The demand to print increasingly smaller microelectronic device features means that the thickness of the polymer films used in the lithographic processes must decrease. The thickness of these films is rapidly approaching the unperturbed dimensions of the polymer, length scales at which confinement deviations and dewetting are a significant concern. We combine specular x-ray reflectivity ͑SXR͒ and incoherent neutron scattering ͑INS͒ to probe the thermal stability and dynamical effects of thin film confinement in poly͑hydroxy styrene͒ ͑PHS͒, a polymer used in a majority of the 248 nm deep UV photoresists. PHS forms stable thin films ͑down to 5 nm͒ that do not dewet over a wide temperature range on Si surfaces ranging from hydrophilic to hydrophobic. The surface energy has a profound influence on the magnitude of the thin film expansion coefficient, especially above the glass transition, in films as thick as 100 nm. Confinement also appears to suppress the mean-square atomic displacements and the level of anharmonicity in the dynamics, primarily above the bulk glass transition.
Sub-100 nm lithography poses strict requirements on photoresist material properties and processing conditions to achieve necessary critical dimension control of patterned structures. As resist thickness and feature linewidth decrease, fundamental materials properties of the confined resist polymer can deviate from bulk values and impact important processing parameters such as the postexposure bake ͑PEB͒ temperature. The effects of these confinement-induced deviations on image or linewidth spread have not been explored. In this work, we characterize the resist thickness dependence of the spatial extent of the reaction-diffusion process in a chemically amplified photoresist system under varying processing conditions. Bilayer samples are prepared with a lower layer of a protected polymer ͑p-tert-butoxycarboxystyrene͒ and a top layer of a de-protected polymer ͓poly͑4-hydroxystyrene͔͒ loaded with a photoacid generator. After flood exposure, PEB, and development, changes in the thickness of the protected polymer provide a measure of the spatial extent of the reaction front between the polymer layers. The velocity of the reaction front is significantly reduced with decreasing thickness of the protected polymer layer under identical processing conditions.
Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) is utilized to provide insight into surface chemical effects in model photoresist films. First, NEXAFS was used to examine the resist/air interface including surface segregation of a photoacid generator (PAG) and the extent of surface deprotection in the film. The concentration of PAG at the resist-air interface was higher than the bulk concentration, which led to a faster deprotection rate at that interface. Second, a NEXAFS depth profiling technique was utilized to probe for compositional gradients in model resist line edge regions. In the model line edge region, the surface composition profile for the developed line edge was dependent on the post exposure bake time.
Cross-linked epoxy network films were cast onto silicon wafers with a variety of surface treatments. X-ray reflectivity was used to characterize their electron density and thermal expansion in the rubbery state. A transition from "bulk" to "confined" expansion occurs in the range of 200-400 Å, where thinner films exhibit smaller rubbery expansion coefficients. The thermal expansion behavior of the epoxy films was independent of the substrate surface treatment, which varied in both surface energy and the strength of bonding interactions with the polymer. The thickest epoxy films displayed typical rubbery thermal expansion values for temperatures above the bulk polymer glass transition temperature. The thinnest epoxy films (<120 Å) exhibited typical glassy expansion values even at temperatures 20-40 °C above the bulk polymer glass transition temperature, independent of the surface treatment.
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