” This novel combination not only boost confidence in the analytical approach but also rapidly assess the contamination to determine where the spectroscopic and imaging analyses should be done. The O-PTIR spectral data and images would not be affected by the auto-fluorescence of the material at the regions of interests.”
FULL PUBLICATION >
Reporting in the IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits, scientists at Photothermal Spectroscopy Corp. demonstrated a novel analytical strategy combining fluorescence imaging with optical photothermal infrared (O-PTIR) spectroscopy for rapid contamination analysis in microelectronics. Contamination analysis poses significant challenges due to small feature sizes and chemical complexities. Conventional FT-IR lacks spatial resolution for features smaller than 50 µm, while Raman microscopy suffers from auto-fluorescence interference that produces strong baseline signals overshadowing weak Raman signal. The fluorescence-guided O-PTIR (FL-OPTIR) technique addresses these limitations by using co-located fluorescence imaging to pre-screen for spatial chemical variations before collecting sub-micron infrared spectra on the same microscope stage.
The researchers presented two compelling case studies demonstrating the technique’s capabilities. In the first case, contamination on a metal pillar was analyzed where fluorescence images revealed chemical boundaries distinguishing epoxy-based underfill regions. The contaminated zone showed brown hue contrast against purple untainted underfill, with O-PTIR spectra confirming higher carboxylic acid signals around 1680 cm⁻¹ consistent with extraneous species. The polyimide-based solder mask displayed strong auto-fluorescence as expected, providing clear contrast. In the second case examining void backfill near a solder bump, fluorescence imaging identified a thin sliver of organic-rich material between the solder bump and acrylate-epoxy solder mask. O-PTIR spectra confirmed relative intensity differences between mineral filler bands, identifying a potential thermal fatigue location that could lead to early device failure.
The FL-OPTIR technique provides sub-micron spatial resolution (approximately 500 nm) determined by the visible probe laser wavelength of 532 nm. Critically, the O-PTIR photothermal detection mechanism remains unaffected by auto-fluorescence, enabling high-quality infrared spectra from strongly fluorescent materials that would be impossible to analyze with Raman spectroscopy. The infrared absorption profiles obtained are comparable to ideal transmission mode FT-IR spectra, allowing direct identification through commercially available spectral databases. The fluorescence images enable rapid assessment of chemical heterogeneity without prior knowledge of contamination chemistry, dramatically improving analysis efficiency by guiding targeted spectral acquisition.
This novel combination of fluorescence imaging and O-PTIR spectroscopy provides a highly capable tool for resolving complex contamination problems in microelectronics failure analysis. The technique enables rapid pre-screening through fluorescence contrast, followed by definitive chemical identification via O-PTIR, all within a single integrated instrument platform. The ability to analyze auto-fluorescent materials that defeat conventional Raman analysis, combined with superior spatial resolution compared to FT-IR, makes FL-OPTIR particularly valuable for identifying heterogeneous organic contamination in confined areas of modern microelectronics devices.
Authors: Michael K. F. Lo
Photothermal Spectroscopy Corp., Santa Barbara, CA, USA
