U.S. patent application number 11/210524 was filed with the patent office on 2005-12-22 for three dimensional flash cell.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Jones, Lyle, Lindsay, Roger W..
Application Number | 20050280071 11/210524 |
Document ID | / |
Family ID | 30770190 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050280071 |
Kind Code |
A1 |
Lindsay, Roger W. ; et
al. |
December 22, 2005 |
Three dimensional flash cell
Abstract
A floating gate memory cell includes isolation regions between
adjacent cells, and a staggered pattern of columns of cells. Word
lines are formed parallel to control gate structures.
Inventors: |
Lindsay, Roger W.; (Boise,
ID) ; Jones, Lyle; (Boise, ID) |
Correspondence
Address: |
LEFFERT JAY & POLGLAZE, P.A.
P.O. BOX 581009
MINNEAPOLIS
MN
55458-1009
US
|
Assignee: |
Micron Technology, Inc.
|
Family ID: |
30770190 |
Appl. No.: |
11/210524 |
Filed: |
August 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11210524 |
Aug 24, 2005 |
|
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10205977 |
Jul 26, 2002 |
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Current U.S.
Class: |
257/314 ;
257/316; 257/E21.693; 257/E27.103; 257/E29.302 |
Current CPC
Class: |
H01L 27/11556 20130101;
H01L 27/115 20130101; H01L 29/7881 20130101 |
Class at
Publication: |
257/314 ;
257/316 |
International
Class: |
H01L 029/788 |
Claims
What is claimed:
1. A flash memory device comprising: an array of floating gate
memory cells arranged in rows having a staggered pattern of cell
pairs between adjacent columns of the device; and control circuitry
to read, write and erase the floating gate memory cells, each
memory cell comprising: a substrate; source and drain regions
located in the substrate and vertically spaced apart to define a
channel region; a floating gate located adjacent the channel
region, the floating gate extending substantially vertically; and a
control gate located adjacent to the floating gate.
2. The flash memory device of claim 1 wherein the floating gate
memory cells are formed on pillars, with two memory cells on each
pillar on opposite sides thereof.
3. The flash memory device of claim 1 and further comprising: a
plurality of word lines extending in a horizontal direction
substantially parallel to the control gates.
4. The flash memory device of claim 1 and further including
isolation regions formed between each row of cells.
5. The flash memory device of claim 1 and further including a
dielectric layer formed over the array of floating gate memory
cells with metal contacts formed through the dielectric layer to
the drain regions of each memory cell.
6. The flash memory device of claim 3 and further including a bit
line formed perpendicular to each word line, each bit line
contacting a drain region.
7. The flash memory device of claim 1 and further including an
oxide-nitride-oxide layer between each floating gate and its
respective control gate.
8. The flash memory device of claim 1 and further including an
oxide layer between each floating gate and its respective control
gate.
9. A flash memory device comprising: an array of floating gate
memory cells arranged in rows having a staggered pattern of cell
pairs between adjacent columns of the device; and control circuitry
to read, write and erase the floating gate memory cells, each
memory cell comprising: a silicon substrate having a plurality of
pillars; a drain region and a plurality of source regions located
in each pillar, each drain region vertically spaced apart from the
source regions to define channel regions therebetween; a floating
gate located adjacent each channel region, the floating gate
extending substantially vertically; and a control gate located
adjacent to the floating gate.
10. The flash memory device of claim 9 wherein the floating gate is
isolated from the pillar by a tunnel insulator.
11. The flash memory device of claim 9 wherein the floating gate is
isolated from the control gate by a gate insulator.
12. The flash memory device of claim 11 wherein the gate insulator
is comprised of one of an oxide material or an oxide-nitride-oxide
layer.
13. The flash memory device of claim 10 wherein the tunnel
insulator is comprised of an oxide material.
14. The flash memory device of claim 9 and further including a
dielectric layer formed over the array such that metal contacts can
be formed through the dielectric layer to couple to each drain
region.
15. The flash memory device of claim 14 wherein the dielectric
layer is comprised of one of boro-phospho-silicate glass or
phospho-silicate glass.
16. A flash memory device comprising: address circuitry for
receiving memory addresses; input/output circuitry for transmitting
and receiving data; control circuitry for controlling operations of
the memory device; and an array of floating gate memory cells
arranged in rows having a staggered pattern of cell pairs between
adjacent columns of the device, each memory cell comprising: a
silicon substrate having a plurality of pillars, each pillar
comprising a pair of cells; a drain region and a plurality of
source regions located in each pillar, each drain region vertically
spaced apart from the source regions to define channel regions
therebetween; a floating gate located adjacent each channel region,
the floating gate extending substantially vertically and isolated
on either side by an insulator material; and a control gate located
adjacent to the floating gate.
17. The flash memory device of claim 16 and further including a
metal contact formed through a dielectric layer and coupled to each
drain region to couple drain regions of adjacent pillars.
18. The flash memory device of claim 16 wherein the insulator
material is an oxide.
19. The flash memory device of claim 16 wherein the control gate is
comprised of polysilicon.
20. The flash memory device of claim 16 and further including a
self-aligned, silicide word line coupled to the control gate.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/205,977 (published) filed Jul. 26, 2002 and titled,
"THREE DIMENSIONAL FLASH CELL," which is commonly assigned and
incorporated by reference in its entirety herein.
FIELD
[0002] The present invention relates generally to memory devices
and in particular the present invention relates to non-volatile
memory cells.
BACKGROUND
[0003] Memory devices are available in a variety of styles and
sizes. Some memory devices are volatile in nature and cannot retain
data without an active power supply. A typical volatile memory is a
DRAM which includes memory cells formed as capacitors. A charge, or
lack of charge, on the capacitors indicate a binary state of data
stored in the memory cell. Dynamic memory devices require more
effort to retain data than non-volatile memories, but are typically
faster to read and write.
[0004] Non-volatile memory devices are also available in different
configurations. For example, floating gate memory devices are
non-volatile memories that use floating gate transistors to store
data. The data is written to the memory cells by changing a
threshold voltage of the transistor and is retained when the power
is removed. The transistors can be erased to restore the threshold
voltage of the transistor. The memory may be arranged in erase
blocks where all of the memory cells in an erase block are erased
at one time. These non-volatile memory devices are commonly
referred to as flash memories.
[0005] The non-volatile memory cells are fabricated as floating
gate memory cells and include a source region and a drain region
that is laterally spaced apart from the source region to form an
intermediate channel region. The source and drain regions are
formed in a common horizontal plane of a silicon substrate. A
floating gate, typically made of doped polysilicon, is disposed
over the channel region and is electrically isolated from the other
cell elements by oxide. For example, gate oxide can be formed
between the floating gate and the channel region. A control gate is
located over the floating gate and can also made of doped
polysilicon. The control gate is electrically separated from the
floating gate by another dielectric layer. Thus, the floating gate
is "floating" in dielectric so that it is insulated from both the
channel and the control gate.
[0006] As semiconductor devices get smaller in size, designers are
faced with problems associated with the production of memory cells
that consume a small enough amount of surface area to meet design
criteria, yet maintain sufficient performance in spite of this
smaller size.
[0007] For the reasons stated above, and for other reasons stated
below which will become apparent to those skilled in the art upon
reading and understanding the present specification, there is a
need in the art for an improved non-volatile memory cell.
SUMMARY
[0008] The above-mentioned problems with non-volatile memory cells
and other problems are addressed by the present invention and will
be understood by reading and studying the following
specification.
[0009] In one embodiment, a memory cell includes a substrate,
source and drain regions located in the substrate and vertically
spaced apart to define a channel region, a floating gate located
adjacent the channel region, the floating gate extending
substantially vertically, and a control gate located adjacent to
the floating gate. The control gate is coupled to a word line, the
word line extending substantially parallel to the control gate in a
horizontal direction.
[0010] In another embodiment, a memory cell includes a substrate
having a pillar of semiconductive material, two source regions and
a drain region formed in the substrate, the source and drain
regions substantially vertically separated, and defining two
channels, one between the first source and the drain and the second
between the second source and the drain. A floating gate structure
is adjacent each channel, formed in a substantially vertical
orientation, and a control gate structure is adjacent each floating
gate, also formed in a substantially vertical orientation.
[0011] In yet another embodiment, an array of memory cells includes
rows of cells, each cell isolated from an adjacent cell by an
isolation region within its row, each row of cells having its cells
staggered from cells in adjacent rows. Each cell includes a
substrate, source and drain regions located in the substrate and
vertically spaced apart to define a channel region, a floating gate
located adjacent the channel region, the floating gate extending
substantially vertically, and a control gate located adjacent to
the floating gate. A number of word lines each contact a number of
control gates in a row of cells, and each word line is
substantially parallel to the control gates.
[0012] In still another embodiment, an array of memory cells
includes rows of memory cell pillars, each pillar having a pair of
memory cells formed thereon, and each memory cell pillar isolated
from an adjacent memory cell pillar by an isolation region within
its row. Each row of pillars has its pillars staggered from pillars
of adjacent rows of pillars to stagger adjacent memory cells. Each
memory cell pillar includes a substrate, two source regions and one
drain region, the drain region vertically spaced apart from the
source regions to define two separate channel regions, a floating
gate located adjacent each channel region, each floating gate
extending substantially vertically, and a control gate located
adjacent to each floating gate.
[0013] In yet another embodiment, a method of fabricating a
floating gate transistor includes fabricating a pillar of substrate
material, fabricating vertically spaced source and drain regions to
define a channel therebetween, and fabricating a first layer of
oxide over the channel. A substantially vertically configured
semiconductive floating gate structure is fabricated over the first
layer of oxide and adjacent to the channel, and a second oxide
layer is fabricated over the floating gate structure. A control
gate structure is formed over the second layer of oxide, and a
horizontally extending word line is formed, coupled to the control
gate structure, the word line contact extending substantially
parallel to the control gate structure.
[0014] Other embodiments are described and claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a cross section view of a transistor according to
one embodiment of the present invention;
[0016] FIG. 2 is a plan view of an array of memory cells according
to another embodiment of the present invention;
[0017] FIG. 2A is a plan view of an array of memory cells according
to yet another embodiment of the present invention;
[0018] FIG. 3 is a cross section view of an in-process transistor
according to one embodiment of the present invention;
[0019] FIG. 4 is a plan view of the embodiment of FIG. 3;
[0020] FIG. 5 is a cross section view of an in-process transistor
according to one embodiment of the present invention;
[0021] FIG. 6 is a cross section view of an in-process transistor
according to one embodiment of the present invention;
[0022] FIG. 7 is a cross section view of an in-process transistor
according to one embodiment of the present invention; and
[0023] FIG. 8 is a block diagram of a memory device according to
another embodiment of the present invention.
DETAILED DESCRIPTION
[0024] In the following detailed description of the invention,
reference is made to the accompanying drawings that form a part
hereof, and in which is shown, by way of illustration, specific
embodiments in which the invention may be practiced. In the
drawings, like numerals describe substantially similar components
throughout the several views. These embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention. Other embodiments may be utilized and structural,
logical, and electrical changes may be made without departing from
the scope of the present invention.
[0025] To aid in interpretation of the description of the
illustrations and claims that follow, the term "semiconductor
substrate" is defined to mean any construction comprising
semiconductive material, including, but not limited to, bulk
semiconductive materials such as a semiconductor wafer (either
alone or in assemblies comprising other materials thereon) and
semiconductive material layers (either alone or in assemblies
comprising other materials). The term "substrate" refers to any
supporting structure, including, but not limited to, the
semiconductor substrates described above. The term substrate is
also used to refer to semiconductor structures during processing,
and may include other layers that have been fabricated thereupon.
Both wafer and substrate include doped and undoped semiconductors,
epitaxial semiconductor layers supported by a base semiconductor or
insulator, as well as other semiconductor structures well known to
one skilled in the art.
[0026] In addition, as the structures formed by embodiments in
accordance with the present invention are described herein, common
semiconductor terminology such as N-type, P-type, N+ and P+ will be
employed to describe the type of conductivity doping used for the
various structures or regions being described. The specific levels
of doping are not believed to be germane to embodiments of the
present invention; thus, it will be understood that while specific
dopant species and concentrations are not mentioned, an appropriate
dopant species with an appropriate concentration to its purpose, is
employed.
[0027] The term conductor is understood to also include
semiconductors, and the term insulator is defined to include any
material that is less electrically conductive than the materials
referred to as conductors. The following detailed description is,
therefore, not to be taken in a limiting sense, and the scope of
the present invention is defined only by the appended claims, along
with the full scope of equivalents to which such claims are
entitled.
[0028] Finally, it will be understood that the number, relative
size and spacing of the structures depicted in the accompanying
figures are exemplary only, and thus were selected for ease of
explanation and understanding. Therefore such representations are
not indicative of the actual number or relative size and spacing of
an operative embodiment in accordance with the present
invention.
[0029] Non-volatile memory cells, as explained above, can be formed
as floating gate transistors. While the area of the transistors can
be reduced, the coupling ratio between the floating gate and the
control gate is also reduced. Generally, as the gate-coupling ratio
between the floating gate and the control gate decreases, the work
voltage necessary to operate the memory transistor increases. As a
consequence, the operational speed and efficiency of the flash
memory decrease tremendously.
[0030] Some methods for increasing the gate-coupling ratio include:
increasing the overlapped area between the floating and the control
gate, reducing the thickness of the dielectric layer between the
floating gate and the control gate, and increasing the dielectric
constant (k) of the dielectric layer between the floating gate and
the control gate. Generally, to achieve an increase in the
overlapped area between the floating and control gates and thus
increase the gate-coupling ratio, the size of the floating gate has
to be increased. However, this is not desirable for the demands of
today's highly-integrated technologies.
[0031] Embodiments of the present invention provide a floating gate
transistor that can occupy less memory die area while maintaining a
large gate-coupling ratio. As explained below, the floating and
control gates are formed substantially in a vertical direction.
[0032] Referring to FIG. 1, a cross section of a floating gate
memory cell structure 100 is shown in detail. Memory cell structure
100 comprises source regions 102, drain 104, floating gates 106,
and control gates 108. The drain region 104 is formed in a pillar
of silicon 110 near its top. The source regions 102 are diffused
near the bottom of the pillar. The region between each source
region 102 and the drain region 104 defines a channel. In this
manner, two source regions are associated with the same drain
region in each of the pillars. Isolation regions 112 isolate cells
100 as shown in greater detail in FIG. 2. Different word lines 114,
described in greater detail herein, access different source regions
through the same drain region, so there are two memory cells on
each pillar.
[0033] Bit lines 116 can then be formed to connect the drain
contacts in a direction perpendicular to the word lines 114. This
is accomplished in one embodiment by depositing a layer of
dielectric 115 over drains 104. This dielectric layer may comprise
boro-phospho-silicate glass (BPSG) or phospho-silicate glass (PSG).
Alternatively, other low dielectric constant materials may be used.
Contact openings are etched through the dielectric layer 115 to the
drains 104, and contacts 118 are formed in the openings. Once the
contacts are formed, a metal layer bit line 116 is formed to join
the drain contacts.
[0034] One embodiment 200 of an array of memory cells such as cells
100 is shown in plan view in FIG. 2. Word lines 114 run in the X
direction, and contact multiple control gates of the various cells
100. Isolation regions 112 isolate transistors 100 in the X
direction. For this description, rows of the array are shown
extending in the X direction of FIG. 2, and columns of the array
are shown extending in the Y direction of the FIG. 2. Each row of
transistors in the array 200 has its cells 100 staggered from the
cells 100 in adjacent rows, forming a staggered pattern of
transistors.
[0035] FIG. 2A shows a plan view of an array 200 with bit lines 116
(shown in greater detail in FIG. 1), which are formed to connect
the drain contacts of pillars in every other row in a direction
perpendicular to the word lines. Each column of memory cells
comprises two transistors per pillar, every other row, as is best
shown in FIG. 2 and FIG. 2A.
[0036] As can be seen in the figures, and especially FIG. 2, each
control gate structure extends along a length in the X direction of
the trench spanning multiple transistor pillars, so that one
control gate structure is associated with multiple floating gates
on alternating adjacent columns of transistors, and therefore is
associated with multiple transistors and memory cells. Each word
line 114 in FIG. 2 contacts alternating cells 100 in adjacent
columns of the array 200. Two different word lines therefore
provide word line contact for the two cells on each pillar. The
cells share a common bit line Addressing with the same bit line but
a different word line allows access to each of the two cells on
each pillar.
[0037] The features of one embodiment of the present invention can
be described in greater detail with reference to a method of
fabricating the transistors. The method is described in sufficient
detail to provide an understanding of one method to form
transistors of the present invention. It will be understood by
those skilled in the art that all process steps are not described
in detail herein, and that extra steps or modifications of the
steps may be needed depending upon the integrated circuit design
and manufacturing equipment.
[0038] FIG. 3 is a cross section of a semiconductor substrate that
has been patterned and etched in an array pattern as shown in the
plan view of FIG. 4. Starting with a semiconductor substrate,
rectangles 402 are etched in the substrate for field isolation, by
patterning deposited nitride 303 on the substrate, and etching into
the silicon a plurality of openings 302 in the desired pattern. The
openings are filled with a dielectric 502, seen in cross section in
FIG. 5, in one embodiment in a high density plasma (HDP) deposition
process. Chemical-mechanical planarization (CMP) follows to create
a uniform top surface.
[0039] The nitride layer 303 is masked and etched to form memory
core trenches 504 between the shallow trench isolation regions 502
as shown in cross section in FIG. 5. As a result, pillars or
islands 506 of substrate material are created. Source region
implants are performed to create diffused source regions 102 near
the base of the islands 506.
[0040] Referring to FIG. 6, a tunnel oxide 602 is deposited in the
memory core trenches to create an isolation of the to be deposited
floating gate structures. Following the deposition of the tunnel
oxide, a floating gate polysilicon layer (poly 1) is deposited,
isolated from the silicon by the tunnel oxide. Another CMP process
is performed to the nitride layer 303 to create a uniform top
surface once again. An etch resistive material such as photoresist
is deposited and patterned, and the exposed polysilicon is etched
to leave the floating gate structures 604 for two transistors shown
in the cross section in FIG. 6. The etch leaves openings 606
between the floating gate structures and the isolation regions
112.
[0041] Referring to FIG. 7, an oxide layer such as an
oxide-nitride-oxide (ONO) layer 702 is then deposited in the
openings 606 to insulate the floating gate structures 604. If
source regions have not been formed in an earlier process, they are
implanted before the ONO layer is deposited. A second polysilicon
layer (poly 2) is deposited to form the control gate structures
704, and another CMP process is performed down to the nitride
layer. The control gate structures 704 extend along multiple
transistor pillars, and control multiple transistors in alternating
columns. A self-aligned silicide layer 706 is deposited on the top
of the control gate structures 704 in the X direction of the array
to form word lines such as those shown in greater detail in FIG. 2.
Once the word lines are in place, a nitride strip operation removes
the nitride layer 303, leaving drain contact areas 204 as shown in
greater detail in FIG. 2. An ion implantation is effected to create
drain regions near the tops of the pillars. Each drain region
serves as the drain for two different source regions accessible by
the same bit line but different word lines.
[0042] Normal back end of line processing is used to form drain
contacts such as those shown in FIG. 1. In one embodiment, a
dielectric layer 115 is formed over the drain contact areas. This
dielectric layer may comprise boro-phospho-silicate glass (BPSG) or
phospho-silicate glass (PSG). Alternatively, other low dielectric
constant materials may be used. Contact openings are etched through
the dielectric layer to the drain contact areas, and contacts are
formed in the openings. Once the contacts are formed, a metal layer
is formed to join the drain contacts.
[0043] FIG. 8 is a functional block diagram of a memory device 800,
of one embodiment of the present invention, which is coupled to a
processor 810. The memory device 800 and the processor 810 may form
part of an electronic system 820. The memory device 800 has been
simplified to focus on features of the memory that are helpful in
understanding the present invention. The memory device includes an
array of memory cells 830. The memory cells are non-volatile
floating-gate memory cells with vertical floating gates as
described above. The memory array 830 is arranged in banks of rows
and columns.
[0044] An address buffer circuit 840 is provided to latch address
signals provided on address input connections A0-Ax 842. Address
signals are received and decoded by row decoder 844 and a column
decoder 846 to access the memory array 830. It will be appreciated
by those skilled in the art, with the benefit of the present
description, that the number of address input connections depends
upon the density and architecture of the memory array. That is, the
number of addresses increases with both increased memory cell
counts and increased bank and block counts.
[0045] The memory device reads data in the array 830 by sensing
voltage or current changes in the memory array columns using
sense/latch circuitry 850. The sense/latch circuitry, in one
embodiment, is coupled to read and latch a row of data from the
memory array. Data input and output buffer circuitry 860 is
included for bi-directional data communication over a plurality of
data (DQ) connections 862 with the processor 810.
[0046] Command control circuit 870 decodes signals provided on
control connections 872 from the processor 810. These signals are
used to control the operations on the memory array 830, including
data read, data write, and erase operations. The flash memory
device has been simplified to facilitate a basic understanding of
the features of the memory. A more detailed understanding of
internal circuitry and functions of flash memories are known to
those skilled in the art.
[0047] The various embodiments of the present invention provide a
three dimensional transistor and memory cell structure with
isolation between adjacent memory cells and in a staggered pattern
to allow closer packing of memory cells. Further, the control gates
of the present embodiments are self aligned because they are formed
in the memory core trenches.
[0048] The various embodiments of the present invention are
amenable to use with periphery gates on or near the edges of memory
arrays, especially those memory arrays using U shaped periphery
transistors.
CONCLUSION
[0049] A floating gate memory cell has been described that includes
isolation regions between adjacent cells, and a staggered pattern
of columns of cells, to allow closer packing of cells into a memory
array or the like. Two cells are placed on one pillar of material
in one embodiment.
[0050] The transistor allows the die real estate occupied by the
transistor to be reduced while maintaining the coupling area
between the floating and control gates. The transistor can be used
in non-volatile memory devices, such as flash memory.
[0051] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement, which is calculated to achieve the
same purpose, may be substituted for the specific embodiment shown.
This application is intended to cover any adaptations or variations
of the present invention. Therefore, it is manifestly intended that
this invention be limited only by the claims and the equivalents
thereof.
* * * * *