Method of fabricating semiconductor device

Details Method of fabricating semiconductor device Method of fabricating semiconductor device

2001-09-20

2003-02-04

Fourson, George (Department: 2826)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

Reexamination Certificate

active

06514883

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device including a gate insulating film made of a material with a high dielectric constant (which will be herein referred to as a “high-dielectric-constant material”) and also relates to a method for fabricating the device.
Recently, there has been a growing demand for high-speed-operating semiconductor devices. To meet this demand, the gate insulating film of MOSFETs has been further thinned for the purpose of increasing the drivability thereof.
However, if the gate insulating film is a thin film of SiO
2
(with a relative dielectric constant ∈ of 3.9), the gate leakage current increases noticeably because a tunneling current flows therethrough.
Thus, to prevent the gate leakage current from increasing while enhancing the drivability of MOSFETs, various methods for increasing the actual thickness of the gate insulating film and obtaining a desired gate capacitance have been researched. For example, according to one of those methods, the gate insulating film is made of a high-dielectric-constant material (high-&kgr;K material) such as HfO
2
(hafnium dioxide with a relative dielectric constant ∈ of about 30) or ZrO
2
(zirconium dioxide with a relative dielectric constant ∈ of about 25).
To deposit a gate insulating film of a high-dielectric-constant material, a reactive sputtering process is performed using a target of Hf or Zr, for example, in a mixed gas ambient containing Ar (argon) and O
2
gases, for example. In this manner, a gate insulating film of a high-dielectric-constant material such as HfO
2
or ZrO
2
can be deposited over a semiconductor substrate.
However, if the gate insulating film of the high-dielectric-constant material is deposited over a silicon substrate by the reactive sputtering method, for example, the surface of the silicon substrate is oxidized by a plasma created from the O
2
gas during the reactive sputtering process. Thus, an unwanted silicon dioxide film is formed between the silicon substrate and gate insulating film. It should be noted that the unwanted film will be herein referred to as a “silicon dioxide film” but can actually be any other silicon oxide film with a non-stoichimetric composition. Consequently, the gate insulating film becomes a stack of the silicon dioxide film with a relatively low dielectric constant and the high-dielectric-constant film. As a result, the gate insulating film has its effective dielectric constant decreased as a whole.
That is to say, the known method for fabricating a semiconductor device cannot obtain the desired gate capacitance. Thus, it is difficult to enhance the drivability of MOSFETs.
FIG. 7
is a cross-sectional view showing the known method for fabricating a semiconductor device.
As shown in
FIG. 7
, a target
80
of Zr is placed in a chamber (not shown) and a silicon substrate
90
is loaded thereto. Then, a reactive sputtering process is performed using the target
80
with a mixed gas ambient containing Ar and O
2
gases created in the chamber. During this process, the surface of the target
80
is oxidized, thereby forming a Zr oxide layer
81
thereon. At the same time, the surface of the silicon substrate
90
is also oxidized to be covered with a silicon dioxide film
91
. Further, as a result of the reactive sputtering process, a Zr oxide film
92
is formed over the silicon substrate
90
with the silicon dioxide film
91
interposed therebetween. Accordingly, the resultant gate insulating film becomes a stack of the silicon dioxide film
91
and Zr oxide film
92
. As a result, the gate capacitance decreases compared to a gate insulating film that has the same thickness but consists essentially of a Zr oxide film alone.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to enhance the drivability of MOSFETs by getting a gate insulating film, consisting essentially of a high-dielectric-constant material alone, formed by a sputtering process without allowing any silicon dioxide film to exist on the surface of a semiconductor substrate.
An inventive method for fabricating a semiconductor device includes the steps of: a) preparing a metal target in a chamber, at least a surface region of the target having been oxidized; b) performing a sputtering process using the metal target with an inert gas ambient created in the chamber, thereby depositing a first metal oxide film as a lower part of a gate insulating film over a semiconductor substrate; and c) performing a reactive sputtering process on the metal target with a mixed gas ambient, containing the inert gas and an oxygen gas, created in the chamber, thereby depositing a second metal oxide film as a middle or upper part of the gate insulating film over the first metal oxide film.
According to the inventive method, in the step of depositing a first metal oxide film over a semiconductor substrate, i.e., the initial stage of a process for forming a gate insulating film, no reactive sputtering process is performed but a sputtering process is performed using a metal target, at least the surface region of which has been oxidized, in an ambient containing no oxygen gas. Thus, the first metal oxide film can be deposited over the semiconductor substrate without allowing any silicon dioxide film to exist on the surface of the semiconductor substrate. Also, in the step of depositing a second metal oxide film over the first metal oxide film, i.e., after the initial stage of the process for forming the gate insulating film is over, a reactive sputtering process is performed in an ambient containing an oxygen gas with the surface of the semiconductor substrate covered with the first metal oxide film. Thus, the second metal oxide film can be deposited over the first metal oxide film without allowing any silicon dioxide film to exist on the surface of the semiconductor substrate. Accordingly, the gate insulating film can be essentially made up of the first and second metal oxide films alone. In other words, a gate insulating film consisting essentially of a high-dielectric-constant material alone can be formed. As a result, the resultant MOSFET can have its gate capacitance increased and its drivability enhanced. In addition, a gate leakage current can be minimized because the gate insulating film can be thick enough with a desired gate capacitance maintained.
In one embodiment of the present invention, the step a) may include the step of performing a provisional reactive sputtering process on the metal target to be oxidized with a mixed gas ambient, containing the inert and oxygen gases, created in the chamber, thereby oxidizing the surface region of the metal target before the semiconductor substrate is loaded into the chamber.
Then, the metal target with the oxidized surface region can be prepared easily.
In this particular embodiment, the provisional reactive sputtering process is preferably performed on another semiconductor substrate that has been loaded into the chamber before the step a) is started.
Then, no insulating metal oxide is deposited on a wafer stage (which will be used as a gas-discharge electrode during the subsequent sputtering process steps) in the chamber when the surface region of the metal target is oxidized. As a result, it is possible to avoid the inability to apply a voltage to the semiconductor substrate in the subsequent process steps.
In another embodiment, the step c) may include the step of introducing the oxygen gas into the chamber with the inert gas, used in the step b), left in the chamber and with a gas-discharge continued from the step b) to carry out the reactive sputtering process.
Then, the steps b) and c) of depositing the first and second metal oxide films can be performed continuously. As a result, the throughput of the process improves.
In an alternative embodiment, the inventive method may further include, between the steps b) and c), the step of introducing the oxygen gas into the chamber with the inert gas, used in the step b), left in the chamber and with a gas-discharge for the sputtering

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