Shallow Boron‐Doped Layer Formation by Boron Diffusion from Poly‐Si Through Thin SiO2

Abstract

This paper discusses boron doping using metal oxide semiconductor structure (poly-Si/SiOs/Si). The thin SiQ layer acts as a stopper to poly-Si removal after doping. When boron implantation is used for po]y-Si doping, shallow borondoped layers suitable for base application can be formed by wet O2-ambient drive-in. When BF5 implantation is used, shallow boron-doped layers can be formed even by N2-ambient drive-in. The surface boron concentration of boron-doped layers increases with dose and saturates, since boron concentration in poly-Si in the region near the interface with SiO2 also increases with dose and saturates. An estimate of the boron diffusion coefficient in SiQ, Dox, shows that it increases by about one order of magnitude both for boron implantation with subsequent wet Q-ambient drive-in and for BF2 implantation with subsequent N2-ambient drive-in.Future large scale integrated (LSIs) will require shallow boron-doped layers as the base layers of bipolar transistors and counter-doped layers of p-metal oxide semiconductor field effector transistors (pMOSFETs). 1' 2 The conventional method of boron-ion implantation cannot provide layers that are shallow enough due to channeling effects and damage-assisted diffusion. ~-6 Moreover, ion-implantation damage may adversely affect device characteristics when relatively high-dose implantation is used to form high-concentration base layers and counter-doped layers. For these reasons, doping methods using diffusion from solid sources are being studied again, since they do not result in channeling effects and implantation damage. 1' 7' 8In doping by diffusion from solid sources, doped oxides (BSG/Si)Y ~9' 1~ and doped poly-Si (poly-Si/Si) 8 are frequently used as diffusion sources. Doped poly-Si is superior in terms of boron-concentration controllability, since the concentration in pely-Si can be controlled by boron-ion implantation. After doping, it is necessary to selectively remove the diffusion sources without etching the silicon substrates to allow applications to base formation and channel doping. A poly-Si diffusion source, however, cannot be removed selectively in a poly-Si/Si system. This paper reports boron doping from poly-Si through thin SiO2, i.e., boron doping using an MOS structure (polySi/SiOs/Si), which makes it possible to selectively remove poly-Si after doping. The thin SiQ layer acts as a stopper to poly-Si removal. In general, it is difficult to form shallow, high-concentration boron-doped layers using the MOS structure because the boron diffusion coefficient in SiQ is very small. The present study, therefore, looks at the diffusion steps in which boron diffusion in SiO2 is enhanced. In fabricating boron-doped poly-Si gate MOSFETs, it has been reported that boron in gate poly-Si penetrates into the channel region through gate oxides when heat-treatment is carried out in wet O5 ambient or when high-dose BF2 implantation is performed, n-13,17 This is due to the enhanced diffusion of boron in SiQ. These phenomena are undesirable in MOS processes sinc...

Study of Nitrogen Effect on the Boron Diffusion during Heat Treatment in Polycrystalline Silicon/Nitrogen-Doped Silicon Thin Films

“…[1][2][3] The miniaturization concept of these components tends to reduce the length of the canal, the thickness of the gate insulator and to maintain a strong capacitive coupling between the gate and the transistor channel. [4][5][6][7] Fulfilling these objectives require not only to shrink the transistor dimensions, but also to introduce new materials and structures able to preserve the quality of the gate/oxide interface and to keep the deep diffusion of B impurities far from the oxide layer. Several solutions have been proposed in MOS structure such as replacing the Si oxide layer by high dielectric permittivity materials, [8][9][10] varying the elaboration method, the doping techniques and thermal annealing [11][12][13] or using multilayer gate transistors.…”