Extreme ultraviolet (EUV) lithography is growing in demand as device feature sizes shrink. With shrinking sizes, reducing line edge roughness (LER) and line width roughness (LWR) becomes critical. While several processes contribute to the LER/LWR of the final structure, understanding the stochastic effects arising from the discrete interaction of photons with the photo-active components in the EUV resist will aid in designing resists and optimizing processes. Infrared photo-induced force microscopy (IR PiFM) is a technique that combines non-contact AFM and IR spectroscopy to non-destructively analyze local chemical bonds on extremely thin samples (~ 1 nm thick) with sub-5 nm spatial resolution. In addition to local IR spectra with high spectral resolution (~ 3 cm-1), it can also generate absorption maps (PiFM images at different wavenumbers associated with chemical components) for visualizing a chemical interface between the exposed and unexposed regions of a resist, providing an unprecedented opportunity to perform chemical metrology. This paper will present IR PiFM data on metal oxide EUV resists, both patterned and un-patterned, that are exposed at different conditions. PiF-IR spectra acquired on exposed but undeveloped resists clearly follow the chemical changes associated with changing dosage, even on a low dose of 10 mJ/cm2. On patterned samples, LER and LWR on chemical images are calculated for different exposure conditions and compared to the values derived from topographical data. The chemical sensitivity and the mapping capability at ultrahigh resolution afforded by IR PiFM will help greatly in developing and optimizing EUV resist composition and processing steps.
With film thicknesses approaching a few monolayers in semiconductor processes, the chemical state and the cleanliness of the surfaces become critical in determining the outcome of many semiconductor processes. Currently available molecular analytical techniques with sufficient surface sensitivity such as XPS and ToF-SIMS lack the spatial resolution to analyze nanoscale defects and residues. While electron microscopy-based EDX can identify many atomic elements, they cannot provide chemical bonding information, which is needed to assess more accurately the nature and origin of the defects. In this paper, a relatively new hyperspectral technique called infrared photo-induced force microscopy (IR PiFM), which combines atomic force microscopy (AFM) and infrared (IR) spectroscopy with ~ 5 nm spatial resolution, is introduced. By utilizing a state-of-the-art tunable broadband IR laser, truly nanoscale PiF-IR spectra that agree with bulk FTIR spectra can be acquired without contact, i.e., it is non-contaminating and non-destructive, on films as thin as ~ 1 nm. PiF-IR spectra can be used to search existing IR databases to unambiguously identify the chemical species (both organic and inorganic molecules) of sub-20 nm defects and sub-monolayer residues via their IR signatures. Examples of defects and residues analyzed by IR PiFM system for 8” wafers and standard 6” photomasks are presented. For both types of samples, the system can automatically navigate to defect locations per defect map to acquire both topographical and chemical map images of the defects. PiF-IR spectra acquired on the defects and residue can be searched against Wiley’s KnowItAll IR database for potential matches.
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