Method for fabrication of a field oxide of the buried inverse T-type using oxygen or nitrogen ion implantation

The present invention relates to a semiconductor and a method for fabrication thereof and particularly to a semiconductor having a field oxide having a shape such that the lower part is wider that the upper part. Therefore, according to the present invention, the ion implantation process for forming a channel stop region becomes unnecessary, because of the effect of accurate insulation between the devices and the pn junction area can be decreased, so that the junction capacitance becomes decreased. Furthermore, because LOCOS edge does not coincide with the junction edge, the leakage current due to the damage of the edge is not generated. Because a field oxide is of the buried inverse T-type, the effective width of the device is increased more than that of a mask. Because the bird's beak is not generated, the problem due to the narrow width can be settled.

BACKGROUND OF THE INVENTION 
The present invention relates to a semiconductor device and a method for 
fabrication thereof having a field oxide of the buried inverse T-type 
suited to sub-micron MOS devices. 
FIG. 1 shows a cross-sectional view of a semiconductor device where a field 
oxide is formed using a conventional LOCOS process (Local Oxidation of 
Silicon). 
In the prior art, when MOS devices are fabricated, the field oxide is 
formed using the conventional LOCOS to isolate devices. 
Thus, as shown in FIG. 1, a field oxide layer 2 is grown on a field region 
of a Si substrate 1 using a nitride layer (not shown herein) and a gate 3 
is formed at a predetermined region, and then predetermined impurities are 
implanted on both sides of gate 3 to form the source and drain regions 4. 
Furthermore, because, when the field oxide 2 is grown, it penetrates only a 
little within Si substrate 1, predetermined impurities are implanted in a 
field region to form a channel stop for insulation transistors. 
However, the conventional fabrication process for forming the field oxide 
has disadvantages in that the effective area of the device is decreased, 
because of the generation of the bird beak and the capacitance is 
increased due to the pn junction. Furthermore, leakage current is 
generated by the damage of the edge portion, so that it is not able to 
completely insulate the device. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor device 
having a field oxide of the buried inverse T-type. 
It is another object of the present invention to provide a method for 
fabrication of the semiconductor device for forming a field oxide of the 
buried inverse T-type using oxygen or nitrogen ion implantation. 
To achieve this object of the present invention, there is provided a 
semiconductor device isolated by a field oxide in the shape of a buried 
inverse T, in which the lower part is wider than the upper part. 
And, there is provided a method for fabrication of the semiconductor 
device, which comprises the steps of: 
successively depositing a pad nitride film, a first nitride film and a 
first oxide film on a Si substrate; 
exposing the Si substrate of field region and forming the side wall of a 
second nitride film; 
implanting ion impurities into the exposed substrate to a predetermined 
depth; 
removing the first oxide film and second nitride film and successively 
depositing a second oxide film and a second nitride film on the whole 
surface; 
etching the third nitride film to expose the fixed portion of the Si 
substrate; 
implanting the impurities with lower energy than the first ion 
implantation; 
forming a field oxide by heat treatment of the impurities implanted within 
the substrate; 
removing the remaining third nitride film, second nitride film, first 
nitride film and pad nitride film; and 
forming a transistor in an active region isolated by the field oxide.

DETAILED DESCRIPTION OF THE INVENTION 
A semiconductor device and a method for fabrication thereof according to 
the present invention will be described in more detail with reference to 
the various views in accompanying FIG. 2. 
FIGS. 2A through 2E are cross-sectional views of the device after 
completion of various steps in a fabrication process for manufacturing a 
semiconductor device having the buried inverse T-type according to a 
preferred embodiment of the present invention. 
A pad nitride film 12, a first nitride film 13 and a first oxide film 14 
are successively deposited on a Si substrate 11. The Si substrate 11 of 
the field region is exposed by using an active mask (not shown herein). 
Then, the second nitride film is thinly deposited and then etched by an RIE 
process to form the side wall 15. 
The fixed impurities, such as the oxygen ions or the nitrogen ions, are 
then implanted into the exposed Si substrate 11 to a predetermined depth. 
Herein, the first ion implantation condition is that the impurities should 
be implanted into the surface of the Si substrate 11 to a depth of about 
0.5 .mu.m with an energy of from about 150 to 250 KeV. 
Furthermore, amount of the dose is on the order of from 10.sup.17 /cm.sup.2 
to 10.sup.19 /cm.sup.2. 
Henceforth, as shown in FIG. 2B, the first oxide film 14 and the side wall 
15 are removed and the second oxide film 16 and the third nitride film 17 
are successively deposited on the whole surface. 
Then, as shown in FIG. 2C, the third nitride film 17 is etched by RIE to 
form the side wall 17a, and the oxygen ions or the nitrogen ions are 
implanted with lower energy against the Si substrate 11. 
Herein, the ion implantation condition is that the impurities should be 
implanted from the surface of the Si substrate 11 to a depth of about 0.5 
.mu.m with energy of from about 150 to 250 KeV. 
Furthermore, amount of the dose is on the order of from 10.sup.17 /cm.sup.2 
to 10.sup.19 /cm.sup.2. 
Henceforth, as shown in FIG. 2B, the first oxide film 14 and the side wall 
15 are removed and a second oxide film 16 and a third nitride film 17 are 
successively deposited over the whole surface. 
Then, as shown in FIG. 2C, the third nitride film 17 is etched by an RIE 
process to form a side wall 17A, and oxygen ions or nitrogen ions are 
implanted with lower energy into the Si substrate 11. 
Herein, the second ion implantation condition is that the impurities should 
be implanted from the surface of the Si substrate 11 to a depth of about 
0.5 .mu.m with energy of from about 50 to 100 KeV. 
Furthermore, the amount of the dose is on the order of from 10.sup.17 
/cm.sup.2 to 10.sup.19 /cm.sup.2. 
Henceforth, as shown in FIG. 2D, a heat treatment process of about 
800.degree. to 950.degree. is performed at the oxygen ion implanted region 
to form a field oxide 18. 
Then the side wall, the second oxide film 16, the first nitride film 13 and 
the pad nitride film 12 are removed. Accordingly, the field oxide 18 of 
the buried inverse T-type is formed. 
Henceforth, as shown in FIG. 2E, the gate 19 and the source and drain 
region 20 are formed in the active region of the isolated field oxide 18 
of the buried inverse T-type according to the present invention. 
According to the present invention use, the ion implantation process for 
forming a channel stop region becomes unnecessary, because of the effect 
of accurate insulation between the devices and the pn junction area can be 
decreased, so that the junction capacitance becomes decreased. 
Furthermore, because the LOCOS edge does not coincide with the junction 
edge, the leakage current due to the damage of the edge is not generated. 
Because the field oxide is of the buried inverse T-type, the effective 
width of the device is increased more than that of a mask. Because the 
bird's beak is not generated, the problem due to the narrow width can be 
settled. It will be understood by those skilled in the art that the 
foregoing description is in terms of a preferred embodiment of the present 
invention, wherein various changes and modifications may be made without 
departing from the spirit and scope of the invention, as set forth in the 
appended claims.