Nanoscale chemical structure variations in nano-patterned and nano-porous low-k dielectrics: A comparative photothermal induced resonance and infrared spectroscopy investigation

The recent development of the photothermal induced resonance (PTIR) technique has enabled atomic force microscope based infrared (AFM-IR) spectroscopy and imaging to be achieved at the nanometer scale. However, a direct correspondance between PTIR/AFM-IR and more traditional Fourier transform IR (FTIR) spectroscopy has been prohibited for nanometer scale features due to Rayleigh diffraction constraints that limit the latter to few micron spatial resolution. In this regard, we have overcome this challenge by fabricating 1 cm2 arrays of 90 nm wide fins in a nano-porous low dielectric constant (i.e. low-k) amorphous hybrid inorganic-organic silicate material using standard nano-electronic fabrication techniques. With these structures, we demonstrate both a general correspondance between AFM-IR, FTIR, and Germanium attenuated total reflection (GATR) IR spectroscopy, as well as differences in the sensitivities that these techniques exhibit to the nanoscale variations in chemical structure induced in the low-k dielectric by the nanopatterning method. To further illustrate the sensitivity of AFM-IR to changes in chemical structure with nanometer resolution, the nanopatterned low-k dielectric was exposed to additional oxidizing plasma ash cleans post patterning. Focusing on the Si-CH3 deformation band at ∼1275 cm−1, both the AFM-IR, FTIR and GATR measurements show a clear reduction in the concentration of terminal methyl groups in the low-k dielectric as the oxidation potential of the plasma ash clean increased. These results further establish the power of AFM-IR to perform nanoscale IR spectroscopy and demonstrates a stronger correspondance between AFM-IR and well-known micron scale IR techniques such as FTIR and GATR.