U.S. patent application number 15/010222 was filed with the patent office on 2016-09-15 for compact reram based fpga.
This patent application is currently assigned to Microsemi SoC Corporation. The applicant listed for this patent is Microsemi SoC Corporation. Invention is credited to Fethi Dhaoui, John L. McCollum.
Application Number | 20160269031 15/010222 |
Document ID | / |
Family ID | 55358142 |
Filed Date | 2016-09-15 |
United States Patent
Application |
20160269031 |
Kind Code |
A1 |
McCollum; John L. ; et
al. |
September 15, 2016 |
COMPACT ReRAM BASED FPGA
Abstract
A push-pull resistive random access memory cell circuit includes
an output node, a word line, a first bit line, and a second bit
line. A first resistive random access memory device is connected
between the first bit line and the output node and a second
resistive random access memory device is connected between the
output node and the second bit line. A first programming transistor
has a gate connected to the word line, a drain connected to the
output node, and a source. A second programming transistor has a
gate connected to the word line, a drain connected to the source of
the first programming transistor, and a source. The first and
second programming transistors have the same pitch, the same
channel length, and the same gate dielectric thickness, the gate
dielectric thickness chosen to withstand programming and erase
potentials encountered during operation of the push-pull ReRAM cell
circuit.
Inventors: |
McCollum; John L.; (Orem,
UT) ; Dhaoui; Fethi; (Mountain House, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Microsemi SoC Corporation |
San Jose |
CA |
US |
|
|
Assignee: |
Microsemi SoC Corporation
San Jose
CA
|
Family ID: |
55358142 |
Appl. No.: |
15/010222 |
Filed: |
January 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62132333 |
Mar 12, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 2213/82 20130101;
G11C 2213/78 20130101; G11C 2213/79 20130101; G11C 2213/74
20130101; H01L 45/16 20130101; G11C 13/0069 20130101; H01L 21/768
20130101; H03K 19/1776 20130101; H01L 27/2454 20130101; G11C 13/003
20130101 |
International
Class: |
H03K 19/177 20060101
H03K019/177; G11C 13/00 20060101 G11C013/00 |
Claims
1. A push-pull resistive random access memory cell circuit
comprising: an output node; a word line; a first bit line; a second
bit line; a first resistive random access memory device connected
between the first bit line and the output node; a second resistive
random access memory device connected between the output node and
the second bit line; a first programming transistor having a gate
connected to the word line, a drain connected to the output node,
and a source; and a second programming transistor having a gate
connected to the word line, a drain connected to the source of the
first programming transistor, and a source, wherein the first and
second programming transistors have the same pitch, the same
channel length, and the same gate dielectric thickness, the gate
dielectric thickness of the first and second programming
transistors chosen to withstand programming and erase potentials
encountered during operation of the push-pull ReRAM cell
circuit.
2. The push-pull resistive random access memory cell circuit of
claim 1 wherein: the push-pull resistive random access memory cell
circuit is fabricated on an integrated circuit having input/output
transistors; and the thickness of the gate dielectric of the first
and second programming transistors is the same as the thickness of
the gate dielectric of the input/output transistors.
3. The push-pull resistive random access memory cell circuit of
claim 2, further comprising: at least one switch transistor having
a gate connected to the output node, a drain connected to a first
logic net node and a source connected to a second logic net node;
and wherein the switch transistor has the same pitch, channel
length, and gate dielectric thickness as the first and second
programming transistors.
4. The push-pull resistive random access memory cell circuit of
claim 1 wherein: the push-pull resistive random access memory cell
circuit is fabricated on an integrated circuit having logic
transistors; and the thickness of the gate dielectric of the first
and second programming transistors is larger than the thickness of
the gate dielectric of the logic transistors.
5. The push-pull resistive random access memory cell circuit of
claim 4, further comprising: at least one switch transistor having
a gate connected to the output node, a drain connected to a first
logic net node and a source connected to a second logic net node;
and wherein the switch transistor has the same pitch, channel
length, and gate dielectric thickness as the first and second
programming transistors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 62/132,333 filed Mar. 12, 2015, the contents
of which are incorporated in this disclosure by reference in its
entirety.
BACKGROUND
[0002] Push-pull resistive random access memory (ReRAM) cells, such
as the ones disclosed in U.S. Pat. No. 8,415,650, are attractive
for use in configuration memory for configurable logic integrated
circuits such as field programmable gate arrays (FPGAs).
[0003] When designing circuits using deep sub-micron (14 nM and
beyond) transistors, any change in transistor pitch forces
designers to use a large transition region to allow
photolithographic production of the pattern. This transition region
can range from 0.2 um to 1 um or more and can be a significant
disadvantage in designing a configurable logic integrated circuit
employing ReRAM push-pull configuration memory cell circuits that
has a compact efficient layout.
[0004] An FPGA requires logic, routing switches, and programming
transistors to be intermingled. To eliminate the transition region
which is required by photolithography processing requirements, all
the above listed devices must have the same pitch, including
channel length pitch. Normally this requirement is not compatible
with devices that are operated at different voltages.
[0005] For ReRAM memory cells, the transistor devices used to
program them will be subjected to higher drain and gate biases, and
will switch at a higher gate bias during programming and operation
as compared to other transistors employed in the integrated
circuit.
[0006] Therefore, there is a need for a design for ReRAM
configuration memory cells which is not associated with these
disadvantages. An objective of the present invention is to provide
ReRAM push-pull configuration memory cell circuits that eliminate
this transition region.
SUMMARY
[0007] According to the present invention, a push-pull ReRAM cell
circuit employs two programming transistors that are cascaded in
series and have the same pitch and channel length. A switch
transistor used in the push-pull ReRAM cell circuit has the same
pitch and channel length as the two programming transistors in
order to maintain the same pitch and channel length for both the
programming devices and the switch transistors that are used to
configure and/or interconnect the logic cells.
[0008] According to the present invention, the switch transistor
whose state is configured by the ReRAM will use the same thick
dielectric that is used in the programming transistors to mitigate
elevated gate stress during programming. Use of a thicker
dielectric also allows the gate of the configuration switch to be
overdriven at higher V.sub.CC during operation, thus allowing the
passage of the full V.sub.CC logic signal.
[0009] According to one aspect of the present invention, a
push-pull resistive random access memory cell circuit includes an
output node, a word line, a first bit line, and a second bit line.
A first resistive random access memory device is connected between
the first bit line and the output node and a second resistive
random access memory device is connected between the output node
and the second bit line. A first programming transistor has a gate
connected to the word line, a drain connected to the output node,
and a source. A second programming transistor has a gate connected
to the word line, a drain connected to the source of the first
programming transistor, and a source. The first and second
programming transistors have the same pitch, the same channel
length, and the same gate dielectric thickness, the gate dielectric
thickness chosen to withstand programming and erase potentials
encountered during operation of the push-pull ReRAM cell
circuit.
[0010] According to another aspect of the present invention, at
least one switch transistor has a gate connected to the output
node, a drain connected to a first logic net node and a source
connected to a second logic net node. The switch transistor has the
same pitch, channel length, and gate dielectric thickness as the
first and second programming transistors.
DRAWINGS
[0011] These and other features, aspects and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0012] FIG. 1 is a schematic diagram of a push-pull ReRAM cell in
accordance with one aspect of the present invention;
[0013] FIG. 2 is a cross-sectional view of an exemplary layout of
the push-pull ReRAM cell of the present invention;
[0014] FIG. 3 is a top view of an exemplary layout of the push-pull
ReRAM cell of the present invention.
DESCRIPTION
[0015] Persons of ordinary skill in the art will realize that the
following description of the present invention is illustrative only
and not in any way limiting. Other embodiments of the invention
will readily suggest themselves to such skilled persons.
[0016] Referring first to FIG. 1, a schematic diagram shows an
illustrative push-pull ReRAM cell circuit 10 in accordance with one
aspect of the present invention. A first ReRAM device 12 is coupled
in series with a second ReRAM device 14 to form a ReRAM cell 16. A
first end of the series connected ReRAM devices 12, 14, at one
terminal of ReRAM device 12, is coupled to a first bit line (BL) 18
and a second end of the series connected ReRAM devices 12, 14, at
one terminal of ReRAM device 14, is coupled to a second bit line
(B1_bar) 20. The ReRAM cell 16 depicted in FIG. 1 is a
front-to-back ReRAM cell, particularly useful for biasing a switch
as shown, but persons of ordinary skill in the art will appreciate
that back-to-back ReRAM cells could also be employed in the present
invention.
[0017] As described above, ReRAM devices 12 and 14 together
comprise a push-pull ReRAM cell 16. The common output node 22
between ReRAM devices 12 and 14 is connected to the gate of one or
more switch transistors. FIG. 1 shows the common output node 22
connected to the gates of two switch transistors 24a and 24b.
Switch transistor 24a is shown connected between two logic net
nodes 26a and 28a. Similarly, switch transistor 24b is shown
connected between two logic net nodes 26b and 28b. Persons of
ordinary skill in the art will appreciate that logic net nodes 26a,
26b, 28a, and 28b can represent logic gates or other devices in a
programmable integrated circuit that are connected together by the
switch transistors 24a and 24b, respectively, and can also
represent circuit nets in a single logic device in such an
integrated circuit that define the function of the logic device, or
can represent a wiring interconnection in a programmable integrated
circuit.
[0018] While FIG. 1 shows multiple switch transistors 24a and 24b
so that more than one logic circuit net can be activated by a
single push-pull ReRAM cell 16, persons of ordinary skill in the
art will appreciate that a single switch transistor could be
connected to the common output node 22.
[0019] According to one aspect of the present invention, push-pull
ReRAM cell 16 is programmed using a pair of n-channel programming
transistors 30 and 32 cascaded in series. N-channel programming
transistor 30 has its drain connected to the common output node 22
of the push-pull ReRAM cell 16, and its source connected to the
drain of the n-channel programming transistor 32. In an actual
embodiment, a single n+ region serves as the source of n-channel
programming transistor 30 and the drain of n-channel programming
transistor 32. The source of n-channel programming transistor 32 is
connected to a word line WLS. By connecting two n-channel
programming transistors 30 and 32 in series, both n-channel
programming transistors 30 and 32 can be designed having the same
pitch and channel length as the n-channel switch transistors 24a
and 24b. The same pitch and channel length used for switch
transistors 24a and 24b are used for the logic devices in the
integrated circuit.
[0020] According to another aspect of the present invention,
n-channel programming transistors 30 and 32 and n-channel switch
transistors 24a and 24b are fabricated having the same gate
dielectric thicknesses. N-channel programming transistors 30 and 32
have gate dielectric thicknesses selected to withstand the
programming and erase potentials that the ReRAM push-pull memory
cell will be subjected to during its operation. Most integrated
circuits include input/output (I/O) transistors used to interface
the integrated circuit with external components. Because these
transistors interface with components that often operate at
voltages higher than the voltages normally found internally in the
integrated circuit, the I/O transistors are usually fabricated
having gate dielectric thicknesses larger than other transistors
used internally in the integrated circuit. It may therefore be
convenient to employ n-channel programming transistors 30 and 32
having the same gate dielectric thicknesses as the I/O
transistors.
[0021] Using the same larger gate dielectric thickness for the
transistors 24a and 24b will mitigate elevated gate stress that
switch transistors 24a and 24b would otherwise be subjected to
during programming, because its gates are connected to the common
node 22 of the push-pull ReRAM memory cell 16, and that node will
experience programming voltages during erase and programming of the
memory cell 16. Use of a thicker dielectric for switch transistors
24a and 24b also allows the gates of the switch transistors 24a and
24b to be overdriven at higher values of V.sub.CC during operation,
thus allowing the switch transistors 24a and 24b to pass the full
V.sub.CC logic signal. As an alternative, thin gate oxides can be
used for the switch transistors 24a and 24b, but care should be
taken to avoid stress during programming and erase operations. This
may be done by raising the source/drain bias to V.sub.CC logic
during programming.
[0022] In normal operation of the programmable integrated circuit,
bit line BL 18 is connected to a voltage source V.sub.CC and BL_bar
20 is connected to a potential such as ground. The WLS line can be
connected to ground or to a slightly positive potential such as
0.9V to limit leakage in the n-channel programming transistors 30
and 32. Push-pull ReRAM cell 16 is programmed so that only one of
the ReRAM devices 12 and 14 is on at any one time, thus either
pulling common node 22 up to the voltage on bitline BL 18 or
pulling common node 22 down to the voltage on bitline BL_bar 20
(usually ground). Push-pull ReRAM cell 16 is shown in FIG. 1 with
ReRAM device 12 turned on and ReRAM device 14 turned off. The
common node 22 is thus pulled up to the voltage on bit line BL 18
(V.sub.CC), thus turning on switch transistors 24a and 24b (shown
in FIG. 1 as n-channel transistors).
[0023] Referring now to both FIG. 2 and FIG. 3, an exemplary layout
of the push-pull ReRAM cell of the present invention is shown. FIG.
2 is a cross-sectional view of an exemplary layout 40 of the
push-pull ReRAM cell circuit 10 of the present invention. FIG. 3 is
a top view of an exemplary layout 40 of the push-pull ReRAM cell
circuit 10 of the present invention. Persons of ordinary skill in
the art will observe that the layouts shown in FIGS. 2 and 3 are
illustrative only and non-limiting.
[0024] Push-pull ReRAM cell circuit 10 (FIG. 1) is formed in a
p-type substrate or well 42 in an integrated circuit. N+ region 44
forms the drain of n-channel programming transistor 30 and n+
region 46 forms its source, as well as acting as the drain of
n-channel programming transistor 32. Polysilicon or metal line 48
forms the gate of n-channel programming transistor 30. N+ region 50
forms the source of n-channel programming transistor 32 and
polysilicon or metal line 52 forms its gate. Contacts 54 connect
the polysilicon gates 48 and 52 of n-channel programming
transistors 30 and 32 to word line 34, shown as being formed from a
first metal interconnect layer (M1). Persons of ordinary skill in
the art will appreciate that p-channel transistors may also be used
in other embodiments of the invention.
[0025] The switch transistor shown in FIG. 3 includes source region
56 and drain region 58, separated by gate 60. It is noted that the
cross sectional view of FIG. 2 is partially taken through the
source region of one of switch transistors 24a and 24b. The switch
transistors may be n-channel or p-channel devices.
[0026] ReRAM device 12 is formed between metal interconnect layers
on the integrated circuit (for example between first and second
metal layers M1 and M2). In FIGS. 2 and 3, ReRAM device 12 is shown
formed between M1 metal segment 62 and M2 metal segment 64. ReRAM
device 12 is formed over metal segment 62 and is connected to M2
metal segment 64 through contact 66. ReRAM device 12 is connected
to bitline BL 18 through contact 68.
[0027] Contact to common node 22 is made from M2 segment 64 to an
M1 metal segment 70 through contact 72. A contact 74 connects M1
metal segment 70 to polysilicon gate 60 of the switch transistor.
ReRAM device 14 is shown formed between M1 metal segment 70 and the
M2 metal segment forming second bitline B1_bar 20. A contact 76
connects M1 ReRAM device 14 to second bitline B1_bar 20. A contact
78 connects M1 metal segment 70 to the n+ region 44 forming the
drain of n-channel programming transistor 30. A metal segment 80
forms the word line WLS and is connected to n+ region 50 forming
the source of programming transistor 32 through contact 82.
[0028] Push-pull ReRAM cell 16 is programmed by turning on the
desired one of ReRAM devices 12 and 14 so as to either turn off, or
turn on, switch transistors 24a and 24b. First, both ReRAM devices
12 and 14 are erased. To erase a ReRAM device means to turn it off
so that it does not pass current. To erase ReRAM device 12, bitline
BL 18 is brought to a high voltage (e.g., 1.8V) and common node 22
is brought to ground. To avoid stressing ReRAM device 14, second
bit line B1_bar 20 is also brought to ground so that there is no
potential impressed across ReRAM device 14. To erase ReRAM device
14, common node 22 is brought to a high voltage (e.g., 1.8V) and
second bit line B1_bar 20 is brought to ground. To avoid stressing
ReRAM device 12, bitline BL 18 is also brought to the high voltage
so that there is no potential impressed across ReRAM device 12.
[0029] After both ReRAM cells 12 and 14 have been erased, the
selected one of the two ReRAM devices 12 and 14 is programmed. To
program ReRAM device 12, bitline BL 18 is brought to ground and
common node 22 is brought to a high voltage (e.g., 1.8V). To avoid
stressing ReRAM device 14 while programming ReRAM device 12, second
bit line B1_bar 20 is also brought to the high voltage so that
there is no potential impressed across ReRAM device 14. To program
ReRAM device 14, common node 22 is brought to ground and second bit
line B1_bar 20 is brought to the high voltage. To avoid stressing
ReRAM device 12, bitline BL 18 is also brought to ground so that
there is no potential impressed across ReRAM device 12.
[0030] According to another aspect of the present invention, two
series connected n-channel programming transistors 30 and 32 are
coupled between common node 22 and word line WLS. The gates of
n-channel transistors 30 and 32 are connected together to word line
WL 34.
[0031] While the present disclosure is directed to the application
of a ReRAM memory device where the logic is switching at a first
voltage and the programming and erasing of the ReRAM cell is
performed at a second voltage, persons of ordinary skill in the art
will appreciate that it is also applicable to other devices where
it is desirable to switch two different voltages in different
operating modes.
[0032] Although the present invention has been discussed in
considerable detail with reference to certain preferred
embodiments, other embodiments are possible. Therefore, the scope
of the appended claims should not be limited to the description of
preferred embodiments contained in this disclosure.
* * * * *