Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-01-04
2002-05-07
Lee, Eddie (Department: 2815)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S153000, C438S154000, C438S163000, C438S282000, C438S517000, C438S519000, C438S521000
Reexamination Certificate
active
06383850
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an SOI (Semiconductor On Insulator) type thin-film transistor, and more particularly to a technique for fixing a potential of its body (referred to as “body potential” hereinafter).
2. Description of the Background Art
FIG. 18
is a cross-sectional view illustrating a structure of a general SOI type thin-film transistor
900
. The thin-film transistor
900
is formed as an n-channel MOS transistor in a semiconductor layer
902
provided on an insulator
901
. The insulator
901
may be formed as a buried layer in a not-shown semiconductor substrate.
In the p
−
type semiconductor layer
902
, a source region
903
and a drain region
904
both of which are n
+
type semiconductor layers are provided at a distance from each other. The semiconductor layer
902
sandwiched by the source region
903
and the drain region
904
is termed “body” of the thin-film transistor
900
. Above the body, a gate electrode
907
made of e.g., polysilicon is provided with a gate insulating film
906
interposed therebetween.
Since the semiconductor layer
902
is provided on the insulator
901
, its body potential is in a floating state in the structure of FIG.
18
. In this state, there are possibilities of generation of leak current and unstable operation of the thin-film transistor
900
due to variations in power-supply level and ground level and parasitic bipolar effect. The parasitic bipolar effect here refers simply to a phenomenon that positive holes created by impact ionization are accumulated in the body and the body potential thereby rises to increase a leak current in the thin-film transistor
900
as an n-channel MOS transistor.
There may be a case where a cosmic ray such as an a ray enters the body to form a pair of an electron and a positive hole. Since the thin-film transistor
900
is used in a state where a channel is formed by inverting a surface of the body into n type, the positive hole is accumulated, though the electron is drawn out, to raise a possibility of inviting a rise of body potential.
To solve the above problem, a technique for fixing the body potential has been proposed.
FIG. 19
is a cross-sectional view of a first technique in the prior art, illustrating a structure of a thin-film transistor
800
and a structure for fixing its body potential. The thin-film transistor
800
comprises a p
−
type semiconductor layer
802
as a body formed on an insulator
801
and a source region
803
and a drain region
804
both of which are of n
+
type and provided thereabove. Above the body, a gate electrode
807
is provided with a gate insulating film
806
interposed therebetween.
Alongside the thin-film transistor
800
, an isolation oxide film
809
is formed by LOCOS oxidization of the semiconductor layer
802
. Below the isolation oxide film
809
, above the insulator
801
, a region
805
a
is formed by enhancing the conductivity of the semiconductor layer
802
. On the opposite side of the thin-film transistor
800
with respect to the isolation oxide film
809
, a p type region
805
b
and a p
+
type region
805
c
are layered on the insulator
801
in this order. The regions
805
a
,
805
b
and
805
c
adjoin the semiconductor layer
802
in this order, and when a potential VB is applied to the region
805
c
, the body potential can be fixed at a position away from the thin-film transistor
800
with the isolation region
809
interposed.
Since the first background-art technique, however, uses the isolation oxide film
809
, it is not suitable for integration. Further, a structure much like that of
FIG. 19
, where the p type semiconductor layer is provided between the source region and the insulator to draw the positive hole, is disclosed in, for example, Japanese Patent Application Laid Open Gazette No. 6-232405.
On the other hand, the thin-film transistor is often used with the potential applied to the source region (referred to simply as “source potential”) and the body potential being equal, and on the premise of such a use, a structure for fixing the body potential can be formed locally in the source region.
FIG. 20
is a plan view illustrating a structure of a thin-film transistor
700
that is advantageous from this viewpoint. The second technique in the background art is disclosed in, for example, “Silicon-on-insulator technology: materials to VLSI” by J. P. Colinge (Kluwer Academic Publishers, 2nd Ed.).
With the gate electrode
707
centered, on the left hand of this figure provided are an n
+
type source region
703
and p
+
type body-potential drawing regions
705
a
and
705
b
which sandwich the region
703
vertically in this figure, and on the right hand of this figure provided is a drain region
704
. A contact structure for supplying the body potential and the source potential is formed at a contact region
310
provided covering part of the body-potential drawing regions
705
a
and
705
b
across the source region
703
. This structure eliminates the necessity of the LOCOS oxide film used in the first background-art technique, thereby being suitable for integration.
The second background-art technique, however, has great problems as follows. The first problem is due to the position of the body-potential drawing region
705
a
. The body is provided in the back of the gate electrode
707
in this figure, though not shown, and a channel is formed mainly in a portion surrounded by the source region
703
, the drain region
704
and the gate electrode
707
. From this portion, the positive hole should be drawn.
In the structure of
FIG. 20
, the body-potential drawing regions
705
a
and
705
b
are positioned at an end of the source region
703
along a direction where the gate electrode
707
extends (in a vertical direction of this figure). Therefore, in order to effectively draw the positive hole from the body, a pair of body-potential drawing regions
705
a
and
705
b
are needed. For example, if the body-potential drawing region
705
a
is not provided, the body-potential drawing region
705
b
can not effectively draw the positive hole from a portion on the upper side of this figure in the body. This needs a larger area for the body-potential drawing regions
705
a
and
705
b
, and a portion which does not function as a channel in a direction (gate width) where the gate electrode
707
extends increases in width. That inhibits integration of the thin-film transistor.
The second problem becomes pronounced in a case where the gate electrode
707
is made of polysilicon and the like. Impurity implantations for forming the source region and the drain region are performed, in general, by using the gate electrode and the gate insulating film provided between the gate electrode and the body as a mask in a self-aligned manner. When an impurity to be implanted into the polysilicon to enhance the conductivity as the gate electrode is equivalent in conductivity to those for the source region and the drain region, the conductivity of the gate electrode is obtained by impurity implantation for forming the source region and the drain region.
If a p type impurity is implanted to also form the p
+
type body-potential drawing regions
705
a
and
705
b
of
FIG. 20
in a self-aligned manner, however, an effect of the n type impurity which the gate electrode
707
has is counter-doped. The gate electrode
707
comprises a straight portion
707
b
which is straight in a direction where the gate electrode
707
extends and a contact portion
707
a
in which a contact structure is formed to apply a predetermined electrical signal to the gate electrode. The conductivity is degraded in portions
401
a
and
401
b
of the straight portion
707
b
near the p
+
type body-potential drawing regions
705
a
and
705
b
at an end (on the left hand of
FIG. 20
) in a direction (horizontal direction of this figure) orthogonal to the direction where the straight portion
707
b
extends. This phenomenon becomes especially pronounc
Brock II Paul E
Lee Eddie
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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