Method for forming a shallow trench with tapered profile and round corners for the application of shallow trench isolation (STI)

The present invention is a method for forming a shallow trench with tapered profile and round corners for the application of shallow trench isolation (STI). This invention utilizes a multiple-step dry etching process with reduced RF power and increased pressure to etch a shallow trench. This takes advantage of different degree of polymer deposition in different steps by varing the pressure and the RF power. Thus, a shallow trench with tapered profile and round corners is achieved.

FIELD OF THE INVENTION 
The present invention relates to a shallow trench isolation process and, 
more particularly, to a method for forming a shallow trench with tapered 
profile and round corners. 
BACKGROUND OF THE INVENTION 
Shallow trench isolation has been an alternative for LOCOS isolation for 
0.25micron technology and beyond. It is known that trench profile is a 
major contributing factor in generating substrate defect during routine 
process steps required for device/circuit fabrication, especially in the 
following trench-fill step. Conventionally, round bottom corners are 
formed to smooth the trench sidewall in order to minimize the generated 
defect and improve the gap-filling ability. Currently, most shallow 
trenches obtained by conventional methods have steep sidewall profiles 
(&gt;80.degree.) and sharp corners. These cause the gap filling problems and 
result in gate oxide thinning and stress concentration problems. The 
aforementioned phenomena become worse as the devices are scaled down, and 
therefore the leakage of the current will further effect the performance 
of the device and the refresh time of the device. 
FIGS. 1-3 are cross section views of a semiconductor wafer illustrating 
various stages of forming shallow trenches according to a conventional 
method. Referring to FIG. 1, an oxide layer 12 is formed on a substrate 
10. The oxide layer 12 serves as a pad oxide layer. The silicon dioxide 
layer 12 is formed to a thickness of approximately 110 angstroms. A 
silicon nitride layer 14 is then formed on the oxide layer 12. The silicon 
nitride layer 14 is about 1700 angstroms thick. Next, the oxide layer 12 
and said nitride layer 14 are patterned and etched to form the trenches on 
the substrate. First, the oxide layer 12 and the nitride layer 14 are 
masked by a patterned mask 15. Then, the oxide layer 12 and the nitride 
layer 14 uncovered by the mask 15 are removed. An etching method 
(indicated by arrows 16) is performed to remove the oxide layer 12 and the 
nitride layer 14. The etching method is a plasma etching method. The 
plasma etching method is performed using CHF.sub.3, CF.sub.4, Ar, and 
O.sub.2 gases. The plasma etching method is achieved using a CHF.sub.3 gas 
flow of about 10 sccm, a CF.sub.4 gas flow of about 15 sccm, an Ar gas 
flow of about 100 sccm, and an O.sub.2 gas flow of about 5 sccm, with a 
radio-frequency(RF) of about 400 W and a process pressure of about 50 mT. 
After that, an etching process 17 is performed to etch the substrate 10 in 
order to form the shallow trenches. The remained oxide layer 12 and the 
remained nitride layer 14 serve as hard masks. The etching process 17 is a 
plasma etching process. The plasma etching method is performed using HBr, 
Cl.sub.2, HeO.sub.2, and CF.sub.4 gases. The plasma etching method is 
performed by changing the reaction condition to following parameters: the 
HBr gas flow is about 100 sccm, the Cl.sub.2 gas flow is about 20 sccm, 
the HeO.sub.2 gas flow is about 30 sccm, and a CF.sub.4 gas flow is about 
20 sccm, the radio-frequency (RF) is about 650 W and the process pressure 
is about 100 mT. The resulting structure is shown in FIG. 3 in which it is 
easy to find that the shallow trenches have steep sidewall profiles 
(&gt;80.degree.) and sharp corners. 
Then, an oxidation process is usually performed to modify the shallow 
trenches. Typically, round corners are achieved by growing a thin oxide 
layer. Unfortunately, if sharp corners are left after the previous etching 
step, the round corners got from the oxidation process usually give 
unsatisfactory roundness because of a limit to the rounding ability of the 
thin oxide layer. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method for forming shallow 
trenches with tapered profile and round corners for the application of 
shallow trench isolation is provided. The method of this invention is to 
form shallow trenches on a wafer, wherein the wafer has a semiconductor 
substrate. Then, an oxide layer is formed on the semiconductor substrate, 
and a nitride layer is formed on the oxide layer. In one embodiment, the 
method includes patterning the oxide layer and the nitride layer; first 
etching the substrate according to the pattern of said oxide layer and 
said nitride layer, wherein the first etching is performed with a first 
set of process parameters; second etching the substrate, wherein the 
second etching process is performed with a second set of process 
parameters, which is performed by reducing the RF power of the first set 
of process parameters and increasing the process pressure of the first set 
of process parameters; third etching said substrate to form shallow 
trenches with tapered profile and round corners, wherein the third etching 
process is performed with a third set of process parameters, wherein the 
third set of process parameters is performed by reducing the RF power of 
the second set of process parameters and increasing the process pressure 
of the second set of process parameters. Unlike the conventional method, a 
shallow trench having round trench top corners and round trench bottom 
corners is obtained by the method of this invention. Thus, the problems of 
gate oxide thinning and the defect generation are eliminated. In addition, 
the sidewall slope of trench profile can be reduced to facilitate gap 
filling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the present invention, the dry etching process module 
consists of two stages. The first stage is to define the trench opening by 
etching through the silicon nitride and pad oxide layers. Then, the second 
stage is to form the shallow trench with the desired sloped sidewall 
profile and round corners by etching into the silicon substrate. A 
multiple-step dry etching is performed in the second stage. Preferably, a 
3-step dry etching is performed in the second stage. Detailed description 
of this invention is described hereafter. 
FIGS. 4-6 are cross section views of a semiconductor wafer illustrating 
various stages of forming shallow trenches according to one embodiment of 
the present invention. Referring to FIG. 4, an oxide layer 32 is formed on 
the substrate 30. The oxide layer 32 serves as a pad oxide layer. In this 
embodiment, the oxide layer 32 is a silicon dioxide layer. The oxide layer 
32 can be formed by any suitable method. The silicon dioxide layer 32 is 
formed by using an oxygen-steam ambient, at a temperature of about 
850.degree.-1000.degree.C. Alternatively, the oxide layer 32 may be formed 
using any suitable oxide chemical compositions and procedures. In this 
embodiment, the silicon dioxide layer 32 is formed to a thickness of 
approximately 110 angstroms. A silicon nitride layer 34 is then formed on 
the oxide layer 32. In this embodiment, the silicon nitride layer 34 is 
deposited by using well-known methods such as a chemical vapor deposition 
process. The silicon nitride layer 34 is about 1700 angstroms thick. Next, 
the oxide layer 32 and the nitride layer 34 are patterned and etched to 
form the trenches on the substrate 30. First, the oxide layer 32 and the 
nitride layer 34 are masked by a patterned mask 35. Then, the oxide layer 
32 and the nitride layer 34 uncovered by the mask 35 are removed. In this 
embodiment, an etching method (indicated by arrows 36) is performed to 
remove the oxide layer 32 and the nitride layer 34. The etching method is 
a plasma etching method. The plasma etching method is performed using 
CHF.sub.3, CF.sub.4, Ar, and O.sub.2 gases. The plasma etching method is 
achieved using a CHF.sub.3 gas flow of about 10 sccm, a CF.sub.4 gas flow 
of about 15 sccm, an Ar gas flow of about 100 sccm, and an O.sub. 2 gas 
flow of about 5 sccm, with a radio-frequency(RF) of about 400 W and a 
process pressure of about 50 mT. The resulting structure is shown in FIG. 
5. 
After that, an etching process 37 is performed to etch the substrate 30 in 
order to form the shallow trenches. The remained oxide layer 32 and the 
remained nitride layer 34 serve as hard masks. The etching process 37 is a 
multiple-step dry etching process. Typically, a 3-step dry-etching process 
is performed to form the desired structure of the shallow trenches. Same 
etchant gases such as HBr, Cl.sub.2, CF.sub.4 and HeO.sub.2, are chosen 
for all these steps to avoid discontinuity in the profile. However, the 
parameters of the RF power and the process pressure among these steps are 
different in order to obtain varied degree of polymer deposition in each 
step. By applying the strongest polymer deposition in the first step, 
round top corners can be achieved using the highest value of the RF power 
about 800 W and lowest value of the process pressure about 50 mT among 
these 3 steps. The third step provides the lowest but enough deposition 
degree to obtain round bottom corners using the lowest value of RF power 
and highest value of process pressure. The amount of polymer forming from 
the 3-step dry-etching process is determined by the gas ratio for HBr to 
Cl.sub.2 and the slope of profile is also determined by the gas ratio for 
HBr to Cl.sub.2. Typical the gas ratio for HBr to Cl.sub.2 is 5. A ratio 
smaller than 4 would lead to vertical profile. On the other hand, in the 
case of ratio larger than 6, etch stop might occur, especially together 
with high flow rate of HeO.sub.2 (larger than 30 sccm). In this 
embodiment, the etching process 37 is a 3-step dry etching process. The 
etching process 37 is a plasma etching process. The plasma etching method 
is performed using HBr, Cl.sub.2, HeO.sub.2, and CF.sub.4 gases. In this 
embodiment, the first step of the etching process 37 is achieved using a 
HBr gas flow of about 100 sccm, a Cl.sub.2 gas flow of about 20 sccm, a 
HeO.sub.2 gas flow of about 30 sccm, a CF.sub.4 gas flow of about 20 sccm, 
with a radio-frequency(RF) of about 800 W and a process pressure of about 
50 mT. The first step of the etching process 37 is performed about 8 
seconds. In the second step of the etching process 37, the parameter of 
the RF power is changed by decreasing about 10% from 800 W down to 700 W 
and the parameter of the process pressure is also changed by increasing 
about 60% from 50 mT to 80 mT. The other parameters of the second step of 
the etching process 37 keep the same conditions as the parameters of the 
second step of the etching process 37. For example, the gas flow of HBr is 
about 100 sccm, the gas flow of Cl.sub.2 is about 20 sccm, the gas flow of 
HeO.sub.2 is about 30 sccm, and the gas flow of CF.sub.4 is about 20 sccm. 
The second step of the etching process 37 is performed about 8 seconds. In 
the third step of the etching process 37, the parameter of the RF power is 
changed by decreasing about 20% from 800 W down to 650 W and the parameter 
of the process pressure is also changed by increasing about 100% from 50 
mT to 100 mT. The other parameters of the third step of the etching 
process 37 keep the same conditions as following: the gas flow of HBr is 
about 100 sccm, the gas flow of Cl.sub.2 is about 20 sccm, the gas flow of 
HeO.sub.2 is about 30 sccm, and the gas flow of CF.sub.4 is about 20 sccm. 
The third step of the etching process 37 is performed about 46 seconds. 
The resulting structure is shown in FIG. 6. 
The method of this invention takes advantage of different degree of the 
polymer deposition in the 3-step etching process by varing the process 
pressure and RF power. Thus, the shallow trenches with round corners are 
obtained. In addition, the sloped trench sidewall profile can be achieved 
by controlling the gas ratio for HBr to Cl.sub.2. A shallow trench having 
round trench top corners and round trench bottom corners is obtained by 
the method of this invention. Therefore, the problems of gate oxide 
thinning and the defect generation are eliminated. In addition, the 
sidewall slope of trench profile can be reduced to facilitate gap filling. 
Although specific embodiment has been illustrated and described, it will be 
obvious to those skilled in the art that various modifications, such as 
changing the parameters of the RF power and the process pressure in the 
etching process, may be made without departing from the which is intended 
to be limited solely by the appended claims.