Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1998-08-24
2001-06-05
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192220, C204S192150, C427S372200, C427S379000, C427S397700, C438S762000, C438S765000, C438S769000
Reexamination Certificate
active
06241859
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming a semiconductor device and a method of forming a self-aligned refractory metal salicide layer over semiconductor diffusion layers and polysilicon layers.
In accordance with highly required scaling down of the semiconductors devices and increasing the density of integration, a design rule of 0.15-0.25 micrometers minimum size has now been on the application to form highly integrated semiconductor devices such as memory devices and logic devices. The increase in the density of the semiconductor device requires substantive reductions in length of the gate electrode and in width of diffusion layers as well as in thicknesses of the diffusion layers and the polysilicon layer as the gate electrode, thereby resulting in increase in electrical resistance of the semiconductor device. The increase in electrical resistance causes a substantive delay in transmission of signals on the circuits. For realizing the substantive reduction in the resistance, it is, therefore, required to form silicide layers, particularly refractory metal salicide layers, on the polysilicon gate electrode and on the diffusion layers of the single crystal silicon substrate. The refractory metal silicide layers are formed by silicidation of a refractory metal layer and a self-alignment technique.
A conventional method of forming a MOS field effect transistor with silicide layers will be described with reference to FIGS.
1
A through
1
E, which are fragmentary cross sectional elevation views illustrative of MOS field effect transistors in sequential steps involved in the first conventional fabrication method.
With reference to
FIG. 1A
, field oxide films
102
are selectively formed on a surface of a silicon substrate
101
by a local oxidation of silicon method, thereby to define an active region surrounded by the field oxide films
102
. An ion-implantation of an impurity into the active region of the silicon substrate
101
is carried out to increase a withstand voltage thereof. A thermal oxidation of silicon is carried out to form a gate oxide film
103
on the active region of the silicon substrate
101
. A chemical vapor deposition method is carried out to deposit a polysilicon film having a thickness of about 150 nanometers on an entire region of the silicon substrate, so that the polysilicon film extends over the field oxide films
102
and the gate oxide film
103
. The polysilicon film is doped with an impurity such as phosphorus to reduce a resistivity of the polysilicon film. The polysilicon film is patterned by a photolithography technique in order to form a polysilicon gate electrode
104
on the gate oxide film
103
. A chemical vapor deposition method is carried out to deposit a silicon oxide film on an entire region of the silicon substrate
101
, so that the silicon oxide film extends over the polysilicon gate electrode
104
, the active region of the silicon substrate
101
and the field oxide films
102
. An anisotropic etching to the silicon oxide film is carried out to leave the silicon oxide film only on side walls of the polysilicon gate electrode
104
, thereby to form side wall spacers
105
on the side walls of the polysilicon gate electrode
104
. An ion-implantation of an impurity such as boron or arsenic into the active region of the silicon substrate
101
is carried out by using the polysilicon gate electrode
104
, the side wall spacers
105
and the field oxide films
102
as masks, thereby to form impurity doped regions in upper regions of the silicon substrate
101
. A heat treatment to the silicon substrate
101
is carried out at a temperature in the range of 800 to 1000° C. to form source/drain diffusion layers
106
in the upper regions of the silicon substrate
101
.
With reference to
FIG. 1B
, a sputtering method is carried out by sputtering a titanium target to deposit titanium on an entire region of the silicon substrate
101
, thereby entirely forming a titanium film
107
having a thickness of 50 nanometers, so that the titanium film
107
extends over the polysilicon gate electrode
104
, the side wall spacers
105
, the source/drain diffusion layers
106
and the field oxide films
102
.
With reference to
FIG. 1C
, a heat treatment to the silicon substrate
101
is carried out by use of a lamp anneal at a temperature in the range of 600 to 650° C. in a nitrogen atmosphere under an atmospheric pressure for a time in the range of 30 to 60 seconds, thereby to cause both a titanium nitration reaction and a titanium silicide reaction, wherein nitrogen atoms are thermally diffused into the titanium layer
107
whereby the titanium film, except for bottom regions thereof on the source/drain diffusion layers
106
and the polysilicon gate electrode
104
, are made into a titanium nitride film
112
, whilst silicon atoms in the source/drain diffusion layers
106
and in the polysilicon gate electrode
104
are also thermally diffused into the bottom region of the titanium layer
107
, whereby titanium atoms in the bottom region of the titanium layer
107
but only on the source/drain diffusion layers
106
and the polysilicon gate electrode
104
are silicided with the thermally diffused silicon atoms from the source/drain diffusion layers
106
and the polysilicon gate electrode
104
. As a result, the bottom regions of the titanium layer
107
but only on the source/drain diffusion layers
106
and the polysilicon gate electrode
104
are made into titanium silicide layers
109
. Namely, the titanium film
107
on the side wall spacers
105
and the field oxide films
102
are completely nitrated. The titanium silicide layers
109
, therefore, extends on the bottom surface of the titanium nitride film
112
and over the source/drain diffusion layers
106
and the polysilicon gate electrode
104
. The titanium silicide layers
109
has a C49-crystal structure hang a relatively high resistivity of about 60 &mgr;&OHgr;cm.
With reference to
FIG. 1D
, the titanium nitride film
112
is completely etched by a wet etching which uses a chemical of a mixture of ammonium solution and a hydrogen peroxide solution, whereby the C49-structured titanium silicide layers
109
remain on the polysilicon gate electrode
104
and on the source/drain diffusion layers
106
.
With reference to
FIG. 1E
, a heat treatment to the silicon substrate
101
is carried out at a temperature of about 850° C. in a nitrogen atmosphere under an atmospheric pressure for 60 seconds to cause a phase transition from the C49 crystal structure to a C54 crystal structure which has a low resistivity of about 20 &mgr;&OHgr;cm. As a result, the C49-structured titanium silicide layers
109
are made into C54-structured titanium silicide layers
111
. Namely, the C54-structured titanium silicide layers
111
are formed on the polysilicon gate electrode
104
and on the source/drain diffusion layers
106
.
The reason why the heat treatment for causing the silicidation reaction is carried out in the nitrogen atmosphere is as follows. During the heat treatment for causing the silicidation reaction of titanium with silicon, silicon may be diffused not only into the titanium film but also onto the silicon oxide films such as the field oxide films
102
. The diffused silicon over the silicon oxide film is then reacted with titanium diffused from the titanium film, thereby to form a titanium silicide layer on an interface between the titanium film and the silicon oxide film. This titanium silicide layer will remain on the silicon oxide film such as the field oxide films, This means it no longer possible to obtain the required insulation. This phenomenon is so called to as “over-growth”. In order to prevent the over-growth of the titanium silicide layer over the silicon oxide film, it is required to carry out the heat treatment for causing the silicidation reaction in the nitrogen atmosphere. The heat treatment in the nitrogen atmosphere causes a diffusion of nitrogen in the nitrogen atmosphere into the titanium film. Further, a nitration react
Ishigami Takashi
Matsubara Yoshihisa
Yamada Yoshiaki
NEC Corporation
Nguyen Nam
VerSteeg Steven H.
Young & Thompson
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