# Patent

## Title
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.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
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
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).
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.
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.
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.
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
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.
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 VCC during operation, thus allowing the passage of the full VCC logic signal.
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.
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
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:
FIG. 1 is a schematic diagram of a push-pull ReRAM cell in accordance with one aspect of the present invention;
FIG. 2 is a cross-sectional view of an exemplary layout of the push-pull ReRAM cell of the present invention;
FIG. 3 is a top view of an exemplary layout of the push-pull ReRAM cell of the present invention.



DESCRIPTION
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.
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 (Bl_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.
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.
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.
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.
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.
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 VCC during operation, thus allowing the switch transistors 24a and 24b to pass the full VCC 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 VCC logic during programming.
In normal operation of the programmable integrated circuit, bit line BL 18 is connected to a voltage source VCC 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 (VCC), thus turning on switch transistors 24a and 24b (shown in FIG. 1 as n-channel transistors).
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.
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.
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.
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.
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 Bl_bar 20. A contact 76 connects M1 ReRAM device 14 to second bitline Bl_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.
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 Bl_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 Bl_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.
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 Bl_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 Bl_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.
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.
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.
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.

## 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.