Sequential infiltration synthesis (SIS) has been recently demonstrated to increase the etch resistance of optical, e-beam, and block copolymer lithography resists for sub-50 nm pattern transfer. Although SIS can dramatically enhance pattern transfer relevant to device applications, the complex processes involved in SIS are not clearly understood. Fundamental knowledge of the chemistry underlying SIS is necessary to ensure a high degree of perfection in large-scale lithography. To this end, we performed in situ Fourier transform infrared (FTIR) spectroscopic measurements during the SIS of Al2O3 using trimethylaluminum (TMA) and H2O into poly­(methyl methacrylate) (PMMA). The FTIR results revealed that TMA reacts quickly with PMMA to form an unstable complex. The subsequent conversion of this intermediate complex into stable AlO linkages is slow and must compete with rapid TMA desorption. We support this interpretation of the FTIR data using density functional theory to calculate plausible structures for the unstable TMA–PMMA complex and the covalently linked species. As a consequence of this two-step reaction between TMA and PMMA, the detailed history of the TMA exposure becomes critical to achieving reliable patterns in SIS lithography. We demonstrate this using scanning electron microscopy to image the patterns resulting from SIS treatment of block copolymer films under different TMA exposure conditions. This better understanding of the SIS reaction dynamics should improve reliability in SIS lithography as well as other SIS applications.

Understanding and controlling the three-dimensional structure of block copolymer (BCP) thin films is critical for utilizing these materials for sub-20 nm nanopatterning in semiconductor devices, as well as in membranes and solar cell applications. Combining an atomic layer deposition (ALD)-based technique for enhancing the contrast of BCPs in transmission electron microscopy (TEM) together with scanning TEM (STEM) tomography reveals and characterizes the three-dimensional structures of poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin films with great clarity. Sequential infiltration synthesis (SIS), a block-selective technique for growing inorganic materials in BCPs films in an ALD tool and an emerging technique for enhancing the etch contrast of BCPs, was harnessed to significantly enhance the high-angle scattering from the polar domains of BCP films in the TEM. The power of combining SIS and STEM tomography for three-dimensional (3D) characterization of BCP films was demonstrated with the following cases: self-assembled cylindrical, lamellar, and spherical PS-b-PMMA thin films. In all cases, STEM tomography has revealed 3D structures that were hidden underneath the surface, including (1) the 3D structure of defects in cylindrical and lamellar phases, (2) the nonperpendicular 3D surface of grain boundaries in the cylindrical phase, and (3) the 3D arrangement of spheres in body-centered-cubic (BCC) and hexagonal-closed-pack (HCP) morphologies in the spherical phase. The 3D data of the spherical morphologies was compared to coarse-grained simulations and assisted in validating the simulations' parameters. STEM tomography of SIS-treated BCP films enables the characterization of the exact structure used for pattern transfer and can lead to a better understating of the physics that is utilized in BCP lithography.

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 Al 2 O 3 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 Al 2 O 3 patterns. Finally, we examine the effects of temperature on Al 2 O 3 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.

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. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al2O3 SIS using trimethyl aluminum (TMA) and H2O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10× slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.

Zinc oxide films derived from drop-coating solutions of zinc acetate in ethanol followed by chemical bath deposition were examined for their suitability as buffer layers for high temperature vapor phase deposition of large area, aligned, zinc oxide nanorod arrays. An X-ray photoelectron spectroscopy analysis of substrates drop coated with zinc acetate solutions clarifies the chemistry of the deposition mechanism of the initial acetate-derived ZnO seeds. Scanning electron microscopy, atomic force microscopy, and white light profilometry studies show that while zinc acetate-derived buffer layers are suitable for chemical bath deposition of aligned zinc oxide nanorod arrays, during high temperature vapor phase depositions these buffer layers undergo substantial changes leading to a loss of nanorod alignment and poor substrate coverage. We present a method to deposit aligned zinc oxide nanorod arrays uniformly over large area substrates, which combines zinc acetate drop coating, chemical bath deposition of buffer layers, and vapor phase transport deposition of nanorods.