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Analyzing sub-100 nm particulate defects and ultrathin (~1 nm thick) residues in semiconductor processes

Background

For semiconductor processes, it is crucial to eliminate sub-100 nm particulate defects and surface contaminants. To do so effectively, it is imperative to be able to discern the molecular identity of the defect/contaminant. While defects as small as 20 nm can be detected with survey tools, the traditional suite of analytical tools such as XPS, ToF-SIMS, or SEM/TEM EDX have difficulty in clearly identifying the contaminating source of defects—especially if they are organic. Organics are difficult because traditional analytical tools either lack the spatial resolution required, or they only provide elemental information. Infrared photo-induced force microscopy (IR PiFM) can fill this gap by offering the ability to identify or name chemical compounds with nanometer-scale resolution. It does this by combining a non-contact AFM with IR spectroscopy to acquire topographical and chemical information concurrently at the nanoscale [1]. Since PiF-IR spectra match FTIR spectra for a given material, existing IR libraries can be used to identify defects analyzed with IR PiFM. Given PiFM’s sub-5 nm spatial resolution, even a multi-component defect can be de-composed into pure components via multivariate data analysis of PiF-IR spectra from different regions of the sub-100 nm defect.

Experimental

Surface contaminants that are common on silicon wafers along with particulate defects found on silicon wafers are characterized by IR PiFM. Additionally, IR PiFM is used to detect the residue of copper-benzotriazole (BTA) polymer, which is formed during copper CMP process, after different cleaning processes.

Results and Discussion

Figure 1 shows two different defects found on silicon surfaces. In Figure 1a, we find a particle (within the red dotted circle) that is about 3 nm in height. The particle appears to be ~20 nm laterally. Since the AFM tip’s radius of curvature is ~20 nm, the particle is most likely a sphere with a diameter of ~3 nm. The blue PiF-IR spectrum acquired on the particle matches the IR spectrum for silica (orange spectrum) from the Wiley IR library [2]. Figure 1b shows another defect that is ~4 nm tall and ~50 nm × ~120 nm laterally. The purple and green PiF-IR spectra on this defect match the black spectrum for PTFE from Wiley KnowItAll.

Figure 1. PiF-IR spectra are used to chemically identify nano-sized defects on silicon surfaces by searching the Wiley IR library. (a) ~3 nm spherical silica particle (b) 3 × 50 × 120 nm3 PTFE particle.

Unwanted residue or surface contaminants need to be controlled since many semiconductor processes involve precursor molecules interacting with the surfaces. With monolayers, it is difficult to even detect its existence, let alone identify it. In Figure 2a below, the PiF-IR spectra acquired on an otherwise clean wafer from a storage container match the pink IR spectrum of perfluoro polyether (PFPE). When the same wafer is plasma cleaned, PFPE is removed and the strong C-F peak ~1250 cm−1 disappears—but the surface now shows a spectrum that indicates hydrocarbon contamination (Figure 1b). An IR library search suggests the contaminant is methylcyclohexane, which is a solvent used in some correction fluids such as White-Out. The search was performed using the average of six spectra taken at the locations shown in the inset topography. Also, the peak associated with the silicon oxide (shaded in blue) was excluded. Note that even though the surfaces are contaminated with different molecules, the micro-roughness of the silicon remains the same.

Figure 2 . PiF-IR spectra are used to chemically identify the contaminant layer on silicon surfaces by searching the Wiley IR library. (a) PFPE (b) methylcyclohexane.

Hybrid bonding is gaining interest as replacement of thermal compression bonding for advanced applications that require higher interconnect density. An important element to ensure successful hybrid bonding is an extremely flat, smooth, clean, and hydrophilic dielectric surface that is properly terminated with Si-OH bonds that can bond together instantaneously upon contact at 25 °C. Additionally, to form good metal-to-metal bonds, the Cu surfaces must be free of organic contaminants and carbides which prevent true Cu interfacial grain growth. It is known that benzotriazole (BTA), which is used as an inhibitor to protect recessed copper, reacts with copper ions to form a thin Cu(II)-BTA polymer on the copper surface that is difficult to remove [3].

Figure 3 shows averaged PiF-IR spectra of four copper surfaces: (a) as electroplated, (b) after the electroplated copper is treated by BTA, (c) after CMP process pf electroplated copper, and (d) after Ar ion cleaning following the CMP process. The as-electroplated copper (Figure 3a) shows a peak at around 730 cm−1, which we attribute to copper oxide (the sharp feature at ~905 cm−1 in all spectra is an artifact of laser power normalization). When the copper is treated with BTA (Figure 3b), two peaks at ~795 and 755 cm−1 appear; these peaks are attributed to the C-H stretching vibration of the benzene ring in BTA, clearly indicating the formation of Cu-BTA polymer [3]. These two peaks are also present in the copper sample that undergoes a CMP process (Figure 3c) even though the peak at 795 cm−1 appears as a shoulder of a new peak. The prominent new peak at ~615 cm−1 is typically associated with Cu2O4. When the same sample is cleaned via Ar ion beam (Figure 3d), the copper surface is mostly clean (except for a small peak at ~805 cm−1, which may be due to the oxide peak associated with the AFM tip). Even though other surface sensitive techniques such as XPS can be used to check for the presence of the Cu-BTA complex, the advantage that PiFM offers is that its spatial resolution allows examination of individual copper pads, which are approaching 1 mm in size.

Figure 3. PiF-IR spectra for different copper surfaces. (a) as electroplated, (b) after the electroplated copper is treated by BTA, (c) after CMP process pf electroplated copper, and (d) after Ar ion cleaning following the CMP process.

Conclusion

IR PiFM can chemically identify sub-100 nm defects and monolayer contaminants efficiently—filling metrology needs that have been lacking. IR PiFM also can be used to check post-CMP residues sensitively even on individual copper pads for Cu-Cu hybrid bonding applications. IR PiFM is available on instruments which support up to 12” wafers with many automation features, including automated defect analysis based on KLARF coordinates. Contact us for a demo on your own samples and register for our webinar for semiconductor applications of IR PiFM.

References

  1. D. Nowak, W. Morrison, H. K. Wickramasinghe, J. Jahng, E. Potma, L. Wan, R. Ruiz, T. R. Albrecht, K. Schmidt, J. Frommer, D. P. Sanders, and S. Park, “Nanoscale chemical imaging by photoinduced force microscopy,” Sci. Adv., 2:e1501571 (2016).
  2. KnowItAll IR Spectral Database Collection – Wiley Science Solutions provides the library and the search algorithm.
  3. Q. Wang, B. Tan, S. Tian, C. Han, L. Yang and B. Gao, “Study on infrared spectrum detection and analysis of BTA residual after copper CMP,” 2019 China Semiconductor Technology International Conference (CSTIC), Shanghai, China, pp. 1-3 (2019).
  4. A. Chen, H. Long, X. Li, Y. Li, G. Yang, and P. Lu, “Controlled growth and characteristics of single-phase Cu2O and CuO films by pulsed laser deposition”, Vacuum 83, pp. 927-930 (2009)

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