Kinetics for the Sequential Infiltration Synthesis of Alumina in Poly(methyl methacrylate): An Infrared Spectroscopic Study

Sequential infiltration synthesis (SIS) is a method for growing inorganic materials within polymers in an atomically controlled fashion. This technique can increase the etch resistance of optical, electron-beam, and block copolymer (BCP) lithography resists and is also a flexible strategy for nanomaterials synthesis. Despite this broad utility, the kinetics of SIS remain poorly understood, and this knowledge gap must be bridged in order to gain firm control over the growth of inorganic materials inside polymer films at a large scale. In this paper, we explore the reaction kinetics for Al2O3 SIS in PMMA using in situ Fourier transform infrared spectroscopy. First, we establish the kinetics for saturation adsorption and desorption of trimethyl aluminum (TMA) in PMMA over a range of PMMA film thicknesses deposited on silicon substrates. These observations guide the selection of TMA dose and purge times during SIS lithography to achieve robust organic/inorganic structures. Next, we examine the effects of TMA desorption on BCP lithography by performing SIS on silicon surfaces coated with polystyrene-block-poly(methyl methacrylate) films. After etching the organic components, the substrates are examined using scanning electron microcopy to evaluate the resulting Al2O3 patterns. Finally, we examine the effects of temperature on Al2O3 SIS in PMMA. to elucidate the infiltration kinetics. The insights provided by these measurements will help extend SIS lithography to larger substrate sizes for eventual commercialization and expand our knowledge of precursor polymer interactions that will benefit the SIS of a wide range of inorganic materials in the future.