Welcome to the nanotechnology era: get small with carbon nanotubes that help fight ESD.

WHEN HIGH-VOLUME production of 90 nm channel length transistors began last year, semiconductor technology officially entered the nanotechnology era. Coincidentally, Hyperion Catalysts' multiwalled carbon nanotubes may be critical in protecting such parts from the potential destruction from electrostatic discharge (ESD).

As device feature sizes and gate oxide thicknesses shrink, sensitivity to ESD will increase. Less than 5 V across a 90 nm gate will result in destruction. If you touch an electronic product anywhere in its lifecycle, you may take a yield hit unless you carefully control ESD.

By their very nature, high voltages, in excess of 1 kV, are generated whenever two different insulators are touched or rubbed. This is called triboelectric charging. Even in high relative humidity, the process of sitting in a chair with urethane foam can raise your voltage to over 1 kV. (1)

Though it is not possible to prevent triboelectric charging in insulators, it is possible to drain them off before they build up voltages large enough to destroy product, but this requires the normally highly insulating plastic be slightly conductive.

How quickly these charges can be drained off, the time constant for dissipating charges around and through a material, is related to its RC time constant. The R is the bulk resistance the excess charges see and C is the capacitance between the charges and the nearest grounding surface. These terms are in turn related to the bulk resistivity and the dielectric constant of the material.

For most polymer materials with a dielectric constant of 4, the discharge time constant is roughly 1 psec x rho, where rho is the bulk resistivity of the material in ohm-cm. Since the mechanical effects which generally give rise to triboelectric charging happen on the time scale of a millisecond, having a bulk resistivity less than about 1 billion ohm-cm will keep static charges from building. This is typically 1 million times more conductivity than found in most insulating polymers. Such materials are called "dissipative" rather than insulating materials.

In the world of ESD prevention, dissipative materials play a critical role. The challenge is reducing the resistivity of an insulator by a factor of a million, without screwing up the other manufacturing or mechanical properties that give the polymer its intrinsic value. This is traditionally done by adding enough conductive filler material to a high enough loading where the conductive particles almost touch. With carbon particles, this filler can be as much as 20% by weight. The big problem in clean room environments is the particulate contamination these filled polymers contribute.

Enter multiwalled carbon nanotubes (FIGURE 1). The cross-section through one tube, trademarked by Hyperion as a carbon FIBRIL, shows the wall to be about 10 atomic layers thick, and about 10 nm in diameter. According to Michael Laine, business development director for Hyperion, these tubes are typically tens of microns long, giving them a 1000:1 aspect ratio. FIGURE 2 shows a collection of these nanotubes as they would be disbursed in a resin.

[FIGURES 1-2 OMITTED]

With these long, floppy strings dispersed in a polymer, much lower loadings are needed to provide resistivities low enough to be useful. Typical loadings for ESD applications are in the 1.5-5% by weight range. This means the mechanical properties of the resin are very similar to the unfilled resin.

Hyperion also blends them into a variety of resin materials, such as polycarbonate, polyetherimide, nylons and even some fluoropolymers. These nano-composites are currently used as fenders on some cars.

In semiconductor and disc drive manufacturing, their ultra clean and dissipative qualities make them attractive as elements in wafer carriers and handlers. According to Laine, other applications in electronics might be as permanent elements in semiconductor packages, sockets, connectors and enclosures for finished electronic equipment. With higher loading, their resistivity can be reduced to the point where these nano-composites can provide some EMI shielding.

REFERENCES

(1.) The electrostatic discharge association, www.esda.org.

DR. ERIC BOGATIN (eric@BeThe Signal.com) is the CTO of IDI and president of Bogatin Enterprises.