Air-filled spaces make swifter chips.

Bubbles don't usually belong in computer chips, but in a new study, researchers at the Georgia Institute of Technology in Atlanta demonstrate the feasibility of using air gaps as insulators in integrated circuits.

At present, layers of easily made silicon dioxide separate the tiny electronic components in chips to prevent signals from heading down the wrong path. To make a new generation of smaller, faster chips with tightly packed features, researchers will need to find better insulating materials.

Air happens to be one of the best insulators around. Although other scientists have considered building air gaps into integrated circuits, it has been difficult to make pockets of air that small, says study coauthor Paul A. Kohl. He and his colleagues now have developed a technique that he says is "the simplest way to make an effective air gap."

The researchers created a test sample by etching a pattern into a special polymer, laid on a rigid foundation. Then they deposited a metal, either gold or copper, in the pattern and laid a coat of silicon dioxide over the whole structure. Heating the sample to 450 [degrees] C burns away the polymer, which escapes through the silicon dioxide overcoat.

The choice of polymer is crucial because "it has to decompose at exactly the right temperature," says Kohl. The silicon dioxide is deposited at 200 [degrees] C, so the polymer must not burn away until a higher temperature is reached. The team modified a polymer made by BF-Goodrich in Brecksville, Ohio.

Although many polymers leave residue behind when heated, this one burns very cleanly and slowly, ensuring that the chip doesn't "blow up like a volcano," Kohl adds. He and his colleagues describe the technique in the July Electrochemical and Solid State Letters.

The amount of insulation provided depends on the air gap's dimensions relative to the thickness of the overcoat. The new system could potentially achieve the insulating ability that the semiconductor industry anticipates needing for a new generation of chips, according to Kohl's analysis.

The decomposition temperature used by Kohl, however, may be "a little on the high side" to be used with new copper chip technology (SN: 9/27/97, p. 196), says Toh-Ming Lu of Rensselaer Polytechnic Institute in Troy, N.Y.

Kohl says that the technique could also provide a way to make other devices, such as thin membranes for pressure sensors.

Scientists are currently investigating other insulating materials, including fluorinated polymers and silicon dioxide foams (SN: 12/14/96, p. 383). Air gaps combined with traditional silicon dioxide could compete favorably with these alternatives, Lu says.