Method for forming an isolation insulating film for internal elements of a semiconductor device

There is provided a method for forming element isolation insulating film of a semiconductor device by employing PBL method for reducing the bird's beak and increasing the length of the effective active region. The method comprising the steps of forming a pad-oxide film, a stack-silicon film, and a nitride film on a semiconductor substrate in sequence; forming an element isolation region by selectively patterning the nitride film with an etching process by using an element isolation mask; and forming an element isolation film by field-oxidizing the element isolation region over the semiconductor substrate.

FIELD OF THE INVENTION 
The present invention relates to a method for forming an isolating film for 
internal elements of a semiconductor device, and more particularly, to a 
technique of using stack-polycrystalline silicon instead of 
polycrystalline silicon of Poly-Buffered LOCOS structure (PBL). 
DESCRIPTION OF THE RELATED ART 
Generally, it is necessary to decrease the dimension of each element and 
also to decrease the width and size of isolation regions present between 
elements in order to improve high-integration of semiconductor devices 
with more-increasing complexity thereof. 
Cell size is greatly affected by the above decreases, and as a result, the 
element isolation technique is very significant in determining memory cell 
size. 
There are several conventional methods for forming an isolation film of 
internal elements including: the LOCal Oxidation of Silicon (LOCOS) which 
is a method of isolating insulating substance, PBL which is a method of 
employing oxide film, polycrystalline silicon layer, and nitride layer on 
a semiconductor substrate, and the Trench method of forming a groove on a 
semiconductor substrate and filling it with insulating substance. However, 
there are disadvantages to each of the above methods. 
The LOCOS method involves problems such as the reduction of the effective 
regions by the bird's beak due to the side spread of oxygen during the 
oxidation process of element isolation insulating film, the phenomenon 
that field oxide film is ungrown in the small area, and the field oxide 
thinning of isolation insulating film of internal elements, etc. 
In the conventional PBL method, the length of the bird's beak can be made 
shorter than in the LOCOS method by using polycrystalline silicon; 
however, the problem of the bird's beak occurs again when it is employed 
in the design rule of the dimension less than 0.35 .mu.m. Therefore, 
reduction of the bird's beak is required in order to apply the 
conventional PBL method on a device with a dimension less than 0.35 .mu.m. 
Recently, the Trench method, or modified LOCOS method, has been employed in 
order to address the above problems of the LOCOS and the PBL methods. 
However, the Trench method results in reduction of the productivity of 
devices because the processing steps thereof are so complicated. 
In respect thereof, FIGS. 1 to 4 show the cross-section of the method of 
forming isolation insulating film of a semiconductor device by PBL 
structure according to the conventional art. 
First, as shown in FIG. 1, a pad oxide film 2, a polycrystalline silicon 
film 3, and a nitride film 4 are formed over a semiconductor substrate 1 
in sequence. 
Here, the polycrystalline silicon film 3 functions to buffer the stress of 
the nitride film 4. 
Then, as shown in FIG. 2, a field region 5 is formed by etching the nitride 
film 4 using an element isolation mask (not shown). Here, the polysilicon 
film 3 is etched by a certain thickness due to the excessive etching in 
the above nitride film 4 etching process. 
Subsequently, as shown in FIG. 3, a field oxide film 6 is formed by 
field-oxidizing the exposed surface of the semiconductor substrate 1. 
Here, the nitride film 4 functions to reduce the length of the bird's beak 
by suppressing the growth of the field oxide film 6. Then, as shown in 
FIG. 4, the nitride film 4, the polycrystalline silicon film 3, and the 
pad oxide film 2 are removed, and an isolation film 7 of elements is 
formed over the semiconductor substrate 1. Here, the field oxide film 7 
forms a smaller bird's beak than in the LOCOS method. 
In addition, FIG. 5 is a top view of FIG. 4, which illustrates an element 
isolation insulating film 7 and an effective active region 8 formed by 0.3 
.mu.m of the design rule. 
"A" of FIG. 4 illustrates the interface of the element isolation region and 
the effective active region in the layout. "a" illustrates the interface 
of the element isolation region and the effective active region after 
element isolation process. "Aa" illustrates the length of the bird's beak 
generated after element isolation process. 
In "a", the interface of the element isolation region and the effective 
active region which forms, is rough. 
FIG. 6 is a cross-sectional view of the structure of the polycrystalline 
silicon film 3 in order to illustrate the rough interface of the above "a" 
as shown in FIG. 5. 
The above polycrystalline silicon film 3 is formed by a desired thickness 
by using SiH4 or Si2H6 gas, as the process of FIG. 1. 
Here, the above polycrystalline silicon film 3 is formed as grain 3a and 
grain boundary 3b. 
In addition, in the oxidation process thereof, the oxide film (t2OX-GB) is 
formed around the grain boundary 3b with greater thickness than around 
grain 3a because the oxidation rate at the grain boundary 3b is higher 
than that at the grain 3a. 
As a result, the interface of element isolation film 7 is rough as shown in 
FIG. 5. 
In the conventional element isolation process, field oxide process is 
carried out by wet oxidation method in order to shorten the processing 
time so as to increase the rate of field oxidation. Because of this, the 
crystal grain boundary of the polycrystalline silicon film is oxidized 
faster than the above crystal grain so that the roughness of the interface 
increases. 
The gate oxide film formed in the following process is degraded if the 
interface of the element isolation region and the effective active region 
is rough as described above. In addition, the oxidation in the grain 
boundary results in the generation of a large bird's beak. 
As described above, there are some problems in the formation of an element 
isolation film of a semiconductor device according to the conventional 
technology. In the conventional method according to the prior art, as the 
design rule decreases, the roughness of the interface increases. 
In addition, the degradation of the characteristics of the gate oxide film 
formed in the following process and the large bird's beak result in the 
reduction in quality and reliability of the semiconductor device. Further, 
it is difficult to expect the highly-integrated semiconductor devices. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing a method of forming element 
isolating film of a semiconductor device, which substantially obviates one 
or more of the problems caused by limitations and disadvantages of the 
related art. 
One object of the present invention is to provide a method of forming 
isolation insulating film of internal elements of a semiconductor device 
by employing PBL method so that the quality and reliability of 
semiconductor devices are improved along with the high-integration 
thereof. 
In accordance with an aspect of the present invention, these objects are 
accomplished by providing a method of forming an element isolation 
insulating film comprises the steps of: forming a pad-oxide film, a 
stack-silicon film, and a nitride film on a semiconductor substrate in 
sequence; forming an element isolation region by selectively patterning 
the nitride film with an etching process by using element isolation mask; 
and forming an element isolation film by field-oxidizing the element 
isolation region over the semiconductor substrate. 
In accordance with an aspect of the present invention, a method of forming 
an element isolation insulating film comprises the steps of: forming a 
pad-oxide film, a stack-amorphous silicon film, and a nitride film on a 
semiconductor substrate in sequence; forming an element isolation region 
by selectively patterning the nitride film with etching process using 
element isolation mask; and field-oxidizing the element isolation region 
disposed over the semiconductor substrate by the wet-oxidation process and 
the dry-oxidation process, to form an element isolation film. 
In accordance with an aspect of the present invention, a method of forming 
an element isolation insulating film comprises the steps of: forming a 
pad-oxide film, a stack-polycrystalline silicon film, and a nitride film 
on a semiconductor substrate in sequence; forming an element isolation 
region by selectively patterning the nitride film with etching process 
using element isolation mask; and field-oxidizing the element isolation 
region disposed over the semiconductor substrate by the wet-oxidation 
process and the dry-oxidation process, to form an element isolation film. 
The present invention is intended to greatly reduce the dimensions of grain 
and grain boundary by periodically shutting off the flow of SiH4 or Si2H6, 
the source gas, for a certain amount of time during the deposition of 
polycrystalline silicon film, in order to prevent the roughness in the 
interface of the element isolation region and the effective active region. 
In addition, a stack-silicon film is formed by forming a native oxide film 
to suppress the growth of grain boundary during the shut-off of the flow 
of SiH4 or Si2H6. The oxidation amount of grain and grain boundary is then 
minimized by using both the dry-oxidation method and the wet-oxidation 
method so as to improve the interface property of the element isolation 
region and the effective active region. 
As a result, the degraded characteristic of gate-oxide film is prevented 
and the effective region is increased by suppressing the growth of the 
bird's beak, and therefore, element isolation insulating film of a 
highly-integrated semiconductor device is formed without using the 
complicated steps of the modified LOCOS method or the Trench method. 
In addition, an amorphous stack-silicon film is formed by depositing the 
stack-silicon film at a low temperature. A stack-polysilicon film having 
grain and grain boundary is formed by phase-transition during the 
following process of field-oxide film formation. Then an element isolation 
insulating film is formed by performing a dry oxidation process so as to 
minimize the oxidation amount of grain and grain boundary and improve the 
interface characteristics of the element isolation insulating region and 
the effective region. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
FIGS. 7 to 10 are cross-sectional views showing a method for forming 
element isolation insulating film of a semiconductor device according to 
one embodiment of the present invention. 
First, as shown in FIG. 7, a pad-oxide film 13 is formed over the 
semiconductor substrate 11, and a stack-polycrystalline silicon film 15 is 
formed over the pad-oxide film 13 by using CVD method. 
Here, the stack-polycrystalline silicon film 15 is formed as multi-layered 
polycrystalline silicon film having a native oxide film formed on the 
interface of each layer, and this is illustrated in the following FIG. 11 
and FIG. 12. 
Then, as shown in FIG. 8, a nitride film 17 is formed over the 
stack-polycrystalline silicon film 15, and PBL structure is formed by the 
stacked structure of a pad-oxide film 13, a stack-polycrystalline silicon 
film 15, and a nitride film 17. 
Then, the nitride film 17 is etched using an element isolation mask (not 
shown). Here, the stack-polycrystalline silicon film 15 is etched by a 
certain thickness by the above excessively-performed etching for the 
nitride film 17. 
Then, field-oxidation process is performed at a temperature of 
900-1200.degree. C. by using the nitride film 17 as an oxidation barrier, 
thereby forming an element isolation film 19. 
The field-oxidation process is carried out by employing the wet-oxidation 
and dry-oxidation processes in turn so as to minimize the difference in 
the oxidation rate of grain and grain boundary of polycrystalline silicon 
generated in the wet oxidation process, and improve the interface 
characteristics of element isolation region and effective active region. 
In addition, the degraded characteristic of a gate oxide film in the 
following process is prevented. 
In addition, as shown in FIGS. 13, 14 and 15, the description for the 
field-oxidation process and the effect thereby is made in detail. 
FIG. 11 is a cross-sectional view showing the stack-polycrystalline silicon 
film 15 formed over the pad-oxide film 13, and FIG. 12 shows a 
stack-polycrystalline silicon film 15. 
First, as shown in FIG. 11, the deposition of the stack-polycrystalline 
silicon film 15 having grain 21 and grain boundary 23 by CVD method is 
carried out under the following deposition conditions. Deposition pressure 
is 0.2-0.6 Torr, and the flow of source gas, SiH4 gas or Si2H6 gas, is 
700-1200 sccm. 
Here, the flow of the SiH4 gas or Si2H6 gas is shut off in the range of 
b-c, d-e for 1-7 min., and the SiH4 gas or Si2H6 gas is flowed in the 
range of a-b, c-d, e-f so as to form stack-polycrystalline silicon film 15 
by a desired thickness. 
Amorphous silicon is formed at a temperature of 450-600.degree. C. for 
silicon-deposition, and polycrystalline silicon is formed at a deposition 
temperature of 600-650.degree. C. 
In the above deposition process, when the flow of the source gas, SiH4 or 
Si2H6 gas, is shut off, a native oxide film 25 is formed due to the 
remaining oxygen inside the processing chamber depositing the 
stack-polycrystalline silicon film 15. The native-oxide film 25 can be 
formed by flowing inert gas, such as nitrogen gas or argon gas by 10-30 
slm in the range of b-c, and d-e. 
Accordingly, the stack-polycrystalline silicon film 15 is formed as the 
1,2,3 stack-polycrystalline silicon film, that is, P1/P2/P3 
stacked-structure, and a native oxide film 25 is formed over the each 
upper side of the P1,P2,P3. 
Meanwhile, when the deposition temperature of silicon is 500-600.degree. 
C., stack-amorphous silicon films are deposited. The stack-amorphous 
silicon film are converted to the stack-polycrystalline silicon film 
having grain and grain boundary during subsequent thermal field oxidation. 
Here, the stack-polycrystalline silicon film having n-layer stacked 
structure is formed by shutting off the SiH4 gas with n-1 times in the 
interface of n (n: integer of two and above) layer so as to form native 
oxide film as shown in FIG. 2. 
Inert gas such as nitrogen gas or argon gas can be injected during shut-off 
of the source gas so as to form oxide film or nitride film. 
FIGS. 13 to 16 show the field oxidation process and its effect of the FIG. 
10. 
FIG. 13 is a cross-sectional view showing the oxide film thickness at the 
grain 21 and the grain boundary 23 of the stack-polycrystalline silicon 
film 15 during the dry and the wet field-oxidation processing of the FIG. 
10. 
According to the FIG. 13, the oxidation rates at the grain 21 and the grain 
boundary 23 are similar so that the oxide film thickness at the grain 21 
(t1OX-G) and the oxide film thickness of the grain boundary 23 (t1OX-GB) 
are similar. 
FIG. 14 is a graph showing the effective active region length according to 
the ratio of wet oxidation process to dry oxidation process after 
field-oxidation process by the dry and the wet method of FIG. 10 when the 
process is applied on the cell having 0.25 .mu.m design rule. 
According to FIG. 14, the (a) and (b) show the effective active region 
length in the case of formation of element isolation insulating film by 
applying the conventional polycrystalline silicon film and the 
stack-polycrystalline silicon film of the present invention respectively. 
Here, (a1) and (b1) show the effective active length when the 
field-oxidation process is carried out by wet-oxidation method, and (a2) 
and (b2) show the effective active length when performing wet-oxidation 
and dry-oxidation processes with 2:1 thickness ratio of field oxide. 
In addition, (a3) and (b3) show the effective active length when performing 
wet-oxidation and dry-oxidation processes with 1:1 thickness ratio of 
field oxide, and (a4) and (b4) show the effective active length when 
performing wet-oxidation and dry-oxidation processes with 1:2 thickness 
ratio of field oxide. 
In addition, (a5) and (b5) show the effective active length when performing 
field-oxidation process with only the dry-oxidation method. 
As shown in the (a1) and (b1), in the field-oxidation process by only 
wet-oxidation method, the rate of the field-oxidation is high and the 
bird's beak is large so that the effective active region length is short. 
In addition, field oxidation is performed slowly in the field-oxidation 
process by only dry-oxidation, and in particular, it shows a shorter 
effective active region length even in the case of applying both of 
wet-oxidation and dry-oxidation appropriately. Therefore, the case of 
employing wet-oxidation and dry-oxidation appropriately together as shown 
in the (a3) and (b3) shows the longest effective active region length. 
In the case where stack-polysilicon is used in the present invention, a 
wider effective active region is shown than when conventional polysilicon 
is used. 
FIGS. 15 and 16 are graphical representations showing the destructive 
electric breakdown field distribution when the PBL process is applied on 
0.25 .mu.m of design rule according to the conventional one and the 
present invention respectively. 
Especially, as shown in FIG. 16, the breakdown voltage of gate oxide film 
by PBL process of the present invention is substantially high unlike the 
case with the PBL process of the prior art. 
According to the method of forming element isolation insulating film of a 
semiconductor device of the present invention, stack-silicon film is 
employed instead of polycrystalline silicon film of PBL structure, and 
wet-field oxidation and dry-field oxidation process are employed 
appropriately together so that the dimension of bird's beak is reduced and 
therefore, the effective active region length is increased. 
Therefore, the characteristics of gate oxide film formed in the following 
process is improved thereby resulting in the improvement of the quality 
and reliability of semiconductor devices and providing highly-integrated 
semiconductor devices. 
While the present invention has been described in detail, it should be 
understood that various changes, substitutions and alterations can be made 
hereto without departing from the spirit and scope of the invention as 
defined by the appended claims.