Technique Uses Lithography and Etch to Dice Arbitrary Shapes

How many times have you been standing on the fab floor, looking at M.C. Escher's Sky and Water I woodcut, lamenting to your coworkers how you wish you could cut die to look like those flying duck and fish shapes, instead of boring squares and rectangles? Although your answer may be "Never," researchers have recently done just that — and for more than just artistic reasons.

Today's conventional methods of dicing wafers — whether cutting or scribing and breaking — allow for only rectangular die. While rectangles may be perfectly suitable for standard circuit designs, they are not well suited for other types of microfabricated devices, such as arrayed waveguide gratings (AWGs), microfluidic devices or MEMS, all of which are not necessarily rectangular. Because cutting and scribing methods only work with rectangular geometries, these arbitrarily shaped devices must be cut into rectangles, fewer fitting onto a given wafer, and thereby limiting yield. Another limitation with conventional dicing is a minimum die size of ~1 mm. Devices smaller than 1 mm must still be cut into the larger die, again causing yield to suffer.

Researchers from Lucent Technologies' Bell Laboratories (Murray Hill, N.J.) have devised a technique that uses photolithography and deep-silicon reactive-ion etch (DRIE) to dice the wafers, enabling any arbitrary shape to be cut into the wafer. In the extreme Escher example, they were able to cut 18,000 die from a single 200 mm wafer.

They describe their work in the June issue of the Journal of Vacuum Science and Technology B. They used a 200 mm wafer polished on both sides to a thickness of 600 µm, then coated with 15 µm of Shipley SJR 5741 photoresist. Semiconductor wafer dicing tape was mounted on the backside of the wafer to hold the pieces in place after etch. Contact lithography exposed the resist — using a Karl Suss MA6 contact proximity printer with backside alignment and 9 in. masks. The pattern was etched into the wafer with an Aspect ICP etcher from Surface Technology Systems, timing the process to stop on the dicing tape. The Bosch process, in which a silicon etching step with SF₆ is alternated by a polymer deposition step with C₄ F₈, was used to protect the sidewalls. Finally, the resist was stripped by dry etch in oxygen.

In this experiment, contact printing offered sufficient dimensions, printing the duck and fish pattern with 30 µm spacing between each die (Figure ). Contact lithography would likewise be sufficient for such practical applications as waveguide gratings, which may fit only two to a 150 mm wafer with conventional dicing methods, according to Stanley Pau of Bell Labs. With this new technique, manufacturers could fit three crescent-shaped waveguide gratings onto the wafer, he noted, increasing yield by 50%.

SEMs show individual die after dicing flying duck and fish shapes from a wafer. In this experiment, 18,000 arbitrary die were cut from a 200 mm wafer. (Source: D.M. Tennant, Lucent Technologies)

But applications and sizes are limited only by lithography techniques. Whether rectangular or not, if devices get too small, dicing becomes difficult. The researchers estimate that the smallest die for a 200 mm wafer is ~2 mm square. "Anything smaller would be difficult to dice because of gap spacing and motion of pieces during cutting, leading to uneven size and unaligned pieces," Pau said. Although the Bell Labs researchers demonstrated non-rectangular pieces of ~1 mm square, he estimates that they could go down to a 0.1 mm square die. "I guess I should have made it even smaller, since the feature size is only limited by lithography."

For example, instead of the thick resist used in the experiments, the process could be done instead with a hardmask and a very thin resist (~1 µm), Pau noted. "Then you could use conventional 248 and 193 nm steppers," he said. "If you're going to do that, you could really get small features."

The etching process could also be varied considerably, depending on the thickness of the wafer and the applications. One variation could use an additional layer of oxide or nitride on the back of the wafer to act as an etch stop, which would provide better uniformity and holding. In another example, rather than etching all the way through from the front, the wafer could be etched half way from the front side, then the tape mounted on the etched side, and the rest of the wafer could be etched from the backside. For some MEMS applications, Pau noted, you may not want anything touching the front of the wafer, so it might make sense to vary the method.

Bell Labs has filed a patent for the new dicing technique, but has no plans for commercial development, since everything can be done on existing industry tools. However, the technique will be available for licensing, Pau said.