Porosity of Low-κ Materials Studied by Slow Positron Beam

Until the past few years, aluminium was used in semiconductor technology as interconnect metal and silicon dioxide (SiO2, κ = 3.9) as the interand intra-level insulator. With the demand of scaling the dimension of advanced integrated circuits well below the 100 nm DRAM 1/2 pitch technology node [1], the signal delay (resistance–capacitance RC delay) becomes one limitation in increasing the speed of the electronic devices. To reduce the RC delay, materials with lower electrical resistance and lower dielectric constant are needed. Copper is replacing aluminium as interconnect metallization [2] for all the advanced integrated circuit. On the other side, recently, some low-κ materials are introduced in electronic devices as insulator in substitutions to SiO2. According to [1] one of the three top interconnect difficult challenges to reach the 45 nm technology node in the next five years is: “Introduction of new materials to meet conductivity requirements and reduce the dielectric permittivity”. The goal is to reduce the dielectric constant below 2. The dielectric constant can be reduced by introducing open space inside the material (pores) and trying to have the skeleton around the pores formed by less polarizable molecules. Total polarization P , in fact, depends through the following equation on N (number of molecules for m), on electronic (αe) polarizability, displacement (αd) polarizability and on the term p0/3kT due to thermal averaging of the dipole moment of molecules, with permanent dipole moment p0,