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Thermally Stable Metal Gate Electrodes

IP.com Disclosure Number: IPCOM000008899D
Original Publication Date: 2002-Jul-22
Included in the Prior Art Database: 2002-Jul-22
Document File: 7 page(s) / 105K

Publishing Venue

Motorola

Related People

J.K. Schaeffer: INVENTOR [+4]

Abstract

A technique for creating thermally stable metal gate electrode materials is described. As the MOSFET gate lengths scale down to 50 nm and below, the series capacitance from poly-silicon gate electrode depletion significantly reduces the gate capacitance as the dielectric thickness is scaled to 10 Å SiO2 equivalent oxide thickness (EOT) or below. Metal gates show promise to solve this problem and address other gate stack scaling concerns like boron penetration and elevated gate resistance. 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).[1] In addition to the electrical requirements, metal gates must be thermally stable up to dopant activation temperatures of nearly 1000°C. A thermally stable electrode must exhibit no inter-diffusion between the electrode and the gate oxide, no chemical reaction at the electrode/oxide interface, no microstructure changes in the electrode, and stable electrical properties. Non-crystalline, ternary and quaternary metal gate electrode films that can satisfy the thermal stability requirements for metal gate electrodes are presented.

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Thermally Stable Metal Gate Electrodes

J.K. Schaeffer, S.B. Samavedam, R.E. Martin, P. Tobin

ABSTRACT

A technique for creating thermally stable metal gate electrode materials is described.  As the MOSFET gate lengths scale down to 50 nm and below, the series capacitance from poly-silicon gate electrode depletion significantly reduces the gate capacitance as the dielectric thickness is scaled to 10 Å SiO2 equivalent oxide thickness (EOT) or below.  Metal gates show promise to solve this problem and address other gate stack scaling concerns like boron penetration and elevated gate resistance. 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).[1]  In addition to the electrical requirements, metal gates must be thermally stable up to dopant activation temperatures of nearly 1000°C.  A thermally stable electrode must exhibit no inter-diffusion between the electrode and the gate oxide, no chemical reaction at the electrode/oxide interface, no microstructure changes in the electrode, and stable electrical properties.  Non-crystalline, ternary and quaternary metal gate electrode films that can satisfy the thermal stability requirements for metal gate electrodes are presented.

Successful implementation of metal gate electrodes depends on the thermal stability of the gate dielectric / gate electrode interface.  Inter-diffusion is a major concern with the use of metal oxide dielectrics and metal gate electrodes.  Inter-diffusion is a phenomena that is more likely to occur in metal gate/metal oxide gate stacks than in poly gate/metal oxide stacks due to the similarities in the electronegativity and radii of metals employed in the gate oxide and the metal gate.[2]  However, depending on the metal gates evaluated, some interfacial reactions are possible.  If such reactions occur, a shift in the device properties with annealing conditions such as CET (Capacitance Equivalent Thickness), Vt (Threshold Voltage), or Ig (Gate Leakage) may be observed.  In addition, significant grain growth, or microstructure changes at source/drain annealing temperatures may roughen the gate/dielectric interface, or change the work function of the metal gate.  Instabilities like these can impact device characteristics such as mobility or Vt

Amorphous ternary metals are a promising material class for satisfying these requirements. These films are metastable, but can retain the amorphous microstructure to high annealing temperatures.  We have demonstrated thermal stability of Ta-Si-N films up to 1025°C.[3, 4]  The amorphous microstructure eliminates grain boundary diffusion and makes these films excellent diffusion barriers (Fig.1).  The most common amorphous ternary films are metal-silicon-nitrogen systems.  Nicolet, et al., demonstrate the metastable amorphous characteristics of TM-Si-N material systems (TM = 3d, 4d, or 5d transition meta...