U.S. patent application number 12/881919 was filed with the patent office on 2011-01-06 for resistance change type memory.
Invention is credited to Yoshiaki Asao, Naoharu SHIMOMURA.
Application Number | 20110002157 12/881919 |
Document ID | / |
Family ID | 40850480 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110002157 |
Kind Code |
A1 |
SHIMOMURA; Naoharu ; et
al. |
January 6, 2011 |
RESISTANCE CHANGE TYPE MEMORY
Abstract
A resistance change type memory includes first, second and third
drive lines, a resistance change element having one end connected
to the third drive line, a first diode having an anode connected to
the first drive line and a cathode connected to other end of the
first resistance change element, a second diode having an anode
connected to other end of the first resistance change element and a
cathode connected to the second drive line, and a driver/sinker
which supplies a write current to the resistance change element. A
write control circuit is arranged such that when first data is
written, the write current is caused to flow in a direction from
the first drive line to the third drive line, and when second data
is written, the write current is caused to flow in a direction from
the third drive line to the second drive line.
Inventors: |
SHIMOMURA; Naoharu;
(Sagamihara-shi, JP) ; Asao; Yoshiaki;
(Sagamihara-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
40850480 |
Appl. No.: |
12/881919 |
Filed: |
September 14, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12350477 |
Jan 8, 2009 |
|
|
|
12881919 |
|
|
|
|
Current U.S.
Class: |
365/148 ;
365/158 |
Current CPC
Class: |
G11C 2213/34 20130101;
G11C 13/0004 20130101; G11C 13/0023 20130101; G11C 13/0011
20130101; G11C 2013/0078 20130101; G11C 2213/32 20130101; G11C
11/1657 20130101; G11C 11/1675 20130101; G11C 2213/72 20130101;
G11C 13/0028 20130101; G11C 2213/74 20130101; G11C 13/0007
20130101; G11C 13/003 20130101; G11C 13/0069 20130101; G11C 11/1659
20130101 |
Class at
Publication: |
365/148 ;
365/158 |
International
Class: |
G11C 11/21 20060101
G11C011/21; G11C 11/02 20060101 G11C011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2008 |
JP |
2008-004452 |
Claims
1. A resistance change type memory comprising: a first drive line
extending in a first direction; a second drive line extending in a
second direction intersecting the first direction; a resistance
change element and a zener diode which are connected in series
between the first and second drive lines; a driver/sinker which
supplies a write current to the resistance change element; and a
write control circuit arranged such that the write current is
caused to flow in a direction from the first drive line to the
second drive line, when first data is written to the resistance
change element, and the write current is caused to flow in a
direction from the second drive line to the first drive line, when
second data is written to the resistance change element.
2. The memory of claim 1, further comprising conductive lines which
extend in the first direction, wherein the first drive line and the
conductive lines function as word lines, and the write control
circuit causes the word lines except the first drive line to float,
when the first and second data are written to the resistance change
element.
3. The memory of claim 1, further comprising conductive lines which
extend in the second direction, wherein the second drive line and
the conductive lines function as bit lines, and the write control
circuit causes the bit lines except the second drive line to float,
when the first and second data are written to the resistance change
element.
4. The memory of claim 1, further comprising conductive lines which
extend in the second direction, wherein the second drive line and
the conductive lines function as bit lines, and the write control
circuit makes electric potentials of the bit lines except the
second drive line equal to an electric potential of the first drive
line, when the first and second data are written to the resistance
change element.
5. The memory of claim 1, wherein the write current is caused to
flow in a reverse direction of the zener diode, when the first data
is written to the resistance change element, and the write current
has a value such that a voltage applied to the zener diode is made
equal to or larger than the zener voltage thereof.
6. The memory of claim 1, wherein a voltage applied to the zener
diode is made less than the zener voltage thereof, when the first
data is not written to the resistance change element.
7. The memory of claim 1, wherein the write current is caused to
flow in a forward direction of the zener diode, when the second
data is written to the resistance change element.
8. The memory of claim 1, wherein a read current is caused to flow
in a forward direction of the zener diode, when data stored in the
resistance change element is read, and the read current is smaller
than the write current.
9. The memory of claim 1, wherein the zener diode is provided in a
semiconductor layer on an insulation layer.
10. The memory of claim 1, wherein the zener diode is provided in a
semiconductor substrate.
11. The memory of claim 1, wherein the resistance change element is
provided above the zener diode.
12. The memory of claim 1, wherein the zener diode is a schottky
diode.
13. The memory of claim 1, wherein the resistance change element
comprises oxides of one or more element selected from Ti, V, Fe,
Co, Y, Zr, Nb, Mo, Hf, Ta, W, Ge, Si.
14. The memory of claim 13, wherein the resistance change element
is interposed between an electrode comprising one of Ag and Cu and
another electrode comprising one of TiW, Ti, and W.
15. The memory of claim 1, wherein the resistance change element
comprises one of Cr.sub.2O.sub.3, CrO.sub.2, MoO.sub.2,
MO.sub.2O.sub.5, WO.sub.2, a mixed crystal of Cr.sub.2O.sub.3 and
CrO.sub.2, a mixed crystal of MoO.sub.2 and Mo.sub.2O.sub.5, and a
mixed crystal of WO.sub.2 and WO.sub.3.
16. The memory of claim 1, wherein the resistance change element
comprises one of NiCr.sub.2O.sub.4, MnCr.sub.2O.sub.4,
FeCr.sub.2O.sub.4, CoCr.sub.2O.sub.4, CuCr.sub.2O.sub.4, and
ZnCr.sub.2O.sub.4.
17. The memory of claim 1, wherein the resistance change element is
a magneto-resistive element.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims the benefit
under 35 U.S.C. .sctn.120 from U.S. application Ser. No.
12/350,477, filed on Jan. 8, 2009, and claims priority under 35
U.S.C. .sctn.119 from Japanese Patent Application No. 2008-004452,
filed Jan. 11, 2008, the entire contents of each of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a resistance change type
memory.
[0004] 2. Description of the Related Art
[0005] Attention has been paid to a resistance change type memory
such as MRAM (Magnetic Random Access Memory), ReRAM (Resistive
Random Access Memory), and the like as a next generation
non-volatile memory succeeding to a NAND type flash memory (refer
to, for example, U.S. Pat. No. 6,256,223).
[0006] Data is written to the resistance change type memory by
causing a write current to flow to, for example, a resistance
change element as a memory cell and changing its state (resistance
value). Further, data is read by causing a read current to flow to
the resistance change element and detecting its resistance value.
The value of the read current is set smaller than that of the write
current.
[0007] There is disturb and retention as elements for evaluating
the performance of a non-volatile memory.
[0008] The disturb means disturbance (erroneous writing) caused to
a memory cell when data is read and written, the retention means a
data-retention period of the memory cell.
[0009] Incidentally, when a memory cell is miniaturized, since a
wiring resistance and the turned-on resistance of a transistor are
increased, when a drive voltage has a predetermined value, the
value of a write current, which can be caused to flow to the memory
cell, is reduced.
[0010] However, the value of the write current affects the
retention. That is, when the value of the write current is reduced,
the data-retention period of a memory cell is generally
shortened.
[0011] Further, a voltage applied to a memory cell when data is
read affects reading sensitivity. Thus, to maintain sufficient
reading sensitivity, the voltage applied to the memory cell when
data is read, i.e., the value of a read current cannot be
sufficiently reduced.
[0012] In this case, when the value of the write current is reduced
as described above, since the voltage applied to the memory cell
when data is written is reduced, the ratio between the voltage
applied to the memory cell when data is read and the voltage
applied to the memory cell when data is written is reduced.
[0013] Accordingly, when the value of the write current is reduced,
read disturb is liable to occur.
BRIEF SUMMARY OF THE INVENTION
[0014] A resistance change type memory according to an aspect of
the present invention comprises first and second drive lines
extending in a first direction, a third drive line extending in a
second direction intersecting the first direction, a first
resistance change element having one end connected to the third
drive line, a first diode having an anode connected to the first
drive line and a cathode connected to other end of the first
resistance change element, a second diode having an anode connected
to other end of the first resistance change element and a cathode
connected to the second drive line, a driver/sinker which supplies
a write current to the first resistance change element, and a write
control circuit arranged such that when first data is written to
the first resistance change element, the write current is caused to
flow in a direction from the first drive line to the third drive
line, and when second data is written to the first resistance
change element, the write current is caused to flow in a direction
from the third drive line to the second drive line.
[0015] A resistance change type memory according to an aspect of
the present invention comprises first and second drive lines
extending in a first direction, third and fourth drive lines
extending in a second direction intersecting the first direction,
first and second resistance change elements having one ends
connected to the third drive line, third and fourth resistance
change elements having one ends connected to the fourth drive line,
a first zener diode having an anode connected to other end of the
first resistance change element and a cathode connected to the
first drive line, a second zener diode having an anode connected to
other end of the second resistance change element and a cathode
connected to the second drive line, a third zener diode having an
anode connected to other end of the third resistance change element
and a cathode connected to the first drive line, a fourth zener
diode having an anode connected to other end of the fourth
resistance change element and a cathode connected to the second
drive line, a driver/sinker which supplies a write current to the
first resistance change element, and a write control circuit
arranged such that when first data is written to the first
resistance change element, the write current is caused to flow in a
direction from the first drive line to the third drive line, and
when second data is written to the first resistance change element,
the write current is caused to flow in a direction from the third
drive line to the first drive line. When the first and second data
are written to the first resistance change element, the write
control circuit causes the second drive line to float. The write
current is set to such a value that a voltage applied to the first
zener diode is made equal to or larger than the zener voltage
thereof and a voltage applied to the third zener diode is made less
than the zener voltage thereof when the first data is written to
the first resistance change element, and that a voltage applied to
the fourth zener diode is made less than the zener voltage thereof
when the second data is written to the first resistance change
element.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0016] FIG. 1 is a view showing a first basic architecture of the
present invention;
[0017] FIG. 2 is a view showing a second basic architecture of the
present invention;
[0018] FIG. 3 is a view showing a third basic architecture of the
present invention;
[0019] FIG. 4 is a view showing a fourth basic architecture of the
present invention;
[0020] FIG. 5 is a view showing a fifth basic architecture of the
present invention;
[0021] FIG. 6 is a view showing a sixth basic architecture of the
present invention;
[0022] FIG. 7 is a view showing a seventh basic architecture of the
present invention;
[0023] FIG. 8 is a view showing an eighth basic architecture of the
present invention;
[0024] FIG. 9 is a view showing a resistance change type memory of
a first embodiment;
[0025] FIG. 10 is a view showing a write control circuit;
[0026] FIG. 11 is a view showing a decoder;
[0027] FIG. 12 is a view showing a resistance change type memory of
the first embodiment;
[0028] FIG. 13 is a view showing the write control circuit;
[0029] FIG. 14 is a view showing a decoder;
[0030] FIG. 15 is a view showing a memory cell array in detail;
[0031] FIG. 16 is a view showing a current path when "1" is
written;
[0032] FIG. 17 is a view showing a current path when "0" is
written;
[0033] FIG. 18 is a view showing a current path when "1" is
written;
[0034] FIG. 19 is a view showing a current path when "0" is
written;
[0035] FIG. 20 is a view showing a first example of a device
structure;
[0036] FIG. 21 is a view showing a second example of the device
structure;
[0037] FIG. 22 is a view showing a third example of the device
structure;
[0038] FIG. 23 is a view showing a fourth example of the device
structure;
[0039] FIG. 24 is a view showing a fifth example of the device
structure;
[0040] FIG. 25 is a view showing a resistance change type memory of
a second embodiment;
[0041] FIG. 26 is a view showing a memory cell array in detail;
[0042] FIG. 27 is a view showing a current path when "1" is
written;
[0043] FIG. 28 is a view showing a current path when "0" is
written;
[0044] FIG. 29 is view showing a current path when "1" is
written;
[0045] FIG. 30 is a view showing a current path when "0" is
written;
[0046] FIG. 31 is a view showing a first example of a device
structure;
[0047] FIG. 32 is a view showing a second example of the device
structure;
[0048] FIG. 33 is a view showing a basic structure of a
magneto-resistive effect element; and
[0049] FIG. 34 is a view showing a basic structure of a resistance
change element.
DETAILED DESCRIPTION OF THE INVENTION
[0050] A resistance change type memory of an aspect of the present
invention will be described below in detail with reference to the
accompanying drawing.
1. Outline
[0051] The present invention proposes the following two cell array
structures as a modified structure of a cross point type cell
array.
[0052] (1) First Cell Array Structure (First to Sixth Basic
Architectures)
[0053] Two diodes, which are disposed in an opposite direction from
each other, are connected to one resistance change element.
[0054] When first data is written to the resistance change element,
a write current is caused to flow to one of the two diodes in a
forward direction, and when second data is written thereto, the
write current is caused to flow to the other of two diodes in the
forward direction.
[0055] (2) Second Cell Array Structure (Seventh and Eighth Basic
Architectures)
[0056] A resistance change element and a zener diode, which are
connected to each other in series, are arranged in an intersecting
portion of each of a plurality of drive lines intersecting with
each other.
[0057] A write current is caused to flow to satisfy the following
condition.
[0058] A voltage applied to a zener diode connected to a selected
resistance change element is made equal to or larger than the zener
voltage thereof as well as a voltage applied to a zener diode
connected to a non-selected resistance change element is made less
than the zener voltage thereof.
[0059] Since these two cell array structures can prevent a sneak
current specific to a cross point type cell array, write disturb
can be suppressed when the write current is increased.
[0060] Therefore, according to the present invention, disturb and
retention can be improved at the same time by increasing the write
current.
2. Basic Architecture
[0061] A basic architecture of the present invention will be
explained.
[0062] (1) First Basic Architecture
[0063] FIG. 1 shows a first basic architecture of a resistance
change type memory of the present invention.
[0064] First and second drive lines L1, L2 extend in a first
direction, and a third drive line L3 extends in a second direction
intersecting the first direction. The first and second directions
are directions, for example, orthogonal to each other.
[0065] The third drive line L3 is one of a bit line and a word
line.
[0066] When the third drive line L3 is the bit line, the first and
second drive lines L1, L2 are the word lines, respectively.
Further, when the third drive line L3 is the word line, the first
and second drive lines L1, L2 are the bit lines, respectively.
[0067] One end of a resistance change element MC is connected to
the third drive line L3.
[0068] The resistance change element MC includes a
magneto-resistive element made by spin-injection writing, a
phase-change element for controlling write data by a current
direction, and the like.
[0069] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0070] A driver/sinker DS is connected to the first and second
drive lines L1, L2, respectively, and the driver/sinker DS is
connected to the third drive line L3 likewise. These elements
supply the write current to the resistance change element MC when
data is written.
[0071] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0072] When binary data is stored to the resistance change element
MC, the first data is, for example, "1" and the second data is, for
example, "0". However, multi-level data such as ternary data or
more may be stored to the resistance change element MC.
[0073] (2) Second Basic Architecture
[0074] FIG. 2 shows a second basic architecture of the resistance
change type memory of the present invention.
[0075] The second basic architecture is different from the first
basic architecture in that a driver/sinker connected to first and
second drive lines L1, L2 has an arrangement different from that of
the first basic architecture.
[0076] The first and second drive lines L1, L2 extend in a first
direction, and a third drive line L3 extends in a second direction
intersecting the first direction. The first and second directions
are directions, for example, orthogonal to each other.
[0077] One end of a resistance change element MC is connected to
the third drive line L3.
[0078] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0079] A driver DRV is connected to the first drive line L1, and a
sinker SK is connected to the second drive line L2. Further, the
driver/sinker DS is connected to the third drive line L3. These
elements supply a write current to the resistance change element MC
when data is written.
[0080] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0081] (3) Third Basic Architecture
[0082] FIG. 3 shows a third basic architecture of the resistance
change type memory of the present invention.
[0083] The third basic architecture is also different from the
first basic architecture in that a driver/sinker connected to first
and second drive lines L1, L2 has an arrangement different from
that of the first basic architecture.
[0084] The first and second drive lines L1, L2 extend in a first
direction, and a third drive line L3 extends in a second direction
intersecting the first direction. The first and second directions
are directions, for example, orthogonal to each other.
[0085] One end of a resistance change element MC is connected to
the third drive line L3.
[0086] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0087] A driver DRV is connected to one ends of the first and
second drive lines L1, L2. A sinker SK is connected to the other
ends of the first and second drive lines L1, L2. A driver/sinker DS
is connected to the third drive line L3. These elements supply a
write current to the resistance change element MC when data is
written.
[0088] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0089] (4) Fourth Basic Architecture
[0090] FIG. 4 shows a fourth basic architecture of the resistance
change type memory of the present invention.
[0091] The fourth basic architecture is different from the first
basic architecture in that a driver/sinker connected to a third
drive lines L3 has an arrangement different from that of the first
basic architecture.
[0092] First and second drive lines L1, L2 extend in a first
direction, and the third drive line L3 extends in a second
direction intersecting the first direction. The first and second
directions are directions, for example, orthogonal to each
other.
[0093] One end of a resistance change element MC is connected to
the third drive line L3.
[0094] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0095] The driver/sinker DS is connected to the first and second
drive lines L1, L2. A driver DRV is connected to one end of the
third drive line L3, and a sinker SK is connected to the other end
of the third drive line L3. These elements supply a write current
to the resistance change element MC when data is written.
[0096] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0097] (5) Fifth Basic Architecture
[0098] FIG. 5 shows a fifth basic architecture of the resistance
change type memory of the present invention.
[0099] The fifth basic architecture is different from the second
basic architecture in that a driver/sinker connected to a third
drive line L3 has an arrangement different from that of the second
basic architecture.
[0100] First and second drive lines L1, L2 extend in a first
direction, and the third drive line L3 extends in a second
direction intersecting the first direction. The first and second
directions are directions, for example, orthogonal to each
other.
[0101] One end of a resistance change element MC is connected to
the third drive line L3.
[0102] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0103] A driver DRV is connected to the first drive line L1, and a
sinker SK is connected to the second drive line L2. Further, the
driver DRV is connected to one end of the third drive line L3, and
the sinker SK is connected to the other end of the third drive line
L3. These elements supply a write current to the resistance change
element MC when data is written.
[0104] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0105] (6) Sixth Basic Architecture
[0106] FIG. 6 shows a sixth basic architecture of the resistance
change type memory of the present invention.
[0107] The sixth basic architecture is different from the third
basic architecture in that a driver/sinker connected to a third
drive line L3 has an arrangement different from that of the third
basic architecture.
[0108] First and second drive lines L1, L2 extend in a first
direction, and the third drive line L3 extends in a second
direction intersecting the first direction. The first and second
directions are directions, for example, orthogonal to each
other.
[0109] One end of a resistance change element MC is connected to
the third drive line L3.
[0110] An anode of a first diode D1 is connected to the first drive
line L1, and a cathode thereof is connected to the other end of the
resistance change element MC. An anode of a second diode D2 is
connected to the other end of the resistance change element MC, and
the cathode thereof is connected to the second drive line L2.
[0111] A driver DRV is connected to one ends of the first and
second drive lines L1, L2. A sinker SK is connected to the other
ends of the first and second drive lines L1, L2. Further, the
driver DRV is connected to one end of the third drive line L3, and
the sinker SK is connected to the other end of the third drive line
L3. These elements supply a write current to the resistance change
element MC when data is written.
[0112] When the first data is written to the resistance change
element MC, a write control circuit CNT causes the write current to
flow in a direction from the first drive line L1 to the third drive
line L3, and when the second data is written to the resistance
change element MC, the write control circuit CNT causes the write
current to flow in a direction from the third drive line L3 to the
second drive line L2.
[0113] (7) Seventh Basic Architecture
[0114] FIG. 7 shows a seventh basic architecture of the resistance
change type memory of the present invention.
[0115] The seventh basic architecture relates to a writing
technology making use of bidirectional energization executed by a
zener diode.
[0116] First and second drive lines L1, L2 extend in a first
direction, and a third and fourth drive lines L3, L4 extend in a
second direction intersecting the first direction. The first and
second directions are directions, for example, orthogonal to each
other.
[0117] When the first and second drive lines L1, L2 are word lines,
the third drive line L3 and a fourth drive line L4 are bit lines.
Further, when the first and second drive lines L1, L2 are the bit
lines, the third and fourth drive lines L3, L4 are word lines.
[0118] One ends of first and second resistance change elements MC1,
MC2 are connected to the third drive line L3, and one ends of the
third and fourth resistance change elements MC3, MC4 are connected
to the fourth drive line L4.
[0119] An anode of a first zener diode ZD1 is connected to the
other end of the first resistance change element MC1 and a cathode
thereof is connected to the first drive line L1, and an anode of a
second zener diode ZD2 is connected to the other end of the second
resistance change element MC2 and a cathode thereof is connected to
the second drive line L2.
[0120] An anode of a third zener diode ZD3 is connected to the
other end of the third resistance change element MC3 and a cathode
thereof is connected to the first drive line L1, and an anode of a
fourth zener diode ZD4 is connected to the other end of the fourth
resistance change element MC4 and a cathode thereof is connected to
the second drive line L2.
[0121] The resistance change elements MC1 to MC4 include
magneto-resistive elements, phase-change elements, and the
like.
[0122] A driver/sinker DS is connected to the first and second
drive lines L1, L2, respectively, and the driver/sinker DS is
connected to the third and fourth drive lines L3, L4, respectively,
likewise. These elements supply a write current to the resistance
change element MC1 when data is written.
[0123] When the first data is written to the resistance change
element MC1, a write control circuit CNT causes the write current
to flow in a direction from the first drive line L1 to the third
drive line L3, and when the second data is written to the
resistance change element MC1, the write control circuit CNT causes
the write current to flow in a direction from the third drive line
L3 to the first drive line L1.
[0124] A write current is set to such values that when the first
data is written to the resistance change element MC1, a voltage
applied to the first zener diode ZD1 is made equal to or more than
the zener voltage thereof and a voltage applied to the second and
third zener diodes ZD2, ZD3 is made less than the zener voltage as
well as when the second data is written to the resistance change
element MC1, a voltage applied to the fourth zener diode ZD4 is
made less than the zener voltage.
[0125] When binary data is stored to the resistance change element
MC1, the first data is, for example, "1" and the second data is,
for example, "0". However, multi-level data such as ternary data or
more may be stored to the resistance change element MC1.
[0126] (8) Eighth Basic Architecture
[0127] FIG. 8 shows an eighth basic architecture of the resistance
change type memory of the present invention.
[0128] The eighth basic architecture is different from the seventh
basic architecture in that a driver/sinker connected to first to
fourth drive lines L1 to L4 has an arrangement different from that
of the seventh basic architecture.
[0129] The first and second drive lines L1, L2 extend in a first
direction, and the third and fourth drive lines L3, L4 extend in a
second direction intersecting the first direction. The first and
second directions are directions, for example, orthogonal to each
other.
[0130] One ends of first and second resistance change elements MC1,
MC2 are connected to the third drive line L3, and one ends of third
and fourth resistance change elements MC3, MC4 are connected to the
fourth drive line L4.
[0131] An anode of a first zener diode ZD1 is connected to the
other end of the first resistance change element MC1 and a cathode
thereof is connected to the first drive line L1, and an anode of a
second zener diode ZD2 is connected to the other end of the second
resistance change element MC2 and a cathode thereof is connected to
the second drive line L2.
[0132] An anode of a third zener diode ZD3 is connected to the
other end of the third resistance change element MC3 and a cathode
thereof is connected to the first drive line L1, and an anode of a
fourth zener diode ZD4 is connected to the other end of the fourth
resistance change element MC4 and a cathode thereof is connected to
the second drive line L2.
[0133] A driver DRV is connected to one ends of the first and
second drive lines L1, L2, and a sinker SK is connected to the
other ends of the first and second drive lines L1, L2. Further, the
driver/sinker DS is connected to one ends of the third and fourth
drive lines L3, L4. These elements supply a write current to the
resistance change element MC1 when data is written.
[0134] When the first data is written to the resistance change
element MC1, a write control circuit CNT causes the write current
to flow in a direction from the first drive line L1 to the third
drive line L3, and when the second data is written to the
resistance change element MC1, the write control circuit CNT causes
the write current to flow in a direction from the third drive line
L3 to the first drive line L1.
[0135] A write current is set to such values that when the first
data is written to the resistance change element MC1, a voltage
applied to the first zener diode ZD1 is made equal to or more than
the zener voltage thereof and a voltage applied to the second and
third zener diodes ZD2, ZD3 is made less than the zener voltage
thereof as well as when the second data is written to the
resistance change element MC1, a voltage applied to the fourth
zener diode ZD4 is made less than the zener voltage thereof.
[0136] (9) Conclusion
[0137] According to the first to eighth basic architectures, since
a sneak current is prevented, an erroneous writing due to write
disturb can be avoided.
[0138] Further, according to these basic architectures, since a
sneak current caused when data is read is prevented at the same
time, sensitivity is also improved when data is read.
3. Embodiments
[0139] Embodiments of the present invention will be explained.
[0140] In the explanation described below, a portion as to a read
circuit is omitted so that a write circuit according to the present
invention can be easily understood.
(1) First Embodiment
A. Circuit Arrangement (First Example)
[0141] The circuit shows a specific arrangement of the second basic
architecture (FIG. 2).
[0142] FIGS. 9 to 11 show a write circuit of the resistance change
type memory of the present invention.
[0143] A memory cell array 11 is composed of a plurality of memory
cells. Four memory cells MC1, MC2, MC3, MC4 are shown here for
simplification.
[0144] The memory cells MC1, MC2, MC3, MC4 are resistance change
elements.
[0145] Word lines WL1a, WL1b, WL2a, WL2b, which extend in a first
direction, are disposed on the memory cell array 11. The word lines
WL1a, WL1b constitute a word line pair and are disposed in
correspondence to the memory cells MC1, MC3. Likewise, the word
lines WL2a, WL2b constitute a word line pair and are disposed in
correspondence to the memory cells MC2, MC4.
[0146] One ends of the word lines WL1a, WL2a are connected to a
word line driver 13 through a word line selector 12. One ends of
the word lines WL1b, WL2b are placed in, for example, an open state
(to which nothing is connected).
[0147] The word line selector 12 is composed of P channel MOS
transistors P1, P2 having gates to which control signals
Axw<s>, Axw<s+1> are input. The word line driver 13 is
composed of a P channel MOS transistor P3 having a gate to which a
control signal SRCw is input.
[0148] A current source I1 generates a write current. Further, the
other ends of the word lines WL1b, WL2b are connected to a word
line sinker 15 through a word line selector 14. The other ends of
the word lines WL1a, WL2a are placed in, for example, an open
state.
[0149] The word line selector 14 is composed of N channel MOS
transistors N1, N2 having gates to which control signals
Axe<s>, Axe<s+1> are input. The word line sinker 15 is
composed of an N channel MOS transistor N3 having a gate to which a
control signal SNKe is input.
[0150] Further, bit lines BL1, BL2, which extend in a second
direction intersecting the first direction, are disposed on the
memory cell array 11. The bit line BL1 is disposed in
correspondence to the memory cells MC1, MC2, and the bit line BL2
is disposed in correspondence to the memory cells MC3, MC4.
[0151] One ends of the bit lines BL1, BL2 are connected to a bit
line driver/sinker 17 through a bit line selector 16. The other
ends of the bit lines BL1, BL2 are placed in, for example, an open
state.
[0152] The bit line selector 16 is composed of P channel MOS
transistors P4, P5 having gates to which control signals
Ayp<t>, Ayp<t+1> are input and N channel MOS
transistors N4, N5 having gates to which control signals
Ayn<t>, Ayn<t+1> are input.
[0153] The word line driver/sinker 17 is composed of a P channel
MOS transistor P6 having a gate to which a control signal SRCn is
input and an N channel MOS transistor N6 having a gate to which a
control signal SNKn is input.
[0154] A current source 12 generates a write current.
[0155] A write control circuit 18 of FIG. 10 generates control
signals SRCw, SRCn, SNKn, SNKe. Further, a decoder 19 of FIG. 11
generates control signals (decode signal) Axw<s>,<s+1>;
Ayp<t>,<t+1>; Ayn<t>,<t+1>;
Axe<s>,<s+1>.
B. Circuit Arrangement (Second Example)
[0156] The circuit shows a specific arrangement of the third basic
architecture (FIG. 3).
[0157] FIGS. 12 to 14 show a write circuit of the resistance change
type memory of the present invention.
[0158] A second example has a feature in word line selectors 12, 14
as compared with the first example. Since the arrangement other
than the word line selectors 12, 14 is the same as the first
example, only the word line selectors 12, 14 will be explained
below.
[0159] One ends of word lines WL1a, WL2a, WL1b, WL2b are connected
to a word line driver 13 through the word line selector 12.
[0160] The word line selector 12 is composed of P channel MOS
transistors P1a, P1b, P2a, P2b having gates to which control
signals Axwa<s>, Axwb<s>, Axwa<s+1>,
Axwb<s+1> are input.
[0161] Further, the other ends of the word lines WL1a, WL2a, WL1b,
WL2b are connected to a word line sinker 15 through the word line
selector 14.
[0162] The word line selector 14 is composed of N channel MOS
transistors N1a, N1b, N2a, N2b having gates to which control
signals Axea<s>, Axeb<s>, Axea<s+1>,
Axeb<s+1> are input.
[0163] A decoder 19 of FIG. 14 generates control signals (decode
signal) Axwa<s>,<s+1>; Axwb<s>,<s+1>;
Ayp<t>,<t+1>; Ayn<t>,<t+1>;
Axea<s>,<s+1>; Axeb<s>, <s+1>.
C. Memory Cell Array
[0164] FIG. 15 shows a memory cell array in detail.
[0165] Two diodes disposed in an opposite direction from each other
are connected to resistance change elements disposed to an
intersecting portion of a word line pair and a bit line.
[0166] Specifically, one ends of the resistance change elements
MC1, MC2 are connected to a bit line BL1, and one ends of the
resistance change elements MC3, MC4 are connected to a bit line
BL2.
[0167] Diodes D1a, D1b are connected to the other end of the
resistance change element MC1 in an opposite direction from each
other. That is, an anode of the diode D1a is connected to a word
line WL1a, and a cathode thereof is connected to the other end of
the resistance change element MC1. An anode of the diode D1b is
connected to the other end of the resistance change element MC1,
and a cathode thereof is connected to the word line WL1b.
[0168] The diodes D2a, D2b are connected to the other end of the
resistance change element MC2 in an opposite direction from each
other. That is, an anode of the diode D2a is connected to a word
line WL2a, and a cathode thereof is connected to the other end of
the resistance change element MC2. An anode of a diode D2b is
connected to the other end of the resistance change element MC2,
and a cathode thereof is connected to a word line WL2b.
[0169] Diodes D3a, D3b are connected to the other end of the
resistance change element MC3 in an opposite direction from each
other. That is, an anode of the diode D3a is connected to a word
line WL1a, and a cathode thereof is connected to the other end of
the resistance change element MC3. An anode of the diode D3b is
connected to the other end of the resistance change element MC3,
and a cathode thereof is connected to the word line WL1b.
[0170] Diodes D4a, D4b are connected to the other end of the
resistance change element MC4 in an opposite direction from each
other. That is, an anode of the diode D4a is connected to the word
line WL2a, and a cathode thereof is connected to the other end of
the resistance change element MC4. An anode of the diode D4b is
connected to the other end of the resistance change element MC4,
and a cathode thereof is connected to the word line WL2b.
D. Operation
[0171] A write operation of the resistance change type memory of
FIGS. 9 to 15 will be explained.
[0172] It is assumed here that the resistance change elements MC1
to MC4 store binary data, and a low resistance state is shown by
"0" and a high resistance state is shown by "1". However, this
definition is only an example.
[0173] Further, the resistance change element MC1 is used as a
selected cell to which data is written. The other resistance change
elements MC2 to MC4 are designated as non-selected cells to which
data is not written.
First Example
[0174] When "1" is written, the word line WL1a is connected to the
current source I1, and the bit line BL1 is grounded. That is, the
word line WL1a is set to "H (high)" and the bit line BL1 is set to
"L (low)".
[0175] In, for example, FIG. 9 as an example, it is sufficient that
the control signals Axw<s>, SRCw are set to "L", the control
signals Ayp<t>, Ayn<t>, SNKn are set to "H", P channel
MOS transistors P1, P3 and N channel MOS transistors N4, N6 are
turned on.
[0176] At the time, as shown in FIG. 16, the write current is
supplied from the current source I1 to the resistance change
element MC1 through the word line WL1a and the diode D1a. Further,
the write current is absorbed by the ground through the bit line
BL1 (PASS "1").
[0177] Further, when "1" is written, for example, the non-selected
word lines WL1b, WL2a, WL2b are caused to float, and the
non-selected bit line BL2 is set to "H".
[0178] In, for example, FIG. 9 as the example, when the control
signal Axw<s+1> is set to "H" and the control signals
Axe<s>, Axe<s+1> are set to "L", since the P channel
MOS transistors P2 and the N channel MOS transistors N1, N2 are
turned off, the word lines WL1b, WL2a, WL2b are caused to
float.
[0179] Further, when the control signals Ayp<t+1>,
Ayn<t+1>, SRCn are set to "L", since the P channel MOS
transistors P5, P6 are turned on and the N channel MOS transistor
N5 is turned off, the bit line BL2 is set to "H".
[0180] As shown in FIG. 16, when "1" is written, there are
generated a current path (SNEAK "1" B), which goes from the word
line WL1a to the bit line BL2 through the resistance change element
MC3 and further goes from the bit line BL2 to the word line WL2b
through the resistance change element MC4, and a current path
(SNEAK "1" A) which goes from the word line WL1a to the word line
WL1b through the diode D3b.
[0181] However, the diode D1b, which is connected in a direction
opposite to the direction of a current flowing in the current path
(SNEAK "1" A), exists between the word line WL1b and the bit line
BL1, and the diode D2b, which is connected in a direction opposite
to the direction of a current flowing in the current path (SNEAK
"1" B), exists between the word line WL2b and the bit line BL1.
Accordingly, since these current paths do not reach the bit line
BL1, a so-called sneak current is prevented.
[0182] Further, when "1" is written, word lines WL1b, WL2a, WL2b
are caused to float, and the electric potential of the bit line BL2
is made equal to the electric potential ("H") of the word line
WL1a.
[0183] A write current, which is generated by the current source
I1, is prevented from flowing to the resistance change element MC3
as a sneak current by setting the bit line BL2 to "H". Further, a
current path from the word line WL2a to the bit line BL1 is shut
off by causing the word line WL2a to float. When the word lines
WL1b, WL2b are caused to float, the electric potentials of the word
lines WL1b, WL2b are made lower than the electric potential ("H")
of the bit line BL2.
[0184] When "0" is written, the bit line BL1 is connected to the
current source I2, and the word line WL1b is grounded. That is, the
bit line BL1 is set to "H", and the word line WL1b is set to
"L".
[0185] In, for example, FIG. 9 as the example, it is sufficient
that the control signals Ayp<t>, Ayn<t>, SRCn are set
to "L", the control signal Axe<s>, SNKe are set to "H", and
the P channel MOS transistors P4, P6 and the N channel MOS
transistors N1, N3 are turned on.
[0186] At the time, as shown in FIG. 17, the write current is
supplied from the current source 12 to the resistance change
element MC1 through the bit line BL1. Further, the write current is
absorbed by the ground through the diode D1b and the word line WL1b
(PASS "0").
[0187] Further, when "0" is written, for example, the non-selected
bit line BL2 is set to "L", and the non-selected word lines WL1a,
WL2a, WL2b are caused to float.
[0188] In, for example, FIG. 9 as the example, when the control
signals Ayp<t+1>, Ayn<t+1>, SNKn are set to "H", since
the N channel MOS transistor N5, N6 are turned on and the P channel
MOS transistor P5 is turned off, the bit line BL2 is set to
"L".
[0189] Further, when the control signal Axw<s>,
Axw<s+1> are set to "H" and the control signal Axe<s+1>
is set to "L", since the P channel MOS transistors P1, P2 and the N
channel MOS transistor N2 are turned off, the word lines WL1a,
WL2a, WL2b are caused to float.
[0190] Here, as shown in FIG. 17, when "0" is written, a current
path (SNEAK "0"), which goes from the bit line BL1 to the word line
WL2b through the resistance change element MC2, is generated.
[0191] However, since the diode D4b, which is connected in a
direction opposite to the direction of a current flowing in the
current path (SNEAK "0"), exists between the word line WL2b and the
bit line BL2, the current path does not reach the bit line BL2, and
thus a so-called sneak current is prevented.
[0192] Further, when "0" is written, the word lines WL1a, WL2a,
WL2b are caused to float, and the electric potential of the bit
line BL2 is made equal to the electric potential ("L") of the word
line WL1b.
[0193] A current path from the bit line BL2 to the word line WL1b
is shut off by setting the bit line BL2 to "L". Further, a current
path from the word line WL1a to the word line WL1b is shut off by
causing the word line WL1a to float, and a current path from the
word line WL2a to the bit line BL2 is shut off by causing the word
line WL2a to float. When the word line WL2b is caused to float, the
electric potential of the word line WL2b is made lower than the
electric potential ("H") of the bit line BL1.
Second Example
[0194] When "1" is written, the word line WL1a is connected to the
current source I1, and the bit line BL1 is grounded. That is, the
word line WL1a is set to "H", and the bit line BL1 is set to
"L".
[0195] In, for example, FIG. 12 as an example, it is sufficient
that the control signals Axwa<s>, Axea<s>, SRCw are set
to "L", the control signals Ayp<t>, Ayn<t>, SNKn are
set to "H", and the P channel MOS transistors P1a, P3 and the N
channel MOS transistors N4, N6 are turned on.
[0196] At the time, as shown in FIG. 18, the write current is
supplied from the current source I1 to the resistance change
element MC1 through the word line WL1a. The write current is
absorbed by the ground through the bit line BL1 (PASS "1").
[0197] Further, when "1" is written, for example, the non-selected
word lines WL1b, WL2b and the non-selected bit line BL2 is set to
"H", and the non-selected word line WL2a is set to "L".
[0198] In, for example, FIG. 12 as the example, when the control
signals Axwb<s>, Axwb<s+1>, Axeb<s>,
Axeb<s+1> are set to "L" and the control signals
Axwa<s+1>, Axea<s+1>, SNKe are set to "H", the word
lines WL1b, WL2b are set to "H" and the word line WL2a is set to
"L". Further, when the control signals Ayp <t+1>, Ayn
<t+1>, SRCn are set to "L", the bit line BL2 is set to
"H".
[0199] As shown in FIG. 18, when "1" is written, there are
generated the current path (SNEAK "1" B), which goes from the word
line WL1a to the bit line BL2 through the resistance change element
MC3 and further goes from the bit line BL2 to the word line WL2b
through the resistance change element MC4, and the current path
(SNEAK "1" A) which goes from the word line WL1a to the word line
WL1b through the diode D3b.
[0200] However, the diode D1b, which is connected in a direction
opposite to the direction of a current flowing in the current path
(SNEAK"1"A), exists between the word line WL1b and the bit line
BL1, and the diode D2b, which is connected in a direction opposite
to the direction of a current flowing in the current path (SNEAK
"1" B), exists between the word line WL2b and the bit line BL1.
Accordingly, since these current paths do not reach the bit line
BL1, a so-called sneak current is prevented.
[0201] Further, when "1" is written, the electric potentials of the
word line WL1b, WL2b and the electric potential of the bit line BL2
are made equal to the electric potential ("H") of the word line
WL1a. Further, the electric potential of the word line WL2a is made
equal to the electric potential ("L") of the bit line BL1.
[0202] A write current generated by the current source I1 is
prevented from flowing to the resistance change element MC3 as a
sneak current by setting the bit line BL2 to "H". Further, a
current path from the word line WL2a to the bit line BL1 is shut
off by setting the word line WL2a to "L". Even if the word lines
WL1b, WL2b are set to "H", since the diodes D1b, D2b exist between
the word lines WL1b, WL2b and the bit line BL1, a current path from
the word lines WL1b, WL2b to the bit line BL1 is shut off.
[0203] When "0" is written, the bit line BL1 is connected to the
current source I2, and the word line WL1b is grounded. That is, the
bit line BL1 is set to "H", and the word line WL1b is set to
"L".
[0204] In, for example, FIG. 12 as the example, the control signals
Axwb<s>, Ayp<t>, Ayn<t>, SRCn are set to "L", the
control signal Axeb<s>, SNKe are set to "H", and the P
channel MOS transistors P4, P6 and the N channel MOS transistors
N1b, N3 are turned on.
[0205] At the time, as shown in FIG. 19, the write current is
supplied from the current source 12 to the resistance change
element MC1 through the bit line BL1. Further, the write current is
absorbed by the ground through the word line WL1b (PASS "0").
[0206] Further, when "0" is written, for example, the non-selected
bit line BL2 and the non-selected word lines WL1a, WL2a are set to
"L", and the non-selected word line WL2b is set to "H".
[0207] In, for example, FIG. 12 as the example, when the control
signals Axwa<s>, Axwa<s+1>, Axea<s>,
Axea<s+1>, SNKe are set to "H" and the control signals
Axwb<s+1>, Axeb<s+1>, SRCw are set to "L", the word
lines WL1a, WL2a are set to "L" and the word line WL2b is set to
"H". Further, when the control signals Ayp<t+1>,
Ayn<t+1>, SNKn are set to "H", the bit line BL2 is set to
"L".
[0208] As shown in FIG. 19, when "0" is written, a current path
(SNEAK "0"), which goes from the bit line BL1 to the word line WL2b
through the resistance change element MC2, is generated.
[0209] However, since the diode D4b, which is connected in a
direction opposite to the direction of a current flowing in the
current path (SNEAK "0"), exists between the word line WL2b and the
bit line BL2, the current path does not reach the bit line BL2,
thereby a so-called sneak current is prevented.
[0210] Further, when "0" is written, the electric potentials of the
word lines WL1a, WL2a and the electric potential of the bit line
BL2 are made equal to the electric potential ("L") of the word line
WL1b, and the electric potential of the word line WL2b is made
equal to the electric potential ("H") of the bit line BL1.
[0211] A current path from the bit line BL2 to the word line WL1b
is shut off by setting the bit line BL2 to "L". Further, a current
path from the word line WL1a to the word line WL1b is shut off by
setting the word line WL1a to "L", and a current path from the word
line WL2a to the bit line BL2 is shut off by setting the word line
WL2a to "L". Even if the word line WL2b is set to "H", since the
diode D4b exists between the word line WL2b and the bit line BL2, a
current path from the word line WL2b to the bit line BL2 is shut
off.
[0212] In the second example, the electric potentials of the word
lines WL1a, WL1b, WL2a, WL2b and the bit lines BL1, BL2 are
controlled by a binary value of "H" and "L". In the first example,
since "floating" is necessary in addition to "H" and "L", the
controllability of the second example is improved as compared with
the first example.
E. Device Structure
[0213] Examples of a device structure will be explained.
First Example
[0214] FIG. 20 shows a first example of the device structure.
[0215] An element separation insulation layer 22 having an STI
(Shallow Trench Isolation) structure is formed in a semiconductor
substrate 21. Diodes D1a, D1b, which are composed of n+ diffusion
layers 23a, 23b and p+ diffusion layers 24a, 24b, are formed in an
element region surrounded by the element separation insulation
layer 22.
[0216] The p+ diffusion layer 24a as an anode of the diode D1a is
connected to a word line WL1a through a plug 25, and the n+
diffusion layer 23a as a cathode thereof is connected to an
intermediate conductive layer 29 through a plug 27.
[0217] The p+ diffusion layer 24b as an anode of the diode D1b is
connected to the intermediate conductive layer 29 through a plug
28, and the n+ diffusion layer 23b as a cathode thereof is
connected to the word line WL1b through a plug 26.
[0218] The intermediate conductive layer 29 is connected to a lower
electrode 31 through a plug 30. A resistance change element MC1 is
formed on the lower electrode 31.
[0219] The resistance change element MC1 is connected to a bit line
BL1 through a cap conductive layer 32.
[0220] The above elements are covered by an insulation layer
33.
Second Example
[0221] FIG. 21 shows a second example of the device structure.
[0222] The second example has a feature in that an SOI (Silicon on
Insulator) substrate is used when compared with the first example.
The other arrangements of the second example are the same as those
of the first example.
[0223] An insulation layer 34 is formed on a semiconductor
substrate 21. An element separation insulation layer 22 having an
STI structure is formed on the insulation layer 34.
[0224] Diodes D1a, D1b are formed in an element region
(semiconductor layer) surrounded by the insulation layers 22,
34.
[0225] The diode D1a is composed of an n+ diffusion layer 23a as a
cathode and a p+ diffusion layer 24a as an anode, and the diode D1b
is composed of an n+ diffusion layer 23b as a cathode and a p+
diffusion layer 24b as an anode.
[0226] In the structure, since the diodes D1a, D1b are completely
surrounded by the insulation layers 22, 34, a leak current
generated in the diodes D1a, D1b is prevented.
Third Example
[0227] FIG. 22 shows a third example of the device structure.
[0228] Diodes D1a, D1b of the third example have a structure
different from that of the first example. The other arrangements of
the third example are the same as those of the first example.
[0229] N-well regions 35a, 35b are formed in an element region
surrounded by an element separation insulation layer 22.
[0230] A diode D1a composed of an n+ diffusion layer 36a and a p+
diffusion layer 37a is formed in the n-well region 35a. Further, a
diode D1b composed of an n+ diffusion layer 36b and a p+ diffusion
layer 37b is formed in the n-well region 35b.
[0231] The p+ diffusion layer 37a as an anode of the diode D1a is
connected to a word line WL1a through a plug 25, and the n+
diffusion layer 36a thereof as a cathode is connected to an
intermediate conductive layer 29 through a plug 27.
[0232] The p+ diffusion layer 37b as an anode of the diode D1b is
connected to the intermediate conductive layer 29 through the plug
28, and the n+ diffusion layer 36b thereof as a cathode is
connected to the word line WL1b through the plug 26.
[0233] The structure is advantageous in that an off-leak is reduced
although the diodes D1a, D1b are increased in size as compared with
the first example.
Fourth Example
[0234] FIG. 23 shows a fourth example of the device structure.
[0235] The device structure of the fourth example is constructed by
combining the device structures of the second and third
examples.
[0236] Diodes D1a, D1b are formed on an SOI substrate.
[0237] The diode D1a is composed of an n+ diffusion layer 36a and a
p+ diffusion layer 37a in an n-well region 35a. The diode D1b
composed of an n+ diffusion layer 36b and a p+ diffusion layer 37b
in an n-well region 35b.
[0238] Since diffusion of impurities can be suppressed by using the
SOI substrate, the distance between the diodes D1a, D1b can be
reduced while sufficiently securing an element separation
function.
Fifth Example
[0239] FIG. 24 shows a fifth example of the device structure.
[0240] The fifth example relates to a diode.
[0241] Word lines WL1a, WL1b are formed on a semiconductor
substrate 21.
[0242] A lower electrode 38a is formed on the word line WL1a, and a
diode D1a, which is composed of a p-type semiconductor and an
n-type semiconductor, is formed on the lower electrode 38a. An
upper electrode 39a is formed on the diode D1a.
[0243] A lower electrode 38b is formed on the word line WL1b, and a
diode D1b, which is composed of an n-type semiconductor and a
p-type semiconductor, is formed on the lower electrode 38b. An
upper electrode 39b is formed on the diode D1b.
[0244] The diodes D1a, D1b are located between the word lines WL1a,
WL1b and the bit line BL1.
[0245] An intermediate conductive layer 40 is formed on the upper
electrode 39a, 39b to connect them. The intermediate conductive
layer 40 is connected to the lower electrode 31 through a plug 30.
A resistance change element MC1 is formed on the lower electrode
31.
[0246] The resistance change element MC1 is connected to the bit
line BL1 through a cap conductive layer 32.
[0247] The above elements are covered by an insulation layer
33.
[0248] The diodes have an advantage in that the size thereof can be
reduced as compared with the diodes formed by the diffusion layers
in the semiconductor substrate as shown by the first to fourth
examples.
F. Conclusion
[0249] As described above, according to the first embodiment, the
two diodes, which are disposed in an opposite direction from each
other, are connected to one resistance change element. Further,
when the first data is written to the resistance change element,
the write current is caused to flow to one of the two diodes in the
forward direction, and when the second data is written thereto, the
write current is caused to flow to the other of the two diodes in
the forward direction. With this arrangement, disturb and retention
can be improved at the same time when data is written.
(2) Second Embodiment
[0250] A second embodiment shows a specific arrangement of the
eighth basic architecture (FIG. 8).
[0251] A. Circuit Arrangement
[0252] FIG. 25 shows a write circuit of the resistance change type
memory of the present invention.
[0253] A memory cell array 11 is composed of a plurality of memory
cells. Four memory cells MC1, MC2, MC3, MC4 are shown here for
simplification.
[0254] The memory cells MC1, MC2, MC3, MC4 are resistance change
elements.
[0255] Word lines WL1, WL2 extending in a first direction are
disposed on the memory cell array 11. The word line WL1 is disposed
in correspondence to the memory cells MC1, MC3, and the word line
WL2 is disposed in correspondence to memory cells MC2, MC4.
[0256] One ends of the word lines WL1, WL2 are connected to a word
line driver 13 through a word line selector 12.
[0257] The word line selector 12 is composed of P channel MOS
transistors P1, P2 having gates to which control signals
Axw<s>, Axw<s+1> are input. The word line driver 13 is
composed of a P channel MOS transistor P3 having a gate to which a
control signal SRCw is input.
[0258] A current source I1 generates a write current.
[0259] Further, the other ends of the word lines WL1, WL2 are
connected to a word line sinker 15 through a word line selector
14.
[0260] The word line selector 14 is composed of N channel MOS
transistors N1, N2 having gates to which control signals
Axe<s>, Axe<s+1> are input. The word line sinker 15 is
composed of an N channel MOS transistor N3 having a gate to which a
control signal SNKe is input.
[0261] Further, bit lines BL1, BL2, which extend in a second
direction intersecting a first direction, are disposed on the
memory cell array 11. The bit line BL1 is disposed in
correspondence to the memory cells MC1, MC2, and the bit line BL2
is disposed in correspondence to the memory cells MC3, MC4.
[0262] One ends of the bit lines BL1, BL2 are connected to a bit
line driver/sinker 17 through a bit line selector 16. The other
ends of the bit lines BL1, BL2 are placed in, for example, an open
state.
[0263] The bit line selector 16 is composed of P channel MOS
transistors P4, P5 having gates to which control signals
Ayp<t>, Ayp<t+1> are input and N channel MOS
transistors N4, N5 having gates to which control signals
Ayn<t>, Ayn<t+1> are input.
[0264] The word line driver/sinker 17 is composed of a P channel
MOS transistors P6 having a gate to which a control signal SRCn is
input and an N channel MOS transistor N6 having a gate to which a
control signal SNKn is input.
[0265] A current source 12 generates a write current.
[0266] The control signals SRCw, SRCn, SNKn, SNKe are created by a
write control circuit 18 of FIG. 10. Further, the control signals
Axw<s>,<s+1>; Ayp<t>,<t+1>;
Ayn<t>,<t+1>; Axe<s>,<s+1> are created by a
decoder 19 of FIG. 11.
[0267] B. Memory Cell Array
[0268] FIG. 26 shows the memory cell array in detail.
[0269] One ends of the resistance change elements MC1, MC2 are
connected to a bit line BL1, and one ends of the resistance change
elements MC3, MC4 are connected to a bit line BL2.
[0270] An anode of a zener diode ZD1 is connected to the other end
of the resistance change element MC1, and a cathode thereof is
connected to the word line WL1. An anode of a zener diode ZD2 is
connected to the other end of the resistance change element MC2,
and a cathode thereof is connected to the word line WL2.
[0271] An anode of a zener diode ZD3 is connected to the other end
of the resistance change element MC3, and a cathode thereof is
connected to the word line WL1. An anode of a zener diode ZD4 is
connected to the other end of the resistance change element MC4,
and a cathode thereof is connected to the word line WL2.
[0272] Note that the zener diodes ZD1 to ZD4 may be connected in an
opposite direction, i.e. the cathodes thereof may be connected to
the resistance change element.
[0273] C. Operation
[0274] A write operation of the resistance change type memory of
FIGS. 25 to 26 will be explained.
[0275] It is assumed here that the resistance change elements MC1
to MC4 store binary data, and a low resistance state is shown by
"0" and a high resistance state is shown by "1". However, this
definition is only an example.
[0276] Further, the resistance change element MC1 is used as a
selected cell to which data is written. The other resistance change
elements MC2 to MC4 are designated as non-selected cells to which
data is not written.
[0277] When "1" is written, the word line WL1 is connected to the
current source I1, and the bit line BL1 is grounded. That is, the
word line WL1 is set to "H", and the bit line BL1 is set to
"L".
[0278] In this case, it is sufficient that the control signals
Axw<s>, SRCw are set to "L", the control signals
Ayp<t>, Ayn<t>, SNKn are set to "H", and the P channel
MOS transistors P1, P3 and the N channel MOS transistors N4, N6 are
turned on.
[0279] At the time, as shown in FIG. 27, the write current is
supplied from the current source I1 to the resistance change
element MC1 through the word line WL1. Since a voltage equal to or
more than the zener voltage thereof is applied to the zener diode
ZD1, the write current flows in an opposite direction by the zener
effect. The write current is absorbed by the ground through the bit
line BL1 (PASS "1").
[0280] Further, when "1" is written, for example, the non-selected
word line WL2 and the non-selected bit line BL2 are caused to
float.
[0281] In this case, the control signal Axw<s+1> is set to
"H", the control signal Axe<s+1> is set to "L", and the P
channel MOS transistor P2 and the N channel MOS transistor N2 are
turned off. Further, the control signal Ayp<t+1> is set to
"H", the control signal Ayn<t+1> is set to "L", and the P
channel MOS transistor P5 and the N channel MOS transistor N5 are
turned off.
[0282] Here, as shown in FIG. 27, when "1" is written, a current
path (PASS "sneak") is generated from the word line WL1 to the
resistance change element MC3.
[0283] Thus, a condition when data is written is set so that the
voltage applied to the zener diodes ZD1 to ZD4 satisfies the
following relation.
[0284] First, it is premised that the potential difference between
the word line WL1 and the bit line BL1 is Vlw, and the forward
voltage of the zener diodes ZD1 to ZD4 is Vldf, and the reverse
voltage thereof is Vldr. Further, the voltage applied to each of
the resistance change elements MC1 to MC4 is Vmtj for
simplification. In this case, the following equation is established
in a current path from the word line WL1 to the bit line BL1
through the resistance change element MC1.
Vlw=Vldr+Vmtj
[0285] Accordingly, it is sufficient to set Vlw, Vzv so that they
satisfy the following equation in order to apply a voltage equal to
or more than the zener voltage Vzv to the zener diode ZD1.
Vldr=Vlw-Vmtj.gtoreq.Vzv
[0286] In contrast, it is sufficient to set Vlw so that a voltage
less than the zener voltage Vzv thereof is applied to the zener
diodes ZD2, ZD3 in order to shut off a current path (sneak pass)
PASS "sneak" from the word line WL1 to the bit line BL1 through the
resistance change element MC3, the bit line BL2, the resistance
change element MC4, the word line WL2, and the resistance change
element MC2 in this order.
[0287] Specifically, in the sneak pass PASS "sneak", the following
equation is established.
Vlw=3Vmtj+2Vldr+Vldf
[0288] Accordingly, it is sufficient to set Vlw, Vzv so that they
satisfy the following equation in order to apply a voltage less
than the zener voltage Vzv thereof to the zener diodes ZD2,
ZD3.
Vldr=Vlw/2-3Vmtj/2-Vldf/2<Vzv
[0289] As a result, the write current can be caused to flow only to
the current path PASS "1". When "0" is written, the bit line BL1 is
connected to the current source 12, and the word line WL1 is
grounded. That is, the bit line BL1 is set to "H", and the word
line WL1 is set to "L".
[0290] In this case, it is sufficient that the control signals
Ayp<t>, Ayn<t>, SRCn are set to "L", the control
signals Axe<s>, SNKe are set to "H", and the P channel MOS
transistors P4, P6 and the N channel MOS transistors N1, N3 are
turned on.
[0291] At the time, as shown in FIG. 28, the write current is
supplied from the current source I2 to the resistance change
element MC1 through the bit line BL1. The write current flows to
the zener diode ZD1 in a forward direction. Further, the write
current is absorbed by the ground through the word line WL1 (PASS
"0").
[0292] Further, when "0" is written, for example, the non-selected
bit line BL2 and the non-selected word line WL2 are caused to
float.
[0293] In this case, the control signal Ayp<t+1> is set to
"H", the control signal Ayn<t+1> is set to "L", and the P
channel MOS transistor P5 and the N channel MOS transistor N5 are
turned off. Further, the control signal Axw<s+1> is set to
"H", the control signal Axe<s+1> is set to "L", and the P
channel MOS transistor P2 and the N channel MOS transistor N2 are
turned off.
[0294] Here, as shown in FIG. 28, when "0" is written, a current
path (PASS "sneak"), which goes from the bit line BL1 to the word
line WL2 through the resistance change element MC2, is
generated.
[0295] Thus, a condition when data is written is set so that the
voltage applied to the zener diodes ZD1 to ZD4 satisfies the
following relation.
[0296] First, it is premised that the potential difference between
the word line WL1 and the bit line BL1 is Vlw, and the forward
voltage of the zener diodes ZD1 to ZD4 is Vldf and the reverse
voltage thereof is Vldr. Further, the voltage applied to each of
the resistance change elements MC1 to MC4 is Vmtj for
simplification.
[0297] In this case, the following equation is established in a
current path from the bit line BL1 to the word line WL1 through the
resistance change element MC1.
Vlw=Vldf+Vmtj
[0298] In contrast, it is sufficient to set Vlw so that a voltage
less than the zener voltage Vzv is applied to the zener diode ZD4
in order to shut off a current path (sneak pass) PASS "sneak" from
the bit line BL1 to the word line WL1 through the resistance change
element MC2, the word line WL2, the resistance change element MC4,
the bit line BL2, and the resistance change element MC3 in this
order.
[0299] Specifically, the following equation is established in the
sneak pass PASS "sneak".
Vlw=3Vmtj+Vldr+2Vldf
[0300] Accordingly, it is sufficient to set Vlw, Vzv so that they
satisfy the following equation in order to apply a voltage less
than the zener voltage Vzv to the zener diode ZD4.
Vldr=Vlw-3Vmtj-2Vldf<Vzv
[0301] As a result, the write current can be caused to flow only to
the current path PASS "0".
[0302] Note that as to the voltage Vmtj applied to the resistance
change elements MC1 to MC4, when "0" is written, a value, by which
the resistance change element MC1 can be changed from a high
resistance state to a low resistance state, is set, and when "1" is
written, a value, by which the resistance change element MC1 can be
changed from the low resistance state to the high resistance state,
is set.
[0303] The value of the zener voltage Vzv can be controlled by the
density of the impurities of a p-type semiconductor layer and an
n-type semiconductor layer that constitute the zener diodes.
[0304] Incidentally, in the present invention, a sneak current in a
read operation can be also prevented by the same principle. In the
read operation, a read current smaller than the write current is
used.
[0305] In this operation, it is essential to cause the non-selected
word line WL2 to float when "1" is written and when "0" is
written.
[0306] However, as shown in FIG. 29, the non-selected bit line BL2
may be set to "H" when "1" is written. In this case, it is
sufficient to set the control signals Ayp<t+1>,
Ayn<t+1>, SRCn to "L". When the non-selected bit line BL2 is
set to "H", since no voltage is applied to the zener diode ZD3, a
current path from the word line WL1 to the bit line BL2 is shut
off.
[0307] Further, as shown in FIG. 30, the non-selected bit line BL2
may be set to "L" when "0" is written. In this case, it is
sufficient to set the control signals Ayp<t+1>,
Ayn<t+1>, SNKn to "H".
[0308] In any of the cases, since a write control can be carried
out by a binary value of "H" and "L", controllability can be more
improved than a case that the write control is carried out by a
ternary value of "H", "L" and "Floating".
[0309] D. Device Structure
[0310] Examples of a device structure will be explained.
First Example
[0311] FIG. 31 is a view showing a first example of the device
structure.
[0312] In a semiconductor substrate 21, an element separation
insulation layer 22 having an STI structure is formed. A zener
diode ZD1, which is composed of an n+ diffusion layer 23 and a p+
diffusion layer 24, is formed in an element region surrounded by an
element separation insulation layer 22.
[0313] The p+ diffusion layer 24 as an anode of the zener diode ZD1
is connected to a lower electrode 31 through a plug 28, and the n+
diffusion layer 23 as a cathode thereof is connected to a word line
WL1 through a plug 26.
[0314] A resistance change element MC1 is formed on the lower
electrode 31.
[0315] The resistance change element MC1 is connected to a bit line
BL1 through a cap conductive layer 32.
[0316] The above elements are covered by an insulation layer
33.
Second Example
[0317] FIG. 32 shows a second example of the device structure.
[0318] The second example relates to a diode.
[0319] A word line WL1 is formed on a semiconductor substrate
21.
[0320] A lower electrode 38 is formed on the word line WL1, and a
zener diode ZD1, which is composed of an n-type semiconductor and a
P-type semiconductor, is formed on the lower electrode 38. An upper
electrode 39 is formed on the zener diode ZD1.
[0321] The zener diode ZD1 is located between the word lines WL1
and the bit line BL1.
[0322] A lower electrode 31 is formed on the upper electrode 39. A
resistance change element MC1 is formed on the lower electrode
31.
[0323] The resistance change element MC1 is connected to the bit
line BL1 through a cap conductive layer 32.
[0324] The above elements are covered by an insulation layer
33.
[0325] The diode is advantageous in that a size can be reduced as
compared with the diode as shown in the first example which is
formed by the diffusion layer in the semiconductor substrate.
[0326] Others
[0327] The zener diode according to the present invention may be
formed on the SOI substrate shown in the second example having the
device structure of the first embodiment or may be formed in the
well region shown in the third example having the device structure
of the first embodiment.
[0328] E. Conclusion
[0329] According to the second embodiment described above, data can
be written to the zener diode by energizing it bidirectionally as
well as a sneak current can be prevented by the zener effect of the
zener diode. With this arrangement, disturb and retention can be
improved at the same time when data is written.
[0330] (3) Others
[0331] The diodes or the zener diodes used in the first and second
embodiment may be schottky diodes.
[0332] 4. Examples to which Present Invention is Applied
[0333] The present invention can be applied to a resistance change
type memory such as MRAM and ReRAM.
[0334] A memory cell of the resistance change type memory has such
characteristics that it exhibits a different resistance value
depending on a direction in which a current flows therein or a
direction in which a voltage is applied thereto, and data is stored
by the different resistance value.
[0335] FIG. 33 shows a basic structure of a memory cell of
MRAM.
[0336] The memory cell (magneto-resistive element) of the MRAM is
composed of a pinned layer 41, a free layer 42, and a tunnel
barrier layer 43 interposed therebetween.
[0337] The memory cell is, for instance, a MTJ (magnetic tunnel
junction) element.
[0338] The pinned layer 41 and the free layer 42 are composed of a
ferromagnetic body. The magnetization direction of the pinned layer
41 and the free layer 42 may be a horizontal direction to a film
surface as shown in a part (a) of the figure or a vertical
direction to the film surface as shown in a part (b) of the
figure.
[0339] The pinned layer 41 and the free layer 42 may be arranged
reversely in a vertical direction.
[0340] In MRAM, a spin-injection writing system, for example, can
be employed as a writing system. In this case, the direction of a
write current is adjusted with respect to write data.
[0341] For instance, to magnetize the pinned layer 41 and the free
layers 42 in the same direction (parallel state: low resistance),
it is sufficient to flow electrons from the pinned layer 41 side to
the free layer 42 side. In this case, since the electrons, which
are spin polarized in the same direction as the magnetization
direction of the pinned layer 41, apply spin torque to the
electrons in the free layer 42, the relation between the pinned
layer 41 and the free layer 42 is placed in a parallel state.
[0342] Further, to magnetize the pinned layer 41 and the free layer
42 in a reverse direction from each other (anti-parallel state:
high resistance), it is sufficient to flow electrons from the free
layer 42 side to the pinned layer 41 side. In this case, since the
electrons, which are spin polarized in a direction reverse to the
magnetization direction of the pinned layer 41, are reflected on
the pinned layer 41 and applies spin torque to the electrons in the
free layer 41, the relation between the pinned layer 41 and the
free layer 42 is placed in an anti-parallel state.
[0343] When the memory cell of FIG. 33 is applied to the devices of
FIGS. 20-24, 31 and 32, the memory cell where the pinned layer 41
is located at a downside (a semiconductor substrate side) and the
free layer 42 is located at an upside is a bottom pin type. In this
case, Electrons flow from the word line to the bit line to
magnetize the pinned layer 41 and the free layers 42 in the
parallel state, and Electrons flow from the bit line to the word
line to magnetize the pinned layer 41 and the free layers 42 in the
anti-parallel state.
[0344] On the contrary, the memory cell where the free layer 42 is
located at a downside and the pinned layer 41 is located at an
upside is a top pin type. In this case, Electrons flow from the bit
line to the word line to magnetize the pinned layer 41 and the free
layers 42 in the parallel state, and Electrons flow from the word
line to the bit line to magnetize the pinned layer 41 and the free
layers 42 in the anti-parallel state.
[0345] FIG. 34 shows a basic structure of a memory cell of
ReRAM.
[0346] The memory cell of ReRAM is composed of a lower electrode
61, an upper electrode 63, and a resistance change film 62
interposed therebetween.
[0347] The following materials can be used as the resistance change
film 62. The materials are, for example, amorphous oxides (oxides
of one or more element selected from, for example, Ti, V, Fe, Co,
Y, Zr, Nb, Mo, Hf, Ta, W, Ge, Si). The resistance change film 62 is
clamped between an electrode of Ag or Cu and an electrode of TiW,
Ti, W, and the like, and a current direction is changed by applying
a voltage having a different polarity thereto. As a result, the
resistance value of the resistance change film 62 can be changed by
ionizing Ag or Cu as an electrode material, diffusing them in a
thin film, and returning them to the electrode.
[0348] More specifically, when a voltage is applied so that the
electrode side of Ag or Cu is set to a positive electric potential,
Ag or Cu is ionized from the electrode and diffused in the
resistance change film 62, coupled with electrons on the opposite
side electrode, and precipitated. With this operation, since a
current path containing a lot of Ag or Cu is formed in the
resistance change film 62, the resistance of the resistance change
film 62 is reduced.
[0349] In contrast, when a voltage is applied so that the electrode
side of Ag or Cu is set to a negative electric potential, since Ag
or Cu, which constitutes the current path formed in the resistance
change film 62, reversely migrates in the resistance change film 62
returns to the electrode of Ag or Cu, the resistance of the
resistance change film 62 is increased.
[0350] Further, an arrangement using the following materials can be
employed in addition to the above example. More specifically, metal
oxides (excluding W0.sub.3) composed of at least one kind of
element of the transition metal elements of group VI is used as the
material of the resistance change film 62. Specifically,
Cr.sub.2O.sub.3, CrO.sub.2, MoO.sub.2, Mo.sub.2O.sub.5, WO.sub.2, a
mixed crystal of Cr.sub.2O.sub.3 and CrO.sub.2, a mixed crystal of
MoO.sub.2 and Mo.sub.2O.sub.5, a mixed crystal of WO.sub.2 and
WO.sub.3, and the like are used.
[0351] Further, metal oxides composed of at least one kind of
element of the transition metal elements of group VI and at least
one kind of element of the transition metal elements of group I,
group II, group VII, and group VIII may be used as the material of
the resistance change film 62. Specifically, NiCr.sub.2O.sub.4,
MnCr.sub.2O.sub.4, FeCr.sub.2O.sub.4, CoCr.sub.2O.sub.4,
CuCr.sub.2O.sub.4, ZnCr.sub.2O.sub.4, and the like are used.
[0352] Note that it is preferable that these metal oxides are not
amorphous and are polycrystal or microcrystal. The resistance of
the resistance change film 62 can be increased or reduced by
changing a current direction by applying a voltage having a
different polarity to these materials.
[0353] ReRAM can employ a writing method of changing a resistance
value by a current or a writing method of changing the resistance
value by a voltage.
[0354] Note that only the basic structure of the memory cell of the
resistance change type memories such as MRAM and ReRAM is shown
above, and the basic structure can be variously modified.
[0355] 5. Conclusion
[0356] According to the present invention, a writing technology for
improving disturb and retention at the same time can be
realized.
[0357] When an element size is made to several tens of nano meters
by miniaturizing a memory cell, since a gate width of a selected
transistor corresponding to the element size is also miniaturized,
a drive current which can be caused to flow is reduced. A diode has
a larger effective sectional area of a channel, to which a current
flows, than a MOS transistor. Accordingly, a current, which can be
caused to flow to a resistance change element, can be increased by
using the diode in place of a selected transistor.
[0358] In general, since a correlation exists between a
data-retention period and a current necessary to writing, an
element having a good data-retention period has an increased
current necessary to write data. Accordingly, the data-retention
period can be increased by increasing a current which can be caused
to flow to the resistance change element.
[0359] Further, since the ratio between a read current and a write
current can be increased, erroneous writing (read disturb) when
data is read can be prevented.
[0360] Further, a problem of deterioration of a reading sensitivity
due to a sneak current and a problem of reduction of an operation
speed can be avoided. High speed writing of about 10 ns or less can
be realized by setting the difference of the electric potentials
between the bit line and the word line to a value sufficiently
larger than a threshold value necessary to writing.
[0361] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *