U.S. patent application number 09/819962 was filed with the patent office on 2002-02-28 for shallow trench isolation type semiconductor device and method of forming the same.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Shin, You-Cheol.
Application Number | 20020024111 09/819962 |
Document ID | / |
Family ID | 19685849 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024111 |
Kind Code |
A1 |
Shin, You-Cheol |
February 28, 2002 |
Shallow trench isolation type semiconductor device and method of
forming the same
Abstract
A shallow trench isolation type semiconductor device is
described, which includes a trench having a flexure in a bottom
thereof. The flexure has a step difference of about 100 .ANG. or
more, and is preferably made at a middle area. Conventionally, a
gate insulating layer includes a thin area of about 100 .ANG. or
less and a thick area of about 200 .ANG. or more. On the basis of a
bottom of a trench peripheral region, a middle part of the flexure
may be concave or convex. Particularly, the foregoing device can
effectively be applied to a self-aligned flash memory in which a
width of a trench between one active region and another is about 3
micrometers or less.
Inventors: |
Shin, You-Cheol; (Suwon,
KR) |
Correspondence
Address: |
Frank Chau, Esq.
F. CHAU & ASSOCIATES, LLP
Suite 501
1900 Hempstead Turnpike
East Meadow
NY
11554
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
Suwon-city
KR
|
Family ID: |
19685849 |
Appl. No.: |
09/819962 |
Filed: |
March 28, 2001 |
Current U.S.
Class: |
257/500 ;
257/510; 257/E21.548; 257/E21.549; 257/E21.628; 438/258; 438/296;
438/424 |
Current CPC
Class: |
H01L 21/76229 20130101;
H01L 21/823481 20130101; H01L 21/76232 20130101 |
Class at
Publication: |
257/500 ;
257/510; 438/258; 438/296; 438/424 |
International
Class: |
H01L 027/10; H01L
021/336; H01L 021/76 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2000 |
KR |
2000-50419 |
Claims
What is claimed is:
1. A shallow trench isolation type semiconductor device including:
at least two regions where gate insulating layers differ in
thickness; and a silicon substrate with at least one flexure trench
having a step difference.
2. The device as claimed in claim 1, wherein there is a thickness
difference of about 100 .ANG. or more between the at least two
regions.
3. The device as claimed in claim 2, wherein the flexure trench is
formed at a region where a gate insulating layer has a thickness of
about 200 .ANG. or more, out of the at least two regions.
4. The device as claimed in claim 1, wherein the silicon substrate
is convex in a middle area of the flexure trench.
5. The device as claimed in claim 1, wherein the silicon substrate
is convex in one of a lower voltage part and a higher voltage part,
wherein the respective flexure trenches are contacted with an
active region.
6. The device as claimed in claim 1, wherein the step difference
has a range of about 100 .ANG. to about 500 .ANG..
7. A shallow trench isolation type flash memory device including a
flexure trench with a flexure having a step difference of about 100
.ANG. to about 500 .ANG. in a bottom thereof.
8. The device as claimed in claim 7, wherein the trench is formed
at a high voltage part of a peripheral region.
9. The device as claimed in claim 7, wherein the flexure trench is
formed using a self-aligned process to pattern a polysilicon layer
in a state that the polysilicon layer being a part of a floating
gate is formed on a gate insulating layer.
10. The device as claimed in claim 7, wherein a width of the
flexure trench is about 3 micrometers or less between one active
region and another, in the high voltage part of the peripheral
region.
11. The device as claimed in claim 10, wherein a silicon substrate
is concave in a middle area of the flexure trench.
12. A method of forming a shallow trench isolation type
semiconductor device comprising the steps of: forming a gate oxide
layer, so that a flexure is different from adjacent parts in
thickness, the flexure being a part of a trench area on a
substrate; forming an etching mask pattern to expose the gate oxide
layer in the trench area; and anisotropically and sequentially
etching the gate oxide layer and a silicon substrate to form a
trench, the gate oxide layer being located on the substrate where
the etching mask pattern is formed.
13. The method of claim 12, wherein the step of anisotropically and
sequentially etching the gate oxide layer and the silicon substrate
is performed by one etchant.
14. The method of claim 12 further comprising a step of stacking a
polysilicon layer between the steps of forming the gate oxide layer
and forming the etching mask pattern.
15. The method of claim 12, wherein the semiconductor device is a
flash memory.
Description
[0001] This application relies for priority upon Korean Patent
Application No. 2000-50419, filed on Aug. 29, 2000, the contents of
which are herein incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present invention generally relates to a shallow trench
isolation type semiconductor device and a method of forming the
same. More specifically, the present invention is directed to a
shallow trench isolation type semiconductor device in which
insulating layers differ in thickness according to regions, and a
method of forming the same.
[0004] 2. Discussion of Related Art
[0005] A common problem encountered in device isolation in a
high-density semiconductor device is a bird's beak effect, which
occurs due to a lateral growth of thermal silicon dioxide in the
form of a bird's beak under a SiN.sub.4 protective layer. The
bird's beak is undesirable since it takes up needed area and has
electrical field effects that permit current leakage. A shallow
trench isolation (STI) technique for device isolation has been
widely used and avoids the problem of the bird's beak effect.
However, to achieve superior device isolation results, the depth
and width of a trench must be increased. Since it is desirable to
have semiconductor devices having small footprints for higher
integration, the depth must be increased while the width for
insulation must be decreased. In a less than ideal scenario, a deep
trench cannot be formed due to the decrease in the width.
[0006] When a high voltage is applied, the component at which a
high voltage is applied must have a structure that is able to
withstand the high voltage. Typically, a gate insulating layer
formed at the part where the high voltage is applied is made
thicker than the gate insulating layer formed at others.
[0007] FIG. 1 is an exemplary cross-sectional view showing the
difference in thickness of a gate insulating layer at a low voltage
part of cell and peripheral regions compared to a gate insulating
layer at a high voltage part of the peripheral region. FIG. 1 also
shows an exemplary formation of a self-aligned trench in each
region of a flash memory device in which the gate insulating layers
differ in thickness. At the peripheral region where the gate
insulating layer 13 is thickly formed, a trench 17 for device
isolation is shallowly formed, which increases the probability that
insulation for device isolation will not be sufficient.
[0008] FIG. 1 includes gate insulating layers 11 and 13 which
differ in thickness according to each region of a substrate 10.
Conventionally, the thickness of the gate insulating layer 11
formed at a low voltage part of cell and peripheral regions is
about 70 .ANG. to about 80 .ANG. and the thickness of the gate
insulating layer 13 formed at a high voltage part of the peripheral
region is about 250 .ANG. to about 350 .ANG.. A polysilicon layer
15 for forming a part of a self-aligned floating gate is stacked on
the gate insulating layers 11 and 13. Based upon the STI technique,
device isolation is then carried out. In other words, an etch-stop
layer made of silicon nitride is deposited. Preferably, a high
temperature oxide (HTO) layer is then deposited for patterning the
etch-stop layer, and an anti-reflection coating (ARC) layer is
additionally deposited.
[0009] Through photoresist coating, exposure using a device
isolation mask pattern, and development, a trench pattern for
device isolation is formed. Subsequently, upper layers are
sequentially etched to be removed. A patterned upper layer can
serve as an etching mask to a lower layer. Generally, the etch-stop
layer is patterned and the photoresist pattern is removed by ashing
and stripping techniques. The polysilicon layer 15, the gate
insulating layers 11 and 13, and the substrate silicon layer 10 are
etched to form a trench. In the etching process to form the trench,
separately etching the gate insulating layer and the substrate
causes problems due to shifting of the etching apparatus. Thus, the
process is carried out in one etching apparatus (i.e., "in-situ").
It takes a great deal of time to etch a gate insulating layer at a
part where the gate insulating layer is thickly formed. With a
conventional silicon oxide layer and silicon etchant for forming a
trench, a depth difference (A) of about 180 .ANG. to about 500
.ANG. is observed in a substrate trench where the gate insulating
layer is thickly formed. Naturally, the depth difference (A) varies
according to the type of etchant used.
[0010] Preferably, after thinly stacking a sidewall oxide layer and
a silicon nitride liner, the trench is filled with a CVD oxide
layer. To complete the trench isolation layer 17, a chemical
mechanical planarization (CMP) process for removing the stacked CVD
oxide layer on a region except the trench, a wet etching process to
remove an etch-stop layer made of silicon nitride, and a cleaning
process are then performed. However, where the gate insulation
layer is thickly formed, a thickness of the isolation layer 17 has
a depth difference (A) as great as a depth of the trench. This is
disadvantageous, since if the isolation layer 17 becomes thin at a
transistor peripheral region to which a high voltage is applied,
device isolation can be incomplete.
[0011] Therefore, a need exists for a shallow trench isolation type
semiconductor device and a method of forming the same, which can
complement a trench isolation layer whose thickness in a silicon
substrate is not sufficient. A need also exists for a shallow
trench isolation type semiconductor device and a method of forming
the same which can complement incomplete device isolation caused by
a trench isolation layer whose thickness in a silicon substrate is
not sufficient in certain regions due to a thicker gate insulating
layer. Additionally, a need exists for a shallow trench isolation
type semiconductor device and a method of forming the same which
can complement a device isolation layer whose thickness is not
sufficient without causing an aligning problem.
SUMMARY OF THE INVENTION
[0012] According to an aspect of the present invention, a shallow
trench isolation type semiconductor device is provided wherein a
flexure with a step difference is made at a bottom of a trench.
Specifically, a shallow trench isolation type semiconductor device
is provided including at least two regions where gate insulating
layers differ in thickness; and a silicon substrate with at least
one flexure trench having a step difference.
[0013] At least in a partial region of the semiconductor device, a
gate insulating layer is deposited at a thickness of about 200
.ANG. or more. Conventionally, the gate insulating layer has a thin
region of about 100 .ANG. or less and a thick region of about 200
.ANG. or more. If a step difference of the gate insulating layer is
about 100 .ANG., the step difference of about 100 .ANG. will remain
in an etching process without an etching selectivity with respect
to a silicon oxide layer and a silicon layer. The step difference
can be expanded to, for example, about 500 .ANG..
[0014] Even though it is preferable that the flexure is made in a
middle area between active and inactive regions, the flexure may
lean toward the active region. At a bottom of a trench peripheral
area, the flexure may be concave and convex. In addition, a
plurality of flexures may be made therein.
[0015] A typical example of an aspect of the present invention can
be shown in a flash memory that has a thick gate oxide layer of
about 300 .ANG. or more in a peripheral region, and a thin gate
oxide layer of about 80 .ANG. or less in a low voltage part of cell
and peripheral regions. Particularly, the typical example is shown
in a self-aligned flash memory wherein a polysilicon layer to form
a part of a floating gate and a trench etching mask are
sequentially deposited on a gate insulating layer. The present
invention is particularly effective in cases where a width of a
trench between one active region and another is 3 micrometers or
less at a region having a thick gate oxide layer.
[0016] A method according to the present invention includes the
steps of making a flexure to have a gate oxide layer whose
thickness is different from that of an adjacent gate oxide layer,
forming an etching mask pattern to expose the gate oxide layer at a
trench area including the flexure, and anisotropically etching the
gate oxide layer and a silicon substrate to form a trench.
[0017] Specifically, a method of forming a shallow trench isolation
type semiconductor device according to an aspect of the present
invention comprises the steps of: forming a gate oxide layer, so
that a flexure is different from adjacent parts in thickness, the
flexure being a part of a trench area on a substrate; forming an
etching mask pattern to expose the gate oxide layer in the trench
area; and anisotropically and sequentially etching the gate oxide
layer and a silicon substrate to form a trench, the gate oxide
layer being located on the substrate where the etching mask pattern
is formed.
[0018] The step of anisotropically etching the gate oxide layer and
the silicon substrate comprises two steps: etching the gate oxide
layer to expose the silicon substrate on the basis of a thin or
thick gate oxide area, and etching the silicon substrate. In at
least one of the two steps, an etchant having a low etching
selectivity is used. To completely remove the gate oxide layer at a
thick gate oxide area, the silicon substrate is etched in a thin
gate oxide area as much as a predetermined thickness according to a
kind of etchant. Alternatively, one etchant is used in the two
steps.
[0019] These and other aspects, features, and advantages of the
present invention will be described or become apparent from the
following detailed description of preferred embodiments, which is
to be read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is an exemplary cross-sectional view showing the
difference in thickness of a gate insulating layer at a low voltage
part of cell and peripheral regions compared to a gate insulating
layer at a high voltage part of the peripheral region.
[0021] FIG. 2 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein gate
insulating layers that differ in thickness are formed.
[0022] FIG. 3 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein the gate
insulating layers are etched.
[0023] FIG. 4 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein a
polysilicon layer is stacked along thin and thick regions of the
gate insulating layers.
[0024] FIG. 5 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein an
etch-stop layer, an oxide layer and an anti-reflection coating
layer are preferably sequentially deposited on the polysilicon
layers.
[0025] FIG. 6 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein a
photoresist pattern is removed.
[0026] FIG. 7 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein the gate
insulating layers and a silicon substrate are preferably
sequentially etched.
[0027] FIG. 8 depicts exemplary cross-sectional views comparing a
deep trench without a flexure, and a trench with a flexure made at
a bottom thereof according to an aspect of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] A system according to the present invention provides a new
and improved shallow trench isolation type semiconductor device and
a method of forming the same. In particular, a shallow trench
isolation type semiconductor device according to the present
invention solves the problem of incomplete device isolation caused
by a trench isolation layer whose thickness in a silicon substrate
is not sufficient due to a thick gate insulating layer.
[0029] FIG. 2 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein gate
insulating layers that differ in thickness are formed. Referring to
FIG. 2, on a substrate 10, a thick gate oxide layer 103 is formed
to a thickness of about 300 .ANG.. To form gate insulating layers
that differ in thickness at each region, a photoresist pattern 21
is formed which exposes a region (such as a low voltage part of
cell and peripheral regions) where a thin gate oxide layer will be
formed. In the low voltage part of the cell and peripheral regions
excluding the regions where the trench is formed, photoresist
remains on a middle area (C) of a trench width (B). In the high
voltage part of the peripheral region to which a high voltage is
applied, the photoresist is removed on a middle area (E) of a
trench width (D).
[0030] For alignment convenience, it is preferable that a width of
a middle area (C) or (E) is about less than half that of a width of
a trench which is to be formed later. For example, if the trench
width is about 1.6 micrometers, a suitable value for patterning is
about 0.3 to about 0.8 micrometers. To make a flexure in the middle
area enhances an alignment margin. For example, in a transistor to
which a high voltage is applied, if a concave flexure is made at a
part that is bounded on an active region that must endure a high
voltage, a gate insulating layer is thinly formed on the active
region in case of misalignment. Thus, insulation breakdown may
occur.
[0031] FIG. 3 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein the gate
insulating layers are etched.
[0032] Referring to FIG. 3, using the photoresist pattern 21 shown
in FIG. 2 as an etching mask, a thin gate oxide layer 103' is
etched. Therefore, in a low voltage area of cell and peripheral
regions, a gate insulating layer remains only at a middle area of a
trench. On the contrary, in a peripheral high voltage area to which
a high voltage is applied, the gate insulating layer is removed
only at a middle area. The photoresist pattern 21 is then removed
using ashing and stripping techniques. Conventionally, after a
cleaning process, a thin gate insulating layer 101 is formed in a
region where a substrate silicon layer undergoes exposure. The gate
insulating layer 101 is a thermal oxide layer that is formed to a
thickness of about 80 .ANG.. A part of the substrate is
additionally oxidized in a region where a thick gate insulating
layer remains, so that a thick gate insulating layer 103 has a
thickness of about 350 .ANG..
[0033] FIG. 4 is an exemplary cross-sectional showing a step of
performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
view according to an aspect of the present invention, wherein a
polysilicon layer is stacked along thin and thick regions of the
gate insulating layers.
[0034] Referring to FIG. 4, by a self-aligned process, a
polysilicon layer 105 is stacked on an entire surface of a
substrate 10 where a thin gate insulating layer 101 and a thick
gate insulating layer 103 are formed. A thickness of the
polysilicon layer 105 is about 300 .ANG. to about 1000 .ANG.;
preferably about 500 .ANG.. The polysilicon layer 105 is preferably
conformally stacked along thin and thick regions of the gate
insulating layers.
[0035] FIG. 5 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein an
etch-stop layer, an oxide layer and an anti-reflection coating
layer are preferably sequentially deposited on the polysilicon
layers.
[0036] Referring to FIG. 5, an etch-stop layer 131, an oxide layer
133 of high temperature, and an anti-reflection coating (ARC) layer
135 are preferably sequentially deposited on a polysilicon layer
105. Using a conventional exposure process, a photoresist pattern
137 is formed to expose a trench area. Using the photoresist
pattern 137 as an etching mask, the ARC layer 135, the oxide layer
133, and the etch-stop layer are preferably sequentially etched to
form a pattern thereof.
[0037] FIG. 6 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein a
photoresist pattern is removed.
[0038] Referring to FIG. 6, the photoresist pattern 137 is removed
using ashing and stripping techniques. Using patterns of an
etch-stop layer 131, an oxide layer 133 of high temperature, and an
ARC layer as an etching mask, the polysilicon layer 105 is etched
to expose gate insulating layers 101 and 103 that each have a
flexure in a trench area. That is, a gate insulating layer having a
concave flexure is formed in a trench area of a high voltage part
of a peripheral region, and a gate insulating layer having a convex
flexure is formed in a trench area of a low voltage part of cell
and peripheral regions.
[0039] In the step of FIG. 6, it is preferable to perform an ion
implantation process for device isolation. In the ion implantation
process, impurity ions opposite to substrate ions are
complementarily implanted to prevent electrical leakage caused by
PN junction at a lower part of a trench. The process is performed
not to an entire surface of a substrate, but to a trench area of a
high voltage part of a peripheral region or a middle area of the
trench. In place of a conventional ion implantation mask for device
isolation at a high voltage part of a peripheral region, an ion
implantation mask is formed of photoresist in the step of FIG. 6.
Then, a trench area of the high voltage part opens up and ion
implantation for device isolation is performed.
[0040] FIG. 7 is an exemplary cross-sectional view showing a step
of performing self-aligned trench device isolation at a low voltage
part in cell and peripheral regions and a high voltage part of the
peripheral region during the formation of a NAND-type flash memory
according to an aspect of the present invention, wherein the gate
insulating layers and a silicon substrate are preferably
sequentially etched.
[0041] Referring to FIG. 7, gate insulating layers and the silicon
substrate are sequentially etched. Preferably, the process of
etching the thick gate insulating layer 103 and a layer of the
silicon substrate 10 are sequentially performed. At the thin gate
insulating layer 101, the thin gate insulating layer 101 is
completely removed and, in the process of removing the insulating
layer, the silicon substrate is etched to a depth of about 500
.ANG.. Then, the silicon substrate is additionally etched about
2000 .ANG. to about 2500 .ANG.. From the part where a thin gate
insulating layer is formed, trenches 141 and 143 are formed having
a depth about 2500 .ANG. to about 3000 .ANG.. Since the trench 143
in the peripheral region is wider than the trench 141, the depth of
the trench 143 may be greater due to a Loading Effect, in which an
etching rate is increased when a pattern density is decreased.
[0042] In the peripheral region to which a high voltage is applied,
a middle part of the trench width has the same depth in the silicon
substrate as the low voltage part of the peripheral region. It is
found that a path a charge carrier must pass to cause an electrical
leakage between devices is identical to that in the case of a deep
trench without a flexure at the bottom thereof. On the other hand,
a trench in the low voltage part of the cell and peripheral regions
also has a convex flexure at the bottom of the trench. Accordingly,
compared with a deep trench without a flexure, the trench of the
low voltage part has an effect to make the path of electrical
leakage two times longer than a flexure step difference. That is,
an effect of the shallow trench isolation is enhanced. For example,
in a flash memory device, given a same trench depth in a low
voltage part and a high voltage part of the cell, a trench depth in
a low voltage part of the cell and peripheral regions is relatively
sufficient for device isolation compared with the trench depth in a
high voltage part of the peripheral region. Accordingly, although
it is not necessary to apply a trench having a middle concave
flexure in a low voltage part of the cell and peripheral regions,
applying such a trench is preferable since device isolation is
enhanced.
[0043] Subsequently, the following STI processes are conventionally
performed. In other words, a thermal oxide layer of a sidewall and
a nitride liner are formed. The trench is then filled with a CVD
oxide layer. Remaining CVD oxide layer is removed using a CMP
technique. Generally, an oxide layer of high temperature is also
removed. Additionally, an etch-stop layer pattern used as an
etching mask is removed as well using a wet etching process.
[0044] As explained above, when a trench lacks sufficient depth in
shallow trench isolation, a depth of the trench can be lengthened
in a middle area thereof according to an aspect of the present
invention. Therefore, it is possible to enhance shallow trench
isolation.
[0045] FIG. 8 depicts exemplary cross-sectional views comparing a
deep trench without a flexure 800, and a trench with a flexure made
at a bottom thereof 805 according to an aspect of the present
invention. FIG. 8 illustrates the fact that there is no distance
difference of a leakage path in a deep trench without a flexure and
another trench with a flexure made at a bottom thereof, even though
a width of the trench 805 is less than a width of the trench
800.
[0046] Although illustrative embodiments of the present invention
have been described herein with reference to the accompanying
drawings, it is to be understood that the present invention is not
limited to those precise embodiments, and that various other
changes and modifications may be affected therein by one skilled in
the art without departing from the scope or spirit of the present
invention. All such changes and modifications are intended to be
included within the scope of the invention as defined by the
appended claims.
* * * * *