Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – Insulated gate formation
Reexamination Certificate
2001-01-08
2001-10-16
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
Insulated gate formation
Reexamination Certificate
active
06303483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the structure of a gate electrode of a semiconductor device such as a field-effect transistor (FET) having a MOS (metal-oxide-semiconductor) structure.
2. Description of the Background Art
Refinement of a field-effect transistor (hereinafter referred to as “MOSFET”) having a MOS structure progresses in recent years. This results in such a problem that thin wire resistance or contact resistance of a gate electrode is increased as the gate length of the gate electrode is reduced, for example. When the thin wire resistance or the contact resistance is increased, the circuit operating speed is disadvantageously slowed down while the number of memory cells capable of sharing a single word line is reduced, the number of divided word lines is increased and the number of peripheral circuits is also increased, leading to an increase of the chip area in a memory such as a DRAM (dynamic random-access memory). When the chip area is increased, the theoretical chip number per wafer is reduced to increase the production cost and reduce the price competitiveness of the chip. Therefore, reduction of the thin wire resistance or the contact resistance, leading to implementation of refinement of the semiconductor device and reduction of the chip area, are important development items in the semiconductor industry.
A gate electrode of a conventional MOSFET is formed by a single-layer electrode consisting of only polysilicon containing an impurity element added in high concentration or a multilayer electrode of a metal silicide/polysilicon structure having relatively small gate resistance, represented by a WSi
x
(x=2.4 to 2.8)/polysilicon structure or a CoSi
2
/polysilicon structure. However, it is difficult to apply the conventional gate electrode having such a structure to a transistor having a fine pattern of not more than 0.12 &mgr;m, for example. This is because the thin wire resistance or the contact resistance of the gate electrode is excessively increased when the gate electrode is refined.
Therefore, a polymetal gate electrode having lower resistance than the conventional gate electrode is watched with interest. For example, Japanese Patent Application Laid-Open No. 11-233451 (1999) discloses a general polymetal gate electrode.
FIG. 21
is a model diagram showing a sectional structure of a conventional polymetal gate electrode
106
. The gate electrode
106
is formed by successively depositing a polysilicon film
102
, a barrier metal film
103
, a metal film
104
and an insulator film
105
on the main surface of a silicon substrate
100
through a gate insulator film
101
. The barrier metal film
103
consists of tungsten nitride (WN) or titanium nitride (TiN), and the metal film
104
consists of tungsten or the like. The polysilicon film
102
is doped with an impurity element in high concentration. This impurity element is prepared from an n-type dopant such as phosphorus or arsenic when forming an NMOSFET (n-channel MOSFET) or from a p-type dopant such as boron or indium when forming a PMOSFET (p-channel MOSFET). A polymetal gate electrode structure stands for such a three-layer structure of a metal film, a barrier metal film and a polysilicon film. When no barrier metal film
103
is interposed between the polysilicon film
102
and the metal film
104
, polysilicon thermally diffusing from the polysilicon film
102
reacts with metal atoms contained in the metal film
104
to form metal silicide and disadvantageously increase the resistance of the gate electrode
106
when the gate electrode
106
is heat-treated after deposition.
FIGS. 22 and 23
illustrate concentration distribution of various types of elements contained in the polysilicon film
102
, the barrier metal film
103
and the metal film
104
when the metal film
104
consists of W and the barrier metal film
103
consists of WN
x
as first prior art of such a polymetal gate electrode. Referring to
FIGS. 22 and 23
illustrating the concentration distribution along the line A
1
-A
2
in
FIG. 21
,
FIG. 22
shows the concentration distribution immediately after depositing the barrier metal film
103
and the metal film
104
, and
FIG. 23
shows the concentration distribution after heat-treating the gate electrode
106
in a later step. Referring to each of
FIGS. 22 and 23
, the horizontal axis shows the distance, and the vertical axis shows the logarithmic scale values of atom numbers per unit cubic centimeter. As the heat treatment, RTA (rapid thermal annealing) is carried out at 1000° C.
Referring to
FIG. 22
, nitrogen atoms (N) and tungsten atoms (W) are substantially homogeneously distributed in the barrier metal film
103
before the heat treatment. After the heat treatment, on the other hand, tungsten nitride (WN
x
) contained in the barrier metal film
103
decomposes into tungsten atoms (W) and nitrogen atoms (N) due to the RTA, and the nitrogen atoms partially evaporate as nitrogen molecules, partially segregate toward the metal film
104
and partially form W
2
N, which is a conductor having lower resistance than WN
x
. It is understood from
FIG. 23
that the concentration distribution of silicon atoms (Si) shifts into the barrier metal film
103
due to the heat treatment and the silicon atoms diffuse into the barrier metal film
103
from the polysilicon film
102
. Hence, it is conceivable that such diffusing Si reacts with W and N in the barrier metal film
103
to form an insulator such as silicon nitride (SiN) or WSiN and tungsten silicide (WSi
x
) having higher resistance than a high melting point metal such as W or Mo. Thus, there is a possibility that the resistance (sheet resistance and contact resistance) of the barrier metal film
103
is increased.
An example employing W for the metal film
104
and two layers of TiN and Ti for the barrier metal film
103
is also proposed as second prior art of the polymetal gate electrode. In conventional manufacturing steps, the polysilicon film
102
, the barrier metal film
103
and the metal film
104
are generally deposited on the overall surface of the silicon substrate
100
through the gate insulator film
101
and thereafter the insulator film
105
of silicon nitride or silicon oxynitride is deposited and the gate electrode
106
is formed by anisotropic etching employing photolithography.
Further, high-temperature heat treatment is performed on the gate electrode
106
in a diluted oxygen atmosphere for forming oxide films on the side walls thereof, in order to recover the gate electrode
106
from damages resulting from the aforementioned anisotropic etching. In this high-temperature heat treatment, titanium and polysilicon react with each other in the barrier metal film
103
to form metal silicide (TiSi
x
). This metal silicide diffuses into the polysilicon film
102
and reaches the gate insulator film
101
during the aforementioned high-temperature heat treatment to remarkably deteriorate the insulation property of the gate insulator film
102
or forms crystal grains and segregates in the polysilicon film
102
to remarkably increase the resistance of the polysilicon film
102
if not reaching the gate insulator film
102
.
The aforementioned problems of the conventional polymetal gate are summarized as follows:
(1) As described with reference to the first prior art, it is apprehended that Si diffuses from the polysilicon film
102
into the barrier film
103
and the diffusing Si reacts with W and N in the barrier film
103
to form SiN, WSi
x
or WSiN in the heat treatment after deposition of the gate electrode
106
, to disadvantageously increase the resistance of the barrier metal film
103
.
(2) As described with reference to the second prior art, titanium reacts with polysilicon in the step of performing the high-temperature heat treatment on the gate electrode
106
to form metal silicide TiSi
x
, which disadvantageously deteriorates the characteristics of the gate insulator film
101
and extremely increases the sheet resistance of t
Hoang Quoc
Mitsubishi Denki & Kabushiki Kaisha
Nelms David
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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