U.S. patent application number 13/479069 was filed with the patent office on 2012-09-13 for highly reliable nand flash memory using a five side enclosed floating gate storage elements.
Invention is credited to Mammen Thomas.
Application Number | 20120228693 13/479069 |
Document ID | / |
Family ID | 39100569 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120228693 |
Kind Code |
A1 |
Thomas; Mammen |
September 13, 2012 |
Highly Reliable NAND Flash memory using a five side enclosed
Floating gate storage elements
Abstract
A NAND flash memory system with an array of individual charge
storage elements, such as floating gates, arranged in a NAND
string, each element being capable of selectively storing data in
the form of charge there-in during a program or an erase operation,
and during a read operation sensing the quantum of charge stored to
provide reconstruction of data. Such a memory made with a floating
gate that is spaced away from the diffusions and covered on all
five sides except the channel side, by the control gate, there by
having increased coupling with the associated advantage of lower
high voltages, reduced impact of the unwanted disturb conditions,
and providing for improved retention and reliability
characteristics at higher operating temperatures is disclosed. The
main emphasis in this technology is to provide a device with
improved retention, endurance, and temperature characteristics
meeting the Automotive specifications even with some area
penalty.
Inventors: |
Thomas; Mammen; (Hercules,
CA) |
Family ID: |
39100569 |
Appl. No.: |
13/479069 |
Filed: |
May 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12082199 |
Apr 10, 2008 |
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13479069 |
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11506026 |
Aug 18, 2006 |
7376014 |
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12082199 |
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Current U.S.
Class: |
257/321 ;
257/314; 257/E29.255; 257/E29.3 |
Current CPC
Class: |
H01L 27/115 20130101;
H01L 27/11524 20130101; H01L 29/42324 20130101; H01L 27/11521
20130101 |
Class at
Publication: |
257/321 ;
257/314; 257/E29.3; 257/E29.255 |
International
Class: |
H01L 29/788 20060101
H01L029/788; H01L 29/78 20060101 H01L029/78 |
Claims
1. A non-Volatile memory cell storage element that is programmed
and erased by FN Tunneling comprising: a. a first connect diffusion
as a source diffusion; b. a second connect diffusion as a drain
diffusion c. a storage element comprising: i. a channel region
between the source and drain diffusions; ii. a tunnel dielectric
region over a channel region; iii. two high voltage dielectric
regions on either side adjacent the tunnel dielectric region over
the channel region; iv. a charge storage floating gate deposed over
the tunnel dielectric region; v. a dielectric layer covering the
floating gate; vi. a conductive control gate deposed over the two
high voltage dielectric regions and covering all five sides of the
floating gate but separated from it by the dielectric layer
covering the floating gate.
2. A non-Volatile NAND Memory cell that is programmed and erased by
FN Tunneling comprising: a. a drain diffusion; b. a drain select
device adjacent the drain diffusion; c. a first storage element; d.
a first connect diffusion acting as a source of the drain select
device and a drain for the first storage element; e. a source
diffusion; f. a source select device adjacent the source diffusion;
g. a last storage element; h. a last connect diffusion acting as a
drain of the source select device and a source for the last storage
element; i. a number of storage elements in series with the first
and the last storage elements, separated from and interconnected to
its neighbor by connect diffusions.
3. A non-Volatile memory cell storage element that is programmed
and erased by FN Tunneling comprising: a. a first connect diffusion
as a source diffusion; b. a second connect diffusion as a drain
diffusion c. a storage element comprising: i. a channel region
between the source and drain diffusions; ii. a tunnel dielectric
region over a channel region; iii. two high voltage dielectric
regions on either side adjacent the tunnel dielectric region; iv. a
charge storage floating gate deposed over the tunnel dielectric
region; v. a dielectric layer covering the floating gate; vi. a
conductive control gate deposed over the two high voltage
dielectric regions and covering all five sides of the floating gate
but separated from it by the dielectric layer covering the floating
gate; vii. the conductive control gate overlaying the high voltage
dielectric and coupling to part of the connect diffusions on either
side of the channel.
Description
CONTINUATION INFORMATION
[0001] This application is a continuation of the filed application
"Highly Reliable NAND Flash memory using a five side enclosed
Floating gate storage elements" having the application Ser. No.
12/082,199 filed on Apr. 10, 2008 which is a continuation of
application Ser. No. 11/506,026 filed on Aug. 18, 2006 named
"Highly Reliable NAND Flash memory using a five side enclosed
Floating gate storage elements" issued as U.S. Pat. No.
7,376,014.
FIELD OF INVENTION
[0002] This invention relates to high density Non-Volatile
Semiconductor memories of the Electrically erasable and
programmable Read Only memory type particularly to high reliability
NAND and related memory cells and arrays having wide temperature
range of operation.
BACKGROUND OF INVENTION
[0003] A number of Flash Memory products exist today. They can be
divided into two categories, the high density components, mainly
used for mass storage, and the high speed products, used for
operational data storage. The Non-volatile field can be hence
sub-divided in terms of the technology as the NOR implementation
which is mainly good for very fast random access with execute in
place capability, and the NAND implementation which is typically
accessed in a serial mode, for program and read of large chunks of
data at a time, that is suitable for high density storage. Today
the NOR and NAND memories share the market, with the NOR being
larger in volume. The NAND memories are denser and the market for
it has been growing at a faster pace than any other types of
memories. Related Memories include architectural variations on the
NAND memory interconnections for program and access using existing
cell structures. This disclosure is a storage element and cell
structure that can be used under different architectures by the
people conversant with NAND and NOR memories, but it will be
described using the well known and standard NAND structure for
simplicity and understanding.
[0004] The NAND implementation is characterized by its capability
to store and move large chunks of data which can be accessed using
a serial interface. In a typical NAND cell, area per storage bit is
smaller than that for the NOR cell, as the NAND consists of
multiple storage elements connected in series in the form of a NAND
string which is in series with a drain select gate and a source
select gate between two contacts. Typical number of storage
elements in the NAND string has increased to 32 or more elements
per cell. These elements in a cell are isolated from the rest of
the cells that are connected to a bit line and the common source
connection by The two select devices one at the drain end (drain
select), between the drain contact and its nearest (the first)
storage element, and another at the source end (source select)
between the source contact and its nearest (the last) storage
element in the string The interlink between the select devices and
the storage elements and between the storage elements in a string
is by a connective diffusion (the connect diffusion).
[0005] The program and erase functions of a typical NAND structure
is by FN Tunneling between the floating gate and the memory well
using voltages of 18 to 22V. This requires that devices that can
withstand High voltage be used, in the peripheral circutry. The use
of these high voltage devices, impact the speeds and make the
devices slow. They also are large devices to prevent punch through
with larger spacing between devices to avoid breakdown and leakage,
thereby making the chip larger and more costly to manufacture.
PRIOR ART
[0006] FIG. 1 shows a typical prior art NAND memory structure. B1
to B4 are memory elements connected in series in a NAND string. A1
is a drain select device and A2 is a source select device. The
drain diffusion (2) with the drain contact is separated from the
nearest (first) memory element B1 by the Drain select device A1.
Similarly the source diffusion (2x) and source connection are
separated from the nearest (last) memory element B4, by the Source
select device A2. The connect diffusions (7) link the memory
element to the Source select and Drain select devices and to each
other to provide an integrated series conductive path from source
to drain in the cell when the select devices and the memory
elements are in the "on" condition. The typical NAND cell in FIG. 1
is formed in a P-well (1) and comprise of N-channel elements and
devices. The memory element of the prior art NAND cell is typically
a channel region (8) controlled by the storage element (Bn), where
`n` is the number of the storage element (1 to 4) in FIG. 1. The
storage element comprise a stack etched element comprising of the
floating gate poly silicon (12) separated from the channel (8) by a
thin tunnel oxide (10). A control gate (14) that also acts as a
word line over lays the floating gate and couples to it through an
inter-poly dielectric or protection oxide (13), as is well known
and used in the Non-volatile memory cells. The NAND (string or
series) connected storage elements B1 to B4 in FIG. 1 are isolated
from the word line drain contact diffusion (2) and source line
diffusion (2x) by the drain select device A1, and the source select
device A2. A1 isolates the first connect diffusion (7) from the
drain (2) by having a select channel (4) controlled by poly-silicon
select gate (6) over laying a gate oxide (5). Similarly A2 isolates
the last connect diffusion (7) from the source (2x) by having a
select channel (4x) controlled by poly-silicon select gate (6x)
over laying a gate oxide (5x). An integrated and interconnected
channel can be formed by the diffusions (7) the channel regions (8)
under the storage elements (B1 to B4), the channel regions under
the select devices (4 and 4x) connecting the drain diffusion (2) to
source diffusion (2x) in each storage cell.
[0007] In a memory, the cells are arranged in columns, each column
having a bit line that connects all the drains of the cells in a
column together. The source is also connected together in a similar
fashion, typically be a diffusion running in a direction
perpendicular to the bit line and parallel to the word direction.
The columns are then arranged together to form an array as will be
well known to those who are involved in the art of memory. Every
row of elements in an array is interconnected by the control gate
that form the word line. The drain select lines and the source
select lines are also interconnected in the row direction in the
array providing selection capability to a row of cells in the
array. The formation of the array and its various interconnection
methods are well known to practitioners in the industry. NAND
arrays are erased as a block of cells, that is, a number of pages
of elements in one or more rows of cells. In a typical NAND array
each column of cells has one or more data registers attached to the
bit line. The data is loaded in serial fashion into, and also read
from, these data registers. The data is then transferred in
parallel to the set of elements selected (a typical page of data at
a time) during program. Similarly data from a set of elements
selected for read are transferred in parallel into the registers
for serial read out during read operation.
[0008] A typical memory array (referred at times as bulk) comprise
of all blocks of memory where a typical block comprise a group of
cells with elements distributed and connected along the rows by the
word lines. Typically each word line interconnects elements in a
row to form a page of memory. Blocks can be one or more cells in
depth along the column though a typical block is a cell in depth.
The bulk is the entire memory array comprising of all the cell
blocks. It should be noted that there are different ways of
interconnecting the elements and cells in an array. Those who are
conversant with memories and their architectures will know the
different interconnection modes within the array.
[0009] The typical write operation of a prior art NAND cell,
comprise both the erase operation where by the electron charge is
removed, are by Fouler-Nordheim Tunneling (FN Tunneling) in a block
or bulk mode, and the program operation, where by electrons are
stored in the floating gate. The first operation is typically the
erase operation when the electron charge in the floating gates of
the storage elements are removed. This is done by generating a
voltage gradient of sufficient magnitude across the tunnel oxides
(10) of the storage elements to remove electrons from the floating
gate (12), by high voltage gradient of the right polarity applied
between the control gate (14), that will be coupled down to the
floating gate from the control gate, and the well (1) to appear
across the floating gate oxide. The necessary field across the
tunnel oxide to achieve erase can be induced in three ways: 1) by
applying a high negative voltage (-Vpp) to the control gate with
the well at `0`V, 2) by applying a lower negative voltage (approx.
-1/2Vpp) to the control gate and applying a positive voltage
(approx. 1/2Vpp) to the well, or 3) by applying a high positive
voltage (Vpp) to the well while keeping the control gates at `0`
potential. Of these the most common is to apply high voltage to the
well with control gate at `0` potential for erase. This mode of
erase is non selective and the memory cells are erased as a block
(group of cells in a row or multiple rows in a specific well) or
bulk (all cells on chip), based on the applied voltage on the
control gate and the well.
[0010] Typically program is the selective operation and it is done
on a page (all storage elements along a row or word line) level.
The data which may be `1` or `0` is loaded into a set of registers
serially and then presented to the cells through the bit lines, the
lines connecting the drains of all the devices in a column of the
array. Typically a self boost scheme is employed to write a `1`,
that is leave the selected storage element, on the selected word
line in the erased state while a `0` allows programming of the
selected storage element.
[0011] Assume that the data register is loaded with a `0`
indicating that the element in the cell needs to get programmed.
The `0` loaded in the register keeps the bit line connected to it
and hence the drain diffusions (2) in the column of cells connected
to the bit line at ground `0` potential. The gate (6) of the Drain
select transistor (A1) of the selected row of cells is pulled to
value approx. equal to Vdd, during write, turning on the drain
select device. The source select transistor (A2) is kept in the off
condition by keeping the gate (6x) of that device at ground or `0`
potential. The unselected word lines in the selected cell are kept
at an intermediate high voltage close to half the programming
voltage (Vpp/2). This turns on the channel (8) of unselected
storage elements by the application of the voltage (1/2Vpp) to the
control gates (14) of the storage elements connected to the
unselected word lines. The coupled down voltage to the floating
gates (12) of the unselected storage elements, is not sufficient to
cause tunneling across the tunnel oxide (10) between the floating
gate and the channel (8), even though the channel is at ground
potential. The selected word line is pulled up to the full
programming voltage (Vpp). This provides the high voltage (Vpp) to
the control gate (14) of the selected storage element. This voltage
applied is sufficient, as the channel (8) of the selected element
is at ground or `0` potential, to produce a coupled down voltage on
the floating gate (12) of the selected storage element, if the
channel is at ground potential, to provide sufficient potential
gradient across the tunnel oxide (10) between the floating gate
(12) and the channel (8) of the selected storage element, to cause
tunneling of carriers across the tunnel oxide (10) and accumulation
of charge in the floating gate from the channel. Since the select
gate (6) for drain select transistor is approx. at Vdd and the bit
line connected to the drain (2) is at ground, and all the series
storage elements in that selected cell are fully on, the ground
voltage from the bit line appears in the channel under the selected
element, and hence cause programming of the element. This
programming process does not generate a current flow in the cell,
other than that required to charge the capacitors and program the
storage element, in fact it is a very low power programming
process.
[0012] Now if the register connected to the bit line is loaded with
a `1`, that is a value close to Vdd, when the word lines in the
selected cell is pulled up, the unselected word lines to approx.
Vpp/2 and selected word line to Vpp, similar to the case explained
earlier, the floating gates in the cell has a coupled up voltage
which in turn tries to pull up the channel voltage by capacitive
coupling. If the channel voltage is not actively pulled to ground
as in the earlier case, but is allowed to rise to a value close to
Vdd value, the drain select device of the selected cell, by virtue
of having its gate (6), its drain diffusion (2) and its source
diffusion, which is the first connect diffusion (7) all at a
voltage close to Vdd, is shut off and the channel remains free to
float to a value of voltage dictated by the capacitive coupling
limited by the leakages. This causes the integrated channel {all
the connect diffusions (7) and the channels beneath the storage
devices (8)} to be pulled up to a value high enough by the coupled
voltage to prevent the potential gradient across the tunnel oxide
(10) of the selected storage element from reaching a value
sufficient to generate a potential gradient across the tunnel oxide
for tunneling to take place. This in turn result in the selected
storage element being left erased. Thus the data stored in the
selected storage element is directly related to the information
stored in the register connected to the related bit line. In a NAND
cell this program operation is done for storage elements connected
to a single word line in a row of selected cells at a time and is
called page program.
[0013] Read back of the data is accomplished by checking the status
of the storage element using a sense circuit and hence getting the
data back and it is well known in the industry. Multiple sense
schemes are available for sensing and retrieving the data stored in
the storage elements of the cell. Typically the read back is also
done for an entire page at a time and data stored in page register
and is serially streamed out from these registers.
[0014] It has been shown that the thickness of the tunnel oxide is
of the order of 70A and cannot be scaled without impacting the
reliability and disturb characteristics of the elements. In order
to cause tunneling typical potential gradient of 10 MeV/cm is
necessary, that is a voltage of over 7 V applied across the tunnel
oxide is essential to initiate tunneling with the 70A oxide. The
prior art NAND cells use the stack etch technology to self align
the control gate and the floating gate. This provides a three-sided
coverage of the floating gate by the control gate. The coupling
between the control gate and the floating gate of the storage
element is typically of the order of 45-50%. This means that only
about half the voltage applied to the control gate is coupled down
to the floating gate to appear across the tunnel oxide. This in
turn mandates relatively high voltage Vpp use during program and
erase operations. In order to improve this coupling ONO, an
Oxide-Nitride-Oxide sandwich inter-poly dielectric, with a Nitride
inter-layer that has a higher dielectric constant than oxide, has
been used in the prior art. This ONO process adds to the mask count
and process complexity and hence the cost of the memory. In return
it reduces the high voltage requirement of the cell marginally by
increasing the coupling to around 60%. The ONO introduces problems
of retention as the Nitride itself conducts charge through hopping
conduction that is well known in the industry. The use of the two
oxide layers on top and bottom of the Nitride of sufficient
thickness is to prevent this leakage, which in turn increase the
thickness of the inter-poly dielectric and hence reduce the
coupling and the ability to scale. It should be noted that if the
coupling ratio of the NAND Storage element, that is the ratio of
the coupling between the control gate and floating gate as against
the total coupling to the floating gate from all sides is low, only
that amount of the applied voltage to the control gate appear on
the floating gate. This also means that voltage gradient due to Vpp
(1--the coupling ratio) appear across the inter-poly dielectric,
causing stress to that dielectric increasing the leakage through
the dielectric ONO layer.
[0015] The Nitride layers are also prone to accumulation of charge
by trapping in the layer. At high voltages used in the program and
erase process of the prior art cell, the Inter-layer dielectric is
subject to high potential gradients. These are usually sufficient
to cause charge transport through the layer and hence trapping.
This trapped charge will cause accelerated window closing during
write cycling and reduce the reliability of the cell in
operation.
[0016] The leakage characteristics and the charge trapping
characteristics of the ONO are made worse under high temperature
and high voltage conditions and impact the high temperature
characteristics of the storage element, typically making it less
suitable for high temperature operation in industrial and
automotive applications.
[0017] Another problem that has been identified in the prior art
cells as the technology shrinks is the problem of coupling of the
floating gate to adjacent memory elements through the open sides of
the floating gates. This can result in voltage swings in the cell
that are not designed in and controlled.
[0018] All these results in problems of cell disturb and problems
of data retention in the memory cells.
[0019] Another problem is the encroachment of the connect diffusion
under the tunnel oxide. This has the effect of reducing the
reliability of the cell by making a high doped region with its
field regions to be under the tunnel oxide and the floating gate
thereby changing the uniformity of tunneling from the channel. This
can reduce the program--erase or write cycling characteristics of
the memory elements by localized degradation. This also has an
impact on the high temperature characteristics of the cells and
retention characteristics of the cell due to generation-
recombination in the depletion region resulting in collection of
charge by the floating gate.
[0020] Some of the problems of the typical present day NAND flash
cell are: [0021] 1. Very high Voltage program and erase operation.
[0022] 2. Low coupling resulting in increased stress on the
inter-poly dielectric. [0023] 3. Lack of ability to scale
vertically the inter-poly dielectric. [0024] 4. Lack of ability to
scale without being affected by coupling to neighboring cells.
[0025] 5. Need for sophisticated program erase controls to prevent
over programming and over erase. [0026] 6. Full processor
controlled reliability screening and redundancy due the cells being
prone to disturb conditions. [0027] 7. Frequent checks and error
correction methods are needed to ensure that the cells retain the
data. [0028] 8. Reduced high temperature capability (typical
commodity operation). [0029] 9. Reduced data retention. [0030] 10.
Reduced program erase cycling. [0031] 11. Difficult to scale
devices with technology node without impacting performance due to
punch trough. [0032] 12. Process complexity increases with ONO.
[0033] What is Proposed
[0034] What is proposed is a cell and technology to be used in a
NAND memory array. Three separate modifications/improvements are
proposed, that improve the operation, the operating temperature
range and increase the reliability, both retention and endurance,
of the cell. The first modification (modification 1) is one that
uses a five side overlapping control gate, to improve coupling
ratio with associated reduction in the program and erase voltages,
and elimination of unwanted disturb conditions due influence of the
neighboring cell through coupling to the floating gate. The high
coupling ratio reduces the stress on the inter-poly dielectric and
hence improves retention and endurance characteristics. The second
modification (modification 2) is by using the five side overlapping
control-gate structure to space away the diffusion edge from the
floating gate and providing a pair of side select gates in parallel
with the floating gate, on either side of the floating gate, to
eliminate the variations in the field due to high doping in the
channel and hence improve the uniformity of tunneling from the
channel through the tunnel oxide. This improvement moves the high
field depletion regions where electron hole pair generation takes
place, especially with increased temperature, away from the
floating gate, there by improving the operating temperature range
of the devices. This improvement also improves the endurance and
retention characteristics of the cell by improving the uniformity
of current flow through the tunnel oxide and hence reducing the
damage to the oxide. The use of the five side coverage of the
floating gate with the improvement in coupling ratio provide a
third improvement, (modification 3) that is the ability to use high
quality grown or deposited oxide or a combination of the two oxides
as inter-poly dielectric instead of ONO. The increased coupling
ratio also allows reduction in the inter-poly oxide thickness as
the voltage appearing across it is reduced. FIG. 2 is a first
implementation of some of the proposed embodiment with the
modification 1 that provides the five side coverage of the floating
gate by the control gate, and FIG. 3 is the preferred
implementation with modifications 1 and 2. Modification 3 can be
implemented in both implementations shown in FIG. 2 and FIG. 3, as
both have high coupling ratios due to the coverage and low
inter-poly voltage drop. Use of Oxide as the inter-poly dielectric
brings with it some additional advantages. First is the
simplification of the manufacturing process due to the elimination
of steps needed for ONO processing. Second is the improvement of
retention characteristics of the cells due to elimination of charge
leakage by hopping conduction through the Nitride. Third is the
improvement in write cycling reliability, by the reduction of
window closing effect due to charge trapping in the Nitride of the
ONO.
[0035] The cell architected is not for high density and is not the
smallest NAND cell, instead it has been architected for improved
manufacturability, quality and reliability. The disturb conditions
have been eliminated by cell and array design and retention and
endurance (write cycling) characteristics are improved. This cell
will also be much more manufaturable and yieldable as ONO issues
are eliminated and lower voltages are used to program and erase.
The use of the side select gates of the embodiment in FIG. 3
enables the cells to be in depletion or enhancement during
operation.
[0036] Some of the advantages of the proposed cell are: [0037] 1. A
lower program erase voltage is used as compared to the existing
NAND structures at the same node due to the improved coupling ratio
from 50 to 80%. [0038] 2. Eliminates the need for ONO and allows
the use of Oxide as inter-poly dielectric, making the element
easier to manufacture and more robust. [0039] 3. The cell with side
select gates is immune to over erase problems. [0040] 4. Eliminates
the unwanted coupling from adjacent cells to the floating gate.
[0041] 5. The lower Program-erase voltage reduces the stress on the
inter poly oxides and hence increases the reliability of the cell.
[0042] 6. Provides cells that are capable of high temperature
operation by moving the junction depletions, where thermal
generation and acceleration take place, away from under the
floating gate. [0043] 7. Very high retention is achieved due to low
and uniform tunnel currents by elimination of junction depletion
overlap of floating gate.
DESCRIPTION OF DRAWINGS
[0044] FIG. 1: Prior Art NAND cell--with Stack etched storage
element.
[0045] FIG. 2: Proposed MEMTEK NAND cell-1 with Five side covered
storage element.
[0046] FIG. 3: Proposed MEMTEK NAND cell-2 with Five side covered
storage element and spaced off connect diffusions.
EXPLANATION OF NUMBERING AND LETTERING IN THE FIGS. 1, 2 AND 3
[0047] A1. Drain Select device. [0048] A2. Source select device.
[0049] B1 to B4. Storage elements of the NAND cell (a 4 element
NAND shown) [0050] 1. Well in substrate. [0051] 2. Drain diffusion.
[0052] 2x. Source diffusion. [0053] 3. Drain contact (contact to
bit line) [0054] 3x. Source Contact. [0055] 4. Drain select device
channel. [0056] 4x. Source select device channel. [0057] 5. Gate
oxide of drain select device. [0058] 5x. Gate oxide of source
select device. [0059] 6. Gate poly of drain select device. [0060]
6x. Gate poly of source select device. [0061] 7. Interconnect
diffusion or connect diffusion. [0062] 8. Channel of storage
element. [0063] 9. Side select gate channels formed on either side
of floating gate controlled by the overlapping control gate
covering the sides of floating gate over the channel. [0064] 10.
Tunnel Oxide. [0065] 11. Oxide under the control gate overlap along
channel region over difusion [0066] 11x. Oxide under the control
side select gate along channel region [0067] 12. Floating gate of
storage element or floating gate [0068] 13. Inter-poly dielectric.
[0069] 14. Control Gate poly-silicon or control gate electrode
[0070] 14x. Control gate poly-silicon region overlapping and
covering the sides along the channel direction of floating gate
over connect diffusion. [0071] 14y. Control gate poly-silicon
region overlapping and covering the sides along the channel
direction of floating gate forming the side select gates.
DESCRIPTION OF THE INVENTION
[0072] Embodiment Shown in FIG. 2:
[0073] FIG. 2 shows one embodiment of the invention in a typical
NAND cell. Like the prior art cell this NAND cell structure
comprise multiple storage elements, four used for the example) B1
to B4, arranged in series with a drain select device A1 separating
the drain diffusion (2) from the first storage element B1, and a
source select device A2 separating the source diffusion (2x) from
the last storage element B4. Typically there can be 8, 16, 32 or 6e
elements in a NAND string. The elements are connected in series
with the drain and the source select devices by connect diffusions
(7). The connect diffusions (7) act as the source diffusion for the
drain select device, drain diffusion for the source select device,
and drain and source diffusions for the storage elements as will be
well known to individuals who understand devices.
[0074] Each storage element is deposed between two connect
diffusions (7) that connect it to the neighboring element or if it
is the last or first element, to the select device. The figure
shows a four storage element cell implementation for simplicity.
The number of elements in a NAND string can be any where up to 64
in a typical cell. This number being limited by the cell current
requirement for read operation. The storage element itself comprise
a floating gate (12) over laying a tunnel oxide (10) on silicon
surface, that floating gate having control of the conductance of a
channel (8) in silicon, in an implanted well (1). [The well
typically being a P-well inside an N-well in a typical n-channel
implementation and the well typically being an N-well in a P
substrate in the case of a P channel implementation. For simplicity
of explanation the N-channel implementation is described here
after.] A control gate (14) separated from the floating gate (12)
by an inter-poly dielectric (13) over lays the floating gate and
couples to it by means of capacitive coupling. In this embodiment
of the disclosure, the floating gate overlaps the floating gate on
all five sides, except the tunnel oxide side, unlike the prior art
where only three sides of the floating gate are covered by the
control gate, the sides along the diffusion being left uncovered
and protected by a dielectric layer only. The overlapping region of
the control gate (14x) along the channel region, in the embodiment
in FIG. 2 over lays the connect diffusion and is separated from the
connect diffusion by a thick oxide (11).
[0075] Operation of Embodiment of FIG. 2:
[0076] The over all operation of the cell and array in this
embodiment shown by FIG. 2 is similar to the prior art cell
described earlier. The higher coupling ratio in the storage
elements, B1 to B4, provide associated advantages, mainly related
to lower applied potentials, and lower stress on the inter-poly
dielectric (13). The covered floating gate (12) is also protected
from influence of neighboring cell coupling by the overlapping
control gate covering it. One of the major advantages of the
architecture of the cell is that the coupling of the control gate
side walls (14x) to the channel and diffusions is higher due to the
poly side gate (14x) coupling through the oxide (11) to the connect
diffusion (7). This will enable a lower voltage on the control gate
(14) to provide the necessary program inhibit voltages for the
selected cell during write. The cell is erased typically by
application of a high voltage to the well (1) and ground potential
to the control gate (14). The coupled down voltage gradient across
the tunnel oxide (10) if sufficient to cause tunneling of negative
carriers from the floating gate to the well, will erase the
element. Since the well is common to a set of cells, it is possible
to erase all elements in a row of cells or multiple rows of cells,
in an array within the well, in one erase operation providing the
necessary flash erase operation. During erase the source select
device and the drain select device are kept in the off state by
application of ground potential to the gates of these devices.
[0077] Programming of an element, of the cell is the selective
operation. If the selected element B2 of the selected cell is to be
programmed, per the data, a "0", a ground potential is impressed on
the drain diffusion (2) of the selected cell with the drain select
transistor channel (4) turned on by application of a suitable
positive voltage to the gate (6) and the channel (4x) of the source
select transistor of the cell in the off condition by application
of ground potential to the gate (6x) of that device. The channels
of all the unselected elements B1, B3 and B4 of the cell will
typically have a voltage approximately equal to Vpp/2 applied to
the control gates (14). This voltage when coupled down to the
floating gates (12) is sufficient to fully turn on the channels (8)
of these elements, but not sufficient to initiate tunneling across
the tunnel oxide (10) in these elements. The selected element B2 to
be programmed will have Vpp applied to the control gate (14). This
will couple down to the floating gate (12) of the element and turn
on the channel (8) under the element. At this point the turned on
channels (4) causes the voltage applied to drain diffusion (2) to
be passed along the on channels, such that all the connected
channels are at the ground potential. The on state of the drain
select device channel (4) with drain diffusion (2) grounded will
pull the connected channels down to ground and will not allow the
channels to float up to higher voltages through coupling from the
control gates and floating gates of the devices to the channel. In
the selected element alone the high coupled down voltage on the
floating gate (12) of the selected element from Vpp applied to the
control gate (14) of that element is sufficient to provide a
potential gradient across the tunnel oxide (10) from the channel
(8) to the floating gate (12) that can sustain tunneling of
carriers from the channel (8) to the floating gate (12) of the
element and hence program the cell.
[0078] In the alternate case when a selected element of a selected
cell in FIG. 2 is to be left un-programmed during the programming
operation, based on the data, then a "1" or a voltage typically
close to Vdd is applied to the drain diffusion (2) of the selected
cell. The condition for programming the selected element is set up
as explained for the case where the drain is connected to "0"
potential. Since the drain is at a potential close to Vdd and the
gate (6) of the drain select device A1 is at a potential close to
Vdd the channels of the elements B1 to B4 tends to the same
potential when the voltages of Vpp/2 are applied to the control
gates (14) of the unselected elements and Vpp is applied to the
control gate (14) of the selected element, to turn on the channel.
Once the channel reaches a potential close to Vdd the channel (4)
of the drain select device is shut off as the gate (6) is no longer
at a Vt higher than the diffusions. This allows the coupled down
voltages from the control gates to the channel and the connect
diffusion across the oxides to pull up or boost the connected
channels to a high voltage. This higher voltage is of a sufficient
value such that the potential across the tunnel oxide (10) from the
channel (8) becomes insufficient to initiate tunneling across the
tunnel oxide (10) even with Vpp applied to the control gate (14) of
the selected element B2. Thus the selected element is left in the
state it was previously, or program operation is inhibited.
[0079] Read back of the data is accomplished by checking the status
of the storage element using a sense circuit and hence getting the
data back as is well known in the industry. Multiple sense schemes
are available for sensing and retrieving the data stored in the
storage elements of the cell. Assume B2 is to read back. Typically
this is done by turning on the channels (8) of all the unselected
elements B1, B3 and B4 in the cell by applying suitable voltages to
the control gates (14) of these elements, with both source select
and drain select channels (4 and 4x) turned on by application of
suitable positive voltages to the gates (6 and 6x). A read voltage
is impressed across the channels from drain diffusion (2) to source
diffusion (2x). The current flowing through the cell is sensed with
the selected control gate (14) at ground potential. If the floating
gate (12) of B2 is in the programmed state, that is charged
negative, there will be no current flow in the cell as the cell
threshold is higher than Vdd/2. If the floating gate of the element
is charged positive, the channel (8) under the element will be in
the on state as the other channels and there will be a current
flow. The selected element may also be sensed with a voltage of the
order of Vdd/2 (close to the neutral Vt of the element) on the
control gate to increase the cell current and improve the read
current and hence the speed. Different sense conditions other than
the two described can be used to sense the state of the element as
is well known in the industry.
[0080] This embodiment allows increased coupling between the
floating gate (12) and control gate (14) by having the control gate
region (14x) cover the floating gate on the sides along the
diffusions (7) over the protective oxide (11), covering the
inter-poly protective dielectric (13) on the side walls of the
floating gate. In the prior art these sides are stack etched and
are only covered by a protective dielectric layer. The additional
covered region increase the area of coupling between the floating
gate and the control gate through the inter-poly dielectric layer
(13). This coverage provide the following advantages to the cell
storage element described and shown in FIG. 2: [0081] 1. There is
increased coupling due to the side wall area coupling between the
floating gate and control gate poly-silicon. [0082] 2. There is
increased coupling between the channel connect diffusion and the
control gate due to the overlap of the control poly over the
diffusion region. [0083] 3. Lower high voltages can be used for
program and erase due to high coupling ratio. [0084] 4. Lower high
voltages used will provide the inhibit voltage necessary through
boosting due to the coupling between the control gate overlap with
the connect diffusion. [0085] 5. The control gate poly-silicon
covered floating gate has more protection from damage and from
external contaminants. [0086] 6. Reduced potential difference
between the floating gate and control gate allow reduction in
thickness of inter-poly dielectric. (improving the coupling
further) [0087] 7. Reduced voltage stress across the inter-poly
dielectric, reduces the damage to the dielectric and hence leakage
of charge across the dielectric. [0088] 8. Five side coverage of
the floating gate eliminate the coupling effects on the floating
gate from the neighboring elements and metal making the cell more
robust. [0089] 9. Lower high voltage requirements enable high speed
designs. [0090] 10. Lower high voltage requirements enable smaller
peripheral high voltage device and reduce overall chip area. [0091]
11. Lower high voltages reduce process complexity and constraints.
[0092] 12. High coupling in the storage element and the low voltage
stress across the inter-poly dielectric enable the use of oxide
instead of the ONO dielectric. This oxide can be grown, deposited
or a combination of the two. [0093] 13. The use of oxide as the
dielectric eliminates the known leakage by hoping conduction
through Nitride. [0094] 14. The use of oxide also eliminated the
charge trapping in the Nitride layer with the associated enhanced
window closing during write cycling. [0095] 15. The elimination of
ONO makes the processing simpler.
[0096] Embodiment Shown in FIG. 3 (Preferred Embodiment with the
Advantages of the Embodiment in FIG. 2 Plus Improved High
Temperature Operation and Higher Reliability):
[0097] FIG. 3 shows an embodiment of the invention that is
different from the flash cell shown in FIG. 2 in that each of the
storage elements (B1 to B4) comprise of three integrated device
regions or controllable channel regions, one channel region
controlled by the floating gate and two side select channel regions
on either side of the floating gate, both controlled by the same
control gate, as explained herein. Each storage element comprise a
floating gate (12) deposed on a tunnel oxide (10) on the silicon
surface of the well (1), an inter-poly dielectric (13) that cover
the floating gate on all five sides, excluding the side of the
tunnel oxide (10), a control gate poly-silicon (14) deposed on the
top and (14Y) on the sides of the floating gate (12), a thick oxide
(11x) separates the silicon surface of the well (1) from the
control gate region (14Y) which form the poly-silicon side cover
for the floating gate and also form two side select gate channels
(9) on either side of the floating gate that is controlled by the
voltage applied to the channel through the control gate over lap at
locations (14Y). The connect diffusions (7) are spaced away from
the edge of the floating gate (12) by the control gate poly silicon
side-wall.
[0098] The control of the channel regions of each element can be
further elaborated as follows. One channel region, under the tunnel
oxide (10) controlled by the voltage impressed or stored on the
floating gate (12) and two channel regions (9), under the thick
gate oxide (11x), on either side of the channel (8) controlled by
the voltage impressed by the control gate through the side select
regions of the control gate (14Y).
[0099] The connect regions (7) in this embodiment is spaced away
from the edge of the tunnel oxide and hence the floating gate. This
has the advantage of moving diffusion (7) away from the floating
gate and hence eliminating the variations in field due to the
diffusions from impacting the tunnel current during write or read
operations. This spacing off of the diffusion from the edge of the
tunnel oxide area makes the write field more uniform in the tunnel
oxide and the cell more robust. It makes the cell capable of higher
temperature operation due to spacing the generation re-combination
region of the doped regions (the depletion regions due to the
doping) away from the floating gate. It also makes the cell more
reliable with respect to data retention and endurance by improving
the uniformity of tunneling of the whole tunnel oxide under the
floating gate.
[0100] The memory cell comprise of multiple storage elements
connected in series with connect diffusions (7) and source and
drain select devices between a drain and a source diffusion as
described previously and shown in FIG. 2 and FIG. 3 of the
embodiments.
[0101] Operation of Embodiment of FIG. 3:
[0102] The operation of the cell in FIG. 3, which is the preferred
embodiment, is similar to that of the implementation in FIG. 2 and
the prior art cells, though some differences exist due to the two
parallel side select devices (9) that exist on either side the
floating gate device, in each storage element (B1 to B4). As with
the embodiment described earlier and shown in FIG. 2, the first
operation is the erase operation.
[0103] Here again the higher coupling ratio in the storage
elements, B1 to B4, provide associated advantages, mainly related
to lower applied potentials, and lower stress on the inter-poly
dielectric (13). The covered floating gate (12) is also protected
from influence of neighboring cell coupling by the overlapping
control gate covering it. One of the major advantages of the
architecture of the cell is that the coupling of the control gate
side walls (14Y) to the channel is higher due to the poly side gate
(14Y) coupling through the oxide (11x) to the channel region
underneath (9). This will enable a lower voltage on the control
gate (14) to provide the necessary program inhibit voltages for the
selected cell during write. The cell is erased typically by
application of a high voltage to the well (1) and ground potential
to the control gate (14). The coupled down voltage gradient across
the tunnel oxide (10) if sufficient to cause tunneling of negative
carriers from the floating gate to the well, will erase the
element. Since the well is common to a set of cells, it is possible
to erase all elements in a row of cells or multiple rows of cells,
in an array within the well, in one erase operation providing the
necessary flash erase operation. During erase the source select
device and the drain select device are kept in the off state by
application of ground potential to the gates of these devices.
[0104] Programming of an element, of the cell is the selective
operation. If the selected element B2 of the selected cell is to be
programmed, per the data, a "0", a ground potential is impressed on
the drain diffusion (2) of the selected cell with the drain select
transistor channel (4) turned on by application of a suitable
positive voltage to the gate (6) and the channel (4x) of the source
select transistor of the cell in the off condition by application
of ground potential to the gate (6x) of that device. The channels
of all the unselected elements B1, B3 and B4 of the cell, will
typically have a voltage approximately equal to Vpp/2 applied to
the control gates (14). This voltage when coupled down to the
floating gates (12) is sufficient to fully turn on the channels (8)
and also turn on the side select channels (9) through the control
gate over lap region (14Y) of these elements, but not sufficient to
initiate tunneling across the tunnel oxide (10) in these elements.
The selected element B2 to be programmed will have Vpp applied to
the control gate (14) and side select regions (14Y). This will
couple down to the floating gate (12) of the element and turn on
the channel (8) under the element. It will also turn on the side
select channels (9) under the thicker oxide (11x) by the control
gate overlap region (14Y). At this point the turned on channel (4)
of the drain select gate causes the voltage applied to drain
diffusion (2) to be passed along the on channels (8 and 9) of the
elements, such that all the connected channels are at the ground
potential. The on state of the drain select device channel (4) with
drain diffusion (2) grounded will pull the connected channels down
to ground and will not allow the channels to float up to higher
voltages through coupling from the control gates and floating gates
of the devices to the channel. In the selected element alone the
high coupled down voltage on the floating gate (12) of the selected
element from Vpp applied to the control gate (14) of that element
is sufficient to provide a potential gradient across the tunnel
oxide (10) from the channel (8) to the floating gate (12) that can
sustain tunneling of carriers from the channel (8) to the floating
gate (12) of the element and hence program the cell.
[0105] In the case where a selected element of a selected cell in
FIG. 2. is to be left un-programmed during the programming
operation, based on the data, then a "1" or a voltage typically
close to Vdd is applied to the drain diffusion (2) of the selected
cell. The condition for programming the selected element is set up
as explained for the case where the drain is connected to "0"
potential. Since the drain is at a potential close to Vdd and the
gate (6) of the drain select device A1 is at a potential close to
Vdd the on channels (8 and 9) of the elements B1 to B4 tends to be
raised to the same potential when the voltages of Vpp/2 are applied
to the control gates (14) with the same voltage appearing across
the thick side select oxide (11x). A higher programming voltage of
Vpp is applied to the control gate (14) of the selected element, to
turn on the channels (8 and 9) of the selected element. Once the
connected channel under B1 to B4 reaches a potential close to Vdd
the channel (4) of the drain select device is shut off as the gate
(6) is no longer at a Vt higher than the diffusions on either side
of the gate. This allows the coupled down voltages from the control
gates to the channel and the connect diffusion across the oxides to
pull up or boost the connected channels to a higher voltage. This
higher voltage is of a sufficient value such that the potential
across the tunnel oxide (10) from the channel (8) becomes
insufficient to initiate tunneling across the tunnel oxide (10)
even with Vpp applied to the control gate (14) of the selected
element B2. Thus the selected element is left in the state it was
previously, or program operation is inhibited.
[0106] Read back of the data is accomplished by checking the status
of the storage element using a sense circuit and hence getting the
data back as is well known in the industry. Multiple sense schemes
are available for sensing and retrieving the data stored in the
storage elements of the cell. Assume B2 is to read back. Typically
this is done by turning on the channels (8) and side select
channels (9) of all the unselected elements B1, B3 and B4 in the
cell by applying suitable voltages to the control gates (14), which
in turn gets applied also to the side select regions of the control
gate (14Y) of these elements, with both source select and drain
select channels (4 and 4x) turned on by application of suitable
positive voltages to the gates (6 and 6x). A read voltage is
impressed across the channels from drain diffusion (2) to source
diffusion (2x). The current flowing through the cell is sensed with
the selected control gate (14) at a potential sufficient to turn on
the side select channels fully but not enough to turn on an off
channel of the floating device. If the floating gate (12) of B2 is
in the programmed state, that is charged negative, there will hence
be no current flow, or low current flow in the cell as the cell
threshold is higher than the applied voltage to the select device,
typically, Vdd/2. If the floating gate of the element is charged
positive, the channel (8) under the element will be in the on state
as the other channels and there will be a current flow. The
selected element current in this case will be limited by the side
select device drive and higher voltages that can be applied to the
control gate will provide higher read current as long as the
impedance of the unselected elements can be made lower than that of
the selected element. Different sense conditions other than the one
described can be used to sense the state of the element as is well
known in the industry.
[0107] This embodiment allows increased coupling between the
floating gate (12) and control gate (14) by having the control gate
region (14Y) cover the floating gate on the sides along the side
select channel regions (9) over the protective oxide (11x),
covering the inter-poly protective dielectric (13) on the side
walls of the floating gate. In the prior art these sides are stack
etched and are only covered by a protective dielectric layer. The
additional covered region increase the area of coupling between the
floating gate and the control gate through the inter-poly
dielectric layer (13). This coverage provides the following
advantages to the cell storage element described and shown in FIG.
3: [0108] 1. There is increased coupling due to the side wall area
coupling between the floating gate and control gate poly-silicon.
[0109] 2. There is increased coupling between the channel and the
control gate due to the overlap of the control poly over the side
select gate region. [0110] 3. Lower high voltages can be used for
program and erase due to high coupling ratio. [0111] 4. Lower high
voltages used will provide the inhibit voltage necessary through
boosting due to the higher coupling between the control gate
overlap with the channel. [0112] 5. The control gate poly-silicon
covered floating gate has more protection from damage and from
external contaminants. [0113] 6. Reduced potential difference
between the floating gate and control gate allow reduction in
thickness of inter-poly dielectric. (improving the coupling
further) [0114] 7. High coupling reduces the voltage stress across
the inter-poly dielectric, resulting in reduce the damage to the
dielectric and hence lower charge loss across the dielectric.
[0115] 8. Five side coverage of the floating gate eliminate the
coupling effects on the floating gate from the neighboring elements
making the cell more robust. [0116] 9. Lower high voltage
requirements enable high speed designs. [0117] 10. Lower high
voltage requirements enable smaller peripheral high voltage device
and reduce overall chip area. [0118] 11. Lower high voltages reduce
process complexity and constraints. [0119] 12. High coupling in the
storage element and the low voltage stress across the inter-poly
dielectric enable the use of high quality oxide instead of the ONO
dielectric. This may be grown, deposited or a combination of the
two. [0120] 13. The use of oxide as the dielectric eliminates the
known leakage by hoping conduction through Nitride. [0121] 14. The
use of oxide also eliminated the charge trapping in the Nitride
layer with the associated enhanced window closing during write
cycling. [0122] 15. The elimination of ONO makes the processing
simpler. [0123] 16. Having uniform oxide cover over the five sides
of the floating gate eliminate localized high fields in the
protection dielectric. This eliminated the degradation and trapping
of charge in the layer improving leakage characteristics and cell
endurance.
[0124] The use of two side select device in parallel with the
floating gate provide additional advantages as follows: [0125] 17.
The connect diffusions are pushed away from the edges of the
floating gate to provide uniform tunneling field and hence uniform
tunnel current over the area of the floating gate. This improves
endurance. [0126] 18. Thermal generation combination at reverse
biased junctions can result in generation of hot electrons and
holes that can get collected by the floating gate and result in
charge loss. This is one reason for the poor retention
characteristics at high temperatures. By moving the edge of
diffusion away from under the floating gate, the collection of
generated carriers from the junction are eliminated resulting in
improved retention. [0127] 19. Having a high doped region with a
different band gap under part of the floating gate during write and
erase can result in localized and selective tunneling and high
current regions in the tunnel oxide. This can result in enhanced
oxide degradation and leakage at those regions with associated
endurance and retention degradations. [0128] 20. The weak bit
issues and tail distribution seen in normal NVM are eliminated by
the use of the side-select gates. The on current is always limited
by the characteristics of the side select device which is a
standard device and the off characteristics is kept close to off
condition and hence the peripheral circuitry and complexity of
program erase is reduced. [0129] 21. The fixed current in the on
state and very low current in the off state of the elements help in
the efficient design of high-speed sense amplifiers.
[0130] It clear that the advantages of the disclosed NAND cell for
embedded applications are sufficient to overcome the size
disadvantage of the cell elements for high reliability applications
like Automotive and high retention applications like smart
cards.
* * * * *