Absorption Sensitivity and
Spatial Resolution in Near-Field Infrared Spectroscopy
C. A. Michaels and S. J. Stranick.
Objective: To benchmark the instrumental capabilities
of an infrared scanning near-field microscope (IR NSOM) that utilizes IR
absorption as a chemical contrast mechanism. Key characteristics to be assessed include absorption sensitivity,
attainable spatial resolution, and the sensitivity of near-field IR absorption
microscopy to topographic artifacts.
Problem: The integration of IR absorption
spectroscopy into near-field scanning optical microscopy is a promising
approach to in-situ, non-destructive, high spatial resolution imaging,
with applications in the chemical characterization of materials and
nanotechnology. The combination of the
sub-diffraction spatial resolution attainable in the near-field with the high
chemical specificity of vibrational spectroscopy promises customers in the US
chemical industry a powerful new analytical instrument that surmounts critical
measurement limitations of both far-field IR microscopes (low spatial resolution) and scanned probe
microscopes (lack of chemical specificity). However, a quantitative assessment of
critical performance characteristics of this microscope is required.
Approach: Spatial resolution and probe quality are
evaluated by analysis of controlled polarization, near-field transmission
images of a thin micropatterned gold film. Absorption sensitivity is estimated from near-field spectra of thin
polymer films in the aliphatic C-H stretching region at 3.4 mm. Analysis of a series of near-field spectra, recorded as a function of
probe-sample separation, yield insight into the degree to which IR absorption
imaging is subject to topographic artifacts.
Results and Future Plans: Near-field transmission
images of a micropatterned thin gold film on silicon allow a quantitative assessment
of both the probe aluminum coating integrity and the demonstrated spatial resolution,
free from complications due to topographic artifacts. Probes are routinely fabricated
which allow imaging with a conservatively estimated spatial resolution of 350
nm to 500 nm and sufficient aperture throughput to allow high signal-to-noise
spectral measurements. Figure 1 is a near-field absorption spectrum of a 2 mm thick polystyrene film recorded in 1 s, within
the broad bandwidth of the laser pulse. This spectra demonstrates that the microscope
sensitivity is sufficiently high to allow spectral measurements of samples of
sub-wavelength thickness, thus meeting the requirement for high spatial resolution,
near-field absorption imaging. The film thickness detection limit for absorption
in the C-H stretch band of polystyrene is estimated at 200 nm. Spectral retraction
curves indicate that baseline corrected, infrared absorption amplitudes are
relatively insensitive to changes in tip-sample separation, suggesting that
near-field absorption imaging may be less sensitive to topographic artifacts
than other near-field imaging modes. Future plans involve the exploration of
apertureless near-field and confocal IR imaging techniques.
Publications:
Scanning Near-Field Infrared Microscopy and Spectroscopy with a Broadband Laser Source,
C. A. Michaels, S. J. Stranick, L. J. Richter and R. R. Cavanagh,
J. Appl. Phys. 88, 4832 (2000).
Chemical Imaging with Scanning Near-Field Infrared
Microscopy and Spectroscopy,
C. A. Michaels, S. J. Stranick, L. J. Richter and R. R. Cavanagh,
Proc. SPIE. 4098, 102 (2000).