Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1998-09-11
2001-05-22
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S900000
Reexamination Certificate
active
06236093
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a gate electrode in which, for example, a refractory metal film and a polysilicon film are used, and more specifically, to a method of manufacturing a semiconductor device including a gate electrode having a polymetal structure, which is a laminate structure of W/WNxSi/Poly-Si or the like.
The performance of MOS LSIs is becoming higher as the gate length of the MOS FETs is reduced. With the reduction of the gate length, the delay of a signal (called “propagation delay” ) when it is transmitted in the gate width direction is becoming impossible to neglect. In order to suppress the propagation delay, it is necessary to use a gate electrode of a low resistance. In general, a gate electrode employs a polycide gate structure in which a refractory metal silicide film such as tungsten silicide film is laminated on a polysilicon film. However, it is presently considered to use the polymetal gate structure, since it can lower the resistance value than the polycide gate structure does. The polymetal gate structure has a structure in which a refractory metal film such as of tungsten, which has a specific resistance smaller than that of a refractory metal silicide film by one digit, is laminated on a polysilicon film.
FIGS. 1
to
4
illustrates the first prior art example. The first prior art example is an cross sectional structure of a general polymetal gate electrode. In
FIG. 4
, a polymetal gate electrode containing a polysilicon film (the lower portion of the gate electrode)
3
, a first tungsten nitride film
16
and a tungsten film (the upper portion of the gate electrode)
5
a
, is formed via a gate insulating film (a gate oxide film)
2
on a P-type semiconductor substrate
1
.
As discussed in Jpn. Pat. Appln. KOKAI Publication No. 7-183513, a thermal oxidation, which is so called post oxidation, is carried out in the conventional method of manufacturing a MOS transistor. In the post oxidation, after the formation of the gate electrode by RIE (reactive ion etching), the polysilicon film and gate insulating film are thermally oxidated. By the post oxidation, the damage caused to the gate insulating film due to the etching can be recovered. Together with this, the lower end portion of the gate electrode is oxidated by the post oxidation to be rounded. Due to the rounded shape, when a voltage is applied to the gate, the concentration of the electrical field at the gate end portion is reduced. Therefore, the reliability of the gate insulating film at the end portion of the gate electrode is improved.
A method of manufacturing a semiconductor device which uses a polymetal gate electrode, according to the first prior art example, will now be described with reference to
FIGS. 1
to
4
.
As can be seen in
FIG. 1
, a gate insulating film
2
is formed on a semiconductor substrate
1
by thermal oxidation. Subsequently, a polysilicon film (the lower portion of a gate electrode)
3
, a first tungsten nitride
16
and a tungsten film (gate electrode)
5
a
are deposited in this order. The first tungsten nitride film
16
serves as a reaction inhibiting film for inhibiting the reaction between the tungsten film
5
a
and the polysilicon film
3
. Then, a first silicon nitride film
6
is deposited on the tungsten film
5
a
. Next, as shown in
FIG. 2
, the first silicon nitride film
6
is etched using a resist, though not shown, as a mask. Further, with use of the first polysilicon nitride film
6
as a mask, the tungsten film
5
a
, the first tungsten nitride film
16
and the polysilicon film
3
are etched, thus forming a gate electrode. Subsequently, as shown in
FIG. 3
, a round portion
8
is formed at the lower end section of the polysilicon film
3
by carrying out the post-oxidation step. Next, ion implantation is carried out while using the gate electrode as a mask, and thus N-type source/drain regions
9
are formed in the substrate
1
. Next, as shown in
FIG. 4
, an interlayer insulating film
10
is formed on an entire surface, and a contact hole
11
is formed in the interlayer insulating film
10
and the first silicon nitride film
6
. In the contact hole
11
, a wiring layer
12
made of a metal is formed.
In the post oxidation step, the tungsten film
5
a
, when oxidated, expands its volume, and the shape of the gate becomes abnormal. Therefore, in this step, it is required to selectively oxidate the polysilicon film
3
only. More specifically, by controlling the partial pressures and flow amounts of H
2
O and H
2
, the oxidation is carried out in an atmosphere which is controlled to oxidate only silicon. At the same time, however, a portion of the tungsten film
5
a
evaporates. Further, depending upon the oxidation condition, the side wall of the tungsten film
5
a
may retreat backwards significantly, and in some cases, the film becomes narrowed by 10 nm or more on one side. As described, in the first prior art example, the oxidation of the tungsten film is avoided as much as possible by controlling the atmosphere of the post oxidation step. However, in the post oxidation step, the oxidation atmosphere is controlled, and therefore a control device, which is very costly, is required. Further, a portion of the tungsten film
5
a
is evaporated so that the tungsten film
5
a
becomes slender, and such a technique may becomes a factor of increasing the wiring resistance. Further, when evaporated tungsten film attaches to the substrate, the contamination of metal occurs, thus increasing the junction leakage. As a result, the deterioration of a property such as the data retention performance of the memory cell occurs.
FIGS. 5
to
10
illustrate the second prior art example. The second prior art example is discussed in Jpn. Pat. Appln. KOKAI Publication No. 7-183513. The second prior art example is not a technique involving a polymetal gate structure as in the first prior art example, but an invention regarding a gate electrode having a so-called polycide structure in which a refractory metal silicide film is used as the upper electrode member of the gate electrode. In addition, the second prior art example discloses not a technique of avoiding the evaporation of the gate electrode member, but a technique of inhibiting the oxidation thereof. More specifically, as shown in
FIG. 10
, the second silicon nitride film
13
is formed on a side wall of the refractory metal silicide film (the upper portion of the gate electrode)
5
b
, and with this structure, the oxidation of the refractory metal silicide film
5
b
is suppressed.
The manufacture method according to the second prior art example will now be described with reference to
FIGS. 5
to
10
. As shown in
FIG. 5
, the method includes a step of depositing not a tungsten film, but a tungsten silicide film
5
b
as the upper portion of the gate electrode. The other processing steps are similar to those of the first prior art example. A gate insulating film
2
is formed on a substrate
1
. On the gate insulating film
2
, a polysilicon film (the lower portion of the gate electrode)
3
, a tungsten silicide film (the upper portion of the gate electrode)
5
b
and a first silicon nitride film
6
are formed in the order. Then, a natural oxide film
4
is created between the polysilicon film
3
and the tungsten silicide film
5
b
. Subsequently, as shown in
FIG. 6
, the first silicon nitride film
6
, the tungsten silicide film
5
b
and the natural oxide film
4
are etched. In this etching, the polysilicon film
3
is left without being etched. Next, as shown in
FIG. 7
, a second silicon nitride film
13
is deposited on the entire surface. Then, as shown in
FIG. 8
, the second silicon nitride film
13
is etched back by RIE, and thus the second silicon nitride film
13
remains on side walls of the first silicon nitride film
6
and the tungsten silicide film
5
b
. Further, the polysilicon film
3
is etched using a side wall made of the second silicon nitride film
13
as a mask. Next, as shown in
FIG. 9
, with the post oxidatio
Kabushiki Kaisha Toshiba
Pillsbury & Winthrop LLP
Prenty Mark V.
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