Preventing gate oxice thinning effect in a recess LOCOS process

Details Preventing gate oxice thinning effect in a recess LOCOS process Preventing gate oxice thinning effect in a recess LOCOS process

2002-08-13

2004-11-02

Chen, Kin-Chan (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Combined with coating step

C438S297000, C438S369000

Reexamination Certificate

active

06812148

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to semiconductor manufacturing and, more particularly, to the formation of gate oxides and preventing or minimizing gate oxide thinning in gate oxides.
An integrated circuit (IC) consists of thousands of transistors such as metal oxide semiconductors (MOSs). A field oxide (FOX) is disposed between two transistors for insulating the transistors in an IC to prevent short circuit. A gate oxide usually underlies the gate of a transistor and the transistor induces charge through the gate oxide. In the conventional method for forming gate oxide, a FOX is formed on a semiconductor substrate by using local oxidation of silicon (LOCOS) and then a well drive-in process is applied. Because the FOX is in ambient nitrogen at high temperatures during the well drive-in process, a residual nitride such as Si
3
N
4
is formed at a partial edge of the FOX. The residual nitride results in gate oxide thinning at the partial edge of the FOX during formation of the gate oxide.
FIGS. 1A
to
1
E show a known method for forming a gate oxide on the semiconductor device in which gate oxide thinning occurs. In
FIG. 1A
, a pad oxide layer
12
and a silicon nitride layer (Si
3
N
4
)
14
are formed on a semiconductor substrate
10
, respectively.
In
FIG. 1B
, lithography and etching form the patterned silicon nitride layer
14
a
and patterned pad oxide layer
12
a
to expose the surfaces of the substrate
10
. Then, the patterned silicon nitride layer
14
a
is used as a hard mask to etch the surfaces to form trenches
11
in the substrate
10
.
In
FIG. 1C
, the FOX (field oxide) layers
16
are formed in the trenches
11
by wet oxidation. Then, the patterned silicon nitride layer
14
a
and patterned pad oxide layer
12
a
are removed to expose the surface of the substrate
10
. The exposed region serves as an active area (AA)
10
a
of the semiconductor device. Subsequently, the ion drive-in process is applied to the active area
10
a
by injecting nitrogen gas at a high temperature.
In
FIG. 1D
, a sacrificial oxide layer
18
such as silicon dioxide(SiO
2
) is formed on the AA
10
a
. Prior to forming the sacrificial oxide layer
18
, silicon nitride (not shown) is formed on the AA
10
a
and the partial edges of the FOX layers
16
(i.e., the regions “A” shown in FIG.
1
D) due to exposure of the AA
10
a
to the nitrogen gas at high temperatures for an extended period of time.
In
FIG. 1E
, the sacrificial oxide layer
18
is removed, thereby removing the silicon nitride on the AA
10
a
. Subsequently, an oxide layer
19
is formed on the AA
10
a
to serve as the gate oxide layer of the semiconductor device by dry oxidation. Only the silicon nitride formed on AA
10
a
is removed with the sacrificial oxide layer
18
. The silicon nitride formed in the partial edge region “A” as seen in
FIG. 1D
often cannot be removed during the removal of the sacrificial oxide layer
18
, but remains as the residual silicon nitride prior to formation of the gate oxide. The oxidation rate of the FOX layers
16
near the residual silicon nitride (i.e., the regions “A” in
FIG. 1D
) is lower than that of the surface of the AA
10
a
when the gate oxide layer
19
is formed. As a result, the gate oxide layer
19
that is formed at the juncture between the FOX layers
16
and the AA
10
a
is thinner, as shown in regions “B” in FIG.
1
E. The thinner gate oxide layer
19
causes the increment of the leakage current and the reduction of the breakdown voltage of the semiconductor device.
BRIEF SUMMARY OF THE INVENTION
Embodiments of the present invention relate to a method for preventing gate oxide thinning in a recess LOCOS process. This is achieved by introducing a mixture of oxygen gas and nitrogen gas, typically at an elevated temperature, into the region in which the substrate is disposed during the ion drive-in to the active area of the semiconductor substrate. The affinity for oxygen with silicon is higher than that for nitrogen with silicon. As a result, oxygen can be used to prevent or reduce silicon nitride formation on the field oxide regions. The amount of oxygen is selected to be sufficient to at least substantially prevent silicon nitride from forming in the field oxide regions, thereby preventing or reducing gate oxide thinning during the subsequent formation of the gate oxide due to the presence of residual silicon nitride. In specific embodiments, an upper oxide layer is formed on the active area during the ion drive-in to the active region. The upper silicon oxide layer
28
can prevent an excess of nitrogen bonds with silicon from forming in the active area to produce a lot of silicon nitride (Si
3
N
4
), which can be difficult to remove before forming the gate oxide layer.
In accordance with an aspect of the present invention, a method of forming a gate oxide on a substrate comprises providing a substrate having thereon a plurality of trenches separated by a patterned pad oxide and a patterned silicon nitride layer disposed thereon and used to form the plurality of trenches. A plurality of first oxide layers are formed in the trenches as field oxides of a semiconductor device. The patterned silicon nitride layer and the patterned pad oxide layer are removed to expose a surface of the substrate as an active area of the semiconductor device. A flow of oxygen and nitrogen with a predetermined flow ratio is introduced for performing an ion drive-in to the active area at a predetermined temperature. The method further comprises forming a second oxide layer on the active area, removing the second oxide layer to expose the active area, and forming a third oxide layer on the active area as a gate oxide of the semiconductor device.
In some embodiments, the predetermined flow ratio of the oxygen to the nitrogen is in a range of about 0.5% to about 2%. The fourth oxide layer is formed by a dry oxidation process. The thickness of the fourth oxide layer is in a range of 100 Å to 200 Å. The first oxide layers are formed by a wet oxidation process. The predetermined temperature for the ion drive-in is in a range of about 800° C. to about 1200° C. The second oxide layer is a sacrificial oxide layer comprising silicon dioxide. The third oxide layer is formed by a dry oxidation process. A fourth oxide layer is formed on the active area during the ion drive-in to the active area. The trenches are formed by using an etching process.
Another aspect of the present invention is directed to a method of preventing or reducing gate oxide thinning in a recess LOCOS process for a substrate including thereon an active area and field oxide regions disposed adjacent the active area. The method comprises performing an ion drive-in to the active area on the substrate by directing a flow of oxygen and nitrogen toward the substrate at a predetermined temperature and with a sufficient amount of oxygen to at least substantially prevent silicon nitride from forming on the field oxide regions. The method further comprises forming a sacrificial oxide layer on the active area, removing the sacrificial oxide layer to expose the active area, and forming a gate oxide layer on the active area.
In some embodiments, the flow of oxygen and nitrogen comprises O
2
and N
2
at a ratio of about 0.5% to about 2% O
2
/N
2
. During the ion drive-in to the active area, a residual nitride is formed on the active area and an upper oxide layer is formed over the residual nitride. The upper oxide layer has a thickness of about 100 Å to about 200 Å. The residual nitride and the upper oxide layer are removed during removal of the sacrificial oxide layer.
In accordance with another aspect of the invention, a method of forming a gate oxide on a substrate comprises providing a substrate having thereon a plurality of trenches having gate oxides formed therein, wherein the plurality of trenches are separated by a patterned pad oxide and a patterned silicon nitride layer disposed thereon and used to form the plurality of trenches. The patterned silicon nitride layer and the patt

Affiliated with

Also associated with

Rating

Preventing gate oxice thinning effect in a recess LOCOS process does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Preventing gate oxice thinning effect in a recess LOCOS process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Preventing gate oxice thinning effect in a recess LOCOS process will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3331435