Sub-10 nm thick microporous membranes made by plasma-defined atomic layer deposition of a bridged silsesquioxane precursor.

Membranes exhibiting high flux and selectivity are important for many applications, including water desalination, greenhouse gas storage and abatement, H2 purification, and selective proton/oxygen conduction in fuel cells. Combined high flux and selectivity is achieved in natural systems by membrane-bound ion and molecular channelsswhose pore size is defined with sub-nanometer precision through protein folding and whose thickness is limited to that of the cellular membrane bilayer, only 4 nm. By comparison, synthetic membranes can seldom be fabricated with similar molecular level precision (the exceptions being carbon nanotubes and zeolites) and are often 100-1000× thicker. This is problematic because membrane flux varies reciprocally with membrane thickness. The inherent issue is that most synthetic approaches require relatively thick membranes to avoid defects or, in the case of zeolites and CNTs, the smallest-sized building blocks used for membrane fabrication are of the order 1 μm.1,2 Here we describe an atomic layer deposition (ALD) approach to construct ultrathin membranes with sub-angstrom control of pore size. Our approach extends the burgeoning ALD field 3-7 in several new directions. First, we perform ALD on a self-assembled nanoporous support, where the internal porosity is protected from deposition. Second, we restrict ALD to the extreme surface by plasma activation. Third, we employ a bridged bis-silsesquioxane precursor to deposit a hybrid organosilicate film, uniformly incorporating organic ligands that serve as molecular templates/porogens upon subsequent removal. Beyond membrane formation, this plasma-directed ALD approach naturally forms a low k dielectric sealing layer needed for future generations of microelectronics. ALD is a self-limiting layer-by-layer thin film deposition technique composed normally of successive steps of adsorption and hydrolysis/activation of metal halide or metal alkoxide precursors.3-6 To date, ALD has focused principally on the formation of dense thin film oxides, metals, or semiconductor alloys on solid substrates. Our previous research introduced plasma-assisted (PA)-ALD as a means to deposit dense oxide films on the immediate surface of a nanoporous film. In PA-ALD, exposure to a remote Ar + O2 plasma, rather than hydrolysis, is used to activate the surface through formation of hydroxyl groups. Because both the plasma Debye length and the radical mean free path exceed greatly the pore diameter (∼3 nm; see Figure 1), deposition does not occur within the interior pores.7 Surface-limited deposition of ultrathin layers on porous supports is important for sealing low k dielectrics. It is also of interest in the formation of high flux membranes. Here we achieve surfacelimited deposition and develop a high flux, high selectivity membrane by an approach combining remote plasma exposure and surface passivation with conventional ALD of an (unconventional) hybrid precursor (see also Supporting Information). We start with a nanoporous silica film (Figure 1), consisting of an ordered cubic arrangement of monosized pores, formed by evaporation-induced self-assembly8 on an underlying anodized alumina support having 20 nm pores aligned normal to the support surface. Following calcination and UV/ozone exposure,9 the nanoporous film has fully hydroxylated 3.2 nm pores as measured by a surface acoustic wave based technique.10 To avoid ALD on any interior porosity, which would detrimentally increase the membrane thickness, we expose this hierarchical membrane support structure to hexamethyldisilazane and then to trimethylchlorosilane vapor at 180 °C for 5 min. This exposure converts the surface and internal hydroxyl groups to trimethylsiloxane groups, which remain inert to hydrolysis reactions and therefore passivate the surface against ALD during subsequent steps. To activate the immediate surface of the nanoporous film to ALD, the sample is exposed to a remote Ar + O2 plasma for 2 s. As reported previously by us, the plasma was designed so that its Debye length (several mm) and radical mean free path (several mm) are much larger than the pore size.7 In this condition, the plasma radicals cannot penetrate the internal porosity, and only trimethylsiloxane groups residing on the immediate surface of the nanoporous film are converted to silanols tSi-OH. These surface silanols are active to halide and alkoxide ALD precursors, M(X)n and M(OR)n, respectively, undergoing condensation reactions to form tSi-O-Mt plus HX and HOR byproducts. Therefore, ALD takes place on the surface of the substrate, while internal, hydrophobic -Si(CH3)3 groups remain unhydrolyzed and do not undergo condensation reactions with ALD precursors. † Sandia National Laboratories. ‡ University of New Mexico. Figure 1. (a) Cross-sectional TEM image of the hybrid membrane supported on mesoporous silica; (b) original mesoporous silica support; (c) support coated with ALD membrane; (d) EELS spectrum of the membrane. Published on Web 11/23/2007