Amorphous carbon based materials as the interconnect dielectric in ULSI chips

Abstract

The shrinkage of the devices and wiring dimensions in the ULSI chips is associated with an increased resistance of the interconnect metallization and increased interlevel and intralevel capacitances, causing corresponding longer signal delays. Low dielectric constant (k) insulators, with k significantly lower than that of presently used SiO2 are needed for reducing these capacitances and improving the switching performances of future ULSI circuits. Integration of low-k insulators in the ULSI circuits will also reduce the power required to operate them. Diamond-like carbon (DLC) has found a variety of applications based on its attractive mechanical, tribological, optical and chemical resistance properties. The films are also dielectrics whose electrical resistivities can reach values of 1016 Ω-cm at low fields. The DLC-type materials are attractive dielectrics because of their isotropic properties and the ability to deposit them by plasma assisted CVD techniques. However, the amorphous carbon materials with diamond-like properties are characterized by dielectric constants that are not lower than that of SiO2 (k=4). It was found that, by adjusting the deposition conditions of plasma deposited hydrogenated DLC (a-C:H), it is possible to reduce its dielectric constant to values between >3.3 and 2.7. Incorporation of the low-k materials in the ULSI structures imposes a significant number of requirements that they have to satisfy, among them stability at the processing temperature of 400°C. While DLC films having dielectric constants k>3.3 appeared to be stable to anneals of 4 h at 400°C in inert ambiance, the thermal stability decreased with decreasing dielectric constant. Incorporation of fluorine in FDLC films produces a material of apparently higher thermal stability and further reduced dielectric constants, to values even lower then 2.4. The as-deposited low-k DLC or FDLC films may be thermally stabilized, in terms of dimensional stability and material loss, by an initial anneal, that also causes a significant reduction in the intrinsic film stress, typical of DLC type materials. The integration of the low-k films in the interconnect structures further requires good adhesion with thermally stable interfaces to materials in contact with the low-k dielectric. Such materials may include processing aids and structural components such as silicon nitride or oxide, and wire cladding metallurgy. The paper discusses the preparation and characterization of the low-k DLC and FDLC films, approaches for their thermal stabilization and evaluation of integration issues.