conventional histograms and convolutions, while the proprietary algorithms perform metrology on "gold" and solder bumps on bumped wafers, Verburgt says.

Click here to enlarge image

FIGURE 4. This defect appears to be a misplaced drop of solder and would be tagged with a red dot by the inspection system. An operator would then call up all the parameters for the die on the wafer to determine whether the die should be classified as a failure. Failed die are routed to the Fail bin.

Other vendor-supplied components include the system's automated Olympus microscope turret, which provides five different inspection magnification levels, and an automated wafer-handling robot, the model ATM107 SCARA robot from PRI Automation Inc. (Billerica, MA). IC Services doesn't use the robot at this time. The precision x, y, z, and theta staging for the NSX-80 system, which uses a combination of off-the-shelf servo and stepper motors, is all custom designed and made by August Technology.

Process flow

IC Services' inspection tasks begin after the company saws a wafer into individual die, and the die, still adjacent to each other as they were on the wafer, are mounted on a film-frame stage to be fed to the NSX-80. "We receive one lot of wafers at a time from the saw, and each wafer is given its unique wafer identification by lot number and wafer number," Miller notes. The NSX-80 tracks each lot, building a database on each.

Before inspection begins, however, the NSX system is "trained" about the wafer to be inspected. That process takes about 30 minutes to manually enter data used as the "recipe" for inspecting that wafer. In addition to the identification information, the data include wafer and die size and die pitch, whether there's a notch or flat on the wafer, as well as the location of features such as alignment marks.

The system's detector is also trained by building a mathematical model or "golden model" of a die against which all the production die are compared. This model is constructed by selecting 30-40 die that are known to be good "and using the data from those to have the NSX create a statistical pixel model of a perfect die," Miller explains. This training process generates the inspection map of the wafer layout, Miller says.

Training also includes adjusting the system's sensitivity level for detecting features such as bonding pads, which aren't required to be as defect-free as the active circuit areas, as well as entering one of the four or five magnification levels required for that wafer.

Inspection recipe

The inspection recipe establishes the die sequence and sampling plans, which can include information such as whether the NSX-80 system needs to import a wafer ink-dot map from a diskette or from a network. Once the golden model and the inspection recipe are completed, the first few wafers in a lot are carefully monitored as they progress through the NSX-80 so that parameters such as sensitivity levels can be adjusted, if necessary.

Then, one wafer at a time is manually loaded onto the film-frame stage. The system proceeds to match the correct inspection recipe for that wafer ID and then initiates the automated inspection process. When each die of that wafer has been inspected, the system generates a colored map of the die, with green indicating a good die and red signaling caution. The system operator examines the red tags and uses hot keys to classify defects such as metal voids and fabrication scratches.

The recipe assesses whether the defect is large or serious enough to reject a red-tagged die, depending on the previously entered parameters. The system then performs a bin sort so that only known good die are sent to the customer.

Click here to enlarge image

FIGURE 5. The menu-based graphical user interface of the NSX-80 inspection system runs the Windows NT operating system. This interface supports numerous user-definable inspection recipes and permits fast and easy system setup.

"The system provides considerable detail," Miller says, "because it is armed with good database software." Defects can be counted by size, and histograms can be generated that are helpful in pinpointing quality drifts in the fabrication process.

IC Services used the NSX-80 as shipped by August Technology. The company runs wafers as large as 8 in. in diameter, although the system can accommodate 12-in. (300-mm) diameter wafers. Although wafers are manually loaded at IC Services, August Technology's Verburgt stresses that the NSX-80 and later models can be equipped with automated wafer-handling equipment, including a dual-cassette station.