Dual work function metal gates by full silicidation of poly-Si with Ni or Ni-Co bi-layers
Abstract
According to the 2005 International Technology Roadmaps for
Semiconductors (ITRS), one of the most important challenges in the semiconductor
industry is the continuously scaling of the gate oxide thickness. However the
depletion of gate poly-Si makes it difficult to continue this scaling especially for sub100
nm CMOS generation. In this work, novel fully silicided (FUSI) metal gates are
proposed to tackle this problem and avoid poly depletion effect. Silicides have much
higher electron concentration than poly Si, which eliminates the polysilicon
depletion effect in these FUSI gates. Thanks to the wide usage of NiSi and Co-NiSi
in the current CMOS processing flow, these two silicides become best candidates of
FUSI gates. In order to achieve surface-channel p- and n- MOSFETs, there are two
main criteria requirements for new gate material: Proper Work functions for two
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types of MOSFET (4.1-4.4eV for NMOS and 4.8-5.1eV for PMOS) and good
thermal stability after high temperature anneals.
In this paper, Co-Ni alloy silicides is used as FUSI gate and Co-Ni ratio is
tuned to manipulate work function. While the method is dopant-free, the work
functions hardly meet the CMOS requirements.
Doped poly Si is used to form silicides in the experiments and silicidation
induced segregation effect (SIIS) is observed and manipulated to tune work function.
High dosage and multiple dopant pre-implantation are proven to push work functions
of FUSI gates to the requirements of CMOS transistors. (4.28eV for NMOS, 5.08eV
for PMOS)
Negative impact of SIIS is observed in B-doped samples due to B penetration.
NiSi-gated MOSFET is studied extensively to minimize the negative impact by the
FUSI processing and the reliability of MOSFET is improved by nitridation of gate
dielectrics.
In order to realize stable NiSi work function in the case of the small dimension
transistors, two-step silicidation process is extensively studied. As-doped sample is
found to have larger reaction rate than B-doped sample. Different thermal conditions
for both As-doped and B-doped samples are found to meet the FUSI requirements.
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