Method of selective oxidation

A method of selective oxidation includes forming a mask layer which includes a silicon oxide film pattern and a silicon nitride film pattern on an active region defined on silicon substrate, a forming a trench using the mask layer in an isolation region defined in the silicon substrate adjoining the active region, forming a buried silicon oxide film in the trench, forming a buried poly-silicon film on the buried silicon oxide film in the trench, converting the buried poly-silicon film to a field oxide film, and removing the mask layer. The occurrence of a bird's beak during selective oxidation of the silicon can be prevented.

BACKGROUND OF THE INVENTION 
1 Field of the Invention 
The present invention generally relates to a method of selective oxidation, 
and more particularly, the present invention relates to a method for 
forming an isolation region. 
This application is a counterpart of Japanese application Serial Number 
238467/1997, filed Sep. 3, 1997, the subject matter of which is 
incorporated herein by reference. 
2 Description of the Related Art 
In general, a local oxidation (LOCOS) method has been used in the formation 
of an isolation region. 
FIGS. 1A-1D are sectional views showing a conventional method of selective 
oxidation. 
As shown in FIG. 1A, a silicon oxide film 12 is formed on a silicon 
substrate 10 using a thermal oxidation method. A silicon nitride film 14 
is formed on the silicon oxide film 12 using a low-pressure chemical vapor 
deposition (LP-CVD) method. 
As shown in FIG. 1B, the silicon oxide film 12 and the silicon nitride film 
14 are patterned so as to remain on a plurality of active regions (not 
shown) defined on the silicon substrate 10 using a photolithography 
technique and an etching technique, and as a result the silicon oxide film 
patterns 12a and the silicon nitride film patterns 14a are formed on each 
active region of the silicon substrate 10. 
As shown in FIG. 1C, a plurality of field oxide films 16 are formed on each 
isolation region defined on the silicon substrate 10 between the active 
regions using a thermal oxidation technique. 
As shown in FIG. 1D, the silicon oxide film patterns 12a and the silicon 
nitride film patterns 14a are removed from the silicon substrate 10. 
In the conventional method, it is desirable to prevent a problem of a 
lateral oxidation producing a so-called bird's beak during selective 
oxidation of the silicon. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of selective 
oxidation that does not produce a bird's beak. 
According to one aspect of the present invention, for achieving the above 
object, there is provided a method of selective oxidation comprising 
forming a mask layer which comprises a silicon oxide film pattern and a 
silicon nitride film pattern on an active region defined on a silicon 
substrate; forming a trench using the mask layer in an isolation region 
defined in the silicon substrate adjoining the active region; forming a 
buried silicon oxide film in the trench; forming a buried poly-silicon 
film on the buried silicon oxide film in the trench; converting the buried 
poly-silicon film to a field oxide film; and removing the mask layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A method of selective oxidation according to a first preferred embodiment 
of the present invention will hereinafter be described in detail with FIG. 
2. 
FIGS. 2A-2K are sectional views showing a method of selective oxidation 
according to a first preferred embodiment of the present invention. 
As shown in FIG. 2A, a first silicon oxide film 12, having a thickness of 
20 nm, is formed on a silicon substrate 10 using a thermal oxidation 
method. A silicon nitride film 14, having a thickness of 150 nm, is formed 
on the first silicon oxide film 12 using an LP-CVD method. 
As shown in FIG. 2B, a plurality of resist patterns 18 are respectively 
formed using a photolithography technique on the silicon nitride film 14 
over a plurality of active regions (not shown ) defined on the silicon 
substrate 10. 
As shown in FIG. 2C, the first silicon oxide film 12 and the silicon 
nitride film 14 are etched using the plurality of resist patterns 18 as a 
plurality of masks to produce silicon oxide film patterns 12a and silicon 
nitride film patterns 14a. Here, CF.sub.4 is used as the etching gas. 
As shown in FIG. 2D, the plurality of resist patterns 18 are removed. The 
silicon oxide film patterns 12a and the silicon nitride film patterns 14a 
are formed on each of the active regions. 
As shown in FIG. 2E, a plurality of trenches 20, each having a depth of 500 
nm, are formed in isolation regions defined in the silicon substrate 10 
between each active region. More specifically, the silicon substrate 10 in 
the isolation regions is dry-etched using the silicon nitride film 
patterns 14a as masks. Here, Cl.sub.2 is used as the etching gas. 
As shown in FIG. 2F, a second silicon oxide film 22 having a predetermined 
thickness, is formed on the silicon substrate 10, for example, a CVD 
process. Here, SiH.sub.4 or TEOS are used as the source gas. The CVD 
method may be used either at atmospheric pressure or at a low pressure. 
The second silicon oxide film 22 has a sufficient thickness so as to 
completely fill in the plurality of trenches 20, and therefore the upper 
surture of the second silicon oxide film 22 is higher than upper surfaces 
of the silicon nitride film patterns 14a. 
As shown in FIG. 2G, a plurality of buried silicon oxide films 22a are 
formed in the plurality of trenches 20 by patterning the second silicon 
oxide film 22. The patterning process is performed by an etch-back 
technique. Here, CF.sub.4 is used as an etching gas for a dry-etching. The 
second silicon oxide film 22 is patterned so that the upper surface of the 
buried silicon oxide films 22a is lower than the interface between the 
silicon substrate 10 and the silicon oxide film patterns 12a Here, the 
distance between the interface and the buried silicon oxide films 22a is 
set to 200 nm.about.300 nm. 
As shown in FIG. 2H, a poly-silicon film 24 is formed on the entire surface 
using an LP-CVD method. As a result, the poly-silicon film 24 is also 
buried in an upper space of the plurality of trenches 20. Here, SiH4 is 
used as source gas. The upper surfaces of the poly-silicon film 24 is 
higher than the upper surfaces of the silicon nitride film patterns 14a. 
As shown in FIG. 2I, a plurality of buried poly-silicon films 24a are 
formed in the plurality of trenches 20 by patterning the poly-silicon film 
24. The patterning process is performed by an etch-back technique using 
the silicon nitride film patterns 14a as masks. Here, CF.sub.4 is used as 
an etching gas for a dry-etching. The poly-silicon film 24 is patterned so 
that the upper surface of the buried poly-silicon films 24a is 
substantially on a same plane as the interface between the silicon 
substrate 10 and the silicon oxide film patterns 12a Therefore, the buried 
poly-silicon films 24a are completely buried in the space of the plurality 
of trenches 20. 
As shown in FIG. 2J, a plurality of field oxide films 16 are formed in the 
isolation region by the thermal oxidation of the buried poly-silicon films 
24a. The thermal oxidation is performed using an mixed gas atmosphere of 
H.sub.2 O and O.sub.2 having a temperature 1000.degree. 
C..about.1050.degree. C. In this time, the plurality of the buried 
poly-silicon films 24a are completely oxidized As a result, the plurality 
of field oxide films 16 are formed in the isolation region. The field 
oxide films 16 and the buried silicon oxide films 22a serve as an 
isolation layer between the active regions. Since the buried poly-silicon 
films 24a expand when they undergo thermal oxidation, the plurality of 
field oxide films 16 are completely buried in the plurality of trenches 20 
even if the poly-silicon films 24 are etched too much in above etch-back 
process. Further, the first preferred embodiment can decrease the thermal 
oxidation rate compared to the conventional LOCOS process. As a result, 
the first preferred embodiment can prevent the lateral oxidation problem 
of the so-called bird's beak produced by the thermal oxidation. 
As shown in FIG. 2K, the silicon oxide film patterns 12a and the silicon 
nitride film patterns 14a are removed. Here, the silicon nitride film 
patterns 14a are removed using H.sub.3 PO.sub.4, and the silicon oxide 
film patterns 12a are removed using 1%.about.5% HF.sub.aq. 
As mentioned above, since the buried poly-silicon films 24a expands when 
performing the thermal oxidation of the buried poly-silicon films 24a, the 
plurality of field oxide films 16 are completely buried in the plurality 
of trenches 20 even if the poly-silicon films 24 are etched too much in 
above etch-back process. Further, the first preferred embodiment can 
decrease the thermal oxidation rate compared to the conventional LOCOS 
process. As a result, the first preferred embodiment can prevent the 
problem of the so-called bird's beak due to lateral oxidation. Further, 
the first preferred embodiment can form suitable isolation layers by 
controlling the depth of the trenches. Although the described first 
preferred embodiment used poly-silicon films 24, an amorphous silicon may 
be used instead 
A method of selective oxidation according to a second preferred embodiment 
of the present invention will hereinafter be described in detail with 
FIGS. 3A-3K. 
FIGS. 3A-3K are sectional views showing a method of selectively oxidation 
according to a second preferred embodiment of the present invention. 
As shown in FIG. 3A, a first silicon oxide film 12, having a thickness of 
20 nm, is formed on a silicon substrate 10 using a thermal oxidation 
method. A silicon nitride film 14, having a thickness of 150 nm, is formed 
on the first silicon oxide film 12 using an LP-CVD method. 
As shown in FIG. 3B, a plurality of resist patterns (not shown) are 
respectively formed on the silicon nitride film 14 over the plurality of 
active regions (not shown) defined on the silicon substrate 10 using a 
photolithography technique. The first silicon oxide film 12 and the 
silicon nitride film 14 are etched using the plurality of resist patterns 
as masks. Here, CF.sub.4 is used as the etching gas. As a result, the 
silicon oxide film patterns 12a and the silicon nitride film patterns 14a 
are formed on each active region. After that, the plurality of resist 
patterns are removed. 
As shown in FIG. 3C, a plurality of trenches 20, having a depth of 500 nm, 
are formed in isolation regions defined in the silicon substrate 10 and 
between each of the active regions. More specifically, the silicon 
substrate 10 is dry-etched in isolation regions using the silicon nitride 
film patterns 14a as masks. Here, Cl.sub.2 is used as the etching gas. 
As shown in FIG. 3D, a second silicon oxide film 22 having a predetermined 
thickness, is formed on the silicon substrate 10 using the CVD method. 
Here, SiH.sub.4 or TEOS are used as source gases. The CVD method may be 
used either at atmospheric pressure or at a low pressure. The thickness of 
the second silicon oxide film 22 is set to equal the sum of the depth of 
the trenches 20 and the thickness of the silicon oxide film patterns 12a 
and the silicon nitride film patterns 14a. The upper surface of the second 
silicon oxide film 22 is higher than the upper surfaces of the silicon 
nitride film patterns 14a. More specifically, a concave surface A of the 
second silicon oxide film 22 is higher than the upper surfaces of the 
silicon nitride film patterns 14a. 
As shown in FIG. 3E, the second silicon oxide film 22 is polished using CMP 
(Chemical Mechanical Polish) to expose the upper surface of the silicon 
nitride film patterns 14a. As a result, a plurality of buried silicon 
oxide films 22b are formed in the plurality of trenches 20. 
As shown in FIG. 3F, a plurality of buried silicon oxide films 22a are 
formed in the plurality of trenches 20 using an etch-back process of the 
second silicon oxide films 22. Here, CF.sub.4 is used as an etching gas 
for dry-etching The second silicon oxide films 22 are etched back so that 
the upper surfaces of the buried silicon oxide films 22a is lower than the 
interface between the silicon substrate 10 and the silicon oxide film 
patterns 12a. Here, the distance between the interface and the buried 
silicon oxide films 22a is set to 200 nm.about.300 nm. 
As shown in FIG. 3G, a poly-silicon film 24 is formed on the entire surface 
using an LP-CVD method As a result, the poly-silicon film 24, is also 
buried in an upper space of the plurality of trenches 20. Here, SiH.sub.4 
is used as source gas. The upper surface of the poly-silicon film 24 is 
higher than the upper surfaces of the silicon nitride film patterns 14a. 
As shown in FIG. 3H, the poly-silicon film 24 is polished using CMP 
(Chemical Mechanical Polish) to expose the upper surface of the silicon 
nitride film patterns 14a. As a result, a plurality of buried poly-silicon 
films 24b are formed in the plurality of trenches 20. 
As shown in FIG. 3I, a plurality of buried poly-silicon films 24a are 
formed in the plurality of trenches 20 by etch-back of the poly-silicon 
films 24. Here, CF.sub.4 is used as an etching gas for a dry-etching. The 
buried poly-silicon films 24b are etched back so that the upper surfaces 
of the buried poly-silicon films 24a are substantially on a same plane as 
the interface between the silicon substrate 10 and the silicon oxide film 
patterns 12a. Therefore, the buried poly-silicon films 24a are completely 
buried in the plurality of trenches 20. 
As shown in FIG. 3J, a plurality of field oxide films 16 are formed in the 
isolation region by thermal oxidation of the buried poly-silicon films 
24a. The thermal oxidation is performed using a mixed gas atmosphere of 
H.sub.2 O and O.sub.2 having a temperature 1000.degree. 
C..about.1050.degree. C. In this time, the plurality of the buried 
poly-silicon films 24a are completely oxidized As a result, the plurality 
of field oxide films 16 are formed in the isolation region. The field 
oxide films 16 and the buried silicon oxide films 22a serve as the 
isolation layer between the active regions. Since the buried poly-silicon 
films 24a expand when performing the thermal oxidation of the buried 
poly-silicon films 24a, the plurality of field oxide films 16 are 
completely buried in the plurality of trenches 20 even if the poly-silicon 
films 24 are etched too much in above etch-back process. Further, the 
second preferred embodiment can decrease the thermal oxidation degree 
compared to the conventional LOCOS process. As a result, the second 
preferred embodiment can prevent the problem of the so-called bird's beak 
due to lateral oxidation. 
As shown in FIG. 3K, the silicon oxide film patterns 12a and the silicon 
nitride film patterns 14a are removed. The silicon nitride film patterns 
14a are removed using H.sub.3 PO.sub.4. The silicon oxide film patterns 
12a are removed using 1%.about.5% HF. 
As mentioned above, since the buried poly-silicon films 24a expand when 
undergoing thermal oxidation, the plurality of field oxide films 16 are 
completely buried in the plurality of trenches 20 even if the poly-silicon 
films 24 are etched too much in above etch-back process. Further, the 
second preferred embodiment can decrease the thermal oxidation rate 
compared to the conventional LOCOS process. As a result, the second 
preferred embodiment can prevent the problem of the so-called bird's beak 
due to the lateral oxidation. Further, the second preferred embodiment can 
form suitable isolation layers by controlling the depth of the trenches. 
Further, the second preferred embodiment can form isolation layers having 
irregular size without having a reliability problem. 
Although the described second preferred embodiment used poly-silicon films 
24, an amorphous silicon may be used instead. 
While the present invention has been described with reference to the 
illustrative embodiments, this description is not intended to be construed 
in a limiting sense. Various modifications of the illustrative 
embodiments, as well as other embodiments of the invention, will be 
apparent to those skilled in the art on reference to this description. It 
is therefore contemplated that the appended claims will cover any such 
modifications or embodiments as fall within the true scope of the 
invention.