Redistribution of Boron Implanted into TaSi 2 /Poly-Si Gates
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REDISTRIBUTION OF BORON IMPLANTED INTO TaSi 2 /POLY-Si GATES U. SCHWALKE, C. MAZURE AND F. NEPPL Siemens AG, Corporate Research and Development, Microelectronics, Otto-Hahn-Ring 6, D-8000 Munich 83, FRG ABSTRACT The redistribution of boron implanted into TaSi 2 /poly-Si gates after high temperature anneals has been studied as a function of the implantation dose and energy as well as implantation scheme, i.e. boron implantation either into the silicide or into the poly-Si prior to TaSi 2 deposition. SIMS measurements indicate that boron can easily diffuse from TaSi2 into poly-Si and vice versa at 900 'C. Independent of the implantation scheme, the boron concentration within poly-Si was found to saturate at approximately 1 x 1019 at/cm3 for boron doses greater than 5 x 1014 at/cm2. The excess of boron always accumulates within the silicide. The observed boron redistribution behavior within polycrystalline TaSi 2 /poly-Si layers is discussed in terms of combined lattice and grain boundary diffusion as well as grain boundary segregation. INTRODUCTION Recently, boron doped silicide/poly-Si ("p--polycide") gates became the focus of considerable interest in VLSI-CMOS technology, because of their ability to improve the performance of CMOS devices: The conductivity of the gate electrode is enhanced by the silicide layer and simultaniously, due to the high workfunction of the boron doped poly-Si, hot carrier degradation in N-channel MOS FETs and short channel effects in P-channel MOS FETs are reduced [1]. However, for realizing p--polycide gate MOS FETs with
stable and
reproducible threshold
voltages, sufficient out-diffusion
of dopants from the silicide into the underlying poly-Si has to occur. Recent investigations on the boron redistribution within TaSi 2 /poly-Si gates at 900 °C have shown that clean silicide/poly-Si interfaces are crucial for p-polycide gates [2]. Thin residual oxides located at the silicide/poly-Si interface were found to act as diffusion barriers inhibiting the boron transport across the silicide/poly-Si interface. In this paper, the influence of different implantation schemes as well as implantation doses and energies on the boron redistribution behavior within TaSi 2 /poly-Si gates (with clean interfaces) will be examined by means of SIMS. EXPERIMENTAL 2 MOS-capacitors with gate areas of 0.01 to 8 mm were fabricated from (100) oriented boron doped (5 to 10 ohm-cm) silicon wafers and thermally grown gate oxides of 25 nm were used. The polycide gates consist of 200 nm TaSi 2 on top of 300 nm thick undoped poly-Si. Special cleaning procedures were applied in order to achieve clean silicide/poly-Si interfaces [2]. Boron was implanted either into the silicide or into the poly-Si prior to TaSi 2 deposition. To prevent dopant loss during subsequent anneals at 900 °C in inert ambients all samples were covered with 100 nm CVD oxide. Prior to SIMS measurements the oxide was removed. TEM measurements revealed TaSi 2 grain sizes in the 80 nm range after annealing at 900 *C. A detailed description of the exp
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