U.S. patent application number 14/524567 was filed with the patent office on 2016-04-28 for apparatuses and methods for setting a signal in variable resistance memory.
The applicant listed for this patent is Micron Technology, Inc.. Invention is credited to Alessandro Sanasi.
Application Number | 20160118101 14/524567 |
Document ID | / |
Family ID | 55792493 |
Filed Date | 2016-04-28 |
United States Patent
Application |
20160118101 |
Kind Code |
A1 |
Sanasi; Alessandro |
April 28, 2016 |
APPARATUSES AND METHODS FOR SETTING A SIGNAL IN VARIABLE RESISTANCE
MEMORY
Abstract
An example of a method reads a spin torque transfer (STT) memory
cell, and writes the STT memory cell using information obtained
during the reading of the STT memory cell to set a pulse to write
the STT memory cell. An example of an apparatus includes a STT
memory cell and read/write circuitry coupled to the STT memory cell
to determine a read current (I.sub.READ) through the STT memory
cell and to set a pulse to write the STT memory cell using
I.sub.READ. Additional embodiments are disclosed.
Inventors: |
Sanasi; Alessandro; (Milano,
IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Micron Technology, Inc. |
Boise |
ID |
US |
|
|
Family ID: |
55792493 |
Appl. No.: |
14/524567 |
Filed: |
October 27, 2014 |
Current U.S.
Class: |
365/158 |
Current CPC
Class: |
G11C 11/1675 20130101;
G11C 11/1673 20130101; G11C 11/1697 20130101; G11C 7/14
20130101 |
International
Class: |
G11C 11/16 20060101
G11C011/16 |
Claims
1. A method, comprising: reading a variable resistance memory cell;
and writing the memory cell using information obtained in relation
to the reading of the memory cell to set a signal to write the
memory cell.
2. The method of claim 1, further comprising determining a
reference current, wherein writing the memory cell using
information obtained in relation to the reading of the memory cell
to set a signal to write the memory cell comprises using the
determined reference current to limit the signal.
3. The method of claim 2, wherein reading the memory cell includes
determining a read current (I.sub.READ) flowing through a data line
coupled to a spin torque transfer memory cell, wherein determining
the reference current includes using the I.sub.READ to determine
the reference current.
4. The method of claim 3, further comprising looking up a value
from a look-up table, wherein determining the reference current
includes summing the I.sub.READ and the looked up value to
determine the reference current.
5. The method of claim 4, wherein the looked up value is a function
of information that includes at least one type of information
selected from the group of information types consisting of: data to
be written into the memory cell; a temperature; a process corner; a
power supply voltage; and a topological location of the memory
cell.
6. The method of claim 2, wherein writing the memory cell includes
applying a current ramp up to the reference current.
7. The method of claim 6, wherein applying the current ramp
includes applying a ramp potential on an access line coupled to a
gate of an access line transistor, comparing a current flowing
through a data line coupled to the cell (I.sub.CELL) to the
reference current, and deselecting the data line responsive to the
I.sub.CELL being equal to or greater than the reference
current.
8. The method of claim 7, wherein comparing the I.sub.CELL to the
reference current includes using a current mirror to compare the
I.sub.CELL to the reference current.
9. The method of claim 7, further comprising compensating the
reference current for data line capacitance.
10. The method of claim 9, further comprising using a dummy data
line to compensate the reference current for the data line
capacitance.
11. An apparatus, comprising: a variable resistance memory cell;
and read/write circuitry coupled to the memory cell to determine a
read current (I.sub.READ) through the memory cell and to set a
pulse to write the memory cell using the I.sub.READ.
12. The apparatus of claim 11, further including an access device,
wherein the memory cell includes a magnetic tunnel junction (MTJ)
coupled in series with the access device, wherein the access device
is configured to control a cell current (I.sub.CELL) through the
MTJ.
13. The apparatus of claim 11, wherein the read/write circuitry is
configured to sum the I.sub.READ and a .DELTA.I value from a
look-up table to provide a reference current, and is configured to
set the pulse to write the memory cell using the I.sub.SW.
14. An apparatus, comprising: a data line; an access device; a
magnetic tunnel junction (MTJ) cell coupled between the data line
and the access device, wherein the access device is configured to
control a cell current (I.sub.CELL) flowing through the MTJ; and a
comparator configured to compare current flowing through the data
line to a reference current set specifically for the cell, and to
cause the data line to be deselected responsive to the current
flowing through the data line being greater than or equal to the
reference current.
15. The apparatus of claim 14, further comprising a reference
current calculator configured to calculate a generated reference
current value using a sensed value of a current flowing through the
MTJ during a read pulse.
16. The apparatus of claim 15, further comprising a look-up table,
wherein the reference current calculator is configured to calculate
the generated reference current value using the sensed value and a
value looked up in the look-up table.
17. The apparatus of claim 16, wherein the access device comprises
a transistor and wherein the apparatus is configured to apply a
ramped potential on an access line coupled to a gate of the
transistor, wherein the ramped potential increases the current
flowing through the data line until the data line is deselected
responsive to the current flowing through the data line being
greater than or equal to the reference current.
18. The apparatus of claim 17, further comprising a dummy data line
configured to conduct current attributable to parasitic capacitance
(I.sub.DUMMY) on the dummy data line, and an I.sub.Dummy current
mirror coupled to the dummy line to mirror the I.sub.Dummy, wherein
a value of the reference current comprises the generated reference
value offset by the value of I.sub.DUMMY.
19. The apparatus of claim 18, further comprising: a regulator
coupled to the data line and to the dummy data line and configured
to provide a regulated potential to the data line; a source,
wherein the access device is coupled between the MTJ and the
source;
20. The apparatus of claim 18, further comprising read/write
circuitry coupled to the data line and the access line, the
read/write circuitry including: a read/write pulse generator; a
sampler to sample a signal on the data line during the read pulse;
and the reference current calculator, wherein the reference current
calculator is configured to calculate the generated reference
current value based on the sampled signal on the data line.
Description
BACKGROUND
[0001] Magnetic Random Access Memory (MRAM) is a non-volatile
computer memory technology based on magnetoresistance. Unlike
typical volatile Random Access Memory (RAM) technologies which
store data as electric charge, data in MRAM is stored by
magnetoresistive elements. Generally, the magnetoresistive elements
are made from two magnetic layers, each of which holds a
magnetization. The magnetization of one layer (the "pinned layer")
is fixed in its magnetic orientation, and the magnetization of the
other layer (the "free layer") can be changed by an external
magnetic field generated by a programming current. Thus, the
magnetic field of the programming current can cause the magnetic
orientations of the two magnetic layers to be either parallel,
giving a lower electrical resistance across the layers, or
antiparallel, giving a higher electrical resistance across the
layers. The switching of the magnetic orientation of the free layer
and the resulting high or low resistance states across the magnetic
layers controls the state of a typical MRAM cell.
[0002] A type of MRAM cell is a spin torque transfer (STT) cell. A
conventional STT cell includes a magnetic tunnel junction (MTJ)
that functions as a magnetoresistive data storing element with a
pinned magnetic layer and a free magnetic layer, and an insulating
layer between the pinned and the free magnetic layers. An example
of an insulating layer is magnesium oxide (MgO). The STT cell is
coupled between an access device and a data line. The MTJ can be
viewed as a multi-state resistor due to different relative
orientations (e.g., parallel and antiparallel) of the magnetic
moments, which can change the magnitude of a current flowing (e.g.,
passing) through the cell. Magnetic fields caused by currents
flowing through the MTJ can be used to switch a magnetic moment
direction of the free magnetic layer of the MTJ, which can place
the device in a high or low resistance state. The pinned layer
polarizes the electron spin of the programming current, and torque
is created as the spin-polarized current flows through the MTJ. The
spin-polarized electron current interacts with the free layer by
exerting a torque on the free layer. When the spin-polarized
electron current flowing through the MTJ is greater than a critical
switching current density (J.sub.C) for writing the cell, the
torque exerted by the spin-polarized electron current is sufficient
to switch the magnetization of the free layer and thus change the
resistance state across the MTJ. A read process can then be used to
determine the state of cell, using a read pulse that causes a
current to flow through the MTJ that has a sufficiently small
magnitude to not disturb the state of the MTJ.
[0003] STT technology has some advantageous characteristics
compared to other MRAM technology. The STT cell does not need an
external magnetic field to switch the free layer but rather uses
the spin-polarized electron current to switch the free layer.
Further, scalability is improved with STT technology as the
programming current decreases with decreasing cell sizes.
Additionally, STT technology can provide a larger ratio between
high and low resistance states, which improves read operations.
[0004] In writing a STT cell, the amplitude of a programming
signal, such as a current or voltage pulse (referred to herein as a
write pulse), is selected to be high enough to reduce or minimize
the bit error rate (BER). However, this write pulse may cause some
over stress of the tunnel barrier between the free and pinned
magnetic layers of the MTJ. The tunnel barrier may be a tunnel
oxide such as magnesium oxide (MgO). This stress may be
attributable to the array topology, or to process spreads that
reflect variability in the manufacturing process that may affect
the MTJ and/or the access device. This stress may reduce the
endurance of the memory cell as the stress may cause writing and
reading failures after fewer write cycles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Embodiments of the invention are illustrated by way of
example and not limitation in the figures of the accompanying
drawings in which:
[0006] FIG. 1 illustrates a portion of a memory with a STT cell
coupled to an access device.
[0007] FIG. 2 illustrates in more detail an example of a STT
cell.
[0008] FIG. 3 illustrates an example of a portion of a memory array
in which STT cells are arranged in column and rows.
[0009] FIG. 4 illustrates an example of read/write circuitry, such
as may be implemented in the read/write circuitry illustrated FIGS.
1 and 3.
[0010] FIG. 5 illustrates an example of a method for setting a
signal to write a STT cell.
[0011] FIG. 6 illustrates a more detailed example of a method for
setting a signal to write a STT cell.
[0012] FIG. 7 illustrates an example of a ramped-up write pulse
limited by a reference current.
[0013] FIG. 8 illustrates, by way of example, an embodiment of
circuitry used in setting a signal to write a STT cell.
[0014] FIG. 9 illustrates, by way of example, a more specific
embodiment of circuitry used in setting a signal to write a STT
cell.
[0015] FIG. 10 illustrates, by way of example, an embodiment of
circuitry used in setting a signal to write a STT cell that is
similar to the circuitry used in FIG. 8, but that also compensates
for data line capacitance.
[0016] FIG. 11 illustrates, by way of example, a more specific
embodiment of circuitry used in setting a signal to write a STT
cell that also compensates for data line capacitance.
DETAILED DESCRIPTION
[0017] The following detailed description refers to the
accompanying drawings that show, by way of illustration, and not
limitation, various embodiments of the invention. These embodiments
are described in sufficient detail to enable those of ordinary
skill in the art to practice these and other embodiments. Other
embodiments may be utilized, and structural, logical, and
electrical changes may be made to these embodiments. The various
embodiments are not necessarily mutually exclusive, as some
embodiments can be combined with one or more other embodiments to
form new embodiments. The following detailed description is,
therefore, not to be taken in a limiting sense.
[0018] A STT cell can be written by switching the magnetization of
the free layer in the cell's MTJ. Switching occurs when the
programming current density flowing through the memory cell is
greater than the critical switching current density (J.sub.C).
Thus, to write the cell, the programming current density need only
be slightly greater than J.sub.C. An embodiment of the present
subject matter can determine the appropriate programming current
density to use based on a cell current sensed in relation to (e.g.,
during) reading of the cell. This can be used to ensure that the
programming current density is greater than J.sub.C and thus
improves the BER and reliability of the write pulse. This can
further be used to ensure that the programming current density is
not unnecessarily greater than J.sub.C, thereby reducing oxide
interface stress. This programming scheme should be insensitive to
process spread (e.g. variability induced by the manufacturing
process) that impacts the access device.
[0019] A STT cell is conventionally read by applying a
low-amplitude current or voltage pulse (e.g. read pulse) to the
cell. The amplitude is sufficiently low to not disturb the data in
the cell. The voltage or current developed on a data line during
the pulse (V.sub.READ/I.sub.READ) may be sampled to obtain (e.g.,
receive) useful information for a subsequent write cycle. The
subsequent write cycle may be, but is not necessarily, the next
write cycle. The useful information may be, for example, the
resistance of the cell (R.sub.READ) (which is dependent from
cross-sectional pillar area (A) of the cell) or may be dependent on
the resistance of the cell. For example, the current flowing
through a data line coupled to the cell (I.sub.READ) is
proportional to R.sub.READ, which is proportional to the
cross-sectional pillar area (A) of the cell. A critical switching
current (I.sub.SW) value is also dependent from pillar area (A) and
the critical switching current density (J.sub.C)
(I.sub.SW.apprxeq.A*Jc). Thus, I.sub.READ can be used to indirectly
provide information about the critical switching current (I.sub.SW)
value of that cell. By way of example, a subsequent write operation
may apply (e.g., force) a ramp voltage on the selected access line
and the resultant current flowing through a data line coupled to
the cell may be sensed. The data line may be deselected responsive
to (e.g., when) the value of the current flowing through the data
line being greater than the I.sub.SW value. In an example, this can
be implemented by setting a value of a reference current to be a
sum of I.sub.READ and a .DELTA.I value, where the .DELTA.I value is
a function of other information that may be contained in a look-up
table. Examples of such other information include information such
as data to be written, temperature, process corner, power supply
voltages, topological location of the cell (access line and data
line addresses) etc., in order to obtain a suitable reference
current. After determining the reference current based on the
useful information obtained during the read of a STT cell, a
subsequent write operation to the STT cell may cause current flow
through the cell to ramp up until the current flowing through the
data line reaches the reference current, at which point the current
flow through the cell can be stopped.
[0020] FIG. 1 illustrates a portion of a memory 100 with a STT cell
101 coupled to an access device 102. The access device 102 can be,
for example, a transistor, diode or other structure used to select
a STT cell. In an embodiment, the access device 102 can be a
vertical FET. The memory 100 can include an array of STT cells, and
the array can include a data line 103, an access line 104, a source
105, read/write circuitry 106, a reference 107, and a sense
amplifier 108. In operation, the STT cell 101 can be selected to be
written. The read/write circuitry may generate and control
application of a write pulse to a selected STT cell 101. The
current flowing through the cell 101 can be spin-polarized by the
pinned ferromagnetic materials of the cell structure such that a
torque is exerted which can switch the magnetization to write (e.g.
program) the cell 101. To read the STT cell, the read/write
circuitry generates and controls application of a read pulse to the
cell 101. The programmed state of the STT cell depends on the
resistance across the cell 101, which may be determined by the
voltage on the data line 103. In one or more embodiments, the
voltage may be compared to the reference 107 and amplified by a
sense amplifier 108. FIG. 2 illustrates in more detail an example
of a STT cell. The STT cell may include an MTJ 209 with a pinned
layer 210 and a free layer 211 separated by a tunnel barrier 212.
An example of a tunnel barrier is MgO.
[0021] FIG. 3 illustrates an example of a portion of a memory array
in which STT cells 101 are arranged in column and rows. A data line
103 is coupled to each STT cell 101 in a column of cells, and an
access line 104 is coupled to each access device corresponding to
each STT cell 101 in a row of cells. The read/write circuitry 106
may be coupled (e.g., operably connected, whether directly or
indirectly, such as by an ohmic connection) to data line selection
circuitry 313 and to access line selection circuitry 314 to select
a data line and an access line corresponding to a selected STT cell
101 to be written or read. The read/write circuitry 106 may also be
coupled to a source 105 that may be below the STT cells 101. Thus,
the read/write circuitry 106 controls the current that flows
through a selected STT cell 101. A selected STT cell 101 may be
read by causing a read current (I.sub.READ) to flow through a data
line coupled to the selected STT cell 101 and sensing a voltage on
a corresponding data line 103. A sense amplifier 108 may be used to
compare the sensed voltage to a reference 107.
[0022] FIG. 4 illustrates an example of read/write circuitry, such
as may be implemented in the read/write circuitry 106 illustrated
FIGS. 1 and 3. The illustrated read/write circuitry 106 includes
selection control circuitry 415 for controlling the data line
selection circuitry 313 illustrated in FIG. 3 to select a data
line, and for controlling the access line selection circuitry 314
illustrated in FIG. 3 to select an access line. A selected data
line and a selected access line enables the read/write circuitry to
access a selected STT cell for writing or reading. The illustrated
read/write circuitry 106 also includes a pulse generator 416
configured to generate an appropriate read pulse to read a selected
STT cell, and to generate an appropriate write pulse to write a
selected STT cell. The read/write circuitry 106 may further include
sampler circuitry 417 configured to sample current or to sample
voltage during a read pulse when a selected STT cell is read. These
samples may be used by a reference current calculator 418 to
determine (e.g., calculate) a value of a generated reference
current to use when writing the selected STT cell. In some
embodiments, the read/write circuitry 106 includes or otherwise is
capable of accessing a .DELTA.I look-up table 419 to find a value
to be added to a value of I.sub.READ to calculate the generated
reference current value. The .DELTA.I value is a function of other
information such as data to be written. For example, some data may
require a lower BER, which may correspond to larger .DELTA.I value
to provide a larger reference current to be used when writing the
STT cell. Other examples of information that may be used to select
a .DELTA.I value may include temperature, power supply voltages,
access line and data line address, and process corners. A process
corner refers to variations in an integrated circuit as applied to
a semiconductor wafer. Process corners represent the extremes of
these parameter variations within which a circuit that has been
formed on or in the wafer must function correctly.
[0023] FIG. 5 illustrates an example of a method for setting a
signal in variable resistance memory, such as a signal used to
write a STT cell. The illustrated method sets a reference current
used when writing a cell based on a parameter sensed during a
previous read of the cell. The reference current can be set to
limit a programming current flowing through the cell to a magnitude
that is effective for that cell, thus reducing BER, while also
avoiding an excessively large magnitude that could contribute to
degradation of the tunnel barrier. At 520, the memory cell is read
to obtain cell-specific information. For example, an I.sub.READ may
be sampled when a voltage is applied across the memory cell. At 521
the memory cell may be written using a write pulse limited in
accordance with cell-specific information obtained during the
previous read. For example, if the cell-specific information
obtained during a read process is I.sub.READ, then the reference
current used to limit the write pulse may be set as a function of
I.sub.READ or otherwise based on I.sub.READ.
[0024] FIG. 6 illustrates a more detailed example of a method for
setting a signal in a variable resistance memory. At 622 the
voltage across the cell or the current flowing through a data
coupled to the cell (I.sub.READ) is sampled during the read
process. For example, if the read process involves placing a read
voltage across the cell to measure a resulting current (I.sub.READ)
to measure a resistive state of the cell, then I.sub.READ may be
sampled. If the read process involves delivering a read current
through the cell to measure a resulting voltage across the cell to
measure a resistive state of the cell, then the read voltage
(V.sub.READ) may be sampled. I.sub.READ is used in this discussion
as an example, and the reference current may be set as a function
of I.sub.READ or otherwise based on I.sub.READ. Some embodiments
use additional information to set the reference current. At 623 a
.DELTA.I value may be looked up, where the .DELTA.I value is based
on this additional information. The .DELTA.I value may be looked up
before, after, or contemporaneously with the sampling 622. A
generated reference current value may be calculated at 624 as the
sum of I.sub.READ and the .DELTA.I value. The STT cell may then be
written using the reference current as illustrated at 625. The
write pulse that programs the cell may be implemented as a
ramped-up current pulse, where the ramped current pulse is limited
by the reference current, such as is generally illustrated by the
ramped pulse signal in FIG. 7.
[0025] FIG. 8 illustrates, by way of example, an embodiment of
circuitry used in setting a reference current used to write the STT
cell. FIG. 8 illustrates a data line 103, an access line 104 and a
MTJ 209. FIG. 8 further illustrates a regulator 826 for providing a
regulated potential on line 827, an access device 102 that
functions as a switch as labeled in FIG. 8, and a data line
deselect switch 828 (which can correspond to the data line
selection circuitry 313 of FIG. 3). If the data line is selected,
such that both 102 and 828 conduct, current can flow from line 827
through the MTJ to the source 105. This current flowing through the
variable resistance memory cell 209 may be referred to as cell
current (I.sub.CELL). A ramped potential may be applied on the
access line 104 to cause the I.sub.CELL through the cell 209 to be
a ramped current pulse. The I.sub.CELL can be sensed as the
potential on the access line ramps up. The voltage on the data line
may be common for all data lines coupled to cells that will be
programmed, but the ramp-up of I.sub.CELL for a particular cell can
be limited to the reference current specifically set for that cell.
For example, FIG. 8 also illustrates current sources, including a
generated reference current source 829 to provide the generated
reference current, a first mirrored current source 830, and a
second mirrored current source 831. A comparator 832 is configured
to compare the mirrored reference current and the mirrored
I.sub.CELL from the mirrored current sources 830 and 831,
respectively, and to cause the data line 103 to be deselected via
switch 828 when I.sub.CELL is greater than or equal to the
reference current. The deselected data line 103 limits the ramped
I.sub.CELL pulse to the reference current, as generally illustrated
by the ramped pulse signal in FIG. 7.
[0026] FIG. 9 illustrates, by way of example, a more specific
embodiment of circuitry used in setting a reference current used to
write a STT cell. FIG. 9 illustrates a data line 103, and access
line 104 and a MTJ 209. FIG. 9 further illustrates a regulator 826
for providing a regulated potential on line 827, an access device
102, and a source 105. Transistor 928 functions as the deselect
switch 828 in FIG. 8. Transistors 931A and 931B function to provide
the mirrored I.sub.CELL current source 831 in FIG. 8, and
transistors 930A and 930B function to provide the mirrored
reference current source 830 in FIG. 8. Transistors 932A-D and a
logic gate 933 can function together to provide the comparator 832
illustrated in FIG. 8 to cause the data line 103 to be deselected
via transistor 928 when I.sub.CELL is greater than or equal to the
reference current. The cross gate connection of transistors 932B
and 932C increases the gain. Initially, the reference current is
greater than I.sub.CELL, such that node 934 will be low. When
I.sub.CELL increases, switching takes place when the drain-source
voltage of transistor 932B is equal to the threshold voltage of
transistor 932C. Transistor 932C begins to take current away from
transistor 932D, decreasing the drain-source voltage of transistor
932D, which starts to turn transistor 932B off. Transistor 932B
shuts off and transistor 932C turns on, such that node 934 is high.
The data line address is high when the data line is selected. When
node 934 is also high, the logic gate 933 has an output to turn off
transistor 928. If transistor 928 is an NFET, then a NAND type
logic gate may be used to provide a low potential to turn off the
NFET when both the data line address and node 934 are high.
[0027] Some embodiments may compensate for the parasitic
capacitance present on the data line. The charge stored by the
parasitic capacitance can cause the actual current flowing through
the data line to vary slightly from the nominal I.sub.CELL. Some
embodiments determine the contribution to the current from the
parasitic capacitance and provide a corresponding current offset to
the generated reference current to compensate for the parasitic
capacitance.
[0028] FIG. 10 illustrates, by way of example, an embodiment of
circuitry used in setting a reference current used to write a STT
cell that is similar to the circuitry used in FIG. 8, but that also
compensates for data line capacitance. Similar to FIG. 8, FIG. 10
illustrates a data line 103, an access line 104, a MTJ 209, a
regulator 826 for providing a regulated potential on line 827, an
access device 102 that functions as a switch as labeled in FIG. 10,
and a data line deselect switch 828. If the data line is selected,
such that both 102 and 828 conduct, current can flow from line 827
through the MTJ to the source 105. This current through the MTJ may
be referred to as the cell current, I.sub.CELL. A ramped potential
may be applied on the access line 104 to cause I.sub.CELL to be a
ramped pulse. The current on the data line can be sensed as the
potential on the access line ramps up. The write pulse (e.g.,
voltage) may be common for all data lines corresponding to cells to
be written, but the ramp-up of I.sub.CELL through each cell can be
limited to the reference current set for that cell. Similar to FIG.
8, FIG. 10 also illustrates current sources, including a reference
current source 829 to provide a generated reference current, a
first mirrored current source 830, and a second mirrored current
source 831, and further illustrates a comparator 832 configured to
compare the mirrored reference current and the mirrored I.sub.CELL
from the mirrored current sources 830 and 831, and to deselect the
data line 103 via a deselect switch 828 when I.sub.CELL is greater
than or equal to the reference current. In addition to the
circuitry illustrated in FIG. 8, FIG. 10 also illustrates a dummy
data line 1044 that mimics the data line 103 with a dummy access
device 1045 and a dummy deselect switch 1046, but without a STT
cell. An I.sub.Dummy current caused by parasitic capacitance may
flow on the dummy data line 1044, and a mirrored I.sub.Dummy
current source 1047 may add this current the generated reference
current, such that the mirrored reference current is adjusted by
the mirrored I.sub.Dummy current. As a result the ramped I.sub.CELL
current through the MTJ can be limited by the sum of the generated
reference current and the I.sub.Dummy current.
[0029] FIG. 11 illustrates, by way of example, a more specific
embodiment of circuitry used in setting a reference current for the
STT cell that also compensates for data line capacitance. FIG. 11
illustrates, by way of example, an embodiment of circuitry used in
setting switching current for the STT cell that is similar to the
circuitry used in FIG. 9, but that also compensates for data line
capacitance. In addition to the circuitry illustrated in FIG. 9,
FIG. 11 also illustrates a dummy data line 1044 with a dummy access
device 1045 and a dummy deselect switch 1046, but without a STT
cell. Parasitic capacitance 1148 may cause an I.sub.Dummy current
to flow on the dummy data line 1044 when the access line 104 is
first selected. The dummy data line 1044 may be selected to allow
the regulated potential to charge the parasitic capacitance, and
then deselected. Subsequently, when the access line 104 is
selected, then the charge associated with the parasitic capacitance
can flow on the dummy data line 1044. Transistors 1149A and 1149B
function to provide the mirrored I.sub.Dummy current source 1047
illustrated in FIG. 10.
[0030] Some advantages of the above-described STT technology is
that it provides a self-referencing programming pulse which can be
effectively high to reduce BER and which can improve reliability by
avoiding excessively high programming pulses, thus improving the
ability to better withstand numerous write cycles. Further, the
above-described STT technology that use a self-referencing
programming pulse is insensitive to process spread reflected by
variations in the manufacturing process that may affect access
devices.
[0031] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement that is calculated to achieve the
same purpose may be substituted for the specific embodiments shown.
Various embodiments use permutations and/or combinations of
embodiments described herein. It is to be understood that the above
description is intended to be illustrative, and not restrictive,
and that the phraseology or terminology employed herein is for the
purpose of description.
* * * * *