Thermal Stability of Fluorinated Amorphous Carbon Films Improved
New work on low-k dielectrics is focused on improving non-electrical properties such as thermal stability and mechanical strength.
Peter Singer, Editor-in-chief -- Semiconductor International, 3/1/1998
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| Company News
| Thermal Stability of Fluorinated Amorphous Carbon Films ImprovedResearchers at Sharp Microelectronics Inc. (Camas, Wash.) have developed a new process that significantly improves the thermal stability of low-k fluorinated amorphous carbon (a-F:C) films, without reducing the fluorine concentration or modifying the cross-linking structure. The details were presented at the Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC) in February. The semiconductor industry is presently evaluating a variety of different low-k materials in an effort to reduce time delays and "cross talk" and minimize the number of levels of interconnects. Fluorinated amorphous carbon is among the leading candidates. As with other fluorinated films, there is a tradeoff between thermal stability and dielectric constant (k) depending on the amount of fluorine in the film and the amount of cross-linking: The more fluorine there is, the lower the dielectric constant, but the worse the thermal stability. The Sharp researchers -- Hongning Yang, Tue Nguyen, S.T. Hsu and Yanjun Ma -- report that over the last two years, efforts have focused on improving the thermal stability of a-F:C by cross-linking the polymer chains by higher temperature deposition and ion bombardment. Such techniques, however, reduce the fluorine concentrations and thereby increase the dielectric constant. The new process developed at Sharp is said to overcome these problems, while still providing stable films at temperatures higher than 450°C, with a dielectric constant of 2.26 to 2.36. Films can also be deposited at much lower temperatures (180-250°C) than conventional films. In the new process, the films are deposited in a single or dual frequency RF discharge and then annealed at a little over 400°C for 30 min. The Table shows how the new process provides a more stable film, with less fluorine loss.
Low-k Porous SOG Material DevelopedA new material called hydrophobic porous SOG (HPS), which has a dielectric constant (k) less than 2.5, has been developed by researchers at Fujitsu's Fine Chemicals Research Center in Japan. Details of the new material were reported at the DUMIC conference in February. The new film is said to compare favorably with better-known aerogels, which are low-density, low-k films obtained by the controlled gelation and drying of the precursor. The problem with aerogel films, according to the Fujitsu researchers, is that they are mechanically weak, and it is difficult to control their pore structure (porosity and pore size) and pore surface character. By comparison, in the case of HPS, it is easy to make the pore surface hydrophobic, since HPS consists of a hydrogen silica sol (a colloidal suspension of a solid in a liquid) that has the Si-H group on its surface. The film strength is controllable by the amount and composition of the binder. The pore size is determined by the size of the sol. | ||||||||||||||||||||
