Method of manufacturing cobalt silicide layer

A method of manufacturing a cobalt suicide layer in the present invention has a silicon layer formation step. The silicon layer is formed at the interface between the cobalt layer and titanium layer, therefore the interface is smoother in this invention than in other conventional methods, and there are no voids formed at the interface. Moreover, consumption of the silicon can be controlled by adjusting the thickness of the silicon layer.

CROSS-REFERENCE TO RELATED APPLICATION 
This application claims the priority benefit of Taiwan application serial 
no. 86119671, filed Dec. 24, 1997, the full disclosure of which is 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates in general to the fabrication of a self-aligned 
silicide and more particularly to a method of manufacturing a cobalt 
silicide (CoSi.sub.2) layer. 
2. Description of the Related Art 
As the level of integration for MOS devices increases, resistance in the 
source/drain terminals of the MOS device gradually rises to a value 
comparable to the channel resistance of the MOS device. To ensure 
integrity at the shallow junction between metallic contacts and the MOS 
terminals, and for the downward adjustment of sheet resistance in the 
source/drain terminals, self-aligned silicide processes are now employed 
in the manufacturing of very large scale integrated (VLSI) circuits, 
especially for the manufacturing of semiconductor devices having a line 
width smaller than about 0.5 .mu.m. 
Titanium is the metal material most frequently used for the self-aligned 
silicide. However, it is not easy to control the silicide step at the high 
temperature needed for the titanium salicide process. Moreover, the 
temperature used to transform the titanium metal into titanium silicide is 
very high. As the size of the titanium silicide is diminished, high 
temperature is needed to transform the phase completely, but high 
temperature bring about thermal unstability. In response to these problems 
cobalt silicide can be used to displace the titanium silicide in the 
future, because it has a low silicide temperature and low resistance. 
However, large amounts of silicon are consumed during the formation of the 
cobalt silicide, and it is not easy to control junction depth. Moreover, 
the formation of cobalt silicide is degraded by the native oxide formed on 
the surface of the silicon substrate. 
FIGS. 1A to 1B are cross-sectional views showing a progression for 
manufacturing a cobalt silicide layer by using a cobalt/silicon structure 
according to a conventional method. 
First, as shown in FIG. 1A, a silicon substrate 10 is provided. The silicon 
substrate 10 is dipped in a buffer oxide etchant (BOE) solution with a 
50:1 concentration to remove the native oxide formed on the surface of the 
silicon substrate 10. Then, a layer of cobalt metal 12 is formed, for 
example, by using a sputtering method, over the silicon substrate 10. The 
sputtering method comprises, for example, a DC magnetically controlled 
sputtering method, at a base pressure of about 2.times.10.sup.-7 torr. 
Next, referring to FIG. 1B, a thermal oxidation method is performed, for 
example, by using a rapid thermal process with nitrogen for about 30 sec, 
so that the cobalt layer 12 reacts with the silicon atoms on the surface 
of the substrate 10, forming a layer of cobalt silicide 14. Then, the 
unreacted and remaining cobalt metal are removed, for example, by using a 
wet etching method. 
A conventional manufacturing method has several drawbacks including the 
roughness at the junction interface of the silicon substrate 10 and cobalt 
silicide layer 14, making it hard to control the junction depth. 
Additionally, the formation of the cobalt silicide is degraded by the 
native oxide formed on the surface of the substrate structure. 
Another conventional manufacturing method comprises adding a layer of 
titanium to the interface between the cobalt layer and silicon substrate, 
thereby preventing the formation of the native oxide. 
FIGS. 2A to 2B are cross-sectional views showing a progression for 
manufacturing a cobalt silicide layer by using a cobalt/titanium/silicon 
structure according to another conventional method. 
First, as shown in FIG. 2A, a silicon substrate 20 is provided. Silicon 
substrate 20 is dipped in a buffer oxide etchant solution with a 50:1 
concentration to remove the native oxide formed on the surface of the 
silicon substrate 20. Then, a layer of titanium metal 22 with a thickness 
of about 50-150 .ANG. is formed, for example, by using a sputtering 
method, over the surface of the silicon substrate 20. Then, a layer of 
cobalt metal 24 is formed, for example, by using a sputtering method, over 
the titanium layer 22. The sputtering method comprises, for example, DC 
magnetically controlled sputtering method, at an base pressure of about 
2.times.10.sup.-7 torr. 
Next, referring to FIG. 2B, a thermal annealing method is performed, for 
example, by using a rapid thermal process with nitrogen for about 30 sec, 
so that the cobalt layer 24 reacts with the silicon atoms on the surface 
of the substrate 20, forming a layer of cobalt silicide 26. 
Simultaneously, a layer of titanium-cobalt-silicon-oxygen mixture 28 is 
formed on the surface. Then, the unreacted and remaining cobalt metal are 
removed, for example, by using a wet etching method. 
There is still a rough interface between the silicon substrate 20 and 
silicon cobalt layer 26 in this conventional method. Moreover, a large 
amount of silicon is consumed and voids are formed at the interface. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide an improved and 
simplified process of manufacturing a cobalt silicide layer to improve the 
drawbacks of the conventional process. 
The invention achieves the above-identified objects by providing a new 
method of manufacturing a cobalt silicide layer. This method comprises the 
following steps. First, a silicon substrate is provided. The substrate is 
cleaned in a buffer oxide etchant solution. Next, three layers are formed 
over the substrate in the following order: a titanium layer, a silicon 
layer and a cobalt layer. Finally, a thermal process is performed, so that 
the cobalt layer is transformed into the cobalt silicide layer. 
A characteristic of this invention is the addition of the silicon layer to 
the position between the cobalt layer and titanium layer, therefore 
forming a smooth interface. Moreover, consumption of the silicon can be 
controlled by adjusting the thickness of the silicon layer. The cobalt 
silicide layer is an epitaxial phase.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 3A to 3B are cross-sectional views showing a progression for 
manufacturing a cobalt silicide layer according to a preferred embodiment 
of this invention. 
First, as shown in FIG. 3A, a silicon substrate 50 is provided. The 
substrate 50 is dipped in a buffer oxide etchant solution with a 50:1 
concentration to clean the silicon substrate 50 by, for example, removing 
the native oxide formed on the surface of the silicon substrate 50. Then, 
a layer of titanium metal 52 with a thickness of about 50-150 .ANG. is 
formed, for example, by using a sputtering method, over the surface of the 
silicon substrate 50. A layer of silicon 54, for example, amorphous 
silicon, polysilicon, or silicon-contained layer is formed, for example, 
by using a chemical vapor deposition method, over the surface of the 
titanium layer 52. 
A characteristic of this invention is that the silicon layer 54 is formed 
over the titanium layer 52, thus the surface of the substrate structure 
smoother, which in turn makes the interface between the substrate 
structure and cobalt layer formed in the subsequent step smoother too. The 
consumption of the silicon can be controlled by adjusting the thickness of 
the silicon layer 54. 
Then, a layer of cobalt metal 56 is formed, for example, by using a 
sputtering method, over the silicon layer 54. The sputtering method 
comprises, for example, a DC magnetically controlled sputtering method, at 
an base pressure of about 2.times.10.sup.-7 torr. 
Referring to FIG. 3B, a thermal oxidation method is performed, for example, 
by using a rapid thermal process with nitrogen for about 30 sec, so that 
the cobalt atoms of the cobalt layer 56 react with the silicon atoms on 
the surface of the deposited silicon layer 54 and substrate 50 to form a 
layer of cobalt silicide 58. Simultaneously, an amorphous phase layer 60 
of titanium-cobalt-silicon-oxygen mixture is formed on the surface. Then, 
the unreacted and remaining cobalt metal are removed, for example, by 
using a wet etching method. The cobalt silicide formed in the present 
invention is an epitaxial metal silicide. 
The interface between the cobalt silicide layer 58 and the silicon 
substrate 50 is smoother in this invention than that formed in the 
conventional method, and there are no voids formed at the interface. 
A characteristic of this invention is the addition of the silicon layer 54 
to the position between the cobalt layer 56 and titanium layer 52, to form 
the smooth interface. Moreover, the consumption of the silicon can be 
controlled by adjusting the thickness of the silicon layer 54, and a 
cobalt silicide layer 58 having an epitaxial phase is formed. 
While the invention has been described by way of example and in terms of a 
preferred embodiment, it is to be understood that the invention is not 
limited thereto. To the contrary, it is intended to cover various 
modifications and similar arrangements and procedures, and the scope of 
the appended claims should therefore be accorded the broadest 
interpretation so as to encompass all such modifications and similar 
arrangements and procedures.