Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2002-07-16
2004-08-10
Nguyen, Thanh (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S591000, C438S769000, C438S776000, C438S777000, C438S786000, C438S792000, C438S981000
Reexamination Certificate
active
06773999
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a method for forming a gate insulating film used in a MIS transistor.
A MOS transistor is a typical MOS device. For example, in a complementary MOS (CMOS) transistor, or the like, a high speed driving transistor, which is required to have a gate insulating film of a relatively thin thickness, and a high breakdown-voltage transistor, which is required to have a gate insulating film of a relatively thick thickness for handling input/output signals of a relatively high voltage, are formed on a single semiconductor substrate.
The high speed driving transistor is required to have a gate insulating film having a thickness of about 1 nm to 3 nm, while it is strongly required to have a high reliability in resisting against dielectric breakdown and to have a low leakage current.
A CMOS transistor employs a so-called “dual gate structure”, in which the gate electrode of the P-channel transistor is a P-type gate electrode obtained by using boron (B) as a dopant, and the gate electrode of the N-channel transistor is an N-type gate electrode obtained by using phosphorus (P) as a dopant. Boron, being a P-type dopant, has a larger diffusion coefficient than that of phosphorus, being an N-type dopant, whereby during a heat treatment after the transistor is formed, boron diffuses through the gate insulating film of the high speed driving transistor to reach the channel region. The diffusion of boron is called “boron penetration”, and causes various problems in the transistor such as a substantial variation in the threshold voltage and a deterioration of the driving ability. The boron penetration is, of course, more pronounced as the thickness of the gate insulating film is reduced, and is particularly pronounced when silicon dioxide (SiO
2
) is used for the gate insulating film.
Furthermore, reducing the thickness of the gate insulating film also causes an increase in the gate leakage current through the gate insulating film. Again, where silicon dioxide is used for the gate insulating film, the conduction mechanism thereof is a Fowler-Nordheim tunneling current if the thickness is 3.5 nm or more, and the direct tunneling current becomes dominant if the thickness is 3.5 nm or less. The gate leakage current increases by an order of magnitude for every 0.2 nm decrease in the thickness of the gate insulating film. If the thickness of the gate insulating film is set to be 2.6 nm or less, the gate leakage current is no longer negligible.
As described above, if a thermal oxide film is used for the gate insulating film, it is no longer possible to suppress the boron penetration and the gate leakage current. In view of this, an oxynitride film into which nitrogen is introduced has been used as a gate insulating film.
A conventional method for forming a gate insulating film of a MOS semiconductor device using a silicon oxynitride film will now be described with reference to the drawings.
FIG. 12A
to
FIG. 12C
are cross-sectional views sequentially illustrating the steps of the conventional method for forming a gate insulating film.
First, a device isolation region
102
that partitions a plurality of device forming regions from one another is formed in an upper portion of a semiconductor substrate
101
made of silicon, and then a first gate oxide film
103
A made of a thermal oxide film having a thickness of about 7.5 nm is formed entirely across the upper surface of the semiconductor substrate
101
. Then, a resist pattern
104
having an opening in a second region
202
is formed on the first gate oxide film
103
A, and then a portion of the first gate oxide film
103
A that is included in the second region
202
is etched away using the resist pattern
104
so that the second region of the semiconductor substrate
101
is exposed, thereby obtaining a structure as illustrated in FIG.
12
A.
Then, as illustrated in
FIG. 12B
, the semiconductor substrate
101
is subjected to a heat treatment so as to form a second gate oxide film
105
A made of a thermal oxide film having a thickness of about 2.6 nm in the second region
202
. In this process, the thickness of the first gate oxide film
103
A increases.
Then, as illustrated in
FIG. 12C
, the semiconductor substrate
101
is subjected to a heat treatment in an oxynitriding atmosphere made of nitrogen monoxide (NO) at a temperature of 900° C. for 30 seconds to several ten minutes so as to introduce nitrogen into the first gate oxide film
103
A and the second gate oxide film
105
A, thereby obtaining a first gate oxynitride film
103
B and a second gate oxynitride film
105
B, respectively. Note that other than nitrogen monoxide (NO), dinitrogen monoxide (N
2
O) or, though rarely, ammonia (NH
3
) may be used in the oxynitriding process using a heat treatment.
When nitrogen monoxide (NO) is used, the oxynitriding process increases the thickness only by 0.3 nm or less. In contrast, when dinitrogen monoxide is used, it is required to perform an oxynitriding process under a high temperature of about 1000° C. to 1150° C. for several ten seconds to several ten minutes, whereby the oxynitriding process with dinitrogen monoxide (N
2
O) increases the thickness by a substantial amount of up to several nanometers. Therefore, with dinitrogen monoxide (N
2
O), care should be taken for the process.
FIG.
13
A and
FIG. 13B
are each a nitrogen concentration profile in a gate oxynitride film which has been oxynitrided by using an oxynitriding atmosphere made of nitrogen monoxide (NO), wherein
FIG. 13A
is for the first gate oxynitride film
103
B and
FIG. 13B
is for the second gate oxynitride film
105
B. As illustrated in
FIG. 13B
, in the second gate oxynitride film
105
B having a thickness of 2.6 nm, the nitrogen atom peak is located near the interface between the second gate oxynitride film
105
B and the semiconductor substrate
101
. The peak concentration is about 4 atm % at maximum, through it varies depending on the oxynitriding temperature. Note that also when the oxynitriding process is performed by using dinitrogen monoxide (N
2
O), the nitrogen concentration profile is as that shown in
FIG. 13B
, and the peak concentration is, at best, 1 atm %.
The second gate oxynitride film
105
B obtained by the conventional oxynitriding process has a nitrogen concentration profile and a nitrogen concentration peak as shown in
FIG. 13B
, whereby boron ion implanted into the p-type gate electrode of the P-channel transistor diffuses through the second gate oxynitride film
105
B relatively easily, though it depends on the heat treatment temperature, and reaches the channel region in the semiconductor substrate
101
. The diffusion of boron is of course suppressed as compared with a gate oxide film made only of silicon dioxide. However, when the thickness is reduced so much as in the second gate oxynitride film
105
B, it is not possible substantially prevent the diffusion of boron with a nitrogen concentration profile in which the nitrogen peak concentration is only about 4 atm % and the peak is located near the interface with the semiconductor substrate
101
. This is the first problem in the prior art.
Furthermore, with such a silicon oxynitride film, in which the nitrogen concentration is only about 4 atm % and the nitrogen atoms are localized near the substrate interface, the nitrogen content of the film as a whole is not sufficient to change the dielectric constant and the refractive index of silicon dioxide (SiO
2
), and thus it is certainly not expected to be sufficient to provide an increase in the electric capacitance or a reduction in the gate leakage current. This is the second problem in the prior art.
SUMMARY OF THE INVENTION
An object of the present invention is to solve these problems in the prior art and to make it possible to reduce the gate leakage current while preventing dopant atoms from diffusing from a gate electrode into a substrate through a gate insulating film having a thickness that
Matsushita Electric - Industrial Co., Ltd.
Nguyen Thanh
Nixon & Peabody LLP
Studebaker Donald R.
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