Prevention of Boron Penetration in a Passivated P+ Polysilicon Gated Mosfet Structure
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2005-Apr-02
Sun, YC: AUTHOR [+3]
Disclosed in this article is a method to avoid boron penetration from P+ polysilicon through thin oxide in polysilicon/oxide/silicon MOSFET structures passivated by LPCVD insulators.
Prevention of Boron Penetration in a Passivated P+
this article is a method to avoid boron
penetration from P+ polysilicon through thin oxide in
polysilicon/oxide/silicon MOSFET structures passivated by LPCVD
diffusion in oxide is enhanced by orders of magnitude in
the presence of hydrogen or water. The deposition of PSG or BPSG for
passivation/gettering/reflow purposes by LPCVD or PECVD methods at
around 400oC can incorporate hydrogen byproduct in the glass as well
as in the oxide between boron doped polysilicon gate electrode and
the silicon substrate. The incorporated hydrogen can enhance boron
diffusion in the gate oxide during subsequent annealing (>800oC) of
the passivation oxide (glass) -*-. Once boron has penetrated the thin
oxide into the silicon substrate, the MOSFET threshold voltage will
shift positively which renders the device inoperative.
technique calls for a medium temperature anneal
after the LPCVD of passivation oxide to evolve hydrogen from the
structure. The idea is based on the fact that hydrogen diffuses
faster in the oxide than boron does, even though the diffusivity of
boron is enhanced in the presence of hydrogen. The temperature for
the hydrogen evolution step can be chosen to minimize the annealing
time with little boron diffusion, e.g., 600oC.
In the case
of nitride capping of p+-poly MOS structures,
windows may have...