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

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.

Effect of the Density of Reactive Sites in P(S‐r‐MMA) Film during Al2O3 Growth by Sequential Infiltration Synthesis

“…These data indicate that, in the presence of MMA units in the polymer matrix, the amount of Al 2 O 3 grown during the SIS process is directly proportional to the concentration of MMA units in the RCP chains. This picture is perfectly consistent with previous experimental results showing that CO functional groups in the MMA units act as seeds for the growth of Al 2 O 3 during the SIS process …”

“…By means of in situ SE analysis, it is possible to measure thickness variation of the polymeric film during the SIS process in real time and to obtain information about TMA diffusion kinetics during the infiltration process . Analysis of ex situ data indicates that the amount of Al 2 O 3 that is grown per unit volume in a P(S‐ r ‐MMA) matrix is independent of the thickness of the polymer film for each specific MMA fraction.…”

“…The thickness evolution as a function of time follows an exponential trend e − t /τ . τ is assumed to be the time constant of the deswelling process associated to the out‐diffusion of nonreacted TMA . Actually, competing processes with different characteristic times occur simultaneously during the deswelling process.…”

Precursor-surface interactions revealed during plasma-enhanced atomic layer deposition of metal oxide thin films by in-situ spectroscopic ellipsometry

“…During the ALD process, the subsequent exposure of the surface to precursors, separated by purging agents leads to cyclic surface modifications, and the control of which may play a critical role in the resulting thin film quality. To the best of our knowledge, unraveling such complex processes have not been attempted previously by in-situ spectroscopic ellipsometry in ALD or PEALD growth of ultra-thin films 41,46,[64][65][66][67][68][69][70][71][72][73][74][75][76][77] . The analysis of the evolution of the optical properties of ultra-thin films during growth cycles in ALD using in-situ SE as reported here in our work using the dynamic dual box model may gain further insight into the kinetics of the surface modifications within individual cycles.…”

“…www.nature.com/scientificreports www.nature.com/scientificreports/ Klaus et al suggested the application of in-situ SE during ALD growth processes 64 , and accurate thickness monitoring was reported for metal nitride thin films 41,65,66 and metal oxide thin films 46,[67][68][69][70][71][72][73][74][75][76][77] , for example. In these previous reports, in-situ SE data was measured once for every ALD cycle in order to determine the thickness GPC.…”