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Mirror Enables 3-D Lithography

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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.

Photonics Spectra
Oct 1998
industrialResearch & TechnologyTech Pulse

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