2, Stanford University, Stanford, California, United States
Silicon-germanium (SiGe) alloys are promising for advanced FinFET channels due to their electronic properties and ease of integration into existing Si CMOS processes. However, Ge-O bonds at the channel/insulator interface introduce defect energy states. Previous investigations into suppressing Ge-O bond formation have involved liquid sulfur treatments or ammonia plasma pre-deposition, or post-deposition N2 plasma nitridation. Sulfur treatment presents reliability concerns, while post-deposition nitridation lacks control over oxide nucleation. In this work, an in-situ downstream RF plasma containing a mixture of N and H species on Si0.7Ge0.3(001) surfaces prior to deposition of high-k oxides Al2O3 and HfO2 by atomic layer deposition (ALD) has been investigated using metal-oxide-semiconductor capacitor (MOSCAP) structures. C-V and I-V characterization was performed, demonstrating improved interface state density (Dit) and leakage current for plasma-cleaned devices. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment at the SiGe/high-k interface.
Al2O3 and HfO2 MOSCAPs were fabricated to compare the in-situ N2/H2 plasma clean with an HF(aq) only preclean. 40 Al2O3 cycles were grown with TMA and H2O precursors at 250 C, and 50 HfO2 cycles were grown with TDMAH and H2O precursors at 250 C. Prior to deposition, SiGe substrates were treated with 2.5 cycles of 2% HF(aq) followed by deionized water, with plasma cleaned devices receiving an exposure to 20s downstream RF plasma at 20 W with 500 mTorr N2, 25 mTorr H2, and 475 mTorr Ar. A significant improvement to Dit was observed for the plasma cleaned devices; HF + plasma Al2O3 MOSCAPs had an EOT of 3.14 nm and peak Dit of 7.2 x 1011 cm-2eV-1, compared with 3.39 nm EOT and 3.6 x 1012 cm-2eV-1 for the HF only device, illustrating Dit improvement without a decrease in EOT. On HfO2 devices, HF + plasma and HF only MOSCAPs had EOT values of 1.61 and 1.77 nm and peak Dit values of 2.9 x 1012 and 4.8 x 1012 cm-2eV-1. Leakage currents at -2 V bias were 100x lower for HF + plasma Al2O3 and HfO2 MOSCAPs, consistent with a more uniform oxide layer after plasma clean.
Using capacitance- and conductance-voltage measurements, a full interface state model across the band-gap was used to find the integrated Dit, demonstrating a 5x improvement in integrated Dit on Al2O3 HF + plasma MOSCAPs compared with HF only, and a 30% improvement compared with sulfur-passivated devices. With HfO2 MOSCAPs, integrated Dit value was decreased 2x for HF + plasma devices compared with only HF.
To investigate the chemical structure at the interface, a thin Al2O3 layer was deposited using 5 ALD cycles on both HF only and HF + N2/H2 plasma cleaned surfaces, and XPS spectra were recorded. Plasma-treated SiGe exhibited lower intensity Si and Ge peaks, consistent with improved Al2O3 nucleation. Si and Ge nitride peaks appear after plasma treatment, consistent with a nitride layer suppressing Ge-O bond formation.