U.S. patent application number 09/837136 was filed with the patent office on 2001-09-20 for shallow trench isolation on a silicon substrate using nitrogen implant into the side wall.
Invention is credited to Leobandung, Effendi.
Application Number | 20010023112 09/837136 |
Document ID | / |
Family ID | 22728325 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010023112 |
Kind Code |
A1 |
Leobandung, Effendi |
September 20, 2001 |
Shallow trench isolation on a silicon substrate using nitrogen
implant into the side wall
Abstract
A process for fabricating a tapered trench on a silicon
substrate. The process comprises the steps of forming an initial
trench in the substrate and implanting nitrogen ions on the initial
trench side walls. More nitrogen ions are implanted adjacent the
exposed surface of the substrate than adjacent the trench bottom.
Finally, the initial trench side walls are oxidized to create the
tapered shape.
Inventors: |
Leobandung, Effendi;
(Wappingers Falls, NY) |
Correspondence
Address: |
Ratner & Prestia
One Westlakes (Berwyn)
Suite 301
P.O. Box 980
Valley Forge
PA
19482-0980
US
|
Family ID: |
22728325 |
Appl. No.: |
09/837136 |
Filed: |
April 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09837136 |
Apr 18, 2001 |
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09197168 |
Nov 20, 1998 |
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6238998 |
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Current U.S.
Class: |
438/423 ;
257/E21.55; 257/E21.551 |
Current CPC
Class: |
H01L 21/76235 20130101;
H01L 21/76237 20130101 |
Class at
Publication: |
438/423 |
International
Class: |
H01L 021/76 |
Claims
What is claimed is:
1. A process for fabricating a trench, having a bottom and side
walls defining a tapered shape, on a silicon substrate, the
substrate having an exposed surface, wherein the trench side walls
are spaced further apart adjacent the exposed surface than adjacent
the trench bottom, the process comprising: forming an initial
trench having initial trench side walls and the trench bottom
extending from the exposed surface into an interior of the
substrate; implanting nitrogen ions on the initial trench side
walls wherein more nitrogen ions are implanted adjacent the exposed
surface than adjacent the trench bottom; and oxidizing the trench
side walls, thereby creating a trench having the tapered shape.
2. The process of claim 1 wherein the initial trench side walls
extend into the substrate in a direction substantially
perpendicular to the exposed surface and are substantially
perpendicular to each other.
3. The process according to claim 2 wherein the step of implanting
the nitrogen ions on the initial trench side walls comprises
implanting the ions at an angle from about 10 degrees to about 60
degrees relative to the exposed surface.
4. The process according to claim 3 wherein the nitrogen ions are
implanted on the initial trench side walls at an angle of about 30
degrees relative to the exposed surface.
5. The process according to claim 1 wherein the step of implanting
the nitrogen ions comprises implanting from about 5 kev to about 10
keV nitrogen ions on the initial trench side walls to a dose of
from about 1.times.10.sup.14/cm.sup.2 to about
1.times.10.sup.15/cm.sup.2.
6. The process according to claim 5 wherein the step of implanting
the nitrogen ions comprises implanting about 10 keV nitrogen ions
on the initial trench side walls to a dose of about
5.times.10.sup.14/cm.sup.2.
7. The process according to claim 1 wherein the step of forming the
initial trench comprises photomasking and etching the
substrate.
8. The process according to claim 7 wherein the etching is dry
etching.
9. The process according to claim 1 wherein the substrate is a
silicon-on-insulator substrate comprising a silicon oxide layer
having a thickness of from about 0.1 .mu.m to about 0.4 .mu.m on a
silicon wafer, a silicon layer having a thickness of from about 0.1
.mu.m to about 0.2 .mu.m on the silicon oxide layer, and a silicon
nitride layer having a thickness of from about 0.1 .mu.m to about
0.2 .mu.m on the silicon layer.
10. The process according to claim 9 wherein the silicon oxide
layer has a thickness of about 0.4 .mu.m, the silicon layer has a
thickness of about 0.18 .mu.m, and the silicon nitride layer has a
thickness of about 0.12 .mu.m.
11. The process according to claim 9 wherein the substrate further
comprises a thin silicon oxide layer having a thickness of from
about 0.006 .mu.m to about 0.01 .mu.m between the silicon layer and
the silicon nitride layer.
12. The process according to claim 1 further comprising the step of
cleaning the initial trench before the step of oxidizing the trench
side walls.
13. A process for fabricating a trench having a bottom and side
walls defining a tapered shape on a silicon-on-insulator substrate,
the substrate having an exposed surface, wherein the trench side
walls are spaced further apart adjacent the exposed surface than
adjacent the trench bottom, the process comprising: forming an
initial trench having initial trench side walls and the trench
bottom extending from the exposed surface into an interior of the
substrate by photomasking and dry etching the substrate; implanting
about 10 keV nitrogen ions to a dose of about 5.times.10.sup.14
cm/cm.sup.2 on the initial trench side walls at an angle from about
10 degrees to about 60 degrees wherein more nitrogen ions are
implanted adjacent the exposed surface than adjacent the trench
bottom; and oxidizing the trench side walls with dry oxygen at
about 1,000.degree. C., thereby creating the tapered shape.
14. The process of claim 13 wherein the substrate comprises a
silicon oxide layer having a thickness of about 0.4 .mu.m on a
silicon wafer, a silicon layer having a thickness of about 0.18
.mu.m on the silicon oxide layer, and a silicon nitride layer
having a thickness of about 0.12 .mu.m on the silicon layer.
15. A silicon substrate having an exposed surface and a trench
having a bottom and side walls defining a tapered shape extending
through the exposed surface into the substrate, wherein the trench
side walls are spaced further apart adjacent the exposed surface
than adjacent the trench bottom, and wherein the trench is
fabricated by: forming an initial trench having initial trench side
walls and the trench bottom extending from the exposed surface into
the substrate; implanting nitrogen ions on the initial trench side
walls wherein more nitrogen ions are implanted adjacent the exposed
surface than adjacent the trench bottom; and oxidizing the trench
side walls, thereby creating a trench having the tapered shape.
16. The substrate according to claim 15 further comprising a
silicon oxide layer on a silicon wafer, a silicon layer on the
silicon oxide layer, and a silicon nitride layer on the silicon
layer.
17. The substrate according to claim 16 further comprising a thin
silicon oxide layer having a thickness of from about 0.006 .mu.m to
about 0.01 .mu.m between the silicon layer and the silicon nitride
layer.
Description
TECHNICAL FIELD
[0001] The present invention relates to a process of fabricating a
trench on a silicon substrate and, more particularly, to a process
of forming a trench having tapered side walls on a
silicon-on-insulator (SOI) substrate.
BACKGROUND OF THE INVENTION
[0002] An integrated circuit or an array of discrete integrated
circuit devices includes numerous semiconductor devices. Current
leakages and parasitic capacitances between devices can interfere
with the intended operation of the circuit. Therefore, in many
circuits, it is necessary to electrically isolate devices from one
another. Several isolation techniques have been developed to meet
that requirement.
[0003] A silicon-on-insulator (SOI) structure is a structure in
which a buried insulating layer electrically isolates a silicon
layer from a silicon substrate. The SOI structure does not always
occupy the entire silicon substrate. Often, the SOI structure
occupies only a portion of the silicon substrate.
[0004] Shallow Trench Isolation (STI) is a process used in
isolating devices formed on SOI substrates. STI involves etching
trenches, having side walls and bottoms, in the SOI substrate.
Following etching, the trenches are filled with an oxide.
[0005] Unfortunately, there is a problem in implementing shallow
trench isolation processing (STI) in SOI substrates. Following
etching, and before filling the trenches with an oxide, the silicon
layer portions of the trench side walls are oxidized. This step
produces a bottleneck-shaped trench in which the trench side walls
are wider adjacent the trench bottom than adjacent the top of the
silicon layer portion of the trench side wall. After the step of
oxidizing the silicon layer portion of the trench side walls, the
next step in STI processing comprises filling the trenches with an
oxide. The bottleneck shape of the trench makes filling the trench
with an oxide difficult. This difficulty results in the formation
of a seam in the center of the trench. This seam may open up during
subsequent processing steps, leading to several problems. For
instance, the seam might cause shorts between subsequently
deposited poly gates if filled with polysilicon during the poly
gate deposition step.
[0006] The deficiencies of the conventional processes of
fabricating a trench on a silicon substrate using shallow trench
isolation (STI) show that a need still exists for a process to
eliminate the bottleneck-shaped trench produced after oxidizing the
trench side walls. To overcome the shortcomings of the conventional
processes, a new process is provided. An object of the present
invention is to provide a process for fabricating a trench using
STI in which the trench side walls are spaced further apart
adjacent the exposed surface than adjacent the trench bottom.
SUMMARY OF THE INVENTION
[0007] To achieve these and other objects, and in view of its
purposes, the present invention provides a process for fabricating
a trench having a tapered shape, side walls, and a bottom on a
silicon substrate. The substrate has an exposed surface. The trench
side walls are spaced further apart adjacent the exposed surface
than adjacent the trench bottom. The process comprises the
following steps:
[0008] forming an initial trench having initial trench side walls
and a trench bottom extending from the exposed surface into an
interior of the substrate;
[0009] implanting nitrogen ions on the initial trench side walls so
that more nitrogen ions are implanted adjacent the exposed surface
than adjacent the trench bottom; and
[0010] oxidizing the trench side walls, thereby creating a trench
having the tapered shape.
[0011] In a preferred embodiment, the nitrogen ions are implanted
on the initial trench side walls at an angle of from about 10
degrees to about 60 degrees relative to the exposed surface.
[0012] It is to be understood that both the foregoing general
description and the following detailed description are exemplary,
but are not restrictive, of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0013] The invention is best understood from the following detailed
description when read in connection with the accompanying drawing.
It is emphasized that, according to common practice, the various
features of the drawing are not to scale. On the contrary, the
dimensions of the various features are arbitrarily expanded or
reduced for clarity. Included in the drawing are the following
figures:
[0014] FIG. 1 shows in schematic representation a silicon substrate
comprising a trench structure having side walls spaced further
apart adjacent the trench bottom than adjacent the top of the
silicon layer portion of the trench side wall;
[0015] FIG. 2A shows the structure of FIG. 1 after the trench has
been filled with an oxide, producing a seam in the center of the
trench;
[0016] FIG. 2B shows the structure of FIG. 2A after the seam has
been filled with polysilicon, causing shorts between deposited poly
gates;
[0017] FIG. 3 shows in schematic representation a silicon
substrate;
[0018] FIG. 4 shows in schematic representation a silicon substrate
having a trench with substantially perpendicular side walls;
[0019] FIG. 5 shows in schematic representation a top view of FIG.
4;
[0020] FIG. 6 illustrates the angle to the exposed surface in which
nitrogen ions are implanted into the trench side walls;
[0021] FIG. 7 shows in schematic representation a silicon substrate
having a tapered trench structure; and
[0022] FIG. 8 shows in schematic representation a tapered trench of
a silicon substrate that has been filled with oxide.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention will next be described with reference
to the drawing in which similar numbers indicate the same elements
in all figures. Such figures are intended to be illustrative rather
than limiting and are included to facilitate the explanation of the
apparatus of the present invention. The apparatus is made by the
Shallow Trench Isolation (STI) process. The STI process comprises
the steps of etching trenches, having side walls and bottoms, in
the SOI substrate, oxidizing the silicon layer portions of the
trench sidewalls, and filling the trenches with an oxide.
[0024] Referring now to the drawing, FIG. 1 shows a side view of a
silicon substrate 1 after the steps of etching a trench 18 and
oxidizing those portions 19, 20 of the side walls 24, 26 of the
trench 18 comprising a silicon layer 14. The substrate 1 comprises
a silicon wafer 10, a buried silicon oxide layer 12 formed on the
silicon wafer 10, the silicon layer 14, a silicon nitride layer 16,
and the trench 18.
[0025] After oxidizing the side walls 24, 26 of the trench 18, the
next step in STI processing comprises filling the trench 18 with an
oxide 32. As a result of the bottleneck-shape of the trench 18, a
shape in which the width of the trench 18 is greater near the
bottom then at the top, it is very difficult to fill the trench 18
with an oxide 32. This difficulty results in the formation of a
seam 22 in the center of the trench 18 as illustrated in FIG.
2A.
[0026] The seam 22 may open up during subsequent processing steps,
leading to several problems. For instance, FIG. 2B shows the
structure of FIG. 2A after the seam 22 has been filled with
polysilicon 40. The polysilicon 40 fills the seam 22 during
deposition of poly gates 42 and may cause shorts between the poly
gates 42.
[0027] The present invention provides a process for fabricating a
trench 18 using shallow trench isolation (STI) in which the side
walls 24, 26 of the trench 18 are spaced further apart adjacent the
exposed surface than adjacent the trench bottom. Forming a trench
18 having this structure eliminates the formation of a seam 22 in
the center of the trench 18 when the trench 18 is filled with an
oxide 32.
[0028] FIG. 3 shows a side view of an embodiment of a silicon
substrate comprising a silicon-on-insulator (SOI) substrate 1. In
this embodiment, the substrate 1, having an exposed surface 8,
comprises a silicon wafer 10, a buried silicon oxide layer 12
formed on the silicon wafer 10, a silicon layer 14, and a silicon
nitride layer 16. A thin oxide layer 15 of from about 0.006 .mu.m
to about 0.01 .mu.m can optionally be deposited between the silicon
layer 14 and the silicon nitride layer 16 to reduce damage to the
silicon layer 14 caused by deposition of the silicon nitride layer
16.
[0029] In a preferred embodiment, the thickness of the silicon
oxide layer 12 is from about 0.1 .mu.m to about 0.4 .mu.m, the
thickness of the silicon layer 14 is from about 0.1 .mu.m to about
0.2 .mu.m, and the thickness of the silicon nitride layer 16 is
from about 0.1 .mu.m to about 0.15 .mu.m. In a more preferred
embodiment, the thickness of the silicon oxide layer 12 is about
0.4 .mu.m, the thickness of the silicon layer 14 is about 0.18
.mu.m, and the thickness of the silicon nitride layer 16 is about
0.12 .mu.m.
[0030] FIG. 4 shows a side view of a substrate 1 having an initial
trench 18 extending into the interior of the substrate 1. In FIG.
4, standard photomasking and etching of the silicon substrate 1,
through the silicon nitride layer 16, thin oxide layer 15 (if
present), and silicon layer 14, results in an initial trench 18
having a bottom 21 and four initial trench side walls 24, 26, 28,
and 30. Side walls 24 and 26 are shown in FIG. 4. FIG. 5 is a top
view of FIG. 4 showing all four side walls 24, 26, 28, and 30 of
initial trench 18. Initial trench side walls 24, 26, 28, and 30 are
substantially perpendicular to the exposed surface 8 of the
substrate 1.
[0031] To form the side walls 24, 26, 28, and 30 of initial trench
18 substantially perpendicular to the exposed surface 8, dry
etching techniques, which are well known in the art, are preferred.
The initial trench 18 can be cleaned using methods commonly known
in the art, which typically include a preliminary step of plasma
oxidation stripping or immersion in an inorganic resist stripper,
followed by the removal of residual organic contaminants and the
desorption of remaining atomic and ionic contaminants. S. Wolf
& R. N. Tauber discuss such cleaning processes on pages 516-517
of their text, titled Silicon Processing for the VLSI Era, Volume
1--Process Technology (1986).
[0032] FIG. 6 shows the step of nitrogen ion implantation into the
side walls 24, 26, 28, and 30 of initial trench 18. The nitrogen
ions 50 are implanted at an angle (.alpha.) to the exposed surface
8 of the substrate 1. Ion implantation is a process in which
energetic, charged atoms or molecules are directly introduced into
a substrate, such as a silicon wafer. At varying orientations,
nitrogen ions 50 are implanted on each side wall 24, 26, 28, and 30
of initial trench 18.
[0033] In one embodiment, a first position of the substrate 1 is
determined so that nitrogen ions 50 are beamed at a direction
parallel to side walls 28 and 30 onto side wall 24. The substrate 1
is then rotated by 90 degrees about a rotation axis parallel to the
top of the substrate 1 and fixed in a second position. Nitrogen
ions 50 are then beamed at a direction parallel to side walls 24
and 26 onto side wall 28. The substrate 1 is then rotated by 90
degrees about a rotation axis parallel to the top of the substrate
1 and fixed in a third position. Nitrogen ions 50 are then beamed
at a direction parallel to side walls 28 and 30 onto side wall 26.
The substrate 1 is then rotated by 90 degrees about a rotation axis
parallel to the top of the substrate 1 and fixed in a fourth
position. Nitrogen ions 50 are then beamed at a direction parallel
to side walls 24 and 26 onto side wall 30.
[0034] The optimal angle (.alpha.) of nitrogen ion implantation
depends on the thickness of the silicon nitride layer 16 and the
size of the trench 18. In a preferred embodiment, the side walls
24, 26, 28, and 30 of trench 18 are implanted with from about 5 keV
to about 15 keV nitrogen ions 50 to a dose of from about
1.times.10.sup.14/cm.sup.2 to about 1.times.10.sup.15/cm.sup.2, and
the nitrogen ions 50 are implanted at an angle (.alpha.) of from
about 10 to about 60 degrees relative to the exposed surface 8. In
a more preferred embodiment, the side walls 24, 26, 28, and 30 of
trench 18 are implanted with about 10 keV nitrogen ions 50 to a
dose of about 5.times.10.sup.14/cm.sup.2 at an angle (.alpha.) of
about 20 degrees. By implanting nitrogen ions 50 at an angle
(.alpha.) relative to the exposed surface 8, more nitrogen ions 50
are implanted adjacent the exposed surface 8 than adjacent the
bottom 21 of the trench 18.
[0035] FIG. 7 shows a side view of a resulting tapered trench 18a
after oxidation of the side walls 24, 26, 28, and 30 of initial
trench 18, resulting in tapered side walls 24a and 26a. The
oxidation rate of the side walls 24, 26, 28, and 30 is affected by
nitrogen ion concentration which proportionally retards trench side
wall oxidation. The high concentration of nitrogen ions in the
silicon nitride layer 16, which forms the portion of the side walls
24 and 26 adjacent the exposed surface 8, inhibits oxidation. In
contrast, the silicon layer 14, which forms the portion of the side
walls 24a and 26a adjacent trench bottom 21, has a lower
concentration of nitrogen ions. Oxidation of the portion of the
side walls 24a and 26a of the trench 18a adjacent the silicon layer
14 results in oxide formation in proportion to nitrogen ion
concentration.
[0036] Because the nitrogen ions 50 are implanted at an angle
(.alpha.) relative to the exposed surface 8, more nitrogen ions 50
are implanted into the portion of the side walls 24a and 26a of the
trench 18a adjacent the silicon layer 14 and the bottom 21 of the
trench 18a than in the silicon nitride layer 16 portion of the side
walls 24 and 26 adjacent the exposed surface 8, thereby producing a
tapered shape upon oxidation. A portion of silicon layer 14
adjacent the side walls 24a and 26a of the trench 18a is also
oxidized. In a preferred embodiment, the side walls 24, 26, 28, and
30 are oxidized using dry oxygen at a temperature of about
1,000.degree. C.
[0037] Following oxidation of the trench sidewalls 24, 26, 28, and
30, the trench is then filled with an oxide 32. FIG. 8 shows a side
view of the tapered trench 18a shown in FIG. 7 after tapered trench
18a has been filled with the oxide 32. Following oxide deposition
to fill the trench 18a, steps known in the art can be used to
complete an STI structure. In addition, the present invention can
be used in bulk STI processes.
[0038] In one embodiment, a silicon substrate comprising an SOI
substrate was formed by the following steps. A 0.4 .mu.m layer of
silicon oxide 12 was formed on a silicon wafer 1 and a 0.18 .mu.m
silicon layer 14 was formed on the silicon oxide layer 12 using the
separation by implantation of oxygen (SIMOX) process. A 0.008 .mu.m
layer of silicon oxide (thin silicon oxide layer 15) was next
formed on the silicon layer 14 followed by deposition of a 0.12
.mu.m layer of silicon nitride 16 on the thin silicon oxide layer
15. An initial trench 18 extending from the exposed surface 8 of
the substrate 1 into an interior of the substrate was formed by
photomasking and plasma etching. The plasma etching was selective
to stop at the silicon oxide layer 15, producing a trench depth of
0.308 .mu.m. The initial trench side walls 24, 26, 28, and 30 were
implanted with 10 keV nitrogen ions 50 to a dose of
5.times.10.sup.14/cm.sup.2 at an angle (.alpha.) of 30 degrees with
respect to the exterior, exposed surface 8 of the substrate 1. The
trench side walls 24, 26, 28, and 30 were next oxidized with dry
oxygen at 1,000.degree. C., resulting in a tapered trench 18a in
which the trench side walls 24, 26, 28, and 30 were spaced further
apart adjacent the exposed surface 8 of the substrate 1 than
adjacent the trench bottom 21.
[0039] Although illustrated and described above with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the spirit
of the invention.
* * * * *