Probing the Metal Oxide/Polymer Molecular Hybrid Interfaces with Nanoscale Resolution Using AFM-IR

Abstract

Obtaining chemical information from the buried interface of an organic coating is not straightforward. In this paper, for the first time, atomic force microscopybased infrared spectroscopy (AFM-IR) is used to probe the chemical interactions at the metal oxide/polymer interface. AFM-IR is a novel technique that provides chemical spectra with spatial resolution below the optical diffraction limits. Poly(acrylic acid) (PAA) on aluminum oxide was chosen as a model system. Two different approaches were used: a thin-layer approach and a cross-section approach. The thin layer confirmed the validity of the AFM-IR to look at the buried interface by comparison with results from the literature. Additionally, with a line scan, the exposed interfaces obtained from cross sections of thick coatings on aluminum substrates were analyzed. A semiquantitative analysis of the spectra from the line scan allowed to identify the chemical contribution coming from the polymer/metal oxide interfaces.

Novel Method for High-Spatial-Resolution Chemical Analysis of Buried Polymer-Metal Interface: Atomic Force Microscopy-Infrared (AFM-IR) Spectroscopy with Low-Angle Microtomy

“…The carboxylate may have been derived from a corrosion inhibitor of the copper wire 54,55 or a reaction of the copper ions diffused from the wire with acrylic acid and/or other carboxylic acid, which may originate from unreacted 56 and/or degraded epoxyacrylate. 9,57 It has been reported that acrylic acid reacts with not only aluminum 2,29,58 and iron 59 but also copper 60 to generate carboxylate. Because the measured interface is approximately 40 µm away from the edge of the copper wire (Fig.…”

“…Recently, atomic force microscopy-based nanoscale infrared spectroscopy (AFM-IR), for which the highest spatial resolution of 10 nm can be achieved, was developed by Dazzi et al. 21–24 and has been applied to various types of systems 21–47 including polymer-metal interfaces. 26–31 The AFM-IR utilizes a sharp AFM probe with a typical tip radius of several tens of nanometers to detect local photothermal expansion of a sample surface induced by absorption of infrared (IR) irradiation, and its spatial resolution depends on the thermal expansion area detected by the apex of an AFM probe tip.…”

“…21–24 and has been applied to various types of systems 21–47 including polymer-metal interfaces. 26–31 The AFM-IR utilizes a sharp AFM probe with a typical tip radius of several tens of nanometers to detect local photothermal expansion of a sample surface induced by absorption of infrared (IR) irradiation, and its spatial resolution depends on the thermal expansion area detected by the apex of an AFM probe tip. A thin sample with a thickness of ∼500 nm or less placed on a substrate that has low IR-absorbance and high thermal conductivity, such as zinc sulfide (ZnS), zinc selenide (ZnSe), metal or metal coated substrates, is ideal for high-spatial-resolution measurements by AFM-IR because of the following reasons: (i) Since the AFM-IR is not surface sensitve 25,32–37 and the penetration depth of IR radiation is more than 1 µm for typical absorption peaks, the spatial resolution may deteriorate for thick samples by detecting the thermal expansion of materials very far below from the tip and also away from the vicinity of the tip, 37 especially when the interface is embedded at a slight tilt from the vertical.…”

“…Focused ion beam preparation 26 can only be applied to limited samples because of the damage from the ion beam irradiation. Therefore, a thick cross-section prepared by mechanical polishing or a cross-section polisher 29 is often measured for buried polymer-metal interfaces/interphases at the expense of spatial resolution in the lateral direction. In this case, it is often necessary to subtract the signals of adjacent layers from the spectrum obtained at an interface arithmetically to obtain a pure spectrum of the interphase layer.…”

“…Therefore, the effectiveness of this method was examined by comparing the results on a low-angle microtomed slope and thick (millimeter scale) vertical cross-section which was frequently measured in AFM-IR for buried polymer-metal interfaces. 2,26,29 FPCs are widely used in many microelectronics devices. 48 Chemical information at the interfaces between the metal materials and coating/packaging polymers in FPCs and other microelectronics parts is critical in terms of adhesion, mechanical strength, and insulation, and there is a great need for this analysis.…”