“The high compatibility of submicron O-PTIR spectra with existing commercial or customized infrared databases significantly enhances confidence in identifying unknown material previously not possible with conventional instrumentation.”
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Reporting in the 2025 20th International Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT), researchers at Photothermal Spectroscopy Corp addressed critical challenges in identifying sub-10 μm foreign matter contamination in advanced semiconductor packages. Modern packages incorporating multiple dies, high bandwidth memories, and chiplets in 3D configurations demand increasingly stringent contamination control as interconnect features push toward the sub-10 μm regime.
Conventional FT-IR microscopy provides practical spatial resolution of only 5-20 μm in the mid-IR range, rendering it ineffective for resolving these critical defects. Additionally, Raman microspectroscopy, while offering sufficient spatial resolution, is hampered by strong fluorescence from resins and chemicals used in microelectronics manufacturing and frequently causes laser-induced sample damage.
The researchers analyzed an open solder joint containing foreign matter wedged between copper lead and solder using co-located fluorescence imaging and O-PTIR (FL-OPTIR). Fluorescence imaging at two excitation/emission wavelength pairs (497/535 nm and 578/641 nm) revealed distinct green and orange regions, indicating at least two different chemical species present.
This qualitative chemical screening was accomplished in seconds, directly guiding precise O-PTIR spectral collection locations to minimize analysis time. Submicron IR spectra were collected at selected points along the contamination with high signal-to-noise ratio from a ~2 μm-wide gap between metallic surfaces.
Spectral analysis using commercial infrared databases revealed the chemically complex nature of the foreign matter. Through spectral de-mixing, researchers identified individual components including epoxy, proteinaceous material, and polyelectrolyte, along with silicon dioxide filler.
The contamination consisted of a spatially varying mixture of these materials distributed along the length of the defect. The O-PTIR spectra showed native IR absorption profiles that could be directly searched and compared with commercially available databases, eliminating the spectral distortions typical of conventional FT-IR measurements on rough or non-ideal sampling surfaces.
O-PTIR’s submicron spatial resolution, derived from its 532 nm visible probe laser, overcomes the diffraction limitations of conventional mid-IR microscopy. The technique’s inherent compatibility with fluorescence imaging enables rapid identification without additional sample preparation, while the probe laser detection scheme ensures sample auto-fluorescence cannot affect IR spectral integrity. This capability proved essential for analyzing auto-fluorescent organic contaminants that would overwhelm Raman spectroscopy.
The high compatibility of O-PTIR spectra with existing infrared databases significantly enhances confidence in identifying unknown materials previously not possible with conventional instrumentation, offering semiconductor manufacturers a powerful tool for root cause analysis and yield improvement.
Author:
Michael K. F. Lo
Photothermal Spectroscopy Corp, Santa Barbara, CA
