U.S. patent application number 11/272685 was filed with the patent office on 2007-03-08 for method for producing a memory with high coupling ratio.
Invention is credited to Chung-Yi Chen, Chih-Ping Chung, Hung-Kwei Liao, Chun-Nan Lin.
Application Number | 20070052003 11/272685 |
Document ID | / |
Family ID | 37829254 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070052003 |
Kind Code |
A1 |
Chung; Chih-Ping ; et
al. |
March 8, 2007 |
Method for producing a memory with high coupling ratio
Abstract
A method for producing a memory with high coupling ratio is
provided. First, a shallow trench isolation is formed on a
substrate to define an active area. Second, a spacer is formed at
the sidewall of the shallow trench isolation. Third, the shallow
trench isolation is etched such that the top of the spacer is
higher than the surface of the shallow trench isolation. Fourth, a
tunnel oxide is formed on the active area. Finally, a floating gate
is formed on the tunnel oxide.
Inventors: |
Chung; Chih-Ping; (Linluo
Township, TW) ; Lin; Chun-Nan; (Changhua City,
TW) ; Chen; Chung-Yi; (Jhonghe City, TW) ;
Liao; Hung-Kwei; (Longtan Township, TW) |
Correspondence
Address: |
ROSENBERG, KLEIN & LEE
3458 ELLICOTT CENTER DRIVE-SUITE 101
ELLICOTT CITY
MD
21043
US
|
Family ID: |
37829254 |
Appl. No.: |
11/272685 |
Filed: |
November 15, 2005 |
Current U.S.
Class: |
257/315 ;
257/E21.209; 257/E29.129; 438/257 |
Current CPC
Class: |
H01L 29/40114 20190801;
H01L 29/42324 20130101 |
Class at
Publication: |
257/315 ;
438/257 |
International
Class: |
H01L 21/336 20060101
H01L021/336 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2005 |
TW |
94130391 |
Claims
1. A method for producing a memory cell, the method comprising:
forming a shallow trench isolation in a substrate to define an
active area; forming a spacer at a sidewall of the shallow trench
isolation; etching the shallow trench isolation such that a top of
the spacer is higher than an upper surface of the shallow trench
isolation; forming a tunnel oxide on the active area; and forming a
floating gate on the tunnel oxide.
2. The method for producing a memory cell of claim 1, wherein the
step of forming the spacer comprises: depositing a spacer material
layer on the substrate; and anisotropically etching the spacer
material layer to form the spacer.
3. The method for producing a memory cell of claim 2, wherein the
step of depositing a spacer material layer is performed by a
chemical vapor deposition process or a high temperature oxidation
process.
4. The method for producing a memory cell of claim 2, wherein the
anisotropic etching is a dry etching.
5. The method for producing a memory cell of claim 2, wherein the
spacer material layer is silicon nitride.
6. The method for producing a memory cell of claim 2, wherein the
spacer material layer is silicon oxynitride.
7. The method for producing a memory cell of claim 2, wherein the
spacer material layer is silicon oxide having an etching rate
slower than the shallow trench isolation.
8. The method for producing a memory cell of claim 2, wherein the
spacer material layer is doped polysilicon.
9. The method for producing a memory cell of claim 1, further
comprising: forming a gate dielectric on the floating gate; and
forming a control gate on the gate dielectric.
10. A method for producing a memory, the method comprising: forming
a shallow trench isolation in a substrate; forming a spacer at a
sidewall of the shallow trench isolation, wherein a top of the
spacer is higher than an upper surface of the shallow trench
isolation; forming a tunnel oxide on the substrate; forming a
floating gate on the tunnel oxide and the spacer; forming a gate
dielectric on the floating gate; and forming a control gate on the
gate dielectric.
11. The method for producing a memory of claim 10, wherein the step
of forming the spacer comprises: depositing a spacer material layer
on the substrate; etching the spacer material layer to form the
spacer at the sidewall of the shallow trench isolation; and
lowering a height of the shallow trench isolation such that the top
of the spacer is higher than the upper surface of the shallow
trench isolation
12. The method for producing a memory of claim 11, wherein the step
of lowering the height of the shallow trench isolation comprises
performing a wet etching process to remove the material on the top
of the shallow trench isolation.
13. The method for producing a memory of claim 10, wherein the
material of the spacer is silicon nitride.
14. The method for producing a memory of claim 10, wherein the
material of the spacer is silicon oxide with an etching rate slower
than the material of the shallow trench isolation.
15. The method for producing a memory of claim 10, wherein the step
of forming a shallow trench isolation comprises: forming a shallow
trench in a substrate; forming a liner layer on an inner surface of
the shallow trench; and filling a dielectric material in the
shallow trench.
Description
RELATED APPLICATIONS
[0001] The present application is based on, and claims priority
from, Taiwan Application Serial Number 94130391, filed Sep. 5,
2005, the disclosure of which is hereby incorporated by reference
herein in its entirety.
BACKGROUND
[0002] 1. Field of Invention
[0003] The present invention relates to a method for producing a
memory. More particularly, the present invention relates to a
method for producing a memory with high coupling ratio.
[0004] 2. Description of Related Art
[0005] Memory-related technology is progressing rapidly. Because of
the high market demand for lighter, thinner and smaller products,
flash memory is extensively used and has become a main nonvolatile
memory nowadays. Because the physical size of the memory is
becoming smaller and smaller, the size of each memory cell within
the memory structures also must be made smaller, which results in
decreasing the overlapping area of a floating gate and a control
gate in each memory cell. Therefore, the coupling ratio of the
floating gate and the control gate decreases. Because of low
coupling ratio, the memory requires a higher voltage applied on its
control gate to function. Not only the efficiency of the memory but
also the reliability of the memory becomes less over a long time.
Moreover, the conventional memory has a serious parasitic
transistor effect that is described in detail below.
[0006] FIG. 1 to FIG. 3 show a conventional nonvolatile memory cell
at different steps of the manufacturing process. In FIG. 1, a
shallow trench isolation 106 in a substrate 102 can be used to
define an active area 111 of a memory cell. A pad oxide 103 is
located on the substrate 102. A liner oxide 104 is located around
the shallow trench isolation 106. The pad oxide 103 functions as a
buffer layer between a hard mask and the substrate 102. After the
hard mask is removed, the surface of the shallow trench isolation
106 is apparently higher than that of the substrate 102.
[0007] Reference is made to FIG. 2. A wet etching process is
performed to remove the pad oxide 103 from the substrate 102, and
the upper surface of the shallow trench isolation 106 is lowered to
approximate to the upper surface of the substrate 102. Then, a
tunnel oxide 105 is formed at the same place where the pad oxide
103 originally is. Since the wet etching process used to remove the
pad oxide 103 is isotropic, the side of the shallow trench
isolation 106 is usually etched to form a concave portion 109.
[0008] In FIG. 3, a polysilicon layer is deposited on the substrate
102. Then, lithography and etching processes are performed on the
polysilicon layer to form a floating gate 108. When the polysilicon
layer is deposited on the substrate 102, some polysilicon material
also fills in the concave portion 109 at the side of the shallow
trench isolation 106. The presence of the polysilicon in the
concave portion 109 causes difficulties in performing the
subsequent etching process. Furthermore, there is likely an
electric leakage in the region of the concave portion 109, which
adversely effects the operation and reliability of the memory.
SUMMARY
[0009] It is therefore an aspect of the present invention to
provide a method for producing a memory. The memory has high
coupling ratio, the memory thus can read and write data faster.
[0010] Another aspect of the present invention relates to a method
for producing a memory structure, wherein a concave portion is not
formed on the edge of shallow trench isolation during wet etching
process, and thus electric leakage is reduced.
[0011] In accordance with the foregoing aspects, one embodiment of
the present invention provides a method for producing a memory.
First, a shallow trench isolation is formed on a substrate to
define an active area. Second, a spacer is formed at the sidewall
of the shallow trench isolation. Third, the shallow trench
isolation is etched such that the top of the spacer is higher than
the surface of the shallow trench isolation. Fourth, a tunnel oxide
is formed on the active area. Finally, a floating gate is formed on
the tunnel oxide.
[0012] Since the spacer protects the edge of the shallow trench
isolation from being etched and from being concave portiond in the
etching process, electric leakage does not occur when the memory is
functioning. Furthermore, the fact that the top of the spacer is
higher than the surface of the shallow trench isolation results in
a curvature on the surface of the floating gate when the floating
gate is formed on the substrate. The surface area of the floating
gate is therefore increased, which allows a greater overlapping
area between the floating gate and the control gate and increases
the coupling ratio of the memory structure. Accordingly, the memory
of the present invention can function more efficiently and is more
reliable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention can be more fully understood by reading the
following detailed description of the preferred embodiments, with
reference made to the accompanying drawings as follows:
[0014] FIG. 1 to FIG. 3 are cross-sectional side views of a
conventional nonvolatile memory cell at different steps of the
manufacturing process.
[0015] FIG. 4 to FIG. 8 are cross-sectional side views of a
nonvolatile memory cell at different steps of the manufacturing
process according to a preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] The memory structure described herein is mainly a
nonvolatile memory, particularly a flash memory. A flash memory is
exemplified below to illustrate the characteristics and the concept
of the present invention.
[0017] A flash memory is composed of a plurality of memory cells.
Each of the memory cells is isolated by an isolation structure. In
a preferred embodiment of the present invention, the isolation
structure is a shallow trench isolation. Since each memory cell has
the same structure, the description below only describes the
structure of a single memory cell.
[0018] FIG. 4 is a cross-sectional side view of a memory cell when
shallow trench isolations are formed. A shallow trench isolation
206 located in a substrate 202 is for defining an active area 211
of a memory cell. Furthermore, a pad oxide 203 is on the surface of
the substrate 202 and serves as a buffer layer between a hard mask
and the substrate 202. Around the shallow trench isolation 206 is a
liner layer, e.g. a liner oxide 204. In a preferred embodiment of
the present invention, the substrate 202 comprises silicon. To form
the structure shown in FIG. 4, a pad oxide 203 is first formed on
the substrate 202 by oxidation at high temperature. Next, a silicon
nitride layer is deposited thereon as a hard mask for a subsequent
etching process. Before the etching process is performed, the
pattern on a photoresist is transferred to the hard mask by
lithography and etching. The patterned hard mask is used as an etch
mask in the etching process, so as to form a shallow trench 207 in
the substrate 202. A liner oxide 204 is then formed on the shallow
trench 207. Finally, the shallow trench 207 is filled with an
isolation material, such as silicon oxide, to form a shallow trench
isolation 206. The shallow trench 207 is generally formed by an
anisotropic etching process.
[0019] Reference is made to FIG. 5, which is a cross-sectional side
view showing a structure after forming a spacer at the sidewall of
the shallow trench isolation. First, a spacer material layer is
deposited on the substrate 202. The spacer material layer is then
etched by anisotropic etching to form a spacer 210 at the sidewall
of the shallow trench isolation 206. The material of the spacer
material layer is a dielectric material like silicon nitride,
silicon oxynitride, doped polysilicon, or silicon oxide having an
etching rate slower than the material of the shallow trench
isolation. The deposition method can be, for example, chemical
vapor deposition (CVD) or high temperature oxidation (HTO). As
shown in FIG. 5, the spacer 210 covers the sidewall of the shallow
trench isolation 206, thereby preventing a concave portion from
forming.
[0020] FIG. 6 shows a cross-sectional side view of a memory cell
when forming a tunnel oxide. First, the pad oxide 203 is removed.
At the same place where the pad oxide 203 is originally, a tunnel
oxide 205 is formed. When removing the pad oxide 203, the upper
surface of the shallow trench isolation 206 is lowered approximate
to the upper surface of the substrate 202, and the spacer 210
therefore protrudes from the surface of the substrate 202 and the
shallow trench isolation 206. The tunnel oxide 205 is formed by,
for instance, a thermal oxidation process.
[0021] In a preferred embodiment of the present invention, the
removal of the pad oxide 203 and the reduction of the height of the
shallow trench isolation 206 are performed by isotropic etching,
preferably by a wet etching. The solution for the wet etching can
be hydrofluoric acid, diluted hydrofluoric acid or buffered
hydrofluoric acid, which is determined according to the required
quality and etching rate.
[0022] FIG. 7 is a cross-sectional side view of a memory cell when
forming a floating gate. The material of the floating gate 208 can
be polysilicon, amorphous silicon, silicon nitride or other
materials that can store electrical charges. In this embodiment,
the material of the floating gate is polysilicon. The floating gate
208 is formed by depositing polysilicon and subsequently performing
lithography and etching processes.
[0023] FIG. 8 shows a cross-sectional side view of a memory cell
when forming a gate dielectric and a control gate. In a preferred
embodiment, the gate dielectric 212 is formed on the floating gate
208. Preferably, the gate dielectric is silicon oxide/silicon
nitride/silicon oxide, and the material of the control gate is
polysilicon. Finally, a control gate 214 is formed by depositing
polysilicon on the substrate 202 and then performing lithography
and etching processes.
[0024] In another embodiment of the present invention, the memory
cell is formed by sequentially depositing a polysilicon layer, a
gate dielectric layer and a polysilicon layer, and then performing
lithography and etching processes to define the floating gate 208,
the gate dielectric 212, and the control gate 214.
[0025] The term "coupling ratio" is used to indicate the
overlapping area of the control gate 214 with the floating gate
208. As the coupling ratio increases, the operation efficiency of
memory is improved and the memory can be erased at a higher speed.
Generally, the coupling ratio can be increased by the thickness
reduction of the gate dielectric 212. However, increasing the
coupling ratio by this method is limited since the thickness of the
gate dielectric 212 generally ranges from a minimum of 80 .ANG. to
90 .ANG..
[0026] In the memory structure of the preferred embodiment, the top
of the spacer 210 is higher than the upper surface of the shallow
trench isolation 206. When the floating gate 208 is formed on the
spacer, the surface of the floating gate 208 is rippled and thus
the surface areas of the floating gate 208, the gate dielectric 212
and the control gate 214 thereon are also increased. Therefore, the
overlapping area between the floating gate 208 and the control gate
214 is increased, and the coupling ratio is increased.
[0027] Accordingly, the present invention has the following
advantages.
[0028] (1) The memory of the embodiment comprises a spacer to
protect the edge of the shallow trench isolation so that a problem
of corner oxide thinning that usually occurs in the concave portion
at the sidewall of the shallow trench isolation can be avoided.
Also, no unwanted materials are left in the concave portion, and
thus electric leakage is reduced.
[0029] (2) The memory of the embodiment has high coupling ratio,
and thus has better efficiency and higher reliability.
[0030] The preferred embodiments of the present invention described
above should not be regarded as limitations to the present
invention. It will be apparent to those skilled in the art that
various modifications and variations can be made to the present
invention without departing from the scope or spirit of the
invention. The scope of the present invention is as defined in the
appended claims.
* * * * *