U.S. patent application number 09/840838 was filed with the patent office on 2001-08-23 for method for operating non-volatile memory cells.
Invention is credited to Chan, Jui-Te, Chen, Kou-Su, Fu, Shih-Chun.
Application Number | 20010015911 09/840838 |
Document ID | / |
Family ID | 23350017 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015911 |
Kind Code |
A1 |
Chen, Kou-Su ; et
al. |
August 23, 2001 |
Method for operating non-volatile memory cells
Abstract
A pre-erase/incremented erase mechanism is employed to reduce
excessive tunnel oxide fields in flash memory cells. A variable
conditioning signal removes charge from "fast" bits in the array,
so that they are configured to have threshold voltages closer to an
ideal initial state in preparation for an erase cycle. In this
manner, the voltage thresholds are tightened and equalized, so that
over-erasure problems associated with Fowler-Nordheim tunneling
erase operations are substantially reduced, and endurance cycles
for the array are maximized. The invention can be used in a device
in the field, or as part of a design process for a flash memory
cell to evaluate device performance.
Inventors: |
Chen, Kou-Su; (Fremont,
CA) ; Fu, Shih-Chun; (Hsin-Chu, TW) ; Chan,
Jui-Te; (Hsin-Chu, TW) |
Correspondence
Address: |
J. Nicholas Gross, Attorney at Law
Suite 240
1385 Mission Street
San Francisco
CA
94103
US
|
Family ID: |
23350017 |
Appl. No.: |
09/840838 |
Filed: |
April 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09840838 |
Apr 24, 2001 |
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09598826 |
Jun 21, 2000 |
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6249459 |
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09598826 |
Jun 21, 2000 |
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09344318 |
Jun 24, 1999 |
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6198662 |
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Current U.S.
Class: |
365/185.26 |
Current CPC
Class: |
G11C 16/3404 20130101;
G11C 16/3477 20130101; G11C 16/3409 20130101; G11C 16/16 20130101;
G11C 16/30 20130101 |
Class at
Publication: |
365/185.26 |
International
Class: |
G11C 011/34 |
Claims
What is claimed is:
1. A method of slowing down erase speeds of fast discharge flash
cells in a memory array, such fast discharge flash cells being
characterized by erase speeds substantially faster than target
erase speeds for flash cells in said array, the method comprising:
(a) generating a conditioning signal to remove a quantity of charge
from the flash cells, said quantity of charge being related to an
erase speed of the flash cell and further being insufficient to
place such flash cells into an erased state; and (b) applying said
conditioning signal to such flash cells while such cells are in a
non-erased state; wherein an erase speed of such fast discharge
flash cells is reduced.
2. The method of claim 1, further including a step (c): measuring
electrical characteristics of the flash cells to determine an
expected erase discharge speed for such flash cells.
3. The method of claim 1, wherein steps (a) and (b) are repeated to
remove further charge from the flash cells.
4. The method of claim 1, wherein properties of said conditioning
signal, including at least magnitude and/or duration, can be varied
to remove more or less charge from the flash cells.
5. The method of claim 3, wherein said conditioning signal is
altered to remove a greater quantity of charge for each time steps
(a) and (b) are repeated.
6. The method of claim 1, further including a step (c): performing
an erase operation on the memory array.
7. The method of claim 6, further including a step (d): verifying
that no cells have been over-erased from said erase operation.
8. The method of claim 7, further including a step (e): performing
a convergence operation on the array if any cells are
over-erased.
9. The method of claim 8, further wherein said convergence
operation is comprised of two independent operations, including:
(a) a global operation performed on the entire array; and (b) a
localized operation performed only on those cells still over-erased
after said global operation is completed.
10. A method of equalizing erase rates of flash cells in a memory
array, the flash cells each having a floating gate with an floating
gate charge value, and which floating gate charge value is intended
to be set to a target initial charge value prior to an erase
operation, the method comprising: (a) generating a flash cell
conditioning signal having a predetermined magnitude and duration;
(b) adjusting the floating gate charge value of each of such flash
cells using said conditioning signal, the floating gate charge
value for any such flash cell being adjusted by said conditioning
signal by a charge amount proportional to the floating gate charge
value; wherein erase rates of flash cells with floating gate charge
values substantially in excess of said target initial charge value
are substantially affected by said flash cell conditioning signal,
while erase rates of flash cells with floating gate charge values
substantially below said target initial charge values are
substantially unaffected by said flash cell conditioning signal;
and further wherein steps (a) and (b) are repeated until the flash
cell floating gate charge values substantially converge to the
target initial charge value so that all of the flash cells can then
be erased at an approximately uniform erase rate by a subsequent
erase operation.
11. The method of claim 10, wherein said conditioning signal is
altered to remove a greater quantity of charge for each time steps
(a) and (b) are repeated.
12. The method of claim 10, wherein characteristics of said
conditioning signal are adjusted during a manufacturing operation
so that they are tailored for such memory array.
13. A method of erasing flash memory cells in a flash memory cell
array, the method including the steps of: (a) modifying electrical
charge on floating gates of the flash memory cells by applying a
partial erase signal having a magnitude and duration configured to
affect substantially less electrical charge on said floating gates
than that electrical charge required to place such memory cells
into a filly erased state; and (b) repeating step (a) for a first
number of pulses n so as to substantially equalize electrical field
intensities across tunnel oxides associated with said floating
gates of the flash memory cells; (c) performing an erase operation
by applying an erase signal for a second number of pulses N (where
N>>n) to place such cells in said filly erased state.
14. The method of claim 13, wherein said electrical charge is
modified by applying said partial erase signal to a floating gate
while keeping a source and drain region for said memory cells
floating.
15. The method of claim 13, wherein said partial erase signal is
applied on a sector by sector basis in the flash memory cell
array.
16. The method of claim 13, wherein said erase signal has a
magnitude equal to or greater than said partial erase signal, and
is applied to said array for N consecutive pulses before a check is
made to see if such array is erased.
17. The method of claim 13, wherein: (i) said partial erase signal
is used for said erase signal; and (ii) said N pulses are allocated
such that said partial erase signal is applied to said array only
for a number of consecutive pulses m, where m<<N; and (iii)
after said partial erase signal is applied for m pulses said array
is checked to see if it is erased; and (iii) repeating (ii) and
(iii) until said array is incrementally erased by removing an
amount of electrical charge required to place such memory cells
into a fully erased state.
18. The method of claim 13, further including a step (d): applying
a ramped voltage to first and second semiconductor well regions in
which said array is situated while said partial erase signal is
applied to said array.
19. A method of erasing flash memory cells in a flash memory cell
array, the method including the steps of: (a) modifying electrical
charge on floating gates of the flash memory cells while they are
in a non-erased state by applying a partial erase signal adapted to
be substantially lower in intensity than a full strength erase
signal; and (b) repeating step (a) if necessary for a first number
of pulses n to substantially reduce threshold voltages of all of
the flash memory cells to be below a target threshold voltage
value; (c) applying an erase signal to such flash cells, which
erase signal is adapted to place such cells in a fully erased
state, for a second number of pulses N, where N>>n,; (d)
determining whether such flash cells are actually in said fully
erased state; and (e) if such flash cells are not in a fully erased
state, performing a convergence cycle on the flash cells to adjust
said threshold voltages to a fully erased state.
20. The method of claim 19, wherein said partial erase signal and
said full strength erase signal are applied globally to such cells
in the array.
21. The method of claim 19, where a magnitude of said partial erase
signal is varied during step (b) for each iteration of step
(a).
22. The method of claim 19, wherein said erase signal has a
magnitude equal to or greater than said partial erase signal, and
is applied to said array for N consecutive pulses before a check is
made to see if such array is erased.
23. The method of claim 19, wherein: (i) said partial erase signal
is used for said erase signal; and (ii) said N pulses are allocated
such that said partial erase signal is applied to said array only
for a number of consecutive pulses m, where m<<N; and (iii)
after said partial erase signal is applied for m pulses said array
is checked to see if it is erased; and (iii) repeating (ii) and
(iii) until said array is incrementally erased by removing an
amount of electrical charge required to place such memory cells
into a fully erased state.
24. A method of operating flash memory cells in a flash memory cell
array, the method including the steps of: (a) placing such flash
memory cells into a programmed state; and (b) modifying electrical
charge on floating gates of the flash memory cells while they are
in said programmed state by applying a partial erase signal adapted
to be substantially lower in intensity than a full strength erase
signal; and (c) repeating step (b) for a number of pulses n so as
to ensure that a threshold voltage of all of said flash memory
cells is below a target threshold voltage value, and said flash
memory cells have approximately equal erase rates; (d) applying an
erase signal for a second number of pulses N (where N>>n) to
such flash cells so as to place such cells in a fully erased
state.
25. The method of claim 24, wherein said partial erase signal and
said full strength erase signal are applied globally to such cells
in the array.
26. The method of claim 24, where a magnitude of said partial erase
signal is varied during step (c) for each iteration of step
(b).
27. The method of claim 24, wherein steps (b) and (c) operate to
counter non-uniformities in electrical field intensities across
tunnel oxides for flash cell in such array.
28. The method of claim 24, wherein said erase signal has a
magnitude equal to or greater than said partial erase signal, and
is applied to said array for N consecutive pulses before a check is
made to see if such array is erased.
29. The method of claim 24, wherein: (i) said partial erase signal
is used for said erase signal; and (ii) said N pulses are allocated
such that said partial erase signal is applied to said array only
for a number of consecutive pulses m, where m<<N; and (iii)
after said partial erase signal is applied for m pulses said array
is checked to see if it is erased; and (iii) repeating (ii) and
(iii) until said array is incrementally erased by removing an
amount of electrical charge required to place such memory cells
into a fully erased state.
30. The method of claim 24, further including a step (e): applying
a ramped voltage to first and second semiconductor well regions in
which said array is situated while said partial erase signal is
applied to said array.
31. A method of making a flash memory integrated circuit, said
method including the steps of: (a) providing an array of flash
memory cells in said integrated circuit, each of said cells having
a conduction threshold voltage, a control gate, a floating gate, a
source and a drain; and (b) providing a control circuit coupled to
said array for adjusting the threshold voltages of the cells prior
to an erase operation, the control circuit being configured such
that: i) electrical charge on floating gates of the flash memory
cells is modified by applying a partial erase signal having a
magnitude and duration configured to affect substantially less
electrical charge on said floating gates than that electrical
charge required to place such memory cells into a fully erased
state; and ii) the modification of said charge during step (i) can
be repeated as desired to substantially equalize electrical field
intensities across tunnel oxides associated with said floating
gates of the flash memory cells; iii) a full strength erase signal
can be applied to such flash cells so as to place such cells in
said fully erased state. wherein a probability of any cell becoming
over-erased from the full strength erase signal is reduced.
32. The method of claim 31, wherein said partial erase signal is
applied on a cell by cell basis, while said full strength erase
signal is applied globally to such cells in the array.
33. The method of claim 31, where a magnitude of said partial erase
signal is varied to remove more charge during each of said steps
(i).
34. The method of claim 31, wherein erase discharge characteristics
of such flash memory cells are made more uniform by said partial
erase signal.
35. The method of claim 31, wherein said partial erase signal
operate to counter non-uniformities in electrical field intensities
across tunnel oxides for flash cell in such array.
36. The method of claim 31, wherein characteristics of said partial
erase signal can be adjusted while the flash memory cell integrated
circuit is being manufactured to tailor such characteristics to the
requirements of such cells in the flash memory.
37. The method of claim 31, wherein said full strength erase signal
consists of repeated applications of said partial erase signal.
38. A control circuit for slowing down erase speeds of fast
discharge flash cells in a memory array, such fast discharge flash
cells being characterized by erase speeds substantially faster than
target erase speeds for flash cells in said array, the circuit
comprising: (a) a pre-erase signal generator for generating a flash
cell pre-erase signal to be applied to the flash cells; and (b) a
pre-erase signal magnitude control circuit for generating a
plurality of control signals, said control signals configuring a
magnitude for said pre-erase signal; (c) a pre-erase voltage supply
circuit for generating a negative voltage source; (d) a pre-erase
voltage supply switching circuit for generating a pre-erase voltage
supply signal that is based on said negative voltage source; (e) a
pre-erase voltage supply switching control circuit configured to
control gating of said pre-erase voltage supply switching circuit
with a stepping signal such that said pre-erase voltage supply
signal is generated as a function of said plurality of control
signals and in a time sequence determined by said pre-erase signal
magnitude control circuit; wherein said flash cell pre-erase signal
is configured to remove a quantity of charge from the flash cells
in preparation for an erase operation, said quantity of charge
being related to an erase speed of the flash cell and further being
insufficient to place such flash cells into an erased state.
39. The circuit of claim 38, wherein said pre-erase signal
generator includes a plurality of FETs connected in parallel and
that are activated by said plurality of control signals to
effectuate said magnitude and said duration of said pre-erase
signal.
40. The circuit of claim 38, wherein said control signals are
varied during said time sequence so that said magnitude of said
pre-erase signal is varied during said defined time sequence.
41. The circuit of claim 38, wherein said pre-erase signal is first
pre-charged to a target reference voltage level before said
plurality of control signals are applied.
42. The circuit of claim 38, further including a plurality of pulse
generator circuits, one for each of said plurality of control
signals, which pulse generator circuits generate a pulsed version
of said control signals that is applied and used to generate said
stepping signal in said pre-erase voltage supply switching control
circuit.
43. The circuit of claim 38, wherein operational characteristics of
said pulse generator circuits can be adjusted during a
manufacturing operation to tailor such operational characteristics
to the pre-erase signal requirements of cells in a flash memory
array associated with said control circuit.
44. The circuit of claim 38, wherein said pre-erase signal is
applied on a cell-by-cell basis.
45. The circuit of claim 44, wherein said pre-erase signal is
applied to a floating gate of said cells, while a source and drain
regions for said cell are left floating.
46. The circuit of claim 38, wherein said pre-erase signal operates
to reduce the likelihood of such cell from becoming over-erased
during a subsequent erase operation.
47. A system for erasing flash memory cells comprising: (a) a
pre-erase controller circuit for controlling a flash cell pre-erase
signal to be applied to the flash cells, said flash cell pre-erase
signal being configured to remove a quantity of charge from the
flash cells that is related to an erase speed of the flash cell and
further being insufficient to place such flash cells into an erased
state; and (b) an erase controller circuit for controlling a flash
cell erase signal to be applied to the flash cells, said flash
erase signal being adapted to place such flash cells into an erased
state.
48. The system of claim 47, further including a flash cell
pre-erase signal generator circuit for generating said flash cell
pre-erase signal under control of said pre-erase controller
circuit.
49. The system of claim 47, wherein said flash cell pre-erase
signal and said flash cell erase signal are applied on a
sector-by-sector basis and/or global basis.
50. The system of claim 47, further including a program controller
circuit for controlling a program signal to be applied to said
flash memory cells.
51. The system of claim 47, wherein said flash cell erase signal
consists of repeated applications of said partial erase signal.
52. The system of claim 47, further including a well bias generator
for providing a bias voltage to first and second well regions
associated with said flash cells while said pre-erase signal is
applied to such cells.
53. A circuit for equalizing erase rates of flash cells in a memory
array, the flash cells each having a floating gate with an floating
gate charge value, and which floating gate charge value is intended
to be set to a target initial charge value prior to an erase
operation, the circuit comprising: (a) a flash cell controller
circuit configured to generate a set of control signals for
controlling a magnitude, duration and sequence associated with a
flash cell conditioning signal; (b) a flash cell conditioning
signal generator for generating said flash cell conditioning signal
in response to said set of control signals; and wherein said flash
cell conditioning signal is used for adjusting the floating gate
charge value of each of such flash cells and operates to adjust the
floating gate charge value for any such flash cell by a charge
amount proportional to an original starting floating gate charge
value; and further wherein erase rates of flash cells with floating
gate charge values substantially in excess of said target initial
charge value are substantially affected by said flash cell
conditioning signal, while erase rates of flash cells with floating
gate charge values substantially below said target initial charge
values are substantially unaffected by said flash cell conditioning
signal.
54. The circuit of claim 53, wherein said set of control signals is
varied during a time sequence so that a series of flash cell
conditioning signals are applied during said time sequence to said
flash cells, and such that the flash cell floating gate charge
values substantially converge to the target initial charge value so
that all of the flash cells can then be erased at an approximately
uniform erase rate by a subsequent erase operation.
55. The circuit of claim 53, wherein a probability of any cell
becoming over-erased from the full strength erase signal is
reduced.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to a pre-erasing/erasing
mechanism for flash EEPROM (Electrically Erasable and Programmable
Read Only Memory) devices. The invention is particularly useful for
flash arrays that utilize Negative Gate Channel Erase (NGCE)
operations because it improves erase operations by reducing field
intensity variations in tunnel oxides caused by processing
deviations, increases the number of endurance cycles, and results
in fewer over-erase errors.
BACKGROUND OF THE INVENTION
[0002] The endurance cycles of an EEPROM, especially the number of
program and erase cycles, is the primary determinant of the life
span of such device. Endurance cycles refer to the number of times
which data can be reliably erased, reprogrammed, and read back
without errors. Accordingly, the number of endurance cycles
dictates in large part the usable life of an EEPROM device.
Consequently, one significant goal of prior art efforts is the
maximization of such endurance cycles through the use of improved
cell architectures which minimize cycle stress, as well as
intelligent erase, program methods designed to be less stressful on
the cell architecture.
[0003] Fowler-Nordheim (FN) tunneling is one of the most
well-known, well-understood and prevalent techniques used in the
art for erasing flash memory cells. A significant problem arises
from the fact that certain cells (bits) in the memory array tend to
be "fast" and others "slow" during an erase operation. In other
words, because such cells tend to over-accumulate, or
under-accumulate charge on their floating gates, or because charge
becomes trapped in various locations, these cells tend to have
threshold voltages that deviate significantly from a target
threshold voltage. For this reason, when a "fast" cell is erased,
it is much more likely to become over-erased. When a cell is
over-erased during an erase operation, this causes additional
administrative overhead, because it must be corrected if at all
possible. This is undesirable, of course, because it slows down the
operation of the device from being re-programmed with new data.
[0004] The existence of excess charge on the "fast" bits caused
them to be over-erased for a couple of reasons. First, the FN erase
mechanism depends strongly on the electrical field across the thin
tunnel oxide layer in the flash cell; with more charge, the field
is proportionately higher. This causes more charge to move off the
floating gate during the erase operation. In other words, the fast
bits have electrical field intensities that deviate significantly
from desired target field strengths. Second, at the beginning of
the FN erase, the floating gates are fully charged (i.e., in a
programmed state) and the electrical field is a maximum. This means
that the rate of erasure (the electrical discharge rate of the
floating gate) is also highest at the onset of the FN erase
operation. Thus, if fast bits are not corrected, they tend to
become over-erased, and this leads to concomitant problems of
excess leakage current and/or data errors. In the long run, these
leaky bits cause failures, reduce endurance cycles, etc. As
mentioned above, the problem is especially acute in (but not
limited to) FN tunneling erase operations used with NGCE
configurations. This is because the electrical field becomes
extremely strong between the negative gate and the substrate
well.
[0005] To date, there are very few practical solutions for dealing
with this phenomenon, and no easy way known to applicants for
compensating for electrical field intensity variations that occur
in flash memory cell arrays. These field variations arise naturally
both from wafer processing operations, which, by their nature,
result in differences in cell structures, tunnel oxide
characteristics (thickness and uniformity), etc., as well as from
cycling of the device in normal operation. As the number of cells
increase in EEPROM devices, and integration density increases, and
cycling increases, the field variations also correspondingly
increase because of the nature of normal distributions.
[0006] A related situation is addressed by U.S. Pat. No. 5,901,089,
incorporated by reference herein. In this reference, the individual
logic levels of a multi-bit cell are kept stable by ensuring that
the threshold voltages of such cell are maintained within
predefined threshold ranges. This is accomplished using what it
refers to as "mini" erase/program operations, where a pulse is
applied so that only enough charge is added or removed from a cell
sufficient to keep it within a safe operating range for that state.
While this approach is beneficial for improving sensing (read)
operations, this technique does not appear to be very practical as
a pre-erase conditioning operation. This is because it requires a
significant amount of administrative overhead to perform this type
of procedure on a cell by cell basis. Moreover, the reference is
primarily directed ensuring that Vt overshoot is reduced for each
programmed cell, and does not really address the issue of how to
ensure that the behavior of cells collectively is relatively
uniform and predictable during a subsequent operation in which they
are to be transitioned from one logical level to another. In other
words, there is no attempt made to treat one segment of the cell
population (the "fast" cells) differently than another as may be
necessary to achieve better operational results. Nor does this
reference teach or suggest using a series of lesser strength erase
signals to completely erase a programmed cell.
[0007] For these reasons, an effective method for dealing with so
called fast bits is not known in the art, but is extremely
desirable. Optimally, the erased set of cells has a relatively
uniform distribution centered around a target erase value. To get
this uniform distribution, however, it is necessary: (1) to place
the cells in a similar state (initial floating gate
voltage/electrical field); and (2) for them to have similar
electrical discharge characteristics. The first step can be
accomplished by a conventional programming step, which, in theory,
is intended to add sufficient charge to put all the cells into an
initial target programmed voltage threshold state V.sub.p. To date,
nonetheless, applicants are unaware of specific and practical
mechanisms for achieving the second step of controlling individual
cell electrical discharge characteristics. To accomplish this goal,
it is necessary to control the initial electric field at the
tunneling oxide. If this electric field is not reduced at the onset
of FN erasure for the fast bit segment of the array, the fast bits
will be erased faster than average bits in the memory array. By the
time the average bits are erased, the fast bits are already in a
state of over-erasure. Correspondingly, if the electric field can
be reduced at the beginning of the erasure for a particular bit,
this will slow down the rate of erasure for the bit in
question.
SUMMARY OF THE INVENTION
[0008] An object of the present invention, therefore, is to
eliminate the problems generally inherent in the aforementioned FN
tunneling erase operations when used with NGCE type EEPROM
devices;
[0009] Another object of the present invention is to provide an
improved circuit and method for erasing flash memory cells so that
threshold voltage distributions of such cells are tightened both
before and after an erase operation;
[0010] A further object of the present invention is to provide a
circuit and method for pre-erasing flash memory cells with a
conditioning signal to adjust and equalize their threshold voltage
distributions prior to an erase operation;
[0011] A related object of the present invention is to provide a
circuit and method for generating such conditioning signal, so that
threshold voltage distributions can be finely controlled by a
series of conditioning signals having predetermined magnitudes,
durations, and similar signal characteristics;
[0012] Another object of the present invention is to provide that
such conditioning signals can be custom tailored based on the
particular processing implementation for such memory cell array,
or, alternatively, that such signals can be determined in the field
during normal operation of a flash EEPROM device;
[0013] Yet another object of the present invention is to control
and equalize electrical field intensities across a tunneling oxide
for flash memory cells in a memory array, and to compensate for
normal manufacturing variations causing such field intensity
deviations;
[0014] A related object of the present invention, is to reduce a
discharge rate of fast bits in a flash memory cell to make such
fast bits discharge at a rate comparable to a an average flash
memory cell having a desired discharge rate;
[0015] Another related object of the present invention is to reduce
the probability of a fast bit from becoming over-erased, by
conditioning such bit to have an electrical field comparable to
that of an average flash memory cell prior to an erase operation,
so that such fast bit behaves essentially like an average memory
cell during such erase operation, and is therefore not over-erased
by a full strength erase pulse; and
[0016] Another object of the present invention is to identify fast
bits in a flash memory array and provide them with conditioning
signals which modify erase behavior of such fast bits but not erase
characteristics of average or slow bits in such array;
[0017] A related object of the present invention is to provide an
erase mechanism that uses a stepped or graduated reduction of
charge for cells in a flash memory, so that erase cycles and
resulting erased voltage distributions are more accurately
controlled;
[0018] Yet a further related object is to reduce cell leakage
current by eliminating the occurrences of over-erased fast memory
bits in a flash memory array;
[0019] Still another object of the present invention is to provide
a memory cell array capable of longer life span, by increasing the
number of endurance cycles;
[0020] An additional object of the present invention is to reduce
the possibility of malfunctions and errors in flash memory cells
caused by excessive numbers of leaky data bits.
[0021] Yet still another objective of the present invention is to
provide a complete integrated circuit that implements the
aforementioned methods.
[0022] Another objective of the present invention is to provide an
improved erase circuit for accomplishing both the regular full
strength Negative Gate Channel Erase and pre-erase operations
described above, using the same charge pump.
[0023] A further object of the present invention is to provide a
programmed logic controller for achieving step-by-step time
sequencing of varying intensity conditioning signals to effectively
reduce the electrical field across the tunneling oxide at the
initial phase of erasure in order to control/equalize the discharge
rate of fast bits in Flash memory array.
[0024] These objectives and other significant advantages are
provided by the novel methods and circuits disclosed herein. A
method of slowing down erase speeds of "fast" discharge flash cells
in a memory array--where the fast discharge flash cells are
generally characterized by erase speeds substantially faster than
target erase speeds for flash cells in the array--generally
includes the following steps: (a) generating a conditioning signal
to remove a quantity of charge from the flash cells, which quantity
of charge is related to an erase speed of the flash cell, but is
insufficient to place such flash cells into an erased state; and
(b) applying such conditioning signal to such flash cells while the
cells are in a non-erased state. The conditioning signal is
preferably applied on a global basis to the control gate while the
source and drain regions are kept floating. In this fashion, the
floating gate charge value is adjusted by the conditioning signal
by a charge amount proportional to the original electrical field
intensity. In other words, the faster cells will experience the
greatest change in charge reduction. From a physical perspective,
therefore, the present invention operates to substantially equalize
electrical field intensities across tunnel oxides associated with
each of the floating gates of the flash memory cells.
[0025] After the conditional signals treat such array, the erase
speed of such fast discharge flash cells is significantly reduced.
Since the other non-fast cells are left relatively unaffected, the
entire distribution of erase speeds (and threshold voltages) in the
cell array is equalized, so that the array can be treated with
greater reliability and predictability in subsequent array
operations (such as an erase operation).
[0026] Prior to performing the above conditioning, of course, a
conventional programming operation can take place, followed by an
additional threshold voltage measuring step. The latter measures
the electrical characteristics of the flash cells to determine an
expected erase discharge speed for such flash cells, and to
determine which ones, if any, may need pre-erase conditioning.
[0027] During the conditioning cycle, the above steps are repeated
as necessary to remove further charge from the flash cells. For
each iteration, the properties of the conditioning signal,
including magnitude and/or duration, can be varied to remove more
or less charge from the flash cells. Usually, the cycle is set up
so that each successive conditioning signal removes a greater
quantity of charge.
[0028] After the array is preconditioned, a normal, full strength
erase operation can be performed. Because the cell discharge
characteristics are modified immediately in advance of the erase
operation, the result is that there are fewer fast bits, and those
that do exist have been slowed down to the point where it is less
likely they will become over-erased.
[0029] In another embodiment of the invention, instead of a full
strength erase cycle, the aforementioned conditioning cycle is
instead repeated as necessary to fully erase the cells. This
approach provides finer, more accurate control of the erase
operation, and helps to ensure that, as the cell array
characteristics change with time, the erase is performed in a
manner that is tailored to such characteristics.
[0030] Thereafter, an over-erase detection operation takes place,
to verify that no cells have been over-erased from said erase
operation. In the event any such cells are detected, a two-stage
convergence operation is preferably performed. This includes
generally two independent sequential operations, including: (a) a
global operation performed on the entire array; and (b) a localized
operation performed only on those cells still over-erased after the
global operation is completed.
[0031] In another variation of the invention, it is possible, for
example, to practice the present methods in the context of a
pre-program conditioning operation, so that the cell charge
acquisition (instead of discharge) characteristics are modified
instead. The only important consideration is that the flash cell
floating gate charge values are made to substantially converge to
an initial target charge value so that all of the flash cells can
then be erased/programmed at an approximately uniform rate by a
subsequent operation.
[0032] Another beneficial aspect of the present invention lies in
the fact that characteristics of the conditioning signal can be
adjusted during a manufacturing operation, so that it will be
specifically tailored for the manufacturing deviations present in
an associated memory array.
[0033] A novel method of erasing flash memory cells in a flash
memory cell array, therefore, utilizes the aforementioned pre-erase
operation in conjunction with a traditional erase operation. Again,
unlike a conventional erase operation, however, the present
invention can "fix" potentially problem cells individually for
maximum control/flexibility of an erase operation. Thus,
non-uniformities in electrical field intensities across tunnel
oxides for flash cell in such array, caused by unpredictable but
inevitable manufacturing process variations, can be substantially
reduced. This results in increased product endurance, less erase
time, better yields, etc.
[0034] A flash memory integrated circuit can be constructed using
conventional wafer processing steps to include the teachings of the
present invention. In such circuit, an array of flash memory cells
is provided, each of such cells having a conduction threshold
voltage, a control gate, a floating gate, a source and a drain. A
logic controller circuit is coupled to the array for adjusting the
threshold voltages of the cells prior to an erase operation, and is
configured such that it can generate control signals in accordance
with the methods described, and so that appropriate signal
generating circuits can apply the aforementioned conditioning
signals in the manner described above as well.
[0035] A preferred embodiment of a preconditioning control circuit
of the present invention effectuates the above processes by the
operation of: a pre-erase signal generator for generating a flash
cell pre-erase signal to be applied to the flash cells; and a
pre-erase signal magnitude control circuit for generating a
plurality of control signals, which control signals determine a
magnitude for the pre-erase signal; a pre-erase voltage supply
circuit for generating a number of necessary supply voltages,
including a negative voltage source in the case of a pre-erase
signal; a pre-erase voltage supply switching circuit for generating
a pre-erase voltage supply signal that is based on the negative
voltage source; and a pre-erase voltage supply switching control
circuit, which is configured to control gating of the pre-erase
voltage supply switching circuit with a stepping signal. The
pre-conditioning control circuit thus generates the pre-erase
voltage supply signal as a function of the plurality of control
signals and in a time sequence determined by the pre-erase signal
magnitude control circuit. This flash cell pre-erase signal is
configured, as noted earlier, to remove a quantity of charge from
the flash cells in preparation for an erase operation, which
quantity of charge is related to an initial erase speed of the
flash cell and further being insufficient to place such flash cells
into a fully erased state.
[0036] Another useful aspect of the inventive pre-conditioning
control circuit includes the fact that the pre-erase signal
generator includes a plurality of FETs connected in parallel, and
that are activated by the plurality of control signals according to
a controllable sequence so that the desired magnitude and duration
of the pre-erase signal is well controlled.
[0037] As noted earlier, the control signals are varied during the
time sequence so that the magnitude of said pre-erase signal is
varied during said defined time sequence. In the typical case, it
is increased from step to step to affect the cells more--i.e., to
remove more and more charge during each step. Again in a preferred
embodiment, a pre-erase signal is first pre-charged to a target
reference voltage level before the plurality of control signals are
applied, to increase speed and reliability.
[0038] To control the pre-erase voltage supply switching control
circuit, a plurality of pulse generator circuits, one for each of
the plurality of control signals, generate a pulsed version of such
control signals that is applied and used to generate the
aforementioned stepping signal.
[0039] The aforementioned pre-conditioning circuit, therefore,
operates as an equalizing circuit that equalizes the erase rates of
the flash cells, and thus makes it more likely that they will not
be over-erased, requiring additional system overhead, loss of data
from faulty bits, etc.
[0040] As alluded to earlier, the operational characteristics of
the pulse generator circuits can be adjusted during a manufacturing
operation, to tailor such operational characteristics to the
pre-erase signal requirements of cells in a flash memory array
associated with the control circuit.
[0041] Thus, a system for erasing flash memory cells in the present
invention includes programmed logic within a flash controller
circuit (usually a state machine), so that erase control logic
includes two logical components, including: (1) a pre-erase
controller circuit controlling a flash cell pre-erase operation,
managing the conditioning signal magnitude, timing and sequencing,
as well as (2) a conventional erase controller circuit for
controlling a normal strength flash cell erase signal.
[0042] The present invention, therefore, is extremely useful in
those environments that rely on FN tunneling, and helps to
eliminate potential problems caused by normal process variations in
tunnel oxide layers, floating gates, cell topographies, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 is a simplified flow chart showing steps used in a
conventional prior art nonvolatile Flash memory erase
operation.
[0044] FIG. 2 is a flow chart showing the steps practiced by a
flash memory logic controller circuit of the present invention,
including a pre-erase operation performed prior to a full
erase.
[0045] FIG. 2B is a cross sectional view of a non-volatile Flash
memory cell structure used in the present invention, which cells
are situated in a P-WELL enclosed by a deep N-WELL.
[0046] FIG. 3 depicts in block diagram format the voltage
generators and signal generators used for performing the pre-erase
and modified erase operation of the present invention.
[0047] FIGS. 4A-4C are more detailed circuit schematics of aspects
of a flash memory conditioning signal generating circuit relevant
to the present invention.
[0048] FIG. 4D is a table showing the sequence and magnitude of
various conditioning signals generated by the present controller
circuit.
[0049] FIG. 5 illustrates a first embodiment for generating a
control pulse in response to the logic controller circuit using a
one-shot delay circuit schematics comprised of a MOSFET device and
capacitor elements.
[0050] FIG. 6 illustrates a second embodiment for generating a
control pulse for the logic controller using a one-shot delay
circuit comprised of resistor and capacitor elements.
[0051] FIG. 7A is a graphical illustration of the effect of a prior
art erase operation on the threshold voltage of a slow flash cell
and a fast flash cell in a memory cell array.
[0052] FIG. 7B is a graphical illustration of the effect of the
pre-erase operation of the present invention on a threshold voltage
for fast flash cells in a memory array.
[0053] FIG. 7C is a graphical depiction of the effect of the
pre-erase operation of the present invention on floating gate
electrical charge discharge characteristics for flash cells in a
memory array.
[0054] FIG. 8 depicts a preferred embodiment of a well bias voltage
circuit used in the present invention;
[0055] FIG. 8A illustrates a timing diagram of a voltage level as
it is generated by the well bias voltage circuit and applied to
various well areas of a flash memory array.
DETAILED DESCRIPTION OF THE INVENTION
[0056] A detailed explanation of the preferred embodiments is now
provided, examples of which are illustrated in the drawings and
discussed herein.
[0057] In a conventional non-volatile Flash EEPROM array, data in
the memory array may be erased electrically all together as a block
while new data can be programmed after on a byte-by-byte basis.
Although the erasure is normally effected on all memory cells in
the array simultaneously, not every cell in the same array will be
erased equally fast, or to the same erased voltage state as
mentioned above. Some bits will be relatively fast, and some bits
will be relatively slow. This is due primarily to normal variations
in the processing technology as well as the topological deviations
associated with the control circuits, which tend to affect the
tunneling oxide characteristics, the floating gate characteristics,
etc. In addition, normal "aging" of a cell from repeated cycling
will also cause it have varying and unpredictable behavior due to
charge trapping, structural deterioration, and the like. These
factors are unavoidable in conventional memory arrays, and are
becoming increasingly more difficult to cope with as such memory
arrays increase in density, size, etc. Thus, owing to the reasons
cited above, controlling the erase operation in a Flash memory
array to achieve uniform memory cell characteristics after
electrical erase is becoming substantially more critical than the
program operation.
[0058] A conventional method of erasing a Flash memory array is
illustrated in FIG. 1. As is known, the memory array is first
pre-programmed at step 20 to set all cells to a known data value
before the erase operation. After confirming at step 30 that all
cells have passed the program verification, the procedure then
proceeds to erase the cells at step 40 using any conventional
method, such as a standard erase pulse of suitable magnitude and
duration applied repeatedly to the entire array. The execution of
an erase operation involves typical logic control circuits such as
a counter, timer, pulse generator logic controller and so forth,
which are well-known in the art. An example of these circuits can
be seen in our pending application Ser. No. 08/988,872 which is
incorporated herein by reference. At step 50, a check is made for
over erased bits; these bits are undesirable for a number of
well-known reasons, including the fact that they tend to cause
current leakage in the memory array. If over erased bits are
detected by the control circuits, step 60 then tries to perform a
correction routine to reduce the leakage current. One exemplary
method for performing such routine is described in detail in the
aforementioned pending application. This correction procedure is
repeated until all bits can pass the leakage current test, or until
there is a determination made that the part has failed due to
uncorrectable current leakage in the Flash memory array.
[0059] As soon as step 50 and step 60 are finished, step 70
executes an erase verification routine to ensure all the memory
cells in the Flash memory array are completely erased. If any cell
in the memory array is not yet erased completely, the control
circuits starts the entire procedure over again as illustrated in
the flow chart of FIG. 1. If for any reason, however, the memory
array can not pass any step as shown in the flow chart, the Flash
memory array is considered defective, and thus the endurance cycles
are compromised to a much lower limit. It is plain also to see that
the existence of over-erased cells significantly slows down the
operation of the device, since routine 60 must be executed to see
if such cells can be corrected. Furthermore, the correction routine
60 and erase operations 40 impose additional stresses on the cells
by way of additional conditioning signals/pulses, and this also
undesirably shortens the life span of such cells.
[0060] Clearly, therefore, it is extremely beneficial to reduce the
need and invocation of such routines; applicant believes that the
present inventions accomplish this goal by taking intelligent
precautions to reduce the probability that any cells will become
over-erased in the first place. This is done primarily by a
strategy that focuses on those cells that are identified as
susceptible or pre-disposed for erratic discharge behavior, and in
particular, so-called "fast" bits which are most likely to cause
such problems.
[0061] FIG. 2 illustrates an embodiment of a preferred erase method
of the present invention, which, in contrast, to the prior art
includes a pre-erase procedure as well. This new erase and
pre-erase method is now explained briefly. Generally speaking, the
primary task of the pre-erase procedure is to correct (as best as
practically possible) the fast bits in the non-volatile Flash
memory array, by reducing an electrical field associated with a
tunneling oxide for such bit. From a simplified perspective, and
with all other things being equal, as the electrical field across
the tunneling oxide becomes greater, so does the rate of charge
accumulation or discharge for the floating gate of the cell.
Accordingly, if in advance of an erase operation this tunnel oxide
field can be reduced for those cells having relatively high initial
starting field values, such cells can be effectively slowed down
and made to behave more like a target cell during the actual erase
operation. Furthermore, if this electrical field treatment can be
controlled with reasonable precision, the pre-erase procedure can
be confined and limited to affect only those cells that are the
most likely to cause a problem during the erase operation. Thus the
present method has little or no adverse effect on any cells having
normal or slow behavior during an erase operation.
[0062] The effect of the pre-erase treatment can be summarized as
follows:
[0063] the electrical field across the tunneling oxide is reduced
by an amount proportional to an initial value for such field; in
other words, those cells having the greatest positive deviation
from a target field value (i.e., those having the greatest
potential for over-erasure) are affected the most;
[0064] this means that the erase speeds of these fast discharge
flash cells are reduced to be more like those of target flash
cells, so that for a given erase pulse magnitude and duration, such
fast cells do not lose as much charge as they would before without
pre-erase treatment;
[0065] the corollary of this is that the overall erase speed of
cells in the memory array is "equalized" in the sense that the fast
bits are made to conform to look like the rest of the cell
population;
[0066] because the overall cell population discharge rate is slowed
down, and the fast cells are not as fast, they do not lose so much
charge that they become over-erased;
[0067] the reduction in probability of a cell becoming over-erased
results in fewer over-erased cells in fact, which in turn means
that fewer additional corrective routines and erase routines are
executed, and which further means increased endurance and life-span
for the device because of the reduction in cycle stressing;
[0068] with the pre-erase cell population more uniformly
distributed, the erase operation has more predictable and
consistent effect, and the resulting erased population also has
better threshold voltage distributions;
[0069] with a more predictable erased threshold voltage
distribution for the cells in the memory array, later operations,
including programming, are also more reliable;
[0070] undesirable processing variations in the tunneling oxide,
cell array topographies, etc. that would otherwise cause premature
failures can be compensated for (to a large extent) in a manner
that is inexpensive, flexible, and easy to implement.
[0071] The routine of FIG. 2 is preferably executed as a control
routine by a logic controller coupled to the flash memory array
(see FIG. 2B). As seen at step 100, this routine starts by
verifying that a command is present for initiating an erase
operation. If the commands are correct, then an internal state
machine of the logic controller sets a variable ERGO=1 valid. The
logic controller circuit and state machine can be constructed in
any number of known ways, and the specifics of the same are not
material to the present invention. This is true, as well, with
other hardware aspects of the present invention with the exception
of the voltage generators (FIG. 3), pulse generators and control
logic (FIGS. 4-6) for which preferred circuit implementations are
specifically provided for herein.
[0072] Referring again to FIG. 2, after the initial erase command
verification step, a conventional pre-programming before erasure
step takes place at 110; the intent of this step is to move the
threshold voltages of the cells to a known state prior to the erase
operation, and to reduce over-erase problems. Thus, at this stage,
the cells are all in a programmed state, which can also be
considered a non-erased state; i.e., the threshold voltage
V.sub.t=V.sub.p, where V.sub.p is a target programmed state value.
Ideally the cells will have the same initial floating gate charge,
and the same threshold voltage value at this point, but, for the
reasons mentioned above, this is not the case in reality. If the
pre-program fails, the routine routes to step 180 for hang-up which
means the memory array is defective, which may be caused by a
number of reasons.
[0073] At step 120, if all the programming procedures are
completed, instead of beginning a conventional erase operation, the
pre-erase procedure of the present invention is executed. During
this pre-erase procedure, a series of conditioning signals (pulses)
are applied to the cells in the memory array. These signals are
configured to remove a quantity of charge from the floating gates
of the cells, which charge quantity is enough to reduce the tunnel
oxide field, but not enough to put the cell into an erased state.
As seen in FIG. 2B, the pre-erase conditioning signals are applied
to a control gate VCG while the source and drain are left floating.
A positive potential is asserted at both the p-well and n-well
regions while the p-substrate is grounded.
[0074] In practical terms, this step can be considered a "partial"
erase, since some charge is removed from the floating gates of the
cells, but not enough to constitute a full strength erase. Unlike a
full strength erase, the conditioning signals in this case are only
applied for short periods of time, for a limited number of cycles,
and can be smaller in magnitude as well. Broadly speaking, the
preferred approach begins with a conditioning signal that causes a
relatively low electrical counter-acting field to be generated
across the tunneling oxide to remove charge from the floating
gates, and this counter-acting field is gradually increased in
subsequent conditioning signals to some maximum value. Notably,
when these conditioning signals are applied to the cells, the cells
with the highest initial fields will also experience a greater loss
of charge, which means they will be affected the most. This
beneficial side effect of the present invention means that
relatively slow bits, and even normal bits, will be only slightly
(if at all) affected by the pre-erase procedure. As mentioned also
above, in the end, the fast bits will be modified to have smaller
electrical fields, and this in turn results in their behavior being
changed to act more like an ideal, target cell in the array.
[0075] Thus, after the pre-erase treatment, the cells will now have
a more uniform, equalized field intensity and correspondingly a
more uniform and equalized erase rate. The cells in the array can
be monitored, after the end of each particular conditioning signal
(or some predetermined sequence) to examine the effects of the
treatment to that point. In other words, the existence, degree, and
characteristics of any remaining fast bits can be determined, by
examining the threshold voltage distributions. If necessary, an
additional conditioning signal (or sequence) can be applied,
followed by a subsequent monitoring step, etc. Thus, a fast,
iterative process can be employed by repeating the above steps to
force the cell population to have a set of threshold values below a
target threshold value, and without concern for placing such cells
accidentally into an erased state, or the possibility of
substantially affecting normal cells. The particular sequencing,
magnitude, duration, and number of cycles of the conditioning
signals will, of course, vary from array to array depending on the
characteristics and number of fast bits, but can be optimized as
necessary and/or desired within the logic controller circuit. A
particular preferred approach used in the present invention is
depicted in FIG. 4D, and is discussed in more detail further
below.
[0076] As soon as the step 120 for pre-erase has completed as
illustrated in FIG. 2, erase verification at step 130 commences to
check if all the memory cells have been completely erased. While no
cells are intended to be fully erased prior to this time, this step
is common practice, and typical of erase procedures known in the
art. If any bit in the memory array does not pass erase
verification (as would be expected to be true at this stage), then
fill strength of erasure at step 140 begins. This erase operation
uses a conventional full strength erase pulse with a magnitude,
duration, and frequency adapted in known fashion for the memory
array in question. At the end of this operation, all the memory
cells should be in an erased state, and, because of the effect of
the pre-erase operations noted above, with reduced numbers of
over-erased cells.
[0077] After each erase operation, cell leakage for each column
will be checked as illustrated at step 150 to ensure that no memory
cell has been over-erased in the memory array. If all the cells are
erased, with no over-erasures, then the erase procedure
terminates.
[0078] However, if current leakage in the memory array is detected,
then the routine transitions to a convergence sub-routine comprised
of global convergence step 170 and column threshold convergence
step 160. A preferred embodiment of this convergence subroutine is
described in applicant's pending Ser. No. 08/988,872. Briefly, when
an over-erase condition is found for any bit, a FLAG=0 condition
exists, so a global convergence cycle 170 utilizes a series of
"soft" program and soft erase conditioning pulses to adjust the
flash cell threshold voltages in the array on a global basis. This
procedure is executed for a configurable, pre-determined length of
time for efficiency and effectiveness based on parameters of the
array that can be programmable and/or determined optimally in
advance by a variety of test and modelling measurements. The soft
program and soft erase pulses are also tailored specifically for
the array in question, and are not of the same type as a full
strength program and erase pulse respectively. The effect of step
170 is to gently pull and tug the edges of the cell threshold
voltages, resulting in a tighter distribution. This is because
cells having relatively high deviations from the target erase
voltage are affected the most by these soft program/erase pulses.
While the details of the global convergence are not material to the
present invention, more information on the same can be found in the
aforementioned pending disclosure.
[0079] After the global convergence cycle step 170, cell leakage
current is checked again at step 150. If step 170 has been
completed and excessive leakage current is still detected in the
memory array, FLAG is set to 1, and a column threshold convergence
cycle 160 is executed to provide even tighter control of cell
threshold distribution in the Flash memory array. This step is more
effective than the global convergence cycle at eliminating single
bit over-erase errors, because it is applied on an individual
column basis. More particularly, this cycle uses a series of
adjustable but well targeted soft program pulses to nudge a
particular cell out of the over-erased state, and into an
acceptable area corresponding to a threshold value for a target
erase state. The duration of the cycle, as well as the number of
soft program pulses, can be customized for specific
applications.
[0080] In the applicant's experience, the combination of these two
cycles substantially reduces the existence of over-erased cells,
should any exist after the erase operation noted above. This
combination is optimal because the global convergence cycle works
very quickly on a rough scale to eliminates a large portion of
leaky cells; the remaining small number of leaky cells are
eliminated on an individual basis by the column threshold cycle,
which, albeit slower overall, should not slow down the operation of
the device since it should not have many errors to fix.
[0081] After step 160 is finished, verification for the completion
of erasure at step 130 is repeated to ensure that no memory cells
were disturbed by the convergence cycles at steps 160 and 170. The
entire sequence of steps beginning at step 130 can be repeated for
a predetermined number of times, based on a programmable parameter
ECOUNT, until the memory array passes erase verification, or until
such parameter maxes out (ECOUNT=Max), which results in a hang-up
at step 180.
[0082] In another variation of the above embodiment, the array
conditioning process is not discontinued prior to an erase
operation. Instead the conditioning process is extended in time so
that the cells in the array are actually finally fully erased, as
they would be from a conventional fill scale erase pulse. In this
fashion, the array is erased in an incremented, controlled
approach, with more and more charge gradually removed from the
cells. This is done as follows: during the pre-erase conditioning
process described above, the conditioning signal is stepped
(incremented) in magnitude after a predetermined number of cycles
(e.g., 8). For the entire pre-erase cycle, the signal magnitude can
be varied from approximately-6.50 to -8.50 volts, with 0.25 volt
increments; however, it is possible that it may be terminated prior
to that time, if the measured threshold voltage distributions
suggests that it is not necessary to continue (i.e., the fast cells
have been slowed down sufficiently). At this point, in lieu of
performing a conventional erase cycle, with a large number of
iterations of a full scale signal, the present embodiment continues
the pre-erase cycle past the normal stopping point, and uses this
process instead to proceed to fully erase the array. Thus, instead
of breaking up the erase operation into two distinct phases (i.e.,
pre-erase and full erase) having different signal characteristics
(as noted earlier), the incremented erase cycle of this variant of
the present invention is actually comprised of a single type of
erase operation in which repeated small intensity signals are used
to gradually reduce the floating gate charge, and, correspondingly,
the threshold voltages of the cells. In this approach, after
stepping through the various signal levels for each invocation of
the process in 0.25 volt increments, a final high intensity voltage
level is reached (-8.50 volts in a preferred embodiment) for the
pre-erase phase. For the "erase" phase, this voltage is kept
constant, and there is no longer any stepping of the
conditioning/erase signal voltage. This is because the lower
intensity signals will no longer be effective to move charge away
from the floating gates anyway. Unlike a full erase operation,
however, the incremented erase of the present embodiment uses a
much smaller number of pulses (for example, 6-10 applications
instead of hundreds for a normal erase) to only adjust the cell
thresholds by a much smaller degree. After each 6-8 pulse
iteration, the cell array is checked to see if it is yet fully
erased. If not, another 6-10 pulses are applied, and the procedure
is repeated as necessary. Other variations are of course possible,
and may be more optimal in some environments. For instance, it may
be preferable to use a scaled number of pulses with each iteration,
so that N are used for the first iteration, N/2 for the second, N/3
for the third, etc., etc. This allows an erase procedure to be
self-adjusting to the behavior of the cells in the array, because,
over time, their erase characteristics will change, and it is
difficult to predict and fix in advance a constant erase parameter
that will satisfy all operating conditions of the array. During
this incremented erase process the number of erase pulses,
therefore, will vary and be dynamically adjusted in accordance with
the requirements of the array.
[0083] Notably, the present method can be used with a device in the
field, or, alternatively, as a design/test tool aid in connection
with monitoring the quality of a non-volatile cell or array
architecture. Examples of particular preferred embodiments for
generating the pre-erase signals, and associated supporting signals
(negative erase supply voltages, control signals, etc.) are
illustrated in FIGS. 3 to 6.
[0084] In FIG. 3, a block diagram is provided showing the circuit
components used with a pre-erase/incremented erase circuit 300 to
generate the necessary voltages, clocks, control and
pre-erase/incremented erase signals of the present invention.
Oscillating clocks Phi and Phi.sub.b are generated from block 305
and supplied to negative pumping circuit 310 to internally generate
the required negative voltage V.sub.NEG that is used for the erase
operation (NGCE) used in the present invention. After the negative
voltage is generated on chip, it is distributed by a voltage switch
control circuit 320 to appropriate sections of the Flash memory
array as signal V.sub.NGI. This signal, as well as a reference
voltage V.sub.REF from voltage generator 350 is used by negative
voltage level control (NVLVC) circuit 330 to generate the pre-erase
conditioning signals described above in connection with step 120
(FIG. 2). Generally speaking, NVLVC circuit 330 divides the
negative voltage V.sub.NGI into several smaller negative voltages,
and these smaller voltages form the basis of the conditioning
signals applied to the flash cells to effectively reduce the
electrical field across the tunneling oxide. While the negative
voltage is divided and distributed to needed sections in the Flash
memory array, comparator circuit block 340 compares the internally
generated voltage V.sub.GER to a predetermined voltage reference to
keep the internally pumped negative voltages at the right voltage
level.
[0085] The aforementioned sections of pre-erase/incremented erase
circuit 300 can be implemented in a variety of forms suitable for
use with the present invention. For the present disclosure only
portions of NVLVC circuit 330 need be explained in more descriptive
detail, since it provides part of the novel and unique
functionality and features of the present invention.
[0086] As seen in FIG. 4A, a pre-erase/incremented erase signal
generator circuit 450 generates output V.sub.GER used for reducing
the tunnel oxide fields in response to a combination of various
voltage reference and logic signals. First, a pre-erase/incremented
erase voltage switch circuit 452 provides a negative input voltage
V.sub.NGI in response to voltage V.sub.NEG from circuit 310, and as
gated by signal ESTEPB. The latter is active in accordance with
signal conditions which can be understood with reference to FIG.
4b. Output V.sub.GER can be seen as a divided version of negative
voltage input V.sub.NGI; in operation, a capacitor divider formed
by capacitor 454, capacitor 460A, and other capacitor stages that
are switched in (460B, 460C, 460D and 460E) depending on the state
of certain logic variables ERLVC<3:0> which in turn activate
associated FET switches (462B and 464B, 462C and 464C, 462D and
464D, and 462E and 464E respectively). For example, if a pre-erase
conditioning pulse V.sub.NGI of approximate magnitude -6.25 volts
is desired, only ERLVC<0> is activated; all other pre-erase
magnitude control signals (ERLVC<1>, ERLVC<2>,
ERLVC<3>) are low. These signals are generated in
conventional fashion by a pre-erase/incremented erase magnitude
control circuit, which is implemented as part of the logic
controller circuit described above within the flash memory
integrated circuit. Thus, with this combination, a negative voltage
V.sub.NGI of approximately -6.25 volts is provided to
pre-erase/incremented signal generator circuit 450, which voltage
is then divided by the serial combination of capacitor 454 taken
with the parallel combination of capacitor 460A and 460B. The
latter is switched in by virtue of the fact that FET 462B is turned
on by the state of ERLVC<0>, and the fact that FET 464B is
switched off (by a signal ERLVCB<0> having opposite logic to
ERLVC<0>). Furthermore, no other capacitor (460C, 460D or
460E) is switched in at this point. Accordingly, a capacitive
divider is formed by 454, 460A and 460B between V.sub.NGI and
ground, with V.sub.GER taken at a potential between capacitor 454
and capacitors 460A and 460B corresponding to approximately -6.25
volts. The other possible combinations of pre-erase/incremented
erase magnitude control signals ERLVC<3:0> needed to generate
larger magnitude pre-erase/incremented erase signals are given in
the table of FIG. 4d. It is apparent, of course, that additional
levels could be used if desired. While the pre-erase/incremented
erase signals are being applied there is no contribution from
V.sub.REF, because FET 456 is not activated by signal ESTEP.
Instead, V.sub.REF is used as a pre-charger for the capacitor
divider, and is controlled by ESTEP generated by
pre-erase/incremented erase voltage switch control circuit 406.
[0087] The origin of signals ESTEP and ESTEPB are shown in FIGS. 4b
and 4c. First, as seen in FIG. 4c, a series of pulse generators
401, 402, 403, 404 and 405 are provided. Inputs to these pulse
generators are provided in the form of pre-erase/incremented erase
magnitude control signals ERLVC<3:0>, and sequence control
signal ER, which is also generated by the logic controller circuit
to control the sequencing (time spacing) between consecutive
pre-erase/incremented erase conditioning for the
pre-erase/incremented erase state machine operations. These pulse
generators govern the stages of the pre-erase/incremented erase
signal V.sub.NGI and can be implemented and configured as described
further below with reference to FIGS. 5 and 6.
[0088] Circuit 406 in FIG. 4b, therefore, can be considered as a
pre-erase/incremented erase voltage switch control circuit for the
pre-erase/incremented erase signal generating circuit 450. In the
beginning, V.sub.GER is pre-charged to a reference voltage, which
is a function of the ERLVC signals selected, and the effect of
ESTEP in activating FET 456. The signal voltage V.sub.GER later on
is coupled by a combination of various capacitors between V.sub.NGI
and ground. This one-shot pre-charge of V.sub.GER is also supported
by the operation of circuit 406.
[0089] As is apparent, the amount of charge removed from the
floating gates of the memory cells is a function of the signal
duration, and, their relative electrical field intensity across the
tunnel oxide. Therefore, the V.sub.NGI control logic of FIG. 4a,
and corresponding signal sequencing logic of FIG. 4c allows for
additional tailoring of the characteristics of the
pre-erase/incremented erase operation for any particular
environment or application.
[0090] FIG. 5 illustrates a first embodiment of a single-shot pulse
generator circuit 500 that can be used with the present invention.
An input signal IN is provided to a first series of conventional
inverters 502, 504 in a first signal path, while a second series of
conventional inverters 506, 508 and 510 along with capacitive
elements 512 and 514 are used in a second, delayed signal path. The
OUT signal of NAND gate will track the IN signal in the manner
illustrated in this figure, and the delay elements can be easily
manufactured using conventional processing to reasonable accuracies
needed to help control the pre-erase/incremented erase conditioning
signal behavior. In FIG. 6, a second embodiment of a pulse
generator circuit 600 is depicted. In this variation, input signal
IN is provided to a first series of conventional inverters 602, 604
in a first signal path, while a second path is comprised of a
variable delay inverter stage formed of PMOS FET 606, NMOS FET 608,
and resistance R. This stage is also followed by conventional
inverters 614, 616, and a capacitive element 612. The OUT signal of
NAND gate will track the IN signal in the manner illustrated in
this figure, and the delay will be a function of the RC constant of
the second signal path. The benefit of this embodiment lies in the
fact that the delay characteristics can be customized on a chip by
chip basis during the manufacturing process by conventional laser
trimming operations performed on a variable resistor.
[0091] FIGS. 7A to 7C illustrate in rough graphical form the effect
of the present invention on a typical flash memory cell array. It
will be understood by those skilled in the art that these graphs
are merely approximations and are not intended to reflect the
actual behavior any particular array. Nevertheless, applicant
believes that these illustrations help clarify the operational
features, effects and benefits of the present inventions.
[0092] First, in FIG. 7A, voltage threshold distributions are
plotted for a flash memory population in an erased state (lower
threshold voltage) and programmed state (higher threshold voltage).
It should be noted first that these terms (erased, programmed) are
merely descriptive labels for the charge condition of the floating
gates, and it is entirely possible that in some applications these
labels will be reversed for the same charge condition. In other
words, the present invention can be practiced even in environments
where the program/erase logic is opposite to that shown herein,
since the operational features and benefits can still be employed
effectively wherever it is desirable to equalize cell discharge
rates. In any event, flash memory cells in a programmed state
ideally have a voltage threshold V.sub.t=V.sub.p, where Vp is a
target programmed voltage. In reality, however, the programmed
threshold voltage distribution looks substantially like that shown
in FIG. 7A, where there is an approximately normal distribution
(solid line) centered around voltage level Vp. So long as the cells
have threshold values between V.sub.pmin and V.sub.pmax,
nonetheless, they will still be adequately discriminated by the
sense amplifiers of the flash device as being in a programmed
state. Similarly, flash memory cells in an erased state ideally
have a voltage threshold V.sub.t=V.sub.e, where V.sub.e is a target
erased voltage. Again, in real world devices, chip designers are
content to operate with the erased threshold voltage distribution
looks substantially like that shown in the FIG. 7A, where there is
an approximately normal distribution (solid line) centered around
voltage level V.sub.e. As above, so long as the cells have
threshold values between V.sub.emin and V.sub.emax, such cells will
still be adequately discriminated by the sense amplifiers of the
flash device as being in an erased state.
[0093] A serious problem, however, lies in the fact that the flash
cells in the array do not behave consistently or uniformly during
an erase operation. A slow bit, identified in FIG. 7a, will change
threshold voltages at a rate far slower than a fast bit identified
in FIG. 7a. These fast bits tend to become over-erased, and the
slow bits tend to be under-erased, as evidenced by the dotted line
distribution shown in FIG. 7a. For a variety of well-known reasons,
the effect is not symmetric, however, so that the slight
under-erasure of the slow bits does not usually present a
significant problem. In other words, they are still discernible as
"erased," so long as their threshold voltage is less than
V.sub.temax, and they do not contribute to any deleterious leakage
currents as is the case with over-erased bits. In contrast, the
fast bits can become over-erased, and therefore the population of
cells within the shaded area of FIG. 7a must normally be dealt with
using the corrective procedures described above. Still, this
after-the-fact correction is undesirable and reduces device life
span.
[0094] The benefits of the present invention on the cell threshold
distribution can be seen in FIG. 7b. This shows the programmed
state of the array, after the pre-erase operations have taken
place. The top end of the voltage thresholds are curtailed,
significantly, so that the distribution is tighter. The same fast
bit has now had a threshold voltage alteration from V.sub.f to
V.sub.f', where V.sub.f'<V.sub.f; this means that this same bit
now acts more like an average cell, and is far less likely to be
over-erased during the subsequent erase operation (shown
approximately as the dotted line). The slow bits and average
programmed bits, in contrast, are hardly affected, and therefore
the overall erase operation is improved significantly from a
variety of perspectives.
[0095] In FIG. 7C, a graphical comparison of cell discharge rates
for fast, normal, and slow bits is given. Because of the intense
electrical fields in fast bits, FN tunneling occurs much faster, so
they discharge much faster compared to normal and slow bits as
well. At the end of an erase operation (time T.sub.e) such bits are
well past being erased (V.sub.e), and in fact are over-erased
(below V.sub.emin). By pushing the threshold voltage Vf to a more
normal threshold voltage V.sub.N at the start of the erase
operation, this bit now is more likely to behave like such normal
bit, and therefore result in a correctly erased bit at the end of
the erase operation.
[0096] FIG. 8 is an embodiment of a well bias voltage circuit 800
that is used in conjunction with the above pre-erase/incremented
erase operations. This circuit is generally responsible for ramping
the voltage state of the N-well and P-well regions of the array
during these operations to facilitate the reduction of the field
intensity across the tunnel oxides of the fast bits. To accomplish
this, a voltage signal V.sub.Gwell is generated and applied by well
bias voltage circuit at the same time the aforementioned V.sub.GER
conditioning signal is applied to the control gates of the cells in
the array. The characteristics of the well bias signal can be
configured to the particular array in the following manner: first,
the overall capacitance of the wells C.sub.L is generally known; to
achieve a particular desired voltage change .DELTA.V in the well in
a particular time .DELTA.T, as shown in FIG. 8A, a constant current
source I can be used, where I=C.sub.L*.DELTA.V/.DELTA.T. Thus,
given these parameters, a well bias voltage circuit 800 can be
constructed in accordance with the circuit elements illustrated in
FIG. 8.
[0097] These include a select gate 805, which is triggered on
either a sector erase (SECERS) or chip erase (CHPERS) signal. A
bias level circuit 808 for generating a bias signal I'
includes:
[0098] a driver 810 coupled to a n type switch 811 to a first
voltage terminal (Vss); the latter is in series with an inverter
stage comprised of p type switch 812 and n type switch 813, and a
p-type switch 814 connected through to a second voltage terminal
(Vdd). A final n type switch 815, connected to Vss, and a capacitor
816 complete this portion of the well bias circuit. A series of
p-channel FETs 820, 821, 822, 823 and 824 receive bias signal I'
and have a first terminal coupled to a high voltage positive pump
signal VPPI (generated in conventional fashion from a charge pump)
from a source. Output VGwell is taken at a second terminal of these
p-channel FETs. In a preferred embodiment, the number and selection
of FETs 820-824 that contribute to this signal is configured during
manufacture of the device based on measured/desired characteristics
of such signal. Thus, in the example shown in FIG. 8, only two of
the p-channel FETs (820 and 821) actually contribute to the
V.sub.Gwell signal in this embodiment, but it is understood that
any or all of such FETs (or a different number of FETs beyond that
shown in this figure) can be configured by suitable processing
selections (laser cutting for example) based on the particular
requirements needed for signal V.sub.Gwell. This circuit operates
generally as follows: when an erase operation is selected, n type
switch 815 is turned on, while p type switch 812 and n type switch
are both turned off This pulls down the gate terminal (node) of p
type FETs 820 and 821 to Vss (ground), turning them on. This event
in turn, couples the output terminals of such FETs to rise based on
the terminal voltage V.sub.PPI, and causing V.sub.Gwell to begin
rising at a relatively constant rate based on the RC time constant
resulting from capacitor 816 and the impedance of the two FETs.
After a time T, therefore, V.sub.Gwell (and therefore the N and P
wells) will attain a final value Vf, which, again, can be tailored
for the specific array. This value of Vf, again, can be empirically
determined based on testing and measurements of the array so that
it contributes to optimize the effects of the aforementioned
conditioning signals V.sub.GER. In a preferred embodiment,
applicants have discovered that a value of approximately--volts
appears to work well as a well bias voltage signal level. As can be
seen in FIG. 8, when there is no erase operation taking place, n
type switch 815 is turned off, but n type switch 811 and p type
switch 814 are turned on. This causes the gate terminal of p type
FETs 820 and 821 to float, which means that such devices will not
be turned on, and the V.sub.Gwell signal will float, and thus also
be essentially isolated from the rest of the circuit 800. Other
variations and implementation for this well bias voltage circuit
will be apparent to those in the art from the above teachings, and
the present invention is certainly not limited in any respect by
any such particular adaptations. It will be immediately understood
by those skilled in the art that a flash memory cell array
integrated circuit can be manufactured using conventional
processing means to include the teachings of the present invention.
Such article of manufacture could include the inventive methods
embodied as replacements for embedded algorithms in known control
circuits, as well as usual combinations of flash memory cell arrays
and conventional supporting peripheral circuitry (power supplies,
address decoders, I/O data buffers, sense amplifiers, reference
arrays, counters, timers, etc.). Such processing means and
peripheral circuitry can be implemented using any of a number of
structures and methods well-known in the art, and are therefore not
described here in substantial detail.
[0099] It will also be appreciated by those skilled in the art that
the above discussion is directed to a preferred embodiment of the
present invention, and that the present teachings can be used in a
number of different environments, applications, etc. For example,
it is possible that other flash memory cell operations (other than
erasures) may benefit from a conditioning operation that adjusts
the electrical field across the tunnel oxide, or the amount of
electrical charge on the floating gates. Similarly, while the
present detailed description illustrates a mechanism for slowing
down fast bits by removing charge, it is of course possible to also
speed up slow bits by adding charge, and this feature, too, may be
attractive for some applications. Accordingly, it is intended that
the all such alterations and modifications be included within the
scope and spirit of the invention as defined by the following
claims.
* * * * *