Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Grooved and refilled with deposited dielectric material
Reexamination Certificate
1998-05-21
2001-08-28
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Grooved and refilled with deposited dielectric material
C257S338000
Reexamination Certificate
active
06281094
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device provided with a trench isolation, particularly to a method of fabricating a semiconductor device capable of reducing an inverse narrow channel effect in which a threshold voltage lowers as the channel width of a transistor decreases in a metal oxide semiconductor field effect transistor (MOSFET).
2. Description of the Related Art
Referring to
FIG. 1A
, description will be made about a conventional method of fabricating a semiconductor device. A trench
103
is first formed on a single-crystal silicon substrate
101
, then the inner surfaces of the trench
103
are entirely oxidized through the chemical vapor deposition (CVD) method, and silicon oxide
105
is piled up as shown in FIG.
1
A.
Then, as shown in
FIG. 1B
, the silicon oxide
105
on the surface is polished and smoothed through the chemical mechanical polishing (CMP) method and thereby, the trench
103
is filled with the silicon oxide
105
, and then the surface of the silicon oxide
105
and the principal surface of the single-crystal silicon substrate
101
are covered with a gate insulating film
111
to be formed between the silicon oxide
105
and a gate electrode
110
.
In this case, when the silicon oxide
105
filling the bottom of the trench
103
is lower than the principal surface of the single-crystal silicon substrate
101
as shown in
FIG. 1B
, a problem occurs that the threshold voltage of a MOSFET lowers by approx. 0.15 V if the channel width of the MOSFET decreases to 0.2 &mgr;m from 10 &mgr;m as shown by the characteristic diagram in FIG.
1
C.
This is because, as described, for example, in IEDM (International Electron Devices Meeting) in 1981, Technical Digest (pp. 380-383), an electric field V in the internal direction of the single-crystal silicon substrate
101
from the gate electrode
110
and an electric field H in the direction parallel with the surface are concentrated nearby a trench shoulder
112
and then the threshold voltage of the trench shoulder
112
lowers.
That is, when the channel width of a MOSFET decreases, the rate of the portion where the threshold voltage lowers to the entire channel increases and the threshold voltage of the entire MOSFET also lowers.
To solve the above problem, there is a method of raising the threshold voltage of an edge portion of the semiconductor device by implanting impurity ions from the side surfaces of a trench.
However, because the impurity concentration becomes higher than that in the single-crystal silicon substrate
101
in the vicinity of the interface between the single-crystal silicon substrate
101
and an oxide film filling the trench
103
, a junction capacity and a junction leak current are increased.
To avoid the above problem, as described in Japanese Unexamined Patent Publication No. 6-177239 (177239/1998) for example, there is a method of forming a tapered trench by etching a separation region of the semiconductor device, that is, a method of controlling the concentration of electric fields by preventing a shoulder shape from being formed at an edge portion of a semiconductor device or chamfering a shoulder.
In the case of the above conventional method of fabricating a semiconductor device, there is a problem that a phenomenon due to the inverse narrow channel effect occurs in which a threshold voltage lowers when the channel width of a transistor is too small even if using the method of controlling the concentration of electric fields by preventing a shoulder shape from being formed at an edge portion of the semiconductor device or chamfering a shoulder.
This is because the boron contained in a channel is piled up on the silicon oxide side filling a trench at the interface between the silicon of a substrate and the silicon oxide due to thermal diffusion and thereby, it diffuses outward, and a region in which the boron concentration lowers is formed nearby the interface between the trench and the substrate. The boron diffusion occurs even at approx. 800° C. because inter-grid silicon is present which is produced due to ion implantation or the like.
Moreover, because phosphorus or arsenic serving as an impurity for forming an n-well is piled up on the silicon side of a substrate, it does not diffuse outward from a channel. Therefore, the above phenomenon does not occur.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor device fabrication method in which a threshold voltage does not lower even if the channel width of a transistor decreases.
Other objects of the present invention will become clear as the description proceeds.
A method to which the present invention is applicable is of fabricating a semiconductor device including a p-well, an n-well, and a trench isolation extending over the p-well and said n-well. The method comprises the steps of preparing a semiconductor substrate and forming a trench for the trench isolation to the semiconductor substrate. The trench is defined by a plurality of side surfaces and a bottom surface extending between said side surfaces. The method further comprises implanting boron ions into the semiconductor substrate through the side surfaces and the bottom surface.
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Stanley Wolf Silicon Processing for the VSLI Era vol. III Lattice Press pp. 222-226, 1995.*
Stanley Wolf Silicon Processing for the VSLI Era vol. 1 Lattice Press p. 25, 1986.*
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T. Izuka et al., Double Threshold MOSFETS in Bird's-Beak Free Structures, IEDM Technical Digest, pp. 380-383, (1981).
Abiko Hitoshi
Higuchi Minoru
Blum David S
Bowers Charles
Foley & Lardner
NEC Corporation
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