Microspectroscopy
Figure 7: O-PTIR technique schematic diagram. Visible and pulsed IR laser beams are combined colinearly through a dichroic mirror (DM1) and focused on a sample via a reflective objective (RO) of a Schwarzschild design. When the IR laser is tuned to a wavelength corresponding to an absorbing band of the sample, the absorbed IR light causes local heating and thermal expansion in the IR-absorbing regions of the sample (zoomed inset). This IR-induced photothermal distortion causes a change in the intensity and/or angular distribution of visible light reflected/scattered from the sample. The visible light can be collected by the same reflective objec- tive (or alternately in transmission, not shown). Reflected visible light passes through a beam splitter (BS) and is measured by a visible detector. The intensity of the detected visible light is synchronously demodulated with a lock-in amplifier at the frequency of the IR laser pulses. IR absorption spectra can be created by measuring the amplitude of demodulated visible light while sweeping the wavelength of the IR laser source. Chemical images can be created by measuring the IR absorption while scanning the sample on an XY stage under the IR and visible laser beams. Submicron spatial resolution can be achieved because the visible beam can be focused to a much smaller spot than the IR radiation. A second dichroic mirror (DM2) enables viewing of the sample via a conventional microscope camera and enables selection of regions of interest for IR measurements. Measurements in this article were performed on a mIRage infrared microscope using the O-PTIR technique.
prevented the solder from wetting the pad and forming a proper weld. Single wavenumber O-PTIR images collected at 1724 cm−1
, where a properly formed weld would have a strong
spectral feature, shows the extent of contamination in the defective device.
32 Tere are many cases where contamination is not directly
accessible with conventional IR instrumentation. Figure 13 shows a 20 μm dark contamination that is underneath the thin metal film, as can be seen from the white circular delamination of the metal film due to the protrusion from
www.microscopy-today.com • 2020 May
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