The New Low-k Candidate: It's A Gas

Peter Singer, Editor-in-Chief -- Semiconductor International, 3/1/1999

Toshiba researchers have come up with an alternative to the various materials with low dielectric constants (low-k) being evaluated in an effort to reduce capacitance and crosstalk in interconnect lines. The surprise is that it is not a solid material, but a gas. The advantage is that it gets very close to achieving the minimum physically possible value for the dielectric constant of 1.0. So far, the researchers have demonstrated feasibility of the concept and presented preliminary process characterization data.

The proposed process (see Figure) is similar to a single damascene process except that the trenches in which metals are filled are formed in a sputter-deposited layer of carbon instead of in an insulator film. The thickness of the carbon film is equal to the desired thickness of the interconnect lines, as in the damascene process. Subsequently, metal is deposited by CVD or PVD so that the trenches are filled with metal. This is followed by CMP. A thin layer of insulator, typically ;50 nm thick, is then deposited over the entire wafer. This forms a thin 'bridge layer' over the metal. Next, a thermal treatment in an oxygen ambient at ;4508C is carried out. This causes oxygen to diffuse through the thin insulator film where it reacts with carbon to form carbon dioxide. This is the gas that insulates the metal lines.

Fig. 1. Proposed gas-dielectric process scheme.

Issues that remain to be investigated include the mechanical stability of the structure, as well as problems associated with the poor thermal conductivity of the gas.

  Silicon-29 Boosts
  Silicon Implant Productivity

Isonics Corp. (San Jose, Calif.) and Voltaix Inc. (North Branch, N.J.) have teamed up to offer a new source of silicon for ion implantation: enriched silicon-29 tetraflouride. Isonics claims the new source can increase implantation productivity by as much as 500%.

Silicon is implanted into GaAs as electrically active dopant atoms, to amorphize silicon to improve the stability of shallow implants in silicon-based MOS devices. Natural silicon consists of three stable (non-radioactive) isotopes of differing atomic masses: silicon-28 (92%), silicon-29 (5%) and silicon-20 (3%). While modern implanters can select any one of the three isotopes of silicon for implantation, silicon-29 is most desirable, since its mass is not the same as the other chemical species (e.g., nitrogen or carbon monoxide) that may be inadvertently implanted along with silicon.

By enriching the silicon-29 to levels above the natural abundance, Isonics claims the productivity of expensive ion implanters can be improved proportionally to the increased concentration. For example, silicon tetrafluoride enriched to 10% silicon-29, rather than the natural 5%, would increase the beam current 100%.