Excimer Laser Produces Nanobump Arrays

Daniel S. Burgess

Two-dimensional arrays of 260-nm-diameter silicon bumps have been fabricated by exposing 1-μm-diameter polystyrene beads on silicon wafers to ultraviolet radiation from a KrF laser. Researchers at East China Normal University in Shanghai and at Data Storage Institute in Singapore suggest that the ability to produce such nanobump arrays may have applications in the production of high-density integrated circuits and data storage media.

Exposing 1-μm-diameter polystyrene beads to 248-nm radiation from an excimer laser yields regular arrays of 260-nm-diameter silicon bumps. Courtesy of Sumei Huang.

The scientists first coated the undoped wafers with a latex suspension of the beads that they had diluted with deionized water. They allowed the water to evaporate, yielding a monolayer atop the silicon surface. They irradiated the regular hexagonal lattice of beads with a single, 23-ns-long pulse of 248-nm radiation from the laser, focused into a 25 × 5-mm spot of uniform intensity.

Half-transparent at 248 nm, the beads nearly completely melt and evaporate at laser fluences greater than 45 mJ/cm². Their brief presence during the exposure, however, leads to an enhancement in the optical near field where they come in contact with the wafer, so that even an incident fluence of 150 mJ/cm² results in a fluence at the contact point well above the 500- to 750-J/cm² melting threshold of silicon.

As a consequence, the silicon becomes molten, and the researchers suggest that competing thermal and chemical capillary forces result in a net inward flow. Moreover, because liquid silicon is more dense than solid silicon, there is an increase in volume at the melt spot as the material cools, and a bulge forms.

The researchers note that the technique enables the adjustment of the distance between the nanoscale bumps through proper selection of bead diameter to control the near-field enhancement.

Applied Physics Letters, April 18, 2005, 161911.