Trimethylaluminum Diffusion in PMMA Thin Films during Sequential Infiltration Synthesis: In Situ Dynamic Spectroscopic Ellipsometric Investigation

TL;DR

This study uses in situ spectroscopic ellipsometry to analyze how trimethylaluminum diffuses into PMMA during sequential infiltration synthesis, revealing how polymer properties influence alumina formation and process control.

Contribution

It provides the first detailed in situ analysis of TMA diffusion in PMMA during SIS, highlighting the effects of polymer type, molecular weight, and process parameters on infiltration.

Findings

PMMA swells more than PS during TMA exposure.

Diffusion coefficients of TMA depend on polymer molecular weight.

In situ SE enables tuning of the SIS process from ALD-like to CVD-like.

Abstract

Sequential infiltration synthesis (SIS) provides a successful route to grow inorganic materials into polymeric films by penetrating of gaseous precursors into the polymer, both in order to enhance the functional properties of the polymer creating an organic-inorganic hybrid material, and to fabricate inorganic nanostructures when infiltrating in patterned polymer films or in selfassembled block copolymers. A SIS process consists in a controlled sequence of metal organic precursor and co-reactant vapor exposure cycles of the polymer films in an atomic layer deposition (ALD) reactor. In this work, we present a study of the SIS process of alumina using trimethylaluminum (TMA) and H2O in various polymer films using in situ dynamic spectroscopic ellipsometry (SE). In situ dynamic SE enables time-resolved monitoring of the swelling of the polymer, which is relevant to the diffusion and retain of the metal precursor into the polymer itself. Different swelling behaviour of poly(methylmethacrylate) (PMMA) and polystyrene (PS) was observed when exposed to TMA vapor. PMMA films swell more significantly than PS films do, resulting in very different infiltrated Al2O3 thickness after polymer removal in O2 plasma. PMMA films reach different swollen states upon TMA exposure and reaction with H2O, depending on the TMA dose and on the purge duration after TMA exposure, which correspond to different amounts of metal precursor retained inside the polymer and converted to alumina. Diffusion coefficients of TMA in PMMA were extracted investigating the swelling of pristine PMMA films during TMA infiltration and shown to be dependent on polymer molecular weight. In situ dynamic SE monitoring allows to control the SIS process tuning it from an ALD-like process for long purge to a chemical vapour deposition - like process selectively confined inside the polymer films