Mirror Enables 3-D Lithography

Michael D. Wheeler

Ball Semiconductor Inc. has set its sights on revolutionizing the computer-chip industry by manufacturing a spherical chip. To accomplish this, it has incorporated a multifaceted mirror into its proprietary lithography system.

The process used to create a spherical microchip may eliminate costly and time-consuming steps in conventional chip processing.

Traditionally, microchips are flat and rectangular, and manufactured on silicon wafers. Engineers cover the wafers with a photoresist and use a lithographic system to etch individual circuits on them. But manufacturing microchips with this method is both time-consuming and expensive. Often, to etch the desired number of circuits on the wafer requires multiple exposures to a light source -- usually a KrF or ArF excimer laser system emitting in the deep-UV.

Novel approach

Researchers at Ball Semiconductor think they have found a better way. Instead of flat wafers measuring 150 or 200 mm, the company has proposed using 1-mm-diameter silicon balls. The manufacturing process begins by dropping molten polysilicon granules down a tube. As the granules cool, they form tiny balls, which travel through sealed tubes for cleaning and processing.

The spheres remain inside the tubes until the circuit-etching stage. This stage posed the biggest challenge to the researchers, who had to find a way to transfer a two-dimensional mask pattern onto a three-dimensional sphere. To do this, they added special algorithms to 2-D design software which, when used in conjunction with two multifaceted mirrors positioned underneath the sphere, enabled the exposure of more than 70 percent of the sphere in a single shot from a 548-nm lamp. The mirrors, designed by the company, measure 1 cm and feature 45 facets.

"We can do it so quickly and in one shot," said Ram Ramamurthi, vice president of research and development. "To get enough information on a typical wafer with normal lithography, you need to send many pulses -- each lasting 2 to 5 seconds," he said.

Because cleaning and processing are done inside the sealed tubes, Ball Semiconductor said it can eliminate the need for vast cleanrooms. It hopes to begin commercial production by 2000.