| We studied the behavior of metallic diborides TiB2, ZrB2 and HfB2 for potential applications as metal gates in Complementary Metal Oxide Semiconductor (CMOS) integrated circuits (IC). The diborides films (20-70 nm) were deposited by e-beam evaporation on thin 20-100 Å thermal oxides. We evaluated work function values of the borides and based on MOS capacitors measurements we found compatibility with p-type Metal Oxide Semiconductor (PMOS) transistors for the annealed gates fabricated with ZrB2. Compatibility of these metal gates with the IC processing was tested using rapid thermal processing (RTP) in temperatures up to 1100°°C. Electrical characterization of the capacitors included capacitance-voltage (C-V), conductance-voltage (G-V), and current-voltage (I-V) measurements. They were complemented by sheet resistance measurements. For material characterization, we used Rutherford Backscattering (RBS), x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), cross-section transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) to determine stoichiometry, composition, and crystallographic structure of the processed films as well as possible B outdiffusion. These techniques confirm very high thermal stability of stoichiometric diborides that allows for preserving the sharp interface of metallic gates with the underlying dielectric layer without its degradation during high temperatures RTP. All films undergo recrystallization, detected by XRD as well as by TEM and electron diffraction, which depends on thermal budget of RTP. Decreasing resistivity also results from the grain growth. However, electrical measurements of the test structures after annealing show positive shifts of flatband voltages, which depend on boride type and process conditions. |
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