U.S. patent application number 13/958415 was filed with the patent office on 2014-09-18 for high speed sensing for advanced nanometer flash memory device.
The applicant listed for this patent is Silicon Storage Technology, Inc.. Invention is credited to Anh Ly, Hung Quoc Nguyen, Vipin Tiwari, Hieu Van Tran, Thuan Vu.
Application Number | 20140269061 13/958415 |
Document ID | / |
Family ID | 51526478 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140269061 |
Kind Code |
A1 |
Tran; Hieu Van ; et
al. |
September 18, 2014 |
High Speed Sensing For Advanced Nanometer Flash Memory Device
Abstract
Improved sensing circuits and improved bit line layouts for
advanced nanometer flash memory devices are disclosed.
Inventors: |
Tran; Hieu Van; (San Jose,
CA) ; Ly; Anh; (San Jose, CA) ; Vu; Thuan;
(San Jose, CA) ; Nguyen; Hung Quoc; (Fremont,
CA) ; Tiwari; Vipin; (Dublin, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Silicon Storage Technology, Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
51526478 |
Appl. No.: |
13/958415 |
Filed: |
August 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61799970 |
Mar 15, 2013 |
|
|
|
Current U.S.
Class: |
365/185.05 ;
365/185.21 |
Current CPC
Class: |
G11C 16/26 20130101;
H01L 27/11519 20130101; G11C 7/062 20130101; G11C 16/06 20130101;
G11C 16/00 20130101; G11C 7/12 20130101; G11C 7/067 20130101; G11C
16/28 20130101; G11C 16/24 20130101; G11C 2207/063 20130101 |
Class at
Publication: |
365/185.05 ;
365/185.21 |
International
Class: |
G11C 16/28 20060101
G11C016/28; G11C 16/06 20060101 G11C016/06 |
Claims
1. A flash memory device, comprising: a first metal layer
comprising a first set of bit lines for accessing flash memory
cells; and a second metal layer comprising a second set of bit
lines for accessing flash memory cells.
2. The flash memory device of claim 1, wherein the each bit line in
the second set of bit lines is coupled to the first metal layer by
one or more vias.
3. The flash memory device of claim 1, wherein the distance between
a first line in the first set of bit lines and the closest bit line
in the first set of bit lines is larger than the distance between
the first line and the closest bit line in the second set of bit
lines.
4. A flash memory device comprising: a first metal layer comprising
a set of bit lines for accessing flash memory cells; and a second
metal layer comprising a plurality of structures parallel to the
set of bit lines, wherein each structure in the plurality of
structures is coupled to a different bit line in the set of bit
lines.
5. The flash memory device of claim 4, wherein each structure in
the plurality of structures is coupled to a different bit line in
the set of bit lines by one or more connectors between the first
metal layer and the second metal layer.
6. The flash memory device of claim 5, wherein each structure in
the plurality of structures is shorter than each bit line in the
set of bit lines.
7. The flash memory device of claim 6, wherein the plurality of
structures are arranged in a staggered formation within the second
metal layer.
8. A sensing circuit, comprising: a memory data read block coupled
to a selected flash memory cell and comprising a bit line, a first
parasitic capacitor from a first adjacent bit line, and a second
parasitic capacitor from a second adjacent bit line; a memory
reference read block coupled to a reference memory cell; and a
differential amplifier block coupled to the memory data read block
and the memory reference read block for determining a value stored
in the selected flash memory cell; wherein the memory data read
block and memory reference read block are coupled to a precharge
circuit to compensate for the first parasitic capacitor and the
second parasitic capacitor.
9. The sensing circuit of claim 8, wherein the memory data read
block comprises a current source, a cascoding sensing NMOS
transistor, a bitline clamp NMOS transistor, a diode connected
sensing load PMOS transistor, and a capacitor.
10. The sensing circuit of claim 9, wherein the memory reference
read block comprises a current source, a reference bitline clamp
NMOS transistor, a cascoding sensing NMOS transistor, an a diode
connected sensing load PMOS transistor, and a capacitor.
11. The sensing circuit of claim 10, wherein the differential
amplifier block comprises an input differential pair of NMOS
transistors, current mirror load PMOS transistors, output PMOS
transistor, current bias NMOS transistor, and an output current
bias NMOS transistor.
12. A sensing circuit comprising: a bit line coupled to a selected
flash memory cell; a first parasitic capacitor coupled to the bit
line and a first adjacent bit line; a second parasitic capacitor
coupled to the bit line and a second adjacent bit line; a precharge
circuit coupled to the bit line for precharging the bit line to a
bias voltage; and a single ended amplifier comprising a PMOS
transistor and an NMOS transistor, wherein a gate of the PMOS
transistor is coupled to the bit line and an output of the
amplifier indicates the value stored in the selected flash memory
cell.
13. The sensing circuit of the claim 12, wherein the output is
generated without the use of a reference memory cell.
14. The sensing circuit of the claim 12, wherein the bias voltage
tracks the position of the memory cells being selected.
15. The sensing circuit of claim 12, wherein a bulk of the PMOS
transistor is forward biased
16. The sensing circuit of claim 12, wherein the precharge circuit
comprises a bit line capacitor coupled to the bit line for storing
a precharge voltage.
17. The sensing circuit of claim 13, wherein the precharge circuit
comprises a PMOS transistor coupled to a voltage source and the bit
line and controlled by a precharge control signal.
18. The sensing circuit of the claim 16, wherein the precharge
voltage tracks the position of the memory cells being selected.
19. The sensing circuit of the claim 17, wherein the precharge
control signal tracks the position of the memory cells being
selected.
20. A sensing circuit comprising: a selected bit line coupled to a
flash memory cell; a first parasitic capacitor coupled to the
selected bit line a first adjacent bit line; a second parasitic
capacitor coupled to the bit line and a second adjacent bit line; a
reference line coupled to a reference memory cell; a third
parasitic capacitor coupled to the reference line and a third
adjacent bit line; a fourth parasitic capacitor coupled to the
reference line and a fourth adjacent bit line, a differential
amplifier coupled to the selected bit line and the reference line
for determining a value stored in the selected flash memory cell;
wherein the selected bit line and the reference line are coupled to
a precharge circuit to compensate for the first parasitic
capacitor, the second parasitic capacitor, the third parasitic
capacitor, and the fourth parasitic capacitor.
21. The sensing circuit of claim 20, wherein the precharge circuit
comprises a bit line capacitor coupled to the selected bit line for
storing a precharge voltage and a reference line capacitor coupled
to the reference line for storing a precharge voltage.
22. The sensing circuit of claim 21, wherein the precharge circuit
comprises a PMOS transistor coupled to a voltage source and the
selected bit line and controlled by a precharge control signal.
23. The sensing circuit of claim 22 wherein the precharge control
signal tracks the position of the memory cells being selected.
24. The sensing circuit of claim 22, wherein a bulk of the PMOS
transistor is forward biased.
Description
PRIORITY CLAIM
[0001] This application claims priority under 35 U.S.C. Section 119
and 120 to U.S. Provisional Patent Application Ser. No. 61/799,970
filed on Mar. 15, 2013, which is incorporated by reference
herein.
TECHNICAL FIELD
[0002] Improved sensing circuits and improved bit line layouts for
advanced nanometer flash memory devices are disclosed.
BACKGROUND OF THE INVENTION
[0003] Flash memory cells using a floating gate to store charges
thereon and memory arrays of such non-volatile memory cells formed
in a semiconductor substrate are well known in the art. Typically,
such floating gate memory cells have been of the split gate type,
or stacked gate type.
[0004] Flash memory devices typically include parallel bit lines,
usually contained within the same metal layer within the
semiconductor, that are used during the reading and writing
operations to select the appropriate memory cell.
[0005] FIG. 1 depicts a typical prior art configuration. Bit lines
10, 20, and 30 are roughly parallel and in relatively close
proximity to one another. Bit lines 10, 20, and 30 typically are
fabricated as part of the same metal layer within the semiconductor
die. Bit lines 10, 20, and 30 connect to other circuit components
through connectors 40.
[0006] FIG. 2 depicts the same prior art configuration from a top
view. Again, bit lines 10, 20, and 30 are roughly parallel to one
another. Their proximity and length result in parasitic
capacitance, which can be modeled as capacitor 15 and capacitor
25.
[0007] As flash memory designs become smaller and denser, parasitic
capacitance between adjacent bit lines will become more
problematic.
[0008] What is needed are improved circuit designs that compensate
for the parasitic capacitance between bit lines.
[0009] What is needed is an improved layout design to reduce the
amount of parasitic capacitance in an advanced nanometer flash
memory device.
SUMMARY OF THE INVENTION
[0010] The aforementioned problems and needs are addressed through
an improved circuit design to compensate for parasitic capacitance
between adjacent bit lines. In addition, improved layout techniques
reduce parasitic capacitance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 depicts an elevated side view of a prior art bit line
layout.
[0012] FIG. 2 depicts a top view of the prior art bit line layout
of FIG. 1.
[0013] FIG. 3 depicts a prior art sensing circuit.
[0014] FIG. 4 depicts a sensing circuit embodiment.
[0015] FIG. 5 depicts another sensing circuit embodiment.
[0016] FIG. 6 depicts another sensing circuit embodiment.
[0017] FIG. 7 depicts an elevated side view of an embodiment of a
bit line layout.
[0018] FIG. 8 depicts a top view of the embodiment of FIG. 7.
[0019] FIG. 9 depicts an elevated side view of an embodiment of a
bit line layout.
[0020] FIG. 10 depicts a top view of the embodiment of FIG. 9.
[0021] FIG. 11 depicts a sensing block diagram.
[0022] FIG. 12 depicts a timing diagram for tracking sensing signal
controls.
[0023] FIG. 13 depicts a graph showing changes in wordline bias and
bitline bias based on position along the bitline.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] FIG. 3 depicts a prior art sensing circuit 100. As can be
seen in FIG. 3, the prior art design does not model the parasitic
capacitance or otherwise take it into account. Sensing circuit 100
comprises memory data read block 110, memory reference read block
120, and differential amplifier block 130. Data read block 110
comprises current source 111, cascoding sensing NMOS transistor
113, bitline clamp NMOS transistor 114, diode connected sensing
load PMOS transistor 112, and capacitor 115.
[0025] Memory reference read block 120 comprises current source
121, reference bitline clamp NMOS transistor 124, cascoding sensing
NMOS transistor 123, and diode connected sensing load PMOS
transistor 122, and capacitor 125.
[0026] Differential amplifier block 130 comprises input
differential pair NMOS transistor 131 and 134, current mirror load
PMOS transistor 132 and 133, output PMOS transistor 135, current
bias NMOS transistor 136, output current bias NMOS transistor 137,
and output 140.
[0027] Node 116 is coupled to the selected memory cell (not shown)
to be read, and node 117 is coupled to the reference memory cell
(not shown) to be used to determine the value of the selected
memory cell.
[0028] Differential amplifier block 130 is used to compare the
signals received from data read block 110 and reference read block
120 to generate output 140 which indicates the value of the data
stored in the selected memory cell. These components are connected
to one another as shown in FIG. 3.
[0029] FIG. 4 depicts an improved sensing circuit 200. Sensing
circuit 200 comprises memory data read block 210, memory reference
read block 220, and differential amplifier block 230. Data read
block 210 comprises current source 211, cascoding sensing NMOS
transistor 213, bitline clamp NMOS transistor 214, diode connected
sensing load PMOS transistor 212, and capacitor 215.
[0030] Memory reference read block 220 comprises current source
221, reference bitline clamp NMOS transistor 224, cascoding sensing
NMOS transistor 223, and diode connected sensing load PMOS
transistor 222, and capacitor 225.
[0031] Differential amplifier block 230 comprises input
differential pair NMOS transistor 231 and 234, current mirror load
PMOS transistor 232 and 233, output PMOS transistor 235, current
bias NMOS transistor 236, output current bias NMOS transistor 237,
and output 240.
[0032] Node 216 is coupled to the selected memory cell (not shown)
to be read, and node 217 is coupled to the reference memory cell
(not shown) to be used to determine the value of the selected
memory cell.
[0033] Node 216 is the selected bit line and is coupled to
capacitor 217 and capacitor 218, which each represents parasitic
capacitance from adjacent bit lines, driven to compensate for
capacitor 215, precharge switch 250 and equalization switch 260 are
selectively turned on. The adjacent bit lines can be driven to a
voltage VB, which is less than or equal to the voltage to which the
selected bit line is driven. Doing so will reduce the effect of the
parasitic capacitance represented by capacitor 217 and capacitor
218.
[0034] Differential amplifier block 230 is used to compare the
signals received from data read block 210 and reference read block
220 to generate output 240 which indicates the value of the data
stored in the selected memory cell. These components are connected
to one another as shown in FIG. 4.
[0035] FIG. 5 depicts another improved sensing circuit 300. Sensing
circuit 300 comprises PMOS transistor 301, cascoding NMOS
transistor 302, output PMOS transistor 308, current bias NMOS
transistor 307, and output 310. Node 304 is coupled to the selected
memory cell (not shown) to be read. The gate of transistor 301
receives pre-charge node voltage 309, which in this example can be
1.2 V or ground. The transistors 307, 308 constitutes single ended
amplifier for the output. These components are connected to one
another as shown in FIG. 5.
[0036] The sensed node (gate of the transistor 308) is precharged
to a bias level through the transistor 301 by the pre-charge node
voltage 309 being ground. Then the pre-charge node voltage 309
going to a voltage level to release (weakly biased or turn off) the
transistor 301. Depending on the state of the memory cell coupled
to the node 304, if there is a current (e.g., erase state of the
split gate cell described in U.S. Pat. No. 8,072,815, which is
incorporated by reference herein and is attached hereto as Appendix
A), the sensed node will go down which turns on the transistor 308
to make the output 310 go high. If there is no current (e.g.,
programmed state of the split gate cell described in U.S. Pat. No.
8,072,815) the sensed node will stay at high which turns off the
transistor 308 to make the output 310 go low. This scheme is called
reference-less sensing.
[0037] The 311 bulk (nwell) substrate terminal of the PMOS
transistor 301 and the 312 bulk (nwell) terminal of the PMOS
transistor 308 are further forward biased (Vsource voltage-bulk
voltage=small positive, e.g. 0.4 v, less than Vp/n forward junction
of .about.0.6 v) to enhance the threshold voltage (lowered) and
high Idsat for lower voltage headroom and higher speed. This bulk
techniques could be applied for other figures.
[0038] Node 304 is coupled to capacitor 305 and capacitor 306,
which each represents parasitic capacitance from an adjacent bit
line, driven to compensate for, capacitor 303 connected to node
304.
[0039] FIG. 6 depicts another improved sensing circuit 400. Sensing
circuit 400 comprises PMOS transistor 401, cascoding NMOS
transistor 403, output PMOS transistor 409, current bias NMOS
transistor 410, and output 420. Node 405 is coupled to the selected
memory cell (not shown) to be read, and node 412 is coupled to a
reference memory cell (not shown).
[0040] The gate of transistor 401 receives pre-charge node voltage
421, which in this example can be 1.2 V or ground. The transistors
409, 410 constitutes single ended amplifier for the output. These
components are connected to one another as shown in FIG. 6.
[0041] The 422 bulk (nwell) substrate terminal of the PMOS
transistor 401 and the 423 bulk (nwell) terminal of the PMOS
transistor 409 are further forward biased (Vsource voltage-bulk
voltage=small positive, e.g. 0.4 v, less than Vp/n forward junction
of .about.0.6 v) to enhance the threshold voltage (lowered) and
high Idsat for lower voltage headroom and higher speed. This bulk
techniques could be applied for other figures.
[0042] Node 405 is coupled to capacitor 406 and capacitor 407,
which each represents parasitic capacitance from an adjacent bit
line. Node 412 is coupled to capacitor 413 and capacitor 414, which
each represents parasitic capacitance from an adjacent bit line,
driven to compensate for capacitors 404 and capacitor 411 and
switches 402 and 408 are selectively turned on.
[0043] FIG. 7 depicts an improved layout 500 of bit lines to reduce
parasitic capacitance between bit lines. Bit lines 510 and 530 are
formed in one metal layer. However, bit line 520 is formed in a
different metal layer. Thus, the distance between bit lines 510 and
520 and between bit lines 520 and 530 is longer than would be the
case if bit line 520 were formed in the same metal layer as bit
lines 510 and 530 as in the prior art. Bit line 520 connects to
other circuit components through vias 560, metal 550, and
connectors 540. Bit lines 510 and 530 connect to other circuit
components through connectors 40.
[0044] FIG. 8 depicts the layout of FIG. 7 from a top view. From
this view, bit lines 510 and 520 and 530 appear adjacent to one
another. However, as indicated by the different shading, bit line
520 and bit lines 510 and 530 are formed in different metal
layers.
[0045] FIG. 9 depicts an improved layout 700 to reduce resistance
of bit lines without increasing parasitic capacitance between bit
lines. Here, bit lines 510, 520, and 530 are adjacent, parallel,
and formed in the same metal layer as in the prior art
configuration. An additional structure 550 is placed above part of
bit line 510 in a different metal layer and connects to bit line
510 through connectors 540. Similarly, an additional structure 560
is placed above part of bit line 520 in a different metal layer and
connects to bit line 520 through connectors 540, and an additional
structure 570 is placed above part of bit line 530 in a different
metal layer and connects to bit line 530 through connectors 540.
Each additional structure 550, 560, and 570 has the effect of
decreasing resistance of the bit line to which it connects but
without increasing parasitic capacitance due to the length and
placement of each additional structure. Specifically, the
additional structures 550, 560, and 570 are placed in a staggered
format so that no significant parasitic capacitance is generated
among or between them and bit lines 510, 520, and 530.
[0046] FIG. 10 depicts the layout of FIG. 9 from a top view. Bit
lines 510 and 520 and 530 are adjacent and parallel to one another.
The staggered formation of additional structures 550, 560, and 570
is evident in this view.
[0047] FIG. 11 depicts flash memory device 900. Flash memory device
900 comprises: an array of memory cells 910, where the cells are
accessed by word lines and bit lines; horizontal decoder 905;
vertical decoder 920; read pulse control block 915; read control
blocks 930; sense amplifier circuit blocks 925; and IO blocks
935.
[0048] Read control blocks 930 are used to generate read timing
pulses that track WL position, BL position, and IOwidth and to
compensate for PVT.
[0049] FIG. 12 depicts a timing diagram 950 showing the operation
of various control signals over time. Signal 951 is the T_SEN-CYC
signal, signal 952 is the T-ATD signal, signal 953 is the T-PRECHa
signal, signal 954 is the T-EQ signal, signal 955 is the T-SENSEa
signal, signal 956 is the T-DOLATCH signal, signal 957 is the
T-BL0,BL1 signal, signal 958 is the T-SO0,SO1 signal, signal 959 is
the T-SOUT signal.
[0050] Signal 953 (T-PRECHa) performs adaptive precharge pulsing.
The pulse is shorter at WL0 (location 0 along the wordline) and
longer at WL-N (location N along the wordline), and it is shorter
at IO0 (location 0 along the IO line) and longer at IO-N (location
N along the IO line). Signal 953 (T-PRECHa) tracks the WL delay and
BL delay, for example, its pulses are shortest for WL0/BL0.
[0051] Signal 955 (T-SENSAa) performs adaptive sensing pulsing. The
pulse is shorter at TOO and longer at IO-N. The pulse is shorter at
WL0 and longer at WL-end. It tracks the WL delay and BL delay, for
example, its pulses are longest for WL-N/BL-N.
[0052] Signal 951 (T-SEN-CYC) performs an automatic power down
after a sense cycle has completed.
[0053] FIG. 13 depicts two graphs. The first graph shows that WL
bias is higher at one end of the bitline than the other, and the
second graph shows that BL bias is higher at one end of the bitline
than the other. This shows the importance of decreasing the
resistance of bitlines without increasing the parasitic capacitance
between bitlines, as discussed previously with reference to FIGS. 9
and 10.
[0054] References to the present invention herein are not intended
to limit the scope of any claim or claim term, but instead merely
make reference to one or more features that may be covered by one
or more of the claims. Materials, processes and numerical examples
described above are exemplary only, and should not be deemed to
limit the claims. It should be noted that, as used herein, the
terms "over" and "on" both inclusively include "directly on" (no
intermediate materials, elements or space disposed there between)
and "indirectly on" (intermediate materials, elements or space
disposed there between). Likewise, the term "adjacent" includes
"directly adjacent" (no intermediate materials, elements or space
disposed there between) and "indirectly adjacent" (intermediate
materials, elements or space disposed there between). For example,
forming an element "over a substrate" can include forming the
element directly on the substrate with no intermediate
materials/elements there between, as well as forming the element
indirectly on the substrate with one or more intermediate
materials/elements there between.
* * * * *