William Witt Bloomstein
As consumer products pack higher and higher levels of functionality into smaller and smaller designs and manufacturers rely on increasingly miniaturized integrated circuits, laser trimming—and its unique ability to eliminate test pads and create more space on the die for circuits—is experiencing a rebirth. More than 70% of the world's analog-semiconductor companies use laser-based trimming and link-blowing in thin-film semiconductor and silicon manufacturing. In addition, advances in areas such as diode-pumped solid-state lasers and software tools have boosted trimming accuracy, as well as system flexibility and reliability, in the current generation of laser-based trimming systems.
"Today's demand for smaller electronic products is driving semiconductor lithography into linewidths in the half-micron range," explains Don Smart, senior scientist, GSI Lumonics (Wilmington, MA). "Closer wiring means smaller circuits on a wafer without having to increase fab costs."
Shedding traditional image
Traditionally, lasers have been associated more with maintenance, accuracy, and cost problems than with anything else. "From both an engineering and reliability standpoint, lasers in the early 1980s were often costly and maintenance-intensive," remembers Ron Erhardt, manager of test engineering, TelCom Semiconductor Inc. (Mountain View, CA). "The older arc-lamp lasers weren't as precise as they needed to be. But with the advent of new diode-pumped laser technology, everything has changed."
TelCom, a precision analog-semiconductor manufacturer, uses several diode-pumped laser systems for laser link-blowing (fuse cutting) for a range of analog- and mixed-signal ICs used in power and thermal management, such as notebooks, cell phones, and other consumer devices. "When we brought in the first diode-pumped laser, there were a lot of fears based on past experience," notes Erhardt. "We had to go through a lengthy cost justification. But it was completely worth it. More than 30% of our products in design are now being planned specifically for laser trim."
Enabling higher densities
Among the many benefits associated with using laser systems, the core is a premise that by using laser link-blowing or trimming, real estate dedicated to test pads can be freed up for higher device functionality and/or die size reduction, lowering overall process costs. "With laser trimming, we're making miniaturized parts that we couldn't otherwise make," says Fred Mapplebeck, manager, trim technology development, Analog Devices Inc. (Norwood, MA).
A manufacturer of precision integrated circuits used in analog- and digital-signal processing, Analog Devices currently deploys a large number of laser systems around the world linked to test equipment (as an integral part of the process) and to their manufacturing network to collect data for continuous quality control. "Using laser trimming (as opposed to link-blowing), we're tightening parameters to specifications that we couldn't guarantee through the traditional wafer-manufacturing process," says Mapplebeck. "To pack so much functionality into such small circuitry, the finishing touch requires laser trimming.
"The difference in lasers today is their exceptional positioning accuracy," he continues. "Previously, we had to constantly fine-tune the laser's position for every device type. Today's lasers offer fourth-generation technology with galvanometer positioning systems that are driven by high-performance digital-to-analog converters (DACs). As DAC technology has improved, so has laser accuracy."
Among the technical advantages of laser trimming and laser link-blowing, resolution can be adjusted with a greater granularity, more circuits can be placed in a given amount of space, and functionality can be boosted while dies continue to shrink. "Take the typical analog circuit," explains TelCom's Erhardt. "It requires six to eight test pads if you blow fuses electrically—each pad about 120 µm. With laser link-blowing, you can eliminate five to seven of those test pads per circuit. If you're working in 6/10-µm geometry, that is 25% to 30% of the die area that is now available for added functionality."
Boosting delivery speeds
For some manufacturers, laser processing offers a solid productivity boost; for others, it is the only practical way of making the device. Erhardt notes a third strategic benefit—faster order shipments. "We can now manufacture a single mask containing the 'generic' circuit designs of a commonly ordered device," he says. "We can fab thousands of these wafers at a single run without determining final device functionality. These wafers then sit in the probe area. When we receive a customer order, we can blow fuses to create the required functionality and get these finished devices to our customers fast. Rather than the traditional six to eight weeks, we can now ship volume orders—built to precise customers' specifications—in one-half the time. This also adds a significant control parameter to our inventory."
For companies such as Analog Devices and TelCom, shipping customer orders on time is often the difference between winning or losing the account. Erhardt says, "We're all fighting the same battle. Our customers want smaller and smaller devices. The only way to go is submicron technology and laser trim."
WILLIAM WITT BLOOMSTEIN, based in Cambridge, MA, is a writer specializing in high technology.