Microspectroscopy
Figure 5: (a) The infrared transmission spectrum of a thin film of PMMA deposited on CaF2
sample. Te sample is first illuminated with an IR beam from a pulsed tunable IR laser source, for example, a QCL or an opti- cal parametric oscillator (OPO) laser. When the IR source is tuned to a wavelength corresponding to an IR absorption band in the sample, a portion of the IR light is absorbed and con- verted to heat causing a temperature increase in the IR-absorb- ing regions. Tis temperature increase results in local heating and thermal expansion that causes changes in the refractive index. Refractive index changes are in turn detected by focus- ing a shorter-wavelength visible laser beam (for example, green at 532 nm) that overlaps the IR-illuminated region onto the sample surface. Changes in the reflected green light intensity are then monitored using a visible light detector. Te spatial resolution achieved by O-PTIR (Equation 1) is set by the wave- length of the visible probe beam rather than the wavelength of the IR beam. Te spatial resolution improvement is determined by the ratio of the IR wavelength to the probe wavelength. Another key advantage of the O-PTIR approach is that
the spectra produced do not contain scattering contributions, thus the intensities are directly proportional to the actual amount of light absorbed by the sample. As a result, the sig- nal from an O-PTIR measurement produces a more “pure” absorbance spectrum, devoid of non-absorbance contributions like dispersive reflectivity responses. Tis means that O-PTIR spectral peak positions, line shapes, and relative intensities are more likely to match those obtained by transmission IR
30
. The optical images (i) and the infrared transmission spectra (ii) of iso- lated (b) 5.5 μm, (c) 10.8 μm, (d) 15.7 μm diameter PMMA microspheres deposited on CaF2. Adapted from Figures 1 and 2 of [1], with permission.
measurements. Tis is true even if these signals are collected in reflection mode (far-field), which has major practical and ease- of-use benefits.
Results Collection of O-PTIR data. Te following examples
show results from a small subset of samples that were char- acterized by Seagate Technology with the mIRage instrument. Multiple samples were loaded and spectra collected at over 60 locations. Figure 8 shows spectra from two contaminant loca- tions. Te top image (Sample A) shows a particle recessed near a feature of the device and the O-PTIR spectrum recorded at the location indicated by the arrow, while the bottom image (Sample B) shows a similar contaminant and its correspond- ing O-PTIR spectrum over a recessed edge. Tese samples could not be measured with traditional FT-IR methods since the ATR objective would have been obstructed before contact with the contamination occurred. Te mIRage instrument can be fitted with different IR pulsed laser sources, allowing measurements over the standard IR spectral range (3600–800 cm−1
of materials that may have similar spectra in the fingerprint region (1800–800 cm−1
). Tis broader spectral range can aid in the identification ). In this example, the O-PTIR spectra
of both contaminant particles are consistent with a polyamide such as nylon, as indicated by comparison to the red nylon ref- erence from the KnowItAll™ IR database (Wiley).
www.microscopy-today.com • 2020 May
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