Implementation of Heterodyne-Detected Tapping-Mode Photothermal Atomic Force Microscopy-Infrared Spectroscopy for...

Atomic force microscopy (AFM) is widely used to characterize the nanoscale electrical, magnetic, mechanical, thermal, electrochemical, and electromechanical properties of materials and devices; however, it does not directly provide chemical identification. In contrast, infrared (IR) spectroscopy probes chemical bonds through their vibrational signatures but is traditionally limited to micron-scale spatial resolution by optical diffraction. Photothermal AFM-IR spectroscopy (AFM-IR) combines AFM and IR spectroscopy to enable nanoscale chemical characterization by measuring localized photothermal expansion following IR absorption. This article presents a protocol for heterodyne-detected tapping-mode photothermal AFM-IR and its application to semiconductor materials and devices. The protocol describes instrument preparation, probe selection and tuning, AFM topography imaging, IR laser alignment and optimization, acquisition of localized IR spectra, and chemical mapping of selected vibrational modes. Particular emphasis is placed on practical implementation of tapping-mode AFM-IR for semiconductor characterization and factors that influence data quality, including probe selection, resonance tuning, and IR beam alignment. Representative examples demonstrate the use of AFM-IR to distinguish material composition in patterned structures, evaluate deposited materials on semiconductor substrates, and identify residual photoresist contamination following processing. The protocol enables acquisition of co-localized topographical and chemical information with nanoscale spatial resolution and illustrates how AFM-IR can be applied to characterization challenges encountered in semiconductor research and manufacturing. To assist new users, a brief overview of AFM-IR development and a comparison of commonly used IR sources and imaging modes are also provided.