We studied the behavior of metallic diborides TiB2, ZrB2 and HfB2 deposited by e-beam evaporation on thin 20-100A thermal oxides. The diborides films (200-500A) were tested for potential applications as metal gates in CMOS integrated circuits (IC). The most important gate parameter, besides low resistivity, is metal work function, which should be within 0.2 eV of the valence or conduction band edges of Si for PMOS and NMOS transistors, respectively. We evaluated work function values of the borides and found compatibility with PMOS transistors based on MOS capacitors and corresponding Kelvin probe measurements.
Compatibility of these metal gates with the IC processing was tested using rapid thermal processing in temperatures up to 1100C. Electrical characterization of the capacitors included C-V tests and I-V measurements. They were complemented by sheet resistance measurements. For material characterization, we used RBS, XRD, SIMS, cross-section TEM, and XPS to determine stoichiometry, composition, and crystallographic structure of the processed films as well as possible B outdiffusion. Kelvin probe combined with depth profiling was used to measure work function uniformity within the films before and after annealing.
Electrical measurements of the test structures 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. This is confirmed by TEM, XPS, RBS, as well as SIMS results. However, annealing causes lateral shifts of C-V characteristics indicating positive shifts of flat band voltages. These changes coincide with Kelvin probe measurements pointing at modifications of borides work functions (ex. 4.71 eV to 4.93 eV for TiB2). They are related to film recrystallization, which is detected by XRD as well as by TEM and electron diffraction. Decreasing resistivity also results from the grain growth.
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1. Formation of contacts and ultra shallow junctions using diborides of Ti, Zr and Hf