Evaluation of Candidate Metals for Dual-Metal Gate CMOS with HfO 2 Gate Dielectric
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Evaluation of Candidate Metals for Dual-Metal Gate CMOS with HfO2 Gate Dielectric S. B. Samavedam, J. K. Schaeffer, D. C. Gilmer, V. Dhandapani, P. J. Tobin, J. Mogab, B-Y. Nguyen, S. Dakshina-Murthy1, R. S. Rai, Z-X. Jiang, R. Martin, M. V. Raymond, M. Zavala, L. B. La, J. A. Smith and R. B. Gregory2. Motorola Digital DNA Laboratories, 3501 Ed Bluestein Blvd, MD:K10, Austin, TX 78721 2 Motorola Digital DNA Laboratories, Mesa, AZ 1 AMD-Motorola Alliance, 3501 Ed Bluestein Blvd, MD:K10, Austin, TX 78721 ABSTRACT As the MOSFET gate lengths are scaled down to 50 nm or below, the expected increase in gate leakage will be countered by the use of a high dielectric constant (high K) material. The series capacitance from polysilicon gate electrode depletion significantly reduces the gate capacitance as the dielectric thickness is scaled down to 10 Å equivalent oxide thickness (EOT) or below. Metal gates promise to solve this problem and address other problems like boron penetration and enhanced gate resistance that will have increased focus as the polysilicon gate thickness is reduced. Extensive simulations have shown that the optimal gate work-functions for the sub-50 nm channel lengths should be 0.2 eV below (above) the conduction (valence) band edge of silicon for n-MOSFETs (p-MOSFETs). This study summarizes the evaluations of TiN, TaSiN, WN, TaN, TaSi, Ir and IrO2 as candidate metals for dual-metal gate CMOS using HfO2 as the gate dielectric. The gate work-function was determined by fabricating MOS capacitors with varying dielectric thicknesses and different post-gate anneals. The metal-dielectric compatibility and thermal stability was studied by annealing the stacks at different temperatures. The gate stacks were characterized using TEM, SIMS and X-ray diffraction. Based on workfunctions and thermal stability, TaSiN and TaN show most promise as metal electrodes for HfO2 n-MOSFETs. INTRODUCTION In sub-50nm gate length MOSFETs, the parasitic series capacitance due to depletion in polycrystalline silicon (poly) gates results in an unacceptable reduction in the gate capacitance and drive current. Metal gates have been studied for several years as replacements for poly gates to address this problem. Boron penetration of gate dielectrics from heavily doped p+ poly electrodes is also a non-issue with metal gates. The lower resistivity of metal gates helps to maintain a low gate resistance as gate heights are scaled down. Several high dielectric constant (high K) materials are being explored as replacements to SiO2-based gate dielectrics as gate leakage becomes a major concern with dielectric scaling. It is quite likely that metal gates will be needed as replacements to n+ and p+ poly as gate electrodes on high K gate dielectrics in CMOS circuits at the 50nm-technology node. Hence, it is essential that metal gate explorations are pursued with high K dielectrics to ensure compatibility when they are both implemented together. Hafnium and Zirconium based dielectrics have been widely reported as promising high K candidates [1].
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