Pore Characterization of Ultralow-k Dielectric Thin Films Using Positronium Annihilation Spectroscopy.

by Ming Liu

Institution: University of Michigan
Department: Physics
Degree: PhD
Year: 2008
Keywords: Positronium Annihilation Lifetime Spectroscopy; Micropore; Ultra Low Dielectric Constant Thin Film; Mesopore; Nanopore Characterization; Porogen; Materials Science and Engineering; Chemistry; Physics; Engineering; Science
Record ID: 1825015
Full text PDF: http://hdl.handle.net/2027.42/61562


In this thesis, a series of nanoporous ultralow dielectric constant (ULK) thin films fabricated using spin-coating and sacrificial pore generators (degradable porogens embedded in a host matrix) were studied as a function of porogen concentration and hence porosity to systematically vary the porogen-porogen and the porogen-matrix interactions. After porogen removal the resulting pore size and pore interconnectivity were characterized by beam-based (depth-profiled) Positronium Annihilation Lifetime Spectroscopy (PALS). In a film series fabricated with the same methyl-silsesquioxane (MSQ)-based matrix, specific porogen-porogen interactions were induced by different functional end-groups of cyclodextrin (CD) porogens and by amphiphilicity of a calix-arene (CA[6]) porogen. Random, linear, and self-assembly growth modes respectively for the tCD, sCD and CA porogen domains were distinguished by PALS depth-profiling. In a sample series fabricated using the same nucleation and growth (N&G) porogen anddifferent MSQ-based matrices, distinct porogen-induced pore structures were observed which exhibit the significant role of porogen-matrix interactions in determining the pore morphology. Furthermore, in-situ thermal curing of nanocomposite hybrid samples to form nanoporous materials extends one???s understanding of microphase separation, porogen degradation, and pore structure evolution with curing temperatures. Monte Carlo simulations were conducted on a three dimensional cubic lattice to simulate the evolution of pore size and pore interconnectivity with increasing porosity. The calculated pore size evolution was found to be very consistent with the experimental results. The calculated pore interconnection length, while consistent with simple percolation concepts, did not follow the typical quadratic growth with porosity observed with PALS. The positronium annihilation intensity in the porogen-induced pores was simulated and compared with experiment. Such measurements appear to be promising as an absolute porosity calibration. In an effort to successfully integrate ULK materials, chemical vapor deposited (CVD) Parylene-N has been found to be an effective sealant for interconnected pores to prevent copper diffusion and moisture uptake. However, in a depth-profiling experiment using PALS, it was found to penetrate the film to a depth of ???200 nm. Characterization of penetrants in permeable systems is a broad area with a promising future for PALS applications.