Silicon process produces pockets

By Kimberly Patch, Technology Research News

Less can give you more, according to a group of Toshiba researchers who have developed a method for creating silicon membranes backed by empty spaces of various shapes. The silicon-on-nothing structure gives silicon new attributes that may lead to better microchip-based sensors.

The researchers came up with the technique after closely examining results from a similar technique that was limited to producing silicon membranes over small, spherical spaces. Microscope images showed that the diameters of the fabricated spaces were larger than those of the trenches used to initiate the spaces, said researcher Ichiro Mizushima, a specialist at Toshiba.

With this in mind, the researchers made larger shapes by arranging the trenches close together, Mizushima said. The silicon-on-nothing structure was not a difficult step once they had proved that the empty spheres could be connected, according to Mizushima.

The researchers are also looking to apply the technique to micromechanics processes, according to Mizushima. Fabricating microelectromechanical systems (MEMS), which are tiny machines built on computer chips, often requires building moving parts on layers of material that have to be removed at the end of the process. The silicon-on-nothing technique could simplify MEMS fabrication.

The silicon-on-nothing structure is more fragile than the usual silicon-on-insulator configuration used to make computer chips, and therefore isn't likely to replace it for most uses, said Sandip Tiwari, a professor of electrical engineering at Cornell University, and director of the school's nanofabrication facility.

However, the attributes of silicon backed by a vacuum makes it better for some specialized uses where a more sensitive material is a plus. "This could be quite useful -- silicon membranes like this are used in many sensor-like applications," Tiwari said.

The silicon-on-nothing structure is more sensitive to changes in electric potential because it has low permittivity, said Tiwari. The permittivity, or dielectric constant of a material, is a measure of how much the material resists the flow of electrons. The silicon-on-nothing structure allows for a "permittivity of one -- the lowest we can get -- underneath the silicon, said Tiwari. Bulk silicon has a dielectric constant of 11, while silicon backed by silicon oxide has a dialectic constant of about four, he said.

Low permittivity allows electrons to flow more easily, and this flow can be measured. The lower a material's permittivity, the more subtle the electrical changes it can sense. This same attribute allows devices made using the silicon-on-nothing structure to run on less power.

"It probably is useful for a number of interesting things that we haven't even thought of yet," said Tiwari. Its higher sensitivity may allow it to amplify higher frequencies, giving rise to higher frequency wireless phones, he said. In addition, a silicon-on-nothing transistor could be filled with liquid to make, for example, a temperature sensor, Tiwari said.

The-silicon-on-nothing technique could be applied to computer chip fabrication sometime in 2005, according to the researchers.

Mizushima's colleagues in the research were Yoshitaka Tsunashima, Tsutomu Sato and Shuichi Taniguchi. They published the work in the November 13, 2000 issue of Applied Physics Letters. The work was funded by Toshiba Corp.

Timeline:   5 years
Funding:   Corporate
TRN Categories:  Semiconductors and Materials; Integrated Circuits
Story Type:   News
Related Elements:   Photo 1, Photo 2; Technical paper, "Empty-Space-in-Silicon Technique for Fabricating a Silicon-on-Nothing Structure," Applied Physics Letters, November 13, 2000


December 20/27, 2000

Page One

Tiny wires store more

Tinier transistors keep Moore's Law on track

3-D geometry adds twists to microfabrication

Virtual annotation fosters understanding

Silicon process produces pockets


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