Group IVB Oxides as High Permittivity Gate Insulators
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thicknesses as low as 2.0 nm1. The national technology roadmap for semiconductors projects this scaling to continue, decreasing to approximately 1.5 nm when the devices reach the 0.1 pm node2 . Such thin gate oxides display direct tunneling between the gate and substrate, and the gate and source/drain diffusions. The leakage current increases by approximately 100x for every 0.5 nm in thickness reduction. 2.0 nm gate oxides leak at about 200 mA/cm2, while 1.5 nm gate oxides leak at about 50 A/cm 2. Power consumption constraints are expected to limit scaling of the gate oxide to approximately this thickness, although reliability concerns may limit the ultimate thickness of SiO2 to about 2.0 nm. Continuation of device scaling will therefore require the replacement of SiO 2 with a higher permittivity material, which will allow 0 Ti, thicker layers at the same equivalent oxide thickness. In this paper we report on the Zr, deposition and characterization of the group IVB binary oxides TiO 2, ZrO2, and HfO 2. The chemical vapor deposition of these films, especially TiO 2, has been practiced for over four decades 3 45 6. The most commonly used precursors, the isopropoxides, can be directly thermolyzed or hydrolyzed. The problem with these reactions is that residual carbon is left in the films, leading to excessive leakage7 . Often a Figure 1 - Structure of nitrato precursor post deposition oxygen anneal is required to 23 Mat. Res. Soc. Symp. Proc. Vol. 606 0 2000 Materials Research Society
obtain low leakage films. This oxidation tends to increase the thickness of the low permittivity
interfacial layer that forms when high permittivity materials are deposited directly on silicon, making the formation of low oxide equivalent thickness films extremely difficult. The column IVB oxides, however, can all be deposited using nitrato precursors (Figure 1), a class of compounds of the form M(O 3N)4, where M is the metal atom (Ti, Zr, or Hf)8 9. We have found that the use of these compounds avoids carbon contamination of the films, obviating the need for any post deposition oxygen anneals. EXPERIMENTAL The metal oxide films were deposited by chemical vapor deposition in three different low pressure CVD reactors. Comparable results were seen in all three systems. Precursors were heated to approximately 50'C (TN), 80'C (ZrN), or 90'C (HfN) to increase their volatility. The transfer lines from the bubbler to the chamber were heated 5 to 10 °C above the bubbler temperature to prevent precursor condensation in the lines. Controlling the precursor temperature and the flow of the carrier gas controlled the precursor flux. UHP Ar was used as a carrier for the TN and ZrN; UHP N2 was used as the HfN carrier. The growth substrate for all experiments was (100) silicon. The TiC 2 and ZrO2 substrate samples were RCA cleaned using an HF last process. HfO2 samples were RCA cleaned and banked. Immediately before deposition the samples were first dipped in 4:1 H2SO4: H202 to remove any organic contaminants, rinsed with de-ionized water and blown d
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