New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors.  During the initial SIS cycles, the organometallic precursor (e.g. trimethyl aluminum, TMA) reacts preferentially with specific functional groups within the polymer, while in later cycles growth occurs in an ALD-like fashion on the inorganic “seeds” that have been planted inside the polymer.  Consequently, it is possible to start with a blend of two polymers, such as a diblock coplymer, and selectively infuse just one of the two components.  As a result of this selective, step-wise infusion, SIS can create novel composites with intermediate or even unique properties compared to those of the pure polymeric and inorganic constituents.  Alternatively, the organic component can be removed through oxidation leaving a purely inorganic material, but with a mesostructure that mimics that of the starting polymer.  One particularly attractive application for SIS is lithography: SIS can harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon.  With this objective in mind, we have performed a series of investigations using in-situ Fourier transform infrared absorption spectroscopy (FTIR), in-situ synchrotron grazing incidence small angle X-ray scattering (GISAXS), and high resolution scanning  transmission electron microscope (STEM) tomography of a model system: Al₂O₃ SIS using TMA and H₂O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA).  While the picture is far from complete, these studies reveal some important details: 1) TMA adsorption on PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, but desorption is 10x slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures.  All of findings impact the resulting composite, and hence will dictate the success of the lithography.