Laser wafer marking tracks IC production

The making of semiconductor integrated-circuit (IC) chips--at one time a labor-intensive operation in which silicon wafers were hand-carried from machine to machine, aligned by eye through a microscope, and tracked by careful technicians--has become a highly automated process where milliseconds count and glitches cannot be tolerated. If a wafer goes astray, or information on the number of manufacturing steps it has gone through disappears, an entire production line may have to be idled while troubleshooters are called in to figure out what went wrong.
To prevent such problems and to keep tabs on the manufacturing process itself, most modern IC fabrication facilities ("fabs") require that each wafer be labeled with its own identification (ID) mark in the form of a string of characters, a barcode, or a two-dimensional (2-D) matrix of pixels (Fab equipment then automatically tracks the wafer through its manufacturing stages to the point at which it is diced into individual IC chips. Any inspection data accumulated along the way can be unambiguously tied to the proper wafer.
Laser marking, with its combination of speed, permanence, and reliability, has become the standard means of marking wafers. Although the technology has been around since the 1970s, it has, through steady improvement and the advent of new applications, continued to serve the semiconductor industry

IC makers weigh improvements

Because silicon has a higher absorption for green light than for near-IR, most manufacturers of laser wafer markers now offer frequency-doubled solid-state lasers as an option--or, as in the case of NEC Corp., as standard equipment. The disadvantages of a frequency-doubled laser--lower power and higher cost--can be offset by improved marking performance resulting from the fact that energy absorption of the doubled light occurs closer to the wafer surface. But the choice between green and near-IR is not clear-cut. Because each IC chip maker has developed its own proprietary methods, what works well at one fab may not pass muster at another.
In the case of backside die marking, the consequence of silicon`s lower near-IR absorption is more obvious. Although opaque to the eye, a silicon wafer transmits enough of the Nd:YAG laser`s 1064-nm fundamental wavelength that a small amount of light can reach all the way to the underside of the die itself, potentially causing damage. But damage of this sort "is uncommon," says Downes of General Scanning. He notes that of all the chips being manufactured at fabs where backside marking is used, only one type of chip at one fab suffered performance degradation due to underside irradiation. Even so, General Scanning offers optional frequency doubling of its lasers, he says.
When operating at high power and slow scan speeds, laser wafer-marking systems are capable of digging pits and trenches in silicon with depths of from a few to more than 100 µm, called "hard" marks. But this sort of marking creates particles that contaminate and ruin chips. In addition, when used for backside die ID, hard marking can produce raised kerfs up to 30 µm high that prevent a finished chip from adequately contacting its heat sink.