U.S. patent application number 11/114360 was filed with the patent office on 2005-11-03 for highly compact eprom and flash eeprom devices.
Invention is credited to Harari, Eliyahou.
Application Number | 20050243601 11/114360 |
Document ID | / |
Family ID | 27394634 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050243601 |
Kind Code |
A1 |
Harari, Eliyahou |
November 3, 2005 |
Highly compact Eprom and flash EEprom devices
Abstract
Structures, methods of manufacturing and methods of use of
electrically programmable read only memories (EPROM) and flash
electrically erasable and programmable read only memories (EEPROM)
include split channel and other cell configurations. An arrangement
of elements and cooperative processes of manufacture provide
self-alignment of the elements. An intelligent programming
technique allows each memory cell to store more than the usual one
bit of information. An intelligent erase algorithm prolongs the
useful life of the memory cells. Use of these various features
provides a memory having a very high storage density and a long
life, making it particularly useful as a solid state memory in
place of magnetic disk storage devices in computer systems.
Inventors: |
Harari, Eliyahou; (Los
Altos, CA) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
595 MARKET STREET
SUITE 1900
SAN FRANCISCO
CA
94105
US
|
Family ID: |
27394634 |
Appl. No.: |
11/114360 |
Filed: |
April 26, 2005 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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11114360 |
Apr 26, 2005 |
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10365686 |
Feb 11, 2003 |
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6914817 |
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10365686 |
Feb 11, 2003 |
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08154162 |
Nov 17, 1993 |
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6570790 |
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08154162 |
Nov 17, 1993 |
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07777673 |
Oct 15, 1991 |
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5268319 |
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07777673 |
Oct 15, 1991 |
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07381139 |
Jul 17, 1989 |
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5198380 |
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07381139 |
Jul 17, 1989 |
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07204175 |
Jun 8, 1988 |
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5095344 |
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Current U.S.
Class: |
365/185.3 ;
257/E21.209; 257/E21.422; 257/E21.682; 257/E27.103;
257/E29.306 |
Current CPC
Class: |
G11C 29/765 20130101;
G11C 2211/5634 20130101; Y10S 438/964 20130101; H01L 2924/00
20130101; G11C 16/0425 20130101; H01L 27/115 20130101; H01L
2924/0002 20130101; G11C 29/00 20130101; H01L 2924/0002 20130101;
G11C 2211/5644 20130101; G11C 16/3495 20130101; H01L 27/11517
20130101; G11C 29/82 20130101; H01L 29/66825 20130101; H01L
27/11519 20130101; G11C 11/5621 20130101; G11C 16/349 20130101;
G11C 11/5642 20130101; H01L 29/7881 20130101; H01L 29/7885
20130101; G11C 11/5635 20130101; H01L 29/40114 20190801; G11C
11/5628 20130101; G11C 2211/5613 20130101; G11C 2211/5631 20130101;
H01L 27/11521 20130101 |
Class at
Publication: |
365/185.3 |
International
Class: |
G11C 011/34 |
Claims
1. A method of forming a split-channel electrically programmable
read only memory transistor on a semiconductor substrate surface,
comprising the steps of: forming on said surface a floating gate
having sidewalls and being electrically isolated by a gate
dielectric layer from said substrate, forming a spacer immediately
adjacent only one sidewall of said floating gate and extending a
controlled distance over said substrate surface, forming source and
drain regions in said substrate by using said floating gate and
said spacer as a mask, whereby a channel region is formed in the
substrate under the masked region between the source and drain
regions, removing said spacer, and forming a control gate extending
over at least a portion of the floating gate and substrate channel
region that was occupied by said spacer, said control gate being
electrically insulated from said floating gate and said substrate,
whereby a split-channel electrically programmable read only memory
transistor is formed.
2. The method according to claim 1 wherein the step of forming a
spacer immediately adjacent only one sidewall of the floating gate
includes the steps of: depositing a thin layer of material over
said floating gate and extending a distance beyond said floating
gate sidewalls, anisotropically etching said layer of material for
a time to remove it except for first and second portions
immediately adjacent opposite sidewalls of said floating gate, and
selectively removing said first portion of material without removal
of said second portion, whereby said second portion remains as said
spacer.
3. The method according to claim 1 wherein the step of forming a
spacer immediately adjacent only one sidewall of the floating gate
includes the steps of: depositing a thin layer of protective
material over said floating gate and extending a distance beyond
said floating gate sidewalls, depositing a relatively thick layer
of spacer material over said thin layer and extending a distance
beyond said floating gate sidewalls, anisotropically etching said
layer of spacer material for a controlled time to remove it except
for first and second portions immediately adjacent opposite
sidewalls of said floating gate, said first portion also being
positioned adjacent the location of said drain region and said
second portion being positioned adjacent the location of said
source region, and selectively removing said first portion of
material without removal of either one of said second portion and
said protective material layer, whereby said second portion remains
as said spacer.
4. The method according to claim 2 wherein the step of selectively
removing said first portion of material includes the steps of:
covering with a masking layer an area including said second portion
of material but not said first portion, etching away said first
portion of material, and removing said masking layer.
5. The method according to claim 1 comprising the additional steps
of: forming regions of a tunnel erase dielectric layer on each of
opposite ends of said floating gate, and forming a pair of parallel
erase gates extending between the source and drain regions and on
the tunnel dielectric layers.
6. The method according to claim 5 comprising the additional step
of forming a second dielectric to insulate the pair of erase gates
from said control gate.
7. The method according to claim 6 wherein the step of forming the
control gate includes forming said control gate to extend over only
a portion of said floating gate, thereby leaving a portion of said
floating gate that is not covered by the control gate, and wherein
the step of forming an erase dielectric layer includes the step of
forming said layer over the portion of the floating gate not
covered by the control gate without forming said layer over a
portion of the floating gate over which the control gate
extends.
8. The method according to claim 7 wherein the steps of forming the
tunnel erase dielectric layer and the erase gates are carried out
prior to the steps of forming the second dielectric layer and the
control gate.
9. The method according to claim 5 wherein the step of forming a
region of a tunnel erase dielectric layer on each of opposite ends
of said floating gate includes forming the layers on a top surface
of the floating gate, and wherein the step of forming a pair of
parallel erase gates includes the step of forming each gate over
said top surface with at least one of the tunnel dielectric layers
therebetween.
10. The method according to claim 5 wherein the step of forming a
region of a tunnel erase dielectric layer on each of opposite ends
of said floating gate includes forming the layers along opposite
sidewalls thereof, and wherein the step of forming a pair of
parallel erase gates includes the step of forming each gate
adjacent one of said sidewalls with one of the tunnel dielectric
layers therebetween.
11. The method according to claim 5 wherein each of the steps of
forming a floating gate, forming a control gate and forming a pair
of erase gates include forming their respective gates in a
conductive layer that is different from the others.
12. The method according to claim 10 which includes an additional
step of forming a thin layer of dielectric on said substrate on at
least the portions where the erase gates are positioned, and
wherein the step of forming the erase gates includes forming said
erase gates over said thin dielectric layer.
13. The method according to claim 1 which includes an additional
step, prior to the step of forming the control gate, of forming an
erase gate extending over the floating gate between its side walls
with a tunnel dielectric therebetween and across the substrate
region that was occupied by the spacer with an insulating layer
therebetween, and wherein the step of forming the control gate
includes forming the control gate over and around the erase gate
over the floating gate and the substrate region that was occupied
by the user.
14. A method of forming a split-channel flash electrically erasable
and programmable read only memory transistor on a semiconductor
substrate surface, comprising the steps of: forming on said surface
a floating gate having opposite sides and opposite ends, said
floating gate being electrically insulated from said substrate by a
gate dielectric layer, forming in said substrate a drain region
adjacent one side of said floating gate and a source region spaced
apart from an opposite side of said floating gate, thereby to form
a channel region between the source and drain that has a first
channel region under the floating gate a second channel region
between the source region and the opposite floating gate side,
forming a control gate extending over at least a portion of the
floating gate and said second channel region, said control gate
being electrically insulated from said floating gate and said
substrate, forming regions of a tunnel erase dielectric layer on
each of opposite ends of said floating gate, and forming a pair of
parallel erase gates extending between the source and drain regions
and across the opposite ends of the floating gate on the tunnel
dielectric layers.
15. The method according to claim 14 wherein the step of forming
the control gate includes forming said control gate to extend over
only a portion of said floating gate, thereby leaving a portion of
said floating gate that is not covered by the control gate, and
wherein the step of forming an erase dielectric layer includes the
step of forming said layer over the portion of the floating gate
not covered by the control gate without forming said layer over a
portion of the floating gate over which the control gate
extends.
16. The method according to claim 14 wherein the step of forming a
region of a tunnel erase dielectric layer on each of opposite ends
of said floating gate includes forming the layers on a top surface
of the floating gate, and wherein the step of forming a pair of
parallel erase gates includes the step of forming each gate over
said top surface with at least one of the tunnel dielectric layers
therebetween.
17. The method according to claim 14 wherein the step of forming a
region of a tunnel erase dielectric layer on each of opposite ends
of said floating gate includes forming the layers along opposite
sidewalls thereof, and wherein the step of forming a pair of
parallel erase gates includes the step of forming each gate
adjacent one of said sidewalls with one of the tunnel dielectric
layers therebetween.
18. The method according to claim 17 which includes an additional
step of forming a thin layer of dielectric on said substrate on at
least the portions where the erase gates are positioned, and
wherein the step of forming the erase gates includes forming said
erase gates over said thin dielectric layer.
19. The method according to claim 17 wherein the step of forming
the floating gate includes forming its said opposite ends to have
edges that are very thin and relatively sharp.
20. A method of forming a split-channel flash electrically
eraseable and programmable read only memory transistor on a
semiconductor substrate surface, comprising the steps of: forming
on said surface a floating gate having opposite sides and opposite
ends, said floating gate being electrically insulated from said
substrate by a gate dielectric layer, forming in said substrate a
drain region adjacent one side of said floating gate and a source
region spaced apart from an opposite side of said floating gate,
thereby to form a channel region between the source and drain that
has a first channel region under the floating gate a second channel
region between the source region and the opposite floating gate
side, forming a region of a tunnel erase dielectric layer on a
portion of the surface of said floating gate, forming an erase gate
extending across the floating gate on the tunnel dielectric layer
and across the second channel region of the substrate with a
dielectric layer therebetween, and forming over and around the
erase gate a control gate extending across the floating gate and
second channel region, said control gate being electrically
insulated from said floating gate and said substrate.
21-149. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to semi-conductor
electrically programmable read only memories (Eprom) and
electrically erasable programmable read only memories (EEprom), and
specifically to semiconductor structures of such memories,
processes of making them, and techniques for using them.
[0002] An electrically programmable read only memory (Eprom)
utilizes a floating (unconnected) conductive gate, in a field
effect transistor structure, positioned over but insulated from a
channel region in a semi-conductor substrate, between source and
drain regions. A control gate is then provided over the floating
gate, but also insulated therefrom. The threshold voltage
characteristic of the transistor is controlled by the amount of
charge that is retained on the floating gate. That is, the minimum
amount of voltage (threshold) that must be applied to the control
gate before the transistor is turned "on" to permit conduction
between its source and drain regions is controlled by the level of
charge on the floating gate. A transistor is programmed to one of
two states by accelerating electrons from the substrate channel
region, through a thin gate dielectric and onto the floating
gate.
[0003] The memory cell transistor's state is read by placing an
operating voltage across its source and drain and on its control
gate, and then detecting the level of current flowing between the
source and drain as to whether the device is programmed to be "on"
or "off" at the control gate voltage selected. A specific, single
cell in a two-dimensional array of Eprom cells is addressed for
reading by application of a source-drain voltage to source and
drain lines in a column containing the cell being addressed, and
application of a control gate voltage to the control gates in a row
containing the cell being addressed.
[0004] This type of Eprom transistor is usually implemented in one
of two basic configurations. One is where the floating gate extends
substantially entirely over the transistor's channel region between
its source and drain. Another type, preferred in many applications,
is where the floating gate extends from the drain region only part
of the way across the channel. The control gate then extends
completely across the channel, over the floating gate and then
across the remaining portion of the channel not occupied by the
floating gate. The control gate is separated from that remaining
channel portion by a thin gate oxide. This second type is termed a
"split-channel" Eprom transistor. This results in a transistor
structure that operates as two transistors in series, one having a
varying threshold in response to the charge level on the floating
gate, and another that is unaffected by the floating gate charge
but rather which operates in response to the voltage on the control
gate as in any normal field effect transistor.
[0005] Early Eprom devices were erasable by exposure to ultraviolet
light. More recently, the transistor cells have been made to be
electrically erasable, and thus termed electrically erasable and
programmable read only memory (EEprom). One way in which the cell
is erased electrically is by transfer of charge from the floating
gate to the transistor drain through a very thin tunnel dielectric.
This is accomplished by application of appropriate voltages to the
transistor's source, drain and control gate. Other EEprom memory
cells are provided with a separate, third gate for accomplishing
the erasing. An erase gate passes through each memory cell
transistor closely adjacent to a surface of the floating gate but
insulated therefrom by a thin tunnel dielectric. Charge is then
removed from the floating gate of a cell to the erase gate, when
appropriate voltages are applied to all the transistor elements. An
array of EEprom cells are generally referred to as a Flash EEprom
array because an entire array of cells, or significant group of
cells, is erased simultaneously (i.e., in a flash).
[0006] EEprom's have been found to have a limited effective life.
The number of cycles of programming and erasing that such a device
can endure before becoming degraded is finite. After a number of
such cycles in excess of 10,000, depending upon its specific
structure, its programmability can be reduced. Often, by the time
the device has been put through such a cycle for over 100,000
times, it can no longer be programmed or erased properly. This is
believed to be the result of electrons being trapped in the
dielectric each time charge is transferred to or away from the
floating gate by programming or erasing, respectively.
[0007] It is the primary object of the present invention to provide
Eprom and EEprom cell and array structures and processes for making
them that result in cells of reduced size so their density on a
semiconductor chip can be increased. It is also an object of the
invention that the structures be highly manufacturable, reliable,
scalable, repeatable and producible with a very high yield.
[0008] It is yet another object of the present invention to provide
EEprom semiconductor chips that are useful for solid state memory
to replace magnetic disk storage devices.
[0009] Another object of the present invention is to provide a
technique for increasing the amount of information that can be
stored in a given size Eprom or EEprom array.
[0010] Further, it is an object of the present invention to provide
a technique for increasing the number of program/read cycles that
an EEprom can endure.
SUMMARY OF THE INVENTION
[0011] These and additional objects are accomplished by the various
aspects of the present invention, either alone or in combination,
the primary aspects being briefly summarized as below:
[0012] 1. The problems associated with prior art split channel
Eprom and split channel Flash EEprom devices are overcome by
providing a split channel memory cell constructed in one of the
following ways:
[0013] (A) In one embodiment, one edge of the floating gate is self
aligned to and overlaps the edge of the drain diffusion and the
second edge of the floating gate is self aligned to but is spaced
apart from the edge of the source diffusion. A sidewall spacer
formed along the second edge of the floating gate facing the source
side is used to define the degree of spacing between the two edges.
Self alignment of both source and drain to the edges of the
floating gate results in a split channel Eprom device having
accurate control of the three most critical device parameters:
Channel segment lengths L1 and L2 control-lable by floating gate
and control gate, respectively, and the extent of overlap between
the floating gate and the drain diffusion. All three parameters are
insensitive to mask misalignment and can be made reproducibly very
small in scaled-down devices.
[0014] (B) In a second embodiment of the split channel Eprom a
heavily doped portion of the channel adjacent to the drain
diffusion is formed by a novel, well-controlled technique. The
length Lp and doping concentration of this channel portion become
the dominant parameters for programming and reading, thereby
permitting the formation of a split channel structure which is
relatively insensitive to misalignments between the floating gate
and the source/drain regions.
[0015] 2. A separate erase gate is provided to transform a Eprom
device into a Flash EEprom device. The area of overlap between the
floating gate and the erase gate is insensitive to mask
misalignment and can therefore be made reproducibly very small.
[0016] 3. In some embodiments of this invention, the erase gate is
also used as a field plate to provide very compact electric
isolation between adjacent cells in a memory array.
[0017] 4. A new erase mechanism is provided which employs tailoring
of the edges of a very thin floating gate so as to enhance their
effectiveness as electron injectors.
[0018] 5. A novel intelligent programming and sensing technique is
provided which permits the practical implementation of multiple
state storage wherein each Eprom or flash EEprom cell stores more
than one bit per cell.
[0019] 6. A novel intelligent erase algorithm is provided which
results in a significant reduction in the electrical stress
experienced by the erase tunnel dielectric and results in much
higher endurance to program/erase cycling.
[0020] The combination of various of these features results in new
split channel Eprom or split channel Flash EEprom devices which are
highly manufacturable, highly scalable, and offering greater
storage density as well as greater reliability than any prior art
Eprom or Flash EEprom devices. Memories that utilize the various
aspects of this invention are especially useful in computer systems
to replace existing magnetic storage media (hard disks and floppy
disks), primarily because of the very high density of information
that may be stored in them.
[0021] Additional objects, features and advantages of the present
invention will be understood from the following description of its
preferred embodiments, which description should be taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a cross section of the split channel Flash EEprom
Samachisa prior art cell which erases by tunneling of electrons
from the floating gate to the drain diffusion.
[0023] FIG. 2a is a cross section of the Flash EEprom Kynett prior
art cell which erases by tunneling of electrons from the floating
gate to the source diffusion.
[0024] FIG. 2b is a cross section of the Flash EEprom Kupec prior
art cell with triple polysilicon.
[0025] FIG. 2c is a schematic of the Kupec cell during erase.
[0026] FIG. 3a is a topological view of the triple polysilicon
split channel Flash EEprom prior art Masuoka cell which erases by
tunneling of electrons from the floating gate to an erase gate.
[0027] FIG. 3b is a schematic view of the Masuoka prior art cell of
FIG. 3a.
[0028] FIG. 3c is a view of the Masuoka prior art cell of FIG. 3a
along cross section AA.
[0029] FIG. 3d is a cross section view of the split channel Eprom
Harari prior art cell.
[0030] FIG. 4a is a cross section view of the split channel Eprom
Eitan prior art cell having a drain diffusion self aligned to one
edge of the floating gate.
[0031] FIG. 4b is a cross section view of the prior art Eitan cell
of FIG. 4a during the process step used in the formation of the
self aligned drain diffusion.
[0032] FIG. 4c is a cross section view of the split channel Eprom
Mizutani prior cell with sidewall spacer forming the floating
gate.
[0033] FIG. 4d is a cross section view of the split channel Eprom
Wu prior art cell with sidewall spacer forming one of two floating
gates.
[0034] FIG. 4e is a cross section view of a stacked gate Eprom
Tanaka prior art cell with heavily doped channel adjacent to the
drain junction.
[0035] FIG. 5a is a cross section of a split channel Eprom cell in
accordance with this invention.
[0036] FIG. 5b through 5f are cross sections of the cell of FIG. 5a
during various stages in the manufacturing process.
[0037] FIG. 6a is a top view of a 2.times.2 array of Flash EEprom
cells formed in a triple layer structure in accordance with one
embodiment of this invention.
[0038] FIG. 6b is a view along cross section AA of the structure of
FIG. 6a.
[0039] FIG. 7a is a top view of a 2.times.2 array of Flash EEprom
cells formed in a triple layer structure in accordance with a
second embodiment of this invention wherein the erase gates also
provide field plate isolation.
[0040] FIG. 7b is a view along cross section AA of the structure of
FIG. 7a.
[0041] FIG. 7c is a view along cross section CC of the structure of
FIG. 7a.
[0042] FIG. 8a is a top view of a 2.times.2 array of Flash EEprom
cells formed in a triple layer structure in accordance with a third
embodiment of this invention wherein the tunnel erase dielectric is
confined to the vertical surfaces at the two edges of the floating
gate.
[0043] FIG. 8b is a view along cross section AA of the structure of
FIG. 8a.
[0044] FIG. 9a is a top view of a 2.times.2 array of Flash EEprom
cells formed in a triple layer structure in accordance with a
fourth embodiment of this invention wherein the erase gate is
sandwiched in between the floating gate and the control gate.
[0045] FIG. 9b is a view along cross section AA of the structure of
FIG. 9a.
[0046] FIG. 9c is a view along cross section DD of the structure of
FIG. 9a.
[0047] FIG. 10 is a schematic representation of the coupling
capacitances associated with the floatings gate of the Flash EEprom
cell of the invention.
[0048] FIG. 11a is a schematic representation of the composite
transistor forming a split channel Eprom device.
[0049] FIG. 11b shows the programming and erase characteristics of
a split channel Flash EEprom device.
[0050] FIG. 11c shows the four conduction states of a split channel
Flash EEprom device in accordance with this invention.
[0051] FIG. 11d shows the program/erase cycling endurance
characteristics of prior art Flash EEprom devices.
[0052] FIG. 11e shows a circuit schematic and programming/read
voltage pulses required to implement multistate storage.
[0053] FIG. 12 outlines the key steps in the new algorithm used to
erase with a minimum stress.
[0054] FIG. 13 shows the program/erase cycling endurance
characteristics of the split channel Flash EEprom device of this
invention using intelligent algorithms for multistate programming
and for reduced stress during erasing.
[0055] FIGS. 14a, 14b and 14c are cross sections of another
embodiment of this invention during critical steps in the
manufacturing flow.
[0056] FIGS. 15a and 15b are schematic representations of two
memory arrays for the Flash EEprom embodiments of this
invention.
[0057] FIGS. 16a and 16b are cross sectional views of Flash EEprom
transistors, illustrating the erase mechanism by asperity injection
(16a) and sharp tip injection (16b).
[0058] FIGS. 16c and 16d are cross sectional views of parts of
Flash EEprom transistors illustrating the formation of sharp-tipped
edges of the floating gate by directional etching to facilitate
high field electronic injection.
[0059] FIG. 17a contains Table I which shows voltage conditions for
all operational modes for the array of FIG. 15a.
[0060] FIG. 17b contains Table II which shows example voltage
conditions for all operational modes for the virtual ground array
of FIG. 15b.
DETAILED DESCRIPTION OF THE PRIOR ART
[0061] There are two distinctly different approaches in the prior
art of Flash EEproms. A triple polysilicon device was described by
J. Kupec et al. in 1980 IEDM Technical Digest, p. 602 in an article
entitled "Triple Level Polysilicon EEprom with Single Transistor
per Bit". An improvement to the Kupec device was proposed by F.
Masuoka and H. Iizuka in U.S. Pat. No. 4,531,203, issued Jul. 23,
1985. Variations on the same cell are described by C. K. Kuo and S.
C. Tsaur in U.S. Pat. No. 4,561,004 issued Dec. 24, 1985, and by F.
Masuoka et al. in an article titled "A 256K Flash EEprom Using
Triple Polysilicon Technology", Digest of Technical Papers, IEEE
International Solid-State Circuits Conference, February 1985, p.
168.
[0062] The second approach is a double polysilicon cell described
by G. Samachisa et al., in an article titled "A 128K Flash EEprom
Using Double Polysilicon Technology", IEEE Journal of Solid State
Circuits, October 1987, Vol. SC-22, No. 5, p. 676. Variations on
this second cell are also described by H. Kume et al. in an article
titled "A Flash-Erase EEprom Cell with an Asymmetric Source and
Drain Structure", Technical Digest of the IEEE International
Electron Devices Meeting, December 1987, p. 560, and by V. N.
Kynett et al. in an article titled "An In-System Reprogrammable
256K CMOS Flash Memory", Digest of Technical Papers, IEEE
International Solid-State Circuits Conference, February 1988, p.
132. A cross-section of the Samachisa cell is shown in FIG. 1.
Transistor 100 is an NMOS transistor with source 101, drain 102,
substrate 103, floating gate 104 and control gate 109. The
transistor has a split channel consisting of a section 112 (L1)
whose conductivity is controlled by floating gate 104, in series
with a section 120 (L2) whose conductivity is controlled by control
gate 109. Programming takes place as in other Eprom cells by
injection of hot electrons 107 from the channel at the pinchoff
region 119 near the drain junction. Injected electrons are trapped
on floating gate 104 and raise the conduction threshold voltage of
channel region 112 and therefore of transistor 100. To erase
transistor 100 the oxide in region 112 separating between the
floating gate 104 and drain diffusion 102 and channel 112 is
thinned to between 15 and 20 nanometers, to allow electronic
tunneling of trapped electrons 108 from the floating gate to the
drain. In the Samachisa cell the appropriate voltages applied to
achieve programming are V.sub.CG=12V, V.sub.D=9V, V.sub.BB=0V,
V.sub.S=0V, and to achieve erase are V.sub.CG=OV, V.sub.D=19V,
V.sub.BB=OV, V.sub.S=floating. Samachisa points out that the
electrical erase is not self-limiting. It is possible to overerase
the cell, leaving the floating gate positively charged, thus
turning the channel portion L1 into a depletion mode transistor.
The series enhancement transistor L2 is needed therefore to prevent
transistor leakage in the overerase condition.
[0063] The Samachisa cell suffers from certain disadvantages. These
are:
[0064] (a) It is difficult to prevent avalanche junction breakdown
or high junction leakage current at the drain junction 102 during
the time the very high erase voltage is applied to the drain;
[0065] (b) It is difficult to-grow with high yields the thin oxide
layer 112 used for tunnel erase;
[0066] (c) Because of the presence of thin oxide layer between the
floating gate and the drain diffusion, it is difficult to prevent
accidental tunneling of electrons from the floating gate to the
drain in what is known as the "program disturb" condition. Under
this condition an unselected cell in a memory array sharing the
same drain (bit line) as a programmed cell may have a drain voltage
of approximately 10 volts and a control gate voltage of 0 volts.
Although this represents a much weaker electric field than that
experienced during tunnel erase (when the drain is at approximately
19 volts), it nevertheless can, over a prolonged period of time
alter by slow tunneling the charge stored on the floating gate.
[0067] The Kynett and Kume cells (FIG. 2a) are similar to the
Samachisa cell except for the elimination of the series enhancement
transistor 120, and the performing of tunnel erase 208 over the
source diffusion 201 rather than over the drain diffusion 202.
Typically the Kynett cell uses during programming voltages
V.sub.CG=12V, V.sub.D=8V, V.sub.S=0V, V.sub.BB=0V, and during erase
voltages V.sub.S=12V, V.sub.BB=OV, V.sub.CG=OV, V.sub.D=Floating.
Kynett achieves a lower erase voltage than Samachisa by thinning
tunnel dielectric 212 to 10 nanometers or less, so that even though
the voltage applied to the source diffusion during erase is
reduced, the electric field across tunnel dielectric 212 remains as
high as in the case of the Samachisa cell.
[0068] The Kynett cell can be contrasted with the Samachisa
cell:
[0069] (a) Kynett is less susceptible to avalanche breakdown of
source diffusion 201 during erase because the voltage is reduced
from 19 volts to 12 volts.
[0070] (b) Kynett's cell is more susceptible to low yields due to
pinholes in the thin dielectric layer 212 because its thickness is
reduced from approximately 20 nanometers to approximately 10
nanometers.
[0071] (c) Because Kynett uses a lower voltage for erase but
essentially the same drain voltage for programming Kynett is far
more susceptible to accidental "program disturb" due to partial
tunnel erase (during programming) occuring from floating gate 204
to drain 202.
[0072] (d) Kynett's cell is highly susceptible to an overerase
condition because it does not have the series enhancement channel
portion 120 of Samachisa's cell. To prevent overerase Kynett et al.
deploy a special erase algorithm. This algorithm applies a short
erase pulse to an array of cells, then measures the threshold
voltage of all cells to ensure that no cell has been overetased
into depletion. It then applies a second erase pulse and repeats
the reading of all cells in the array. This cycle is stopped as
soon as the last cell in the array has been erased to a reference
enhancement voltage threshold level. The problem with this approach
is that the first cell to have been adequately erased continues to
receive erase pulses until the last cell has been adequately
erased, and may therefore be susceptible to overerase into a
depletion threshold state.
[0073] Kupec's cell employs essentially the Kynett cell without a
thin tunnel dielectric over the source, channel, or drain, and with
a third polysilicon plate covering the entire transistor and acting
as an erase plate. A cross sectional view of the Kupec device is
shown in FIG. 2b. Transistor 200b consists of a stacked floating
gate 204b and control gate 209b with source 201b and drain 202b
self aligned to the edges of the floating gate. Gate dielectric 212
is relatively thick and does not permit tunnel erase from floatina
gate to source or drain. An erase plate 230b overlies the control
gate and covers the sidewalls of both the control gate and the
floating gate. Erase takes place by tunneling across the relatively
thick oxide 231b between the edges of floating gate 204b and erase
plate 230b. Kupec attempts to overcome the overerase condition by
connecting the erase plate during high voltage erase to drain 202b
and through a high impedance resistor R (FIG. 2c) to the erase
supply voltage V.sub.ERASE. As soon as the cell is erased into
depletion the drain to source transistor conduction current drops
most of the erase voltage across the resistor, reducing the voltage
on the erase plate 230b to below the tunneling voltage. This
approach is extremely difficult to implement in a block erase of a
large array because different transistors begin conduction at
different times.
[0074] Masuoka's approach to Flash EEprom overcomes most of the
disadvantages of the Samachisa, Kynett and Kupec cells. FIG. 3a
provides a top view of the Masuoka prior art cell, FIG. 3b shows
the schematic representation of the same cell, and FIG. 3c provides
a cross section view along the channel from source to drain.
Transistor 300 consists of a split channel Eprom transistor having
a source 301, a drain 302, a floating gate 304 controlling channel
conduction along section L1 (312) of the channel, a control gate
309 capacitively coupled to the floating gate and also controlling
the conduction along the series portion of the channel L2 (320),
which has enhancement threshold voltage.
[0075] The transistor channel width (W), as well as the edges of
the source and drain diffusions are defined by the edges 305 of a
thick field oxide formed by isoplanar oxidation. Oxide 332 of
thickness in the 25 to 40 nanometers range is used as isolation
between the floating gate and the substrate. Masuoka adds an erase
gate 330 disposed underneath the floating gate along one of its
edges. This erase gate is used to electrically erase floating gate
304 in an area of tunnel dielectric 331 where the floating gate
overlaps the erase gate. Tunnel dielectric 331 is of thickness
between 30 and 60 nanometers.
[0076] Masuoka specifies the following voltages during erase:
V.sub.S=OV, V.sub.D=OV, V.sub.CG=OV, V.sub.BB=OV, V.sub.ERASE=20V
to 30V.
[0077] Comparing the Masuoka cell with the Samachisa and Kynett
cells:
[0078] (a) Masuoka's cell does not erase by using either the source
diffusion or the drain diffusion for tunnel erase. Therefore these
diffusions never experience a voltage higher than during Eprom
programming. The junction avalanche breakdown and junction leakage
problems therefore do not exist.
[0079] (b) Masuoka's cell uses a relatively thick tunnel dielectric
and therefore does not need to use thin tunnel dielectrics for
erase. Therefore it is less susceptible to oxide pinholes
introduced during the manufacturing cycle.
[0080] (c) Masuoka's cell does not have a "program disturb" problem
because programming and tunnel erase involve two different
mechanisms occuring at two different regions of the transistor.
[0081] (d) Masuoka's cell is not susceptible to the overerase
condition because of the presence of the series enhancement
transistor channel 320 (L2).
[0082] (e) Masuoka's cell requires a third layer of polysilicon,
which complicates the process as well as aggravates the surface
topology. Because the erase gate consumes surface area over the
field oxide 305 it results in a larger cell.
[0083] (f) The overlap area 331 in Masuoka's cell is sensitive to
mask misalignment between the two masks defining this overlap.
Since the overlap area is nominally very small, even small
misalignments can result in large variations in the area used for
tunnel erase. This results in severe variations from wafer to
wafer.
[0084] From the foregoing analysis it is clear that while the
Masuoka prior art cell successfully addresses most of the problems
encountered by Samachisa and Kynett, it itself has disadvantages
not encountered by Samachisa or Kynett.
[0085] Masuoka and Samachisa both use a split channel Eprom
transistor for programming. In the split channel eprom transistor,
the portion L2 of the channel length controlled by control gate
109, 309 has a fixed enhancement threshold voltage determined by
the p+ channel doping concentration 360. The portion L1 of the
channel length controlled by floating gate 104 (Samachisa) and 304
(Masuoka) has a variable threshold voltage determined by the net
charge stored on the floating gate.
[0086] Other prior art split channel Eprom transistors are
described by E. Harari in U.S. Pat. No. 4,328,565, May 4, 1982 and
by B. Eitan in U.S. Pat. No. 4,639,893, Jan. 27, 1987. The Harari
split channel Eprom transistor 300d is shown in cross section in
FIG. 3d. Source 301d and drain 302d are formed prior to formation
of the floating gate 304d. Therefore, the total channel length
L1+L2 is insensitive to mask misalignment. However, both L1 and L2
are sensitive to misalignment between floating gate 304d and drain
diffusion 302d.
[0087] The Eitan split channel Eprom transistor 400 is shown in
cross sections in FIG. 4a. The Eitan patent highlights the main
reasons for using a split channel architecture rather than the
standard self aligned stacked gate Eprom transistor 200 (FIG. 2).
These reasons can be summarized as follows:
[0088] The addition of a fixed threshold enhancement transistor in
series with the floating gate transistor decouples the floating
gate from the source diffusion. This allows the channel length L1
to be made very small without encountering punchthrough between
source and drain. Furthermore, transistor drain-turnon due to the
parasitic capacitive coupling between the drain diffusion and the
floating gate is eliminated because the enhancement channel portion
L2 remains off.
[0089] Eitan shows that the shorter the length L1 the greater the
programming efficiency and the greater the read current of the
split channel Eprom transistor. For Flash EEprom devices the series
enhancement channel L2 acquires additional importance because it
allows the floating gate portion L1 to be overerased into depletion
thereshold voltage without turning on the composite split channel
transistor.
[0090] The disadvantages incurred by the addition of the series
enhancement channel L2 are an increase in cell area, a decrease in
transistor transconductance, an increase in control gate
capacitance, and an increase in variability of device
characteristics for programming and reading brought about by the
fact that L1 or L2 or both are not precisely controlled in the
manufacturing process of the prior art split channel devices.
Samachisa, Masuoka and Eitan each adopt a different approach to
reduce the variability of L1 and L2:
[0091] Samachisa's transistor 100 (FIG. 1) uses the two edges 140,
143 of control gate 109 to define (by a self aligned ion implant)
drain diffusion 102 and source diffusion 101. Edge 141 of floating
gate 104 is etched prior to ion implant, using edge 140 of control
gate 109 as an etch mask. This results in a split channel
transistor where (L1+L2) is accurately controlled by the length
between the two edges 140, 143 of the control gate. However, L1 and
L2 are both sensitive to misalignment between the mask defining
edge 142 and the mask defining edges 140, 143.
[0092] Masuoka's transistor 300 (FIG. 3c) forms both edges 341, 342
of floating gate 304 in a single masking step. Therefore L1 is
insensitive to mask misalignment. L2, which is formed by ion
implant of source diffusion 301 to be self aligned to edge 343 of
control gate 309, is sensitive to misalignment between the mask
defining edge 342 and the mask defining edge 343. Furthermore the
Masuoka transistor 300 may. form a third channel region, L3, if
edge 340 of control gate 309 is misaligned in a direction away from
edge 341 of floating gate 304. the formation of L3 will severely
degrade the programming efficiency of such a cell.
[0093] Eitan's transistor 400 (FIGS. 4a, 4b) uses a separate mask
layer 480 to expose the edge of floating gate 404 to allow drain
diffusion 402 to be self aligned (by ion implantation) to edge 441
of floating gate 404. Therefore L1 can be accurately controlled and
is not sensitive to mask misalignment. L2 however is sensitive to
the misalignment between edge 482 of photoresist 480 and edge 442
of the floating gate. Eitan claims that the variability in L2 due
to this mask misalignment, can be as much as 1.0 micron or more
without affecting the performance of the device (see claims 3, 4 of
the above-referenced Eitan patent).
[0094] It should be pointed out that even with the most advanced
optical lithography systems available today in a production
environment it is difficult to achieve an alignment accuracy of
better than .+-.0.25 microns between any two mask. layers.
Therefore the variability in L2 or L1 inherent to any structure
which is alignment sensitive can be as much as approximately 0.5
microns from one extreme to the other.
[0095] Another prior art split channel Eprom device which attempts
to achieve the objective of accurately establishing L1 and L2 is
disclosed by Y. Mizutani and K. Makita in the 1985 IEDM Technical
Digest, p. 63, shown in cross section in FIG. 4c. Transistor 400c
has a floating gate 404c formed along the sidewall 440c of control
gate 409c. In this way both L1 and L2 can be independently
established and are not sensitive to mask misalignment. Transistor
400c has the drawback that the capacitive coupling between control
gate 409c and floating gate 404c is limited to the capacitor area
of the sidewall shared between them, which is relatively a small
area. Therefore there is a very weak capacitive coupling between
the control gate and the floating gate either during programming or
during read. Therefore, although the device achieves good control
of L1 and L2 it is of rather low efficiency for both modes of
operation.
[0096] Yet another prior art device which has a split channel with
a well controlled L1 and L2 is disclosed by A. T. Wu et al. in the
1986 IEDM Technical Digest, p. 584 in an article entitled "A Novel
High-Speed, 5-Volt Programming Eprom Structure with Source-Side
Injection". A cross section of the Wu prior art transistor is shown
in FIG. 4d (FIG. 2 in the above-referenced article). This
transistor has a floating gate 404d coupled to a control gate 409d,
extending over channel region L1 (412d), in series with a second
floating gate 492d formed in a sidewall adjacent to source
diffusion 401d and overlying channel region L2 (420d). This second
floating gate is capacitively coupled to the control gate 409d
through the relatively small area of the sidewall 493d shared
between them and is therefore only marginally better than the
Mizutani prior art device, although it does achieve a good control
of both L1 and L2.
[0097] Another prior art Eprom transistor which does not have a
split channel structure but which seeks to achieve two distinct
channel regions to optimize the Eprom programming performance is
disclosed by S. Tanaka et al. in 1984 ISSCC Digest of Technical
Papers, p. 148 in an article entitled "A Programmable 256K CMOS
Eprom with On Chip Test Circuits". A cross section of this device
is shown in FIG. 4e (corresponding to FIG. 3 in the Tanaka
article). Transistor 400e is a stacked gate Eprom transistor (not
split channel) with source 401e and drain 402e self aligned to both
edges of floating gate 404e and control gate 409e. The channel
region is more heavily p doped 460e than the p substrate 463e, but
in addition there is a second p+ region 477e which is even more
heavily p-doped than region 460e. This region 477e is formed by
diffusion of boron down and sideways from the top surface on the
drain side only, and is formed after formation of the floating gate
so as to be self aligned to the floating gate on the drain side.
The extent of sideway diffusion of boron ahead of the sideway
diffusion of arsenic from the N+ drain junction defines a channel
region Lp (478e) adjacent to the drain. This is a DMOS type
structure, called DSA (Diffusion Self Aligned) by Tanaka. The
presence of the p+ region 478e reduces considerably the width of
the drain depletion region during high voltage programming. A
shorter depletion layer width results in greater energy being
imparted to channel electrons entering the depletion region, which
in turn results in significant increase in programming efficiency
through hot electron injection. Transistor 400e has proven
difficult to manufacture because it is rather difficult to control
the length Lp and the surface channel concentration p+ through a
double diffusion step. Furthermore, it is rather difficult to
obtain value of Lp bigger than approximately 0.3 microns by
diffusion because device scaling dictates the use of rather low
temperature diffusion cycles. Still further, the DSA Eprom device
suffers from an excessively high transistor threshold voltage in
the unprogrammed (conducting) state, as well as from high drain
junction capacitance. Both these effects can increase substantially
the read access time.
Detailed Description of Specific Embodiments of the Invention
[0098] I.a. Split Channel Eprom Transistor with Self Aligned Drain
Diffusion and Self Aligned Spaced Apart Source Diffusion
[0099] FIG. 5a presents a cross sectional view of a split channel
Eprom transistor in accordance with. a first embodiment of this
invention. Transistor 500a consists of a p type silicon substrate
563 (which can alternatively be a p type epitaxial layer grown on
top of a p++ doped silicon substrate), N+ source diffusion 501a, N+
drain diffusion 502a, a channel region 560a which is more heavily
p-doped than the surrounding substrate, a floating gate 504a
overlying a portion L1 of the channel, 512a, and a control gate 509
overlying the remaining portion L2 of the channel, 520a as well as
the floating gate. Floating gate 504a is dielectrically isolated
from the surface of the silicon substrate by dielectric film 564a,
which is thermally grown Silicon Dioxide. Control gate 509 is
capacitively coupled to floating gate 504a through dielectric film
567a, which can either be thermally grown Silicon Dioxide or a
combination of thin layers of Silicon Dioxide and Silicon Nitride.
Control gate 509 is also insulated from the silicon surface in
channel portion L2 as well as over the source and drain diffusions
by dielectric film 565a, which is made of the same material as
dielectric 567a.
[0100] P-type substrate 563 is typically 5 to 50 Ohms centimeter,
p+ channel doping 560a is typically in the range of
1.times.10.sup.16 cm.sup.-3 to 2.times.10.sup.17 cm.sup.-3,
dielectric film 564a is typically 20 to 40 nanometers thick,
dielectric film 567a is typically 20 to 50 nanometers thick,
floating gate 504a is usually a heavily N+ doped film of
polysilicon of thickness which can be as low as 25 nanometers (this
thickness will be discussed in Section VII) or as high as 400
nanometers. Control gate 509 is either a heavily N+ doped film of
polysilicon or a low resistivity interconnect material such as a
silicide or a refractory metal. Of importance, edge 523a of N+
drain diffusion 502a formed by ion implantation of Arsenic or
Phosphorus is self aligned to edge 522a of floating gate 504a,
while edge 521a of N+ source diffusion 501a formed by the same ion
implantation step is self aligned to, but is spaced apart from,
edge 550a of the same floating gate 504a, using a sidewall spacer
(not shown in FIG. 5a) which is removed after the ion implantation
but prior to formation of control gate 509. The implant dose used
to form diffusions 501a, 502a, is typically in the range of
1.times.10.sup.15 cm.sup.-2 to 1.times.10.sup.16 cm.sup.-2.
[0101] The key steps for the formation of channel portions L1 and
L2 are illustrated in FIGS. 5b through 5f. In the structure of FIG.
5b floating gates 504a, 504b are formed in a layer of N+ doped
polysilicon on top of a thin gate oxide 564a, by anisotropic
reactive ion etchings, using photoresist layer 590 as a mask. In
FIG. 5c a thin protective film 566a is deposited or thermally
grown, followed by the deposition of a thick spacer layer 570. The
purpose of film 566a is to protect the underlying structure such as
layer 565a from being etched or attacked when the spacer film is
etched back. The spacer film is now etched back in an anisotropic
reaction ion etch step with carefully controlled timing. The
conditions for etchback must have no significant undercutting and
must have a differential etch rate of 20:1 or higher between the
spacer material and the material of protective film 566a. Spacer
layer 570 can be a conformal film of undoped LPCVD polysilicon
while protective film 566a can be silicon dioxide or silicon
nitride. Alternatively, spacer layer 570 can be a conformal film of
LPCVD silicon dioxide while protective film 566a can be either
LPCVD silicon nitride or LPCVD polysilicon. The thickness of
protective film 566a should be as thin as possible, typically in
the range of 10 to 30 nanometers, so as to to allow penetration of
the subsequent Arsenic implantation to form the source and drain
diffusions.
[0102] The thickness of the conformal spacer layer determines the
width of the sidewall spacer, and therefore also the length of
channel portion L2. Typically for an L2 of 400 nanometers a spacer
layer of approximately 600 nanometers thickness is used.
[0103] In FIG. 5d spacers 592a, 593a and 592b, 593b are formed
along the vertical edges of floating gates 504a and 504b
respectively at the completion of the timed reactive ion etch step.
These spacers result from the fact that the thickness of layer 570
is greater adjacent to the vertical walls of the floating gates
than it is on flat surfaces. Therefore a carefully timed
anisotropic reactive ion etchback will etch through layer 570 in
areas of flat surface topology while not completely etching through
it along each edge, forming the spacers. The technique for
formation of narrow sidewall spacers along both edges of the gate
of MOS transistors is well known in the industry, and is commonly
used to form lightly doped drains (LDD) in short channel MOSFETS.
(See, for example, FIG. 1 in an article in 1984 IEDM Technical
Digest, p. 59 by S. Meguro et al. titled "Hi-CMOS III
Technology".)
[0104] In the present invention, the spacer can be significantly
wider, it is used along one edge only, and it is used not to define
a lightly doped source or drain but rather to define the series
enhancement transistor channel portion L2.
[0105] The next step is a masking step. Photoresist 591a, 591b
(FIG. 5d) is used as a mask to protect spacers 592a, 592b while
exposing spacers 593a, 593b. The latter are etched away, preferably
with a wet chemical etch (which should be chosen so as to not etch
protective film 566a), and the photoresist is stripped.
[0106] In FIG. 5e ion implantation of Arsenic through dielectric
films 566a and 565a is used to form N+ source diffusions 501a, 501b
and N+ drain diffusions 502a, 502b. On the drain side these
diffusions are self aligned to edges 522a and 522b of the floating
gates. On the source side the diffusions are self aligned to edges
550a and 550b of the floating gates but are spaced apart from these
edges by the width of spacers 592a and 592b less the sideways
diffusion in subsequent high temperature process steps.
[0107] Next, spacers 592a, 592b and the protective film 566a are
removed (FIG. 5f), preferably with wet etches which will not attack
the underlying layers 565a and 504a. Dielectric film 567a is grown
by thermal oxidation or deposited by LPCVD on the exposed surfaces
of the floating gates and substrate. A conductive layer is then
deposited and control gates 509a, 509b are formed through etching
of long narrow strips which constitute the word lines in rows of
memory cells in an array.
[0108] The remaining part of the process is standard:
[0109] The surface of the structure is covered with a thick
passivation layer 568, usually phosphorous doped glass or a
Borophosphosilicate glass (BPSG). This passivation is made to flow
in a high temperature anneal step. Contact vias are etched (not
shown in FIG. 5f) to allow electrical access to the source and
drain diffusions. Metallic interconnect strips 569a, 569b are
provided on top of passivation layer 568, accessing the source and
drain diffusions through the via openings (not shown).
[0110] Comparing split channel transistor 500a of FIG. 5f with the
Samachisa, Masuoka, Harari and Eitan prior art split channel
transistors 100, 300, 300d and 400, the advantages of transistor
500a can be summarized as follows:
[0111] a) L1 and L2 are insensitive to mark misalignment. Therefore
they can be controlled much more accurately and reproducibly than
the prior art.
[0112] b) Because all four prior art transistors 100, 300, 300d and
400 define L2 through a mask alignment tolerance whereas transistor
500a defines L2 through control of the width of a sidewall spacer
it is possible in transistor 500a to achieve controllably a much
shorter channel portion L2 than possible through a mask alignment.
This becomes an important consideration in highly scaled split
channel Eprom and Flash EEprom transistors.
[0113] I.b. Split Channel Eprom Transistor with Heavily Doped
Channel Adjacent to the Drain Junction
[0114] FIG. 14c presents a cross sectional view of a non self
aligned split channel Eprom transistor in accordance with a second
embodiment of this invention. FIGS. 14a and 14b illustrate the
critical process steps in the manufacturing process of this device.
Transistor 1400 consists of a p type silicon substrate 1463 (which
can also be a p type epitaxial layer grown on a p++ substrate).
Shallow N+ source diffusions 1401 and N+ drain diffusions 1402 are
formed prior to formation of floating gate 1404, in contrast with
the embodiment of section Ia above. The channel region between the
source and drain diffusions is split into two portions: a portion
L1 (1412) which is lying directly underneath the floating gate, and
a portion L2 (1420) which is lying directly underneath the control
gate 1409. The improvement over the Harari prior art split channel
transistor 300d (FIG. 3d) consists of a heavily p+ doped narrow
region 1460 adjacent to drain diffusion 1402. The width Lp (1413)
and doping concentration of this region at the top surface where
the field effect transistor. channel is formed, become the
controlling parameters for device programming and reading
efficiency, provided that p+ is sufficiently high. Typically, p
substrate 1463 may have a p type doping concentration of
1.times.10.sup.16 cm.sup.-3 whereas p+ region 1460 may have a p+
type doping concentration of between 1.times.10.sup.17 cm.sup.-3
and 1.times.10.sup.18 cm.sup.-3. In the preferred manufacturing
process the length Lp and doping concentration of region 1460 are
chosen so that the depletion region width at the drain junction
under programming voltage conditions is less than the width Lp. So
long as that condition is satisfied, and so long as L1 is bigger
than Lp, then the actual value of L1 is of secondary importance to
the device performance. Since L1 in this device is determined
through a mask alignment between the floating gate and the drain it
is not as well controlled as in the Eitan prior art transistor 400.
However, to the extent that region 1460 can be made to be self
aligned to the drain so that parameter Lp is not sensitive to mask
alignment, then any variability in L1 is of secondary importance,
Lp being the controlling parameter.
[0115] A new method is disclosed for manufacturing the split
channel Eprom transistor 1400 which results in much better control
of the parameter Lp and of the surface channel doping concentration
1413 than is provided by the DSA (Diffusion Self Align) approach of
the Tanaka prior art transistor 400e (FIG. 4e).
[0116] The main steps in this new method for the fabrication of a
memory array of transistors 1400 are as follows:
[0117] 1. In the structure of FIG. 14a a thin oxide layer 1475,
typically 50 nanometers of silicon dioxide, is covered with a layer
1474 of silicon nitride, approximately 100 nanometers thick. This
in turn is covered with a second layer 1473 of deposited silicon
dioxide, approximately 100 nanometers thick. Oxide 1475 and nitride
1474 can, for example, be the same films used to form isoplanar
isolation regions in the periphery of the memory array.
[0118] 2. A photoresist mask P.R.1 (1470) is used to define source
and drain regions in long parallel strips extending in width
between edges 1471, 1472 of openings in the photoresist. Exposed
oxide layer 1473 is now wet etched in a carefully controlled and
timed etch step which includes substantial undercutting of
photoresist 1470. The extent of undercutting, which is measured by
the distance Lx between oxide edges 1476 and 1478, will eventually
determine the magnitude of parameter Lp. Typically, Lx is chosen
between 300 nanometers and 700 nanometers. The three parameters
critical for a reproducible Lx are the concentration and
temperature of the etch solution (hydrofluoric acid) and the
density (i.e., lack of porosity) of the oxide 1473 being etched.
These can be well controlled sufficiently so that a timed
undercutting etch step results in well controlled etched strips of
width Lx and running parallel to edges 1471, 1472 of the long
openings in the photoresist. In fact, for values of Lx below
approximately 500 nanometers, it is easier to achieve a
reproducible Lx through controlled sideway etching than by
controlling the line width of long, narrow line in a photoresist
layer. An example of the use of sideway etching self aligned to an
edge in a similar fashion (but to achieve the different purpose of
forming a very narrow guard ring) can be found in the prior art
article by S. Kim titled "A Very Small Schottky Barrier Diode with
Self-Aligned Guard Ring for VLSI Application", appearing in the
1979 IEDM Technical Digest, p. 49.
[0119] 3. At the completion of the sideway etch step a second,
anisotropic etch is performed, using the same photoresist mask
P.R.1 to etch away long strips of the exposed silicon nitride film
1474. Edges 1471, 1472 of P.R.1 (1470) are used to form edges 1480,
1481 respectively in the etched strips of nitride layers.
[0120] 4. Arsenic ion implantation with an ion dose of
approximately 5.times.10.sup.15 cm.sup.-2 is performed with an
energy sufficient to penetrate oxide film 1475 and dope the surface
in long strips of N+ doped regions (1402, 1401). Photoresist mask
P.R.1 can be used as the mask for this step, but nitride layer 1474
can serve equally well as the implant mask. P.R.1 is stripped at
the completion of this step.
[0121] 5. An implant damage anneal and surface oxidation step
follows, resulting in 200 to 300 nanometers of silicon dioxide 1462
grown over the source and drain diffusion strips. The temperature
for this oxidation should be below 1000.degree. C. to minimize the
lateral diffusion of the N+ dopants in regions 1402, 1401. If
desired it is possible through an extra masking step to remove
nitride layer 1474 also from the field regions between adjacent
channels, so as to grow oxide film 1462 not only over the source
and drain regions but also over the field isolation regions.
[0122] 6. In FIG. 14b a second photoresist mask P.R.2 (1482) is
used to protect the source-side (1401) of the substrate during the
subsequent implant step. This implant of boron can be performed at
relatively high energy sufficient to penetrate through nitride
layer 1474 and oxide layer 1475 but not high enough to penetrate
top oxide 1473, nitride 1474 and oxide 1475. Alternatively, nitride
layer 1474 can first be etched along edge 1482, using edge 1478 of
the top oxide 1473 as a mask. The boron implant dose is in the
range of 1.times.10.sup.13 cm.sup.-2 and 1.times.10.sup.14
cm.sup.-2. The surface area of heavy p+ doping 1460 is confined to
the very narrow and long strip of width extending between edge 1478
of the top oxide and the edge of the N+ diffusion 1402, and running
the length of the drain diffusion strip. Note that the thick oxide
1462 prevents penetration of the boron implant into the drain
diffusion strip. This greatly reduces the drain junction
capacitance, which is highly desirable for fast reading. Note also
that p+ region 1460 is automatically self aligned to drain region
1402 through this process.
[0123] 7. Top oxide 1473, nitride 1474 and thin oxide 1475 are now
removed by etching. This etching also reduces the thickness of the
oxide layer 1462 protecting the source and drain diffusions. It is
desirable to leave this film thickness at not less than
approximately 100 nanometers at the completion of this etch
step.
[0124] 8. The remaining steps can be understood in relation to the
structure of FIG. 14c: A gate oxide 1464 is grown over the surface,
including the channel regions, separating between the long
source/drain diffusion strips (typical oxide thickness between 15
and 40 nanometers). A layer of polysilicon is deposited (thickness
between 25 and 400 nanometers), doped N+, masked and etched to form
continuous narrow strips of floating gates 1404 mask aligned to run
parallel to drain diffusion strips 1402 and to overlap p+ regions
1460.
[0125] 9. A second dielectric 1466, 1411 is grown or deposited on
top of the substrate and floating gate strips, respectively. This
can be a layer of silicon dioxide or a combination of thin films of
silicon dioxide and silicon nitride, of combined thickness in the
range between 20 and 50 nanometers.
[0126] 10. A second layer of polysilicon is deposited, doped N+ (or
silicided for lower resistivity), masked and etched to form control
gates 1409 in long strips running perpendicular to the strips of
floating gates and source/drain strips. Each control gate strip is
capacitively coupled to the floating gate strips it crosses over
through dielectric film 1411 in the areas where the strips overlap
each other. Control gates 1409 also control the channel conduction
in channel portions L2 not covered by the floating gate strips.
Each strip of control gates is now covered by a dielectric
isolation film (can be thermally grown oxide).
[0127] 11. Using the strips of control gates as a mask, exposed
areas of dielectric 1466, 1411 and of the strips of first
polysilicon floating gates are etched away. The resulting structure
has long strips, or rows, of control gates, each row overlying
several floating gates 1404 where the outer edges of each floating
gate are essentially self aligned to the edges defining the width
of the control gate strip. These edges are now oxidized or covered
with a deposited dielectric to completely insulate each floating
gate. Field areas between adjacent rows of cells or between
adjacent strips of source and drain regions are now automatically
self aligned to the active device areas and do not require space
consuming isoplanar oxidation isolation regions. (Of course, it is
also possible to fabricate transistor 1400 with source, drain and
channel regions defined by the edges of a thick isoplanar oxidation
isolation layer, or to rely for field isolation on oxide 1462 grown
also in the field regions, see the option described in step 5
above.)
[0128] The Eprom cell of this embodiment has several advantages
over the prior art Eprom cells:
[0129] a) Control gate 1409 now runs over a relatively thick oxide
1462 over the source and drain regions. Such a thick oxide is not
possible for example with the prior art Eitan cell, where these
source and drain regions are formed after, not before, the floating
gate is formed. This improves the protection from oxide breakdowns
and reduces the parasitic capacitance between control gate and
drain.
[0130] b) Control of parameter Lp and of the surface P+ doping
concentration in region 1460 is superior to that afforded by the
DSA prior art Tanaka cell.
[0131] c) The device sensitivity to misalignment between floating
gate and drain is far less than that experienced with the prior art
Harari, Samachisa and Masuoka cells.
[0132] d) For a given p+ concentration in the channel region, drain
junction capacitance is less with this cell than with all other
prior art devices, because p+ region 1460 is very narrowly confined
near the drain diffusion.
[0133] e) It is possible to dope p+ region 1460 to very high levels
(which significantly enhances the programming efficiency) without
unduly raising the conduction threshold voltage in the enhancement
series channel region L2. This is particularly useful for Flash
EEprom embodiments using this cell for the Eprom part. In such a
Flash EEprom, the high initial threshold volage in region Lp
controlled by floating gate 1404 (initial Vt can be as high as
+5.0V, the supply voltage, or higher), can be easily overcome by
erasing the cell to lower threshold voltages. As an Eprom device
the initial Vt in the unprogrammed state must not be higher than
the control-gate voltage during read, and this requirement sets an
upper limit on how high the p+ doping concentration can be. Another
limit on the magnitude of p+ doping concentration 1460 is
established by the minimum drain voltage necessary for programming.
The drain junction avalanche breakdown voltage must be at least as
high as this minimum programming voltage.
[0134] II. Self Aligned Split Channel Flash EEprom Cell with
Isoplaner Field Isolation
[0135] FIG. 6a presents a topological view of a 2.times.2 memory
array consisting of four Flash EEprom transistors 600a, 600b, 600c
and 600d in accordance with one embodiment of this invention. FIG.
6b presents a cross section view of the same structure along AA of
FIG. 6a. A second cross section along BB results in the Eprom
transistor 500a shown in FIG. 5a.
[0136] Transistor 600a of FIG. 6a is a split channel Eprom
transistor which has added to it erase gates 530, 535, which
overlap edges 532a, 562a of floating gate 504a. Transistor 600a is
programmed as a split channel Eprom transistor having a source
diffusion 501a, a drain diffusion 502a, and a control gate 509.
Floating gate 504a and channel portions L1 and L2 are formed in
accordance with the split channel Eprom transistor 500aof section
I.a. or the split channel Eprom transistor 1400 of section I.b.
However other split channel Eprom devices (such as the Eitan,
Harari, Masuoka or Samachisa prior art Eprom) can also be used for
the Eprom structure. The transistor channel width W is defined by
the edges 505, 505a of a thick field oxide 562.
[0137] Transistor 600a is erased by tunneling of electrons from
floating gate 504a to erase gates 530, 535, across tunnel
dielectrics 531a, 561a on the sidewalls and top surface of the
floating gate where it is overlapped by the erase gate.
[0138] Tunnel dielectric film 531a, 561a is normally a layer of
Silicon Dioxide grown through thermal oxidation of the heavily N+
doped and textured polycrystalline silicon comprising the floating
gate. It is well known in the industry (see for example an article
by H.A.R. Wegener titled "Endurance Model for textured-poly
floating gate memories", Technical Digest of the IEEE International
Electron Device Meeting, December 1984, p. 480) that such a film,
when grown under the appropriate oxidation conditions over properly
textured doped polysilicon allows an increase by several orders of
magnitude of the conduction by electron tunneling even when the
film is several times thicker than tunnel dielectric films grown on
single crystal silicon (such as the tunnel dielectric films used in
the prior art Samachisa and Kynett devices). For example, a tunnel
dielectric oxide grown to a thickness of 40 nanometers on N+ doped
and textured polysilicon can conduct by electronic tunneling
approximately the same current density as a tunnel dielectric oxide
of 10 nanometers thickness grown on N+ doped single crystal silicon
under identical voltage bias conditions. It is believed that this
highly efficient tunneling mechanism is a result of sharp
asperities at the grain boundaries of the polysilicon which is
specially textured to enhance the areal density of such asperities.
A commonly practices technique is to first oxidize the surface of
the polysilicon at a high temperature to accentuate the texturing,
then stripping that oxide and regrowing a tunnel oxide at a lower
temperature. The oxide film capping such an asperity experiences a
local amplification by a factor of four to five of the applied
electric field resulting in an efficient localized tunnel injector.
The advantage provided by the thicker films of tunnel dielectric is
that they are much easier to grow in uniform and defect-free
layers. Furthermore the electric field stress during tunneling in
the thick (40 nanometer) tunnel dielectric is only 25 percent of
the stress in the thin (10 nanometer) tunnel dielectric, assuming
the same voltage bias conditions. This reduced stress translates
into higher reliability and greater endurance to write/erase
cycling. For these reasons, all Flash EEprom embodiments of this
invention rely on polypoly erase through a relatively thick tunnel
dielectric.
[0139] In the embodiment of FIGS. 6a, 6b floating gate 504a is
formed in a first layer of heavily N+ doped polysilicon of
thickness between 25 and 400 nanometers, erase gates 530, 535 are
formed in a second layer of N+ doped polysilicon of thickness
between 50 and 300 nanometers, and control gate 509 is formed in a
third conductive layer of thickness between 200 and 500 nanometers,
which may be N+ doped polysilicon or a polycide, a silicide, or a
refractory metal. The erase gate can be formed in a relatively thin
layer because a relatively high sheet resistivity (e.g., 100 Ohm
per square) can be tolerated since almost no current is carried in
this gate during tunnel erase.
[0140] The manufacturing process can be somewhat simplified by
implementing erase gates 530, 535 in the same conductive layer as
that used for control gate 509. However the spacing Z between the
edges of the control gate and the erase gate (and hence the cell
size) would then have to be significantly greater than is the case
when the control gate and erase gates are implemented in two
different conductive layers insulated from each other by dielectric
film 567a. In fact, in the triple layer structure 600a of FIG. 6a
it is even possible to have control gate 509 slightly overlap one
or both of the erase gates 530 and 535 (i.e., spacing Z can be zero
or negative.) Transistor 600a employs a field isolation oxide 562
(FIG. 6b) of thickness between 200 and 1000 nanometers. Gate oxide
564a protecting channel portion L1 (512a) is thermally grown
silicon dioxide of thickness between 15 and 40 nanometers.
Dielectric film 567a which serves to strongly capacitively couple
control gate 509 and floating gate 504a is grown or deposited. It
may be silicon dioxide or a combination of thin films of silicon
dioxide and oxidized silicon nitride of combined thickness of
between 20 and 50 nanometers. This dielectric also serves as part
of the gate oxide protecting channel portion L2 (520a) as well as
insulation 565a (FIG. 5a) over the source and drain diffusions.
Erase dielectric 531a, 561a is thermally grown Silicon Dioxide or
other deposited dielectrics possessing the appropriate
characteristics for efficient erase conduction, such as Silicon
Nitride. Its thickness is between 30 and 60 nanometers.
[0141] A point of significance is the fact that the tunnel
dielectric area contributing to erase in each cell consisting of
the combined areas of 531a and 561a, is insensitive to the mask
misalignment between edges 532a, 562a of floating gate 504a and
erase gates 530, 535. (Note that each erase gate, such as 535, is
shared between two adjacent cells, such as 600a and 600c in this
case). Any such misalignment will result in a reduction of the area
of the tunnel dielectric at one edge of the floating gate, but also
in an increase of equal magnitude in the area available for
tunneling at the other edge of the floating gate. This feature
permits the construction of a cell with very small area of tunnel
dielectric. By contrast the prior art triple layer Flash EEprom
cells of Masuoka and Kuo referenced above are sensitive to mask
misalignment and therefore require a structure wherein the nominal
area provided for tunnel erase may be much larger than the optimum
such area, in order to accommodate for the worst case misalignment
condition.
[0142] Another distinguishing feature of this embodiment relative
to the Masuoka cell of FIGS. 3a and 3b is that Masuoka implements
the erase gate in a first conductive layer 330. and the floating
gate in a second conductive layer 304, i.e., in a reverse order to
that used in this invention. This results in a far less efficient
tunnel erase in the Masuoka cell because the asperities in
Masuoka's tunnel dielectric 331 are at the surface of the erase
gate (collector) rather than at the injecting surface of the
floating gate. Therefore Masuoka's cell requires higher electric
fields (and therefore higher V.sub.ERASE voltages) than the
structure of this invention.
[0143] Typical bias voltage conditions necessary to erase memory
cells 600a, 600b, 600c and 600d are:
[0144] V.sub.ERASE (on all erase gates 530, 535, 536)=15V to 25V
applied for between 100 milliseconds and 10 seconds (the pulse
duration is strongly dependent on the magnitude of V.sub.ERASE),
V.sub.CG=0V , V.sub.BB=0V. V.sub.D and V.sub.S can be held at 0V or
at a higher voltage between 5V and 10V, so as to reduce the net
voltage experienced during erase across dielectric film 565a in
areas such as 563 (FIG. 6a) where erase gate 530 crosses over drain
diffusion 502.
[0145] III. Self Aligned Split Channel Flash EEprom Cell with Field
Plate Isolation
[0146] A 2.times.2 array of Flash EEprom cells in accordance with
another embodiment of this invention is shown in topological view
in FIG. 7a and in two cross sectional views AA and CC in FIGS. 7b
and 7c respectively. Cross sectional view BB is essentially the
same as the split channel Eprom transistor of FIG. 5a.
[0147] Split channel Flash EEprom transistor 700a employs three
conductive layers (floating gate 704 erase gates 730, 735 and
control gate 709) formed in the same sequence as described in
section II in conjunction with the Flash EEprom transistor 600a of
FIGS. 6a, 6b. The major distinguishing feature of transistor 700a
is that erase gates 730, 735, 736 are used not only for tunnel
erase but also as the switched off gates of isolation field
transistors formed outside the active transistor regions. Thus, the
thick isoplaner isolation oxide 562 of cell 600a (FIG. 6b) is not
necessary, and is replaced inside the array of memory cells 700a,
700b, 700c and 700d by a much thinner oxide 762 (FIGS. 7b, 7c)
capped with field plates 730, 735, 736, which are held at OV at all
times except during erasing.
[0148] The elimination of the thick isoplanar oxide inside the
array of memory cells (this isoplanar oxide may still be retained
for isolation between peripheral logic transistors) has several
advantages:
[0149] 1. The surface stress at the silicon-silicon dioxide
boundary due to a prolonged thermal isoplanar oxidation cycle is
eliminated inside the array, resulting in less leaky source and
drain junctions and in higher quality gate oxides.
[0150] 2. For a given cell width, the elimination of the isoplanar
oxide allows the effective channel width W.sub.1 under floating
gate 704 to extend all the way between the two edges 732a, 762a of
the floating gate. By comparison, effective channel width W of
transistor 600a (FIG. 6b) is determined by the edges 505 of the
isoplanar oxide and is therefore substantially smaller. This
difference results in a higher read signal for cell 700a, or a
narrower, smaller cell.
[0151] 3. From capacitive coupling considerations (to be discussed
in section VI below) the efficiency of tunnel erase is higher in
cells where coupling of the floating gate to the silicon substrate
763 is greatest. In transistor 700a the entire bottom surface area
of the floating gate is tightly coupled to the substrate 763
through the thin gate dielectric 764. By contrast, in transistor
600a (FIG. 6b) much of the bottom surface area of floating gate
504a overlies the thick field oxide 562 and is therefore not
strongly capacitively coupled to substrate 563.
[0152] 4. The width of control gate 709 between its edges 744 and
774 defines channel width W.sub.2 of the series enhancement channel
portion L2 (FIG. 7c). This permits the reduction in overall cell
width due to removal of the requirement for the control gate to
overlap the edges of the isoplaner oxide. One precaution necessary
in the fabrication of cell 700a is that any misalignment between
the mask layers defining edge 732a of Eloatin g ate 704a, edge 784
of erase gate 730, and edge 744 of control gate 709 must not be
allowed to create a situation where a narrow parasitic edge
transistor is created under control gate 709 in parallel with the
split channel L1 and L2. However, as with cell 600a, since erase
gates 730, 736 and control gate 709 are formed in two separate
conductive layers which are isolated from each other by dielectric
insulator film 767 (FIG. 7b) there is no requirement placed on the
magnitude of the spatial separation Z between edge 784 and edge
744. In fact, the two edges can be allowed to overlap each other
through oversizing or through misalignment, i.e., Z can be zero or
negative. Dielectric insulator 767 also forms part of the gate
dielectric 766 (FIG. 7c) over channel portion L2.
[0153] In a memory array source diffusion 701 and drain diffusion
702 can be formed in long strips. If transistor 500a is used as the
Eprom transistor, then source diffusion edge 721 is self aligned to
the previously discussed sidewall spacer (not shown) while drain
diffusion edge 723 is self aligned to edge 722 of floating gate
704a. In areas between adjacent floating gates 704a, 704c the
source and drain diffusion edges (721x, 723x in FIG. 7a)
respectively must be prevented from merging with one another. This
can be accomplished by for example first forming floating gates
704a, 704c as part of a long continuous strip of polysilicon, then
using this strip with an associated long continuous strip of
sidewall spacer to form by ion implantation long diffusion strips
701, 702, removing the spacer strip, and only then etching the long
continuous strip of polysilicon along edges 732a, 762a to form
isolated floating gates 704a, 704c. As with the prior Flash EEprom
embodiment it is possible to form this embodiment also in
conjunction with Eprom cell 1400 (FIG. 14c) or with any other prior
art split channel Eproms so long as they do not have their
isoplanar isolation oxide inside the memory array.
[0154] IV. Self Aligned Split Channel Flash EEprom Cell with Erase
Confined to the Vertical Edges of the Floating Gate.
[0155] Another embodiment of the self aligned split channel Flash
EEprom of this invention can result in a cell which has smaller
area than cells 600a and 700a of the embodiments described in
Sections II and III respectively. In this third embodiment the area
for tunnel erase between the floating gate and the erase gate is
confined essentially to the surfaces of the vertical sidewalls
along the two edges of each floating gate. To best understand how
cell 800a of this embodiment differs from cell 700a a 2.times.2
array of cells 800a, 800b, 800c and 800d are shown in FIG. 8a in
topological view and in FIG. 8b along the same cross section
direction AA as is the case in FIG. 7b for cells 700a, 700c.
[0156] Cell 800a has a floating gate 804a formed in a first layer
of heavily N+ doped polysilicon. This gate controls the transistor
conduction in channel portion L1 (FIG. 8a) through gate oxide
insulation film 864. Control gate 809 is formed in the second
conductive layer, and is insulated from the floating gate by
dielectric film 867, which may be a thermally grown oxide or a
combination of thin silicon dioxide and silicon nitride films.
Edges 874, 844 of control gate 809 are used as a mask to define by
self aligned etching the edges 862a, 832a respectively of floating
gate 804a. Erase gates 830, 835 are formed in a third conductive
layer and are made to overlap edges 832a, 862a of floating gate
804a. Each erase gate such as 830 is shared by two adjacent cells
(such as 800a, 800c).
[0157] The erase gates are insulated from control gate 809 by
dielectric insulator 897 which is grown or deposited prior to
deposition of erase gates 830, 835, 836. Tunnel erase dielectrics
831a, 861a are confined to the surface of the vertical edges 832a,
862a of the floating gate 804a. Erase gate 830 also provides a
field plate isolation over oxide 862 in the field between adjacent
devices.
[0158] The thickness of all conducting and insulating layers in
structure 800 are approximately the same as those used in structure
700a. However, because the erase gate is implemented here after,
rather than before the control gate, the fabrication process
sequence is some-what different. Specifically (see FIGS. 8a,
8b):
[0159] 1. Floating gates 804a, 804c are formed in long continuous
and narrow strips on top of gate oxide 864. The width of each such
strip is L1 plus the extent of overlap of the floating gate over
the drain diffusion.
[0160] 2. Dielectric 867 is formed and the second conductive layer
(N+ doped polysilicon or a silicide) is deposited.
[0161] 3. Control gates 809 are defined in long narrow strips in a
direction perpendicular to the direction of the strips of floating
gates. The strips are etched along edges 844, 874, and insulated
with relatively thick dielectric 897.
[0162] 4. Edges 844, 874 (or the edges of insulator spacer 899
formed at both edges of control gate strip 809) are then used to
etch dielectric 867 and then, in a self aligned manner to also etch
vertical edges 832a and 862a of the underlying floating gate
strips, resulting in isolated floating gates which have exposed
edges of polysilicon only along these vertical walls.
[0163] 5. Tunnel dielectric films 831a, 861a are formed by thermal
oxidation of these exposed surfaces.
[0164] 6. A third conductive layer is deposited, from which are
formed erase gates 830 in long strips running in between and
parallel to adjacent strips of control gates. These erase gates
also serve as field isolation plates to electrically isolate
between adjacent regions in the memory array.
[0165] Flash EEprom transistor 800a can be implemented in
conjunction with any of the split channel Eprom transistors of this
invention (transistors 500a and 1400) or with any of the prior art
split gate Eprom transistors of Eitan, Samachisa, Masuoka or
Harari. For example, an array of Flash EEprom transistors 800a can
be fabricated by adding a few process steps to the fabrication
process for the split channel Eprom transistor 1400. (FIG. 14c), as
follows:
[0166] Steps 1 through 10 are identical to steps 1 through 10
described in Section I.b. in conjunction with the manufacturing
process for split channel Eprom transistor 1400.
[0167] Steps 11, 12, and 13 are the process steps 4, 5, and 6
respectivly described in this section IV in conjunction with split
channel Flash EEprom transistor 800a.
[0168] Cell 800a results in a very small area of tunnel erase,
which is also relatively easy to control (it is not defined by a
mask dimension, but rather by the thickness of the deposited layer
constituting the floating gates). For this reason, this cell is the
most highly scalable embodiment of this invention.
[0169] V. Self Aligned Split Channel Flash EEprom Cell with a
Buried Erase Gate.
[0170] A 2.times.2 array of Flash EEprom cells 900a, 900b, 900c and
900d in accordance with a fourth embodiment of this invention is
shown in topological view in FIG. 9a and in two cross sectional
views AA and DD in FIGS. 9b and 9c respectively. Cross section BB
of FIG. 9ayields the split channel Eprom structure 500a of FIG.
5a.
[0171] Transistor 900a is a split channel Flash EEprom transistor
having channel portions L1 and L2 formed by self alignment as in
Eprom transistor 500a or in a non self aligned manner as in Eprom
transistor 1400. Erase gate 930 is a narrow conductive strip
sandwiched between floating gate 904a on the bottom and control
gate 909 on top. Erase gate 930 is located away from edges 932a,
962a of the floating gate. These edges therefore play no role in
the tunnel erase, which takes place through tunnel dielectric 931
confined to the area where erase gate 930 overlaps floating gate
904a. Erase gate 930 also overlaps a width W.sub.e of the series
enhancement channel portion L2. During read or programming, erase
gate 930 is held at OV, and therefore the channel portion of width
W.sub.e does not contribute to the read or program current. The
only contribution to conduction in channel portion L2 comes from
widths W.sub.p and W.sub.q where the channel is controlled directly
by control gate 909. Channel portion L1 however sees conduction
contributions from all three widths, W.sub.p, W.sub.q and W.sub.e.
Edges 932a, 962a of floating gate 904a can be etched to be self
aligned to edges 944, 974 respectively of control gate 909. This
then permits the formation of channel stop field isolation 998, by
implanting a p type dopant in the field regions not protected by
the control gate or floating gate (FIG. 9b).
[0172] One advantage of cell 900a is that erase gate strips 930,
936 can be made very narrow by taking advantage of controlled
undercutting by for example isotropic etchings of the conductive
layer forming these strips. This results in a small area of tunnel
erase, which is insensitive to mask misalignment. Furthermore the
channel width W.sub.p and W.sub.q is also insensitive to mask
misalignment. This embodiment of Flash EEprom can also be
implemented in conjunction with prior art split channel Eproms
cells such as the Eitan, Harari, Samachisa or Masuoka cells.
[0173] VI. Device Optimization
[0174] FIG. 10 represents a schematic of the major capacitances
which couple the floating gate of the split channel Flash EEprom
cells of this invention to the surrounding electrodes.
[0175] Specifically these are:
[0176] C.sub.G=Capacitance between Floating gate 1104 and control
gate 1109.
[0177] C.sub.D=Capacitance between Floating gate 1104 and drain
diffusion 1102.
[0178] C.sub.B=Capacitance between Floating gate 1104 and substrate
1163.
[0179] C.sub.E=Capacitance between Floating gate 1104 and erase
gate 1130.
[0180] C.sub.T=C.sub.G+C.sub.D+C.sub.B+C.sub.E is the total
capacitance. Q is the net charge stored on the floating gate. In a
virgin device, Q=0. In a programmed device Q is negative (excess
electrons) and in an erased device Q is positive (excess
holes).
[0181] The voltage V.sub.FG on Floating gate 1104 is proportional
to voltages V.sub.CG, V.sub.ERASE, V.sub.D, V.sub.BB and to the
charge Q according to the following equation: 1 V FG = Q + V CG C G
+ V ERASE C E + V D C D + V BB C B C T ( 1 )
[0182] In all prior art Eprom and Flash EEprom devices as well as
in embodiment 600a of this invention, the dominant factor in
C.sub.T is C.sub.G, the coupling to the control gate. However, in
embodiments 700a, 800a and 900a C.sub.B is also a major contributor
by virtue of the fact that the entire bottom surface of the
floating gate is strongly coupled to the substrate.
[0183] a. Electrical Erase
[0184] During erase, the typical voltage conditions are
V.sub.CG=OV, V.sub.D=OV, V.sub.S=OV, V.sub.BB=OV and
V.sub.ERASE=20V. Therefore, substituting in equation (1),
V.sub.FG=Q/C.sub.T+20C.sub.E/C.sub.T (2)
[0185] The electric field for tunnel erase is given by
E.sub.ERASE=V.sub.ERASE/t-V.sub.FG/t (3)
[0186] where t is the thickness of the tunnel dielectric. For a
given V.sub.ERASE, E.sub.ERASE is maximized by making V.sub.FG
small, which, from equation (2) is possible if C.sub.E/C.sub.T is
small. Embodiments 700a, 800a and 900a allow this condition to be
readily met: C.sub.E is small since the area of tunnel dielectric
is small, and C.sub.T is large because both C.sub.G and C.sub.B are
large. These embodiments are therefore particularly well suited for
efficiently coupling the erase voltage across the tunnel
dielectric.
[0187] b. Multistate Storage
[0188] The split channel Flash EEprom device can be viewed as a
composite transistor consisting of two transistors T1 and T2 in
series--FIG. 11a. Transistor T1 is a floating gate transistor of
effective channel length L1 and having a variable threshold voltage
V.sub.T1. Transistor T2 has a fixed (enhancement) threshold voltage
V.sub.T2 and an effective channel length L2. The Eprom programming
characteristics of the composite transistor are shown in curve (a)
of FIG. 11b. The programmed threshold voltage V.sub.tx is plotted
as a function of the time t during which the programming conditions
are applied. These programming conditions typically are
V.sub.CG=12V, V.sub.D=9V, V.sub.S=V.sub.BB=OV. No programming can
occur if either one of V.sub.CG or V.sub.D is at OV. A virgin
(unprogrammed, unerased) device has V.sub.T1=+1.5V and
V.sub.T2=+1.0V. After programming for approximately 100
microseconds the device reaches a threshold voltage
V.sub.tx.gtoreq.+6.0 volts. This represents the off ("0") state
because the composite device does not conduct at V.sub.CG=+5.0V.
Prior art devices employ a so called "intelligent programming"
algorithm whereby programming pulses are applied, each of typically
100 microseconds to 1 millisecond duration, followed by a sensing
(read) operation. Pulses are applied until the device is sensed to
be fully in the off state, and then one to three more programming
pulses are applied to ensure solid programmability.
[0189] Prior art split channel Flash EEprom devices erase with a
single pulse of sufficient voltage V.sub.ERASE and sufficient
duration to ensure that V.sub.T1 is erased to a voltage below
V.sub.T2 (curve b) in FIG. 11b). Although the floating gate
transistor may continue to erase into depletion mode operation
(line (C) in FIG. 11b), the presence of the series T2 transistor
obscures this depletion threshold voltage. Therefore the erased on
("1") state is represented by the threshold voltage
V.sub.tx=V.sub.T2=+1.0V. The memory storage "window" is given by
.DELTA.V=V.sub.tx("0")-V.sub.tx("1")=6.0-1.0=5.0V. However, the
true memory storage window should be represented by the full swing
of V.sub.tx for transistor T1. For example, if T1 is erased into
depletion threshold voltage V.sub.T1=-3.0V, then the true window
should be given by .DELTA.V=6.0-(-3.0)=9.0V. None of the prior art
Flash EEprom devices take advantage of the true memory window. In
fact they ignore altogether the region of device operation (hatched
region D in FIG. 11b) where V.sub.T1 is more negative than
V.sub.T2.
[0190] This invention proposes for the first time a scheme to take
advantage of the full memory window. This is done by using the
wider memory window to store more than two binary. states and
therefore more than a single bit per cell. For example, it is
possible to store 4, rather than 2 states per cell, with these
states having the following threshold voltage:
[0191] State "3":--V.sub.T1=-3.0V, V.sub.T2=+1.0V (highest
conduction)=1, 1.
[0192] State "2":--V.sub.T1=-0.5V, V.sub.T2 =+1.0V (intermediate
conduction)=1, 0.
[0193] State "1":--V.sub.T1=+2.0V, V.sub.T2=+1.0V (lower
conduction)=0, 1.
[0194] State "0":--V.sub.T1=+4.5V, V.sub.T2=+1.0V (no
conduction)=0, 0.
[0195] To sense any one of these four states, the control gate is
raised to V.sub.CG=+5.0V and the source-drain current I.sub.DS is
sensed through the composite device. Since V.sub.T2=+1.OV for all
four threshold states transistor T.sub.2 behaves simply as a series
resistor. The conduction current I.sub.DS of the composite
transistor for all 4 states is shown as a function of V.sub.CG in
FIG. 11c. A current sensing amplifier is capable of easily
distinguishing between these four conduction states. The maximum
number of states which is realistically feasible is influenced by
the noise sensitivity of the sense amplifier as well as by any
charge loss which can be expected over time at elevated
temperatures. Eight distinct conduction states are necessary for 3
bit storage per cell, and 16 distinct conduction states are
required for 4 bit storage per cell.
[0196] Multistate memory cells have previously been proposed in
conjunction with ROM (Read only Memory) devices and DRAM (Dynamic
Random Access Memory). In ROM, each storage transistor can have one
of several fixed conduction states by having different channel ion
implant doses to establish more than two permanent threshold
voltage states. Alternatively, more than two conduction states per
ROM cell can be achieved by establishing with two photolithographic
masks one of several values of transistor channel width or
transistor channel length. For example, each transistor in a ROM
array may be fabricated with one of two channel widths and with one
of two channel lengths, resulting in four distinct combinations of
channel width and length, and therefore in four distinct Conductive
states. Prior art multistate DRAM cells have also been proposed
where each cell in the array is physically identical to all other
cells. However, the charge stored at the capacitor of each cell may
be quantized, resulting in several distinct read signal levels. An
example of such prior art multistate DRAm storage is described in
IEEE Journal of Solid-State Circuits, February 1988, p. 27 in an
article by M. Horiguchi et al. entitled "An Experimental
Large-Capacity Semi-conductor File Memory Using 16-Levels/Cell
Storage". A second example of prior art multistate DRAM is provided
in IEEE Custom Integrated Circuits Conference, May 1988, p. 4.4.1
in an article entitled "An Experimental 2-Bit/Cell Storage DRAM for
Macrocell or Memory-on-Logic Applications" by T. Furuyama et
al.
[0197] To take full advantage of multistate storage in Eproms it is
necessary that the programming algorithm allow programming of the
device into any one of several conduction states. First it is
required that the device be erased to a voltage V.sub.T1 more
negative than the "3" state (-3.0V in this example). Then the
device is programmed in a short programming pulse, typically one to
ten microseconds in., duration. Programming conditions are selected
such that no single pulse can shift the device threshold by more
than one half of the threshold voltage difference between two
successive states. The device is then sensed by comparing its
conduction current I.sub.DS with that of a reference current source
I.sub.REF, i (i=0, 1, 2, 3) corresponding to the desired conduction
state (four distinct reference levels must be provided
corresponding to the four states). Programming pulses are continued
until the sensed current (solid lines in FIG. 11c) drops slightly
below the reference current corresponding to the desired one of
four states (dashed lines in FIG. 11c). To better illustrate this
point, assume that each programming pulse raises V.sub.tx linearly
by 200 millivolts, and assume further that the device was first
erased to V.sub.T1=-3.2V. Then the number of programming/sensing
pulses required is:
[0198] For state "3" (V.sub.T1=-3.0V)
[0199] No. of pulses=(3.2-3.0)/0.2=1
[0200] For state "2" (V.sub.T1=-0.5V)
[0201] No. of pulses=(3.2-0.5)/0.2=14
[0202] For state "1" (V.sub.T1=+2.0V)
[0203] No. of pulses=(3.2-(-2.0))/0.2=26
[0204] and for state "0" (V.sub.T1=+4.5V)
[0205] No. of pulses=(3.2-(-4.5))/0.2=39.
[0206] In actual fact shifts in V.sub.tx are not linear in time, as
shown in FIG. 11b (curve (a)), therefore requiring more pulses than
indicated for states "1" and "0". If 2 microseconds is the
programming pulse width and 0.1 microseconds is the time required
for sensing, then the maximum time required to program the device
into any of the 4 states is approximately
39.times.2+39.times.0.1=81.9 microseconds. This is less than the
time required by. "intelligent programming algorithms" of prior art
devices. In fact, with the new programming algorithm only carefully
metered packets of electrons are injected during programming. A
further benefit of this approach is that the sensing during reading
is the same sensing as that during programming/sensing, and the
same reference current sources are used in both programming and
reading operations. That means that each and every memory cell in
the array is read relative to the same reference level as used
during program/sense. This provides excellent tracking even in very
large memory arrays.
[0207] Large memory systems typically incorporate error detection
and correction schemes which can tolerate a small number of hard
failures i.e. bad Flash EEprom cells. For this reason the
programming/sensing cycling algorithm can be automatically halted
after a certain maximum number of programming cycles has been
applied even if the cell being programmed has not reached the
desired threshold voltage state, indicating a faulty memory
cell.
[0208] There are several ways to implement the multistate storage
concept in conjunction with an array of Flash EEprom transistors.
An example of one such circuit is shown in FIG. 11e. In this
circuit an array of memory cells has decoded word lines and decoded
bit lines connected to the control gates and drains respectively of
rows and columns of cells. Each bit line is normally precharged to
a voltage of between 1.0 V and 2.0 V during the time between read,
program or erase. For a four state storage, four sense amplifiers,
each with its own distinct current reference levels IREF, 0, IREF,
1, IREF, 2, and IREF, 3 are attached to each decoded output of the
bit line. During read, the current through the Flash EEprom
transistor is compared simultaneously (i.e., in parallel) with
these four reference levels (this operation can also be performed
in four consecutive read cycles using a single sense amplifier with
a different reference applied at each cycle, if the attendant
additional time required for reading is not a concern). The data
output is provided from the four sense amplifiers through four Di
buffers (D0, D1, D2 and D3).
[0209] During programming, the four data inputs Ii (I0, I1, I2 and
I3) are presented to a comparator circuit which also has presented
to it the four sense amp outputs for the accessed cell. If Di match
Ii, then the cell is in the correct state and no programming is
required. If however all four Di do not match all four Ii, then the
comparator output activates a programming control: circuit. This
circuit in turn controls the bit line (VPBL) and word line (VPWL)
programming pulse generators. A single short.programming pulse is
applied to both the selected word line and the selected bit line.
This is followed by a second read cycle to determine if a match
between Di and Ii has been established. This sequence is repeated
through multiple programming/reading pulses and is stopped only
when a match is established (or earlier if no match has been
established but after a preset maximum number of pulses has been
reached).
[0210] The result of such multistate programming algorithim is that
each cell is programmed into any one of the four conduction states
in direct correlation with the reference conduction states
I.sub.REF, i. In fact, the same sense amplifiers used during
programming/reading pulsing are also used during sensing (i.e.,
during normal reading). This allows excellent tracking between the
reference levels (dashed lines in FIG. 11c) and the programmed
conduction levels (solid lines in FIG. 11c), across large memory
arrays and also for a very wide range of operating temperatures.
Furthermore, because only a carefully metered number of electrons
is introduced onto the floating gate during programming or removed
during erasing, the device experiences the minimum amount of
endurance-related stress possible.
[0211] In actual fact, although four reference levels and four
sense amplifiers are used to program the cell into one of four
distinct conduction states, only three sense amplifiers and three
reference levels are required to sense the correct one of four
stored states. For example, in FIG. 11c, I.sub.REF ("2") can
differentiate correctly between conduction states "3" and "2",
I.sub.REF("1") can differentiate correctly between conduction
states "2" and "1", and I.sub.REF("0") can differentiate correctly
between conduction states "1" and "0". In a practical
implementation of the circuit of FIG. 11e the reference levels
I.sub.REF, i (i=0, 1, 2) may be somewhat shifted by a fixed amount
during sensing to place them closer to the midpoint between the
corresponding lower and higher conduction states of the cell being
sensed.
[0212] Note that the same principle employed in the circuit of FIG.
11e can be used also with binary storage, or with storage of more
than four states per cell. Of course, circuits other than the one
shown in FIG. 11e are also possible. For example, voltage level
sensing rather than conduction level sensing can be employed.
[0213] c. Improved Charge Retention
[0214] In the example above, states "3" and "2" are the result of
net positive charge (holes) on the floating gate while states "1"
and "0" are the result of net negative charge (electrons) on the
floating gate. To properly sense the correct conduction state
during the lifetime of the device (which may be specified as 10
years at 125.degree. C.) it is necessary for this charge not to
leak off the floating gate by more than the equivalent of
approximately 200 millivolts shift in V.sub.T1. This condition is
readily met for stored electrons in this as well as all prior art
Eprom and Flash EEprom devices. There is no data in the literature
on charge retention for stored holes, because, as has been pointed
out above, none of the prior art devices concern themselves with
the value V.sub.T1 when it is more negative than VT.sub.T2, i.e.,
when holes are stored on th floating gate. From device physics
considerations alone it is expected that retention of holes trapped
on the floating gate should be significantly superior to the
retention of trapped electrons. This is because trapped holes can
only be neutralized by the injection of electrons onto the floating
gate. So long as the conditions for such injection do not exist it
is almost impossible for the holes to overcome the potential
barrier of approximately 5.0 electronvolts at the silicon-silicon
dioxide interface (compared to a 3.1 electron volts potential
barrier for trapped electrons).
[0215] Therefore it is possible to improve the retention of this
device by assigning more of the conduction states to states which
involve trapped holes. For example, in the example above state "1"
had V.sub.T1=+2.0V, which involved trapped electrons since V.sub.T1
for the virgin device was made to be V.sub.T1=+1.5V. If however
V.sub.T1 of the virgin device is raised to a higher threshold
voltage, say to V.sub.T1=+3.0V (e.g. by increasing the p-type
doping concentration in the channel region 560a in FIG. 5a), then
the same state "1" with V.sub.T1=+2.0V will involve trapped holes,
and will therefore better retain this value of V.sub.T1. Of course
it is also possible to set the reference levels so that most or all
states will have values of V.sub.T1 which are lower than the
V.sub.T1 of the virgin device.
[0216] d. Intelligent Erase for Improved Endurance
[0217] The endurance of Flash EEprom devices is their ability to
withstand a given number of program/erase cycles. The physical
phenomenon limiting the endurance of prior art Flash EEprom devices
is trapping of electrons in the active dielectric films of the
device (see the Wegener article referenced above). During
programming the dielectric used during hot electron channel
injection traps part of the injected electrons. During erasing the
tunnel erase dielectric likewise traps some of the tunneled
electrons. For example, in prior art transistor 200 (FIG. 2)
dielectric 212 traps electrons in region 207 during programming and
in region 208 during erasing. The trapped electrons oppose the
applied electric field in subsequent write/erase cycles thereby
causing a reduction in the threshold voltage shift of V.sub.tx.
This can be seen in a gradual closure (FIG. 11d) in the voltage
"window" between the "0" and "1" states of prior art devices.
Beyond approximately 1.times.10.sup.4 program/erase cycles the
window closure can become sufficiently severe to cause the sensing
circuitry to malfunction. If cycling is continued the device
eventually experiences catastrophic failure due to a ruptured
dielectric. This typically occurs at between 1.times.10.sup.6 and
1.times.10.sup.7 cycles, and is known as the intrinsic breakdown of
the device. In memory arrays of prior art devices the window
closure is what limits the practical endurance to approximately
1.times.10.sup.4 cycles. At a given erase voltage, V.sub.ERASE, the
time required to adequately erase the device can stretch out from
100 milliseconds initially (i.e. in a virgin device) to 10 seconds
in a device which has been cycled through 1.times.10.sup.4 cycles.
In anticipation of such degradation prior art Flash EEprom devices
specify a sufficiently long erase pulse duration to allow proper
erase after 1.times.10.sup.4 cycles. However this also results in
virgin devices being overerased and therefore being unnecessarily
over-stressed.
[0218] A second problem with prior art devices is that during the
erase pulse the tunnel dielectric may be exposed to an excessively
high peak stress. This occurs in a device which has previously been
programmed to state "0" (V.sub.T1=+4.5V or higher). This device has
a large negative Q (see equation (2)). When V.sub.ERASE is applied
the tunnel dielectric is momentarily exposed to a peak electric
field with components from V.sub.ERASE as well as from Q/C.sub.T
(equations (2) and (3)). This peak field is eventually reduced when
Q is reduced to zero as a consequence of the tunnel erase.
Nevertheless, permanent and cumulative damage is inflicted through
this erase procedure, which brings about premature device
failure.
[0219] To overcome the two problems of overstress and window
closure a new erase algorithm is disclosed, which can also be
applied equally well to any prior art Flash EEprom device. Without
such new erase algorithm it would be difficult to have a multistate
device since, from curve (b) in FIG. 11d, conduction states having
V.sub.T1 more negative than V.sub.T2 may be eliminated after
1.times.10.sup.4 to 1.times.10.sup.5 write/erase cycles.
[0220] FIG. 12 outlines the main steps in the sequence of the new
erase algorithm. Assume that a block array of m.times.n memory
cells is to be fully erased (Flash erase) to state "3" (highest
conductivity and lowest V.sub.T1 state). Certain parameters are
established in conjunction with the erase algorithm. They are
listed in FIG. 12: V.sub.1 is the erase voltage of the first erase
pulse. V.sub.1 is lower by perhaps 5 volts from the erase voltage
required to erase a virgin device to state "3" in a one second
erase pulse. t is chosen to be approximately {fraction (1/10)}th of
the time required to fully erase a virgin device to state "3".
Typically, V.sub.1 may be between 10 and 20 volts while t may be
between 10 and 100 milliseconds. The algorithm assumes that a
certain small number, X, of bad bits can be tolerated by the system
(through for example error detection and correction schemes
implemented at the system level. If no error detection and
correction is implemented then X=0). These would be bits which may
have a shorted or leaky tunnel dielectric which prevents them from
being erased even after a very long erase pulse. To avoid excessive
erasing the total number of erase pulses in a complete block erase
cycling can be limited to a preset number, n.sub.max. .DELTA.V is
the voltage by which each successive erase pulse is incremented.
Typically, .DELTA.V is in the range between 0.25V and 1.0V. For
example, if V.sub.1=15.0V and .DELTA.V=1.0V, then the seventh erase
pulse will be of magnitude V.sub.ERASE=21.0V and duration t. A cell
is considered to be fully erased when its read conductance is
greater than I.sub."3". The number S of complete erase cyclings
experienced by each block is an important information at the system
level. If S is known for each block then a block can be replaced
automatically with a new redundant block once S reaches
1.times.10.sup.6 (or any other set number) of program/erase cycles.
S is set at zero initially, and is incremented by one for each
complete block erase multiple pulse cycle. The value of S at any
one time can be stored by using for example twenty bits (2.sup.20
equals approximately 1.times.10.sup.6) in each block. That way each
block carries its own endurance history. Alternatively the S value
can be stored off chip as part of the system.
[0221] The sequence for a complete erase cycle of the new algorithm
is as follows (see FIG. 12):
[0222] 1. Read S. This value can be stored in a register file.
(This step can be omitted if S is not expected to approach the
endurance limit during the operating lifetime of the device).
[0223] 1a. Apply a first erase pule with
V.sub.ERASE=V.sub.1+n.DELTA.V, n=0, pulse duration=t. This pulse
(and the next few successive pulses) is insufficient to fully erase
all memory cells, but it serves to reduce the charge Q on
programmed cells at a relatively low erase field stress, i.e., it
is equivalent to a "conditioning" pulse.
[0224] 1b. Read a sparse pattern of cells in the array. A diagonal
read pattern for example will read m+n cells (rather than m.times.n
cells for a complete read) and will have at least one cell from
each row and one cell from each column in the array. The number N
of cells not fully erased to state "3" is counted and compared with
X.
[0225] 1c. If N is greater than x (array not adequately erased) a
second erase pulse is applied of magnitude greater by .DELTA.V than
the magnitude of the first pulse, with the same pulse duration, t.
Read diagonal cells, count N.
[0226] This cycling of erase pulse/read/increment erase pulse is
continued until either N.ltoreq.X or the number n of erase pulses
exceed n.sub.max. The first one of these two conditions to occur
leads to a final erase pulse.
[0227] 2a. The final erase pulse is applied to assure that the
array is solidly and fully erased. The magnitude of V.sub.ERASE can
be the same as in the previous pulse or higher by another increment
.DELTA.V. The duration can be between 1t; and 5t.
[0228] 2b. 100% of the array is read. The number N of cells not
fully erased is counted. If N is less than or equal to X, then the
erase pulsing is completed at this point.
[0229] 2c. If N is greater than X, then address locations of the N
unerased bits are generated, possibly for substitution with
redundant good bits at the system level. If N is significantly
larger than X (for example, if N represents perhaps 5% of the total
number of cells), then a flag may be raised, to.indicate to the
user that the array may have reached its endurance end of life.
[0230] 2d. Erase pulsing is ended.
[0231] 3a. S is incremented by one and the new S is stored for
future reference. This step is optional. The new S can be stored
either by writing it into the newly erased block or off chip in a
separate register file.
[0232] 3b. The erase cycle is ended. The complete cycle is expected
to be completed with between 10 to 20 erase pulses and to last a
total of approximately one second.
[0233] The new algorithm has the following advantages:
[0234] (a) No cell in the array experiences the peak electric field
stress. By the time V.sub.ERASE is incremented to a relatively high
voltage any charge Q on the floating gates has already been removed
in previous lower voltage erase pulses.
[0235] (b) The total erase time is significantly shorter than the
fixed V.sub.ERASE pulse of the prior art. Virgin devices see the
minimum pulse duration necessary to erase. Devices which have
undergone more than 1.times.10.sup.3 cycles require only several
more .DELTA.V voltage increments to overcome dielectric trapped
charge, which only adds several hundred milliseconds to their total
erase time.
[0236] (c) The window closure on the erase side (curve (b) in FIG.
11d) is avoided indefinitely (until the device experiences failure
by a catastrophic breakdown) because V.sub.ERASE is simply
incremented until the device is erased properly to state "3". Thus,
the new erase algorithm preserves the full memory window.
[0237] FIG. 13 shows the four conduction states of the Flash EEprom
devices of this invention as a function of the number of
program/erase cycles. Since all four states are always accomplished
by programming or erasing to fixed reference conduction states,
there is no window closure for any of these states at least until
1.times.10.sup.6 cycles.
[0238] In a Flash EEprom memory chip it is possible to implement
efficiently the new erase algorithm by providing on chip (or
alternatively on a separate controller chip) a voltage multiplier
to provide the necessary voltage V1 and voltage increments .DELTA.V
to n.DELTA.V, timing circuitry to time the erase and sense pulse
duration, counting circuitry to count N and compare it with the
stored value for X, registers to store address locations of bad
bits, and control and sequencing circuitry, including the
instruction set to execute the erase sequence outlined above.
[0239] VII. Edge Tailored Flash EEprom with New Erase Mechanism
[0240] Flash EEprom embodiments 600a, 700a, 800a, and 900a of this
invention use tunnel erase across a relatively thick dielectric
oxide grown on the textured surface of the polysilicon floating
gate. Wegener (see article referenced above) has postulated that
asperities--small, bump-like, curved surfaces of diameter of
approximately 30 nanometers, enhance the electric field at the
injector surface (in this case, the floating gate) by a factor of 4
to 5, thereby allowing efficient tunnel conduction to occur even
across a relatively thick tunnel dielectric film (30 to 70
nanometers). Accordingly, there have been in the prior art efforts,
through process steps such as high temperature oxidation of the
polysilicon surface, to shape the surface of the polysilicon so as
to accentuate these asperities. Although such steps are
reproducible, they are empirical in nature, somewhat costly to
implement, and not well understood.
[0241] A new approach is disclosed in this invention which results
in a highly reproducible, enhanced electric field tunnel erase
which is more efficient than the asperities method yet simpler to
implement in several EEprom and Flash EEprom devices. In this
approach, the floating gate layer is deposited in a very thin
layer, typically in the range between 25 and 200 nanometers. This
is much thinner than floating gates of all prior art Eprom, EEprom
or Flash EEprom devices, which typically use a layer of polysilicon
of thickness at least 200 nanometers, and usually more like 350 to
450 nanometers. The prior art polysilicon thickness is chosen to be
higher than 200 nanometers primarily because of the lower sheet
resistivity and better quality polyoxides provided by the thicker
polysilicon. In certain prior art devices such as the Eitan split
channel Eprom the floating gate also serves as an implant mask
(FIG. 4b) and must therefore be sufficiently thick to prevent
penetration of the implant ions. Likewise, in the split channel
Eproir embodiment 500a (FIG. 5a) the spacer formation (FIGS. 5b
through 5f) cannot be readily implemented if floating gate 504a is
100 nanometers or less in thickness. However, Eprom transistor 1400
(FIG. 14c) and Flash EEprom transistors 600a (FIG. 6a), 700a (FIG.
7a), 800a (FIG. 8a) and 900a (FIG. 9a) as well as the Kupec prior
art transistor 200b (FIG. 2b) can all be implemented with a
floating gate of thickness 100 nanometers or less to achieve a
significant improvement in erase efficiency.
[0242] The reason for going to such a thin layer of polysilicon is
that the edges of the floating gate in such a thin layer can be
tailored through oxidation to form extremely sharp-tipped edges.
The radius of curvature of these tipped edges can be made extremely
small and is dictated by the thickness of the thin polysilicon film
as well as the thickness of the tunnel dielectric grown. Therefore,
tunnel erase from these sharp tips no longer depends on surface
asperities but instead is dominated by the tip itself.
[0243] As an illustration of this modification, consider Flash
EEprom transistor 800a (FIG. 8a) in two different embodiments, a
relatively thick floating gate (transistor 800a shown in FIG. 8b
and FIG. 16a) and the same transistor modified to have a very thin
floating gate (transistor 800M shown in FIG. 16b). In the cross
section view of FIG. 16a (corresponding to direction AA of FIG.
8a), floating gate 804a is approximately 300 nanometers thick. Its
vertical edges 862a, 832a are shown having a multitude of small
asperities at the surface. Each asperity acts as an electron
injector during tunnel erase (shown by the direction of the arrows
across tunnel dielectric layers 861a, 831a). Injected electrons are
collected by erase gates 835, 830 which overlap vertical edges
862a, 832a.
[0244] By contrast, the cross section view of modified transistor
800M is shown in FIG. 16b (along the same cross section AA of FIG.
8a) shows a transistor with floating gate 804M of thickness 100
nanometers or less. Dielectric layers 864 and 867 as well as
control gate 809 can be the same as in transistor 800a.
[0245] During oxidation of the thin vertical edges of floating gate
804M to form tunnel dielectric layers 861M, 831M, both top and
bottom surfaces of the thin floating gate at its exposed edges are
oxidized. This results in extremely sharp tips 870l, 870r being
formed. These tips serve as very efficient electron injectors
(shown by arrows across tunnel dielectrics 861M, 831M). Injected
electrons are collected as in transistor 800a by erase gates 835,
830, which overlap these sharp-tipped edges.
[0246] Apart from the very efficient and highly reproducible
injector characteristics inherent to the thin floating gate of
transistor 800M there is an additional benefit. in that the
capacitance between the floating gate at its tip and the erase gate
is much smaller than the corresponding capacitance in all other
embodiments, including transistor 800a. Therefore, from equations
(1), (2) and (3) in section VI.a., since
C.sub.E<<C.sub.T,
Therefore, V.sub.FG=Q/C.sub.T, and
E.sub.ERASE=(V.sub.ERASE-Q/C.sub.T)/t.
[0247] When Q=0 (virgin device), then
E.sub.ERASE=V.sub.ERASE/t (4)
[0248] Equation (4) basically states that when C.sub.E is very
small relative to C.sub.T, then essentially 100% of the erase
voltage V.sub.ERASE is effectively applied across the tunnel
dielectric layer of thickness t. This allows a reduction of the
magnitude of V.sub.ERASE necessary to erase the device. Also, a
very small C.sub.E allows all other device capacitances
contributing to C.sub.T (in FIG. 10) to be made small, which leads
to a highly scalable Flash EEprom device. The thinner floating gate
also helps to improve metalization step coverage and to reduce the
propensity to form polysilicon stringers in the manufacturing
process.
[0249] Two other points are worth noting. First, the very thin
floating gate should not be overly heavily doped, to avoid
penetration of the N+ dopant through polysilicon 804M and gate
dielectric 864. Since floating gate 804M is never used as a current
conductor, a sheet resistivity of between 100 and 10,000 Ohms per
square in quite acceptable.
[0250] Secondly, it is necessary to ensure that the sharp tips of
the floating gate are adequately spaced apart or isolated from
control gate 809M as well as substrate 860 or the source or drain
diffusions (not shown in FIG. 16b). This is because the sharp tip
injection mechanism can be so highly effective that unintended
partial erase to these surfaces may take place under the voltage
conditions prevailing during device programming (i.e., a "program
disturbance" condition). This problem is not necessarily a severe
one because, looking again at equations (1), (2) and (3),
capacitance components C.sub.G, C.sub.D and C.sub.B are each much
larger than C.sub.E and therefore the electric field between the
floating gate at its edges and any of these three surfaces is much
less than E.sub.ERASE. Nevertheless, this should be an important
consideration in the actual geometrical layout of any floating gate
transistor using a very thin floating gate for edge erase.
[0251] Although a thin floating gate layer provides a relatively
straight forward approach to achieving after oxidation sharp-tipped
edges, other approaches are possible to achieve sharp-tipped edges
even in a relatively thick floating gate layer. For example, in
FIG. 16c a relatively thick layer forming floating gate 804 is
etched with a reentrant angle of etching. After oxidation, a sharp
tip 870 is formed at the top edge, facilitating high field
tunneling 861 to the erase gate 830 deposited on top of the tunnel
erase dielectric 831.
[0252] In the device of FIG. 16d the erase gate is deposited before
the floating gate. Erase gate 830 is etched so as to create a
reentrant cavity close to its bottom surface. Tunnel erase
dielectric 831 is then grown, followed by deposition and formation
of floating gate 804. Floating gate 804 fills the narrow reentrant
cavity where a sharp tip 870 is formed, which facilitates the high
field tunneling 861. Note that the device of FIG. 16d has
asperities formed at the surface of the erase gate whereas all
other devices described in this invention have asperities formed at
the surfaces of their floating gate.
[0253] VIII. Flash EEprom Memory Array Implementations
[0254] The Flash EEprom cells of this invention can be implemented
in dense memory arrays in several different array architectures.
The first architecture, shown in FIG. 15a, is the one commonly used
in the industry for Eprom arrays. The 3.times.2 array of FIG. 15a
shows two rows and three columns of Flash EEprom transistors.
Transistors T10, T11, T12 along the first row share a common
control gate (word line) and a commmon source S. Each transistor in
the row has its own drain D connected to a column bit line which is
shared with the drains of all other transistors in the same column.
The floating gates of all transistors are adjacent their drains,
away from their sources. Erase lines are shown running in the bit
line direction (can also run in the word line direction), with each
erase line coupled (through the erase dielectric) to the floating
gates of the transistors to the left and to the right of the erase
line. The voltage conditions for the different modes of operation
are shown in Table I (FIG. 17a) for the selected cell as well as
for unselected cells sharing either the same row (word line) or the
same column (bit line). During block erase of all the cells in the
array, all erase lines are brought high. However, it is also
possible to erase only sectors of the array by taking V.sub.ERASE
high for pairs of erase gates only in these sectors, keeping all
other erase lines at OV.
[0255] A second Flash EEprom memory array architecture which lends
itself to better packing density than the array of FIG. 15a is
known as the virtual ground array (for a detailed description of
this array architecture, see the Harari patent referenced herein).
A topological view of such an array of cells was provided in FIGS.
6a, 7a, 8a and 9a. A schematic representation of a 2.times.2
virtual ground memory array corresponding to the array of FIG. 6a
is shown in FIG. 15b. In a virtual ground array, the source and
drain regions are used interchangeably. For example, diffusion 502
is used as the drain of transistor 600a and as the source of
transistor 600b. The term "virtual ground comes from the fact that
the ground supply applied to the source is decoded rather than
hard-wired. This decoding allows the source to be used
interchangeably as ground line or drain. The operating conditions
in the virtual ground array are given in Table II (FIG. 17b). They
are essentially the same as that for the standard architecture
array, except that all source and drain columns of unselected cells
are left floating during programming to prevent accidental program
disturbance. During reading all columns are pulled up to a low
voltage (about 1.5V) and the selected cell alone has its source
diffusion pulled down close to ground potential so that its current
can be sensed.
[0256] The array can be erased in a block, or in entire rows by
decoding the erase voltage to the corresponding erase lines.
[0257] While the embodiments of this invention that have been
described are the preferred implementations, those skilled in the
art will understand that variations thereof may also be possible.
In particular, the split channel Flash EEprom devices 600a, 700a,
800a and 900a can equally well be formed in conjunction with a
split channel Eprom composite transistor 500a having channel
portions L1 and L2 formed in accordance with the one-sided spacer
sequence outlined in FIGS. 5b through 5f, or in accordance with
Eprom transistor 1400, or with Eprom transistors formed in
accordance with other self-aligning process techniques or,
altogether in non self-aligning methods such as the ones employed
in the prior art by Eitan, Samachisa, Masuoka and Harari.
Therefore, the invention is entitled to protection within the full
scope of the appended claims.
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