Metallic diborides TiB₂, ZrB₂ and HfB₂ were used for the formation of contacts integrated with ultra shallow junctions required in deep submicron devices in integrated circuits. The films were deposited by electron-beam evaporation on an oxide free surface of both p- and n-type Si. Rapid thermal processing (RTP) in N₂ ambient was performed using temperatures up to 1100C and time up to 30s. Test structures on p-type Si were fabricated to determine contact resistance before and after the annealing processes. The borides form ohmic contacts on p-type Si after deposition and improve their ohmic behavior after annealing; specific contact resistivity decreases with temperature (from 9.3E-05Ωcm² at 800C to 4.3E-6Ωcm² at 1100C for TiB₂). On n-type Si, the formation of Schottky diodes was determined for as deposited structures and those annealed at low temperatures. Barrier heights and ideality coefficients n were determined. With increasing thermal budget p-n junctions were identified that had low leakage currents. To evaluate possible doping of the silicon beneath the borides contacts and specific contact resistivity we used measurements of transmission line model resistor.
A set of complementary material testing including XPS, XRD, SIMS, cross-section TEM, and RBS was done to determine stoichiometry, composition, and crystallographic structure of the processed films. Electrical characterization of the films was done using sheet resistance measurements, which show decreasing resistance with increasing thermal budget and Kelvin probe, which indicated changes in the work function values during annealing. These films showed recrystallization as identified by XRD and by TEM with electron diffraction. Materials studies show very high stability of the diborides if their composition is stoichiometric. However, SIMS results combined with XRD provide evidence that nonstoichiometry deteriorates this stability and results in larger than desired B outdiffusion.
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