Fluorine ion implantation into silicon dioxide to form stable low-k intermetal dielectric films

For 0.25 μm and smaller ULSI technology, materials having a dielectric constant (k) of less than 3.0 are required. These low-k dielectric materials reduce the parasitic capacitance between adjacent metal lines that can give rise to cross-talk. F incorporation into SiO ₂ films by PECVD processes results in weakly bonded F atoms which may evolve as a gas during subsequent thermal processing. This may produce defects in the SiO₂ which allow water to penetrate into the films, causing an increase in k and corrosion of metal lines. This paper describes an intermetal dielectric (IMD) structure formed by implantation of F ions into SiO₂. Fluorosilicate glass (FSG) films with k as low as 2.9 were formed by the implantation. Triple F implants (13, 33, and 54 keV) were made into 1500 A SiO₂ films which had k=4.0 and an index of refraction (n) of 1.46 before implantation. The doses were chosen, using TRIM software, to give F atomic concentrations of 1%, 2%, and 4%. The k of the 1%, 2%, and 4% F films were 3.40, 3.15, and 2.90, respectively. The n of the 1%, 2%, and 4% F films were 1.41, 1.38, and 1.36, respectively. The films were thermally stable. For the 2% F films, the k and n data remained constant after 3 weeks in a clean room environment. The data also did not change when the films were subjected to a 450°C, 2 hour thermal cycle. Implant conditions required to form a 6000 A thick, F-implanted SiO₂ IMD layer would consume too much implant processing time to be practical using existing high current implanter technology. Therefore, an IMD structure is proposed utilizing two 500 A implanted liners that sandwich a 5000 A thick PECVD FSG layer