Serendipitous applications: Ball Semi sells litho tool and MEMS sensor

08/01/2002

After years of skepticism over its unusual plan to make spherical semiconductors, Ball Semiconductor is about to start shipping real product to real customers. But it won't be a tiny ball-shaped semiconductor device after all.

Instead, the Allen, TX, company has found markets in the high density packaging and LCD sectors for the direct-write lithography tool it developed to pattern its little spheres. It is also shipping evaluation samples of MEMS three-axis accelerometers, which are actually made by processing tiny balls of silicon as they flow through pipes.

Customers are testing these units for applications ranging from tire vibration monitors to security systems to earthquake sensors. The maskless lithography tool makes relatively wide 10-20μm lines, but makes them uniformly over a wide area. "We didn't want to spend money for masks," explains Ball Exec. VP and COO Hideshi Nakano. "So we came up with the idea to use TI technology as a digital mask to expose the whole 1mm sphere at one shot."

Texas Instruments' digital mirror device technology makes digital images on mirrors with drivers for a pseudo mask, using up to 500,000 individually controlled mirrors, and it is directly scalable to cover large areas by just adding on more units. People attending Ball's technical conferences liked the company's approach for printing its spheres — but wanted to use it for planar processing, for big flip-chip substrates and big black matrices for color filters for ever-larger LCDs.

Fast growth of flip-chip technology is pushing the high-end printed circuit board market from printing to lithography, as the roadmap for flip chips heads towards 20μm lines in 2002 to 2003. "We believe PCB lithography will be a half billion dollar market in 2003," says Nakano. One of the digital mirror engines can serve as a desktop lithography system for a lab, while a commercial tool combines eight to 100 of the units. He targets sales of $10 million within a year. Ball will sell its optical engines on an OEM basis to two substrate lithography vendors in Japan, including market leader ORC Manufacturing.

Ball will make the tools in China, contracting production to Japanese optical device maker Moritex at its new plant in Shenzhen, where the labor-intensive assembling and adjusting can be done for 5% of the cost in the US. The company aims to make 100 of the engines/month, for some 200 systems/year. The next step is to bring resolution down to the submicron range and speed up throughput with a new light source. Ball is working on replacing high-pressure mercury lamps with an array of high power violet laser diodes, now being pre-marketed to display makers in Japan and Taiwan.

Meanwhile, Ball is actually fabricating devices on its weird little spheres — but they're MEMS sensors instead of semiconductors. "We'd essentially developed our spherical semiconductor production technology and were ready to fabricate something," says Nakano. "But the number of gates per chip was low, so the value of a 1mm sphere was very small, only a few cents. So we adjusted our direction to the MEMS sensor area."

The company is now selling evaluation samples of three-directional spherical sensors, set in standard 16-pin packages. First samples were distributed in late March. The sensors are fabricated individually on 1mm spheres of silicon as they flow through pipes. With no wafers, no cleanroom, no batch requirements, and no long cycle times, the company figures its process should be much cheaper than the conventional approach.

The initial spherical accelerometer also has the advantage of sensing in three dimensions instead of just the two that conventional flat sensors can measure, as the spherical unit can tell whenever a small movement knocks its inner sphere against its outer shell.

The sacrificial manufacturing process puts oxidization and metallization layers on the 1mm silicon balls, exposing and etching circuit patterns in the metal. Then the balls are coated with polysilicon and dipped in a porous ceramic material. Pressurized gas passes through the ceramic and etches away the polysilicon layer, leaving an inner ball floating by electrostatic force within an outer shell. The process also transfers the metal pattern to the inside of the shell. Sixteen bonding pads on the bottom side of the sphere attach it to a 16-pin package.

Potential users are testing the highly sensitive multidimensional sensors to monitor the vibration in tires, tell if someone is trying to steal a car, or continually adjust the display in a handheld device.
—Paula Doe, Contributing Editor