Characterization of Porosity in Porous Films
Porous oxide films have a very low dielectric constant (<2), which is significant in producing high speed electronic devices such as the interconnect structure shown in Figure 1. The goal of positron research is to characterize the pore size and structure in these films. Sample films were provided by Lucent Technologies. The matrix material is methyl-silsesquioxane, MSSQ, (SiOC structure, CH3SiO1.5). Porosity was achieved by mixing the MSSQ with a sacrificial volatile polymer (porogen) which is later thermally decomposed leaving voids behind.
Figure 1 - Illustration of a low-k dielectric material located in an interconnect structure.
Figure 2 shows positronium lifetime spectra for MSSQ films with and without pores. The spectrum from the oxide containing pores yields two positron lifetimes of 54 ns and 140 ns. The 54 ns lifetime corresponds to a pore size of 5 nm in diameter. The 140 ns lifetime indicates that some of these pores are connected. Both lifetimes are much longer that that for the MSSQ matrix without pores. The Lucent films were also studied by measuring the positronium concentration as a function of depth. The results provide a guideline for Lucent to select a procedure in which closed pores can be generated in MSSQ films without interconnections.
Figure 2 - Positron lifetime spectra for MSSQ films with and without pores.
